In the context of renewable energies and the energy mix, dihydrogen production is particularly important. The production of dihydrogen by steam electrolysis at high temperature (700-900°C) using Solid Oxide Cell technology (SOC), has the advantage of producing dihydrogen with a better efficiency as compared to low temperature water electrolysis. In addition, this technology is able to operate in reversible mode, which can ensure electricity production in reverse mode.
The SOC high temperature electrolysis technology is based on a successive stack of single repeat units (SRU) made of cells and metallic interconnects ensuring both the collection of oxygen and hydrogen and the electronic conduction between the different SRUs. In such complex systems, ensuring the sealing of this ceramic/metallic multilayer assembly fed with different gases is technically very challenging. The specifications that seals must meet are particularly demanding. In addition to sealing, the material must have mechanical, thermomechanical and chemical properties, as well as sufficient electrical resistivity (Figure 1) [1].
The high temperature characteristics of glass oxides and glass-ceramic seals make this type of material one of the few candidates for use as sealants. Furthermore, the geometries are complex and the lengths to be sealed are important. These numerous constraints lead to the use of glass powder suspended in organic solvents. This process allows for the installation of the seal by simply depositing the suspension on the areas to be sealed. A heat treatment is then applied to shape the material and ensure the sealing of the system. The use of a composition that tends to crystallize allows, a priori, improving the thermal and mechanical properties [2].
The usual thermal treatment consists in a first temperature rise followed by a crystallization stage. As the temperature rises, the organic solvents evaporate producing CO2 and gas present in the furnace atmosphere may be trapped during sintering, which causes porosity to appear. Early surface crystallization can also compete with the sintering phenomenon and prevent the maximum densification of the glass ceramic by freezing the structure. Crystallization stage will then occur.
The general objective of the study is to control the material microstructure (crystallization, porosity) in order optimize sealing properties. A detailed knowledge of the material microstructure is necessary in order to optimize the thermal treatment of the glass-ceramic formation and anticipate the possible microstructural evolution of the glass-ceramic seal operating in the stack. Two studies were conducted in parallel on a specific sealant.
The first one focuses on the evolution of the porosity (percentage, pores mean diameter and density) as a function of time and temperature. Swelling, coalescence and rising of the pores are observed at high temperature.
The second study focuses on the material crystallization: phases identification, evolution of crystals morphology as a function of temperature and crystalline surface fraction analysis as a function of time and temperature. These data allow to obtain the equilibrium crystalline fraction as a function of temperature for further modelling.
This paper will first recall the specifications that seals must meet in Solid Oxide Cells stacks. It will then present an overview of our methodology and preliminary results of our studies on the material microstructure.
References
[1] D. Tulyaganov et. al., Aluminosilicate-based sealants for SOFCs and other electrochemical applications – A brief review, Journal of Power Sources, 242, 486-502 (2013)
[2] K. Gurbinder, Solid Oxide Fuel Cell Components, Chapter 5, Sealing Concepts: Glasses and sealants, Springer International Publishing (2016)