For all electrolytes tested (0.5 M KHCO3, 1 M KHCO3, and simulated seawater), the coulombic efficiency initially is 0% due to the dominance of salt-crossover at low current densities. Nonetheless, the efficiency rapidly increases to ~80% for all electrolytes due to the onset of the water dissociation reaction. Crucially, water dissociation in all electrolytes rapidly reaches ~99% efficiency (Figure ), consistent with prior study of the Fumasep BPM, meaning that deviations coulombic efficiencies lower than 99% are due to inefficiencies in conversion of the generated H + . For the simulated seawater BPM, the coulombic efficiency monotonically decreases as current density increases. The rapid decrease of coulombic efficiency for simulated seawater is due to complete consumption of the meager 0.00211 M HCO3 -in the catholyte (i.e., the CO2 regeneration is HCO3 -limited). For the simulated seawater, once all the HCO3 -in the catholyte diffusion boundary layer is consumed, the H + flux from the BPM exits the boundary layer unreacted, acidifying the bulk electrolyte. The impact of unreacted H + can be seen due to the effective transference number of H + in the catholyte reaching 1 at high current densities (Figure ). It is important to note that the coulombic efficiency defined herein is the coulombic efficiency of H + liberating CO2 within the diffusion boundary layer. Unreacted H + could theoretically react with further dissolved inorganic carbon to liberate greater amounts of CO2 by feeding more electrolyte downstream. However, in this work, we seek to consider just the fluxes in the direct vicinity of the BPM to explore the effects of mass transfer at the micrometer scale.