Elucidating electrochemical nitrate and nitrite reduction over atomically-dispersed transition metal sites

Electrocatalytic reduction of waste nitrates (NO₃⁻) enables the synthesis of ammonia (NH₃) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts demonstrate a high catalytic activity and uniquely favor mono-nitrogen products. However, the reaction fundamentals remain largely underexplored. Herein, we report a set of 14; 3d-, 4d-, 5d- and f-block M-N-C catalysts. The selectivity and activity of NO₃⁻ reduction to NH₃ in neutral media, with a specific focus on deciphering the role of the NO₂⁻ intermediate in the reaction cascade, reveals strong correlations (R=0.9) between the NO₂⁻ reduction activity and NO₃⁻ reduction selectivity for NH₃. Moreover, theoretical computations reveal the associative/dissociative adsorption pathways for NO₂⁻ evolution, over the normal M-N₄ sites and their oxo-form (O-M-N₄) for oxyphilic metals. This work provides a platform for designing multi-element NO₃RR cascades with single-atom sites or their hybridization with extended catalytic surfaces.