2.1.5 Cellulose/MXene paper based composite electrode
As a 2D material with high specific surface area and high conductivity, MXene has shown great prospects in the field of supercapacitor energy storage[78]. In recent years, the composite paper based electrode materials prepared by vacuum filtration of cellulose and Mxene have been fully studied. This section summarizes cellulose/MXene paper based electrode materials. When 2DMxene chips are used as supercapacitors and electrode materials of supercapacitors, stacking between layers is easy to occur. In order to solve this problem, scientists also tried to use 1D carbon nanotubes and other carbon materials as spacers to prevent their stacking. However, the preparation process of these carbon materials is complex and costly. Cellulose, as a green and sustainable renewable resource, is the first choice as a 2D active material spacer. For example, Jiao et al. [79] first prepared BC/MXene composite paper with excellent new mechanical properties and electrochemical properties through vacuum pumping power. In addition, stretchable and patterned paper based electrode materials are prepared by laser cutting process to adapt to various use environments (Figure5A). Thanks to the close stacking structure between BC fiber and Mxene lamellae (Figure5B). The prepared paper based electrode was assembled into an all solid state paper based supercapacitor, showing an area capacitance of 115mF cm-2 and flexible mechanical properties. This work provides a promising method for designing and manufacturing flexible electronic equipment used in different environments through the patterned scheme of laser cutting. Similarly, Tian et al. [80] prepared CNF/Mexene paper-based electrodes by vacuum filtration, which also showed excellent electrochemical performance (298F g-1). The above two works show that the cellulose/MXene paper-based electrode prepared by simple vacuum filtration process has a very broad application prospect.
Although the paper based electrode obtained by direct filtration of cellulose and Mxene has good performance, it can not meet people’s needs under certain conditions. Therefore, it is very necessary to improve the energy storage performance of paper-based materials through certain methods. For example, the introduction of the third component or the surface modification of MXene can improve its energy storage performance. In order to improve the electron transfer rate of MXene in paper based materials, as shown in Figure 5C, it is inspired by the structure of plant leaves. Tang et al. [81] prepared a paper based composite material with leaf like structure by vacuum filtration of cellulose, Mxene and silver nanowires (AgNWs). As shown in Figure5D, the structure diagram of natural leaf veins and prepared cellulose/Mxene/AgNWs paper based electrode (PMxAg) is similar to leaf veins. Among them, cellulose as the main pulse provides excellent mechanical properties for paper based materials and provides a transport channel for electrolyte ions. Secondly, AgNWs as the secondary pulse provides a fast channel for electron transfer. Finally, as the ”mesophyll”, Mxene provides sufficient energy storage capacity for composite materials. Thanks to the ”blade like structure”, the paper based electrode shows a specific capacitance of up to 505F g-1. In addition, the author also successfully prepared large area composite paper by using paper machine. The batch preparation of this flexible supercapacitor provides a solution. In addition, surface treatment of MXene is also an effective method to improve the electrochemical performance. Previous research shows that the energy storage performance of MXene mainly comes from the=O group, while the -F and -OH groups in it have no contribution to the energy storage performance, and even more content will affect the energy storage performance. For this method, Chen et al. [82] modified the surface of MXene by KOH and high temperature treatment to reduce the -F and -OH groups on its surface. Subsequently, NFC/Mxene paper based composite electrode was prepared by combining nano fibrillar cellulose (NFC) prepared from soybean straw with modified Mxene through vacuum filtration process. The preparation process is shown in Figure 5E. The test results show that the paper electrode based supercapacitor has a specific capacitance of 303.1F g-1 at 1mA cm-2 and a capacitance retention of 92.84% after 10000 cycles. As shown in Figure 5F, Cai et al. [83] prepared SnS2 on the surface of MXene by hydrothermal method Mxene@SnS2 Nanosheets, then CNF was prepared by vacuum filtration/ Mxene@SnS2 Paper based composite materials. To illustrate Mxene@SnS2 The contribution of nano sheets to the electrochemical performance of composite materials is shown in Figure5G. The author compares CNF/MXene, CNF/MXene/SnS2and CNF/ Mxene@SnS2 Energy storage performance among the three. CNF/ Mxene@SnS2 The specific capacitance of 171.6F g-1 is significantly higher than that of CNF/Mxene (163.3F g-1) and CNF/Mxene/SnS2 (130F g-1). At the same time, because of the light heat conversion function of SnS2, the assembled supercapacitor has the function of solar energy driving. As shown in Figure 5H, the discharge time of the paper-based supercapacitor increases with the increase of light intensity (under the light intensity of 1kW m-2, the energy storage performance of the device is improved by 60%).