ReviewElectrochemical approaches to the production of graphene flakes and their potential applications
Introduction
Graphene (represented as GN) has raised a surge of interest amongst the scientific community since its experimental recognition and elegant, ‘sticky tape’ isolation by Geim and Novoselov (in 2004) [1], [2]. Both physicists were awarded the Nobel Prize in Physics (2010) due to this discovery [3]. GN-based materials are inherently nontoxic, chemically and thermally tolerant, and mechanically robust [4], [5], [6]. GN exhibits superior electrical conductivity and a high charge carrier mobility (20 m2 V−1 s−1) because electron tunnelling occurs within its structure allowing electron movement at relativistic speeds. It has been reported to have the fastest electron and hole mobility than any other material [7]. It also has a high specific surface area (2630 m2 g−1, which could be likened to that of a soccer pitch per gram), excellent mechanical strength and stiffness, good elasticity, superior thermal conductivity, a broad electrochemical window and can offer both optical transparency and high electrical conductivity [6], [8], [9], [10].
GN is usually produced as a mixture of monolayers, bi-layers and multilayers (3–10 monolayers), in the form of irregular structured flakes or flat (or folded) sheets [11]. GN can also be produced as 0 D buckyballs (fullerenes) or as 1 D nanotubes. However, a key challenge in the synthesis and processing of bulk-quantities of GN is to surmount the strong exfoliation energy of the p-stacked layers in graphite, that is, the high cohesive van der Waals energy (5.9 kJ mol−1 carbon) that result in the adherence of graphitic sheets [12], [13]. So far, the common techniques applied to GN production include mechanical exfoliation of graphite [10], epitaxial growth of a GN sheet on a substrate by chemical vapour deposition [14], solvothermal synthesis and pyrolysis [15], wet-chemical synthesis from graphite particles [16], liquid phase exfoliation [17], thermal decomposition of a SiC (silicon carbide) wafer under ultrahigh vacuum conditions [6] and oxidation of graphite [18] but none of these methods is able to produce the material in bulk or at a reasonable cost. In addition, all these production processes have been proven in the laboratory [19] as described in several review articles [20], [21], [22] and a high-throughput approach to determine the number of atomic planes in GN samples has also been recently put forward [23]. A summary of different production processes for GN is given in Fig. 1 [24].
There are other processing limitations to these synthesis techniques. For example, processing of GN products can be time-consuming (for example, the wet-chemical method can take tens of hours to days to complete and can cause severe damage to the honeycomb lattices of GN) [19], high temperatures are required during chemical reduction of graphite oxides and GN films produced have high resistances ranging from 1 to 70 kΩ per square area of the GN sheet (<80% transmittance) [25]. This high resistance is typically reported for GN sheets produced by liquid-phase exfoliation or intercalation of graphite that results in sizes that are normally less than 1 μm2. This is caused by damage during the exfoliation process that results in large amounts of intersheet junctions [21]. Other drawbacks of most of the synthesis methods have been detailed elsewhere [19]. For widespread engineering applications of GN-based technology, simple processes for its production on the scale of milligrams for laboratory tests to tens-of-kilograms for pilot- and industrial-scale are essential.
The electrochemical approach has the advantages of being single-step, easy to operate, environmentally friendly (if using ionic liquid electrolytes or aqueous surfactants) and operates at ambient conditions. Highly controllable flakes can be formed without the need for volatile solvents or reducing agents [26]. Recently, electrochemical methods have been demonstrated by a number of research groups to produce GN flakes in milligram and gram quantities [4], [27], [28], [29], [30], [31], [32]. The production is readily scaled-up using known principles of electrochemical cell design and engineering [33]. The process can take several minutes to hours to complete and the reported results are encouraging for the fast-processing of large quantities of GN flakes [6]. The electrochemical method utilises a liquid solution (electrolyte) and an electrical current to drive structural expansion (oxidation or reduction), intercalation and exfoliation at a piece of graphite (rod, plate, wire) to produce GN flakes [27]. The experimental arrangement uses a monopolar, undivided electrolysis cell. The yield, productivity and properties of GN flakes can be tuned by controlling the electrolysis parameters and electrolytes [29].
