Polyoxometalate-based metal–organic framework-derived bimetallic hybrid materials for upgraded electrochemical reduction of nitrogen

Abstract

The development of high-efficiency noble-metal-free catalysts for the electrochemical nitrogen reduction reaction (NRR) to ammonia under ambient conditions has great significance in fertilizer production and energy storage. Considering the major role of Mo-nitrogenase with the FeMo cofactor in the biological N₂ fixation process, the design and preparation of Mo and Fe bi-active metal based hybrid materials for the NRR under ambient conditions is proposed in this work. By using PMo₁₂@MOF-100(Fe)@PVP (polyvinylpyrrolidone) as the precursor, two cost efficient FeMo-based electrocatalysts Fe_1.89Mo_4.11O₇/FeS₂@C and FeMoO₄/FeS₂@C were designed and fabricated for the NRR under room temperature and pressure conditions (RTP) via the easy-to-implement hydrothermal sulfuration method. The experiment results confirm that Fe_1.89Mo_4.11O₇/FeS₂@C (NH₃ yield rate of 105.3 μg h⁻¹ mg_cat.⁻¹, FE of 54.7% at −0.4 V vs. RHE) is more efficient towards the NRR than FeMoO₄/FeS₂@C (NH₃ yield of 51.0 μg h⁻¹ mg_cat.⁻¹, FE of 43.9% at −0.5 V vs. RHE) in acidic electrolytes; moreover they are all superior to most of the electrocatalysts reported to date. Further electrocatalysis of Fe_1.89Mo_4.11O₇/FeS₂@C in alkaline electrolytes (NH₃ yield of 86.3 μg h⁻¹ mg_cat.⁻¹, FE of 53.6% at −0.4 V vs. RHE) reveals the extensive NRR catalytic activity of this hybrid material. Density functional theory (DFT) calculation indicates that the NRR on Fe_1.89Mo_4.11O₇/FeS₂ has optimized nitrogen binding which facilitates the fast kinetics process through an enzymatic mechanism, and the protonation of N₂ to form *N₂H species is the potential-determining step (PDS) with the maximum ΔG values (+0.61 eV). This work opens up a significant opportunity to develop a family of efficient and robust FeMo-based electrocatalysts for the NRR under ambient conditions by using polyoxometalate-based metal–organic frameworks (POMOFs) as precursors by tuning metal sources.