**2. Porous carbon materials as counter electrodes for DSSCs**

As counter electrode (CE) is one of the most crucial components regulating the efficiency of DSSCs by catalyzing the reduction of the redox couples used as mediators to regenerate the sensitizer after electron injection, it is important to find a low-cost, high-efficiency, easy scalability, and corrosion-stable counter electrode. Platinum (Pt) has been widely employed as the standard CE in DSSCs due to its high catalytic reduction for redox shuttles, good chemical stability, and high conductivity. However, Pt is an expensive and scarce noble metal, which causes a problem for its large-scale production.

Carbon materials are one of the promising substitutes of Pt CE due to their low cost, environmentally friendly, scale availability, high surface area, high catalytic activity, high electrical conductivity, high thermal stability, good corrosion resistance toward iodine, high reactivity for triiodide reduction, etc. In 1996, Kay and Grätzel first explored graphite-carbon black mixture as CE and achieved a power conversion efficiency of 6.7% [8]. Thereafter, intensive research efforts have been focused on carbonaceous materials, such as carbon black, mesoporous carbon, graphite [9], graphene [10, 11], carbon nanotubes [12–15], and carbon nanofibers [16–18], and they have been successfully employed as counter electrodes.

Among the carbonaceous materials, porous carbon materials have captured extensive attention as CE materials for DSSCs, owing to its exceptional properties, including ultrahigh surface areas, large pore volumes, and tunable pore sizes and shapes, and also exhibit nanoscale effects in their mesopore channels and on their pore walls. High surface areas provide a large number of reaction or interaction sites for surface- or interface-related processes such as adsorption and catalysis. Moreover, mesopore channels facilitate the transport of atoms, ions, and large molecules through the bulk of the material and assist to overcome charge transfer resistance [19, 20].

## **2.1. Synthetic methods of porous carbon materials and photovoltaic performance in DSSCs**

In their experiment, PANI nanorods were firstly prepared by polymerization of aniline in aqueous solution using ammonium persulfate (APS) as the initiator, as shown in **Figure 2**. Ten grams of aniline and 7 g of citric acid were mixed in 500 mL of distilled water, and then 20 g of ammonium persulfate was dissolved in 150 mL of distilled water. Afterward, ammonium persulfate solution was added into the mixture solution of aniline and citric acid with vigorous stirring. The resulting solution was left standing at about 3°C for 20 h. The obtained dark green sample was filtered and washed with distilled water and then dried under vacuum at 50°C. The as-prepared PANI nanorods were pyrolyzed at 600°C under nitrogen atmosphere for 3 h to obtain nitrogen-doped carbon nanorods (N-CNRs). Finally, N-PCNRs were pre-

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

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63

**Figure 2.** Preparation of nitrogen-doped porous carbon nanorods (N-PCNRs) [27] (copyright (2017) Elsevier).

Recently, H. K. Kim group synthesized anchovy-derived nitrogen and sulfur Co-doped porous carbons (AnCs) by a simple carbonization and alkali activation method for DSSC CEs, as shown in **Figure 3**. Typically, a three-step strategy was used to prepare the activated carbon. Firstly, a dried anchovy powder was weighed and transferred into a quartz tubular furnace and then pre-carbonized at 300°C for 1 h under atmosphere, yielding a pre-carbonized material. An activation and carbonization process was sequentially carried out as follows: The pre-carbonized material was mixed with various mass ratios of KOH and then directly heated at an elevated temperature (700, 800, 900°C) for 2 h under inert nitrogen atmosphere. Finally,

**Figure 3.** Approximate composition of a dried anchovy powder and schematic illustration for the preparation of anchovyderived nitrogen and sulfur co-doped porous carbon materials [28] (copyright (2017) Royal Society of Chemistry).

pared by activating N-CNRs with KOH [27].

Porous carbon materials are mainly produced by activation [21, 22] and templating [19, 23–25] methods from different precursors, and the efficiency of porous carbon electrodes depends on synthesis methods and precursors. The synthetic process of porous carbons by activation is simple, low-cost, and environmentally friendly. Also, activated carbons (ACs) have excellent chemical and thermal stability as well as relatively good electrical conductivity. ACs are derived from different types of carbon-rich organic precursors (coffee waste, wood, pitch, coal, polymers, etc.) by carbonization in inert atmosphere with subsequent physical and/or chemical activation or plasma surface treatment. Depending on the activation process and the used carbon precursors, a variety of ACs with different physicochemical properties and welldeveloped specific surface area (SSA) from 500 to 3000 m<sup>2</sup> g−1 have been prepared and used as CEs for DSSCs. The high surface area of activated carbons can considerably contribute to improving the catalytic activity of the CEs in DSSCs.

Imoto et al. firstly reported that porous carbon materials prepared with an activation process were superior to a Pt-sputtered electrode as the DSSC CEs. Moreover, they observed that the photovoltaic performance was strongly influenced by the roughness factor and the apparent charge transfer resistance of DSSC CEs. Generally, the roughness factor of the carbon electrode becomes larger when a porous carbon material with larger surface area was used. Thus, the photovoltaic performance was improved with increasing the roughness factor of the carbon-based CE. Also, the apparent charge transfer resistance for redox reaction of I− /I3 − couple on the porous carbon CE decreased with increasing the roughness factor. Therefore, back transfer of the photoinjected electrons to the oxidized dyes and/or I3 − ions was inhibited which in turns improved the FF value significantly. Furthermore, the *Voc* value for the carbon counter electrode compared to the Pt counter electrode was increased by about 60 mV, maybe because of a positive shift of the formal potential for I− /I3 − couple [26].

Nitrogen-doped porous carbon nanorods (N-PCNRs) with high accessible surface area were prepared by Wang group through carbonization of polyaniline (PANI) nanorods and consequent chemical activation and explored as the DSSC CEs. The unique combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure endows the N-PCNR electrode with an excellent electrocatalytic activity for the I<sup>3</sup> − reduction, which is illuminated by electrochemical measurement. Under simulated AM 1.5 illumination (100 mW cm−2), the DSSC based on N-PCNR counter electrode achieves a conversion efficiency of 7.01%, which is nearly close to that of the cell based on Pt CE (7.25%).

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells http://dx.doi.org/10.5772/intechopen.75398 63

**Figure 2.** Preparation of nitrogen-doped porous carbon nanorods (N-PCNRs) [27] (copyright (2017) Elsevier).

Among the carbonaceous materials, porous carbon materials have captured extensive attention as CE materials for DSSCs, owing to its exceptional properties, including ultrahigh surface areas, large pore volumes, and tunable pore sizes and shapes, and also exhibit nanoscale effects in their mesopore channels and on their pore walls. High surface areas provide a large number of reaction or interaction sites for surface- or interface-related processes such as adsorption and catalysis. Moreover, mesopore channels facilitate the transport of atoms, ions, and large molecules through the bulk of the material and assist to overcome charge transfer

**2.1. Synthetic methods of porous carbon materials and photovoltaic performance in** 

improving the catalytic activity of the CEs in DSSCs.

because of a positive shift of the formal potential for I−

Porous carbon materials are mainly produced by activation [21, 22] and templating [19, 23–25] methods from different precursors, and the efficiency of porous carbon electrodes depends on synthesis methods and precursors. The synthetic process of porous carbons by activation is simple, low-cost, and environmentally friendly. Also, activated carbons (ACs) have excellent chemical and thermal stability as well as relatively good electrical conductivity. ACs are derived from different types of carbon-rich organic precursors (coffee waste, wood, pitch, coal, polymers, etc.) by carbonization in inert atmosphere with subsequent physical and/or chemical activation or plasma surface treatment. Depending on the activation process and the used carbon precursors, a variety of ACs with different physicochemical properties and welldeveloped specific surface area (SSA) from 500 to 3000 m<sup>2</sup> g−1 have been prepared and used as CEs for DSSCs. The high surface area of activated carbons can considerably contribute to

Imoto et al. firstly reported that porous carbon materials prepared with an activation process were superior to a Pt-sputtered electrode as the DSSC CEs. Moreover, they observed that the photovoltaic performance was strongly influenced by the roughness factor and the apparent charge transfer resistance of DSSC CEs. Generally, the roughness factor of the carbon electrode becomes larger when a porous carbon material with larger surface area was used. Thus, the photovoltaic performance was improved with increasing the roughness factor of the carbon-based CE. Also, the apparent charge transfer resistance for redox reaction of I−

couple on the porous carbon CE decreased with increasing the roughness factor. Therefore,

which in turns improved the FF value significantly. Furthermore, the *Voc* value for the carbon counter electrode compared to the Pt counter electrode was increased by about 60 mV, maybe

Nitrogen-doped porous carbon nanorods (N-PCNRs) with high accessible surface area were prepared by Wang group through carbonization of polyaniline (PANI) nanorods and consequent chemical activation and explored as the DSSC CEs. The unique combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure endows

is illuminated by electrochemical measurement. Under simulated AM 1.5 illumination (100 mW cm−2), the DSSC based on N-PCNR counter electrode achieves a conversion effi-

/I3 −

couple [26].

back transfer of the photoinjected electrons to the oxidized dyes and/or I3

the N-PCNR electrode with an excellent electrocatalytic activity for the I<sup>3</sup>

ciency of 7.01%, which is nearly close to that of the cell based on Pt CE (7.25%).

