Investigation Of Low-Temperature Ceramic Fuel Cells With Novel Composite Electrolytes

Low-temperature (400-600oC) ceramic fuel cell (CFC) has received much attention in recent years due to its promising prospect for commercialization. The key to develop practical low-temperature CFC is to develop materials and techniques with high performance and low cost. In this thesis, based on the innovative composite electrolyte materials, compatible electrode materials were developed, and suitable techniques were explored to fabricate single cell, furthermore, the structure of the fuel cell stack was designed. In addition, a theoretical electrochemical model was proposed to describe the fuel cell with a co-ionic conducting composite electrolyte. All these have laid a foundation for further R & D in low temperature CFC.Beginning from Ce0.8Sm0.2O1.9(SDC)-carbonate composite electrolytes, the material systems with various compositions were prepared, and the structure and electrical properties of these materials were studied. The results show that SDC and carbonates have formed a composite, and its conductivity behavior has two typical features: conductivity transition temperature and conductivity enhancement effect. The conductivity transition temperature is mainly dependent of the type of carbonates, and it is normally 20-40oC lower than the eutectic melting point of the carbonates. Around the conductivity transition temperature, the conductivities of these composite electrolytes are several to several ten times to that of SDC. The conductivity enhancement effect is related to the SDC powder prepared by different methods, the content of the carbonates, the mole ratio of the carbonates and the type of the carbonates.Single cells with SDC-carbonate composite electrolytes were fabricated by a cold-pressing technique, and the effects of the above factors, cell structure and operating conditions on the cell performances were investigated. The results show that all cells with SDC-carbonate composite electrolytes have exhibited excellent performances. Among them, the best performances of 690 mWcm-2 at 500oC and more than 300 mWcm-2 at 400oC were achieved by the hydrogen-air fuel cells with SDC-LiNaCO3 composite electrolytes. It has found for the first time that the ionic conduction properties of such composite electrolytes varied with the content of the carbonates. A new integrated hot-pressing technique has been developed to fabricate single cell with larger size. The cell was operated steadily at 500-600oC, and it discharged at 500oC for more than 12h with an output of about 500 mWcm-2. However, it became unsteady below 475oC. Short stacks with 2 or 3 cells were constructed with designed structure. A maximum output power of 1.55W was achieved at 550oC for a three-cell stack. The stack performances are expected to be improved by solving the sealing on the gas manifold.Novel Ce0.8Zn0.2O1.9(ZDC)-carbonate composite electrolytes, pervoskite-like structured La2NiO4+δ(LNO) based cathodes, and NiO based binary anodes were prepared and tested in fuel cells. It shows that the composite electrolyte fuel cells using these new materials are suitable to be operated at low temperatures, and they are compatible with existing material systems.Based on the ionic-electronic conducting system, a one-dimension electrochemical model was set up to describe the fuel cell with a co-ionic conducting composite electrolyte in combination with the theory of defect chemistry. The operating characteristics of the fuel cell with a SDC based composite electrolyte and the fuel cell with a SDC electrolyte were compared, which showed that the former is superior to the later.