Woo-kum Lee

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.

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.

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

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.

Effect of Reformate Components on PEMFC Performance Dilution and Reverse Water Gas Shift Reaction

bW.L. Gore & Associates, Incorporated, Elkton, Maryland 21922, USA Data are presented to quantify the effect of reformate components on the performance of proton exchange membrane fuel cells ~PEMFCs! with Pt and Pt/Ru alloy anodes. The performance deviates by 10-70 mV from Nernst behavior at current densities above 0.3 A/cm 2 when the cell is operated with N2 /H2 mixtures. The same deviations were observed with CO2 /H2 mixtures for a PEMFC with a Pt/Ru anode. However, for a PEMFC with a Pt anode, CO 2-diluted H2 gives significantly larger polarizations than N2-diluted H2 . Also, cyclic voltammetry indicates that, after the anode is exposed to the CO 2 /H2 mixture, some CO is produced through the reverse water gas shift ~RWGS! reaction. These data are consistent with equilibrium calculations showing that the CO concentration can reach between 10 to 170 ppm. Consistent with CO production from CO2 , the CO stripping areas ~which are a measure of CO coverage! also showed dependence on the inlet composition of the anode gas as well as the presence of O2 on the cathode side. The CO coverage resulting from the RWGS reaction approaches 5 3 10 27 mol/cm 2 for a 0.4 mg/cm 2

Degradation of Gasket Materials in a Simulated Fuel Cell Environment

A Polymer Electrolyte Membrane (PEM) fuel cell stack requires elastomeric gaskets in each cell to keep the reactant gases within their respective regions. If any gasket degrades or fails, the reactant gases (O2 and H2 ) can leak overboard or mix with each other directly during operation or during standby, and affect the overall operation and performance of the fuel cell. The degradation of four commercial gasket materials was investigated in a simulated fuel cell environment in this study. In an effort towards predicting lifetime of fuel cells, two solutions and two temperatures were used in the short-term, accelerated aging tests. Bend-strip environment crack resistance tests were performed on samples with various bend angles. Weight loss was monitored and surface structure changes were examined using optical microscopy on the samples exposed to the simulated fuel cell environment for selected periods of time. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy was employed to study surface chemistry of the gasket materials before and after exposure to the simulated fuel cell environment over time. Stress and strain analysis was conducted using finite element method (FEM) to quantify the stress/state in test samples. The test results reveal that two silicone materials were degraded significantly while the other two did not show much degradation up to 42 weeks exposure to the simulated fuel cell environment. Optical microscopy and ATR-FTIR spectroscopy analysis indicate that the surface chemistry altered gradually via mechanisms involving de-cross linking and chain scission in the backbone. From experimental and numerical results, it is concluded that there is an interaction between chemistry and stress that appears to accelerate the degradation of the gasket materials in fuel cell environment.Copyright