Jeremy P. Meyers

The Impedance Response of a Porous Electrode Composed of Intercalation Particles

A mathematical model is developed to describe the impedance response of a porous electrode composed of spherical intercalation particles. The model considers a porous electrode without solution‐phase diffusion limitations. The model is developed by first deriving the impedance response of a single intercalation particle, obtained by solving a set of governing equations which describe charge‐transfer and double‐layer charging at the surface, solid‐phase diffusion inside the particle, and an open‐circuit potential which varies as a function of intercalant concentration. The model also considers the effect of an insulating film surrounding the particle. The governing equations are linearized to take advantage of the small amplitude of the perturbing current in impedance analysis. Once the impedance of a single particle is determined, this result is incorporated into a model which describes a porous electrode limited by ohmic drop in the solution and solid phases, and by the impedance of the particles of which the porous electrode is composed. The model can be used to examine the effect of physical properties and particle‐size distributions in the porous electrode, and the usefulness of impedance analysis to measure solid‐phase diffusion coefficients is scrutinized.

Simulation of the Direct Methanol Fuel Cell II. Modeling and Data Analysis of Transport and Kinetic Phenomena

A mathematical model that describes the transport of species in a multicomponent membrane is presented. The transport is described by concentrated-solution theory, and the electrochemical potential driving forces are described by a thermodynamic framework set forth in the first paper in this series. A kinetic model is developed to describe methanol oxidation kinetics on Pt-Ru catalysts. Physical properties are estimated by correlation of data in the literature and by simulation of methanol electrolysis experiments. This model provides the framework for direct methanol fuel cell simulation and design in the third paper in this series.

Computer Simulations of the Impedance Response of Lithium Rechargeable Batteries

A mathematical model is developed to simulate the impedance response of a wide range of lithium rechargeable battery systems. The mathematical model is a macroscopic model of a full-cell sandwich utilizing porous electrode theory to treat the electrode region and concentrated solution theory for transport processes in solution. Insertion processes are described with charge-transfer kinetic expressions and solid-phase diffusion of lithium into the active electrode material. The impedance model assumes steady-state conditions and a linear response with the perturbation applied about the open-circuit condition for the battery. The simulated impedance response of a specific system, the lithium-polymer cell Li{vert{underscore}bar}PEO{sub 18}LiCF{sub 3}So{sub 3}{vert{underscore}bar}LiTiS{sub 2}, is analyzed in more detail to illustrate several features of the impedance behavior. Particular attention is paid to the measurement of solid-phase lithium-ion diffusion coefficients in the insertion electrodes using impedance techniques. A number of complications that can lead to errors in diffusion-coefficient measurements based on impedance techniques are discussed.

Simulation of the Direct Methanol Fuel Cell I. Thermodynamic Framework for a Multicomponent Membrane

A theoretical framework that describes the equilibrium of species in a multicomponent membrane is presented. This framework considers explicitly first-order nonidealities that describe the interactions between pairs of species in a multicomponent membrane (e.g., Nafion). These binary interaction parameters are fit to methanol and water uptake data for liquid methanol solutions. A chemical model is combined with this framework to describe uptake of water vapor by the membrane over the entire range of relative humidity. The framework established here provides a means to describe the gradients in electrochemical potential for species in the membrane when describing the driving forces for multicomponent transport in a second companion paper. This paper describes equilibrium conditions; the second paper considers nonequilibrium conditions (transport and reaction kinetics). The modeling aspects are combined in a third paper to simulate the direct methanol fuel cell and quantify aspects of its design.

Simulation of the Direct Methanol Fuel Cell III. Design and Optimization

In this paper, a mathematical model developed in two preceding papers in this series is used to simulate the performance of the direct methanol fuel cell. By mapping methanol concentration and potential profiles under steady-state operating conditions, we gain insight into the behavior and limitation of the cell performance. Methanol fuel efficiency, catalyst layer design, and the effect of methanol crossover on cell performance are examined, and the optimization of membrane separator thickness and methanol feed concentration is discussed.

Evaporatively-Cooled PEM Fuel-Cell Stack and System

Jeremy P. Meyers, Robert M. Darling, Craig Evans, Ryan Balliet, and Mike L. Perry UTC Power, LLC, 195 Governor’s Highway, South Windsor, CT 06074 USA In this paper we describe a new design and mode of operation for a PEM fuel cell. A fuel cell can be configured to operate with humidification of the membrane-electrode assembly (MEA) over the entire face of the cell, which minimizes local variations in relative humidity that can lead to mechanical stress on the membrane. Normally, using this humidification approach requires a rather large volume of circulating water to keep the stack in water balance and provide cooling to the cell by sensibly heating the circulating water. This external water volume can be a liability when attempting to start up a fuel cell power plant from the frozen state. By designing a system which allows the cell to be cooled evaporatively, the system water volume can be reduced significantly, and moving mechanical parts in contact with liquid water can be eliminated. We show a simple system schematic and some example results to demonstrate the viability of this novel system.

Manufacturing of Direct Methanol Fuel Cell Electrodes by Spraying

Spraying is a well-established coating process used to fabricate electrodes for both PEMFC and DMFC fuel cells, and also for the fabrication of gas-diffusion media (GDM) used in fuel cells (1-7). Despite its popularity as a process there is little basic research on how spray parameters and nozzle characteristics affect the drop sizes of catalyst inks, and how those droplet sizes, affect the electrode structure, porosity, conductance and eventually the overall MEA performance. We present results from an experimental study to systematically answer these questions, characterizing the spray nozzle and measuring drop diameters, and then investigating the microstructural effects on electrode performance. For this purpose, first a spraying apparatus was developed and calibrated, and MEAs were fabricated with fixed electrode loadings but with different droplet sizes. Droplet sizes were controlled by characterizing the spray nozzle and measuring the spray by two methods, optically by utilizing high-speed photography and by the hot wire method (8-12).