Energy - Research - Department of Materials Science & Engineering

Three-dimensional reconstruction of anode

Adapted from: Wilson, J.R., et al., Three-dimensional reconstruction of a solid-oxide fuel-cell anode. Nature Materials, 2006. 5(7): p. 541-544.

James R. Wilson¹, Worawarit Kobsiriphat¹, Roberto Mendoza^1,3, Hsun-Yi Chen³, Jon M. Hiller², Dean J. Miller², Katsuyo Thornton³, Peter W. Voorhees¹, Stuart B. Adler⁴, and Scott A. Barnett¹

1) Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208
2) Argonne National Laboratory, Argonne, Illinois 60439
3) Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109
4) Department of Chemical Engineering, University of Washington, Seattle, Washington 98195

ABSTRACT:

The drive toward increased energy efficiency and reduced air pollution has led to accelerated worldwide development of fuel cells. As the performance and cost of fuel cells have improved, the materials comprising them have become increasingly sophisticated, both in composition and microstructure. In particular, state-of-the-art fuel cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytically active surfaces. Determining this microstructure is a critical, yet usually missing, link between materials properties/processing and electrode performance. Current methods of microstructural analysis, such as scanning electron microscopy, provide only two-dimensional anecdotes of the microstructure, and thus limited information about how regions are interconnected in three-dimensional space. Here we have demonstrated the use of dual-beam focused ion beam scanning electron microscopy (FIB-SEM) for making a complete 3D reconstruction of a solid oxide fuel cell (SOFC) electrode. We use this data to calculate critical microstructural features such as volume fractions and surface areas of specific phases, three-phase boundary length, and the connectivity and tortuosity of specific sub-phases.

RESULTS:

The focus of this initial study was the Ni-yttria-stabilized zirconia (Ni-YSZ) composite anode, typical of an anode-supported SOFC. This system was chosen because it typifies the problem of connecting microstructure to performance in a 3-D microstructural network. The cell consisted of a thin yttria-stabilized zirconia (YSZ) electrolyte layer cast onto a thick NiO-YSZ anode support, with a composite cathode of La_0.8Sr_0.2MnO₃ and YSZ. A maximum power density of ≈ 1.2 W/cm² was produced by the cell at 800°C. This power density is fairly typical of SOFCs tested under these conditions. Thus, the microstructures examined below are believed to be representative of state-of-the-art Ni-YSZ SOFC anodes.

The first step in 3D microstructure reconstruction is to obtain volumetric information that can be interrogated to provide 3D information. The advent of dual-beam focused ion beam – scanning electron microscopy (FIB-SEM) has greatly facilitated this process by providing high-quality volumetric data. The FIB is first used to ion mill a trench in the SOFC. Figure 1 shows the FIB-SEM configuration schematically. Thin (50 nm) sections are removed from an exposed surface by the FIB, followed by SEM imaging of the surface, and the process repeated to yield a series of consecutive SEM images.

Figure 1: Schematic drawing showing the FIB-SEM geometry used to reconstruct the SOFC.

A three-dimensional reconstruction of the anode is obtained by stacking the 2D SEM images in 3D space. Figure 2 shows a portion of the resulting 3D reconstruction showing the Ni (green), YSZ (transparent), and pore phases (blue).

Figure 2: Three-Dimensional Reconstruction of the SOFC showing the Ni (green), YSZ (transparent), and pore (blue) phases.

An important application of 3D reconstruction is to quantitatively connect microstructure to electrochemical performance. Three-phase boundary (TPB) length is widely viewed as a key electrode structural parameter. For a SOFC anode, this concept can be understood by considering the electrochemical reaction of H₂ with an oxygen ion O^2- to produce the product H₂O:

H₂(gas) + O^2-(YSZ) → H₂O(gas) + 2e^-(Ni)

This reaction involves gas phase species (present in pores), free electrons (in Ni), and oxygen ions (in the YSZ phase), and hence is expected to occur at a rate correlated to the TPB length. Figure 3 shows an inventory of TPBs obtained by analyzing the 3D anode map (Fig. 2). The volume-specific TPB length was found to be 4.28x106 m/cm³. Although TPB lengths have previously been estimated from 2D images of LSM-YSZ cathodes, this is the first direct measurement known to the authors.

This material is based upon work supported by the National Science Foundation under Grant No. 0542619.

Oxide Anodes

La_0.8Sr_0.2Cr_0.82Ru_0.18O_3-_d powders for composite anodes, prepared by a solid state reaction, are a promising candidate for new SOFC anodes. In reducing conditions, Ru catalyst nano-particles precipitate onto the surface of the oxide. Anode polarization resistances were shown to decrease by a factor of 3 after 50h of operation, due to the increased Ru precipitation.

