2.4.1 Papermaking process
Thanks to the long history of paper making technology, the paper making
technology in modern society is mature enough. At present, all kinds of
special paper are developed by people through papermaking process. As
shown in Figure 14A, Huang et al. [119] prepared a kind of cellulose
based on the disposal arrangement of polyaniline, and then prepared a
paper based composite electrode through the papermaking process. The
preparation of the electrode is mainly divided into three steps: first,
a large number of small branches are generated by high-speed mechanical
stirring of cellulose pulp, so that more hydroxyl groups conducive to
polyaniline polymerization are exposed on more cellulose surfaces.
Secondly, polyaniline is polymerized on the cellulose surface to form
PANi@cellulose. Finally, the PANi@paper composite electrode is obtained
through an industrial paper forming machine. As shown in Figure 14B, the
TEM image of PANi fiber shows that polyaniline grows vertically and
tightly on the surface of cellulose. This provides a faster redox
kinetics for electron/ion transfer and charge storage in the
electrochemical process. And the vertically arranged PANi structure also
gives the paper based electrode excellent capacitive characteristics.
The PANi@paper electrode exhibits an excellent specific capacitance of
296F g-1 at 1A g-1. More
importantly, the solid paper based energy storage device assembled by
the PANi @ paper electrode also has a specific capacitance of 282F
g-1, and still maintains excellent and stable
electrochemical performance under different bending angles (Figure 14C).
The large-scale preparation of paper-based electrode materials by
papermaking technology is exciting. However, the prepared paper-based
materials also require manual assembly of solid-state devices when
applied, which undoubtedly increases production costs and time. In
response to this problem, the team achieved large-scale production of
integrated paper-based supercapacitors by improving the process
technology [120]. As shown in Figure 14D, this technique uses
cellulose from orderly stacking PPy as the raw material for paper-based
electrodes. After continuous molding, pressing and drying, an integrated
supercapacitor is obtained. Figure 14E shows a schematic diagram of the
detailed preparation process. Firstly, the 1DPPy/cellulose pulp fiber is
passed through the sheet molding machine to obtain the non-dry state
PPy@paper, and then the two pieces of PPy@paper are assembled
sequentially with the cellulose diaphragm obtained by pure cellulose
pulp to form a sandwich structure. Finally, an all-paper-based
supercapacitor device is obtained by hot pressing, drying, and
impregnating the electrolyte (Figure 14F). The all-in-one paper-based
device has a mass ratio capacitance of 360F g-1 at
0.1A g-1 and a capacitance retention rate of 81.7%
after 1000 cycles. It is worth mentioning that by controlling the
thickness of the pulp raw material in different electrodes, an
integrated device with different areas of capacitance can be obtained. A
series of paper-based devices in the 562-2507mF cm-2range of area ratio capacitance were designed. This provides a fast and
efficient way to industrialize mass customization of paper-based
devices.
Chen et al. [121] prepared a carbon fiber (CFs)-reinforced
cellulose-based activated carbon paper-based electrode (cellulose-based
ACFPs) by wet papermaking, carbonization and activation. Figure 14G
shows the preparation process of this electrode. Firstly, CFs were mixed
with fibrillated pulp fibers evenly, and carbonized precursor paper
matrix composites were prepared by papermaking process. This composite
is then soaked in H3PO4 solution.
Finally, cellulose-based ACFPs paper-based electrodes were obtained by
H3PO4 and CO2 double
activation process. Among them, CFs in paper-based materials have a low
coefficient of thermal expansion and high chemical stability, which
provides excellent mechanical properties for paper-based electrodes
(Figure 14H).