Investigation of crossover processes in a unitized bidirectional vanadium/air redox flow battery

• Crossover in a VARFB is investigated using ICP-MS and in situ UV/Vis spectroscopy.
• Coulombic efficiency of a VARFB is lowered by crossover (mainly O2 permeation).
• Vanadium crossover which limits the cycle life of VARFB is quantified.
• Diffusion coefficients of V2+ and V3+ through Nafion® 117 are determined.

In this paper the losses in coulombic efficiency are investigated for a vanadium/air redox flow battery (VARFB) comprising a two-layered positive electrode. Ultraviolet/visible (UV/Vis) spectroscopy is used to monitor the concentrations cV2+cV2+ and cV3+cV3+ during operation. The most likely cause for the largest part of the coulombic losses is the permeation of oxygen from the positive to the negative electrode followed by an oxidation of V2+ to V3+. The total vanadium crossover is followed by inductively coupled plasma mass spectroscopy (ICP-MS) analysis of the positive electrolyte after one VARFB cycle. During one cycle 6% of the vanadium species initially present in the negative electrolyte are transferred to the positive electrolyte, which can account at most for 20% of the coulombic losses. The diffusion coefficients of V2+ and V3+ through Nafion® 117 are determined as DV2+,N117=9.05·10−6 cm2 min−1 and DV3+,N117=4.35·10−6 cm2 min−1 and are used to calculate vanadium crossover due to diffusion which allows differentiation between vanadium crossover due to diffusion and migration/electroosmotic convection. In order to optimize coulombic efficiency of VARFB, membranes need to be designed with reduced oxygen permeation and vanadium crossover.