PDF Publication Title:
Text from PDF Page: 008
D.F. Carrasco, J.I. Paredes, S. Villar-Rodil et al. repeated imaging of the same area. The physicochemical characteristics of the EOG-H holey prod- ucts were consistent with the TEM observations and the expected etching behavior of their precursors. More to the point, after the etching process the components of the high resolution C 1s core level band associated to graphitic carbon, i. e., both the most intense component centered at ~284.6 eV and the p/p* satellite band, were left unchanged (Fig. 3b), indicating that the aromatic domains in the precursor graphene NSs were not altered for the most part during the etching process, as anticipated. Only subtle changes were noticed in the C 1s XPS spectrum of EOG following hydrogen peroxide treatment (Fig. 3b, blue trace), namely, (1) the component located at a binding energy of 286.5 eV became somewhat weaker, and (2) a faint but nonetheless discernible component developed at a binding energy of 288.5 eV. In EOG, the former component (carbon atoms in an oxidation state of þ1, i.e., single-bonded to oxygen) can be mostly ascribed to hydroxyl and epoxide groups bound to the basal plane in the highly oxidized domains of the graphene lattice [23,25]. These groups are expected to be largely removed upon etching with hydrogen peroxide, thus contributing to a reduction in the 286.5 eV component, but the holes generated in the etched NSs also provided new edge sites where oxygen atoms in other C-O configurations could be stabilized (e.g., phenol-type hydroxyls and ethers). For this reason, the component at 286.5 eV cannot be expected to vanish in the holey graphene products, even if all the original oxygen groups were abstracted during the etching process. On the other hand, the C 1s component located at 288.5 eV was attributed to carboxylic acid groups, which are normally found at edge sites of sp2-based carbon materials [23,47]. Therefore, the rise of this component in EOG-H could be correlated with the creation of new (internal) edges in the holey graphene NSs. We also note that the p plasmon band of EOG (component centered at ~291 eV) remained unchanged after the chemical treatment, further corroborating the idea that the electronically conjugated, aromatic areas of the material were not affected by such a treatment to a large extent. A similar evolution was observed for MRGO-H relative to its precursor (see Fig. 3c, green trace), i.e., a decrease and an increase of the 286.5 and 288.5 eV components, respectively. However, in this case the changes were somewhat more apparent, especially for the 286.5 eV component, which could be attributed to the more extensive perforation of the MRGO NSs upon treatment with hydrogen peroxide. It is also worth noting that the 288.5 eV component was already present in the starting MRGO sample (Fig. 3c, orange trace), whereas it was not in its EOG counterpart (Fig. 3b, red trace). This observation was in line with previous re- ports [24] and suggested that (1) contrary to the case of standard graphene oxides, oxidation of the graphite anode during the elec- trochemical exfoliation and subsequent work-up processes did not trigger any substantial cleavage of C-C bonds within the sp2-hy- bridized lattice, where carboxylic acid groups could have been accommodated, and (2) as-prepared EOG is chemically more ho- mogeneous overall than MRGO. The Raman spectra of EOG-H (Fig. 3d, blue trace) and MRGO-H (Fig. 3e, green trace) were not substantially different to those of their respective precursors (red traces). In particular, the measured ID/IG ratios (1.18 ± 0.08 for EOG-H and 1.042 ± 0.003 for MRGO-H) were similar to the values determined for their corresponding precursor graphenes, again suggesting that the aromatic areas of the NSs were not altered to a large extent during the hydrogen peroxide etching. On the other hand, significant changes were observed in the TPD profiles of the holey products relative to those of their parent materials, as shown in Fig. 3f for EOG-H (CO2: blue solid traces; CO: blue dotted traces) and Fig. 3g for MRGO-H (CO2: green solid traces; CO: green dotted traces). In all cases, the intense Carbon 195 (2022) 57e68 evolution of carbon oxides at low temperatures (100e300 C), which was characteristic of the starting graphenes, was drastically reduced after the hydrogen peroxide treatment. This result corroborated that etching by