Energetics of Water Oxidation Catalyzed by Cobalt Oxide Nanoparticles: Assessing the Accuracy of DFT and DFT+U Approaches against Coupled Cluster Methods.

Some of the most promising catalysts for water oxidation rely on crystalline and amorphous cobalt oxide nanoparticles. Density functional theory (DFT) calculations are routinely used to study the electronic and atomic structures of these materials as well as the thermodynamics and mechanisms of the electrochemical oxygen evolution reaction. The accuracy of these theoretical predictions has never been compared to high-level quantum chemistry methods. We perform coupled cluster (CC) quantum chemistry calculations on model cobalt oxide surface sites and use them to benchmark the accuracy of the most popular exchange and correlation functionals. Hybrid B3LYP and PBE0 functionals lead to fair agreement with the CC energies, while standard gradient-corrected functionals show important discrepancies. The inclusion of on-site electronic repulsion (DFT+U) substantially improves the calculated electronic and structural properties, but no value of the U parameter reproduces the CC results. We discuss the implications of these findings for amorphous cobalt phosphate nanoparticles, showing that the reactivity of these catalysts is not altered by surface phosphate groups.