Electrochemistry of graphene: The current state of the art

Transcription

1 Electrochemistry of graphene: The current state of the art Sabine Szunerits and Rabah Boukherroub DOI: / Introduction Carbon-based materials such as glassy carbon, diamond, diamond-like carbon, carbon nanotubes, fullerenes, and graphene have attracted much interest, because of their marked structural differences and their related variety of electronic and electrochemical properties. 1 4 Their low cost, chemical stability, wide potential window, rich surface chemistry and enhanced electro-catalytic activity for a variety of redox reactions have made these materials of high importance for analytical and industrial electrochemistry. Different forms of carbon nanomaterials, including carbon nanotubes, carbon nanofibers and fullerenes have been more recently employed for electroanalytical applications and have shown to outperform the classical carbon materials based on graphite, glassy carbon, diamond and carbon black. 5 In contrast to other carbon-based electrodes, carbon nanotubes are typically grown from carbon-containing gases with the use of catalytic metal nanoparticles, which remain in the nanostructures even after extensive purification procedures. As a consequence, the electrochemical behaviour of carbon nanotubes is dominated by these metal nanoparticles. 6 These metallic impurities are not only causing problems for the construction of reliable sensors and energy devices, 7,8 but are also responsible for toxicological hazards within biological samples. 8 The demonstration by K. S. Novoselov and A. K. Geim, the 2010 Nobel Laureates in Physics, that single layers of graphene can be isolated from graphite and identified by microscopy has pushed graphene to the forefront of research in the design of electrochemical sensors. 9,10 By measuring the electrical properties of a few-layer graphene, Novoselov et al. found some remarkable behaviour, such as the fact that graphene behaves like a zero-gap semiconductor with ballistic transport of charge carriers, which could be electrons or holes, depending on the sign of the applied gate voltage. 9,10 These and other intriguing properties (e.g. high transparency, hardness, large surface area, etc.) offer exciting prospects for graphene to be used in creating nanometre scale transistors, biosensors as well as electroanalytical devices. In this chapter, the electrochemical properties of graphene and graphene derived materials will be examined. With the steadily increasing number of experimental and theoretical studies on graphene, several reviews on the electrochemistry of graphene and its electrochemical applications exist It is, however, only very recently that the electrochemical behaviour of graphene and its derivatives has been exploited in more depth taking into account the source of graphene, together with defects and amount of Institut de Recherche Interdisciplinaire (IRI, USR 3078), Universite Lille1, Parc de la Haute Borne, 50 Avenue de Halley BP 70478, Villeneuve d Ascq France. sabine.szunerits@iri.univ-lille1.fr; rabah.boukherroub@iri.univ-lille1.fr Electrochemistry, 2013,12, c The Royal Society of Chemistry 2014

2 oxygen-functions present on its surface. Indeed, the influence of the synthesis or fabrication route of graphene on the electrochemical behaviour of graphene electrodes has been neglected for some time, but has emerged as a crucial parameter which has to be optimized and controlled. Many of the methods used to produce graphene induce defects, which tend to be active sites for electrochemical reactions. While the introduction of defects often results during the fabrication process of graphene (e.g. formation of new edges during micromechanical peeling of graphene sheets from graphite), the formation of defects and functional groups can be more deliberate as is the case in the oxidation process of graphite to graphene oxide (GO). Chemical or thermal post-treatment of GO can partially restore the original graphene structure, substantial amounts of defects and impurities remain however. 18 Each parameter will have a different effect on the electrochemical activity of graphene. The materials science part of graphene has thus to be considered to overcome the ambiguity about the electrochemical properties of graphene. Here we will not describe in details the different fabrication methods employed so far. Some recent reviews are merely devoted to the synthesis of graphene and the reader is referred to them Before looking more closely into the electrochemical behaviour of graphene and its derivatives, some words concerning the nomenclature of graphene. 2 The structural difference between graphene, graphene oxide (GO) and reduced graphene oxide (rgo) Strictly speaking, graphene is a carbon monolayer. Fig. 1 shows the idealized two-dimensional structure of graphene, comprising a single View Online Fig. 1 (A) Schematic image of graphene s hexagonal crystal structure together with graphene derived structures. (B) Raman spectra of single and double layer graphene (with courtesy provided from E. Pichonat). 25 (C) Tapping-mode AFM images of graphene synthesized from GO through photochemical irradiation at l=312 nm for 6 h (with reprint permission from ref. 26). 212 Electrochemistry, 2013, 12,

3 layer of carbon atoms joined by sp 2 covalent bonds to form a flat hexagonal lattice. 3 It is essentially a very large polyaromatic hydrocarbon and the building block for all fullerene allotropic dimensionalities. Thus, in addition to its planar form, graphene can be rolled into carbon nanotubes or stacked into graphite (Fig. 1). Indeed, graphene may simply be described as an individual layer of graphite, which is made up of many graphene layers stacked one atop another and held together by weak van der Waals forces and p p stacking. There is still some disagreement in the literature regarding the point at which graphene becomes graphite. Geim and Novoselov have reported that structures composed of up to around ten graphitic sheets have electronic properties sufficiently different from bulk graphite to be classified as graphene. 22 Pumera suggested that structures consisting of up to 100 layers should be thought of as graphene. 12 One has to bear in mind that the current techniques available for the production of graphene hardly allow the formation of only single layers of graphene. Most of the preparation methods of graphene actually result in a few-layer graphene (3 10 layers) or even more approaching the characteristics of highly ordered pyrolytic graphite (HOPG). To embrace somehow the number of layers in graphene, terms such as single-layer graphene, double-layer, bilayer, few-layer (3 9 layers), multilayer graphene and graphene platelets ( layers) are used. 12 The number of graphene layers N can be determined from the peak shapes in Raman spectra, characteristic of the specific numbers of layers in a few-layer graphene (Fig. 1B) or from thickness measurements using AFM (Fig. 1C). An additional complexity in the structure of graphene is that single-layer graphene is not completely flat and the flexible sheets have a tendency to fold, buckle and corrugate. 3 When working with graphene, invariably some of the individual layers will fold and buckle during casting processes (e.g. drop casting, dip coating, spin-coating etc.) onto electrode surfaces. 3,23,24 Pumera in addition pointed out, that the x-axes parallel edges exhibit a zig-zag structure, while the y-axes parallel edges have an armchair configuration (Fig. 2A). 12 The difference will have an important influence View Online Fig. 2 (A) Zig-zag and armchair orientation of graphene edges. (B) Presentation of graphene s edge and basal sites. Electrochemistry, 2013,12,

