Effect of Carbon Dioxide on the Cathodic Performance of Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are ideally suited for environmentally benign conversion of chemical energy in hydrocarbons to electricity.  While medium scale SOFC power systems in the hundreds of kWe have been demonstrated, larger scale application of such systems have been hobbled by still too high cost.  In order to bring down the cost of these systems, manufacturing costs have to be decreased while the power densities have to be improved.  Achieving these twin goals will require the reduction of operating temperatures of the cells. On a single cell level, the largest source of performance loss at lower operating temperatures occurs in the cathode.  Thus many different groups are presently investigating a slew of new cathode materials.  While the performance of the new cathode materials are impressive under controlled operating conditions, it is presently not clear whether they will stand up to the rigorous and punishing operating environments of actual fuel cell stacks.  In particular the interfacial and bulk stability of the new generation of cathode materials such as strontium doped lanthanum cobaltites, strontium doped lanthanum cobaltites , and lanthanum nickelates when exposed to anodic gases such as carbon dioxide and water vapor is presently unknown.  Exposure to at least trace amounts of anodic gases during the lifetime of the stack is a likely occurrence and could arise from back diffusion, pinholes in the electrolyte and/or interconnection, or compromised seals.  Thus, it is important to have a clear understanding of the thermodynamic and kinetic stability of both the interfaces and the bulk of the cathode material under exposure to the aforementioned anodic gases.  In this paper, we present results from a study that combines electrochemical and x-ray measurements of the impact of carbon dioxide exposure on strontium doped lanthanum cobalt iron oxide (LSCF), as a function of strontium dopant concentration, gas composition, and temperature.  These are combined with computational studies of the stability of the LSCF cathode to carbon dioxide exposure.  The longer term goal of this study is to identify cathode compositions which have long term stability to exposure to trace anode side gases.