[1] B.D. Madsen, W. Kobsiriphat, Y. Wang, L.D. Marks and S.A.Barnett, Journal of Power Sources. 166 (2007) 64-67.

This project is currently supported by the DoE (Basic Energy Sciences Program) and the California Energy Commission

Interconnect Materials

J.A. Scott, Y. Boonyongmaneerat, J.D. DeFouw, and D.C. Dunand

Figure 1. SEM micrograph of 50% porous Fe-based E-BRITE foam prepared by powder metallurgy with a sacrificial salt placeholder.

We are currently developing Fe- and Ni-based metallic foams to replace non-porous interconnects in solid-oxide fuel cells (SOFCs), thus providing significant weight savings and facilitating their use in transportation applications. We use temporary spaceholders (salts or oxides) to create a composite via casting or powder metallurgy and subsequently remove the spaceholder to create open porosity. Alternatively, we embed ceramic hollow spheres in a metallic matrix to create a closed porosity foam. To verify their suitability as SOFC interconnects, foam mechanical properties are being examined at room and elevated temperatures for comparison to analytical and finite-element models.

Y. Boonyongmaneerat, D.C. Dunand
“Ni-Mo-Cr Foams Processed by Casting Replication of Sodium Aluminate Preforms”
Advanced Engineering Materials, 10, 4, 379-383 (2008).

This work is funded by GE Global Research and NASA.

Direct Hydrocarbon SOFCs

Stable, coke-free operation on methane and other hydrocarbons has been achieved with Ni-YSZ anode supported cells over a wide range of cell operating conditions. Cell stability, for methane fuel, is influenced primarily by operating temperature and cell current. Additionally, cell stability can be greatly enhanced by presence of a diffusion barrier layer. One promising application of direct hydrocarbon SOFCs is electrochemical partial oxidation of natural gas for syngas production.

1. Lin, Y.B., et al., Direct operation of solid oxide fuel cells with methane fuel. Solid State Ionics, 2005. 176(23-24): p. 1827-1835.

2. Lin, Y.B., Z.L. Zhan, and S.A. Barnett, Improving the stability of direct-methane solid oxide fuel cells using anode barrier layers. Journal of Power Sources, 2006. 158(2): p. 1313-1316.

3. Zhan, Z.L., et al., High-rate electrochemical partial oxidation of methane in solid oxide fuel cells. Journal of Power Sources, 2006. 161(1): p. 460-465.

4. Zhan, Z. and S.A. Barnett, Use of a catalyst layer for propane partial oxidation in solid oxide fuel cells. Solid State Ionics, 2005. 176(9-10): p. 871-879.

5. Zhan, Z., J. Liu, and S.A. Barnett, Operation of anode-supported solid oxide fuel cells on propane-air fuel mixtures. Applied Catalysis A: General, 2004. 262(2): p. 255-259.

6. Pillai, M.R., D.M. Bierschenk, and S.A. Barnett, Electrochemical partial oxidation of methane in solid oxide fuel cells: Effect of anode reforming activity. Catalysis Letters, 2008. 121(1-2): p. 19-23.

This project is currently funded by the Petroleum Research Fund

Optimized electrode microstructure

Work of Dr. Jason D. Nicholas (Scott A Barnett, PI)

The newest aspect of my work focuses on modeling the behavior of solid oxide fuel cell electrodes in an attempt to understand the rate limiting processes and optimize electrode microstructure. As shown in the figure, which is a composite cathode comprised of a Ce_0.9Gd_0.1O_1.95(CGO) backbone infiltrated with a completely uniform Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ (BSCF) nano-particle coating, by combining the results of fundamental property measurements with Finite Element Modeling, the voltage distribution (and therefore the electrode polarization resistance) of a spatially complicated solid oxide fuel cell electrode can be determined. For the modeling, LaPlace's Equation was solved after a 1V potential was placed on the surface of the cathode and a 0V potential was placed in the center of the SOFC electrolyte (which is located at the bottom of the pictured repeat unit). The fact that most of the resistance in a solid oxide fuel cell electrode comes from the surface exchange resistance can be seen in the relatively low magnitude of the potential within the CGO backbone, compared to what was applied to the surface.

This work is the first time finite element modeling has been performed on branched solid oxide fuel cell electrodes, and the branches can lead to a significant reduction in electrode polarization resistance. For instance, the electrode polarization resistance for the electrode shown in the figure is 0.060 Ohm*cm^2 at 700C, whereas the same cathode without the 50nm wide CGO arms is 0.287 Ohm*cm^2.
For a complete description of this and other work I'm performing, please visit: http://jdnicholas.mccormick.northwestern.edu/