Judith Valerio

A Comparative Study of Water Uptake By and Transport Through Ionomeric Fuel Cell Membranes

Water uptake and transport parameters measured at 30 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 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 one 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.

The water content dependence of electro-osmotic drag in proton-conducting polymer electrolytes

Abstract The water transport numbers for protons in a variety of available poly (perfluorosulfonic acid) membranes are presented as a function of water content. The data indicate that, for membranes equilibrated with water vapor over a wide range of activities, a water drag coefficient of unity is observed. Several implications of these results, both fundamental and for fuel cell applications, are discussed.

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.

PEMFC Reconfigured Anodes for Enhancing CO Tolerance with Air Bleed

Practical PEM fuel cells based on perfluorinated ionomer membranes (eg Nafion), most probably will use reformed fuel as primary source for the anode feed. The reformate, besides hydrogen, may contain trace amounts of carbon monoxide (CO. from a few to hundreds ppm), whose presence is detrimental to the cell performance. Energy conversion at fuel cells depends on highly dispersed carbon-supported Pt, where the hydrogen electro-oxidatisn takes place. However, CO strongly adsorbs on the Pt surface leading to a decreasing of the Pt active Surface area and consequently to losses in electrical current that are unacceptable for a practical device.

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.

Application of conducting polymer precoats for the metallization of insulators

Abstract We describe a process for the metallization of insulating materials based on the application of a conducting polymer precoat, followed by direct metal electrodeposition. The precoat is applied in a single step by immersing the substrate ( e.g. printed circuit board material, polyimide or nylon) in an aqueous bath containing the monomer ( e.g pyrrole), an oxidizing agent ( e.g. FeCl 3 ) and an appropriate dopant. The conducting polymer coats obtained are typically 0.2 to 0.5 μm thick and have sheet resistivities of several hundred Ω/square. This level of sheet resistivity is sufficient for direct metal electrodeposition onto the polymer-coated substrate using standard metal plating baths. The dependences of polypyrrole film conductivities on parameters such as substrate immersion time, the bath temperature, the oxidation potential in the bath, and the nature of the dopant have been investigated. We demonstrate that the electrodeposition of copper onto polypyrrole precoats propagates along the surface from the ohmic contact by a nucleation and dendritic growth mechanism.