Modeling the Effect of the Electrolyte on Standard Reduction Potentials of Polyoxometalates

The electrochemistry of transition metal oxide systems is gaining much interest in the context of energy storage. Yet, predicting the redox behavior of such systems remains very challenging for computational chemistry. In this work, we examined instead a computational strategy for related nano-sized molecular transition metal polyoxoanions, as such polyoxometalates (POMs) can be treated at manageable computational costs. As an example, we addressed the effects of an aqueous electrolyte at the atomic scale for estimating the standard reduction potentials Mn(IV/III) and Mn(III/II) of the tri-Mn-substituted W-based Keggin ion. The electrolyte model involves explicitly solvated Li⁺ counterions and accounts for the fluctuating aqueous medium, described in first-principles molecular dynamics simulations. After equilibration, the systems showed different local structures of the electrolyte around the POM, depending on the oxidation state of the Mn centers. These varying local structures affect the Mn reduction potentials differently for the redox couples under study. Hybrid DFT calculations yield rather accurate absolute redox potentials for Mn, in good agreement with experiment, i.e., within 0.1 eV. This is in strong contrast to analogous results from an implicit solvation model, where redox potentials were notably underestimated, whereas models with counterions added, but without explicit solvation, notably overestimated the redox potentials, by up to 1 eV. Only by taking into account the full atomistic structure of the multicomponent system, solute, and surrounding electrolyte is one able to estimate the electrochemical properties of nanostructured transition metal oxide systems with acceptable accuracy.