The electrochemical reduction of oxidized nitrogen species, nitrate (NO₃^-) and nitrite (NO₂^-), both of which are environmental pollutants, to benign nitrogen gas (N₂) has been extensively researched for water remediation applications. Recently, a large research focus has shifted to reducing these oxidized nitrogen species to ammonia (NH₃).¹ In this way, a dual benefit is achieved by eliminating the environmentally hazardous nitrates, while also creating a value-added product, ammonia. Moreover, the nitrate reduction reaction (NO₃RR) can aid in closing the nitrogen cycle, by reforming nitrates introduced into the environment through nitrate-based fertilizers originating from the Haber-Bosch process, back into NH₃, in a carbon neutral fashion.

The NO₃RR is a complex, 8e^-, 9H⁺ transfer process with multiple possible intermediate species (NO₂^-, NO, N₂O, N₂H₄, NH₂OH, NH₃) and rate limiting steps. The NO₃RR mechanism and its activity descriptors over various metals is not fully understood, and this is especially true over atomically dispersed sites, where less than a handful of studies exist.^2,3 Our previous work found that atomically dispersed nitrogen coordinated Fe-N₄ and Mo-N₄ sites displayed distinct associative adsorption and dissociative adsorption of the nitrate molecule in the NO₃RR pathways, respectively. Specifically, Fe-N₄ sites adsorb and reduce NO₃^- into NH₃through an 8e^- pathway on the surface, while Mo-N₄ sites dissociate NO₃^- and release NO₂^- into the bulk electrolyte. These active sites were then integrated into a single bi-metallic catalyst (FeMo-N-C), creating a catalytic cascade yielding a significantly improved yield rate and Faradaic efficiency for NH₃.⁴

In this talk, building on our previous work, we will present a series of transition metal atomically dispersed nitrogen doped carbon (M-N-C) electrocatalysts (M = Mn, Fe, Co, Ni, Cu...) for the systematic investigation of NO₃RR activities over the different metal centers. In this work we leverage physical (ICP-MS, gas physisorption), electrochemical (e.g., NO₃RR) and computational (DFT) characterization to create a set of NO₃RR activity descriptors. A detailed set of activity descriptors can aid in the selection of atomically dispersed metals for creating efficient reaction cascades to synergize favorable reaction pathways.