Numerical study of the electrochemical exfoliation of graphite
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In this study, graphene was prepared using electrochemical exfoliation method. Numerical study of the exfoliation process of graphite was carried out using COMSEL Multiphysics 5. The homemade graphene was characterized using various microscopy, spectroscopy and X-ray diffraction techniques. The prepared graphene oxide consists of few layers of graphene. The numerical study showed that the concentration of sulfate anions was transported through graphite rod in the range (9.77 × 10−7– 4.58 × 10−5) mol/m3 and stresses distribution which resulted from sulfate anions interaction along the graphite rod in the range (3.43 × 10−8– 5.77 × 10−7) N/m2.
KeywordsGraphene Electrochemical exfoliation COMSOL Multiphysics
Graphene, a two-dimensional honeycomb sp2 carbon lattice, shows a potential use in the next-generation of electronic devices [1, 2], composite materials , energy storage and conversion , drug delivery, sensor and catalysis due to its extraordinary properties [5, 6]. Graphene has a high charge carrier mobility (2 × 105 cm2/Vs) , high thermal conductivity (greater than 2000 W/m.K) , exceptional young’s modulus (~ 1 TPa) , large specific theoretical surface area (2630 m2/g)  and high transparency 97.7% .
Since the graphene discovery in 2004, many fabrication methods were explored. Graphene could be prepared by exfoliation, epitaxial growth, chemical vapor deposition (CVD), solvent- and/or surfactant assisted liquid-phase exfoliation of graphite, and the thermal/chemical reduction of graphene oxide [12, 13]. Electrochemical exfoliation of graphite has attracted specific attention due to its simplicity, fast, controllable and environmentally friendly in producing high-quality graphene [14, 15, 16]. It involved shear deformation of graphite electrode by applying an electrical potential either anodic polarization to oxidize the graphite rod to produce graphene oxide or cathodic polarization to obtain graphene then allowed the cations or anions to intercalate between graphene layers to form graphite intercalated compounds (GICs) . Then as the process evolves into exfoliation stage, graphene is produced. The electrolyte played a major role in the electrochemical exfoliation process due to its influence on the composition, structure and properties of resulting graphene sheets . Different kinds of electrolytes have been used, including ionic liquid [19, 20], high molecular polymer , polar solvent , surfactant  and inorganic solution [24, 25, 26]. Among them H2SO4 solution, has been more frequently investigated because of its high yield, which is essential for the practical use of graphene. The quality of graphene is strongly depended on the potential applied as well as by the electrolyte employed. The use of high voltages facilitates the generation of oxygen groups on the exfoliated graphene as well as structural damage. For this reason, the application of negative potentials under cathodic conditions in the presence of positive intercalation ions such as tetraalkylammonium cations was attempted to avoid the formation of oxygen functionalities and to obtain graphene of better quality [27, 28]. Abdelkader et al.  prepared graphene via cathodic exfoliation of graphite in dimethyl sulfoxide containing lithium and triethylammonium (Et3NH+). They obtained few layers of graphene with a diameter ranging from 1 to 20 µm without using sonication or centrifugation processes. Kakaei and Zhiani  prepared graphene nanosheets through electrochemical exfoliation using Urea choline chloride/water solution at (1:1) ratio then fabrication composite material consisted of platinum nanoparticles/graphene nanosheets rod to enhance methanol oxidation of fuel cell. They stated that the prepared composite material is a promising method for improving efficiency of direct methanol fuel cells. Huang et al.  utilized two-steps electrochemical exfoliation using NaOH to expand graphite foil and H2SO4 for the exfoliation. They reported that the two-step electrochemical exfoliation produced high quality graphene. Kakaei and Hasanpour investigated the role of cetyltrimethylammonium bromide (CTAB) as an electrolyte on the mechanism of electrochemical exfoliation of graphite. They reported that the CTAB intercalate into the graphite rod then followed by exfoliation process to produce graphene . Ambrosi and Pumera  studied electrochemical exfoliation of graphite using three different electrolytes H2SO4, Na2SO4, and LiClO4. They reported that graphene produced in H2SO4 had higher density of defects than other electrolytes. However, LiClO4 seems very suitable electrolyte to prepare graphene oxide. Because of a high amount of oxygen functionalities, the grapherne oxide can be used as additives for fabrication of polymer-based nanocomposite. Najafabadi et al.  used iso-molded graphite as anode and a novel electrolyte that consist of ionic liquids and acetonitrile (1:50 vol. IL/solvent ratios) for graphene production. They showed that higher rate of graphene exfoliation in BMPyrr BTA/acetonitrile solution as compared to other ionic liquids because of large oxygenated BTA anions. However, few