Research On The Electrolyte Material And The Preparation Of Thin Electrolyte Film Of IT-SOFC

The operating temperature at 750℃in SOFC can decrease materials degradation, improve the reliability and stability of SOFC, and facilitate the use of low cost metallic components such as an interconnect instead of the more expensive ceramic interconnect. There are two mainly approaches to reduce the: 1.use electrolyte materials with high ipnic conductivity at low temperature; 2.decrease the electrolyte thickness. For these reasons, anode support SOFCs with different electrolyte materials and different electrolyte thickness were fabricated by dual dry pressing and co-firing method.Conventionl YSZ was studied as electrolyte materials.Three single SOFCs with different electrolyte thickness were prepared, which thickness were controlled by the amount of electrolyte powder. The open circuit voltage of single cells with 0.3g,0.2g amd 0.1g YSZ were 1.02V,0.994V and 0.99V. A peak current density of 489mA/cm²,526 mA/cm² and 548 mA/cm², and a peak power density of 114.2m W/cm²,122.9 mW/cm² and 141.7 mW/cm² were achieved at 900℃,respectively. With the decrease of electrolyte thickness, the electrical performance increased, which agreed well with the theoretical calculation results. The peak current density and the peak power density of cell with 0.1g YSZ were decreased from 548 mA/cm² and 141.7 mW/cm² at 950℃to 124 mA/cm² and 30.4mW/cm² at 700℃.All these show that YSZ depends too much on operating temperature which can't be electrolyte material of IT-SOFC.Ce_0.85Gd_0.15O_2-δ(CGO) which exhibits higher ionic conductivity in air than YSZ at intermediate temperature was choosed as electrolyte material. CGO and La_0.6Sr_0.4Fe_0.8Co_0.2O_3-δ(LSCF) were prepared by citric acid sol-gel method. XRD and TEM analysis were made to check the phase composition and particle size of the sample. Three cells with different electrolyte thickness were fabricated by the similar method as YSZ cell. The quality of CGO were 0.1 g,0.2g and 0.3g, respectively. The peak current densities of these cells were 385mA/cm²,315mA/cm² and 262mA/cm², and the peak power densities were 93.2mW/cm²,69 mW/cm² and 45.4mW/cm² at 900℃,respectively. The open-circuit voltage of the cells were 0.81V,0.76Vand 0.63V respectively, which were much depressed by comparison with the theoretical open-circuit potential of a hydrogen fuel cell of 1.05-1.10V. This is as a result of the electronic leakage through the CGO electrolyte at open circuit, which induced by by the conversion of some Ce⁴⁺ ions to Ce³⁺ ions in CGO electrolyte. When a cell was operated at different temperature, the electronic leakage current density increases with increased temperature,which slower the reduction of electrical performance and the OCV increases with the decreased temperature. This change has proved that CGO is a very well electrolyte material of IT-SOFC.In this study, in an attempt to. suppress the reduction of ceria, a thin ScSZ layer was inserted between the CGO electrolyte film and NiO-CGO anode. ScSZ were prepared by citrate Sol-Gel method and co-precipitation method, respectively. XRD and TEM analysis were made to check the phase composition and particle size of the sample. A peak current density of 677.5mA/cm²,605.6 mA/cm²,534.8 mA/cm² and 435.3mA/cm² and a peak power density of 197.3 mW/cm²,156.4 mW/cm²,147.6 mW/cm² and 108.8 mW/cm² were achieved at 800℃,750℃,700℃and 650℃, respectively. The measured OCV of the SOFC with CGO/ScSZ electrolyte were 1.02V to 1.05V at 800℃to 650℃, much higher than the values of SOFCs with CGO (CGO =0.1,0.2 and 0.3g) electrolytes. This indicates that the ScSZ electrolyte film was so compacted which could block the electronic current. A similar effect is applicable to the ScSZ layer incorporated between anode and CGO electrolyte films for improvement of the OCV.