**5. Carbon nanotube (CNT)-based aerogels for supercapacitors**

Porous interconnects in 3 dimensions with carbon nanotubes as skeleton constitute this category. Different processes are employed to synthesize carbon nanotubebased aerogels, such as chemical vapor deposition (CVD), [36] freeze-drying, [37, 38] and critical-point-drying [39–41].

The CNT-based aerogels possess benefits of the carbon nanotubes, like excellent electrical conductivities, good mechanical resilience and superior thermal conductivity, and show the special characteristics of aerogels, too; 3D network with pores, less density, porous nature and high specific surface areas. These attractive characters direct carbon nanotube aerogels for applications as supercapacitor electrodes. Also, as these 3D networks possess number of pores facilitate substrate for holding other active materials like metal oxides, carbon, and polymers with conducting nature, improving storage capacities.

Most extensively used method to fabricate CNT-based aerogels is CVD. Bordjiba and coworkers synthesized CNT aerogels by CVD method with surface area 1059 m2 /g and modified with microfibrous carbon offered 524 F/g in 5 M KOH [36]. Polyaniline were made use to modify CNT aerogel by Zhong et al., to improvise the specific capacitance with the contribution from pseudocapacitive conducting polymer. It offered 189 F/g in in 1 M H2SO4 medium [42]. In an attempt Fang et al., Ni microfiber supported CNT aerogels were designed which showed 348 F/g in 5 M KOH electrolyte [43]. Preparing carbon nanotube layers on other three-dimensional networks using CVD process also results in the synthesis of CNT-based composite aerogels which can be expected to perform better. Bordjiba et al., targeted this kind of composite of CNT aerogel with carbon aerogel by CVD method [44]. This material with around 700 m2 /g of surface area delivered 524 F/g of specific capacitance in 5 M KOH electrolyte. Freeze-drying method was employed to design CNT-based aerogel using wet-gel precursors [45]. The aerogels prepared so, exhibit superior properties which include mechanical, thermal, etc. Authors of these report the suitability of cellulose-CNT hybrid aerogels for sensor applications for gases and other volatile organic compounds. Sun et al., demonstrated [46] direct freeze-drying synthesis of CNT-graphene composite aerogels which exhibit superior thermal property, electrical conductivity, and good adsorption characteristic, etc., which advocate suitability for electrodes in supercapacitors. Li and co-workers prepared

**87**

249 m<sup>2</sup>

area of about 365 m2

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

volumetric energy 70 Wh/cm3

for charge storage applications.

CNT aerogel using CVD process which successfully demonstrated capacitive behavior [47]. Under 50% of compressive strain, capacity retention was about 90% and it was 70% under 80% of strain. This demonstrates the quality of electrode as compressible and deformation sustainable electrodes which is of unique quality. A concept of decorating CNT aerogel with a conducting polymer was successful for superior quality supercapacitor electrodes. Lee et al., [48] were successful in doing so as CNT aerogel coating by polyl(3, 4-ethylenedioxythiophene) which success-

mechanical flexibility and strength. It will be high interest if a carbon material is composed with CNT aerogels which can eventually improve the storage capacity. There are reports which advocate this fact by fabricating composite materials of CNT aerogels with cellulose nanofibers [49] and mesoporous carbon [50] which successfully showed enhanced properties. Though CNT-based aerogels pose superior electrochemical properties, the high production cost hinders their application.

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

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

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

at 100 V/s and high

/g of surface area. Up

/g [58]. He et al., designed

along with added superior properties including high

ful by delivering a volumetric capacitance of about 40 F/cm3

with L-ascorbic acid by Zhang et al., measure to be 512 m<sup>2</sup>

was having a surface area in the range of 361–763 m<sup>2</sup>

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

/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

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

aerogel of polypyrrole graphene with 3D hierarchical applied as supercapacitor

/g. Pyrolization was employed to fabricate graphene aerogel

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

*Colloids - Types, Preparation and Applications*

38] and critical-point-drying [39–41].

nature, improving storage capacities.

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].

**5. Carbon nanotube (CNT)-based aerogels for supercapacitors**

Porous interconnects in 3 dimensions with carbon nanotubes as skeleton constitute this category. Different processes are employed to synthesize carbon nanotubebased aerogels, such as chemical vapor deposition (CVD), [36] freeze-drying, [37,

The CNT-based aerogels possess benefits of the carbon nanotubes, like excellent electrical conductivities, good mechanical resilience and superior thermal conductivity, and show the special characteristics of aerogels, too; 3D network with pores, less density, porous nature and high specific surface areas. These attractive characters direct carbon nanotube aerogels for applications as supercapacitor electrodes. Also, as these 3D networks possess number of pores facilitate substrate for holding other active materials like metal oxides, carbon, and polymers with conducting

Most extensively used method to fabricate CNT-based aerogels is CVD. Bordjiba

/g and modified with microfibrous carbon offered 524 F/g in 5 M KOH [36].

/g of surface area delivered 524 F/g of specific capacitance

and coworkers synthesized CNT aerogels by CVD method with surface area

Polyaniline were made use to modify CNT aerogel by Zhong et al., to improvise the specific capacitance with the contribution from pseudocapacitive conducting polymer. It offered 189 F/g in in 1 M H2SO4 medium [42]. In an attempt Fang et al., Ni microfiber supported CNT aerogels were designed which showed 348 F/g in 5 M KOH electrolyte [43]. Preparing carbon nanotube layers on other three-dimensional networks using CVD process also results in the synthesis of CNT-based composite aerogels which can be expected to perform better. Bordjiba et al., targeted this kind of composite of CNT aerogel with carbon aerogel by CVD method [44]. This mate-

in 5 M KOH electrolyte. Freeze-drying method was employed to design CNT-based aerogel using wet-gel precursors [45]. The aerogels prepared so, exhibit superior properties which include mechanical, thermal, etc. Authors of these report the suitability of cellulose-CNT hybrid aerogels for sensor applications for gases and other volatile organic compounds. Sun et al., demonstrated [46] direct freeze-drying synthesis of CNT-graphene composite aerogels which exhibit superior thermal property, electrical conductivity, and good adsorption characteristic, etc., which advocate suitability for electrodes in supercapacitors. Li and co-workers prepared

**86**

1059 m2

rial with around 700 m2

CNT aerogel using CVD process which successfully demonstrated capacitive behavior [47]. Under 50% of compressive strain, capacity retention was about 90% and it was 70% under 80% of strain. This demonstrates the quality of electrode as compressible and deformation sustainable electrodes which is of unique quality. A concept of decorating CNT aerogel with a conducting polymer was successful for superior quality supercapacitor electrodes. Lee et al., [48] were successful in doing so as CNT aerogel coating by polyl(3, 4-ethylenedioxythiophene) which successful by delivering a volumetric capacitance of about 40 F/cm3 at 100 V/s and high volumetric energy 70 Wh/cm3 along with added superior properties including high mechanical flexibility and strength. It will be high interest if a carbon material is composed with CNT aerogels which can eventually improve the storage capacity. There are reports which advocate this fact by fabricating composite materials of CNT aerogels with cellulose nanofibers [49] and mesoporous carbon [50] which successfully showed enhanced properties. Though CNT-based aerogels pose superior electrochemical properties, the high production cost hinders their application.
