Characterizing the Impact of Oligomerization on Redox Flow Cell Performance

Abstract

Redox flow batteries (RFBs) are hindered by complex failure modes, particularly crossover through the membrane, resulting in capacity fade and reduced cycling efficiencies. Redox‐active oligomers (RAOs) have recently been proposed for mitigating this phenomenon while maintaining sufficient transport properties; however, to date, few studies have quantified how the chemical and electrochemical properties of RAOs influence their performance in redox flow cells. Here, we demonstrate that oligomeric derivatives of 2,2,6,6‐tetramethylpiperidine 1‐oxyl (TEMPO) exhibit lower diffusivities than the monomeric species but retain facile charge transfer characteristics. The size‐dependent variations in mass transport rates directly translate to differences in flow cell polarization and symmetric cycling performance. Post‐mortem analyses reveal that oligomerization does not meaningfully alter decay processes as evinced by similar capacity fade across all species. Broadly, these findings corroborate and extend upon previously developed relationships between molecular size, electrochemical properties, and flow cell performance.

Membrane–Electrolyte System Approach to Understanding Ionic Conductivity and Crossover in Alkaline Flow Cells

“…Redox flow batteries (RFBs) constitute a case in which this approach could be particularly significant. The all-vanadium redox flow battery, with the V 4+ /V 5+ redox couple in the posolyte and V 3+ /V 2+ in the negolyte, aqueous acidic supporting electrolyte, and typically a proton-conducting cation-exchange membrane, is presently the most developed RFB technology. , Water-soluble synthetic redox reactants, including organic molecules, − oligomers, , and polymers, as well as metalorganic − and inorganic complexes, − attract research and commercial attention to replace or supplement incumbent vanadium technology. The synthetic redox reactants offer tunable properties, potentially sustainable synthesis and feedstocks, and recently, extremely low capacity fade rates .…”