The potential at peak phosphate-mediated HER (E -iR @ jPeak,PO4-HER; squares in Figure ) exhibits a volcano-style trend with literature H chemisorption energies (Figure ), mirroring the well-known relationship between HER exchange current and H chemisorption energy in acidic electrolytes (Supplementary Figure ). HER activity on TMs binding H weakly (e.g. Ag, Cu) is limited by proton adsorption, while TMs binding H strongly (e.g. Fe, Ti) are limited by H-H bond formation kinetics. HER activity is also well-described by work function, where higher work function TMs (e.g. Ni and other Group X TMs) demonstrate greater HER activity than those of lower work function TMs (Supplementary Figure ). For CVs in the presence of 0.1 M sodium nitrate (Figure , solid lines), nitrate reduction current becomes apparent at potentials nominally less-cathodic than HER (Supplementary Figures and). NO3RR current manifests as either a shift in the phosphate deprotonation peak to less cathodic potentials (e.g. Ni), or as a second mass-transfer limited peak (e.g. Ag).