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D.F. Carrasco, J.I. Paredes, S. Villar-Rodil et al. Carbon 195 (2022) 57e68 Fig. 5. (a) Cyclic voltammograms recorded at a potential scan rate of 10 mV s1 for the EOG (red trace) and EOG-H (blue trace). (b,c) Cyclic voltammograms recorded at different potential scan rates for (b) EOG-H and (c) EOG. (d) Nyquist plots for EOG (red trace) and EOG-H (blue trace). The inset shows a detailed view of the high frequency region of the plots. (e) Imaginary part of the capacitance versus frequency for EOG (red trace) and EOG-H (blue trace). The black and gray dotted lines correspond to the deconvolution of EOG-H profile into two components. (f) Capacitance values measured at different current densities for EOG-H (blue symbols), EOG-H (red symbols) and for electrodes prepared from EOG treated with different amounts of the etchant, namely, 0.18 (orange symbols), 0.37 (green), 0.55 (cyan), 1.84 (gray) and 4.55 wt% (black). (A colour version of this figure can be viewed online.) Gaussian bands, which is indicative of two underlying phenomena, the resulting relaxation times being ~82 and ~6 s. The shorter relaxation time measured with EOG-H revealed a faster response of the holey graphene-based electrode, which again pointed to an improved accessibility of this material by the electrolyte, relative to that of EOG. For comparison purposes, the corresponding voltammetric and EIS data recorded for the Hummers-derived graphenes (MRGO and MRGO-H samples) are shown in Fig. S5 of the ESM. As expected, in this case the obtained results were also consistent with a faster diffusion of the electrolyte throughout the holey graphene elec- trode, i.e., MRGO-H yielded a more vertical Nyquist plot profile and a shorter relaxation time than those of MRGO (~68 s vs. ~82 s; if the former is deconvoluted into two components, it yields ~82 and ~6 s). More significantly, comparison of the electrochemical- and Hummers-derived holey graphenes (i.e., EOG-H vs. MRGO-H) ten- ded to favor the former material. Specifically, although EOG-H showed a similar relaxation time to that of MRGO-H (both mate- rials being porous), it boasted a somewhat broader voltage window (~0.7 vs. 0.5 V; see, e.g., corresponding voltammograms in Figs. 5a and S5a). When the voltage window of MRGO-H was extended above (below) 0.4 (0.1) V vs. the Hg/HgO reference electrode, sharp increases in the measured current were observed, indicative of substantial water electrolysis taking place in the cell. By contrast, the same effect was noticed in EOG-H only above (below) 0.5 (0.2) V vs. Hg/HgO. Water electrolysis and other electro-induced re- actions are known to be catalyzed by different types of active (defect) sites present in sp2-based carbon materials, in particular carbon edges saturated by heteroatoms (oxygen, nitrogen, etc) [64,65]. Exhibiting a larger fraction of holes and, thus, of oxygen- 65PDF Image | electrochemical route to holey graphene nanosheets
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