Influence of Fe and Ni Doping on the OER Performance at the Co<sub>3</sub>O<sub>4</sub>(001) Surface: Insights from DFT+<i>U</i>Calculations
Yuman Peng, Hamidreza Hajiyani, Rossitza Pentcheva
Abstract
Using density functional theory calculations with an on-site Hubbard U term, we study the oxygen evolution reaction (OER) at the Co3O4(001) surface. The stability of different surface terminations as a function of oxygen partial pressure as well as pH and applied voltage is compiled in a Pourbaix diagram. The termination with octahedral Co and O ions (B-layer) is found to have the lowest overpotential of 0.46 V for an octahedral Co reaction site, associated with its p-type conducting character and the higher oxidation state of the active site (+4) during OER. Furthermore, we systematically investigated the effect of Fe and Ni doping on the overpotential. Our results indicate that Ni doping at an octahedral site in the surface layer reduces the overpotential from 0.46 to 0.34 V. Likewise, Fe doping at an octahedral site at the tetrahedral Co termination (A-layer) lowers η from 0.63 to 0.37 V with octahedral Co remaining in the active site. We note that the potential determining step changes from *OH (B-layer) to *OOH formation (A-layer). While implicit solvation increases the overpotential by 0.2 V (B-layer) and 0.4 V (A-layer), which is attributed to enhanced binding energies of the intermediates, the general trends with respect to doping remain unchanged. The scaling relationship of the binding energies of *OOH and *OH is overall satisfied, with the doped systems lying close to the top of the volcano plot of the overpotential versus (ΔG*Ob – ΔG*OHb). A further insight into the origin of this behavior is gained by analyzing the changes in oxidation states of surface ions and, in particular, the Co active site during OER.