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D.F. Carrasco, J.I. Paredes, S. Villar-Rodil et al. Carbon 195 (2022) 57e68 decorated internal edges, MRGO-H was expected to be electro- catalytically more active overall towards water splitting than EOG- H. Such a higher activity implied lower overpotentials for the hydrogen and oxygen evolution reactions and, consequently, a narrower operating voltage window when MRGO-H was used as an electrode for charge storage, as it was indeed observed. Fig. 5f shows the capacitance values for the EOG-H (blue sym- bols) and EOG electrodes (red features) calculated from their gal- vanostatic charge-discharge profiles recorded at different current densities between 1 and 20 A g1 (shown in Fig. S6 in the ESM). As could be anticipated from the results presented above, EOG-H featured substantially larger capacitance values and a better rate capability compared to the case of EOG, e.g., a capacitance of 269 F g1 (EOG-H) vs. 122 F g1 (EOG) was measured at a gravi- metric current intensity of 1 A g1, together with a capacitance retention of 33% (EOG-H) vs. 14% (EOG) at 20 A g1. The positive effect brought about by generating holes within the EOG NSs via hydrogen peroxide etching was made more apparent by investi- gating samples treated with different amounts of the etchant, the capacitance values of which are given in Fig. 5f. Using both higher and lower amounts of hydrogen peroxide than that of the bench- mark EOG-H material (0.92 wt% H2O2; see Experimental section for details) led in all cases to graphene electrodes with better elec- trochemical performance than that of the non-etched EOG product. However, the best overall results in terms of combined capacitance and rate capability were obtained with the benchmark EOG-H material, and etching with other amounts of hydrogen peroxide was not able to improve on the values attained with EOG-H to any significant extent (except for EOG etched with 1.84 wt% H2O2 when measured at the lowest current intensities). Table S1 of the ESM compares capacitance values measured for the present EOG-H material with those of other holey graphenes previously reported in the literature (mostly obtained from standard graphene oxides), where it is noticed that a competitive performance was achieved with the electrochemically derived holey graphene. We note that, even in cases where EOG-H loses out in the comparison of capac- itance values, it is still advantageous from the point of view of its simpler, less time-consuming and more environmentally friendly preparation process. The galvanostatic profiles measured at different current densities for MRGO-H and MRGO are presented in the ESM (Figs. S7a and b, respectively) together with their corre- sponding capacitances (Fig. S7c). It is worth noting that while the capacitance values of MRGO-H were similar to those of its EOG-H counterpart (also shown in Fig. S7c for a better comparison), the total charges stored by the latter (i.e., the capacity values) were substantially larger on account of its wider operating voltage window. This point is illustrated in Fig. S7d, which compares the capacities of the EOG, EOG-H, MRGO and MRGO-H samples and highlights the advantage of using the holey graphene derived by electrochemical means. Thermal annealing under air atmosphere, which is known to be another effective method to create holes in both standard graphene oxide [11,12] and reduced graphene oxide [66], was also investi- gated on the EOG material (see Experimental section for details). The capacitance values measured at different current intensities for electrodes prepared from EOG annealed at temperatures ranging between 400 and 650 C are presented in Fig. S5f. In this case, while the thermal treatments were generally seen to boost the electro- chemical behavior of EOG, the performance of the annealed sam- ples tended to be lower than that of their hydrogen peroxide- etched counterparts. This result can be tentatively ascribed to a more indiscriminate attack of the carbon lattice in the oxidized graphenes by molecular oxygen at high temperatures compared to the etching attained with hydrogen peroxide at a moderate tem- perature (100 C). Such an aggressive attack by molecular oxygen was expected to not only remove the highly oxidized domains of the NSs, but also much of their pristine, aromatic areas via edge recession. In fact, it is well documented from past studies that even the edges of highly crystalline graphites recede at a fast rate (affording CO and CO2 molecules) upon exposure to air at tem- peratures of 500 C and above [67,68]. Evidence for the extensive abstraction of carbon atoms from the EOG NSs by this type of treatment was obtained by measuring their product yields, which were as low as ~15 wt% after thermal annealing at 650 C. For comparison, typical product yields for hydrogen peroxide etching were ~72 wt%. This result was further supported by the observation of very irregular, jagged edge profiles in the annealed NSs (see Fig. S8 of the ESM). Such irregular profiles were a clear sign of extensive edge recession and were not seen in the hydrogen peroxide-etched EOG NSs. Finally, the holey graphene materials were also tested as elec- trodes in a two-electrode, symmetric supercapacitor configuration. Fig. S9 of the ESM shows cyclic voltammograms (a,c) and galva- nostatic charge-discharge profiles (b,d) for EOG-H (a,b) and MRGO- H (c,d) symmetric devices in 6 M KOH electrolyte. Similar to the situation noted above for the three-electrode measurements, a wider operating voltage window could be used with the electro- chemically derived holey graphene, i.e., 1.2 V for EOG-H vs. 0.6 V for MRGO-H. Such a difference was translated into a better perfor- mance of the former in terms of energy and power densities, as can be noticed in the Ragone plots of Fig. 6, based on gravimetric values (orange symbols for EOG-H, green for MRGO-H). For instance, the EOG-H device delivered a gravimetric energy density of ~8.7 Wh kg1 at a power density of ~600 W kg1, to be compared with 0.46 Wh kg1 at 600 W kg1 for the MRGO-H device. These figures were relatively modest in the context of carbon-based aqueous supercapacitors [69e71]. Furthermore, in both cases the energy density fell sharply with increasing power density in the 103e104 W kg1 range. Likewise, using a concentrated water-in-salt electrolyte for EOG-H (14 m NaClO4 [72]) to broaden the operating voltage window (up to 2.4 V) was only able to marginally improve the device performance (gray symbols in Fig. 6). For example, the water-in-salt device delivered an energy density of 9.4 Wh kg1 at a 66 Fig. 6. Ragone plots, based on gravimetric values, for EOG-H (orange symbols), MRGO- H (green) symmetric devices in 6 M KOH electrolyte, as well as for EOG-H using 14 m NaClO4 water in salt electrolyte (gray symbols). (A colour version of this figure can be viewed online.)PDF Image | electrochemical route to holey graphene nanosheets
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