(Invited) Ex-Situ Tracking of Microstructural Evolution in Solid Oxide Cell Electrodes with Ptychographic Nanotomography
Authors: Bowen Jacob Ross , De Angelis Salvatore , Jørgensen Peter Stanley , Tsai Esther Hsiao Rho , Holler Mirko , Abdellahi Ebtisam , Kreka Kosova , Fevola Giovanni 
Published: February 2020

The 3D morphology of solid oxide cell (SOC) electrode microstructure and its evolution has a strong impact on the SOC electrochemical performance during their lifetime. The morphology of the nickel network, in nickel – yttria-stabilized-zirconia (Ni-YSZ) hydrogen electrodes is particularly influenced by: their initial start-up reduction profile, high temperature annealing, high current density operation and exposure to oxidation-reduction cycles. Thus, the development of 3D microstructure characterisation techniques to quantify the 3D microstructure parameters has been an intense topic of research. What remains challenging, due to SOC operating conditions, is to track non-destructively the evolution of the same volume of electrode material with high 3D spatial resolution from before, after and during processes which alter initial SOC microstructures[1–3]. Here we summarise the advantage of X-ray ptychographic nanotomography to image Ni-YSZ electrodes before and after oxidation[4], reduction[5] and annealing[6] with voxel resolutions below 55 nm. Ptychography provides outstanding data quality containing quantitative electron density image contrast. The image quality extrapolates to phase segmentations which in turn provide 3D microstructure parameters of sufficient quality to track the details of e.g. phase fractions, interfacial areas and triple phase boundary density of the changing electrode. Further to this, the image quality enables the possibility to observe the details of structural change of individual particles and interfaces as the basis for gaining fundamental insights into the operating mechanisms. In particular we draw attention to the benefits of identifying isolated particles as models of closed systems that can provide empirical control points into physically based 3D models of electrode evolution. Examples of: redox induced YSZ backbone mechanical failure, resulting change in the Ni particle size distribution and interfacial areas (see Fig. 1a); as well as the effects of annealing on Ni and pore phase percolation on the activity of individual TPBs and the coarsening of isolated Ni particles will be summarised (see Fig. 1b). The importance of appropriate sample preparation will also be discussed.

Figure 1: a) Magnified view of a region of Ni-YSZ electrode evolving from the pristine state via the oxidised state to the re-reduced state illustrating the effects volumetric expansion and contraction of Ni/NiO on the YSZ backbone. Reprinted from [4], with permission from Elsevier. b) A magnified view of the Ni network in a Ni-YSZ electrode illustrating the detachment of a Ni particle from a thinly necked region of the Ni network during annealing. Reprinted from [6], with permission from Elsevier.

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Figure 1