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An ultrathin (thickness 1.0 nm) 2D nitrogen-doped hierarchically porous carbon (2DNHPC) film is developed by the nanocasting method; for comparison, a 3D nitrogen-doped ordered mesoporous carbon (3DNOMC) is also synthesized. Characterizations reveal that 2DNHPC is featured by an extremely high aspect ratio (several hundred) and a bimodal pore distribution. Such a 2D hierarchically porous structure is found to facilitate both the mass transfer of the reactive species and the utilization of active site in the electrode. First, 2DNHPC yields a larger limiting current than does 3DNOMC for the oxygen reduction reaction (ORR), revealing the key role of the low dimensional structure to facilitate the mass transfer. Second, at the loading of 500 μg cm −2 , 2DNHPC shows the same kinetic current with 3DNOMC, indicating that the two catalysts have the same active site and turnover frequency. In comparison, at a lower loading of 250 μg cm −2 , the kinetic current of 2DNHPC remains unchanged, which however seriously deteriorates for 3DNOMC. This result strongly highlights the effect of the carbon dimension on the utilization efficiency of the active site. Finally, it is noted that 2DNHPC yields a comparable ORR electrocatalytic activity and long-term stability with commercial Pt catalyst in both alkaline and acid media. [ABSTRACT FROM AUTHOR]

Nitrogen-doped ordered mesoporous carbon (NOMC) is studied as the support to synthesize a Pt/C N-doped catalyst for methanol oxidation reaction (MOR). The effects of carbon dimension and metal loading are investigated by nitrogen ad/desorption isotherms, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Both TEM and XRD results show that platinum nanoparticles (Pt NPs) are highly dispersed on NOMC with a uniform and narrow distribution, while the dimension of NOMC has a considerable effect on the dispersion of Pt. For 10 wt% metal loading, the average diameter of Pt NPs on 2D-NOMC is 1.40 nm; smaller than that on 3D-NOMC (1.90 nm). XPS results reveal that a strong electronic interaction exists between Pt and NOMC, indicating the formation of Pt–N bonds at the interface. Such an interaction gets more pronounced in the case of low Pt loadings and low-dimensional supports; the reason is that the 2D support is more accessible to load Pt and the metal-support interface is better developed at low metal loadings. The electrochemical results are well correlated with the physiochemical characterizations. 1) The electrochemically active surface area of Pt is higher on the 2D-support than that on the 3D-one, confirming the better dispersion of Pt on NOMC-2D. 2) A positive shift in the potential is observed for the adsorption of oxygen-containing species onto Pt, which is indicative of the charge transfer from Pt to the support and the formation of Pt–N bonds. 3) Both onset and peak potentials are negatively shifted by ca . 50 mV for MOR on 10 wt% Pt/NOMC-2D, as compared with the commercial Pt/XC-72. The ratio of forward to backward current, a measure of poisoning tolerance, is 1.1 on Pt/NOMC-2D and 0.80 on Pt/XC-72R. This enhancement can be attributed to the bifunctional mechanism at the Pt/C N-doped interface. In MOR, CO-like intermediate species on Pt can be effectively stripped off by the adjacent active –OH species on carbon, generated at lower potentials. [ABSTRACT FROM AUTHOR]