Mechanistic insights into the stepwise lithium-mediated electrochemical nitrogen reduction for enhanced ammonia synthesis

Abstract

The lithium-mediated electrochemical nitrogen reduction reaction (Li-NRR) is a sustainable route for green NH3 synthesis. It accelerates the N2 reduction by reacting the inert N ≡ N with active Li metal. However, the study on the mechanism of the Li-NRR process remains limited. Herein, a novel stepwise Li-NRR system is established to separately optimize the major steps of Li-NRR: lithium-ion reduction, lithium nitridation, and Li3N protonation. It is revealed that during the Solid Electrolyte Interface (SEI) decomposition, the carbon chains in organic species become shorter, and LixBFy is converted to LiF, which is similar to the continuous Li-NRR. Contrary to the continuous Li-NRR, the O/C ratio decreases as the SEI decomposes due to the absence of ethanol. Commercial Cu, Ni, and Zn are employed as cathodes in the experiments at various currents. The highest Faraday efficiency of 33.2 % and ammonia yield rate of 1404.5 μg h−1 cm−2 are achieved on Zn electrode owing to its superior electron transfer and enhanced Li+ diffusion than those of Cu and Ni. The current has also been found to affect Faraday efficiency through the portion of active lithium in coulomb efficiency experiments. Moreover, the LiCl generated in protonation process is found to facilitate lithium cycling.