Ceria‑iron electrocatalysts for ammonia electrosynthesis using protonic ceramic electrolysis cells

Abstract

Iron has proven to be a simple yet high-performing electrode for ammonia electrosynthesis, particularly when used with protonic ceramic electrolysis cells. On a proton-conducting BaCe_0.9Y_0.1O_3−δ (BCY) electrolyte, iron oxide forms an interfacial layer during sintering due to solid-state cation diffusion. In this work, we found that the ceria‑iron layer that is formed in-situ both enables electrode adhesion and is active for ammonia electrosynthesis. Cells with electrodes fabricated from CeO₂-Fe₂O₃ at a weight ratio of 1:1 (CeFe11) and 6:1 (CeFe61), designed to replicate the composition of the interfacial layer, resulted in ammonia formation rates similar to those of cells with pure Fe electrodes, reaching 1.1–1.2 × 10⁻⁸ mol s⁻¹ cm⁻² at an applied voltage of −1 V at 600 °C. The ceria‑iron catalysts exhibited higher catalytic activity and a moderate electrochemical activity. A comparison of these electrodes suggests that the regions where ceria and iron are in proximity are the most active for ammonia electrosynthesis. Furthermore, CeFe11 demonstrates similar ammonia formation rates on BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb1711) as on BCY; as BZCYYb is more stable than BCY in the presence of water vapor, the development of ceria‑iron electrodes could widen the application of iron-based electrodes to ammonia electrosynthesis combined with water electrolysis.