As discussed in the Introduction section, the ammonium-formation mechanism abides by one of two pathways: PCET of associatively-adsorbed NO * (Ref. ) or dissociative adsorption of nitric oxide and subsequent hydrogenation of N * adatoms by H * . Activation of a Langmuir-Hinshelwood like NO * hydrogenation pathway (e.g. NO * + H * ) may explain the source of enhanced ammonium selectivity on Cu at HER-relevant potentials. Nitric oxide adorbs associatively on low H * chemisorption energy Cu (Supplementary Figure ), where ammonium formation by PCET is likely predominant, only activating the Langmuir-Hinshelwood mechanism after appreciable H * coverage is achieved (e.g. cathodic of HER onset). In contrast, Ni-preferring dissociative nitric oxide adsorption-likely only forms ammonium by a Langmuir-Hinshelwood like N * adatom hydrogenation, where our rate-order analysis and microkinetic modelling indicate high H * affinity at all potentials measured here. with NO3RR, and is used to compare NO3RR FE and selectivity to ammonium for all TMs in Figures and.