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

The poor electronic conductivity of MnO x 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 MnO x 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 MnO x, but also leads to obvious structural transformation from highly aggregated and poorly crystallized MnO x nanorobs and nanoparticles into uniformly orientated and highly crystallized MnO x nanosheets via the Ostwald ripening process. Compared with pristine MnO x on carbon fiber (CF-MnO x), the reduced CF-MnO x 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 MnO x 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-MnO x as a cathode and electrochemically activated graphene oxide on carbon fiber (CF-ArGO) as an anode is fabricated. The MnO x//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.