**2. Basics of symmetric supercapacitors (SSCs)**

have excellent recyclability, typically >5000 times [5]. The past decade has witnessed significant progresses in SCs researches, many types of SCs have come to existence, including but not limited to electrochemical double-layer capacitors (EDLCs) and asymmetric supercapacitors (ASSCs), and they can be further sorted by applicable electrolytes (mainly three types electrolytes: aqueous, organic, and ionic liquid electrolytes) [2]. Still, they are not satisfactory enough from the perspectives of energy stored, which is mainly due to low capacitances or narrow potential windows especially in aqueous electrolytes [6, 7]. Coupling SCs with pseudo-capacitive electrode materials such as transition metal-based materials or electronically conductive polymers is feasible to enlarge the specific energy; however, the lifetime of pseudo-capacitors normally goes down quickly in a few hundred cycles [8, 9]. Currently, it is still very hard to simultaneously obtain large capacitance, high-operating voltage, and high-cycling stability.

118 Supercapacitors - Theoretical and Practical Solutions

Symmetric supercapacitors (SSCs), mainly including carbon-based EDLCs and a few SSCs with identical metallic component- or conductive polymer-based electrodes, supply much higher specific power and cycling stability than pseudo-capacitors, due to the interfacial charging/ discharging mechanism [2, 10]. Their energy are given by the equation (1), in which E is the energy stored in capacitor cell, CT is the total capacitance, and V is the operating voltage. E is proportional to total capacitance and square voltage, which means that specific energy E can be improved via two ways: increasing specific capacitance and expanding operating voltage [11]. Both of these aforementioned two aspects are highly related to the interfacial chemistry and phenomenon [5]. According to the electrochemical double-layer phenomenon established by Helmholtz, the electrochemical interface consists of electrode surface and thin layered electrolyte (containing ions or cations) adjacent to electrode surface. In the first place, this thin layer of electrolyte plays the fundamental roles of conducting ions and facilitating charge compensation on electrode interface; additionally, they can be decomposed or transformed to supply non-Faradaic current once charge transfer occurs when the electrons arrive in or depart from the conduction band of electrode [6]. Before that, electrolyte ions and molecules are forced to strongly absorb onto electrode surface to form a tightly packed stern layer [6, 7]. According to previous literatures, the efficiency and strength of absorption are highly depend on the surface properties of electrode materials, doping, defect, and functionality and can significantly alter the interactions between electrolyte and electrode interface [11–13]. Therefore, in order to achieve high operating voltage as well as high energy, it is critical to address the interface issues regarding both the surface properties of electrode materials and the applicable electrolytes.

E = 1/2CTV2 (1)

In the past few decades, many review articles have discussed the investigations on materials selections and device fabrications for developing high-performance SCs, but few accounted for the interface designing and engineering. Also, research progresses from different angles (material synthesis, electrolyte selections, and device fabrications) have come to the point calling for a generic summary for improving the integrated performance of SCs on the clear understanding of electrode interfacial phenomenon. This chapter aims to present and discuss a number of relevant issues, including fundamentals of interfacial (mainly electric double layer (EDL)) capacitance, nanoscale charge transfer, discussions on a few benchmarked Symmetric supercapacitors (SSCs) are mainly built on electrochemical double-layer configured identical positive and negative electrodes; most applicable electrode materials are carbon-based due to their high chemical stability of carbon materials [1, 14]. The electrochemical doublelayer model, first established by Helmholtz, reveals that two oppositely charged ionic layers are formed at electrode-electrolyte interfaces under electrochemical forces driven. Afterward, Stern recognized that there are two regions of ion distribution at the electrode-electrolyte interfaces: one inner layer and one outer layer, as schematically depicted in **Scheme 1**. The inner region, where ions are strongly absorbed onto the electrode surface, is called the compact layer (or Stern layer); and the outer layer consists of a continuous distribution of ions in solution [10]. The capacitance at electrode-electrolyte interface (CEDL) with electrochemical double-layer configuration can be divided into capacitance from the inner compact layer (CH) and capacitance from the diffuse layer (CDiff), as described in equation (2)

$$\frac{1}{\overline{C}\_{\text{LTM}}} = \frac{1}{\overline{C}\_{\text{H}}} + \frac{1}{\overline{C}\_{\text{Dgf}}} \tag{2}$$

