Intrinsic Alkaline OER Activity in Mechanochemically Activated WO3 from Commercial and Single-Crystal Precursors

Abstract

This study uncovers the interplay between phase-induced interfaces and mechanochemical activation on the alkaline oxygen evolution reaction (OER) performance of tungsten oxide (WO₃). Commercial polycrystalline (WO₃-C) and synthesized single-crystal (WO₃-S) monoclinic WO₃ precursors were subjected to identical mechanochemical processing by planetary ball milling (PBM). Ball-milled single-crystal WO₃ (WO₃-S-PBM) yielded nanocrystalline powders (~20-50 nm) with enriched γ-monoclinic phase content (40% vs. 28% for WO₃-C-PBM), suppressed surface W⁵⁺ defects, and enhanced hydroxylation. Using Hansen solubility parameters, powders were formulated into stable inks to fabricate OER anodes on Ni substrates. Multi-technique characterization revealed that anodes made from WO₃-S-PBM resulted in continuous, crack-free coatings with uniform nanoscale roughness (5-fold lower variability) and enhanced wettability (initial contact angle 40° vs. 82°), establishing dense WO₃/Ni interfacial contacts that were not achievable for WO₃-C-PBM. Electrochemically, WO₃-S-PBM/Ni delivered a current density of 84.8 mA cm^-2 at 1.65 V vs. RHE with 328 mV overpotential a 3-fold enhancement over WO₃-C-PBM (375 mV) despite nearly identical active surface areas (<3% difference). Cyclic voltammetry demonstrates that this superior performance arises from phase-selective stabilization of the intrinsically more active γ-NiOOH (1.45 V vs. RHE) at uniform WO₃-S-PBM interfaces, whereas heterogeneous WO₃-C-PBM coatings permit mixed β/γ phase coexistence. These results establish the interplay of extrinsic mechanochemical activation with intrinsic phase and crystallinity as a critical, previously overlooked design parameter governing interfacial phase selectivity and OER activity in metal-oxide composite electrocatalysts.