papers were published depicting the exfoliation mechanism of graphite. Shinde et al.  studied electrochemical exfoliation of graphite by shear assisting to exfoliate graphene and simulated this process using COMSOL Multiphysics. They stated that the shear rate played a main role on the yield, thickness and quality of exfoliated graphene. Pupysheva et al.  simulated graphene nanoplatelets exfoliation via ultrasonication process using gaussian and universal force field. They reported that a nonresonant locking mechanism involving surfactant molecules is shown to be a feasible exfoliation mechanism through direct ultrasonication. Yang and Liu  studied the mechanism of liquid exfoliation of graphene using molecular dynamics. They reported that the sliding-away mechanism is played a main role for liquid exfoliation process because the surfactant-graphene interaction is maximized which is critical to the separation process. Shao et al.  studied exfoliation mechanism of graphite for modified Hummers method and they stated that the layer spacing of graphene sheets 3.334Å calculated by molecular dynamics simulations coincided with the X-ray diffraction results. Gai et al.  exfoliated graphite using rotor–stator in different solvents (supercritical CO2, water, NMP) and they used computational fluid dynamics to simulate exfoliation mechanism. They stated that the volume of the active region, which is the gap between the stator and the rotor, and the effective exfoliation time influenced on graphene yield. Fu and Yang  simulated the liquid phase exfoliation of graphite using different solvents, N-methyl pyrrolidine (NMP), dimethyl sulfoxide (DMSO) and water via molecular dynamic simulation. They stated that NMP and DMSO had stronger affinity with graphene surface and parallel exfoliation of graphene is preferred rather than vertical exfoliation due to it required less external power. In this work, we prepared graphene oxide via anodically electrochemical exfoliation method and we used COMSOL Multiphysics 5 to simulate this phenomenon employing transport of diluted species and fluid–structure interaction physics for numerical study of this process.
2 Experimental procedure
The starting and prepared materials were characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction and Raman spectroscopy.
2.2 Numerical approach
COMSOL Multiphysics 5 used to simulate the electrochemical exfoliation of graphite. We used tridimensional model to build the electrochemical exfoliation cell. The dimensions of the graphite rod domain (radius 2 μm, height 4 μm) and diluted sulfuric acid domain (radius 4 μm, height 5 μm). We simulated the intercalation of sulfate anions into graphite via transport of diluted species physics using diffusion equations. The physics which used to simulate this phenomenon were transport of diluted species and fluid structure interaction physics to simulate the intercalation and exfoliation processes.
2.3 Governing equations
3 Results and discussion
On the other side, the intensity ratio of (I2D/IG) indicated the quantity of graphene layers which is 0.22 for exfoliated graphene oxide, that means it is consisted of few layers of graphene. The intensity ratio of (I2D/IG) value is lower than other work done by Copper et al. which was 0.68 and they estimated the graphene layers between 2 and 5 layers .
In summary, graphene oxide prepared using anodically electrochemical exfoliation technique. XRD’s pattern for exfoliated graphene oxide indicated appearing a peak at 10.8° with d-spacing 8.19Å which is related to graphene oxide and Raman spectrum showed that the intensity ratio (I2D/IG) 0.22 therefore the prepared graphene oxide consisted of few layers with crystallite size 22.8 nm. The numerical study indicated that the concentration gradient through graphite rod is in the range (9.77 × 10−7–4.58 × 10−5) mol/m3 and the magnitude of stresses applied along graphite rod is between 3.43 × 10−8–5.77 × 10−7 N/m2.These stresses along graphite rod are appropriate to overcome the weak van der Waals band to exfoliate graphite.
Compliance with ethical standards
Conflict of interest
The author(s) declare that they have no competing interests.
- 3.Wu D, Zhang F, Liang H, Feng X (2012) Nanocomposite and macrosopic materials: assembly of chemically modified graphene sheets. Chem Soc Rev 18:6141–6160Google Scholar
- 15.Singh VV, Gupta G, Batra A, Nigam AK, Boopathi M, Gutch PK, Tripathi BK, Srivastava A, Samuel M, Agarwal GS, Singh B, Vijayaraghavan R (2012) Greener electrochemical synthesis of high quality graphene nanosheets directly from pencil and its SPR sensing application. Adv Funct Mater 22:2352–2362CrossRefGoogle Scholar
- 36.Pupysheva OV, Farajian AA, Knick CR, Zhamu A, Jang BZ (2010) Modeling direct exfoliation of nanoscale graphene platelets. J Phys Chem 114:21083–21087Google Scholar
- 41.COMSOL Multiphysics (2014) Chemical species transport, version 5Google Scholar
- 42.Vanýsek P (1992) Ionic conductivity and diffusion at infinite dilution. CRC Press, Boca Raton, pp 111–113Google Scholar
- 43.COMSOL Multiphysics (2014) Fluid flow,version 5Google Scholar
- 44.Lide DR (2005) Handbook of chemistry and physics, 86th edn. CRC Press, Boca RatonGoogle Scholar