Phase and Vacancy Modulation in Tungsten Oxide: Electrochemical Hydrogen Evolution

Abstract Defects in crystal structures often contribute to an increase in the number of catalytically active sites and, in turn, enhance the electrochemical performances. In the present work, we modulate the phase transition of tungsten oxide from hexagonal to the monoclinic by tuning annealing conditions in vacuum environment. This phase transition also accompanies the incorporation of oxygen vacancies in crystal structures. WO 3‐ x annealed at 550 °C (Vac‐550) shows a transition from heaxagonal to monoclinic with superior hydrogen evolution activity, exhibiting a Tafel slope of only 30 mV/dec, which is comparable that of commercial Pt/C. XRD, in situ TEM, EPR, and XPS analysis provide detailed insights into the structural transition as well as oxygen vacancy concentration of the annealed WO 3‐ x samples in a vacuum environment. This work paves the way towards a generalized methodology for enhancing electrochemical activity by the vacancy modulation in transition‐metal oxides.