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