**Table 1.** *Polymeric optical sensing materials based on benzimidazole derivatives.*

**163**

**Figure 1.**

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives*

can also be easily modified by tuning the ICT character of fluorescent molecules, which is often achieved by introducing electron donating groups (e.g. N,N-diethyl amino) and strong electron withdrawing moieties (e.g. -CN and –NO2) on the

As an alternative to dye-impregnation of polymers, fluorescent molecules can be covalently attached to polymeric materials, as demonstrated, for example, in a fluorescence solid sensor for the mercury detection based on a photocrosslinked membrane functionalised with (benzimidazolyl)methyl-piperazine derivative of 1,8-naphthalimide [22]. Benzimidazole, linked to a piperazine moiety by a methylene spacer, is responsible for the specific recognition of Hg2+ ions by forming a stable complex structure, that resulted in a strong fluorescence (**Figure 1B**). Materials developed by the covalent attachment of the sensing molecules usually have more advantages that those utilising physical entrapment, in which active molecules may easily leach out of the matrix. Stability and duration of covalently functionalised polymer materials are much better, and they even often provide

Another approach to obtain fluorescent sensing materials is a clever design and synthesis of novel luminescent polymers. For instance, conjugated polymers are the constant trend in the development of novel functional materials [23, 24]. They effectively coordinate with many organic compounds or transition metals, which is very well conjoined with their excellent optical properties and exploited in optical chemical sensors. Detection methods are mostly relying on the fluorescence techniques, particularly the quenching effect ('superquenching') described by Stern-Volmer relationships. Fluorescent conjugated polymers also offer many advantages in regard to simple organic fluorophores, such as amplified sensitivity and the possibility of simple introduction of desired functional groups in order to achieve better interactions with the analyte. Benzimidazole is often found as a constituent of conjugated polymers [25–28]. Optical sensing ability of benzimidazole-based fluorescent polymers is demonstrated for the detection of pH [29, 30], metal ions [28] or inorganic anions [27], where benzimidazole moiety often plays a crucial

> O N O O O

**Reversible sensor weak**

PET N N <sup>N</sup> HN

**fluorescence**

*(A) Fluorescent pH-sensitive bulk optodes based on immobilised Schiff base derivatives in plasticised PVC matrix. Tuneable fluorescent response of the optodes is a result of different substituents on the benzimidazole moiety. Reprinted from [21]. Copyright (2018), with permission from Elsevier. (B) Selective fluorescence solid sensor for Hg2+ based on N-(2-hydroxyethyl)-4-(4-(1Hbenzo[d]imidazol-2-yl)methyl) piperazine-1-yl)-1,8 naphthalimide, here presented by the author's courtesy. Fluorescence sensor undergoes fluorescence enhancement upon binding mercuric ion due to the inhibition of photo-induced electron transfer (PET) process from the* 

**strong fluorescence**

Hg

O N O O O

PET <sup>N</sup> N <sup>N</sup> HN

**+**

**Hg2+ detection**

*DOI: http://dx.doi.org/10.5772/intechopen.85643*

opposite parts of molecular system (**Figure 1A**).

**2.2 Covalently attached benzimidazole derivatives**

improved analytical parameters of chemical sensor.

A B

*piperazine to the naphthalimide moiety [22].*

**2.3 Luminescent polymers**

#### *Chemistry and Applications of Benzimidazole and its Derivatives*

can also be easily modified by tuning the ICT character of fluorescent molecules, which is often achieved by introducing electron donating groups (e.g. N,N-diethyl amino) and strong electron withdrawing moieties (e.g. -CN and –NO2) on the opposite parts of molecular system (**Figure 1A**).

#### **2.2 Covalently attached benzimidazole derivatives**

As an alternative to dye-impregnation of polymers, fluorescent molecules can be covalently attached to polymeric materials, as demonstrated, for example, in a fluorescence solid sensor for the mercury detection based on a photocrosslinked membrane functionalised with (benzimidazolyl)methyl-piperazine derivative of 1,8-naphthalimide [22]. Benzimidazole, linked to a piperazine moiety by a methylene spacer, is responsible for the specific recognition of Hg2+ ions by forming a stable complex structure, that resulted in a strong fluorescence (**Figure 1B**). Materials developed by the covalent attachment of the sensing molecules usually have more advantages that those utilising physical entrapment, in which active molecules may easily leach out of the matrix. Stability and duration of covalently functionalised polymer materials are much better, and they even often provide improved analytical parameters of chemical sensor.

#### **2.3 Luminescent polymers**

*Chemistry and Applications of Benzimidazole and its Derivatives*

**162**

**Material**

PVC PVC PVC PVC Photocrosslinked

membrane

Amphiphilic copolymer

Hydrophilic copolymer

Conjugated polymer

Conjugated polymer

Coordination polymer

Metal organic framework

Metal organic framework

Coordination polymer

Coordination polymer

Coordination polymer

Coordination polymer

Coordination polymer

Zr-UiO-66 nanocrystals

pH Fe3+ Fe3+ Fe3+

Cr

O2 7 Fe3+ ion and

nitroaromatics

Multi-analyte

2−

Humidity and

formaldehyde

pH pH Cu2+ Fe3+ and PO4

Fe3+

3−

**Analyte**

Hg2+

Ag+ pH pH Hg2+

**BI-based sensing molecule**

Crown-based ionophore

Crown-based ionophore

Acrylonitrile derivative

Schiff bases 1,8-naphthalimide derivative

Vinyl monomer

Pyridyl substituted benzimidazole derivative

Pendant benzimidazolyl moieties

Pendant benzimidazolyl moieties

Bis(benzimidazole) derivative

Benzimidazolyl-attached bent organic ligand

1,6-Bis(benzimidazol-1-yl)hexane ligand

Benzimidazole ligand

Benzimidazole-appended tripodal tridentate

Fluorimetric

—

Tripathi et al. [37]

ligand

Benzimidazole-functionalized organic ligand

Benzimidazole-functionalized organic ligand

Benzimidazole-functionalized organic ligand

Benzimidazole-functionalized organic ligand

Fluorimetric

Fluorimetric

Phosphorescence

Fluorimetric

—

2.53 × 10−6 2.72 × 10−5

—

Yang et al. [38]

Zhao et al. [39]

Wei et al. [40]

Dong et al. [34]

Fluorimetric Fluorimetric

2.16 × 10−6 3.70 × 10−7

Li et al. [31]

Zhou et al. [33]

Colorimetric

Fluorimetric

Fluorimetric

Fluorimetric Fluorimetric

Fluorimetric

—

—

—

3.38 × 10−6

3.2 × 10−6

—

**Detection** 

**Limit of detection** 

**Ref.**

**(mol L−1**

3.5 × 10−13 2.8 × 10−12

—

—

2.5 × 10 −6

Firooz et al. [18]

Firooz et al. [19]

Horak et al. [20]

Horak et al. [21]

Fernández-Alonso et al.

[22]

Han et al. [29]

Shen et al. [30]

Wu et al. [28]

Saikia *et al* (2011) [27]

Hao et al. [36]

Yu et al. (2014) [32]

**)**

**method**

Fluorimetric Fluorimetric Colorimetric

Fluorimetric

Fluorimetric

> Another approach to obtain fluorescent sensing materials is a clever design and synthesis of novel luminescent polymers. For instance, conjugated polymers are the constant trend in the development of novel functional materials [23, 24]. They effectively coordinate with many organic compounds or transition metals, which is very well conjoined with their excellent optical properties and exploited in optical chemical sensors. Detection methods are mostly relying on the fluorescence techniques, particularly the quenching effect ('superquenching') described by Stern-Volmer relationships. Fluorescent conjugated polymers also offer many advantages in regard to simple organic fluorophores, such as amplified sensitivity and the possibility of simple introduction of desired functional groups in order to achieve better interactions with the analyte. Benzimidazole is often found as a constituent of conjugated polymers [25–28]. Optical sensing ability of benzimidazole-based fluorescent polymers is demonstrated for the detection of pH [29, 30], metal ions [28] or inorganic anions [27], where benzimidazole moiety often plays a crucial

#### **Figure 1.**

*(A) Fluorescent pH-sensitive bulk optodes based on immobilised Schiff base derivatives in plasticised PVC matrix. Tuneable fluorescent response of the optodes is a result of different substituents on the benzimidazole moiety. Reprinted from [21]. Copyright (2018), with permission from Elsevier. (B) Selective fluorescence solid sensor for Hg2+ based on N-(2-hydroxyethyl)-4-(4-(1Hbenzo[d]imidazol-2-yl)methyl) piperazine-1-yl)-1,8 naphthalimide, here presented by the author's courtesy. Fluorescence sensor undergoes fluorescence enhancement upon binding mercuric ion due to the inhibition of photo-induced electron transfer (PET) process from the piperazine to the naphthalimide moiety [22].*

**Table 1.**

*Polymeric optical sensing materials based on benzimidazole derivatives.*

role in the sensing mechanism. For example, a copolymer built from *N*-(1-ethyl-2-(pyridin-4-yl)-1*H*benzo[*d*]imidazol-5-yl)methacrylamide and 2-hydroxyethyl methacrylate exhibits a pH sensitivity due to acid-base equilibria on the heteroatom of pyridyl-substituted benzimidazole moiety [30] (**Figure 2A**).

Besides conjugated polymers, benzimidazole-based materials can be developed as luminescent metal organic frameworks (MOFs) [31–35] or coordination polymers [36–42]. Such advanced functional materials have been extensively applied in the field of luminescence sensing due to their diverse structural characteristics and tunable pore sizes. For example, luminescence sensing of iron is achieved by a coordination polymer employing the linear 2,5-dichloroterephthalic acid ligand and the flexible bis(benzimidazole) derivatives. Ligand affords the capacity to strongly bind metal atoms, while bis(benzimidazole) derivatives can freely twist around two methylene -CH2 groups with disparate angles to generate different conformations [36]. Luminescent MOFs have been exploited for the development of sensing materials for humidity and formaldehyde, such as a porous Cu(I)-MOF, constructed from CuI and 1-benzimidazolyl-3,5-bis(4-pyridyl)benzene (**Figure 3**) [32]. 3D cadmium metal-organic framework was demonstrated as sensing material for the detection of Cr2O7 <sup>2</sup><sup>−</sup> in water [31], while diamond-like coordination polymer exhibits selective emission quenching responses towards the Fe3+ ion and nitroaromatics [33].

Interesting to note is the emerging trend in the development of the so-called 'smart' materials, where the final product exhibit multistimuli-responsive photoluminescence sensing properties. Tripathi et al. developed Hg(II) coordination polymer with benzimidazole-appended tripodal tridentate ligand, 1,3,5-tris(benzimidazolylmethyl)benzene. Luminescent material is the first example of Hg(II) coordination polymer with multistimuli-responsive properties (**Figure 2B**). Luminescence quenching response is observed to a range of stimuli, including anions, solvents and nitroaromatic compounds [37].

#### **2.4 Inorganic polymers**

Inorganic polymers, such as networks of metal oxides obtained by sol-gel process, are also attractive substrates for immobilising sensing molecules. Sol-gel materials are very popular for the development of optical sensors, especially nanosized

#### **Figure 2.**

*(A) Chemical structure of pH-responsive copolymer of N-(1-ethyl-2-(pyridin-4-yl)-1Hbenzo[d]imidazol-5-yl) methacrylamide and 2-hydroxyethyl methacrylate, here presented by the author's courtesy. pH sensitivity is achieved by pyridyl substituted benzimidazole moiety [30]. (B) Representation of multistimuli-responsive 'smart' mercury(II) coordination polymer and different possible conformations of benzimidazole-based ligand in metal complexes. Adapted with permission from [37]. Copyright (2018) American Chemical Society.*

**165**

**Figure 3.**

*of Chemistry.*

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives*

probes [43]. Basically, the sol-gel process is a method for the synthesis of ceramic or glass materials at low temperature, starting from the colloidal suspension ('sol'). Hydrolysis of alkoxy metal groups in the precursors followed by polycondensation results in a network structure ('gel'). Meantime, fluorescent indicators can be easily incorporated in sol-gel by impregnation, chemical or covalent immobilisation. These materials are porous, so that the analyte can freely diffuse. They are robust and biocompatible, which makes them suitable for intracellular sensing. Hoffman et al. have developed novel benzimidazole-based fluorescent materials using the sol-gel process [44]. Tetraethylorthosilicate (TEOS) was used as an inorganic precursor for the development of new silica hybrid materials. Although sol-gel chemistry is firmly embedded in the field of chemical sensors, there is a lack of benzimidazole-based sol-gel materials. The reason can be poor solubility and self-assembly properties of many benzimidazole derivatives, often inducing gelation process and thus, making the development of novel sol-gel materials, in a classical manner described above, a challenging task. However, the gelation of such compounds has been shown as an excellent method for preparing new sensing

*(A) Highly sensitive naked eye colorimetric sensor for water and formaldehyde detection based on a porous Cu(I)-MOF constructed from CuI and 1-benzimidazolyl-3,5-bis(4-pyridyl)benzene. (B) The colour change of the bulk crystal samples of MOF in atmospheres with different relative humidity (RH 33–78.5%) and the corresponding solid-state emission spectra. Adapted with permission from [32]. Copyright (2013) Royal Society* 

membranes, which will be discussed in further sections.

*DOI: http://dx.doi.org/10.5772/intechopen.85643*

A

B

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives DOI: http://dx.doi.org/10.5772/intechopen.85643*

#### **Figure 3.**

*Chemistry and Applications of Benzimidazole and its Derivatives*

of pyridyl-substituted benzimidazole moiety [30] (**Figure 2A**).

role in the sensing mechanism. For example, a copolymer built from *N*-(1-ethyl-2-(pyridin-4-yl)-1*H*benzo[*d*]imidazol-5-yl)methacrylamide and 2-hydroxyethyl methacrylate exhibits a pH sensitivity due to acid-base equilibria on the heteroatom

Besides conjugated polymers, benzimidazole-based materials can be developed as luminescent metal organic frameworks (MOFs) [31–35] or coordination polymers [36–42]. Such advanced functional materials have been extensively applied in the field of luminescence sensing due to their diverse structural characteristics and tunable pore sizes. For example, luminescence sensing of iron is achieved by a coordination polymer employing the linear 2,5-dichloroterephthalic acid ligand and the flexible bis(benzimidazole) derivatives. Ligand affords the capacity to strongly bind metal atoms, while bis(benzimidazole) derivatives can freely twist around two methylene -CH2 groups with disparate angles to generate different conformations [36]. Luminescent MOFs have been exploited for the development of sensing materials for humidity and formaldehyde, such as a porous Cu(I)-MOF, constructed from CuI and 1-benzimidazolyl-3,5-bis(4-pyridyl)benzene (**Figure 3**) [32]. 3D cadmium metal-organic framework was demonstrated as sensing material for the detection of

<sup>2</sup><sup>−</sup> in water [31], while diamond-like coordination polymer exhibits selective

Interesting to note is the emerging trend in the development of the so-called

Inorganic polymers, such as networks of metal oxides obtained by sol-gel process, are also attractive substrates for immobilising sensing molecules. Sol-gel materials are very popular for the development of optical sensors, especially nanosized

*(A) Chemical structure of pH-responsive copolymer of N-(1-ethyl-2-(pyridin-4-yl)-1Hbenzo[d]imidazol-5-yl) methacrylamide and 2-hydroxyethyl methacrylate, here presented by the author's courtesy. pH sensitivity is achieved by pyridyl substituted benzimidazole moiety [30]. (B) Representation of multistimuli-responsive 'smart' mercury(II) coordination polymer and different possible conformations of benzimidazole-based ligand in metal complexes. Adapted with permission from [37]. Copyright (2018) American Chemical Society.*

emission quenching responses towards the Fe3+ ion and nitroaromatics [33].

'smart' materials, where the final product exhibit multistimuli-responsive photoluminescence sensing properties. Tripathi et al. developed Hg(II) coordination polymer with benzimidazole-appended tripodal tridentate ligand, 1,3,5-tris(benzimidazolylmethyl)benzene. Luminescent material is the first example of Hg(II) coordination polymer with multistimuli-responsive properties (**Figure 2B**). Luminescence quenching response is observed to a range of stimuli,

including anions, solvents and nitroaromatic compounds [37].

**164**

**Figure 2.**

Cr2O7

**2.4 Inorganic polymers**

A B

*(A) Highly sensitive naked eye colorimetric sensor for water and formaldehyde detection based on a porous Cu(I)-MOF constructed from CuI and 1-benzimidazolyl-3,5-bis(4-pyridyl)benzene. (B) The colour change of the bulk crystal samples of MOF in atmospheres with different relative humidity (RH 33–78.5%) and the corresponding solid-state emission spectra. Adapted with permission from [32]. Copyright (2013) Royal Society of Chemistry.*

probes [43]. Basically, the sol-gel process is a method for the synthesis of ceramic or glass materials at low temperature, starting from the colloidal suspension ('sol'). Hydrolysis of alkoxy metal groups in the precursors followed by polycondensation results in a network structure ('gel'). Meantime, fluorescent indicators can be easily incorporated in sol-gel by impregnation, chemical or covalent immobilisation. These materials are porous, so that the analyte can freely diffuse. They are robust and biocompatible, which makes them suitable for intracellular sensing. Hoffman et al. have developed novel benzimidazole-based fluorescent materials using the sol-gel process [44]. Tetraethylorthosilicate (TEOS) was used as an inorganic precursor for the development of new silica hybrid materials. Although sol-gel chemistry is firmly embedded in the field of chemical sensors, there is a lack of benzimidazole-based sol-gel materials. The reason can be poor solubility and self-assembly properties of many benzimidazole derivatives, often inducing gelation process and thus, making the development of novel sol-gel materials, in a classical manner described above, a challenging task. However, the gelation of such compounds has been shown as an excellent method for preparing new sensing membranes, which will be discussed in further sections.

To conclude this section, we can highlight several facts. Literature shows that polymer-based materials are the most common substrates for the preparation of novel optical sensing platforms based on benzimidazole derivatives. Benzimidazole moiety retained its functional properties upon immobilisation in presented polymeric platforms. Although they are thoroughly explored and their potential for sensing applications is often emphasised, luminescent polymers that incorporate benzimidazole moiety are not adequately exploited in optical sensors. Even though polymer-based sensing materials are still relatively rare, a recent advance in developing benzimidazole-based ultralong-persistent room temperature phosphorescence (RTP) materials that exhibit reversible pH-responsive emission [38] represents a significant breakthrough of benzimidazole derivatives in materials science. Unfortunately, the biggest disadvantages of most polymer-based sensing materials are still very limited, such as selectivity, poor photostability and often leaching of indicator dyes.

#### **3. Self-assembled sensing materials**

#### **3.1 Gels**

Soft matter research and supramolecular organogels are one of the emerging scientific areas in the last decade. Functional materials based on supramolecular organogels are very attractive for the applications in tissue engineering, medical implants, controlled drug release, environmental studies etc. Small organic molecules have often been investigated as *π*-gelators, including benzimidazole derivatives [46, 47]. Utilising their fluorescence and self-assembling properties, benzimidazole-based gels are successfully demonstrated as novel functional materials. For example, a family of alkylpyridinylium benzimidazole derivatives was synthesised in order to examine its gelation properties [48], while several fluorescent *π*-gelators based on benzimidazole are presented as stimuli responsive systems and sensors [49–53]. Ghosh et al. presented sensing system for Ag<sup>+</sup> based on the cholesterol-appended benzimidazole. Benzimidazole moiety with conformational flexibility can exhibit different alignments upon metal ion chelation, while the cholesterol is likely oriented to exert hydrophobic-hydrophobic interaction for establishing cross-linked network for solvent trapping. The addition of Ag+ ions to the solution of presented molecules in DMF:H2O (1:1, *v*/*v*) at room temperature causes instant gelation and the change of colour, visible by a naked eye [51]. Another example of multi-analyte sensor array based on benzimidazole and acylhydrazone naphthol moities was demonstrated by Yao et al. [52]. The latter sensing system is able to detect many analytes such as CN<sup>−</sup>, Al3+, Fe3+ and L-Cys with a possibility for the selective identification of Fe3+ and Al3+ in the gel state (**Figure 6**).

#### **3.2 Aggregation-induced emitters**

Self-assembly of benzimidazole derivatives takes a great role in emerging mechanisms and designs of novel optical sensing materials. One of the research directions of the self-assembled molecules are the sensing materials based on the emissive (nano)aggregates. Aggregation of organic fluorophores is mostly investigated as an undesirable side effect in many biological or chemical applications due to fluorescence quenching. However, development of novel organic luminophores with aggregation-induced emission (AIE) changed the aspect of aggregation phenomena and the AIE was introduced as an analytical tool in a wide range of application, such as bioimaging, optoelectronics and chemosensors [54]. AIE or AIEE

**167**

**Figure 4.**

organophosphate.

materials [61–65].

