Preparation And Characterization Of Electrolyte Film Materials For IT-SOFC

Been the third system of fuel cells, Solid Oxide Fuel Cells(SOFCs) have the ability of converting chemical energy to electrical energy directly and efficiency. The outstanding advantages of fuel flexibility, environmental friendly, full solid structure and long-term stability making them be the first alternative use of clean and high efficiency power generation technology. At present, YSZ (yttria-stabilized zirconia) as electrolyte used in SOFCs need high operating temperature(～1000℃), resulting in a series troubles of interfacial reaction, electrode sintering and limit of component materials, putting off the commercialization of SOFCs. If the operating temperature can be lowered below800℃, the problems above can be overcome. The development trend of SOFCs is the management of reducing the operating temperature, making intermediate SOFCs possible. The main approach performed is to develop new solid electrolyte materials having higher ionic conductivity being more profitable for intermediate temperature application. Another way is to make thinner electrolyte membranes.The particular structure and conducting mechanism of apatite-type lanthanum silicates provide a brand new idea on researching and developing electrolyte. Different from fluorite and perovskite, the apatite lattice consist covalent SiO4and ionic-like La/O channels. The La-site cations occupy cavities created by SiO4units with four distinct oxygen positions; additional oxygen sites form channels through the lattice. The open structure of lanthanum silicate apatite suggests that this materials should be appropriate for the electrolyte applications for intermediate temperature SOFCs. The apatite structured oxides have advantages of lower activation energy for ionic conduction as compared with conventional solid electrolytes. Recent years, many efforts and research focused on the synthesis and properties of pure or substituted materials, and have obtained many experimental and theoretical achievements.Based on La10Si6O27, our works concentrate on the synthesis of pre-powders, preparation of sintering samples and the measurement of properties. It consists of:l) analysis the difference between apatite-type electrolytes comprised of different rare earth elements Sm3+、Gd3+、La3+, making decision on the investigation system;2) under the condition of our laboratory, synthesizing precursors via different methods, optimizing technology, preparing high performance powders;3) the doping of electrolyte materials;4)the preparation of IT-SOFCs films. The results are showed as following:1. La10Si6O27synthesized via Sol-gel process shows the best performance in Ln10Si6027(Ln=Sm、Gd、La) series. With convenient experimental operation, the synthetic powders have high activity, good sintering properties. The conductivity of sintered samples increased with the rise of sintering temperature. Higher temperature can increase the volume diffusion ability of grain, decrease the porosity, improving the relative density and the ionic conductivity. The relative density of La10Si6O27sintered at1600℃for6h can be96%, lowering about200℃compared with traditional solid state methods.2. In the series of La10-xSi6027-1.5X (0<x≤0.67), the increase of La3+vacancy decrease the interstitial oxide ionics, which result in the decrease of electrical conductivity. As a result, the conductivity of La10Si6O27is the maximum within the series under the same sintering condition, which can be0.08S/cm at800℃measuring temperature after sintered6h at1600℃. ln(σT) and1000T-1of the samples are in linear approximately. All the materials exhibit excellent thermal expansion coefficient with the value9.16-9.61×10-6/K.3. The substitution of Al3+for Si4+can improve the sintering property and increase ionic conductivity of apatite-type lanthanum silicate effectively. The powders can obtain after calcined dried gels of LaioSi6-xAlx027-0.5x(0<x≤1) synthesized via Sol-gel at900℃, which is lower about100℃compared with La10Si6O27. The doping amount of Al3+has a remarkable influence on ionic conductivity. A little adding increase the conductivity and reach the maximum at x=0.5. After that the conductivity decrease with the increase of x. The relative density of the sintered bulk increases with the rise of sintering temperature, so as the conductivity. After sintered6h at1600℃, the relative density of La10Si5.5Al0.5O26.75can reach a value of97%and the conductivity value can be2.3×10-2S/cm. All the materials exhibit excellent thermal expansion coefficient with the value of9.5×10-6/K、9.42×10-6/K、9.38×10-6/K、9.2×10-6/K and9.2×10-6/K respectively, which is close to traditional electrolyte YSZ.4. The substitution of Cu2+for Si4+can increase ionic conductivity of apatite-type lanthanum silicate effectively, but it requaries high temperatures compared with undoped silicate. The powders can obtain after calcined dried gels of La10Si6-xCuxO27-x synthesized via Sol-gel at1100℃, which is higher about100℃than. The doping amount of Cu2+has a remarkable influence on ionic conductivity. A little doping decrease the conductivity to the minimum when x=1. With the doping amount increase, the ionic conductivity increase again, and reach the maximum at x=1.5. The maximum value is the La10Si4·5Cu1.5O25.5(x=1.5) sintered at1600℃for6h with a value of σ800℃=4.8×10-2S/cm, which is a little higher than La10Si6027(σ800℃=3.0×10-2S/cm). All the La10Si6-xCuxO27-x (0≤x≤2) materials exhibit excellent thermal expansion coefficient with the value of about9.0×10-6/K.5. The apatite-type powders can obtain after calcining dried gels of La10-xBaxSi6-yAly027±σ(0≤x≤1,y=1) synthesized via Sol-gel at1100℃. Within the series, the samples of x=0.4exhibit outstanding performance under the same experiment conditions. The sinterability of La9.6Ba0.4Si5Al1O26.3improved compared with La10Si5Al1026.5.The relative density of sintered samples at1600℃for6h improved from84%to93%. The EIS analysis of300℃to800℃demonstrate that doping Ba on La site for aluminium lanthanum oxyapatite reduces the grain boundary resistance and improves the grain boundary conductivity, thus enhancing the total conductivity. After sintering at1600℃for6h, the conductivity of La9.6Ba0.4Si5Al1O26.3(σ800℃=2.17×10-2S/cm) is higher than La10Si5Al1O26.5(σ800℃=1.38×10-2S/cm). The materials also exhibit excellent thermal expansion coefficient with the value of about8.8×10-6/K from room temperature to800℃6. By means of PLD depositing SDC electrolyte film on LSCF cathode substrate to optimizate the performance of electrolyte. Thinner electrolyte will reduce the transportation resistance of oxide ionics and reduce the resistance between electrolyte and electrode interface. The powders of SDC and LSCF synthesized via sol-gel method both present well phase with single fluorite and peroskite structure respectively. At the same time, the temperature of substrate has an abouvious influence on the film, the substrate of650℃is more conductive and crystallization to the growth of thin film. But higher temperature can make film over growth. And the temperature of substrate is generally controned at650℃.