2.1.2 Cellulose/graphene paper based composite electrode
Graphene is a two-dimensional carbon material with hexagonal honeycomb lattice structure, which is connected by carbon atoms with sp2 hybridization[61]. Because of its excellent conductivity, ultra-high theoretical specific surface area, high mechanical strength and good thermal conductivity, it has attracted extensive attention from researchers around the world. At present, graphene has been widely used in many fields such as energy storage, sensing, electromagnetic shielding, wave absorption and environment. However, the van der Waals force and π-π bond force exist between 2D graphene sheets[62]. The electrode plates directly prepared from graphene into supercapacitors will have stacking effect, which will lead to the reduction of electrochemical reaction sites. And it is not conducive to the rapid movement of electrolyte ions, which seriously affects the performance of energy storage devices. Cellulose has a large number of hydrophilic groups, such as hydroxyl and carboxyl groups[63,64], which can interact with graphene to promote the dispersion of graphene in the solute and act as a perfect spacer between 2D graphene sheets. He et al.[65]prepared a self supporting flexible paper based electrode material with excellent mechanical properties and energy storage properties by vacuum filtration of CNF and reduced graphene oxide (RGO). Figure 3A is a schematic diagram of the preparation process of CNF/RGO paper-based electrical materials. The influence of different mass ratios of CNF to RGO on the energy storage performance of paper based electrode materials was discussed. As shown in Figure 3B, when the ratio between CNF and RGO is 1:2, the composite material shows an excellent laminated structure. This shows that at this ratio, the existence of CNF weakens the force between graphene layers to the greatest extent and limits the accumulation of RGO. And as expected, the composite also showed the best performance, with a specific capacitance of 146mF cm-2, tensile strength of 83Mpa and excellent conductivity (202.94S m-1) at a current density of 5mA cm-2. Since RGO can only provide certain energy storage characteristics of double electric layers, the paper based energy storage materials prepared in this study have room for improvement. For example, the introduction of the third component with excellent energy storage performance. On the basis of the former research, Qiang et al.[66]prepared CNF/RGO/polypyrrole (PPy) paper electrode materials with sandwich structure by vacuum filtration. Figure 3C shows the preparation process of the composite. First, CNF/RGO paper matrix composite was obtained by vacuum filtration. Then immerse it in Fe(ClO4) solution. Finally, the pyrrole monomer was dispersed on the CNF/RGO surface by spraying and polymerized for 30 min to obtain the CNF/RGO/PPy paper based electrode. Thanks to the porous structure of the CNF/RGO layer and the pseudocapacitance characteristics of PPy, the paper based electrode shows excellent specific capacitance (195.8F g-1, 915mF cm-1), excellent energy density (13.04 Wh kg-1) and power density (200.6 Wkg-1). Figure 3D shows the ion and electron transfer model inside CNF/RGO/PPy electrode. The author points out that thanks to the porous structure of CNF/RGO, the paper based electrode material shortens the diffusion channel of electrolyte ions in the electrochemical reaction. Moreover, the highly conductive polypyrrole layers assembled layer by layer provide sufficient pseudo capacitance. Using CNF/RGO paper-based materials obtained by vacuum filtration as the substrate, the process of introducing pseudocapacitors through in-situ polymerization similar to the above work is relatively cumbersome. The electropolymerization law is a relatively quick method. For example, Xiong et al.[67]used CNF/RGO prepared by vacuum filtration as the substrate to introduce polyaniline into composite paper based materials by means of electropolymerization (Figure 3E). Figure 3F shows the CV curve of the composite. It can be clearly seen that due to the introduction of high pseudo capacitance PANi, the paper based electrode shows greater energy storage characteristics. From the above two works, it can be seen that the cellulose/graphene paper based composite obtained by vacuum filtration can not only be directly used as the electrode material of supercapacitors, but also be used as an effective substrate material to introduce other components to prepare cellulose paper based electrode materials with better performance.
In addition to conducting polymers, metal oxides also exhibit excellent pseudo capacitive properties, which can show high energy density through reversible redox reactions. Therefore, introducing metal oxides into CNF/RGO paper based materials is also a way to improve their electrochemical performance. For example, Zou et al.[68]prepared cellulose/RGO/Ag paper matrix composites with high conductivity through vacuum filtration. Then, a paper based electrode material with high conductivity and double electroactive substances was obtained by depositing Fe2O3 on its surface (Figure 3G). The test results show that the symmetrical supercapacitor device assembled with this paper based electrode achieves a large area capacitance (1132mF cm-1) and energy density (226.4μWh cm-2).Moreover, the device does not need additional collectors, and it also shows excellent flexibility. Therefore, the material has broad application prospects in the field of flexible supercapacitor. In a word, the addition of metal oxides has an obvious effect on improving the electrochemical performance of paper based supercapacitors.