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives*

(aggregation-induced emission enhancement) can be observed in the so called 'poor' solvents, in crystalline or powder forms. With that in mind, benzimidazolebased fluorophores capable of emitting intense fluorescence in the aggregated form (AIE emitters) can also be classified as novel sensing materials [55–58]. For example, self-assembled nanoaggregates of benzimidazole-based acrylonitrile derivative are presented as sensing system for pH, based on aggregation-deaggregation mechanism and aggregation-induced emission (AIE) [59]. 2-Benzimidazolyl-substituted acrylonitrile dye exhibits fluorescence emission in the red, green or cyan spectral regions, depending on its protonation degree. The neutral form is capable of self-assembly in the aqueous environment (pH between 5 and 9), exhibiting stable red-orange fluorescence emission at 600 nm. Thus, due to the aggregation-induced emission (AIE), from the single molecular entity, tri-state system (RGB) is derived. The aggregation and emission are pH switchable and fully-reversible. Gogoi et al. presented a novel AIE system based on a benzimidazole derivative for the detection of pyrophosphate (PPi) (**Figure 4**) [56]. The benzimidazole moiety has a functional role in assembling aggregate structures and the recognition of Ppi. Molecules of benzimidazole derivative self-assemble in nanostructures when so-called 'bad' solvent, H2O, is added into the THF solution ('good' solvent). Self-assembled nanostructures exhibit pronounced emission at *λ* = 530 nm. Their *π*-*π* stacking is affected by the Ppi presence, thus assembled aggregates are of different emission properties and sizes. Another example is offered by Singh et al. by the preparation of fluorescent aggregates for sensing chemical warfare agents (diethylchlorophosphate) from benzimidazolium-based receptors containing 2-mercaptobenzimidazole and 2-mercaptobenzthiazole as functional groups, using anionic surfactants [60]. Authors presented receptors with benzimidazolium moiety in the centre, as well as receptors with two fluorescent arms as binding- and signalling units that initially interacts with chemical warfare agents and captures the hydrolysed product of an

An AIE phenomenon is especially exploited in solid-state, where the concentration effect commonly causes fluorescence quenching. Having in mind that fluorophores emitting in the solid states are extremely rare, especially red ones, benzimidazole-based AIE molecular systems show a great prospect for future applications of pristine powder samples or crystals as solid-state sensors and 'smart'

430 nm 430 nm

*Aggregation-induced emission of benzimidazole-based derivative and detection of pyrophosphate (PPi).* 

*Reprinted with permission from [56]. Copyright (2015) American Chemical Society.*

*DOI: http://dx.doi.org/10.5772/intechopen.85643*

#### *Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives DOI: http://dx.doi.org/10.5772/intechopen.85643*

*Chemistry and Applications of Benzimidazole and its Derivatives*

leaching of indicator dyes.

**3.1 Gels**

**3. Self-assembled sensing materials**

**3.2 Aggregation-induced emitters**

To conclude this section, we can highlight several facts. Literature shows that polymer-based materials are the most common substrates for the preparation of novel optical sensing platforms based on benzimidazole derivatives. Benzimidazole

Soft matter research and supramolecular organogels are one of the emerging scientific areas in the last decade. Functional materials based on supramolecular organogels are very attractive for the applications in tissue engineering, medical implants, controlled drug release, environmental studies etc. Small organic molecules have often been investigated as *π*-gelators, including benzimidazole derivatives [46, 47]. Utilising their fluorescence and self-assembling properties, benzimidazole-based gels are successfully demonstrated as novel functional materials. For example, a family of alkylpyridinylium benzimidazole derivatives was synthesised in order to examine its gelation properties [48], while several fluorescent *π*-gelators based on benzimidazole are presented as stimuli responsive systems

cholesterol-appended benzimidazole. Benzimidazole moiety with conformational flexibility can exhibit different alignments upon metal ion chelation, while the cholesterol is likely oriented to exert hydrophobic-hydrophobic interaction for estab-

solution of presented molecules in DMF:H2O (1:1, *v*/*v*) at room temperature causes instant gelation and the change of colour, visible by a naked eye [51]. Another example of multi-analyte sensor array based on benzimidazole and acylhydrazone naphthol moities was demonstrated by Yao et al. [52]. The latter sensing system is able to detect many analytes such as CN<sup>−</sup>, Al3+, Fe3+ and L-Cys with a possibility for

Self-assembly of benzimidazole derivatives takes a great role in emerging mechanisms and designs of novel optical sensing materials. One of the research directions of the self-assembled molecules are the sensing materials based on the emissive (nano)aggregates. Aggregation of organic fluorophores is mostly investigated as an undesirable side effect in many biological or chemical applications due to fluorescence quenching. However, development of novel organic luminophores with aggregation-induced emission (AIE) changed the aspect of aggregation phenomena and the AIE was introduced as an analytical tool in a wide range of application, such as bioimaging, optoelectronics and chemosensors [54]. AIE or AIEE

based on the

ions to the

and sensors [49–53]. Ghosh et al. presented sensing system for Ag<sup>+</sup>

lishing cross-linked network for solvent trapping. The addition of Ag+

the selective identification of Fe3+ and Al3+ in the gel state (**Figure 6**).

moiety retained its functional properties upon immobilisation in presented polymeric platforms. Although they are thoroughly explored and their potential for sensing applications is often emphasised, luminescent polymers that incorporate benzimidazole moiety are not adequately exploited in optical sensors. Even though polymer-based sensing materials are still relatively rare, a recent advance in developing benzimidazole-based ultralong-persistent room temperature phosphorescence (RTP) materials that exhibit reversible pH-responsive emission [38] represents a significant breakthrough of benzimidazole derivatives in materials science. Unfortunately, the biggest disadvantages of most polymer-based sensing materials are still very limited, such as selectivity, poor photostability and often

**166**

(aggregation-induced emission enhancement) can be observed in the so called 'poor' solvents, in crystalline or powder forms. With that in mind, benzimidazolebased fluorophores capable of emitting intense fluorescence in the aggregated form (AIE emitters) can also be classified as novel sensing materials [55–58]. For example, self-assembled nanoaggregates of benzimidazole-based acrylonitrile derivative are presented as sensing system for pH, based on aggregation-deaggregation mechanism and aggregation-induced emission (AIE) [59]. 2-Benzimidazolyl-substituted acrylonitrile dye exhibits fluorescence emission in the red, green or cyan spectral regions, depending on its protonation degree. The neutral form is capable of self-assembly in the aqueous environment (pH between 5 and 9), exhibiting stable red-orange fluorescence emission at 600 nm. Thus, due to the aggregation-induced emission (AIE), from the single molecular entity, tri-state system (RGB) is derived. The aggregation and emission are pH switchable and fully-reversible. Gogoi et al. presented a novel AIE system based on a benzimidazole derivative for the detection of pyrophosphate (PPi) (**Figure 4**) [56]. The benzimidazole moiety has a functional role in assembling aggregate structures and the recognition of Ppi. Molecules of benzimidazole derivative self-assemble in nanostructures when so-called 'bad' solvent, H2O, is added into the THF solution ('good' solvent). Self-assembled nanostructures exhibit pronounced emission at *λ* = 530 nm. Their *π*-*π* stacking is affected by the Ppi presence, thus assembled aggregates are of different emission properties and sizes. Another example is offered by Singh et al. by the preparation of fluorescent aggregates for sensing chemical warfare agents (diethylchlorophosphate) from benzimidazolium-based receptors containing 2-mercaptobenzimidazole and 2-mercaptobenzthiazole as functional groups, using anionic surfactants [60]. Authors presented receptors with benzimidazolium moiety in the centre, as well as receptors with two fluorescent arms as binding- and signalling units that initially interacts with chemical warfare agents and captures the hydrolysed product of an organophosphate.

An AIE phenomenon is especially exploited in solid-state, where the concentration effect commonly causes fluorescence quenching. Having in mind that fluorophores emitting in the solid states are extremely rare, especially red ones, benzimidazole-based AIE molecular systems show a great prospect for future applications of pristine powder samples or crystals as solid-state sensors and 'smart' materials [61–65].

#### **Figure 4.**

*Aggregation-induced emission of benzimidazole-based derivative and detection of pyrophosphate (PPi). Reprinted with permission from [56]. Copyright (2015) American Chemical Society.*

In conclusion, due to its emerging and multidisciplinary character, further research on optical sensing applications of benzimidazole-based gel membranes and self-assembled structures is strongly encouraged. Gelation is proven and widely investigated effect in many benzimidazole-based compounds, yet not exploited enough for the preparation of novel chemical sensors. Meanwhile, research on the aggregation-induced emission phenomena has taken momentum in all areas of application. Beside the fact that certain benzimidazole-based derivatives exhibit AIE property, which is not often found within small heterocyclic molecular systems, optical chemical sensors based on this principle are rarely found. Selfassembled materials for optical sensing that incorporate benzimidazole unit are summarised in **Table 2**.

#### **4. Nanomaterials for optical sensing**

Nowadays, the term *nano* appears in all aspects of our life, technology and science, including the optical chemical sensors. In most general way, an optical nanosensor can be defined as a device smaller than 1 μm that is continuously tracking an analyte and simultaneously converting optical information into an analytically useful signal [66]. Fluorescence is the most commonly applied detection technique, due to its high sensitivity and relative simplicity of measurement [67]. Nanosensors can be macromolecular nanostructures, nano-sized polymer materials and sol-gels, multi-functional core-shell systems, multi-functional magnetic beads or nanosensors based on quantum dots or metal beads. We have previously mentioned the nanoaggregates formed by the self-assembly process. Although such type of nanomaterial can be classified as nanosensors, the emphasis in this section is placed on synthesis of nano-sized substrate materials functionalised with benzimidazole derivatives. Most commonly used method for the preparation of nanosensors is previously mentioned sol-gel process resulting in silica nanoparticles [68, 69]. Some other methods, such as precipitation, are often utilised for the preparation of polymer nanoparticles [70]. Research in the field of benzimidazole-based nanosensors is still in the early stages. Benzimidazole-based nanomaterials for optical sensing of metal ions are so far demonstrated as hybrid silica materials [44, 68, 71], ZnO nanoparticles decorated with benzimidazolebased organic ligand [72] or self-assembled nano hyperbranched polymer [73]. For example, Badiei et al. recently presented SBA-15 nanoporous silica functionalised with 2,6-bis(2-benzimidazolyl) pyridine for the selective recognition of mercury (**Figure 5**) [71]. Fluorescence intensity of the SBA-15 functionalized material quenched in the presence of Hg2+ ions, wherein the sensor is applicable in the physiological pH range of 6–8.

#### **5. Other benzimidazole-based materials for optical sensing**

A simple, fast and economic determination of target analyte, on-site and without a reference device is one of the key challenges of modern analytical chemistry. As mentioned in previous sections, the response to this challenge came forth in the form of optical chemical sensors. In addition, design and development of sensing materials as straightforward optical sensors enable countless possibilities of their applications, especially in modern technology where the emphasis is put on mobile, wearable and wireless devices. Simple, yet effective materials for colorimetric or fluorimetric detection of analytes can easily be achieved using filter paper or TLC plates. Paper substrates themselves are an attractive platform for the

**169**

**Material**

Gel Gel Gel Gel Gel AIEgen AIEgen AIEgen Powder SBA-15 nanoporous silica

ZnO nanoparticles

Nano hyperbranched

polyester

Filter paper Filter paper Filter paper

TLC plates

**Table 2.**

*Benzimidazole-based materials for optical sensing.*

TNT CN− Ni2+ Acid/amine

vapours

Hg2+ Zn2+ Fe3+

2,6-bis(2-benzimidazolyl) derivative

Benzimidazole-based organic ligand

Benzimidazole end groups

Pyrene-substituted

benzimidazole-isoquinolinones

Acrylonitrile-embedded

benzimidazole-anthraquinone

2-(2′-hydroxyphenyl)benzimidazole

Carbazole-based benzimidazole derivatives

Colorimetric

Colorimetric

—

—

Dhaka et al. [77]

Aich et al. [75]

Fluorimetric Colorimetric Fluorimetric

Colorimetric Colorimetric

37 × 10−9

Kumar et al. [76]

50 × 10−6

Boonsri et al. [74]

2.6 × 10−6 4.09 × 10−9

—

**Analyte** Multi-analyte

Ag+ pH and anions

Picric acid

Na2S Pyrophosphate

pH Warfare agents

F− and COO−

**BI-based sensing molecule**

Benzimidazole and acylhydrazone naphthol

moities

Cholesterol appended benzimidazole

Benzimidazole moiety and four amide units

Cholesterol-based anthraquinone-coupled

imidazole

Carboxylic acid functionalized benzimidazole

Dipodal benzimidazole- functionalized sensor

Acrylonitrile derivative

Benzimidazolium-based dipodal receptors

Benzimidazole derivative

Fluorimetric

Fluorimetric Fluorimetric

Fluorimetric Colorimetric

—

1.67 × 10−9

—

10 × 10−9 0.38 × 10−3

Yao et al. [53]

Gogoi et al. [56]

Horak et al. [59]

Singh et al. [60]

Chaudhuri et al.

[64]

Badiei et al. [71]

Kaur et al. [72]

Wang et al. [73]

Colorimetric Colorimetric /

Fluorimetric

Colorimetric

4.30 × 10−6

4.31 × 10−5

—

Ghosh et al. [51]

Xue et al. [49]

**Detection method**

Fluorimetric

**Limit of detection (mol L−1**

**Ref.**

**)**

—

Yao et al. [52]

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives*

*DOI: http://dx.doi.org/10.5772/intechopen.85643*

Mondal et al. [50]


#### *Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives DOI: http://dx.doi.org/10.5772/intechopen.85643*

*Chemistry and Applications of Benzimidazole and its Derivatives*

summarised in **Table 2**.

physiological pH range of 6–8.

**4. Nanomaterials for optical sensing**

In conclusion, due to its emerging and multidisciplinary character, further research on optical sensing applications of benzimidazole-based gel membranes and self-assembled structures is strongly encouraged. Gelation is proven and widely investigated effect in many benzimidazole-based compounds, yet not exploited enough for the preparation of novel chemical sensors. Meanwhile, research on the aggregation-induced emission phenomena has taken momentum in all areas of application. Beside the fact that certain benzimidazole-based derivatives exhibit AIE property, which is not often found within small heterocyclic molecular systems, optical chemical sensors based on this principle are rarely found. Selfassembled materials for optical sensing that incorporate benzimidazole unit are

Nowadays, the term *nano* appears in all aspects of our life, technology and science, including the optical chemical sensors. In most general way, an optical nanosensor can be defined as a device smaller than 1 μm that is continuously tracking an analyte and simultaneously converting optical information into an analytically useful signal [66]. Fluorescence is the most commonly applied detection technique, due to its high sensitivity and relative simplicity of measurement [67]. Nanosensors can be macromolecular nanostructures, nano-sized polymer materials and sol-gels, multi-functional core-shell systems, multi-functional magnetic beads or nanosensors based on quantum dots or metal beads. We have previously mentioned the nanoaggregates formed by the self-assembly process. Although such type of nanomaterial can be classified as nanosensors, the emphasis

in this section is placed on synthesis of nano-sized substrate materials functionalised with benzimidazole derivatives. Most commonly used method for the preparation of nanosensors is previously mentioned sol-gel process resulting in silica nanoparticles [68, 69]. Some other methods, such as precipitation, are often utilised for the preparation of polymer nanoparticles [70]. Research in the field of benzimidazole-based nanosensors is still in the early stages. Benzimidazole-based nanomaterials for optical sensing of metal ions are so far demonstrated as hybrid silica materials [44, 68, 71], ZnO nanoparticles decorated with benzimidazolebased organic ligand [72] or self-assembled nano hyperbranched polymer [73]. For example, Badiei et al. recently presented SBA-15 nanoporous silica functionalised with 2,6-bis(2-benzimidazolyl) pyridine for the selective recognition of mercury (**Figure 5**) [71]. Fluorescence intensity of the SBA-15 functionalized material quenched in the presence of Hg2+ ions, wherein the sensor is applicable in the

**5. Other benzimidazole-based materials for optical sensing**

A simple, fast and economic determination of target analyte, on-site and without a reference device is one of the key challenges of modern analytical chemistry. As mentioned in previous sections, the response to this challenge came forth in the form of optical chemical sensors. In addition, design and development of sensing materials as straightforward optical sensors enable countless possibilities of their applications, especially in modern technology where the emphasis is put on mobile, wearable and wireless devices. Simple, yet effective materials for colorimetric or fluorimetric detection of analytes can easily be achieved using filter paper or TLC plates. Paper substrates themselves are an attractive platform for the

**168**

**Table 2.** *Benzimidazole-based materials for optical sensing.*

**Figure 5.**

*Synthesis procedure of benzimidazole functionalized SBA-15 material and fluorescence emission of the aqueous suspended nanoparticles (0.4 g L<sup>−</sup><sup>1</sup> ) upon titration of increasing amount of Hg2+ ions. Inset: Stern-Volmer plot, λexc = 353 nm. Reprinted with permission from [71]. Copyright (2018) Springer Nature.*

use in a wide range of optical sensing, due to the possibility of a passive sample manipulation by capillary forces. So far, paper-based optical chemical sensors for neutral molecules, anions and cations, relying on benzimidazole derivatives as recognition element, have been successfully presented by several research groups. For example, Boonsri et al. demonstrated paper-based sensors for the trinitrotoluene (TNT) detection [74]. Sensing material prepared from pyrene-substituted benzimidazole-isoquinolinones can readily detect TNT in aqueous media by a naked-eye observation at concentrations as low as 50 μM. Optical sensing of acid/ amine vapours with three carbazole-based benzimidazole derivatives in the solid state was also demonstrated using TLC plates [75]. Plates were immersed with benzimidazole-based dyes and then exposed to trifluoroacetic acid (TFA) vapours for 1 minute. In following step, the TLC plates which were exposed with TFA vapours were further revealed to triethyl amine vapours and the restored colour was observed in each case (**Figure 6**).

Anion detection was demonstrated by the ratiometric detection of CN<sup>−</sup> based on acrylonitrile embedded benzimidazole-anthraquinone coated on the filter paper [76]. Paper strips coated with the sensing molecule showed a distinct colour change from yellow-greenish to red under UV light in the presence of the CN<sup>−</sup> ions. Dhaka et al. demonstrated a 'bare-eye' probe for the detection of Ni2+ based on 2-(2′-hydroxyphenyl)benzimidazole. Colourimetric sensing of Ni2+ was demonstrated on filter paper. Paper test strips exhibit distinct visual change from colourless to yellow-gold [77]. Other materials for optical sensing that incorporate benzimidazole unit are summarised in **Table 2**.

Besides optical sensing, paper-based materials coated with functional benzimidazole derivatives are also presented as 'smart', stimuli responsive materials with potential applications in security, optoelectronic or fluorescent imaging [62]. Simple sensing substrates such as paper and textile materials are perfectly suited

**171**

can perfectly respond to.

**6. Conclusion**

**Figure 6.**

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives*

for applications in emerging mobile and wearable chemical sensors. Design and development of compatible 'sensing chemistries' that operate in the background of such devices is a constant challenge. The multifunctional nature of materials based

*(A) Proposed self-assembly mechanism of a supramolecular AIE gel and its multiple-stimuli responsive behaviour (a) and fluorescence responses of the multi-analyte sensor array to the presence of various anions, cations and amino acids. Reprinted with permission from [52]. Copyright (2018) Royal Society of Chemistry. (B) TLC plates immersed with carbazole benzimidazole-based dyes observed under the UV light (λexc = 365 nm) before (a) and after exposure to TFA vapours (b). Adapted with permission from [75].* 

Benzimidazole unit represents an important multifunctional building block in optical chemical sensors, with proven potential for the development of novel functional (nano)materials. Solid-state optical sensing systems incorporating benzimidazole derivatives are reviewed and discussed. Polymers are most commonly used substrates for the development of optical chemical sensors. Materials for optical sensing based on benzimidazole are also demonstrated as gels, sol-gel matrices, silica or polymer nanoparticles, (nano)aggregates and TLC or paper-based strips. The role of benzimidazole moiety in optical sensing (nano)materials is important and crucial, since it maintains the function of the system and plays a key role in the formation of the analytical signal in the majority of chemical sensing systems reviewed here. Besides, the planar moiety significantly contributes to the conjugation of the chromo/fluorophore system. Although benzimidazole derivatives reviewed in the literature are mostly fluorescent sensors, several probes based on colourimetric switches are also demonstrated. It is very challenging to transfer the sensing chemistry from a solution to the solid state, which is successfully comprehended for the benzimidazole derivatives. It is even observed for some classes of chromophores with relatively unattractive sensing properties in aqueous solution (such as low quantum yield, decomposition upon protonation) to be drastically

Although examples of sensing materials presented in the literature show that benzimidazole derivatives can be successfully and easily applied in optical chemical sensors, they are yet insufficiently explored. Challenges in development of novel optical sensing (nano)materials are constantly emerging, since the scientific and industrial field of mobile and wearable sensors are experiencing great progress. Simple, fast and economic determination of target analyte, on-site and without reference device is a request that a multifunctional molecule such as benzimidazole

on molecules such as benzimidazole can perfectly respond to this challenge.

improved upon immobilisation in a polymer matrix.

*DOI: http://dx.doi.org/10.5772/intechopen.85643*

*Copyright (2016) Royal Society of Chemistry.*

A B

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives DOI: http://dx.doi.org/10.5772/intechopen.85643*

#### **Figure 6.**

*Chemistry and Applications of Benzimidazole and its Derivatives*

use in a wide range of optical sensing, due to the possibility of a passive sample manipulation by capillary forces. So far, paper-based optical chemical sensors for neutral molecules, anions and cations, relying on benzimidazole derivatives as recognition element, have been successfully presented by several research groups. For example, Boonsri et al. demonstrated paper-based sensors for the trinitrotoluene (TNT) detection [74]. Sensing material prepared from pyrene-substituted benzimidazole-isoquinolinones can readily detect TNT in aqueous media by a naked-eye observation at concentrations as low as 50 μM. Optical sensing of acid/ amine vapours with three carbazole-based benzimidazole derivatives in the solid state was also demonstrated using TLC plates [75]. Plates were immersed with benzimidazole-based dyes and then exposed to trifluoroacetic acid (TFA) vapours for 1 minute. In following step, the TLC plates which were exposed with TFA vapours were further revealed to triethyl amine vapours and the restored colour

*λexc = 353 nm. Reprinted with permission from [71]. Copyright (2018) Springer Nature.*

*Synthesis procedure of benzimidazole functionalized SBA-15 material and fluorescence emission of the aqueous* 

*) upon titration of increasing amount of Hg2+ ions. Inset: Stern-Volmer plot,* 

Anion detection was demonstrated by the ratiometric detection of CN<sup>−</sup> based on acrylonitrile embedded benzimidazole-anthraquinone coated on the filter paper [76]. Paper strips coated with the sensing molecule showed a distinct colour change from yellow-greenish to red under UV light in the presence of the CN<sup>−</sup> ions. Dhaka et al. demonstrated a 'bare-eye' probe for the detection of Ni2+ based on 2-(2′-hydroxyphenyl)benzimidazole. Colourimetric sensing of Ni2+ was demonstrated on filter paper. Paper test strips exhibit distinct visual change from colourless to yellow-gold [77]. Other materials for optical sensing that incorporate

Besides optical sensing, paper-based materials coated with functional benzimidazole derivatives are also presented as 'smart', stimuli responsive materials with potential applications in security, optoelectronic or fluorescent imaging [62]. Simple sensing substrates such as paper and textile materials are perfectly suited

**170**

**Figure 5.**

*suspended nanoparticles (0.4 g L<sup>−</sup><sup>1</sup>*

was observed in each case (**Figure 6**).

benzimidazole unit are summarised in **Table 2**.

