Synthesis of ammonia directly from wet nitrogen using a redox stable La0.75Sr0.25Cr0.5Fe0.5O3−δ–Ce0.8Gd0.18Ca0.02O2−δ composite cathode

Abstract

Ammonia was directly synthesised from wet nitrogen at an intermediate temperature (375–425 °C) based on the oxygen-ion conduction of the Ce_0.8Gd_0.18Ca_0.02O_2−δ–((Li/Na/K)₂CO₃) composite electrolyte. A redox stable perovskite-based catalyst, La_0.75Sr_0.25Cr_0.5Fe_0.5O_3−δ (LSCrF), was synthesised via a combined EDTA–citrate complexing sol–gel process to be used as a component of the La_0.75Sr_0.25Cr_0.5Fe_0.5O_3−δ–Ce_0.8Gd_0.18Ca_0.02O_2−δ composite cathode for ammonia synthesis. Ammonia formation was studied at 375, 400 and 425 °C and a maximum ammonia formation rate of 4.0 × 10⁻¹⁰ mol s⁻¹ cm⁻² with corresponding Faradaic efficiency of 3.87% was observed at 375 °C when the applied voltage was 1.4 V. This is much higher than 7.0 × 10⁻¹¹ mol s⁻¹ cm⁻² at 1.4 V and 400 °C when Cr-free Sr-doped LaFeO_3−δ, La_0.6Sr_0.4FeO_3−δ was used as the catalyst for the electrochemical synthesis of ammonia, indicating LSCrF is potentially a better catalyst. Ammonia was successfully synthesised using a redox stable cathode with higher formation rates at reduced temperature. Introduction of Cr³⁺ ions at the B-site of doped LaFeO₃ improves both the chemical stability and catalytic activity for ammonia synthesis.