4 View Online on the electrochemical properties of x-y axes constrained graphene, called nanoribbons and stacked graphene platelet nanofibers. Graphene nanoribbons can be imagined as open single-walled carbon nanotubes (SWCNTs) in the case of a single-layered graphene nanoribbon and as open multi-walled carbon nanotubes in the case of few-layer graphene nanoribbons. If the dimension of the z-axes exceeds those of the x-y-axes, the multilayered graphene nanoribbons are often called stacked graphene platelet nanofibers. Such nanofibers possess an exceptionally high ratio of exposed graphene edge sites vs. basal sites (Fig. 2B) and should be preferentially used for electrochemical applications since the edges of the graphene sheets are the potential electroactive sites. For the integration of graphene into electrochemical devices, it is essential to have a simple, reproducible and controllable technique to produce large-area graphene sheets on a large scale. The use of reduced graphene oxide (rgo) rather than graphene appears to be a promising alternative (Fig. 3). Reduced graphene oxide is obtained through reduction of GO, which by nature is electrically insulting and thus cannot be used without further processing as a conductive nanomaterial. Hydrazine and its derivatives have been widely used for the formation of rgo from GO, 27 as the resulting rgo has a C/O ratio of as high as 10 and exhibits good electrical conductivity. 19 The following section will describe in more details different aspects concerning the preparation of rgo. The synthesized rgo contains a large amount of defects even after reduction compared to graphene obtained by other methods and shows significantly lower electrical conductance in comparison to pristine graphene, prepared by the scotch tape method reported by Geim and Novoselov. It is thus not of major use for high performance electrical devices. However, it proved to be well adapted for electrochemical studies. Van der Waals interactions are so strong and rgo sheets have a strong tendency to aggregate, leading to more of a graphite layered structure. Some care has to be thus taken concerning the definition of rgo modified interfaces, with in most cases might show graphitic- rather than graphene-like characteristics. From the above discussion, it is evident that the graphene morphology and its properties depend strongly on the synthesis method. Before discussion the electrochemical related aspects of graphene, the technological advances of graphene production will be presented. Fig. 3 Formation of reduced graphene oxide (rgo) from graphene oxide (GO) using strong oxidizing agents. 214 Electrochemistry, 2013, 12,

5 3 Preparation of graphene, graphene oxide and reduced graphene oxide The preparation of graphene is a crucial issue. There are at present a large number of fabrication routes which can be divided into two main categories: 12,28,29 (i) Physical methods (ii) Chemical methods The physical methods include epitaxial growth of graphene on a substrate, 30 mechanical exfoliation 9,10 and chemical vapour deposition (CVD). 31 Single, free standing films are formed through mechanical exfoliation processes, while CVD results in multilayered graphene structures or even stacked graphene platelet nanofibers with micrometer in length. 29 Graphene synthesis via CVD on nickel and copper films appears to be ideally suited for applications in electrochemistry with regards to the prevalence of copious manufacturing volumes, large surface area, uniform graphene sheets. Large-area graphene films, with about 95% single-layered graphene, in the order of centimetres can be nowadays grown on copper substrates by CVD using methane as carbon source. 32,33 A further appealing character of CVD grown graphene is that it can be transferred after its growth to any type of interface. 34 It is indeed currently not possible to grow graphene directly on substrates such as glass, silicon dioxide, gold, silver or carbon. Methods have been developed to separate graphene from the nickel/copper surface and transfer it to the required substrates Typically, following its growth on copper, the graphene layer is coated with a thin polymer film such as polymethylmethacrylate (PMMA) 32 or PDMS 36 or a thermal release tape 34 to maintain mechanical stability during the transfer (Fig. 4). As seen in Fig. 4A, coating a CVD grown graphene on Cu with first a 1.4 mm thick photoresist (AZ5214) to protect the graphene, placing a flat elastomeric polydimethylsiloxane (PDMS) stamp on the top and etching the copper foil in iron chloride (1 M) reveals protected graphene stamps. The stamp can then be applied onto a substrate (in this case gold) and the photoresist is thermally released. The use of the dry transfer technique employing a commercial thermal release tape is a fast and highly reproducible alternative to fabricate graphene-modified interfaces (Fig. 4B). The tape is released by annealing the interface at 120 1C for 2 3 min and then rinsed with acetone followed by a high temperature annealing at 250 1C for 360 min to remove any remaining traces of the tape. Between 2 4 graphene sheets are normally transferred by these techniques. The high quality of the CVD grown graphene was evidenced by the small intensity of the D-band in the Raman spectrum. 34 While the transfer of CVD graphene has the advantage of using high quality material and offers a control over the number of graphene layers, the strategy implies that one has access to CVD grown graphene. 38 An alternative preparation method that is commonly used, owing to the ease of production, high yield and low cost, is based on the chemical oxidation of graphite to GO via one of the three principle methods developed by Brodie, 33 Staudenmeier, 39 and Hummers. 40 The Hummers method is View Online Electrochemistry, 2013,12,

6 Fig. 4 (A) Wet chemical transfer using a polymer (with reprint permission from ref. 36). (B) Dry transfer process of graphene onto a substrate using a thermal release tape (with reprint permission from ref. 34) probably the most widely used and involves soaking graphite into a solution of sulphuric acid and potassium permanganate (Fig. 5A). 40 Stirring or sonication of the resulting graphite oxide is then performed to exfoliate single layers of GO. The obtained GO consists of oxidized graphene sheets having the basal planes decorated mostly with epoxide and hydroxyl groups, in addition to carbonyl and carboxyl groups located preferentially at the edge. 41,42 These oxygen functions render GO layers hydrophilic and water molecules can readily intercalate into the GO interlayers. As GO is insulating for electrochemical applications, it is necessary to restore its conductivity (Fig. 3). The level of reduction depends strongly on the reducing agents and the experimental conditions employed. 43 Comparison of the high resolution C1s core level X-ray photoelectron spectroscopy (XPS) spectrum of GO and chemically reduced GO reveals the significant structural changes occurring on the surface upon reduction process (Fig. 5A). Chemically exfoliated GO contains a variety of functional groups with binding energies at 283.8, 284.7, and ev due to sp 2 hybridized carbon, C-H/C-C, C-O and C¼O species, respectively. 44 The C1s XPS spectrum of exfoliated GO after chemical reduction using hydrazine exhibits the same oxygen functionalities, but with decreased peak intensities. The C1s core level can be deconvoluted into three bands with binding energies at 284.9, 285.9, and ev due to sp 2 carbon, C-O/C-N and C¼O bonds, respectively. The presence of C-N functions is due to hydrazine incorporation during the reduction process Electrochemistry, 2013, 12,

