The need for cleaner and more efficient alternative energy sources is becoming urgent as concerns mount about climate change wrought by greenhouse gas emissions. Solid oxide fuel cells (SOFCs) are one of the most efficient options if the goal is to reduce emissions while still operating on fossil energy resources. One of the foremost problems in SOFCs that causes efficiency loss is the polarization resistance associated with the oxygen reduction reaction(ORR) at the cathodes. Hence, improving the cathode design will greatly enhance the overall performance of SOFCs. Lanthanum nickelate, La2NiO4+δ (LNO), is a mixed ionic and electronic conductor that has competitive surface oxygen exchange and transport properties and excellent electrical conductivity compared to perovskite-type oxides. This makes it an excellent candidate for solid oxide fuel cell (SOFC) applications. It has been previously shown that composites of LNO with Sm0.2Ce0.8O2-δ (SDC20) as cathode materials lead to higher performance than standalone LNO. However, in contact with lanthanide-doped ceria, LNO decomposes resulting in free NiO and ceria with higher lanthanide dopant concentration. In this study, the aforementioned instability of LNO has been addressed by compositional tailoring of LNO: lanthanide doped ceria (LnxCe1-xO2,LnDC)composite. By increasing the lanthanide dopant concentration in the ceria phase close to its solubility limit, the LNO phase has been stabilized in the LNO:LnDC composites. Electrical conductivity of the composites as a function of LNO volume fraction and temperature has been measured, and analyzed using a resistive network model which allows the identification of a percolation threshold for the LNO phase. The thermomechanical compatibility of these composites has been investigated with SOFC systems through measurement of the coefficients of thermal expansion. LNO:LDC40 composites containing LNO lower than 50 vol%and higher than 40 vol% were identified as being suitable to incorporate into full button cell configuration from the standpoint of thermomechanical stability and adequate electrical conductivity. Proof-of-concept performance comparison for SOFC button cells manufactured using LNO: La0.4Ce0.6O2-δ composite to the conventional composite cathode materials has also been provided. This thermodynamics-based phase stabilization strategy can be applied to a wider range of materials in the same crystallographic family, thus providing the SOFC community with alternate material options for high performance devices.

The US navy has a critical need for air independent advanced electric power sources to replace batteries in unmanned undersea vehicles (UUVs). Solid oxide fuel cells (SOFCs) are being considered as one potential replacement option. However, SOFCs typically operate using atmospheric air as their oxidant which is not an option for this underwater application. For this application, pure pressurized oxygen would be used as the oxidant which motivates the search for a cathode material which would be optimal for a high oxygen partial pressure environments. Specifically, this research focuses on cathode materials which can exploit the unique operating conditions required for UUVs. The operation in 100% oxygen atmosphere rather than air provides a significant opportunity. This is because oxygen surface exchange and bulk transport through the cathode is mediated through point defects whose concentrations are sensitive to the partial pressure of oxygen in the atmosphere surrounding the cathode. Oxygen bulk transport along with oxygen surface exchange are the rate controlling steps in oxygen reduction and incorporation at the cathode. The focus of this research is to examine the relationship between oxygen partial pressure and its effect on SOFC cathode performance for two different families of cathode materials, namely strontium doped lanthanum manganite, and a relatively new class of cathode materials, rare-earth nickelates. The experimentally measured relationship between cathode polarization and oxygen partial pressure will be correlated with the underlying transport and surface exchange processes in both families of materials.