Electrical and Structural Dual Function of Oxygen Vacancies for Promoting Electrochemical Capacitance in Tungsten Oxide

Intrinsic defects, including oxygen vacancies, can efficiently modify the electrochemical performance of metal oxides. There is, however, a limited understanding of how vacancies influence charge storage properties. Here, using tungsten oxide as a model system, an extensive study of the effects of structure, electrical properties, and charge storage properties of oxygen vacancies is carried out using both experimental and computational techniques. The results provide direct evidence that oxygen vacancies increase the interlayer spacing in the oxide, which suppress the structural pulverization of the material during electrolyte ion insertion and removal in prolonged stability tests. Specifically, no capacitive decay is detected after 30 000 cycles. The medium states and charge storage mechanism of oxygen-deficient tungsten oxide throughout electrochemical charging/discharging processes is studied. The enhanced rate capability of the oxygen-deficient WO_{3- x} is attributed to improved charge storage kinetics in the bulk material. The WO_{3- x} electrode exhibits the highest capacitance in reported tungsten-oxide based electrodes with comparable mass loadings. The capability to improve electrochemical capacitance performance of redox-active materials is expected to open up new opportunities for ultrafast supercapacitive electrodes.