7 A Binding energy/ev B binding energy/ev Fig. 5 (A) Preparation of GO from graphite using the Hummers method. (B) high resolution XPS C1s core level spectrum of GO before (left) and after reduction using hydrazine (right). 45 Another approach involves the intercalation of small molecules between the graphene layers in graphite and subsequent separation of the graphene sheets by ultrasound sonication The success of the ultrasonic cleavage depends on the correct choice of solvents and surfactants as well as sonication frequency, amplitude and time. As for mechanical exfoliation, the quality of the obtained graphene varies strongly with remaining graphitic impurities. 50 The graphene nanomaterials produced by these methods have a multilayerd structure resembling those of graphene platelets rather than graphene. Recently, a method producing more than 90% single-layer graphene sheets from bulk graphite has been reported. This method combines nitric acid oxidation and small ion intercalation within graphene sheets with subsequent exfoliation. 51 In conclusion, careful characterization of the graphene materials has to be performed before any electrochemical experiment as the quality of the material has important consequences on the overall electrochemical response. Before looking closely into electron transfer related issues on graphene-based electrodes, it has to be pointed out that GO has intrinsic electrochemical behaviour showing a significant reduction wave on gold from 0.6 V vs. Ag/AgCl onwards (Fig. 6), decreasing significantly after several scans. The wave is assigned to the reduction of GO into rgo following a ph dependent mechanism: 52,53 GO þ ah þ þ be! rgo þ ch 2 O Electrochemistry, 2013,12,

8 i/µa ,8 0,6 0,4 0,2 0 E/V vs. Ag/AgCl Scan 1 Scan 5 Scan 10 Fig. 6 (A) Cyclic voltammogram of an aqueous solution of GO (0.5 mg/ml) in 0.25 M NaCl, scan rate: 0.1 mv/s 1. 4 Electron transfer at graphene electrodes Heterogeneous electron transfer is the fundamental process of electrochemical reactions. Given the enormous potential of graphene as electrode material, the question of where heterogeneous electron transfer occurs and what are the parameters influencing it are of uttermost importance. The electron transfer on graphite and in particular highly ordered pyrolytic graphite (HOPG) is often used as a reference point for graphene electrochemistry. The question is whether cyclic voltammetric measurements with common redox molecules will show more rapid electron transfer on graphene functionalized electrodes or not? The answer to this question depends strongly on the type of graphene as will be illustrated in several examples in the following. One of the first electrochemical investigations of rgo modified glassy carbon (GC) was reported by Zhou et al. 54 rgo was obtained from GO by hydrazine reduction and was deposited onto GC electrodes by drop casting, resulting in film of B100 nm in thickness (about 295 nanosheets). According to graphene nomenclature, this structure is more of graphitic than graphene type. However, as seen from Fig. 7A, the charge transfer resistance on rgo/gc as determined from AC impedance spectra is much lower than of graphite and GC electrodes. Tang et al. investigated more systematically the electrochemical properties of rgo films (obtained from GO via hydrazine reduction) deposited by drop casting onto GC with respect to unmodified GC electrodes. Fig. 7B shows cyclic voltammograms recorded on rgo/gc and GC electrodes for several redox systems: rutheniumhexamine chloride ([Ru(NH 3 ) 6 ] 3þ/2þ ), potassium ferrocyanide ([Fe(CN) 6 ] 3 /4 ), Fe 3þ/2þ and dopamine. As a simple outer-sphere redox system, Ru(NH 3 ) 6 ] 3þ/2þ is relatively insensitive to the surface microstructures, most surface defects and surface oxides. It is the density of the electronic states near the formal potential of the redox system which is the most important factor affecting the reaction rate. 1 The Fe(CN) 6 ] 3 /4 redox couple is, on the other hand, surface-sensitive but 218 Electrochemistry, 2013, 12,

9 Fig. 7 (A) Nyquist plots at rgo/gc (d), graphite/gc (e) and GC electrodes (f) in 5 mm Fe(CN) 3 /4 6 containing 0.1 M KCl. The frequency range is from 1 Hz to 10 khz. Inset is the equivalent circuit (reprint with permission from ref. 54). (B) Cyclic voltammograms for four kinds of redox systems at unmodified GC (dashed line) and rgo/gc (solid line) electrodes deoxygenated with Ar: a) 1.0 mm Ru(NH 3 ) 3þ/2þ 6 in 1 M KCl, b) 1.0 mm Fe(CN) 3 /4 6 in 1 M KCl, c) 1.0 mm Fe 2þ/3þ in 0.1 M HClO 4, d) 1 mm dopamine in 1 M HClO 4. Data shown are for the second scan. Scan rate: 100 mv s 1 (reprint with permission from ref. 55). not oxide-sensitive, while Fe 3þ/2þ is both surface and oxide sensitive. The apparent electron transfer rates (k 0 ) calculated from cyclic voltammograms on rgo/gc and GC electrodes are in all cases larger on rgo/gc, indicating that that the electronic structure and the surface physicochemistry of graphene are beneficial for electron transfer. The k 0 enhancement is particularly pronounced with Fe 3þ/2þ, where the electron transfer rate is very sensitive to the presence of surface carbonyl groups on sp 2 carbon electrodes. The k 0 enhancement and the better electrochemical reversibility with dopamine are due to the involvement of an additional chemical amplification effect. The aromatic ring of dopamine interacts strongly with rgo through p p stacking interactions, being responsible for enhanced sensitivity as well as selectivity in electrochemical experiments. As we will see later, this will be of uttermost importance in the design of graphenebased electrochemical sensors. The electrochemical behavior of the aforementioned graphene-modified GC electrodes are however representative of the electrochemical behavior from numerous graphene fragments with ill-defined coverage, layer numbers and orientation, with possible interference from exposed areas of the underlying electrode. One of the first attempts to investigate the electrochemical trends observed on these ill-defined interfaces was undertaken by Papakonstantinou and co-workers. 31 Multilayered graphene flakes were grown by microwave plasma-enhanced chemical vapour deposition on heavily doped Si wafers, so that the sharp edges of the graphene sheets were easily accessible to the electrolyte solution. Cyclic voltammetry of the graphene-flake/silicon electrode in the presence of Fe(CN) 3 /4 6 exhibited smooth oxidation and reduction peaks with a peak separation of 61.5 mv at 10 mv s 1, indicting rapid electron transfer between graphene layers and the hexacyanoferrate ions. 31 Impressive electrocatalytic performance was obtained on these graphene interfaces in response to electroactive Electrochemistry, 2013,12,

