Non-Nernstian effects on MNC single-atom catalysts for Oxygen Reduction: A Grand Canonical Density Functional Theory Study

Metal single-atoms embedded in Nitrogen-doped graphene (so-called MNCs) are a promising group of electrocatalysts for the Oxygen Reduction Reaction (ORR). Previous theoretical calculations on these materials have simplified the influence of applied potential on electrocatalyst activity to extrapolation from neutral with a linear, Nernstian term. Here we utilize Grand-Canonical Density Functional Theory (GCDFT) to account for non-ideal charging behavior and evaluate reaction energies at applied potentials for 5 3d metal atoms (Mn/Fe/Co/Ni/Cu). We briefly consider the correspondence between Density of States and some features of grand canonical free energy curves. We also explore the effect of considering adsorbed *OO on predicted activity, as treatment of that state has been inconsistent in literature. We find significant deviations between Nernstian and non-Nernstian treatments, with Co having the greatest overpotential difference of 0.36 V. The stability of the *OO state results in significant overpotential differences for Fe and Co. We predict variance in the rate-determining step (RDS) for each metal, changing from *OH hydrogenation for Mn/Fe, to *OO hydrogenation for Co, and finally simultaneous O2 adsorption and hydrogenation for Ni/Cu. Co possessed the lowest theoretical overpotential. This work highlights the importance of explicit consideration of potential when modeling electrochemical systems.