Ammonia Synthesis Via Electrochemical Atomic Hydrogen Permeation between 25 and 100 C
Authors: Ripepi Davide , Zaffaroni Riccardo , Schreuders Heman , Boshuizen Bart , Mulder Fokko 
Published: July 2022

Ammonia is an energy-rich molecule, primarily used as fertiliser and produced at large scale via the intensive Haber-Bosh process, which is responsible for 1-2% of the CO2 emitted globally.1 In recent years, the electroreduction of N2 to ammonia registered a bursting interest, motivated by the urgent necessity to meet the fast-growing demand for carbon neutral fertilisers and sustainable fuels. However, ammonia synthesis in conventional electrochemical aqueous electrolyte systems is facing serious challenges, such as the competing hydrogen evolution reaction that regularly prevails during electrocatalysis.

Herein we propose a strategy to decouple the hydrogen generation from the activation and hydrogenation of nitrogen (Figure 1). Atomic hydrogen is electrochemically generated from water reduction and inserted in the nickel electrode/membrane lattice. The dense metallic electrode provides a selective and controlled access of protons and electrons to the nitrogen active site, while ensuring a complete separation from the electrolyte. Our results demonstrate the unprecedented hydrogenation reaction of activated surface nitrogen to ammonia by electrochemical permeating atomic hydrogen. Ammonia synthesis and quantification were carried out following rigorous protocols and control experiments, including careful check of contamination level, purification of feed gases and 15N2 experiments. A gas chromatography method was developed to continuously detect in situ the produced ammonia, with a limit of quantification of 150 ppb.2 Quantitative 15N labelling experiments confirm that ammonia is produced directly in the gas phase at room temperature and atmospheric pressure from gaseous N2. Experimental observations reveal that the presence of N-vacancies, formed upon the hydrogenation of surface nitrides, plays a key role in the catalytic process facilitating gaseous nitrogen adsorption via a Mars-van Krevelen mechanism. This work provides an alternative pathway for the development of an efficient direct electrolytic ammonia production at ambient conditions from water, nitrogen and renewable electricity.

We are currently extending our knowledge on the reaction mechanisms and considerably improving the systems performances. Preliminary results reveal that the formation of nitrogen vacancies via electrochemical hydrogen permeation at 100 C and atmospheric pressure enables a direct hydrogenation process, in which H2, N2 and N-vacancies react to form NH3 at conditions otherwise not favourable to ammonia synthesis.

In this talk our latest progress on ammonia synthesis via electrochemical permeating hydrogen will be reviewed.

References

D. R. MacFarlane, P. V. Cherepanov, J. Choi, B. H. R. Suryanto, R. Y. Hodgetts, J. M. Bakker, F. M. Ferrero Vallana and A. N. Simonov, Joule, 2020, 4, 1186-1205.

R. Zaffaroni, D. Ripepi, J. Middelkoop and F. M. Mulder, ACS Energy Letters, 2020, 5, 3773-3777.

D. Ripepi, R. Zaffaroni, H. Schreuders, B. Boshuizen and F. M. Mulder, ACS Energy Letters, 2021, DOI: 10.1021/acsenergylett.1c01568, 3817-3823.

Figure 1