Oxygen evolution reaction: Bifunctional mechanism breaking the linear scaling relationship

The bifunctional mechanism for the oxygen evolution reaction (OER) involving two distinct reaction sites is studied through the computational hydrogen electrode method for a set of catalyst materials including rutile TiO2(110), anatase TiO2(101), SnO2(110), RuO2(110), IrO2(110), Ni2P(0001), and BiVO4(001). The calculations are performed both at the semilocal level and at the hybrid functional level. Moreover, anodic conditions are modeled and their effect on the OER free energy steps is evaluated. The free energies of the reaction steps indicate that for specific combinations of catalysts, the limitations due to the linear scaling relationship can be overcome, leading to smaller overpotentials for the overall OER. At the same time, a detailed analysis of the results reveals a strong dependence on the adopted functional. For both functionals, it is shown that the energy level of the highest occupied electronic state can serve as a descriptor to guide the search for the optimal catalyst acting as a hydrogen acceptor. These results support the bifunctional mechanism as a means to break the linear scaling relationship and to further reduce the overpotential of the OER. I. INTRODUCTION