Understanding the Enhancement of the Catalytic Properties of Goethite by Transition Metal Doping: Critical Role of O* Formation Energy Relative to OH* and OOH*

Goethite (α-FeOOH), thermodynamically the most stable phase among various iron (oxy)hydroxides, is getting attention as an oxygen evolution reaction (OER) catalyst due to its terrestrial abundance. But goethite suffers from an inferior catalytic activity like other iron-based oxides. To enhance its catalytic performance, doping has been applied universally. However, due to the lack of a systematic approach to doping, the choice of dopant element has been carried out without standards. Herein, we provide a comprehensive study on a critical factor to evaluate the activity of an introduced dopant at the goethite surface based on both theoretical investigation and experimental verification. For the pristine goethite, the most dominant surface for OER is determined. To enhance the catalytic property of pristine goethite, transition metals (TM = Cr, Mn, Co, and Ni) are substituted with the surface layer iron atom, and substituted dopants are all confirmed to be the active site of OER. The Co-doped goethite has the oxygen-adsorbed state (O*) formation energy near the optimal value. Lowered overpotential in doped goethite mainly originates from the O* formation energy, which is proportional to the occupied p-band center of adsorbed oxygen. To verify the calculation result, pristine and TM-doped goethite is synthesized. The measured overpotential value has the same tendency as the calculated overpotential value.