**2. Fundamentals of charge storage in electrochemical supercapacitors**

Three categories of supercapacitors are made with respect to the mechanism involved in energy storage in them. The very first category is electrical double-layer capacitor (EDLC), wherein electrostatic charge gathered at the electrode electrolyte interface results in capacitance. Here capacitance is directly proportional to surface area accessed by the ions from electrolyte. Pseudocapacitor is the second category, where in reversible redox reactions by electroactive compounds are considered. In third type, combination of both of these kinds are made use to extract electrical energy. The type of material chosen plays a crucial role in the energy delivery. The challenges posed by supercapacitors are low energy density, low workable potential window, economy and self-discharge. The strategy to overcome these limitations is to design newer energy materials. Popularly, designing hybrid of a carbon material, pseudocapacitive metal compounds and conducting polymers. Synergistic effects, high surface area from carbon materials, high specific capacitance, redox processes from pseudocapacitive materials contribute to achieve high capacitance with good rate capability [1–4].

The first category supercapacitors, which are dependent on the formation of electric double layer, can be fabricated with the help of two carbon related electrode materials, an electrolyte and a separator. There will be no electron exchanges between electrolyte and electrode being non-faradaic. There occurs population of charges on electrodes when voltage is applied which drives the ion diffusion in the electrolytic solution. An electric double layer with oppositely charged ions on the electrode surface is formed to skip ionic recombination. Because of this mechanism, charge take up will be very fast and energy delivery too. Also, electrodes are benefitted with no swelling during charge and discharge cycles similar to the batteries [1, 5–8].

Considering pseudocapacitive materials as electrodes in supercapacitors, redox reactions take place between electrode and electrolyte which stores the charge. Oxidation or reduction reactions occur when a required voltage is applied, on the electrodes involving charge passage across double layer generating faradaic current. Metallic compounds and conducting polymers are best examples for this class of electrodes which suffer with lack of stability during cycling leading to lowering in power density [1]. As these faradaic reactions involve redox reactions, are slower which makes them to exhibit lower power density and poor cycling stability compared to EDLC type [1, 6–9].

There are few criteria to design a high performance supercapacitor electrode materials. To list out, high specific capacitance stands first. To achieve this, electrode material should pose a very high specific surface area which will eventually store high energy per unit mass and volume. So nanomaterials and porous materials can be expected to satisfy this criterion. Large rate capability and high cycle stability are major characteristic to be possessed by an ideal electrode material, which signifies capacitance retention at high scan rate and/or current density. Additionally economically viable and non-toxic materials are preferred.

Majorly the factors that determine the characteristics of high specific capacitance, rate capability and cycle stability are Surface area of the electrode, electronic/ ionic conductivity and mechanical/chemical stability. As the charge storage mechanism involves the adsorption and desorption of the ions on the electrode surface, more the surface area higher will be energy storage. Specific capacitance and rate

**85**

m2

*Aerogels Utilization in Electrochemical Capacitors DOI: http://dx.doi.org/10.5772/intechopen.93421*

**3. Aerogels as supercapacitor electrode materials**

derived from graphene, and (4) aerogels sourced from biomass.

**4. Polymer-derived aerogels for supercapacitors**

design the desired structure of CAs.

coworkers with surface area 368 m2

Interestingly it possesses 577 m<sup>2</sup>

was not so high, i.e., 206 m<sup>2</sup>

materials in electrochemical supercapacitors.

capability are highly relative to electronic and ionic conductivity. Higher values of these can maintain a rectangular shape of cyclic voltammogram which is typical for an ideal capacitor and symmetric profiles in galvanostatic charge discharge cycles.

Carbonaceous aerogels extended to three-dimensional structures are very much potential for high performance electrode materials in supercapacitor because of superior characteristics such as vast surface area and porous nature, facilitating uninterrupted paths for ionic movement by shortening diffusion pathways.

Based on the source, the carbonaceous gels are typically classified as (1) Aerogels derived from polymers (2) aerogels derived from carbon nanotubes, (3) Aerogels

Carbon aerogels (CAs) derived from polymers belong to wide category of carbonaceous aerogels. Pekala and Kong in 1989 firstly recorded the synthesis of CAs by inert atmosphere pyrolysis of resorcinol–formaldehyde (RF) organic aerogel [10, 11]. The organic aerogel from RF can be synthesized by aqueous mediated sol-gel-based resorcinol and formaldehyde monomeric poly-condensation followed by supercritical drying [12]. Alternatively, polymers of sol–gel source too utilized as starting materials for synthesizing carbon aerogels, such as phenol– melamine–formaldehyde gel [13], poly-benzoxazine gel [14], cresol, resorcinol and formaldehyde gel [15], resorcinol and pyrocatechol gel [16], cresol and formaldehyde gel [17], gel of resorcinol–methanol [18] and poly(vinyl chloride) gel [19]. Additionally, microwave drying [20] air drying [21, 22] and freeze-drying [23] too are preferred as the synthetic procedures for dried organic gels. The process of pyrolysis can also be employed to convert dried organic gel into CAs. Precursor configuration and conditions of pyrolysis process are very crucial parameters to

