While the d-band model provides a first approximation for understanding trends in ammonium selectivity, the ammonium selectivity of Co is exceptionally high (>95%) across the range of potentials measured here (-0.2 to -0.7 VRHE; Supplementary Figures and) compared to materials of similar Ed vs EF. To better understand the exceptional ammonium selectivity of Co we consider the calculated reaction free energies of nitrite reduction to nitric oxide and its further dissociation (Figure ). Selectivity to nitrite nominally decreases as Δ(𝐺 𝑁𝑂 𝐺 𝑁𝑂 2 -) decreases (Figure ). However, while nitrite reduction is more favorable on Ni than Co, Ni demonstrates a poorer ammonium selectivity than Co. In this case, Ni has a much lower driving force for nitric oxide dissociation than Co (Figure ), where we hypothesize N * selectively reduces to ammonium at the potentials considered here. In contrast, while nitric oxide dissociation is more favorable on Fe compared to Co, nitrite reduction on Fe is less favorable (Figure ), resulting in greater nitrite selectivity. This explanation produces a Sabatier-like trend where ammonium selectivity is determined by the free energy of nitrite reduction to either nitric oxide or dissociated N * (Figure ): materials with insufficient driving force for either nitric oxide dissociation (e.g. Ag, Cu, Ni0.68Cu0.32, Ni; Figure ) or nitrite reduction (e.g. Fe) provide sub-optimal ammonium selectivity. Thus, Co represents an optimum where nitric oxide is bound strong enough to prefer dissociation while still maintaining an appreciable driving force for nitrite reduction.