MAX-derived B-doped Mo1.33C MXene for ambient electrocatalytic conversion of nitrate to ammonia

Abstract

Electrochemical conversion of nitrate to ammonia under ambient conditions, a potential substitute for the conventional Haber–Bosch process, features a considerably sluggish reaction kinetics and thermodynamic complexity due to its eight-electron process coupled with nine protons, which should be responsible for the poor ammonia yield rate and low faradaic efficiency. Consequently, exploring and developing highly efficient electrocatalysts with high selectivity towards ammonia from nitrate is necessary to accelerate the cleanliness and efficiency of the ammonia industry. Herein, we propose a typical defect-rich Mo_1.33C_0.9B_0.1 MXene electrocatalyst synthesized through selective etching of the (Mo_2/3Y_1/3)₂AlC_0.9B_0.1 MAX phase, which exhibits an excellent faradaic efficiency of 91% and ammonia yield rate of 1.7 mmol h⁻¹ mg_cat.⁻¹ in 1 M KOH containing 0.1 M KNO₃ at −0.35 V vs. RHE, superior to the majority of electrocatalysts reported to our knowledge. The results demonstrate that the synergistic effect between high-valence Mo sites and the empty-orbital-rich B dopant boosts the electrocatalytic performance of Mo_1.33C_0.9B_0.1 by enhanced NO₃⁻ adsorption and activation, which can significantly accelerate the multiple-electron transfer dynamics. The novel understanding based on the active sites paves a new way toward designing and modulating electrocatalysts for industrial ammonia production.