Activating the Highly Reversible Mo4+/Mo5+ Redox Couple in Amorphous Molybdenum Oxide for High-Performance Supercapacitors

Molybdenum oxide provides high theoretical capacity, but the moderate charge transfer kinetics and cycle life have limited the applications for capacitive energy storage. Herein, the electrochemical performance of molybdenum oxide is modified by tuning the active redox couple through an electrochemical activation process. The activated electrode K-MoO_x with a 10 mg cm^-2 loading shows excellent pseudocapacitive behavior within the large potential range of -1.2-0 V and delivers 313 F g^-1 capacitance at 5 mA cm^-2. It also presents an ultrastable cycle life, with a 98% capacitance retention after 10,000 cycles. The activation process involves the insertion of K⁺ into MoO_x, which modifies the Mo electronic structure and introduces Mo⁴⁺ sites according to X-ray photoelectron spectroscopy. As a result, the charge storage redox couple shifts from Mo⁵⁺/Mo⁶⁺ to Mo⁴⁺/Mo⁵⁺, with the latter delivering higher electrochemical activity due to improved conductivity. Electrochemical impedance spectroscopy also suggests faster ion diffusions and thus higher power capability in K-MoO_x, resulting in the enhanced performance. A 2.4 V asymmetric supercapacitor is assembled using K-MoO_x as the anode with a MnO_x cathode. The work demonstrates a feasible and facile strategy to promote the pseudocapacitive behavior of metal oxide materials.