Tri-chamber Polysulfide/Iodide-Based Redox Flow Batteries with Improved Coulombic Efficiency

The polysulfide/iodide flow battery is a promising candidate for large-scale energy storage systems, but the technology suffers from a notably low Coulombic efficiency. This phenomenon arises because the crossover of hydroxide ions triggers the disproportionation reaction of I₃^- ions in the posolyte. To mitigate these issues, a middle chamber filled with Na₂SO₄ solution is created between two cation-exchange membranes to separate the posolyte and negolyte chambers of the PSIFB, thereby constructing a trichamber reactor. This middle chamber can serve as a reservoir for the cross-transportation OH^- ions during charge-discharge processes, thereby minimizing their adverse effects on the posolyte. Experimental results reveal that the introduction of the middle chamber leads to a 97.3% reduction in the concentration of OH^- ions within the posolyte. Furthermore, finite element analysis demonstrates that the trichamber configuration significantly reduces the flux of OH^- ions entering the posolyte, achieving levels even lower than half of those in the dual-chamber structure. This innovative architecture markedly enhances the Coulombic efficiency from approximately 78% in the dual-chamber structure to around 91% in the trichamber structure with a high-concentration electrolyte (4 M NaI and 3 M Na₂S₂), while concurrently sustaining a highly competitive energy density of 20.05 W h L_{posolyte+negolyte}^-1 at 75% state-of-charge. These findings demonstrate an effective strategy for improving the performance of the polysulfide/iodide flow battery through OH^- management, showing potential for developing more efficient flow battery systems for renewable energy storage applications.