J.W. Van Zee

Numerical prediction of mass-exchange between cathode and anode channels in a PEM fuel cell

Abstract A numerical model is developed to predict the mass flow between channels in a Polymer Electrolyte Membrane (PEM) fuel cell with a serpentine flow path. The complete three-dimensional Navier–Stokes equations with multi-species mixture are solved and electro-chemical reactions are modeled as mass source/sink terms in the control volumes. The results indicate that flow distribution in both anode and cathode channels are significantly affected by the mass consumption patterns on the Membrane Electrode Assembly (MEA). The water transport is governed by both electro-osmosis and diffusion processes. Further, the overall pressure drop is less than that expected for a regular straight channel flow.

The effects of compression and gas diffusion layers on the performance of a PEM fuel cell

Abstract Changes in the performance of a PEM fuel cell are presented as a function of the compression pressure resulting from torque on the bolts that clamp the fuel cell. Three types of gas diffusion layers were studied at 202 kPa and 353°K. An optimum bolt torque was observed when ELAT® or a combination of CARBEL® and TORAY™ gas diffusion media were used as diffusion layers. The optimum is related to the gasket thickness and the measured compression pressure on the diffusion layer. The performance changes may also be related to changes in the porosity, the electrical contact resistance, and the excluded water at the membrane.

Three-dimensional numerical simulation of straight channel PEM fuel cells

The need to model three-dimensional flow in polymer electrolyte membrane (PEM) fuel cells is discussed by developing an integrated flow and current density model to predict current density distributions in two dimensions on the membrane in a straight channel PEM fuel cell. The geometrical model includes diffusion layers on both the anode and cathode sides and the numerical model solves the same primary flow related variables in the main flow channel and the diffusion layer. A control volume approach is used and source terms for transport equations are presented to facilitate their incorporation in commercial flow solvers. Predictions reveal that the inclusion of a diffusion layer creates a lower and more uniform current density compared to cases without diffusion layers. The results also show that the membrane thickness and cell voltage have a significant effect on the axial distribution of the current density and net rate of water transport. The predictions of the water transport between cathode and anode across the width of the flow channel show the delicate balance of diffusion and electroosmosis and their effect on the current distribution along channel.

Effects of Hydrogen Sulfide on the Performance of a PEMFC

An exploratory study of H 2 S poisoning of membrane electrode assemblies (MEAs) in proton exchange membrane fuel cells (PEMFCs) consisting of Pt and Pt-Ru alloy electrodes is presented. Steady-state polarization curves arereported for each electrode after exposure to 50 ppm H 2 S at 70°C. Significant findings include (i) partial recovery of the MEA after 3.8 h of exposure to H 2 S: (ii) the degree of the recovery is influenced by the electrochemical oxidation of two surface species observed during cyclic voltammetry experiments; (iii) in contrast to CO poisoning, Ru has no effect on increasing MEA tolerance toward H 2 S poisoning; and (iv) increasing the Pt loading by 60% appears to quadruple the partially recovered current density at 0.6 V (i.e., 0.125 A/cm 2 for Pt-Ru alloy and 0.575 A/cm 2 for Pt electrodes) after exposure to neat H 2 for 24 h.

Verifying Predictions of Water and Current Distributions in a Serpentine Flow Field Polymer Electrolyte Membrane Fuel Cell

A three-dimensional numerical model is used to predict the local current density and membrane conductivity inside a 10 cm 2 polymer electrolyte membrane fuel cell with a serpentine flow path. The model includes the gas diffusion layer, and it accounts for the area hidden from direct contact with the flowing gases by the current collector contacts (i.e., the ribs). The predictions agree with experimental data for various operating cell temperatures at a fixed inlet humidity condition. Data on closure of the water balances are presented and are also shown to be consistent with the numerical predictions of water transport by electro-osmotic drag and back diffusion. The data and the model show conditions where insufficient water lowers the conductivity of the membrane and yields low currents at a fixed voltage. Finally, predictions of the distributions of current density, water flux per proton, and membrane conductivity are presented.

Effect of Transient Ammonia Concentrations on PEMFC Performance

bW. L. Gore and Associates, Incorporated, Elkton, Maryland 21922-1488, USA Data are shown to indicate the effect of high NH3 concentrations on the membrane electrode assembly ~MEA! performance in proton exchange membrane fuel cells ~PEMFCs!. Steady-state tests were performed with different NH3 mixtures: 200 ppm NH3 /H2 ; 500 ppm NH3 /H2 ; and 1000 ppm NH3 /H2 . Also, transient tests were performed with 200 ppm NH3 /H2 and 1000 ppm NH3 /H2 and data show that poisoning and recovery rates with NH3 are much slower than with CO and that there may be two mechanisms occurring in series during recovery. These slow rates may be exploited to allow for treatments and control schemes that maintain PEMFC performance should the anode stream be exposed to high transient levels of NH3 . The significant findings include ~i! the ability of a MEA to recover completely in neat H2 after exposure to 200 ppm NH3 for 10 h ~i.e. ,5 .7 3 10 24 mol exposure!; ~ii! the performance is lower for 1000 than for 500 ppm at the same dosage ~i.e., 5.7 3 10 24

