Metal Nitrides for Electrochemical Ammonia Synthesis in Molten Salts

Abstract

Ammonia is one of the most important base chemicals in the world being a quintessential ingredient in most fertilizers. It is produced on the mega ton per year scale by the Haber-Bosch process from hydrogen and nitrogen at temperatures of around 400-500°C and pressures of up to 200 atmospheres contributing approximately 2% to global CO2 emissions.1 A promising alternative for small scale applications is electrochemical ammonia production utilizing molten salts as electrolytes.2 Such systems work at ambient pressure, moderate temperature and use renewable hydrogen sources like water enabling small local production of ammonia for energy storage and as building block chemical for fertilizer synthesis.2 Few materials have been evaluated as electrocatalysts for ammonia synthesis in molten salts, mainly transition metals.2Metal nitrides, especially Co3Mo3N, are highly active in classical ammonia synthesis with a Mars-van-Krevelen-type mechanism being suggested as the mechanism at play.3 Recently, computational studies have suggested using metal nitrides as electrocatalysts for ammonia formation assuming a similar mechanism.4 In this work, we explore the possibilities of using more complex gas electrode materials in alkali chloride melt to increase the rate and current efficiency of electrochemical ammonia synthesis. Furthermore, we hope to gain a deeper understanding of the mechanism of nitrogen reduction and more generally ammonia formation in molten salts. We synthesized several ternary metal nitrides using temperature-programmed nitridation of oxide precursor and characterized them by powder XRD, BET, XPS, UV-Vis, SEM, TEM and SQUID. Furthermore, the stability of the nitride phase in the chloride melt was tested and the necessary conditions to inhibit decomposition were optimized. A range of electrochemical tests (CV, amperometry, etc.) were carried out and XPS, as well as SEM were used to monitor compositional and structural changes on the surface. Finally, metal nitride electrodes were used for nitrogen reduction. Lan, R.; Irvine, J. T. S.; Tao, S.Sci. Rep. 2013, 3.Giddey, S.; Badwal, S. P. S.; Kulkarni, A. Int. J. Hydrogen Energy 2013, 38 (34), 14576.Abghoui, Y.; Skúlasson, E. Procedia Computer Science 2015, 51, 1897.Howalt, J. G.; Vegge, T. Phys. Chem. Chem. Phys. 2013, 15 (48), 20957.