ZIF-derived CoS@CN with a hollow cage structure for improved electrochemical nitrate reduction to synthesize ammonia

Electrocatalytic nitrate (NO3‒) reduction reaction (eNO3RR) is a promising route for ammonia (NH3) synthesis to alternate Haber-Bosch process. It is important to design catalysts with high efficiency and strong selectivity for the preparation of NH3 via the eNO3RR. Herein, a ZIF-derived nitrogen-doped carbon/cobalt sulfide electrocatalyst, CoS@CN, with a hollow cage structure synthesized by vulcanization etching, is presented as a high-efficiency electrocatalyst for eNO3RR. CoS@CN shows outstanding selectivity and stability with a large NH3 yield of 4.74 mg h-1 mcat-1 at -0.75 V vs. RHE and a high Faradaic efficiency of 97.88% at -0.35 V vs. RHE in 0.1 M NaOH with 200 ppm NO3‒. Density functional theory calculations demonstrate that CoS@CN has a narrower bond gap (0.16 eV) and lower barrier potential determining step (0.71 eV). The S atoms function as bridges connecting the charge repository C atoms to the active site Co atoms, as is more favorable for NH3 synthesis through the eNO3RR. It was demonstrated both experimentally and theoretically that the S element plays an important role. The introduction of the S element could not only modulate the electronic structure of the catalysts, but also modulate the morphology of the catalysts to a hollow cage structure, as facilitates the proceeding of eNO3RR. This work offers a promising strategy for designing high-performance eNO3RR catalysts for electrocatalytic NH3 synthesis and the waste water treatment.