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. 2023 Jan 18;8(2):1003-1009.
doi: 10.1021/acsenergylett.2c02697. eCollection 2023 Feb 10.

Nonaqueous Li-Mediated Nitrogen Reduction: Taking Control of Potentials

Romain Tort  1   2 , Olivia Westhead  2 , Matthew Spry  2 , Bethan J V Davies  2 , Mary P Ryan  2 , Maria-Magdalena Titirici  1 , Ifan E L Stephens  2
Affiliations

Nonaqueous Li-Mediated Nitrogen Reduction: Taking Control of Potentials

Romain Tort et al. ACS Energy Lett. .

Abstract

The performance of the Li-mediated ammonia synthesis has progressed dramatically since its recent reintroduction. However, fundamental understanding of this reaction is slower paced, due to the many uncontrolled variables influencing it. To address this, we developed a true nonaqueous LiFePO4 reference electrode, providing both a redox anchor from which to measure potentials against and estimates of sources of energy efficiency loss. We demonstrate its stable electrochemical potential in operation using different N2- and H2-saturated electrolytes. Using this reference, we uncover the relation between partial current density and potentials. While the counter electrode potential increases linearly with current, the working electrode remains stable at lithium plating, suggesting it to be the only electrochemical step involved in this process. We also use the LiFePO4/Li+ equilibrium as a tool to probe Li-ion activity changes in situ. We hope to drive the field toward more defined systems to allow a holistic understanding of this reaction.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Illustration of the Li-mediated N2 reduction electrochemical interphase. Formation of a passivating layer from electrolyte degradation products is expected to slow down H+ diffusion to the active surface, using them more wisely to form NH3 rather than H2.
Figure 2
Figure 2
(a) Typical cyclic voltammogram at .s−1 of Ferrocene 10 mM with LiNTf2 1 M in THF/EtOH 99:1 v/v, used to monitor changes in reference electrode potentials. Insert: scheme for the Ferrocene–Ferrocenium equilibrium. (b and c) Comparison of LiFePO4, Li4Ti5O12 and Pt electrodes stability at open circuit in 1 M LiNTf2 (blue squares and orange triangles, respectively) or 1 M LiClO4 (green circles), in THF/EtOH 99:1 v/v, saturated with (b) N2 or (c) H2 gas. (d) Reproducibility (blue) and stability (orange) comparison over the course of an electrolysis passing 10 C of charge at a constant current of −2 mA·cmgeo–2 for 1 h 23 min 20 s, in LiNTf2 1 M in THF/EtOH 99:1 v/v.
Figure 3
Figure 3
Working and counter electrode potentials recorded when passing 10 Coulomb charge at constant current densities ranging from 0.1 to 10 mA·cmgeo–2, using a 1 cm2 Mo working electrode and Pt mesh counter electrode parallel to each other, parallel and separated by 3.6 cm with the LiFePO4 reference electrode midway. The produced ammonia remaining in the electrolyte post electrolysis was quantified using the Salicylate method (further details of the electrochemical cell and quantification are in the Supporting Information). Potential measurements and ohmic drop corrections are described in Figure S5. The polarization analysis displays a constant working electrode potential of −3.423 ± 0.019 V vs LiFePO4 and a linear increase in counter electrode potential.
Figure 4
Figure 4
(a) Change in LiFePO4/Li+ equilibrium potential with respect to LiNTf2 concentration in THF/EtOH 99:1 v/v, plotted in comparison to the slope of the Nernst equation at constant activity coefficients. (b) Nernst equation for the LiFePO4/Li+ equilibrium, omitting (left) or considering (right) activity coefficient variations with concentration, the latter explaining the observed experimental deviation at higher concentrations.
See this image and copyright information in PMC

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