The Role of Atomically Dispersed Transition Metal Centers for the Electrochemical Nitrate Reduction Reaction Towards Ammonia Synthesis

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. Figure 1