2.2.1.2 Cellulose/conductive polymer paper based composite electrode
1D composite fiber prepared by composite of conductive polymer and 1D cellulose has been reported so far. Electrodes depositing polyaniline (PANi), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) on the surface of 1D cellulose by in-situ polymerization have been prepared. However, due to the low solubility and unstable structure of conductive polymers in solvents, the energy storage performance of electrodes directly compounded with cellulose is not excellent. At present, scientists prepare paper based electrodes with excellent electrochemical performance by adding surfactants or blending 1D cellulose composite fibers based on conductive polymers with other pseudo capacitive materials. As shown in Figure 7A, Song et al. [88] used the in-situ polymerization method to uniformly deposit PPy on the surface of BC fiber to obtain 1DPPy@BC composite fiber, and then prepared PPy@BC/Mxene composite paper-based electrode by vacuum filtration of the dispersion of PPy@BC and Mxene. Firstly, 1DBC fibers as effective templates provide abundant space for the deposition of Ppy. Secondly, the hierarchical porous structure formed by the assembly of PPy@BC fibers and Mxene provides sufficient sites for electrochemical reactions. This is very important for the electrochemical performance of this paper-based electrode. Thanks to the above two points, the PPy@BC/Mxene composite paper-based electrode exhibits a mass ratio capacitance of 550F g-1 and an area ratio capacitance of 879mF cm-2. The work is to prepare high-performance paper-based electrodes by blending 1D composite fibers with Mxene. Although satisfactory electrochemical performance can be obtained by this method, the thickness of the electrode will always be increased by blending the two components, which is detrimental to flexible microelectronic devices. It is a feasible method to introduce the second component directly into the 1D composite fiber by in-situ polymerization. Yang et al. [89] prepared 1DPPy@ cobalt hydroxyoxide/cellulose composite fiber (1DPCC fiber) by liquid phase reduction. Figure 7B shows the preparation process of this paper-based electrode. First, cobalt hydroxyoxide is reduced to CoCl2ยท6H2O by NaBH4 and cobalt hydroxyoxide is loaded on the surface of 1D cellulose in situ. 1DPCC fibers are then obtained by in situ oxidative polymerization of PPy. Finally, PCC paper-based composite electrode was obtained by vacuum filtration. Electrochemical test results show that PCC paper-based electrodes have a specific capacitance of up to 571.3F g-1 and a capacitance retention rate of more than 93% (1000 cycles). In this work, 1D cellulose composite fibers with excellent performance were prepared by two-step in-situ polymerization. The paper-based electrode also exhibited excellent energy storage characteristics, which provided a simple and fast method for preparing paper-based composite flexible energy storage electrode.
Recent studies have found that the use of redox small molecules to treat paper-based composite electrodes can also improve electrochemical energy storage performance. Small organic molecules with specific intramolecular conjugated structures, such as quinones, can be a substance that enhances the electrochemical performance of composite materials with an ideal valence. As shown in Figure 7C, Chang et al. [90] effectively improved the energy storage performance of PEDOT paper-based electrode by sodium alizarin sulfonate (sodium 1,2-dihydroxyanthraquinone-3-sulfonate) (ARS). ARS/H2SO4 composite electrolyte was prepared. The results show that the supercapacitor device assembled by ARS-treated electrode and electrolyte shows excellent electrochemical performance (area ratio capacitance 2191.3mF cm-2, energy density 4.87mWh cm-3). This is a significant improvement over the 348 mF cm-2 area ratio capacitance of a paper-based electrode without ARS treatment. Figure 7D shows the energy storage mechanism in the charging and discharging process of the device. Among them, the addition of ARS promotes the regeneration of PEDOT and reuses in redox reaction, which plays an important role in improving the electrochemical performance of the device. The water-soluble sulfonated polymer surfactant polystyrene sulfonate (PSS) has attracted extensive attention from researchers due to its excellent mechanical flexibility and adjustable conductivity. In this paper, three recent works on the improvement of conductive polymer 1D cellulose composite fibers using PSS are summarized. As shown in Figure 7E, Zhang et al. [91] added PSS to a mixed solution of pyrrole monomer and CNF, and improved the loading of polypyrrole on the surface of CNF through PSS. The results show that the paper-based electrode PPy:PSS/CNP with PSS exhibits a specific capacitance of 240F g-1. Similarly, Zhang et al. [92,93] also used PSS to improve the loading of PANi as well as PEDOT on the CNF surface. Figure 7F and 7G are schematic diagrams of the preparation process of PANi: PSS/CNP and PEDOT:PSS/CNP paper-based electrodes. It is worth mentioning that the addition of PSS in these three works effectively improves the cellulose loading of conductive polymers and promotes the construction of high-performance paper-based electrodes. It provides a good idea for the preparation of conductive polymer-based paper-based composite electrodes.