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D.F. Carrasco, J.I. Paredes, S. Villar-Rodil et al. Carbon 195 (2022) 57e68 former involves the creation of micrometer-to nanometer-sized voids between neighboring graphene NSs, for example, through their assembly into aerogels. In-plane porosity, on the other hand, relates to the presence of nanometer-sized holes within the NSs, which can be produced by either physical or chemical means and give rise to what is usually referred to as holey graphene [11e14]. Even with substantial nanosheet re-stacking, mass/ion transport through an electrode made up of holey graphene can still be allowed to a substantial extent by the presence of in-plane porosity [12,15]. Many of the approaches developed so far to access holey gra- phene in considerable quantities make use of graphene oxide as the starting material [11,14]. As both experimental observation [16,17] and theoretical modeling [18] have revealed, from a structural point of view graphene oxide is a patchwork of heavily oxidized graphene domains a few to several nanometers in size interspersed with essentially pristine domains. Due to their much higher chemical reactivity and structural instability, the oxidized domains can be selectively etched away by means of, e.g., oxidative and/or heat treatments, thus leaving behind holes encircled by the largely unreactive, pristine graphene domains of the NSs. Nonetheless, while standard graphene oxide, i.e., that obtained from the oxida- tion of graphite by the Hummers or Brodie methods, is widely used as a precursor to holey graphene, such a type of oxidized graphene may not be the best option as a starting material, and instead other oxidation routes could potentially offer some advantages for this purpose. For example, the oxidized and pristine domains of stan- dard graphene oxide tend to be very small (typically 1e3 nm) [16]. As a result, the percolated carbon network in the corresponding holey NSs could be dominated by narrow graphene constrictions, which act as a barrier to the flow of electrical charge and thus could impair the overall electrical conductivity of the material. Further- more, graphene oxides prepared by the Hummers and Brodie methods require the use of harsh acids and oxidants, making such methods unattractive from an environmental and practical stand- point. It is thus clear that resorting to highly oxidized graphene NSs obtained by proper alternative routes (i.e., routes other than those typical of standard graphene oxide) as a precursor to holey gra- phene could positively impact the use of the latter as an electrode material and also in other applications. However, to the best of our knowledge this issue has not yet been addressed. As a prospective replacement for standard graphene oxide, we have turned our attention to oxidized graphene obtained by elec- trochemical exfoliation of graphite, particularly aqueous anodic exfoliation. The electrolytic route to graphene has been shown to be fast, easy to implement, efficient, potentially scalable at low cost [19,20], as well as versatile enough to afford NSs with controllable degrees of oxidation [21,22], including highly oxidized NSs that could be suitable as a precursor to holey graphene [23e25]. Because the oxidizing conditions are generated by the application of an anodic potential in aqueous electrolyte that obviates the need of strong chemical oxidants, the preparation of highly oxidized graphene by electrochemical exfoliation can be regarded as more environmentally friendly than the traditional routes based on chemical oxidation [25]. Moreover, some experimental evidence suggests the electrical conductivity and overall structural quality of electrochemically derived, oxidized graphene NSs to be higher than those of their (reduced) standard graphene oxide counterparts of similar oxidation degree [26e28], thus hinting at the idea that the former possess larger pristine graphene domains. Here, oxidized anodic graphene is used for the first time as a precursor to holey graphene and proved to be an advantageous alternative to standard graphene oxide in such a role. Moreover, when used as an electrode material for supercapacitors, the holey NSs prepared from anodic graphene are shown to possess improved performance compared to that of their counterpart stemming from traditional graphene oxide, which can be put down to the superior electrical and structural characteristics of the former. Overall, the present work introduces a new-generation holey graphene as an advantageous electrode material that could be used in a variety of electrochemical energy storage applications, e.g., either as the active material or as a support for the active material in different types of supercapacitor and battery devices. 2. Experimental section 2.1. Materials and reagents High purity graphite foil with a thickness of 0.5 mm (Papyex I980) was purchased from Mersen. Highly concentrated sulfuric acid (H2SO4, 95e97 wt%), hydrogen peroxide (H2O2, 30 wt% solu- tion), potassium hydroxide (KOH, powder form) and platinum foil (dimensions: 25 25 0.025 mm3) were obtained from Sigma- Aldrich and used as received. Ultrapure deionized water (re- sistivity: 18.2 MU cm; Milli-Q Reference water purification system, from Millipore Corporation) was used throughout the experiments. 2.2. Preparation of highly oxidized graphene by anodic intercalation and exfoliation of graphite Highly oxidized graphene was obtained by anodic treatment of graphite foil in a two-electrode set-up via a two-step process pre- viously described elsewhere [25] that comprised (1) intercalation of the graphite anode in highly concentrated sulfuric acid and (2) delamination and oxidation of the intercalated anode in a more diluted solution of the same acid as the electrolyte. To this end, a piece of graphite foil (lateral dimensions: 25 50 mm2, with only 25 40 mm2 immersed in the electrolytic medium) as the working electrode (anode) and platinum foil as the counter electrode were first immersed in 80 mL of concentrated sulfuric acid in a face-to- face configuration at a distance of ~2 cm from each other, and then a bias voltage of þ3 V was applied to the graphite electrode for 20 min by means of a power supply (E3633A apparatus, from Keysight Technologies). During this stage, the graphite foil anode was seen to acquire a bluish tone and swell to a small extent, which was indicative of its extensive intercalation by sulfate anions. Subsequently, the electrolytic medium of the cell was changed to 50 wt% (z7.1 M) sulfuric acid solution and a bias voltage of þ3 V was applied to the intercalated electrode for another 20 min. Such a treatment led to a fast and very pronounced expansion of the graphite foil piece, with some small expanded fragments even detaching from the anode and building up on the surface of the electrolytic solution. Upon completion of this step, the expanded material (both the detached fragments and the expanded fraction remaining on the anode) was collected, extensively washed with water and finally dried under a vacuum overnight at room tem- perature. The resulting dried product was then stored for further use and is henceforth denoted as EOG. 2.3. Preparation of holey graphene by treatment of electrochemically derived, highly oxidized graphene To obtain holey graphene, the highly oxidized graphene product derived from the electrochemical process was subjected to chem- ical treatment with hydrogen peroxide at a moderate temperature. In a typical standard procedure, 36 mg of highly oxidized graphene were added to 36 mL of an aqueous 0.92 wt% hydrogen peroxide solution (i.e., 1 mL of 30 wt% H2O2 solution mixed with 35 mL of deionized water) in a Teflon-lined autoclave, which was heated at 100 C for 10 h (heating ramp of 1 C min1). The hydrogen 58PDF Image | electrochemical route to holey graphene nanosheets
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