Proton and oxygen ion-mixed-conducting ceramic composites and fuel cells

This work focuses on the behaviour and role of protons and oxygen ions for the electrical properties of salt-oxide composites based on halides, more specifically on fluorides e.g. MF_x (M=Li, Na, Ca, Ba, Sr, Mg, x=1, 2)-based composites with oxides. The electrical properties were studied by using DC measurements, employing either gas concentration cell or fuel cell techniques. The conductivity of the fluoride-based composite electrolytes is 10^-3-10^-2 S/cm at temperatures between 600 and 800°C. Proton and oxygen ion defects, their generation and transport, showed a very strong dependence on the employed gas resources. Possessing both proton and oxygen ion conduction, on one hand, enhances the material total conductivity and, on the other hand, may promote the electrode reaction and the kinetics at two interfaces between the electrolyte and electrodes for fuel cells, thus, enhancing the current exchange rate and fuel cell performance. Using these composite electrolytes, a typical power density of 120-160 mW/cm² under a current density 300-800 mA/cm² was obtained in a fuel cell. Although both proton and oxygen ion conduction are possible, the proton defect concentration and related conduction generally predominate as the basic feature of the halide-based composite materials.