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Research Papers

Characterization and Quantification of Electronic and Ionic Ohmic Overpotential and Heat Generation in a Solid Oxide Fuel Cell Anode

[+] Author and Article Information
Kyle N. Grew, John R. Izzo

Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269-3139

Wilson K. S. Chiu2

Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269-3139wchiu@engr.uconn.edu

2

Corresponding author.

J. Fuel Cell Sci. Technol 8(3), 031001 (Feb 15, 2011) (12 pages) doi:10.1115/1.4002226 History: Received July 27, 2007; Revised June 26, 2010; Published February 15, 2011; Online February 15, 2011

The development of a solid oxide fuel cell (SOFC) with a higher efficiency and power density requires an improved understanding and treatment of the irreversibilities. Losses due to the electronic and ionic resistances, which are also known as ohmic losses in the form of Joule heating, can hinder the SOFC’s performance. Ohmic losses can result from the bulk material resistivities as well as the complexities introduced by the cell’s microstructure. In this work, two-dimensional (2D), electronic and ionic transport models are used to develop a method of quantification of the ohmic losses within the SOFC anode microstructure. This quantification is completed as a function of properties determined from a detailed microstructure characterization, namely, the tortuosity of the electronic and ionic phases, phase volume fraction, contiguity, and mean free path. A direct modeling approach at the level of the pore-scale microstructure is achieved through the use of a representative volume element (RVE) method. The correlation of these ohmic losses with the quantification of the SOFC anode microstructure are examined. It is found with this analysis that the contributions of the SOFC anode microstructure on ohmic losses can be correlated with the volume fraction, contiguity, and mean free path.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the 1D SOFC button cell model used to model experimental work in literature (10). The electronic RVE is at an arbitrary location within the anode support while the ionic RVE is localized to the anode/electrolyte interface.

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Figure 2

Validation of 1D SOFC button cell model (lines) against experimental data (symbols) (10). (a) Voltage versus current density curves for different fuel feed streams. (b) Local cell potential, normalized at anode current collector to Nernst OCV and local electronic, and ionic current densities. It can be noted that total current density is conservative and current exchange occurs a finite distance into the respective electrodes.

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Figure 3

A sample Auger electron spectroscopy map of polished SOFC anode sample. Note distinct phases: red is Ni, white is YSZ, and black is pore.

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Figure 4

Definition of the phase tortuosity, defined as the primary type-A conductor, which represents a continuous path of the conducting phase across the structure. (a) 1D domain considering type-A cluster continuity with remainder of domain. The gray region is the remainder of the domain that is considered continuous with the light blue type-A conductor in the first Laplace solution, the dotted line is a continuity boundary, and the solid black lines at top/bottom of domain are insulation boundaries. (b) Local Laplace solutions, where the cross-hatched region is not considered, and the solid black lines represent insulation boundaries between bulk type-A cluster and the rest of the RVE.

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Figure 5

Phenomenological electronic conduction geometries considered. Red: Ni, white: YSZ, and black: pore. All geometries have a symbol associated with them that will be used during analysis of results. For each, the left denotes the base case containing only Ni and pores while the right case denotes a variation on the case with inclusions of secondary phases. Complementary cases are considered for ionic conduction in YSZ. (a) Straight Ni phase with pore constriction, (b) straight Ni phase with a blind branch, and (c) tortuous pore cutting through Ni substrate. All geometries are 10×10 μm2 in size.

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Figure 6

Normalized distributions of current density in anode samples for 2.0×104 A/m2 net current density: (a) Current density magnitude for electronic conduction in Ni phase normalized to area corrected left Ni phase boundary current density of 7.65×104 A/m2, (b) constant electronic current density flux lines, (c) current density magnitude for ionic conduction in YSZ phase normalized to area corrected left YSZ phase boundary current density of 4.85×105 A/m2, and (d) constant ionic current density flux lines

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Figure 7

Electronic and ionic tortuosity corrected effective heat release as a function of characterization parameters: (a) electronic corrected effective heat release versus Ni volume fraction, (b) electronic corrected effective heat release versus Ni–Ni contiguity for mean particle size of 1 μm, 3 μm, and 5 μm, (c) electronic corrected effective heat release versus Ni mean free path, (d) ionic corrected effective heat release versus YSZ volume fraction, (e) ionic corrected effective heat release versus YSZ-YSZ contiguity for mean particle size of 0.5 μm, 5.4 μm, and 25 μm, and (f) ionic corrected effective heat release versus YSZ mean free path

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Figure 8

Fit of parameterized results based on quantification parameters shown with 95% confidence interval bands: (a) tortuosity corrected effective electronic heat release versus correlated characterization parameter fit and (b) tortuosity corrected effective ionic heat release versus correlated characterization parameter fit

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