There are several critical factors that give significant impact on CEDL, mainly including the conductivity of electrode material, the surface area of electrode materials, the accessibility to the inner electrochemical surface, the electric field across the electrode, and the electrolyte/solvent properties [15]. For instance, SCs with high-surface area porous carbon electrode materials (such as activated carbon) can store much more capacitances by several orders of magnitude. There are also a few other ways that are feasible to enlarge the capacitances including doping heteroatom elements and compositing stable metal oxides or conductive polymers [11, 16]. Heteroatom doping is able to break down the high symmetry of graphitic carbon, creating a large amount of defects, leading to the easy formation of compact inner absorption layer [16]. The advantages of using stable metal oxides or conductive polymers as electrochemical interfaces instead of carbon materials are obvious; they are capable of supplying much more capacitance through pseudo-capacitive absorption besides the EDL capacitance [17]. However, the greatly enlarged capacitances are often obtained on the basis of compromising the efficiency and cycling stability, and only a minor few metal oxides and conductive polymers are qualified due to conductivity and cost issues, which we will summarize later in Section 3.

Besides the electrode materials selection, it is also important to choose a proper electrolyte and solvent to form a robust electrode-electrolyte interface since energy stored in a SSCs

used in SSCs due to their abundant sources, low costs, high porosity, excellent chemical stability, and conductivity [1, 13]. Considering the mechanism of energy stored in a SSC to be mainly dominated by electrical double-layer configurations, the overall performance of full SSCs is restricted below this value. Therefore, exploring high-performance electrode materials is urgent in order to achieve high-energy SSCs. There are mainly two types of commonly used materials: carbon-based materials and pseudo-capacitive materials, which we will sepa-

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As mentioned in Section 2, EDL-configured SCs are mainly based on various kinds of carbon materials such as activated carbon (AC). In this specific part, the general ways of enlarging capacitance of carbon-based materials are first summarized via giving representative example. Generally speaking, three ways can be utilized to achieve these goals: (1) increasing the electrochemically accessible surface area (EASA), (2) creating more anchoring sites for charge carriers, and (3) involving few amounts of pseudo-capacitance on the basis of EDL capacitance [4, 15, 24]. As followed, we will give explanations on the latter two strategies; the foremost strategy of enlarging EASA is only for listing because it involves a very few interfacial

The charming feature of carbon materials is mostly founded on the largely conjugated aromatic six-atom ringed carbon that create huge amount of delocalized electrons, for instance, graphene, one typical allotrope of carbon, have an ultrahigh charge carrier mobility of

sically generates highly symmetric molecular structure. The superlarge conjugation system, on one hand, enables fast transportation of charge carriers and, on the other hand, leaves very few sites for ions to absorb. From this specific aspect, doping heteroatom species and creating defects in graphitic carbon lattice can significantly boost the apparent capacitance by providing more sites for ions anchoring, as depicted in **Scheme 2** [26]. In some cases, very strong chemical binding on heavily doped carbon sites can even generate charge transfer to achieve much larger capacitance [16, 27]. Therefore, if let a small proportion of conductivity sacrificed, desirably larger capacitances can be obtained via simply involving more heteroatom sites

Activated carbons (AC) are the most commonly used electrode materials in SSCs because of their large surface areas and low costs. ACs are usually converted from carbon-rich chemicals and biomasses under the high-temperature activation of chemicals such as KOH, ZnCl<sup>2</sup>

 [28, 29]. Although, very high porosity is obtained, most of the heteroatom-containing functionalities and moieties are forced to be removed by the disturbance of high temperature [30]. The as-formed clean carbon surface may probably leave very limited sites that are readily to absorb ions, so the most obtainable capacitance using AC electrode materials are falling in 100–200 F g−1. Thanks to the abundant chemistry of carbon species, most of inorganic elements can connect with carbon to break the symmetry of π-π conjugation and also to include partial pseudo-capacitive sites for higher capacitance [11]. For instance, a nitrogen-doped porous carbon (N content ~ 12 at.%) reported by Huang et al. delivered an ultrahigh capacitance of 855 F g−1 and a high specific energy of 41 Wh kg−1 in aqueous electrolytes. The improvement

, and

v−1 s−1 [25]. Meanwhile, the large conjugation system of graphitized carbon intrin-

rately describe in the following paragraphs.

especially containing nitrogen species [11].

chemistries, which is not our main topic here in this chapter.

