Assessing the Activity of Transition Metal Oxides for the Electrochemical N₂ Oxidation to Nitrate

The electrochemical oxidation of dinitrogen (N2) to nitrate (NO3–) is an attractive method for decentralized fertilizer production. Yet, scarce experimental evidence with trace NO3– produced in reported catalysts hints at the kinetics challenge and motivates a search for reliable electrocatalysts. We addressed the gaps in the understanding of N2 oxidation by computing three pathways: the (1) direct electrochemical pathway that extends all the way to NO3–, (2) surface lattice oxygen pathway on perovskites, and (3) surface-adsorbed oxygen pathway. These computations revealed the unfavorable trade-off between N2 activation and NO2 desorption/O vacancy filling step energies, which potentially limit the N2 oxidation activity and render the parasitic OER dominant. However, several oxides which possess reactive surface oxygen and inert/moderate OER activity were identified as more promising for experimental assessment. We then experimentally examined 20+ transition metal oxides, namely, ABO3 perovskites (A = La, Sr, Ca, Bi and B = Co, Mn, Fe) and MO2 rutiles (IrO2, RuO2, TiO2, SnO2, and Fe- and Ir-doped TiO2 and SnO2) in alkaline and neutral electrolytes. Electrochemical measurements via up to 22 h chronoamperometry showed minimal NO3– concentrations of <1 ppmN via UV–vis spectroscopy, which were comparable to those measured in the absence of N2. Time-dependent investigations of different substrates (i.e., carbon paper and Ti foil), increasing catalyst loadings in H-cells and flow cells, as well as high-surface-area La0.5Sr0.5CoO3 and La0.5Sr0.5MnO3 showed that the observed NO3– concentrations were not greater than those measured without N2 with experimental certainty. This work underscores the need for proliferating NOx production (massprod) beyond system size (masssys) or rigorous quantitative 15N2-labeling to provide concrete evidence for true N2 oxidation and encourages exploration of ambient N2 oxidation beyond conventional approaches.