Abstract
Non-aqueous redox flow batteries have emerged as promising systems for large-capacity, reversible energy storage, capable of meeting the variable demands of the electrical grid. Here, we investigate the potential for a series of Lindqvist polyoxovanadate-alkoxide (POV-alkoxide) clusters, [V6O7(OR)12] (R = CH3, C2H5), to serve as the electroactive species for a symmetric, non-aqueous redox flow battery. We demonstrate that the physical and electrochemical properties of these POV-alkoxides make them suitable for applications in redox flow batteries, as well as the ability for ligand modification at the bridging alkoxide moieties to yield significant improvements in cluster stability during charge-discharge cycling. Indeed, the metal-oxide core remains intact upon deep charge-discharge cycling, enabling extremely high coulombic efficiencies (∼97%) with minimal overpotential losses (∼0.3 V). Furthermore, the bulky POV-alkoxide demonstrates significant resistance to deleterious crossover, which will lead to improved lifetime and efficiency in a redox flow battery.
Highlights
Growing global energy demands drive the need for technologies that can address energy storage at the grid and microgrid scale, thereby enabling the incorporation of distributed renewable resources such as solar and wind.[1]
Unlike secondary batteries that contain solid-phase electrolytes and migratory ions as charge carriers, RFBs take advantage of electrolyte solutions typically consisting of a solvent, a supporting electrolyte, and an electroactive species that can cycle in its redox states.[1,2,3]
To serve as the electroactive species in an energy storage device, the molecules in various redox states must be highly soluble in organic media and simultaneously stable with respect to chemical decomposition, membrane crossover, and selfdischarge
Summary
Whereas the methoxide-functionalized polyoxovanadate cluster results in only modest energy storage potential, owing to its oxidative instability that limits the practical Vcell, molecular modi cation to the ethoxide derivative provides access to the full multi-electron redox chemistry of the Lindqvist core. The ability for 1-V6O7(OMe)[12] to undergo both oxidative and reductive processes enables its use in a symmetric RFB scheme, wherein a single molecule serves as both catholyte and anolyte.[44,45] The wide separation between the outermost redox events (DE 1⁄4 1.6 V), coupled with the ability of the cluster to hold four electrons, frames this POV-alkoxide as an effective charge carrier for ow batteries, provided that all redox states remain soluble and stable on times scales of relevance to electrochemical energy storage.
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