Performance Characteristics of Composite Film Electrolytes for Intermediate-Temperature Solid Oxide Fuel Cells

This paper reports on the estimated performance of a cell with a three-layer electrolyte, consisting of one gadolinia-doped ceria (GDC) layer, one yttria-stabilised zirconia (YSZ) electron-blocking layer and one CGO-YSZ solid solution interlayer, the latter being used to avoid solid-state reaction and interdiffusion between YSZ and GDC, in comparison to a cell with a double-layer YSZ-CGO composite electrolyte. For a constant temperature and overall cell oxygen potential as boundary conditions, the open circuit voltage, the voltage under operating conditions and the oxygen potential profile inside the electrolyte are related to the ionic and electronic transport properties of the materials involved and are calculated as a function of the thickness of the layers involved and the relative positions of the YSZ and GDC layers. Thermodynamic stability of the electrolyte is shown to depend upon the transport properties of the materials and primarily the electronic conductivity of the air-side layers. To determine the particular ionic and electronic contributions for conduction of the materials involved, conductivity was measured as a function of the oxygen partial pressure and temperature, using the standard four point d.c. method. Based on the calculations, performed the conditions are discussed under which a functionally graded composite electrolyte YSZ-CGO can be effective for intermediate-temperature solid oxide fuel cells (SOFCs).