Ni Foil 8 mA/cm 2 geo measured here (Figure ). A qualitative assessment of CVs in stirred electrolyte (top panels of Figure ) reinforce this observation, where the ratio of current in the presence and absence of nitrate on Ni is much lower than that of Cu. However, electrons contributing to NO3RR on Ni primarily form ammonium (50-90%). Ammonium selectivity on Ni decreases with increasingly cathodic potentials, unlike all other TMs measured here (Supplementary Figures and). This suggests adsorbed H * are more likely to form dihydrogen than to hydrogenate nitrate as cathodic potential is increased, or that mass-transfer limited phosphate deprotonation (top panel of Figure ) hinders the formation of proton-rich ammonium (10 H + /NH4 + from nitrate).