*(A) Proposed self-assembly mechanism of a supramolecular AIE gel and its multiple-stimuli responsive behaviour (a) and fluorescence responses of the multi-analyte sensor array to the presence of various anions, cations and amino acids. Reprinted with permission from [52]. Copyright (2018) Royal Society of Chemistry. (B) TLC plates immersed with carbazole benzimidazole-based dyes observed under the UV light (λexc = 365 nm) before (a) and after exposure to TFA vapours (b). Adapted with permission from [75]. Copyright (2016) Royal Society of Chemistry.*

for applications in emerging mobile and wearable chemical sensors. Design and development of compatible 'sensing chemistries' that operate in the background of such devices is a constant challenge. The multifunctional nature of materials based on molecules such as benzimidazole can perfectly respond to this challenge.

#### **6. Conclusion**

Benzimidazole unit represents an important multifunctional building block in optical chemical sensors, with proven potential for the development of novel functional (nano)materials. Solid-state optical sensing systems incorporating benzimidazole derivatives are reviewed and discussed. Polymers are most commonly used substrates for the development of optical chemical sensors. Materials for optical sensing based on benzimidazole are also demonstrated as gels, sol-gel matrices, silica or polymer nanoparticles, (nano)aggregates and TLC or paper-based strips.

The role of benzimidazole moiety in optical sensing (nano)materials is important and crucial, since it maintains the function of the system and plays a key role in the formation of the analytical signal in the majority of chemical sensing systems reviewed here. Besides, the planar moiety significantly contributes to the conjugation of the chromo/fluorophore system. Although benzimidazole derivatives reviewed in the literature are mostly fluorescent sensors, several probes based on colourimetric switches are also demonstrated. It is very challenging to transfer the sensing chemistry from a solution to the solid state, which is successfully comprehended for the benzimidazole derivatives. It is even observed for some classes of chromophores with relatively unattractive sensing properties in aqueous solution (such as low quantum yield, decomposition upon protonation) to be drastically improved upon immobilisation in a polymer matrix.

Although examples of sensing materials presented in the literature show that benzimidazole derivatives can be successfully and easily applied in optical chemical sensors, they are yet insufficiently explored. Challenges in development of novel optical sensing (nano)materials are constantly emerging, since the scientific and industrial field of mobile and wearable sensors are experiencing great progress. Simple, fast and economic determination of target analyte, on-site and without reference device is a request that a multifunctional molecule such as benzimidazole can perfectly respond to.

## **Acknowledgements**

This work was supported by the Croatian Science Foundation under grant number IP-2014-09-3386 entitled 'Design and synthesis of novel nitrogen-containing heterocyclic fluorophores and fluorescent nanomaterials for pH and metal-ion sensing' which is gratefully acknowledged.

#### **Conflict of interest**

Authors have no conflict of interest to declare.

#### **Abbreviations**


**173**

**Author details**

Institute, Zagreb, Croatia

Ema Horak1

provided the original work is properly cited.

\*, Robert Vianello1

Engineering and Technology, Zagreb, Croatia

\*Address all correspondence to: Ema.Horak@irb.hr

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives*

*DOI: http://dx.doi.org/10.5772/intechopen.85643*

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

1 Computational Organic Chemistry and Biochemistry Group, Ruđer Bošković

2 Department of General and Inorganic Chemistry, Faculty of Chemical

and Ivana Murković Steinberg2

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives DOI: http://dx.doi.org/10.5772/intechopen.85643*

#### **Author details**

*Chemistry and Applications of Benzimidazole and its Derivatives*

sensing' which is gratefully acknowledged.

Authors have no conflict of interest to declare.

AIE aggregation-induced emission

ICT intramolecular charge transfer MOF metal organic framework

RGB red, green and cyan spectrum

TEOS tetraethylorthosilicate TFA trifluoroacetic acid THF tetrahydrofuran

TLC thin layer chromatography

pHEMA poly(2-hydroxyethyl methacrylate)

DMF dimethylformamide

NLO non-linear optics

PPi pyrophosphate PVC poly(vinyl chloride)

SBA-15 porous silica

TNT trinitrotoluene UV ultraviolet

AIEE aggregation-induced emission enhancement

RTP ultralong-persistent room temperature phosphorescence

This work was supported by the Croatian Science Foundation under grant number IP-2014-09-3386 entitled 'Design and synthesis of novel nitrogen-containing heterocyclic fluorophores and fluorescent nanomaterials for pH and metal-ion

**Acknowledgements**

**Conflict of interest**

**Abbreviations**

**172**

Ema Horak1 \*, Robert Vianello1 and Ivana Murković Steinberg2

1 Computational Organic Chemistry and Biochemistry Group, Ruđer Bošković Institute, Zagreb, Croatia

2 Department of General and Inorganic Chemistry, Faculty of Chemical Engineering and Technology, Zagreb, Croatia

\*Address all correspondence to: Ema.Horak@irb.hr

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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substituent groups of the ligands on the structures. Crystal Growth and Design.

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[36] Hao SY, Hou SX, Hao ZC, Cui GH. A new three-dimensional

Spectroscopy. 2018;**189**:613-620

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2016;**8**(24):15489-15496

2018;**156**:80-88

2019;**1175**:253-260

2015;**17**(11):2279-2293

temperature phosphorescence of metal coordination polymers exhibiting reversible pH-responsive emission. Acs Applied Materials and Interfaces.

[39] Zhao XX, Liu D, Li YH, Cui GH. Bifunctional silver(I) coordination polymer exhibiting selective adsorptive

of Congo red and luminescent sensing for ferric ion. Polyhedron.

[40] Wei XJ, Li YH, Qin ZB, Cui GH. Two zinc(II) coordination polymers for selective luminescence

sensing of iron(III) ions and

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photocatalytic degradation of methylene blue. Journal of Molecular Structure.

coordination polymers based on a flexible bis(2-methylbenzimidazole) ligand and different carboxylates: Synthesis, structures, photoluminescence and catalytic properties. CrystEngComm.

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[29] Han B, Zhou NC, Zhang W, Cheng ZP, Zhu J, Zhu XL. Fluorescence emission of amphiphilic copolymers bearing benzimidazole groups: Stimuli-responsive behaviors in aqueous solution. Journal of Polymer Science Part A: Polymer Chemistry.

2012;**3**(12):3308-3317

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[31] Li JX, Liu D, Hao ZC, Cui GH. An

cadmium metal-organic framework as a luminescent sensor for detection of

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[34] Dong Y, Zhang H, Lei F, Liang M, Qian X, Shen P, et al. Benzimidazolefunctionalized Zr-UiO-66 nanocrystals for luminescent sensing of Fe3+ in water. Journal of Solid State Chemistry.

<sup>2</sup><sup>−</sup> in water. Inorganic Chemistry

unusual (3,4,5)-connected 3D

Communications. 2018;**97**:79-82

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2014;**50**(12):1444-1446

2018;**89**:68-72

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copolymer sensor based on benzimidazole chromophore for microbioreactors. Dyes and Pigments.

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Cr2O7

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[45] Babu SS, Praveen VK, Ajayaghosh A. Functional π-gelators and their applications. Chemical Reviews. 2014;**114**(4):1973-2129

[46] Ma XX, Xie JS, Tang N, Wu JC. AIE-caused luminescence of a thermally-responsive supramolecular organogel. New Journal of Chemistry. 2016;**40**(8):6584-6587

[47] Yu H, Kawanishi H, Koshima H. Preparation and photophysical properties of benzimidazole-based gels. Journal of Photochemistry and Photobiology A: Chemistry. 2006;**178**(1):62-69

[48] Shen X, Jiao T, Zhang Q, Guo H, Lv Y, Zhou J, et al. Nanostructures and self-assembly of organogels via benzimidazole/benzothiazole imide derivatives with different alkyl substituent chains. Journal of Nanomaterials. 2013;**2013**:8

[49] Xue P, Lu R, Jia J, Takafuji M, Ihara H. A smart gelator as a chemosensor: Application to integrated logic gates in solution, gel, and film. Chemistry – A European Journal. 2012;**18**(12):3549-3558

[50] Mondal S, Ghosh K. Anthraquinone derived cholesterol linked imidazole gelator in visual sensing of picric acid. Chemistry Select. 2017;**2**(17):4800-4806

[51] Ghosh K, Panja S, Bhattacharya S. Visual sensing of Ag<sup>+</sup> ions through gelation of cholesterol-appended benzimidazole and associated ion conducting behaviour. Chemistry Select. 2017;**2**(3):959-966

[52] Yao H, Wang J, Song S-S, Fan Y-Q, Guan X-W, Zhou Q, et al. A novel supramolecular AIE gel acts as a multianalyte sensor array. New Journal of Chemistry. 2018;**42**(22):18059-18065

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[57] Malakar A, Kumar M, Reddy A, Biswal HT, Mandal BB, Krishnamoorthy G. Aggregation induced enhanced emission of 2-(2 '- hydroxyphenyl) benzimidazole. Photochemical

and Photobiological Sciences. 2016;**15**(7):937-948

[58] Wu Z, Sun JB, Zhang ZQ, Gong P, Xue PC, Lu R. Organogelation of cyanovinylcarbazole with terminal benzimidazole: AIE and response for gaseous acid. RSC Advances. 2016;**6**(99):97293-97301

[59] Horak E, Hranjec M, Vianello R, Steinberg IM. Reversible pH switchable aggregation-induced emission of self-assembled benzimidazole-based acrylonitrile dye in aqueous solution. Dyes and Pigments. 2017;**142**:108-115

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[62] Horak E, Robić M, Šimanović A, Mandić V, Vianello R, Hranjec M, et al. Tuneable solid-state emitters based on benzimidazole derivatives: Aggregation induced red emission and mechanochromism of D-π-a fluorophores. Dyes and Pigments. 2019;**162**:688-696

[63] Benelhadj K, Massue J, Retailleau P, Ulrich G, Ziessel R. 2-(2 '-Hydroxyphenyl)benzimidazole and 9,10-Phenanthroimidazole chelates and borate complexes: Solution- and solid-state emitters. Organic Letters. 2013;**15**(12):2918-2921

[64] Chaudhuri T, Mondal A, Mukhopadhyay C. Benzimidazole: A solid state colorimetric chemosensor for fluoride and acetate. Journal of Molecular Liquids. 2018;**251**:35-39

[65] Maeda C, Todaka T, Ueda T, Ema T. Color-Tunable solid-state fluorescence emission from Carbazolebased BODIPYs. Chemistry-A European Journal. 2016;**22**(22):7508-7513

[66] Borisov SM, Klimant I. Optical nanosensors–Smart tools in bioanalytics. The Analyst. 2008;**133**(10):1302-1307

[67] Wolfbeis OS. An overview of nanoparticles commonly used in fluorescent bioimaging. Chemical Society Reviews. 2015;**44**(14):4743-4768

[68] Montalti M, Rampazzo E, Zaccheroni N, Prodi L. Luminescent chemosensors based on silica nanoparticles for the detection of ionic species. New Journal of Chemistry. 2013;**37**(1):28-34

[69] Song X, Li F, Ma J, Jia N, Xu J, Shen H. Synthesis of fluorescent silica nanoparticles and their applications as fluorescence probes. Journal of Fluorescence. 2011;**21**(3):1205-1212

[70] Borisov SM, Mayr T, Mistlberger G, Waich K, Koren K, Chojnacki P, et al. Precipitation as a simple and versatile method for preparation of optical nanochemosensors. Talanta. 2009;**79**(5):1322-1330

[71] Badiei A, Razavi BV, Goldooz H, Ziarani GM, Faridbod F, Ganjali MR. A novel fluorescent chemosensor assembled with 2,6-Bis(2-Benzimidazolyl)pyridinefunctionalized Nanoporous silica-type SBA-15 for recognition of Hg2+ ion in aqueous media. International Journal of Environmental Research. 2018;**12**(1):109-115

[72] Kaur N, Raj P, Kaur N, Kim DY, Singh N. Supramolecular hybrid of ZnO nanoparticles with benzimidazole

**179**

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives*

*DOI: http://dx.doi.org/10.5772/intechopen.85643*

based organic ligand for the recognition

of Zn2+ ions in semi-aqueous media. Journal of Photochemistry and Photobiology A: Chemistry.

[73] Wang XX, Zeng FY, Ma ZY, Jiang YL, Han QR, Wang BX. Selfassembly of benzimidazole-ended nano hyperbranched polyester and its host-guest response. Materials Letters.

[74] Boonsri M, Vongnam K, Namuangruk S, Sukwattanasinitt M, Rashatasakhon P. Pyrenyl benzimidazole-isoquinolinones: Aggregation-induced emission enhancement property and

application as TNT fluorescent sensor. Sensors and Actuators B: Chemical.

[75] Aich K, Das S, Goswami S, Quah CK, Sarkar D, Mondal TK, et al. Carbazole-benzimidazole based dyes for acid responsive ratiometric emissive switches. New Journal of Chemistry.

2017;**347**:41-48

2016;**173**:191-194

2017;**248**:665-672

2016;**40**(8):6907-6915

2018;**267**:549-558

2017;**464**:18-22

[76] Kumar G, Gupta N, Paul K, Luxami V. Acrylonitrile embedded benzimidazole-anthraquinone based chromofluorescent sensor for ratiometric detection of CN<sup>−</sup> ions in bovine serum albumin. Sensors and Actuators B-Chemical.

[77] Dhaka G, Kaur N, Singh J. Spectral studies on benzimidazole-based "bare-eye" probe for the detection of Ni2+: Application as a solid state sensor. Inorganica Chimica Acta.

*Optical Sensing (Nano)Materials Based on Benzimidazole Derivatives DOI: http://dx.doi.org/10.5772/intechopen.85643*

based organic ligand for the recognition of Zn2+ ions in semi-aqueous media. Journal of Photochemistry and Photobiology A: Chemistry. 2017;**347**:41-48

*Chemistry and Applications of Benzimidazole and its Derivatives*

for fluoride and acetate. Journal of Molecular Liquids. 2018;**251**:35-39

[65] Maeda C, Todaka T, Ueda T, Ema T. Color-Tunable solid-state fluorescence emission from Carbazolebased BODIPYs. Chemistry-A European

Journal. 2016;**22**(22):7508-7513

[67] Wolfbeis OS. An overview of nanoparticles commonly used in fluorescent bioimaging. Chemical Society Reviews. 2015;**44**(14):4743-4768

[68] Montalti M, Rampazzo E, Zaccheroni N, Prodi L. Luminescent

chemosensors based on silica

2013;**37**(1):28-34

2009;**79**(5):1322-1330

2018;**12**(1):109-115

nanoparticles for the detection of ionic species. New Journal of Chemistry.

[69] Song X, Li F, Ma J, Jia N, Xu J, Shen H. Synthesis of fluorescent silica nanoparticles and their applications as fluorescence probes. Journal of Fluorescence. 2011;**21**(3):1205-1212

[70] Borisov SM, Mayr T, Mistlberger G, Waich K, Koren K, Chojnacki P, et al. Precipitation as a simple and versatile method for preparation of optical nanochemosensors. Talanta.

[71] Badiei A, Razavi BV, Goldooz H, Ziarani GM, Faridbod F, Ganjali MR. A novel fluorescent chemosensor assembled with 2,6-Bis(2-Benzimidazolyl)pyridinefunctionalized Nanoporous silica-type SBA-15 for recognition of Hg2+ ion in aqueous media. International Journal of Environmental Research.

[72] Kaur N, Raj P, Kaur N, Kim DY, Singh N. Supramolecular hybrid of ZnO nanoparticles with benzimidazole

[66] Borisov SM, Klimant I. Optical nanosensors–Smart tools in bioanalytics. The Analyst.

2008;**133**(10):1302-1307

and Photobiological Sciences.

[58] Wu Z, Sun JB, Zhang ZQ, Gong P, Xue PC, Lu R. Organogelation of cyanovinylcarbazole with terminal benzimidazole: AIE and response for gaseous acid. RSC Advances.

[59] Horak E, Hranjec M, Vianello R, Steinberg IM. Reversible pH switchable aggregation-induced emission of self-assembled benzimidazole-based acrylonitrile dye in aqueous solution. Dyes and Pigments. 2017;**142**:108-115

2016;**15**(7):937-948

2016;**6**(99):97293-97301

[60] Singh A, Raj P, Singh N. Benzimidazolium-based selfassembled fluorescent aggregates for sensing and catalytic degradation of diethylchlorophosphate. Acs Applied Materials and Interfaces.

2016;**8**(42):28641-28651

[61] Zhan Y, Wei Q, Zhao J, Zhang X. Reversible mechanofluorochromism

[62] Horak E, Robić M, Šimanović A, Mandić V, Vianello R, Hranjec M, et al. Tuneable solid-state emitters based on benzimidazole derivatives: Aggregation induced red emission and mechanochromism of D-π-a fluorophores. Dyes and Pigments.

and acidochromism using a cyanostyrylbenzimidazole derivative with aggregationinduced emission. RSC Advances.

2017;**7**(77):48777-48784

2019;**162**:688-696

[63] Benelhadj K, Massue J,

2013;**15**(12):2918-2921

[64] Chaudhuri T, Mondal A,

Mukhopadhyay C. Benzimidazole: A solid state colorimetric chemosensor

Retailleau P, Ulrich G, Ziessel R. 2-(2 '-Hydroxyphenyl)benzimidazole and 9,10-Phenanthroimidazole chelates and borate complexes: Solution- and solid-state emitters. Organic Letters.

**178**

[73] Wang XX, Zeng FY, Ma ZY, Jiang YL, Han QR, Wang BX. Selfassembly of benzimidazole-ended nano hyperbranched polyester and its host-guest response. Materials Letters. 2016;**173**:191-194

[74] Boonsri M, Vongnam K, Namuangruk S, Sukwattanasinitt M, Rashatasakhon P. Pyrenyl benzimidazole-isoquinolinones: Aggregation-induced emission enhancement property and application as TNT fluorescent sensor. Sensors and Actuators B: Chemical. 2017;**248**:665-672

[75] Aich K, Das S, Goswami S, Quah CK, Sarkar D, Mondal TK, et al. Carbazole-benzimidazole based dyes for acid responsive ratiometric emissive switches. New Journal of Chemistry. 2016;**40**(8):6907-6915

[76] Kumar G, Gupta N, Paul K, Luxami V. Acrylonitrile embedded benzimidazole-anthraquinone based chromofluorescent sensor for ratiometric detection of CN<sup>−</sup> ions in bovine serum albumin. Sensors and Actuators B-Chemical. 2018;**267**:549-558

[77] Dhaka G, Kaur N, Singh J. Spectral studies on benzimidazole-based "bare-eye" probe for the detection of Ni2+: Application as a solid state sensor. Inorganica Chimica Acta. 2017;**464**:18-22

Chapter 10

Abstract

of poly[2,2<sup>0</sup>

working nowadays.

1. Introduction

chemical materials.

181

anion exchange membrane

Benzimidazole as Solid Electrolyte

This chapter is focused in the application of benzimidazole, mainly in the form

azole) (ABPBI), in the fuel cell technology. A short introduction is given of the fuel cell principles, explaining both the theory and the high importance of this technology. PBI and ABPBI are used in a certain type of fuel cells: the polymer electrolyte fuel cells and are key materials in the composition of some of the electrolyte membranes used. Commercially available membranes composed of PBI are indicated in order to give an overview of their potential performance. The synthesis of the polymers is explained. Moreover, the preparation of the different kinds of membranes, both in proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs) is studied. A deep description is given about the properties that make this family of compounds so interesting for the fuel cell technology as well as an how these polymers have been characterized with the corresponding analysis. The comparison with other ion exchange membranes is also discussed. Special attention will be given to the state of the art of different kinds of PBI/ABPBI fuel cell electrolyte membranes, in which our group and others are

Keywords: polybenzimidazole, electrolyte, fuel cells, proton exchange membrane,

Benzimidazole and its family can be used in the energy world easily in the form of polymers, since these materials have the possibility to create designed structures for many applications. The fuel cells and electrolyzers are emerging technologies with wonderful potential. In these technologies, an electrolyte is needed to separate two electrodes where electrochemical reactions occur. The separation must be physical and electrical, but the electrolyte allows the ionic conduction of ions in order to close the circuit (so the current goes through the external circuit and can be used) and to make possible the continuity of the reactions at the electrodes. Here is where benzimidazoles (in the form of polybenzimidazole, e.g.) play a key role, in the conformation of a solid polymer electrolyte membrane, alone or with other

But, what is a fuel cell? What do we understand for membranes in this field? Fuel cells are electrochemical devices that convert directly the chemical energy of the reagents into electrical energy and side-products via an electrochemical


Material for Fuel Cells

Daniel Herranz and Pilar Ocón


#### Chapter 10

## Benzimidazole as Solid Electrolyte Material for Fuel Cells

Daniel Herranz and Pilar Ocón

#### Abstract

This chapter is focused in the application of benzimidazole, mainly in the form of poly[2,2<sup>0</sup> -(m-phenylene)-5,5<sup>0</sup> -bisbenzimidazole] (PBI) and poly(2,5-benzimidazole) (ABPBI), in the fuel cell technology. A short introduction is given of the fuel cell principles, explaining both the theory and the high importance of this technology. PBI and ABPBI are used in a certain type of fuel cells: the polymer electrolyte fuel cells and are key materials in the composition of some of the electrolyte membranes used. Commercially available membranes composed of PBI are indicated in order to give an overview of their potential performance. The synthesis of the polymers is explained. Moreover, the preparation of the different kinds of membranes, both in proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs) is studied. A deep description is given about the properties that make this family of compounds so interesting for the fuel cell technology as well as an how these polymers have been characterized with the corresponding analysis. The comparison with other ion exchange membranes is also discussed. Special attention will be given to the state of the art of different kinds of PBI/ABPBI fuel cell electrolyte membranes, in which our group and others are working nowadays.