**3.1. Carbon-based materials**

200,000 cm<sup>2</sup>

H3 PO<sup>4</sup>

**Scheme 1.** Schematic representations of the cross-sectional EDL structure of electrode/electrolyte interface.

device is governed by the square of operating voltage multiplying capacitance, as described in equation (1). Relative to capacitance, operating voltage actually gives bigger impact on the specific energy of the whole SSCs device. Therefore, pushing the operating voltage to the limit by means of choosing proper electrolyte solution is of significant practical meanings, a robust electrode-electrolyte interface can efficiently prevent the nanoscale charge transfer at the interfaces, thus to largely expand the applicable operating voltage and to enhance the specific energy [6]. In general, electrode-electrolyte interfaces formed in ionic liquid are more stable than in organic solvents, and then in aqueous solution, which is governed by the chemical stable window of solvents [6]. Typically, the chemical stability of the abovementioned three types of electrolytes follows the sequence of ionic liquid > organic electrolyte > aqueous electrolyte. For instance, an ionic liquid electrolyte can deliver a potential window as wide as 4.0 V, while aqueous electrolyte can only supply a 1.23-V wide potential window due to water splitting at 1.23 V [18, 19]. However, in some aqueous cases, by suppressing the activity of water splitting such as in neutral electrolytes, the safe operating voltage can be largely expanded to 1.8 V [20, 21]. The operating voltage can be further enlarged by applying electrode material that has much less activity toward water splitting, leading to a 2.0-V safe operating voltage, which is clearly achieved by mediating the interface properties [22]. The discussion on interface engineering for safe voltage expansion is placed in Section 4.

#### **3. High-capacitance electrode materials for SSCs**

Symmetric supercapacitors (SSCs), as one typical sort of SCs, though are able to provide charging/discharging in several minutes, deliver incomparable capacitance as to secondary batteries (such as Li-ion battery) [23]. Therefore, it is crucial to enlarge the specific energy stored in each electrode by either optimizing existed electrode materials or developing new types of materials. For now, carbon-based materials contribute to most of electrode materials used in SSCs due to their abundant sources, low costs, high porosity, excellent chemical stability, and conductivity [1, 13]. Considering the mechanism of energy stored in a SSC to be mainly dominated by electrical double-layer configurations, the overall performance of full SSCs is restricted below this value. Therefore, exploring high-performance electrode materials is urgent in order to achieve high-energy SSCs. There are mainly two types of commonly used materials: carbon-based materials and pseudo-capacitive materials, which we will separately describe in the following paragraphs.

#### **3.1. Carbon-based materials**

device is governed by the square of operating voltage multiplying capacitance, as described in equation (1). Relative to capacitance, operating voltage actually gives bigger impact on the specific energy of the whole SSCs device. Therefore, pushing the operating voltage to the limit by means of choosing proper electrolyte solution is of significant practical meanings, a robust electrode-electrolyte interface can efficiently prevent the nanoscale charge transfer at the interfaces, thus to largely expand the applicable operating voltage and to enhance the specific energy [6]. In general, electrode-electrolyte interfaces formed in ionic liquid are more stable than in organic solvents, and then in aqueous solution, which is governed by the chemical stable window of solvents [6]. Typically, the chemical stability of the abovementioned three types of electrolytes follows the sequence of ionic liquid > organic electrolyte > aqueous electrolyte. For instance, an ionic liquid electrolyte can deliver a potential window as wide as 4.0 V, while aqueous electrolyte can only supply a 1.23-V wide potential window due to water splitting at 1.23 V [18, 19]. However, in some aqueous cases, by suppressing the activity of water splitting such as in neutral electrolytes, the safe operating voltage can be largely expanded to 1.8 V [20, 21]. The operating voltage can be further enlarged by applying electrode material that has much less activity toward water splitting, leading to a 2.0-V safe operating voltage, which is clearly achieved by mediating the interface properties [22]. The discussion on interface engineering for safe voltage expansion is

**Scheme 1.** Schematic representations of the cross-sectional EDL structure of electrode/electrolyte interface.

Symmetric supercapacitors (SSCs), as one typical sort of SCs, though are able to provide charging/discharging in several minutes, deliver incomparable capacitance as to secondary batteries (such as Li-ion battery) [23]. Therefore, it is crucial to enlarge the specific energy stored in each electrode by either optimizing existed electrode materials or developing new types of materials. For now, carbon-based materials contribute to most of electrode materials

placed in Section 4.

