Spectroelectrochemistry of next-generation redox flow battery electrolytes: A survey of active species from four representative classes

Abstract

The spectroelectrochemistry of five active electrolyte species from four representative classes of compounds used in aqueous, near-neutral pH redox flow batteries (RFBs) is reported. The electrolytes include two anolytes, di-sulfonatopropyl viologen ((SPr)2V) and polysulfide (PS), and three catholytes, the nitroxyl radical 4-hydroxy-TEMPO (TEMPO-OH), ammonium-substituted ferrocene ([FcN2]2+), and iron 2,2′-tris-bipyridine ([Fe(bpy)3]2+). Time-resolved UV–vis absorbance spectra monitor redox transformations of these species in situ during cyclic voltammetry and chronoamperometry experiments. Molar attenuation coefficients at peak wavelengths are reported for both redox forms, representing the charged and discharged species in RFBs. The method is validated using the well-studied ferr-cyanide redox couple, also with relevance to RFBs. Anomalous and as-yet unreported observations on the reduced viologen species implicate formation of a pimer complex and new insights are presented on aqueous polysulfide speciation, both with the aid of time-dependent density functional theory (TD-DFT) calculations. The findings are applicable fundamentally – most of the charged-form spectral data have not been reported – as well as practically, for the in and ex situ monitoring of electrolyte chemistry to track performance, state of charge, and degradation in redox flow batteries.