**6. Graphene-based hydrogels and aerogels for supercapacitors**

2D carbon material with one atom thick graphene has been very popular among scientific community because of its unique properties like superior thermal and electrical conductivity, appreciable flexibility and high mobility of charge carriers, very high specific surface area, mechanical and chemical stability make it potential for charge storage applications.

Graphene layers are basic skeleton for aerogels of graphene. The aerogels of graphene not only possess the merits of graphene but carry the inherited characters of aerogels. If water is replaced by air from graphene hydrogels, can result in threedimensional cross-link, graphene aerogels. Mostly used method to prepare graphene aerogel is supercritical freeze-drying of the wet gels of graphene. There are few recent reports on the fabrication of graphene aerogels and their proven ability to behave as superior supercapacitor electrode materials. Liu and coworkers utilized freeze-drying method to prepare graphene aerogel which studied for electrochemical properties. It exhibited 172 F/g of capacitance when utilized as supercapacitor electrode in 1 M H2SO4 [51]. Supercritical-drying also adapted as synthetic route for graphene aerogels by Wu and Si, in a two different studies. The performance of these electrodes were fair enough which exhibited 153 F/g and 279 F/g respectively, where in corresponding electrolytes were ionic liquid and 1 M H2SO4 [52, 53]. Wu et al., [54] fabricated metal oxide composite with graphene aerogel which delivers a specific capacitance of 226 F/g by synergistic contribution from pseudocapacitive material, in 1 M H2SO4. Graphene synthesized via freeze-drying modified with L-ascorbic acid by Zhang et al., measure to be 512 m<sup>2</sup> /g of surface area. Up on using it as supercapacitor electrodes, it exhibits 128 F/g as a full cell in 6 M KOH electrolyte [55]. Aerogels with modification of nitrogen and some atoms also attempted by scientific community with a hope that to have improved capacitance. Wu et al., were successful in doping nitrogen and Boron which eventually delivers 62 F/g at 5 mV/s in sulfuric acid-PVA medium. The material had a surface area of 249 m<sup>2</sup> /g [56]. Carbohydrate modified graphene aerogel in neutral medium, i.e., Na2SO4 shows 162 F/g at 0.5 A/g [57]. However, this doped graphene had a surface area of about 365 m2 /g. Pyrolization was employed to fabricate graphene aerogel aiming to be used as electrode material for supercapacitor. Carbon modified such a graphene aerogel exhibited 122 F/g at 0.05 A/g in 6 M potassium hydroxide which was having a surface area in the range of 361–763 m<sup>2</sup> /g [58]. He et al., designed aerogel of polypyrrole graphene with 3D hierarchical applied as supercapacitor

electrodes [59]. It showed a considerably high 418 F/g at 0.5 A/g with an appreciable cyclability with 74% capacity retention in 1 M KOH. It can be noticed some literatures on rGO-based aerogel and their hybrids. In an attempt related to this, Boota's research group, utilized 2,5-dimethoxy-1,4-benoquinone and rGO to synthesize an electrode material which showed up to 650 F/g of specific capacitance at 5 mV/s in an acidic environment and interestingly 99% of initial capacity retained even after 25,000 cycles [60].
