Huayang Zhu

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.

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.

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.

Catalytic combustion of premixed methane-in-air on a high-temperature hexaaluminate stagnation surface

Abstract Microprobe mass-spectrometric sampling is used to measure major species in a stagnation-point flow over a heated La0.267Sr0.333Mn0.4Al11O18 hexaaluminate catalyst surface. The temperature of the catalytic surface is controlled, and the flow is composed of high-purity methane and air. A chemically-reacting flow model with a global surface-reaction mechanism and GRI-Mech 3.0 gas-phase chemistry is developed and used to assist interpretation of the experimental data. Comparison of experimental data with numerical models that include both surface and gas-phase chemistry suggests that the catalyst surface tends to quench gas-phase combustion by removing methyl radicals. Over a range of conditions, surface temperature (680°C ≤ Ts ≤ 1110°C), and equivalence ratio (0.2 ≤ φ ≤ 0.4), the numerical model is able to achieve good agreement with the measurements.

Catalytic combustion of premixed methane/air on a palladium-substituted hexaluminate stagnation surface

This paper is an experimental and modeling study of catalytic combustion of lean methane/air mixtures in stagnation flow over a strontium-palladium-substituted hexaluminate catalyst surface. It reports gasphase profiles in the stagnation-flow boundary layer as measured by microprobe mass-spectrometric sampling. A chemically reacting flow model is developed and used to assist interpretation of the experimental data. A new surface-reaction mechanism does an excellent job of representing the effect of surface temperature (400 °C≤ T s ≤760 °C) and equivalence ratio (0.2≤≤0.8).

Solid oxide fuel cell with oxide anode-side support

Solid oxide fuel cells (SOFCs) with Sr 0.8 La 0.2 TiO 3 anode-side supports, along with Ni-Y 2 O 4 -stabilized ZrO 2 (YSZ) anode, YSZ electrolyte, and LSM-YSZ (LSM = La 0.8 Sr 0.2 MnO 3 ) cathode, were prepared. Button cells yielded a power density up to 1.0 W/cm 2 in humidified H 2 and air at 800°C. The cells showed much-improved stability against coking in methane compared with conventional Ni-YSZ anode-supported SOFCs. A detailed model was used to fit the SOFC electrical results and to show that the good stability in methane was a result of the high 0-to-C ratio in the Ni-YSZ anode due to the Sr 0.8 La 0.2 TiO 3 support.