Oxygen-Enhanced Chemical Stability of Lithium-Mediated Electrochemical Ammonia Synthesis

Abstract

Although oxygen added to nonaqueous lithium-mediated electrochemical ammonia synthesis (LiMEAS) enhances Faradaic efficiency, its effect on chemical stability and byproducts requires understanding. Therefore, standardized high-resolution gas chromatography–mass spectrometry and nuclear magnetic resonance were employed. Different volatile degradation products have been qualitatively analyzed and quantified in tetrahydrofuran electrolyte by adding some oxygen to LiMEAS. Electrodeposited lithium and reduction/oxidation of the solvent on the electrodes produced organic byproducts to different extents, depending on the oxygen concentration, and resulted in less decomposition products after LiMEAS with oxygen. The main organic component in solid-electrolyte interphase was polytetrahydrofuran, which disappeared by adding an excess of oxygen (3 mol %) to LiMEAS. The total number of byproducts detected was 14, 9, and 8 with oxygen concentrations of 0, 0.8, and 3 mol %, respectively. The Faradaic efficiency and chemical stability of the LiMEAS have been greatly improved with addition of optimal 0.8 mol % oxygen at 20 bar total pressure.

The state-of-the-art in the electroreduction of NOx for the production of ammonia in aqueous and nonaqueous media at ambient conditions: a review

“…However, the reaction rate of phosphorus salt was low. It has been recently proposed that adding oxygen (0.8 mol%) to the system can stabilize the electrolyte and prevent the generation of by-products 144 (Fig. 20d).…”

“…Fig. 20 Hydrophobic GDE with (a) aqueous electrolyte and (b) proton donor cycling; 141 (c) ylide structure produced by reversible deprotonation of phosphorus cations; 143 (d) lithium-mediated electrochemical ammonia synthesis (LiMEAS) with oxygen 144. …”

Water Increases the Faradaic Selectivity of Li-Mediated Nitrogen Reduction

“… 20 They suggested that Li 2 O species formed in the SEI reduced Li + diffusivity in the SEI, 19 thus kinetically suppressing excessive lithium plating, a significant parasitic side-reaction, as well as other electrolyte-degrading side reactions. 24 Quantitative electrochemical measurements of suppression of Li plating will be possible given recent developments of more accurate reference electrodes for the Li-mediated system. 25 , 26 Our own XPS results are remarkably similar to those of Li et al, as shown in Figure 3 a-d: increasing water generally shifts the peaks away from those corresponding to LiClO n under dry conditions (i.e., 533 eV for O 1s and 56.9 eV for Li 1 s) to those corresponding to toward Li 2 O under moist conditions (i.e., 531.5 eV for O 1s and 55.3 eV for Li 1 s. Our results suggest that the replacement of LiClO n , LiCl, and related species with nonchlorinated species such as Li 2 O, consistent with earlier reports from the battery literature on the effect of trace H 2 O on the SEI.…”

A Versatile Li0.5FePO4 Reference Electrode for Nonaqueous Electrochemical Conversion Technologies

“…The Li-O 2 battery, for example, fundamentally relies on O 2 reduction in a nonaqueous electrolyte, and methods to improve the reversibility of Li 2 O 2 formation and oxidation have been thoroughly studied . In N 2 R studies, the addition of ∼1 mol% O 2 has also recently been demonstrated to improve the Faradaic efficiency to NH 3 due to the deposition of Li 2 O and LiOH species in the solid–electrolyte interphase. , With this in mind, we conducted an experiment in which Pt, Ag/AgCl, and LFP electrodes were submerged in 0.5 M LiClO 4 in 99 vol% THF and 1 vol% EtOH electrolyte in the aforementioned glass electrochemical cell (shown in the SI, section S1). We purged a series of gases through the electrolyte in the following order: (i) Ar, (ii) 5% O 2 in Ar, and (iii) Ar.…”

“…We note that the choice of RE was relatively inconsequential in understanding the Li-mediated N 2 R process in the early stages of the field, as early studies featured total cell potentials of ∼20 V, making fluctuations of hundreds of millivolts in the RE potential negligible. However, recent advances in optimizing electrolyte degradation, studying solid–electrolyte interphase formation, , identifying prominent reactions at the anode, and the application of high surface area electrodes , have enabled the decrease of the full cell potential to about 4–5 V, making a stable RE much more crucial. Owing to the advantages of LFP, we recommend that researchers consider transitioning from Pt and Ag/AgCl to LFP REs when studying nonaqueous electrochemical systems.…”