We describe a polymer electrolyte fuel cell model emphasizing operation on hydrocarbon reformate, i.e., the anode feed stream consists of dry H-2 concentrations as low as 40%, inlet CO levels of 10-100 ppm, and hydrogen fuel utilization as high as 90%. Refinements of interfacial kinetics equations used in our previous work on CO effects in H-2 anodes have yielded a better quantitative ft to the measured dependence of voltage loss on inlet CO level [in Electrode Materials and Processes for Energy Conversion and Storage, J. McBreen, S. Mukerjee, and S. Srinivasan, Editors, PV 97-13, pp. 15-24, The Electrochemical Society Proceedings Series, Pennington, NJ (1997)]. We calculate anode potential losses by coupling such interfacial kinetic processes to reactant diffusion limitations and ionic resistance in the catalyst layer, and by accounting for the drop in local hydrogen concentration along the flow channel due to significant fuel utilization. As a result of internal readjustment of cell overpotentials when hydrogen concentration drops along the flow channel, we show that loss of current, or power, under the realistic condition of constant cell voltage is smaller than loss of current at constant anode potential. We show that voltage losses associated with CO poisoning are significantly amplified with diluted hydrogen feed streams and particularly so under high fuel utilization. We make projections on improvements required, qualitative and quantitative, in the physical parameters of the anode catalyst surface chemistry to significantly improve "CO tolerance". (C) 2000 The Electrochemical Society. S0013-4651(00)03-119-0. All rights reserved.

This paper compares direct methanol fuel cells (DMFCs) employing two types of Nafion(R) (E.I. DuPont de Nemours and Company) membranes of different equivalent weight (EW). Methanol and water uptakes in 1100 and 1200 EW Nafion membranes were determined by weighing P2O5-dried and methanol solution-equilibrated membranes. Both methanol and water uptakes in the 1200 EW membrane were about 70-74% of those in the 1100 EW membrane. The methanol crossover rate corresponding to that in a DMFC at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions. After accounting for the thickness difference between the membrane samples, the methanol crossover rate through a 1200 EW membrane was 52% of that through an 1100 EW membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode was used as a reference electrode. Results show that in an operating DMFC the cathode can be easily flooded. as shown in a DMFC using 1100 EW membrane. An increase in methanol crossover rare decreases the DMFC cathode potential at open circuit. At a high cell current density, the DMFC cathode potential can approach that of a H-2/air cull. (C) 2000 The Electrochemical Society. S0013-4651(99)05-093-4. All rights reserved.

This paper describes methanol flux measurements across Nafion, 1100 equivalent weight membranes under conditions of a direct methanol fuel cell but in which methanol is completely electro-oxidized on the opposite side in an inert atmosphere at sufficiently high electrode potential. Both the diffusion coefficient and the methanol concentration in the membrane were determined from the measured transient limiting current density following a potential step. Corrections for electro-osmotic drag effects are developed and found necessary even for low MeOH concentrations. The results agree well with those obtained from nuclear magnetic resonance measurements. The partition coefficient [p = [MeOH](membrane)/[MeOH](solution)] was approximately constant for the membranes in contact with methanol solutions of various concentration and from room temperature to 90 degrees C. The activation energy of methanol diffusion in a fully hydrated Nafion membrane between 30 and 130 degrees C is 4.8 kcal mol(-1), and that for protonic conduction under the same conditions is 2.3 kcal mol(-1). For a membrane dried in vacuum at above 100 degrees C, lower values of methanol permeation rate and protonic conductance were found. (C) 2000 The Electrochemical Society. S0013-4651(99)03-015-3. All rights reserved.

AC impedance has provided a useful diagnostic tool in the Los Alamos polymer electrolyte fuel cell (PEFC) program. The techniques we have used in ac impedance modeling and the corresponding results are reviewed. These techniques include equation implementation, model simplification and verification, least squares fitting, application of two-dimensional Laplace equation solvers handling complex interfacial boundary conditions, and interpretation of impedance features.

Methanol and water absorption in 1100 and 1200 e.w. Nafion(R) membranes was determined by weighing P2O5 dried and methanol solution equilibrated membranes. Both methanol and water absorption in the 1200 e.w. membrane is about 70-74 % of that in the 1100 e.w. membrane. The methanol cross-over rate corresponding to that in a direct methanol fuel cell (DMFC) at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions (temperature, humidification and concentration of feed methanol solution). Accounting for the thickness difference between the membrane samples, the methanol cross-over rate through a 1200 e.w. membrane is 52 % of that through a 1100 e.w. membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode (DHE) was used as a reference electrode. Results show that in DMFC operation the cathode could be flooded due to the high water and methanol cross-over rates, especially through the 1100 e.w. membrane at a cell temperature below 80 degrees C. An increase in methanol cross-over rate as incurred by increasing the concentration of the feed methanol solution, increasing the cell operating temperature or using a membrane more permeable to methanol decreases the cathode potential of the DMFC at open circuit. As the cell current density is increased, the cathode potential of the DMFC can approach the cathode potential of a H-2/air cell, thanks to the consumption of methanol at the anode and consequent decrease in methanol cross-over rate.

