Water Increases the Faradaic Selectivity of Li-Mediated Nitrogen Reduction

Abstract

The lithium-mediated system catalyzes nitrogen to ammonia under ambient conditions. Herein we discover that trace amount of water as an electrolyte additive—in contrast to prior reports from the literature–can effect a dramatic improvement in the Faradaic selectivity of N 2 reduction to NH 3 . We report that an optimal water concentration of 35.9 mM and LiClO 4 salt concentration of 0.8 M allows a Faradaic efficiency up to 27.9 ± 2.5% at ambient pressure. We attribute the increase in Faradaic efficiency to the incorporation of Li 2 O in the solid electrolyte interphase, as suggested by our X-ray photoelectron spectroscopy measurements. Our results highlight the extreme sensitivity of lithium-mediated N 2 reduction to small changes in the experimental conditions.

Combined, time-resolved, in situ neutron reflectometry and X-ray diffraction analysis of dynamic SEI formation during electrochemical N2 reduction

“…45 Thus, the presence of a LiOH film due to the small quantities of water that are likely in this system could promote the formation of Li 3 N within the deposited Li layer, thereby promoting the additional formation of LiOH via the decomposition of the nitride. 46,47…”

“…At higher current densities, it could be possible to observe the formation of other Li-containing species such as Li 2 CO 3 due to an increase in degradation of the carbonaceous solvent as Li metal is deposited at the electrode surface more quickly. 47,54 Thus, a combination of in situ experimental methods will be critically useful to develop an understanding of the behavior of the system at current densities other than those studied in this work.…”

Searching for the Rules of Electrochemical Nitrogen Fixation

“…Like in Li batteries and nitrogen reduction, ,,, an excessive number of parameters can influence the performance of beyond-lithium electrolytes making a holistic screening challenging. For comparative purposes, we put our theoretical screening to the test by revisiting the Li-based electrolytes with their Na, Ca, and Mg equivalents using a model electrolyte: a NTf 2 salt dissolved in an ether solvent (tetrahydrofuran (THF) or 1,2-dimethoxyethane (DME) for Mg­(NTf 2 ) 2 ) at different concentrations (0.1 to 1 M) and EtOH contents (0.2 to 5% v/v).…”

“…Why is that? It is likely that Mg and Ca chemistries are as sensitive as Li toward electrolyte modifications, , and a restricted experimental parameter space may contribute to false-negative observations.…”

“…On a solid electrode, lithium-mediated ammonia synthesis remains the only system–thus far–capable of reducing nitrogen to ammonia selectively . Since it was rigorously verified, significant breakthroughs were made, − alongside reports uncovering the fundamental reasons behind its selectivity, likely due to the solid-electrolyte interphase, or SEI. − This interphase is commonly associated with Li-ion batteries, where a Li-ion conducting but electrically insulating layer forms on the negative electrode from the partial breakdown of the electrolyte, stabilizing the electrolyte kinetically. − For ammonia synthesis, it is hypothesized that a similar layer forms and selectively slows down reagents’ diffusion to the electrode, preventing substantial parasitic H 2 evolution. ,, In other words, a selective barrier on the electrode controls the delivery of reagents to the active surface and provides ideal species activities for NH 3 to form. This last sentence depicts a concept that sounds transferable to alternative chemistries.…”