Improving the interface adherence in solid oxide cell stacks: Glass seals and oxygen electrode contact layers

Solid oxide cells (SOC) are electrochemical devices that allow an efficient conversion of electrical energy into chemical electrical energy, and vice-versa. The possibility of converting and storing excess energy from wind and solar energy production makes SOCs a promising solution for the future sustainable energy and transport sector. Notwithstanding, this technology is not fully commercialized yet due to economic reasons and a wish for higher durability. Production costs are still high and the lifetime is limited to some extend by the degradation of the different stack components. The interfaces between the different components are the mechanical “weak spots” in SOC stacks. Here, reactions between the joined materials can occur or thermal expansion mismatches can create thermal stresses resulting in interface delamination. Especially two interfaces are contributing to the degradation of SOC stacks: i) the interface between the seal and the interconnect and ii) the interface between the oxygen electrode contact layer and the interconnect. The seal has to be gas tight and keep the gasses (fuel and air) separated, consequently, a delamination at this interface could lead to catastrophic stack failure. The contact layer is the “electrical link” between the electrodes and the interconnect, and a delamination at this interface would cause a contact loss and consequently a drop in performance, local heating and accelerated degradation. The goal of this PhD project was to improve the robustness of these two interfaces. The first part of the project was focused on the seal-interconnect interface. An improved glass-ceramic seal was developed and characterized, and the chemical compatibility with relevant stack components/materials was investigated. Gadolinium doped ceria (CGO) and yttria stabilized zirconia (YSZ), materials used as the electrolyte in the SOC, and aluminized, pre-oxidized or MnCo2O (MCO) coated Crofer 22 APU, materials commonly used as interconnect, were selected as joining partners. Characterization of the interfaces between these materials and the seal did not reveal any undesired reactions, even after long term testing. The interface adherence in terms fracture energy was measured for samples of the glass seal joined to aluminized, pre-oxidized or MCO coated interconnects. The results showed that the type of coating/pre-treatment of the steel had a strong influence on the measured toughness. The effective fracture energy of the glass seal was measured on the aluminized interconnect, and was as high as 23.7 J/m2. In this type of assembly, the fracture occurred within the seal layer, showing that the seal/interconnect interface is no longer the weakest spot. When the glass seal was interfaced with MCO coated or pre-oxidized Crofer 22 APU, the fracture occurred within the oxide layers of the steel. In these cases, the fracture energies (13.6 J/m2 and 15.9 J/m2, respectively) were significant lower. Moreover, the fracture energy was measured again, but this time on an in-house alumina coating, and it resulted in an significantly higher fracture energy of approximately 100 J/m2. In this test, the alumina coating was ripped off from the steel, in most of the fracture area. The tougher value obtained here is likely due to an optimization of the coating roughness. The second part of the project was focused on the oxygen electrode-interconnect interface. Two spinels, Cu1.3Mn1.7O4 and MnCo2O4, were selected as contact materials, based on their thermal expansion and high electrical conductivity. Cu-Mn and Co-Mn metallic powders were used as the starting materials. During heat treatment, the metallic powders oxidized and formed the conductive spinels. After 250 h at 750°C, the conversion from metallic powder to the oxide phases was completed. The electrical and mechanical properties of the contact layers were investigated by measuring the area specific resistance (ASR) and fracture toughness, respectively. After 3000 h aging at 750°C the ASR for an uncoated 441 interconnect with the Cu-Mn or Co-Mn contact layer was low (<25 mΩcm2) and stable. Moreover, the fracture energy measured for a contact layer-interconnect interface aged 250 h at 750°C was 6.0 J/m2 and 3.9 J/m2 for the Cu-Mn and the Co-Mn contact layer, respectively. The values obtained in this study are four times higher compared to state-of-the-art.