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

Abstract

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 ppm_N 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 La_0.5Sr_0.5CoO₃ and La_0.5Sr_0.5MnO₃ showed that the observed NO₃^– concentrations were not greater than those measured without N₂ with experimental certainty. This work underscores the need for proliferating NO_x production (mass_prod) beyond system size (mass_sys) or rigorous quantitative ¹⁵N₂-labeling to provide concrete evidence for true N₂ oxidation and encourages exploration of ambient N₂ oxidation beyond conventional approaches.