John R. Izzo

Nondestructive Reconstruction and Analysis of SOFC Anodes Using X-ray Computed Tomography at Sub-50 nm Resolution

A high-resolution, nondestructive X-ray computed tomography (XCT) technique is applied to image the three-dimensional (3D) microstructure of a solid oxide fuel cell (SOFC) composed of a solid yttria-stabilized zirconia (YSZ) electrolyte and a porous nickel YSZ (Ni-YSZ) anode. The X-ray microscope uses the 8 keV Cu Kα line from a laboratory X-ray source, with a reflective condenser optic lens providing a spatial resolution of 42.7 nm. The reconstructed volume data is visualized as 3D images and further postprocessed in binary-image format to obtain structural parameters. The porosity is calculated using a voxel counting method, and tortuosity is evaluated by solving the Laplace equation. A 3D representation of the microstructure is used to calculate true structural parameters and carry out a detailed study of the gas transport within an SOFC electrode at the pore scale. Simulation of multicomponent mass transport and electrochemical reactions in the anode microstructure using the XCT data as geometric input illustrate the impact of this technique on SOFC modeling.

Nondestructive Nanoscale 3D Elemental Mapping and Analysis of a Solid Oxide Fuel Cell Anode

Present solid oxide fuel cells (SOFCs) use complex materials to provide (i) sufficient stability and support, (ii) electronic, ionic, and mass transport, and (iii) electrocatalytic activity. However, there is a limited quantitative understanding of the effect of the SOFCs three dimensional (3D) nano/microstructure on electronic, ionic, and mass-transfer-related losses. Here, a nondestructive tomographic imaging technique at 38.5 nm spatial resolution is used along with numerical models to examine the phase and pore networks within an SOFC anode and to provide insight into the heterogeneous microstructures contributions to the origins of transport-related losses. The microstructure produces substantial localized structure-induced losses, with approximately 50% of those losses arising from phase cross-sectional diameters of 0.2 μm or less.

Comparison of X-ray Nanotomography and FIB-SEM in Quantifying the Composite LSM/YSZ SOFC Cathode Microstructure

Department of Mechanical Engineering, University of Connecticut, Storrs, CT, USA Electrochemical Engineering Group (GGEC), Centre for Interdisciplinary Electron Microscopy, Industrial Energy Systems Laboratory École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland Stanford Synchrotron Radiation Lightsource, Stanford Linear Accelerator Center Menlo Park, CA 94025, USA National Synchrotron Light Source II, Brookhaven National Laboratory Upton, NY 11973, USA

Nondestructive Imaging and Analysis of Transport Processes in the Solid Oxide Fuel Cell Anode

Three-dimensional reconstruction methods, such as the nondestructive transmission x-ray imaging with tomographic reconstruction, have enabled the micro- to nano-scale characterization of the porous solid oxide fuel cell electrode structures. This work provides an examination on the use of several methods being developed by the authors to quantitatively characterize and examine electrode structures in the SOFC. The porous Ni-YSZ cermet anode is used as a framework for this study. Specific attention is paid to the effect these types of structures may have on the functional electrochemical behavior that must be supported by the SOFC; including transport phenomena in the electrode structure in addition to accounts of the interfaces associated with the electrochemical and heterogeneous catalytic phenomena. Phenomenological structures are used to support these efforts. Further, a quantitative description of the characteristic lengths of the electrode structure is discussed in this work.

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

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.

Analysis of Solid Oxide Fuel Cell LSM-YSZ Composite Cathodes With Varying Starting Powder Sizes

Solid oxide fuel cell cathodes have been examined using non-destructive x-ray nanotomography. The cathodes examined were a composite of strontium-doped lanthanum manganite (LSM) and yttria-stabilized zirconia (YSZ), with three different starting powder sizes of 0.3 μm, 0.5 μm, and 1 μm. Differential absorption contrast imaging was performed over the manganese K-edge (6539 eV) for the identification of the LSM, YSZ, and pore phases. The three phases were each segmented from reconstruction of the tomography data. Three dimensional volumes of the segmented phases were used to calculate structural characterization parameters of the sample including porosity, pore size distributions, and mean phase sizes. These parameters are reported and some correlations are drawn to the starting powder size.Copyright

3D Imaging of Nickel Oxidation States using Full Field X-ray Absorption Near Edge Structure Nanotomography

Reduction-oxidation (redox) cycling of the nickel electrocatalyst phase in the solid oxide fuel cell (SOFC) anode can lead to performance degradation and cell failure. A greater understanding of nickel redox mechanisms at the microstructural level is vital to future SOFC development. Transmission x-ray microscopy (TXM) provides several key techniques for exploring oxidation states within SOFC electrode microstructure. Specifically, x-ray nanotomography and x-ray absorption near edge structure (XANES) spectroscopy have been applied to study samples of varying nickel (Ni) and nickel oxide (NiO) compositions. The imaged samples are treated as mock SOFC anodes containing distinct regions of the materials in question. XANES spectra presented for the individual materials provide a basis for the further processing and analysis of mixed samples. Images of composite samples obtained are segmented, and the distinct nickel and nickel oxide phases are uniquely identified using full field XANES spectroscopy. Applications to SOFC analysis are discussed.

Analysis of the X-Ray Imaged Solid Oxide Fuel Cell Anode Microstructure

Solid oxide fuel cell (SOFC) anodes are comprised of heterogeneous functional materials that include a pore phase which supports gas transport and solid phases which support ionic and electronic charge transport. A more detailed understanding of the contributions each of these phases makes to overall anode performance is critical for the design and development of next generation SOFCs. In the present work, three dimensional tomographic reconstructions of SOFC anodes are addressed with consideration given to the characterization of distinct pore, ionic and electronic conducting phases. These reconstructions are produced from transmission x-ray microscope (TXM) images taken at 38 nm spatial resolutions. Elemental mapping enabled by the TXM is used to determine the distribution of pore and solid ionic and electronic conducting phases within the anode. The results presented provide key insights into the composition and morphology of SOFC microstructures. The application of x-ray computed tomography (XCT) to ex situ SOFC micrsostructural characterization is demonstrated, and further applications of this technique are discussed.Copyright

Strontium-Doped Lanthanum Manganate Cathode Degradation Due to a Decomposed Hydrogen Peroxide Oxidant Feed Stream

Decomposed hydrogen peroxide (H2 O2 ) contains excess moisture and stabilizers that can deteriorate SOFC performance and durability markedly in the cathode. A numerical study is performed for a strontium-doped lanthanum manganate and yttria stabilized zirconia (LSM-YSZ) composite cathode using an oxidant stream consisting of a 20% O2 and 80% H2 O mixture to study the detailed reaction mechanism and local transport and polarization phenomena. Specifically the 1-D cathode model couples multi-component gas and charge transport with an oxygen reduction mechanism. The model is validated with data from the literature and used to study the transport effect only of H2 O in the cathode. Future work will consider the effect of H2 O on the electrochemical reaction mechanism.Copyright