10 biomolecules such as dopamine, ascorbic acid and uric acid and was attributed to the large area of edge planes that are available on the multilayer graphene flasks to allow rapid heterogeneous electron transfer. 31 Pumera and co-workers found similar results on platelet graphite nanofibres, which can be considered as graphene disks with a diameter of nm, stacked atop each other, perpendicular to the axis of the fibres to expose only edge planes on the fibre surface. 29,56 Edge plane pyrolytic graphite (EPPG), GC, cleaned graphite microparticles and cleaned multiwalled carbon nanotubes electrodes were run for comparison. The platelet graphite nanofibres showed the fastest heterogeneous electron transfer and best electrochemical performance to a variety of electroactive species such as ascorbic acid, NADH and DNA bases. The high performance was attributed to the high density of edge-plane sites. One might be tempted to suggest that the amount of edge-like sites is one of the determining factors in the electrochemical response. Indeed, it has been shown by the group of Banks that the origin of electron transfer in graphene is from its peripherical edge, as opposed to its side, where the former acts electrochemically akin to that of edge plane- and the latter to that of basal plane-like sites/defects of HOPG. 57 Fig. 8A depicts the cyclic voltammetric responses obtained at edge plane and basal plane electrodes of highly ordered pyrolytic graphite (HOPG) recorded in a solution of potassium ferrocyanide. It is clear that the edge plane pyrolytic graphite (EPPG) electrode exhibits a peak-to-peak potential separation of 67 mv, while the basal plane pyrolytic graphite (BPPG) electrode displays a peakto-peak separation of 238 mv. The response of the BPPG electrode is due to the low proportion of edge plane like sites/defects, which are the origin of the electrochemical activity. 58 The cyclic voltammetric response of a graphene modified BPPG electrode where the graphene was entirely pristine graphene platelets (27% single-layered, 48% double layered, 20% triple and 5% of 4 þ layered graphene) shows a peak-to-peak potential separation of 122 mv, slower than that of EPPG electrode, but faster than that of BPPG electrode (Fig. 8A). The difference was linked to the presence of surfactants View Online Fig. 8 (B) Cyclic voltammetric profiles recorded in 1 mm potassium ferrocyanide/1 M KCl using bare basal plane (dotted line) and edge plane (red solid line) pyrolytic graphite electrodes and 2 mg graphene (solid black line) modified BPPG electrode. Scan rate: 100 mv s 1. (B) CV recorded in 1 mm epinephrine/phosphate buffer solution (ph 7) using bare basal plane (black dotted line) and edge plane (blue solid line) pyrolytic graphite electrodes and a 2 mg graphene modified BPPG electrode (red dashed line). Scan rate: 100 mv s 1 vs. SCE. (C) Conceptual image showing the electron transfer sites on graphene (reprinted with permission from ref. 57). 220 Electrochemistry, 2013, 12,

11 in the commercial graphene, causing decreased electron transfer rates and hindrance of diffusion species to the electrode surface. 59 When epinephrine was used as the electrochemical probe, graphene modified BPPG electrode exhibits a voltammetric peak at þ 0.21 V, which is nearly identical to that observed at the EPPG electrode (Fig. 8B). This suggests that the reported electro-catalytic properties of graphene are edge plane sites, which occur at the edge of the graphene (Fig. 8C). The same team explored recently the electrochemical behaviour of pristine graphene monolayer flakes. Drop casting was used to modify EPPG and BPPG electodes. 60 Fig. 9A shows the cyclic voltammetric profiles of EPPG and BPPG electrodes recorded in potassium ferrocyanide/kcl aqueous solution. The EPPG electrode exhibits a peak-to-peak potential separation of 60 mv (at 100 mv s 1 ) due to the global high coverage of edge plane sites, while BPPG shows an enlarged peak separation of 242 mv due to its low edge plan site coverage and hence poor voltammetric activity. 61 Addition of increasing amounts of graphene nanosheets onto both interfaces changes the electrochemical behaviour rather differently. Deposition of graphene onto EPPG results in a decrease in the voltammetric peak height as well as in the electrochemical reversibility of the redox probe, as View Online Fig. 9 (A) Cyclic voltammograms of an EPPG (black line) and a BPPG (grey line) electrode in potassium ferrocyanide (1 mm) þ KCl (1 M), scan rate: 100 mv s 1. (B) Cyclic voltammogram of an EPPG (dotted line) electrode in potassium ferrocyanide (1 mm) þ KCl (1 M) with the addition of increasing amounts of graphene (solid lines, 10, 20, 30, 40 ng), scan rate: 100 mv s 1. (C) Cyclic voltammograms of an EPPG (dotted line) electrode in potassium ferrocyanide (1 mm) þ KCl (1 M) with the addition of increasing amounts of graphene (solid lines, 0.5, 1, 2 mg), scan rate: 100 mv s 1. (E) Effect of the global coverage of graphene on the electron transfer rate for an underlying electrode substrate assuming to possess fast electron transfer rate kinetics with an outer-sphere redox probe such as potassium ferrocyanide (print with permission from ref. 60). Electrochemistry, 2013,12,

