Development of Electrochemical Capacitors

Electrode active materials play an important role in determining the electrochemical performance of supercapacitors. Various active materials are found out to exhibit capacitive behaviors. In general, these materials can be divided into three main categories: (1) carbon-based materials; (2) metal oxides; and (3) conducting polymers.

Carbon-based materials

Carbon-based materials have attracted much attention on the applications of electric double-layer capacitors due to their high surface area (over 1000 m²/g), convenient synthesis method, non-toxicity, low cost, good electronic conductivity, high chemical stability, and wide operating temperature range [9]. The high specific capacitance of the carbon-based supercapacitors is typically arising from their high surface area and the large pore volume for ions accommodation during charging/discharging process.

Generally, activated carbon, glassy carbon and carbon fiber are major electrode materials. By adopting different precursors and activation processes, the surface properties and microtextures (more or less ordered) of derived carbon could be significantly changed [7]. Activated carbon is the mostly widely used electrode material due to its large surface area, relatively good electrical properties, and moderate cost. It has been reported to possess various physicochemical properties with well-developed surface areas as high as 3000 m²/g [10-13]. However, several researchers have pointed out the discrepancy between the capacitance of the activated carbon and their specific surface area. With a ultrahigh surface area, only a relatively small specific capacitance (< 10 μF/cm²) was obtained, indicating that not all pores are effective in charge accumulation [14]. As a result, Efforts have been made to search for the relationship

doi:10.6342/NTU201602172

15

between the nanoporous structure of activated carbon and its capacitance performance in different electrolytes. Recently, 3-D hierarchical porous carbon monoliths have been proposed as promising supercapacitor electrodes. A textured morphology composed of a bicontinuous macroporous carbon network and interconnected microporous carbon colloids is revealed. The macropores let the electrolyte ions transport easily to the microporous area. The specific capacitance is about 200 F/g in H2SO4 electrolyte [15].

In addition, carbon nanotubes present a unique structure and unusually high conductivity, stability, surface area, and large accessible mesopores which are suitable for supercapacitor applications [16, 17]. Different forms of carbon nanotubes have been investigated as the electrode materials for supercapacitors including porous tablets, aligned carbon nanotubes, and entangled carbon nanotubes. In 2004, graphene, which is basically composed of a 2-D hexagonal lattice of sp² covalently bonded carbon atoms, was discovered [18]. Owing to its outstanding conductivity and ultrahigh specific area, the electrochemical properties of graphene has attracted much interest especially for serving as additive or supporter for metal oxide to obtain enhanced cyclability [19].

Metal oxides

Metal oxides have drawn much attention for supercapacitor applications due to its high volumetric capacitance. Until now, several transition metal-based materials including RuO2 [20-26], V2O5 [27, 28], CoOx [29, 30], MnO2 [31-41], and Fe3O4 [42-44]

have been found out to exhibit pseudocapacitive behaviors. Moreover, the diversity in synthesis routes, which contain sol-gel method, precipitation, and electrochemical synthesis, makes it more attractive in preparing electrode materials with highly porous structure and large surface area. However, in view of the low conductivity of metal oxides, either additional carbon black or conducting polymer will be employed to

doi:10.6342/NTU201602172

16

increase the overall conductivity of the electrode. Amorphous ruthenium oxide is the prototype of pseudocapacitor, which provides an ultra-high specific capacitance up to 700 F/g in H2SO4 electrolyte within the voltage window of 1.4 V [45, 46]. However, its extraordinary cost limits the potential for commercial applications. As an alternative approach, MnO2 was found out to be a promising candidate for supercapacitors due to its low cost, environmentally safety, and high theoretical capacities ranging from 1100 to 1300 F/g [9, 47, 48]. Unfortunately, MnO2 suffers from challenges such as dissolution problem of manganese ions during cycling and poor electronic conductivity. Recently, some intensive studies on nanostructured MnO2 and its composite materials have been carried out [49, 50]. A superior electrochemical performance of MnO2 could be derived by shortening the diffusion distance of ions and electrons in the former study while the latter one demonstrated an increased total conductivity by combining MnO2 with graphite or CNTs.

Conducting polymers

Conducting polymers [51], one kind of polymers containing conjugated double bonds in the backbone of the structure, are promising electrode materials for supercapacitor applications. The conjugated double bond allows free movement of electrons within conjugation length. In recent years, various conducting polymers such as polyaniline (PAN) [52], polypyrrole (PPy) [53-55], and poly-methyl-thiophene (PMeT) [55, 56] have been reported to serve as active materials for pseudocapacitors. The pseudocapacitance is arising from the doping/de-doping process, where the anions and cations are introduced into the polymer chains during the electron-transfer process. Such conducting polymers demonstrate a high power delivery and a large capacitance over 400 F/g. However, the unstable redox sites in the polymer backbone limit their cycling

doi:10.6342/NTU201602172

17

performances.

2.2.2 Electrolyte

Compared to electrode materials, the innovation of electrolyte is also in progress.

Until now, liquid, gel-type [57-60], and ionic liquid [61, 62] electrolytes were employed for supercapacitor applications. Depending on the physical properties, they can be divided into aqueous and non-aqueous systems, each exhibits their own advantages and limitations in practical applications.

For aqueous system, inorganic acids, bases, and salts mainly act as electrolytes while water is employed as solvent. These electrolytes present superior conductivity due to their high solubilities and diffusivities [63]. For electric double layer capacitor, the derived capacitance is highly related to the ion adsorption/desorption processes which is sensitive to ionic sizes. Therefore, H2SO4 and KOH are commonly used as the electrolyte because of the small ionic size of H⁺ and OH^-. The fast diffusion rate of these ions makes it more capable of high-power delivery. However, the major limitation for aqueous electrolyte is the restricted potential window of 1.23V owing to the decomposition of water.

The solvent used for non-aqueous system, on the other hand, usually belongs to one of organic solutions, gel-solutions, melted-salts, and solid-state electrolytes. For organic solutions, propylene carbonate (PC), ethylene carbonate (EC) and acetonitrile are commonly employed as solvents while the corresponding salts are usually quaternary ammonium (R4N⁺), phosphonium (R4P⁺) or lithium salts [64, 65]. The R group is typically alkyl groups such as C2H5, C3H7, and C4H9 for cations, but are mostly

doi:10.6342/NTU201602172

18

BF4-, PF6-, and CF3SO3- for anions. Due to the higher decomposition potential of organic electrolytes, the energy density benefits greatly from the enlarged operation voltage window which can usually be operated over 3 V.

doi:10.6342/NTU201602172

19