Enhancing Performance Efficiency in Mn-Fe Redox Flow Batteries Through Electrolyte Optimization

Among redox flow batteries for grid energy applications, Mn-Fe redox flow batteries are considered a promising option due to their low-cost materials. However, the unstable formation of Mn(III) during charging leads to the precipitation of MnO2, which is the primary factor limiting their performance. In this study, we employed three different Mn precursors for the catholyte in Mn-Fe redox flow batteries: MnSO₄, MnCl₂-4H₂O, and Mn(CH₃COO)₂ (Mn(Ac)₂). We propose Mn(Ac)₂ as a Mn precursor for the catholyte, as it demonstrated a distinct reaction compared to MnSO₄ and MnCl₂-4H₂O due to the coordination effect of Ac⁻. In the Mn(Ac)₂ reaction, the formation of MnO₂ differs significantly from that seen with MnSO₄ and FeCl₂, producing a highly reversible reaction with no Mn(III) disproportionation.
The effect of acids on electrolyte conductivity and stability was evaluated by studying different concentrations of isolated and mixed H₂SO₄ and HCl with the Mn precursors. The results showed that a mixture of H₂SO₄ and HCl more effectively inhibits electrolyte precipitation than either acid alone. To further understand the reaction mechanisms of the different catholytes, electrochemical impedance spectroscopy and cyclic voltammetry were performed. These studies help evaluate the charge transfer processes between the electrode and electrolyte, with impedance analysis particularly helping to predict the limiting steps for redox reactions.
Additionally, electrolyte mixing is a significant challenge in hybrid batteries, as it can disrupt electrolyte balance and degrade battery performance. Therefore, various membranes, including Nafion 212, Nafion 115, and FAP-450, are tested for their impact on electrolyte mixing. It is found that a thin membrane like Nafion 212 can result in maximum electrolyte imbalance due to more feasible electrolyte corssover. This research represents an important step toward realizing the full potential of Mn-Fe redox flow batteries.