Tailoring Phase-Dependent Energy Storage in Nonstoichiometric Tungsten Oxide Nanostructures for Enhanced Supercapacitor Performance.

The multivalent states of tungsten oxide offer multiple stoichiometric (WO3 and WO2) and nonstoichiometric WO x (2.72 ≤ x < 3) crystal structures. As x decreases in WO x , the reduced oxygen stoichiometry coincides with enhanced electronic conductivity and the emergence of structural channels within the unit cell. Despite these promising properties, most research has focused on WO3 due to the significant challenges in precisely synthesizing the various WO x phases. This study overcomes this limitation by employing a versatile resistive hot-wire oxidation (RHWO) method, enabling the precise engineering of tungsten-to-oxygen ratios and morphologies in WO x phases. Electrochemical characterization of these compounds indicates that energy storage performance is strongly influenced by crystal phase, morphology, and electrolyte cation size. In particular, clear performance distinction emerged between WO2.90 which shows enhanced capacitance (electric double-layer capacitance (EDLC)) due to fine nanostructures, whereas WO2.72 exhibits both capacitive and diffusion-controlled energy storage behavior, attributed to its larger open channels, higher carrier concentration, and larger concentration of W5+ and W4+ oxidation states. These findings highlight the correlation between crystal structure, morphology, redox behavior, and ionic transport in WO x , providing a foundation for advanced energy storage systems and an eco-friendly pathway for practical supercapacitor (SCs) applications.