The electrochemical procedure for manufacturing GN flakes has been briefly reviewed in the literature [21], [30], [34] among other synthesis steps such as chemical or micromechanical exfoliation. It is now timely to provide a detailed overview of the principles and practice of electrochemical methods for the production of GN flakes. This review aims to highlight electrochemical approaches for the controlled production of GN flakes, including approaches such as anodic oxidation, cathodic reduction and the use of aqueous and non-aqueous solvents as electrolytes. A summary of the properties of GN flakes produced electrochemically is provided, which include structural, chemical and electrical aspects that have been determined by analytical techniques such as atomic force microscopy (AFM), tunnelling electron microscopy (TEM), Raman spectroscopy and others. Some applications of electrochemically produced GN flakes are considered here before a perspective is provided on the technology. This review aims to stimulate new and existing research groups to advance the science and technology of electrochemical methods for the large-scale production of high quality GN flakes.
Section snippets
Summary of chemical methods of producing GN and their drawbacks
At present, chemical conversion of graphite to GO (GN oxide) has emerged to be a viable route to produce GN-based single sheets in considerable quantities [16], [20]. Graphite oxide is usually synthesized through the oxidation of graphite using oxidants including concentrated sulphuric acid, nitric acid and potassium permanganate based on the Hummers method [35]. It is important to note that although graphite oxide and GO share similar chemical properties (i.e., surface functional groups),
History of electrochemical production of GN flakes
Although GN was not known until the 20th century, it was effectively being exfoliated onto paper when the lead pencil (containing graphite) was used for writing purposes since the mid-1500s. However, a scientific method towards its exfoliation was not discovered until the first GIC and GO compounds were reported in the early 1840s (Fig. 2) [22], [52]. The turn of the 20th century saw a growth in the application of GICs because they possessed much better electrical, electronic and catalytic
Principles of electrochemical approaches to produce GN flakes
The electrochemical methods of preparing GN flakes involve the application of cathodic or anodic potentials or currents in either aqueous (acidic or other media) or non-aqueous electrolytes (Table 1). One of the most important parameters for consideration of scaling-up the electrochemical technology is the yield of GN flakes. Table 1 shows this along with other important parameters determined by sophisticated analytical techniques. From the results shown, it appears that the procedure used by
Properties of electrochemically produced GN flakes
Post-treatment procedures on electrochemically prepared GN flakes (such as washing with ethanol or de-ionised water) are performed prior to determination of their properties by SEM (scanning electron microscopy), TEM, FTIR (Fourier transform infra-red spectroscopy), Raman, UV/Vis (Ultra-violet and visible spectroscopy) and other analytical techniques. Thermal and electrical conductivities are also measured due to their significance toward applications of GN and GO flakes in sensors and energy
Some applications of electrochemically produced GN flakes
GN flakes have been applied in different energy storage devices [12], [121]. The highly conductive property and enormous active area of GN allow it to be used as both a lithium ion and an electronic conductor to reduce both the size and weight of batteries without sacrificing the energy capacity [87], [89]. GN has the potential to make the construction of transparent or semi-transparent energy storage devices feasible in the future [6]. For instance, electrochemically symmetrical
Summary
GN can be produced electrochemically by three main routes that include electrochemical oxidation (or reduction) of the graphite host followed by intercalation of ions from the electrolyte (leading to structural expansion of the graphite intercalated compound) and ultimate exfoliation of GN flakes. In general, the electrochemical synthesis involves a one-step simple procedure to attain the objective and has the potential to be scaled up easily at relatively low costs. GN can be produced within 30
Future directions and their challenges
In all cases, the economical production of GN flakes and sheets in bulk and in an environmentally friendly manner has been the most important milestone in the scientific community. The electrochemical method appears to provide a solution and a careful study on its scaling-up issues may be required to understand the process further. It is expected that after complete development of GN flakes, on a large scale, with desired electrical properties, GN may become more attractive than silicon-based
Acknowledgements
The authors are grateful to the University of Malaya and the Ministry of Higher Education in Malaysia for supporting this collaborative work by means of the research Grant UM.C/HIR/MOHE/ENG/18 which made possible an extended visit of M.H.C. to the University of Southampton and Imperial College London. M.H.C. is also grateful to Prof. N.P. Brandon for providing full access to College facilities.
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