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.
Microporous carbons, with nitrogen groups or without, were synthesized and tested as ORR catalysts. A strong dependence of the ORR efficiency on the combined V<0.7nm, ECSA, and the number of dissociating groups (expressed as PIF) was found.
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