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.