Experimental Analysis of Solid Oxide Fuel Cells under Dry Reforming of Methane

During the last decades, the increase in world population, living standard, along with the industrial and transportation sectors led to a pronounced increase in the energy consumption. A consistent improvement of energy production systems is needed to satisfy the world energy demands in terms of fuel and energy. The increasing attention to reduce emissions of pollutants and the concerns about global warming motivate the development of environmental friendly technologies and the enhancement of energy conversion systems efficiency. Although renewable energies are becoming more and more relevant, fossil fuels are still the predominant resources used for stationary and mobile devices. In the last years, many efforts have been devoted to the pollution control and improvements in energy efficiency, with the achievement of remarkable progresses in the electric power sector. Nevertheless, a further improvement can be reached with the development of relatively new technologies, which show great potential in terms of electrical efficiency and low level of pollutant [1-3]. In this context, fuel cells are interesting energy conversion systems, which operate without combusting fuel and directly converting chemical energy to electrical energy. They show electrical efficiency higher than the traditional power generating systems and do not produce significant quantities of pollutants as power plants or internal combustion engines. Hence, fuel cells are very appealing from both energy and environmental point of view. Furthermore, the fuel flexibility of some kind of fuel cells is a very interesting feature for the prospective possibility to use hydrocarbon or alcohol fuels. In particular, the direct feeding of fuel cells with biogas represents a very attractive option for coupling a renewable source with a high efficiency energy conversion system [4-5]. In this PhD thesis, a detailed experimental analysis of Solid Oxide Fuel Cells (SOFC) fed by a mixture of methane and carbon dioxide (which are the main components of biogas) is presented. A brief summary of the contributions of this thesis is reported below: • In Chapter 1, a brief introduction on the state-of-the-art of SOFCs is provided. An overview on the state-of-the-art materials and the potential alternative materials to be used as anodes is given. Some typical issues related to the direct internal reforming of hydrocarbon and alcohol fuels on the anode surface (in particular, carbon deposition build-up on the anode surface of the cell) are reported, along with some common techniques and processes performed in order to prevent these deleterious phenomena. • In Chapter 2, the performances of a Ni/YSZ anode supported tubular SOFC running on direct internal dry forming of methane were studied through polarization curves and performance maps under different carbon dioxide-methane volumetric ratios in a carbon safe region; • In Chapter 3, planar anode-supported SOFCs under direct dry reforming of methane were studied. Aging tests, EIS measurements and post-mortem SEM and FE-SEM analysis of the cells were performed for different carbon dioxide-methane volumetric ratios. A detailed analysis of carbon formation mechanism over the anode surface is reported; • In Chapter 4, the catalytic properties of commercial Ni/YSZ anode supports under different mixture of methane and carbon dioxide were investigated. A particular attention was devoted to the investigation and the explanation of the conversion mechanism: the kinetics and stability of the reactions over the catalyst were investigated by means of Temperature-programmed Techniques. The relevance and the contribution of the chemical reactions (i.e. without the contribution of the electrochemical reactions occurring under current load) involved in these processes on the anode surface were analyzed. This part of the thesis is the most original and was carried out to provide a deeper insight on the conversion mechanism and the catalytic properties for this combination of gases and anode material. As a matter of fact, this topic was only partially explored in literature from an experimental standpoint. The “optimal” operating conditions were indentified for these conditions; • In Chapter 5, the main results are summarized. Some practical conclusions on the optimization of the operation of Solid Oxide Fuel Cells under direct internal dry reforming of methane are given.