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

[Image: see text] 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(3)}(y) to absorb y electrons in aqueous solution, focusing mechanistically on the Wells–Dawson structure X(6)[P(2)W(18)O(62)], which comprises 18 metal centers and can uptake up to 18 electrons reversibly (y = 18) per cluster in aqueous solution when the countercations are lithium. 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(2) when the solution is diluted. Finally, we show that this understanding is transferrable to other salts of [P(5)W(30)O(110)](15–) and [P(8)W(48)O(184)](40–) anions, which can be charged to 23 and 27 electrons per cluster, respectively.