Regarded as an important feedstock, ammonia is predominantly produced by the century-old Haber-Bosch process operating at high temperature and high pressure. By comparison, electrochemical nitrogen reduction reaction (eNRR) is capable of producing ammonia at ambient condition and thus shows great potential as an alternative to the Haber-Bosch process. However, its practical use is hampered by sluggish production rate and low Faradaic efficiency. Recently, lithium-mediated eNRR has received renewed attention due to its prominent performance and reproducibility. Nevertheless, further improvement of the system is restricted by limited recognition of its mechanism.

Herein, we demonstrate that lithium-mediated eNRR began with electrochemical deposition of lithium, followed by two chemical processes of dinitrogen splitting and protonation to ammonia. Furthermore, we also prove that the freshly-deposited active lithium gradually lost its activity toward eNRR due to a passivation reaction between lithium and solvent (tetrahydrofuran as the most widely used). Hence, protecting fresh metallic lithium should be a strategy to effective N2 reduction. And such “fresh lithium strategy” also rationalizes how current density affects performance. In electrolysis at large plating current, lithium generation had an advantage over passivation, resulting in fresher lithium deposits and thus exposing more active lithium to nitrogen, while in electrolysis at low current, passivation prevailed over lithium generation, leading to deterioration of active lithium and thus losing its reactivity toward eNRR. A high ammonia yield of 0.410± 0.038 μg s-1 cmgeo-2 (corresponding to 1476 ± 137 μg h-1 cmgeo-2) and Faradaic efficiency of 39.5 ± 1.7% were achieved at 20 mA cmgeo-2 and 10 mA cmgeo-2 respectively, reaching top-level class in the field of eNRR.

Regarded as an important feedstock, ammonia is predominantly produced by the century-old Haber-Bosch process operating at high temperature and high pressure. By comparison, electrochemical nitrogen reduction reaction (eNRR) is capable of producing ammonia at ambient condition and thus shows great potential as an alternative to the Haber-Bosch process. However, its practical use is hampered by sluggish production rate and low Faradaic efficiency. Recently, lithium-mediated eNRR has received renewed attention due to its prominent performance and reproducibility. Nevertheless, further improvement of the system is restricted by limited recognition of its mechanism.

Herein, we demonstrate that lithium-mediated eNRR began with electrochemical deposition of lithium, followed by two chemical processes of dinitrogen splitting and protonation to ammonia. Furthermore, we also prove that the freshly-deposited active lithium gradually lost its activity toward eNRR due to a passivation reaction between lithium and solvent (tetrahydrofuran as the most widely used). Hence, protecting fresh metallic lithium should be a strategy to effective N~2~ reduction. And such “fresh lithium strategy” also rationalizes how current density affects performance. In electrolysis at large plating current, lithium generation had an advantage over passivation, resulting in fresher lithium deposits and thus exposing more active lithium to nitrogen, while in electrolysis at low current, passivation prevailed over lithium generation, leading to deterioration of active lithium and thus losing its reactivity toward eNRR. A high ammonia yield of 0.410± 0.038 μg s^-1^ cm~geo~^-2^ (corresponding to 1476 ± 137 μg h^-1^ cm~geo~^-2^) and Faradaic efficiency of 39.5 ± 1.7% were achieved at 20 mA cm~geo~^-2^ and 10 mA cm~geo~^-2^ respectively, reaching top-level class in the field of eNRR.

Mechanism study on lithium-mediated electrochemical nitrogen reduction and fresh lithium strategy to better performance

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