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A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements

Nature volume 570, pages 504–508 (2019)Cite this article

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An Author Correction to this article was published on 26 September 2019

This article has been updated

Abstract

The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1,2,3,4 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 fixation7,8,9 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.

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Fig. 1: Potential range for chronoamperometric measurements of pure-metal catalysts and the detected amounts of ammonia in each measurement.
Fig. 2: An overview of literature data on the electrochemical synthesis of ammonia.
Fig. 3: Suggested protocol for the benchmarking of electrochemical nitrogen reduction.
Fig. 4: Ammonia synthesized at −3.1 V and more negative potentials vs SHE.

Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Change history

  • 26 September 2019

    An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

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Acknowledgements

This work was supported by the Villum Foundation V-SUSTAIN grant 9455 to the Villum Center for the Science of Sustainable Fuels and Chemicals. 400 MHz and 800 MHz NMR spectra were recorded on the spectrometers of the NMR Center at the Technical University of Denmark supported by the Villum Foundation. We acknowledge the contribution of Albert Kravos in setting up the analytical methods to detect ammonia.

Reviewer information

Nature thanks Lauren Greenlee, Mark Symes and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Author notes

  1. Sungeun Yang

    Present address: Center for Energy Materials Research, Korea Institute of Science and Technology (KIST), Seoul, South Korea

  2. These authors contributed equally: Suzanne Z. Andersen, Viktor Čolić, Sungeun Yang.

Authors and Affiliations

  1. Department of Physics, Technical University of Denmark, Kongens Lyngby, Denmark

    Suzanne Z. Andersen, Viktor Čolić, Sungeun Yang, Jakob Kibsgaard, Peter C. K. Vesborg & Ib Chorkendorff

  2. SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, CA, USA

    Jay A. Schwalbe, Adam C. Nielander, Joshua M. McEnaney, Jon G. Baker, Aayush R. Singh, Brian A. Rohr, Michael J. Statt, Sarah J. Blair, Matteo Cargnello, Stacey F. Bent, Thomas F. Jaramillo & Jens K. Nørskov

  3. Department of Chemistry, Technical University of Denmark, Kongens Lyngby, Denmark

    Kasper Enemark-Rasmussen

  4. Department of Materials, Imperial College London, London, UK

    Stefano Mezzavilla & Ifan E. L. Stephens

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Contributions

S.Z.A., V.C., S.Y., J.K., S.M., P.C.K.V., I.E.L.S. and I.C. conceived the idea for the protocol and experimental design. S.Z.A., V.C., S.M. and S.Y. performed the experiments; V.C. performed potential scale determination and Ohmic-drop correction; S.Y. performed isotope measurements; K.E.-R. performed NMR measurements and wrote the NMR section; J.A.S., A.C.N. and J.M.M. performed Fourier transform infrared spectroscopy and wrote the corresponding section; J.K., P.C.K.V., I.E.L.S. and I.C. supervised the work; S.Z.A., V.C. and S.Y. drafted the manuscript; and all authors contributed to the editing of the manuscript.

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Correspondence to Ib Chorkendorff.

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Extended data figures and tables

Extended Data Fig. 1 Concentrations of ammonia produced by various possible contamination sources.

All measurements were performed three times; the top of the bars show the mean, the errors show the standard deviation. The following were tested for contamination: a, new and treated Nafion membranes; b, new Celgard membrane; c, 2 ml of 0.1 M solutions or MilliQ H2O left open overnight; d, 2 ml 0.1 M KOH, into which one person had breathed 3 l of air through a glass straw.

Extended Data Fig. 2 NMR data.

Spectra were acquired using a Bruker AVANCE III HD 800 MHz spectrometer equipped with a 5 mm TCI CryoProbe. Solutions contain equal concentrations of 14NH4+ and 15NH4+ from NH4Cl. a, Spectrum of a solution of 600 µl 0.1 M KOH, acidified with 0.5 M H2SO4 to a pH of 1. CH3OH (200 μM) was added as an internal reference. b, Integrated peak areas from a for both 14NH4 and 15NH4. c, Spectrum of a solution of 500 µl 0.1 M LiClO4 in THF and ethanol with a ratio of 99:1, respectively, with 2 µl 4 M HCl and 50 µl THF-d8. d, Integrated peak areas from c for both 14NH4 and 15NH4. The variation of chemical shift of the NMR peaks is due to slight differences in the pH, volume, and/or temperature of the samples. e, Magnified view of the samples in a that contain the lowest concentrations of 14NH4+ and 15NH4+. f, Spectra of samples with the same concentrations as those in a, using a Bruker AVANCE III HD 400 MHz spectrometer equipped with a 5 mm Prodigy probe.

Extended Data Fig. 3 Gas phase Fourier-transform infrared spectra of labelled and unlabelled ammonia.

Spectra were acquired on a Nicolet iS50 spectrometer fitted with a 2-m path length gas cell heated to 135 °C. The total volume of the vaporized sample was 100 μl. The ammonia concentration was 1,000 p.p.m. in H2O before vaporization.

Extended Data Fig. 4 The set-up for gas (15N2, 14N2, Ar or 10% H2 in Ar) circulation through the electrochemical cell.

a, Schematic of the set-up. b, Photograph of the set-up with the components labelled. The pump that circulates the gas through the set-up is situated behind the metal panel, and its position is outlined by the white dashed rectangle.

Extended Data Fig. 5 1H NMR spectra of 0.1 M KOH samples after purging the solution with 15N2 gas.

Spectra were acquired using a Bruker AVANCE III HD 800 MHz spectrometer equipped with a 5-mm TCI CryoProbe. No 15NH4+ was seen in the measurements.

Extended Data Fig. 6 Concentration of ammonia, nitrite and hydrazine in the gas supply.

a–e, Three sequentially repeated measurements of the concentrations of: ammonia (using the indophenol method) without cleaning the gas stream (a); ammonia (using the indophenol method) with the described cleaning procedure (b); nitrite concentration without cleaning the gas stream (c); nitrite concentration with the described cleaning procedure (d); and hydrazine without cleaning the gas stream (e).

Extended Data Table 1 Literature data on the electrochemical reduction of N2 to ammonia in near-ambient conditions
Full size table
Extended Data Table 2 Electrochemical reduction of N2 to ammonia at high temperature or pressure
Full size table
Extended Data Table 3 Electrochemical reduction of N2 to ammonia in non-aqueous electrolytes
Full size table
Extended Data Table 4 Photochemical reduction of N2 to ammonia in non-aqueous electrolytes
Full size table

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Andersen, S.Z., Čolić, V., Yang, S. et al. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 570, 504–508 (2019). https://doi.org/10.1038/s41586-019-1260-x

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