Electrochemical synthesis of ammonia was performed at 500 degrees C using a mixed protonic-electronic conducting cathode, Ru-doped BaCe0.9Y0.1O3 (BCYR) in a proton-conducting electrolysis cell (PCEC) using a BaCe0.9Y0.1O3 (BCY) electrolyte. Ru nanoparticles were formed in situ on the surface of the BCYR particles after a heat-treatment in a reducing atmosphere, as determined by TEM and XPS measurements. The BCYR cathode exhibited activity toward electrochemical ammonia formation, which indicates that the Ru nanoparticles were active sites for the electrochemical synthesis of ammonia. We found that altering the reduction temperature could be used to control the exsolved Ru-nanoparticle size; decreasing the Ru particle size contributes to an improvement in electrochemical ammonia formation due to an increased triple-phase-boundary active length. The ammonia formation rate per amount of exsolved Ru nanoparticles achieved with BCYR was higher than that achieved with a previously reported Ru-doped La1-xSrxTiO3 (LSTR) cathode. Our results suggest that the mixed protonic-electronic conduction in the BCYR cathode may contribute to the increased Ru-nanoparticles activity toward electrochemical ammonia synthesis. The dependence of the ammonia formation rate on the applied voltage and the reaction mechanism were discussed based on kinetic analysis. (c) 2017 The Electrochemical Society. All rights reserved.