/I3 −

ions was inhibited

reduction, which

−

−

resistance [19, 20].

62 Emerging Solar Energy Materials

**DSSCs**

In their experiment, PANI nanorods were firstly prepared by polymerization of aniline in aqueous solution using ammonium persulfate (APS) as the initiator, as shown in **Figure 2**. Ten grams of aniline and 7 g of citric acid were mixed in 500 mL of distilled water, and then 20 g of ammonium persulfate was dissolved in 150 mL of distilled water. Afterward, ammonium persulfate solution was added into the mixture solution of aniline and citric acid with vigorous stirring. The resulting solution was left standing at about 3°C for 20 h. The obtained dark green sample was filtered and washed with distilled water and then dried under vacuum at 50°C. The as-prepared PANI nanorods were pyrolyzed at 600°C under nitrogen atmosphere for 3 h to obtain nitrogen-doped carbon nanorods (N-CNRs). Finally, N-PCNRs were prepared by activating N-CNRs with KOH [27].

Recently, H. K. Kim group synthesized anchovy-derived nitrogen and sulfur Co-doped porous carbons (AnCs) by a simple carbonization and alkali activation method for DSSC CEs, as shown in **Figure 3**. Typically, a three-step strategy was used to prepare the activated carbon. Firstly, a dried anchovy powder was weighed and transferred into a quartz tubular furnace and then pre-carbonized at 300°C for 1 h under atmosphere, yielding a pre-carbonized material. An activation and carbonization process was sequentially carried out as follows: The pre-carbonized material was mixed with various mass ratios of KOH and then directly heated at an elevated temperature (700, 800, 900°C) for 2 h under inert nitrogen atmosphere. Finally,

**Figure 3.** Approximate composition of a dried anchovy powder and schematic illustration for the preparation of anchovyderived nitrogen and sulfur co-doped porous carbon materials [28] (copyright (2017) Royal Society of Chemistry).

the anchovy-derived activated carbon obtained was washed several times with aqueous 2 M HCl solution and deionized water in order to remove the potassium compounds and any impurities. After filtration, the samples were dried at 70°C in an electric vacuum oven.

The anchovy-derived activated carbons as a DSSC CE exhibited superior PCE of 12.72% to the Pt CE with 12.23%. It is ascribed to an improved fill factor caused by its better electrocatalytic ability [28]. To the best of our knowledge, this PCE is the highest efficiency value reported for DSSCs based on carbon nanomaterial-based CEs. **Table 1** summarizes porous carbon materials produced by activation and their precursors with photovoltaic performance.

Although activated carbon possesses high surface area, but most of the pores are micropore which may increase diffusion impedance of electrolytes into the pore of CEs especially for the bulkier redox couples like Co(bpy)3 2+/3+. Templating method is used to synthesize mesopore and macropore carbon materials with connected channel. Depending on the template used, templating method can be classified into two classes: hard template and soft template. Typically, silica nanoparticles, zeolites, anodic aluminum oxide (AAO) films, and mesoporous silica have been used as the hard templates. The synthetic process involves the impregnation of carbon precursor into the porous structure of pre-synthesis template following the carbonization and removal of the template. On the other hand, soft templating involves the self-assembly of copolymers and their direct removal through carbonization. Various commercially available triblock copolymers PEO-PPO-PEO (PO, propylene oxide; EO, ethylene oxide), such as F127 (EO106PO70EO106), P123 (EO20PO70EO20), and F108 (EO132PO50EO132), have been extensively used as the soft templates.

structural characteristics, particularly, the unique hierarchical core/shell nanostructure along with 3D large interconnected interstitial volume with large specific surface area and mesoporous volume, were observed compared with other porous carbon counterparts such as activated carbons and ordered mesoporous carbon CMK-3 (fabricated by replication through nano-casting of SBA-15 silica and using phenol as the carbon source) and Pt counter electrode. This hierarchical core/shell nanostructural feature with 3D large interconnected interstitial volume facilitates mass transportation in hollow macroporous core/mesoporous shell carbon (HCMSC), which was synthesized by replicating through nano-casting of solid core/mesoporous shell silica and phenol and paraformaldehyde as precursor, and enables HCMSC to have

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

**Figure 4.** Typical synthetic scheme for HCMSC capsule [33] (copyright (2010) American Chemical Society).

photovoltaic performance. HCMSC as a DSSC CE displays a *Voc* of 0.74 V, which is superior to that of the Pt CE by 20 mV. Moreover, it also exhibits a fill factor of 0.67 and an energy conversion efficiency of 7.56%, which are markedly higher than those of its carbon counterparts and comparable to that of Pt (i.e., fill factor of 0.70 and conversion efficiency of 7.79%). In addition, superb chemical stability was observed by HCMSC in the liquid electrolyte containing

redox couples, and its initial efficiency of ca. 87% was attained by the HCMSC counter

Gokhale et al. introduced a laser photochemical process to synthesize broccoli-type hierarchical morphology for use as a metal-free CE in DSSCs. The method includes pulsed excimer laser irradiation on a thin layer of liquid halo-aromatic organic solvent containing o-dichlorobenzene (DCB). The obtained coating reflects a self-assembled carbon nanoparticle and process-controlled morphology that yields a PCE of 5.1% which was comparable

Wang et al. designed and fabricated a novel bioinspired Pt- and FTO-free integrated pure carbon counter electrode for DSSCs with orderly mesoporous carbon (OMC) as the catalytic layer and a porous carbon sheet as a conducting substrate (see in **Figure 5**). On a porous conducting substrate, a stiff, crustose lichen-like, integrated carbon-carbon architectured composite as a catalytic layer was made by a sequential process of spin coating, infiltration, and pyrolysis of polymer precursor. To fabricate the integrated pure carbon electrode, porous carbon plates (PCPs) were firstly prepared by a bulk molding-carbonization process using cheap coal powder as precursor. The permeation of electrolyte through the porous substrate has been fairly resolved by impermeable pretreatment through a repeat pitch impregnation-carbonization treatment of paraffin-protected PCPs. A phenol-formaldehyde resin solution including

to the PCE of 6.2% with the conventional Pt-based counter electrode [24].

−

and so substantially enhanced

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65

highly boosted catalytic activity toward the reduction of I<sup>3</sup>

electrode-based solar cell, even after 60 days of aging.

I− /I3 −

Hierarchical nanostructured carbon with a hollow macroporous core of ca. 60 nm in diameter in combination with mesoporous shell of ca. 30 nm in thickness was synthesized and explored as CE in metal-free organic dye-sensitized solar cells exhibited in **Figure 4** [33]. The superior


**Table 1.** Carbon materials produced by activation and their precursors with photovoltaic performance.

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells http://dx.doi.org/10.5772/intechopen.75398 65

**Figure 4.** Typical synthetic scheme for HCMSC capsule [33] (copyright (2010) American Chemical Society).

the anchovy-derived activated carbon obtained was washed several times with aqueous 2 M HCl solution and deionized water in order to remove the potassium compounds and any impurities. After filtration, the samples were dried at 70°C in an electric vacuum oven.

