Therefore, with hydroxide crossover into posolyte, dimerized complex would accumulate and cannot be discharged. We added HCl solution to reverse the dimerization, as shown in Fig. . Each time after adding acid, we achieve a few cycles of higher Coulombic efficiency, contributed by dimer discharging. The capacity will go back to initial value with just enough acid provided. According to the pH measurement, any time when pH reached 4, any hydroxide crossing in the Fe complex chamber will cause dimerization. Precipitation formed is shown in Fig. . And this fading can always be recovered by adding acid. When pH for Fe(Bhmbpy)3 side is higher than 2, the center chamber salt solution (neutral pH) can remain colorless, indicating slow crossover (Fig. ). Thus, for optimized operation, the upper bound of the pH for Fe(Bhmbpy)3 is 4. Extra ligands as pH buffer (Fig. )