Operando Analysis of the Electrolyte’s Influence upon Degradation in Protonic-Ceramic Solid Oxide Fuel Cells Fed with CO/H ₂

Yttrium-and-cerium-doped barium zirconates represent common electrolyte materials used in protonic-ceramic solid oxide fuel cells (H-SOFCs) due to their high ionic conductivities at intermediate temperatures. Although protons serve as primary charge carriers, these materials also exhibit oxide conductivity that increases with the cerium content as well as temperature. For H-SOFCs operating with carbonaceous fuels, the latter phenomenon could be potentially harnessed for mitigating the deleterious effects of anode coking (carbon accumulation) by providing an oxygen source to directly or indirectly oxidize carbon while providing a faradaic contribution. In this study, three H-SOFCs differing by electrolyte support (yttrium-doped barium zirconate, BZY; cerium- and yttrium-doped barium zirconate, BZCY, and BZY/BZCY bilayer) were fabricated and operated galvanically using fuel mixtures of CO and H2 (0:1, 1:1, or 2:1 CO/H2) and at temperatures ranging from ∼ 560 °C–720 °C. With decreasing temperature and increasing CO content, voltage losses over multihour chronopotentiometry experiments became increasingly severe and were associated with carbon deposition at the Ni-BZY anode. Additionally, the electrolyte clearly influenced carbon-induced degradation, generally in the order of BZY > bilayer > BZCY. To understand the electrolyte’s influence on the anode’s chemistry, operando Fourier transform infrared emission spectroscopy was used to directly probe the anode headspace during operation. Real-time spectra combined with exhaust analysis (mass spectrometry) showed that CO2 produced in trace quantities (<1%) clearly depended on temperature and more subtly on electrolyte composition and suggest that (1) CO2 is a direct or indirect product of carbon removal at the anode, (2) oxides supplied by the electrolyte are responsible for carbon removal, and (3) the electrolyte impacts the extent of carbon deposition. Taken together, this study highlights the application and benefit of operando spectroscopic measurements combined with electrochemical analysis to probe electrocatalytic processes responsible for performance losses in H-SOFCs.