The anchovy-derived activated carbons as a DSSC CE exhibited superior PCE of 12.72% to the Pt CE with 12.23%. It is ascribed to an improved fill factor caused by its better electrocatalytic ability [28]. To the best of our knowledge, this PCE is the highest efficiency value reported for DSSCs based on carbon nanomaterial-based CEs. **Table 1** summarizes porous carbon materi-

Although activated carbon possesses high surface area, but most of the pores are micropore which may increase diffusion impedance of electrolytes into the pore of CEs especially

size mesopore and macropore carbon materials with connected channel. Depending on the template used, templating method can be classified into two classes: hard template and soft template. Typically, silica nanoparticles, zeolites, anodic aluminum oxide (AAO) films, and mesoporous silica have been used as the hard templates. The synthetic process involves the impregnation of carbon precursor into the porous structure of pre-synthesis template following the carbonization and removal of the template. On the other hand, soft templating involves the self-assembly of copolymers and their direct removal through carbonization. Various commercially available triblock copolymers PEO-PPO-PEO (PO, propylene oxide; EO, ethylene oxide), such as F127 (EO106PO70EO106), P123 (EO20PO70EO20),

Hierarchical nanostructured carbon with a hollow macroporous core of ca. 60 nm in diameter in combination with mesoporous shell of ca. 30 nm in thickness was synthesized and explored as CE in metal-free organic dye-sensitized solar cells exhibited in **Figure 4** [33]. The superior

> **FF (%)**

*Jsc* **(mA cm−2)**

7.56 75.9 18.78 12.72 12.23 [28]

<sup>−</sup> 0.46 72.6 15.09 8.32 8.07 [21]

<sup>−</sup> 2.2 63.0 15.85 7.01 7.25 [27]

<sup>−</sup> 51.6 13.51 4.98 6.25 [29]

<sup>−</sup> 7.5 55.0 17.92 6.9 7.1 [30]

<sup>−</sup> 2.2 67.5 15.5 6.1 7.0 [31]

<sup>−</sup> 0.9 67.3 13.3 6.17 6.37 [32]

<sup>−</sup> 60.7 7:93 3.89 3.61 [26]

2+/3+. Templating method is used to synthe-

**PCE (%) PCE-Pt (%)**

**Reference**

als produced by activation and their precursors with photovoltaic performance.

and F108 (EO132PO50EO132), have been extensively used as the soft templates.

*Rct* **( cm2 )**

**electrolyte**

Co(bpy)3 2+/3+

/I3

/I3

/I3

/I3

/I3

/I3

/I3

**Table 1.** Carbon materials produced by activation and their precursors with photovoltaic performance.

for the bulkier redox couples like Co(bpy)3

64 Emerging Solar Energy Materials

**CE Precursor Dye-**

AnCs Anchovy SM-315-

N-PCNRs Polyaniline N719-I−

HPC Pine cone N719-I−

a-NCs Resol N719-I−

Bellfine AP Commercial N719-I−

Coffee waste N719-I−

Commercial N719-I−

Commercial N719-I−

Carbon chunks

Porous carbon

Flexible carbon

structural characteristics, particularly, the unique hierarchical core/shell nanostructure along with 3D large interconnected interstitial volume with large specific surface area and mesoporous volume, were observed compared with other porous carbon counterparts such as activated carbons and ordered mesoporous carbon CMK-3 (fabricated by replication through nano-casting of SBA-15 silica and using phenol as the carbon source) and Pt counter electrode. This hierarchical core/shell nanostructural feature with 3D large interconnected interstitial volume facilitates mass transportation in hollow macroporous core/mesoporous shell carbon (HCMSC), which was synthesized by replicating through nano-casting of solid core/mesoporous shell silica and phenol and paraformaldehyde as precursor, and enables HCMSC to have highly boosted catalytic activity toward the reduction of I<sup>3</sup> − and so substantially enhanced photovoltaic performance. HCMSC as a DSSC CE displays a *Voc* of 0.74 V, which is superior to that of the Pt CE by 20 mV. Moreover, it also exhibits a fill factor of 0.67 and an energy conversion efficiency of 7.56%, which are markedly higher than those of its carbon counterparts and comparable to that of Pt (i.e., fill factor of 0.70 and conversion efficiency of 7.79%). In addition, superb chemical stability was observed by HCMSC in the liquid electrolyte containing I− /I3 − redox couples, and its initial efficiency of ca. 87% was attained by the HCMSC counter electrode-based solar cell, even after 60 days of aging.

Gokhale et al. introduced a laser photochemical process to synthesize broccoli-type hierarchical morphology for use as a metal-free CE in DSSCs. The method includes pulsed excimer laser irradiation on a thin layer of liquid halo-aromatic organic solvent containing o-dichlorobenzene (DCB). The obtained coating reflects a self-assembled carbon nanoparticle and process-controlled morphology that yields a PCE of 5.1% which was comparable to the PCE of 6.2% with the conventional Pt-based counter electrode [24].

Wang et al. designed and fabricated a novel bioinspired Pt- and FTO-free integrated pure carbon counter electrode for DSSCs with orderly mesoporous carbon (OMC) as the catalytic layer and a porous carbon sheet as a conducting substrate (see in **Figure 5**). On a porous conducting substrate, a stiff, crustose lichen-like, integrated carbon-carbon architectured composite as a catalytic layer was made by a sequential process of spin coating, infiltration, and pyrolysis of polymer precursor. To fabricate the integrated pure carbon electrode, porous carbon plates (PCPs) were firstly prepared by a bulk molding-carbonization process using cheap coal powder as precursor. The permeation of electrolyte through the porous substrate has been fairly resolved by impermeable pretreatment through a repeat pitch impregnation-carbonization treatment of paraffin-protected PCPs. A phenol-formaldehyde resin solution including

optimum activation temperature of 700°C, the obtained sample, labeled as a-NC700, possesses a satisfactory graphitization, a specific surface area of 1302 m<sup>2</sup> g−1 and an N fraction of 4.5%. A PCE of 6.9% is reached, when as-prepared a-NC700 used as a DSSC CE, which is

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

H. K. Kim group prepared copolymer-templated nitrogen-enriched nanocarbons (CTNCs)

at a heating rate of 10°C min−1, after stabilization through the cross-linking of the PAN block at 280°C for 1 h under air flow (150 mL min−1) at a heating rate of 1°C min−1, purged with N2 gas for 1 h during cooling. Superior performance in catalytic activity toward the reduction

activity of CTNCs is owing to their distinctive electronic properties stemming from the presence of nitrogen heteroatoms placed on the edges of nanographitic domains in combination with high specific surface area delivered by a three-dimensional, hierarchical pore structure. Overall, the application of CTNC CEs enhanced the efficiency and fill factor (FF) of JK-306 dye

respectively, suggesting the substantial potential of these materials as an striking substitute

Hasin group prepared Co or Ni species incorporated N-doped mesoporous carbon (Co-N-MC or Ni-N-MC) with high specific surface area and observed brilliant electrocatalytic activity

Co-N-MC and Ni-N-MC were synthesized on the basis of replication through nano-casting of SBA-15 mesoporous silica. The polymerization of aniline was conducted in the pore of SBA-15 mesoporous silica functionalized with alkyldiamine group. Cobalt(II) or nickel(II) ion was doped in the polyaniline (PANI) within the pores of SBA-15 followed by pyrolyzing in

/I3 −

**Figure 6.** Schematic representation of preparation of CTNC [42] (copyright (2015) Royal Society of Chemistry).

and Co(bpy)3

2+/3+ compared to the conventional Pt CE was observed. The detected remarkable

2+/3+ redox couple-based DSSCs at one sun illumination, up to 10.32 and 73.5%,

2+/3+ redox couples. CTNCs were syn-

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67

redox couple in DSSC systems as compared with

gas flow (150 mL min−1)

/I3 −

thesized by the pyrolysis of PAN-b-PBA copolymer as shown in **Figure 6**.

The block copolymer was pyrolyzed at 600–1000°C for 0.5 h under N<sup>2</sup>

comparable to that of the Pt CE-based DSSC (7.1%) [30].

and observed the better PCEs for both I−

of Co(bpy)3

and Co(bpy)3

to expensive Pt-based CEs [42].

toward the electrochemical reaction of I<sup>−</sup>

N-doped mesoporous carbon (N-MC) displayed in **Figure 7**.

**Figure 5.** The fabrication procedure of the integrated pure carbon counter electrode for DSSCs [34] (copyright (2013) Royal Society of Chemistry).

Pluronic F127 (EO106PO70EO106) as a template was spin coated onto the open pore side of the conductive substrate and then carbonized at 800°C under inert nitrogen atmosphere. By an evaporation-induced self-assembly (EISA) mechanism, an identical mesostructured resin film was formed. Successively, a catalytic mesoporous carbon layer that was embedded in the porous carbon plate was produced by carbonization. The architecturally integrated carbonbased CE displays very low charge transfer resistance (*Rct*), owing to the large specific surface area of the OMC layer that is accessible to the redox couple, and low series resistance (*Rs* ), due to the high conductivity of the carbon sheet (sheet resistance of 488 m cm−1). The values of *Rs* and *Rct* are far lower than those of the platinized fluorine-doped tin oxide glass (Pt/FTO) electrode. DSSCs with this CE show higher PCE of 8.11% than the Pt/FTO-based devices with the PCE of 8.16% [34].

