Supramaniam Srinivasan

The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, P tMn/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 pr imary phase of the type P t3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution f rom this phase) being of the type P tM (LIo structure with tetragonal lattice) as evidenced f rom x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95 ~ and 5 a tm 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 P tCr /C alloy exhibited the best performance. I n s i tu 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 a tom (with the exception of P tMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances wh ich con-f i rmed the results f rom ex s i tu 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 f rom the electrolyte following a Temkin isotherm behavior, wh ich does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond

A knowledge of the temperature dependence of the electrode‐kinetic parameters for oxygen reduction at the platinum/ proton exchange membrane (PEM) interface and of mass‐transport parameters of oxygen in the PEM is of vital importance in analyzing the performance of proton exchange‐membrane fuel cells. The microelectrode technique which was previously developed to determine these parameters at the platinum/Nafion® interface at 25°C was used in the present investigation at the same electrode/electrolyte interface. 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 (TFMSA) interface but less than values obtained at the and interfaces. The diffusion coefficient of oxygen in Nafion increases with temperature while its solubility decreases with temperature. These parameters also depend on the water content of the membrane. The conductivity of the membrane increases with temperature until it reaches a plateau at a temperature of 80°C; the dependence of the conductivity on temperature was correlated with the variation of water content of Nafion with temperature.

An empirical equation [E = E0- b log i- R i- m exp (ni)] 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 E0 and b yield the lectrode kinetic parameters for oxygen reduction in the PEMFC and R represents he 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 f these parameters i needed.