(392f) X-MAS: Redox-Metal-Mediated Electrocatalytic Ammonia Synthesis

Electrochemical conversion of nitrogen (N₂) to ammonia (NH₃) would enable sustainable production of food and carbon-free fuel. Direct electrochemical N₂ reduction is unfortunately hindered by the hydrogen evolution reaction (HER); as a result, state-of-the-art N₂ reduction reactors utilize the complex, poorly understood, and inefficient lithium-mediated NH₃ synthesis (Li-MAS) process. The essence of this process lies in the electrodeposition of Li⁺, a critical phase demanding current oscillations to fortify the solid-electrolyte interface (SEI) and ensure voltage stability. This distinctive operational cadence orchestrates Li nitridation and Li₃N protolysis, profoundly influencing NH₃ selectivity. The increase in pressure up to 20 bar amplifies NH₃ Faradaic efficiency (FE) up to a certain point, where proton availability becomes a linchpin in steering Li nitridation and Li₃N protolysis. The pKa of proton donors, particularly alcohols, emerges as a pivotal factor, with 1-butanol standing out for the highest NH₃ FE. The anion selection in Li salt becomes another lever to tweak, wherein larger anions, exemplified by BF₄^-, fortify SEI stability, showcasing a direct impact on LiMAS efficacy. Notably, achieving a peak NH₃ FE of ~70% and a staggering NH₃ current density of ~-100 mA/cm² mandates a delicate balance of process conditions, encompassing N₂ pressure, proton availability, and Li salt concentration. Given the exceptional efficiency of the HB process (~60-65%), the upper bound on energy efficiency (~27%) of Li-MAS suggests that there is a critical need to identify other possible catalytic mediators for NH₃ electrosynthesis. In this talk, we also explore mediators beyond Li such as Ca,Mg,Sr,Y, and V using DFT. The Ca-mediated NH₃ synthesis shows FEs and current density comparable to Li-MAS. This work serves as the proof of concept for Ca-mediated NH₃ synthesis and would motivate further research to improve the performance for Li-free ammonia synthesis.