(Invited) Nonaqueous Redoxmer Flow Batteries: New Opportunities for Size-Exclusion Using Nanoporous Separators

Abstract

Nonaqueous redox flow batteries (NRFBs) are attractive devices for energy storage due to an increased potential window and the manifold possibilities they offer in terms of redox-active architectures and chemistries available for increasing energy density and battery performance. Our collaborations in the context of the Joint Center for Energy Storage Research (JCESR) have shown that pairing commercial nanoporous separators with redoxmer architectures[1] such as Redox-Active Polymers (RAPs),[2] Redox-Active Colloids (RACs),[3] and Redox-Active Oligomers (RAOs),[4] effectively prevent crossover of the active component within the flow system. The choice of redoxmer size and chemical makeup impact profoundly the electrochemical dynamics of the NRFB, and it is clear that optimal performance is only attained with a balanced pairing between redoxmer and separator membrane. In this presentation, I will discuss the design principles underlying size-exclusion NRFBs and some current directions on how nanoporous separators enable new strategies for long-term performance in the flow battery. I will highlight how the progression from high polymers to oligomers modifies the charge transfer characteristics of NRFB electrolytes and the implications of the dynamics of these redoxmer solutions on the size-exclusion approach.[5] Unlike small molecules, the electrochemical reactivity of solution-phase redoxmers is strongly modulated by the properties of both surface confined and freely diffusing species. Building from this knowledge, the recent introduction of Polymers of Intrinsic Microporosity (PIM) separators[4] into the NRFB scene area allows new opportunities in the design of redoxmers with improved charge transfer characteristics and diffusivity without sacrificing their size-exclusion performance. Finally, the use of a size-exclusion approach enables advanced concepts for long-term maintenance of a battery system. I will discuss recent results where controlled de-polymerization reactions are used for maintaining the state-of-health of NRFB components, highlighting the role of component separation using our strategy. Altogether, the combination of novel structural motifs and the use of new nanoporous materials, results in new directions to make better NRFBs that exploit the versatility of a new type of reacting redoxmer fluids for energy storage.