Highly Porous Expanded Graphite: Thermal Shock vs. Programmable Heating

Abstract

Highly porous expanded graphite was synthesized by the programmable heating technique using heating with a constant rate (20 °C/min) from room temperature to 400–700 °C. The samples obtained were analyzed by scanning electron microscopy, energy-dispersive spectroscopy, low-temperature nitrogen adsorption, X-ray photoelectron spectroscopy, Raman spectroscopy, thermogravimetry, and differential scanning calorimetry. A comparison between programmable heating and thermal shock as methods of producing expanded graphite showed efficiency of the first one at a temperature 400 °C, and the surface area reached 699 and 184 m2/g, respectively. The proposed technique made it possible to obtain a relatively higher yield of expanded graphite (78–90%) from intercalated graphite. The experiments showed the advantages of programmable heating in terms of its flexibility and the possibility to manage the textural properties, yield, disorder degree, and bulk density of expanded graphite.

Effect of Process Parameters on the Graphite Expansion Produced by a Green Modification of the Hummers Method

“…From the widening observed in the plane (002), the corresponding FWHM was calculated ( Figure 3 b), and we can notice that this value is increased as the milling time is increased; this behavior resulted from some factors: the intercalation of oxygen functional groups such as epoxy, hydroxyl, carbonyl, and carboxyl groups during the chemical reaction with carbon conjugated with the drawdown of the crystalline structure and defects generation during the HEBM [ 34 ]. Some authors described some possible causes of the chemical broadening of the IGs: Bannov et al mentioned that after chemical intercalation with sulfuric acid, the XRD spectrum of IGs contains a mixture of two peaks: one corresponding to pure graphite and the other shifted to lower angles d (002) = 0.3368 nm with a lower interlayer distance, corresponding to an intercalation compound described as graphite bisulfate in the form of an asymmetrical peak appearing in the spectrum below 26° [ 12 ]. On the other hand, Hou et al stated that some (002) diffraction peaks appearing at 26.59 and 26.73° suggest that some acid species remain in the interlayer spaces as residual compounds damaging the regular stacking of the graphite layers [ 2 ].…”

“…On the contrary, as seen in the thermogram of Figure 6 a, untreated graphite (NGr) has no expansion capacity under heating. Usually, the expansion process of IGs is conducted by putting the IGs in a furnace at a high temperature (~1000 °C) for a period below one min, leading to the decomposition of the IGs, generating a porous, cellular structure [ 12 ].…”

“…Figure 9 shows the reached EV as a function of microwave heating time, and peroxide concentration, the samples prepared with 50% H 2 O 2 concentration achieved almost 500 mL/g (5–2); meanwhile, with 30% of peroxide concentration, the sample with the highest value was 0–1 reaching 400 mL/g, as can be seen in the Figure 9 a, the mentioned evidence suggests that samples prepared with more concentrated peroxide reach higher values compared to the lower concentration counterparts. As a clear comparison of the reached highly efficient process, other works reported values of 150 [ 48 ], 11.4 [ 8 ], ranges from 17 to 62 [ 12 ], and 110 mL/g [ 35 ]. As can be noticed, most of the samples presented an optimum EV after 30 s of microwave heating.…”

“…The same happened with special molecules such as carbon nanotubes and graphene, which have successfully proven their extraordinary performance but, at present, are too expensive to be used in oil clean-up [ 9 ]. Expanded graphite (EG) has been receiving global attention due to the number of potential applications based on its intrinsic properties, such as low density, high porosity, and electrical conductivity, making it a promising material for fuel cells, electromagnetic interference shielding, catalyst, vibration damping, supercapacitors, biomedical materials and, as spilled oil adsorbent [ 10 , 11 , 12 ]. The high effectiveness of EG for oil on water or pure oil sorption was early described by Inagaki and Toyoda two decades ago [ 13 ].…”

The potential of thermally expanded graphite in oil sorption applications

“…The researchers also discovered that their novel method of production, involving intercalated graphite, enabled them to achieve a relatively higher yield ranging from 78% to 90%. 95 achieved a high expansion volume of 375 mL per gram and a good surface area of 100 m 2 per gram. The optimized ratios of graphite akes, KMnO 4 , and H 2 SO 4 were 1 : 0.15 : 2.5, respectively, with a 65% mass concentration.…”