CHARACTERIZATION OF SOLID OXIDE FUEL CELL CERAMIC COMPOSITE ELECTROLYTES

Kurzfassung

The increased use of fossil fuels has significantly amplified greenhouse gas emissions, particularly of carbon dioxide, creating an enhanced greenhouse effect known as global warming. Energy use from fossil fuels is also a primary source of air, water, and soil pollution producing carbon monoxide, sulphur dioxide, nitrogen dioxide, particulate matter, and lead. Pollution and global warming pose major health risks. Air pollution contributes to lung disease, including asthma, lung cancer, and respiratory tract infections, whilst the long-term effects associated with global warming may be even more devastating, leading to climate changes we are not able to control or evaluate with precision. Another issue regarding fossil fuels is their availability: fossil fuels are not unlimited. It has taken nature millions of years to store underground energy captured from the sun by plants and other organic matter and to turn it into coal, oil and natural gas. We are consuming very quickly what nature has been putting aside for geological eras. According to the best analysts of the petroleum industry the world oil and gas supplies will start to run out between 2020 and 2060. Coal is available in larger quantities and therefore could last longer, but it is not really desirable to think about a world energy demand fulfilled by coal, as this is the most pollutant of the three main fossil fuel types (although technology is moving forward and developing cleaner ways to burn coal, such as coal gasification). Another answer to the energy demand could be nuclear energy, but in this field, apart from the availability of sufficient fuel, which could be bypassed by the adoption of fast reactors, the main concern is the environmental one. As a by-product, nuclear energy produces radioactive waste that continues to be dangerous for thousands of years. The treatment and storage of this kind of waste requires a social stability that permits information to be passed on over generations and generations. It is just not possible to predict what will happen in such a long time range, so it can be regarded as not sensible to leave such a dangerous heritage behind us. Against this background, efforts are being made to find sources of energy that are non-polluting and available in great or possibly unlimited quantities. These sources are called renewable energy sources. Man has being using wind and water mills for many thousands of years, and has been using the energy of wind to move ships, and the energy of the sun to warm up water. All the energy that is behind these processes comes directly or indirectly from the sun. We know that the sun produces its energy by nuclear fusion reactions and that it also will not last forever, but its life will be so long that we can think of it as relatively eternal when we are concerned with our energy demand problems. Modern science is developing ways of converting energy from sun, wind and water into mechanical and electrical energy. The main energy conversion devices are hydraulic turbines, wind turbines, photovoltaic and thermal solar panels. However, all the renewable energies have two significant drawbacks: they have a low energy density and the amount of energy that they produce is susceptible to casual fluctuations due to the different amount of sunlight, wind or water flow available. It is therefore necessary to couple these systems with energy storage. It is difficult to store electrical energy, and so heat or pressure storages have been considered, but they are not really satisfactory solutions. It is at this point that hydrogen comes into play. Hydrogen, the simplest element, is composed of one proton and one electron. It makes up more than 90% of the composition of the universe. It is the third most abundant element on the earth's surface, and is found mostly in water. Under ordinary (earthly) conditions, hydrogen is a colourless, odourless, tasteless, and non-poisonous gas composed of diatomic molecules. When it reacts with oxygen, only water and heat are produced, so its employment has no effect on global warming. Hydrogen is not available on earth in its atomic form; it is only to be found bounded with other elements, especially with oxygen to form water. Hydrogen can be extracted from water by electrolysis when sufficient energy is provided. The energy provided by renewable sources can be employed to create Hydrogen via water electrolysis. Hydrogen can be stored as a gas or a liquid, and thus provides energy storage capacity. The most efficient way to convert hydrogen’s chemical energy into electrical energy is to use a fuel cell. A fuel cell converts the chemical energy of the fuel into electrical energy without going through the heat and mechanical energy conversion steps. As each step results in a loss of energy, avoiding these two steps allows the fuel cell to obtain a better efficiency. There are many kinds of fuel cells, each with its own advantages. However, Solid Oxide Fuel Cells (SOFC) in particular have the features to be a reasonable competitor, especially in the static electrical energy production field. Their advantages are the fuel flexibility, as the high temperature allows an internal reforming of many fuels; high efficiency; and the capability of being employed in cogeneration configurations due to the high quality of the waste heat that is generated. The efficiency of the SOFC is about 50-60%, and as high as 90% in combined electrical-heat applications. To enhance the long-term performance stability and to widen the material selection, research is moving towards lowering the operating temperature of SOFCs from the traditional 1000 °C to a 500–700 °C temperature range. At operating temperature below 700 °C, low-cost ferrite stainless steels could be used as the components of fuel cell systems such as interconnects, gas manifolds and heat exchangers. When the operating temperature of SOFCs is lowered the conductivity of traditional electrolytes decreases, affecting negatively the power density of the fuel cell. There are two possible ways of reducing the electrolyte resistance: reducing the thickness of the electrolyte or using a material with a higher ionic conductivity. There is a limit to the thickness reduction that can be carried out, as the electrolyte must be gas tight and must avoid the mixing of the fuel and oxidant. The traditional material used as electrolyte in SOFCs is yttria-stabilized zirconia (YSZ), because of its relatively low cost and good stability in the anode and cathode operating atmospheres. A material that presents a higher ionic conductivity at lower temperatures is scandia-stabilized zirconia (ScSZ).The main drawback of ScSZ is its higher price. The purpose of this research project is to produce samples of a SOFC composite ceramic fuel cell electrolyte by means of sintering. The electrolyte is produced by combining the YSZ and ScSZ materials, and the effects of different compositions and production cycles on the material characteristics and especially on the ionic conductivity of the material are investigated. The goal is to obtain a material with a better ionic conductivity at low operating temperatures than the YSZ, but less expensive than the ScSZ. The rest of this thesis is organized as follows: Chapter II explains the working principles of fuel cells, Chapter III deals with the kinetics of fuel cell reactions, Chapter IV gives an overview of the different kinds of fuel cells, Chapter V analyses the different techniques used to investigate fuel cell characteristics and behaviour, Chapter VI examines the theory of sintering, Chapter VII concerns in particular the SOFC fuel cell electrolyte component and the different materials it is made from, Chapter VIII illustrates the Experimental work that has been done and Chapter IX analyses the results that have been achieved.