2.3.1 Writing
The pencil as a very common writing instrument in life. Due to friction, the graphite particles generated by writing on the paper by the graphite rod inside it adhere to the surface of the cellulose paper. And with the pencil you can draw a variety of shapes of conductive paper, this conductive paper substrate provides an easy way to build. This pencil-based conductive paper has been used to prepare flexible supercapacitors. Zang et al. [104] used waste newspaper as a flexible substrate and bought a patterned graphite substrate through a pencil on its watch. Finally, PPy was deposited on its surface by electrochemical polymerization to obtain a paper-based composite electrode of newspaper/graphite/PPy (NGP) (Figure 10A). The electrode drawn by pencil exhibits a specific capacitance of 270.32mF cm-2 and an energy density of 24.03 μWh cm-2. This work shows that graphite layer paper constructed by pencil drawing can serve as a substrate for the active substance. Since then, Yeasmin et al. [105] have successfully introduced polyaniline on the surface of a paper-based material drawn by electrochemical polymerization, and the electrode exhibits an area capacitance of 93.64mF cm-2 and a specific capacitance of 28.37F g-1. With the popularity of flexible energy storage equipment, it is inevitable that there will be damage and deformation during use. This can seriously affect the performance of flexible energy storage devices. Therefore, sustainable use is particularly important for flexible devices. In response to this problem, Xiong et al. [106] introduced Vitrimer with shape memory and self-healing function into paper-based electrode, and prepared the first paper-based electrode material with shape memory and self-healing function. Figure 10B shows the preparation process of the paper-based material. A OPD@PN-V paper-based electrode material is obtained by polymerizing Vitrimer on conductive paper (OPD@PN) supported with polyaniline. The original shape can be restored by giving the initial paper-based electrode a certain shape and then processing it with a blower (Figure 10C). And after external force damage, it can also be self-repaired by blower treatment (Figure 10D). In addition, the composite exhibits excellent sensing properties. In conclusion, the introduction of Vitrimer into paper-based electrodes is a novel method for the preparation of multifunctional intelligent energy storage devices.
In order to increase the service life of the device and realize the sustainable use of flexible energy storage devices. Ma et al. [107] integrated TENG with a miniature supercapacitor (MSC) on a sheet of paper. Figure 10E shows the preparation process of the integrated device. First, interfinger-like 200 nm thick gold nanoparticles are deposited on the paper-based surface by magnetron sputtering. For the MSC unit, the gold surface is applied to functionalize its surface. The pencil-drawn paper-based electrode is then soaked in 8-Amino-2-naphthol solution for 1h. Finally, a PCA/H2SO4gel electrolyte is applied to the patterning unit to obtain MCS. The TENG consists of a sliding module and a stator (the stator on a piece of paper with the MCS). When the stator moves across the stator covered by the FEP membrane, the TENG releases current to charge the MSC for continuous use of the device. As shown in Figure 10F, the TENG can charge three MSCs in series to 2.6V in 465s and successfully drive the electronics to operate. In conclusion, this work successfully prepared self-charging paper-based supercapacitors. It provides a good design idea for the integration of paper-based self-charging power supply and paper-based energy storage device in the future.
In addition to the introduction of active materials after building a conductive layer by drawing with a pencil, it is inspired by writing ink pens. Guang et al. [108] prepared an energy storage ink that can be written directly. The ink consists of CNT and Ag. As shown in Figure 10G, by assembling the ink in a ballpoint pen, the device controls its writing and provides pressure to it. Finally, the written electrode is placed at 60 °C to completely dry the ink. The results show that the prepared CNT/Ag paper-based electrode has a resistivity of 5.1×10-4Ω cm-1 and a specific capacitance of 72.8F cm-3. And after bending test, there is no effect on the electrochemical properties (Figure 10H), showing excellent mechanical stability.