Robert J. Kee

Modeling Elementary Heterogeneous Chemistry and Electrochemistry in Solid-Oxide Fuel Cells

This paper presents a new computational framework for modeling chemically reacting flow in anode-supported solid-oxide fuel cells (SOFC). Depending on materials and operating conditions, SOFC anodes afford a possibility for internal reforming or catalytic partial oxidation of hydrocarbon fuels. An important new element of the model is the capability to represent elementary heterogeneous chemical kinetics in the form of multistep reaction mechanisms. Porous-media transport in the electrodes is represented with a dusty-gas model. Charge-transfer chemistry is represented in a modified Butler-Volmer setting that is derived from elementary reactions, but assuming a single rate-limiting step. The model is discussed in terms of systems with defined flow channels and planar membrane-electrode assemblies. However, the underlying theory is independent of the particular geometry. Examples are given to illustrate the model.

A general mathematical model for analyzing the performance of fuel-cell membrane-electrode assemblies

Abstract We have developed a general mathematical model to represent the membrane-electrode assembly (MEA) of fuel-cell systems. The model is used to analyze the effects of various polarization resistances on cell performance. The model accommodates arbitrary gas mixtures on the anode and cathode sides of the MEA. Moreover, it accommodates a variety of porous electrode and electrolyte structures. Concentration overpotentials are based on a dusty-gas representation of transport through porous electrodes. The activation overpotentials are represented using the Butler–Volmer equation. Although the model is general, the emphasis in this paper is on solid-oxide fuel-cell (SOFC) systems for the direct electrochemical oxidation (DECO) of hydrocarbons.

A critical evaluation of Navier–Stokes, boundary-layer, and plug-flow models of the flow and chemistry in a catalytic-combustion monolith

Abstract The objective of this paper is to evaluate three alternative formulations for simulating the steady-state flow and chemistry in a honeycomb channel for conditions that are typical of the catalytic combustion of natural gas. In developing simulation capabilities, it is important to understand the physical and computational accuracy that a model can deliver and the computational resources required to do so. Direct comparison of the solutions, using three different model formulations, reveal the range of validity of the various approximations. Computation times range from hours for the Navier–Stokes formulation to seconds for the plug-flow models.

Modeling Distributed-Charge-Transfer Processes in SOFC Membrane-Electrode Assemblies

A model is developed to represent chemistry and transport in porous mixed ionic-and-electronic conducting composite electrode structures in solid oxide fuel cells (SOFC). The model considers the coupled behavior of a full membrane electrode assembly (MEA, i.e., cathode, electrolyte, and anode), which is an important advance compared to earlier models that consider only a single electrode structure. Within each electrode the model represents parallel conduction of electrons and ions, as well as porous-media, chemically reacting gas transport. The model predicts electric-potential distributions in both phases. Charge-transfer chemistry is handled via a modified Butler-Volmer formalism, which depends on the local electric-potential difference between phases. Heterogeneous chemistry (e.g., reforming or partial oxidation) is handled via a detailed surface-reaction mechanism. For typical composite-electrode structures the charge-transfer region extends about 10-20 μm from the dense electrolyte. The results show cell performance depends upon particle sizes within the porous electrodes. Smaller particles generally improve cell performance as a result of expanded three-phase-boundary length. However, smaller particle sizes impede gas transport. Cell performance can be optimized as a function of functional-layer thickness and particle sizes. A distributed charge-transfer formulation is especially important in advanced thin-film electrode structures (e.g., segmented-in-series architectures) where the entire MEA is only a few tens of micrometers thick. The model is formulated as continuum differential equations, which are solved computationally on a discrete mesh network. The paper illustrates the model with examples comparing alternative MEA structures.

Modeling Electrochemical Oxidation of Hydrogen on Ni–YSZ Pattern Anodes

A computational model is developed to represent the coupled behavior of elementary chemistry, electrochemistry, and transport in the vicinity of solid-oxide fuel cell three-phase boundaries. The model is applied to assist the development and evaluation of H_2 charge-transfer reaction mechanisms for Ni–yttria-stabilized zirconia (YSZ) anodes. Elementary chemistry and surface transport for the Ni and YSZ surfaces are derived from prior literature. Previously published patterned-anode experiments [J. Mizusaki et al., Solid State Ionics, 70/71, 52 (1994)] are used to evaluate alternative electrochemical charge-transfer mechanisms. The results show that a hydrogen-spillover mechanism can explain the Mizusaki polarization measurements over wide ranges of gas-phase composition with both anodic and cathodic biases.

