Insight into the Mechanism of Oxygen Reduction Reaction on Micro/Mesoporous Carbons: Ultramicropores versus Nitrogen-Containing Catalytic Centers in Ordered Pore Structure

Ordered mesoporous/microporous carbon was synthesized from sucrose. The carbon was oxidized and modified with urea at 600, 800, and 950 °C. The obtained carbons differed in the pore size distributions in the meso- and micropore ranges. The carbons, after an extensive surface characterization, were used as ORR catalysts in a KOH electrolyte. Kinetic current densities and n reached 12.5 mA cm–2 and 3.94, respectively. The results suggested that small micropores increased the efficiency of ORR. The density of surface oxygen in mesopores had also a positive effect on ORR by increasing the hydrophilicity and the electrochemically active surface area (ECSA) and, thus, the accessibility of oxygen dissolved in the electrolyte to the pores of the highest adsorption potential. From the viewpoint of the number of electron transfers, current density, and onset potential, the carbons with small pore sizes and with only traces of nitrogen performed equally, or even better, in ORR process as those containing the catalytic nitrogen sites.