Redox Potentials of Polyoxometalates from an Implicit Solvent Model and QM/MM Molecular Dynamics

The ability to electrochemically store energy is crucial for the integration of intermittent renewable energy sources such as wind and solar power into modern energy grids. Among a wide variety of possible solutions, redox flow batteries (RFBs) are especially attractive as their energy content and power output can be scaled independently, offering a high degree of flexibility. For RFBs, polyoxometalates (POMs) are very appealing as these transition metal oxide nanoclusters exhibit the ability to store multiple electrons in a reversible manner. However, despite the interest in their properties, the link between the POM structure and its redox properties remains unclear. In this contribution, we study the redox potentials of [SiW12O40](4-) (SiW12) and [PV14O42](9-) (PV14) using a number of different theoretical methods. We first adopt the thermodynamic cycle approach combined with quantum chemistry and implicit solvation to estimate the redox potentials. Subsequently, we use molecular dynamics to facilitate an explicit description of the solvent environment. The implicit solvation model is semiquantitative, and problems arise when strong solute-solvent interactions are present. Using two approaches, thermodynamic integration and fractional number of electrons methods, we show that explicitly including the solvent environment can improve the calculated redox potentials for strong solute-solvent interactions and also gives important atomistic insights into its nature and how it changes upon reduction. Our results illustrate the performance of these approaches for addressing the challenging problem of simulating the redox potentials in POMs and provides the framework to develop a more detailed understanding of the structure-property relationships that exist.