Phase-Dependent Electrocatalytic Activity in Fe−Cu Alloys toward Efficient Alkaline Water Splitting

Elucidating how phase evolution and interfacial interactions govern catalytic activity is key to the rational design of efficient bimetallic electrocatalysts for alkaline water splitting. Here, we systematically investigate a series of FexCu100−x bimetallic alloys synthesized via high-energy ball milling to elucidate composition-dependent phase transitions and their impact on electrocatalytic performance. Structural analysis reveals a transition from single-phase FCC alloys at low Fe contents (x ≤50) to dual-phase FCC+BCC compositions (x = 55−65), followed by stabilization into a single-phase BCC structure at higher Fe fractions. This phase evolution is accompanied by distinct surface oxide transformations with Cu2O dominating Cu-rich alloys and Fe3O4 emerging prominently in phase-coexisting compositions together with enhanced oxygen vacancy concentrations and modified electronic states. The alloys exhibit a nanocrystalline and defect-rich microstructure, giving rise to pronounced interfacial effects as evidenced by exchange bias behavior in Fe25Cu75, indicative of coupling between Fe-rich metallic regions and oxide-derived magnetic phases. Benefiting from these synergistic structural and electronic features, the Fe25Cu75 supported on nickel foam delivers excellent HER activity, achieving 10 mA/cm2 at an overpotential of only 144 mV, whereas Fe65Cu35 exhibits superior OER performance with an overpotential of 300 mV at the same current density. A full alkaline water-splitting cell assembled with Fe25Cu75 (cathode) and Fe65Cu35 (anode) delivers a low cell voltage of 1.61 V at 10 mA/cm2. These results demonstrate that composition-driven phase evolution enables phase-selective catalytic functionality, providing general design guidelines for efficient bimetallic electrocatalysts.