An empirical equation [E = E 0-b log i-Rim 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 regwns of th~ E vs. z 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 prevwusly shown, the terms E 0 ~nd b yield the electrode kmetic parameters for oxygen reduction in the PEMFC and R represents the resistance, predommantly ohmic and, to a small extent, the chargetransfer resistance of the electro-oxidation of hydrogen. The exponential term charactenzesthe mass-transport regwn of theE vs. i plot. The parameter n has more pronounced effects than the parameter m m this regwn. A physicochemical interpretation of these parameters is needed.

No abstract

Ordered mesoporous carbon-carbon nanotube (OMC-CNT) nanocomposites were prepared and used as catalyst supports for polymer electrolyte fuel cells. The OMC-CNT composites were synthesized via a nanocasting method that used ordered mesoporous silica as a template and Ni-phthalocyanine as a carbon source. For comparison, sucrose and phthalocyanine were used to generate two other OMCs, OMC(Suc) and OMC(Pc), respectively. All three carbons exhibited hexagonally ordered mesostructures and uniform mesopores. Among the three carbons the OMC-CNT nanocomposites showed the highest electrical conductivity, which was due to the nature of their graphitic framework as well as their lower interfacial resistance. The three carbons were then used as fuel cell catalyst supports. It was found that highly dispersed Pt nanoparticles (ca. $1.5 nm in size) could be dispersed on the OMCs via a simple impregnation-reduction method. The activity and kinetics of the oxygen reduction reaction (ORR), measured by the rotating ring-disk electrode technique revealed that the ORR over the Pt/OMC catalysts followed a four-electron pathway. Among the three Pt/OMC catalysts, the Pt/OMC-CNT catalyst resulted in the highest ORR activity, and after an accelerated durability test the differences in the ORR activities of the three catalysts became more pronounced. In single cell tests, the Pt/OMC-CNTbased cathode showed a current density markedly greater than those of the other two cathodes after a high-voltage degradation test. These results were supported by the fact that the Pt/OMC-CNT-based cathode had the lowest resistance, which was probed by electrochemical impedance spectroscopy (EIS).The results of the single cell tests as well as those of the EIS-based measurements indicate that the rigidly interconnected structure of the OMC-CNT as well as their highly conductive frameworks are concomitantly responsible for the OMC-CNT nanocomposites exhibiting higher current density and durability than the other two carbons.

No abstract