Improving the mechanical properties and stability of solid oxide fuel and electrolysis cells

Solid oxide fuel and electrolysis cells are subjected to mechanical stresses during their manufacturing, stacking, and during the operation and thermal cycles. These stresses can cause mechanical failure of the cells and stacks. Improving the mechanical robustness of the cells is thus imperative to address the concerns regarding their reliability.

Fuel electrode supported cell design, in which the relatively thin electrochemically active layers (cathode, anode and electrolyte) are supported by a rather thick substrate, is widely applied. Ni(O)‒3Y-TZP (6 mol% yttria-doped zirconia) is the state-of-the-art substrate in this design. In this PhD project, the possibility for improving the mechanical robustness of the fuel electrode supports was investigated.

This was carried out by tailoring the stabilizer type and concentration in the zirconia phase following two approaches: First, to increase the transformation toughening effect through using a metastable tetragonal zirconia phase, and second, to benefit from the so-called ferroelastic toughening mechanism using a stable tetragonal zirconia phase.

Transformation toughening: In a transformation toughened ceramic, lowering the stabilizer content can increase the transformability of the metastable tetragonal phase, consequently the toughness. Lower stabilizer concentration on the other hand means a lower stability of the tetragonal phase, and can result in undesirable tetragonal to monoclinic phase transformation.

To find the optimum stabilizer content, the stability of the tetragonal phase in Ce-Y co-doped zirconia were studied. From this tetragonal phase stability diagrams were developed as a function of heat treatment temperature and particle packing. The stabilizing effect of YO1.5 was found to be approximately twice that of CeO2, varying slightly with sintering temperature and density.

Based on the developed phase stability diagrams, several Y-doped and Ce-Y co-doped zirconia compounds were chosen and used to prepare fuel electrode supports. Fracture toughness, strength and low- and high-temperature aging of the supports were investigated. NiO‒5YO1.5-SZ and NiO‒1.5CeO2 4.5YO1.5-SZ showed improved fracture toughness over the state-of-the-art support (NiO‒6YO1.5-SZ).

In the NiO‒1.5CeO2 4.5YO1.5-SZ support an enhancement as much as 30% in the room temperature fracture toughness was achieved. At 800°C the fracture toughness of all the metastable tetragonal zirconia based supports decreased. Yet, the co-doped zirconia based support exhibited ~10% improvement over the state-of-the-art support.

Microstructural studies showed that the metastable tetragonal zirconia based supports had a significantly smaller grain size compared to the cubic zirconia based support. The superior mechanical properties of the metastable tetragonal zirconia based supports (compared to their cubic zirconia based counterparts) were then proposed to be resulting from both the transformation toughening effect and their finer grained microstructure.

In contrast to the oxidized samples, the tetragonal and cubic zirconia based supports had comparable fracture toughness in the reduced state. The fracture toughness of the reduced samples decreased dramatically at 800°C, probably due to the lower yield stress and stiffness of the Ni phase. In addition, an improved strength was observed in the newly studied zirconia compounds.

The NiO‒5YO1.5-SZ showed the highest enhancement, i.e. ~30% compared to the state-of-the-art support. The crystalline phase and the fracture toughness of the metastable tetragonal zirconia based supports were studied after aging at 800°C. The 5YO1.5 based supports degraded very fast at the studied temperature, with around 35% monoclinic phase formed after aging for 850 hours.

On the other hand, the crystalline phase of Ce-Y co-doped zirconia based supports remained intact. The fracture toughness of both 1.5CeO2 4.5YO1.5-SZ and 6YO1.5-SZ based supports degraded due to the aging. Nevertheless, the co-doped zirconia based support exhibited ~18% higher fracture toughness over the state-of-the-art support.

It was concluded that the strategy of lowering the stabilizer content to increase the transformation toughening effect in yttria-doped zirconia should be avoided, despite it enhancing the mechanical properties. Co-doping with Ce-Y is on the other hand the recommended approach. 1.5CeO2 4.5YO1.5-SZ was concluded as a promising alternative to 3Y-TZP, as it can provide supports with improved mechanical properties and stability.

The low temperature aging studies showed that the substrates based on metastable tetragonal zirconia in both oxidized and reduced states are prone to the degradation. As the low temperature degradation is significant at temperatures (typically) below 500°C, a low humidity environment is thus crucial during the cooling step in the sintering of the materials, for storage and more importantly during thermal cycles.

Ferroelastic toughening: In a ferroelastic toughened zirconia, a non-transformable tetragonal zirconia with high so-called tetragonality is desirable. Here, the type and concentration of stabilizer(s) should provide such a tetragonal phase. For this reason, several Ti-Y and Ti-Ce co-doped zirconia compounds were synthesized and their crystalline phase was investigated.

As a preliminary evaluation, NiO‒Ti0.2 Y0.08 Zr0.72 support (with t'-zirconia) was prepared and its fracture toughness evaluated. The support had an interestingly high toughness, comparable to that of the state-of-the-art support. Moreover, its toughness was observed to be temperature invariant. The t'-zirconia based supports are thus interesting for further evaluation.

These supports are promising alternatives for the current metastable tetragonal zirconia based supports due to their high toughness and their expected excellent aging resistance. As slow (subcritical) crack growth can occur in zirconia based ceramics, the double torsion method was evaluated as a tool to study the slow crack growth behavior of thin porous zirconia samples - the typical geometry of the supports.

The results showed that the method is applicable if the dependency of the stress intensity factor on the crack length is taken into account, even for thin samples (thickness : width : length ~ 1 : 75 : 130). This dependency decreased with increasing sample porosity.