A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements

Suzanne Z. Andersen, Viktor Čolić, Sungeun Yang, Jay A. Schwalbe, Adam C. Nielander, Joshua M. McEnaney, Kasper Enemark-Rasmussen, Jon G. Baker, Aayush R. Singh, Brian A. Rohr, Michael J. Statt, Sarah J. Blair, Stefano Mezzavilla, Jakob Kibsgaard, Peter C. K. Vesborg, Matteo Cargnello, Stacey F. Bent, Thomas F. Jaramillo, Ifan E. L. Stephens, Jens K. Nørskov and Ib Chorkendorff ()
Additional contact information
Suzanne Z. Andersen: Technical University of Denmark
Viktor Čolić: Technical University of Denmark
Sungeun Yang: Technical University of Denmark
Jay A. Schwalbe: Stanford University
Adam C. Nielander: Stanford University
Joshua M. McEnaney: Stanford University
Kasper Enemark-Rasmussen: Technical University of Denmark
Jon G. Baker: Stanford University
Aayush R. Singh: Stanford University
Brian A. Rohr: Stanford University
Michael J. Statt: Stanford University
Sarah J. Blair: Stanford University
Stefano Mezzavilla: Imperial College London
Jakob Kibsgaard: Technical University of Denmark
Peter C. K. Vesborg: Technical University of Denmark
Matteo Cargnello: Stanford University
Stacey F. Bent: Stanford University
Thomas F. Jaramillo: Stanford University
Ifan E. L. Stephens: Imperial College London
Jens K. Nørskov: Stanford University
Ib Chorkendorff: Technical University of Denmark

Nature, 2019, vol. 570, issue 7762, 504-508

Abstract: Abstract The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1–4 to the energy-intensive Haber–Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges5,6 facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation7–9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochemical reduction of nitrogen to ammonia.

Date: 2019
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DOI: 10.1038/s41586-019-1260-x

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