Aldo A. Peracchio

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.

Extension of anisotropic effective medium theory to account for an arbitrary number of inclusion types

The purpose of this work is to extend, to multi-components, a previously reported theory for calculating the effective conductivity of a two component mixture. The previously reported theory involved preferentially oriented spheroidal inclusions contained in a continuous matrix, with inclusions oriented relative to a principle axis. This approach was based on Bruggemans unsymmetrical theory, and is extended to account for an arbitrary number of different inclusion types. The development begins from two well-known starting points; the Maxwell approach and the Maxwell-Garnett approach for dilute mixtures. It is shown that despite these two different starting points, the final Bruggeman type equation is the same. As a means of validating the developed expression, comparisons are made to several existing effective medium theories. It is shown that these existing theories coincide with the developed equations for the appropriate parameter set. Finally, a few example mixtures are considered to demonstrate the ...

Lattice Boltzmann method for continuum, multi-component mass diffusion in complex 2D geometries

Multi-component gas diffusion in the continuum flow regime is often modelled using the Stefan–Maxwell (SM) equations. Recent advances in lattice Boltzmann (LB) mass diffusion models have made it possible to directly compare LB predictions with solutions to the SM equations. In this work, one-dimensional (1D) and two-dimensional (2D), equi-molar counter-diffusion of two gases in the presence of a third, inert gas is studied. The work is an extension and validation of a recently proposed binary LB model for components having dissimilar molecular weights. The treatment of inflow and outflow boundary conditions (for specifying species mole fractions or mole flux) is developed via the averaging of component velocities before and after collisions. Results for one and two spatial dimensions have been compared with analytic and numerical solutions to the SM equations and good agreement has been found for a wide range of parameters and for large variations in molecular weights. A novel molecular weight tuning strategy for increasing the accuracy has been demonstrated. The model developed can be used to model continuum, multi-component mass transfer in complex geometries such as porous media without empirical modification of diffusion coefficients based on porosity and tortuosity values. An envisioned application of this technique is to model gas diffusion in porous solid oxide fuel cell electrodes.

Lattice Boltzmann method for multi-component, non-continuum mass diffusion

Recently, there has been a great deal of interest in extending the lattice Boltzmann method (LBM) to model transport phenomena in the non-continuum regime. Most of these studies have focused on single-component flows through simple geometries. This work examines an ad hoc extension of a recently developed LBM model for multi-component mass diffusion (Joshi et al 2007 J. Phys. D: Appl. Phys. 40 2961) to model mass diffusion in the non-continuum regime. In order to validate the method, LBM results for ternary diffusion in a two-dimensional channel are compared with predictions of the dusty gas model (DGM) over a range of Knudsen numbers. A calibration factor based on the DGM is used in the LBM to correlate Knudsen diffusivity to pore size. Results indicate that the LBM can be a useful tool for predicting non-continuum mass diffusion (Kn > 0.001), but additional research is needed to extend the range of applicability of the algorithm for a larger parameter space. Guidelines are given on using the methodology described in this work to model non-continuum mass transport in more complex geometries where the DGM is not easily applicable. In addition, the non-continuum LBM methodology can be extended to three-dimensions. An envisioned application of this technique is to model non-continuum mass transport in porous solid oxide fuel cell electrodes.

Effect of orientation anisotropy on calculating effective electrical conductivities

This paper develops an analytical effective medium theory (EMT) equation for calculating the effective conductivity of a mixture based on Maxwells and Maxwell-Garnett’s theories, extended to higher volume fractions using Bruggemans unsymmetrical treatment (BUT), with a long term goal of extending the treatment to mixtures more representative of real materials in order to calculate their effective electrical conductivity. The development accounts for spheroid shaped inclusions of varying degrees of anisotropic orientation. The orientation is described by the introduction of a distribution function. Two methodologies valid for the inclusion dilute limit were used to evaluate the effective conductivity: one based on Maxwells far field approach, and the other based on the Maxwell-Garnett in the matrix approach. It was found that while the dilute limit equations for the effective conductivity were different, the final EMT equations derived by applying BUT collapsed to the same formula which was generalized ...

