This effect may vary near the terminal electrodes in a BPM-ED stack depending on the consumption of protons and hydroxides by the redox reactions. For the 0.5 M and 1 M KHCO3 electrolytes, the coulombic efficiency similarly decreases after achieving a maximum of ~80%. However, instead of being HCO3 -limited, the reduction in coulombic efficiency at intermediate current densities is limited by the consumption of H + by CO3 2-to form HCO3 -(Figures S38-S39, Section S17). Once all of the CO3 2-in the catholyte has been consumed, there is an inflection point in the coulombic efficiency at which the CO2 regeneration increases again, achieving higher coulombic efficiencies approaching 90% for 1 M KHCO3 and 95% for 0.5 M KHCO3 at current densities approaching 100 mA cm -2 . The coulombic efficiencies reported in these electrolytes are also consistent with prior work by Eisaman et al. Between 0.5 and 1 M KHCO3, 0.5 M KHCO3 possesses a higher coulombic efficiency at high current density due to its lower bulk pH, which inhibits the loss of water dissociation generated H + to H + -OH -recombination and H + -CO3 2-recombination relative to the 1 M KHCO3 case. The above analysis demonstrates that the regeneration of CO2 in the diffusion boundary layer is dictated by complex interactions between H + , HCO3 -, and CO3 2-and their associated buffer kinetics.