Brant A. Peppley

Development and application of a generalised steady-state electrochemical model for a PEM fuel cell

Abstract Models have previously been developed and published to predict the steady-state performance of solid polymer electrolyte membrane fuel cells (PEMFC). In general, such models have been formulated for particular fuel cells and have not been easily applicable to cells with different characteristics, dimensions, etc. The development of a generic model is described here that will accept as input not only values of the operating variables such as anode and cathode feed gas, pressure and compositions, cell temperature and current density, but also cell parameters including active area and membrane thickness. A further feature of the model is the addition of a term to account for membrane ageing. This term is based on the idea that the water-carrying capacity of the membrane deteriorates with time in service. The resulting model is largely mechanistic, with most terms being derived from theory or including coefficients that have a theoretical basis. The major nonmechanistic term is the ohmic overvoltage that is primarily empirically based. The model is applied to several sets of published data for various cells which used platinum as the anode catalyst. Data for various PEM cell designs were well correlated by the model. The lack of agreement of the model predictions with some experimental results may be due to differences in the characteristics of the electrocatalyst. The value of such a generic model to predict or correlate PEM fuel cell voltages is discussed.

Methanol–steam reforming on Cu/ZnO/Al2O3. Part 1: the reaction network

Abstract On-board generation of hydrogen by methanol–steam reforming on Cu/ZnO/Al 2 O 3 catalyst is being used in the development of fuel-cell engines for various transportation applications. There has been disagreement concerning the reactions that must be included in the kinetic model of the process. Previous studies have proposed that the process can be modelled as either the decomposition of methanol followed by the water-gas shift reaction or the reaction of methanol and steam, to form CO 2 and hydrogen, perhaps followed by the reverse water-gas shift reaction. Experimental results are presented which clearly show that, in order to explain the complete range of observed product compositions, rate expressions for all three reactions (methanol–steam reforming, water-gas shift and methanol decomposition) must be included in the kinetic analysis. Furthermore, variations in the selectivity and activity of the catalyst indicate that the decomposition reaction occurs on a different type of active site than the other two reactions. Although the decomposition reaction is much slower than the reaction between methanol and steam, it must be included in the kinetic model since the small amount of CO that is produced can drastically reduce the performance of the anode electrocatalyst in low temperature fuel cells.

A model predicting transient responses of proton exchange membrane fuel cells

Abstract There has been a recent interest in modelling the transient behaviour of proton exchange membrane (PEM) fuel cells. In the past, there have been several electrochemical models which predicted the steady-state behaviour of fuel cells by estimating the equilibrium cell voltage for a particular set of operating conditions. These operating conditions included reactant gas concentrations and pressures, and operating current. Steady-state behaviour is very common and in some cases is considered as the normal operating standard. Unsteady-state behaviour, however, is becoming more of an issue, especially for the transportation applications of fuel cells where the operating conditions will normally change with time. For example, system start-up, system shut-down, and large changes in the power level may be accompanied by changes in the stack temperature, as well as changes in the reactant gas concentrations at the electrode surface. Therefore, both mass and heat transfer transient features must be incorporated into an electrochemical model to form an overall model predicting transient responses by the stack. A thermal model for a Ballard Mark V 35-cell 5 kW PEM fuel cell stack has been developed by performing mass and energy balances on the stack. The thermal characterization of the stack included determining the changes in the sensible heat of the anode, cathode, and water circulation streams, the theoretical energy release due to the reaction, the electrical energy produced by the fuel cell, and the heat loss from the surface of the stack. This thermal model was coupled to a previously-developed electrochemical model linking the power produced by the stack and the stack temperature to the amount and method of heat removal from the stack. This electrochemical model calculates the power output of a PEM fuel cells stack through the prediction of the cell voltage as a complex function of operating current, stack temperature, hydrogen and oxygen gas flowrates and partial pressures. Initially, a steady-state overall dynamic model (electrochemical model coupled with the thermal model) was developed. This was then transformed into a transient model which predicts fuel cell performance in terms of cell voltage output and heat losses as a function of time due to various changes imposed on the system.

Ionic conductivity of chitosan membranes

AbstractChitosan membranes with various degrees of deacetylation and different molecular weights (MW) were prepared by film casting withaqueous solutions of chitosan and acetic acid. Ultraviolet (UV) spectrometry and infrared (IR) spectrometry were used to determine thedegree of deacetylation (DDA) of chitosan. The viscosity–average MW of chitosan was measured in an aqueous solvent system of 0.25 MCH 3 COOH/0.25 M CH 3 COONa. The intrinsic ionic conductivities of the hydrated chitosan membranes were investigated using impedancespectroscopy. It was found that the intrinsic ionic conductivity was as high as 10 24 Scm 1 after hydration for 1 h. The tensile strength andbreaking elongation of the membranes were evaluated according to standard ASTM methods. The crystallinity and swelling ratio of themembranes were examined. A tentative mechanism for the ionic conductivity of chitosan membranes is also suggested.q 2002 Published by Elsevier Science Ltd. Keywords: Chitosan membrane; Hydration; Ionic conductivity

Hydrogen production by steam reforming of methanol for polymer electrolyte fuel cells

Abstract Catalytic steam reforming of methanol has been studied as a means of generating hydrogen for a polymer electrolyte membrane fuel cell. A review of previous work has revealed a lack of understanding of the process for operation at elevated pressures. Also, earlier models of the process do not consider the rate of production of carbon monoxide. A semi-empirical model of the kinetics of the catalytic steam reforming of methanol over CuO/ZnO/Al2O3 catalyst has been developed. This model is able to predict the performance of the reformer with respect to the various parameters important in developing an integrated reformer-polymer fuel cell system. A set of sample calculations of reformer temperature and CO production are given. The impact of the performance of the reformer catalyst on the design of the overall fuel cell power system is discussed. The selectivity of the catalyst to minimize CO content in the fuel gas is shown to be more critical than was previously believed.

