Redox Flow Batteries (RFBs) are inherently well suited for large-scale electrical-energy-storage (EES) applications [1]. RFBs are entering a period of renaissance, buoyed by both the increasing need for affordable long-duration EES solutions, as well as recent substantial advancements in cell performance that leverage state-of-the-art (SOA) flow-cell technologies, such as those originally developed for polymer-electrolyte fuel cells (PEFCs) [2, 3]. A good example of this approach has been the recent dramatic improvements in RFB power density, illustrated in Fig. 1. There are multiple opportunities for advanced RFB materials, especially cell-stack components and RFB active materials, since the remaining components of a RFB system are typically comprised of commercial off-the-shelf parts [2]. This talk will focus on the key requirements for advanced materials for SOA RFB cells, since high power density cells enable inherently lower cell-stack cost and size.
First-generation RFB chemistries have been based on single-element active materials dissolved in aqueous electrolytes. Next-generation RFB chemistries are likely to be engineered molecules or complexes. Both aqueous and non-aqueous options are being pursued because non-aqueous electrolytes enable a broader window of electrochemical stability, which is obviously advantageous from both an energy-density and cell-voltage perspective. However, non-aqueous electrolytes also have significant disadvantages, such as higher solvent costs, higher viscosities, and lower ionic conductivities. Detailed techno-economic analysis have recently made a quantitative assessment of these trade-offs [5, 6], and a brief summary of the key requirements for RFB active materials will be briefly summarized. In addition, the key requirement for some less conventional RFB systems will be discussed (e.g., mediated RFB systems with solid-phase storage [7]).
Most RFBs today utilize ion-exchange membranes (IEMs), similar to those used in PEFCs. IEMs provide high ionic conductivities, good selectivity for the transport of the desired charge carrier relative to the active materials, and good mechanical and chemical stability. However, IEMs are inherently expensive materials, especially fully-fluorinated IEMs, which are typically used in RFB cells since hydrocarbon-based IEMs are generally not sufficiently stable when exposed to the highly oxidative conditions present in the positive reactant solution (e.g., see [8]). SOA RFB cells employ relatively thin IEMs to reduce material cost and to enable higher ionic conductivities; however, ion selectivity is also key requirement, which is highly dependent on the type of RFB chemistry and what happens to active molecules at the counter electrode [9]. A fundamental understanding of the different causes of crossover in RFB cells (i.e., diffusion, migration, and electro-osmosis) under a various operating conditions [10], as well as how these are related to the physical properties of the separator and the active materials, is also required to intelligently develop alternative RFB separators. Transport-property requirements for RFB separators have been derived for both aqueous and non-aqueous RFB chemistries [9].
Many first-generation RFB chemistries are able to utilize simple carbon electrodes because simple redox reactions are facile and involve outer-sphere electrocatalysis. However, the fundamental reaction kinetics of even the relatively mature all-vanadium RFB is not well understood [11]. A major contributing factor to the complexity of RFB reactions on carbon electrodes is the fact that carbon is an extremely complex material. There are many types of carbons, and pretreatments of even the same carbon material can yield very different surface species, which can dramatically impact catalytic activity. Additionally, carbon itself is electrochemically active in the potential window of interest for most RFBs. Therefore, a better understanding of fundamental carbon properties, and stability in an electrochemical environment, is required for RFB cells that rely on carbon as a catalyst. A major conclusion of a recent review article on carbon materials in RFBs was that additional studies on degradation mechanisms are needed [12]. Catalyst materials for redox reactions, beyond carbon, also deserve more attention.
Acknowledgements
Thanks to the organizers of this Symposia for the invitation to present. The author is also grateful to many collaborators, especially at Vionx Energy and UTRC.
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