Intermediate Temperature Solid Ionic Conductors and Fuel Cells

Abstract: Novel solid proton conductors based on oxyacid salts and salt-ceramic composites and intermediate temperature fuel cells have been developed in this thesis work. The discovery of proton conduction in these materials introduces a new concept of proton conductors and starts a new research field - Intermediate Temperature Solid Proton Conductors and Fuel Cells. In these materials highly mobile protons are present in very low concentrations resulting in a high proton conductivity, 10^-2 to 10^-1 S/cm, between 300 and 600 °C. Most materials investigated have a face centred cubic structure or are two-phase materials.

Two types of proton bond states and proton conduction mechanisms have been suggested and are discussed. The first type is a single proton jump accompanying the hydrogen bond reorientation due to a "Paddle wheel" mechanism for the sulphates and sulphate-ceramic composites and the second type is interfacial proton conduction for the nitrate-ceramic composites. The theory for the fuel cell process, protonic conduction and diffusion processes in these types of materials is discussed.

Fuel cells using these materials as electrolytes have been successfully demonstrated. A typical fuel cell performance is a current density of 300 mA/cm² at 0.75 V, which is suitable for the requirement of commercialisation.