The redox potential of Fe(Bhmbpy)3 is pH-independent, but that of BHPC, which undergoes proton-coupled electron transfer, is pH-dependent. Their redox potentials at different pH values are indicated by solid lines in Fig. . Operational conditions for Fe(Bhmbpy)3 and BHPC were carefully optimized through extensive evaluation (Fig. ). In our optimized AFRB, Fe(Bhmbpy)3 was employed at pH 2 -4 and BHPC at pH 13. Furthermore, we chose Fe(Bhmbpy)3 to be the capacity-limiting side because of its higher fade rate (Fig. ). Fig. and 3b show the temporal evolution of the pH of the reservoirs and the cell capacity and Coulombic efficiency, measured for a test cell in which the negolyte contained 0.1 M Na2BHPC at pH 13 (20 mL), while the posolyte contained 0.1 M Fe(Bhmbpy)3Cl2 at pH 3 (10 mL). 1.0 M NaCl was used as a supporting electrolyte on both sides, with added NaOH or HCl to adjust the pH. 20 mL 1.3 M NaCl was cycled in the middle chamber. An E-620K CEM separated the negolyte from the middle chamber and a DSV-N AEM separated the posolyte from the middle chamber. This cell was cycled with the standard constant-current followed by constant-voltage (CCCV) protocol at 20 mA cm -2 with voltage cut-off values of 2.0 V and 0.7 V during charge and discharge, respectively, held until the current density dropped below a current cutoff value. The cell accessed >95% of its theoretical capacity and exhibited an average Coulombic efficiency of 99%, presumably limited by the self-discharging of the iron-based posolyte. Based on the measured crossover fluxes, we estimated that, for the operating pH and cell area, replenishment of the acid and base levels of the electrolyte reservoirs should be performed every 2-3 days to maintain the pH difference within the cell for long-term operation. Starting with the simple approach of restoring the initial pH difference through the simultaneous addition of acid and base, 0.1 mL of 1 M HCl was added to the posolyte side and 0.1 mL of 1 M NaOH was added to the negolyte side when pH had drifted significantly from its initial value. The amount and time of adding exogeneous acid-base can be estimated according to the net acid-base crossover result. These addition events are apparent from the discontinuities in the pH data shown in Fig. . The cell was cycled over 18 days, and high Coulombic efficiency and a low capacity fade rate were observed (Fig. ). The voltage-capacity profiles of this battery during three representative cycles are shown in Fig. , demonstrating its excellent stability. The plateau near 0.8 V during discharge has been identified as the reversal of a dimerization side-reaction of Fe(Bhmbpy)3. Because the Coulombic efficiency (CE) of the system is limited by the posolyte side, the cell drifted from perfect charge balance toward reduction of both sides over roughly 13 days (Fig. , day 13).