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

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 WO_2.90 which shows enhanced capacitance (electric double-layer capacitance (EDLC)) due to fine nanostructures, whereas WO_2.72 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.