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

Abstract

A free-standing, lightweight, highly porous, and highly flexible cellulose nanofibril (CNF)–reduced graphene oxide (RGO)–molybdenum oxynitride (MoO_xN_y) 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/MoO_xN_y 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/MoO_xN_y 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.