The high fuel flexibility of solid oxide fuel cells allow them to be operated using carbonaceous fuels. This work focuses on a supply of carbon monoxide on large planar single cells, since detailed investigations thereof are rarely found in literature. In the course of this project, stable cell operation was achieved with CO/CO2/N-2 gas compositions at 800 degrees C and 750 degrees C as well as acceptable performance, though test results with H-2/H2O/N-2 gas mixtures were not reached. Electrochemical impedance measurements revealed high polarization resistances to be the cause of this differences in performance. The combination of carbon monoxide and hydrogen as reactants in fuel mixtures results in rapid performance degradation: a performance reduction of 27% was recognizable within 6 h. Additionally, a 100 h test at 700 degrees C with CO/CO2/N-2 = 20/10/70 vol% in polarization conditions (50 mAcm(-2)) is also presented, wherein there was continuous degradation at a rate of 17.2% over 100 h. Scanning electron investigations showed significant influences on the microstructure as carbon led to an enlargement of the nickel components and the coverage of the catalyst surfaces, respectively. (C) 2018 Elsevier Ltd. All rights reserved.