Nitrogen-doped carbon (N/C) and graphene (N/G) were synthesized by the established conventional heat-treatment method, and the incorporation of nitrogen into the carbon matrix was confirmed by CHN analysis, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electrochemical impedance spectroscopy (EIS) of the prepared catalysts in argon-saturated 0.1 M KOH was performed in a three-electrode rotating disk electrode (RDE) configuration. The capacitance derived from the low-frequency region of the EIS patterns was used to estimate the effective density of states [D(EF)] of carbon and its nitrogen-doped counterparts. Moreover, the carrier concentrations (ND) and flat band potentials of the samples were obtained by Mott-Schottky analysis. The metal-free catalyst samples were tested for possible oxygen reduction reaction (ORR) activity in oxygen-saturated 0.1 M KOH electrolyte, and the origin of the activity improvement with nitrogen doping of carbon/graphene can be explained on the basis of the effective density of states [D(EF)], carrier concentration (ND), and flat band potential. The results suggest that N/C-900 has the highest carrier concentration and maximum flat band potential and, therefore, the highest activity for the ORR. (Graph Presented).