120 Supercapacitors - Theoretical and Practical Solutions

**3. High-capacitance electrode materials for SSCs**

As mentioned in Section 2, EDL-configured SCs are mainly based on various kinds of carbon materials such as activated carbon (AC). In this specific part, the general ways of enlarging capacitance of carbon-based materials are first summarized via giving representative example. Generally speaking, three ways can be utilized to achieve these goals: (1) increasing the electrochemically accessible surface area (EASA), (2) creating more anchoring sites for charge carriers, and (3) involving few amounts of pseudo-capacitance on the basis of EDL capacitance [4, 15, 24]. As followed, we will give explanations on the latter two strategies; the foremost strategy of enlarging EASA is only for listing because it involves a very few interfacial chemistries, which is not our main topic here in this chapter.

The charming feature of carbon materials is mostly founded on the largely conjugated aromatic six-atom ringed carbon that create huge amount of delocalized electrons, for instance, graphene, one typical allotrope of carbon, have an ultrahigh charge carrier mobility of 200,000 cm<sup>2</sup> v−1 s−1 [25]. Meanwhile, the large conjugation system of graphitized carbon intrinsically generates highly symmetric molecular structure. The superlarge conjugation system, on one hand, enables fast transportation of charge carriers and, on the other hand, leaves very few sites for ions to absorb. From this specific aspect, doping heteroatom species and creating defects in graphitic carbon lattice can significantly boost the apparent capacitance by providing more sites for ions anchoring, as depicted in **Scheme 2** [26]. In some cases, very strong chemical binding on heavily doped carbon sites can even generate charge transfer to achieve much larger capacitance [16, 27]. Therefore, if let a small proportion of conductivity sacrificed, desirably larger capacitances can be obtained via simply involving more heteroatom sites especially containing nitrogen species [11].

Activated carbons (AC) are the most commonly used electrode materials in SSCs because of their large surface areas and low costs. ACs are usually converted from carbon-rich chemicals and biomasses under the high-temperature activation of chemicals such as KOH, ZnCl<sup>2</sup> , and H3 PO<sup>4</sup> [28, 29]. Although, very high porosity is obtained, most of the heteroatom-containing functionalities and moieties are forced to be removed by the disturbance of high temperature [30]. The as-formed clean carbon surface may probably leave very limited sites that are readily to absorb ions, so the most obtainable capacitance using AC electrode materials are falling in 100–200 F g−1. Thanks to the abundant chemistry of carbon species, most of inorganic elements can connect with carbon to break the symmetry of π-π conjugation and also to include partial pseudo-capacitive sites for higher capacitance [11]. For instance, a nitrogen-doped porous carbon (N content ~ 12 at.%) reported by Huang et al. delivered an ultrahigh capacitance of 855 F g−1 and a high specific energy of 41 Wh kg−1 in aqueous electrolytes. The improvement

**3.2. Interfacial pseudo-capacitive materials**

*3.2.1. Electronically conducting polymers (ECPs)*

*CP* → *CPn*<sup>+</sup> (*A*<sup>−</sup>)

*CP* + *n e*<sup>−</sup> → (*C*+)

).

PEDOT is poly (3,4-ethylenedioxythiophene).

**Table 1.** Theoretical specific capacitance of conducting polymers.

of square voltage (V<sup>2</sup>

**Conducting polymer**

good rate/cycling performance.

