Toward Explicit Solvation for Simulations of Electrocatalytic Reactions: AIMD for p<i>K</i>_a and Redox Potentials of Transition Metal Compounds and Catalyst Models

In this work, we study the possibility to extend electronic structure simulations for electrocatalysis by explicit solvation models. In previous work, we proposed a simulation scheme that explicitly includes the effects of pH and electrochemical potential in density functional theory (DFT) simulations with implicit solvation. Based on calculations of protonation and oxidation reactions, the pH and electrochemical potential can be included given appropriate reference values. In this work, we compute the p<i>K</i>_a values and oxidation potentials for a series of transition metal aquo complexes and compare the results including implicit, explicit static and explicit dynamic (AIMD) models for the aqueous solvent and compare vs experimental p<i>K</i>_a and redox potential data. This allows the construction of a p<i>K</i>_a/redox potential scale that can in principle be extrapolated to the simulation of other transition metal-based materials. An explicit dynamic solvent model is then proposed and applied to a model system for iridium oxide-based catalysts for the oxygen evolution reaction. We outline the advantages and disadvantages of the different approaches and demonstrate that, at the expense of a larger computational effort, the microsolvation environment of a given model can be described in a robust way using a limited amount of solvent molecules and AIMD. Especially for reactions in which water is solvent and reactant like the oxygen evolution reaction (OER) or oxygen reduction reaction (ORR), this model provides a more detailed and complete description that can be exploited in mechanistic studies.