Tetragonal Tungsten Oxide for Supercapacitor Electrodes: Study of Phase‐Driven Charge Storage Mechanism and Work Function Control

Abstract The crystal phase of pseudocapacitive materials significantly influences charge storage kinetics and capacitance; yet, the underlying mechanisms remain poorly understood. This study focuses on tungsten oxide (WO 3 ), a material exhibiting multiple crystal phases with potential for energy storage. Despite extensive research on WO 3 , the impact of different crystal structures on charge storage properties remains largely unexplored. Here, the successful synthesis and electrochemical characterization of tetragonal WO 3 are reported. This investigation demonstrates that tetragonal WO 3 exhibits superior energy storage capabilities compared to other WO 3 polymorphs. According to in situ Raman spectroscopy and ultraviolet photoelectron spectroscopy combined with in‐depth electrochemical analyses, this enhancement is attributed to a unique charge storage mechanism and an expanded potential window facilitated by an engineered electrode work function. This study highlights the critical role of the crystal phase in optimizing the performance of pseudocapacitive materials and provides valuable insights for the development of next‐generation energy storage devices.