R.F. Mann

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.

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.

Dynamic interaction of a proton exchange membrane fuel cell and a lead-acid battery

An air-independent fuel cell/lead-acid battery power system could extend the submerged endurance of conventional submarines. This load sharing system requires a deep understanding of how the systems will interact, both from an operational perspective and to avoid high battery voltages that may result in excessive hydrogen production in a completely contained environment. A methodology for predicting the response of the coupled systems was developed to predict the transient behaviour under various loads at standard operating conditions.

Carbon monoxide poisoning of proton-exchange membrane fuel cells

The platinum-alloy catalyst used in proton-exchange membrane (PEM) fuel cell anodes is highly susceptible to carbon monoxide (CO) poisoning. CO reduces the catalyst activity by blocking active catalyst sites normally available for hydrogen chemisorption and dissociation. The reaction kinetics at the anode catalyst surface can be used to estimate the decrease in cell voltage due to various levels of CO contamination in the inlet fuel stream. Literature data on the effects of CO-contaminated fuel streams on PEM fuel cell performance have been reviewed and analysed in an attempt to further understand the electrochemical properties of the CO adsorption process. A fuel cell performance model of a bipolar, Nafion 117 PEM fuel cell stack has been developed which predicts equilibrium cell output voltage as a function of current density and partial pressure of CO. The model contains both empirical and mechanistic parameters and evolved from a steady-state electrochemical model for a PEM fuel cell fed with a CO-free anode gas. Reaction kinetics and equilibrium surface coverage have been incorporated into the electrochemical model to predict the decrease in fuel cell performance at equilibrium. The effects of CO were studied at various concentrations of CO in hydrogen as the anode feed gas. Literature data were used to develop the model parameters and the resulting model is used to compare the model-predicted voltages, with and without CO, to data found in the literature.

Deactivation of Cu/ZnO/Al2O3 catalyst: evolution of site concentrations with time

The reforming of methanol is acknowledged as a convenient means to generate hydrogen for a PEFC due to the low temperature (<280 °C) at which the reaction occurs and the low CO content of the reformate. However, the catalyst is prone to deactivation in the upper range of its operating temperatures. This paper presents a method of analysis in which relative changes in site concentrations are deduced from a series of long-duration rate measurements. The sites involved in the reforming reaction are shown to evolve independently of each other. In particular, concentration of sites for hydrogen adsorption decline at a greater rate than those responsible for the adsorption of oxygenated species. A key process in the deactivation of the catalyst is the decrease in its capacity to adsorb and dissociate hydrogen.