Heteroatom-doped mesoporous carbons have received a great attention because of their enhanced electrocatalytic activity resulted from heteroatoms [35, 36]. The electrocatalytic properties of these systems are typically attributed to high charge polarization arising from the difference in electronegativity between carbon and heteroatom leading to enhanced charge transfer capability and thus increased catalytic activity. Also, to enhance the ordered graphitic layer and stability, porous carbon materials are mixed with polymer, transition metal, and other nanocarbon materials like graphene and CNTs [37–41].

Wang et al. synthesized activated N-doped porous carbons (a-NCs) by pyrolysis and alkali activation of graphene-incorporated melamine formaldehyde resin (MF). To prepare a-NCs, firstly a homogeneous mixture of activated graphene (a-G) with melamine and formaldehyde was prepared and pre-carbonized in Teflon-lined autoclave at 140°C for 4 h to form graphene-incorporated MF resin. The as-prepared graphene-incorporated MF resin composite then carbonized at 800°C for 2 h in N<sup>2</sup> atmosphere followed by activation with KOH, yielding a-NCs. a-NCs with the moderate N-doping level, mesopore-rich porous texture, and incorporation of graphene enable the applications of a-NCs in electrode materials with high specific surface area and good conductivity for dye-sensitized solar cell (DSSCs). At the optimum activation temperature of 700°C, the obtained sample, labeled as a-NC700, possesses a satisfactory graphitization, a specific surface area of 1302 m<sup>2</sup> g−1 and an N fraction of 4.5%. A PCE of 6.9% is reached, when as-prepared a-NC700 used as a DSSC CE, which is comparable to that of the Pt CE-based DSSC (7.1%) [30].

H. K. Kim group prepared copolymer-templated nitrogen-enriched nanocarbons (CTNCs) and observed the better PCEs for both I− /I3 − and Co(bpy)3 2+/3+ redox couples. CTNCs were synthesized by the pyrolysis of PAN-b-PBA copolymer as shown in **Figure 6**.

The block copolymer was pyrolyzed at 600–1000°C for 0.5 h under N<sup>2</sup> gas flow (150 mL min−1) at a heating rate of 10°C min−1, after stabilization through the cross-linking of the PAN block at 280°C for 1 h under air flow (150 mL min−1) at a heating rate of 1°C min−1, purged with N2 gas for 1 h during cooling. Superior performance in catalytic activity toward the reduction of Co(bpy)3 2+/3+ compared to the conventional Pt CE was observed. The detected remarkable activity of CTNCs is owing to their distinctive electronic properties stemming from the presence of nitrogen heteroatoms placed on the edges of nanographitic domains in combination with high specific surface area delivered by a three-dimensional, hierarchical pore structure. Overall, the application of CTNC CEs enhanced the efficiency and fill factor (FF) of JK-306 dye and Co(bpy)3 2+/3+ redox couple-based DSSCs at one sun illumination, up to 10.32 and 73.5%, respectively, suggesting the substantial potential of these materials as an striking substitute to expensive Pt-based CEs [42].

Pluronic F127 (EO106PO70EO106) as a template was spin coated onto the open pore side of the conductive substrate and then carbonized at 800°C under inert nitrogen atmosphere. By an evaporation-induced self-assembly (EISA) mechanism, an identical mesostructured resin film was formed. Successively, a catalytic mesoporous carbon layer that was embedded in the porous carbon plate was produced by carbonization. The architecturally integrated carbonbased CE displays very low charge transfer resistance (*Rct*), owing to the large specific surface area of the OMC layer that is accessible to the redox couple, and low series resistance (*Rs*

**Figure 5.** The fabrication procedure of the integrated pure carbon counter electrode for DSSCs [34] (copyright (2013)

to the high conductivity of the carbon sheet (sheet resistance of 488 m cm−1). The values of

Heteroatom-doped mesoporous carbons have received a great attention because of their enhanced electrocatalytic activity resulted from heteroatoms [35, 36]. The electrocatalytic properties of these systems are typically attributed to high charge polarization arising from the difference in electronegativity between carbon and heteroatom leading to enhanced charge transfer capability and thus increased catalytic activity. Also, to enhance the ordered graphitic layer and stability, porous carbon materials are mixed with polymer, transition metal, and

Wang et al. synthesized activated N-doped porous carbons (a-NCs) by pyrolysis and alkali activation of graphene-incorporated melamine formaldehyde resin (MF). To prepare a-NCs, firstly a homogeneous mixture of activated graphene (a-G) with melamine and formaldehyde was prepared and pre-carbonized in Teflon-lined autoclave at 140°C for 4 h to form graphene-incorporated MF resin. The as-prepared graphene-incorporated MF resin compos-

yielding a-NCs. a-NCs with the moderate N-doping level, mesopore-rich porous texture, and incorporation of graphene enable the applications of a-NCs in electrode materials with high specific surface area and good conductivity for dye-sensitized solar cell (DSSCs). At the

atmosphere followed by activation with KOH,

other nanocarbon materials like graphene and CNTs [37–41].

ite then carbonized at 800°C for 2 h in N<sup>2</sup>

 and *Rct* are far lower than those of the platinized fluorine-doped tin oxide glass (Pt/FTO) electrode. DSSCs with this CE show higher PCE of 8.11% than the Pt/FTO-based devices with

*Rs*

the PCE of 8.16% [34].

Royal Society of Chemistry).

66 Emerging Solar Energy Materials

), due

Hasin group prepared Co or Ni species incorporated N-doped mesoporous carbon (Co-N-MC or Ni-N-MC) with high specific surface area and observed brilliant electrocatalytic activity toward the electrochemical reaction of I<sup>−</sup> /I3 − redox couple in DSSC systems as compared with N-doped mesoporous carbon (N-MC) displayed in **Figure 7**.

Co-N-MC and Ni-N-MC were synthesized on the basis of replication through nano-casting of SBA-15 mesoporous silica. The polymerization of aniline was conducted in the pore of SBA-15 mesoporous silica functionalized with alkyldiamine group. Cobalt(II) or nickel(II) ion was doped in the polyaniline (PANI) within the pores of SBA-15 followed by pyrolyzing in

**Figure 6.** Schematic representation of preparation of CTNC [42] (copyright (2015) Royal Society of Chemistry).

**Figure 7.** Immobilized Co or Ni on N-doped mesoporous carbons [43] (copyright (2017) Elsevier).

quartz tube. The considerable photoelectric conversion efficiency of Co-N-MC- and Ni-N-MC-based DSSCs is ascribed to the good electrical conductivity in their frameworks and the dominating contents of pyridinic and quaternary N species over pyrrolic N in their structural configuration. Furthermore, significantly lower charge transfer resistance (*Rct*) associated with metal doping of the Co-N-MC and Ni-N- MC and excellent structural surface properties can also be responsible to enhance the photovoltaic performance. The fill factor (FF) and power conversion efficiency (*η*) of DSSC utilizing Ni-N-MC counter electrode (FF = 0.70 and *η* = 8.42%) were higher compared to the DSSC using platinized counter electrode (FF = 0.66 and *η* = 8.22%). Additionally, Ni-N-MC counter electrode exhibited good electrochemical stability after experiencing ten CV cycles. Moreover, compared to platinized electrode in catalyzing the T2 /T<sup>−</sup> organic redox system, all N-MC, Co-N-MC, and Ni-N-MC show better performance. The integration of electrocatalytic Co or Ni species and N-doped mesoporous carbon matrix is an alternative approach for counter electrode electrocatalyst to diminish the cost of DSSCs [43]. Precursors and photovoltaic performance of porous carbon materials synthesized by soft- and hard-templating methods are summaries in **Tables 2** and **3**, respectively.

101 (2θ = 44°) crystal planes as shown in **Figure 9**(**a**) [50]. To identify the degree of graphitization, Raman spectra are used as follows: Two peaks at 1340 and 1590 cm−1 that is assigned to

**Table 2.** Porous carbon material synthesis by soft template and their precursors with photovoltaic performance.

**( cm2 ) FF (%)** *Jsc* **(mA cm−2)**

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

<sup>−</sup> 4.68 65.0 14.97 7.22 7.25 [25]

<sup>−</sup> 17.2 61.0 14.32 6.06 6.29 [19]

<sup>−</sup> 6.99 73.0 14.26 8.11 8.16 [34]

<sup>−</sup> 46.1 13.86 5.1 6.2 [24]

<sup>−</sup> 0.3 67.0 15.44 6.48 6.45 [44]

<sup>−</sup> 0.7 65.0 15.5 6.18 6.26 [46]

6.5 68.0 14.13 7.69 8.25 [45]

<sup>−</sup> 72.9 14.58 7.88

**PCE (%)**

0.31 73.5 14.57 10.32 9.80 [42]

**PCE-Pt (%)**

http://dx.doi.org/10.5772/intechopen.75398

**Reference**

69

The breathing mode vibration of A1g, associated with the disordered carbon or defective graphitic carbon appeared at around 1353 cm−1 corresponding to the D band. Another peak related to the G band appeared at around 1590 cm−1, which specifies the in-plane stretching

> *Rct* **( cm2 )**

C–C bond vibrations, as in graphitic phase [51].

