Electrolyte engineering beyond the conventional alkaline concentration for high-capacity retention of aqueous redox flow batteries

Electrolyte formulation significantly impacts the electrochemical performance of redox flow batteries (RFBs) in an alkaline medium. We demonstrate that the voltage, reversibility, and galvanostatic cyclic stability of RFBs improve with a potassium hydroxide (KOH) concentration beyond 1 M (pH 14). The study shows that a high KOH concentration is more critical for catholyte-potassium ferrocyanide (PF) than anolyte-Alizarin to maintain high capacity retention in full-cell, confirmed via symmetric (x M KOH-Alizarin || x M KOH-PF) and asymmetric (x M KOH-Alizarin || y M KOH-PF) KOH concentrations. Asymmetric high KOH concentration (4 M KOH-Alizarin || 3 M KOH-PF) compared to conventional symmetric (1 M KOH-Alizarin || 1 M KOH-PF) exhibits higher discharge capacity retention (∼ 32%) after 200 cycles at 100% state of charge (SOC) with ∼ 5% higher energy efficiency at 30°C. Furthermore, our cyclic voltammetry experiments reveal that the separation (ΔE) between cathodic and anodic peaks decreases significantly with high KOH concentration at the graphite felt compared to the glassy carbon electrode, enhancing the redox couple's reversibility, therefore emphasizing its crucial role in the RFBs.