High Energy Density, Asymmetric, Nonaqueous Redox Flow Batteries without a Supporting Electrolyte

Identifying structure-function relationships to modulate crossover in nonaqueous redox flow batteries

Nonaqueous redox flow batteries (NARFBs) offer a promising solution for large-scale storage of renewable energy. However, crossover of redox active molecules between the two sides of the cell is a major factor limiting their development, as most selective separators are designed for deployment in water, rather than organic solvents. This report describes a systematic investigation of the crossover rates of redox active organic molecules through an anion exchange separator under RFB-relevant non-aqueous conditions (in acetonitrile/KPF6) using a combination of experimental and computational methods. A structurally diverse set of neutral and cationic molecules was selected, and their rates of crossover were determined experimentally with the organic solvent-compatible anion exchange separator Fumasep FAP-375-PP. The resulting data were then fit to various descriptors of molecular size, charge, and hydrophobicity (overall charge, solution diffusion coefficient, globularity, dynamic volume, dynamic surface area, clogP). This analysis resulted in multiple statistical models of crossover rates for this separator. These models were then used to predict tether groups that dramatically slow the crossover of small organic molecules in this system.

A Physical Organic Chemistry Approach to Developing Cyclopropenium-Based Energy Storage Materials for Redox Flow Batteries

ConspectusRedox flow batteries (RFBs) represent a promising modality for electrical energy storage. In these systems, energy is stored via paired redox reactions of molecules on opposite sides of an electrochemical cell. Thus, a central objective for the field is to design molecules with the optimal combination of properties to serve as energy storage materials in RFBs. The ideal molecules should undergo reversible redox reactions at relatively high potentials (for the molecule that is oxidized during battery charging, called the catholyte) or low potentials (for the species that is reduced during battery charging, called the anolyte). Furthermore, anolytes and catholytes must be highly soluble in the electrolyte solution and stable to extended electrochemical cycling in all battery-relevant redox states. The ideal candidates would undergo more than one reversible electron transfer event. Finally, the optimal structures should be resistant to crossover through a selective separator in order to maintain isolation of the two sides of the cell. This Account describes our design and optimization of organic molecules for this application. We first provide background for the metrics and experiments used to characterize anolytes/catholytes and to progress them toward deployment in flow batteries. We then use our studies of aminocyclopropenium-based catholytes to illustrate this workflow and approach.We identified tris(dimethylamino) cyclopropenium hexafluorophosphate as a first-generation catholyte for nonaqueous RFBs based on literature reports from the 1970s describing its reversible chemical and electrochemical oxidation. Cyclic voltammetry and electrochemical cycling experiments in acetonitrile/LiPF₆ confirmed that this molecule undergoes oxidation at relatively high potential (0.86 V versus ferrocene/ferrocenium) and exhibits moderate stability toward charge-discharge cycling. Replacing the methyl groups with isopropyl substituents led to enhanced cycling stability but poor solubility of the radical dication (<0.1 M in acetonitrile). Solubility was optimized using quantitative structure-property relationship modeling, which predicted derivatives with ≥10-fold enhanced solubility. Cyclopropeniums with 300-500 mV higher redox potentials were identified by replacing one of the dialkylamino substituents with a less electron-donating thioalkyl or aryl group. Multielectron catholytes were developed by creating hybrid structures that contain a di(amino) cyclopropenium conjugated with a phenothiazine moeity. Finally, oligomeric tris(amino) cyclopropeniums were designed as crossover resistant catholytes. Optimization of their solubility enabled the deployment of these oligomers in high concentration asymmetric redox flow batteries with energy densities that are comparable to the state-of-the-art commercial aqueous inorganic systems.