Green synthesis of ammonia from steam and air using solid oxide electrolysis cells composed of ruthenium-modified perovskite catalyst

Abstract

The green electrochemical synthesis of ammonia (NH3) through solid electrolysis has been intensively investigated. This research reported an improvement of NH3 production rate using in situ exsolution of ruthenium (Ru) atoms into lanthanum strontium chromium ferrite perovskite (LaxSr1−xCryFe1−yO3−δ) catalyst. The in situ Ru exsolution was achieved by reducing the optimized stoichiometric La0.33Sr0.67Cr0.33Fe0.52Ru0.15O3−δ (LSCrFRu) powders obtained in 10-vol% H2/Ar at 800°C for 1.0 h. These Ru nanoparticles (NPs) were embedded on the surface of the LSCrFRu matrix, evenly distributed with a size varying from 4.4 ± 0.5 nm. With the exsolution of Ru atoms, greater oxygen vacancies were formed in ex-LSCrFRu and gadolinium-doped ceria (GDC) composite than those in LSCrF-GDC electrocatalyst, beneficial to N2 gas adsorption and triple bond cleavage. These Ru-modified LSCrFRu-GDC catalysts showed a synthesis rate of 4.73 × 10−10 mol s−1 cm−2 at 550°C under 1.6 V, doubling the rate using LSCrF-GDC catalyst. The improved ammonia synthesis kinetic is mainly attributed to embedded Ru NPs and the increased oxygen vacancies formed during the in situ exsolution process. More active sites and higher activity for H2O and N2 adsorption and activation collectively advanced facile transportation of O2− that further promotes the cleavage of covalent bonds in H2O, providing more H+ for the hydrogenation in the nitrogen reduction reaction. This research will open a new paradigm for the electrochemical synthesis of ammonia with mitigating the drawbacks of traditional NH3 production.