(6he) Design Principles and Performance Metrics for Realizing Cost-Effective Electrochemical Technologies for Energy Storage

The widespread integration of locally-available intermittent renewable electricity generators, such as solar photovoltaic and wind turbines, necessitates advances in the science and engineering of energy storage devices for cost-effective deployment.  Redox flow batteries (RFBs), sometime referred to as regenerative fuel cells, have shown a favorable balance of cost, safety, and performance for meeting this storage need.  RFBs are rechargeable electrochemical devices that utilize the reversible redox reactions of two soluble electroactive species for energy storage.  Aqueous RFBs have been the subject of the vast majority of research effort to date.  Shifting from aqueous to non-aqueous electrolytes may promise a higher cell voltage due to the extended electrochemically stable window (~ 4 V) and an enriched selection of redox materials due to the broader variety of organic solvents.  A key RFB component is the separator which is used in keep the positive and negative electrolytes from mixing while selectively enabling working ion transfer between the two fluids to maintain electroneutrality.  The ideal membrane should have negligible electronic conductivity, high ionic conductivity, good chemical stability and mechanical strength, and low active species crossover.  However, to date, no membrane has been identified that simultaneously fulfills all of these requirements, especially high ionic conductivity and low crossover rate.  In fact, most aqueous and non-aqueous RFBs adopt ion-selective Nafion^® membranes,  originally designed for chlor-alkali production or size-selective separators designed for lithium-ion batteries.  To date, few membranes have been specifically designed for RFB operation and performance requirements for this application have yet to be established.  Therefore, these effort seeks to define the performance metrics of separators for both aqueous and non-aqueous RFBs using stringent cost targets established by the US Department of Energy.  The metrics are based on a detailed analysis of the electrochemical and transport phenomena within RFBs and the impact of rechargeable operation.  Building on these benchmarks, design principles for high-performance separators are determined by a combination of experimental and computational analysis.  Of particular interest is the behavior of state-of-the-art aqueous separators under non-aqueous conditions.  The results from these studies provide a framework for understanding and optimizing membranes and electrolytes for not only RFBs but also a range of electrochemical technologies and processes of societal and industrial relevance (e.g., fuel cells, electrolyzers).