Highly dispersive CoO nanoparticles 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 (CoO) nanoparticles. However, the low intrinsic electronic conductivity and aggregation of CoO 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 CoO 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 at 1 A g with remarkable cycling behavior (∼94% even after 5000 cycles) compared to that of pristine CNF and CoO electrodes in an aqueous electrolyte. Furthermore, the binder-free nature of 1D CoO@CNF (which was carbonized at 200 °C for about 20 min under a H/Ar atmosphere) shows great potential as a hybrid flexible paper-like electrode and provides a high specific capacitance of 80 F g at 1 A g with a superior energy density of 10 W h kg in the gel electrolyte. This study provides a novel pathway, using a hydrophilic 1D CNF, for realizing the full potential of CoO nanoparticles as advanced electrode materials for next generation flexible electronic devices.