Synthesis And Characterization Of Proton Conducting BaZr_0.1 Ce_0.7 Y_0.2 O₃ Electrolyte

High temperature proton conductors with perovskite structure have become hot for electrolyte of IT-SOFCs because of their high ionic conductivity, high ionic transferring number and low conducting activation energy. In this thesis, investigation is focused on BaZr_0.1Ce_0.7Y_0.2O_{3–δ} (BZCY). To improve the sintering property and conductivity, ZnO is incorporated as an additive. Single cell with BZCY electrolyte is fabricated and characterized.BZCY has been synthesized successfully using glycine-nitrate process. Conductivity tests at various temperature point out BZCY displays a fairly high conductivity in the intermediate-low temperature range, and it is around 0.039S/cm at 700℃. Fitting results of the conductivity data in terms of Arrhenius equation reveal the activation energy of BZCY is 67.68kJ/mol at 500⁶50℃, 89.23 kJ/mol at 650⁷50℃respectively. It proves a fairly good conductive property of BZCY at intermediaerature region.According to sintering shrinkage curves, the sintering of BZCY is not occurred until the sintering temperature ascends to 1073℃, and a sintering temperature as high as 1400℃even cannot finalize the sintering of BZCY. Meanwhile, SEM images indicate the sintering of a specimen calcined at 1300℃for 6h is not satisfactory yet. To improve the sintering behavior of BZCY, ZnO is incorporated to the specimen as an additive. And the effect of different amount of incorporated ZnO and how to add it to BZCY are investigated. Results show, when ZnO powder is used as additive which was mixed with BZCY directly, the addition of only 2mol% ZnO has facilitated BZCY sintering drastically and lowered the starting sintering temperature from 1073℃to 900℃. The conductivity of BZCY ascends to its peak with the addition of 4mol% ZnO. On the other hand, when ZnO is added to BZCY by decomposition of Zn(NO₃)₂ which is mixed with BZCY at firsit could improve both sintering ability and conductive property of BZCY much more.Cells using BZCY as electrolyte are fabricated and characterized. The influence of three methods for manufacturing electrolyte layer, namely slurry spin coating, dry pressing and spray pressing is examined. For the slurry spin coating method, the electrolyte membrane is very thin, about 17μm. Due to the low green density of electrolyte layer, it is difficult to densify BZCY electrolyte, which results in a low open-circuit voltage and large Ohmic resistance. Maximum power densities of the cell are 125 and 220mW/cm² at 650 and 700℃, respectively. The dry pressing method produces an excellent sintering of BZCY, which is attributed to the high green density of electrolyte. And the fuel cell displays an OCV close to the theoretical value. However the h but also very dense, then resulting in the best cell performance among three 40⁶0μm thickness of the electrolyte membrane is so big and uneven that the Ohmic resistance is very large. Maximum power densities of the cell fabricated by dry pressing were 165 and 212mW/cm² at 650 and 700℃, respectively. The spray pressing is a simple and effective one for fabricating BZCY films. First, the BZCY suspension is sprinkled on the anode surface to form a thin and even BZCY layer. Then the electrolyte layer and the anode pellet are pressed together in a high pressure in order to increase the green density of such bi-layers. The high green density enhances the sintering of BZCY effectively, so that an increased OCV and decreased Ohmic resistance are achieved. Maximum power densities of the cell fabricated by spray pressing are 375 and 527mW/cm² at 650 and 700℃, respectively.In conclusion, the use of ZnO as an additive to facilitate sintering ability of BZCY has decreased the starting sintering temperature significantly and improved the sintering property of BZCY. Comparison of three different methods for making electrolyte indicate that spray pressing method produces a high-quality electrolyte layer which is not only thin enougmethods.