Ostwald Ripening Improves Rate Capability of High Mass Loading Manganese Oxide for Supercapacitors

Realizing fast charging–discharging for high mass loading pseudocapacitive materials has been a great challenge in the field of supercapacitors because of the sluggish electron and ion migration kinetics through the thick electrode materials. Here we demonstrate for the first time a facile hydrothermal treatment that can substantially enhance the rate capability of a highly loaded manganese oxide electrode via the Ostwald ripening process. Hydrothermal treatment improves not only the electrical conductivity of manganese oxide but also the ion diffusion rate in the thick oxide film. At slow scan rates below 40 mV s–1, the capacitance of the hydrothermally treated manganese oxide electrode increases linearly with mass loading (up to 23.5 mg cm–2) as expected for a capacitor under the non-diffusion-limited conditions. At high scan rates beyond 100 mV s–1, capacitive saturation is observed only at a high mass loading of ∼9 mg cm–2, which is significantly greater than the values reported for other manganese oxide electrodes. The electrode achieves an areal capacitance of 618 mF cm–2 at a high scan rate of 200 mV s–1, which is 3 times greater than that of the untreated sample. An asymmetric supercapacitor assembled with a hydrothermally treated manganese oxide cathode and a vanadium oxide/exfoliated carbon cloth anode can deliver a good volumetric energy density of 5 mWh cm–3. This value is 2–10 times greater than the values obtained from supercapacitors with comparable dimensions.