A Mathematical Model for Intercalation Electrode Behavior I. Effect of Particle‐Size Distribution on Discharge Capacity

A mathematical model is presented to study the effect of the particle size distribution (PSD) on the galvanostatic discharge behavior of the lithium/separator/intercalation electrode system. A recently developed packing theory has been incorporated into a first-principles model of an intercalation electrode to provide a rational basis for including the effect of PSD on packing density. The model is used to investigate how binary mixtures of spherical particles affect electrode capacity. The electrode capacity of an insertion electrode is calculated for various parameters including applied current density, thickness of the electrode, and volume fraction, size, and size ratio of the particles. The model shows that an electrode comprised of two different sized particles can have a significantly higher capacity than an electrode consisting of single-sized particles. However, increasing the tacking density increases the liquid-phase diffusion resistance. As a result of the trade-off between packing density and liquid-phase diffusion resistance, discharge capacity can be optimized by adjusting the particle size, volume fraction of large and small particles, and the size ratio. Pulse discharge of an intercalation electrode comprised of two different sized particles shows a marked difference in transient behavior from that of an electrode which has single-sized particles. Since there are many parameters which control the performance of the electrode, use of this model should aid greatly in making superior electrodes.

Measurement of MacMullin Numbers for PEMFC Gas-Diffusion Media

A technique for measuring the MacMullin number is described and the values are reported for different carbon-cloth and carbon-paper gas-diffusion media GDM. This number provides the missing data to test the applicability of the commonly used Bruggeman equation for these porous structures. Analysis of the MacMullin number and porosity data show that only carbon-cloth GDM follow the Bruggeman equation and that carbon-paper GDM has a different relationship with porosity. These differences are

The Effect of Temperature and Pressure on the Performance of a PEMFC Exposed to Transient CO Concentrations

Data are reported for Gores advanced PRIMEA® membrane electrode assembly (MEA) series 5561 exposed to relatively high concentrations (500, 3,000 and 10,000 ppm) of CO in hydrogen at 202 kPa and at 70 and 90°C. The steady-state and transient measurements obtained in this study at low reactant stoichiometry and 202 kPa are compared with earlier results 1 for atmospheric conditions to show the effect of temperature and pressure on the poisoning and recovery rates. All data are reported for a 25 cm 2 laboratory-scale proton exchange membrane fuel cell (PEMFC) using CARBEL GDM CL gas diffusion media (GDM) for conditions with and without air-bleed treatments. For 500 ppm CO/H 2 mixtures without air-bleed, the performance at 202 kPa and 0.6 V provides a steady-state current density of 1.0 A/cm 2 at 90°C but only 0.4 A/cm 2 at 70°C. At 101 kPa and 70°C, exposure to 500 ppm CO/H 2 mixtures requires 5% air-bleed to obtain this performance. Transient experiments with these CO levels indicate that there is up to a four time decrease in the poisoning rates at 202 kPa vs. 101 kPa. Further at 202 kPa, increasing the cell temperature from 70 to 90°C results in approximately a fourteen time decrease in the poisoning rate for 3.000 ppm CO/H 2 mixtures and approximately four time decrease for 10,000 ppm CO/H 2 mixtures. The data discussed in this paper are suitable for verifying numerical models of a PEMFC and establishing a baseline for new recovery schemes using new MEAs with enhanced CO tolerance. In addition, the results have implications for the design of reformate fuel-processing systems and the use of effective control schemes to prevent CO transients.

Performance of a Polymer Electrolyte Membrane Fuel Cell Exposed to Transient CO Concentrations

The response of Gores advanced PRIMEA® Series 5561 membrane electrode assembly (MEA) exposed to transient concentrations of CO in the anode feed was studied for a 25 cm 2 laboratory-scale polymer electrolyte membrane fuel cell (PEMFC). The data include relatively high (500 and 3000 ppm) CO levels at 70°C cell temperature, low reactant stoichiometry, and atmospheric pressure, conditions that may be typical for stationary PEMFC applications. Poisoning and recovery rates are reported far saturated conditions and these rates are compared for two types of gas diffusion media [single-sided ELAT® and CARBEL CL gas diffusion media (GDM)] and for conditions with and without air-bleed treatments. It is shown that a 5% air bleed provides a current density of 1.0 A/cm 2 at 0.6 V for CARBEL CL GDM exposed to 500 ppm CO/H 2 mixtures. The data show that the transient performance at 0.6 A/cm 2 with this MEA and relatively high concentrations of CO is a result of an interaction of CO kinetics and mass transfer through the GDM. Indirect evidence of electrochemical oxidation of CO during the transient pulses with 3000 ppm CO is presented. The data discussed in this paper are suitable for verifying numerical models of a PEMFC and establishing a baseline for new recovery schemes using new MEAs with enhanced CO tolerance. In addition, the results have implications for the design of reformate fuel processing systems and the use of effective control schemes to prevent CO transients.