Charge-Dependent Crossover in Aqueous Organic Redox Flow Batteries Revealed Using On-Line NMR Spectroscopy

Aqueous organic redox-flow batteries (AORFBs) are promising candidates for the low-cost grid-level energy storage. However, their widescale deployment is limited by crossover of redox-active material through the separator membrane, which causes capacity decay. Traditional membrane permeability measurements do not capture all contributions to crossover in working batteries, including migration and changes to ion size and charge. Here we present a new method for characterising crossover in operating AORFBs, using on-line 1H NMR spectroscopy. By introducing a separate pump to decouple NMR and battery flow rates, this method opens a route to quantitative time-resolved monitoring of redox-flow batteries under real operating conditions. In this proof-of-concept study of a 2,6-dihydroxyantharquinone(2,6-DHAQ)/ferrocyanide model system, we observed a doubling of 2,6-DHAQ crossover during battery charging, which we attribute to migration effects. This new membrane testing methodology will advance our understanding of crossover and accelerate the development of improved redox-flow batteries.