3. Summary and outlook
As a biomass resource with biorenewable, easily degradable and excellent
mechanical flexibility, cellulose is considered to be the most
attractive green material for the preparation of flexible supercapacitor
electrodes. Especially in the context of global efforts to reduce
environmental pollution, cellulose paper-based supercapacitor energy
storage devices have become a current research hotspot. Among them, the
porous structure of cellulose gives paper-based supercapacitors unique
advantages, including high energy density, designability of thickness,
design of porous structure conducive to the rapid movement of
electrolyte ions, and preparation of integrated devices. In addition,
the application of printing technology makes the large-scale and
commercialization of paper-based energy storage devices a possibility.
This paper summarizes the latest research progress of electrode related
preparation methods of cellulose paper-based supercapacitors in recent
years, and classifies and summarizes them to provide a rich reference
for their development. Although scientists have done a lot of excellent
research in the field of the preparation of cellulose paper-based
electrodes. However, many problems and challenges require further
research and breakthroughs (Figure 16). (1) Although cellulose
paper-based electrodes prepared by different preparation methods have
been used in many fields. But these methods have a common problem with
energy storage. As a flexible energy storage device, the improvement of
electrochemical energy storage performance (specific capacitance, energy
density and power density) and designability are problems that need to
be further explored and studied in these preparation processes. (2)
Large-scale preparation is the most important problem restricting the
development of flexible supercapacitors. The relevant research work
reported so far is limited to laboratories and has not been demonstrated
on a large scale. Among them, mature papermaking technology and low-cost
printing technology have become the most effective way to achieve
large-scale preparation. In future research, the preparation of pulp
composite active fiber by blending pulp cellulose with active materials
or by in-situ polymerization is a research direction with high
feasibility of large-scale preparation. In addition, the development of
electroactive inks in printing technology is also a highly feasible
method for large-scale preparation of high-performance paper-based
supercapacitor devices. (3) In the preparation process of paper-based
electrodes other than flat printing technology and LIG technology, the
control of the material structure and pore size of the electrode by the
long-diameter ratio of cellulose is also a very worthy of study. For
example, cellulose materials with different length-diameter ratios and
composite forms with active materials have an impact on the pore
structure inside the electrode material. Therefore, controlling these
porosities by choosing the appropriate preparation method remains a
challenge. (4) The preparation of high-performance paper-based
electrodes assisted by computational simulation theory has also been
gradually applied. For example, COMSOL is used to carry out theoretical
simulation calculations on the movement of electrolyte ions in the pores
inside the electrode, and the influence of pore structure on energy
storage performance is studied more intuitively. And with the help of
simulation, the preparation process of paper-based electrodes can be
continuously optimized and referenced. In addition, advanced finite
element simulation can also be used to establish the relationship
between paper-based electrode structure and mechanical properties. (5)
The designability of the energy storage performance of paper-based
electrodes by related preparation methods is a topic that has not been
widely studied. For example, the change of thickness during vacuum
filtration, the control of polymerization conditions in in-situ
polymerization, and the thickness of ink in printing technology will
have a certain impact on the energy storage performance of paper-based
devices. Therefore, the controllable design of paper-based electrode
energy storage performance by using these technologies also has an
important impact on the preparation of high-performance paper-based
supercapacitor energy storage devices. (6) The development trend of
flexible electronics in the future when intelligent integrated
equipment. The introduction of self-charging, electrochromics,
self-healing and shape memory into paper-based supercapacitors, and the
development of integrated multifunction devices are critical to the
future development of paper-based supercapacitors. In summary, the
development of cellulose paper-based electrodes in the field of energy
storage of flexible supercapacitors is worthy of further exploration and
breakthrough. Finally, the future of cellulose paper-based
supercapacitor devices will definitely have a very promising market. We
also need to continue to work hard and expect paper-based energy storage
devices to enter an era of widespread application.