To achieve ion type selectivity, we utilized anion exchange membranes (AEMs) and cation exchange membranes (CEMs) to separate the acid and base sides, respectively. In this system, sodium cations and chloride anions serve as the major charge carriers. The main charge carrier concentration is almost identical across the liquid junctions of the two membranes, and thus the battery open circuit voltage is directly determined by the difference between the two electrode potentials. Although hydroxides and protons are mainly excluded by the CEM and AEM, respectively, the pH gradient that exists in the cell nevertheless causes undesired crossover of these ions. 20 A cross-sectional view of the ion concentration profile, and transport expected during operation of the cell is shown in Fig. . Most hydroxide ions are blocked by the CEM. However, small amounts of hydroxide ions can crossover and increase the center chamber pH before eventually exchanging through the AEM. The symmetric process can occur for protons. The crossover rate of protons and hydroxides can be described by Equations 1 and 2. [H + ]A, [H + ]C, and [H + ]B represent the proton concentrations in acidic, center, and basic chambers, respectively; their chemical compositions are assumed to be spatially uniform. Amem represents the membrane area, VA represents acidic chamber volume, and l AEM represents the thickness of AEM. P represents the permeability of ions though the membrane. M represents the ion-migration coefficient inside the membrane due to the applied electric field. j represents the signed ionic current, with direction pointing from acid to base chamber as positive.