Suppression of the hydrogen evolution reaction of Iron–chromium flow batteries by organic compounds containing the imidazole group

Iron–chromium redox flow batteries (ICRFBs) are attractive potential long-duration energy storage facilities because of their extensive sources and low cost. However, the hydrogen evolution reaction (HER) causes irreversible capacity loss and limits its application. Herein, we explore the influence of organic compounds containing imidazole groups, such as l-histidine (l-his) and 2-methylimidazole (2-mIm), on the performance of negative iron–chromium electrolytes. The results of molecular dynamics and density functional theory calculations revealed that both additives can interact with chromium ions to regulate the solvation shell and that the new complex structures have greater hydrogen evolution barriers. The electrochemical test results show that 2-mIm has a more significant influence than l-his dose. The findings of the single battery tests with two additives indicate that both additives improve the coulombic efficiency (CE) and average decay rate of ICRFB. The capacity decay rate of ICRFB with the electrolyte containing 0.2 M 2-mIm reached 1.79 %. Compared with that of the pure electrolyte, the capacity decay rate is reduced by 76 % in the 0.2 M 2-mIm electrolyte. It achieves a CE of 97.8 % at a current density of 100 mA·cm⁻². Furthermore, UV–Vis spectroscopy and long-cycle tests revealed that new complex structures are present and stable during battery operation. Finally, the in situ Raman results show that additives can reduce the amount of water and disrupt the hydrogen bond network around the surface of the electrode during energization. The improvement in the hydrogen evolution barrier and Raman results explain the mechanism by which the HER is suppressed. This study provides a feasible rationale for additive selection.