Redox Flow Battery Membranes: Improving Battery Performance by Leveraging Structure–Property Relationships

Membranes are a critical component of redox flow batteries (RFBs), and their major purpose is to keep the redox-active species in the two half cells separate and allow the passage of charge-balancing ions. Despite significant performance enhancements in RFB membranes, further developments are still needed that holistically consider conductivity, selectivity, stability, sustainability, and cost. In this Focus Review, structure–property relationships that have led to advances in membranes for various RFB types (vanadium, zinc, iron, etc.) are analyzed. First, two strategies to increase conductivity are highlighted: tuning membrane microstructure and controlling electrolyte uptake. Next, selectivity improvements through size and/or Donnan exclusion are reviewed. With respect to stability, methods to enhance the mechanical robustness of membranes and factors that affect chemical stability are discussed. Additionally, avenues to reduce battery cost and increase sustainability are explored. Future directions are suggested, which include how more in-depth theoretical studies, microstructure optimization, and enhanced characterization will push the field of RFB membranes forward.