*Jsc* **(mA cm−2)**

<sup>−</sup> 0.14 69.0 16.54 8.75 7.55 [36]

<sup>−</sup> 3.99 67.0 14.98 6.97 7.28 [47]

<sup>−</sup> 60.0 15.46 7.02 7.26 [35]

<sup>−</sup> 8.75 70.0 14.95 8.18 8.85 [48]

<sup>−</sup> 0.5 67.0 16.86 7.56 7.79 [49]

<sup>−</sup> 69.0 14.70 7.89 [23]

**PCE (%)**

**PCE-Pt (%)**

**Reference**

**FF (%)**

the D band and G band are observed as shown in **Figure 9**(**b**).

**electrolyte**

/I3

/I3

/I3

/I3

/I3

/I3

**Table 3.** Porous carbon material synthesis by hard template and their precursors with photovoltaic performance.

I− /I3 −

**CE Precursor Dye-electrolyte** *Rct*

Co(bpy)3 2+/3+

JK-306-I− /I3

/I3

/I3

/I3

/I3

/I3

/I3

CTNC PBA-b-PAN JK-306-

HPCs Resol N719-I−

MC Resol N719-I−

OMC Coal powder N719-I−

HPC Resol N719-I−

MC Resol N3-I<sup>−</sup>

OMC Resol Thiocyanate-

O-dichlorobenzene N719-I−

Broccolicarbon

Carbon framework

vibration mode of E2g in sp2 sp<sup>2</sup>

**CE Precursor Dye-**

S-PC Pitch N719-I−

NMC Resol N719-I−

MSU-F–C Resol N719-I−

HCMSC Resol N719-I−

Fe-MCF-C Divinylbenzene N719-I−

Quinoline N719-I−

#### **2.2. Morphological and structural characterization of porous carbon materials**

The morphology of porous carbon materials is usually characterized by using scanning electron microscopy (SEM) or field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). **Figure 8**(**a)**–(**c**) shows the FE-SEM images for the surface morphology and microstructures of honeycomb-like activated porous carbons (HPCs) [29]. As shown in **Figure 8**(**a**), the micro-sized particles of the as-prepared sample revealed a honeycomb-like morphology containing a huge number of minute holes over its entire frameworks. Magnified view of FE-SEM image of the sample confirmed the interconnected macroporous structures with sizes of ~100–600 nm (in **Figure 8**(**b**)). Moreover, a close inspection reveals that the pore and surface wall of HPC sample comprised of many mesopores and micropores as shown in **Figure 8**(**c**). In addition, the TEM image in **Figure 8**(**d**) also confirmed the honeycomb-like morphology with the porous property.

X-ray diffraction (XRD) and Raman spectroscopy are used to ascertain the extent of crystallinity in the structure of porous carbon samples. Porous carbon materials display diffraction peaks at about 24 and 44°, which are the equivalent of hexagonal graphitic 002 (2θ = 24°) and


**Table 2.** Porous carbon material synthesis by soft template and their precursors with photovoltaic performance.

quartz tube. The considerable photoelectric conversion efficiency of Co-N-MC- and Ni-N-MC-based DSSCs is ascribed to the good electrical conductivity in their frameworks and the dominating contents of pyridinic and quaternary N species over pyrrolic N in their structural configuration. Furthermore, significantly lower charge transfer resistance (*Rct*) associated with metal doping of the Co-N-MC and Ni-N- MC and excellent structural surface properties can also be responsible to enhance the photovoltaic performance. The fill factor (FF) and power conversion efficiency (*η*) of DSSC utilizing Ni-N-MC counter electrode (FF = 0.70 and *η* = 8.42%) were higher compared to the DSSC using platinized counter electrode (FF = 0.66 and *η* = 8.22%). Additionally, Ni-N-MC counter electrode exhibited good electrochemical stability after experiencing ten CV cycles. Moreover, compared to platinized electrode in

**Figure 7.** Immobilized Co or Ni on N-doped mesoporous carbons [43] (copyright (2017) Elsevier).

performance. The integration of electrocatalytic Co or Ni species and N-doped mesoporous carbon matrix is an alternative approach for counter electrode electrocatalyst to diminish the cost of DSSCs [43]. Precursors and photovoltaic performance of porous carbon materials synthesized by soft- and hard-templating methods are summaries in **Tables 2** and **3**, respectively.

The morphology of porous carbon materials is usually characterized by using scanning electron microscopy (SEM) or field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). **Figure 8**(**a)**–(**c**) shows the FE-SEM images for the surface morphology and microstructures of honeycomb-like activated porous carbons (HPCs) [29]. As shown in **Figure 8**(**a**), the micro-sized particles of the as-prepared sample revealed a honeycomb-like morphology containing a huge number of minute holes over its entire frameworks. Magnified view of FE-SEM image of the sample confirmed the interconnected macroporous structures with sizes of ~100–600 nm (in **Figure 8**(**b**)). Moreover, a close inspection reveals that the pore and surface wall of HPC sample comprised of many mesopores and micropores as shown in **Figure 8**(**c**). In addition, the TEM image in **Figure 8**(**d**) also confirmed

X-ray diffraction (XRD) and Raman spectroscopy are used to ascertain the extent of crystallinity in the structure of porous carbon samples. Porous carbon materials display diffraction peaks at about 24 and 44°, which are the equivalent of hexagonal graphitic 002 (2θ = 24°) and

**2.2. Morphological and structural characterization of porous carbon materials**

the honeycomb-like morphology with the porous property.

organic redox system, all N-MC, Co-N-MC, and Ni-N-MC show better

catalyzing the T2

68 Emerging Solar Energy Materials

/T<sup>−</sup>

101 (2θ = 44°) crystal planes as shown in **Figure 9**(**a**) [50]. To identify the degree of graphitization, Raman spectra are used as follows: Two peaks at 1340 and 1590 cm−1 that is assigned to the D band and G band are observed as shown in **Figure 9**(**b**).

The breathing mode vibration of A1g, associated with the disordered carbon or defective graphitic carbon appeared at around 1353 cm−1 corresponding to the D band. Another peak related to the G band appeared at around 1590 cm−1, which specifies the in-plane stretching vibration mode of E2g in sp2 sp<sup>2</sup> C–C bond vibrations, as in graphitic phase [51].


**Table 3.** Porous carbon material synthesis by hard template and their precursors with photovoltaic performance.

**Figure 8.** (a–c) FE-SEM images and (d) TEM images of the carbonized HPC sample under inert gas atmosphere [29] (copyright (2017) Elsevier).

Additionally, X-ray photoelectron spectroscopy (XPS) analysis gives the information about the presence of surface functional groups in the porous carbon. In **Figure 9**(**c**), the high-resolution XPS spectrum of C1s in the binding energy range between 280 and 288 eV revealed several carbon-based functional groups, which are assigned to the C–OH, C–C/ C–H, C–O, and C=O bonds, respectively. Meanwhile, as shown in **Figure 9**(**d**), the deconvoluted XPS spectra of the O 1 s exhibited the three distinct peaks at the binding energies of 526–538 eV, which are attributed to the C–OH, C=O groups and chemisorbed water molecules, respectively [29].

The surface area and pore volume of porous carbon materials are measured through the experiments of adsorption-desorption isotherms, as shown inset in **Figures 9**(**a**) and **10**(**b**). Porous carbon materials typically exhibit III/IV isotherms with pronounced hysteresis loops, indicating the larger surface area.

The specific surface area is calculated from the Brunauer-Emmett-Teller (BET) equation, and the pore size distribution is derived from the desorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) method (**Figure 10**(**b**)) [52].

