Thermo- vs. electrocatalysis on Fe- and Ru-based cathodes for ammonia synthesis in proton-conducting ceramic cells

Electrochemical ammonia synthesis in proton-conducting ceramic cells offers an electricity-assisted alternative to the energy-intensive Haber-Bosch process and may enable decentralized NH$_3$ production. This study compares the performances of Fe- and Ru-based cathodes with similar catalyst loadings on BCZY721 membranes and an active area of 12.57 cm$^{2}$. Under open-circuit voltage (OCV) conditions, Ru exhibited NH$_3$ synthesis rates nearly two orders of magnitude higher than those of Fe. Under polarization, rates of 5.52 × 10$^{-8}$ mol cm$^{-2}$ s$^{-1}$ for Ru and 3.03 × 10$^{-8}$ mol cm$^{-2}$ s$^{-1}$ for Fe were achieved, while increased Ru loading further raised the rate to 7.70 × 10$^{-8}$ mol cm$^{-2}$ s$^{-1}$ at 475◦C. Despite its lower intrinsic activity, Fe showed a substantially stronger relative response to polarization and approached the performance of Ru. These distinct behaviors provide a basis for electrode concepts combining high intrinsic activity with strong polarization responsiveness. Further mechanistic studies are required to clarify the origin of the observed performance differences.