A solid oxide fuel cells (SOFC) technology is one of the most promising energy conversion device due to its high conversion efficiency, low environmental pollution and high flexibility to various fuel types (1). To overcome all challenges concerning materials’ long term stability at operating temperatures, current research efforts are aimed at intermediate temperature SOFCs (IT-SOFCs) (2). From this point of view samaria-doped ceria (SDC) is a promising ceramic material, with superb ionic conductivity, which can be used as an anode material when combined with Ni in Ni-SDC cermet (2). To ensure electronic and ionic conductivity as well as gas permeability, anode cermet should exhibit carefully tailored microstructure where metallic Ni, ceramic SDC and pores form continuous phases. In an operating cell fuel gas is electrochemically oxidized at the Ni/SDC/fuel interface, called the triple phase boundary (TPB) region. Therefore, both the activity of Ni-SDC and their stability are strongly influenced by the cermet’s morphology and microstructures such as volume fractions, grain connectivity, grain size, pore size, pore distribution and TPB length (3).

For this purpose, exact and accurate microstructural determination is crucial in predicting material’s performances in an operating cell. In such a cermet composite critical topological features such as connectivity and the tortuosity of transport pathways in the pores can only be established based on 3D microstructural information (4). In this work we present quantitative characterization of Ni-Sm_0.2Ce_0.8O_2-δ cermets, sintered at 1400°C, using high-resolution FIB-SEM tomography. Initially, a layer of platinum was deposited ontop the region of interest to protect the surface and prevent rounding of top edges of cross section during milling. The volume of interest was separated using an optimised U-pattern pre-milling procedure to prevent material redeposition and shadowing of the signals used for imaging and microanalysis. The sample was then serial sectioned using an automated slicing procedure with drift correction algorithms to obtain a series of 2D images with narrow and reproducible spacing between the individual image planes (5). Experimental milling and imaging parameters have been optimised in order to obtain a high quality 3D reconstruction with phase contrast information. Individual phases were identified from EDXS elemental maps and further segmented according to their grey level. 3D reconstructed volume is a base for determination of: volume fractions of individual phases, grain connectivity, porosity, tortuosity and TPB length.

The presented analytical method will serve as a tool for quantitative characterization of primary microstructural parameters and complex topological features, during microstructure evolution of Ni-Sm_0.2Ce_0.8O_2-δ anode cermets using various sintering procedure.

Literature:

[1] S.C. Singhal, K. Kendall, High Temperature Solid Oxide Fuel Cells, Elsevier, 2002

[2] Liu Q, Dong X, Yang C, Ma S, Chen F, J Power Sources 2010; 195: 1543-1550

[3] S.D. Kim, H. Moon, S.H. Hyun, J. Moon, J. Kim, H.W. Lee, J Power Sources, 163 (2006) 392-397

[4] Brus, G., Miyawaki, K., Iwai, H., Saito, M., & Yoshida, H., Solid State Ionics, 265(2014), 13–21

[5] M. Schaffer, J. Wagner, B. Schaffer and M. Schimed, Ultramicroscopy, 107, pp. 587-597 (2007)

Figures:

Figure1: Experimental approach for three-dimensional characterization of Ni-Sm0.2Ce0.8O2-δ cermet, sintered at 1400°C: a) FIB-SEM cross-section using imaging parameters for optimal Z-contrast between SDC-phase (bright grey), Ni-phase (dark grey) and pore-phase (black), b) cube preparation for automated serial sectioning, c) example of 3D reconstructed data presenting Ni- and SDC-phase.

Figure2: Quantification of microstructure features obtained from reconstructed 3D volume of Ni-Sm0.2Ce0.8O2-δ cermet, sintered at 1400°C: a) volume fractions of individual phases, b) enlarged part of 3D reconstruction, reveling grain connectivity and TPB regions, c) example of one possible gas transport path, through connected pore, for the explanation of tortuosity.

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EMC Abstracts - https://emc-proceedings.com/abstract/three-dimensional-characterization-of-ni-sm0-2ce0-8o2-%ce%b4-cermet-for-sofc-anodes-by-high-resolution-fib-sem-tomography/