2.2.1.1 Cellulose/metal paper based composite electrode
Metals and their compounds are considered as promising electrode
materials for supercapacitors because of their excellent point and
storage application prospects. Due to the large number of modifiable
hydroxyl groups in cellulose molecules, it provides a good platform for
the construction of polymerization and deposition of metals and their
metal compounds. Recent studies have shown that metals and their
compounds can be well combined with cellulose fibers or polymerized and
deposited on their surfaces to form 1D cellulose composite fibers with
excellent energy storage properties. Finally, paper based composite
electrode materials can be obtained by vacuum filtration. Zhou et al.
[85] used CNF as a substrate and deposited a conductive
metal-organic framework(c-MOF) with excellent electrical conductivity
through interfacial polymerization on its surface, and obtained
CNF@c-MOF nanofibers. Figure 6A shows the preparation process of
CNF@c-MOF, first introducing carboxyl groups to the surface of CNF
through TEMPO oxidation, then exchanging carboxylated CNF with
Ni2+ ions, and finally adding organic ligands to
obtain CNF@c-MOF nanofibers. CNF@c-MOF paper-based electrodes are
obtained by vacuum filtration. In addition, the authors discuss the
differences in the electrochemical performance of CNF@c-MOF paper-based
electrodes prepared by two different organic ligands, HITP
(2,3,6,7,10,11-hexaiminotriphenylene) and HHTP
(2,3,6,7,10,11-hexahydroxytriphenylene). Figure 6B shows a comparison of
the weight ratio capacitance of two different CNF@c-MOF paper-based
electrodes. It is evident that the CNF@Ni-HITP electrode prepared by
HITP has a weight capacitance of 125F g-1. This is
mainly due to the continuous Ni-HITP nanolayers tightly wrapped around
the CNFs (Figure 6C). The authors note that the continuous nucleation of
c-MOF nanometers on CNF and the hierarchical porous high conductivity
structure provide a fast pathway for electrolyte transport and charge
transfer for CNF@c-MOF paper-based electrodes (Figure 6D). It is worth
mentioning that the authors also discuss the performance of CNF@c-MOF
paper-based electrodes of different thicknesses, and the results show
that the increase in thickness has almost no effect on the
electrochemical performance of the paper-based electrode. This opens up
the possibility of customizing paper-based electrodes of different
thicknesses.
Huang et al.[86]used quaternized chitosan (QCS) as the connection
stabilizer between CNF and copper sulfate nanocrystals (CuS-NCs). Figure
6E shows the contribution of QCS to CuS NCs deposition on CNF fiber
surface. In the absence of QCS, the deposition of CuS is mainly through
the electrostatic adsorption between the electronegative CNF and
[Cu(NH3)4]2+in the
precursor solution of CuS-NCs. At the same time, S2-will lead to rapid prototyping of CuS NCs, which will lead to uneven
distribution of CuS NCs on the CNF surface (Fig. 6F). When
S2- and QCS are added to the CNF/[Cu
(NH3)4]2+precursor
solution, CuS NCs are well anchored on the CNF surface (Fig. 6G) due to
the electrostatic attraction formed between the positively charged QCS
and CNF. In order to more intuitively check the contribution of QCS to
the stable deposition of CuS NCs on the CNF surface, as shown in Figure
6H, CuS-NCs/CNF and CuS-NCs/QCD/CNF were respectively immersed in KOH
electrolyte for 48h. It can be clearly observed from the figure that the
paper-based electronic base shows excellent stability due to the
addition of QCD. However, CuS NCs in the paper-based electrode without
QCD fell off obviously. The test results showed that CuS-NCs/QCD/CNF
paper based electrode exhibited 314.3F g-1 high specific capacitance and
high cycle stability (the capacitance retention rate was 88.8% after
5000 cycles). Rabani et al. [87] used the sol-gel method to deposit
zinc oxide (ZnO) in 1D-CNF to obtain 1D-CNF@ZnO nanometer composite
fibers, and then further assembled the 1D-CNF@ZnO into an
all-solid-state flexible paper-based supercapacitor. As shown in Figure
6I, ZnO nanoparticles perfectly cover the 1D-CNF surface at suitable
concentrations of ZnO precursors, which is essential for their
application to energy storage electrodes. In order to better illustrate
the excellent electrochemical performance of this paper-based electrode.
As shown in Figure 6J, the authors compared 1D-CNF, ZnO, and 1D-CNF@ZnO
paper-based electrodes. It is clear from the figure that the 1D-CNF@ZnO
electrode has a larger CV area and energy storage characteristics for
discharge time.
Based on the above summary of the work related to metal compounds
deposited on 1D cellulose, we can see that the current work is mainly
focused on non-metallic oxide pseudocapacitor materials. This may be due
to the non conductivity of metal oxides. As an excellent pseudo
capacitor material, metal oxides can be used to prepare cellulose/metal
oxide composite fibers through the above process in future research.
Then, the paper based electrode material with excellent performance is
prepared by compounding with the electroactive material with excellent
conductivity.