Incorporation of voltage degradation into a generalised steady state electrochemical model for a PEM fuel cell

Currently there has been very little reliability or end-of-life analysis conducted for polymer electrolyte membrane fuel cell (PEM) stacks, and detailed designs of PEM systems are still in a rapid evolutionary stage. Voltage degradation as a fuel cell ages is a widely observed phenomenon and results in a significant reduction in the electrical power produced by the stack. Little systematic information has been reported, however, and this phenomenon has not been included in electrochemical models. An earlier work described the development of the generalised steady state electrochemical model (GSSEM) which accepts as input the values of the operating variables (anode and cathode feed gas pressure and compositions, cell temperature and current density), and cell design parameters such as the active area and Nafion membrane thickness. This work will introduce new terms to the model to account for membrane electrode assembly (MEA) ageing, which is a factor in the durability of the stack. One term is based on the concept that the water-carrying capacity (a principal factor in membrane resistance) of the membrane deteriorates with time-in-service. A second term involves the apparent catalytic rate constants associated with the reactions on the anode and cathode side, and the changes in catalytic activity or active site density due to catalyst degradation. A third term deals with the decrease in the rate of mass transfer within the MEA. The resulting model is largely mechanistic, with most terms being derived from theory or including coefficients that have a theoretical basis, but includes empirical parameters to deal with the changing performance. Changes in the polarisation curve predicted by the generalised steady state electrochemical degradation model (GSSEDM) are demonstrated from the data for the performance of typical PEM fuel cell hardware.

On board hydrogen purification for steam reformation/ PEM fuel cell vehicle power plants

The design of the fuel conditioning system for an electrochemical engine using a methanol steam reformer/proton exchange membrane (PEM) fuel cell stack for terrestrial vehicle applications is discussed. The current requirements for PEM anode feed gas quality are described. A comparison of the various alternatives to the fuel purification sub-system is given. The advantages and disadvantages of a number of purification schemes are discussed.

Parametric modelling of the performance of a 5-kW proton-exchange membrane fuel cell stack

Abstract A parametric model predicting the performance of a solid polymer electrolyte, proton-exchange membrane fuel cell has been developed using a combination of mechanistic and empirical modelling techniques. Mass-transport properties, thermodynamics equilibrium potentials, activation overvoltages, and internal resistance were defined by fundamental relations. But the mechanistic model, however, could not completely model fuel cell performance, since several simplifying approximations had been used to facilitate model development. Additionally, certain properties likely to be observed in operational fuel cells, such as thermal gradients have not been considered. Nonetheless, the insights gained from the mechanistic assessment of fuel cell processes were found to give the resulting empirical model a firmer theoretical basis than many of the models presently available in the literature. Correlation of the empirical model to actual experimental data was very good. The performance of a Ballard Mark V 35-cell stack, using a Nafion™ electrolyte membrane, and operating on inlet feeds of air (150% excess) and hydrogen (15% excess) has been modelled parametrically, based on a model previously developed for a Ballard Mark IV single cell.

Simulation of a 250 kW diesel fuel processor/PEM fuel cell system

Polymer-electrolyte membrane (PEM) fuel cell systems offer a potential power source for utility and mobile applications. Practical fuel cell systems use fuel processors for the production of hydrogen-rich gas. Liquid fuels, such as diesel or other related fuels, are attractive options as feeds to a fuel processor. The generation of hydrogen gas for fuel cells, in most cases, becomes the crucial design issue with respect to weight and volume in these applications. Furthermore, these systems will require a gas clean-up system to insure that the fuel quality meets the demands of the cell anode. The endothermic nature of the reformer will have a significant affect on the overall system efficiency. The gas clean-up system may also significantly effect the overall heat balance. To optimize the performance of this integrated system, therefore, waste heat must be used effectively. Previously, we have concentrated on catalytic methanol-steam reforming. A model of a methanol steam reformer has been previously developed and has been used as the basis for a new, higher temperature model for liquid hydrocarbon fuels. Similarly, our fuel cell evaluation program previously led to the development of a steady-state electrochemical fuel cell model (SSEM). The hydrocarbon fuel processor model and the SSEM have now been incorporated in the development of a process simulation of a 250 kW diesel-fueled reformer/fuel cell system using a process simulator. The performance of this system has been investigated for a variety of operating conditions and a preliminary assessment of thermal integration issues has been carried out. This study demonstrates the application of a process simulation model as a design analysis tool for the development of a 250 kW fuel cell system.

An Experimental Investigation of Water Transport in PEMFCs The Role of Microporous Layers

This experimental study was undertaken to resolve the contrasting viewpoints on the role of a microporous layer (MPL), attached to carbon paper porous transport layer (PTL), on the net water transport in a polymer electrolyte membrane fuel cell (PEMFC). Experimental results on single cells with and without cathode MPL show no statistically significant change in the net drag coefficient that could be attributed to the presence of the MPL. In contrast to the two prevailing but contrasting viewpoints, our results indicate that the MPL on the cathode neither enhances back-diffusion nor increases water removal from the cathode catalyst layer to the PTL.