2.4.1 Papermaking process
Thanks to the long history of paper making technology, the paper making technology in modern society is mature enough. At present, all kinds of special paper are developed by people through papermaking process. As shown in Figure 14A, Huang et al. [119] prepared a kind of cellulose based on the disposal arrangement of polyaniline, and then prepared a paper based composite electrode through the papermaking process. The preparation of the electrode is mainly divided into three steps: first, a large number of small branches are generated by high-speed mechanical stirring of cellulose pulp, so that more hydroxyl groups conducive to polyaniline polymerization are exposed on more cellulose surfaces. Secondly, polyaniline is polymerized on the cellulose surface to form PANi@cellulose. Finally, the PANi@paper composite electrode is obtained through an industrial paper forming machine. As shown in Figure 14B, the TEM image of PANi fiber shows that polyaniline grows vertically and tightly on the surface of cellulose. This provides a faster redox kinetics for electron/ion transfer and charge storage in the electrochemical process. And the vertically arranged PANi structure also gives the paper based electrode excellent capacitive characteristics. The PANi@paper electrode exhibits an excellent specific capacitance of 296F g-1 at 1A g-1. More importantly, the solid paper based energy storage device assembled by the PANi @ paper electrode also has a specific capacitance of 282F g-1, and still maintains excellent and stable electrochemical performance under different bending angles (Figure 14C). The large-scale preparation of paper-based electrode materials by papermaking technology is exciting. However, the prepared paper-based materials also require manual assembly of solid-state devices when applied, which undoubtedly increases production costs and time. In response to this problem, the team achieved large-scale production of integrated paper-based supercapacitors by improving the process technology [120]. As shown in Figure 14D, this technique uses cellulose from orderly stacking PPy as the raw material for paper-based electrodes. After continuous molding, pressing and drying, an integrated supercapacitor is obtained. Figure 14E shows a schematic diagram of the detailed preparation process. Firstly, the 1DPPy/cellulose pulp fiber is passed through the sheet molding machine to obtain the non-dry state PPy@paper, and then the two pieces of PPy@paper are assembled sequentially with the cellulose diaphragm obtained by pure cellulose pulp to form a sandwich structure. Finally, an all-paper-based supercapacitor device is obtained by hot pressing, drying, and impregnating the electrolyte (Figure 14F). The all-in-one paper-based device has a mass ratio capacitance of 360F g-1 at 0.1A g-1 and a capacitance retention rate of 81.7% after 1000 cycles. It is worth mentioning that by controlling the thickness of the pulp raw material in different electrodes, an integrated device with different areas of capacitance can be obtained. A series of paper-based devices in the 562-2507mF cm-2range of area ratio capacitance were designed. This provides a fast and efficient way to industrialize mass customization of paper-based devices.
Chen et al. [121] prepared a carbon fiber (CFs)-reinforced cellulose-based activated carbon paper-based electrode (cellulose-based ACFPs) by wet papermaking, carbonization and activation. Figure 14G shows the preparation process of this electrode. Firstly, CFs were mixed with fibrillated pulp fibers evenly, and carbonized precursor paper matrix composites were prepared by papermaking process. This composite is then soaked in H3PO4 solution. Finally, cellulose-based ACFPs paper-based electrodes were obtained by H3PO4 and CO2 double activation process. Among them, CFs in paper-based materials have a low coefficient of thermal expansion and high chemical stability, which provides excellent mechanical properties for paper-based electrodes (Figure 14H).