Curvature of planar solid oxide fuel cells during sealing and cooling of stacks

Thin solid oxide fuel cells (SOFCs), with a planar multi-layer structure, typically exhibit curvature behavior and develop residual stresses originating from the thermal mismatch of the materials involved. The curvature effects oppose successful stack operation, which also relies on the permanent physical contact of geometrically stable planar cells. Curvature studies have been carried out as part of the project "Component Reliability in Solid Oxide Fuel Cell Systems for Commercial Operation" (CORE-SOFC). Various aspects of the curvature of unconstrained cells and cells fixed in stacks were considered. The experiments and theoretical analyses included the curvature changes due to anode reduction and the suppression of cell curvature, either by a compensation layer or by an additional surface load. The critical stresses in the layers are addressed and results are reported for different cell thicknesses and lengths. Cell flatness can be achieved with a compensation layer but the average residual tensile stress in the anode increases. On the other hand finite element analysis (FEA) indicates that relatively high loads are necessary to completely suppress cell curvature. Furthermore, analytical and FEA results on the curvature changes of sealed cells during cooling are presented and the impact of variations in sealing geometry is illustrated. The results are compared to in-situ observations of model stacks, where curvature and strain are analyzed using image correlation.