We next consider the role of TM Ed vs EF on selectivity towards ammonium (Figure ). For TMs with Ed well below EF (e.g. Cu, Ag) selectivity towards ammonium is very low (typically <20%), though increases with H * coverage under more cathodic applied potentials (Supplementary Figures and). In contrast, selectivity towards ammonium is nominally high for Ed approaching EF (e.g. Ni, Co, Fe), remaining high for TMs with Ed well above EF (e.g. Ti). This observation can be explained by the d-band model: the antibonding molecular orbital formed between adsorbed nitric oxide and the TM surface becomes increasingly unoccupied as Ed approaches and overcomes EF , manifesting as stronger binding (more negative Δ𝐺 𝑁𝑂 ) and a preference towards dissociative adsorption (Δ𝐺 𝑁 +𝑂 < 𝐺 𝑁𝑂 , Figure ). Increased ammonium selectivity for TMs that favor NO dissociation leads us to propose that N * selectively forms ammonium at the potentials considered here. While the dband model includes an adsorption energy penalty proportional to the overlap integral squared (𝑉 𝑎𝑑 ), we find tabulated values of 𝑉 𝑎𝑑 do not describe well the trends in either NO3RR FE (Supplementary Figure ) or selectivity towards ammonium (Supplementary Figure ), suggesting Ed vs EF predominates this description.