Quantifying Influence of the Solid-Electrolyte Interphase in Ammonia Electrosynthesis

Abstract

The solid-electrolyte interphase (SEI) is often invoked to rationalize trends observed during lithium-mediated electrochemical ammonia synthesis (LiMEAS), but quantitative analysis of the SEI constituents is lacking. In this work, we quantified lithium alkoxide SEI species (derived from proton donor reduction) under a variety of electrolyte compositions, using ethanol as the standard proton donor along with five alternative linear and branched alcohol proton donors. With ethanol, we showed that Faradaic efficiency linearly increased from ∼10 to ∼20% as the lithium ethoxide layer thickened. When comparing different proton donors, we found that FE exhibited a "volcano plot" dependence with lithium alkoxide SEI layer thickness, such that FE increased with increasing SEI thickness up to ∼4 μm and decreased with increasing SEI thickness thereafter. We concluded the lithium alkoxide SEI layer limited proton donor and solvated Li + transport, resulting in an optimal SEI thickness at which proton donor, solvated Li + , and N 2 transport were appropriately tuned to maximize NH 3 selectivity.

Pathway toward Scalable Energy-Efficient Li-Mediated Ammonia Synthesis

“…A recent report by Cargnello et al. finds that the SEI thickness changes with varying alcohol chain lengths, which affects the transport of both N 2 and alcohol proton donors . The proton donor could influence the SEI by allowing optimal diffusion of N 2 and the proton donor to reach the surface of the electrode, thereby promoting NH 3 FE.…”

“…We find that the NH 3 FE also follows the same trend, yielding maximum FE for 1-butanol. The decrease in the diffusion coefficient with increasing alkyl chain length can also have some effect on the mass transfer of proton donor and NH 3 FE, particularly in light of recent reports suggesting that the SEI thickness can be significantly affected by the alcohol chosen as the proton donor …”

“…Li + dissolution is an unlikely step at reducing potentials (i.e., more reducing than Li electrodeposition); instead, the N can be substituted by X to form lithium alkoxide (LiX) in reactions and [ normalSEI via Li 3 N p rotolysis ] goodbreak0em1em⁣ Li 3 N false( normals false) + 3 HX false( sol . false) → NH 3 ( s o l . ) + 3 LiX false( normals false) [ SEI via Li p rotolysis ] goodbreak0em1em⁣ 2 Li ( s ) + 2 HX ( sol . ) → normalH 2 false( normalg false) + 2 LiX ( s ) It has been recently reported that LiX forms the solid-electrolyte interface (SEI) on the cathode . In addition to LiX, the cathodic reduction of electrolyte degradation products can also contribute to the SEI.…”

“…The first step is the electrochemical step, whereas the rest are chemical steps, including Li + dissolution during Li 3 N protolysis. Li + dissolution is an unlikely step at reducing potentials (i.e., more reducing than Li electrodeposition); instead, the N can be substituted by X to form lithium alkoxide (LiX) in reactions and It has been recently reported that LiX forms the solid-electrolyte interface (SEI) on the cathode . In addition to LiX, the cathodic reduction of electrolyte degradation products can also contribute to the SEI.…”

Operando investigations of the solid electrolyte interphase in the lithium mediated nitrogen reduction reaction

“…There have been substantial efforts aiming at a better understanding of the role of the SEI, though most studies rely on ex situ techniques. 12–16 In situ investigations of the SEI by means of neutron reflectometry and grazing incidence wide-angle X-ray scattering (GI WAXS) have been conducted only with LiClO 4 as electrolyte salt at current densities below 1 mA cm −2 . 17,18…”