Study On The Materials Of Electrolytes And Electrodes For Ceria-based Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are one of the solid-state electrochemicaldevices which directly convert the energy of chemical reactions into that ofelectricity. Being a new green energy in the world for 21st century, theypossess many distinct advantages, such as environmental friendship, higherenergy conversion efficiency and broad adaptive fuels, etc. The technology ofhigh temperature solid oxide fuel cells (operating at 800-1000oC) have beendeveloped to some extent in the world and several commercial power stationsalso been built on account of effective catalysis of the electrodes and highcomprehensive energy utilization. However, there are many unconquerablebarriers in SOFCs operating at relatively higher temperatures, such as sealingof the cells and interface reactions between the components, etc., whichrestricts the choices of materials and reduces the life-span of the fuel cells.Lowering the operating temperature to 500-800oC (so-called intermediatetemperature solid oxide fuel cells, or ITSOFCs) would overcome thesedifficulties and enhance the reliability, prolong the life and reduce theproduction costs. The key problem for developing ITSOFC is how to chooseproper materials for electrolytes and electrodes. To date, there are two kindsof electrolyte materials accepted extensively, i.e. doped LaGaO₃ and dopedceria. However, practical use of the former is questionable because of the costof gallium and the possibility of reaction with electrodes. Compared with theformer, the latter is a promising candidate for their low cost and higheroxygen ion conductivity at intermediate temperatures. The correspondingcathode materials commonly used is perovskite oxides, mainly dopedLaMnO3 and doped LaCoO3. Nickel is widely used as anode materialsbecause of its low cost. Nevertheless, coarsening of the nickel particles andmismatch of the thermal expansion coefficient with electrolytes at the fuelcell operating temperatures maybe cause the decrease of catalysis andshelling off the electrolytes. In this thesis, several representative materials areselected to investigate their synthesizing method and behavior used asITSOFC electrolytes and electrodes.During the fabrication process of the SOFC raw materials are vital toattain excellent fuel cell output. In this work a citrate/nitrate combustionmethod is adopted to prepare electrolyte and electrode powders. Comparedwith solid-state reaction, co-precipitation and sol-gel method the advantagesof this means include high-yield, short-period, pure-ingredient andultrafine-particles, etc. The detailed approaches will be illuminatedhereinafter.1) (CeO2)0.8-x(GdO1.5)x(SmO1.5)0.2(x=0-0.3) used as electrolytesAccording to the stoichiometry the staring materials, nitrate hydrates ofcerium, samarium and gadolinium were weighted, respectively, and certainproportional citrate was added, too. After mixing with deionized waterammonia water was added to adjust pH value. The solution (pH=5-6) wasvaporized in water bath at 80oC, and the gel was obtained. Then the gel wastorrified in oven at 130oC. When the dry gel was heated up to 200-300oC inair, it burns in a self-propagating combustion manner until all of them arecompletely burnt out to form loose powders.During the course of synthesis, citrate has an important effect on thetechnique. On one hand, as a complex citrate influence on the stability of thegel, if no citrate was added to the solution, it was difficult to obtain stable andhomogeneous gel from the solution. When adding the citrate at ratio 1.5 tometal nitrates, the amorphous gels were achieved with pH value of 5-6, andthe dry gel have the self propagation combustion characteristic after beingheated up to as low as 170-200oC, the highest reaction temperature reaches to1000oC. On the other hand, citrate acts as reducer and the ratio of citrate tometal nitrate has a great influence on the reaction. The gel wouldn't exhibitself-propagating combustion behavior when the ratio is below 1:1. The gelswith ratio 1:1 have the most rapid combustion rate, which maximum reactiontemperature reaches to above 1000oC, and the powder obtained aftercombustion have a coarse crystallite size (26.4nm). When the ratio equals to1.5:1, the combustion rate decreases, yet the gels were still completely burntout. However, with ratio above 1.5:1, the gels wouldn't react completely, thecombustion temperature decreases with the increasing of citric acid, thereexists part of carbon remained after reaction. As a whole, the optimal ratio is1.5 in our experiment.As demonstrated by XRD, TG-DTA, TEM and EDS, homogenous, singlephase solid solutions with a fluorite structure and a particle size of 16-28nmdoped ceria ultrafine powders can be achieved after the dry gel combustion.The powder was dry-pressed into pellets as electrolyte with a dimensionof Φ13mm×1mm.The operation properties of a single fuel cell, with platinumas both anode and cathode, hydrogen and air as fuel and oxidizer, respectively,were measured at different temperatures and loads. The fuel cell withelectrolyte (CeO2)0.6(SmO1.5)0.2(GdO1.5)0.2 has the best output at 500oC,which open circuit voltage (OCV) is 0.833V, and the maximum powerdensity up to 250mW/cm2 at 0.6V.2) Three Cathode Materials By means of the same route describedabove three cathode materials, La0.6Sr0.4Co0.8Fe0.2O3, La0.8Sr0.2Mn0.8Co0.2O3and Sm0.5Sr0.5CoO3 were synthesized via citrate-nitrate combustion method,too. During the precursors of the three perovskite oxides preparation,adjustment of the concentration, temperature and pH of the starting solutiondo great effect on the properties of sol and gel. However, not