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capability of these graphene films in the more convenient Li2SO4. This is confirmed by the CVs in Figure 9b, which show ideal capacitive behavior up to a scan rate of 2 V s-1, with an electrode areal density of 36 mF cm-2 (measured at 0.8 V). Further evaluation of the supercapacitor performance by means of galvanostatic charge/discharge cycling indicates a capacitance retention of 60 % for a 1000-fold increase in discharge current (from 0.2 to 200 mA cm-2), as depicted in Figure 9c. In spite of the moderate capacitance of this material (e.g. 44 mF cm-2 at 0.2 mA cm-2), the large cell voltage of 1.6 V (see Figure S5a) endows the supercapacitor with a maximum areal energy density of ca. 15 μWh cm-2 (0.25 mW cm-2) and still delivers ca. 5 μWh cm-2 at a high power density of ~ 287 mW cm-2 (see Figure S5b). Stability cycling tests show a decrease in capacitance of 8 % during the first 5000 cycles at 5 A g-1, but only 3 % after another 5000 cycles (see Figure 9d), confirming the robustness of these graphene films. 3. Conclusions In summary, we report an electrochemically assisted process for producing stable dispersions of graphene in water in three simple steps: (a) the electrochemical exfoliation of graphite, (b) sonication to separate the oxidized graphene layers and (c) the reduction of the oxidized graphene to graphene. The dispersed graphene sheets have a C/O atomic ratio of 11.7 and are made up of only a few graphene layers, though bilayer sheets are the most abundant. Around 30 wt.% of the initial graphite is converted to graphene, which forms stable aqueous dispersions in concentrations of around 2-3 mg/mL. The processability of the aqueous dispersions of graphene allows them to be used directly for fabricating a variety of macroscopic graphene structures. For 15PDF Image | Graphene from Electrochemically Exfoliated Graphite
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