Arvind Parthasarathy

Temperature dependence of the electrode kinetics of oxygen reduction at the platinum/Nafion interface - A microelectrode investigation

Results of a study of the temperature dependence of the oxygen reduction kinetics at the Pt/Nafion interface are presented. This study was carried out in the temperature range of 30-80 C and at 5 atm of oxygen pressure. The results showed a linear increase of the Tafel slope with temperature in the low current density region, but the Tafel slope was found to be independent of temperature in the high current density region. The values of the activation energy for oxygen reduction at the platinum/Nafion interface are nearly the same as those obtained at the platinum/trifluoromethane sulfonic acid interface but less than values obtained at the Pt/H3PO4 and Pt/HClO4 interfaces. The diffusion coefficient of oxygen in Nafion increases with temperature while its solubility decreases with temperature. These temperatures also depend on the water content of the membrane.

Pressure Dependence of the Oxygen Reduction Reaction at the Platinum Microelectrode/Nafion Interface: Electrode Kinetics and Mass Transport

The investigation of oxygen reduction kinetics at the platinum/Nafion interface is of great importance in the advancement of proton-exchange-membrane (PEM) fuel-cell technology. This study focuses on the dependence of the oxygen reduction kinetics on oxygen pressure. Conventional Tafel analysis of the data shows that the reaction order with respect to oxygen is unity at both high and low current densities. Chronoamperometric measurements of the transport parameters for oxygen in Nafion show that oxygen dissolution follows Henrys isotherm. The diffusion coefficient of oxygen is invariant with pressure; however, the diffusion coefficient for oxygen is lower when air is used as the equilibrating gas as compared to when oxygen is used for equilibration. These results are of value in understanding the influence of O2 partial pressure on the performance of PEM fuel cells and also in elucidating the mechanism of oxygen reduction at the platinum/Nafion interface.

The Platinum Microelectrode/Nafion Interface: An Electrochemical Impedance Spectroscopic Analysis of Oxygen Reduction Kinetics and Nafion Characteristics

The kinetics of oxygen reduction at the platinum/proton‐exchange‐membrane interface is of direct importance in solid‐polymer‐electrolyte fuel‐cell research and development. Previous studies at a platinum microelectrode/Nafion® interface yielded electrode‐kinetic parameters for this reaction. The objectives of this study were to use electrochemical impedance spectroscopy (EIS) to study the oxygen‐reduction reaction under lower humidification conditions than previously studied. The EIS technique permits the discrimination of electrode kinetics of oxygen reduction, mass transport of in the membrane, and the electrical characteristics of the membrane. Electrode‐kinetic parameters for the oxygen‐reduction reaction, corrosion current densities for Pt, and double‐layer capacitances were calculated. The production of water due to electrochemical reduction of oxygen greatly influenced the EIS response and the electrode kinetics at the Pt/Nafion interface. From the finite‐length Warburg behavior, a measure of the diffusion coefficient of oxygen in Nafion and diffusion‐layer thickness was obtained. An analysis of the EIS data in the high‐frequency domain yielded membrane and interfacial characteristics such as ionic conductivity of the membrane, membrane grain‐boundary capacitance and resistance, and the uncompensated resistance.

Electrode kinetics of oxygen reduction at carbon-supported and unsupported platinum microcrystallite/Nafion® interfaces

The electrode kinetics of oxygen reduction at the platinum microcrystallite/Nafion® interface was investigated as a function of temperature and pressure. These studies were conducted using porous gas-diffusion electrodes, containing unsupported platinum (10 Mg cm−2) or carbon-supported platinum (0.4 mg cm−2), in proton exchange membrane fuel cells, using H2 and O2 as reactants. The effects of platinum loading and of Nafion impregnation on the electrode kinetic parameters were elucidated. Over the range of current densities from 1 to 1000 mA cm−2 (geometric), the Tafel slope was ca. −60 mV per decade and was practically independent of temperature, O2 pressure, and type of electrode. The platinum electrode was still in the potential regime where oxygenated species are present on the surface (Temkin conditions). The reaction order with respect to O2 is unity and the activation energy for O2-reduction was ca. 80 kJ mol−1. These electrode kinetic parameters are in good agreement with those obtained at the platinum microelectrode/Nafion interface, as previously determined in our laboratories.

Electronically conductive polymers as chemically-selective layers for membrane-based separations

Abstract We show that electronically conductive polymers are promising new materials for membrane-based separations, including gas separations and pervaporation. The approach we have taken is to use interfacial polymerization to synthesize thin films of the desired electronically conductive polymer (e.g. polypyrrole, poly(N-methylpyrrole), polyaniline) onto the surfaces of microporous support membranes. These interfacial polymerizations yield thin film composite membranes in which the microporous support provides the requisite mechanical strength and the conductive polymer provides the chemical selectivity. Results of gas-transport and pervaporation experiments on such conductive polymer-based thin film composite membranes will be described.

Interfacial polymerization of thin polymer films onto the surface of a microporous hollow-fiber membrane

Abstract Two methods for synthesizing thin films of chemically selective polymers onto the surface of microporous hollow-fiber membranes are described. The first involves interfacial redox polymerization and yields thin films of electronically conductive polymers (e.g., polypyrrole,poly (N-methylpyrrole), poly-aniline) on the outside surface of the hollow-fiber membrane. The second method entails interfacial photopolymerization of styrenic monomers on the surface of the hollow-fiber membrane. Such coated hollow-fiber membranes are potentially useful in membrane-based separation technologies such as pervaporation and industrial gas separations. We also investigated the transport properties of the coated hollow-fiber membranes described here using a simple pervaporation-type experiment.