Solid oxide fuel cells (SOFCs) provide a means for direct conversion of hydrogen or hydrocarbon fuel to electricity, thereby overcoming limitations of the Carnot cycle. Due to their high energy conversion efficiency (>50%) and flexibility in fuel choice, SOFCs reduce carbon dioxide output when operating on hydrocarbon fuels available in current infrastructure (e.g. kerosene) and will eliminate greenhouse gas emissions when hydrogen fuel becomes widely available. In addition, SOFCs can be operated in reverse, producing hydrogen or carbon monoxide through steam and carbon dioxide electrolysis, respectively, and acting as buffers to smooth out power oscillations from such renewable resources such as solar and wind.
Mixed ionic electronic conductor (MIEC) materials, which conduct both electricity and oxide ions, have been shown to reduce improve SOFC electrode performance, especially at lower temperatures (~600 oC). Often, MIEC materials exhibit significant changes in oxygen content during operation which results in a lattice dilation termed chemical expansion, which in turn has been shown to result in mechanical failure of SOFC composite systems (see figure) where large oxygen chemical potential gradients exist.
In this research, new materials with mixed ionic electronic conductivity (improving electrode kinetics) while at the same time achieving a breakthrough in maximizing durability by minimizing chemical expansion using stable oxides is investigated. A recent investigation has shown that chemical expansion arises from two effects, the change in radius of multivalent cations and a contraction from oxygen loss [1], thereby giving two ways to reduce chemical expansion. In this work, multivalent elements are being introduced into cerium oxide, resulting in MIEC behavior and potentially a reduced chemical expansion coefficient (alphac, see figure), analogous to the thermal expansion coefficient.
