Ammonia is an important chemical and a possible energy carrier. The Haber–Bosch process, currently used to produce ammonia industrially, is carbon intensive and so various alternatives that could be powered by renewable electricity are being investigated. One such approach that has been rigorously verified to make ammonia from N2 is based on Li electrochemistry in organic electrolytes. However, this method has so far been limited to Li as mediator and it has remained unclear as to whether it could be extended to other metals. Now, Jens Nørskov, Thomas Jaramillo, Ib Chorkendorff and colleagues at the Technical University of Denmark, Stanford University and SLAC National Accelerator Laboratory demonstrate that Ca can also mediate N2 reduction to form ammonia.
The researchers carried out experiments in an electrochemical flow cell at room temperature and 1 bar pressure, using various Ca salts as electrolytes. At the anode, H2 is flowed through a gas-diffusion electrode, where it is oxidized over a Pt-Au catalyst. This provides protons, which are transported by ethanol — acting as a shuttle molecule — to the cathode. At the cathode, N2 enters via a gas diffusion electrode and Ca2+ from the electrolyte is reduced to Ca0, electrodepositing on the surface. This metallic Ca reacts with N2 to form CaxNyHz species, which are then protonated by ethanol to make ammonia. The team report a Faradaic efficiency of 40% when using an electrolyte of calcium tetrakis(hexafluoroisopropyloxy)borate dissolved in tetrahydrofuran. The work shows that this kind of metal-mediated approach is not restricted only to Li, paving the way for future exploration of other candidates.
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