Design and development of an innovative barrier layer to mitigate crossover in vanadium redox flow batteries

Capacity loss induced by the undesired transport of vanadium ions across the ion-exchange membrane (i.e. crossover) is one of the most critical issues associated with vanadium redox flow batteries. This work reports on the manufacturing and testing of an innovative barrier layer to mitigate crossover. The barrier layer conceptual design is described in detail in the patent application WO 2019/197917. The barrier was deposited directly onto Nafion® 212 using the Reactive Spray Deposition Technology, in which carbon-rich particles (∼4–10 nm in diameter) formed in the flame were deposited simultaneously with a mixture of 1100EW Nafion® and Vulcan® XC-72R (∼40 nm diameter) that was sprayed from air-assisted secondary nozzles. During cycles at fixed capacity, the presence of the barrier layer significantly reduced battery self-discharge; the average variation of battery state of charge compared to a reference cell with Nafion® 115 was reduced from 21% to 7%. Moreover, battery energy efficiency was increased by nearly 5%, indicating that the barrier layer does not significantly hinder proton transport. During cycles at 50 mA cm⁻² with fixed cut-off voltages, the barrier layer exhibited stable operation, maintaining a coulombic efficiency around 99.4%. Additionally, the use of the barrier layer projects to a 30% reduction of stack-specific cost.