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.