Lithium-mediated electrochemical nitrogen reduction: Mechanistic insights to enhance performance

Abstract

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.

Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase

“…11 Although the accurate mechanisms are still not entirely understood, it is broadly believed that this LiNR process relies on the metallic lithium reduced from Li + to dissociate N 2 followed by a sequence of electron and proton transfers to form NH 3 with suitable proton donors or so-called sources (Figure 1A). 13,14 The LiNR process was revisited by several groups recently, 11,[13][14][15][16][17][18][19][20] and the typical reported faradaic efficiency (FE) is around 5%-20% at ambient conditions with NH 3 production rate less than 0.01 mmol s À1 cm geo À2 . 11,14,16,17 Recently, Suryanto et al has reported 69% FE at a current density of À0.022 A cm geo À2 and an NH 3 production rate of 0.053 mmol s À1 cm geo À2 by using phosphonium salt as a proton carrier under 20 bar pressure.…”

Evaluation of Electrocatalytic Activity of Noble Metal Catalysts Toward Nitrogen Reduction Reaction in Aqueous Solutions under Ambient Conditions

“…[3,4,36] The principle of lithium-mediated NRR was summarized in Figure S13, in which the sluggish N�N was split by active metallic lithium, followed by protonation process to produce ammonia. [37][38][39][40] Besides, nonaqueous electrolyte permits the use of more inert proton source to suppress potential HER. The effective activation of N 2 (by metallic lithium) and suppressed HER (by inert proton source) endowed the lithium-mediated system with outstanding performance.…”

“…To break the limitation of nitrogen mass transfer and compare the intrinsic NRR performance of lithium-mediated system with that of noble catalysts, a home-made gas diffusion electrocatalytic cell was employed in our experiments. [40] Stainless-steel cloth with wire diameter of 30 μm (Figure S14) Pt foil were adopted as working electrode and counter electrode, respectively. Tetrahydrofuran (THF) electrolyte containing 1 m LiBF 4 and 0.11 m EtOH was used as electrolyte, which was injected in the electrolytic cell and saturated by continuous nitrogen flow before electrolysis.…”

“…The lithium-mediated nitrogen reduction reaction was conducted with a home-made gas diffusion electrolytic cell. [40] Stainless-steel cloth and Pt foil was employed as working electrode and counter electrode respectively. Dehydrated THF solution containing 1 m LiBF 4 and 0.11 m EtOH was used as electrolyte.…”

Ammonia as Green Fuel in Internal Combustion Engines: State-of-the-Art and Future Perspectives

“…Recently, lithium-mediated electrochemical NRR has received renewed attention due to its reproducibility. Lithium-mediated NRR begins with electrochemical deposition of lithium, followed by two chemical processes of dinitrogen splitting and protonation to ammonia (Cai et al, 2021).…”