In this work, several aspects related to the limiting polarization behavior of polymer electrolyte fuel cells were studied using ac impedance spectroscopy. The results were analyzed taking into account the different types of potential losses caused by the interfacial reaction kinetics, the conductance of the electrolyte in the catalyst layer, the oxygen diffusion in the gas phase, in the thin film and in the distributed agglomerate regions of the gas diffusion electrodes and the balance of water in the membrane. The main conclusion is that the water transport in the membrane plays an important role in establishing the limiting polarization behavior of polymer electrolyte fuel cells, corresponding to the only limiting factor when pure oxygen is employed as the cathodic reagent. When the oxygen source is air, the diffusion of oxygen in the gas phase becomes a limiting factor, but for pressurized systems the diffusion of water can also be observed, especially for thick membranes.

The ac impedance spectra of polymer electrolyte fuel cell (PEFC) cathodes measured under various experimental conditions are analyzed. The measurements were carried out in the presence of large de currents. The impedance spectrum of the air cathode is shown to contain two features: a higher frequency loop or are determined by interfacial charge-transfer resistance and catalyst layer properties and a lower frequency loop determined by gas-phase transport limitations in the backing. The lower frequency loop is absent from the spectrum of cathodes operating on pure oxygen. Properties of measured impedance spectra are analyzed by a PEFC model to probe the effect of ac perturbation. Comparison of model predictions to observed data is made by simultaneous least squares fitting of a set of spectra measured for several cathode potentials. The spectra reveal various charge and mass-transfer effects in the cathode catalyst layer and in the hydrophobic cathode backing. Three different types of losses caused by insufficient cell hydration, having to do with interfacial kinetics, catalyst layer proton conductivity, and membrane conductivity, are clearly resolved in these impedance spectra. The data reveal that the effective tortuous path length for gas diffusion in the cathode backing is about 2.6 times the backing thickness.

Water uptake and transport properties of Nation((R) a) 117 membranes at 30-degrees-C are reported here. Specifically, we have determined the amount of water taken up by membranes immersed in liquid water and by membranes exposed to water vapor of variable water activity. Transport parameters measured are the diffusion coefficient and relaxation time of water in the membrane and the protonic conductivity of the membrane as functions of membrane water content. The ratio of water molecules carried across the membrane per proton transported, the electro-osmotic drag coefficient, also was determined for a limited number of membrane water contents. The drag coefficient is contrasted with the experimentally determined net water transport across an operating PEM fuel cell.

This paper presents a fit between model and experiment for well-humidified polymer electrolyte fuel cells operated to maximum current density with a range of cathode gas compositions. The model considers, in detail, losses caused by: (i) interfacial kinetics at the Pt/ionomer interface, (ii) gas-transport and ionic-conductivity Limitations in the catalyst layer, and (iii) gas-transport limitations in the cathode backing. Our experimental data were collected with cells that utilized thin-film catalyst layers bonded directly to the membrane, and a separate catalyst-free hydrophobic backing layer. This structure allows a clearer resolution of the processes taking place in each of these distinguishable parts of the cathode. In our final comparison of model predictions with the experimental data, we stress the simultaneous fit of a family of complete polarization curves obtained for gas compositions ranging from 5 atm O-2 to a mixture of 5% O-2 in N-2, employing in each case the same model parameters for interfacial kinetics, catalyst-layer transport, and backing-layer transport. This approach allowed us to evaluate losses in the cathode backing and in the cathode catalyst layer, and thus identify the improvements required to enhance the performance of air cathodes in polymer electrolyte fuel cells. Finally, we show that effects of graded depletion in oxygen along the gas flow channel can be accurately modeled using a uniform effective oxygen concentration in the flow channel, equal to the average of inlet and exit concentrations. This approach has enabled simplified and accurate consideration of oxygen utilization effects.

Water uptake and transport parameters measured at 30-degrees-C for several available perfluorosulfonic acid membranes are compared. The water sorption characteristics, diffusion coefficient of water, electroosmotic drag, and protonic conductivity were determined for Nafion(R) 117, Membrane C, and Dow XUS 13204.10 developmental fuel cell membrane. The diffusion coefficient and conductivity of each of these membranes were determined as functions of membrane water content. Experimental determination of transport parameters, enables us to compare membranes without the skewing effects of extensive features such as membrane thickness which contributes in a nonlinear fashion to performance in polymer electrolyte fuel cells.

