Molecular Engineering of Polyoxovanadate-Alkoxide Clusters and Microporous Polymer Membranes to Prevent Crossover in Redox-Flow Batteries

Abstract

The ongoing development of redox-active charge carriers for nonaqueous redox-flow batteries has led to energy-dense storage concepts and chemistries with high cell voltages. However, rarely are these candidates for flowable energy storage evaluated in tandem with cell separators compatible with organic solvent, limiting progress in the identification of suitable charge carrier–separator pairings. This is important, as the efficiency of a redox-flow battery is dictated by extent of active species crossover through a separator, dividing the two cells, and the contribution of the separator to cell resistance. Here, we report the size-dependent crossover behavior of a series of redox-active vanadium­(III) acetoacetonate, and two polyoxovanadate-alkoxide clusters, [V6O7(OR)12] (R = CH3, C5H11) through separators derived from polymers of intrinsic microporosity (PIMs). We find that highly efficacious active-material blocking requires both increasing the size of the vanadium species and restricting pore swelling of the PIMs in nonaqueous electrolyte. Notably, increasing the size of the vanadium species does not significantly affect its redox reversibility, and reducing swelling decreases the conductivity of the separator by only 50%. By pairing polyoxometalate clusters with PIM membranes in nonaqueous redox-flow batteries, more efficient systems may well be within reach.

Suppressing Crossover in Nonaqueous Redox Flow Batteries with Polyethylene-Based Anion-Exchange Membranes

“…The first involves the use of a porous size-exclusion separator, such as Daramic and Celgard, and high molecular weight redox-active species (oligomers, polymers, and colloids), allowing for transport selectivity based solely upon steric bulk of electrolytes. This approach has been explored in the literature, both from a molecular and a membrane perspective, and has proven effective at mitigating crossover. − However, augmenting the bulk of the electrolytes decreases their intrinsic capacity and can introduce other drawbacks such as increased synthetic rigor and system complexity, decreased solubility, and slower free diffusion in solution. The second strategy is to employ permanently charged redox-active small organic molecules (SOMs) as anolyte and catholyte species paired with an ion-exchange membrane to avoid crossover through Coulombic repulsion (Donnan exclusion) .…”

Decaniobate: The Fruit Fly of Niobium Polyoxometalate Chemistry

“…Applications exploiting rich redox behavior, supramolecular assembly, interactions with biomolecules, and POMs as inorganic ligands were summarized in a 1998 Chemical Reviews special issue . Currently, these applications continue to grow, with a focus on emergent societal issues of clean energy production, , energy storage, − and health sciences . Concurrently in the 20th century, Nb/Ta POM chemistry was largely limited to the Lindqvist ion (Nb 6 ) and the substitution of Nb into W-POM clusters.…”

On the Characterization of Membrane Transport Phenomena and Ion Exchange Capacity for Non-Aqueous Redox Flow Batteries

“…Some research is still ongoing on how to limit performance losses. 12,13 Non-aqueous redox flow batteries.-In pursuit of systems with higher energy and power densities, non-aqueous redox flow batteries (NAqRFBs) have become attractive. Although NAqRFBs must overcome some challenges related to their low ionic-conductivity and toxicity, organic active molecules in non-aqueous electrolytes offer a vast range of design possibilities.…”