(428g) Cyclic Stabilization Method for Lithium Mediated Electrochemical Nitrogen Reduction

Lithium mediated nitrogen reduction is a proven method to electrochemically synthesize ammonia (NH₃). However the process has so far been unstable, and the continuous deposition of lithium limits the practical applicability of the process. In addition, very little is understood about the mechanism. To address these issues, this report sets out to achieve two goals. Firstly, we present a general molecular-level model to describe the mechanism of lithium mediated N₂ reduction to NH₃. Despite its simplicity, this model can explain a number of mechanistic features that have been observed experimentally, such as the dependence of faradaic efficiency (FE) upon N₂ partial pressure and EtOH concentration. Based on this understanding, we develop a new strategy for increasing the FE by cycling the applied potential between a regime where Li deposition is favored and one where Li dissolution is favored. This strategy leads to a significant improvement to both the system’s stability and catalytic efficiency. We show experimentally that the Li cycling process is stable for up to (at least) 125 hours of operation and yields a substantial increase in FE from 21% during constant Li deposition to 37% during potential switching. Importantly, we additionally measure an energy efficiency of 7.1% that is well beyond the previously highest reported value of 1.5% in the literature.