Improved interfaces and innovative joining strategies for high-pressure Solid Oxide Electrolyzer integration

Green hydrogen is emerging as a versatile chemical and energy carrier in a context promoting decarbonisation and transition to renewable energy sources. Power-to-H2 systems can be successfully integrated with renewable energy systems by using excess electricity for H_2 production. Solid Oxide Electrolyzers Cells (SOECs) are one of the most attractive options due to their high electrical conversion efficiency and potential for CO_2 co-electrolysis. This advantage is related to their elevated operating temperatures, ranging from 600 to 900 °C, which allow a significant increase in the transport processes involved in the stack operation. Nevertheless, high temperatures have a strong impact on system’s durability due to thermal degradation issues. Therefore, the stack components must comply with severe thermal, chemical and mechanical conditions. A key component for long-term reliable operation is the interface between the metallic interconnects and the ceramic electrolyte, which must fulfil the important tasks of joining cell components, providing electrical insulation and preventing gas mixing. Glass-ceramics (GCs) are among the most promising sealing materials due to their thermochemical and thermomechanical compatibility with other stack components and their mechanical properties. From the perspective of large-scale production and commercialization, integrating stack fabrication with Additive Manufacturing (AM)-3D printing techniques offers significant benefits, including reduced energy consumption, material wastage, and machining steps. This thesis was developed in the context of the HyP3D Horizon Europe project (GA 101101274) aiming at the realization of ultra-high power density SOEC stacks of 2.14kW. New strategies to achieve high-pressure difference resistant joints, with a surface engineering approach, such as laser modification of the interconnect, thus improving the overall joining reliability, will be reviewed and discussed in this MSc thesis. Since the reliability of the joined materials over a pressure difference plays a key role in preventing gas mixing and consequent stack failure, 3D-printed corrugated cells with embedded functionality will be joined to enhanced surface-modified interconnect-sealant interfaces. Two barium-based and one strontium-based commercial glass systems are characterized for pressurized SOEC applications at 850 °C and 5 bar. Thermochemical and thermomechanical compatibility of the GCs with the interconnector and electrolyte materials are evaluated through microstructural and compositional evaluation. Rheological characterization of glass powder pastes is performed for Robocasting deposition, and the effect of printing parameters variation is analysed observing the extruded filaments. The shear strength of the Crofer22APU-to-3YSZ joint is estimated through a single lap offset (SLO) testing. Finally, the effect of increased surface roughness, measured through profilometric analysis, on substrates' wettability and mechanical properties is investigated through laser modification on Crofer22APU. An Infrared Nanosecond fibre laser treatment leading to a suitable roughness on the interconnect surface is demonstrated to be a feasible approach to obtain mechanical interlocking effect, thus enhancing the adhesion of the sealing system. The shear strength results of the improved joined interfaces will be discussed, showing the positive effects of the optimal surface processing on the joining strength and reliability.