The performance of solid oxide electrolysis cells (SOEC) is affected by microstructural changes inside the cell. In this work detailed post-mortem characterization is conducted on an SOEC stack tested for electrolysis of steam over 9000 h [1]. The stack, produced by Topsoe Fuel Cell A/S, consists of 25 identical repeating units connected in serial. Each repeating unit includes an SOEC cell and neighboring interconnects (ICs). The SOEC cell has a Ni-YSZ support layer and fuel electrode, a YSZ electrolyte, a LSCF-CGO (LSCF: (La,Sr)(Co,Fe)O_3-δ, CGO:(Ce,Gd)O₂) oxygen electrode and a CGO barrier layer between the electrolyte and the oxygen electrode. The objective of the present investigation is to identify modifications in the active Ni-YSZ electrode microstructure, which are believed to strongly affect the SOEC cell and stack performance. These microstructural changes are quantified thorough 2D and 3D microstructure analyses.

Based on micrographs recorded for the 9 kh tested stack and a reference cell (i.e. a cell from the same production batch as those in the stack, but not long-term tested), a relative quantification of Ni, YSZ and pore phase fractions and particle sizes has been obtained through scanning electron microscopy (SEM) imaging analysis. The mean intercept length principle has been used to investigate 2D SEM images recorded on different cell locations. The ease of this type of characterization makes it possible to investigate several locations in the long term tested stack: both fuel electrode inlet and outlet, directly under the contact point between the IC and at the point where the cell is in contact with the gas channel. By comparing results of the reference and tested cell, significant changes in Ni and pore size are observed while the YSZ backbone does not show substantial changes. The 9000 h electrolysis testing results in a decrease in the Ni phase fraction and an increase in Ni particle size. Compared to the reference cell, the biggest microstructural change is observed at the location where the cell is directly in contact with the IC, where the Ni phase fraction is decreased from 28 % (reference) to 18 % and the Ni mean intercept length is increased from ~1 μm (reference) to ~1.4 μm.

Based on the 2D image analysis locations of special interest were selected for 3D reconstruction of the electrode structure by focused-ion-beam (FIB) serial sectioning. The 3D data provides information on the true particle size distribution, avoiding the assumptions that need to be made for the 2D data. Besides, the 3D analysis allows the transport pathways to be characterized through each of the phase networks [2]. A comparison between 2D and 3D data is carried out to concisely investigated microstructural changes and relate them with the cell performance during long-term operation.

[1] A. Brisse, J. Schefold, G. Corre, Q. Fu. Performance and Lifetime of Solid Oxide Electrolyzer Cells and Stacks. 11^thEuropean SOFC & SOEC Forum, July 2014

[2] Peter Stanley Jørgensen, Søren Lyng Ebbehøj, and Anne Hauch. "Triple phase boundary specific pathway analysis for quantitative characterization of solid oxide cell electrode microstructure." Journal of Power Sources 279 (2015): 686-693.