Researchers from Tohoku University have identified a promising alternative to the conventional means of synthesizing ammonia, one that is more environmentally friendly. Details of their research are published in an open-access paper in the Journal of Materials Chemistry A.

The Haber-Bosch synthesis, developed in the early 20^th century to synthesize ammonia from nitrogen and hydrogen gas remains the dominant means of producing ammonia. However, the process is energy- and resource-intensive, and producing hydrogen gas often involves natural gas, which releases carbon dioxide as a by-product.

The electrochemical nitrogen reduction reaction (ENRR), where nitrogen gas from the air can be converted into ammonia using an electrical current, is seen as a promising and sustainable alternative. Pursuing high-performance and cost-effective ENRR catalysts, however, is an open challenge for achieving commercial-scale ambient ammonia production.

We explored the potential of less-precious transition metal disulfides (TMS₂) as catalysts for ENRR. Through meticulous analysis of electrochemistry-induced surface states, we uncovered a previously unrecognized factor contributing to their high ENRR performance: S-vacancy generation.

—Hao Li, corresponding author

Li and his colleagues started with a typical ENRR TMS₂ catalyst—iron disulfide (FeS₂). They observed that under ENRR conditions, S-vacancies can be easily generated on the catalyst surface. Through advanced computational simulations, they demonstrated that this electrochemistry-driven in situ generation of S-vacancies significantly enhances ENRR activity by promoting stronger N-N adsorption and activation.

Experimental observations confirmed their findings, which were also consistent with recent literature on ENRR potential windows reaching maximum Faradaic efficiency—the measure of the effectiveness of an electrochemical process in converting electrical energy into chemical energy or vice versa.

(a) 1D and (b) 2D surface Pourbaix diagrams of FeS₂(111) considering different coverages of SV, O^*, H^*, and HO^*. The experimental potentials at the highest faradaic efficiencies of reported FeS₂-based catalysts are plotted for a direct comparison. ©Hao Li et al.

Their analysis also extended to other TMS₂ catalysts (SnS₂, MoS₂, NiS₂, and VS₂), revealing a universal phenomenon of in situ S-vacancy generation under ENRR potentials.

Our research underscores the critical importance of considering surface states in the design of ENRR catalysts. By shedding light on the role of S-vacancies, we have provided a valuable roadmap for enhancing ENRR performance and accelerating the transition towards sustainable ammonia production.

—Hao Li

(a) 1D surface Pourbaix diagrams of different transition metal disulfide (TMS₂) surfaces. (b) 2D surface Pourbaix diagram of SV formation as a function of pH and potential. The color-coded segments demarcate the potential windows of SV generation on the respective instances of TMS₂. ©Hao Li et al.

This work was supported by AIMR Fusion Research and also received substantial support for the JSPS Postdoctoral Fellowship for Dr. Tianyi Wang in the Hao Li Lab.

Resources

• Tianyi Wang, Zhongyuan Guo, Hirofumi Oka, Akichika Kumatani, Chuangwei Liu, and Hao Li (2024) “Origin of electrocatalytic nitrogen reduction activity over transition metal disulfides: critical role of in situ generation of S vacancy” Journal of Materials Chemistry A doi: 10.1039/D4TA00307A