Hierarchical WO3 nanostructures via hydrothermal strategy: optical bandgap tuning and high-rate electrochemical response for next-generation supercapacitors

Hexagonal tungsten trioxide (h-WO₃) is a noteworthy electrode material because of its tunnel-like lattice, multivalent tungsten states, and defect-mediated redox chemistry. This paper identifies hydrothermal duration as a significant factor in determining phase purity, oxygen vacancy density, and crystallite coherence, which directly influences electrochemical performance. By varying the synthesis time systematically (12–72 h), we discover a transition from disordered nanosheets (W12) to hierarchically organized nanosheet nanorod frameworks (W72), the latter of which displays optimal W⁶⁺/W⁵⁺ ratios, effective defect management, and improved electron–phonon coupling (16.6 eV). These enhancements from atomic to mesoscale are reflected in the performance metrics: the W72 electrode delivers 473.45 F g⁻¹ at 1 A g⁻¹, retaining 160.34 F g⁻¹ at 3 A g⁻¹, with minimal charge-transfer resistance (R_ct = 4.99 Ω) and low series resistance (R_s = 0.319 Ω). The asymmetric configuration (W72//AC) achieves an energy density of 15.3 Wh k g⁻¹ and a power density of 2461.5 W k g⁻¹, sustaining 82.3% capacitance retention after 1000 cycles and a quick relaxation time (τ = 0.602 s). This work demonstrates that hydrothermal time-engineering is a robust method for optimizing defects and crystallinity, positioning h-WO₃ as a scalable platform for high-rate, environmentally sustainable supercapacitors and hybrid energy storage systems.