BPM sub-cell pH recovery was conducted in a 2.5 cm 2 flow battery with a BPM sandwiched in 2 layers of baked SGL-AA carbon paper. The sub-cell electrolyte inlets and outlets shared the electrolyte tank of the three-chamber cell. The sub-cell was connected to an independent potentiostat. During recovery, the cell cycling of the three-chamber cell was paused. Then we opened the valve and started the pump connecting sub-cell, with partially charged electrolytes (> 50% SOC) pumped through. Electrolytes are discharged in sub-cell using a constant current of 40 mA cm -2 for 3 -4 minutes to recover the pH gradient loss. Afterwards, electrolytes were pumped out of the sub-cell, with valve closed and three-chamber cell normal cycling restarted. BPM polarization of certain pairs of electrolytes were evaluated by LSV with a sweeping rate of 3 mV s-1, scanning from a voltage close to OCV to lower voltage.