Synthesis And Electrical Conductivity Of SrCe_1-xEr_xO_3-α Ceramics

High-temperature protonic conductors have potential applications in fuel cells, gas sensors, membrane separators and membrane reactors, etc. Protonic conduction in ABO₃ perovskites was first discovered by Iwahara, and has aroused great attention. Among the ABO₃ perovskites studied, doped SrCeO₃, SrCe_1-yM_yO_3-α(M is some rare earth element; y is less than its upper limit of solid solution formation range, usually less than 0.20;αis the oxygen deficiency concentration per unit formula which depends on the concentration of dopant M and surrounding atmosphere), exhibits excellent protonic conduction under hydrogen-containing atmosphere at high temperature. In addition, it also exhibits oxide ionic conduction to some extent under certain conditions, thus have attracted great interest.In the present study, a series of proton-conducting perovskite-type oxide ceramics, SrCe_1-xEr_xO_3-α (x = 0.05, 0.10, 0.15, 0.20), were synthesized by high-temperature solid state reaction. Proton-conducting perovskite-type oxide ceramic SrCe_0.95Er_0.05O_3-α was also synthesized by sol-gel method. X-ray powder diffraction patterns show that the ceramics are of a single orthorhombic phase of perovskite-type SrCeO₃. Using the ceramics as solid electrolytes and porous platinum as electrodes, the measurements of conductivities and ionic transport numbers of the ceramic samples were performed by using ac impedance spectroscopy and gas concentration cell methods in the temperature range of 600 - 1000℃under the different atmospheres, respectively. We also systematically studied the relation between the dopant content x and the ionic conductivity of ceramic samples. The main results are as follows: (1) In wet hydrogen, the sample of x = 0.05 is almost a pure protonic conductor, whereas the other samples are pure protonic conductors at 600 - 800℃, and are protonic and electric mixed conductors above 900℃. The maximum protonic conductivities for the every samples are over 10^-2S·cm^-1. (2) In dry air, the samples are a mixed conductor of oxide-ion and hole. (3) In wet air, the samples are a mixed conductor of oxide-ion,the proton and hole. (4) The dopant content x in the samples also remarkably influences the conduction properties, SrCe_0.85Er_0.15O_3-α has the highest ionic conductivity among the samples. (5) SrCe_0.95Er_0.05O_3-α synthesized by sol-gel method is a pure proton conductor.