Keywords: polybenzimidazole, electrolyte, fuel cells, proton exchange membrane, anion exchange membrane

#### 1. Introduction

Benzimidazole and its family can be used in the energy world easily in the form of polymers, since these materials have the possibility to create designed structures for many applications. The fuel cells and electrolyzers are emerging technologies with wonderful potential. In these technologies, an electrolyte is needed to separate two electrodes where electrochemical reactions occur. The separation must be physical and electrical, but the electrolyte allows the ionic conduction of ions in order to close the circuit (so the current goes through the external circuit and can be used) and to make possible the continuity of the reactions at the electrodes. Here is where benzimidazoles (in the form of polybenzimidazole, e.g.) play a key role, in the conformation of a solid polymer electrolyte membrane, alone or with other chemical materials.

But, what is a fuel cell? What do we understand for membranes in this field? Fuel cells are electrochemical devices that convert directly the chemical energy of the reagents into electrical energy and side-products via an electrochemical

reaction. This process allows theoretical efficiencies as high as 80% [1], which is a wonderful advantage compared to the thermal machines limited thermodynamically by the Carnot cycle. There are many types of fuel cells, the most relevant are alkaline fuel cells (AFCs), polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs).

Polymer electrolyte membrane fuel cells have as principal characteristics the low operation temperature (<120°C), high power density, and easy scale-up, making them a promising technology for power generation. Their main application fields are backup power, portable power, distributed generation, and transportation [1]. It is relevant to note the role of transition energy technology, since they can play an important function in the near future in order to overcome the fossil fuel depletion and mitigate the climate change. The reason is that fuels like hydrogen or alcohols, which are produced by unsustainable ways, could be produced with renewable energies. An example of this is the actual production of hydrogen mainly from catalytic reforming of methane and just some from electrolysis [2]. The hydrogen can be produced from electrolysis powered with electricity coming from renewables. This should be done when production is higher than the demand, allowing to store chemically the energy and later use it when needed with a PEMFC; this is known as the "hydrogen economy system." It is also possible to accumulate energy in short-chain alcohols like methanol or ethanol and use them to power PEMFCs [3, 4], mainly used in the portable applications. A great advantage of this technology is the low pollution associated with the process. For example, when hydrogen is used as fuel, the only products are electricity and water. The potential of PEMFCs is really promising but still drawbacks as high cost (mainly from the expensive catalysts based in Pt) and low durability have to be overcome for a general commercialization [1].

fuel is oxidized, and the cathode, where the oxidant (O2) is reduced. It also involves an electrolyte, which acts as an ionic conductor and electrical insulator. The electrons obtained in the anode are addressed directly to the cathode through the external circuit, generating an electric current directly usable. In addition, the protons produced in the anode go through the electrolyte, up to the cathode to reduce O2, generating water as the only product of the reaction. The reaction is exothermic

(v). Although this is a spontaneous reaction, it needs to be catalyzed to be opera-

and the maximum potential difference, obtained in the fuel cell, E0

<sup>E</sup><sup>0</sup> <sup>¼</sup> �ΔG<sup>0</sup>

where n is the number of electrons exchanged and F is Faraday's constant. At 298 K and 1 atm, <sup>Δ</sup>G0 <sup>=</sup> �237.340 J/mol and therefore E0 = 1.23 V. For an operating temperature of 80°C, the values of ΔH and ΔS change, but slightly, and the decrease in ΔG will be mainly due to the temperature, resulting in a theoretical potential difference of 1.18 V approximately. However, in practice this potential, called the open circuit potential, is significantly lower than this potential value, usually less than 1 V. This suggests that some losses appear in the fuel cell even when no external current is generated. The potential difference of the fuel cell in operation, that is, when the current is passing through the system, Efuel cell (I), will be given by the sum of thermodynamic or reversible value (I = 0), minus the anode and cathode activation overvoltage and the ohmic losses or overvoltage. The electrode kinetics was represented by the Butler-Volmer equation, the mass transport process was described by the multicomponent Stefan-Maxwell equations and Fick's law, and the

At atmospheric pressure, the maximum potential difference obtained by the fuel cell will be determined by the difference of energy between the initial and final state of the system. The Gibbs free energy variation of the process, ΔG, can be calculated from the operation temperature (T) and changes with both enthalpy (ΔH) and

tional, since the kinetics of the process is too slow otherwise.

Polymer exchange membrane fuel cell working with H2 and O2.

Benzimidazole as Solid Electrolyte Material for Fuel Cells

DOI: http://dx.doi.org/10.5772/intechopen.85430

entropy (ΔS) of the reaction. Under standard conditions

<sup>r</sup> = �285.83 kJ/mol for H2O (l) and � 241.862 kJ/mol for H2O

<sup>Δ</sup>G<sup>0</sup> <sup>¼</sup> <sup>Δ</sup>S<sup>0</sup> � <sup>T</sup> <sup>Δ</sup>S<sup>0</sup> (1)

nF <sup>¼</sup> <sup>1</sup>:23<sup>V</sup> (2)

theoric, will be

and has a value of ΔH0

Figure 1.

183

In PEMFCs one of the most important components is the polymeric ion exchange membrane (IEM) that works as an electrolyte. It has to be an electrical insulator to force the produced electrons to go through the external circuit, it also has to avoid the mixture of the reagents supplied in anode and cathode, and it is responsible of the adequate ionic conductivity of the ions traveling through it. Depending on the ion movement, two types of IEMs can be distinguished: anion exchange membranes (AEMs), where the ionic charge carriers are the hydroxide ions (OH) that travel from cathode to anode, and cation exchange membranes (CEMs) where generally the proton ion (H+ ) moves from anode to cathode in the fuel cell. For that reason, the last ones are also called proton exchange membranes (PEMs). The AEMs are used in alkaline media and the others in acid media. The proton exchange membrane fuel cells (PEMFCs) have been historically more used because of the discovery of the Nafion® membrane that has good ionic conductivity and durability and has been the standard so far [5]. The higher mobility of the H<sup>+</sup> ion compared to OH in aqueous media has also been a relevant factor [6]. The alkaline media in the other hand does not have a standard membrane and presents relevant advantages that have produced high interest in the last years. Some of them are the faster electrochemical kinetics in the alkaline media, possible absence of noble metals as catalysts, minimized corrosion problems, and cogeneration of electricity and valuable chemicals [7].

Independently of the media, membranes are expected to have good ionic conductivity, long-term chemical and electrochemical stability, adequate mechanical strength, good moisture control, low fuel or oxygen crossover, and production costs compatible with intended application [5, 6].

In the FCs, the active materials (fuel and oxidant) are continuously fed and extracted. The fuel cell, Figure 1, is made up of two electrodes: the anode, where the Benzimidazole as Solid Electrolyte Material for Fuel Cells DOI: http://dx.doi.org/10.5772/intechopen.85430

reaction. This process allows theoretical efficiencies as high as 80% [1], which is a wonderful advantage compared to the thermal machines limited thermodynamically by the Carnot cycle. There are many types of fuel cells, the most relevant are alkaline fuel cells (AFCs), polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), and solid

Chemistry and Applications of Benzimidazole and its Derivatives

Polymer electrolyte membrane fuel cells have as principal characteristics the low operation temperature (<120°C), high power density, and easy scale-up, making them a promising technology for power generation. Their main application fields are backup power, portable power, distributed generation, and transportation [1]. It is relevant to note the role of transition energy technology, since they can play an important function in the near future in order to overcome the fossil fuel depletion and mitigate the climate change. The reason is that fuels like hydrogen or alcohols, which are produced by unsustainable ways, could be produced with renewable energies. An example of this is the actual production of hydrogen mainly from catalytic reforming of methane and just some from electrolysis [2]. The hydrogen can be produced from electrolysis powered with electricity coming from renewables. This should be done when production is higher than the demand, allowing to store chemically the energy and later use it when needed with a PEMFC; this is known as the "hydrogen economy system." It is also possible to accumulate energy in short-chain alcohols like methanol or ethanol and use them to power PEMFCs [3, 4], mainly used in the portable applications. A great advantage of this technology is the low pollution associated with the process. For example, when hydrogen is used as fuel, the only products are electricity and water. The potential of PEMFCs is really promising but still drawbacks as high cost (mainly from the expensive catalysts based in Pt) and low durability have to be overcome for a general

In PEMFCs one of the most important components is the polymeric ion exchange membrane (IEM) that works as an electrolyte. It has to be an electrical insulator to force the produced electrons to go through the external circuit, it also has to avoid the mixture of the reagents supplied in anode and cathode, and it is responsible of the adequate ionic conductivity of the ions traveling through it. Depending on the ion movement, two types of IEMs can be distinguished: anion exchange membranes (AEMs), where the ionic charge carriers are the hydroxide ions (OH) that travel from cathode to anode, and cation exchange membranes

fuel cell. For that reason, the last ones are also called proton exchange membranes (PEMs). The AEMs are used in alkaline media and the others in acid media. The proton exchange membrane fuel cells (PEMFCs) have been historically more used because of the discovery of the Nafion® membrane that has good ionic conductivity and durability and has been the standard so far [5]. The higher mobility of the H<sup>+</sup> ion compared to OH in aqueous media has also been a relevant factor [6]. The alkaline media in the other hand does not have a standard membrane and presents relevant advantages that have produced high interest in the last years. Some of them are the faster electrochemical kinetics in the alkaline media, possible absence of noble metals as catalysts, minimized corrosion problems, and cogeneration of

Independently of the media, membranes are expected to have good ionic conductivity, long-term chemical and electrochemical stability, adequate mechanical strength, good moisture control, low fuel or oxygen crossover, and production costs

In the FCs, the active materials (fuel and oxidant) are continuously fed and extracted. The fuel cell, Figure 1, is made up of two electrodes: the anode, where the

) moves from anode to cathode in the

oxide fuel cells (SOFCs).

commercialization [1].

(CEMs) where generally the proton ion (H+

electricity and valuable chemicals [7].

182

compatible with intended application [5, 6].

Figure 1. Polymer exchange membrane fuel cell working with H2 and O2.

fuel is oxidized, and the cathode, where the oxidant (O2) is reduced. It also involves an electrolyte, which acts as an ionic conductor and electrical insulator. The electrons obtained in the anode are addressed directly to the cathode through the external circuit, generating an electric current directly usable. In addition, the protons produced in the anode go through the electrolyte, up to the cathode to reduce O2, generating water as the only product of the reaction. The reaction is exothermic and has a value of ΔH0 <sup>r</sup> = �285.83 kJ/mol for H2O (l) and � 241.862 kJ/mol for H2O (v). Although this is a spontaneous reaction, it needs to be catalyzed to be operational, since the kinetics of the process is too slow otherwise.

At atmospheric pressure, the maximum potential difference obtained by the fuel cell will be determined by the difference of energy between the initial and final state of the system. The Gibbs free energy variation of the process, ΔG, can be calculated from the operation temperature (T) and changes with both enthalpy (ΔH) and entropy (ΔS) of the reaction. Under standard conditions

$$
\Delta G^0 = \Delta S^0 - T\,\Delta S^0 \tag{1}
$$

and the maximum potential difference, obtained in the fuel cell, E0 theoric, will be

$$E^0 = \frac{-\Delta G^0}{nF} = 1.23V \tag{2}$$

where n is the number of electrons exchanged and F is Faraday's constant. At 298 K and 1 atm, <sup>Δ</sup>G0 <sup>=</sup> �237.340 J/mol and therefore E0 = 1.23 V. For an operating temperature of 80°C, the values of ΔH and ΔS change, but slightly, and the decrease in ΔG will be mainly due to the temperature, resulting in a theoretical potential difference of 1.18 V approximately. However, in practice this potential, called the open circuit potential, is significantly lower than this potential value, usually less than 1 V. This suggests that some losses appear in the fuel cell even when no external current is generated. The potential difference of the fuel cell in operation, that is, when the current is passing through the system, Efuel cell (I), will be given by the sum of thermodynamic or reversible value (I = 0), minus the anode and cathode activation overvoltage and the ohmic losses or overvoltage. The electrode kinetics was represented by the Butler-Volmer equation, the mass transport process was described by the multicomponent Stefan-Maxwell equations and Fick's law, and the ionic and electronic resistances are described by Ohm's law. The E fuel cell (I) value could be obtained by

$$E\_{\text{fuel cell } (I)} = E\_{\text{Rereversible } (I=0)} - \eta\_{\text{activation}} - \eta\_{\text{ohmic}} \tag{3}$$

side. Each hydrogen molecule on the cathode side reacts with oxygen on the surface of the catalyst resulting in two fewer electrons in the generated current that travels through the external circuit and thus in a reduction of cathode and the overall fuel cell potential. These losses are not big in fuel cell operation, but when the fuel cell is at open circuit potential or at very low current densities, this situation may have a dramatic effect on fuel cell potential. At least, all these losses have to be taken into account when the device works and have a lot to do with good fuel cell

Polybenzimidazoles are synthesized by the repetitive reaction of aromatic amino groups with carboxyl groups using a 1:2 molar ratio by the process of step-grow polymerization [9]. Usually the monomer reagents are a diacid and a tetra-amine, like the example in Figure 3. There are many polybenzimidazoles but the ones that

known as ABPBI. Both were first synthesized by Vogel and Marvel in 1961 [10]. For PBI the synthesis was a two-step process with an intermediate prepolymer that prevented the production of high molecular weight polymer. Cho et al. [11, 12]

(IPA) to do the synthesis in a single step obtaining high molecular weight, in the presence of catalyzers and at temperatures higher than 350°C. It is important to know the molecular weight of the polymer, which is obtained by the measurement

sulfuric acid. For membrane application, usually casted from solution, it is interesting to have high molecular weight in order to achieve mechanically stable membranes that can support higher doping and thus obtain better ionic conductivity. The previously described method of Vogel and Marvel and Cho et al. can be classified in the heterogeneous molten/solid state synthesis [13, 14]. The other synthesis

polyphosphoric acid (PPA) [15]; this method allows to use moderate temperature and more stable monomers and is excellent to synthesize linear high molecular weight polymers at laboratory or small batch scale. These advantages make this synthesis method the most commonly used. Another example of solvent is Eaton's reagent, a mixture of phosphorus pentoxide (P2O5) and methanesulfonic acid (MSA) proposed by Eaton et al. [16], which has low viscosity making it suitable for the homogeneous solution synthesis and the acid washing after it [17, 18]. A shorter reaction time with high molecular weight has been obtained using homogeneous solution microwave-assisted synthesis recently, both for PBI and ABPBI [14].



) of the polymer dissolved in concentrated



have presented better application and have been more studied are poly(2,2<sup>0</sup>

,4,4<sup>0</sup>

method used is the homogeneous solution synthesis, using solvents as


performance.

phenylene)-5,5<sup>0</sup>

Figure 3.

185

Example synthesis of (top) poly(2,2<sup>0</sup>

(bottom) poly(2,5-benzimidazole) (ABPBI).

discovered a process with 3,3<sup>0</sup>

of the inherent viscosity (IV, in dLg�<sup>1</sup>

2. Synthesis of polybenzimidazole materials

Benzimidazole as Solid Electrolyte Material for Fuel Cells

DOI: http://dx.doi.org/10.5772/intechopen.85430

The losses considered are in relation to the activation overvoltages, and they are dependent on the kinetics of the processes involved and therefore directly related to the goodness of catalyst used for the process. Thus, ηactivation is related directed with both the oxidation kinetic reaction and the reduction kinetic reaction of the reagent involved in the catalysts surface materials. The ηactivation for an H2/O2 fed in PEMFC will come mainly determined by the slow kinetics of oxygen reduction reaction (ORR) on the catalyst material in comparison to H2 oxidation, while ηactivation (transport) is the consequence of material transport. This overpotential considers the combination of the flow of reactants and products in the fuel cell. The polarization from concentration gradients occurs when a reactant is rapidly consumed at the electrode by the electrochemical reaction so that gradients are established. The ηohmic = iR will be due to the combination of resistors provided by internal/external electrical contacts and ionic resistance due to ion motion through the membrane. Therefore, the fuel cell when current is not zero has an Efuel cell(I) expression like this:

$$E\_{\text{fuel cell (I)}} = E\_{\text{rener (I = 0)}} - \frac{RT}{a\_c} \ln\left(\frac{i}{i\_{0,c}}\right) - \frac{RT}{a\_a} \ln\left(\frac{i}{i\_{0,a}}\right) - \frac{RT}{a\_c} \ln\left(\frac{i\_{I,c}}{i\_{I,c} - i}\right) - \frac{RT}{a\_a} \ln\left(\frac{i\_{I,a}}{i\_{I,a} - i}\right) \tag{4}$$

$$E\_{\text{re} (I = 0)} = 1.229 - \left(8.5 \times 10^{-4}\right)(T - 298.15) + \left(4.308 \times 10^{-5}\right)T[\ln\left(P\_{H2}\right) + 0.5 \ln\left(P\_{O2}\right)] \tag{5}$$

being ioc, αc, iLc and ioa, αa, iLa the exchange current density, transfer coefficient, and limit current density of the cathodic and anodic processes, respectively [8]. The polarization curve of the device can be found in Figure 2, where the different losses mentioned above are indicated.

It was previously stated that the ion exchange polymer membrane is electrically insulator and practically impermeable to reactant gases, but some small amount of mainly H2 will crossover from anode to cathode. Hydrogen that permeates through the membrane does not participate in the electrochemical reaction on the anode

Figure 2. Polarization curves with voltage losses of a fuel cell.

ionic and electronic resistances are described by Ohm's law. The E fuel cell (I) value

Chemistry and Applications of Benzimidazole and its Derivatives

The losses considered are in relation to the activation overvoltages, and they are dependent on the kinetics of the processes involved and therefore directly related to the goodness of catalyst used for the process. Thus, ηactivation is related directed with both the oxidation kinetic reaction and the reduction kinetic reaction of the reagent involved in the catalysts surface materials. The ηactivation for an H2/O2 fed in PEMFC will come mainly determined by the slow kinetics of oxygen reduction reaction (ORR) on the catalyst material in comparison to H2 oxidation, while ηactivation (transport) is the consequence of material transport. This overpotential considers the combination of the flow of reactants and products in the fuel cell. The polarization from concentration gradients occurs when a reactant is rapidly consumed at the electrode by the electrochemical reaction so that gradients are established. The ηohmic = iR will be due to the combination of resistors provided by internal/external electrical contacts and ionic resistance due to ion motion through the membrane. Therefore, the fuel cell when current is not zero has an Efuel cell(I)

Efuel cell Ið Þ ¼ EReversible Ið Þ <sup>¼</sup><sup>0</sup> � ηactivation � ηohmic (3)

could be obtained by

expression like this:

Efuel cell Ið Þ <sup>¼</sup> Erever Ið Þ <sup>¼</sup><sup>0</sup> � RT

mentioned above are indicated.

Polarization curves with voltage losses of a fuel cell.

Figure 2.

184

αc

ln <sup>i</sup> i0,c 

� RT αa

ln <sup>i</sup> i0,a 

Erev Ið Þ <sup>¼</sup><sup>0</sup> <sup>¼</sup> <sup>1</sup>:<sup>229</sup> � <sup>8</sup>:<sup>5</sup> � <sup>10</sup>�<sup>4</sup> ð Þþ <sup>T</sup> � <sup>298</sup>:<sup>15</sup> <sup>4</sup>:<sup>308</sup> � <sup>10</sup>�<sup>5</sup> <sup>T</sup>½ � ln ð Þþ PH<sup>2</sup> <sup>0</sup>:5ln ð Þ PO<sup>2</sup>

being ioc, αc, iLc and ioa, αa, iLa the exchange current density, transfer coefficient, and limit current density of the cathodic and anodic processes, respectively [8]. The polarization curve of the device can be found in Figure 2, where the different losses

It was previously stated that the ion exchange polymer membrane is electrically insulator and practically impermeable to reactant gases, but some small amount of mainly H2 will crossover from anode to cathode. Hydrogen that permeates through the membrane does not participate in the electrochemical reaction on the anode

� RT αc

ln iI,c iI,c � i 

� RT αa

ln iI,a iI,a � i 

(4)

(5)

side. Each hydrogen molecule on the cathode side reacts with oxygen on the surface of the catalyst resulting in two fewer electrons in the generated current that travels through the external circuit and thus in a reduction of cathode and the overall fuel cell potential. These losses are not big in fuel cell operation, but when the fuel cell is at open circuit potential or at very low current densities, this situation may have a dramatic effect on fuel cell potential. At least, all these losses have to be taken into account when the device works and have a lot to do with good fuel cell performance.

#### 2. Synthesis of polybenzimidazole materials

Polybenzimidazoles are synthesized by the repetitive reaction of aromatic amino groups with carboxyl groups using a 1:2 molar ratio by the process of step-grow polymerization [9]. Usually the monomer reagents are a diacid and a tetra-amine, like the example in Figure 3. There are many polybenzimidazoles but the ones that have presented better application and have been more studied are poly(2,2<sup>0</sup> -(mphenylene)-5,5<sup>0</sup> -bibenzimidazole), known as PBI, and poly(2,5-benzimidazole), known as ABPBI. Both were first synthesized by Vogel and Marvel in 1961 [10]. For PBI the synthesis was a two-step process with an intermediate prepolymer that prevented the production of high molecular weight polymer. Cho et al. [11, 12] discovered a process with 3,3<sup>0</sup> ,4,4<sup>0</sup> -tetraaminobiphenyl (TAB) and isophthalic acid (IPA) to do the synthesis in a single step obtaining high molecular weight, in the presence of catalyzers and at temperatures higher than 350°C. It is important to know the molecular weight of the polymer, which is obtained by the measurement of the inherent viscosity (IV, in dLg�<sup>1</sup> ) of the polymer dissolved in concentrated sulfuric acid. For membrane application, usually casted from solution, it is interesting to have high molecular weight in order to achieve mechanically stable membranes that can support higher doping and thus obtain better ionic conductivity. The previously described method of Vogel and Marvel and Cho et al. can be classified in the heterogeneous molten/solid state synthesis [13, 14]. The other synthesis method used is the homogeneous solution synthesis, using solvents as polyphosphoric acid (PPA) [15]; this method allows to use moderate temperature and more stable monomers and is excellent to synthesize linear high molecular weight polymers at laboratory or small batch scale. These advantages make this synthesis method the most commonly used. Another example of solvent is Eaton's reagent, a mixture of phosphorus pentoxide (P2O5) and methanesulfonic acid (MSA) proposed by Eaton et al. [16], which has low viscosity making it suitable for the homogeneous solution synthesis and the acid washing after it [17, 18]. A shorter reaction time with high molecular weight has been obtained using homogeneous solution microwave-assisted synthesis recently, both for PBI and ABPBI [14].

