Supramaniam Srinivasan

Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction An In Situ XANES and EXAFS Investigation

The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the primary phase of the type Pt3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution from this phase) being of the type PtM (LIo structure with tetragonal lattice) as evidenced from x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95~ and 5 atm pressure showed a two- to threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em -2 relative to Pt/C eleetrocatalyst. The PtCr/C alloy exhibited the best performance. In situ EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per atom (with the exception of PtMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances which confirmed the results from ex situ XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species from the electrolyte following a Temkin isotherm behavior, which does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond distance) with the electrochemical performance characteristics exhibits a volcano type behavior with the PtCr/C alloy being at the top of the curve. The enhanced electrocatalysis by the alloys therefore can be rationalized on the basis of the interplay between the electronic and geometric factors on one hand and their effect on the chemisorption behavior of OH species from the electrolyte. The role of Pt/C and Pt alloys on the mechanism of the oxygen reduction reaction (ORR) has been investigated previously, 1-4 however the mechanism still remains elusive. One of the first investigations I of the ORR on Pt alloy electrocatalysts was in phosphoric acid; the effect of changes in the Pt-Pt interatomic distances, caused by alloying, was examined. The strength of the [M-HO2]aas bond, the intermediate formed in the rate-determining step of the molecular dioxygen reduction, was shown to depend on the Pt-Pt bond distance in the alloys. A plot of the electrocatalytic activity vs. adsorbate bond strength exhibited a volcano type behavior. 5 It was shown that the lattice contractions due to alloying resulted in a more favorable Pt-Pt distance (while maintaining the favorable Pt electronic properties) for dissociative adsorption of 02. This view was disputed by Glass et al. ~ in their investigation on bulk alloys of PtCr (the binary alloy at the top of the volcano plot) of different compositions. The latter investigation showed no activity enhancement for the ORR in phosphoric acid. This study therefore suggested the possibility of differences in electrochemical properties of bulk vs. supported alloy electrocatalysts (small particles of 35-85 A). A recent study on supported PtCo electrocatalysts ~ revealed the possibility that particle termination, primarily at the vicinal planes in the supported alloy electrocatalyst, is the reason for the enhanced ORR electrocatalysis (i.e., vicinal planes are more active than ). Paffett et al., 3 attributed higher activities for the ORR on bulk PtCr alloys in phosphoric acid to surface roughening, and hence increased Pt surface area, caused by the dissolution of the more oxidizable alloying component Cr. In contrast to these findings on bulk alloys, the supported alloy electrocatalysts have been reported to retain their nonnoble alloying element in the electrode during long periods (6000-9000 h) of operation in phosphoric acid fuel cells (PAFCs) 6 and proton exchange membrane fuel ceils (PEMFCs). 7 Based on these previous investigations and in the context of the ORR mechanisms, the principle explanations for the

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.

Enhanced electrocatalysis of oxygen reduction on platinum alloys in proton exchange membrane fuel cells

Enhanced electrocatalysis of the oxygen reduction reaction (ORR) on carbon-supported binary and ternary alloys of Pt in phosphoric acid fuel cells has been reported previously. This investigation focuses on the electrocatalysis of the ORR on some binary alloys of Pt (Pt+Ni, Pt+Cr and Pt+Co) at interfaces with proton exchange membranes (Dow perfluorinated sulfonic acids). Comparison of the results of these studies with those on carbon-supported Pt electrocatalysts (electrodes containing same Pt loading of 0.3 mg/cm2) revealed enhanced activities, lower activation energies and different reaction orders for all the alloys. X-ray powder diffraction showed lattice contractions for the alloys, the predominant phase being Pt3M (LI2) f.c.c. crystalline. X-ray photoelectron spectroscopy studies on the constituent elements of the electrocatalyst showed no chemical energy shifts owing to alloying and/or the presence of oxides on the surface. Lifetime evaluations of proton exchange membrane fuel cells, using both electrochemical as well as scanning electron microscopy/energy-dispersive X-ray analysis techniques, revealed only small amounts of dissolution of the more oxidizable component during the testing periods, which ranged from 400 to 1200 h. Therefore, the enhanced electrocatalysis exhibited by the binary Pt alloys appears to originate primarily as a result of changes in the lattice structure owing to alloying and the unique environment of the supported catalyst in the particle size range 35–75 A.

Modeling of Proton Exchange Membrane Fuel Cell Performance with an Empirical Equation

An empirical equation [E = E{sub 0} {minus} b log i {minus} Ri {minus} m exp (in)] was shown to fit the experimental cell potential (E) vs. current density (i) data for proton exchange membrane fuel cells (PEMFCs), at several temperatures, pressures, and oxygen compositions in the cathode gas mixture. The exponential term compensates for the mass-transport regions of the E vs. i plot; i.e., the increase in slope of the pseudolinear region and the subsequent rapid fall-off of the cell potential with increasing current density. As has been previously shown, the terms E{sub 0} and b yield the electrode kinetic parameters for oxygen reduction in the PEMFC and R represents the resistance, predominantly ohmic and, to a small extent, the charge-transfer resistance of the electro-oxidation of hydrogen. The exponential term characterizes the mass-transport region of the E vs. i plot. The parameter n has more pronounced effects than the parameter m in this region. A physicochemical interpretation of these parameters is needed. The PEMFC is the most promising candidate fuel cell power source for a zero emission vehicle, because of its desirable characteristics, such as quick start capability, low operating temperature, high energy efficiency, and high power density.

