Growing global demands on supply of fuels stimulate extensive investigations towards sustainable energy sources and more efficient energy converting methods. Fuel cells exhibit by far higher efficiency than internal combustion engines and therefore attract worldwide attention as promising chemical-to-electrical energy transformation devices. Solid oxide fuel cells (SOFC) are considered as one of the most promising type of fuel cells for stationary, combined heat and power, as well as automotive applications, as they can be operated with wide range of fuels, are resistant to poisoning by CO and H 2 S, and do not require expensive noble metals as electrodes [1,2]. Most of profits, as well as disadvantages of SOFC (mainly due to component degradation), stem from the fact that operating temperature is relatively high, typically in the range of 600 -1000°C. This type of fuel cell usually consists of Ni-yttrium stabilized zirconia composite as anode, (LaSr)MnO 3 or (LaSr)(Co,Fe)O 3 as cathode, and yttrium stabilized zirconia, gadolinium doped ceria, or other oxygen conducting material as solid electrolyte [3,4]. Since resistance of electrolyte layer is a major source of overall overpotential and limits the cell's efficiency, many efforts are dedicated towards its optimization. A relatively new idea is application of highly conducting solid oxide proton-conducting membrane instead of well-established oxygen-ion conducting one. Such modification possesses certain benefits, because in such an arrangement water, the product of the reaction, generated at the air-electrode (cathode) side, does not dilute fuel, allowing for more complete fuel utilization. Additionally lower activation energy of proton conduction than oxygen-ion conduction implies higher conductivity at the intermediate temperature range. Interestingly, high performance of SOFC with thin-film proton-conducting Four proton conducting oxides of perovskite structure: BaZrO 3 , SrZrO 3 , BaCeO 3 and SrCeO 3 doped with 5 mol.% of gadolinium are compared in terms of crystal structure, microstructure, sinterability, water sorption ability, ionic transference number, electrical conductivity and stability towards CO 2 . Relations between proton conductivity, structural and chemical parameters: pseudo-cubic unit cell volume, lattice free volume, tolerance factor, crystal symmetry and electronegativity are discussed. The grain boundary resistance is shown to be the limiting factor of total proton-conductivity for the materials examined. The highest proton conductivity was observed for BaCeO 3 , however, it turned out to be prone to degradation in CO 2 -containing atmosphere and reduction at high temperatures. On the other hand, Ba and Sr zirconates are found to be more chemically stable, but exhibit low electrical conductivity. Electrical conductivity relaxation upon hydration is used to calculate proton diffusion coefficient. Selected materials were tested as electrolytes in solid oxide fuel cells.
