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(a) The capacitance values of the samples drastically decrease after the first 10 cycles, only the sample with un-doped graphene (mass ratio urea/GO = 0) seems to maintain high capacitance (ca. above 100 F/g); (b) Capacitance values normalized as F/m2 show that the nitrogen doping actually improves the cycle life when samples surface area is considered, reaching a maximum for the sample prepared with an urea/GO mass ratio of 0.25.
