Preparation And Electrochemical Performances Of Novel Composite Cathode Materials For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?has attracted worldwide attentions because of its high energy conversion efficiency,wide fuel choice and environmental friendship.To realize a practical SOFC,it is necessary to reduce the operation temperature and cost of the present SOFC.However,the area specific resistance?ASR?of the cathode increases dramatically with the reduction of operation temperature,degrading the performance of SOFC.Therefore,the cathode material has become one of the hot spots in SOFC researches.It has been shown that the oxide reduction reaction?ORR?can be effectively enhanced and the ASR reduced by mixing cathode materials with some catalysts or electrolyte materials.Ion impregnation method is a novel technique to prepare porous composite electrode of SOFC,in which porous cathode skeletons were first prepared,and then impregnated with the solution contained catalytic compositions and driving of capillaries force.By firing the precursors at a low temperature,target oxide formed and adhered to the surfaces of the cathode materials.The composite cathodes prepared by this process have high catalytic activity and good performance at relative low temperature,which can improve the performances and reduce the operation temperature of a SOFC.In the second chapter of this dissertation,porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3-?-Sm_0.2Ce_0.8O_1.9?LSCF-SDC?composite cathodes were loaded with Co₃O₄ catalyst by hydrothermal method,and the phase constitution,morphology,electrochemical performances of the samples before and after modification were investigated.The results show that Co₃O₄ particles with needle shape grow onto the surfaces of the LSCF-SDC cathode.The modification of LSCF-SDC cathodes by Co₃O₄ can reduce the ASR from 0.49 ?·cm² to 0.19 ?·cm² at 700 °C,and the oxide reduction reaction activation energy from 1.52 eV to 1.03 eV.This reduction may be ascribed to the extending of trible phase boundary?TPB?from the interfaces of cathode and electrolyte to the cathode.In addition,impregnating of the LSCF-SDC cathode with Co₃O₄ increases the power density of the single cells from 185 m W·cm-2 to 259 mW·cm-2 at 700 °C.The preliminary results have demonstrated that impregnating LSCF-SDC composite cathodes with Co₃O₄ can effectively improve the electrochemical performances.In the third chapter of this dissertation,porous Mn_1.5Co_1.5O₄?MCO?cathodes with SDC nanoparticles on the surface were prepared by ion impregnation method.Microstructure,chemical stability,electrical conductivity and electrochemical performances of the MCO-SDC composite cathodes were investigated.The results show that SDC is chemical compatible with MCO even after co-fired at 1200 °C for 2 h.Porous MCO cathodes were impregnated with nitrate solution of Sm and Ce,and a layer of SDC nano-particles forms on the surfaces of the cathode after firing at 700 °C for 2 h.Compared with the MCO-SDC composite cathodes prepared by the traditional mechanical blending process,impregnating MCO cathodes with SDC nano-particles can reduce the ASR from 5.23 ?·cm-2 to 1.63 ?·cm-2 and increase the power density of single cells from 252 mW·cm-2 to 366 mW·cm-2.This is associated with the SDC nano-particles adhering on the surface of MCO cathode,which increases the areas of TPB and promotes the process of ORR.In the fourth chapter of the dissertation,Sr2Fe1.5-xCuxMo0.5O6-??x=0,0.05,0.1,0.2,0.3??SFCxM?powders were synthesized by the citric acid process to investigate the influence of Cu on the microstructure,sinterability,thermal expansion coefficient?TEC?and electrochemical performance of the SFCxM materials.XRD patterns showed that all of the SFCxM powders have pure cubic perovskites structure without impurity,and the crystal cell size grows up with the increase of Cu content.The tests of thermal expansion performance test showed that TEC increases with the increase of Cu content,which is about 14.5^-16.1 K^-1.Electrical conductivity measurements of the SFCxM samples demonstrate that SFC0.1M has the highest electronic conductivity of 49.3 S·cm^-1 at 450 °C,which is 2.5 times higher than that of the SFM materials.SFC0.1M samples show the lowest ASR of 0.26 ?·cm² at 800 °C,which is about 60% reduction than the SFM.Furthermore,the SFC0.1M-SDC composite cathodes show the lowest ASR of 0.15 ?·cm² at the mass ratio of SFC0.1M to SDC of 60:40.