1. Introduction
With the development of electronic devices and people’s yearning for electronic devices in the intelligent era [1-6], flexible electronic devices have shown great application potential in the future portable consumer electronics and wearable devices [7-12]. Traditional flexible electronic equipment uses flexible substrates such as metal [13-15] (copper foil, aluminum foil, etc.) and plastic [16-18] (such as polyethylene terephthalate, polyethylene, polypropylene, etc.). Although these flexible substrates can endow flexible energy storage equipment with excellent mechanical properties, metal and plastic substrates have their own shortcomings. For example, metal and plastic are both hard materials with poor bending performance. Moreover, due to the weak binding force between the substrate and the active material, the active material will fall off during the use of flexible devices, resulting in poor cycle stability, which is not friendly to the life of flexible devices. Therefore, it is very important to develop new flexible materials with excellent flexibility and strong adhesion with active materials.
Cellulose is passed by glucose unit β-1,4 glycosidic bond linked natural polymer [19-21]. Its general molecular formula is (C6H10O5)n, which is the most extensive and renewable biomass resource in nature and can be extracted from trees, algae, bacteria and other substances. Its structure can be seen from atomic, molecular and macro scales (Figure1A). Its excellent aspect ratio and porous structure have important application value in the energy storage field of cellulose paper based supercapacitors [22-24]. Cellulose has attracted extensive attention in the field of flexible supercapacitors [25-27]. On the one hand, it is because cellulose is a kind of material with rich content, excellent mechanical properties, sustainable and cheap. On the other hand, the cellulose based paper supercapacitor electrode has a controllable porous structure and pore size distribution, which is very important for the rapid transfer of electrolyte ions during energy storage. For the flexible supercapacitor, the most important thing is the design and preparation of its electrode material structure. In recent years, cellulose, including nano cellulose, bacterial cellulose, cellulose fiber pulp and cellulose paper, has been used to prepare high-performance cellulose paper based supercapacitor electrode materials [28-32]. In this field, different paper based supercapacitors show different emphasis on performance. Therefore, it is very important to choose an appropriate preparation method to prepare paper based electrodes. In this paper, the preparation methods of cellulose based paper based supercapacitors in recent years are systematically summarized and classified for the first time. As shown in Figure1B, vacuum filtration [33,34], in-situ polymerization [35,36] and printing technology [37, 38] based on the division of multi-component mixing are the main technologies for preparing cellulose paper-based electrodes at present. In addition, commercially mature papermaking process [39,40], carbonization [41,42], impregnation [43,44] and laser induced graphene technology [45,46] are also used to prepare paper based supercapacitors. In conclusion, this review is intended to provide convenient reference materials for researchers in the field of paper-based supercapacitors, so as to promote the rapid development of paper-based energy storage devices.