2.2.1.2 Cellulose/conductive polymer paper based composite
electrode
1D composite fiber prepared by composite of conductive polymer and 1D
cellulose has been reported so far. Electrodes depositing polyaniline
(PANi), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT)
on the surface of 1D cellulose by in-situ polymerization have been
prepared. However, due to the low solubility and unstable structure of
conductive polymers in solvents, the energy storage performance of
electrodes directly compounded with cellulose is not excellent. At
present, scientists prepare paper based electrodes with excellent
electrochemical performance by adding surfactants or blending 1D
cellulose composite fibers based on conductive polymers with other
pseudo capacitive materials. As shown in Figure 7A, Song et al. [88]
used the in-situ polymerization method to uniformly deposit PPy on the
surface of BC fiber to obtain 1DPPy@BC composite fiber, and then
prepared PPy@BC/Mxene composite paper-based electrode by vacuum
filtration of the dispersion of PPy@BC and Mxene. Firstly, 1DBC fibers
as effective templates provide abundant space for the deposition of Ppy.
Secondly, the hierarchical porous structure formed by the assembly of
PPy@BC fibers and Mxene provides sufficient sites for electrochemical
reactions. This is very important for the electrochemical performance of
this paper-based electrode. Thanks to the above two points, the
PPy@BC/Mxene composite paper-based electrode exhibits a mass ratio
capacitance of 550F g-1 and an area ratio capacitance
of 879mF cm-2. The work is to prepare high-performance
paper-based electrodes by blending 1D composite fibers with Mxene.
Although satisfactory electrochemical performance can be obtained by
this method, the thickness of the electrode will always be increased by
blending the two components, which is detrimental to flexible
microelectronic devices. It is a feasible method to introduce the second
component directly into the 1D composite fiber by in-situ
polymerization. Yang et al. [89] prepared 1DPPy@ cobalt
hydroxyoxide/cellulose composite fiber (1DPCC fiber) by liquid phase
reduction. Figure 7B shows the preparation process of this paper-based
electrode. First, cobalt hydroxyoxide is reduced to
CoCl2ยท6H2O by NaBH4 and
cobalt hydroxyoxide is loaded on the surface of 1D cellulose in situ.
1DPCC fibers are then obtained by in situ oxidative polymerization of
PPy. Finally, PCC paper-based composite electrode was obtained by vacuum
filtration. Electrochemical test results show that PCC paper-based
electrodes have a specific capacitance of up to 571.3F
g-1 and a capacitance retention rate of more than 93%
(1000 cycles). In this work, 1D cellulose composite fibers with
excellent performance were prepared by two-step in-situ polymerization.
The paper-based electrode also exhibited excellent energy storage
characteristics, which provided a simple and fast method for preparing
paper-based composite flexible energy storage electrode.
Recent studies have found that the use of redox small molecules to treat
paper-based composite electrodes can also improve electrochemical energy
storage performance. Small organic molecules with specific
intramolecular conjugated structures, such as quinones, can be a
substance that enhances the electrochemical performance of composite
materials with an ideal valence. As shown in Figure 7C, Chang et al.
[90] effectively improved the energy storage performance of PEDOT
paper-based electrode by sodium alizarin sulfonate (sodium
1,2-dihydroxyanthraquinone-3-sulfonate) (ARS).
ARS/H2SO4 composite electrolyte was
prepared. The results show that the supercapacitor device assembled by
ARS-treated electrode and electrolyte shows excellent electrochemical
performance (area ratio capacitance 2191.3mF cm-2,
energy density 4.87mWh cm-3). This is a significant
improvement over the 348 mF cm-2 area ratio
capacitance of a paper-based electrode without ARS treatment. Figure 7D
shows the energy storage mechanism in the charging and discharging
process of the device. Among them, the addition of ARS promotes the
regeneration of PEDOT and reuses in redox reaction, which plays an
important role in improving the electrochemical performance of the
device. The water-soluble sulfonated polymer surfactant polystyrene
sulfonate (PSS) has attracted extensive attention from researchers due
to its excellent mechanical flexibility and adjustable conductivity. In
this paper, three recent works on the improvement of conductive polymer
1D cellulose composite fibers using PSS are summarized. As shown in
Figure 7E, Zhang et al. [91] added PSS to a mixed solution of
pyrrole monomer and CNF, and improved the loading of polypyrrole on the
surface of CNF through PSS. The results show that the paper-based
electrode PPy:PSS/CNP with PSS exhibits a specific capacitance of 240F
g-1. Similarly, Zhang et al. [92,93] also used PSS
to improve the loading of PANi as well as PEDOT on the CNF surface.
Figure 7F and 7G are schematic diagrams of the preparation process of
PANi: PSS/CNP and PEDOT:PSS/CNP paper-based electrodes. It is worth
mentioning that the addition of PSS in these three works effectively
improves the cellulose loading of conductive polymers and promotes the
construction of high-performance paper-based electrodes. It provides a
good idea for the preparation of conductive polymer-based paper-based
composite electrodes.