Functionalized MXenes for efficient electrocatalytic nitrate reduction to ammonia

Abstract

The electrocatalytic reduction of nitrate to ammonia is of scientific and practical significance. The feasibility of using two-dimensional transition metal carbides, M₃C₂ MXenes, as an electrocatalyst for converting NO₃⁻ to NH₃, is demonstrated by using density functional theory calculations. Reaction active center analysis reveals that the reaction prefers to occur on the basal plane rather than the edge plane. By systematically analyzing thermodynamic and kinetic aspects, the most probable reaction pathway was determined to be consecutive deoxygenation followed by hydrogenation: NO₃⁻ → *NO₃ → *NO₂ → *NO → *N → *NH → *NH₂ → *NH₃ → NH₃(g). Furthermore, it's found that the deoxygenation processes are exothermic while the hydrogenation processes are endothermic. Both catalytic deoxygenation and hydrogenation processes in NRA are substantially affected by pH. Thus, the rate-determining step and overpotential exhibit pH dependent characteristics. For unfunctionalized MXenes, the NRA is suppressed due to the strong hydrogen evolution reaction (HER). By functionalization, the NRA catalytic activity of Ti₃C₂ and transition metal doped Ti₃C₂ MXenes increases effectively. This improvement is attributed to the high oxidation states of Ti atoms at catalytic centers and weakening of *NH_x adsorption on the MXene surfaces, thereby facilitating easy hydrogenation. In particular, partially O-vacant Ti₃C₂O₂ is recognized as one promising NRA electrocatalyst, with the free energy change of every reaction step being negative. The findings of this work provide new strategies for the rational design of MXene-based NRA electrocatalysts with universal significance.