Electrochemical Nitrate Production via Nitrogen Oxidation with Atomically Dispersed Fe on N-Doped Carbon Nanosheets.

Abstract

Electrocatalytic N2 oxidation (NOR) into nitrate is a potential alternative to the emerging electrochemical N2 reduction (NRR) into ammonia to achieve a higher efficiency and selectivity of artificial N2 fixation, as O2 from the competing oxygen evolution reaction (OER) potentially favors the oxygenation of NOR, which is different from the parasitic hydrogen evolution reaction (HER) for NRR. Here, we develop an atomically dispersed Fe-based catalyst on N-doped carbon nanosheets (AD-Fe NS) which exhibits an exceptional catalytic NOR capability with a record-high nitrate yield of 6.12 μ mol mg-1 h-1 (2.45 μ mol cm-2 h-1) and Faraday efficiency of 35.63%, outperforming all reported NOR catalysts and most well-developed NRR catalysts. The isotopic labeling NOR test validates the N source of the resultant nitrate from the N2 electro-oxidation catalyzed by AD-Fe NS. Experimental and theoretical investigations identify Fe atoms in AD-Fe NS as active centers for NOR, which can effectively capture N2 molecules and elongate the N≡N bond by the hybridization between Fe 3d orbitals and N 2p orbitals. This hybridization activates N2 molecules and triggers the subsequent NOR. In addition, a NOR-related pathway has been proposed that reveals the positive effect of O2 derived from the parasitic OER on the NO3- formation.