Pseudo-capacitance is achieved by Faradaic electron charge-transfer with redox reactions, intercalation or electro-sorption. Unlike electrochemical double-layer supercapacitor, pseudosupercapacitor use electronically conducting polymers (ECPs) or stable metal oxides electrodes can achieve balanced capacitive performance of much enhanced capacitance and fairly

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123

ECPs are one typical sort of pseudo-capacitive materials that can engage electrochemical doping or redox reaction with anions and cations, many ECPs such as polyaniline (PANI), polypyrrole (PPy), and derivatives of polythiophene (PTh) have been widely applied in supercapacitors due to their large capacitances, flexibility, and low costs. The typical dopant level for these polymers as well as typical specific capacitances and applicable voltage ranges are given in **Table 1**. The mechanism of ECPs storing charges can be described by the follow-

ECPs-made SSCs can be sorted into two categories: Type I-using the same *p*-doped ECPs for both electrodes and Type II-using the same ECPs but using different forms of ECPs as electrodes (*p*-doped form as positive electrode and *n*-doped form as negative electrode). When the Type I SSCs is charged, the positive electrode is completely oxidized while the negative electrode remains neutral, which supplies 0.5–0.75 V potential difference. When it is discharged, both electrodes are semi-oxidized, which makes only 50% of the *p*-doped capacitance can be used. In addition, the supplied potential of Type II SSCs is higher compared with Type I SSCs. This improvement would result in high specific energy due to much larger difference

**Mw (g mol−1) Conductivity (S cm−1) Dopant level Potential range (V) Theoretical specific** 

Mw is molecular weight per unit monomer (g mol−1) PANI is polyaniline, PPy is polypyrrole, PTh is polythiophene and

*<sup>n</sup>* + *n e*<sup>−</sup> (*p* − *doping*) (3)

*<sup>n</sup> CPn*<sup>−</sup> (*n* − *doping*) (4)

**capacitance (F g−1)**

ing two formulas, *i.e*., *p*-doping upon oxidization and *n*-doping upon reduction.

**PANI** 93 0.01–5 0.5 0.7 750 **PPy** 67 0.3–100 0.33 0.8 620 **PTh** 84 2–150 0.33 0.8 485 **PEDOT** 142 300–500 0.33 1.2 210

**Scheme 2.** Aromatic carbon lattices can embrace many types of heteroatom doping such as nitrogen (blue), oxygen (green), sulfur (pink), and so on. Defective carbon sites (red) that are unsaturated can also be treated as anchoring sites for ions.

mostly originates from robust redox reactions at electrochemically active nitrogen-containing sites that transform inert graphene-like layered carbon [16]. A nitrogen, sulfur-co-doped carbon fabricated by Sun et al. exhibited a very high specific capacitance of 427 F g−1 at 1.0 A g−1 and still showed an excellent capacitance of 270 F g−1 at superhigh current density of 100 A g−1 (**Figure 1**) [31]. However, the dopant content shall be controlled under an optimistic level; otherwise, the capacitance will not be increased because of severely damaged conductivity. For instance, Cheng et al. fabricated a high level of N, S, or B doping graphene, also called superdoping, achieving 29.82, 17.55, and 10.79 at% for N-, S-, and B-doping, respectively [32]. However, the 29.82 at%-N-doped graphene achieved a medium specific capacitance of 354 F g−1 while the pristine graphene without any doping obtained a specific capacitance of 213 F g−1 (taking 60% of heavily N-doped graphene).

**Figure 1.** (a) SEM image and (b) XPS survey of NS-co-doped carbon materials. (c) Gravimetric capacitance of NS-co-doped carbon materials made SSCs in three aqueous electrolytes. Adapted with permission [31]. Copyright 2017, American Chemical Society.

#### **3.2. Interfacial pseudo-capacitive materials**

Pseudo-capacitance is achieved by Faradaic electron charge-transfer with redox reactions, intercalation or electro-sorption. Unlike electrochemical double-layer supercapacitor, pseudosupercapacitor use electronically conducting polymers (ECPs) or stable metal oxides electrodes can achieve balanced capacitive performance of much enhanced capacitance and fairly good rate/cycling performance.

