Modification of Anode Microstructure to Improve Redox Stability of Solid Oxide Fuel Cells (SOFCs)

In the last decade the increase in energy demand, the awareness of limited availability of fossil fuels and the need to reduce green house gases emission impelled governments and research institutions to focus on the study of renewable energy sources such as solar, wind and biomass derived energy and on the increase in the energy production devices efficiency. Within such scenario a relevant contribution is given by fuel cells technologies as advanced power generation system. Fuel cells are high efficiency devices and comply with the request of environmental friendly source of energy. They convert directly fuel energy into power and heat by electrochemical reactions without the need for combustion as intermediate step. The possibility to use Hydrogen makes fuel cells virtually zero-emission devices being water the only reaction product; nevertheless, also the use of hydrocarbons as fuel reduces considerable CO2 emissions. Among the different systems, solid oxide fuel cells (SOFCs) operate at high temperatures (650-1000°C) and allow to achieve the highest electrical efficiency, from 45 to 60% for common fuels, values not attainable by traditional electrical power generation methods, and up to 70% in combination with a gas turbine for Hybrid Power System generation, with an overall electrical and thermal efficiency higher than 90%. Moreover, such technology presents many advantages such as the possibility to be fed with different fuels, the absence of moving parts, modularity and limited emissions. These characteristics make SOFC suitable for application in the distributed generation market. Despite all the mentioned advantages, SOFCs show problems that make these devices not suitable for the production on industrial scale yet. In particular they present low reliability and are not competitive with traditional powers sources. SOFCs are constituted by single cells (consisting of an anode and a cathode separated by a solid electrolyte) that are collected together in a stack by interconnects in order to obtain the required power. This means that a stack is a multilayer assembly of materials with different thermal, mechanical and chemical properties that need to fulfil many prerequisites for their own function. Moreover, some of these properties must match for other connected components; for instance they have to show similar thermal expansion coefficients, to be stable at high temperatures and during thermal transients. Due to the high working temperature, stack components are necessarily subjected to degradation phenomena, which reduce their long term reliability. Among them, poisoning of cathode by Chromium evaporation from metallic interconnects, chemical interactions between glass–ceramic sealants and ferritic steel interconnects, anode poisoning caused by carbon or sulphur deposition, reduction of electrical conductivity are worthy of mention. Furthermore, various cycling conditions such as thermal cycle, redox cycle, and load cycle affect stability of SOFCs. All these degradation phenomena must be minimized in order to increase SOFC reliability. All these issues are object of intense research. The research work of the present thesis has been focused on the increase of redox stability of anode supported cells, which is considered one of the key point to improve stack reliability. The state of the art materials for the anode is Ni/YSZ cermet due to its high performance. Nevertheless, this cermet is prone to severe degradation upon redox cycling. Due to the high operating temperatures, Nickel particles tend to coalesce and coarsen. Fuel supply interruptions, over-potentials and leakages can cause the re-oxidation of Ni to NiO with a consequent volumetric expansion that can generate internal stresses and lead to cracks formation within the YSZ network and the electrolyte resulting in cell failure. Different approaches can be taken in account in order to minimize redox instability. In order to study redox phenomena and produce redox stable cells many aspects related to the modification of anodic microstructure were analyzed. Among these, one of the most promising method is to modify the anode microstructure by increasing its porosity. The present thesis is divided in two parts. In the first section the theoretical background of fuel cells, specifically SOFCs, is reported. A particular attention is dedicated to describe redox phenomenon and the state of the art of the research in this field. The second experimental part concerns with the production of anodes with improved microstructure. The modification of microstructure was realized by using different powders and by adding different pore formers and doping elements. A detailed study of the effects on redox stability of the microstructure modifications induced by the addition of each of the aforesaid substances is described.