2.1.1 Cellulose/activated carbon paper based composite electrode
In the field of supercapacitors, typical activated carbon materials with double electric layers have been widely used due to their low cost, extensive sources and mature preparation process. However, when traditional activated carbon based electrode materials are assembled into devices, it is inevitable to combine them with inactive binders (such as polytetrafluoroethylene), conductive substances (such as carbon black) and collectors (such as foam nickel). The preparation process is not only complicated, but also the prepared electrode materials lack a certain flexibility, which limits its application in flexible electronic devices. To solve this problem, Chen et al. [59] prepared porous carbon (PC)/MXene/nano cellulose (CNF) paper based composite electrode materials through simple vacuum filtration. As shown in Figure 2A, the preparation process of CNF/PC/Mxene paper-based material is shown. The paper-based electrode is first prepared by simple mixing and dispersion, and then by vacuum assisted filtration process. As shown in Fig. 2B, the author points out that the densified structure formed by vacuum filtration is the key to the excellent performance of the material. This provides a strong interface interaction between the oxygen-containing groups on the surfaces of CNF, PC and MXene, which makes paper based materials have obvious mechanical advantages. It can be seen from the microscopic image of the composite (Figure2C) that it is inside the material. Mxene provides excellent conductivity (83.1S cm-1), ensuring fast electron transfer. On the other hand, PC provides excellent double layer capacitors for the composite. And the PCs are evenly distributed between the network structures built by MXene and CNF. This also further avoids the stacking effect between two-dimensional MXene and CNF, which makes the paper based material form an intercalation structure extremely conducive to the rapid movement of electrolyte ions, and shortens the diffusion distance of ions. The test results show that the excellent structure of the paper-based material gives it excellent electrochemical performance, such as the area specific capacitance of 143mF cm-2 (current density 0.1mA cm-2, thickness 0.2mm) and 2.4μWh cm-2 area energy density.
As mentioned above, the adhesive similar to PTFE will coat the surface of some activated carbon, resulting in insufficient activation of electrochemical performance. In the cellulose/activated carbon paper based composite electrode materials prepared by vacuum filtration similar to the above research work, cellulose molecules are only used as the adhesive and structural support of an active substance in the device. In the electrochemical reaction process, it can not provide a transfer path for electron transfer. Therefore, in paper based electrode materials, it is particularly important to build cellulose with excellent electrical conductivity and mechanical properties. Luo et al. [60] constructed BC/AB/AC paper based composite electrode materials by vacuum filtration of bacterial cellulose (BC), acetylene black (AB) and activated carbon (AC). Figure 2D shows the TEM image of the paper-based material. It can be seen that due to the existence of a large number of hydroxyl and carboxyl groups with electronegativity on the molecular face of BC, the electrically neutral AB particles can be adsorbed on its surface during the ultrasonic dispersion process, thus building conductive cellulose on the microstructure. This provides an excellent path for electron transfer in the electrochemical reaction process (Figure 2E), which greatly enhances the energy storage performance of paper based materials. As shown in Figure 2F, the author also discussed the performance difference between PVDF as the binder and BC as the binder. It can be seen from the figure that compared with PVDF, the paper based electrode material constructed by BC has excellent porous structure and AC has more active sites for electrochemical reaction. The paper based electrode shows excellent energy storage performance (the specific capacitance is 275F g-1 at current density of 1A g-1). This work provides a simple and efficient method for the construction of paper based electrode materials with excellent mechanical properties by non carbonization method, and provides an idea for the wide application of paper based electrode materials.