Understanding of nitrogen fixation electro catalyzed by molybdenum-iron carbide through the experiment and theory

The electrochemical method is considered being a sustainable alternative to the industrial Haber-Bosch process (150-350 atm, 350-550 degrees C) because it can produce ammonia (NH3) from nitrogen (N-2) and water (H2O) at room temperature and pressure. However, since the N equivalent to N triple bond in N-2 is one of the strongest bonds in nature, it requires a more negative potential for N-2 reduction, which often leads to violent hydrogen evolution reaction (HER) in aqueous electrolysis systems. Therefore, it is a great challenge for the electrocatalytic N-2 reduction reaction (ENRR) to find catalysts that can reduce the energy barrier of N-2 fixation and inhibit the HER. Herein, inspired by the Mo-Fe site in the biological nitrogenase, we found that the catalyst containing Mo3Fe3C active material has excellent N-2-fixing catalytic performance and can effectively inhibit the HER. At 0.05 V vs RHE, the Faraday efficiency (FE) of ENRR was as high as 27.0%. In addition, we innovatively used the Fourier-transformed alternating current voltammetry (FTACV) to explore the electron transfer process in ENRR, indicating that Mo3Fe3C is more conducive to reducing N-2 at low potential. According to density functional theory (DFT) calculations, compared with Mo2C and Fe3C, Mo3Fe3C is more helpful in promoting N-2 activation and hydrogenation. Due to the synergistic effect of the Mo-Fe site, N-2 hydrogenation needs to overcome a lower energy barrier in potential-determining step (PDS). Our research extends the knowledge into bimetallic active sites in ENRR and provides a new insight for the subsequent synthesis of high selectivity catalysts.