### **2.3. Electrochemical characterization of porous carbon material-based counter electrodes**

The catalytic properties of counter electrodes in DSSCs are usually characterized by electrochemical impedance spectroscopy (EIS) [53, 54]. EIS measurements are performed in a symmetric thin-layer cell (called dummy cell) comprising two identical electrodes and same electrolyte used in the full DSSCs. The Nyquist plots for porous carbon CE consist of three

semicircles, which are assigned in the order of increasing frequency to the Nernst diffusion impedance in the bulk electrolyte (˂ 10 Hz), the charge transfer processes at the electrode/ electrolyte interface (2500–25 Hz), and the Nernst diffusion impedance in the pores of elec-

**Figure 10.** Nitrogen adsorption-desorption isotherms (a) and BJH pore-sized distributions (b) of the S-PC [47] (copyright

**Figure 9.** (a) XRD pattern, (b) Raman spectrum, and (c-d) XPS spectra of the HPC sample. The inset of (a) shows the

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

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71

nitrogen adsorption–desorption isotherm of the HPC sample [29] (copyright (2017) Elsevier).

trode materials (100–2.5 KHz) in **Figure 11**(**a**).

(2015) Elsevier).

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells http://dx.doi.org/10.5772/intechopen.75398 71

**Figure 9.** (a) XRD pattern, (b) Raman spectrum, and (c-d) XPS spectra of the HPC sample. The inset of (a) shows the nitrogen adsorption–desorption isotherm of the HPC sample [29] (copyright (2017) Elsevier).

Additionally, X-ray photoelectron spectroscopy (XPS) analysis gives the information about the presence of surface functional groups in the porous carbon. In **Figure 9**(**c**), the high-resolution XPS spectrum of C1s in the binding energy range between 280 and 288 eV revealed several carbon-based functional groups, which are assigned to the C–OH, C–C/ C–H, C–O, and C=O bonds, respectively. Meanwhile, as shown in **Figure 9**(**d**), the deconvoluted XPS spectra of the O 1 s exhibited the three distinct peaks at the binding energies of 526–538 eV, which are attributed to the C–OH, C=O groups and chemisorbed water

**Figure 8.** (a–c) FE-SEM images and (d) TEM images of the carbonized HPC sample under inert gas atmosphere [29]

The surface area and pore volume of porous carbon materials are measured through the experiments of adsorption-desorption isotherms, as shown inset in **Figures 9**(**a**) and **10**(**b**). Porous carbon materials typically exhibit III/IV isotherms with pronounced hysteresis loops,

The specific surface area is calculated from the Brunauer-Emmett-Teller (BET) equation, and the pore size distribution is derived from the desorption branches of the isotherms using the

The catalytic properties of counter electrodes in DSSCs are usually characterized by electrochemical impedance spectroscopy (EIS) [53, 54]. EIS measurements are performed in a symmetric thin-layer cell (called dummy cell) comprising two identical electrodes and same electrolyte used in the full DSSCs. The Nyquist plots for porous carbon CE consist of three

**2.3. Electrochemical characterization of porous carbon material-based counter** 

molecules, respectively [29].

(copyright (2017) Elsevier).

70 Emerging Solar Energy Materials

indicating the larger surface area.

**electrodes**

Barrett-Joyner-Halenda (BJH) method (**Figure 10**(**b**)) [52].

**Figure 10.** Nitrogen adsorption-desorption isotherms (a) and BJH pore-sized distributions (b) of the S-PC [47] (copyright (2015) Elsevier).

semicircles, which are assigned in the order of increasing frequency to the Nernst diffusion impedance in the bulk electrolyte (˂ 10 Hz), the charge transfer processes at the electrode/ electrolyte interface (2500–25 Hz), and the Nernst diffusion impedance in the pores of electrode materials (100–2.5 KHz) in **Figure 11**(**a**).

The equivalent circuit elements for fitting the EIS data, which is well known as a Randles-type equivalent circuit, comprise of the ohmic serial resistance (*Rs* ), the resistance-capacitance (RC) network consisting of the charge transfer resistance (*Rct*) and the corresponding capacitance (C) at the electrode/electrolyte interface, and the Nernst diffusion impedance in the bulk electrolyte (Nbulk) between electrodes and the pores of electrode materials (Npore) as shown in **Figure 11**(**b**) [55]. The impedance spectrum of the Pt/electrolyte interface diverges to some extent from the behavior of a RC network with an ideal capacitance owing to the roughness of the TCO substrate surface. This effect can be defined by a term called as "constant phase element (CPE)." The impedance ZCPE of a CPE is given by Eq. (3) [53, 56]:

$$Z\_{\rm CPE} = \mathcal{B} \, (i\alpha)^{\perp \beta} \tag{3}$$

diameter of the high-frequency semicircle, *Rct* value of the Pt CE can be estimated directly. The charge transfer resistance, *Rct*, of the porous carbon CEs depends on the thickness of electrodes. The data listed in **Table 4** show that the *Rct* of NMC-3 electrode decreases with increasing thickness of NMC-3 layer. *Rct* value of NMC-3 electrode decreases by 10.36 cm2

4.39 m). This result is mainly owing to the increase in active surface area of NMC-3 film with

Nevertheless, *Rct* value barely changes with additional increase of the thickness of NMC-3 layer. The optimum thickness of NMC-3 layer is 4.39 m, which is considerably lower than that

significantly with the thickness of NMC-3 layer. This is due to the preparation of NMC-3 elec-

properties of the FTO substrate. Capacitance, *C*, increases systematically with the thickness of

Lower *Rct* denotes more efficient reduction of redox couples at the CE-electrolyte interface, which in turn reduces charge recombination process and increases the dye-regeneration

(EASAs) were estimated from the results of chronoamperometric (CA) measurements according to the Randles-Sevcik equation to expose the question of whether the lower *Rct* originates from an intrinsically higher catalytic activity or the high surface area of porous carbon [42]. It was found that the intrinsically higher catalytic activity of porous carbon electrodes was

Tafel polarization measurements are carried out in a symmetric cell similar to the one used in EIS experiment. Usually, the Tafel curve comprises of three distinguishable zones, the polarization zone at low potential (voltage range of −120 to 120 mV), the Tafel zone at middle potential (with a sharp decrease), and the diffusion zone at high potential (horizontal part).

of an electrode, can be calculated by extrapolating the intercept of the anodic and cathodic branches of the corresponding curves in the Tafel zone. **Figure 11**(**c**) displays the Tafel polar-

**Table 4.** Electrochemical impedance parameters for NMC-3 electrode with various thicknesses of carbon layer (35)

of the commonly studied carbon nanoparticle CE in DSSCs. Also, *Rs*

NMC-3 layer as expected due to an increase in active surface area.

mainly responsible for lower charge transfer at the interface.

*o*

0.95 15.41 11.81 15.8 1.68 15.44 7.75 26.9 3.16 15.45 4.09 34.6 4.39 15.52 1.45 42.6 6.53 15.57 1.5 49.7

trodes on the same FTO substrate, and also the *Rs*

), once the thickness of NMC-3 layer increases by 3.44 m (from 0.95 to

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

/electrolyte interface. Electrochemically active surface areas

), which is directly associated to the electrocatalytic ability

**)** *C* **(μF cm−2)**

 **()** *Rct* **( cm2**

value does not change

value is controlled mainly by the electrical

http://dx.doi.org/10.5772/intechopen.75398

73

(from 11.81 to 1.45 cm2

the thicker NMC-3 layer.

process at the dye-coated TiO2

The exchange current density (*J*

(copyright (2013) Elsevier).

**Thickness of NMC layer (**μ**m)** *R<sup>s</sup>*

where *ω* is the angular frequency, B is the CPE parameter, and *β* is the CPE exponent (0≤≤1, for ideal capacitance = 1).

The parameter Npore can be omitted for Pt CE where the catalytic reaction occurs on virtually nonporous surface (the equivalent circuit for the symmetric cell comprising of Pt CEs is shown in the inset of **Figure 11**(**a**)). As a result, the Nyquist plot for Pt electrode exhibits two semicircles, allocating in the order of decreasing frequency to the charge transfer process at Pt electrode/electrolyte interface and Nernst diffusion of redox couple in electrolyte. From the

**Figure 11.** (a) Nyquist plots of the symmetric cells consisting of FTO glass, NMC-3, and Pt electrodes, respectively, equivalent circuit element for Pt CE in the inset. (b) Equivalent circuit element for NMC-3 porous carbon. (c) Tafel polarization curves of the symmetric cells fabricated with two identical Pt and NMC-3 electrodes [35] (copyright (2013) Elsevier).

diameter of the high-frequency semicircle, *Rct* value of the Pt CE can be estimated directly. The charge transfer resistance, *Rct*, of the porous carbon CEs depends on the thickness of electrodes. The data listed in **Table 4** show that the *Rct* of NMC-3 electrode decreases with increasing thickness of NMC-3 layer. *Rct* value of NMC-3 electrode decreases by 10.36 cm2 (from 11.81 to 1.45 cm2 ), once the thickness of NMC-3 layer increases by 3.44 m (from 0.95 to 4.39 m). This result is mainly owing to the increase in active surface area of NMC-3 film with the thicker NMC-3 layer.

