Operando optical and quantitative electrochemical studies of solid oxide fuel cell anode degradation and regeneration

Abstract

Solid Oxide Fuel Cells (SOFCs) are high temperature (600-1000 °C) devices that can generate electricity with extremely high efficiencies from a wide variety of fuels, including H 2, CH 4, Biogas, and crushed coal. Unfortunately, the SOFC anodes are highly sensitive to gas phase contaminants, including sulfur and carbon containing fuels. Sulfur is ubiquitous in all carbon containing fuels, with concentrations as low as a few parts per million to as high as 1% by mass. At all concentrations sulfur substantially decreases SOFC performance. Conventional models propose that sulfur decreases fuel cell performance by blocking anode active sites, preventing electrochemical reactions, and reducing surface area for heterogeneous catalysis. Carbon containing fuels can rapidly degrade SOFCs due to graphitic carbon formation. Graphite blocks active sites on the anode, causes damage within the anode microstructure, and removes electrocatalytic material via metal dusting. Studies presented in this work used operando optical techniques and quantitative electrochemistry to study degradation and remediation of SOFC anodes. First, since typical electrochemical techniques infer microstructural changes rather than directly measuring surface area, a traditional electrochemical technique, chronocoulometry (CC), was adapted to SOFCs for the first time to measure the electrochemically active area of the anode. This technique showed that active area is temperature dependent, and that sulfur participates in electrochemical reactions, decreasing performance with sluggish oxidation kinetics, rather than simply blocking active sites. Carbon monoxide, on the other hand, decreased the number of active sites, rather than participating in electrochemical reactions, either by blocking active sites or forming carbon. Then, a comparative study was undertaken of different methodologies of carbon remediation, comparing electrochemical oxidation, molecular oxygen, and steam as methods to remove graphite accumulated on SOFC anodes. This study found that with all methods, CO 2 played a key role in removing carbon, that both electrochemical oxidation and steam removed carbon more globally than oxygen, and that imaging the entire cell is critical for understanding the complex, spatially and temporally heterogeneous chemistry occurring across SOFC anodes. Finally, sulfur was employed to passivate SOFC anodes operating on dry methane, significantly reducing carbon formation with only slight decreases in electrochemical performance.