Influence of iron doping on tetravalent nickel content in catalytic oxygen evolving films

Significance Iron-doped nickel oxide films are the most active nonnoble metal oxygen evolution reaction (OER) catalysts in alkaline electrolyte. Since Corrigan’s original discovery of enhanced activity with Fe doping in nickel oxides, the chemical basis for this synergy remains unclear. Recent studies suggest iron to assume a high valent oxidation state, thus promoting OER. We provide evidence for an alternative role of Fe 3+ as a Lewis acid in the host nickel oxide. We observe that Fe 3+ promotes the formation of Ni 4+ , which leads to enhanced catalytic activity. This result is consistent with Fe 3+ to be one of the strongest Lewis acidic metals by any measure of Lewis acidity, including hard–soft acid base theory, metal ion pK a s, and chemical inertness.