Water transport is an important consideration in the optimization of polymer electrolyte fuel cell (PEFC) performance, affecting both internal resistance and cathode polarization losses. Novel experiments are described using neutron radiography to measure water gradient profiles within Nafion® in an operating PEFC. Preliminary neutron intensity gradients show qualitative agreement with the expected response of membrane water content to changes in feed gas humidification and fuel cell current. Previous experimental measurements of similar water gradients have generally relied on “integral” measurements such as ac impedance spectroscopy which cannot probe details of the gradient within the membrane. This is one of the first “differential” measurements of water gradients within Nafion. © 1999 The Electrochemical Society. All rights reserved.

Polymer electrolyte fuel cells operate at high efficiencies using pure H2 fuel. H2 produced by reforming hydrocarbons or alcohols contains CO and CO2 (CO x ) impurities which readily adsorb onto anode Pt electrocatalysts, reducing the efficiency for H2 electrooxidation. A CO x inventory model is useful for describing the behavior of adsorbed CO on Pt electrocatalyst surfaces. The model compares three fluxes:  (1) direct adsorption of CO, (2) the electroreduction of CO2, and (3) electrooxidation of adsorbed CO. The fluxes of CO2 electroreduction and CO electrooxidation were measured electrochemically on polycrystalline Pt electrodes (surface roughness = 30−100) in 1 N H2SO4 between 23 and 66 °C. This study concludes that, under polymer electrolyte fuel cell conditions, CO tolerance is achieved when the flux of CO electrooxidation balances the combined adsorption fluxes from both CO and CO2. Furthermore, under most conditions, CO adsorption will generally exceed the fluxes of CO2 electroreduction.

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ExperimentalThe membrane electrode assembly (MEA) was prepared by General Motor's Global Alternative Propulsion Center using a 40 wt % Pt/Vulcan XC-72 at a loading of 400 µg Pt/cm 2 on both sides of a Nafion 112 membrane. The MEA was mounted in a Globetech test fixture with a 5 cm 2 active area.Performance tests were run using a commercial Globetech GT-120 test station which controls gas flow rates, pressures, and humidifier temperatures. The cell was run at 80˚C and ambient pressure at both electrodes. Humidifiers were approximately 60-70% filled with liquid. Initial anode humidification used Globetech's standard 1000 mL heated stainless steel humidifier bottle. In subsequent testing, this bottle was replaced by a heated polypropylene (PP) humidifier chamber. The PP humidifier was made from a Nalgene 250 mL narrow-mouthed square bottle using Jayco 0.25 in. PP fittings. The temperature of the PP humidifier was controlled by partially immersing it in a water bath heated on a hot plate. The cathode humidifier was operated at 82˚C and the anode humidifier was controlled between 25˚ and 45˚C in various experiments. House H 2 was used for anode feed after passing through a butyllithium getter (Supelco, OMI 2 indicator tube). Cylinders of impure H 2 were research grade, prepared by MG Products. House air was used for the cathode feed.Performance curves for the fuel cell were obtained using a power supply and an HP 6050A system dc electronic load. The test protocol held the anode and cathode flow rates constant during performance measurements. The cell was held at constant potential for 60 s prior to recording each current. Transients were allowed to stabilize for at least 30 min, after switching from pure to impure H 2 or changing the H 2 O 2 concentration in the anode humidifier. H 2 O 2 (J.T. Baker, 30 wt %, reagent grade) was diluted with deionized H 2 O between 0.5 and 15 wt %.Gas analysis was performed on a Wasson modified HP 5890 gas chromatograph (GC). Anode feed samples were collected in a double-valved stainless steel sample bomb. Anode gases were allowed to flow through the humidifier for at least 90 min prior to sampling. GC analysis for N 2 confirmed that the sampling procedure had not been contaminated with traces of air.Results and Discussion Performance curves for pure and impure H 2 , with and without H 2 O 2 , are plotted in Fig. 1. Without H 2 O 2 , the performance on impure H 2 containing 96 ppm CO was severely degraded. However, with 0.75 vol % H 2 O 2 in the stainless steel humidifier, performance was restored, approaching that for pure H 2 . This performance mitigation is comparable to that reported previously by We observed small gas bubbles in the sight glass of the stainless steel anode humidifier. This suggested that H 2 O 2 might be decomposing via reaction 3 to form O 2 in the humidifier, with the reaction being catalyzed by the metallic walls in the humidifier. (Schmidt et CO poisoning in proton exchange membrane fuel cells (PEMFCs) can be mitigated by using dilute H 2 O 2 in the anode hum...