#### *3.2.1. Electronically conducting polymers (ECPs)*

mostly originates from robust redox reactions at electrochemically active nitrogen-containing sites that transform inert graphene-like layered carbon [16]. A nitrogen, sulfur-co-doped carbon fabricated by Sun et al. exhibited a very high specific capacitance of 427 F g−1 at 1.0 A g−1 and still showed an excellent capacitance of 270 F g−1 at superhigh current density of 100 A g−1 (**Figure 1**) [31]. However, the dopant content shall be controlled under an optimistic level; otherwise, the capacitance will not be increased because of severely damaged conductivity. For instance, Cheng et al. fabricated a high level of N, S, or B doping graphene, also called superdoping, achieving 29.82, 17.55, and 10.79 at% for N-, S-, and B-doping, respectively [32]. However, the 29.82 at%-N-doped graphene achieved a medium specific capacitance of 354 F g−1 while the pristine graphene without any doping obtained a specific capacitance of 213 F g−1

**Scheme 2.** Aromatic carbon lattices can embrace many types of heteroatom doping such as nitrogen (blue), oxygen (green), sulfur (pink), and so on. Defective carbon sites (red) that are unsaturated can also be treated as anchoring sites for ions.

**Figure 1.** (a) SEM image and (b) XPS survey of NS-co-doped carbon materials. (c) Gravimetric capacitance of NS-co-doped carbon materials made SSCs in three aqueous electrolytes. Adapted with permission [31]. Copyright 2017, American

(taking 60% of heavily N-doped graphene).

122 Supercapacitors - Theoretical and Practical Solutions

Chemical Society.

ECPs are one typical sort of pseudo-capacitive materials that can engage electrochemical doping or redox reaction with anions and cations, many ECPs such as polyaniline (PANI), polypyrrole (PPy), and derivatives of polythiophene (PTh) have been widely applied in supercapacitors due to their large capacitances, flexibility, and low costs. The typical dopant level for these polymers as well as typical specific capacitances and applicable voltage ranges are given in **Table 1**. The mechanism of ECPs storing charges can be described by the following two formulas, *i.e*., *p*-doping upon oxidization and *n*-doping upon reduction.

$$\text{CP} \rightarrow \text{CP}^{\bullet+} \left( A^{-} \right)\_{\text{\tiny n}} \star n \, e^{-} \text{ (\$p-doping)}\tag{3}$$

$$\text{CP} + n \, e^- \to \left(\text{C}^\*\right)\_n \text{CP}^{n-} \text{ ( $n-d$ oping)}\tag{4}$$

ECPs-made SSCs can be sorted into two categories: Type I-using the same *p*-doped ECPs for both electrodes and Type II-using the same ECPs but using different forms of ECPs as electrodes (*p*-doped form as positive electrode and *n*-doped form as negative electrode). When the Type I SSCs is charged, the positive electrode is completely oxidized while the negative electrode remains neutral, which supplies 0.5–0.75 V potential difference. When it is discharged, both electrodes are semi-oxidized, which makes only 50% of the *p*-doped capacitance can be used. In addition, the supplied potential of Type II SSCs is higher compared with Type I SSCs. This improvement would result in high specific energy due to much larger difference of square voltage (V<sup>2</sup> ).


Mw is molecular weight per unit monomer (g mol−1) PANI is polyaniline, PPy is polypyrrole, PTh is polythiophene and PEDOT is poly (3,4-ethylenedioxythiophene).

**Table 1.** Theoretical specific capacitance of conducting polymers.

Many ECPs such as PANI and PPy can only be *p*-doped due to the very negative potentials required for *n*-doping, if compared with the reduction potential limit of molecular solvent-based electrolytes. For instance, Pan et al. synthesized PANI hydrogel with high surface area and threedimensional porous nanostructures and demonstrated that the as-obtained PANI-based supercapacitor could supply a very large specific capacitance of 480 F g−1, excellent rate capability, and very good cycling stability of 83% capacitance retention after 10,000 cycles but only provide a safe operating voltage of 0.8 V for SSCs. Unique three-dimensional (3D) microstructure by interconnected polymer (**Figure 2**) by Yu et al. exhibit good mechanical properties and high rate performance with specific capacitance of 400 F g−1, excellent rate capability [33]. On the contrary, PTh and its derivatives can be used as *n*-doped ECPs; however, the conductivity of these ECPs after *n*-doping is not very high in the reduced state and thus leads to a low capacitance in the negative potential region [34]. For example, Stenger-smith et al. developed poly (3,4-propylenedioxythiophene) and poly(3,4-ethylenedioxythiophene) as electrode couples show good cycle life [35].

reversible redox reaction, and long cycling life. For instance, RuO<sup>2</sup>

The main mechanism of the pseudo-capacitive energy storage in MnO<sup>2</sup>

electrolytes [39, 41]. These can be expressed by the following formula:

represents the protons and alkali metal cations (Li<sup>+</sup>

high flexibility, and high electrical conductivity [45].

