The capital costs of 4-h, 6-h and 10-h projects with different cell voltage were approximated at different acid-base crossover fluxes (Fig. ). The solid line shows the cost of the whole system with zero pH crossover. Higher cell voltage will increase the energy stored in a certain amount of active molecules. Higher OCV will also increase the cell power density and thereby decrease the required area for a cell stack. Therefore, the capital cost of the battery system drops when the cell voltage goes higher. Meanwhile, in a pH-decoupling cell, higher acid-base crossover will lead to higher energy cost for pH recovery and higher capital cost associated with a larger BPM sub-cell. For example, when the cell voltage is around 1.7 V, a crossover flux of 20 nmol s -1 cm -2 would increase the capital cost by around 3% (dashed line). When the net crossover reaches 100 (alternating dashed-dotted line) or 200 (dotted line) nmol s -1 cm -2 , capital cost increases by 15% and 35%, respectively. Acid-base crossover will be less impactful when using a cell stack with smaller area and higher current density. Because the required battery area for a certain power output decreases fast due to roughly an (ASR)*(OCV -2 ) dependence, an increased OCV will decrease the BPM cell costs and the energy cost of the BPM recovery sub-cell, making the costs of the BPM recovery sub-cell acceptable if the increased OCV more than offsets the increased ASR associated with an additional membrane and an additional electrolyte chamber. FeCl2 -BHPC high concentration cell (Fig. ) FeCl2 -BHPC high concentration cell was operated using 10 mL 2 M FeCl2, with HCl adjusting initial pH to 2, and 10 mL 1 M BHPC, with NaOH adjusting initial pH to 13. PEIS was used to measure three-chamber cell resistance. PEIS also shows slower kinetic of iron ions as posolyte (Fig. ). Cell was cycled with CCCV: 1.8 V charge cutoff, with voltage hold until 1 mA/cm 2 cutoff, and 1 V discharge cutoff, with voltage hold until 0.2 mA/cm 2 cutoff (Fig. ). pH recovery (indicated by black arrows) was operated by discharging the electrolyte in the BPM sub-cell for about 6 mAh after the second cycle. pH afterwards showed clear higher gradient, indicating successful pH recovery (Fig. ). The capacity fading was caused by fast iron ion crossover, forming precipitation in center chamber. This calls for further development of mild acidic posolyte that has low crossover, high voltage, low cost, low fade rate and high solubility.