#### Figure 3.

Example synthesis of (top) poly(2,2<sup>0</sup> -(m-phenylene)-5,5<sup>0</sup> -bibenzimidazole), abbreviated as PBI, and (bottom) poly(2,5-benzimidazole) (ABPBI).

ABPBI is synthesized from a single monomer, (3,4-diaminobenzoic acid) (DABA), which as the advantages of being less expensive, commercially available, and non-carcinogenic. The scheme is shown in Figure 3. Different syntheses have been done by the homogeneous solution method in PPA or Eaton's reagent, and inherent viscosity values as high as 7.33 have been reached, as reported by Li et al. by using recrystallized DABA [19]. This is essential for the direct casting of ABPBI membranes since it has been suggested by Asensio and Gómez-Romero that values of at least 2.3 dL g�<sup>1</sup> are necessary to cast good membranes [13].

compounds or the repeating unit of a polymer. Solvents that can be used include deuterated dimethyl sulfoxide (DMSO- d6) and deuterated sulfuric acid (D2SO4).

D2SO4, the fast exchange interaction with the proton in the imine of the imidazole rings (-NH-) causes the chemical shift of that hydrogen to be often indiscernible

3145 cm<sup>1</sup> has been attributed to the stretching vibrations of N–H groups selfassociated by hydrogen bonds, and the peak at 3145 cm<sup>1</sup> is assigned to the N–H

observed come from the vibration of C=C and C=N bonds [27]. In the Raman spectra of PBI, the most significant absorption band comes from the benzene ring vibration and is located around 1000 cm<sup>1</sup> [28]. For the measurement of the Raman spectra, it is relevant to use an excitation wavelength of 785 nm (red laser) since it gives much less fluorescence than the 532 nm (green laser) [29]. Because the structure and functional groups are the same, ABPBI presents the same IR peaks than PBI, as reported by Asensio et al. [30]. They also investigated the bands appearing when the polymer membrane is doped with phosphoric acid: in the N–H stretching zone, they found the evolution of nitrogen protonation by the acid, and

stronger, while the nonassociated imidazole protons decreases. In polybenzimidazoles doped with alkaline media for anion conductivity purposes, the structure changes are also clearly identified. Aili et al. [31] investigated PBI with different degrees of KOH doping and found that in the IR spectra, at KOH concentrations higher than 15 wt.%, the N–H stretching band at 3415 cm<sup>1</sup> disappear as well as the broad band around 3100 cm<sup>1</sup> of shelf-associated hydrogen bonded N–H groups. They concluded that the IR data indicated the predominance of the deprotonated form of PBI with KOH concentrations of the bulk solution around 15–20 wt.%. In

bulk KOH concentration, and most signals from the aromatic protons showed upfield shift compared to pristine PBI, indicating complete ionization. This full ionization of the polymer releases the extensive intermolecular hydrogen bonding allowing for high swelling behavior and water and KOH uptake and therefore enhanced ion conductivity. This study corroborates the knowledge that the introduction of species that interact with imidazole groups by hydrogen bonding decreases the intermolecular polybenzimidazole cohesion, causing a strong plasticizing effect observed in the great decay of the tensile strength and enhanced elongation at break when the doping level increases, especially when full ionization of the polymer is reached. Using an even higher concentration doping solution, they found that a higher crystallinity structure was obtained, as observed by XRD, mechanical test, and swelling behavior measurements. X-ray photoelectron spectroscopy (XPS) is also a helpful technique for the characterization of polybenzimidazoles, concretely for the capacity to distinguish the oxidation states of the elements present and allow their quantification in the surface of the membrane [29]. Other fundamental measurements usually performed on the synthesized membranes are the determination of the ionic conductivity, the swelling behavior with water and in acidic/alkaline media, or the thermogravimetric analysis (TGA). In conclusion, a full set of characterization analysis have been studied and are used

H-NMR spectrum the signal at 13.3 ppm of the N–H proton disappeared at high

H-NMR PBI characteristic signals in DMSO-d6 are at 13.2 (2H), 9.1 (1H), 8.3 (2H), and 8.0–7.6 (7H) ppm, the first of them attributed to the imidazole protons and the others to the aromatic protons [25, 26]. IR and Raman spectroscopy are also used, mainly to identify different functional groups and obtain or corroborate the chemical structure of the polymers [24, 27, 28]. In PBI, the IR spectrum region from 2000 to 4000 cm<sup>1</sup> is interesting since N–H stretching modes occur in this range,

H-NMR spectra is DMSO-d6 because with

. The broad band around

, the peaks

–H vibration becomes

The most commonly used to record <sup>1</sup>

DOI: http://dx.doi.org/10.5772/intechopen.85430

Benzimidazole as Solid Electrolyte Material for Fuel Cells

showing three typical bands at 3415, 3145, and 3063 cm<sup>1</sup>

groups stretching vibration. In the region from 1630 to 1500 cm<sup>1</sup>

in the medium and high doped samples, the broad band of N+

[24]. <sup>1</sup>

the <sup>1</sup>

187

In the case of ABPBI, since there is only a monomer, its purity is not as critical as in PBI; however, the use of high purity monomer produces polymers of high molecular weight [20]. Since polybenzimidazoles have to be doped in order to become ionic conductors, two methods are used to prepare the membranes: direct casting from the polymerization solution, as the work developed by Asensio et al. [21], or dissolving the previously synthesized polymer and then doing the casting of the membrane. The casting process consists in the formation of a thin film by the deposition of the polymer by evaporation of the solvent in the solution. To solubilize PBI or ABPBI, usually strong bases or acids are needed; only a few organic solvents can also do it; one of them is the N,N-dimethylacetamide (DMAc) [13, 22]. There is also an alternative way to cast ABPBI membranes from a mixture of NaOH and ethanol [23].

#### 3. Properties of the materials and characterization

The structure of polybenzimidazoles has a good degree of flexibility and chemical and thermal resistance compared to other polymers with more single bonds in their main chain between aromatic units. The presence of aromatic units in the main chain to have higher thermal stability than the aliphatic analogs is also important [9]. In order to characterize polybenzimidazoles, one of the most important parameters is the molecular weight of the polymer, which will be highly related to the final membranes properties. The common way to obtain the molecular weight is by measurement of the intrinsic viscosity of the polymer (ηIV) at a certain temperature (normally 25–30°C). From the plotting of the specific viscosity (ηsp) as function of the polymer concentration, the intrinsic viscosity is calculated extrapolating to zero concentration. A simpler measurement process was proposed to do the calculation with a single-point method using Eq. (6), where C is the polymer concentration in a concentrated acid like 96 wt% H2SO4:

$$\eta\_{IV} = (\eta\_{SP} + \mathfrak{Z}\ln\left(\mathfrak{1} + \eta\_{SP}\right)) / \mathfrak{4C} \tag{6}$$

The protocol test is to calculate the ηsp of a polymer solution 5 g L�<sup>1</sup> in concentrated sulfuric acid at 30°C using an Ubbelohde viscometer. From the ηIV value, the average molecular weight is calculated with the Mark-Houwink- Sakurada expression:

$$
\eta\_{IV} = K \ast \mathcal{M}\_W^a \tag{7}
$$

where the Mark-Houwink constants depend on the molecular weight range and distribution. Values often used for this constants are K = 1.94 � <sup>10</sup>�<sup>4</sup> dL g�<sup>1</sup> , and α = 0.791, obtained from Buckley et al. by light scattering measurements. Other solvents as formic acid or MSA can also be used to measure the viscosity of polybenzimidazoles [14].

There are various techniques in order to investigate the structure of polybenzimidazoles. Nuclear magnetic resonance is very powerful for pure organic

#### Benzimidazole as Solid Electrolyte Material for Fuel Cells DOI: http://dx.doi.org/10.5772/intechopen.85430

ABPBI is synthesized from a single monomer, (3,4-diaminobenzoic acid) (DABA), which as the advantages of being less expensive, commercially available, and non-carcinogenic. The scheme is shown in Figure 3. Different syntheses have been done by the homogeneous solution method in PPA or Eaton's reagent, and inherent viscosity values as high as 7.33 have been reached, as reported by Li et al. by using recrystallized DABA [19]. This is essential for the direct casting of ABPBI membranes since it has been suggested by Asensio and Gómez-Romero that values

In the case of ABPBI, since there is only a monomer, its purity is not as critical as

The structure of polybenzimidazoles has a good degree of flexibility and chemical and thermal resistance compared to other polymers with more single bonds in their main chain between aromatic units. The presence of aromatic units in the main chain to have higher thermal stability than the aliphatic analogs is also important [9]. In order to characterize polybenzimidazoles, one of the most important parameters is the molecular weight of the polymer, which will be highly related to the final membranes properties. The common way to obtain the molecular weight is by measurement of the intrinsic viscosity of the polymer (ηIV) at a certain temperature (normally 25–30°C). From the plotting of the specific viscosity (ηsp) as function of the polymer concentration, the intrinsic viscosity is calculated extrapolating to zero concentration. A simpler measurement process was proposed to do the calculation with a single-point method using Eq. (6), where C is the polymer

The protocol test is to calculate the ηsp of a polymer solution 5 g L�<sup>1</sup> in concentrated sulfuric acid at 30°C using an Ubbelohde viscometer. From the ηIV value, the average molecular weight is calculated with the Mark-Houwink- Sakurada

<sup>η</sup>IV <sup>¼</sup> <sup>K</sup> <sup>∗</sup> <sup>M</sup><sup>α</sup>

distribution. Values often used for this constants are K = 1.94 � <sup>10</sup>�<sup>4</sup> dL g�<sup>1</sup>

midazoles. Nuclear magnetic resonance is very powerful for pure organic

α = 0.791, obtained from Buckley et al. by light scattering measurements. Other solvents as formic acid or MSA can also be used to measure the viscosity of

where the Mark-Houwink constants depend on the molecular weight range and

There are various techniques in order to investigate the structure of polybenzi-

ηIV ¼ ηSP þ 3 ln 1 þ ηSP ð Þ ð Þ =4C (6)

<sup>W</sup> (7)

, and

in PBI; however, the use of high purity monomer produces polymers of high molecular weight [20]. Since polybenzimidazoles have to be doped in order to become ionic conductors, two methods are used to prepare the membranes: direct casting from the polymerization solution, as the work developed by Asensio et al. [21], or dissolving the previously synthesized polymer and then doing the casting of the membrane. The casting process consists in the formation of a thin film by the deposition of the polymer by evaporation of the solvent in the solution. To solubilize PBI or ABPBI, usually strong bases or acids are needed; only a few organic solvents can also do it; one of them is the N,N-dimethylacetamide (DMAc) [13, 22]. There is also an alternative way to cast ABPBI membranes from a mixture of NaOH

of at least 2.3 dL g�<sup>1</sup> are necessary to cast good membranes [13].

Chemistry and Applications of Benzimidazole and its Derivatives

3. Properties of the materials and characterization

concentration in a concentrated acid like 96 wt% H2SO4:

and ethanol [23].

expression:

186

polybenzimidazoles [14].

compounds or the repeating unit of a polymer. Solvents that can be used include deuterated dimethyl sulfoxide (DMSO- d6) and deuterated sulfuric acid (D2SO4). The most commonly used to record <sup>1</sup> H-NMR spectra is DMSO-d6 because with D2SO4, the fast exchange interaction with the proton in the imine of the imidazole rings (-NH-) causes the chemical shift of that hydrogen to be often indiscernible [24]. <sup>1</sup> H-NMR PBI characteristic signals in DMSO-d6 are at 13.2 (2H), 9.1 (1H), 8.3 (2H), and 8.0–7.6 (7H) ppm, the first of them attributed to the imidazole protons and the others to the aromatic protons [25, 26]. IR and Raman spectroscopy are also used, mainly to identify different functional groups and obtain or corroborate the chemical structure of the polymers [24, 27, 28]. In PBI, the IR spectrum region from 2000 to 4000 cm<sup>1</sup> is interesting since N–H stretching modes occur in this range, showing three typical bands at 3415, 3145, and 3063 cm<sup>1</sup> . The broad band around 3145 cm<sup>1</sup> has been attributed to the stretching vibrations of N–H groups selfassociated by hydrogen bonds, and the peak at 3145 cm<sup>1</sup> is assigned to the N–H groups stretching vibration. In the region from 1630 to 1500 cm<sup>1</sup> , the peaks observed come from the vibration of C=C and C=N bonds [27]. In the Raman spectra of PBI, the most significant absorption band comes from the benzene ring vibration and is located around 1000 cm<sup>1</sup> [28]. For the measurement of the Raman spectra, it is relevant to use an excitation wavelength of 785 nm (red laser) since it gives much less fluorescence than the 532 nm (green laser) [29]. Because the structure and functional groups are the same, ABPBI presents the same IR peaks than PBI, as reported by Asensio et al. [30]. They also investigated the bands appearing when the polymer membrane is doped with phosphoric acid: in the N–H stretching zone, they found the evolution of nitrogen protonation by the acid, and in the medium and high doped samples, the broad band of N+ –H vibration becomes stronger, while the nonassociated imidazole protons decreases. In polybenzimidazoles doped with alkaline media for anion conductivity purposes, the structure changes are also clearly identified. Aili et al. [31] investigated PBI with different degrees of KOH doping and found that in the IR spectra, at KOH concentrations higher than 15 wt.%, the N–H stretching band at 3415 cm<sup>1</sup> disappear as well as the broad band around 3100 cm<sup>1</sup> of shelf-associated hydrogen bonded N–H groups. They concluded that the IR data indicated the predominance of the deprotonated form of PBI with KOH concentrations of the bulk solution around 15–20 wt.%. In the <sup>1</sup> H-NMR spectrum the signal at 13.3 ppm of the N–H proton disappeared at high bulk KOH concentration, and most signals from the aromatic protons showed upfield shift compared to pristine PBI, indicating complete ionization. This full ionization of the polymer releases the extensive intermolecular hydrogen bonding allowing for high swelling behavior and water and KOH uptake and therefore enhanced ion conductivity. This study corroborates the knowledge that the introduction of species that interact with imidazole groups by hydrogen bonding decreases the intermolecular polybenzimidazole cohesion, causing a strong plasticizing effect observed in the great decay of the tensile strength and enhanced elongation at break when the doping level increases, especially when full ionization of the polymer is reached. Using an even higher concentration doping solution, they found that a higher crystallinity structure was obtained, as observed by XRD, mechanical test, and swelling behavior measurements. X-ray photoelectron spectroscopy (XPS) is also a helpful technique for the characterization of polybenzimidazoles, concretely for the capacity to distinguish the oxidation states of the elements present and allow their quantification in the surface of the membrane [29]. Other fundamental measurements usually performed on the synthesized membranes are the determination of the ionic conductivity, the swelling behavior with water and in acidic/alkaline media, or the thermogravimetric analysis (TGA). In conclusion, a full set of characterization analysis have been studied and are used

to identify and test the properties of the synthesized polybenzimidazoles and the membranes prepared with them.

heat with a simpler cooling system. If the fuel cell is working with reformed natural gas as a power source, the device does not require humidification of reactants due to the simple water management; that is why all these features greatly

In the PBI/H3PO4 system, the polybenzimidazole acts not only as a matrix polymer but also as proton acceptor [39]. For HT-PEMFCs, PBI/H3PO4 is considered a reasonably successful solid electrolyte because the excellent conductivity and thermochemical stability. Phosphoric acid has been widely employed as an anhydrous proton conductor because of its high proton conductivity, low cost, and thermal stability. At temperatures above 150°C, the dehydration of the acid occurs and yields pyrophosphoric acid or higher oligomers, which exhibit worse proton conductivity. On the other hand, the long-running operation leads to the release and dilution of H3PO4 from the membranes, which results in a loss of the acid into the fuel cell gas/vapor exhaust streams, the decrease of membrane ionic conductivity, and thus a lower fuel cell performance occurs. The high proton conductivity of the membranes was proved only when the polymer holds a large excess of phosphoric acid [40]. The optimum doping level was around 5 moles H3PO4 per PBI repeat unit, where a compromise between conductivity and mechanical properties was

A thick membrane is not usually advantageous because it is mainly responsible

The problems of HT-PEMFCs operating at temperatures up to 100°C are not solved yet and demonstrate the necessity of research on new and more satisfactory alternatives. In this context, the ionic liquids (ILs) have been used as nonaqueous and low-volatility proton carriers in replacement of aqueous electrolytes. The protic ILs for example are able to transport protons due to their acid-base character and their capability to form complex or intermolecular hydrogen bonds [43] even in nonaqueous conditions. This type of materials tries to overcome the formation of unstable materials in the operating conditions and then to improve the performance of the PEMFC at high temperatures. The first research team working in this subject was Watanabe and colleagues, who identified the potential electroactive use of ILs in fuel cell reactions [44]. Sometimes, polymer phase substrate and the IL result in nonhomogeneous and unmanageable membranes when both components are integrated together. In general, ILs and polymers dissolved in a common solvent and later are casted as a film. In this way hybrid membranes are obtained, and the materials may be studied once the solvent has been removed. PBI-based hybrid membranes holding ILs are examples of this methodology. Greenbaum et al. [45] demonstrated that the composite gel-type membranes obtained from H3PO4 and aprotic hydrophilic IL, namely, 1-propyl-3-methylimidazolium dihydrogen phosphate [PMI][H2PO4] and PBI, can be operated as ion exchange membrane up to 150°C in a PEMFC. The composite membranes were homogeneous and both chemically and thermally stable with wide temperature range. Nevertheless, phase separation occurred when mixing the 1-ethyl-3-methylimidazolium triflate [EMI][Tf] or 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMI][TFSI] ILs with H3PO4 and PBI, resulting in homogeneous membranes. Schauer et al. [46] investigated the use of aprotic ionic liquid 1-butyl-3-methylimidazoliumtrifluoromethanesulfonate [BMIM][TfO] and protic ionic liquid 1-ethylimidazoliumtrifluoromethanesulfonate [EIM][TfO] to prepare membranes with several different

for the large ohmic polarization and modest power performance of HT-MEA. However, approx. 100 μm has been implemented with the intention of improving their mechanical properties [41]. The acid doping is an essential process, but it softens the PBI membrane, causing membrane ripping in MEA fabrication. The mechanical stability of the doped PBI membrane can be improved by lowering the

H3PO4 doping level; however, the proton conductivity is reduced [42].

simplify design of HT-PEMFC stack [38].

DOI: http://dx.doi.org/10.5772/intechopen.85430

Benzimidazole as Solid Electrolyte Material for Fuel Cells

achieved.

189

#### 4. Commercial availability

There have been different companies relevant in the fuel cell membrane field, probably the most known one is DuPont for developing the Nafion® membrane made of a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer with excellent thermal and mechanical stability as well as high proton conductivity in low-temperature fuel cells. Companies like Solvay, Gore, and others have also commercialized membranes with this chemistry. This membrane has been the standard for fuel cells used in low-temperature and acidic media, but at temperatures higher than 100°C, Nafion® performance drops dramatically due to the lower hydration level. It is in these conditions where membranes made of polybenzimidazoles have shown good performance and promising applicability, and production for commercialization has occurred. BASF Fuel Cell (formerly PEMEAS Fuel Cell), a part of one of the larger chemistry industries, has developed a product line about a membrane electrode assembly (MEA) based in a PBI membrane, Celtec® [32, 33]. These MEAs optimal operation conditions are between 120 and 180°C, doped in phosphoric acid. They have shown relevant advantages working as high temperature PMFCs, like high tolerance to fuel gas impurities such as CO (up to 3%), H2S (up to 10 ppm), NH3, or methanol, no humidification required, far simpler system due to elimination of water, and a less complex reformer technology. In addition, several advantages can be obtained for the electrocatalysis, but it is necessary to be especially careful at the high stability toward corrosion needed to ensure long fuel cell lifetimes, apart from high activity for the oxidation of the fuels and the oxygen reduction reaction. Other companies that commercialize PBI- and PBI-based membranes are "PBI Performance Products" with their Celazole® PBI PEM [22, 34] and Danish Power Systems with their Dapozol® membranes and MEAs [35]. Membranes based on PBI are of high applicability as it can be observed, both for the fuel cell technology in development and also for other applications as carbon capture, pervaporation dehydration processes, or electrochemical hydrogen separation, among others.

#### 5. Proton exchange membrane fuel cells (PEMFCs)

Polybenzimidazole (PBI) as ionic exchange membrane can be used as proton exchange if the material is doped with phosphoric acid (H3PO4), sulfuric acid (H2SO4), and nitric acid (HNO3) solvent media. The PBI has benzimidazole units in the polymer chain and bears the pKa = 5.5 that is responsible for the weak acid character, and they have excellent oxidative and thermal stability [36]. The acid molecules penetrate the membranes during doping process, due to the acid-base interaction between them and gradually swelling of PBI membrane. Therefore, PBI can be easily doped with different types of strong acids, which act as predominant protonation through the PBI membranes.

