Extending the cycle life of high mass loading MoOx electrode for supercapacitor applications

Highlights: • Electrochemical deposition of MoO_x with mass loading of 13.8 mg cm⁻². • 101% of initial capacitance over 30000 cycles within −1 to −0.4 V (vs. SCE). • Mo⁴⁺/Mo⁵⁺ redox couple worked within −1 to −0.4 V (vs. SCE) have good reversibility. • Mo⁶⁺ accumulates above −0.4 V (vs. SCE) resulting in capacitance decay. -- Abstract: Molybdenum oxides are promising pseudo-capacitive electrode materials, but their capacitance and cycling stability require improvement. Herein, we demonstrate a facile electrochemical method to fabricate a high mass loading (14 mg cm⁻²) MoO_x electrode. It provides a high areal capacitance of 3.3 F cm⁻² at 5 mA cm⁻² and maintains 1.9 F cm⁻² at the high current density of 100 mA cm⁻². The preferential selection of charge storage redox couples – realized via tuning the cycling potential – effectively promotes an ultra-long cycle life. In the optimal potential window of −1 V to −0.4 V, 101% of the initial capacitance is maintained after 30 000 cycles, in direct contrast to the −1 V to 0 V window which shows a fast degradation. X-ray photoelectron spectroscopy (XPS) reveals the good reversibility of the Mo⁴⁺/Mo⁵⁺ redox couple within −1 V to −0.4 V window, while Mo⁶⁺ accumulates above −0.4 V. Electrochemical impedance spectroscopy (EIS) suggests the poorer electric and more sluggish ion diffusion with the increase amount of Mo⁶⁺-components in electrodes, which is responsible for the poorer electrochemical activity and shorter cycle life. Our work demonstrates the utilization of desirable redox couples can effectively enhance the cycling stability of molybdenum oxide electrodes, which could also be applied to other pseudo-capacitive electrode materials.