Synthesis of ammonia directly from wet nitrogen using a redox stable La 0.75 Sr 0.25 Cr 0.5 Fe 0.5 O 3− δ–Ce 0.8 Gd 0.18 Ca 0.02 O 2− δ composite cathode

Ammonia was directly synthesised from wet nitrogen at an intermediate temperature (375–425 °C) based on the oxygen-ion conduction of the Ce0.8Gd0.18Ca0.02O2−δ–((Li/Na/K)2CO3) composite electrolyte. A redox stable perovskite-based catalyst, La0.75Sr0.25Cr0.5Fe0.5O3−δ (LSCrF), was synthesised via a combined EDTA–citrate complexing sol–gel process to be used as a component of the La0.75Sr0.25Cr0.5Fe0.5O3−δ–Ce0.8Gd0.18Ca0.02O2−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 LaFeO3−δ, La0.6Sr0.4FeO3−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.

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