Effective Storage of Electrons in Water by the Formation of Highly Reduced Polyoxometalate Clusters.

Aqueous solutions of polyoxometalates (POMs) have been shown to have potential as high-capacity energy storage materials due to their potential for multi-electron redox processes, yet the mechanism of reduction and practical limits are currently unknown. Herein, we explore the mechanism of multi-electron redox processes that allow the highly reduced POM clusters of the form {MO} to absorb electrons in aqueous solution, focusing mechanistically on the Wells-Dawson structure X[PWO], which comprises 18 metal centers and can uptake up to 18 electrons reversibly ( = 18) per cluster in aqueous solution when the countercations are . This unconventional redox activity is rationalized by density functional theory, molecular dynamics simulations, UV-vis, electron paramagnetic resonance spectroscopy, and small-angle X-ray scattering spectra. These data point to a new phenomenon showing that cluster protonation and aggregation allow the formation of highly electron-rich meta-stable systems in aqueous solution, which produce H when the solution is diluted. Finally, we show that this understanding is transferrable to other salts of [PWO] and [PWO] anions, which can be charged to 23 and 27 electrons per cluster, respectively.