Solid Oxide Fuel Cells Operating Conditions Research Articles

A Controlled Solid State Diffusion Process to form Cermet Anodes for Solid Oxide Fuel Cells

ABSTRACTThe interfacing of thin film vapor deposition technologies and solid state electrochemistry has led to the recent development of polarized electrochemical vapor deposition (PEVD). In this study, PEVD was applied to deposit a thin layer of yttria stabilized zirconia (YSZ) over a porous metallic electrode to form the cermet anode of a solid oxide fuel cell (SOFC). During PEVD, oxygen ions are transported through the solid electrolyte of an SOFC under an electrical potential gradient provided by an external dc source. At the metallic electrode (anode) surface, oxygen ions react electrochemically with ZrCl4and YCl3 in the vapor phase to deposit YSZ. The growth of YSZ resembles the mechanisms illustrated in Wagner's tarnishing theory. However, modification has been made to the initial growth of YSZ at both electronically and ionically shorted paths along the metallic electrode and solid electrolyte surfaces, respectively. The initial experimental results in the present study showed that PEVD is capable of depositing a thin layer of YSZ on a porous metallic electrode to form a cermet anode. This layer not only provides a continuous ionic conducting path in the anode to reduce the overpotential loss, but also protects the metallic electrode from further sintering, vapor loss and poisoning in the harsh SOFC operating conditions.

Nonstoichiometry and Defect Chemistry of Ceria Solid Solutions

Ceria solid solutions are potential electrolytes for solid oxidefuel cells (SOFC). At the anode side of SOFCs at low oxygen partialpressures ceria is partially reduced. The nonstoichiometry ofCeO2-x solid solutions with 20 mol%SmO1.5, 20 mol% GdO1.5 and 10 mol%CaO were investigated by isothermal coulometric titration in a zirconia cellin the temperature range from 700 to 900°C and in the oxygen partialpressure range from 10-12 to 10-25 atm. Theresults of the 3 different compositions at 900°C are comparable to thosefor pure ceria. At lower temperatures, pure ceria is more stable againstreduction than ceria solid solutions. The dependence of nonstoichiometry onoxygen partial pressure suggests a defect model with oxygen vacancies andtheir associates with reduced cerium cations and dopant cations. Low oxygennonstoichiometries of x = 0.005 to 0.008 can be expected for ceria solidsolutions at 700° under SOFC operating conditions.