KEYWORDS
capacitance, microfibrillated cellulose, tin oxide, thin film
│ INTRODUCTION
Supercapacitors with high-power output, long-term cyclability, and rapid
charging-discharging cycles in electrochemical energy storage systems
have been extensively developed in the past decade1.
Conventional batteries with such attractive features cannot be produced
because charging-discharging mechanisms involve electrochemical
reactions2. However, supercapacitors also have
pitfalls since the poor energy density and limited energy capacitance
can affect the overall performance of electrochemical energy storage
systems. As the key element in a supercapacitor, electrode materials
play a crucial role in the overall electrochemical
performance3. The electrode must also possess a highly
interconnected porous structure to allow the diffusion of ions in the
material during charging and discharging, as well as a high surface area
with active sites for charge storage to overcome mentioned
drawbacks4. Besides the highly efficient, cheap and
sustainable supercapacitors are in great demand5.
Cellulose-based electrodes have been developed to meet the increasing
demand for supercapacitors. Cellulose contains a large number of
hydroxyl groups6 on its reactive sites and provides
various hybridization opportunities with different active nanomaterials
to form conductive nanocomposites7. Cellulose
nanofiber (CNF) and microfibrillated cellulose (MFC) are particularly
appealing for flexible energy storage systems because of their high
aspect ratio, outstanding mechanical properties and excellent physical
qualities8. They can form hydrogel with a
three-dimensional structure and abundant hydrated groups, as well as
aerogel with a highly porous network. More importantly, CNF and MFC can
also be easily transformed into flexible thin films due to fiber
entanglement. Conductive thin films with supercapacitance were
successfully developed by incorporating carbonaceous nanomaterials into
cellulose. Deng et al.9reported on the electrospun
cellulose acetate thin film with an electrochemical capacitance of 105
F/g after incorporating multiwalled carbon nanotubes and carbonization
in one of the earliest works. Without MWCNTs, the carbonized cellulose
nanofiber-activated carbon thin film only attained a capacitance up to
88 F/g10. Conductive polymers such as polyaniline
(PANI)11, polyvinyl alcohol-borax
(PVAB)12, and polypyrrole
(PPy)12–15were further incorporated to improve film
conductivity. In addition to MWCNTs and carbon nanofibers (CNFs),
reduced graphene oxide (rGO) was extensively used to produce CNF-based
supercapacitors in recent works 16,17. Other
conductive fillers such as tin oxide
(SnO2)18, molybdenum disulfide
(MoS2)12, cobalt oxide
(Co3O4)19, lithium
chloride (LiCl)20, and ZnO nanoparticles were reported
in the development of CNF-based supercapacitors as well. Compared to
CNF, MFC possesses a wider size distribution that includes nano and
microfibers. MFC can be produced at a lower cost than CNF since MFC
production can be sustainably accomplished by mechanical treatment
without enzymatic or chemical pre-treatment. MFC based supercapacitors
were also developed, although they were not widely reported in the
literature. Liu et al. 21impregnated carbonized
chitosan into the porous MFC framework before drying it into a flexible
thin film with a specific capacitor of 139.6 F/g. Without the formation
of a porous framework through freeze-drying, a specific capacitance of
53 F/g was only obtained even rGO-SnO2 nanocomposite was
blended into MFC to form a supercapacitor 22.
Differently, carbon aerogel with a capacitance as high as 330 F/g was
successfully produced from MCF, carbon quantum dots and graphene oxide
(GO) through blending, bidirectional freeze-casting, freeze drying and
carbonation23.
Herein, the flexible and thin supercapacitor was fabricated from MFC and
SnO2-cellulose nanocomposite.
SnO2-cellulose nanocomposite was first hydrothermally
produced before being coated on the MFC thin film. Hu et
al.24reported that the hydrothermal treatment of rice
husk nanocellulose in the presence of SnO2 not only resulted in
homogenous dispersion of SnO2 nanoparticles, but also improved
crystallinity of SnO2 nanoparticles. In addition, MFC
was stabilized in the organic solvent using LiCl before hydrothermal
synthesis of nanocomposite and fabrication of MFC thin films. LiCl could
stabilize MFC in the organic solvent20 and improve
conductivity24. The flexible
SnO2-cellullose thin films were characterized to
understand the chemical and physical properties before the evaluation of
specific capacitance.
│ RESULTS AND DISCUSSION