Three-chamber two-membrane cell cycling was conducted in a 5 cm 2 flow battery, using 10 mL 0.1 M Fe(Bhmbpy)3 as capacity limiting side, pairing with 20 mL 0.1 M negolyte, if not clarified. Electrolytes were pumped using a flow rate of 60 mL/min. 20 mL 1.3 M NaCl was cycled as center chamber electrolyte. Center solution was pumped using 10 mL/min. Three pH sensors were inserted in posolyte, negolyte and center solution, respectively. pH reading out was collected by an Arduino during cell operation and manually calibrated after. The cell was operated with a constant current (20 mA cm -2 ) followed by constant potentials of 2 V (charging) and 0.7 V (discharging). During charging, the potential was held until the current density dropped lower than 1 mA cm -2 . During discharge, the current density cutoff was 0.2 mA cm -2 to ensure full discharge of the iron complex and its dimers. During operation, we would manually add 0.1 mL 1 M HCl and NaOH to compensate the acid-base crossover when the posolyte pH reaches 5. Or in pH recovery experiments, we would conduct BPM sub-cell pH recovery.