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This work is focussed on establishing the effect of the nanostructure of a series of ordered mesoporous carbon (OMC) support materials, after Pt loading, on the oxygen reduction reaction (ORR) performance, for application in proton exchange membrane fuel cells (PEMFCs). Hexagonal mesoporous silica (HMS) templates were prepared using alkylamine surfactants with varying carbon chain lengths, producing wormhole pore diameters of 1.5-3.1 nm and silica wall thicknesses of ca. 2.3 nm. The HMS pores were then filled either with sucrose or an aromatic carbon precursor (anthracene or naphthalene), followed by carbonization and removal of the HMS in NaOH, leaving behind an interconnected carbon structure ("nano-strings"), 1.5-3.1 nm in diameter. These OMCs all have a similar, bimodal, pore size distribution, with the smaller pores (1.8 nm) attributed to removal of the HMS walls and the larger pores ($3.5 nm) arising from incomplete filling of the HMS pores with carbon precursor. The OMCs were loaded with 20 wt% Pt, resulting in very similar Pt particle sizes (ca. 5 nm), as confirmed by XRD, TEM, and electrochemical surface area measurements. The ORR activity was found to decrease as the carbon nano-string diameter decreased, proposed to be due to a higher electronic resistance, while the degree of OMC graphitization, determined by XRD analysis, had only a minor impact on the ORR activity.

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Over the past decade; a significant amount of research has been performed on novel carbon supports for use in proton exchange membrane fuel cells (PEMFCs). Specifically, carbon nanotubes, ordered mesoporous carbon, and colloid imprinted carbons have shown great promise for improving the activity and/or stability of Pt-based nanoparticle catalysts. In this work, a brief overview of these materials is given, followed by an in-depth discussion of our recent work highlighting the importance of carbon wall thickness when designing novel carbon supports for PEMFC applications. Four colloid imprinted carbons (CICs) were synthesized using a silica colloid imprinting method, with the resulting CICs having pores of 15 (CIC-15), 26 (CIC-26), 50 (CIC-50) and 80 (CIC-80) nm. These four CICs were loaded with 10 wt. % Pt and then evaluated as oxygen reduction (ORR) catalysts for use in proton exchange membrane fuel cells. To gain insight into the poorer performance of Pt/CIC-26 vs. the other three Pt/CICs, TEM tomography was performed, indicating that CIC-26 had much thinner walls (0-3 nm) than the other CICs and resulting in a higher OPEN ACCESSCatalysts 2015, 5 1047 resistance (leading to distributed potentials) through the catalyst layer during operation. This explanation for the poorer performance of Pt/CIC-26 was supported by theoretical calculations, suggesting that the internal wall thickness of these nanoporous CICs is critical to the future design of porous carbon supports.