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 Charge and Heat Transfer in a Solid Oxide Fuel Cell Anode

The development of a more efficient and power dense solid oxide fuel cell (SOFC) requires better treatment of irreversibilities that exist in present SOFC designs. Loss due to material resistance, or Joule heating, is one such loss that substantially hinders present SOFC designs. This work examines two dimensional (2D) continuum electronic charge and heat transfer models of the SOFC to examine how Joule heating can be characterized and quantified in terms of the common and easily measured microstructure properties of tortuosity, Ni mean free path, and Ni contiguity. By modeling these processes at the pore scale, presently lacking in the literature, this work uses a direct approach to develop a working understanding of how to quantify the SOFC microstructure geometric features with respect to ohmic performance. It has been found that while the pore tortuosity has little correlation to ohmic performance, the Ni mean free path and contiguity were found to be excellent measures of a 2D anode microstructure’s ohmic performance.Copyright

Backing Out Diffusion Coefficients in Alkaline Anion Exchange Membranes

Water diffusion coefficients for electrolyte membranes are a subject of interest due to the relationship between water content and ionic conductivity. These diffusion coefficients are functionally dependent on water content. Many existing techniques for measuring the diffusion coefficient suffer from experimental complexity or a lack the diffusion coefficients detailed behavior. This paper uses a simple technique for determining the water diffusion coefficient involving the use of experimental data coupled with a numeric model. This back out algorithm was validated through the use of Nafion®, a proton exchange membrane, with the results comparing well with NMR data from the literature. The process has also been applied to SnowPure ExcellionTM, an alkaline exchange membrane, to obtain the diffusion coefficient as a function of water content. It was found that these membranes are much less permeable to water with the diffusion coefficient being almost an order of magnitude less than that of Nafion®.

Detailed Electrochemistry and Gas Transport in a SOFC Anode Using the Lattice Boltzmann Method

A coupled electrochemical reaction and diffusion model has been developed and verified for investigation of mass transport processes in Solid Oxide Fuel Cell (SOFC) anode triple-phase boundary (TPB) regions. The coupled model utilizes a two-dimensional (2D), multi-species Lattice Boltzmann Method (LBM) to model the diffusion process. The electrochemical model is coupled through localized flux boundary conditions and is a function of applied activation overpotential and the localized hydrogen and water mole fractions. This model is designed so that the effects of the anode microstructure within TPB regions can be examined in detail. Results are provided for the independent validation of the electrochemical and diffusion sub-models, as well as for the coupled model. An analysis on a single closed pore is completed and validated with a Ficks law solution. A competition between the electrochemical reaction rate and the rate of mass transfer is observed to be dependent on inlet hydrogen mole fraction. The developed model is presented such that future studies on SOFC anode microstructures can be completed.Copyright

Characterization of Solid Oxide Fuel Cell Materials Based on Microstructural Skeletonization

The understanding of the relationship between the microstructure of materials for energy applications and their transport and electrochemical properties is needed to optimize their long-term performance. The improvements of 3D imaging techniques such as x-ray nanotomography allow access to geometrical and elemental information with ever increasing accuracy and details. These advances warrant determining new relevant metrics for material characterization, the calculation of which will require adaptations of the methodologies for parameter extraction.This study presents the development of a tool for the characterization of porous, heterogenous materials that provides coherent geometrical and topological information. We illustrate the relevance of the methodology by discussing the differences between geometrical concepts for estimating phase size distributions of real heterogeneous materials investigated using x-ray nanotomography and how research between different scales and physics can be bridged. This is achieved by providing, on the one hand, inputs to classical continuum models and, on the other hand, by synergetic combination with discrete element methods.Copyright