Mass-Transport Effects in the Lithium Redox-Mediated Nitrogen Reduction Reaction

Design of high-performance ammonia electrosynthesis systems based on the lithium redox-mediated nitrogen reduction reaction (Li-NRR) requires an understanding of the interplay between the reaction kinetics and mass transport. To advance this currently insufficient understanding, we developed a wall-jet electrode flow cell and used it to investigate the Li-NRR and key processes occurring in the system, namely, the reduction of Li⁺, formation of Li₃N and LiH, and protonation of the lithium-based electrodeposits, as a function of mass-transport conditions. The effects of the latter were found to be electrolyte-dependent when tetrahydrofuran was used as a solvent and 0.1 M C₂H₅OH as a proton carrier. In 1 M LiBF₄, the Li-NRR showed high mass-transport sensitivity, achieving the highest faradaic efficiency of 75 ± 2% when the optimal Li₃N-to-LiH ratio was provided. The performance with 2 M LiNTf₂ was also defined by the Li₃N:LiH ratio but followed opposite dependencies on the flow rate as compared to LiBF₄. The overall reduction rate with 2 M LiNTf₂ exhibited minimal sensitivity to mass transport, likely due to the intensified passivation of the cathode at high flow rates. The 1 M LiNTf₂ electrolyte demonstrated pronounced effects of the mass transport on the overall reduction rate, which, however, was dominated by side reactions rather than the Li-NRR under examined conditions. Our study shows that intensifying mass transport only cannot improve the ammonia production rates while maintaining high faradaic efficiency. Adjusting the electrolyte solution flow rate in the Li-NRR systems necessitates reoptimization of N₂ pressure, proton carrier concentration, and/or electrolyte composition.