**4. Electrolyte-mediated operating voltage**

ible intercalation/de-intercalation of protons or an adsorption of cations such as Li<sup>+</sup>

environmental friendliness, and high theoretical capacitances (up to 1100–1300 F g−1) [38–40].

*Mn O*<sup>2</sup> + *X*<sup>+</sup> + *e*<sup>−</sup> ↔ *MnOOX* (5)

and coworkers believed that the pseudo-capacitances are mainly occurring in the interfaces due to the difficulty of the protons or cations transportation into the bulk phase materials [38, 42–44].

Usually, the specific capacitance can be improved by tuning morphology, surface area, and porous structure of active material. The most common electrode materials are carbon materials, metal oxides, and ECPs. To maximize the advantages of these materials, composite materials are of great technological advantages due to the combination of the intrinsic properties of each component as well as the synergistic effect resulting from the hybrids. The composites of carbon materials with other materials such as ECPs and metal oxides, which often use carbon materials as substrate, for example, carbon nanotubes, carbon fibers, graphene materials, activated carbon, etc. As depicted in Section 3.2, ECPs and metal oxides are highly promising active electrode materials but these materials suffer from severe cycling stability problems because of the structure collapse caused by swelling and shrinking during charging/discharging. Hybridizing carbon materials with ECPs and/or metal oxides can synergistically boost nearly all the aspects of capacitive performance including conductivity, capacitance, and cycling stability. For instance, Peng et al. fabricated hollow fiber electrodes using reduced graphene oxide (RGO)/ECPs, simultaneously achieving large areal capacitance (304.5 mF cm−2),

Electrolyte is one of the key components of SCs, basically, conveying ionic current and leading the formation of electrical double layer, more importantly, under certain circumstances of matched electrode materials and electrolytes, and facilitating reversible redox processes for larger amount of charges stored in the interfaces. In general, electrolytes used in SSCs can be sorted into two main categories: liquid electrolytes and solid/quasi-solid state electrolytes. Liquid electrolytes can be further sorted into three groups: aqueous electrolytes, organic electrolytes, and ionic liquids (ILs), while solid/quasi-solid state electrolytes can be divided into organic electrolytes and inorganic electrolytes [7]. There is no once-for-all solution for electrolyte selection, each electrolyte has its own advantages and disadvantages. For

voltage of 0.7 V in SSCs [37]. Due to the very high costs, RuO<sup>2</sup>

military application. While MnO<sup>2</sup>

where X<sup>+</sup>

*3.2.3. Composites materials*

can supply a safe operating

125

is attributed to a revers-

) in the electrolyte. Toupin

, Na<sup>+</sup> , K<sup>+</sup> from

is often limited in aerospace and

http://dx.doi.org/10.5772/intechopen.73131

is generally studied because of their low cost, low toxicity,

Toward High-Voltage/Energy Symmetric Supercapacitors via Interface Engineering

, Na<sup>+</sup> , K<sup>+</sup>

#### *3.2.2. Transition metal oxides*

Transition metal oxides show high pseudo-capacitive behavior with redox chemistry both on and in the interfaces, which have been extensively studied due to high specific capacitance. There are several commonly studied stable metal oxide materials such as RuO<sup>2</sup> (theoretical specific capacitance ~ 1000 F g−1) and MnO<sup>2</sup> (theoretical specific capacitance: 1100–1300 F g−1) [36]. RuO<sup>2</sup> is one of the most explored electrode materials due to the high specific pseudo-capacitance,

**Figure 2.** (a) Illustration depicting controlled synthesis of the CuPcTs doped PPy hydrogel. (b) SEM images of nanostructured PPy hydrogel. (c) Specific capacitance as a function of current density for CuPcTs-PPy and pristine PPy. Adapted with permission [31]. Copyright 2017, American Chemical Society.