Diffusion barrier effects on diffusion and flow mapping by NMR microscopy were simulated for the general case of an interface between two compartments with different relaxation properties and diffusion coefficients. The extreme case of a completely impermeable barrier (i.e. a glass wall) shows substantial changes in signal intensity and phase at distances of a few micrometers from the surface. Permeable boundaries show varying degrees of intensity and phase changes that can be used for deducing permeability properties of barriers that would otherwise be below the true spatial resolution of the image. The simulations were done for the case of a solid, water-imbibing polymer immersed in water and for water diffusion between intra- and extracellular compartments across a biological membrane. These specific boundary effects have substantial implications for the determination of image resolution and for the separation and measurement of coherent flow and random diffusion.

We describe a study of the oxygen reduction kinetics at the Pt/recast Nafion(R) interface employing a minicell based on a filmed microelectrode. The experimental conditions are identical to those in polymer-electrolyte fuel cells in that the recast ionomer electrolyte is exposed only to water vapor. We find that the interfacial rate of oxygen reduction near 0.9 V is similar for the Pt/recast ionomer interface and for Pt immersed in dilute aqueous acid solutions. A significant loss of oxygen reduction activity occurs when the recast ionomer electrolyte loses water. We discuss a possible evaluation of catalyst utilization in polymer electrolyte fuel cells.

We review here our recent work on polymer electrolyte fuel cells emphasizing membrane transport issues. Transport parameters measured at 30°C for several available perfluorosulfonic acid membranes are compared. The diffusion coefficient and conductivity of each of these membranes were determined as functions of membrane water content. Data on water sorption and conductivity are reported for an experimental membrane which is a modified form of NAFION®. Contact angle measurements indicate that the surface of a perfluorosulfonic acid membrane exposed to water vapor is quite hydrophobic, even in the presence of saturated water vapor. Modeling of water distribution in PEFCs based on the uptake and transport data shows that membrane thickness contributes in a nonlinear fashion to performance in PEM fuel cells.

We have conducted a search for neutron emission from cold fusion systems of the electrochemical type and, to a lesser extent, the high‐pressure gas cell type. Using a high‐efficiency well counter and an NE 213 scintillator, the experiments were conducted on the earth’s surface and in a shielded cave approximately 50 ft underground. After approximately 6500 h of counting time, we have obtained no evidence for cold fusion processes leading to neutron production. However, we have observed all three types of neutron data that have been presented as evidence for cold fusion: large positive fluctuations in the neutron counting rate, weak peaks near 2.5 MeV in the neutron energy spectrum, and bursts of up to 140 neutrons in 500‐μs intervals. The data were obtained under circumstances that clearly show our results to be data encountered as a part of the naturally occurring neutron background, which is due primarily to cosmic rays. Thus, observing these types of data does not, of itself, provide evidence for the existence of cold fusion processes. Artifacts in the data that were due to counter misbehavior were also observed to lead to long‐term ‘‘neutron bursts’’ whose time duration varied from several hours to several days. We conclude that any experiments which attempt to observed neutron emission must include strong steps to ensure that the experiments deal adequately with both cosmic‐ray processes and counter misbehavior.

The dc and small‐signal ac impedances of a flooded‐agglomerate element of a porous gas‐diffusion electrode have been calculated. The complete solution involves a nonlinear concentration profile within the agglomerate and requires numerical integration. An approximate solution has been developed leading to a simple equivalent circuit that is valid for most practical situations. The complete agglomerate model was combined with a thin‐film diffusion element and the total impedance was calculated. Transfer functions were developed that also allow the simplified agglomerate representation to be combined with the thin‐film element, greatly facilitating numerical fitting of the model to measured impedance data.

Platinum gauze electrodes coated with the proton‐conducting polymer, Nafion, were investigated with respect to the kinetics of the oxygen reduction and hydrogen oxidation reactions. The coated electrodes were in contact with a Nafion membrane electrolyte. The current‐potential behavior of the coated electrodes depended upon the thickness of the coating. The electrodes with thicker coatings exhibited less activation and ohmic control but encountered diffusion limitations. Those with thinner coatings showed significant ohmic control. A theoretical analysis was made for the current‐potential relation and the current distribution using a model of a Nafion‐coated wire in contact with the membrane. The present analysis confirms the necessity of optimizing the amount of Nafion impregnation in low‐Pt‐loaded, porous gas‐diffusion electrodes, as was experimentally observed in another study in our laboratory.

We report impulse response measurements on InP:Fe photoconductors excited by laser and electron beam radiation. Measurements are reported on crystals with Fe concentrations from 2×1015 cm−3 to 4×1016 cm−3 and with excited electron-hole-pair densities of ∼1012 cm−3 and 9×1017 cm−3. Measured signal decays are purely exponential in character, and decay times are inversely related to Fe concentration. No long-lived tails are observed. Decay times show no dependence on excitation level for excited carrier concentrations that are well above and well below the Fe concentrations. The magnitude of the photoresponse indicates that electrons and not holes are the primary current carriers. The data suggest that for impulse excitation photoconductance decay in InP:Fe is due to trap-assisted recombination of electrons and holes at the Fe sites, with a rate determined by the species with the slower capture rate.