In these circumstances, the material can work as solid electrolyte in a fuel cell in temperature range between 100 and 200°C, overcoming the dehydration problem that the Nafion® membrane has in operation condition at around 100°C and in consequence the dramatically reduction of its proton conductivity, presenting a near zero electro-osmotic drag [37]. High temperature makes HT-PEMFC more tolerant to impurities in feed gases (CO, e.g.) and simplifies elimination of waste

#### Benzimidazole as Solid Electrolyte Material for Fuel Cells DOI: http://dx.doi.org/10.5772/intechopen.85430

to identify and test the properties of the synthesized polybenzimidazoles and the

There have been different companies relevant in the fuel cell membrane field, probably the most known one is DuPont for developing the Nafion® membrane made of a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer with excellent thermal and mechanical stability as well as high proton conductivity in low-temperature fuel cells. Companies like Solvay, Gore, and others have also commercialized membranes with this chemistry. This membrane has been the standard for fuel cells used in low-temperature and acidic media, but at temperatures higher than 100°C, Nafion® performance drops dramatically due to the lower hydration level. It is in these conditions where membranes made of polybenzimidazoles have shown good performance and promising applicability, and production for commercialization has occurred. BASF Fuel Cell (formerly PEMEAS Fuel Cell), a part of one of the larger chemistry industries, has developed a product line about a membrane electrode assembly (MEA) based in a PBI membrane, Celtec® [32, 33]. These MEAs optimal operation conditions are between 120 and 180°C, doped in phosphoric acid. They have shown relevant advantages working as high temperature PMFCs, like high tolerance to fuel gas impurities such as CO (up to 3%), H2S (up to 10 ppm), NH3, or methanol, no humidification required, far simpler system due to elimination of water, and a less complex reformer technology. In addition, several advantages can be obtained for the electrocatalysis, but it is necessary to be especially careful at the high stability toward corrosion needed to ensure long fuel cell lifetimes, apart from high activity for the oxidation of the fuels and the oxygen reduction reaction. Other companies that commercialize PBI- and PBI-based membranes are "PBI Performance Products" with their Celazole® PBI PEM [22, 34] and Danish Power Systems with their Dapozol® membranes and MEAs [35]. Membranes based on PBI are of high applicability as it can be observed, both for the fuel cell technology in development and also for other applications as carbon capture, pervaporation dehydration processes, or electrochemical hydrogen

membranes prepared with them.

Chemistry and Applications of Benzimidazole and its Derivatives

4. Commercial availability

separation, among others.

188

5. Proton exchange membrane fuel cells (PEMFCs)

protonation through the PBI membranes.

Polybenzimidazole (PBI) as ionic exchange membrane can be used as proton exchange if the material is doped with phosphoric acid (H3PO4), sulfuric acid (H2SO4), and nitric acid (HNO3) solvent media. The PBI has benzimidazole units in the polymer chain and bears the pKa = 5.5 that is responsible for the weak acid character, and they have excellent oxidative and thermal stability [36]. The acid molecules penetrate the membranes during doping process, due to the acid-base interaction between them and gradually swelling of PBI membrane. Therefore, PBI can be easily doped with different types of strong acids, which act as predominant

In these circumstances, the material can work as solid electrolyte in a fuel cell in temperature range between 100 and 200°C, overcoming the dehydration problem that the Nafion® membrane has in operation condition at around 100°C and in consequence the dramatically reduction of its proton conductivity, presenting a near zero electro-osmotic drag [37]. High temperature makes HT-PEMFC more tolerant to impurities in feed gases (CO, e.g.) and simplifies elimination of waste

heat with a simpler cooling system. If the fuel cell is working with reformed natural gas as a power source, the device does not require humidification of reactants due to the simple water management; that is why all these features greatly simplify design of HT-PEMFC stack [38].

In the PBI/H3PO4 system, the polybenzimidazole acts not only as a matrix polymer but also as proton acceptor [39]. For HT-PEMFCs, PBI/H3PO4 is considered a reasonably successful solid electrolyte because the excellent conductivity and thermochemical stability. Phosphoric acid has been widely employed as an anhydrous proton conductor because of its high proton conductivity, low cost, and thermal stability. At temperatures above 150°C, the dehydration of the acid occurs and yields pyrophosphoric acid or higher oligomers, which exhibit worse proton conductivity. On the other hand, the long-running operation leads to the release and dilution of H3PO4 from the membranes, which results in a loss of the acid into the fuel cell gas/vapor exhaust streams, the decrease of membrane ionic conductivity, and thus a lower fuel cell performance occurs. The high proton conductivity of the membranes was proved only when the polymer holds a large excess of phosphoric acid [40]. The optimum doping level was around 5 moles H3PO4 per PBI repeat unit, where a compromise between conductivity and mechanical properties was achieved.

A thick membrane is not usually advantageous because it is mainly responsible for the large ohmic polarization and modest power performance of HT-MEA. However, approx. 100 μm has been implemented with the intention of improving their mechanical properties [41]. The acid doping is an essential process, but it softens the PBI membrane, causing membrane ripping in MEA fabrication. The mechanical stability of the doped PBI membrane can be improved by lowering the H3PO4 doping level; however, the proton conductivity is reduced [42].

The problems of HT-PEMFCs operating at temperatures up to 100°C are not solved yet and demonstrate the necessity of research on new and more satisfactory alternatives. In this context, the ionic liquids (ILs) have been used as nonaqueous and low-volatility proton carriers in replacement of aqueous electrolytes. The protic ILs for example are able to transport protons due to their acid-base character and their capability to form complex or intermolecular hydrogen bonds [43] even in nonaqueous conditions. This type of materials tries to overcome the formation of unstable materials in the operating conditions and then to improve the performance of the PEMFC at high temperatures. The first research team working in this subject was Watanabe and colleagues, who identified the potential electroactive use of ILs in fuel cell reactions [44]. Sometimes, polymer phase substrate and the IL result in nonhomogeneous and unmanageable membranes when both components are integrated together. In general, ILs and polymers dissolved in a common solvent and later are casted as a film. In this way hybrid membranes are obtained, and the materials may be studied once the solvent has been removed. PBI-based hybrid membranes holding ILs are examples of this methodology. Greenbaum et al. [45] demonstrated that the composite gel-type membranes obtained from H3PO4 and aprotic hydrophilic IL, namely, 1-propyl-3-methylimidazolium dihydrogen phosphate [PMI][H2PO4] and PBI, can be operated as ion exchange membrane up to 150°C in a PEMFC. The composite membranes were homogeneous and both chemically and thermally stable with wide temperature range. Nevertheless, phase separation occurred when mixing the 1-ethyl-3-methylimidazolium triflate [EMI][Tf] or 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMI][TFSI] ILs with H3PO4 and PBI, resulting in homogeneous membranes. Schauer et al. [46] investigated the use of aprotic ionic liquid 1-butyl-3-methylimidazoliumtrifluoromethanesulfonate [BMIM][TfO] and protic ionic liquid 1-ethylimidazoliumtrifluoromethanesulfonate [EIM][TfO] to prepare membranes with several different

polymers: a polybenzimidazole derivative with benzofuranone (PBI-O-Ph), Udel® type polysulfone (Udel® PSU), and poly(vinylidene fluoride-co-hexafluoropropene) fluoroelastomer. The proton conductivity of the membranes was a function of the temperature and the ionic liquid amount in the membrane and the polymeric matrix itself. For PBI-O-Ph-based membranes, the conductivity was very low up to 90°C. Wang et al. [47] studied the PBI/IL composite membranes where the IL was 1-hexyl-3-methylimidazolium trifluoromethanesulfonate [HMI][Tf], an organosoluble fluorine ionic liquid. The ionic conductivity reached a value as high as 1.6 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> at 250°C under anhydrous conditions, and the results depended on temperature and IL content. The IL [HMI][Tf] works simultaneously as plasticizer and ion carrier. On the other hand, the major drawback related to the IL addition is a loss of membranes' mechanical properties, resulting in a good solid electrolyte to carry out the functions of HT-PEMFC at temperature > 200°C.

It is necessary to keep in mind that the requirements of cell lifetime vary for different applications, that is, 5000 h for cars, 20,000 h for buses, and 40,000 h for stationary application with continuous operation [43]. This means that the development of ionic exchange membranes with a long operating life is a challenge to

Many electrochemical systems use ion exchange membranes, such as fuel cells, electrolyzers, or redox flow batteries. Traditionally cation exchange membranes have been used in these systems due to the idea that anion exchange membranes had too low conductivity and stability. However, in the last years, many advances have been made, and anion exchange membranes (AEMs) are demonstrating to have performances comparable to acid ones, showing promising application in several technologies [2]. These membranes conduct negatively charged ions like OH or Cl and usually have positive-charged groups in the polymer structure, which could be directly present in the polymer backbone or more commonly fixed to it by extended side chains of varying lengths and chemistries. Varcoe et al. [2] investigated a deep review about the different chemistries of polymer backbones and head groups and their current state of research. The use of alkaline media, compared to acid media, has some advantages like the better electrochemical kinetics of the oxygen reduction reaction (ORR). This allows the possibility of using nonnoble metals in the electrocatalysts reducing the fuel cell system cost. Other advantages are the minimized corrosion problems and the cogeneration of electricity and valuable chemicals [7, 50]. Compared to classical alkaline fuel cells (AFCs) where the electrolyte is in aqueous phase, the use of AEMs solves the carbonation problems and the difficulties of the liquid electrolyte management. The fuels commonly used in anion exchange membrane fuel cells (AEMFCs) are hydrogen and alcohols. Hydrogen is the common fuel in commercialization and research and gives the higher power densities. On the other hand, alcohols like methanol or ethanol have the advantages of easier handle, store, and transport and can be acquired from abundant biomass, which is environmentally friendly considering the process is

Among all the polymers available and tested for AEMFCs, polybenzimidazoles have demonstrated good applicability, and the most commonly used and studied are PBI and ABPBI. Some of their advantages remain in the properties previously described, as excellent thermal stability, which allows to use them at higher temperatures, superior mechanical properties that can withstand the performance conditions, and the presence of amine and imine groups which form strong hydrogen bonding interactions and can be further functionalized. The great stability properties have also encouraged many studies combining polybenzimidazoles with other polymers, creating blend or crosslinked membranes with excellent performances. Membranes based on polybenzimidazoles alone or with other polymers have also demonstrated low alcohols crossover, making them adequate electrolytes in alcohol fuel cells. In the alkaline media, the pristine form of PBI can be equilibrated in aqueous solutions of alkali metal hydroxides forming homogeneous systems with the hydroxide salt and water dissolved in the polymer matrix. These materials have shown high ion conductivity and great chemical stability at low alkali concentrations and have been tested as anion-conducting electrolytes in fuel cells with hydrogen or alcohol and in water electrolyzers. In order to understand the physical and chemical properties of polybenzimidazoles in alkaline media, Aili et al. have made a study with thin films of PBI in aqueous KOH solution with concentrations

6. Anion exchange membrane fuel cells (AEMFCs)

Benzimidazole as Solid Electrolyte Material for Fuel Cells

DOI: http://dx.doi.org/10.5772/intechopen.85430

develop.

carbon-neutral.

191

In many cases imidazolium salts are the most investigated as ILs in these applications; composite membranes with good specific conductivity have been found for their application as electrolytes in PEMFCs; however low performances (maximum power densities of around 1 mW cm<sup>2</sup> [48]) have been obtained.

Another example of composite hybrid membranes is the use of PBI as matrix and the diethlyaminebisulfate/sulfate IL, [DE][SH], in different compositional ratios, PBI/[DE][SHx], as was published by Ocón et al. [49]. In this case, the composite membranes were obtained using a solution casting method. The interaction between the IL and the PBI was analyzed by FTIR spectroscopy. The imine group from the imidazole ring of PBI composite membranes showed no evidence of protonation, and consequently, the interaction between the IL and PBI was weak, remaining free inside of the PBI structure and allowing for the ionic conduction. The mechanical properties and tensile stress of pristine PBI was deteriorated dramatically on increasing the IL content, despite the fact that the conductivity values were very acceptable for the described application. For demanding fuel cell operation conditions, such as 200°C, and low humidity conditions, the PBI/[DE][SHx] membranes exhibited acceptable ionic conductivity values, higher than 0.01 S cm<sup>1</sup> . In addition to high proton conductivity in anhydrous environment, which is an indispensable condition for potential HT-PEMFC membrane candidates, other requisites must also be fulfilled: barrier to the reagent gases, thermal and dimensional stability under operating conditions, electrochemical stability under reducing and oxidizing potentials, and compatibility with the electrocatalyst. In this particular case, low open-circuit voltage (OCV) of the cell, 0.8 V, was obtained. This suggests a mixed potential, although no crossover was detected in the experiments. The authors suggested that kinetic complication could show up like additional oxidation and reduction reactions simultaneously with the corresponding oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR), respectively; furthermore, the poisoning effect of the H2S generated at the anode should not be ignored.

On the other side, the beneficial effect on the decrease of the IL viscosity was observed in the performance of the fuel cell. The optimum performance was obtained with no limiting current, being the maximum current density ca. 70 mA cm<sup>2</sup> and 13.5 mW cm<sup>2</sup> , using 100% relative humidity at 80°C. At temperature higher than 80°C, the system starts to dehydrate, whereas the IL viscosity increases and the proton diffusion was hindered. The performance at 150°C wasn't good showing clear evidences of the system dehydration at temperatures beyond 80°C. The migration of the IL from anode to cathode was demonstrated in postmortem analysis of PBI/[DE][SHx] composite-based electrodes. The IL went out of the composite membrane, and in consequence the cell resistivity increased by a factor of six times after polarization measurements.

polymers: a polybenzimidazole derivative with benzofuranone (PBI-O-Ph), Udel®-

opropene) fluoroelastomer. The proton conductivity of the membranes was a function of the temperature and the ionic liquid amount in the membrane and the polymeric matrix itself. For PBI-O-Ph-based membranes, the conductivity was very low up to 90°C. Wang et al. [47] studied the PBI/IL composite membranes where the IL was 1-hexyl-3-methylimidazolium trifluoromethanesulfonate [HMI][Tf], an organosoluble fluorine ionic liquid. The ionic conductivity reached a value as high

depended on temperature and IL content. The IL [HMI][Tf] works simultaneously as plasticizer and ion carrier. On the other hand, the major drawback related to the IL addition is a loss of membranes' mechanical properties, resulting in a good solid electrolyte to carry out the functions of HT-PEMFC at temperature > 200°C.

In many cases imidazolium salts are the most investigated as ILs in these applications; composite membranes with good specific conductivity have been found for their application as electrolytes in PEMFCs; however low performances (maximum

Another example of composite hybrid membranes is the use of PBI as matrix and the diethlyaminebisulfate/sulfate IL, [DE][SH], in different compositional ratios, PBI/[DE][SHx], as was published by Ocón et al. [49]. In this case, the composite membranes were obtained using a solution casting method. The interaction between the IL and the PBI was analyzed by FTIR spectroscopy. The imine group from the imidazole ring of PBI composite membranes showed no evidence of protonation, and consequently, the interaction between the IL and PBI was weak, remaining free inside of the PBI structure and allowing for the ionic conduction. The mechanical properties and tensile stress of pristine PBI was deteriorated dramatically on increasing the IL content, despite the fact that the conductivity values were very acceptable for the described application. For demanding fuel cell operation conditions, such as 200°C, and low humidity conditions, the PBI/[DE][SHx]

. In addition to high proton conductivity in anhydrous environment,

type polysulfone (Udel® PSU), and poly(vinylidene fluoride-co-hexafluor-

Chemistry and Applications of Benzimidazole and its Derivatives

as 1.6 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> at 250°C under anhydrous conditions, and the results

power densities of around 1 mW cm<sup>2</sup> [48]) have been obtained.

membranes exhibited acceptable ionic conductivity values, higher than

which is an indispensable condition for potential HT-PEMFC membrane candidates, other requisites must also be fulfilled: barrier to the reagent gases, thermal and dimensional stability under operating conditions, electrochemical stability under reducing and oxidizing potentials, and compatibility with the electrocatalyst. In this particular case, low open-circuit voltage (OCV) of the cell, 0.8 V, was obtained. This suggests a mixed potential, although no crossover was detected in the experiments. The authors suggested that kinetic complication could show up like additional oxidation and reduction reactions simultaneously with the corresponding oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR), respectively; furthermore, the poisoning effect of the H2S generated at the anode should

On the other side, the beneficial effect on the decrease of the IL viscosity was observed in the performance of the fuel cell. The optimum performance was obtained with no limiting current, being the maximum current density ca.

perature higher than 80°C, the system starts to dehydrate, whereas the IL viscosity increases and the proton diffusion was hindered. The performance at 150°C wasn't good showing clear evidences of the system dehydration at temperatures beyond 80°C. The migration of the IL from anode to cathode was demonstrated in postmortem analysis of PBI/[DE][SHx] composite-based electrodes. The IL went out of the composite membrane, and in consequence the cell resistivity increased by a factor

, using 100% relative humidity at 80°C. At tem-

0.01 S cm<sup>1</sup>

not be ignored.

190

70 mA cm<sup>2</sup> and 13.5 mW cm<sup>2</sup>

of six times after polarization measurements.

It is necessary to keep in mind that the requirements of cell lifetime vary for different applications, that is, 5000 h for cars, 20,000 h for buses, and 40,000 h for stationary application with continuous operation [43]. This means that the development of ionic exchange membranes with a long operating life is a challenge to develop.

#### 6. Anion exchange membrane fuel cells (AEMFCs)

Many electrochemical systems use ion exchange membranes, such as fuel cells, electrolyzers, or redox flow batteries. Traditionally cation exchange membranes have been used in these systems due to the idea that anion exchange membranes had too low conductivity and stability. However, in the last years, many advances have been made, and anion exchange membranes (AEMs) are demonstrating to have performances comparable to acid ones, showing promising application in several technologies [2]. These membranes conduct negatively charged ions like OH or Cl and usually have positive-charged groups in the polymer structure, which could be directly present in the polymer backbone or more commonly fixed to it by extended side chains of varying lengths and chemistries. Varcoe et al. [2] investigated a deep review about the different chemistries of polymer backbones and head groups and their current state of research. The use of alkaline media, compared to acid media, has some advantages like the better electrochemical kinetics of the oxygen reduction reaction (ORR). This allows the possibility of using nonnoble metals in the electrocatalysts reducing the fuel cell system cost. Other advantages are the minimized corrosion problems and the cogeneration of electricity and valuable chemicals [7, 50]. Compared to classical alkaline fuel cells (AFCs) where the electrolyte is in aqueous phase, the use of AEMs solves the carbonation problems and the difficulties of the liquid electrolyte management. The fuels commonly used in anion exchange membrane fuel cells (AEMFCs) are hydrogen and alcohols. Hydrogen is the common fuel in commercialization and research and gives the higher power densities. On the other hand, alcohols like methanol or ethanol have the advantages of easier handle, store, and transport and can be acquired from abundant biomass, which is environmentally friendly considering the process is carbon-neutral.

Among all the polymers available and tested for AEMFCs, polybenzimidazoles have demonstrated good applicability, and the most commonly used and studied are PBI and ABPBI. Some of their advantages remain in the properties previously described, as excellent thermal stability, which allows to use them at higher temperatures, superior mechanical properties that can withstand the performance conditions, and the presence of amine and imine groups which form strong hydrogen bonding interactions and can be further functionalized. The great stability properties have also encouraged many studies combining polybenzimidazoles with other polymers, creating blend or crosslinked membranes with excellent performances. Membranes based on polybenzimidazoles alone or with other polymers have also demonstrated low alcohols crossover, making them adequate electrolytes in alcohol fuel cells. In the alkaline media, the pristine form of PBI can be equilibrated in aqueous solutions of alkali metal hydroxides forming homogeneous systems with the hydroxide salt and water dissolved in the polymer matrix. These materials have shown high ion conductivity and great chemical stability at low alkali concentrations and have been tested as anion-conducting electrolytes in fuel cells with hydrogen or alcohol and in water electrolyzers. In order to understand the physical and chemical properties of polybenzimidazoles in alkaline media, Aili et al. have made a study with thin films of PBI in aqueous KOH solution with concentrations

from 0 to 50 wt.% [31]. They observed by the EDS cross-sectional maps that the dissolved KOH is evenly distributed in the electrolyte membrane. The polymer has strong water affinity through hydrogen bonding with the imidazole groups, absorbing around the water molecules per repeating unit (r.u.), and KOH forms various hydrated complexes when dissolved in water. The degree of ionization of the polymer is determined by the position of the acid-base equilibrium presented in Figure 4. They observed that it depends on the KOH concentration as was expected, increasing the KOH content per PBI r.u. with the higher concentration of the bulk solution, reaching 2.6 KOH molecules/r.u. at bulk concentration of 25 wt.%. dissociation of the acidic proton. The result was that the deprotonated form of PBI predominates when the KOH concentration of the bulk solution is around 15–20%. In order to discuss the different membranes based on polybenzimidazoles, the classification of anion exchange membranes made by Merle et al. will be useful [6].

Alkali-doped PBI was investigated by Xing et al. for use in AEMFCs [51]. They obtained very interesting results, like conductivity as high as 9 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> at 25° C, higher than 2 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> of a H2SO4-doped PBI membrane, or the similar performance in hydrogen/oxygen fuel cells with alkali-doped PBI membrane and Nafion®117 membrane. Since that pioneering work, extensive attention has been paid to the alkali-doped PBI membranes, and thus great progress has been made. However, relevant issues are still remaining such as alkali leakage, fuel permeability, and mechanical stability. The single-cell performance of alkali-doped PBIs has been extensively studied with various fuels [52], such as hydrogen, methanol,

Using hydrogen as fuel, Zarrin et al. [53] have developed a stable and highly ionconductive porous membrane doped with KOH. They found enhanced ionic conductivity by introducing the porosity in the membrane and obtained around twice better cell performance and conductivity compared with a commercial Fumapem® FAA membrane. Moreover, the KOH-doped PBI membrane maintained the ionic conductivity after 14 days of stability test, far more than the 3 h of the commercial one. The peak power density obtained with the porous PBI membrane of porosity

PBI membrane and the commercial FAA membrane, respectively. This better performance was demonstrated to be ascribed to the fact that the porous structure offered a higher ion transport rate through the membrane. One of the previously mentioned issues is the gradual alkali leakage during the cell operation. To solve it Zeng et al. [54] synthesized a sandwiched porous PBI membrane doped with KOH.

interconnected macropores, improving the interaction between the PBI and the doping alkali, indicating that both anionic conductivity and alkali retention could be enhanced by this method. Using this sandwiched porous PBI membrane doped with KOH in an AEMFC, they obtained an open-circuit voltage (OCV) of 1.0 V and a peak power density of 544 mW cm<sup>2</sup> at 90°C, which was higher than using the conventional membrane structure. They also investigated the durability of the fuel cell at a constant current density of 700 mW cm<sup>2</sup> and found that the conventional fuel cell had a dramatic voltage drop after short operation time, which was ascribed

The pore-forming method rendered numerous sponge-like walls and

, better than the 41 and 45 mW cm<sup>2</sup> obtained with a dense

ethanol, ethylene glycol, glycerol, formate, and borohydrides.

