Deep Reconstruction of Ni(OH) ₂ via Magnetic‐Field Regulation of Fe(OH) ₃ Colloids for Efficient Oxygen Evolution Reaction

Trace Fe impurities are ubiquitous in alkaline water electrolysis and commonly regarded as beneficial promoters for activating Ni‐based oxygen evolution catalysts. However, the non‐monotonic dependence of catalytic activity on Fe 3+ concentration suggests a complex role beyond simple ionic promotion. Here, we show that Fe 3+ predominantly exists as negatively charged Fe(OH) 3 colloids instead of free cations in alkaline electrolytes. Their accumulation at the positively biased electrode induces interfacial heterogeneities that limit the depth and uniformity of Ni(OH) 2 pre‐catalyst reconstruction. We further demonstrate that an external magnetic field regulates the Fe(OH) 3 colloidal interface in a potential‐dependent manner. During the initial activation stage, the magnetic field perturbs interfacial water matrix and weakens intermolecular interactions, thereby promoting Fe(OH) 3 interfacial adsorption and Fe incorporation, which accelerates the early‐stage Ni(OH) 2 reconstruction. In the subsequent OER regime, the magnetic field sustains a dynamically restructured interface that enables thorough Ni oxidation. Consequently, the Ni(OH) 2 pre‐catalysts evolve into Fe‐rich and porous NiOOH structures with intrinsically higher catalytic activity, leading to markedly improved oxygen evolution performance. This work identifies Fe 3+ colloidal chemistry as a previously overlooked factor governing Ni‐based catalyst activation and establishes magnetic field modulation as an effective strategy to control interfacial reconstruction.