Unlocking enhanced electrolyte performance in vanadium redox flow batteries through organic acid-based reduction

Organic acid is introduced as a novel, cost-effective reducing agent for vanadium electrolytes. • Citric acid enhances VO 2+ chemical stability with 98% concentration retention over 7 days. • Citric acid-based electrolytes exhibit improved electrochemical performance with 70% energy efficiency and 75.6% capacity retention over 200 cycles. • Citric acid reduces reagent costs by 60% at a low stoichiometric requirement (0.6:1 molar ratio). The preparation of vanadium electrolytes for vanadium redox flow batteries (VRFBs) remains a critical bottleneck for commercial deployment due to high costs, complex processes, and stability issues. This study presents a systematic comparison of citric acid, lactic acid, and traditional oxalic acid as reducing agents for the ambient-temperature reduction of V 2 O 5 to VO 2+ . Citric acid demonstrates superior performance with a 99% conversion rate at a 0.6:1 molar ratio, significantly outperforming lactic acid (59%) and oxalic acid (50%). The resulting electrolyte exhibits optimal physicochemical properties: conductivity of 323.5 mS cm –1 and viscosity of 3.16 mm 2 s –1 , attributed to citric acid's multidentate coordination structure. UV–Vis analysis confirms 98% VO 2+ concentration retention over 7 days, far exceeding oxalic acid (77%). Cyclic voltammetry reveals enhanced diffusion coefficients (1.85 × 10 –6 cm 2 s –1 ) and redox activity. In single-cell VRFB tests, the citric acid-based electrolyte achieves 70% energy efficiency and 75.6% capacity retention over 200 cycles, with coulombic efficiency consistently above 90%. Mechanistic studies via Raman spectroscopy demonstrate that citric acid’s three carboxyl and one hydroxyl groups form stable multidentate coordination with VO 2+ , inhibiting polymerization and precipitation. This work establishes a cost-effective, scalable protocol for high-performance VRFB electrolyte preparation. Download: Download high-res image (247KB) Download: Download full-size image