(55c) High-Temperature Electrocatalytic NH3 Production from N2 and H20: Investigation of Mixed Anionic (Oxide-Nitride) Electrocatalytic Systems

Ammonia is the key chemical for the production of fertilizers to feed the World’s growing population. The conventional Haber-Bosch (HB) process consumes ~2% of the total energy supply and releases 2.2 tons of CO₂ per ton of NH₃ produced. Therefore, for a sustainable future, alternative NH₃ production processes need to be developed. Among the alternatives, electrocatalytic-route has the potential to enable on-site/sustainable/scalable/energy-efficient ammonia production.

In this study, fundamental investigation of the double perovskite (DP) oxynitrides is targeted at the electronic/atomic/molecular levels. In particular, this study focuses on the development/characterization of iron-based DP oxynitrides for high-temperature electrocatalytic-NH₃ production from N₂ and H₂O at atmospheric pressure in an oxide-ion-conducting solid-oxide-electrochemical-cell (SOEC). The cell functions as an electrolyzer, reducing/dissociating H₂O to O^2- and H⁺/H* at the cathode where H⁺/H* activates lattice nitrogen within the structure of the DP oxynitrides to form NH₃. The resulting nitrogen vacancies are replenished by gaseous N₂ while O^2- ions are conducted to the anode where they form oxygen.

This project targets acquiring a fundamental understanding of the high-temperature electrocatalytic-nitrogen-reduction-reaction (e-NRR) using DP oxynitrides and manipulation of their anionic/cationic ordering through synthesis parameters. It is aiming to gain insight into the relationship between the DP oxynitride structure and their (electro)-chemical and physical properties, as well as their potential in SOEC applications involving nitrogen fixation/activation.

The DP oxynitrides were designed/characterized using the lab-based-techniques including XRD, XPS, Mössbauer, temperature-programmed desorption/reaction, DRIFTS, and Raman spectroscopy and synchrotron-techniques including XANES/EXAFS, NAP-XPS and NEXAFS. The electrocatalytic-activity experiments showed that the presence of mixed anions in the electrode structure significantly enhances the NH₃ production rate. It was also confirmed by an electrochemical-activity-control-experiment with 3% H₂O/Ar (absence of nitrogen) that the major source of nitrogen was N₂ fed to the reactor and the amount of NH₃ produced as a result of lattice nitrogen depletion is negligible.