Evolution of 3-D Transport Pathways and Triple-Phase Boundaries in the Ni-YSZ Hydrogen Electrode upon Fuel Cell or Electrolysis Cell Operation
Authors: Nakajo Arata , Cocco Alex P. , DeGostin M. B. , Burdet Pierre , Peracchio Aldo A. , Cassenti Brice N. , Cantoni Marco , Van herle Jan , Chiu Wilson K. S. 
Published: February 2020

The understanding of the relationships between the microstructure and performance of heterogeneous materials for electrochemical energy conversion and storage have significantly benefited from the advances of 3-D imaging methods. The Ni and yttria-stabilized zirconia (YSZ) hydrogen electrode in solid oxide fuel/electrolysis cells (SOFC/SOEC) is for instance heterogeneous, comprising an assembly of reticulated phases that form and provide transport pathways to electrocatalytic sites. The morphology and topology of the phases is known to evolve during long-term operation. Despite the aggressive environment, the performance degradation has been reduced to less than a few percent per year. Yet, further efforts are required to guarantee the commercial viability. The remaining degradation stems from subtle morphological, chemical and crystallographic alterations that are increasingly difficult to detect and quantify. Analyses are commonly based upon the measurement of the evolution of the effective transport properties of the phases and of the connected triple-phase boundary length measured on 3-D reconstructions obtained by x-ray or electron microscopy. The properties are then implemented in continuum electrode models for the estimating and analyzing of material performance. One limitation of such averaged property-based approaches is that all the TPBs are assumed equally accessible. Therefore, information on the effects of 3-D microstructural features may be only partially retrieved.
In this study, Ni-YSZ samples were extracted from cells in pristine state and after SOFC or SOEC operation in short-stacks and segmented-cells for up to 10,000 h. Regions including or close to the interface with the YSZ electrolyte were imaged by focused ion beam-scanning electron microscopy (FIB-SEM). For a sample aged for 10,000 h in SOEC mode, the data from the secondary electron/secondary ion (SESI), in-lens secondary electrons and energy-selective backscattered (EsB) detectors was complemented with 3-D energy dispersive x-ray spectroscopy (EDS) elemental maps by switching automatically the acceleration voltage from 1.7 kV to 10 kV every 10 slices. The 3-D reconstructions with an isometric voxel size of 10 nm were segmented for measuring a series of structural, material and electrochemical properties relevant for degradation analyses, such as tortuosity, polarization resistance and accessible TPB length. The latter is a newly proposed performance metric that combines geometrical and physical considerations to quantify the access to TPB sites. The measurement method consists in applying an analytical electrochemical fin model (ECF) to a 3-D discrete representation of the heterogeneous structure provided by skeleton-based partitioning to probe the resistance of the pathways to each TPB, within each phase separately.
The measurement of structural properties highlights as expected a significant coarsening of the Ni phase upon aging, which modifies the transport properties of the Ni and pore phases and decreases the total TPB length. The degradation of the polarization resistance computed by ECF is in line with electrochemical impedance spectroscopy (EIS) measurements on segmented-cells. In all the phases, the TPBs were not observed to be equally accessible, even in the pristine sample. The variation exceeds one order of magnitude and a few connected TPBs can be even passivated because of diffusion limitations. Preferential pathways are clearly detected, which suggests a non-uniform utilization of the phases that is potentially detrimental for the performance and the resilience of the material to alterations caused by degradation operation. However, the accessible TPB remains almost unchanged after 4,700 h of SOFC operation. This is because the YSZ phase provides the best access to the TPBs, followed by Ni and pore, and is not significantly affected by 4,700 h of SOFC operation. Further, the changes in the spatial distribution of the TPBs caused by the coarsening of the Ni phase do not modify the access through YSZ. Therefore, during this operation window, the degradation scales with the decrease in the connected TPB. In contrast, micro-cracks were detected in the YSZ scaffold after 10,000 in SOEC mode, close to the YSZ electrolyte. The accessible TPB can capture the effects of such localized alterations, whereas standard volume-averaged measurements remain insensitive, showcasing the need for new 3-D measurements for understanding the long-term degradation in SOFC/SOEC materials.