Ammonia offers potential as a carbon-free energy carrier with a high hydrogen content and if fully oxidized, the products nitrogen and water are environmentally benign. It also serves as the major feedstock for fertilizer production and a building block for nitrogen containing chemicals and polymers. Ammonia is readily liquefied and conveniently stored in low cost steel tanks. It may function as a high energy density storage medium for hydrogen produced by renewable energy derived electricity. Ammonia synthesis is currently carried out in a few very large Haber-Bosch plants, mostly fueled from natural gas. The current large-scale Haber-Bosch (H-B) technology needs to run at constant inputs of energy and reactants. The economics dictate very large process reactors and separation systems, consequently the technology does not downscale well to small sized plants, nor does it work with interruptible energy supplies. Even though the ammonia synthesis efficiency can be considered low at ~70%, these efficiencies are only obtained in large-scale integrated facilities. Highly efficient ammonia synthesis in small-scale H-B systems does not exist, thus stimulating research towards a low temperature and pressure processes. Electrosynthetic pathways for NH₃ production theoretically offer higher efficiency and scalability but, the challenges are quite foreboding as the dinitrogen triple bound is extremely stable and most known nitrogen dissociation catalysts are deactivated by oxygen or water [1].

We are currently engaged in a DOE ARPA-e funded program to develop electrochemical methods for ammonia synthesis. We are developing and evaluating intermediate NH₃ synthesis processes, one based on the spontaneous electro-reduction of nitrogen in lithium ion systems where the potential for lithium is close to metal deposition and the subsequent formation of lithium nitride. The other utilizes proton conducting, anhydrous metal pyrophosphate/polymer composite membranes for protonation of nitrogen [2]. The advantage of the lithium process is the reaction is autocatalytic, however the reaction of Li₃N with water to form NH₃ and LiOH is exothermic, with an amount of enthalpy of -581.62 kJ mol^-1 and thus presents challenges for the design of an energy efficient process.

The direct protonation process is thermodynamically favorable at modest potentials near hydrogen evolution potentials, however it requires suitable electrocatalysts that dissociate dinitrogen but inhibit the parasitic hydrogen evolution reaction (HER). Using proprietary Li ion solid electrolytes, we have demonstrated lithium nitride formation at high coulombic efficiency and ammonia evolution upon hydrolysis. We have also demonstrated intermittent ammonia production at >8% efficiency using Pt catalysts, however the coulombic efficiency is limited by HER. We are exploring the use of carbon supported Ru, Mo_xC and Ru/Mo_xC catalysts to limit current efficiency losses due to HER.

References:

1. J. Van der Ham, M. T. Koper , D. G. Hetterscheid , Chem. Soc. Rev., 2014, 43, 5183
2. F. Garzon, C. R. Kreller, M. S. Wilson, R. Mukundan, H. Pham, N. J. Henson, M. Hartl, L. Daemen, ECS Transactions 2014, 61, 159-168.