12 evidenced by the increase in the peak-to-peak separation (Fig. 9B). The rate constant of the unmodified EPPG electrode was found to be cm s 1, while the addition of graphene decreased the observed rate constant to cm s 1 (10 ng) and cm s 1 (40 ng). Deposition of larger quantities of graphene induced an increase of the electrochemical reversibility and heterogeneous electron transfer rate (Fig. 9C). Banks proposed a zone diagram for an underlying electrode substrate that is assumed to possess fast electron transfer rate kinetics with an outer-sphere redox probe (Fig. 9D) for the electrochemical utilization of graphene: i) Zone I corresponds to an incomplete surface coverage of the underlying electrode with graphene, where the underlying electrode material will contribute to the overall electrochemical response. The decreased edge plane contribution of EPPG with addition of graphene results in reduced electron transfer kinetics and electrochemical activity. ii) Zone II corresponds to complete single-layer coverage and an improvement of the electrochemical response. In the case of modification of BPPG electrode with graphene, a contrasting behaviour is observed as the addition of small and larger quantities of graphene resulted both in a decreased electron transfer kinetics and even in completely blocked interfaces. While the importance for electrochemistry of the control of edges and basal planes in graphene has been understood, the electrochemistry of open and folded graphene edges has only been recently investigated (Fig. 10A). Pumera used electrochemical impedance spectroscopy to show that electrodes of stacked graphene nanofibres consisting almost entirely of edge planes (99.5% of the total surface) with open edges perform better than stacked graphene nanofibres with folded edges formed by thermal treatment of open stacked graphene nanofibres (Fig. 10B). Lower charge transfer resistance is observed for open edged stacked graphene nanofibres with a higher capacitance of 56 F/g compared to 6.6 F/g for folded edged stacked graphene nanofibres. View Online Fig. 10 (A) Schematic drawing and HR-TEM images of open and folded graphene edge nanostructures. (B) Nyquist diagrams and EIS measurements of folded (red) and open edged stacked graphene nanofibres modified electrodes: 5 mm Fe(CN) 3 /4 6 /KCl (0.1 M) (print with permission from ref. 62). 222 Electrochemistry, 2013, 12,

13 Fig. 11 (A) CV of CVD epitaxial graphene (pristine EG) and anodized pristine EG for 200 s and 500 s in Fe(CN) 3 /4 6 (1 mm)/kcl (1 M), scan rate 100 mv s 1. (B) Comparison of the CV of anodized EG (500 s), GC and BDD in Fe(CN) 3 /4 6 (1 mm)/kcl (1 M), scan rate 100 mv s 1 (with permission from ref. 63). Beside the influence of the orientation of graphene on the electro-catalytic response, other recent studies also hint that the oxygen-containing groups play a significant role. Lim at al. compared the electrochemical behaviour of pristine epitaxial CVD grown graphene with that of anodized epitaxial graphene. 63 They showed that pristine graphene exhibits a low heterogeneous transfer kinetic (Fig. 11A). Indeed, this high quality crystalline graphene is markedly different in chemical composition and structure from rgo flakes, which consist of a high density of reactive edges. As graphene has a high electron density around its edges as opposed to its centre, 60 pristine graphene, owing to its low portion of edge surface area, 16,60 shows slow heterogeneous electron transfer rates compared to graphite. 64 However with electrochemically anodized graphene, displaying higher degree of oxygen-related edge plane defects, superior electron transfer rates, surpassing those observed on pristine graphene, GC and BDD electrodes, were determined (Fig. 11B). Anodization induces formation of defects such as kinks, steps and vacancies on the edge planes of epitaxial graphene. These defects produce localized edge states, resulting in high density of electronic states near the Fermi level. This leads to increased electrochemical reactivity and explains the observation that the sample with the highest defect density on the edge planes displays the fastest electron transfer kinetics. The anodized pristine epitaxial graphene behaves thus similar to rgo coated electrodes, which is hardly surprising given the apparent chemical similarities of the two materials. 54 The defect density has in addition an important influence on non-faradic processes. In fact, the presence of ionisable hydroxyl and carboxylic functionalities contributes to an increased capacitive charging current. 63 Anodised epitaxial graphene electrodes showed also high level of performance for electrochemical impedance spectroscopy (EIS) with a strongly decreased charge transfer resistance after anodisation (Fig. 12A). 65 The electrical characteristic of the electrode changes from a metallic- to a semiconductor-like behaviour (Fig. 12B). Before anodisation, the admittance plot is independent of the applied potential at high frequencies, while after anodisation strong dispersion with applied voltage is observed, characteristic of semiconductor-like behaviour. Electrochemistry, 2013,12,

14 Fig. 12 (A) Nynquist plot of pristine graphene before and after anodisation at 2 V for 500 s. (B) Admittance plot for different potentials for pristine graphene before and after anodisation (reprinted with permission from ref. 65). Similarly, Keeley et al. found that pyrolytic carbon films, prepared by a CVD process, have sluggish electron transfer unless the carbon surface had previously been activated by a ten minute etch in oxygen plasma. 66 This creates many edge-plane-like sites on the nanocrystalline surface of the films as well as a high coverage of oxygen-containing moieties. It is believed that the interplay between the creation of edge-plane-like defects and the heavy surface functionalization is responsible for the outstanding electrochemical activity of the electrode. To aid in the understanding of electron transfer process on graphene, Li et al. recently studied the electrochemical behaviour of individual monolayer CVD graphene sheets. 67 Mechanically exfoliated graphene and CVD graphene were examined and compared with a focus on investigating the interaction between graphene and the one electron outer-sphere redox couple, ferrocenemethanol (FcMeOH). The structure of the device is based on lithographically connecting a piece of graphene to two metal leads as schematically shown in Fig. 13A. Sigmoidal voltammograms and scan-rate independent limiting steady-state currents were observed for mechanical exfoliated graphene with a geometric area 130 mm 2, characteristic of radial diffusion at ultramicroelectrodes (Fig. 13B). Peak-shaped CVs were observed on CVD graphene given the larger surface area of 0.19 mm 2 (Fig. 13C). The heterogeneous reaction rate of FcMeOH at the CVD graphene was determined as k 0 =0.042 cm/s, about 1 order of magnitude higher than on freshly prepared bulk graphite electrode with k 0 = cm/s. Mechanically exfoliated graphene shows an exceptional electrochemical reaction rate of as high as k 0 =0.5 cm/s. This enhancement is believed to be a consequence of the large intrinsic corrugations of graphene sheets that are not present on the atomically flat surface of bulk graphite. The corrugations lead to considerable curvature and strain in graphene sheets at the atomic scale, which, in turn, activate the graphene surface towards chemical reactions. Beside these reports, others start to emerge demonstrating that graphene might not provide a significant advance over existing electrode materials. 16,64 Pumera et al. has in fact shown that single-, few-, and multi-layer graphene does not exhibit a significant advantage over graphite 224 Electrochemistry, 2013, 12,

