Francisco A. Uribe

Characterization of polymer electrolytes for fuel cell applications

Abstract 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.

Effect of Ammonia as Potential Fuel Impurity on Proton Exchange Membrane Fuel Cell Performance

Effects of NH 3 on proton exchange membrane fuel cell performance are reported. Traces of NH 3 in the anode feedstream cause a decrease in cell current. The extent of the effect depends on NH 3 concentration and time of exposure of the anode to NH 3 . High trace levels and long time of exposure result in severe and irreversible disability. We discuss possible mechanisms by which NH 3 affects cell performance. A method for trapping ammonia from contaminated fuel streams using a H + form ion exchange resin is presented.

Characterization and long-term performance of polyaniline-based electrochemical capacitors

The performance of polyaniline-based electrochemical capacitors was evaluated under various experimental conditions. The capacitor consisted of two platinized tantalum foils coated with polyaniline as the active material, a separator, and an appropriate aqueous electrolyte solution. The polyaniline coatings were formed galvanostatically 5 mA cm -2 from a 0.1 M aniline +1.0 M HCl aqueous solution. With a polyaniline loading formed by a deposition charge of 20 C cm -2 on each electrode and with a 4.0 M HBF 4 aqueous solution as the electrolyte for an optimized capacitor, energy and power densities of 2.7 Wh kg -1 and 1.0 kW kg -1 (of active polymer) were achieved, respectively. Cyclic voltammograms for both positive and negative polyaniline electrodes of the capacitor before and after 20,000 cycles showed only a 5% loss of polyaniline electroactivity, which was smaller than the observed 33% decrease in the discharge capacity of the capacitor. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the capacitors and to understand their initial performance loss upon constant-current cycling.

A Microelectrode Study of Oxygen Reduction at the Platinum/Recast‐Nafion Film Interface

We describe a study of the oxygen reduction kinetics at the Pt/recast Nafion ® 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 Pr/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

A study of polymer electrolyte fuel cell performance at high voltages. Dependence on cathode catalyst layer composition and on voltage conditioning

We report polymer electrolyte fuel cell (PEFC) performances at high cell voltages (>0.7 V). We have tested various carbon supported PtM alloys (M: Cr, Ni, Rh, Sn) as cathode catalysts and compared performances with Pt/C. PtNi and PtCr provided the best results. Fuel cell performance at high constant voltages (>0.8 V) decreases rapidly with time. Within 1 h currents may drop to one half of their original value. Experiments strongly suggest that, at these voltages, there is an inherent Pt activity loss for oxygen reduction reaction (ORR) due to adsorption of oxygenated species from water. We also demonstrate that OH− species (from water oxidation), which block Pt active sites, adsorb at more positive voltages on PtM than on Pt surfaces. Thus, we conclude that the Pt surfaces of these alloys are more active for the ORR because of slower OH− species adsorption at high cell voltages. We have also found that short time cell voltage excursions from 0.8 to 0.5 V or lower at regular intervals, completely recovers the loss of performance. Consistent with our other results, pulsing to lower voltages reactivates the Pt surface by electrodesorption of OH− or other anions.

A novel N-substituted polyaniline derivative

Abstract We report the chemical synthesis of an N -substituted water-soluble polyaniline derivative. This novel processable conducting polymer has been characterized by a wide range of techniques including visible absorption, FT i.r. and 1 H n.m.r. spectroscopy, scanning electron microscopy, cyclic voltammetry in non-aqueous media, room temperature d.c. conductivity measurements, photon correlation spectroscopy and thermogravimetry.

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.

The Application of a Polypyrrole Precoat for the Metallization of Printed Circuit Boards

We describe a printed circuit (PC) board metallization process starting with the formation of a precoat of polypyrrole (PPY) on the board followed by the direct electrodeposition of copper onto the polypyrrole-coated substrate. The polypyrrole film is applied to the insulating substrate by a single chemical polymerization step from an aqueous solution. The sheet resistivity of the polypyrrole precoat is typically of the order of a few hundred ohms/square, but this turns out to be a sufficiently low resistance to enable direct metal electrodeposition onto the PPY-coated substrate.

Platinum submonolayer–monolayer electrocatalysts: an electrochemical and X-ray absorption spectroscopy study

A new Pt monolayer electrocatalyst concept is described and the results of electrochemical and X-ray absorption spectroscopy (XAS) studies are presented. Two new methods that facilitate the application of this concept in obtaining ultra-low-Pt-content electrocatalysts have been developed. One is the electroless (spontaneous) deposition of a Pt submonolayer on Ru nanoparticles, and the other is a deposition of a Pt monolayer on Pd nanoparticles by redox displacement of a Cu adlayer. The Pt submonolayer on Ru (PtRu20) electrocatalyst demonstrated higher CO tolerance than commercial catalysts under conditions of rotating disk experiments. The long-term stability test showed no loss in performance over 870 h using a fuel cell operating under real conditions, even though the Pt loading was approximately 10% of that of the standard Pt loading. In situ XAS indicated an increase in d-band vacancy of deposited Pt, which may facilitate partly the reduced susceptibility to CO poisoning for this catalyst. The kinetics of O2 reduction on a Pt monolayer on Pd nanoparticles showed a small enhancement in comparison with that from a Pt nanoparticle electrocatalyst. The increase in catalytic activity is partly attributed to decreased formation of PtOH, as shown by XAS experiments.

The Effect of Electrode Ink Processing and Composition on Catalyst Utilization

Platinum surface area was chemically and electrochemically characterized using X-ray diffraction (XRD), H2S adsorption, and electrochemical techniques including half-cells and fuel cells. Electrode ink processing and composition were found to have a strong impact on observed surface area with increased processing leading to a reduction in platinum surface area. Fuel cell electrode measurements show a significant portion of the platinum may not be available. This unutilized Pt may be due to either ionic or electronic connectivity as evidence exists to support both ionic and electronic isolation.