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

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 design the desired structure of CAs.

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 materials in electrochemical supercapacitors.

An aerogel derived from pyrolysis of polybenoxazine by Katanyoota and coworkers with surface area 368 m2 /g with 2–5 nm pore size exhibits 56 F/g of 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]. Interestingly it possesses 577 m<sup>2</sup> /g of surface area associated with 18 nm pore size. 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 was not so high, i.e., 206 m<sup>2</sup> /g, the doping of pseudocapacitive material NiCo2O4 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 m2 /g [27]. Conducting polymer, polyaniline being a doping material in a CA for

which RF were the precursors, shows of 710 F/g in acidic electrolyte 1 M H2SO4 [28]. There are some research efforts wherein secondary materials were used to modify the carbon aerogel derived from resorcinol-formaldehyde gel. Wang and co-workers doped nickel oxide particles to enhance the activity which resulted in exhibiting 356 F/g at 1 A/g in 6 M KOH medium [29]. In an alternate piece of work, carbon nanotubes were used as dopants to activate the CAs and delivered 141 F/g at 5 mV/s in 30% potassium hydroxide solution [30]. Also there are some reports wherein CAs are activated by CO2 and KOH to enhance the electrochemical behavior [31]. These activated CAs possess hierarchical porous network structures with microporous, mesoporous and large pores with <2 nm, 2–4 nm and >30 nm correspondingly. These CAs deliver 250 F/g after KOH activation and 8.4 Wh/kg at 0.5 A/g of current density in 6 M potassium hydroxide as electrolyte solution. Doping with metal also found to influence the performance of CAs. Lee et al., [22] doped a series of CAs with different metals. They found metal doped CAs with higher capacitance comparing to pristine ones. Mn doping showed higher capacitance compared to those of Cu, Fe. The metal compounds doped CAs are also studied including Mn3O4 [18], NiCo2O4 [26], ZnO [32], FeOx [33], MnO2 [27], SnO2 [34], NiO [29] and RuO2 [35].
