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Non-aqueous redox flow batteries constitute a promising solution for grid-scale energy storage due to the ability to achieve larger cell voltages than can be readily accessed in water.

 

Emergent, flowable electrochemical energy storage technologies suitable for grid-scale applications are often limited by sluggish electron transfer kinetics that impede overall energy conversion efficiencies. To improve our understanding of these kinetic limitations in heterometallic charge carriers, we study the role of solvent in influencing the rates of heterogeneous electron transfer, demonstrating its impact on the kinetics of di-titanium substituted polyoxovanadate-alkoxide cluster, [Ti 2 V 4 O 5 (OMe) 14 ]. Our studies also illustrate that the one electron reduction and oxidation processes exhibit characteristically different rates, suggesting that different mechanisms of electron transfer are operative. We report that a 1 : 4 v/v mixture of propylene carbonate and acetonitrile can lead to a three-fold increase in the rate of electron transfer for one electron oxidation, and a two-fold increase in the one electron reduction process as compared to pure acetonitrile. We attribute this behavior to solvent–solvent interactions that lead to a deviation from ideal solution behavior. Coulombic efficiencies ≥90% are maintained in MeCN–PC mixtures over 20 charge/discharge cycles, greater than the efficiencies that are obtained for individual solvents. The results provide insight into the role of solvent in improving the rate of charge transfer and paves a way to systematically tune solvent composition to yield faster electron transfer kinetics. 

Redox flow batteries are attractive for grid-scale energy storage, but ongoing work on materials discovery is hampered by the difficulty of measuring electron-transfer rates under battery-relevant conditions. We have developed an experimental approach for collecting continuous voltammetric measurements of flow battery electrolytes by placing a 3-electrode cell containing an ultramicroelectrode into the flow loop of a functioning redox flow battery. We further developed an empirical approach for extracting electron-transfer rate constants from each voltammetric cycle, thereby enabling continuous measurements as a function of state of charge and cycle time. Benchmarking these approaches with iron-based aqueous flow battery electrolytes using platinum and carbon fiber ultramicroelectrodes yielded rate constants that varied in the order Pt > electrochemically oxidized carbon > pristine carbon, in good agreement with prior work. We also found that Pt electrodes become more catalytically active upon cycling for several hours, whereas carbon fiber electrodes with and without oxidative pretreatments remained stable over the same interval. We expect these experimental approaches can be used to measure kinetics and other figures of merit for most electrodes and electrolytes of interest for redox flow batteries as well as in other systems where it is useful to evaluate the properties of a flowing electrolyte in real time. 

The selective uptake of lithium ions is of great interest for chemists and engineers because of the numerous uses of this element for energy storage and other applications. However, increasing demand requires improved strategies for the extraction of this element from mixtures containing high concentrations of alkaline impurities. Here, we study solution phase interactions of lithium, sodium, and potassium cations with polyoxovanadate-alkoxide clusters, [V 6 O 7 (OR) 12 ] (R = CH 3 , C 3 H 7 , C 5 H 11 ), using square wave voltammetry and cyclic voltammetry. In all cases, the most reducing event of the cluster shifts anodically as the ionic radius of the cation decreases, indicating increased stability of the reduced cluster and further suggesting that these assemblies might be useful for the selective uptake of Li + . Exploring the consequence of ligand length, we found that the short-chain cluster, [V 6 O 7 (OCH 3 ) 12 ], irreversibly binds Li + in the presence of excess potassium (K + ) and exhibits an electrochemical response in titration experiments similar to that observed upon the addition of Li + to the POV–alkoxide in the presence of non-coordinating tetrabutylammonium ions. However, in the presence of excess sodium (Na + ), the cluster showed only a modest preference for lithium, with exchange between sodium and lithium ions governed by equilibrium. Extending these studies to [V 6 O 7 (OC 5 H 11 ) 12 ], we found that the presence of the pentyl ligands allows the assembly to irreversibly bind Li + in the presence of Na + or K + . The change in mechanism caused by surface functionalization of the clusters increases the differential binding affinity for more compact cations, translating to improved selectivity for Li + uptake in these molecular assemblies.