The final voltage collected at each current step was reported in the current density-voltage plots, with the exception of some of the higher current density steps. During the 1 M KHCO3 and 0.5 M KHCO3 experiments, CO2 bubbles formed at the surface of the electrode at higher current densities (≥ 20 mA cm -2 ) causing a significant amount of noise in the data. For these measurements, the voltage reported was taken as the average over the current step.