Enhancing Battery Performance through Solvation Structure Modulation of Iron–Chromium Electrolytes Using Guanidine Hydrochloride

Iron-chromium redox flow batteries (ICRFBs) are promising, cost-effective options for grid-scale energy storage, but the sluggish reaction kinetics in chromium ions continues to hinder their performance. In this study, guanidine hydrochloride is chosen as an additive for the negative electrolyte to enhance energy efficiency and suppress the hydrogen evolution reaction. First, the effect of the concentration of the additive is investigated, and the electrolyte with 0.4 M guanidine hydrochloride shows the lowest overpotential and achieves the highest energy efficiency of 80% at 55 °C, surpassing that of the pristine electrolyte by 6%. Next, the EIS and Tafel curves show that guanidine hydrochloride improves the reactivity of chromium ions. Finally, molecular dynamic simulation and UV–vis spectroscopy show that the existence of guanidine hydrochloride enhances the number of Cl– in the first hydration layer of chromium ions, which increases the activity of the chromium ion reduction reaction. Furthermore, in situ Raman spectroscopy shows that the hydrogen-bonding network of water near the electrode is broken, and the transfer of hydrogen ions is hindered when guanidine hydrochloride is added, which explains its mechanism of suppressing the hydrogen evolution reaction. As a result, guanidine hydrochloride enhances the reactivity of chromium ions and improves the performance of ICRFB, and the mechanisms discovered in this study provide a perspective for explaining the mechanisms of other similar additives.