Membranes are classified in three main groups: heterogeneous membranes, interpenetrating polymer networks, and homogeneous membranes. The heterogeneous membranes are composed by an anion exchange material embedded in an inert compound and can be divided in ion-solvating polymers if the inert compound is a salt or hybrid membranes in it is an inorganic segment. Polybenzimidazoles alone or blended with other polymers would fall into the category of ion-solvating polymers. The interpenetrating polymer network is a combination of two polymers in which one or both are synthesized or crosslinked in the presence of the other without any covalent bonds between them. The homogeneous membranes are composed only by the anion exchange material, forming a one-phase system, where the cationic charges are covalently bonded to the polymer backbone. Mobile counter ions are associated with the ionic sites to preserve the electroneutrality of the polymer. Examples of the cationic sites are the quaternary ammonium (QA) groups commonly used in AEMs. Depending on the production method and the starting materials, homogeneous membranes are divided into three types: (co) polymerization of monomers, modification into a polymer, and modification on a

Benzimidazole as Solid Electrolyte Material for Fuel Cells

DOI: http://dx.doi.org/10.5772/intechopen.85430

preformed film.

0.7 was 72 mW cm<sup>2</sup>

193

A similar trend was observed for the water molecules, reaching more than 20 H2O molecules/r.u. at KOH concentration around 20–25 wt.% in the bulk solution. In the polymer phase, the number of water molecules per KOH decreased while increasing the bulk solution concentration, showing a concentrating effect of KOH in the polymer. They did the measurements by titration and gravimetrically, getting consistent results that corroborate previous knowledge. They also observed the anisotropic swelling behavior of the polymer at different KOH concentrations that had been previously reported and performed X-ray diffraction (XRD) measurements to explain it. The explanation they found was that the increasing of surface area and thickness up to 15 wt.% concentration was due to the uptake of water and KOH, but further increasing the concentration leads to full ionization of the polymer, breaking many of the hydrogen bonds and separating the layered structure. This separation is easier in the interlayer dimension than in the intra-layer one, causing high thickness increase and area decrease. When KOH bulk solution concentration reached 50 wt.%, sharp peaks appeared in the XRD and were attributed to a crystalline phase of a poly(potassiumbenzimidazolide) hydrate with a symmetric and highly regular structure with crystallite size in the range of 70–120 nm. These crystalline peaks were vanished after washing in water until neutral pH. They also observed that the previously described effect of the introduction of water and KOH that disturbs the polymer hydrogen bonding of imidazole groups affected the mechanical properties, causing great decay in the tensile strength and enhanced elongation at break. When full ionization of the polymer was reached, at 20–25 wt. %, more than 200% elongation at break and 0.3 GPa elastic modulus were obtained, which compared with the 80% elongation at break and 3.0 GPa in pure water, showing the great differences. The IR measurements showed clearly that the chemical environment of the benzimidazole moieties changed greatly from the

Figure 4. Scheme showing the amphoteric nature of PBI in acidic (left) and alkaline (right) environments.

#### Benzimidazole as Solid Electrolyte Material for Fuel Cells DOI: http://dx.doi.org/10.5772/intechopen.85430

from 0 to 50 wt.% [31]. They observed by the EDS cross-sectional maps that the dissolved KOH is evenly distributed in the electrolyte membrane. The polymer has strong water affinity through hydrogen bonding with the imidazole groups, absorbing around the water molecules per repeating unit (r.u.), and KOH forms various hydrated complexes when dissolved in water. The degree of ionization of the polymer is determined by the position of the acid-base equilibrium presented in

Chemistry and Applications of Benzimidazole and its Derivatives

Figure 4. They observed that it depends on the KOH concentration as was

ical environment of the benzimidazole moieties changed greatly from the

Scheme showing the amphoteric nature of PBI in acidic (left) and alkaline (right) environments.

Figure 4.

192

expected, increasing the KOH content per PBI r.u. with the higher concentration of the bulk solution, reaching 2.6 KOH molecules/r.u. at bulk concentration of 25 wt.%. A similar trend was observed for the water molecules, reaching more than 20 H2O molecules/r.u. at KOH concentration around 20–25 wt.% in the bulk solution. In the polymer phase, the number of water molecules per KOH decreased while increasing the bulk solution concentration, showing a concentrating effect of KOH in the polymer. They did the measurements by titration and gravimetrically, getting consistent results that corroborate previous knowledge. They also observed the anisotropic swelling behavior of the polymer at different KOH concentrations that had been previously reported and performed X-ray diffraction (XRD) measurements to explain it. The explanation they found was that the increasing of surface area and thickness up to 15 wt.% concentration was due to the uptake of water and KOH, but further increasing the concentration leads to full ionization of the polymer, breaking many of the hydrogen bonds and separating the layered structure. This separation is easier in the interlayer dimension than in the intra-layer one, causing high thickness increase and area decrease. When KOH bulk solution concentration reached 50 wt.%, sharp peaks appeared in the XRD and were attributed to a crystalline phase of a poly(potassiumbenzimidazolide) hydrate with a symmetric and highly regular structure with crystallite size in the range of 70–120 nm. These crystalline peaks were vanished after washing in water until neutral pH. They also observed that the previously described effect of the introduction of water and KOH that disturbs the polymer hydrogen bonding of imidazole groups affected the mechanical properties, causing great decay in the tensile strength and enhanced elongation at break. When full ionization of the polymer was reached, at 20–25 wt. %, more than 200% elongation at break and 0.3 GPa elastic modulus were obtained, which compared with the 80% elongation at break and 3.0 GPa in pure water, showing the great differences. The IR measurements showed clearly that the chemdissociation of the acidic proton. The result was that the deprotonated form of PBI predominates when the KOH concentration of the bulk solution is around 15–20%.

In order to discuss the different membranes based on polybenzimidazoles, the classification of anion exchange membranes made by Merle et al. will be useful [6]. Membranes are classified in three main groups: heterogeneous membranes, interpenetrating polymer networks, and homogeneous membranes. The heterogeneous membranes are composed by an anion exchange material embedded in an inert compound and can be divided in ion-solvating polymers if the inert compound is a salt or hybrid membranes in it is an inorganic segment. Polybenzimidazoles alone or blended with other polymers would fall into the category of ion-solvating polymers. The interpenetrating polymer network is a combination of two polymers in which one or both are synthesized or crosslinked in the presence of the other without any covalent bonds between them. The homogeneous membranes are composed only by the anion exchange material, forming a one-phase system, where the cationic charges are covalently bonded to the polymer backbone. Mobile counter ions are associated with the ionic sites to preserve the electroneutrality of the polymer. Examples of the cationic sites are the quaternary ammonium (QA) groups commonly used in AEMs. Depending on the production method and the starting materials, homogeneous membranes are divided into three types: (co) polymerization of monomers, modification into a polymer, and modification on a preformed film.

Alkali-doped PBI was investigated by Xing et al. for use in AEMFCs [51]. They obtained very interesting results, like conductivity as high as 9 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> at 25° C, higher than 2 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> of a H2SO4-doped PBI membrane, or the similar performance in hydrogen/oxygen fuel cells with alkali-doped PBI membrane and Nafion®117 membrane. Since that pioneering work, extensive attention has been paid to the alkali-doped PBI membranes, and thus great progress has been made. However, relevant issues are still remaining such as alkali leakage, fuel permeability, and mechanical stability. The single-cell performance of alkali-doped PBIs has been extensively studied with various fuels [52], such as hydrogen, methanol, ethanol, ethylene glycol, glycerol, formate, and borohydrides.

Using hydrogen as fuel, Zarrin et al. [53] have developed a stable and highly ionconductive porous membrane doped with KOH. They found enhanced ionic conductivity by introducing the porosity in the membrane and obtained around twice better cell performance and conductivity compared with a commercial Fumapem® FAA membrane. Moreover, the KOH-doped PBI membrane maintained the ionic conductivity after 14 days of stability test, far more than the 3 h of the commercial one. The peak power density obtained with the porous PBI membrane of porosity 0.7 was 72 mW cm<sup>2</sup> , better than the 41 and 45 mW cm<sup>2</sup> obtained with a dense PBI membrane and the commercial FAA membrane, respectively. This better performance was demonstrated to be ascribed to the fact that the porous structure offered a higher ion transport rate through the membrane. One of the previously mentioned issues is the gradual alkali leakage during the cell operation. To solve it Zeng et al. [54] synthesized a sandwiched porous PBI membrane doped with KOH. The pore-forming method rendered numerous sponge-like walls and interconnected macropores, improving the interaction between the PBI and the doping alkali, indicating that both anionic conductivity and alkali retention could be enhanced by this method. Using this sandwiched porous PBI membrane doped with KOH in an AEMFC, they obtained an open-circuit voltage (OCV) of 1.0 V and a peak power density of 544 mW cm<sup>2</sup> at 90°C, which was higher than using the conventional membrane structure. They also investigated the durability of the fuel cell at a constant current density of 700 mW cm<sup>2</sup> and found that the conventional fuel cell had a dramatic voltage drop after short operation time, which was ascribed to the progressive release of the alkali solution. On the other hand, the sandwiched porous membranes performed with improved stability; the voltages reduced gradually to 0.1 V and remained there for another 25 h approximately. They explained that the performance enhancement was attributed to the retarding in the release of the alkali solution from the sponge-shaped wall, maintaining the high conductivity of the membrane. However, finally the leakage occurred, but as the authors indicated, the membrane could be reused after doping with KOH solution again.

+2.0 M ethanol as fuel, while in the cathode they used air flow. With these conditions and at temperature of 80°C, a peak power density of 100 mW cm<sup>2</sup> was obtained at a voltage of 0.4 V. It was also found that by operating the fuel cell with pure oxygen, the current density was improved by 10%. Also using ethanol as fuel, recently Herranz et al. [29] tested the fuel cell performance of membranes synthesized with PBI and poly(vinyl alcohol) (PVA) with different weight ratios. PVA alcohol groups interacted with PBI by hydrogen bonding as well as allowing

enhanced conductivity of the hydroxyl anion through the membranes. The increasing content in the PVA blend membrane leads to higher conductivities but if excessive could bring structural problems since PBI demonstrated to be essential for the membrane integrity. PVA:PBI 4:1 membrane obtained the best performance with a peak power density of 76 mW cm<sup>2</sup> at 90°C, 50% higher than a pristine

ABPBI has also been widely investigated for AEMs synthesis and application. Luo et al. [64] synthesized ABPBI and prepared the pristine membranes by the solution casting method. They studied the conductivity of the membranes at various alkali doping levels. They found high conductivity values for the membranes as 2.3 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> at 25°C and 7.3 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> at 100°C in the ABPBI membrane with alkali doping level of 0.37. They also founded the membranes have great thermal stability and excellent chemical stability, demonstrated by maintaining the

Other alcohols and fuels have also been tested in AEMFCs using polybenzimidazoles in the membrane structure, showing promising results [65, 66]. Overall, the applicability and interest of benzimidazoles as AEMs are actual and will con-

Polybenzimidazoles have been deeply studied in the last decades, and great advancements have been done in their synthesis, making them economical materials with excellent thermal and mechanical properties as well as high chemical resistance in acidic and alkaline media. Their special structure with imidazole moieties and high intermolecular hydrogen bonding make them excellent materials to be used and ion exchange membranes for fuel cells. They can be used alone or in combination with other polymers or compounds, like the ionic liquids, as has been demonstrated many times. With them, it is possible to reach performances similar to other fuel cells and allow the application at higher temperatures, with all the benefits that implies. In the acidic media temperatures in the range of 120–200° C are used with good performances and easier water management, but still issues like structural stability with high doping level have to be solved. In order to help with the conductivity, ionic liquids have been investigated because of their nonaqueous and low-volatility properties as proton carriers. Interesting developments have been done but further research is necessary. In the alkaline media, their application has also attracted great interest. The ionization of the structure has been clearly identified at certain doping levels and the plasticizing effects it has. Pristine polybenzimidazole membranes have been directly doped with alkali solu-

tions obtaining very good conductivity values, and other strategies like

crosslinking with other polymers or synthesis of blend membranes have reported also promising results. The fuel cell performance is not yet as good as in the acidic media, but good results around 100 mW cm<sup>2</sup> have been obtained. Commercialization of membranes and MEAs based on PBI shows the potential they have, and research continues nowadays to develop them even more and better understand

conductivity values in alkaline media at 100°C for more than 1000 h.

KOH-doped PBI tested in the same conditions.

Benzimidazole as Solid Electrolyte Material for Fuel Cells

DOI: http://dx.doi.org/10.5772/intechopen.85430

tinue to increase due to their excellent properties.

7. Conclusions

195

Another approach was that used by Lu et al. [55]. They used PBI to react with poly(vinylbenzyl chloride) (PVBC), a polymer commonly used by other groups as for example Varcoe et al. in their grafted PTFE membranes [56, 57]. PVBC has the advantage of reacting with the imidazole rings of PBI creating a crosslinking connection with remaining -CH2Cl groups unreacted that can be later functionalized as desired. For the functionalization of these groups, they decided to use the diamine 1,4-diazabicyclo (2.2.2) octane (DABCO), a very stable amine in alkaline media especially when only one of the two nitrogen is quaternized as previously reported [2, 6]. This method had the advantage that quaternization is done in the already casted membrane so it can be ensured that only one of the nitrogens react with PVBC obtaining the stability desired. Thanks to the good mechanical properties of PBI, they obtained membranes with good flexibility and strength both in dry conditions and saturated in water as well as high hydroxide conductivity (>25 mS cm<sup>1</sup> at room temperature) and superior chemical stability in alkaline environment. They tested the membrane in the H2/O2 fuel cell obtaining a peak power density of 230 mW cm<sup>2</sup> at 50°C and performed stability test, which showed high durability both in the constant current and continuous open-circuit voltage.

In addition to being used as an anion exchange membrane, alkali-doped PBI can work as ionomer, serving as ion-conductive pathway in the catalyst layer as well as a binder. Matsumoto et al. [58] developed a well-structured electrocatalyst for AEMFCs composed of carbon nanotubes (CNT), KOH-doped PBI ionomer, and platinum nanoparticles. This allowed them to obtain highly effective diffusivity and improved electrochemical activity, and they obtained a peak power density of 256 mW cm<sup>2</sup> at 50°C when tested in a H2/O2 fuel cell.

For fuel cells running on methanol, Hou et al. [59] tested a direct methanol fuel cell with a KOH-doped PBI membrane and observed that when a mixed solution of 2.0 M methanol and 2.0 M KOH was used as fuel, the OCV was around 1.0 V, and the peak power density was 31 mW cm<sup>2</sup> at 90°C. Wu et al. [60] prepared a membrane of KOH-doped PBI with CNT nanocomposites and obtained maximum power densities of 67 mW cm<sup>2</sup> and 104 mW cm<sup>2</sup> at 60 and 90°C, respectively, with a fuel composition of 2.0 M methanol + 6.0 M KOH and humidified oxygen. Li et al. [61] worked with pristine PBI membrane synthesized by solution casting method and treated it separately with 2.0 M H3PO4 and 6.0 M KOH to prepare a PEM and an AEM, respectively. They also studied several parameters of the structure design and operating parameters. They found that the conductivity of the KOH-doped PBI membrane was higher than the phosphoric acid membrane, 21.6 and 7.9 mS cm<sup>1</sup> , respectively. They also obtained a higher peak power density with the KOH-doped PBI membrane, 117.9 mW cm<sup>2</sup> at 90°C, than with the acid one, 46.5 mW cm<sup>2</sup> . They even reached a peak power density of 158.9 mW cm<sup>2</sup> at 90°C when using free-microporous layer electrodes and tripled the fuel flow rate.

In fuel cells running on ethanol, Hou et al. [62] developed a KOH-doped PBI membrane and found that with fuel composition of 2.0 M ethanol +2.0 M KOH, they obtained OCV of 0.92 V and maximum power density of 42.9 mW cm<sup>2</sup> at 75° C and 0.97 V and 60.9 mW cm<sup>2</sup> at 90°C. Modestov et al. [63] fabricated a membrane electrode assembly (MEA) employing non-platinum electrocatalysts and a KOH-doped membrane. In the anode they used a mixed solution of 3.0 M KOH

Benzimidazole as Solid Electrolyte Material for Fuel Cells DOI: http://dx.doi.org/10.5772/intechopen.85430

+2.0 M ethanol as fuel, while in the cathode they used air flow. With these conditions and at temperature of 80°C, a peak power density of 100 mW cm<sup>2</sup> was obtained at a voltage of 0.4 V. It was also found that by operating the fuel cell with pure oxygen, the current density was improved by 10%. Also using ethanol as fuel, recently Herranz et al. [29] tested the fuel cell performance of membranes synthesized with PBI and poly(vinyl alcohol) (PVA) with different weight ratios. PVA alcohol groups interacted with PBI by hydrogen bonding as well as allowing enhanced conductivity of the hydroxyl anion through the membranes. The increasing content in the PVA blend membrane leads to higher conductivities but if excessive could bring structural problems since PBI demonstrated to be essential for the membrane integrity. PVA:PBI 4:1 membrane obtained the best performance with a peak power density of 76 mW cm<sup>2</sup> at 90°C, 50% higher than a pristine KOH-doped PBI tested in the same conditions.

ABPBI has also been widely investigated for AEMs synthesis and application. Luo et al. [64] synthesized ABPBI and prepared the pristine membranes by the solution casting method. They studied the conductivity of the membranes at various alkali doping levels. They found high conductivity values for the membranes as 2.3 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> at 25°C and 7.3 <sup>10</sup><sup>2</sup> S cm<sup>1</sup> at 100°C in the ABPBI membrane with alkali doping level of 0.37. They also founded the membranes have great thermal stability and excellent chemical stability, demonstrated by maintaining the conductivity values in alkaline media at 100°C for more than 1000 h.

Other alcohols and fuels have also been tested in AEMFCs using polybenzimidazoles in the membrane structure, showing promising results [65, 66]. Overall, the applicability and interest of benzimidazoles as AEMs are actual and will continue to increase due to their excellent properties.

#### 7. Conclusions

to the progressive release of the alkali solution. On the other hand, the sandwiched porous membranes performed with improved stability; the voltages reduced gradually to 0.1 V and remained there for another 25 h approximately. They explained that the performance enhancement was attributed to the retarding in the release of the alkali solution from the sponge-shaped wall, maintaining the high conductivity of the membrane. However, finally the leakage occurred, but as the authors indicated, the membrane could be reused after doping with KOH solution again.

Chemistry and Applications of Benzimidazole and its Derivatives

Another approach was that used by Lu et al. [55]. They used PBI to react with poly(vinylbenzyl chloride) (PVBC), a polymer commonly used by other groups as for example Varcoe et al. in their grafted PTFE membranes [56, 57]. PVBC has the advantage of reacting with the imidazole rings of PBI creating a crosslinking connection with remaining -CH2Cl groups unreacted that can be later functionalized as desired. For the functionalization of these groups, they decided to use the diamine 1,4-diazabicyclo (2.2.2) octane (DABCO), a very stable amine in alkaline media especially when only one of the two nitrogen is quaternized as previously reported [2, 6]. This method had the advantage that quaternization is done in the already casted membrane so it can be ensured that only one of the nitrogens react with PVBC obtaining the stability desired. Thanks to the good mechanical properties of PBI, they obtained membranes with good flexibility and strength both in dry conditions and saturated in water as well as high hydroxide conductivity (>25 mS cm<sup>1</sup> at room temperature) and superior chemical stability in alkaline environment. They tested the membrane in the H2/O2 fuel cell obtaining a peak power density of 230 mW cm<sup>2</sup> at 50°C and performed stability test, which showed high durability

In addition to being used as an anion exchange membrane, alkali-doped PBI can work as ionomer, serving as ion-conductive pathway in the catalyst layer as well as a binder. Matsumoto et al. [58] developed a well-structured electrocatalyst for AEMFCs composed of carbon nanotubes (CNT), KOH-doped PBI ionomer, and platinum nanoparticles. This allowed them to obtain highly effective diffusivity and improved electrochemical activity, and they obtained a peak power density of

For fuel cells running on methanol, Hou et al. [59] tested a direct methanol fuel cell with a KOH-doped PBI membrane and observed that when a mixed solution of 2.0 M methanol and 2.0 M KOH was used as fuel, the OCV was around 1.0 V, and the peak power density was 31 mW cm<sup>2</sup> at 90°C. Wu et al. [60] prepared a membrane of KOH-doped PBI with CNT nanocomposites and obtained maximum power densities of 67 mW cm<sup>2</sup> and 104 mW cm<sup>2</sup> at 60 and 90°C, respectively, with a fuel composition of 2.0 M methanol + 6.0 M KOH and humidified oxygen. Li et al. [61] worked with pristine PBI membrane synthesized by solution casting method and treated it separately with 2.0 M H3PO4 and 6.0 M KOH to prepare a PEM and an AEM, respectively. They also studied several parameters of the structure design and operating parameters. They found that the conductivity of the KOH-doped PBI membrane was higher than the phosphoric acid membrane, 21.6

the KOH-doped PBI membrane, 117.9 mW cm<sup>2</sup> at 90°C, than with the acid one,

when using free-microporous layer electrodes and tripled the fuel flow rate. In fuel cells running on ethanol, Hou et al. [62] developed a KOH-doped PBI membrane and found that with fuel composition of 2.0 M ethanol +2.0 M KOH, they obtained OCV of 0.92 V and maximum power density of 42.9 mW cm<sup>2</sup> at 75°

C and 0.97 V and 60.9 mW cm<sup>2</sup> at 90°C. Modestov et al. [63] fabricated a

membrane electrode assembly (MEA) employing non-platinum electrocatalysts and a KOH-doped membrane. In the anode they used a mixed solution of 3.0 M KOH

, respectively. They also obtained a higher peak power density with

. They even reached a peak power density of 158.9 mW cm<sup>2</sup> at 90°C

both in the constant current and continuous open-circuit voltage.

