Surface Functionalization of Hexagonal WO ₃ for Enhanced Pseudocapacitive Energy Storage

The development of sustainable and high‐performance electrochemical energy‐storage systems requires electrode materials capable of combining fast charge transfer, high power capability and long‐term stability. In this context, pseudocapacitive transition metal oxides have attracted increasing attention due to their ability to bridge the gap between conventional capacitors and batteries. Herein, a sustainable surface‐engineering strategy is proposed to enhance the pseudocapacitive response of hexagonal WO ₃ nanoparticles through in situ functionalization with N‐ and S‐containing species during a simple aqueous hydrothermal synthesis. The introduced surface functionalities improve the electrochemical performance without altering the bulk crystal structure or morphology of WO ₃ . In particular, the functionalized materials exhibit enhanced specific capacitance, improved coulombic efficiency, and remarkable cycling stability over 10,000 cycles, with S‐functionalized WO ₃ showing the best overall response. Spectroscopic analyses suggest that surface electronic modulation and more favorable interfacial charge–transfer processes contribute to the improved pseudocapacitive behavior. Moreover, the exclusive use of aqueous synthesis, water‐based electrode processing, and aqueous electrolytes provides a greener route toward electrode fabrication. These findings highlight surface functionalization as a promising and sustainable strategy for tuning the interfacial properties of WO ₃ ‐based electrodes for next‐generation energy‐storage technologies.