Figure 2: Electrochemical characterization of graphene electrodes.

(a) EIS results for 50 × 50 μm² Au, G and doped-graphene samples. Measurement results are shown with symbols and fitting results are shown with solid lines for impedance magnitude (top figure) and phase (bottom figure) plots. The impedance magnitude (top figure) significantly decreased with doping of graphene, more prominently for frequencies lower than 1 kHz. (b) Cyclic voltammogram showing enhanced charge storage capacity for doped-graphene electrode. Electrode size is 50 × 50 μm². Legend shows calculated charge by integrating the area under the CV curve. (c) Schematic for the equivalent circuit model used to fit EIS measurement results. C_PE is the constant phase element representing the double-layer capacitance; R_CT is the charge transfer resistance; Z_W is the Warburg element representing the diffusion of charges species to the interface and R_S is the solution resistance. The models for circuit elements are provided in Supplementary Materials.