Importantly, the periods of enhanced 2,6-DHAQ crossover correspond to the charging steps where state-of-charge is increasing (Figure ). These increases in crossover rate are most likely explained by additional transport driven by 2,6-DHAQ migration: During charging, electrons are supplied to the anolyte side of the cell, and in return K + ions migrate into the anolyte side to balance the charge. However, the charge balance can also be achieved by the movement of negative ions into the catholyte side. In the 2,6-DHAQ/ferrocyanide battery, such a mechanism would manifest as the 2,6-DHAQ crossover rates being positively correlated with charging current. Previous electrochemical and theoretical studies estimate that crossover is increased by this effect, though this has not yet been measured directly, and the contribution to crossover was predicted to be small at currents below 100 mA cm -2 . To explore the relationship between charging current and 2,6-DHAQ crossover, the crossover rate was measured (moles 2,6-DHAQ, per area of membrane, per hour) for each charging current used (Figure ). Note that as we now have a polarised system, Fick's law of diffusion law is no longer appropriate, and crossover rates are therefore used instead of permeabilities (see Methods