Statistical benchmarking of chloride and sulphate posolytes for all-iron redox flow batteries

• A 2 2 factorial design with 3 replicates was used. • The electrolyte type and acidity strongly influence Fe 3+ |Fe 2+ electrochemical behavior. • The electrolyte type is the dominant factor in charge-transfer kinetics. • Acid concentration primarily controls mass transfer and charge transport. • Acidic chloride electrolyte provides the best overall performance as AIRFB posolyte. All-iron Redox Flow Batteries (AIRFBs) are promising large-scale energy storage systems due to their low cost and chemical symmetry. However, their performance is largely limited by the slow kinetics and instability of the Fe 3+ |Fe 2+ redox couple in the posolyte. A good electrolyte for AIRFBs must combine fast and reversible charge-transfer kinetics, high selectivity toward the Fe 3+ |Fe 2+ reaction, high ionic conductivity, and efficient mass transport. This study systematically investigates how the electrolyte type (chloride and sulphate) and acidity affect the key electrochemical parameters of the Fe 3+ |Fe 2+ redox couple, namely the standard heterogeneous rate constant ( k 0 ), the charge transfer coefficient ( α ), the diffusion coefficient ( D ), and the conductivity (Λ). A multifactorial experimental design was employed, and an ANalysis Of VAriance (ANOVA) was used to identify the statistically significant effects of both factors and their interaction on the aforementioned electrochemical parameters. The ANOVA results show a statistically significant difference in the charge-transfer process between chloride- and sulphate-based electrolytes: the electrolyte type primarily controls electron-transfer kinetics, with chloride medium markedly enhancing k 0 ; while acid concentration mainly affects D and Λ. Among the tested formulations, 1 M HCl exhibited the best compromise between Fe 3+ |Fe 2+ kinetics, mass transport, and an adequate stability window. These insights provide a quantitative basis for electrolyte optimization and support the rational design of improved AIRFB posolytes.