2.2.1.1 Cellulose/metal paper based composite electrode
Metals and their compounds are considered as promising electrode materials for supercapacitors because of their excellent point and storage application prospects. Due to the large number of modifiable hydroxyl groups in cellulose molecules, it provides a good platform for the construction of polymerization and deposition of metals and their metal compounds. Recent studies have shown that metals and their compounds can be well combined with cellulose fibers or polymerized and deposited on their surfaces to form 1D cellulose composite fibers with excellent energy storage properties. Finally, paper based composite electrode materials can be obtained by vacuum filtration. Zhou et al. [85] used CNF as a substrate and deposited a conductive metal-organic framework(c-MOF) with excellent electrical conductivity through interfacial polymerization on its surface, and obtained CNF@c-MOF nanofibers. Figure 6A shows the preparation process of CNF@c-MOF, first introducing carboxyl groups to the surface of CNF through TEMPO oxidation, then exchanging carboxylated CNF with Ni2+ ions, and finally adding organic ligands to obtain CNF@c-MOF nanofibers. CNF@c-MOF paper-based electrodes are obtained by vacuum filtration. In addition, the authors discuss the differences in the electrochemical performance of CNF@c-MOF paper-based electrodes prepared by two different organic ligands, HITP (2,3,6,7,10,11-hexaiminotriphenylene) and HHTP (2,3,6,7,10,11-hexahydroxytriphenylene). Figure 6B shows a comparison of the weight ratio capacitance of two different CNF@c-MOF paper-based electrodes. It is evident that the CNF@Ni-HITP electrode prepared by HITP has a weight capacitance of 125F g-1. This is mainly due to the continuous Ni-HITP nanolayers tightly wrapped around the CNFs (Figure 6C). The authors note that the continuous nucleation of c-MOF nanometers on CNF and the hierarchical porous high conductivity structure provide a fast pathway for electrolyte transport and charge transfer for CNF@c-MOF paper-based electrodes (Figure 6D). It is worth mentioning that the authors also discuss the performance of CNF@c-MOF paper-based electrodes of different thicknesses, and the results show that the increase in thickness has almost no effect on the electrochemical performance of the paper-based electrode. This opens up the possibility of customizing paper-based electrodes of different thicknesses.
Huang et al.[86]used quaternized chitosan (QCS) as the connection stabilizer between CNF and copper sulfate nanocrystals (CuS-NCs). Figure 6E shows the contribution of QCS to CuS NCs deposition on CNF fiber surface. In the absence of QCS, the deposition of CuS is mainly through the electrostatic adsorption between the electronegative CNF and [Cu(NH3)4]2+in the precursor solution of CuS-NCs. At the same time, S2-will lead to rapid prototyping of CuS NCs, which will lead to uneven distribution of CuS NCs on the CNF surface (Fig. 6F). When S2- and QCS are added to the CNF/[Cu (NH3)4]2+precursor solution, CuS NCs are well anchored on the CNF surface (Fig. 6G) due to the electrostatic attraction formed between the positively charged QCS and CNF. In order to more intuitively check the contribution of QCS to the stable deposition of CuS NCs on the CNF surface, as shown in Figure 6H, CuS-NCs/CNF and CuS-NCs/QCD/CNF were respectively immersed in KOH electrolyte for 48h. It can be clearly observed from the figure that the paper-based electronic base shows excellent stability due to the addition of QCD. However, CuS NCs in the paper-based electrode without QCD fell off obviously. The test results showed that CuS-NCs/QCD/CNF paper based electrode exhibited 314.3F g-1 high specific capacitance and high cycle stability (the capacitance retention rate was 88.8% after 5000 cycles). Rabani et al. [87] used the sol-gel method to deposit zinc oxide (ZnO) in 1D-CNF to obtain 1D-CNF@ZnO nanometer composite fibers, and then further assembled the 1D-CNF@ZnO into an all-solid-state flexible paper-based supercapacitor. As shown in Figure 6I, ZnO nanoparticles perfectly cover the 1D-CNF surface at suitable concentrations of ZnO precursors, which is essential for their application to energy storage electrodes. In order to better illustrate the excellent electrochemical performance of this paper-based electrode. As shown in Figure 6J, the authors compared 1D-CNF, ZnO, and 1D-CNF@ZnO paper-based electrodes. It is clear from the figure that the 1D-CNF@ZnO electrode has a larger CV area and energy storage characteristics for discharge time.
Based on the above summary of the work related to metal compounds deposited on 1D cellulose, we can see that the current work is mainly focused on non-metallic oxide pseudocapacitor materials. This may be due to the non conductivity of metal oxides. As an excellent pseudo capacitor material, metal oxides can be used to prepare cellulose/metal oxide composite fibers through the above process in future research. Then, the paper based electrode material with excellent performance is prepared by compounding with the electroactive material with excellent conductivity.