Improved thermo-mechanical reliability of anode-supported solid oxide fuel cells

Solid oxide fuel cell (SOFC) based systems are co-generators of electricity and heat that excel for their high efficiency, low pollutant emissions and fuel flexibility. Their reliability is close to meet the requirements for market deployment. Recent research improved the reliability during both thermal cycling and polarisation, mostly by series of incremental modifications. However, the transition from small- to large-scale production imposes ever more severe costs requirements, which from the perspective of manufacturing and operation, largely depend on thermo-mechanical aspects. The end of life of SOFC stacks still often occurs because of mechanical failure, which therefore remains a major hurdle for commercialization.

The purpose of this study is to advance the understanding of the mechanical failures in SOFC stacks. It has two principal scopes: (a) the characterisation of the mechanical properties of SOFC materials and (b) investigations by thermo-mechanical modelling of the failure modes. The two activities are interrelated, because the prediction of failure mode by modelling relies on the knowledge of the mechanical properties.

Two setups for mechanical testing were designed and fabricated. Both are capable of testing multiple samples in one experimental stage, under the adequate temperature and atmosphere. A model-based parameters estimation approach was developed to measure the elastic and both primary and secondary creep properties and overcome the limitations of analytical solutions. The experimental and numerical developments were applied to the characterization of the NiO/Ni-YSZ electrode material and complemented with analyses using 3-D imaging and computational homogenization. The strength was further analysed by the Weibull statistics approach. An extension of thermo-elastic computational homogenisation was finally implemented to estimate the anisotropic visco-elastic properties of the gas-diffusion layers. A detailed continuum thermo-mechanical model for SOFC stack analysis was developed. Differences in modelling assumptions, selected design modifications and operation conditions were examined. Further, the effects on the stack reliability of component geometrical imperfections caused by manufacturing were for the first time modelled.

Parameters for the modelling of both primary and secondary creep in Ni-YSZ were identified, which is needed for advancing stack modelling capabilities. The thermo-elastic properties were not found to vary significantly during operation. The analysis of the strength measurements highlighted the combined beneficial effects of residual stress and plasticity of the Ni phase in the heterogeneous Ni-YSZ structure. The stack simulations allowed the analysis of cell and sealing failures and distribution of contact pressure at electrically conducting interfaces. The effects of operation conditions were found to overall dominate, and worst for counter-flow with high internal reforming. Component imperfections altered the distribution of the contact pressure in the short-term, which was alleviated by creep relaxation during operation, without leading to strong history-dependence. The effect of the geometrical imperfections considered in this work remained mild, compared to that of the mechanical properties. The risk of cell failure is strongly affected by the operation conditions, whereas that of the sealants by the design and position in the stack.