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

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)(2)V) and polysulfide (PS), and three catholytes, the nitroxyl radical 4-hydroxy-TEMPO (TEMPO-OH), ammonium-substituted ferrocene ([FcN(2)](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 wave-lengths 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 in-sights 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.

Automated summary

This study reports the spectroelectrochemistry of five active electrolyte species from different classes of compounds in aqueous redox flow batteries. UV-vis absorbance spectra were used to monitor the redox transformations of the electrolytes during cyclic voltammetry and chronoamperometry experiments. The findings provide insights into the performance and degradation of the electrolyte chemistry in redox flow batteries.