Polymer Ion-Sieving Membranes Enable High-Performance Aqueous Organic Redox Flow Batteries

Redox flow battery (RFB) technology is promising for grid-scale long-duration energy storage owing to its decoupled power and energy, long discharge duration and long cycle life. In recent years, aqueous organic RFBs based on organic redox-active molecules have attracted significant interest owing to their potential low cost compared to conventional vanadium RFBs. The widespread adoption of aqueous organic RFBs will require low-cost ion-selective membranes that could replace expensive Nafion membranes while achieving high ionic conductivity and selectivity towards redox-active materials. As yet, inexpensive ion-selective membranes are still lacking. Despite the pressing need for high-performance membranes, it remains a significant scientific challenge to develop alternative cost-effective materials and to achieve precise control over their water channels at a sub-nanometre scale for selective ionic and molecular transport.<br/>Here, I will discuss our efforts to develop next-generation ion-selective membranes comprising sub-nanometer pores through modular synthesis of polymers of intrinsic microporosity (PIMs), whose structural diversity is controlled by choice of monomer, polymerization reaction and post-synthetic modification. The interconnected sub-nanometer pores in these functionalized PIMs allow fast, selective transport of ionic and molecular species, overcoming the conventional upper bounds of ion permeability and selectivity found in all existing membrane materials. These PIM membranes enable significant improvements in the efficiency, power density and lifetime of redox flow batteries over several thousand charge-discharge cycles in lab-scale demonstrations.