2.2.2.1 Cellulose/metal paper-based composite electrodes
Due to its excellent porous structure, low cost, easy degradation and
renewable, 2D cellulose paper is considered to be an ideal flexible
substrate in the field of flexible supercapacitors. Zhang et al.
[94] prepared cellulose paper (CP)/Ni/Au paper-based electrodes by
loading nickel/gold bimetals on vitamin paper. Figure 8A shows the
preparation process of this paper-based electrode. Thanks to the
hydrophilic group and porous structure of CP, CP immersion in nickel
salt solution can adsorb a large number of nickel ions. The metal Ni is
then loaded in CP by NaBH4 reduction. Then, CP loaded with nickel metal
is entered into the electroless plating solution of Au to obtain
CP/Ni/Au paper-based current collector. Finally, the carbon-based active
material is deposited on its surface by electrophoretic deposition to
obtain a paper-based supercapacitor electrode. The results show that
thanks to the flexibility of cellulose paper, the CP/Ni/Au paper-based
electrode still maintains 92.1% of the initial capacitance after 2000
bending cycles, showing excellent mechanical properties. Li et al.
[95] also prepared Ni-paper by loading the filter paper with Ni
metal, and then soaked it in KMnO4/HCl solution to
obtain a Ni-paper-MnO2 paper-based electrode (Figure
8B). In order to show the excellent mechanical properties of Ni-paper as
an electrode material. After 5000 consecutive bending of the Ni-paper,
the resistance of the film only increased from the initial 0.8 Ω
cm-2 to 2.7 Ω cm-2. When
MnO2 is used as the active material, this paper-based
electrode exhibits an area specific capacitance of 1095mF cm-2 at a
current density of 1mA cm-2. This is mainly due to the hierarchical
porous fiber structure in the Ni-paper-MnO2 electrode and the high
conductivity of Ni, which greatly contribute to the electrochemical
performance of the electrode.
General flexible energy storage devices must assemble independent
electrodes, current collectors, and diaphragms. This is not only complex
but also increases the size of the device. Combining all units of the
device in a single substrate to build an all-in-one flexible energy
storage device is a promising approach. Recently, Chang et al.
[96-98] from Chung-Ang University in South Korea cleverly designed a
series of integrated paper-based supercapacitors with excellent
performance using paraffin heating-assisted in situ polymerization of
Au. As shown in Figure 8C, the team first designed a single-layer
integrated porous paper-based supercapacitor. The preparation of the
device is divided into three steps, the first step is to fill the inside
of cellulose paper by printing solid paraffin wax on the surface of the
paper (Figure 8D(i)), then heating it to a molten state and filling it
inside the cellulose paper (Figure 8D(ii)). In the second step, AgNPs
are deposited on the upper and lower surfaces of the cellulose paper by
drop-casting process (Figure 8D(iii)). The paraffin is then removed by
methanol (Figure 8D(iv)). In the third step, the paper loaded with AgNPs
gold source is soaked in Ag growth solution (HauCl4 and hydroxylamine
hydrochloride mixed solution) to obtain the Au-Paper electrode (Figure
8D(vi)). Finally, MnO2-Au-Paper paper-based integrated
electrode is obtained by electrodeposition by loading
MnO2 on the surface of Au-Paper (Figure 8E). Devices
assembled by combining a paper-based electric base with a gel
electrolyte exhibit a specific capacitance of 252.2F
g-1. To achieve a high potential window of the device,
the team used the technique to fabricate five supercapacitors connected
in series on a sheet of paper. The devices connected in series can
exhibit a high voltage window of 4.0V, easily lighting the blue LED
operating at 2.65V. As shown in Figure 8F, this technique is used to
print the pattern by changing the paraffin wax as well as the heating
temperature. The team prepared three vertically arranged parallel
interdigital electrodes on a sheet of cellulose paper. Compared to
devices assembled with single-layer interdigital electrodes. The three
devices connected in parallel exhibit a larger CV curve area and a
specific capacitance for discharge time. The detailed preparation
process invites the reader to read the original article, which makes it
easier to understand the working principle of the device. Building on
the former’s work, the team fabricated vertically integrated planar
multielectrode devices on a single sheet of paper. The highly integrated
multi-electrode, diaphragm, and current collector exist only on one
sheet of paper, and once again a device that expands the electrochemical
window is cleverly designed on a single sheet. Figure 8G shows a
schematic diagram of multilayer device integration and its corresponding
circuit diagram. Multilayer paper-based devices can exhibit greater
discharge times and CV curve areas in the same voltage window than
during a single layer. And it can still maintain excellent energy
storage performance under a higher electrochemical window. This series
of work provides an innovative idea for the ingenious design of
paper-based electrodes, which is of great significance to the
development of paper-based supercapacitors.