Increasing Ammonia Formation Rates of Li-Mediated Ammonia Synthesis with High Surface Area Copper Electrodes

Abstract

The Haber-Bosch process, which industrially produces ammonia, is one of the most important inventions of the 20th century. It is argued that without the ability to mass produce ammonia and therefore fertilizer, we would not be able to feed half of the current population. However, the Haber-Bosch process is harmful to the environment as it runs at high pressures and temperatures and is therefore very energy intensive. Furthermore, it utilizes H2 from steam reforming which causes large CO2 emissions. To mitigate the environmental strain of the Haber-Bosch process, electrochemical ammonia synthesis from renewable electricity sources would be an alternative, however the large activation barrier for dinitrogen splitting and the competition of hydrogen evolution reaction (HER) makes the electrochemical nitrogen reduction very difficult. (1)As of now, only the Li-mediated ammonia synthesis has been successfully proven by several labs to consistently produce ammonia at rates and faradaic efficiencies that could be industrially relevant. The exact mechanism is yet to be elucidated but it widely accepted that the first step is electrochemical plating of metallic Li from Li+ ions. The freshly plated Li metal is very reactive and is then believed to react with N2 in the electrolyte which produces an intermediate Li-N species. Lastly, ammonia is being formed by protonation of this Li-N species (2). Recent breakthroughs in the field managed to push the faradaic efficiencies to 78 %, however that was achieved at low current densities of -4 mA/cm2 (3). To make the process industrially relevant the current densities and therefore ammonia formation rates need to be increased significantly. The Department of Energy has stated in their REFUEL program a goal of 300 mA/cm2 at faradaic efficiencies of 90 % (4). In our latest publication, we have reached current densities of -100 mA/cm2 with high surface area Cu electrodes, however at relatively low faradaic efficiencies of 13 %. To synthesize the high surface area Cu electrodes we used the hydrogen bubble templating procedure that deposits metals at high overpotentials where the competing HER makes a templating structure and forms porous metal foams. We have further improved upon the deposition method by changing the substrate, which not only increased the physical stability but also the electrochemical performance. By varying the deposition conditions and optimizing the electrolyte for ammonia synthesis, we achieved a current density of -1 A/cm2 and high faradaic efficiencies of 75 %. The increase in faradaic efficiency is speculated to be due to changes in the solid electrolyte interface (SEI) layer, which we probe with X-ray photoelectron spectroscopy with the help of our in-house build transfer system that limits contact to air and moisture. The results are supported by theoretical models that calculate the Li+ conductivity of different constituents of the SEI layer. J. Kibsgaard, J. K. Nørskov, I. Chorkendorff, The Difficulty of Proving Electrochemical Ammonia Synthesis. ACS Energy Lett. 4, 2986–2988 (2019).N. Lazouski, Z. J. Schiffer, K. Williams, K. Manthiram, Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction. Joule. 3, 1127–1139 (2019).K. Li, S. Z. Andersen, M. J. Statt, M. Saccoccio, V. J. Bukas, K. Krempl, R. Sažinas, J. B. Pedersen, V. Shadravan, Y. Zhou, D. Chakraborty, J. Kibsgaard, P. C. K. Vesborg, J. K. Nørskov, I. Chorkendorff, Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science (6575). 374, 1593–1597 (2021).G. Soloveichik, in 2019 AIChE Annual Meeting: Topical Conference - Ammonia Energy (AIChE, 2019).