(Invited) Electrolytic Synthesis of Ammonia at Ambient Conditions from Nitrogen and Acidic Water By a Catalyst-Free, Plasma Process

• Published in: Meeting abstracts (Electrochemical Society) Vol. MA2019-01; no. 32; p. 1683
• Main Authors: Hawtof, Ryan, Toth, Joseph, Ghosh, Souvik, Guarr, Evan, Xu, Cheyan, Renner, Julie N., Sankaran, R. Mohan
• Format: Journal Article
• Language: English
• Published: 01.05.2019
• ISSN: 2151-2043; 2151-2035
• DOI: 10.1149/MA2019-01/32/1683

Industrially, nitrogen reduction to ammonia is performed via the Haber-Bosch process at high pressure and high temperature which makes it difficult to conduct at small scales, and uses hydrogen gas as a reactant that currently comes from steam methane reforming which results in significant carbon dioxide emission. Electrolytic synthesis of ammonia is attractive because it can take place at or near ambient conditions, enables remote operation and integration with renewable sources of energy such as solar and wind, and can rely on water (typically acidic) as the source of hydrogen atoms. However, to date, the selectivity for electrolytic ammonia formation has been low, mostly because of issues associated with the catalyst materials. Specifically, large overpotentials are required to reduce the stable nitrogen molecule, and at these potentials hydrogen preferentially adsorbs on the catalyst surface and leads to hydrogen gas evolution instead of ammonia formation. In this talk, we will present a new electrolytic approach to ammonia synthesis in which the solid cathode is replaced by a gas discharge (plasma). Similar to the more typical electrochemical systems, ammonia is formed at ambient conditions from nitrogen and water using only electricity. However, nitrogen is reduced without a catalyst by the unique interactions of the plasma and water. Specifically, electrons in the plasma potentially excite or even dissociate the nitrogen molecule in the gas phase, and solvate to produce one of the strongest reducing agents known. Remarkably, we find that at certain processing conditions, ammonia is synthesized with 100% charge-transfer (faradaic) efficiency. Scavenger experiments show that solvated electrons and hydrogen radicals are key intermediates. The dependence of ammonia formation on current, time, and pH, as well as potential reasons for the highly selective synthesis of ammonia in our process will also be discussed.