15 Fig. 13 (A) Procedure for the fabrication of monolayer graphene sheets into working electrodes. (B) CV of an exfoliated graphene electrode in FcMeOH (5.2 mm) in water/kcl (1 M) at different scan rates. (C) CV of CVD graphene electrode (reprinted with permission from ref. 67). micro-particles in terms of sensitivity, linearity and repeatability towards the electroanalytical detection of uric acid. 64 In principle, greater electrochemical activity can be introduced to the basal planes of graphene by oxidation or by doping and functionalization processes. Oxidation is typically achieved by converting many of the sp 2 bonds in graphene to sp 3 bonds through the formation of hydroxyl and epoxide groups on the basal planes, and carboxyl and carbonyl groups on the edges. 19 These groups provide electroactive sites on GO and thus increase the density of states for heterogeneous electron transfer. While oxidation of graphene is deleterious to its electrical conductivity, oxygen-containing groups and structural defects are beneficial for electrochemistry. It is believed that these defects are likely to be the major sites for rapid heterogeneous electron transfer. However, rapid electron transfer is of little use without a reasonable rate of charge transport within GO to move the electrons to and from the supporting electrode. Thus there has to be an optimal balance between the level of oxidation and reduction that can give a reasonable number of functional groups and defects for electron-transfer reactions while maintaining an appropriate high level of conductivity of GO. The processes used to oxidize graphene change simultaneously the amount of edge-plane sites and defects in GO and rgo making it difficult to separate the electrochemical effects of the functional groups from those caused by addition or removal of edgeplane-like sites and defects, known for their high electrochemical activity. Finally, the influence of the functional groups depends on the redox system Electrochemistry, 2013,12,

16 involved. Electrostatic attraction between negatively charged carboxylate groups and positively charged redox species can cause positive electron transfer effects. 68,69 Consequently, the kind of graphene used and the method of its production will critically determine the final electrochemistry. However, as a guide line, graphene with a high density of edge-plane defects, oxygen functions and of several layers seem to be the most promising material for electrochemistry. 5 Graphene for sensing Given the ease of producing graphene and its electrochemical properties, it comes as no surprise that graphene-based electrochemical sensors have started to revolutionize sensing. The most straightforward electrochemical sensors are macroelectrodes coated with graphene or rgo. Kang et al. have reported on the electrocatalytic sensing of paracetamol on rgo/gc, prepared from graphite oxide and exfoliated by rapid heating. 70 The rgo/gc electrode displayed quasi-reversible oxidation and reduction peaks with a peak-to-peak separation of just 42 mv (scan rate 50 mv s 1 ), contrasting with the response on a plain GC electrode, where irreversible redox peaks with peak-to-peak separation of 297 mv are seen with less than one-fifth of peak current for a 20 mm solution of paracetamol (Fig. 14A). The excellent electrochemical response of rgo/gc electrode was attributed to defect sites in GO, accelerating the electrochemical oxidation of paracetamol and to the p p stacking between the aromatic ring of paracetamol and GO, resulting in a detection limit of 32 nm. Another notable example is the electrochemical sensing of cytochrome c. 71 Alwarappan et al. coated a GC electrode with rgo (prepared from GO by hydrazine reduction). The interface showed well-defined redox behaviour for cytochrome c with a peak-to-peak separation of 70 mv (scan rate 20 mv s 1 ), while no peak occurred when only a GC electrode was used (Fig. 14B). An important contribution towards understanding the electrochemical reactivity of graphene is that by Tsai et al. They compared naked basal (BPPG) and edge plane pyrolytic graphite (EPPG) electrodes with View Online Fig. 14 (A) Electrochemical sensing of paracetamol at a glassy carbon electrode (a, 100 mm) and on a rgo/gc electrode (b, 20 mm) in NH 3d H 2 0-NH 4 Cl (0.1 M), ph 9.3, scan rate: 50 mv s 1 (print with permission from ref. 70); (B) CV of 500 mm Cytochrome c in PBS (ph 7.4) at GC (a) and rgo/gc (b) electrode, scan rate: 20 mv s 1. (print with permission from ref. 71). 226 Electrochemistry, 2013, 12,

17 Fig. 15 (A) CV recorded on bare BPPG (a), rgo/bppg (b), bare EPPG (c), and rgo/eggp (d) electrodes in NADH (2 mm)/pbs (0.1 M, ph 7) (print with permission from ref. 72). (B) CV of GC electrode modified with chitosan (a), glucose oxidase-chitosan (b), GO-chitosan (c), and glucose oxidase-go-chitosan (d) films in PBS with N 2 saturation, scan rate: 100 mv s 1 (print with permission from ref. 75). rgo/bppg and rgo/eppg for the electrocatalytic oxidation of hydrogen peroxide (H 2 O 2 ) and NADH (Fig. 15A). 72 They found that the application of rgo on the electrode surface has the ability to lower the electrooxidation potentials of hydrogen peroxide and NADH in comparison to bare basal and edge plane pyrolytic graphite electrodes. Graphene modified electrodes have also been found ideal for the realization of direct electrochemistry of redox enzyme linked to the electrode. Direct electrochemistry of enzymes refers to the direct communication between the electrode and the active center of the enzyme without the participation of mediators or other reagents. 73 This field of research is highly active for the development of biosensors and biofuel cells. The realization of direct electrochemistry of redox enzymes on common electrodes is very difficult as the active center of most redox enzymes is located deeply in the hydrophobic cavity of the molecules. Shan et al. reported the first graphene-based glucose biosensor with graphene/polyethyleneiminefunctionalized ionic liquid/glucose oxidase nanocomposites modified electrode which exhibits a wide linear glucose response (2 14 mm). 74 Zhou et al. reported a glucose sensor based on chemically reduced GO modified glassy carbon electode (GC/rGO/glucose oxidase) with a linear range of mm and a detection limit of 2 mm. In addition, the response of the GC/ rgo/glucose oxidase electrode to glucose is very fast (about 9s) and highly stable, making this electrode a fast and highly stable biosensor to continuously measure the plasma glucose level for the diagnostic of diabetes. 54 Kang et al. employed biocompatible chitosan to disperse graphene and construct a glucose sensor of long term stability and excellent sensitivity (37 ma mm 1 cm 1 ). 75 A glucose oxidas-chitosan-graphene modified GC shows a redox wave with a standard potential of vs. Ag/AgCl, close to that of FAD/FADH 2. FAD is part of the glucose oxidase molecule and is known to undergo a redox reaction where two protons and two electrons are exchanged. The CV in Fig. 15B suggests that the GOD molecules retain their bioactivity after adsorption on the graphene-chitosan sheets and that the electrochemistry response of GOD is due to the redox reaction of FAD. Electrochemistry, 2013,12,