hydrogen peroxide preferentially removed the most labile and reactive oxygen groups from both types of graphene. By contrast, the evolution of CO2 and CO was not seen to decrease for the most part at higher temperatures, or even increased somewhat in the case of CO for MRGO-H. The release of carbon oxides from sp2-based carbon materials at such high tem- peratures is usually associated to stable oxygen groups bound to edges, and not basal planes, of the carbon lattice (e.g., phenols, ethers or carbonyls) [47]. Therefore, the creation of holes, and thus of internal edges, should promote the presence of such stable ox- ygen groups in the hydrogen peroxide-treated NSs, which in turn should contribute to the persistence of carbon oxide evolution in the TPD profiles at these high temperatures. Another expected effect of the generation of holes in graphene is the enhancement of specific surface area [58]. Indeed, nitrogen physisorption isotherms revealed a development of porosity upon hydrogen peroxide treatment (Fig. S3), which led to an increase in the specific surface area from 6 m2 g1 (EOG) to 107 m2 g1 (EOG- H). 3.3. Electrochemical charge storage behavior of the holey graphenes The introduction of holes in graphene enhances the specific surface area and introduces channels for ion transport, which is expected to be beneficial for its use in electrochemical charge storage applications [58]. The holey graphene materials were tested as electrodes for electrochemical charge storage in a three- electrode configuration using aqueous 6 M KOH as the electrolyte (see Experimental section for details). Fig. 5a shows cyclic vol- tammograms recorded at a potential scan rate of 10 mV s1 for the EOG (red trace) and EOG-H (blue trace) samples, where it can be seen that substantially higher gravimetric current densities were measured for the latter. The enhanced ability of EOG-H to store electrical charge relative to EOG could be reasonably ascribed to a better access of the electrolyte due to the presence of nanometer- sized openings within the holey graphene NSs, affording compar- atively faster ionic diffusion in the holey graphene electrode, which in turn would be afforded by shorter ion transport channels asso- ciated to the holes in the NSs [12,15]. Indeed, the EOG-H voltam- mograms showed rectangular shape (Fig. 5a) except for the highest potential scan rates (see Fig. 5b; voltammograms for EOG at different scan rates are also shown in Fig. 5c). Additional support for this conclusion was obtained from electrochemical impedance spectroscopy (EIS) measurements. Fig. 5d shows Nyquist plots for EOG (red trace) and EOG-H (blue trace). Although the overall appearance of such plots was similar, that of EOG-H displayed a more vertical profile in the medium-low frequency range, including the Warburg region, indicative of a lower resistance to electrolyte diffusion compared to the case of EOG (shorter ion diffusion lengths) [59,60]. As for the high frequency range of the plots (see inset to Fig. 5d), EOG-H showed a lower intercept with the real part of the impedance axis than EOG. Such intercept values are inter- preted as the ohmnic resistance of the solution and separator, and any internal resistance of the electrode material [61]. As the two former factors are equivalent for both electrodes, this observation points to a lower internal resistance of porous EOG-H. The char- acteristic relaxation time of the electrochemical cell derived from the EIS data, t0, determined as t0 1⁄4 1/f0, where f0 is the frequency at which the imaginary part of the capacitance reaches its maximum value [62,63] (see corresponding plots in Fig. 5e), was calculated to be 31 s both for EOG and EOG-H, respectively. However, the latter showed a complex profile resulting from convolution of two 64PDF Image | electrochemical route to holey graphene nanosheets
PDF Search Title:
electrochemical route to holey graphene nanosheetsOriginal File Name Searched:
Electrochemical route-Carrasco-Carbon.pdfDIY PDF Search: Google It | Yahoo | Bing
Salgenx Redox Flow Battery Technology: Power up your energy storage game with Salgenx Salt Water Battery. With its advanced technology, the flow battery provides reliable, scalable, and sustainable energy storage for utility-scale projects. Upgrade to a Salgenx flow battery today and take control of your energy future.
| CONTACT TEL: 608-238-6001 Email: greg@infinityturbine.com | RSS | AMP |