Theory-driven design of high-valence metal sites for water oxidation confirmed using in situ soft X-ray absorption

The efficiency with which renewable fuels and feedstocks are synthesized from electrical sources is limited at present by the sluggish oxygen evolution reaction (OER) in pH-neutral media. We took the view that generating transition-metal sites with high valence at low applied bias should improve the activity of neutral OER catalysts. Here, using density functional theory, we find that the formation energy of desired Ni⁴⁺ sites is systematically modulated by incorporating judicious combinations of Co, Fe and non-metal P. We therefore synthesized NiCoFeP oxyhydroxides and probed their oxidation kinetics with in situ soft X-ray absorption spectroscopy (sXAS). In situ sXAS studies of neutral-pH OER catalysts indicate ready promotion of Ni⁴⁺ under low overpotential conditions. The NiCoFeP catalyst outperforms IrO₂ and retains its performance following 100 h of operation. We showcase NiCoFeP in a membrane-free CO₂ electroreduction system that achieves a 1.99 V cell voltage at 10 mA cm^-2, reducing CO₂ into CO and oxidizing H₂O to O₂ with a 64% electricity-to-chemical-fuel efficiency.