Molybdenum and boron synergistically boosting efficient electrochemical nitrogen fixation

Abstract

Ammonia production consumes ~2% of the annual worldwide energy supply, therefore strategic alternatives for the energy-intensive ammonia synthesis through the Haber-Bosch process are of great importance to reduce our carbon footprint. Inspired by MoFe-nitrogenase and the energy-efficient and industrially feasible electrocatalytic synthesis of ammonia, we herein establish a catalytic electrode for artificial nitrogen fixation, featuring a carbon fiber cloth fully grafted by boron-doped molybdenum disulfide ( B-MoS 2 /CFC) nanosheets. An excellent ammonia production rate of 44.09 μg h –1 cm –2 is obtained at −0.2 V versus the reversible hydrogen electrode (RHE), whilst maintaining one of the best reported Faradaic efficiency (FE) of 21.72% in acidic aqueous electrolyte (0.1 M HCl). Further applying a more negative potential of −0.25 V renders the best ammonia production rate of 50.51 μg h –1 cm –2 . A strong-weak electron polarization (SWEP) pair from the different electron accepting and back-donating capacities of boron and molybdenum (2 p shell for boron and 5 d shell for molybdenum) is proposed to facilitate greatly the adsorption of non-polar dinitrogen gas via N ≡ N bond polarization and the first protonation with large driving force. In addition, for the first time a visible light driven photo-electrochemical (PEC) cell for overall production of ammonia, hydrogen and oxygen from water + nitrogen, is demonstrated by coupling a bismuth vanadate BiVO 4 photo-anode with the B-MoS 2 /CFC catalytic cathode. A catalytic electrode B-MoS 2 /CFC is established for artificial nitrogen fixation, displaying the excellent ammonia production rate of 44.09 μg h –1 cm –2 and Faradaic efficiency of 21.72% in acidic aqueous electrolyte at −0.2 V. A supposed strong-weak electron polarization pair is inferred from the different electron accepting and back-donating capacities of boron and molybdenum (2 p shell for boron and 5 d shell for molybdenum), which may facilitate the adsorption of nonpolar dinitrogen and its first protonation greatly. • A carbon fiber cloth fully grafted by boron-doped molybdenum disulfide ( B-MoS 2 /CFC) nanosheets was prepared. • An excellent ammonia production rate of 44.09 μg h –1 cm –2 was obtained in 0.1 M HCl. • A strong-weak electron polarization pair of 2 p shell for B and 5 d shell for Mo was proposed to facilitate the N ≡ N bond polarization. • A visible light driven photo-electrochemical cell for overall production of NH 3 , H 2 and O 2 from H 2 O + N 2 , was demonstrated.