Summary Green synthesis of ammonia by electrochemical nitrogen reduction reaction (NRR) shows great potential as an alternative to the Haber-Bosch process but is hampered by sluggish production rate and low Faradaic efficiency. Recently, lithium-mediated electrochemical NRR has received renewed attention due to its reproducibility. However, further improvement of the system is restricted by limited recognition of its mechanism. Herein, we demonstrate that lithium-mediated NRR began with electrochemical deposition of lithium, followed by two chemical processes of dinitrogen splitting and protonation to ammonia. Furthermore, we quantified the extent to which the freshly deposited active lithium lost its activity toward NRR due to a parasitic reaction between lithium and electrolyte. A high ammonia yield of 0.410 ± 0.038 μg s −1 cm −2 geo and Faradaic efficiency of 39.5 ± 1.7% were achieved at 20 mA cm −2 geo and 10 mA cm −2 geo, respectively, which can be attributed to fresher lithium obtained at high current density.
Aqueous electrochemical nitrogen reduction (ENR) to ammonia (NH 3 ) under ambient conditions is considered as an alternative to the energy-intensive Haber-Bosch process for ammonia production. Many metal, non-metal, carbon-based materials along with metal-chalcogenides, metal-nitrides have been explored for their ENR activity. The reported NH 3 production through ENR is still in the micro-gram level and often falls in the range of NH 3 and NO x contaminations from the surrounding. The quantification of NH 3 at very low concentration possess enormous challenge in this field and thus many reported ENR electrocatalysts suffer from reproducibility issue. This review highlights in detail the challenges associated with ENR in aqueous medium and necessitates standardization of protocols to quantify the low concentration of NH 3 free of false-positives. It concludes the prospects of electrochemical NH 3 production through lithium-mediated N 2 reduction.
Zhaojun Meng,Ke Wang,Haotian Bai,Lijun Han,Zhiwen Yang,Yanrong Liu
Lithium-mediated nitrogen reduction reaction (Li-NRR) is a promising reaction pathway to realize green ammonia production. However, there is a lack of clear understanding on how solid electrolyte interphase (SEI) membranes affects Li-NRR performance. The effects of different lithium salts on the SEI layer were investigated in this work. Among lithium tetrafluoroborate (LiBF 4 ), lithium perchlorate (LiClO 4 ), and lithium trifluoromethanesulfonate (LiOTf), LiOTf showed better ammonia synthesis performance. At 1 bar N 2 and −6 mA cm −2 current density, the faraday efficiency (FE) and ammonia production rate reached 16.94 ± 0.67 % and 3.51 ± 0.14 nmol s −1 cm −2 , respectively. Conversely, the LiBF 4 electrolyte has the lowest NH 3 selectivity and rate under identical operating conditions, which are only 2.41 ± 0.16 % and 0.50 ± 0.03 nmol s −1 cm −2 . Cryo-electron microscopy reveals the influence of SEI nanostructures and chemical properties on the performance of lithium-mediated electrochemical synthesis of ammonia. The results of Cryo-electron microscopy showed that the SEI membranes of LiOTf electrolyte exhibited a reticular structure, which was favorable for N 2 adsorption, while the SEIs of LiClO 4 and LiBF 4 electrolytes had a lamellar and irregular planar structure, respectively. Molecular dynamics (MD) simulations and density functional theory (DFT) results showed that the LiOTf electrolyte can more easily generate a stable SEI membrane on the electrode surface. These findings highlight the importance of SEI structure in enhancing the reaction process, and thus provide valuable suggestions for the development of artificial SEI with specific structure.
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