Long-term evolution of solid oxide fuel and electrolysis cell 3-D microstructure

Solid oxide fuel cells (SOFC) are highly efficient electrochemical conversion devices for the production of electricity that combine fuel flexibility with low emissions of pollutants. The reverse operation in electrolysis mode (SOEC) allows storing electrical energy into fuels, such as hydrogen and synthetic methane.

The mitigation of the detrimental microstructural alterations upon long-term operation requires the understanding of the phenomena causing the changes and of the effects on the electrochemical performance. The standard electrodes fabrication procedure involves the sintering of randomly mixed powders, which results in an interconnected microstructure characterized by relatively high initial density of three phase boundaries (TPB) sites. However, this configuration is prone to significant rearrangement, due to material mobility at high temperature. Coarsening, interdiffusion and formation of secondary phases are commonly observed. Moreover, although the performance is known dependent upon the density of connected TPBs, the effects of the nearby morphology are yet to be clarified, as well as the dependence upon the operation mode.

The objective of the present Thesis is to clarify and discern the causes of degradation induced by microstructural rearrangement in SOFC and SOEC modes. The study is therefore focused on two main areas of focus: (i) the characterization of the morphology near the TPB sites to investigate the surfaces available for diffusion and the effect on performance and (ii) microstructural evolution in the cell upon long-term operation, i.e., up to 15000 h, with the emphasis placed on the Ni-yttria-stabilized zirconia (YSZ) cermet and the interface between the gadolinia-doped ceria (GDC) compatibility layer and YSZ electrolyte. A set of quantification methods based on 3-D imaging was developed to improve the understanding of selected issues observed for these heterogeneous materials. The time evolution analysis was performed on a series of samples imaged by focused ion beam-scanning electron microscopy (FIB-SEM) serial sectioning and extracted from stacks operated under different conditions and for varying operation times. The evolution of microstructural properties was tracked using existing and newly-developed algorithms, to advance the capability to describe quantitatively fine alterations of the microstructure. The focus was on measurements that directly relate to the expected driving forces for material alterations or electro-catalytic properties of the TPB sites, such as dihedral angle, curvature, estimate of the internal energy and the available length, a concept proposed within this Thesis for the characterization of surfaces available for diffusion near the electrochemical reaction sites.

The investigations and capabilities developed within this Thesis for the understanding of the relations between differences in the morphology and topology of heterogeneous microstructures and degradation mechanisms are expected relevant for supporting future improvements of the design and operation of solid oxide electrodes.