Boosting Electrochemistry of Manganese Oxide Nanosheets by Ostwald Ripening during Reduction for Fiber Electrochemical Energy Storage Device

The poor electronic conductivity of MnOx severely limits the practical application as high-performance electrode materials for faradaic pseudocapacitors. Herein, a facile vapor reduction method is demonstrated for the treatment of MnOx with hydrazine hydrate (HH) to improve the electronic conductivity. The HH vapor treatment without annealing process not only introduces oxygen vacancies to form oxygen-deficient MnOx, but also leads to obvious structural transformation from highly aggregated and poorly crystallized MnOx nanorobs and nanoparticles into uniformly orientated and highly crystallized MnOx nanosheets via the Ostwald ripening process. Compared with pristine MnOx on carbon fiber (CF-MnOx), the reduced CF-MnOx exhibits a highly improved specific capacitance of 1130 mF cm–1 (434 F g–1) with excellent rate capability and cycling stability. Our results have shown that the moderate concentration of oxygen vacancies and highly uniform orientation of reduced MnOx endow the electrode with a fast electron and ion transport, respectively. Moreover, a flexible fiber asymmetric supercapacitor (ASC) device with high-energy and power density based on the as-prepared reduced CF-MnOx as a cathode and electrochemically activated graphene oxide on carbon fiber (CF-ArGO) as an anode is fabricated. The MnOx//ArGO ASC device delivers a high volumetric capacitance of 1.9 F cm–3, a maximum energy density of 1.06 mWh cm–3, and a volumetric power density of 371.3 mW cm–3. The present work opens a new way for oxygen vacancy introduction and structural modification of metal oxide as high-performance materials for energy storage applications.