A freestanding cellulose nanofibril-reduced graphene oxide-molybdenum oxynitride aerogel film electrode for all-solid-state supercapacitors with ultrahigh energy density

A free-standing, lightweight, highly porous, and highly flexible cellulose nanofibril (CNF)-reduced graphene oxide (RGO)-molybdenum oxynitride (MoOxNy) aerogel film electrode was synthesized by freeze-drying a CNF/GO/MoO3 aqueous dispersion followed by subsequent in situ hydrazine reduction. Due to the partial reduction and nitrogen doping of the MoO3 nanobelts and the highly open and continuous porous structure, the free-standing CNF/RGO/MoOxNy aerogel film electrode delivered an outstanding specific capacitance of 680 F g(-1) in an aqueous electrolyte and 518 F g(-1) in an ionic liquid electrolyte in a three-electrode configuration at a current density of 1.0 A g(-1). Furthermore, highly flexible and all-solid-state supercapacitors using CNF/RGO/MoOxNy as electrodes and either a hydrogel or ionogel as the electrolyte were fabricated and both types of supercapacitors demonstrated high specific capacitance and excellent long-term cycling stability. Remarkably, the solid-state supercapacitor devices fabricated using an ionogel electrolyte demonstrated an energy density of 114 W h kg(-1) (corresponding to a volumetric energy density of 18.8 W h L-1), which is among the highest values achieved for any type of solid-state supercapacitor and is even comparable to the energy densities of Li-ion batteries. Therefore, this study provides a simple and cost-efficient method for fabricating flexible energy storage devices with excellent electrochemical performance.