18 Graphene-based electrodes have shown to be of high importance for the selective and sensitive detection of dopamine (DA). Dopamine, an important neurotransmitter, plays significant role in the central nervous system, cardiovascular, renal and hormonal systems as well as in drug addiction and Parkinson s disease. The quantification of DA becomes more and more important in clinical tests, serum and urine. DA often coexists with high concentration of ascorbic acid ( times) in biological samples, which results in poor selectivity and sensitivity for DA detection. Furthermore, ascorbic acid (AA) is oxidized at almost the same potential as DA, resulting in an overlapping voltammetric response for the oxidation of a mixture of DA and AA. The development of a simple and rapid method for the determination of DA with high selectivity and sensitivity is desirable for diagnostic applications. Alwarappan et al. reported that chemically reduced GO modified electrodes can effectively distinguish ascorbic acid from dopamine and serotonine and exhibit better sensing performance towards dopamine than single walled carbon nanotubes (Fig. 16A). 68 This was attributed to the presence of sp 2 -like planes and various edge defects on the surface of rgo. Similar results were reported by Shang et al. 31 using multilayer graphene nanoflake film electrodes, formed through microwave plasma-enhanced chemical vapor deposition, for the simultaneous discrimination of ascorbic acid, dopamine and uric acid with a detection limit of 0.17 mm for dopamine (Fig. 16B). Our group has recently demonstrated the possibility of analysing L-dopa and carbidopa, two important catecholamines found in pharmaceutical products, separately and simultaneously by differential pulse voltammetry View Online Fig. 16 (A) CV of 2.5 mm dopamine on (a) rgo/gc, (b) single walled carbon nanotubes modified GC electrode (reprint permission from ref. 68). (B) CV of graphene/gc (left) and bare GC electrode (right) in a solution of PBS 50 mm (ph 7) with 1 mm ascorbic acid, 0.1 mm dopamine and 0.1 mm uric acid (reprint with permission from ref. 31). 228 Electrochemistry, 2013, 12,

19 A i/ A (a) L-dopa carbidopa 0 0,2 0 0,2 0,4 0,6 0,8 1 E/V vs. Ag/AgCl utilizing chemically rgo modified GC interfaces. 45 The detection limit was about two times lower for L-dopa than carbidopa being 0.8 mm and 1.8 mm, respectively. In addition, the presence of L-dopa with concentrations 4 times higher than carbidopa has no influence on the voltammetric profile (Fig. 17). The possibility of 4-aminophenylboronic acid modified rgo was recently demonstrated. 76 Graphene has also found interest in electrochemical DNA sensors. Electrochemical DNA sensors offer high sensitivity, high selectivity and low cost for the detection of selected DNA sequences or mutated genes associated with human disease and promise to provide a simple, accurate and inexpensive platform of patients diagnosis. 77 Zhou et al. reported an electrochemical DNA sensor based on rgo. 54 As seen in Fig. 18A, the current signals of the four free bases of DNA (guanine, adenine, thymine and cytosine) on rgo/gc electrodes are all nicely separated, indicating that rgo/gc can simultaneously detect four free bases, neither possible on GC or graphite/gc. The accelerated electron transfer kinetics on rgo/gc is attributed as usual to the high density of edge-plane-like defective sites and oxygen-containing functional groups on rgo providing many catalytic sites for accelerated electron transfer between the electrode and the species in solution. More recently, Loh and co-workers have shown that anodized epitaxial grown graphene, consisting of oxygen-related defects, is a superior biosensing platform for the detection of nucleic acids as well as for dopamine and uric acid. 63,65 Anodized epitaxial grown graphene allows the simultaneous detection of all four DNA bases in double stranded DNA without a prehydrolysis step (Fig. 18B, left) and could also differentiate single stranded DNA from double stranded one from the relatively higher oxidation peaks for A and C and the lower energy shift of the C oxidation peak in the latter (Fig. 18B, right). 63 Such a differentiation is currently unique for anodized epitaxal grown graphene interfaces. Pristidine graphene, GC and boron doped diamond (BDD) as well as other carbon based materials suffer from problems such as narrow electrochemical potential window (except for BDD and graphene), slow electron transfer kinetics, B i/ A L-dopa (b) 50 0,2 0 0,2 0,4 0,6 0,8 1 E/V vs. Ag/AgCl carbidopa Fig. 17 Differential pulse voltammogramms of 1 mm aqueous solutions of L-dopa (black) and carbidopa (grey) recorded on rgo/gc electrode in KCl (0.1 M): (A) anodic scan, (B) cathodic scan, scan rate 50 mv s 1 (reprinted with permission from ref. 45). Electrochemistry, 2013,12,

