Ammonia synthesis using Fe/BZY–RuO2 catalysts and a caesium dihydrogen phosphate-based electrolyte at intermediate temperatures

Abstract

In this study, we developed an Fe₂O₃/BZY (yttrium-doped barium zirconate)–RuO₂ (Fe/BZY–RuO₂) cathode catalyst, which was applied to the electrochemical synthesis of NH₃ using a proton-conducting electrolyte, CsH₂PO₄/SiP₂O₇, at 220 °C and ambient pressure. The highest faradaic efficiency of 7.1% was achieved at −0.4 V (vs. open-circuit voltage (OCV)) and the highest NH₃ yield rate of 4.5 × 10⁻¹⁰ mol (s cm²)⁻¹ was achieved at −1.5 V (vs. OCV). We also successfully detected N₂H₄ and NH₃ at −0.2 V (vs. OCV), which indicated that the N₂ reduction proceeded via an associative mechanism. A potentiostatic pulse experiment was conducted under a feed of Ar or N₂ in the cathode at different applied voltages to investigate the N₂ reduction reaction (NRR) mechanism. A model was developed to fit the current response of the potentiostatic pulse experiment, which comprised the decomposition of adsorbed intermediates on the surface of the cathode catalyst, diffusion of H in the cathode catalyst, and an electrical double layer. The results revealed that the rate constant estimated by the model for the decomposition of intermediates, such as NH or N₂H, was lowest at −0.2 V, where N₂H₄ was detected. The fitting results also indicated that the NRR proceeded via an associative mechanism at lower applied voltages, while a dissociative mechanism dominated at higher applied voltages.