2.1.2 Cellulose/graphene paper based composite electrode
Graphene is a two-dimensional carbon material with hexagonal honeycomb
lattice structure, which is connected by carbon atoms with
sp2 hybridization[61]. Because of its excellent
conductivity, ultra-high theoretical specific surface area, high
mechanical strength and good thermal conductivity, it has attracted
extensive attention from researchers around the world. At present,
graphene has been widely used in many fields such as energy storage,
sensing, electromagnetic shielding, wave absorption and environment.
However, the van der Waals force and π-π bond force exist between 2D
graphene sheets[62]. The electrode plates directly prepared from
graphene into supercapacitors will have stacking effect, which will lead
to the reduction of electrochemical reaction sites. And it is not
conducive to the rapid movement of electrolyte ions, which seriously
affects the performance of energy storage devices. Cellulose has a large
number of hydrophilic groups, such as hydroxyl and carboxyl
groups[63,64], which can interact with graphene to promote the
dispersion of graphene in the solute and act as a perfect spacer between
2D graphene sheets. He et al.[65]prepared a self supporting flexible
paper based electrode material with excellent mechanical properties and
energy storage properties by vacuum filtration of CNF and reduced
graphene oxide (RGO). Figure 3A is a schematic diagram of the
preparation process of CNF/RGO paper-based electrical materials. The
influence of different mass ratios of CNF to RGO on the energy storage
performance of paper based electrode materials was discussed. As shown
in Figure 3B, when the ratio between CNF and RGO is 1:2, the composite
material shows an excellent laminated structure. This shows that at this
ratio, the existence of CNF weakens the force between graphene layers to
the greatest extent and limits the accumulation of RGO. And as expected,
the composite also showed the best performance, with a specific
capacitance of 146mF cm-2, tensile strength of 83Mpa
and excellent conductivity (202.94S m-1) at a current
density of 5mA cm-2. Since RGO can only provide
certain energy storage characteristics of double electric layers, the
paper based energy storage materials prepared in this study have room
for improvement. For example, the introduction of the third component
with excellent energy storage performance. On the basis of the former
research, Qiang et al.[66]prepared CNF/RGO/polypyrrole (PPy) paper
electrode materials with sandwich structure by vacuum filtration. Figure
3C shows the preparation process of the composite. First, CNF/RGO paper
matrix composite was obtained by vacuum filtration. Then immerse it in
Fe(ClO4) solution. Finally, the pyrrole monomer was
dispersed on the CNF/RGO surface by spraying and polymerized for 30 min
to obtain the CNF/RGO/PPy paper based electrode. Thanks to the porous
structure of the CNF/RGO layer and the pseudocapacitance characteristics
of PPy, the paper based electrode shows excellent specific capacitance
(195.8F g-1, 915mF cm-1), excellent
energy density (13.04 Wh kg-1) and power density
(200.6 Wkg-1). Figure 3D shows the ion and electron
transfer model inside CNF/RGO/PPy electrode. The author points out that
thanks to the porous structure of CNF/RGO, the paper based electrode
material shortens the diffusion channel of electrolyte ions in the
electrochemical reaction. Moreover, the highly conductive polypyrrole
layers assembled layer by layer provide sufficient pseudo capacitance.
Using CNF/RGO paper-based materials obtained by vacuum filtration as the
substrate, the process of introducing pseudocapacitors through in-situ
polymerization similar to the above work is relatively cumbersome. The
electropolymerization law is a relatively quick method. For example,
Xiong et al.[67]used CNF/RGO prepared by vacuum filtration as the
substrate to introduce polyaniline into composite paper based materials
by means of electropolymerization (Figure 3E). Figure 3F shows the CV
curve of the composite. It can be clearly seen that due to the
introduction of high pseudo capacitance PANi, the paper based electrode
shows greater energy storage characteristics. From the above two works,
it can be seen that the cellulose/graphene paper based composite
obtained by vacuum filtration can not only be directly used as the
electrode material of supercapacitors, but also be used as an effective
substrate material to introduce other components to prepare cellulose
paper based electrode materials with better performance.
In addition to conducting polymers, metal oxides also exhibit excellent
pseudo capacitive properties, which can show high energy density through
reversible redox reactions. Therefore, introducing metal oxides into
CNF/RGO paper based materials is also a way to improve their
electrochemical performance. For example, Zou et al.[68]prepared
cellulose/RGO/Ag paper matrix composites with high conductivity through
vacuum filtration. Then, a paper based electrode material with high
conductivity and double electroactive substances was obtained by
depositing Fe2O3 on its surface (Figure 3G). The test results show that
the symmetrical supercapacitor device assembled with this paper based
electrode achieves a large area capacitance (1132mF
cm-1) and energy density (226.4μWh
cm-2).Moreover, the device does not need additional
collectors, and it also shows excellent flexibility. Therefore, the
material has broad application prospects in the field of flexible
supercapacitor. In a word, the addition of metal oxides has an obvious
effect on improving the electrochemical performance of paper based
supercapacitors.