For vanadium-based redox flow battery systems, vanadium crossover occurs through the ion exchange membrane, which results in self discharge and variations in electrolyte concentration. VO₂⁺ crossover rate through various Nafion membranes was measured directly with a hydrogen-vanadium reversible fuel cell. Measuring crossover of electrolyte species directly with a fuel cell vice an idealized dual-chamber system, allows for determining diffusivity under actual fuel cell testing conditions. The crossover rate in an actual fuel cell is expected to be different due to the migrational effect across the membrane, the reduction in the membrane area due to the solid phase of the electrodes, the compressive forces exerted on the membrane during fuel cell assembly, the effect of the membrane electrode assembly hot pressing technique on membrane thickness and surface chemistry, and the fuel cell operating conditions (i.e. electrolyte flow and pressure on each side of the membrane). This new in-situ technique for measuring crossover with an actual fuel cell is shown to be reliable and easy to use. Electrospun Nafion/PVDF single-fiber blend membranes were also fabricated and tested in the hydrogen-vanadium reversible fuel cell. The electrospun membranes led to lower crossover rates and high fuel cell performance.