**2. Graphene**

Graphene is an emerging carbon nanomaterial with an ideal 2D structure and unique electronic properties. On the other hand, the word graphene wasn't coined until 1986. Graphene is a single layer (2D) honeycomb-arranged carbon atom connected with sp2 bonds. Graphene serves as the fundamental building block for the structure of all other carbon allotropes. Geim and Novoselov made the groundbreaking discovery in 2004 that single-layer and two to three-layer graphene nanosheets can stably survive in the environment [8, 9]. The exceptional qualities of graphene include its high electrical conductivity, high thermal conductivity (5000 W m<sup>−</sup><sup>1</sup> K<sup>−</sup><sup>1</sup> ), high intrinsic charge mobility (250,000 cm2 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> ), and very high surface area (2630 m<sup>2</sup> g<sup>−</sup><sup>1</sup> ), and high Young's modulus. Graphene also has a very high surface area (1.0 TPa).

Due to its low mass density, extremely large surface area, great conductivity, and high flexibility, graphene has widespread use in various sectors, including energy storage and conversion, electronic devices, sensors, polymer additives, and biological applications [10–15]. Different graphene synthesis approaches are reported as mechanical filiation, epitaxial growth, chemical vapor deposition, and reduction of graphene oxide [9]. The use of graphene and graphene composites as supercapacitor materials was the subject of numerous publications. The following are examples of recent research on graphene-based supercapacitors.

A high-performance supercapacitor was prepared based on a composite of carbonized wood cell chamber-reduced graphene oxide@PVA (CWCC-rGO@PVA) [16]. CWCC-rGO@PVA revealed a high specific capacitance of 288F g<sup>−</sup><sup>1</sup> , capacitance retention of 91%, energy density of 36 Wh kg<sup>−</sup><sup>1</sup> , and power density of 3600 W kg<sup>−</sup><sup>1</sup> .

The Co3O4/CoO nanoparticles were attached to reduced graphene oxide (rGO) nanosheets by microwave irradiation. The rGO@Co3O4/CoO electrode showed excellent electrochemical performance of specific capacitance of 276.1 F *g* −1 and 82.37% capacitance retention after 10,000 cycles [17].

As supercapacitor electrodes, 3D flower-like spheres of NiCo2S4@Ni-Mo layered double hydroxide (LDH) nanocomposites grown in situ on reduced graphene oxide (RGO) were developed using a simple hydrothermal method [18]. For comparison, RGO@NiCo2S4 and RGO@NiMo-LDH electrodes were also prepared. The redox peaks in CV curves for the RGO@NiCo2S4@NiMo-LDH electrode were symmetric and had identical profiles as the scanning rate increased, demonstrating excellent pseudocapacitance behavior and rate capacity of the electrode. Charge–discharge curve platforms were more pronounced at varying current densities, suggesting the presence of a Faraday redox reaction. Capacity retention was very good for the RGO@NiCo2S4@NiMo-LDH electrode, with specific capacitances of 1346, 1336, 1305, 1294, 1283, and 1272 F g<sup>−</sup><sup>1</sup> at 1, 2, 4, 6, 8, and 10 A g<sup>−</sup><sup>1</sup> , respectively. The rated capacity of RGO@NiCo2S4@NiMo-LDH was higher than that of RGO@NiCo2S4.

*Carbon Nanomaterials Based Supercapacitors: Recent Trends DOI: http://dx.doi.org/10.5772/intechopen.106730*

RGO@NiCo2S4@NiMo-specific LDH's capacitance was greater than that of RGO@ NiCo2S4 and RGO@NiMo-LDH taken separately, suggesting that the presence of several NiCo2S4 nanosheets on graphene sheets may give more growth spots for NiMo-LDH nanosheets than a smooth graphene skeleton. It can be seen from the symmetrical charge–discharge curve that it has good electrochemical reversibility. The device can obtain a maximum energy density of 59.38 Wh kg<sup>−</sup><sup>1</sup> at a power density of 808.19 W kg<sup>−</sup><sup>1</sup> and maintain an energy density of 25.24 Wh kg<sup>−</sup><sup>1</sup> at a high power density of 8055.32 W kg<sup>−</sup><sup>1</sup> . The capacitance of the RGO@NiCo2S4@NiMo-LDH electrode retained 80% of its initial capacitance after 10,000 cycles.