reversible redox reaction, and long cycling life. For instance, RuO<sup>2</sup> can supply a safe operating voltage of 0.7 V in SSCs [37]. Due to the very high costs, RuO<sup>2</sup> is often limited in aerospace and military application. While MnO<sup>2</sup> is generally studied because of their low cost, low toxicity, environmental friendliness, and high theoretical capacitances (up to 1100–1300 F g−1) [38–40]. The main mechanism of the pseudo-capacitive energy storage in MnO<sup>2</sup> is attributed to a reversible intercalation/de-intercalation of protons or an adsorption of cations such as Li<sup>+</sup> , Na<sup>+</sup> , K<sup>+</sup> from electrolytes [39, 41]. These can be expressed by the following formula:

$$Mn\text{ }O\_2 + X^\* + e^- \leftrightarrow Mn\text{ }OOX\tag{5}$$

where X<sup>+</sup> represents the protons and alkali metal cations (Li<sup>+</sup> , Na<sup>+</sup> , K<sup>+</sup> ) in the electrolyte. Toupin and coworkers believed that the pseudo-capacitances are mainly occurring in the interfaces due to the difficulty of the protons or cations transportation into the bulk phase materials [38, 42–44].

#### *3.2.3. Composites materials*

Many ECPs such as PANI and PPy can only be *p*-doped due to the very negative potentials required for *n*-doping, if compared with the reduction potential limit of molecular solvent-based electrolytes. For instance, Pan et al. synthesized PANI hydrogel with high surface area and threedimensional porous nanostructures and demonstrated that the as-obtained PANI-based supercapacitor could supply a very large specific capacitance of 480 F g−1, excellent rate capability, and very good cycling stability of 83% capacitance retention after 10,000 cycles but only provide a safe operating voltage of 0.8 V for SSCs. Unique three-dimensional (3D) microstructure by interconnected polymer (**Figure 2**) by Yu et al. exhibit good mechanical properties and high rate performance with specific capacitance of 400 F g−1, excellent rate capability [33]. On the contrary, PTh and its derivatives can be used as *n*-doped ECPs; however, the conductivity of these ECPs after *n*-doping is not very high in the reduced state and thus leads to a low capacitance in the negative potential region [34]. For example, Stenger-smith et al. developed poly (3,4-propylenedioxythiophene) and poly(3,4-ethylenedioxythiophene) as electrode couples show good cycle life [35].

Transition metal oxides show high pseudo-capacitive behavior with redox chemistry both on and in the interfaces, which have been extensively studied due to high specific capacitance.

**Figure 2.** (a) Illustration depicting controlled synthesis of the CuPcTs doped PPy hydrogel. (b) SEM images of nanostructured PPy hydrogel. (c) Specific capacitance as a function of current density for CuPcTs-PPy and pristine PPy.

Adapted with permission [31]. Copyright 2017, American Chemical Society.

is one of the most explored electrode materials due to the high specific pseudo-capacitance,

(theoretical spe-

(theoretical specific capacitance: 1100–1300 F g−1) [36].

There are several commonly studied stable metal oxide materials such as RuO<sup>2</sup>

*3.2.2. Transition metal oxides*

124 Supercapacitors - Theoretical and Practical Solutions

RuO<sup>2</sup>

cific capacitance ~ 1000 F g−1) and MnO<sup>2</sup>

Usually, the specific capacitance can be improved by tuning morphology, surface area, and porous structure of active material. The most common electrode materials are carbon materials, metal oxides, and ECPs. To maximize the advantages of these materials, composite materials are of great technological advantages due to the combination of the intrinsic properties of each component as well as the synergistic effect resulting from the hybrids. The composites of carbon materials with other materials such as ECPs and metal oxides, which often use carbon materials as substrate, for example, carbon nanotubes, carbon fibers, graphene materials, activated carbon, etc. As depicted in Section 3.2, ECPs and metal oxides are highly promising active electrode materials but these materials suffer from severe cycling stability problems because of the structure collapse caused by swelling and shrinking during charging/discharging. Hybridizing carbon materials with ECPs and/or metal oxides can synergistically boost nearly all the aspects of capacitive performance including conductivity, capacitance, and cycling stability. For instance, Peng et al. fabricated hollow fiber electrodes using reduced graphene oxide (RGO)/ECPs, simultaneously achieving large areal capacitance (304.5 mF cm−2), high flexibility, and high electrical conductivity [45].
