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D.F. Carrasco, J.I. Paredes, S. Villar-Rodil et al. peroxide-treated product was then collected, washed with water and dried under a vacuum overnight at room temperature. This material is henceforth designated as EOG-H. For comparison pur- poses, samples were also prepared by treating the highly oxidized graphene material with hydrogen peroxide solutions of different concentrations, namely, 0.18, 0.37, 0.55, 1.84 and 4.55 wt%, which amounted to adding 0.2, 0.4, 0.6, 2 and 5 mL of 30 wt% H2O2 so- lution to 35.8, 35.6, 35.4, 34 and 31 mL of deionized water, respectively (the density of 30 wt% H2O2 solution was taken as 1.11 g mL1). Also for comparison, the highly oxidized graphene material was subjected to thermal treatment under air atmosphere. To this end, 10 mg of the oxidized graphene were placed on a ceramic boat and introduced into a furnace (Thermolyne from ThermoScientific) that had been pre-heated to a given temperature. The dwelling time in the furnace was 30 min. Different heat treatment temperatures, namely, 400, 450, 500, 550 and 600 C, were tested. 2.4. Characterization techniques The morphological, structural, thermal and chemical charac- teristics of the materials were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spec- troscopy, elemental analysis, UVeVis absorption spectroscopy, X- ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD) and nitrogen physisorption. FE-SEM images were recorded with a Quanta FEG apparatus (FEI Company) oper- ated at 25 kV. TEM measurements were carried out in a JEOL JEM 2100F microscope at an acceleration voltage of 200 kV, with specimens prepared by drop-casting 20 mL of a dispersion of the sample (0.01 mg mL1) in water or a water/ethanol mixture onto a copper grid (200 square mesh) covered with a lacey carbon film and allowing it to dry at room temperature. AFM imaging was per- formed in a Nanoscope IIIa Multimode system in the tapping mode of operation and using rectangular silicon cantilevers with reso- nance frequencies of 250e300 kHz and nominal spring constant of 40 N m1. Specimens for AFM were prepared by drop-casting small volumes (~20 mL) of an aqueous dispersion of the graphene sample (~0.01 mg mL1) onto freshly cleaved highly oriented pyrolytic graphite substrates that were pre-heated to 50e60 C. Raman spectra were obtained with a Renishaw inVia Qontor instrument, using a green laser excitation line (532 nm) at an incident power below 0.5 mW. For elemental analysis, the measurement of carbon and hydrogen was done in a LECO CHN-2000 instrument, whereas oxygen and sulfur were measured with a LECO Truspec Micro O accessory and a LECO S632 analyzer, respectively. Carbon, hydrogen and sulfur were determined, respectively, from the amount of CO2, H2O and SO2 generated upon combustion of the sample, and oxy- gen was inferred from the amount of CO and CO2 produced upon pyrolysis of the sample at high temperature. UVevis absorption spectra of graphene dispersions were recorded with a double-beam Genesys 180 spectrophotometer (Thermo Fisher Scientific). XPS analysis was performed in a SPECS instrument, working at a pres- sure of 107 Pa with a monochromatic Al Ka X-ray source (14 kV, 175 W). TPD measurements were carried out with an Autochem II chemisorption analyzer (Micromeritics) under argon flow (50 mL min1) at a heating rate of 10 C min1. A mass spectrometer (Omnistar apparatus, from Pfeiffer Vacuum) was used to measure the amount of H2O, CO and CO2 released from the sample during the heating cycle, by determining the intensity of the m/z 1⁄4 18, 28 and 44 signals, respectively. The electrical conductivity of paper- like graphene films was also evaluated. To this end, graphene pa- pers were first prepared by vacuum filtration of aqueous graphene suspensions (250 mL, ~0.1 mg mL1) through polycarbonate Carbon 195 (2022) 57e68 membrane filters (47 mm in diameter, 0.2 mm of pore size, from Whatman). Rectangular strips (0.5 cm in width and about 3 cm in length) were then cut from the papers and their electrical re- sistivity was measured at different fixed distances by means of a hand-held digital multimeter (Fluke 45). These data, together with the thickness of the graphene paper (estimated with a Mitutoyo digital micrometer), were used to calculate the electrical conduc- tivity of the films. Nitrogen adsorption isotherms were recorded at 196 C in an ASAP 2420 volumetric apparatus (Micromeritics), after degassing overnight the samples under vacuum at 110 C. The specific surface areas (SBET) were obtained from the adsorption branch of the nitrogen isotherms by the standard Brunauer- Emmett-Teller (BET) method in the relative pressure range from 0.06 to 0.30. 2.5. Electrochemical measurements The electrochemically derived, holey graphene materials were tested as electrodes for electrochemical charge storage in three- and two-electrode cell configurations. The tests were carried out with a BioLogic VSP potentiostat in Swagelok-type cells, recording cyclic voltammograms at different potential scan rates and galva- nostatic charge-discharge (GCD) profiles at different current den- sities, as well as electrochemical impedance spectroscopy (EIS) data. For the three-electrode cells, holey graphene and a commer- cial activated carbon (YP-50F, from Kuraray) were used as the active material for the working electrode and counter electrode, respec- tively, whereas Hg/HgO (1 M NaOH) was used as the reference electrode in aqueous 6 M KOH electrolyte. To prepare the working electrodes, an amount of holey graphene sample was placed on a circular piece of graphite foil at a typical mass loading of ~1 mg cm2 and pressed under 1 ton for a few seconds with a hydraulic press. The counter electrodes were obtained in the form of a paste by mixing the commercial activated carbon, carbon black (acquired from Cabot) as a conductive additive and polytetra- fluoroethylene (PTFE; powder form, from Aldrich) as a binder in a mortar. The weight ratio of these components in the paste was 90:5:5 (activated carbon:carbon black:PTFE). For the two-electrode cells, a symmetric configuration was employed, whereby the abovementioned pressed holey graphene served as both the posi- tive and negative electrode. In this case, in addition to the aqueous 6 M KOH electrolyte, a water-in-salt electrolyte, namely 14 m NaClO4, was also tested. Two stacked pieces of nylon membrane filter (0.45 mm of pore size, from Whatman) were used as electrode separator in all cases. Gravimetric capacitance values were derived from the GCD profiles according to the following equation: C1⁄4IDt/mDV (1) where C (F g1) is the gravimetric capacitance, I (A) is the current intensity, Dt (s) is the discharge time, m (g) is the mass of holey graphene in the electrode (only the mass of the working electrode for the three-electrode cell and the mass of the two symmetric electrodes for the two-electrode cell) and DV (V) is the potential window. Gravimetric energy (E) and power (P) densities were calculated for the two-electrode cells from the following equations: E 1⁄4 C DV2/2 (2) P 1⁄4 E/Dt (3) Volumetric values of these parameters were also estimated by taking into account the density of holey graphene in the electrodes. To determine such a density, circular pellets (12 mm in diameter) were prepared by pressing a given amount of holey graphene 59PDF Image | electrochemical route to holey graphene nanosheets
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