CAs derived from polymers have been studied exclusively as supercapacitor electrode. First, Pekala in 1994 demonstrated capacity of CAs electrodes in supercapacitors [24]. The 3D CAs consist of nanoparticle assembly associated with variable sized pores. CAs possess the merits like of good electrical conductivity, high porosity, high surface area and tunable pore sizes [24, 25]. These unique structure and superior properties, make CAs potential candidates for the application as electrode

An aerogel derived from pyrolysis of polybenoxazine by Katanyoota and

specific capacitance in 3 M sulfuric acid [14]. CAs modified with pseudocapacitive materials such as metal oxides or conducting polymers can be expected to offer higher performances compared to their pristine counterparts. Resorcinol-methanolderived CA modified with Mn3O4 doping shows 503 F/g in 0.5 M Na2SO4 [18].

CA from RF precursor developed by Chien et al., [26] shows an excellent electrochemical behavior with 1700 F/g in 1 M potassium hydroxide. Though surface area

contributes towards high capacity. In an alternate report MnO2 doped RF-derived CA offers 515 F/g in neutral 1 M Na2SO4 and corresponding surface area was 120

/g [27]. Conducting polymer, polyaniline being a doping material in a CA for

/g with 2–5 nm pore size exhibits 56 F/g of

/g of surface area associated with 18 nm pore size.

/g, the doping of pseudocapacitive material NiCo2O4

*Colloids - Types, Preparation and Applications*

electrodes [2, 3].

rate capability [1–4].

batteries [1, 5–8].

pared to EDLC type [1, 6–9].

metals, e.g., CoOx, MnOx, and NiOx are extensively being studied as supercapacitor

**2. Fundamentals of charge storage in electrochemical supercapacitors**

Three categories of supercapacitors are made with respect to the mechanism involved in energy storage in them. The very first category is electrical double-layer capacitor (EDLC), wherein electrostatic charge gathered at the electrode electrolyte interface results in capacitance. Here capacitance is directly proportional to surface area accessed by the ions from electrolyte. Pseudocapacitor is the second category, where in reversible redox reactions by electroactive compounds are considered. In third type, combination of both of these kinds are made use to extract electrical energy. The type of material chosen plays a crucial role in the energy delivery. The challenges posed by supercapacitors are low energy density, low workable potential window, economy and self-discharge. The strategy to overcome these limitations is to design newer energy materials. Popularly, designing hybrid of a carbon material, pseudocapacitive metal compounds and conducting polymers. Synergistic effects, high surface area from carbon materials, high specific capacitance, redox processes from pseudocapacitive materials contribute to achieve high capacitance with good

The first category supercapacitors, which are dependent on the formation of electric double layer, can be fabricated with the help of two carbon related electrode materials, an electrolyte and a separator. There will be no electron exchanges between electrolyte and electrode being non-faradaic. There occurs population of charges on electrodes when voltage is applied which drives the ion diffusion in the electrolytic solution. An electric double layer with oppositely charged ions on the electrode surface is formed to skip ionic recombination. Because of this mechanism, charge take up will be very fast and energy delivery too. Also, electrodes are benefitted with no swelling during charge and discharge cycles similar to the

Considering pseudocapacitive materials as electrodes in supercapacitors, redox reactions take place between electrode and electrolyte which stores the charge. Oxidation or reduction reactions occur when a required voltage is applied, on the electrodes involving charge passage across double layer generating faradaic current. Metallic compounds and conducting polymers are best examples for this class of electrodes which suffer with lack of stability during cycling leading to lowering in power density [1]. As these faradaic reactions involve redox reactions, are slower which makes them to exhibit lower power density and poor cycling stability com-

There are few criteria to design a high performance supercapacitor electrode materials. To list out, high specific capacitance stands first. To achieve this, electrode material should pose a very high specific surface area which will eventually store high energy per unit mass and volume. So nanomaterials and porous materials can be expected to satisfy this criterion. Large rate capability and high cycle stability are major characteristic to be possessed by an ideal electrode material, which signifies capacitance retention at high scan rate and/or current density. Additionally

Majorly the factors that determine the characteristics of high specific capacitance, rate capability and cycle stability are Surface area of the electrode, electronic/ ionic conductivity and mechanical/chemical stability. As the charge storage mechanism involves the adsorption and desorption of the ions on the electrode surface, more the surface area higher will be energy storage. Specific capacitance and rate

economically viable and non-toxic materials are preferred.

**84**

capability are highly relative to electronic and ionic conductivity. Higher values of these can maintain a rectangular shape of cyclic voltammogram which is typical for an ideal capacitor and symmetric profiles in galvanostatic charge discharge cycles.