In another paper, graphene/MnV2O6 nanocomposite was prepared using solvothermal and liquid phase exfoliation processes. A maximum specific capacitance of 348 Fg<sup>−</sup><sup>1</sup> and capacitance retention of 88% was achieved after 3000 cycles for an optimal graphene/manganese vanadate ratio (1:8) sample [19].

A hybrid 2D platform was constructed from polypyrrole (PPy) /rGO and nickeltungsten metal oxides. The prepared electrode showed excellent specific capacitance of 597 F.g<sup>−</sup><sup>1</sup> with capacitance retention of 98.2% after 5000 cycles. The twoelectrode device using the same electrode platform showed a specific capacitance of 361 F.g<sup>−</sup><sup>1</sup> [20].

New hierarchical porous hybrid architecture consists of biomass-based porous carbon derived from Ganoderma lucidum residues (DDLG)/graphene composite aerogel were synthesized by chemical self-assembly and Vitamin C as a reducing agent [21]. Composites with 2.1, 3:1, 4:1, and 8:1 porous carbon ratios to GO were prepared. The large interconnected pores of DDLG were confirmed from SEM images. In addition, graphene aerogel retains the conventional three-dimensional network structure, and the sheet-like form of graphene is orientated unpredictably. Porous carbon/graphene composites feature a new three-dimensional hierarchical porous structure when the ratio of porous carbon to graphene is between 1:1 and 3:1. This ratio creates a densely packed structure. This is due to the graphene oxide sheet reduction process to conductive reduced graphene oxide resulting in forming a porous three-dimensional network structure around the BPC. When the ratio of porous carbon to graphene is exactly one to one, a system of porous carbon and graphene tightly packed together is produced.

Furthermore, the graphene self-assembled aerogel's structure dominates throughout the self-assembly process since just a few porous carbons are exposed owing to the high graphene concentration, and graphene nanoflakes cover the porous carbon. When the porous carbon to graphene ratio approaches 4:1, there are still two different types of porous structures, and the pore structure of porous carbon becomes more visible as the percentage increases. Furthermore, when the mass ratio of porous carbon increases to 8:1, graphene is shown to be distributed evenly throughout the porous carbon.

EIS measurements were used to analyze and compare the resistance characteristics of DDLGC and DDLGC/GO8. The DDLGC and DDLGC/GO8 ESRs were 0.53 and 0.46, demonstrating that the graphene-enhanced composite aerogel had significantly improved conductivity. The DDLGC/GO8-based electrode has a lower interfacial charge transfer resistance since the semicircle has a smaller diameter. A vertical line indicates capacitive behavior near to ideal [22]. A virtually vertical line was seen in the low-frequency region, suggesting high charge storage, rapid ion transport/diffusion, and excellent electrical double layer capacitor (EDLC) properties.

CV curves of DDLGC/GO8 at 5–100 mV s <sup>−</sup><sup>1</sup> show the creation of EDLC with rectangular curve shapes. The electrode of DDLGC/GO8 exhibited isosceles triangle shapes in the GCD plots at different current densities, demonstrating that the material's energy storage mechanism is a double-layer storage energy mechanism with good electrochemical reversibility. The specific capacitances of DDLGC and DDLGC/ GO8 at different current densities were calculated. At a current density of 1 A g <sup>−</sup><sup>1</sup> , the specific capacitances of DDLGC and DDLGC/GO8 were determined to be 365.6 F g<sup>−</sup><sup>1</sup> and 366 F g<sup>−</sup><sup>1</sup> , respectively. DDLGC/GO8 has a substantially greater rate capacity at high current density than DDLGC, which may be attributed to the material's increased electron transfer efficiency at high scan rates [23] and the bigger average pore size and higher effective surface area. These findings show that adding graphene, another carbon element, may greatly enhance the capacitance characteristics of biomass-based porous materials.
