2.2.2.2 Cellulose/conductive polymer paper-based composite electrode
In view of the excellent flexibility and modifiable properties of 2D cellulose paper, two conductive polymers, PEDOT and PPy, have been widely used in 2D cellulose paper by in-situ polymerization.Li et al. [99] constructed a PEDOT/CP paper-based composite electrode by successfully loading PEDOT in cellulose paper (CP) by gas phase polymerization. Figure 9A shows a schematic diagram of the preparation of a paper-based electrode. The Fe3+ oxidant is first added dropwise to the CP, and then the CP adsorbed Fe3+ is placed in a vapor phase polymerization vessel. Finally, PEDOT/CP paper-based electrode was obtained by washing and drying. The authors prepared paper-based electrodes with different PEDOT loading amounts by controlling the number of gas-phase polymerization. The results show that the electrode (PEDOT/CP-10) after 10 polymerization shows excellent conductivity (14 Ω/square) and a volume ratio capacitance of 13.7F cm-3. Similarly, Heo et al. [100] obtained conductive KHO (C-KHP) by loading Korean conventional paper (KHP) with a loaded conductive ink. Then the PEODT is loaded on C-KHP by gas phase polymerization to obtain PC-KHP paper-based composite electrode material. Finally,1D paper-based yarn electrode was prepared by mechanical winding of PC-KHP (Figure 9B). A power supply unit formed by connecting six independent devices in series successfully charges a smart device operating at 3.6V (Figure 9C). This work provides a conceptual reference for the practical application and large-scale production of 2D paper-based electrodes, and promotes the development of a new generation of flexible paper-based electronic devices.
In recent years, simple paper-based flexible energy storage devices have been difficult to meet people’s living needs. As a device for collecting tiny energy, nanogenerators have received widespread attention. Shi et al. [101] prepared PPy/Cellulose Paper (PCC) by in-situ polymerization as the main component units of flexible supercapacitors and friction nanogenerators, respectively. As shown in Figure 9D, the TENG consists of a Cellulose Paper substrate, a negative triboelectric layer Nitrocellulose Membrane, and a PCC that acts as both a positive triboelectric layer and an electrode. Supercapacitors consist of two symmetrical PCCs, PVA/H2SO4 electrolytes, and a Cellulose Paper substrate. The TENG exhibits an output voltage of up to 60V and a power density of 0.83W m-2. SC exhibits a specific capacitance of 90.1mF cm-2. The two devices were successfully assembled into a miniaturized electronic energy storage device (Figure 9E). To obtain paper-based high-performance electrodes under high-quality loads. After the traditional in-situ polymerization process, the addition of electric double-layer graphene can improve energy storage performance (Figure 9F) [102]. The paper-based electrode exhibits a specific capacitance of 1685mF cm-2 and a cycle stability of 92.8% (5000 cycles). In addition, the assembled solid-state paper-based device exhibits an area capacitance of 1408mF cm-2 and a high area energy density of 147μWh cm-2.
Aramid fiber has attracted the attention of researchers due to its excellent mechanical properties and chemical stability. In recent years, multifunctional paper-based materials prepared from aramid fibers have been widely used in various fields of social life. Yu et al. [103] formed ACF/PPS/MWCNT composites by papermaking processes with adhesive polyphenylene sulfide (PPS), MWCNTs and aramid chopped fibers. The PPS is then used as a binder for ACF and MWCNT after heating. Finally, the ACF/PPS/MWCNT-PPy paper-based electrode was obtained by in situ polymerization of PPy (Figure 9G). The synergies between PPy thanks to flexible ACF fibers, highly conductive MWCNTs and excellent pseudocapacitance properties. The paper-based electrode exhibits high specific capacitance (3205mF cm-2 current density at 5mA cm-2) and excellent cycle stability (93% capacitance retention after 5000 cycles). In addition, ACF/PPS/MWCNT-PPy also exhibits excellent flame retardant properties (Figure 9H). In conclusion, bifunctional paper-based materials with excellent energy storage performance and good flame retardant performance were prepared.