Understanding the mechanical properties of Solid Oxide Fuel Cell anode materials by measurement and image-based modelling
Creators: name: Cui, Guansen
Publisher: name: Imperial College London
Year: 2018

description: Solid Oxide Fuel Cells (SOFCs) have the potential to meet the critical energy needs of our modern civilisation and minimise the adverse environmental impacts from growing energy consumption. They are highly efficient, clean and can run on a variety of fuels. Nonetheless the high temperature operation of SOFCs places stringent requirements on the materials used for cell construction and hinders their widespread usage. So the key technical challenges are the development of suitable materials and the fabrication techniques to increase the mechanical properties (eg. elastic modulus and mechanical toughness) of the cell/electrode materials. Of the material requirements, the most important consideration is the matching of the thermal expansion coefficients of the electrode materials with those of the electrolyte to prevent cracking or delamination of SOFC components either during high temperature operation or heating/cooling cycles. However, the stresses generated inside the microstructure might be very different than those at macro-scale. Therefore micro-level modeling is required for an accurate estimation of stress generation points and the performance of SOFCs. The present research work is based on the flat planar design owing to its ease of fabrication and potential for providing high power densities compared to other cell configurations. This research combines 3D imaging microstructural characterisation techniques to determine microstructural properties, and to correlate the mechanical properties of the anode thick films with their microstructure through experimental approaches such as nanoindentation, 3D quantitative microstructural analysis and numerical finite element methods (FEM). In the first part of the thesis, thick films of Sc2O3-ZrO2 (ScSZ) have been studied. The thick film scaffold was fabricated by multi-layer tape casting method and sintered at temperatures from 1050 to 1450℃. The room temperature mechanical properties were measured using nanoindentation techniques. A spherical indenter was used as the contact area was much greater than the scale of the porous microstructure. It was shown that severe discontinuous bursts occurred during the nanoindentation measurements, hence the Joslin parameter was utilised to minimise effects from both surface roughness and sample substrate. The estimated elastic modulus of the scaffold samples was found to increase from 9.5±3.1 to 23.4±5.7 GPa, and hardness from 0.18±0.02 to 0.37±0.05 GPa, as the sintering temperature varied from 1250 to 1450℃ associated with a porosity decrease from 66.2 to 55.6 vol%. In the second part of the thesis, pre-sintered conventional Ni-ScSZ anode cermet materials (with different Ni solid contents from 30 to 60 vol%) were studied. A non-linear behaviour between porosity and elastic moduli of the studied sample materials was observed. Good consistency of the elastic modulus for the studied cermet materials between the 3D FEM and the nanoindentation results was observed, showing that the developed compression model produced reliable predictions of the mechanical properties (eg elastic modulus) of electrode microstructures. The model further enabled the visualisation of the compression process generated stress distribution map occurring within the electrode microstructure, with the stress concentration locations suggesting a potential failure and hence redesign for these regions. Therefore, the entire effort of the present research work is aimed at the development of better designs of the electrode microstructure by correlating the microstructure (such as porosity, particle size distributions and neck sizes) with the mechanical performance.