Residual stress and thermal cycling of planar solid oxide fuel cells

Abstract

The published literature relating to damage to planar solid oxide fuel cells caused by thermally induced stresses and thermal cycling is reviewed. This covers reported studies of thermal cycling performance and stresses induced by temperature gradients and differences in thermal expansion coefficients in typical planar SOFC configurations, namely electrolyte supported; anode supported and inert substrate supported cells. Generally good agreement is found between electrolyte residual stresses measured by X-ray diffraction or cell curvature and stresses calculated from simple thermo-elastic analysis. Finite element modelling of temperature distributions in cells and stacks in steady state operation are well advanced and capable of being extended to compute stress distributions. Failure criteria are then discussed for laminated cell structures based on critical energy release rate fracture mechanics models developed originally for coatings. However, in most cases the data required to apply the models quantitatively (such as elastic moduli of actual laminated material and fracture energies of materials and interfaces) are not available. Where data are available there are inconsistencies that require resolution. Seals are critical components in many planar solid oxide fuel cell configurations, but again there are discrepancies in experimental mechanical properties and the role of internal stresses in their fracture. In addition, there is as yet no firm evidence that thermal cycling damage involves any true materials fatigue process. Effect of thermal cycling on SOFC integrityTaniguchi et al. 7 investigated the thermal cycling performance of electrolyte supported planar SOFCs

Sintering of 4YSZ (ZrO2 + 4mol% Y2O3) nanoceramics for solid oxide fuel cells (SOFCs), their structure and ionic conductivity

“…Refs. [1][2][3][4][5][6][7][8][9][10][11] contain some recent reviews of this work, much of which has been focused on reducing the size of the microstructure of the cell components and therefore of the particles in the powders used to produce the components.…”

Some recent studies on glass/glass-ceramics for use as sealants with special emphasis for high temperature applications

“…Achieving a sealant that fulfils these conditions is important to long term stability of SOFC stacks. Most SOFC sealants reported so far have been based on the alkaline earth aluminosilicate system such as barium alumino-silicate due to good thermal expansion matching and the ability to wet materials such as the interconnect allowing good sealing [21,29,30,31,33,34]. While the chemical durability of these glass-ceramics is generally good, it is necessary to further improve phase stability, crystallization behaviour and adhesion behaviour of the glass-ceramics with other SOFC components.…”

“…One of the recent applications of glass-ceramics, which places great demand on materials performance, is sealing of solid oxide fuel cells (SOFCs) using glass/glassceramics, to effectively channel fuel and oxidant (commonly air) [14][15][16][17][18][19][20]. SOFC based power generation presents many advantages, including higher efficiency, lower environmental impact and ability to co-generate heat from the exhaust gas to name a few, as compared to conventional power generation [21][22][23][24][25][26][27][28]. The glass-ceramics used for sealing must show good matching of thermal expansion coefficient (TEC) with other SOFC components, low reactivity with other fuel cell components and long term stability at elevated temperature in either oxidizing or wet reducing atmosphere [21,29,[30][31][32][33].…”

Metal foam heat exchangers for thermal management of fuel cell systems

“…Air-cooled fuel cells, on the other hand, use the ambient air as the coolant. They can be best described by the above two figures with the only difference that in most cases the air flow channels are vertical [4,7,9,23,24]. They do not have the aforementioned drawbacks associated with the wet cooling systems.…”