Electrochemical approaches to the production of graphene flakes and their potential applications

Abstract

Graphene (GN) has many beneficial properties that encourage wide applications. Various manufacturing procedures are detailed in the literature but most are unable to produce GN flakes in bulk and usually result in toxic discharges. These techniques are also time-consuming and involve operations at high temperatures. A ‘greener’, simpler and a one-step synthesis of the material may be realised by electrochemical oxidation (or reduction) of the graphite host leading to intercalation of ions from the electrolyte (which may be aqueous, organic or an ionic liquid) followed by electrochemical exfoliation. Single- or multi-layered GN flakes can easily be produced in short periods of time, typically within 30 min. This paper reviews the state-of-the-art methods reported in the literature regarding electrochemical synthesis of GN flakes as well as their properties (determined via sophisticated analytical methods such as AFM, TEM, SEM or Raman spectroscopy). This is followed by a discussion on the applications of electrochemically prepared GN flakes. Challenges and opportunities are briefly considered leading to the conclusion that the cathodic intercalation of lithium ions into graphite can produce the highest yield (>70%) of pristine GN flakes in organic electrolytes. Future work is recommended with ternary eutectic melts as electrolytes.

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.