Unraveling the coordination behavior and transformation mechanism of Cr3+ in Fe–Cr redox flow battery electrolytes

Currently, the iron chromium redox flow battery (ICRFB) has become a research hotspot in the energy storage field owing to its low cost and easily-scaled-up. However, the activity of electrolyte is still ambiguous due to its complicated solution environment. Herein, we performed a pioneering investigation on the coordination behavior and transformation mechanism of Cr3+ in electrolyte and prediction of impurity ions impact through quantum chemistry computations. Based on the structure and symmetry of electrostatic potential distribution, the activity of different Cr3+ complex ions is confirmed as [Cr(H2O)5Cl]2+ > [Cr(H2O)4Cl2]+ > [Cr(H2O)6]3+. The transformation mechanism between [Cr(H2O)6]3+ and [Cr(H2O)5Cl]2+ is revealed. We find the metal impurity ions (especially Mg2+) can exacerbate the electrolyte deactivation by reducing the transformation energy barrier from [Cr(H2O)5Cl]2+ (24.38 kcal mol−1) to [Cr(H2O)6]3+ (16.23 kcal mol−1). The solvent radial distribution and mean square displacement in different solvent environments are discussed and we conclude that the coordination configuration limits the diffusivity of Cr3+. This work provides new insights into the activity of electrolyte, laying a fundamental sense for the electrolyte in ICRFB.