Fabrication And Performances Of Anode-Supported Intermediate Temperature Solid Oxide Fuel Cells With Perovskite Electrodes

Solid oxide fuel cells?SOFCs?are all-solid and high-efficiency devices which convert the chemical energy in fuels into electrical power in an environmentally friendly way.The intermediate temperature SOFCs?IT-SOFCs?are promising technology for commercialization due to reduction in system cost and degradations.However,development of anode-supported IT-SOFCs faces a major challenge to deal with high polarization which mainly occurs at the cathode.Therefore,the cathode is required to have high activity and durability for electrochemical oxygen reduction and low area specific resistances at intermediate temperatures.Recently,Ln₂NiO_4+??Ln=La,Pr,Nd?oxides with high surface oxygen exchange and bulk diffusion coeffiecients,high oxygen hyper-stoichiometry under oxidizing conditions,interesting mixed ionic-electronic conduction and catalytic activity have been reported as cathode materials for IT-SOFCs.Meanwhile,perovskite oxide materials offer excellent thermal and mechanical stability,physical compatibility with electrolyte materials,and relatively low cost and have attracted interest in their application as anode in SOFC designs,strontium titanate shows the potentials for this application..In this thesis,Nd₂NiO4+?-Ce_0.8Gd_0.2O_2-??NNO-CGO?composites and Nd₂Ni_0.9M_0.1O₄+_??M=Ni,Co,Cu,Fe,and Mn?are systematically investigatigated for planar Ni-YSZ anode-supported cells with YSZ as the electrolyte in commercial size of 48 mm×48 mm.The Nd₂NiO4+?cathode material is synthesized at 1150°C for 12h by modified Pechini method.NNO showed good chemical compatibility with YSZ and CGO at?1000°C.NNO-CGO composite cathodes present a good adhesion to YSZ electrolyte layer as well as CGO interlayer after sintering at 1000°C for 2 h.The chemical compatibility problem of NNO with YSZ/CGO can be avoided through sintering process at low temperature.The power densities of each series of cells with NNO-CGO composite cathodes increase with the increase in CGO content from 0 to30 wt%,and then decrease with the further increase in CGO content.By using La_0.6Sr_0.4CoO_3-??LSC?as current collector,the cell with 70NNO-30CGO composite cathode sintered on YSZ electrolyte at 1000°C exhibits the best performance.The highest power density is 620 mW cm^-2 at 0.7 V,800°C.The corresponding ohmic resistance,polarization resistance and area specific resistance are 0.08,0.32 and 0.4?cm²,respectively.The cell with 70NNO-30CGO composite cathode sintered on CGO interlayer at 1000°C exhibits the power density of 385 mW cm^-2 at 0.7V,800°C.The ohmic resistance,polarization resistance and area specific resistance are 0.31,0.266and 0.576?cm²,respectively.After short-term durability test at 700°C,70NNO-30CGO composite cathode still have a good adhesion to YSZ electrolyte and CGO interlayer,no cation migration occurs from composite cathode layer or current collector layer towards the electrolyte.It can be concluded that adding a CGO interlayer in the single cells leads to an increase in the ohmic resistances and a decrease in the power densities,whereas the values of the polarization resistances decrease.The results indicate that NNO-CGO composites can be good candidates of cathode materials for IT-SOFCs.Meanwhile,the effect of partial substitution of the transition metal M=Co,Cu,Fe and Mn for Ni in NNO on the crystal chemistry,electrical conductivity,thermal stability,chemical reactivity with YSZ electrolyte and electrochemical performance has been systematically investigated.Compared to NNO,each NNMO?M=Co,Cu,Fe,Mn?material shows a decrease in electrical conductivity and an increase in lattice volume,oxygen vacancy concentration and chemical reactivity with YSZ electrolyte.The cell with NNCoO cathode achieves the maximum power density of 530 mW cm²at 0.7 V,800°C.The corresponding ohmic resistance,polarization resistance and area specific resistance are 0.08,0.38 and 0.46?cm²,respectively.The cell with NNCuO cathode is the most stable in short-term test.A small amount of B-site doping in NNO does not cause any change in the cathode microstructure.The study demonstrates that a tradeoff between oxygen transport properties and electrical conductivity can be accomplished by appropriate selection of transition metal and doping level at cation B-site in NNO.The all above results indicate that NNO-CGO composites cathode can more effectively improve the electrochemical performance of the cathode,compared to NNMO?M=Co,Cu,Fe,Mn?cathodes.Subsequently,a novel fabrication process for SOFC with La_0.2Sr_0.7TiO_3-?(LST_A-)as anode support and La₂NiO_4+??LNO?as cathode,which avoids high ohmic resistance from anode and complicated impregnation process of cathode,is designed and investigated.LNO is successfully sintered on the YSZ at 1200°C in nitrogen atmosphere and then annealed in-situ at 850°C in air.By using LSC as current collector,the cell with LNO cathode sintered in nitrogen atmosphere exhibits the power density of 235 mW cm^-2 at 0.7 V,800°C.The ohmic resistance?R_S?and polarization resistance?R_P?are 0.373 and 0.452?cm²,respectively.Compared to the in-situ reduction process,the reduction process at 1200°C in hydrogen atmosphere for the LST_A-anode-supported half cell decreased the value of R_S and improved the performance of the single cell,whereas the sintering processes of the cell with the LNO cathode sintered in nitrogen atmosphere led to an increase in R_P.A tradeoff between the decrease in R_S and the increase in R_P for the single cell was achieved.It can be concluded that the value of R_S from the anode of SOFC with LST_A-as anode support played a very important role in the performance of the cell.The present work demonstrated that the promising performance of the cell with LST_A-as anode support can be achieved by cost effective and simple processes including tape casting and screen printing.In conclusion,the thesis has a certain guiding significance to the exploitation of electrode materials and the preparation of single cell for IT-SOFCs,but many bottlenecks need breakthrough to realize commercial application of IT-SOFCs.