Three-Dimensional Reconstruction of Porous LSCF Cathodes

Abstract

In this initial study the electrochemically active region of a La 0.8 Sr 0.2 Co 0.2 Fe 0.8 O 3−␦ ͑LSCF͒ cathode was reconstructed in three dimensions using a focused ion beam/scanning electron microscope. The reconstructed volume totaled 1065 m 3 from the free air surface to the dense yttria-stabilized zirconia electrolyte interface. Various microstructural properties were measured, including overall porosity, closed porosity, graded porosity, surface area, tortuosity, triple-phase boundary length, and pore size. Electrochemical impedance spectroscopy data was correlated to microstructure. © 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2794672͔ All rights reserved. Solid oxide fuel cells ͑SOFCs͒ are efficient, environmentally friendly, and fuel-flexible electrochemical devices for the generation of electrical power and heat.1 They consist of three basic layers: cathode, electrolyte, and anode. The cathode is a porous, conductive catalyst for the reduction of O 2 and for the oxidation of fuel. Between the cathode and anode is the dense electrolyte. The circuit is completed via cathode and anode contacts to an external load.The basic chemical formula for the cathodic reduction reaction isCurrent SOFC performance is limited by cathode polarization, which increases with decreasing operational temperatures. 2,3 Cathode microstructure and morphology have a strong effect on this polarization. [2][3][4] In this initial study a dual-beam focused ion beam/scanning electron microscope ͑FIB/SEM͒ was utilized to reconstruct an actual three-dimensional ͑3D͒ model of a La 0.8 Sr 0.2 Co 0.2 Fe 0.8 O 3−␦ ͑LSCF͒ cathode and its interface with a dense yttrium-stabilized zirconia ͑YSZ͒ electrolyte. This highresolution, 3D technique advances the understanding of the cathode microstructure's effect on performance. The identification of critical microstructural properties such as surface area, tortuosity, and interfacial porosity may be correlated with the ionic, electronic, and catalytic processes for a better fundamental understanding of electrochemical performance. With this tool, SOFC material and microstructural design can be more effective in reducing cathodic polarization at lower operational temperatures.The semiconductor industry has used the FIB since the 1980s to deposit, etch, micromachine, and image specimens during different stages of circuit processing. 5,6 This technology was brought forward to reconstruct 3D, geometrically complex submicrometer structures. [7][8][9][10][11] With the advent of 3D modeling software, nanotomography utilizing the dual-beam FIB/SEM technique was used to quantify nanoceramic suspended powders. [10][11][12] This technique was applied to SOFC cermet anodes to quantify microstructural properties such as porosity, triple-phase-boundary ͑TPB͒ length, and degree of anisotropy via tortuosity.13 Such a technique has never before been applied to reconstruct a cathode and the cathode/ electrolyte interface. ExperimentalSquare LSCF symmetric cell cathodes ͑8 ϫ 8 mm͒ were screen printed using prem...

Contradictory concepts in tortuosity determination in porous media in electrochemical devices

“…2) The MATLAB application TauFactor uses an iterative over-relaxation method to solve the diffusion equation and, again, compares the flux through the porous phase to the flux through the dense volume of equal dimensions Brown et al, 2016b;Finegan et al, 2016). 3) The pore centroid method calculates the path length through a porous structure by following the centre of mass of pores of each 2D image slice along the in-plane direction of the volume and dividing it by the thickness of the volume (Gostovic et al, 2007;Smith et al, 2009;Shearing et al, 2012;Cooper et al, 2013;Cooper et al, 2014). 4) The fast marching method calculates the shortest path through the analysed phase by creating a distance map between the starting and end boundary of a propagating front.…”

A Simple Expression for the Tortuosity of Gas Transport Paths in Solid Oxide Fuel Cells’ Porous Electrodes

“…Berson et al [16] reported that Bruggeman correction, which was only valid for ε 0.6  , introduced big errors in low [15,[19][20][21][22][23][24][25][26][27], line + solid triangle symbols denote the results calculated by Equation (9), solid square symbols denote the results calculated by the 3D sphere packing model [17]. Figure 4 shows the dependence of fpε, τqp" D eff {D 0 q on porosity.…”

La0.6Sr0.4Co0.2Fe0.8O3 Perovskite: A Stable Anode Catalyst for Direct Methane Solid Oxide Fuel Cells

“…LSCF has found its application as oxygen membrane [1][2][3][4], catalyst for combustion and oxidative coupling of hydrocarbons [5][6][7], and electrocatalyst for redox reactions in solid oxide fuel cells (SOFCs) [8][9][10][11][12]. LSCF as an electrode for SOFC exhibits high mixed electronic-ionic conductivity, high thermal and chemical stability, and also compatibility with other fuel cell materials.…”