The equivalent circuit elements for fitting the EIS data, which is well known as a Randles-type

network consisting of the charge transfer resistance (*Rct*) and the corresponding capacitance (C) at the electrode/electrolyte interface, and the Nernst diffusion impedance in the bulk electrolyte (Nbulk) between electrodes and the pores of electrode materials (Npore) as shown in **Figure 11**(**b**) [55]. The impedance spectrum of the Pt/electrolyte interface diverges to some extent from the behavior of a RC network with an ideal capacitance owing to the roughness of the TCO substrate surface. This effect can be defined by a term called as "constant phase

*ZCPE* = *B* (*i*)−*<sup>β</sup>* (3)

where *ω* is the angular frequency, B is the CPE parameter, and *β* is the CPE exponent (0≤≤1,

The parameter Npore can be omitted for Pt CE where the catalytic reaction occurs on virtually nonporous surface (the equivalent circuit for the symmetric cell comprising of Pt CEs is shown in the inset of **Figure 11**(**a**)). As a result, the Nyquist plot for Pt electrode exhibits two semicircles, allocating in the order of decreasing frequency to the charge transfer process at Pt electrode/electrolyte interface and Nernst diffusion of redox couple in electrolyte. From the

**Figure 11.** (a) Nyquist plots of the symmetric cells consisting of FTO glass, NMC-3, and Pt electrodes, respectively, equivalent circuit element for Pt CE in the inset. (b) Equivalent circuit element for NMC-3 porous carbon. (c) Tafel polarization curves of the symmetric cells fabricated with two identical Pt and NMC-3 electrodes [35] (copyright (2013)

), the resistance-capacitance (RC)

equivalent circuit, comprise of the ohmic serial resistance (*Rs*

for ideal capacitance = 1).

72 Emerging Solar Energy Materials

Elsevier).

element (CPE)." The impedance ZCPE of a CPE is given by Eq. (3) [53, 56]:

Nevertheless, *Rct* value barely changes with additional increase of the thickness of NMC-3 layer. The optimum thickness of NMC-3 layer is 4.39 m, which is considerably lower than that of the commonly studied carbon nanoparticle CE in DSSCs. Also, *Rs* value does not change significantly with the thickness of NMC-3 layer. This is due to the preparation of NMC-3 electrodes on the same FTO substrate, and also the *Rs* value is controlled mainly by the electrical properties of the FTO substrate. Capacitance, *C*, increases systematically with the thickness of NMC-3 layer as expected due to an increase in active surface area.

Lower *Rct* denotes more efficient reduction of redox couples at the CE-electrolyte interface, which in turn reduces charge recombination process and increases the dye-regeneration process at the dye-coated TiO2 /electrolyte interface. Electrochemically active surface areas (EASAs) were estimated from the results of chronoamperometric (CA) measurements according to the Randles-Sevcik equation to expose the question of whether the lower *Rct* originates from an intrinsically higher catalytic activity or the high surface area of porous carbon [42]. It was found that the intrinsically higher catalytic activity of porous carbon electrodes was mainly responsible for lower charge transfer at the interface.

Tafel polarization measurements are carried out in a symmetric cell similar to the one used in EIS experiment. Usually, the Tafel curve comprises of three distinguishable zones, the polarization zone at low potential (voltage range of −120 to 120 mV), the Tafel zone at middle potential (with a sharp decrease), and the diffusion zone at high potential (horizontal part). The exchange current density (*J o* ), which is directly associated to the electrocatalytic ability of an electrode, can be calculated by extrapolating the intercept of the anodic and cathodic branches of the corresponding curves in the Tafel zone. **Figure 11**(**c**) displays the Tafel polar-


**Table 4.** Electrochemical impedance parameters for NMC-3 electrode with various thicknesses of carbon layer (35) (copyright (2013) Elsevier).

ization curves of the NMC-3 and electrodepositing Pt electrodes. The cathodic and anodic branches of the Tafel curves show a steep slope for NMC-3 and Pt electrodes, indicating a high *J o* on the surface of NMC-3 and Pt electrodes. This implies that NMC-3 can have an electrocatalytic ability as effective as Pt electrode toward the I− /I3 − redox reaction. Furthermore, the limiting diffusion current density of the NMC-3 and Pt electrodes is observed to be of the same magnitude. This result describes that the bimodal mesopore structure can deliver an advantageous network for the diffusion of the electrolyte.

CTNC-800 was also confirmed by the EIS measurements of dummy cells after aging the two electrodes at room temperature and open circuit in **Figure 13(d)**. The EIS curve of the CTNC-800 electrode over the period of 14 days was nearly unchanged, while the EIS curves for Pt

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

The greater slopes of the linear dependence of the reduction and oxidation currents against the square root of potential and higher peak current of CTNC-800 electrodes at all scan rate

To evaluate the photovoltaic performance of DSSCs employing porous carbon CEs, generally a prescribed amount of carbon materials is grounded in a mortar with 0.1 ml tetrabutyl titanate and 8 ml n-butanol to obtain the carbon paste. Then, the electrode is prepared by coating carbon paste on FTO glass using doctor-blade method and heated at a certain temperature for

of 10 μm on FTO glass substrate by doctor-blade method and then sintered at a certain tem-

**Figure 13.** Electrochemical stability under potential-cycling on dummy cells with Pt (a), CTNC-800 (b) in acetonitrile solution of . The sequence of measurements was as follows: 10 × CV scans (from 0V → 1V → 1V → 0V → 1 V, scan rate 50 ) followed by 30 s relaxation at 0 V followed by an EIS measurement at 0 V from Hz to0.1 Hz. This sequence of electrochemical stability testing was repeated 10 times. (c) changes in the Pt and CTNC-800 according to the potential cycling. (d) Nyquist plots of symmetrical dummy cells consisting of Pt and CTNC-800 (inset) collected after 3 days (black), 7 days (red) and 14 days (green) of aging at room temperate and open circuit [42]. Copyright (2015) Royal Society

2+/3+ than

75

film with the thickness

http://dx.doi.org/10.5772/intechopen.75398

confirm the better catalytic activity of CTNC-800 toward the reduction of Co(bpy)<sup>3</sup>

**2.4. Photovoltaic performance evaluation of DSSCs with porous carbon CEs**

30 min. On the other hand, photoanode is prepared by depositing TiO<sup>2</sup>

showed evidence of a progressive increase of the resistance upon aging.

the Pt CE.

of Chemistry.

Furthermore, cyclic voltammetry of dummy cell is used to verify the electrocatalytic activity of porous carbon. The peak-to-peak separation (*Epp*) and the peak current are two important parameters for comparing catalytic activities of both CEs. A smaller *Epp* and larger peak currents indicate higher catalytic performance. **Figure 12**(**a)–(b**) shows the comparison of the CV curves of CTNC-800 and Pt electrodes in the Co(bpy)<sup>3</sup> 2+/3+ system acquired at different scan rates (25–200 mV s−1).

EIS is also used to check the stability of porous carbon CEs. **Figure 13**(**a**) and (**b**) shows the electrochemical stability of the dummy cells with CTNC-800 and Pt CEs under cumulative potential cycling with Co(bpy)3 2+/3+ in acetonitrile. After 100 cycles, although only a very modest increase of *Rct* values was observed from 0.35 to 0.45 cm2 for CTNC-800 CEs, but for Pt CEs, the *Rct* values increased from 3.10 to 11.26 cm2 by nearly 400%. The sluggish decline of *Rct* for CTNC-800 compared to Pt clearly implies the higher stability of CTNC-800 in the acetonitrile Co(bpy)3 2+/3+ electrolyte as shown in **Figure 13(c)**. Superior electrochemical stability of

**Figure 12.** Cyclic voltammograms using CTNC-800 (a) and Pt (b) cathodes in with various scanning rates. The oxidation (c) and reduction (d) peak currents with respect to the square root of scanning rate [42]. Copyright (2015) Royal Society of Chemistry.

CTNC-800 was also confirmed by the EIS measurements of dummy cells after aging the two electrodes at room temperature and open circuit in **Figure 13(d)**. The EIS curve of the CTNC-800 electrode over the period of 14 days was nearly unchanged, while the EIS curves for Pt showed evidence of a progressive increase of the resistance upon aging.

The greater slopes of the linear dependence of the reduction and oxidation currents against the square root of potential and higher peak current of CTNC-800 electrodes at all scan rate confirm the better catalytic activity of CTNC-800 toward the reduction of Co(bpy)<sup>3</sup> 2+/3+ than the Pt CE.

