Challenges like global warming, environmental pollution, the increasing demand for energy and the finite nature of fossil fuels presents us with the challenge of finding alternative renewable energy systems. One aspect involves the improvement of energy conversion efficiency of new technologies that are not yet fully developed. Fuel cells and electrolyser cells are promising devices as they are environmentally friendly, cost-effective and allow higher efficiencies than conventional combustion-based systems. This work deals with protonic ceramic fuel cells (PCFC), a type of fuel cell that utilises proton-conducting oxides as electrolyte material. One reason why this technology has not yet found widespread application is the limited power density of PCFCs. The poor rate of oxygen reduction at the cathode is the main reason why the performance does not reach the expectations. As the performance of the cathode depends on component materials, the development of innovative cathode materials with optimised properties is an important part of the improvement of PCFCs. In this thesis, three different composites, prepared by in-situ phase decomposition are investigated in terms of their properties for the application as cathode material. The composites produced are based on the idea of triple conducting oxides (TCO), which can conduct protons, oxygen-ions and electrons simultaneously. As it is difficult to combine these properties in just one material, our approach is to create a composite in which two different phases together fulfil this task. The first part of this work deals with the preparation of the materials using the one-pot synthesis, a new method based on in-situ phase separation of suitable precursors. The second part addresses the fundamental characterisation of the synthesised composites. Therefore, structural and chemical analyses, as well as investigations of mass- and charge transport properties are carried out. The methods applied include X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectrometry (EDX), electrical conductivity measurements by van der Pauw method and conductivity relaxation measurements. Post-test analyses by STEM-EDX are employed to obtain detailed insights into effects that occur during the measurements. The results of the present study are then compared with literature data.