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.