Operando Investigation of the Solid Electrolyte Interface in the Electrochemical Lithium-Mediated Ammonia Synthesis

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Ammonia plays a crucial role in fertilizer production, but its conventional synthesis via the Haber-Bosch process is marked by high energy consumption, centralization, and dependence on non-renewable resources, resulting in considerable carbon emissions. In contrast, the lithium-mediated electrochemical nitrogen reduction reaction (Li-NRR) emerges as an innovative solution, enabling ammonia synthesis at room pressure and temperature with the potential for renewable energy integration [1]. This approach also promotes the decentralization of ammonia manufacturing. Within the Li-NRR framework, metallic lithium undergoes electrochemical deposition in an organic electrolyte and is believed to subsequently interact with inert nitrogen molecules, facilitating its protonation and the formation of ammonia [2].In recent years, significant research efforts aimed at boosting the selectivity and efficiency of the Li-NRR through the modification of electrolyte salt composition and concentration, alongside the implementation of potential cycling techniques [3-4]. These advancements are often attributed to alterations in the structure and composition of the solid electrolyte interface (SEI) formed on the cathode's surface, analogous to phenomena observed in lithium-ion batteries. Nonetheless, our current understanding of SEI properties primarily stems from ex situ analyses of the catalysts and the bulk electrolyte, leaving the behavior of this interface under reaction conditions largely unexplored.In this study, our aim is to deepen our understanding of the SEI's structure and composition under Li-NRR reaction conditions. Therefore, we employed cutting-edge operando spectroscopic techniques, such as surface-enhanced Raman spectroscopy, to closely examine the electrochemical interface during operational conditions. Taking inspiration from our expertise in lithium batteries we studied various electrolyte compositions and concentrations, including LiClO4 and LiFSI in tetrahydrofuran and ethanol to correlate the SEI’s properties with the performance of Li-NRR for ammonia production [5]. This approach enabled us to monitor the early stages of lithium plating and the subsequent formation and evolution of the SEI. Consequently, we could observe the changes and stability of various inorganic and organic SEI species, influenced by the initial composition of the electrolyte, and link them to the ammonia performance. Importantly, our investigations revealed ethanol's critical role in forming the organic SEI layer. By tracking nitrogen and hydrogen intermediates, we could further establish their correlation to the Li-NRR's efficiency, underscoring the competition with the hydrogen evolution reaction (HER). The mechanistic insights garnered from this study will aid in fine-tuning and enhancing the electrolyte composition for the improvement of Li-NRR in industrial applications.