Current-Driven Vanadium Crossover as a Function of SOC and SOD in the Vanadium Redox Flow Battery

The performance of vanadium redox flow batteries (VRFBs) is impacted by the diffusion and migration of the vanadium species across the separator. In this work, the vanadium crossover as a function of current density for vanadium-containing electrolytes of various state of charge (SOC) and state of discharge (SOD) is measured. Experiments conducted with electrolytes at complete charge/discharge yielded direct measurements of the transference numbers of the vanadium species. The transference numbers of V2+, V3+, VO2+, and VO2+ were estimated to be 0.064 0.002, 0.087 0.003, 0.068 0.003, and 0.018 0.002, respectively. Experiments conducted with electrolytes at intermediate states of charge/discharge yielded direct measurements of the sum of transport numbers of the vanadium species. The transport number estimates are quantitatively related to faradaic efficiency loss and capacity fade of a working VRFB.