Progress in the Design of Polyoxovanadate-Alkoxides as Charge Carriers for Nonaqueous Redox Flow Batteries

Abstract

ABSTRACTInnovation in the development of electrochemical energy storage methods is essential if these technologies are to meet the variable needs of the electrical grid. Nonaqueous redox flow batteries represent an underdeveloped area of research in energy storage—one which has seen a recent spike in interest owing to the potential for modular, energy-dense electrochemical energy conversion. Here, we summarize our recent work, focused on the design of polyoxovanadate-alkoxide clusters [V6O7(OR)12] as a new class of charge carrier for nonaqueous energy storage. The synthetic strategies we have employed, including homoleptic ligand substitution, selective ligand functionalization, and heterometal installation, demonstrate the flexibility of this hexametalate platform, and result in significant improvement of molecular properties with relevance to flow battery energy density. The identified homoleptic surface modifications (substituting R = CH3 for R = C2H5) to the polyoxovanadate-alkoxide scaffold yield an increase in stable operating voltage window (from 0.6 V to 1.8 V), as well as an increase in the stoichiometric number of electrons that can be stored at each battery electrode (from 1 to 2). Targeted functionalization at the cluster surface with an ether-based ligand affords [V6O7(OC2H5)9(OCH2)3CCH2OC2H4OCH3], which demonstrates a 12-fold increase in solubility over its homoleptic congener, from 0.05 to 0.60 M in acetonitrile. The substitution of a titanium center into the hexametalate core to generate [V5TiO6(OCH3)13]− further increases the voltage window to 2.3 V, as new heterometal-based redox events are introduced to the profile. Through these studies, we have gained valuable insights into the molecular parameters that determine the energy storage capabilities of multimetallic charge carriers. Collectively, our results highlight new opportunities for polynuclear charge carriers, and emphasize the critical role that synthetic inorganic chemistry plays in the development of effective nonaqueous redox flow battery technologies.