Intermediate And Low Temperature Solid Oxide Fuel Cell Based On Barium Cerate-Barium Zirconate Proton Conductor Electrolyte

Proton conductor solid oxide fuel cell(PCFC)is a green and efficient energy conversion technology which can directly convert chemical energy stored in fuel into electrical energy.It uses proton conductor as electrolyte.Because the conduction activation energy of proton is lower than that of oxygen ion,it can operate at medium and low temperature(600?300°C),overcoming the disadvantages of high operating temperature(800?1000°C)of traditional oxygen ion conductor solid oxide fuel cell.It has been widely concerned by researchers all over the world.This thesis mainly focuses on the synthesis of proton conductor oxide and the preparation of proton conductor solid oxide fuel cell.The general opetating features and problems of PCFCs are discussed.The effects of solid-state reaction method,co-precipitation method and EDTA-citric acid chelating-combustion method on the properties of the synthesized powder are compared.The preparation process of the button-type PCFC's electrolyte is improved by the slurry spin-coating method,which enhanced the cell's performance.The tubular-type PCFC with larger effective area is prepared with dip-coating method,and its high output performance at medium and low temperature was maintained at the same time.The causes of performance degradation during the stability test are investigated.Among all kinds of electrolyte materials of PCFC,ABO₃type perovskite oxide is the most popular research object,such as Ba Ce O₃,Ba Zr O₃,and so on.Their proton conductivity and chemical stability can be further improved by single doping or co-doping of B-site elements with lower valence transition metal and rare earth metal elements.Ba Ce_0.7Zr_0.1Y_0.2O_3-?(BCZY7)is the Ba Ce O₃-Ba Zr O₃based solid solution doped with 20mol.%Y³⁺.It has both the characteristics of high proton conductivity of Ba Ce O₃and good H₂O and CO₂resistance of Ba Zr O₃and is a kind of early developed proton conducting electrolyte materials.In this paper,the BCZY7 powder synthesized by high temperature solid state reaction method shows cubic perovskite phase.Anode-supported Ni O-BCZY7/BCZY7/La_0.6Sr_0.4Co_0.2Fe_0.8O_3-?(LSCF)-BCZY7 button cell is successfully prepared by dip-coating and co-sintered methods with effective area of 0.2 cm²and electrolyte thickness of 25?m.The cell's electrochemical performance was tested using humidified hydrogen(with 3 vol.%H₂O)as fuel and air as oxidant.The results show that at 600°C,550°C and 500°C,the maximum power density of single cell are 203 m W?cm^-2,123m W?cm^-2and 92 m W?cm^-2,respectively.The cell shows good performance at medium and low temperature.Solid state reaction method is the most commonly used to prepare proton conductor oxide and has the features of simple process,high yield and easily obtained raw material.However,it needs high calcination temperature and long calcination time.The prepared powder has larger particle size and is easy to agglomerate,which decrease its sintering activity.Some sintering aids are usually needed to increase the density of electrolyte.In order to avoid the potential adverse effects of sintering aids on proton conducting of electrolyte,co-precipitation method and EDTA-citric acid chelating-combustion method are used to prepare BCZY7,respectively.In this case,the required calcination temperature is lower than that of solid state reaction method,and the synthesized powder has more pure phase,better dispersion and particle size of several hundred nanometers.Thereinto,the powder synthesized by co-precipitation method has the smallest particle size and best sintering activity.A thin BCZY7 electrolyte film is prepared on the Ni O-BCZY7 anode support by spin-coating with thickness of about 6?m.It is sintered dense without sintering aids.Then the button cell is prepared using LSCF-BCZY7 composite cathode.The electrochemical performance of single cell is also tested under humidified H₂fuel.The results show that at 650°C,600°C and550°C,the cell's open circuit voltages are 1.03 V,1.04 V and 1.06 V,respectively,and the maximum power densities are 989 m W?cm^-2,675 m W?cm^-2and 333 m W?cm^-2,respectively.These indicate that the performance of cell could be significantly improved by modified powder and electrolyte preparation process.The button-type cells prepared in the above experiment have small effective area and are suitable for laboratory research.However,only high-power cell stack systems can meet the needs of practical life.They are mainly composed of plate-type or tubular-type single cells.Compared with plate-type structure,tubular one is easier to seal,faster to start,and has better thermal shock resistance.In this paper,a semi-closed tubular PCFC was successfully prepared by dip-coating method with BCZY7 as electrolyte.The effective area is 2.3 cm².Under humidified H₂fuel,the peak power densities of single cell are 465 m W?cm^-2and 118 m W?cm^-2,respectively,at 600°C and 450°C.In addition,total power output of 3 m W is detected at200°C.During the discharge stability test at 0.7 V constant voltage and 600°C,the power output of the cell decreases first,and then stabilizes at about 300 m W?cm^-2.The tubular PCFC prepared in this paper not only has large effective area,but also shows high performance at medium and low temperature,which provides helpful reference for the development of tubular PCFC in the future.