Preparation And Performance Evaluation Of SrCo_0.8Ti_0.2-xNb_xO_3-? Cathode Materials In SOFC

The Solid Oxide Fuel Cell?SOFC?is an efficient,environmentally friendly,versatile power generation device that can convert chemical energy directly into electrical energy.The traditional SOFC is operated at high temperatures of 800?-1000?,which greatly limits the choices of materials and cost high prices so that make against to its commercial development.So there is a main trend to research the intermediate temperature solid oxidation fuel cell?IT-SOFC?by reducing its operating temperatures down to 600?-800?.Nevertheless the low operating temperatures will lead to sluggish the ability about transport oxygen of the cathode,so it is essential to explore a higher efficient cathode material for IT-SOFC.Among many kinds of cathodic materials,ABO₃-type strontium cobaltate(SrCoO_3-?)cubic perovskite oxide has extremely high conductivity and oxygen permeability,but it also exists the defect of unstable perovskite phase structure.So this thesis mainly discusses about the B-bit doping method to improve its performance.This thesis used solid phase method to synthesis SrCo_0.8Ti_0.2-xNb_xO_3-??SCTN_x?cathode powder,investigated its phase structure by using X-ray diffraction?XRD?founding that the SCTN_x cathode powder synthesized perovskite structure completely by sintering at 1150? for 10 hours.In addition,with the increase of the Nb doping ratio,the unit cell volume becomes larger.In order to investigate the compatibility of the cathode material with the electrolyte SNDC,the composite cathode SrCo_0.8Ti_0.1Nb_0.1O_3-?-SNDC was characterized by mixing and grinding SCTN_0.1 and SNDC at the ratio of 1:1.It was seen from the XRD pattern that the composite cathode only maintains the phase structure of the electrolyte and the single phase cathode.The diffraction peaks were superimposed by both the SrCo_0.8Ti_0.1Nb_0.1O_3-? and SNDC phases,and no other peaks were observed,indicating that there was no chemical reaction between the cathode material and the electrolyte,so that they have a good chemical compatibility.Afterward the cathode material was made into a symmetrical half-cell of SrCo_0.8Ti_0.2-xNb_xO_3-?/SNDC/SrCo_0.8Ti_0.2-xNb_xO_3-? by screen printing and coating.The effects of sintering temperatures and different doping ratios of Nb and Ti on the cathode materials were investigated by the AC impedance spectroscopy and microscopic morphology test,suggested that the symmetrical half-cell had a minimum polarization resistance about 0.07595?·cm² when sintering at 850? for 5 hours.It is shown that the sintering temperatures had a great influence on the electrochemical performance of the cathode,and the optimum sintering temperature is favorable for the growth of the cathode particles,resulting in proper porosity and increasing electrochemical reactivity.As the amount of Nb doping increase,the AC impedance of the symmetrical cell decreased initially and increased afterwards.When the doping amount is x =0.05,the AC impedance of the symmetrical cell got the minimum value about 0.03665?· cm².It was known that the optimum SrCo_0.8Ti_0.2-xNb_xO_3-? cathode material was obtained when doping content was x = 0.05 and the sintering temperature was 850?.In order to further demonstrate the feasibility of this cathode material,the electrolyte-supported single cell is produced.The polarization resistance of the single cell is 0.0905?·cm2 at 800?,and the maximum power density is 268.51mW·cm-2,which has a good electrochemical performance.Therefore,it is proved that SrCo_0.8Ti_0.2-xNbXO_3-? cathode material is a promising IT-SOFC cathode material.