Operating Proton Exchange Membrane Fuel Cells Without External Humidification of the Reactant Gases Fundamental Aspects

Operation of proton exchange membrane fuel cells (PEMFC) without external humidification of the reactant gases is advantageous for the PEMFC system, because it eliminates the need of a gas-humidification subsystem. The gas-humidification subsystem is a burden in the fuel cell system with respect to weight, complexity, cost, and parasitic power. A model for the operation of PEMFC with internal humidification of the gases is presented and the range of operating conditions for a PEMFC using dry H 2 /air was investigated. The model predicts that dry air, entering at the cathode, can be fully internally humidified by the water produced by the electrochemical reaction at temperatures up to 70°C. This model was experimentally verified for cell temperatures up to 60°C by long-term operation of a PEMFC with dry gases for up to 1800 h. The current densities, obtained at 0.6 V, were 20 to 40% lower than those measured when both gases were humidified. The water distribution in the cell, while operating with dry gases, was investigated by measuring the amount of product water on the anode and cathode sides. It was found that the back-diffusion of product water to the anode is the dominant process for water management in the cell over a wide range of operating conditions. The dominating water back-diffusion also allows internal humidification of the hydrogen reactant and prevents drying out of the anode.

High performance proton exchange membrane fuel cells with sputter-deposited Pt layer electrodes

An ultra-low platinum (Pt) loading (0.1 mg cm2) electrocatalyst layer (1 μm thick) was deposited on an uncatalyzed gas diffusion electrode utilizing the sputter-deposition technique. The performance of the proton exchange membrane fuel cell (PEMFC) with this electrode for the cathode (the anode was a conventional low Pt loading E-TEK electrode, 20% PtC, 0.4 mg cm2) showed a good oxygen electrode performance, even at high current densities. The performance of the oxygen electrode was comparable to that of a standard E-TEK electrode, but was somewhat lower than the standard E-TEK electrode with a sputtered Pt layer. However, the potential of the PEMFC with this cathode showed mass transport limitations because of the unexpectedly high anode overpotentials at higher current densities.

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.

Analysis of proton exchange membrane fuel cell performance with alternate membranes

Abstract Renewed interest in proton exchange membrane fuel cell technology for space and terrestrial (particularly electric vehicles) was stimulated by the demonstration, in the mid 1980s, of high energy efficiencies and high power densities. One of the most vital components of the PEMFC is the proton conducting membrane. In this paper, an analysis is made of the performances of PEMFCs with Duponts Nafion®, Dows experimental, and Asahi Chemicals Aciplex-S® membranes. Attempts were also made to draw correlations between the PEMFC performances with the three types of membranes and their physico-chemical characteristics. Practically identical levels of performances (energy efficiency, power density, and lifetime) were achieved in PEMFCs with the Dow and the Aciplex-S® membranes and these performances were better than in the PEMFCs with the Nafion®-115 membrane. The electrode kinetic parameters for oxygen reduction are better for the PEMFCs with the Aciplex-S® and Nafion® membranes than with the Dow membranes. The PEMFCs with the Aciplex-S® and Dow membranes have nearly the same internal resistances which are considerably lower than for the PEMFC with the Nafion® membrane. The desired membrane characteristics to obtain high levels of performance are low equivalent weight and high water content.

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.

Alloys for hydrogen storage in nickel/hydrogen and nickel/metal hydride batteries

Since 1990, there has been an ongoing collaboration among the authors in the three laboratories (i) to prepare alloys of the AB5 and AB2 types, using arc-melting/annealing and mechanical alloying/annealing techniques; (ii) to examine their physiochemical characteristics (morphology, composition; (iii) to determine the hydrogen absorption/desorption behavior (pressure-composition isotherm as a function of temperature), and (iv) to evaluate their performance characteristics as hydride electrodes (charge/discharge, capacity retention, cycle life, high-rate capability). This review article presents the work carried out on representative AB5 and AB2 type modified alloys (by partial substitution of with small additives of other elements). The purpose of the modification was to optimize the thermodynamics and kinetics of the hydriding/dehydriding reactions and to enhance the stabilities of the alloys for the desired battery applications. The results of our collaboration, to date, demonstrate that: (i) alloys prepared by arc-melting/annealing and mechanical alloying/ annealing techniques exhibit similar morphology, composition and hydriding/dehydriding characteristics; (ii) alloys with the appropriate small amounts of substituent or additive elements — retain the single phase structure, improve the hydriding/dehydriding reactions for the battery applications, and enhance the stability in the battery environment — and (iii) the AB2 type alloys exhibit higher energy densities than the AB5 type alloy but the state-of-the-art, commercialized batteries are predominantly manufactured using AB5 type alloys.