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A highly efficient microwave irradiation method was employed for doping oxygen into the g-C3N4 framework using H2O2, which entailed 10 min compared to energy- and time-intensive calcination and hydrothermal methods. HR-XPS revealed that O doping preferentially occurred through replacement of two coordinated N atoms of the triazine units. The effects of the ensuing electronic structure modulation were investigated with electrochemical tests where the metal-free catalyst displayed a specific capacitance of 262.5 F/g at a 1 A/g current density and a satisfactory capacitance retention of 73.17% at 6 A/g after 2000 cycles, which is comparable to those of various metals containing g-C3N4-based catalysts. The designed asymmetric supercapacitor device revealed a specific energy of 36.45 Wh/kg at a specific power of 2.5 kW/kg. The microwave-irradiated O-doped g-C3N4 showed an 8-fold increase in specific capacitance compared with bulk g-C3N4 in both CV and GCD analyses. The improved surface structure and O content enhanced the electrode–electrolyte contact area, thus making ion transfer highly efficient for EDLCs. These results also opened the potential of microwave-synthesized O-doped g-C3N4 as a base material for designing binary and ternary electrode materials.

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