3. Summary and outlook
As a biomass resource with biorenewable, easily degradable and excellent mechanical flexibility, cellulose is considered to be the most attractive green material for the preparation of flexible supercapacitor electrodes. Especially in the context of global efforts to reduce environmental pollution, cellulose paper-based supercapacitor energy storage devices have become a current research hotspot. Among them, the porous structure of cellulose gives paper-based supercapacitors unique advantages, including high energy density, designability of thickness, design of porous structure conducive to the rapid movement of electrolyte ions, and preparation of integrated devices. In addition, the application of printing technology makes the large-scale and commercialization of paper-based energy storage devices a possibility. This paper summarizes the latest research progress of electrode related preparation methods of cellulose paper-based supercapacitors in recent years, and classifies and summarizes them to provide a rich reference for their development. Although scientists have done a lot of excellent research in the field of the preparation of cellulose paper-based electrodes. However, many problems and challenges require further research and breakthroughs (Figure 16). (1) Although cellulose paper-based electrodes prepared by different preparation methods have been used in many fields. But these methods have a common problem with energy storage. As a flexible energy storage device, the improvement of electrochemical energy storage performance (specific capacitance, energy density and power density) and designability are problems that need to be further explored and studied in these preparation processes. (2) Large-scale preparation is the most important problem restricting the development of flexible supercapacitors. The relevant research work reported so far is limited to laboratories and has not been demonstrated on a large scale. Among them, mature papermaking technology and low-cost printing technology have become the most effective way to achieve large-scale preparation. In future research, the preparation of pulp composite active fiber by blending pulp cellulose with active materials or by in-situ polymerization is a research direction with high feasibility of large-scale preparation. In addition, the development of electroactive inks in printing technology is also a highly feasible method for large-scale preparation of high-performance paper-based supercapacitor devices. (3) In the preparation process of paper-based electrodes other than flat printing technology and LIG technology, the control of the material structure and pore size of the electrode by the long-diameter ratio of cellulose is also a very worthy of study. For example, cellulose materials with different length-diameter ratios and composite forms with active materials have an impact on the pore structure inside the electrode material. Therefore, controlling these porosities by choosing the appropriate preparation method remains a challenge. (4) The preparation of high-performance paper-based electrodes assisted by computational simulation theory has also been gradually applied. For example, COMSOL is used to carry out theoretical simulation calculations on the movement of electrolyte ions in the pores inside the electrode, and the influence of pore structure on energy storage performance is studied more intuitively. And with the help of simulation, the preparation process of paper-based electrodes can be continuously optimized and referenced. In addition, advanced finite element simulation can also be used to establish the relationship between paper-based electrode structure and mechanical properties. (5) The designability of the energy storage performance of paper-based electrodes by related preparation methods is a topic that has not been widely studied. For example, the change of thickness during vacuum filtration, the control of polymerization conditions in in-situ polymerization, and the thickness of ink in printing technology will have a certain impact on the energy storage performance of paper-based devices. Therefore, the controllable design of paper-based electrode energy storage performance by using these technologies also has an important impact on the preparation of high-performance paper-based supercapacitor energy storage devices. (6) The development trend of flexible electronics in the future when intelligent integrated equipment. The introduction of self-charging, electrochromics, self-healing and shape memory into paper-based supercapacitors, and the development of integrated multifunction devices are critical to the future development of paper-based supercapacitors. In summary, the development of cellulose paper-based electrodes in the field of energy storage of flexible supercapacitors is worthy of further exploration and breakthrough. Finally, the future of cellulose paper-based supercapacitor devices will definitely have a very promising market. We also need to continue to work hard and expect paper-based energy storage devices to enter an era of widespread application.