alike theelectrolytes, the ashes of all of the three cathodes obtained after combustionhave to be baked at 900oC to gain target powders.XRD results showed that all of the three powders have single phase withperovskite oxide structure and average crystallized size is 20nm. EDSconfirmed that the elements distribution is homogenous and the atomic ratioof the elements is close to that of experimental operation. However,morphology of the powders under TEM indicated that the particle size isbetween 100nm and 200nm. It is obvious that there is agglomerationphenomena occurred during powder preparation.Analysis of the three crystal structure demonstrated that all of the threetolerance factors are close to 1. That is to say, introducing of doped elementsin the crystal lattice did not bring apparent deformation on the acceptorcrystal structure. All of the three structures have more lattice free volume andmoderate critical radius corresponding to the opening between A-site cationand B-site cation through which the mobile oxygen must pass. Theparameters make the three materials superior candidates for SOFC cathodeswith ionic and electronic mixed conduction.After mixing the three cathode powder and electrolyte powderCe0.8Gd0.2O1.9 (with mass ratio 1:1), respectively, the Φ13mm pellets weredry-pressed with the mixed powders and then sintered at 1000oC for 10h.XRD testing for these samples indicated that there is no other phase occurredthan a cathode and electrolyte phases. That is to say, the interface reactionsbetween doped ceria electrolyte and the three cathodes will not occur duringoperation of the fuel cells.Three kinds of single SOFC pellet (Φ13×1mm)with Ce0.8Gd0.2O1.9 aselectrolyte, NiO/ Ce0.8Gd0.2O1.9 (mass ratio 1:1) as anode, cathode/electrolyte(mass ratio 1:1) as buffer layer and one of the three perovskite oxides ascathode, respectively were prepared via sintering at 900oC for 5h afterdry-pressing. Electric output behavior of the fuel cells assembled with thepellets was tested via hydrogen as fuel and air as oxidant operating atdifferent temperatures. The maximum open circuit voltage of the fuel cellswas above 1V, and the maximum output power density with 230mW/cm2 wasachieved when La0.6Sr0.4Co0.8Fe0.2O3 used as cathode and operatingtemperature was at 650oC.3) Preparation of compound powder NiO/ Ce0.8Gd0.2O1.9 Proper ratioof mixing NiO and Ce0.8Gd0.2O1.9 as compound anode not only can avoidcoarsening of the nickel anode at fuel cell operating temperatures but alsocould adjust the difference between anode and electrolyte to alleviatedelamination coming of the mismatch of thermal expansion coefficientbetween them. The compound powder NiO/ Ce0.8Gd0.2O1.9 could also beprepared via citrate-nitrate combustion method stated above. However, itshould be suggested that the molar ratio of citric acid to metallic ions(MRCM) of raw materials intensely influence on the preparing process andconsequential products. In order to illuminate the statement six experimentswere operated of which different MRCM by 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0was adopted, respectively. X-Y function register was adopted to keep thecurves of temperature changing during combustions of the dry gel. XRD wasused to detect phases and crystallized sizes of the obtained powders.Morphology of the powders was observed under TEM, and the elementdistribution came from EDS. According to the experiments the conclusionsbelow can be drawn:There is an inverse relation between the molar quantity of citric acid andammonia to gain transparent sol. Grain sizes of the powders decreases firstlyas MRCM increases from 0.5 to 1.5, and then increases as MRCM increasesfrom 1.5 to 3.0. The minimum grain sizes of 18nm and 13nm for both NiOand Ce0.8Gd0.2O1.9, respectively were produced with the MRCM being 1.5.Comparatively lower MRCM resulted in the dry gel slow and unstablecombustion of which temperature was not so high. The powders producedconglomerates seriously of which average crystallized sizes was 20nm.Higher MRCM will lead to the dry gel violent and higher temperaturecombustion. The powders obtained conglomerates more seriously of whichgrain sizes was even larger. Only proper MRCM causes the gel to combuststeadily of which temperature was not too high. Grain sizes of the powders soprepared were thin (Ni 18nm and Ce0.8Gd0.2O1.9 13nm) and the powdersagglomeration was not so serious. In our experiments when MRCM equals to1.5 homogeneously double-phase nano crystalline NiO/ Ce0.8Gd0.2O1.9compound powders can be synthesized via citrate-nitrate combustion method.In a word, homogeneous nanosized fluorite-structured doped ceria,peroveskite type oxides and double-phase NiO/ Ce0.8Gd0.2O1.9 compoundpowders would be produced via the combustion method described above. Thesimple technique has the advantages of large-yield, short-period and low-costover many other technologies, such as solid state reaction, co-precipitationand conventional sol-gel, etc.Among the electrolytes (CeO2)0.8-x (GdO1.5) x (SmO1.5)0.2(x=0-0.3) the bestoutput of the fuel cell came from x=0.2. La0.6Sr0.4Co0.8Fe0.2O3,La0.8Sr0.2Mn0.8Co0.2O3 and Sm0.5Sr0.5CoO3 all can be used as cathodes of thedoped ceria based ITSOFCs. There is no reactions occurred between thesecathodes and the electrolyte during fuel cell operation. The maximum powerdensity output of the fuel cell made of Ce0.8Gd0.2O1.9 as electrolyte,dual-phased complex NiO/ Ce0.8Gd0.2O1.9 as anode and three perovskiteoxides as cathode, separately came from that of La0.6Sr0.4Co0.8Fe0.2O3 ascathode one.Further investigations on doped ceria based intermediate temperaturesolid oxide fuel cells will be conducted on the basis of the study stated in thisthesis.