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 N₂ reduction to NH₃. We report that an optimal water concentration of 35.9 mM and LiClO₄ 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 Li₂O in the solid electrolyte interphase, as suggested by our X-ray photoelectron spectroscopy measurements. Our results highlight the extreme sensitivity of lithium-mediated N₂ reduction to small changes in the experimental conditions.

Figure 1

(a) Effect of initial water concentration on Faradaic efficiency in electrolytes of 0.2 M, 0.6 M, 0.8 M, 1 M LiClO₄ in THF with 1% v/v ethanol. In each experiment, 10 C was passed at a current density of −2 mA cm^–2. Each data point represents a single experimental measurement. Faradaic efficiency data can be found in Table S1. Error bars represent the standard error calculated from the standard addition method of ammonia quantification (see Supporting Information, Figure S1). (b) A heat map showing the variation in Faradaic efficiency with LiClO₄ concentration and water concentration, using data from panel a. Intermediate values have been obtained by linear interpolation between measured values. Measured values are shown as black circles.

Figure 2

A comparison of the maximum Faradaic efficiencies reported for different strategies at ambient pressure.^,,,, Each of these systems uses THF as the solvent, and 1% v/v ethanol as a proton source, unless otherwise stated.

Figure 3

X-ray Photoelectron Spectroscopy measurements: (a) O 1s core level spectra, (b) Li 1s core level spectra, measured for electrode SEIs following experiments in 0.2 and 1 M LiClO₄ electrolytes, under dry (red line), optimum initial water concentration (blue line) and wetter than optimum (green line) conditions. (c-d) Relative elemental compositions of C, O, Li, and Cl under dry and wet conditions for 0.2 and 1 M LiClO₄ electrolytes, respectively. Other elements observed in trace amounts have been excluded. H cannot be detected by XPS so is also excluded but will be present in the SEI. Data for dry samples is replotted from previous work. All samples were gently rinsed in 0.1 mL of THF after the experiment to remove dried electrolyte from the surface. Spectra are all normalized to their respective peak maxima. Quantitative peak fitting is not presented here due to small differences in binding energy peak positions for Li₂O and Li₂CO₃ and LiClO_n species. Attempts at peak fitting to determine specific Li and O species can be found in the Supporting Information (Figures S5, S6).