Concluding, here we have developed a new on-line NMR spectroscopy approach for studying electrolyte crossover in operating redox-flow batteries. We found that 2,6-DHAQ crossover rates through Nafion depended on the charging current, and that they are doubled during a 50 mA constant-current charge compared to when the cell is at rest. This demonstrates that the transport mechanisms governing crossover vary throughout battery charging cycles and are not adequately captured by simple diffusional permeability studies that do not take the electrochemistry into account. Considering that the charging currents used in industry (~100 mA cm -2 ) are over an order of magnitude higher than those used here (2-10 mA cm -2 ), the impact of this additional transport on the total crossover rate in commercial redox-flow batteries is likely large. Importantly, our experiments were all carried out using a commercially available flow NMR tube and widespread standard solution-state NMR equipment. No longer bound by the equipment requirements of previous in situ NMR configurations, this from of rigorous RFB analysis could be performed routinely in most laboratories. Determining key parameters for each contribution to crossover