PROTON CONDUCTIVE PROPERTIES OF GADOLINIUM-DOPED BARIUM CERATES AT HIGH-TEMPERATURES

Proton conductive properties of Gd-doped BaCeO3 ceramics were investigated by electrochemical methods (hydrogen permeation test and hydrogen-oxygen fuel cell test) in the temperature range of 600-degrees-C to 1000-degrees-C. As a result of electrochemical hydrogen-permeation tests, it was verified that the proton was the only conductive species in BaCe1-xGdxO3-alpha in the absence of oxygen at 600-degrees-C-1000-degrees-C. Next the relationship between conductivity and Gd-substitution value of x BaCe1-xGdxO3-alpha (0.5 < x < 3.0) was determined in various atmospheres. The highest conductivity was observed for 0.20 < x < 0.25. A fuel cell using BaCe0.8Gd0.2O3-alpha ceramics with a thickness of 0.5 mm as a solid electrolyte was constructed. The short-circuit currents of this cell were 1.2 A cm-2 at 1000-degrees-C, and 0.8 A cm-2 at 800-degrees-C, which were the best performances in the fuel cell using perovskite-type electrolyte instead of YSZ. The transference numbers of mobile ions (protons and oxide ions) were determined from the evolution rate of water vapor at each electrode when the fuel cells were discharged. Regarding the temperature dependence on the transport numbers of proton and oxide ions, it was found that protonic conduction was fully dominant at 600-degrees-C. On the other hand, oxide ions were the main charge carriers at 1000-degrees-C and mixed conduction was observed in the temperature range of 600-1000-degrees-C.