#### **2.4. Photovoltaic performance evaluation of DSSCs with porous carbon CEs**

ization curves of the NMC-3 and electrodepositing Pt electrodes. The cathodic and anodic branches of the Tafel curves show a steep slope for NMC-3 and Pt electrodes, indicating a

the limiting diffusion current density of the NMC-3 and Pt electrodes is observed to be of the same magnitude. This result describes that the bimodal mesopore structure can deliver an

Furthermore, cyclic voltammetry of dummy cell is used to verify the electrocatalytic activity of porous carbon. The peak-to-peak separation (*Epp*) and the peak current are two important parameters for comparing catalytic activities of both CEs. A smaller *Epp* and larger peak currents indicate higher catalytic performance. **Figure 12**(**a)–(b**) shows the comparison of the CV

EIS is also used to check the stability of porous carbon CEs. **Figure 13**(**a**) and (**b**) shows the electrochemical stability of the dummy cells with CTNC-800 and Pt CEs under cumulative

*Rct* for CTNC-800 compared to Pt clearly implies the higher stability of CTNC-800 in the ace-

**Figure 12.** Cyclic voltammograms using CTNC-800 (a) and Pt (b) cathodes in with various scanning rates. The oxidation (c) and reduction (d) peak currents with respect to the square root of scanning rate [42]. Copyright (2015) Royal Society

trocatalytic ability as effective as Pt electrode toward the I−

advantageous network for the diffusion of the electrolyte.

curves of CTNC-800 and Pt electrodes in the Co(bpy)<sup>3</sup>

CEs, the *Rct* values increased from 3.10 to 11.26 cm2

est increase of *Rct* values was observed from 0.35 to 0.45 cm2

on the surface of NMC-3 and Pt electrodes. This implies that NMC-3 can have an elec-

/I3 −

2+/3+ in acetonitrile. After 100 cycles, although only a very mod-

2+/3+ electrolyte as shown in **Figure 13(c)**. Superior electrochemical stability of

redox reaction. Furthermore,

2+/3+ system acquired at different scan

by nearly 400%. The sluggish decline of

for CTNC-800 CEs, but for Pt

high *J o*

rates (25–200 mV s−1).

74 Emerging Solar Energy Materials

tonitrile Co(bpy)3

of Chemistry.

potential cycling with Co(bpy)3

To evaluate the photovoltaic performance of DSSCs employing porous carbon CEs, generally a prescribed amount of carbon materials is grounded in a mortar with 0.1 ml tetrabutyl titanate and 8 ml n-butanol to obtain the carbon paste. Then, the electrode is prepared by coating carbon paste on FTO glass using doctor-blade method and heated at a certain temperature for 30 min. On the other hand, photoanode is prepared by depositing TiO<sup>2</sup> film with the thickness of 10 μm on FTO glass substrate by doctor-blade method and then sintered at a certain tem-

**Figure 13.** Electrochemical stability under potential-cycling on dummy cells with Pt (a), CTNC-800 (b) in acetonitrile solution of . The sequence of measurements was as follows: 10 × CV scans (from 0V → 1V → 1V → 0V → 1 V, scan rate 50 ) followed by 30 s relaxation at 0 V followed by an EIS measurement at 0 V from Hz to0.1 Hz. This sequence of electrochemical stability testing was repeated 10 times. (c) changes in the Pt and CTNC-800 according to the potential cycling. (d) Nyquist plots of symmetrical dummy cells consisting of Pt and CTNC-800 (inset) collected after 3 days (black), 7 days (red) and 14 days (green) of aging at room temperate and open circuit [42]. Copyright (2015) Royal Society of Chemistry.

perature for 30 min. After that, the sintered TiO<sup>2</sup> electrodes are immersed into a solution containing dye in ethanol for a long time at room temperature to adsorb dye on TiO2 photoanode.

**3. Conclusions and outlook**

term instability to the I<sup>−</sup>

tance of CEs increase FF and *J*

CEs in Co(bpy)3

tion of DSSCs.

**Acknowledgements**

No. KK1702-A00).

and higher PCE than the Pt CEs in Co(bpy)<sup>3</sup>

/I3 −

Dye-sensitized solar cells (DSSCs) have aroused intense interest and been regarded as one of the most prospective solar cells, due to low-cost, flexibility, simple device fabrication, and high conversion efficiency under weak light, in comparison to the conventional photovoltaic devices. Very recently, G2E in Swiss and G24i in the UK including Korean and Japan companies have demonstrated commercial and prototyped components based on DSSC technology with liquid electrolytes. However, the unit costs, long-term device stability, and power conversion efficiency must be further improved for real-life applications. For this purpose, considerable efforts have been devoted to the search for low-cost Pt-free catalysts that exhibit high electrochemical activity and fast electron transfer kinetics, while a platinum (Pt) metal is still known as the highly efficient and extensively used counter electrode (CE) in DSSCs; however, it has more or less problems that make it improper for the real-life application in DSSCs, such as its high manufacturing cost owing to its natural scarcity and insufficient long-

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

http://dx.doi.org/10.5772/intechopen.75398

77

devoted to finding possible alternatives to Pt, including carbon blacks, carbon nanotubes, functionalized graphene, and heteroatom-doped graphene nanoplatelets as efficient metalfree electrocatalysts. An ideal counter electrode in DSSCs must possess the following properties: high electrocatalytic ability and high conductivity, optimum thickness, high surface area and porous nature, low charge transfer resistance, high electrochemical and mechanical stability, energy level that matches the potential of the redox couple electrolyte, high reflectivity, and good adhesivity with TCO. High electrocatalytic ability and low charge transfer resis-

carbon materials have been considered as one of the promising candidates for the alternative to PT CE, since they have high surface area and porous nature, chemical corrosion resistance, electrochemical and mechanical stability, low cost, and simple preparation methods compared to Pt counter electrodes. Especially, heteroatom-doped porous carbon materials, such as CTNCs and AnCs, exhibited better electrocatalytic ability, lower charge transfer resistance,

ing candidates as metal-free CE for DSSCs and open a new research area for porous carbon

lytes, which are more common and desirable electrolytes. In the future, better electrocatalytic ability and electrochemical stability of carbon-based materials toward both redox couples of iodide and cobalt electrolytes still need to be significantly improved for the practical applica-

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) through the Mid-career Researcher Program (2017R1A2A1A17069374), the Functional Districts of the Science Belt Support Program (2017K000494), and the Korea Research Institute of Chemical Technology (KRICT & Project

2+/3+-based DSSCs. However, they are not sufficiently active in iodide electro-

redox couple in DSSCs. As a result, significant efforts have been

*sc* and subsequently PCE of the DSSC. In this chapter, porous

2+/3+-based electrolyte which make them promis-

A DSSC device is fabricated by clamping a dye-sensitized TiO<sup>2</sup> photoanode, a drop of electrolyte, and a counter electrode. For I− /I3 − redox shuttle, a mixture of 0.5 M 1-methyl-3-propylimidazolium iodide, 0.3 MLiI, 0.06 M I<sup>2</sup> , and 0.4 M 4-tert-butylpyridine in 3-methoxypropionitrile is used as the electrolyte of DSSCs [35]. Finally, the photovoltaic performance of DSSC device is evaluated under AM 1.5 solar simulator illumination at 100 mW cm−2.

**Figure 14** compares the photocurrent density-voltage (*J-V*) curves of the DSSCs with NMC-3, Pt, and FTO glass counter electrodes. The photovoltaic parameters of the DSSCs, including short-circuit current density (*J sc*),), open-circuit voltage (*Voc*), fill factor (FF), and power conversion efficiency (*η*), are summarized in **Table 5**. The DSSC with FTO glass as counter electrode exhibits a *Voc* of 0.49 V, a *J sc* of 4.13 mA cm−2, and a FF of 0.10, leading to a poor power conversion efficiency of 0.20%. When NMC-3 porous carbon material is employed as the counter electrode in DSSC, the DSSC attains a *Voc* of 0.71 V, *J sc* of 15.46 mA cm−2, FF of 0.64, and *η* of 7.02%. This power conversion efficiency is considerably comparable to 7.26% of the DSSC with Pt counter electrode. The outstanding photovoltaic performances of the DSSCs with NMC-3 counter electrode mostly originate from the vivid electrocatalytic performance of NMC-3 electrode associated with the nitrogen doping, bimodal mesopore structure, and large surface area.

**Figure 14.** Photocurrent density-voltage curves of DSSCs with Pt, NMC-3, and FTO glass counter electrodes [35] (copyright (2013) Elsevier).


**Table 5.** Photovoltaic parameters of the DSSCs using Pt, NMC-3, and FTO glass counter electrode [35] (copyright (2013) Elsevier).
