Polyoxometalates have been identified as a class of nano-sized compounds that normally are formed by linking transition metal polyhedra through oxygen centers at their vertices. These compounds exist in a number of sizes, compositions, and shapes. They are stable over a range of pH values as well as temperatures, and capable of undergoing reversible redox processes. Polyoxometalates have been referred to as molecular electron sponges owing to their ability to undergo multiple redox reactions depending on the number and the nature of the involved metal centers. Polyoxometalates can also be employed in electrocatalysis such as hydrogen evolution reactions. In addition, they have been suggested as building blocks for a cathode material in high-performance rechargeable batteries.
Polyoxometalates have recently been proposed for large-scale energy storage, specifically in redox flow batteries. For this application, researchers have focused on substituted Keggin ions with two types of metals, whose redox potentials are separated by up to a few Volts. For example, vanadium-exchanged tungsten-based Keggin ions yield an aqueous battery with a cell voltage of 0.8 V, although with very low current densities. Inspired by these achievements a different substituted polyoxometalate, a tri-Mn-substituted W-based Keggin ion, has been introduced and evaluated electrochemically.
The latter experimental work motivated a team of researchers, led by Professor Notker Rösch, at the Technical University of Munich (TUM), to examine computationally (with density functional methods) the reversible half-wave redox potentials for two types of redox reactions: the cation-coupled electron transfer in the case of manganese (III/II) and the proton-coupled electron transfer for the case of manganese (IV/III). Their research work is published in Electrochimica Acta. These calculations are able to reproduce the experimentally determined dependency on pH of the manganese (IV/III) potential.
Taking into account the high charge of bare polyoxometalates, up to -7e, an adequate description of the electrolyte environment is crucial. Following the experiments, the computational research team introduced lithium counterions that were necessary for charge neutralization, and embedded such neutral quantum models in a polarizable continuum. As a further approximation, the team considered a number of lithium ion arrangements around the polyoxometalates as well as their impact on structural aspects and electrochemical attributes of the polyoxometalates.
With that model approach, hybrid density functionals overestimated the experimental reduction potentials. It was observed that the larger the exact-exchange contribution, the larger the obtained reduction potential. The best agreement with the experiment was obtained with the (non-hybrid) PBE method, but that finding likely has to be attributed to a fortuitous error cancellation.
The outcome of their computational study suggests that a more sophisticated representation of the electrolyte environment is necessary for predicting redox potentials of polyoxometalates in better agreement with experiment. “Likely, a more elaborate representation of the electrolyte structure, as provided by an ensemble of snapshots from molecular dynamics simulations, will be required to describe the screening of the electrostatic potential around a highly charged polyoxometalate due to the (at least partially) solvated counterions,” Professor Rösch remarked. His team is currently carrying out such first-principles molecular dynamics simulations for tri-Mn-substituted W-based Keggin polyoxoanions for various oxidation states of the Mn centers surrounded by water molecules and with various concentrations of Li cations. The authors are planning to report on these simulations in the near future.
Alena Kremleva, Pablo A. Aparicio, Alexander Genest, and Notker Rösch. Quantum chemical modeling of tri-Mn-substituted W-based Keggin polyoxoanions. Electrochimica Acta, volume 231 (2017), pages 659–669.Go To Electrochimica Acta