Design and characterisation of glass-based sealants for solid oxide technology

Hydrogen offers a green and flexible option to face the challenges for energy transition, thus allowing to meet objectives of European and international policies for decarbonization. H2 can be produced from fossil fuels and biomass via steam methane reforming (SMR), partial oxidation (oil), gasification (coal), and by water electrolysis. Solid oxide cells (SOCs) devices are of great interest for clean and efficient power generation and for highly productive conversion of electricity to hydrogen, fuels and chemicals using high temperature electrolysis. Enhanced efficiency and durable electrochemical energy conversion in SOC systems can be achieved by suitable material choice and processing a reliable integration technology and an effective stack design. Research is now moving toward the upgrading of cell performances to boost durability and cost-competitiveness, in order to further increase the TRL for this technology[1]. Due to the high operational temperatures (750-850°C), materials must show thermal and mechanical compatibility in with adjoining stack elements. To avoid fuel and oxidant mixing, gas tightness must be ensured by the use of sealants. A key aspect in solid oxide cell devices is given by the design of reliable sealants, which must be used to join dissimilar materials in order to assembly a SOC stack. The role of the sealant is crucial in order to assure high stack efficiency and durability. The sealants must show high gas tightness, thermo-chemical and thermo-mechanical compatibility with the adjacent materials, as well as stability in the relevant operating conditions (temperature range of 700-850 °C) for thousands of hours. Thanks to their versatility in terms of composition and thermomechanical properties, glass-based materials are likely to be the materials of choice as sealants in solid oxide cells devices. Studies on advanced ceramics are proving important developments resulting in performances improvements. This MSc thesis analyses different glass-ceramic sealant compositions, focusing on the thermal properties, the thermomechanical matching with other components and good adherence with the substrates. The formation of crystalline phases with undesirable coefficient of thermal expansion has been avoided by using suitable ratio between glass formers and modifiers. The sealant is thermally and chemically stable for high temperature and long-term operations under conditions typical for reversible SOC stacks. The developed glass-ceramic sealing composition can be used to join ceramics-to-ceramics, ceramics-to-metals and metals-to-metals. Furthermore, the optimisation of robocasting extrusion for a printable ink made of SiO2-based glass powders was also processed, in order to quickly deposit the seal on the support before sintering step and to improve the quality of the system by standardising height and width of the sintered glass-ceramic. Once this procedure is optimised, the printed samples will be tested in dual atmosphere in order to see the long-term compatibility between the sealant and the C22APU interconnect on one side, and 3YSZ spacer on the other side. These research findings provide important insights for future sealing development, their characterization, deposition and validation in a reversible SOC stack.