Enhancement of Lithium-Mediated Ammonia Synthesis By Addition of Oxygen
Authors: Li Katja , Andersen Suzanne Zamany , Statt Michael , Saccoccio Mattia , Bukas Vanessa Jane , Krempl Kevin , Sazinas Rokas , Bruun Pedersen Jakob , Shadravan Vahid , Zhou Yuanyuan , Chakraborty Debasish , Kibsgaard Jakob , Vesborg Peter , Norskov Jens , Chorkendorff Ib 
Published: July 2022

Ammonia is one of the most produced chemicals worldwide and is currently synthesized by the Haber-Bosch process, which is a thermally catalyzed method that requires high pressures and temperatures. These harsh conditions, in addition to the prerequisite steam reforming process, leads to about 1 % of the annual energy consumption and 1.4 % of the global CO2 emission. One way to mitigate some of the Haber-Bosch process is to produce ammonia electrochemically, utilizing renewable energy sources.

The electrochemical synthesis of ammonia faces several big issues. One being the selectivity, since the more facile hydrogen evolution reaction (HER) will always dominate over the nitrogen reduction reaction (NRR), and another issue being the activity, given that the nitrogen triple bond is very stable and therefore hard to split. Hence, most often the reported ammonia contents are in the low ppm regime, which makes it very susceptible to contaminations both from the gas stream (NH3 and NOx impurities) and the system itself (catalyst, cell, chemicals, nitrile gloves, etc.). To avoid misleading results, several protocols have been published on how to correctly perform NRR experiments [1, 2]. One of these confirm that only the Li-mediated ammonia synthesis (LiMeAS) is currently able to produce ammonia electrochemically [3]. The actual mechanism is not fully understood, but it is generally believed that the first step is Li plating from a Li salt containing non-aqueous electrolyte. The very reactive Li will then react with N2 solvated in the electrolyte to form Li3N, which is believed to hydrolyze to ammonia when in contact with a proton source. The currently highest archived faradaic efficiency (FE) is at 69 % at 20 bar N2 pressure when applying an ionic liquid as a proton shuttle [4].

In this work, we achieved up to 79 % FE at 20 bar N2 by the addition of 0.8 mol. % O2 in the reaction atmosphere. The positive effect of O2 is a very counterintuitive observation, since the original work by Tsuneto et al. [5] showed that the use of synthetic air significantly hindered the reaction, as it was postulated that O2 inhibits the reaction due to LiO2 formation and/or leading primarily to the oxygen reduction reaction (ORR). We will present experimental results obtained at 10 and 20 bar with varying O2 contents, which were measured accurately by a mass spectrometer probing the atmosphere just above the electrolyte inside the pressure vessel. By combining experimental observations with theoretical modelling, we conclude that the unexpectedly beneficial role of small O2 concentrations has a positive influences the solid electrolyte interface (SEI), which is of great importance in our system. Additional ex-situ X-Ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS) measurements were conducted without exposure to air and moisture, to analyze the SEI layer and deposition after electrochemistry. We believe that this study will not only be beneficial for industrializing the LiMeAS, but will also bring us a step further in understanding the complex mechanism behind this process.

[1] S. Z. Andersen et al., "A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements," Nature, vol. 570, pp. 504-508, 2019.

[2] H. Iriawan et al., "Methods for nitrogen activation by reduction and oxidation," Nature Reviews Methods Primers, vol. 1, no. 1, pp. 1-26, 2021.

[3] J. Choi et al., "Identification and elimination of false positives in electrochemical nitrogen reduction studies," Nature communications, vol. 11, no. 1, pp. 1-10, 2020.

[4] B. H. Suryanto et al., "Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle," Science, vol. 372, no. 6547, pp. 1187-1191, 2021.

[5] A. Tsuneto, A. Kudo, and T. Sakata, "Lithium-mediated electrochemical reduction of high pressure N2 to NH3," Journal of Electroanalytical Chemistry, vol. 367, no. 1-2, pp. 183-188, 1994.