A zoomed-in view of the voltage and pH profile for a few cycles is shown in Fig. . During cell cycling, the pH of the negolyte oscillated due to proton-coupled electron transfer of BHPC. We attribute the oscillating pH of the posolyte to the reversible dimerization side-reaction of the oxidized Fe(Bhmbpy)3. The posolyte pH oscillates between 2 and 5, representing a small change in proton concentration (10 -2 mol L -1 ). Repeated addition of exogenous acid and base into the reservoirs to maintain the pH difference is not viable in the long term. Over months of cell operation, dissolved salts in the cell will accumulate through the net acid-base crossover reaction: HCl + NaOH = NaCl + H2O. Because these salts are difficult to remove, they will eventually reach saturation and precipitate, leading to cell failure. To avoid this trajectory, we used a BPM sub-cell to recover the lost pH difference by dissociating water into protons and hydroxide ions during discharging of the electrolytes. In this way, no exogenous materials are introduced into the system.