Mahlon S. Wilson

Characterization of Polymer Electrolyte Fuel Cells Using AC Impedance Spectroscopy

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 dc 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.

High Performance Direct Methanol Polymer Electrolyte Fuel Cells

Direct methanol fuel cells (DMFCs) using Pt-Ru electrocatalysts and perfluorosulfonic acid membranes provide high performances if operated above 100 C with optimized catalyst layers. A decal transfer method is used to apply thin-film catalyst/ionomer composite layers to Nafion{reg_sign} membranes. A Nafion 112 membrane/electrode assembly operating on 5 atm oxygen at 130 C yields a current of 670 mA/cm{sup 2} at 0.5 V cell voltage. Peak power density is 400 mW/cm{sup 2}. The same cell operating on 3 atm air at 110 C yields 370 mA/cm{sup 2} at 0.5 V and provides a maximum power density of 250 mW/cm{sup 2}.

Low platinum loading electrodes for polymer electrolyte fuel cells fabricated using thermoplastic ionomers

Low platinum loading catalyst layers for polymer electrolyte fuel cells (PEFCs) consist of a thin film of highly inter-mixed ionomer and catalyst that is applied to the electrolyte membrane. High performances are achieved with loadings as low as 0.12 mg Pt cm−2 at the cathode and even lower loadings are required at the anode. However, the long-term performance of these fuel cells depends upon the structural integrity of the recast, ionomer-bound catalyst layers. The discovery that the inclusion of large cations through a simple ion-exchange process renders perfluorosulfonate ionomers moderately melt-processable is exploited to significantly improve the structural integrity of the catalyst layers. When the thermoplastic form of the solubilized ionomer is used in the membrane catalyzation process, the reproducibility is greatly improved and the long-term performance losses are quite low. Overall, the fuel cells demonstrate less than 10% loss in maximum power over almost 4000 h. An indication of the durability of the catalyst layer and the integrity of the catalyst layer/membrane interface is provided by the high tolerance of such fuel cells to shut-down/start-up and freeze-thaw cycles. Various other aspects of endurance testing and overall operation of such PEFCs are also discussed.

High Performance Catalyzed Membranes of Ultra‐low Pt Loadings for Polymer Electrolyte Fuel Cells

This paper reports on polymer electrolyte membranes that are catalyzed by the direct application of thin film catalyst layers cast from solutions of suspended Pt/C catalyst and solubilized Nafion ionomer. Both the ionomeric membrane and the solubilized ionmer are in the Na{sup +} form during casting to enable higher curing temperatures, which results in more robust catalyst layers. In addition to simplifying the fabrication process, the direct application apparently provides enhanced bonding at the interface between the membrane and the catalyst layer. Consequently, the performances of fuel cells utilizing these catalyzed membranes with ultra-low platinum loading are superior to those achieved with other approaches to polymer electrolyte membranes of low Pt loading.

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Life tests of polymer-electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4,000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects in majority of the platinum to conditions that sustain particle growth. X-ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m[sup 2]/g Pt eventually stabilize to about 40 to 50 m[sup 2]/g in the cathode and 60 to 70 m[sup 2]/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributed to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.

A printed circuit board approach to measuring current distribution in a fuel cell

A new method of measuring current distribution in a polymer electrolyte fuel cell of active area 100cm2 has been demonstrated, using a printed circuit board (PCB) technology to segment the current collector and flow field. The PCB technique was demonstrated to be an effective approach to fabricating a segmented electrode and provide a useful tool for analysing cell performance at different reactant gas flow rates and humidification strategies. In this initial chapter of work with the segmented cell, we describe measured effects on current distribution of cathode and anode gas stream humidification levels in a hydrogen/air cell, utilizing a NafionTM 117 membrane and single serpentine channel flow fields, and operating at relatively high gas flow rates. Effects of the stoichiometric flow of air are also shown. A clear trend is seen, apparently typical for a thick ionomeric membrane, of lowering in membrane resistance down the flow channel, bringing about the highest local current density near the air outlet. This trend is reversed at low stoichiometric flows of air. At an air flow rate less than three times stoichiometry, the local performance starts to drop significantly from inlet to outlet, as local oxygen concentration drop overshadows the lowering in resistance along the direction of flow.

PEM fuel cells for transportation and stationary power generation applications

We describe recent activities at Los Alamos National Laboratory devoted to polymer electrolyte fuel cells in the contexts of stationary power generation and transportation applications. A low cost/high performance hydrogen or reformate/air stack technology is being developed based on ultra-low platinum loadings and non-machined, inexpensive elements for flow-fields and bipolar plates. On-board methanol reforming is compared to the option of direct methanol fuel cells in light of recent significant power density increases demonstrated in the latter.

Further refinements in the segmented cell approach to diagnosing performance in polymer electrolyte fuel cells

Described is the most recent configuration of a segmented cell used to measure current distribution across the surface of an electrode in a polymer electrolyte fuel cell (PEFC). In this fourth generation cell design, measurement and data collection capabilities have been modified to significantly improve ease of use and quality of information obtained. The current configuration allows examination of spatial resolution of the cell current and cell voltage with respect to well-defined baseline reference measurements, as well as measurement of the high frequency resistance (HFR) distribution and spatial ac impedance spectroscopy. This specially designed cell is intended for use in studies on time and location resolved carbon monoxide poisoning, humidification and flow-field design effects on fuel cell performance.

Direct observation of surface oxide formation and reduction on platinum clusters by time-resolved X-ray absorption spectroscopy

Abstract Dispersive X-ray absorption fine structure (XAFS) spectroscopy was used to monitor changes in the Pt charge and the number of O and Pt nearest neighbors during the electrochemical oxidation and reduction of a dispersed Pt catalyst in real time. The oxidation at 1.20 V/SHE follows logarithmic kinetics over a period of 5 min with all three XAFS features ( N O , N Pt and the absorption peak) changing identically. However, the reduction at 0.10 V is well fitted by a single exponential on a similar time-scale with N Pt changing at half the rate at which N O and absorption peak are changing. When combined with the direct quantitative structural information obtained from XAFS, we find that these reactions on clusters apparently proceed by a different mechanism from that on bulk platinum electrodes in aqueous solution and also suggest a mechanism for the platinum restructuring during these reactions. Therefore the behavior of the clusters may differ from that of bulk electrode surfaces.

In situ structural characterization of a platinum electrocatalyst by dispersive X-ray absorption spectroscopy

Using in-situ dispersive EXAFS spectroscopy, we have monitored the numbers of PtO and PtPt bonds and the charge on the Pt in parallel with cyclic voltammetry on Pt clusters in a polymer electrolyte fuel cell. Due to the increased sensitivity of this method, we detect small structural changes not previously reported for these clusters and can correlate these changes with specific reactions, ie H desorption or O adsorption in the double-layer region.