Robert M. Darling

Model of Carbon Corrosion in PEM Fuel Cells

This paper presents a mathematical model of the corrosion of carbon catalyst supports in polymer electrolyte membrane (PEM) fuel cells. The model describes how a maldistribution of hydrogen across the fuel electrode can induce both oxygen evolution and carbon corrosion on the positive electrode of the fuel cell in the fuel-starved region. Implications of this reverse-current mechanism are explored by simulating a cell with a nonuniform distribution of hydrogen along the fuel channel in both steady-state and transient operation.

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.

Modeling Two-Phase Behavior in PEFCs

UTC Fuel Cells, South Windsor, Connecticut 06074, USAA model is developed to examine quantitatively the effects of flooding on the operation of polymer-electrolyte fuel cells ~PEFCs!.Specifically, the change in the maximum power as a function of the structural properties of the diffusion media, including the bulkporosity, wettability, thickness, and pore-size distribution, is described. The porous-medium model developed includes analyticexpressions and a modeling methodology for handling both liquid and gas flow. The model is used in combination with ourprevious membrane model to simulate transport in typical gas diffusion layers and examine the effect of layer hydrophobicity onthe maximum power.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1792891# All rights reserved.Manuscript submitted November 3, 2003; revised manuscript received March 24, 2004. Available electronically September 27,2004.

Mathematical Model of Platinum Movement in PEM Fuel Cells

This paper presents a mathematical model of platinum dissolution and movement through the layers of a polymer electrolyte membrane (PEM) fuel cell. The model is based on dilute-solution theory. Dilute solution equations are used to describe the movement of soluble platinum through the PEM.

Damage to the Cathode Catalyst of a PEM Fuel Cell Caused by Localized Fuel Starvation

This paper examines damage to the cathode catalyst layer of a polymer electrolyte membrane (PEM) fuel cell caused by restriction of hydrogen access to a portion of the anode catalyst layer. The results show severe damage to the majority of the cathode catalyst behind the obstruction after 100 h of operation. A portion of the cathode catalyst layer near the outside edge of the obstruction remains undamaged.

Modeling side reactions in composite LiyMn2O4 electrodes

A Li/1 M LiClO 4 in propylene carbonate (PC)/Li y Mn 2 O 4 cell is used to investigate the influence of side reactions on the current-potential behavior of intercalation electrodes. Slow cyclic voltammograms and self-discharge data are combined to estimate, simultaneously, the reversible potential vs state-of-charge curve for the host material and the kinetic parameters for the side reaction. This information is then used, together with estimates of the solid-state diffusion coefficient and main reaction exchange current density, in a mathematical model of the system. Predictions from the model compare favorably with continuous cycling results and galvanostatic experiments with periodic current interruptions.

Modeling a Porous Intercalation Electrode with Two Characteristic Particle Sizes

A mathematical model of a complete cell containing a porous intercalation electrode with two characteristic particle sizes is presented. Galvanostatic cycling and relaxation phenomena on open-circuit are compared to a cell with a single particle size. Electrodes with a particle-size distribution show modestly inferior capacity-rate behavior in all cases considered in this work. The cycling results exhibit a mismatch in the states-of-charge of the surfaces of the different particle sizes located at the same position in the electrode. The magnitude of this mismatch correlates with the slope of the open-circuit potential vs. state-of-charge curve of the intercalation material. The relaxation on open circuit is substantially faster when the particles are uniformly sized. Asymptotic solutions were developed to aid in the description of the open-circuit behavior in the cases with nonuniform particle sizes. The particle-size distribution has a more pronounced influence on the open-circuit results than on the galvanostatic results.

Dynamic Monte Carlo simulations of diffusion in Li{sub y}Mn{sub 2}O{sub 4}

Monte Carlo techniques are used to simulate the thermodynamics and diffusion of Li in the intercalation compound Li{sub y}Mn{sub 2}O{sub 4}. Results are presented for stoichiometric Li{sub y}Mn{sub 2}O{sub 4} and for Li-rich Li{sub y}Mn{sub 2}O{sub 4} containing pinned Li. The predicted theoretical open circuit potential compares favorably with literature results. The influence of Li-Li interactions on the activation energy leads to a diffusion coefficient that depends upon concentration. The diffusion coefficient is interpreted in terms of a thermodynamic factor and a binary interaction parameter.

On the Short‐Time Behavior of Porous Intercalation Electrodes

The reaction-rate distribution in a porous electrode containing an intercalation compound is examined theoretically. Particular attention is paid to the influence of the exchange current density on the propagation of the reaction through the depth of the electrode at short times. The governing differential equations are nondimensionalized, linearized, and applied to semi-infinite electrode and separator regions. A Laplace transform of the solution to this problem is presented, and asymptotes are developed for short and moderate times. These limiting forms are compared to the results of numerical simulations. The short-time reaction-rate distribution consistent with the assumption of an arbitrarily large exchange current density is presented and analyzed

Capillary Pressure Saturation Relations for PEM Fuel Cell Gas Diffusion Layers

Capillary pressure saturation relations (CPSRs) are presented for Toray TGP-H-060 and Mitsubishi rayon carbon fiber paper which can both be used as gas diffusion layers (GDLs) in proton-exchange membrane fuel cells (PEMFCs). The saturation is measured using water over a range of capillary pressures. Boundary and scanning curves for imbibition and drainage are measured to further understand the hysteresis observed during PEMFC operation. The primary source of hysteresis in CPSRs is attributed to the difference in advancing and receding contact angles. The measured hysteresis is predicted to have a significant effect on mass transport in the GDL and thus performance in PEMFCs.