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

Abstract

The electrocatalytic nitrate (NO₃⁻) reduction reaction (eNO₃RR) is a promising route for ammonia (NH₃) synthesis, serving as an alternative to the 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 the eNO₃RR. CoS@CN shows outstanding selectivity and stability with a large NH₃ yield of 4.74 mg h⁻¹ mg_cat⁻¹ at −0.75 V vs. the reversible hydrogen electrode (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 the introduction of the S element destroys the electronic structure and reduces the potential determining step (PDS) barrier, and is more favorable for NH₃ synthesis through the eNO₃RR. It is demonstrated both experimentally and theoretically that the S element plays an important role. The introduction of the S element can not only modulate the electronic structure of the catalysts, but also modulate the morphology of the catalysts to a hollow cage structure, facilitating the proceeding of the eNO₃RR. This work offers a promising strategy for designing high-performance eNO₃RR catalysts for electrocatalytic NH₃ synthesis and waste water treatment.