2.2.2.1 Cellulose/metal paper-based composite electrodes
Due to its excellent porous structure, low cost, easy degradation and renewable, 2D cellulose paper is considered to be an ideal flexible substrate in the field of flexible supercapacitors. Zhang et al. [94] prepared cellulose paper (CP)/Ni/Au paper-based electrodes by loading nickel/gold bimetals on vitamin paper. Figure 8A shows the preparation process of this paper-based electrode. Thanks to the hydrophilic group and porous structure of CP, CP immersion in nickel salt solution can adsorb a large number of nickel ions. The metal Ni is then loaded in CP by NaBH4 reduction. Then, CP loaded with nickel metal is entered into the electroless plating solution of Au to obtain CP/Ni/Au paper-based current collector. Finally, the carbon-based active material is deposited on its surface by electrophoretic deposition to obtain a paper-based supercapacitor electrode. The results show that thanks to the flexibility of cellulose paper, the CP/Ni/Au paper-based electrode still maintains 92.1% of the initial capacitance after 2000 bending cycles, showing excellent mechanical properties. Li et al. [95] also prepared Ni-paper by loading the filter paper with Ni metal, and then soaked it in KMnO4/HCl solution to obtain a Ni-paper-MnO2 paper-based electrode (Figure 8B). In order to show the excellent mechanical properties of Ni-paper as an electrode material. After 5000 consecutive bending of the Ni-paper, the resistance of the film only increased from the initial 0.8 Ω cm-2 to 2.7 Ω cm-2. When MnO2 is used as the active material, this paper-based electrode exhibits an area specific capacitance of 1095mF cm-2 at a current density of 1mA cm-2. This is mainly due to the hierarchical porous fiber structure in the Ni-paper-MnO2 electrode and the high conductivity of Ni, which greatly contribute to the electrochemical performance of the electrode.
General flexible energy storage devices must assemble independent electrodes, current collectors, and diaphragms. This is not only complex but also increases the size of the device. Combining all units of the device in a single substrate to build an all-in-one flexible energy storage device is a promising approach. Recently, Chang et al. [96-98] from Chung-Ang University in South Korea cleverly designed a series of integrated paper-based supercapacitors with excellent performance using paraffin heating-assisted in situ polymerization of Au. As shown in Figure 8C, the team first designed a single-layer integrated porous paper-based supercapacitor. The preparation of the device is divided into three steps, the first step is to fill the inside of cellulose paper by printing solid paraffin wax on the surface of the paper (Figure 8D(i)), then heating it to a molten state and filling it inside the cellulose paper (Figure 8D(ii)). In the second step, AgNPs are deposited on the upper and lower surfaces of the cellulose paper by drop-casting process (Figure 8D(iii)). The paraffin is then removed by methanol (Figure 8D(iv)). In the third step, the paper loaded with AgNPs gold source is soaked in Ag growth solution (HauCl4 and hydroxylamine hydrochloride mixed solution) to obtain the Au-Paper electrode (Figure 8D(vi)). Finally, MnO2-Au-Paper paper-based integrated electrode is obtained by electrodeposition by loading MnO2 on the surface of Au-Paper (Figure 8E). Devices assembled by combining a paper-based electric base with a gel electrolyte exhibit a specific capacitance of 252.2F g-1. To achieve a high potential window of the device, the team used the technique to fabricate five supercapacitors connected in series on a sheet of paper. The devices connected in series can exhibit a high voltage window of 4.0V, easily lighting the blue LED operating at 2.65V. As shown in Figure 8F, this technique is used to print the pattern by changing the paraffin wax as well as the heating temperature. The team prepared three vertically arranged parallel interdigital electrodes on a sheet of cellulose paper. Compared to devices assembled with single-layer interdigital electrodes. The three devices connected in parallel exhibit a larger CV curve area and a specific capacitance for discharge time. The detailed preparation process invites the reader to read the original article, which makes it easier to understand the working principle of the device. Building on the former’s work, the team fabricated vertically integrated planar multielectrode devices on a single sheet of paper. The highly integrated multi-electrode, diaphragm, and current collector exist only on one sheet of paper, and once again a device that expands the electrochemical window is cleverly designed on a single sheet. Figure 8G shows a schematic diagram of multilayer device integration and its corresponding circuit diagram. Multilayer paper-based devices can exhibit greater discharge times and CV curve areas in the same voltage window than during a single layer. And it can still maintain excellent energy storage performance under a higher electrochemical window. This series of work provides an innovative idea for the ingenious design of paper-based electrodes, which is of great significance to the development of paper-based supercapacitors.