256 mW cm<sup>2</sup> at 50°C when tested in a H2/O2 fuel cell.

and 7.9 mS cm<sup>1</sup>

46.5 mW cm<sup>2</sup>

194

Polybenzimidazoles have been deeply studied in the last decades, and great advancements have been done in their synthesis, making them economical materials with excellent thermal and mechanical properties as well as high chemical resistance in acidic and alkaline media. Their special structure with imidazole moieties and high intermolecular hydrogen bonding make them excellent materials to be used and ion exchange membranes for fuel cells. They can be used alone or in combination with other polymers or compounds, like the ionic liquids, as has been demonstrated many times. With them, it is possible to reach performances similar to other fuel cells and allow the application at higher temperatures, with all the benefits that implies. In the acidic media temperatures in the range of 120–200° C are used with good performances and easier water management, but still issues like structural stability with high doping level have to be solved. In order to help with the conductivity, ionic liquids have been investigated because of their nonaqueous and low-volatility properties as proton carriers. Interesting developments have been done but further research is necessary. In the alkaline media, their application has also attracted great interest. The ionization of the structure has been clearly identified at certain doping levels and the plasticizing effects it has. Pristine polybenzimidazole membranes have been directly doped with alkali solutions obtaining very good conductivity values, and other strategies like crosslinking with other polymers or synthesis of blend membranes have reported also promising results. The fuel cell performance is not yet as good as in the acidic media, but good results around 100 mW cm<sup>2</sup> have been obtained. Commercialization of membranes and MEAs based on PBI shows the potential they have, and research continues nowadays to develop them even more and better understand

the possibilities of these wonderful materials in the fuel cell technology and the energy applications.

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b000000x

94-007-7708-8

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DOI: http://dx.doi.org/10.5772/intechopen.85430

Benzimidazole as Solid Electrolyte Material for Fuel Cells

Fundamentals. Hoboken, New Jersey: John Wiley & Sons; 2006. DOI: https://

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Polybenzimidazoles, new thermally stable polymers. Journal of Polymer Science. 1961;50:511-539. DOI: https:// doi.org/10.1002/pol.1961.1205015419

doi.org/10.1002/9781119191766

[10] Vogel H, Marvel CS.

[11] Choe E. Catalysts for the preparation of Polybenzimidazoles. Journal of Applied Polymer Science. 1994;53:497-506. DOI: https://doi.org/

10.1002/app.1994.070530504

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progpolymsci.2008.12.003

[15] Iwakura Y, Uno K, Imai Y.

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[16] Eaton PE, Carlson GR, Lee JT. Phosphorus pentoxide-methanesulfonic

[14] Yang J, He R, Aili D. Synthesis of Polybenzimidazoles. High Temperature Polymer Electrolyte Membrane Fuel Cells. Switzerland: Springer; 2016. DOI: https://doi.org/10.1007/978-3-319-

90434-3

(312):976

17082-4\_7

100020611

Technology, applications, and needs on fundamental research. Applied Energy. 2011;88:981-1007. DOI: https://doi.org/ 10.1016/j.apenergy.2010.09.030

[2] Varcoe JR, Atanassov P, Dekel DR, Herring AM, Hickner MA, Kohl PA, et al. Anion-exchange membranes in electrochemical energy systems. Energy & Environmental Science. 2014;7:3135- 3191. DOI: https://doi.org/10.1039/

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197

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[7] Pan ZF, An L, Zhao TS, Tang ZK. Advances and challenges in alkaline anion exchange membrane fuel cells. Progress in Energy and Combustion Science. 2018;66:141-175. DOI: https://doi.org/10.1016/j.pecs.2018.

[8] O'Hayre RP, Cha S-W, Colella W, Prinz FB. editors. In: Fuel Cell

fuel cells: A review. Journal of

## Acknowledgements

The authors want to acknowledge the Spanish Ministry of Economy Industry and Competitiveness (MINECO) project ENE2016-77055-C3-1-R and to Madrid Regional Research Council (CAM) project P2018/EMT-4344 (BIOTRES-CM).

## Conflict of interest

The authors declare that they have no conflict of interest.

#### Nomenclature


### Author details

Daniel Herranz and Pilar Ocón\* Department of Applied Physic Chemistry, University Autonomous of Madrid, Madrid, Spain

\*Address all correspondence to: pilar.ocon@uam.es

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Benzimidazole as Solid Electrolyte Material for Fuel Cells DOI: http://dx.doi.org/10.5772/intechopen.85430

#### References

the possibilities of these wonderful materials in the fuel cell technology and the

The authors want to acknowledge the Spanish Ministry of Economy Industry and Competitiveness (MINECO) project ENE2016-77055-C3-1-R and to Madrid Regional Research Council (CAM) project P2018/EMT-4344 (BIOTRES-CM).

The authors declare that they have no conflict of interest.

Chemistry and Applications of Benzimidazole and its Derivatives


mer electrolyte membrane fuel cells

PEMFCs Proton exchange membrane fuel cells. Also used for general poly-

Department of Applied Physic Chemistry, University Autonomous of Madrid,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,


energy applications.

Acknowledgements

Conflict of interest

PBI Poly[2,2<sup>0</sup>

ABPBI Poly(2,5-benzimidazole)

IEM Ion exchange membrane

ORR Oxygen reduction reaction

MEA Membrane electrode assembly

\*Address all correspondence to: pilar.ocon@uam.es

provided the original work is properly cited.

IV Inherent viscosity PPA Polyphosphoric acid

OCV Open-circuit voltage QA Quaternary ammonium

ILs Ionic liquids

Daniel Herranz and Pilar Ocón\*

Author details

Madrid, Spain

196

AEMFCs Anion exchange membrane fuel cells

AEMs/CEMs Anion/cation exchange membranes

Nomenclature

[1] Wang Y, Chen KS, Mishler J, Cho SC, Adroher XC. A review of polymer electrolyte membrane fuel cells: Technology, applications, and needs on fundamental research. Applied Energy. 2011;88:981-1007. DOI: https://doi.org/ 10.1016/j.apenergy.2010.09.030

[2] Varcoe JR, Atanassov P, Dekel DR, Herring AM, Hickner MA, Kohl PA, et al. Anion-exchange membranes in electrochemical energy systems. Energy & Environmental Science. 2014;7:3135- 3191. DOI: https://doi.org/10.1039/ b000000x

[3] Corti HR, Gonzalez ER. Direct Alcohol Fuel Cells. Dordrecht: Springer; 2014. DOI: https://doi.org/10.1007/978- 94-007-7708-8

[4] Zakaria Z, Kamarudin SK, Timmiati SN. Membranes for direct ethanol fuel cells: An overview. Applied Energy. 2016;163:334-342. DOI: https://doi.org/ 10.1016/j.apenergy.2015.10.124

[5] Smitha B, Sridhar S, Khan AA. Solid polymer electrolyte membranes for fuel cell applications—A review. Journal of Membrane Science. 2005;259:10-26. DOI: https://doi.org/10.1016/j. memsci.2005.01.035

[6] Merle G, Wessling M, Nijmeijer K. Anion exchange membranes for alkaline fuel cells: A review. Journal of Membrane Science. 2011;377:1-35. DOI: https://doi.org/10.1016/j.memsci. 2011.04.043

[7] Pan ZF, An L, Zhao TS, Tang ZK. Advances and challenges in alkaline anion exchange membrane fuel cells. Progress in Energy and Combustion Science. 2018;66:141-175. DOI: https://doi.org/10.1016/j.pecs.2018. 01.001

[8] O'Hayre RP, Cha S-W, Colella W, Prinz FB. editors. In: Fuel Cell

Fundamentals. Hoboken, New Jersey: John Wiley & Sons; 2006. DOI: https:// doi.org/10.1002/9781119191766

[9] Ebewele RO. Polymer Science and Technology. Vol. 74. Boca Raton, New York: CRC Press LLC; 1985. DOI: https://doi.org/10.1016/0025-5416(85) 90434-3

[10] Vogel H, Marvel CS. Polybenzimidazoles, new thermally stable polymers. Journal of Polymer Science. 1961;50:511-539. DOI: https:// doi.org/10.1002/pol.1961.1205015419

[11] Choe E. Catalysts for the preparation of Polybenzimidazoles. Journal of Applied Polymer Science. 1994;53:497-506. DOI: https://doi.org/ 10.1002/app.1994.070530504

[12] Choe EW. Single-stage melt polymerization process for the production of high molecular weight polybenzimidazole. US patent. 1982;4 (312):976

[13] Li Q, Jensen JO, Savinell RF, Bjerrum NJ. High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Progress in Polymer Science. 2009;34: 449-477. DOI: https://doi.org/10.1016/j. progpolymsci.2008.12.003

[14] Yang J, He R, Aili D. Synthesis of Polybenzimidazoles. High Temperature Polymer Electrolyte Membrane Fuel Cells. Switzerland: Springer; 2016. DOI: https://doi.org/10.1007/978-3-319- 17082-4\_7

[15] Iwakura Y, Uno K, Imai Y. Polyphenylenebenzimidazoles. Journal of Polymer Science. 1964;2:2605-2615. DOI: https://doi.org/10.1002/pol.1964. 100020611

[16] Eaton PE, Carlson GR, Lee JT. Phosphorus pentoxide-methanesulfonic acid. Convenient alternative to polyphosphoric acid. The Journal of Organic Chemistry. 1973;38:4071-4073. DOI: https://doi.org/10.1021/ jo00987a028

[17] Kim H, Cho SY, An SJ, Eun YC, Kim J, Yoon H, et al. Synthesis of poly (2,5 benzimidazole) for use as a Fuel-cell membrane. Macromolecular Rapid Communications. 2004;25:894-897. DOI: https://doi.org/10.1002/ marc.200300288

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[20] Asensio JA, Borro S, Gómez-Romero P. Polymer electrolyte fuel cells based on phosphoric acid-impregnated poly(2,5-benzimidazole) membranes. Journal of the Electrochemical Society. 2004;151:304-310. DOI: https://doi.org/ 10.1149/1.1640628

[21] Asensio JA, Borrós S, Gómez-Romero P. Proton-conducting membranes based on poly (2,5 benzimidazole) (ABPBI) and phosphoric acid prepared by direct acid casting. Journal of Membrane Science. 2004;241:89-93. DOI: https://doi.org/10.1016/j. memsci.2004.03.044

[22] Fishel KJ, Gulledge AL, Pingitore AT, Hoffman JP, Steckle WP, Benicewicz BC. Solution polymerization of polybenzimidazole. Journal of Polymer Science, Part A: Polymer Chemistry. 2016;54:1795-1802. DOI: https://doi.org/10.1002/pola.28041

[23] Litt M, Ameri R, Wang Y, Savinell R, Wainwright J. Polybenzimidazoles/ phosphoric acid solid polymer electrolytes: Mechanical and electrical properties. Materials Research Society Symposium Proceedings. 1999;548:313- 323. DOI: https://doi.org/10.1557/PROC-548-313

(benzimidazole) blend membranes for high performance alkaline direct ethanol fuel cells. Renewable Energy. 2018;127:883-895. DOI: https://doi.org/

DOI: http://dx.doi.org/10.5772/intechopen.85430

Benzimidazole as Solid Electrolyte Material for Fuel Cells

L123. DOI: https://doi.org/10.1149/

[37] Weng D, Wainright JS, Landau U, Savinell RF. Electro-osmotic drag coefficient of water and methanol in polymer electrolytes at elevated temperatures. Journal of the

Electrochemical Society. 1996;143:1260- 1263. DOI: https://doi.org/10.1149/

[38] Chandan A, Hattenberger M, El-Kharouf A, Du S, Dhir A, Self V, et al. High temperature (HT) polymer electrolyte membrane fuel cells

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[41] Vielstich W, Lamm A, Gasteiger HA, editors. In: Handbook of Fuel Cells:

Applications. United States: John Wiley

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[42] Aili D, Allward T, Alfaro SM, Hartmann-Thompson C, Steenberg T, Hjuler HA, et al. Polybenzimidazole and

oligosilsesquioxane composite membranes for high temperature polymer electrolyte membrane fuel cells. Electrochimica Acta. 2014;140: 182-190. DOI: https://doi.org/10.1016/j.

Fundamentals, Technology,

& Sons, Ltd.; 2009

sulfonated polyhedral

electacta.2014.03.047

S0013-4686(97)10031-7

1.1630037

1.2044337

1.1836626

10.1007/978-3-642-20487-6

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[31] Aili D, Jankova K, Han J, Bjerrum NJ, Jensen JO, Li Q. Understanding

[32] Mader J, Xiao L, Schmidt TJ, Fuel B,

benzimidazolide)-based polymer electrolytes. Polymer. 2016;84:304-310.

DOI: https://doi.org/10.1016/j.

Ave V. Polybenzimidazole/acid complexes as high-temperature membranes. Advances in Polymer Science. 2008;216:63-124. DOI: https://

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Benzimidazole as Solid Electrolyte Material for Fuel Cells DOI: http://dx.doi.org/10.5772/intechopen.85430

(benzimidazole) blend membranes for high performance alkaline direct ethanol fuel cells. Renewable Energy. 2018;127:883-895. DOI: https://doi.org/ 10.1007/978-3-642-20487-6

acid. Convenient alternative to polyphosphoric acid. The Journal of Organic Chemistry. 1973;38:4071-4073.

DOI: https://doi.org/10.1021/

[17] Kim H, Cho SY, An SJ, Eun YC, Kim J, Yoon H, et al. Synthesis of poly (2,5 benzimidazole) for use as a Fuel-cell membrane. Macromolecular Rapid Communications. 2004;25:894-897. DOI: https://doi.org/10.1002/

Chemistry and Applications of Benzimidazole and its Derivatives

[23] Litt M, Ameri R, Wang Y, Savinell R, Wainwright J. Polybenzimidazoles/

electrolytes: Mechanical and electrical properties. Materials Research Society Symposium Proceedings. 1999;548:313- 323. DOI: https://doi.org/10.1557/PROC-

[24] He RH, Sun BY, Yang JS, Che QT.


[25] Yang J, He R, Che Q, Gao X, Shi L. A


[26] Conti F, Willbold S, Mammi S, Korte C, Lehnert W, Stolten D. Carbon

polybenzimidazole–dimethylacetamide interactions in membranes for fuel cells. New Journal of Chemistry. 2013;37:152.

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[29] Herranz D, Escudero-Cid R, Montiel M, Palacio C, Fatás E, Ocón P. Poly

NMR investigation of the

DOI: https://doi.org/10.1039/

benzimidazole) by microwave irradiation. Chemical Research in Chinese Universities. 2009;25:585-589



phosphoric acid solid polymer

548-313

5,50

5,5<sup>0</sup>

pi.2906

c2nj40728k

(89)90072-4

ssi.2004.02.013

(vinyl alcohol) and poly

Synthesis of poly[2,20

copolymer of poly[2,2<sup>0</sup>

[18] Jouanneau J, Mercier R, Gonon L, Gebel G. Synthesis of sulfonated

monomers: Preparation of ionic

Macromolecules. 2007;40:983-990. DOI: https://doi.org/10.1021/

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conducting membranes.

[19] JS W, MH L. S RF. High

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of polybenzimidazole. Journal of Polymer Science, Part A: Polymer Chemistry. 2016;54:1795-1802. DOI: https://doi.org/10.1002/pola.28041

198

Benicewicz BC. Solution polymerization

Ltd. 2003. pp. 436-446

10.1149/1.1640628

polybenzimidazoles from functionalized

jo00987a028

marc.200300288

ma0614139

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[64] Luo H, Vaivars G, Agboola B, Mu S, Mathe M. Anion exchange membrane based on alkali doped poly(2,5-

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[66] Zeng L, Zhao TS, An L, Zhao G, Yan XH. Physicochemical properties of alkaline doped polybenzimidazole membranes for anion exchange membrane fuel cells. Journal of

Membrane Science. 2015;493:340-348.

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[55] Lu W, Zhang G, Li J, Hao J, Wei F,

Li W, et al. Polybenzimidazolecrosslinked poly(vinylbenzyl chloride) with quaternary 1,4-diazabicyclo (2.2.2) octane groups as high-performance anion exchange membrane for fuel cells.

polybenzimidazole membranes for fuel cell applications. Renewable and Sustainable Energy Reviews. 2018;89: 168-183. DOI: https://doi.org/10.1016/j.

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[57] Poynton SD, Slade RCT, Omasta TJ, Mustain WE, Escudero-Cid R, Ocón P, et al. Preparation of radiation-grafted powders for use as anion exchange ionomers in alkaline polymer electrolyte fuel cells. Journal of Materials Chemistry A. 2014;2:5124-5130. DOI: https://doi.org/10.1039/c4ta00558a

[58] Matsumoto K, Fujigaya T, Yanagi H, Nakashima N. Very high performance alkali anion-exchange membrane fuel cells. Advanced Functional Materials. 2011;21:1089-1094. DOI: https://doi. org/10.1002/adfm.201001806

[59] Hou H, Sun G, He R, Sun B, Jin W, Liu H, et al. Alkali doped polybenzimidazole membrane for alkaline direct methanol fuel cell. International Journal of Hydrogen Energy. 2008;33:7172-7176. DOI: https:// doi.org/10.1016/j.ijhydene.2008.09.023

[60] Wu JF, Lo CF, Li LY, Li HY, Chang CM, Liao KS, et al. Thermally stable polybenzimidazole/carbon nano-tube composites for alkaline direct methanol fuel cell applications. Journal of Power Sources. 2014;246:39-48. DOI: https:// doi.org/10.1016/j.jpowsour.2013.05.171

[61] Li L-Y, Yu B-C, Shih C-M, Lue SJ. Polybenzimidazole membranes for direct methanol fuel cell: Acid-doped or alkali-doped? Journal of Power Sources. 2015;287:386-395. DOI: https://doi.org/ 10.1016/j.jpowsour.2015.04.018

[62] Hou H, Sun G, He R, Wu Z, Sun B. Alkali doped polybenzimidazole membrane for high performance alkaline direct ethanol fuel cell. Journal of Power Sources. 2008;182:95-99. DOI: https://doi.org/10.1016/j.jpowsour. 2008.04.010

[63] Modestov AD, Tarasevich MR, Leykin AY, Filimonov VY. MEA for alkaline direct ethanol fuel cell with alkali doped PBI membrane and nonplatinum electrodes. Journal of Power Sources. 2009;188:502-506. DOI: https://doi.org/10.1016/j.jpowsour. 2008.11.118

[64] Luo H, Vaivars G, Agboola B, Mu S, Mathe M. Anion exchange membrane based on alkali doped poly(2,5 benzimidazole) for fuel cell. Solid State Ionics. 2012;208:52-55. DOI: https://doi. org/10.1016/j.ssi.2011.11.029

[65] Couto RN, Linares JJ. KOH-doped polybenzimidazole for alkaline direct glycerol fuel cells. Journal of Membrane Science. 2015;486:239-247. DOI: https:// doi.org/10.1016/j.memsci.2015.03.031

[66] Zeng L, Zhao TS, An L, Zhao G, Yan XH. Physicochemical properties of alkaline doped polybenzimidazole membranes for anion exchange membrane fuel cells. Journal of Membrane Science. 2015;493:340-348. DOI: https://doi.org/10.1016/j. memsci.2015.06.013

**203**

**Chapter 11**

*Ana Beloqui*

**Abstract**

Applications

fabrication of new nanostructures.

π-π stacking interactions

**1. Introduction**

Supramolecular Assembly of

Benzimidazole Derivatives and

Herein, we focus on the chemical and physical properties of benzimidazole and its derivatives used for the synthesis of supramolecular materials. The design and modification of benzimidazole opens the scope of the diversity of structures (different sizes and morphologies) that can be built. The synthesized materials include not only small coordination complexes but also isolated crystals, metal-organic frameworks, metal-coordination polymers, smart nanocontainers, and more advanced macrostructures such as microflowers and nanowires. These supramolecular structures are based on noncovalent interactions, mostly on metal coordination chemistry and π-π stacking interactions. Moreover, the same molecule, due to its chemical structure, can undergo both sorts of interactions in order to induce the self-assembly into supramolecular materials. In this process, as it is shown in this chapter, the conditions used for the assembly determine the final structure and morphology of the fabricated macromolecule. Finally, we show most recent applications of these materials in the field of sensing, photoluminescence, fuel cell, and

**Keywords:** self-assembly, supramolecular interactions, metal-imidazole coordination,

Benzimidazole and its derivatives are mostly known by their role in therapeutic drugs and by their pharmacological activities, for example, antimicrobial, analgesic, and anti-inflammatory [1]. Moreover, they are part of essential biomolecules as vitamin B12 [2]. Thus, the biological activity of benzimidazole and its derivatives is unquestionable. However, there is a growing research interest in using benzimidazole derivatives for their assembly into supramolecular structures for technological applications. This implies the formation of well-defined complex bond through noncovalent interactions. In this regard, the interest on benzimidazole molecule is twofold (**Figure 1**). On the one hand, benzimidazole is a popular N-donor ligand that is often used in coordination chemistry, meaning that it can through metal- or small-molecule coordination to the assembly of molecules. Indeed, the imidazole ring is commonly found as part of essential components of biological products,

#### **Chapter 11**
