Electrochemical Ammonia Synthesis in Barium-Cerium Zirconate Cells

Abstract

Fixation of nitrogen to ammonia (N2 (g) + 3H2 (g) ⇌ 2NH3 (g)) is firmly established as one of the most important chemical processes for humanity. Global production in 2016 reached 144 million tons, with 88% being used for fertiliser production.1 Ammonia is currently produced through the Haber process, requiring temperatures of 300-500 oC and pressures of 150-200 bar. This is associated with high CO2 emissions, made worse by the fact that the hydrogen for the process comes from steam reformation of methane. A suggestion to decarbonise the process uses ‘agile’ synthesis powered by offshore wind farms. In this scenario, the hydrogen for the process would be provided by electrolysis of water. The term ‘agile’ refers to the fact that it should be easy to start and stop the process to cope with wind intermittency. This would require new catalysts or systems capable of operating at significantly lower pressures. Ammonia also shows much promise as an energy storage material. Ammonia contains 17.6 wt % hydrogen and has an energy density of 13.6 GJ m-3 (pressurized tank at 10 bar).2 Electrochemical ammonia synthesis has been demonstrated in numerous different systems, involving many types of electrolytes.3–7 However most studies report current efficiencies of 10% or less. This highlights the challenge of reducing N2 in the presence of more easily reducible species. On this basis, it should be possible to improve both the rate and efficiency by developing new catalysts which can selectively reduce nitrogen, but not other species. In this work, BaCe0.2Zr0.7Y0.1O3- δ perovskite proton conductor-based electrolysis cells are studied for application of reduction of nitrogen gas to ammonia. Specifically, we focus on modifying electrodes to improve the cell performance. Some interesting results have already been obtained, showing drastically different currents when designing electrodes from mixed-metal systems. These currents, however, do not seem to translate into improved synthesis rates, which is also explored. Apodaca, L. E. Mineral Commodity Summaries. 116–117 (2018).Zamfirescu, C. & Dincer, I. Using ammonia as a sustainable fuel. J. Power Sources 185, 459–465 (2008).Kim, K., Yoo, C.-Y., Kim, J.-N., Yoon, H. C. & Han, J.-I. Electrochemical Synthesis of Ammonia from Water and Nitrogen in Ethylenediamine under Ambient Temperature and Pressure. J. Electrochem. Soc. 163, F1523–F1526 (2016).Skodra, A. & Stoukides, M. Electrocatalytic synthesis of ammonia from steam and nitrogen at atmospheric pressure. Solid State Ion. 180, 1332–1336 (2009).Zhou, F. et al. Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids. Energy Environ. Sci. 10, 2516–2520 (2017).Lan, R. & Tao, S. Electrochemical synthesis of ammonia directly from air and water using a Li+/H+/NH4+ mixed conducting electrolyte. RSC Adv. 3, 18016–18021 (2013).Murakami, T., Nishikiori, T., Nohira, T. & Ito, Y. Electrolytic Synthesis of Ammonia in Molten Salts under Atmospheric Pressure. J. Am. Chem. Soc. 125, 334–335 (2003).