Effect of carbon support nanostructure on the oxygen reduction activity of Pt/C catalysts

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 (similar to 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.