Ordered mesoporous carbon-carbon nanotube nanocomposites as highly conductive and durable cathode catalyst supports for polymer electrolyte fuel cells

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. similar to 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.