Highly dispersive Co₃O₄nanoparticles incorporated into a cellulose nanofiber for a high-performance flexible supercapacitor

Transition metal oxides used as electrode materials for flexible supercapacitors have attracted huge attention due to their high specific capacitance and surface-to-volume ratio, specifically for cobalt oxide (Co₃O₄) nanoparticles. However, the low intrinsic electronic conductivity and aggregation of Co₃O₄ nanoparticles restrict their electrochemical performance and prevent these electrode materials from being commercialized. Herein, a facile, advantageous, and cost effective sol-gel synthetic route for growing Co₃O₄ nanoparticles uniformly over a low cost and eco-friendly one-dimensional (1D) hydrophilic cellulose nanofiber (CNF) surface has been reported. This exhibits high conductivity, which enables the symmetric electrode to deliver a high specific capacitance of ∼214 F g⁻¹ at 1 A g⁻¹ with remarkable cycling behavior (∼94% even after 5000 cycles) compared to that of pristine CNF and Co₃O₄ electrodes in an aqueous electrolyte. Furthermore, the binder-free nature of 1D Co₃O₄@CNF (which was carbonized at 200 °C for about 20 min under a H₂/Ar atmosphere) shows great potential as a hybrid flexible paper-like electrode and provides a high specific capacitance of 80 F g⁻¹ at 1 A g⁻¹ with a superior energy density of 10 W h kg⁻¹ in the gel electrolyte. This study provides a novel pathway, using a hydrophilic 1D CNF, for realizing the full potential of Co₃O₄ nanoparticles as advanced electrode materials for next generation flexible electronic devices.