Tuning Cobalt Electronic States to Engineer a Strong and Oriented Built-In Electric Field in NiFeOOH Heterojunctions for Oxygen Evolution and Water Splitting

Nickel iron oxyhydroxide (NiFeOOH) is a promising nonprecious oxygen evolution reaction (OER) electrocatalyst, but its activity is impeded by the high energy barriers for adsorbing key reaction intermediates. To overcome this, we constructed NiFeOOH/cobalt oxide heterojunctions to introduce a built-in electric field (BIEF). By precisely modulating the electron-donating ability of cobalt ions, we achieve directional control over the BIEF's strength and orientation, as demonstrated in two model systems: NiFeOOH/Co₃O₄ versus NiFeOOH/CuCo₂O₄, and NiFeOOH/CoO versus NiFeOOH/CuCoO. Comparative analysis reveals that cobalt ions with stronger electron-donating ability (in Co₃O₄ and CoO) generate a more intense and favorably oriented BIEF than their weaker counterparts (in CuCo₂O₄, CuCoO). This optimized BIEF effectively reduces the work function, narrows the bandgap, and increases the in situ generation of the highly active γ-NiFeOOH phase, thus lowering the OER barrier. Consequently, these heterojunctions achieve remarkably low overpotentials (279 mV for NiFeOOH/Co₃O₄ and 299 mV for NiFeOOH/CoO at 100 mA cm⁻²). In a full water-splitting cell, NiFeOOH/Co₃O₄ requires only 1.57 V to reach 50 mA cm⁻² and operates stably for over 100 h. This work establishes electronic-state-mediated BIEF engineering as a universal strategy for the design of efficient and durable water-splitting catalysts.