George J. Nelson

Three-dimensional mapping of nickel oxidation states using full field x-ray absorption near edge structure nanotomography

The reduction-oxidation cycling of the nickel-based oxides in composite solid oxide fuel cells and battery electrodes is directly related to cell performance. A greater understanding of nickel redox mechanisms at the microstructural level can be achieved in part using transmission x-ray microscopy (TXM) to explore material oxidation states. X-ray nanotomography combined with x-ray absorption near edge structure (XANES) spectroscopy has been applied to study samples containing distinct regions of nickel and nickel oxide (NiO) compositions. Digitally processed images obtained using TXM demonstrate the three-dimensional chemical mapping and microstructural distribution capabilities of full-field XANES nanotomography.

Three-Dimensional Microstructural Imaging of Sulfur Poisoning-Induced Degradation in a Ni-YSZ Anode of Solid Oxide Fuel Cells

Following exposure to ppm-level hydrogen sulfide at elevated temperatures, a section of a solid oxide fuel cell (SOFC) Ni-YSZ anode was examined using a combination of synchrotron-based x-ray nanotomography and x-ray fluorescence techniques. While fluorescence measurements provided elemental identification and coarse spatial mapping, x-ray nanotomography was used to map the detailed 3-D spatial distribution of Ni, YSZ, and a nickel-sulfur poisoning phase. The nickel-sulfur layer was found to form a scale covering most of the exposed nickel surface, blocking most fuel reformation and hydrogen oxidation reaction sites. Although the exposure conditions precluded the ability to develop a detailed kinetic description of the nickel-sulfur phase formation, the results provide strong evidence of the detrimental effects of 100 ppm hydrogen sulfide on typical Ni-YSZ anode materials.

HeteroFoaMs: Electrode Modeling in Nanostructured Heterogeneous Materials for Energy Systems

Hetero geneous F unctio na l M aterials, e.g., “HeteroFoaMs” are at the heart of countless energy systems, including (from left to right below) heat storage materials (a), batteries (b), solid oxide fuel cells (c), and polymer electrolyte fuel cells (d). HeteroFoaMs are generally nano-structured and porous to accommodate transport of gasses or fluids, and must be multi-functional (i.e., active operators on mass, momentum, energy or charge, in combinations). This paper will discuss several aspects of modeling the relationships between the constituents and microstructure of these material systems and their device functionalities. Technical advances based on these relationships will also be identified and discussed. Three major elements of the general problem of how to model HeteroFoaM electrodes will be addressed. Modeling approaches for ionic charge transfer with electrochemistry in the nano-structured porosity of the electrode will be discussed. Second, the effect of morphology and space charge on conduction through porous doped ceria particle assemblies, and their role in electrode processes will be modeled and described. And third, the effect of local heterogeneity and morphology on charge distributions and polarization in porous dielectric electrode materials will be analyzed using multi-physics field equations set on the details of local morphology. Several new analysis methods and results, as well as experimental data relating to these approaches will be presented. The value, capabilities, and limitations of the approaches will be evaluated.Copyright

Redox instability, mechanical deformation, and heterogeneous damage accumulation in solid oxide fuel cell anodes

Mechanical integrity and damage tolerance represent two key challenges in the design of solid oxide fuel cells (SOFCs). In particular, reduction and oxidation (redox) cycles, and the associated large transformation strains have a notable impact on the mechanical stability and failure mode of SOFC anodes. In this study, the deformation behavior under redox cycling is investigated computationally with an approach that provides a detailed, microstructurally based view of heterogeneous damage accumulation behavior within an experimentally obtained nickel/yttria stabilized zirconia SOFC anode microstructure. Simulation results underscore the critical role that the microstructure plays in the mechanical deformation behavior of and failure within such materials.

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

Computed Tomography Characterization of a Porous Hybrid Motor Grain

X-ray computed tomography (XCT) has been used to non-destructively examine specimens in many applications. This research investigation details the use of X-ray computed tomography (XCT) to examine the structure of porous hybrid motor grains. This technique will support examination of the effects of porosity and chamber pressure on the combustion process in these hybrid fuel grains. XCT was used on a polyethylene sample to determine its void fraction. The sample was thresholded, and a voxel counting method was used to find the void space and total volume of the grain in voxels, yielding a void fraction. The sample was submerged in alcohol, and the liquid displacement was also used to determine a void fraction to validate the XCT results. Plans for future tests on other samples of different porosities, post-burn specimens, and the use of contrast agents are detailed.

Exergy Based Analysis for the Environmental Control and Life Support Systems of the International Space Station

When optimizing the performance of complex systems, a logical area for concern is improving the efficiency of useful energy. The energy available for a system to perform work is defined as a systems energy content. Interactions between a systems subsystems and the surrounding environment can be accounted for by understanding various subsystem energy efficiencies. Energy balance of reactants and products, and enthalpies and entropies, can be used to represent a chemical process. Heat transfer energy represents heat loads, and flow energy represents system flows and filters. These elements allow for a system level energy balance. The energy balance equations are developed for the subsystems of the Environmental Control and Life Support (ECLS) system aboard the International Space Station (ISS). The use of these equations with system information would allow for the calculation of the energy efficiency of the system, enabling comparisons of the ISS ECLS system to other systems as well as allows for an integrated systems analysis for system optimization.

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

Localized Constriction Resistance Effects Upon SOFC Transport Phenomena

Continuum level mass and electronic transport through solid oxide fuel cell (SOFC) anodes are addressed employing the analytic solution of the Laplace equation. The constriction resistance effects of conventional interconnect design upon mass and electronic transport within SOFC anodes are emphasized. Mass transfer resistance effects are created by changes in cross-sectional area between the fuel stream-anode and anode-electrolyte interfaces. Similarly, increased ohmic losses are created by the reduction in cross-sectional area from the electrolyte-anode interface to the interconnect-anode interface. These resistance effects create competing losses that may require consideration in future SOFC component designs. The following paper has pilot quantification of such effects as an introduction to performance trends and an illustration of the analytic approach applied.Copyright

Solid Oxide Cell Microstructural Performance in Hydrogen and Carbon Monoxide Reactant Streams

A distributed charge transfer (DCT) model has been developed to analyze solid oxide fuel cells (SOFCs) and electrolyzers operating in H2–H2O and CO–CO2 atmospheres. The model couples mass transport based on the dusty-gas model (DGM), ion and electron transport in terms of charged species electrochemical potentials, and electrochemical reactions defined by Butler–Volmer kinetics. The model is validated by comparison to published experimental data, particularly cell polarization curves for both fuel cell and electrolyzer operation. Parametric studies have been performed to compare the effects of microstructure on the performance of SOFCs and solid oxide electrolysis cells (SOECs) operating in H2–H2O and CO–CO2 gas streams. Compared to the H2–H2O system, the power density of the CO–CO2 system shows a greater sensitivity to pore microstructure, characterized by the porosity and tortuosity. Analysis of the pore diameter concurs with the porosity and tortuosity parametric studies that CO–CO2 systems are more sensitive to microstructural changes than H2–H2O systems. However, the concentration losses of the CO–CO2 system are significantly higher than those of the H2–H2O system for the pore sizes analyzed. While both systems can be shown to improve in performance with higher porosity, lower tortuosity, and larger pore sizes, the results of these parametric studies imply that CO–CO2 systems would benefit more from such microstructural changes. These results further suggest that objectives for tailoring microstructure in solid oxide cells (SOCs) operating in CO–CO2 are distinct from objectives for more common H2-focused systems. [DOI: 10.1115/1.4034114]