Iron-Nickel Based Catalysts for Oxygen-Evolution Reaction: Mechanism, Stability, and Potential-Dependent Role of Iron

The oxygen-evolution reaction (OER) represents the principal kinetic bottleneck in electrochemical water splitting, necessitating catalysts that balance high activity with long-term stability. Nickel oxyhydroxide (NiO(OH)) is notable for its ultralow overpotential OER activity in alkaline media. Yet, the role of iron doping─a common strategy to enhance performance─introduces complex, potential-dependent behavior that remains poorly understood across operating conditions. Here, we systematically investigate Fe's potential-dependent behavior in NiO(OH) catalysts, revealing a bifunctional role. While trace Fe incorporation enhances OER currents at moderate overpotentials (200-400 mV) by improving conductivity and active site density, it suppresses activity at ultralow overpotentials (≈100 mV) by shifting the Ni(II)/Ni(III) redox transition to higher potentials, limiting charge accumulation. Fe undergoes progressive dissolution at extreme overpotentials (>500 mV), degrading the catalyst into a less active Fe-depleted phase. Through electrochemical and spectroscopic analyses, we demonstrate that Fe's inherent instability is not exclusively a detrimental property; rather, this dissolution can be harnessed as an electrochemical purification method for NiO(OH), whereby Fe impurities are eliminated, thereby restoring its intrinsic catalytic activity. These findings resolve longstanding ambiguities about Fe's role in Ni-based systems, offering a roadmap to optimize Fe content and operating windows for durable, high-efficiency water-splitting technologies.