Colocalized Raman spectroscopy – scanning electrochemical microscopy investigation of redox flow battery dialkoxybenzene redoxmer degradation pathways

Non-aqueous redox flow batteries offer high voltages for grid-level energy storage technologies. However, decomposition of the redoxmers – redox-active molecules that make up the anolyte and catholyte in the negative and positive cell compartments – is an important challenge to overcome for long-term storage. Here, we present a spectroelectrochemical study of the catholyte candidate 2,3-dimethyl-1,4-dialkoxybenzene ( C7 ) and its decomposition mechanisms in the presence of a model nucleophilic base, pyridine. We utilize colocalized Raman microscopy and scanning electrochemical microscopy (Raman-SECM) to quantify the chemical rates of decomposition and qualitatively identify the reaction intermediates and products. A detailed study on how the Raman-SECM parameters (electrode distance, substrate electrode, and laser focal height) influence the Raman signal of the charged catholyte C7 •+ is presented. Using optimized conditions, we monitored the deprotonation of C7 •+ via Raman spectroscopy and the subsequent hydrogen abstraction from solvent molecules to regenerate C7 via electrochemistry. Finite element modeling was used to fit electrochemical and spectroscopic data, quantifying the deprotonation rates as kdep  = 2000 and 700 L mol −1 s − 1 and abstraction rates as kabs  = 0.5 and 0.2 s − 1 in propylene carbonate and acetonitrile solvents, respectively. Our results show the value of spatiotemporal resolution in evaluating the chemical and electrochemical behavior of materials for redox flow batteries.