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.