Solid Oxide Fuel Cells: Operating Principles, Current Challenges, and the Role of Syngas

Syngas mixtures are excellent fuels for solid-oxide fuel cells (SOFC). Depending on the primary feedstock and the processing technology to produce the syngas, the composition (primarily mixtures of H2 and CO, but often including CH4, H2O, CO2, N2, and other impurities) can vary considerably. Thus, it is important to understand how SOFCs perform with alternative syngas mixtures. Syngas composition can affect materials selection, system design, and operating conditions. To assist understanding and interpreting performance, the article first reviews the basic principles governing SOFC chemistry and electrochemistry. The article also discusses alternative materials and system architectures, especially in the context of syngas fuels. A detailed computational model for a particular tubular, anode-supported, cell is used to compare SOFC performance using different syngas compositions. The syngas mixtures are derived from several processes, including partial oxidation (CPOx) or steam reforming of methane and dodecane, and gasification of coal or biomass.

Modeling Electrochemical Impedance Spectra in SOFC Button Cells with Internal Methane Reforming

A time-accurate transient model of an anode-supported solid-oxide-fuel-cell (SOFC) membrane-electrode assembly (MEA) is developed and used as the basis for simulating electrochemical impedance spectra (EIS). The one-dimensional model includes porous-media transport, elementary heterogeneous chemical reaction, ion conduction, and electrochemical charge transfer. Porous-media transport is represented by the dusty-gas model and electrochemical charge-transfer is modeled with a modified Butler-Volmer formulation. A button-cell configuration is used, with the fuel and air flows modeled as perfectly stirred reactors. Impedance spectra are determined by imposing oscillating electric currents over a range of frequencies and observing the resulting cell voltage. Results are discussed for hydrogen and methane fuels, including the effects of internal methane reforming chemistry.

The effect of monodispersed water mists on the structure, burning velocity, and extinction behavior of freely propagating, stoichiometric, premixed, methane-air flames

We present a computational model to describe the two-phase thermal and chemical interactions between a freely propagating premixed flame and fine droplets of water. The objective is to develop a fundamental understanding of flame structure and extinction in the presence of a water mist. The model and the computational algorithm must accommodate strong coupling between the droplet dynamics and the gaseous flow. The gas-phase conservation equations, which include elementary chemistry, are discretized and solved on an adaptive Eulerian mesh, while the droplet dynamics are represented in a Lagrangian framework. A modified arclength-continuation method is used to follow the solutions through the extinction turning point and thus predict flame-extinction limits. The model predicts how burning velocity and extinction conditions depend on droplet size and number density. The results compare very favorably with previously published theoretical analyses.

Ammonia conversion and NOx formation in laminar coflowing nonpremixed methane-air flames

This paper reports on a combined experimental and modeling investigation of NOx formation in nitrogen-diluted laminar methane diffusion flames seeded with ammonia. The methane-ammonia mixture is a surrogate for biomass fuels which contain significant fuel-bound nitrogen. The experiments use flue-gas sampling to measure the concentration of stable species in the exhaust gas, including NO, O2, CO, and CO2. The computations evolve a two-dimensional low Mach number model using a solution-adaptive projection algorithm to capture fine-scale features of the flame. The model includes detailed thermodynamics and chemical kinetics, differential diffusion, buoyancy, and radiative losses. The model shows good agreement with the measurements over the full range of experimental NH3 seeding amounts. As more NH3 is added, a greater percentage is converted to N2 rather than to NO. The simulation results are further analyzed to trace the changes in NO formation mechanisms with increasing amounts of ammonia in the fuel.

Polarization Characteristics and Chemistry in Reversible Tubular Solid-Oxide Cells Operating on Mixtures of H2, CO, H2O, and CO2

This paper reports the results of combined experimental and modeling studies of reversible solid-oxide cells. The tubular cells are fabricated using a Ni-YSZ (yttria-stabilized zirconia) fuel-electrode support, a dense YSZ electrolyte membrane, and a strontium-doped lanthanum manganate-YSZ composite air electrode. Experiments are designed to systematically vary gas-phase species partial pressures and operating temperatures. The fuels are mixtures of H 2 , CO, H 2 O, CO 2 , and Ar. Performance is measured under anodic (fuel cell) and cathodic (electrolysis) polarization. The models consider reactive porous-media transport within the composite electrodes, thermal chemistry on Ni and YSZ surfaces, and charge-transfer chemistry. All chemistry is modeled with elementary reversible reactions. Close coupling between experimental measurements and model-based interpretation provides a basis for establishing reaction pathways and rates. In addition to advancing fundamental understanding, the resulting detailed reaction mechanisms are valuable for incorporation into predictive models that can be used for design and optimization of fuel-cell and electrolysis systems.