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