While an understanding of the local concentrations and environments within the BPM provides valuable information regarding the phenomena occurring within it, knowledge of the fluxes of the various ionic species in these membranes is just as crucial. The primary charge carrier within an ion-exchange membrane dictates its conductivity, and understanding charge transport through the BPM is key to mitigating detrimental phenomena such as salt crossover. Plotting the effective transference numbers, defined here as the fraction of current carried by a given species, of all ions as a function of applied potential throughout the BPM (Figure ; Figure -32) reveals the nature of charge transport within the BPM operated with 1 M KHCO3. At low applied potentials (0 V and 0.2 V), the current density is primarily due to co-ion crossover (Figure ) (K + crossing from the anolyte to catholyte or HCO3 -crossing from the catholyte to anolyte), with K + the dominant carrier of current, due to its higher diffusion coefficient in the BPM. As the applied potential is increased and electric-field-enhanced water dissociation occurs, H + rapidly becomes the primary charge carrier in the CEL (Figure ). For the AEL, on the other hand, CO3 2-becomes the primary charge carrier at moderate potentials (0.4 -0.7 V), and HCO3 -possesses a negative effective transference number (Figure ). This flux behavior is consistent with the interpretation that HCO3 -transports as a counter-ion through the AEL to the CL, where it is either directly dissociated by the large electric field or reacts with waterdissociation-generated OH -to form CO3 2-that can then transports directly out of the CL and serves as the major carrier of charge in the AEL. At higher potentials (> 0.7 V), where HCO3 -is depleted, OH -becomes the primary charge carrier in the AEL (Figure ). Interestingly, H + and OH -do not immediately become the primary carrier of charge in the BPM when water dissociation begins at ~0.5 V, and there appears to be a penetration depth moving out from the CL through which the fronts of H + and OH -flux dominate. This penetration depth is dependent on the current density and is due to out-of-equilibrium buffer reactions that convert the generated OH -or H + to alternative charge carrying ions within the BPM domain. At large enough current densities, the buffering species within the BPM are depleted, and H + and OH -become the dominant charge carriers throughout the entirety of the BPM. This flux analysis was also carried out for the simulated seawater case and similar trends were observed (Figures ). However, due to the small concentration of reactive carbon in the seawater, OH -and H + much more rapidly become the primary charge carriers in the BPM.