Surface segregation and poisoning in materials for low-temperature SOFCs
- PDF / 1,268,785 Bytes
- 7 Pages / 585 x 783 pts Page_size
- 48 Downloads / 186 Views
troduction As commercial implementation of solid-oxide fuel cells (SOFCs) approaches, a better understanding of the surface chemistry and the processes leading to long-term performance degradation is required. The surface composition governs the kinetics of oxygen exchange between the gas phase and the mixed ionic-electronic conducting electrodes (see the Introductory article in this issue). This reaction, involving the adsorption of an oxygen molecule, its subsequent dissociation, ionization, and finally incorporation of oxygen ions into the ceramic is often the rate-limiting step for performance. Moreover, mass transport across the electrode-electrolyte interface also contributes a non-negligible resistance to the overall cell behavior, which points to the importance of the surface composition of the electrolyte for the operation of the SOFC. However, the electrochemically active surface and interfaces in SOFCs are generally very different from the bulk chemical compositions for virtually all materials commonly used for SOFCs, and ultimately will determine the performance and long-term durability of the devices. Several processes might contribute to the changes in the surface chemistry at low and intermediate operating temperatures (500–800°C), such as cation segregation and external surface poisoning, affecting all the components of the SOFCs
(electrolytes, cathodes, and anodes). In this article, we discuss these processes and how they might affect the electrocatalytically active surfaces and interfaces, with strong implications for SOFC performance and durability.
Electrolytes By far, the most widely employed electrolytes in SOFC technologies are those with a fluorite structure, such as Zr1–xYxO2–x/2 (YSZ) and substituted cerium dioxide (i.e., Ce1–xRExO2–x/2), where RE is a rare-earth element. Perovskite structured electrolytes, such as those based on substituted lanthanum gallate (e.g., (La,Sr)(Ga,Mg)O3–δ, (LSGM)), are also of interest. However, these perovskite-based electrolytes show similar trends (e.g., segregation of Sr2+ upon annealing1) to those that will be discussed for the electrode materials (also typically perovskite-based) later. Similar to the well-studied phenomenon of impurity segregation to grain boundaries in polycrystalline ceramic samples,2 the surfaces of fluorite-structured electrolytes after hightemperature treatment are coated with impurities, typically Ca, but also Na, Si, Al, as well as some transition metals.3–6 Studies using low energy ion scattering spectroscopy, which selectively analyzes only the very outer layer of atoms of a surface, showed that after calcination at 1000°C for five hours (i.e., comparable to the heat treatment that may be used to
John Druce, International Institute for Carbon Neutral Energy Research, Kyushu University, Japan; [email protected] Helena Téllez, International Institute for Carbon Neutral Energy Research, Kyushu University, Japan; [email protected] Junji Hyodo, Department of Applied Chemistry, Kyushu University, Japan; hyodo_
Data Loading...
