Disentangling Electronic and Geometric Effects in Ternary NiFeX Layered Double Hydroxide Catalysts for Oxygen...

Ternary NiFeX layered double hydroxide (LDH) electrocatalysts incorporating high-valence metals X from groups 5 and 6 have shown improved performance for the alkaline oxygen evolution reaction (OER), yet the molecular origin has remained speculative. Here, we synthesize a comprehensive series of phase-pure NiaFe1X1 LDHs (X = V, Nb, Ta, Cr, Mo, W) with controlled stoichiometry and unravel distinct catalytic enhancement mechanisms for different metals X. By combining impedance spectroscopy-based real ECSA determination, in situ high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS), and density functional theory (DFT) calculations, we show that the geometric activity improvements of NiFeMo and NiFeV LDH arise from fundamentally different mechanisms: increased electrochemically active surface area for Mo, and a previously overlooked electronic promotional effect for V. In situ HERFD-XAS reveals that V uniquely modifies the activation of the NiFe LDH host, shifting the Ni K-edge prepeak of the active γ-phase to lower energies. DFT calculations demonstrate that surface V5+ acts as an electronic modulator at the V5+-OH-Ni2+ bridge site, enabling a favorable Ni-based redox pathway and lowering the overpotential to 0.37 V─an improvement of 0.08 V over γ-NiFe LDH. Both catalysts were validated in 25 cm2 anion exchange membrane electrolyzer cells, confirming their promise for large-scale alkaline water electrolysis.