Origin of electrocatalytic nitrogen reduction activity over transition metal disulfides: critical role of in situ generation of S vacancy

The electrochemical nitrogen reduction reaction (ENRR) is a promising and sustainable alternative to conventional Haber-Bosch ammonia (NH₃) synthesis. Pursuing high-performance and cost-effective ENRR catalysts is an open challenge for achieving commercial-scale ambient NH₃ production. Less-precious transition metal disulfides (TMS₂) are a class of promising catalysts that can be highly active for ENRR. However, the origin of their high ENRR performance is not well understood. Herein, we analyze the origin of their activity by probing their electrochemistry-induced surface states. Starting with a typical ENRR TMS₂ catalyst, iron disulfide (FeS₂), from our calculated surface Pourbaix diagrams we found that S-vacancies can be easily generated under an ENRR potential. Our subsequent spin-polarized density functional theory (DFT) calculations show that this electrochemistry-driven “in situ” generation of S-vacancies shows significantly higher ENRR activity than a stoichiometric pristine FeS₂ surface due to the stronger N-N adsorption and activation capacity of a lower-coordination-number S-vacancy site. This finding is in excellent agreement with experimental observations published in recent years regarding potential windows reaching the maximum faradaic efficiency. We then expanded our analysis to other typical TMS₂ that had shown promising ENRR performance in recent experimental literature (SnS₂, MoS₂, NiS₂, and VS₂), and found that such an “in situ” S-vacancy generation phenomenon is universal under ENRR potentials, with results in good agreement with many experimental observations reported to date. We conclude that, though S-vacancy engineering during synthesis is a promising strategy to enhance the ENRR performance on TMS₂ catalysts, the “in situ” generation of S-vacancies will also endow pristine TMS₂ with a measurable ENRR performance. This study shows that the surface states of ENRR catalysts should not be dismissed before analyzing the activity of an ENRR catalyst. Most importantly, we found that when designing a promising TMS₂ catalyst for ENRR, its capacity to form S-vacancies is a key performance indicator that needs to be analyzed.