Tailoring the performance of ZnO for oxygen evolution by effective transition metal doping

In the quest for active and inexpensive (photo)electrocatalysts, atomistic simulations of the oxygen evolution reaction (OER) are essential for understanding the catalytic process of water splitting at solid surfaces. In this paper, we study the enhancement of the OER by first-row transition-metal (TM) doping of the abundant semiconductor ZnO, using density functional theory (DFT) calculations on a substantial number of possible structures and bonding geometries. The calculated overpotential for undoped ZnO is 1.0 V. For TM dopants in the 3d series from Mn to Ni, the overpotentials decrease from 0.9 V for Mn, and 0.6 V for Fe, down to 0.4 V for Co, and rise again to 0.5 V for Ni and 0.8 eV for Cu. We analyze the overpotentials in terms of the binding to the surface of the species involved in the four reaction steps of the OER. The Gibbs free energies associated with the adsorption of these intermediate species increase down the series from Mn to Zn, but the difference between OH and OOH adsorption (the species involved in the first, respectively the third reaction step) is always in the range 3.0-3.3 eV, despite a considerable variation in possible bonding geometries. The bonding of the O intermediate species (involved in the second reaction step), which is optimal for Co, and to a somewhat lesser extend for Ni, then ultimately determines the overpotential. These results imply that both Co and Ni are promising dopants for increasing the activity of ZnO-based anodes for the OER.