20 Fig. 18 Differential pulse voltammetry for G (blue), A (orange), T (violet) and C (magenta) on GC (A), graphite/gc (B), and rgo/gc (C); conc: 10 mg/ml in PBS (0.1 M, ph 7) (reprint permission from ref. 54). (B) DVP profiles for pristine EG, anodized EG, GC and boron-doped diamond electrodes in 30 mg/ml double stranded DNA (left) and on anodized EG in 30 mg/ml double and single stranded DNA (supporting electrolyte 10 mm KCl/10 mm PBS, ph 7) (reprinted with permission from ref. 63). and/or high background currents, which preclude distinct detection of individual bases in intact DNA by voltammetric sensing. Graphene-based electrodes have also been employed for the detection of small molecules such as hydrogen peroxide. Hydrogen peroxide is a general enzymatic product of oxidases and a substrate of peroxidises. Therefore, it is of great importance to detect hydrogen peroxide. Currently, the key issue in the development of electrodes H 2 O 2 detection is to decrease the oxidation/reduction overpotentials. Zhou et al. studied the electrochemical behaviour of hydrogen peroxide on chemically rgo/gc electrodes and found a remarkable increase in electron transfer rate compared with graphite/gc and bare GC electrodes. 54 As seen in Fig. 19A, the onset potentials of H 2 O 2 oxidation/reduction on rgo/gc is 0.2/ 0.1 V (compared to 0.8/ 0.35 V for graphite and 0.70/ 0.25 V for GC), indicating superior electrocatalytic activity of graphene towards H 2 O 2. The high sensitivity of rgo towards H 2 O 2 has been recently used for the construction of an ultrasensitive electrochemical immunosensing platform. 78 The sensor is based on electrochemically reduced GO, which was coated with an N-acryloxysuccinimide-activated amphiphilic polymer through p-p stacking 230 Electrochemistry, 2013, 12,

21 Fig. 19 (A) Background subtracted CVs on rgo/gc (a1), graphite/gc (b1) and GC electrode (c1) in 4 mm H 2 O 2 /PBS (0.1 M, ph 7), scan rate: 50 mv s 1 (reprint with permission from ref. 78). (B) Electrochemical reduction of enzymatically produced hydroquinone (HQ) on the rgo/gc interface modified with anti-mouse IgG for the detection of mouse-igg, revealed by interaction by anti-mouse IgG-HRP (reprint with permission from ref. 78). (C) Fabrication processes of the gold nanoparticle based graphene/chitosan bionanolabel with integrated HRP-modified carcinoembryonic antigen (HRP-anti-CEA) and measurements protocol, PB=Prussian Blue (reprint with permission from ref. 79). interactions between the benzene ring tethered to the polymer and rgo. After immobilization of anti-mouse IgG on the polymer modified interface, mouse IgG was detected by using a HRP-labelled secondary antibody in an electrochemical based detection scheme employing hydroquinone (HQ) as the HRP substrate; the formed quinone (BQ) species are electrochemically reduced back to HQ (Fig. 19B). The estimated detection limit was 100 fg/ml of mouse IgG. When the molecular mass of mouse IgG (150 kda) is considered, the detection limit corresponds to 700 am, the lowest reported thus far for electrochemically based immunosensors. The high sensitivity and wide linear range of the immunosensor were attributed to the synergistic contribution of several factors: i) enhanced electrocataytic activity of electrochemically reduced GO owing to its large surface area, high electrical conductivity and high density of edge-plane-like defects, which allow a rapid heterogeneous electron transfer and a decrease of the reduction potential of hydroquinone while increasing the redox current; ii) minimized background current level due to reduced nonspecific protein adsorption due to the multiple PEG groups present in the polymer. Zhong and co-workers have demonstrated how graphene can be utilized within an immunosensor (Fig. 19C). 79 A highly sensitive electrochemical immunosensor that was designed to quantify carcinoembryonic antigens using nanogold-wrapped graphene nanocomposites as trace labels has been Electrochemistry, 2013,12,

22 constructed. The device consisted of a GC electrode coated with Prussian Blue (PB), on whose surface the graphene nanocomposites were electrochemically deposited and then further modified with the specific analytecapturing molecule, the anti-cea antibodies. The results indicated that the method showed high signal amplification and exhibited a dynamic working range of ng/ml with a detection limit of 0.01 ng/ml. Recently, environmental problems have also been approached using graphene-based electrochemical sensors in particular the detection of heavy metals. 80,81 Graphene nanosheets dispersed in Nafion solution were used in combination with an in situ plated bismuth film electrode for the sensing of lead and cadmium via differential pulse voltammetry (Fig. 20A). The composite electrode not only exhibited improved sensitivity for metal ion detection, but also alleviated the interferences due to the synergistic effects of graphene nanosheets and Nafion. The detection limit was estimated to be 0.02 mg/l for both cations. The analytical performance of the Nafion film coated GC electrode and that of Nafion graphene coated GC both plated with a bismuth film is seen in Fig. 20B. Sharper and higher peak currents for the target metal ions were seen on the Nafion graphene interfaces. The improvement was explained by the rough and stratified structures of the graphene composite allowing a more effective area for the nucleation of bismuth. Impedance changes of the electrode surfaces were conducted and it was found that when compared with that of pure Nafion modified electrodes, the electron-transfer resistance of the graphene/nafion modified electrode was greatly reduced, suggesting the interfusion of the graphene into the Nafion film. The described examples for the use of graphene for biosensing are only a handful of examples reported in the literature. The development of composite materials involving the addition of biopolymers, ionic liquids and metallic particles is advancing fast and these graphene composite materials show in many cases even better electrochemical characteristics. For example, Sahn et al. 82 introduced gold nanoparticles onto poly(n-vinyl-2- pyrrolidone) stabilized rgo by redcuing HAuCl 4 in a suspension of rgo View Online Fig. 20 (A) CV of bare (solid line), Nafion (dashed line) and Nafion-graphene (dotted line) modified GC in Fe(CN) 3þ 2 (1 mm)/kcl (1 M, scan rate 50 mv s 1. (B) Differential pulse anodic stripping voltammetry for 20 mg/l each of Cd 2þ and Pb 2þ on an in situ plated Nafiongraphene-BFE and Nafion-BFE in solution containing 1 mg/l Bi 3þ (reprint with permission from ref. 80). 232 Electrochemistry, 2013, 12,

23 Fig. 21 (A) UV/Vis absorption spectra of (a) GO, (b) PVP-protected rgo and (c) rgo-gold NPs in water. (B) CV of (a) graphene/au NPs/chitosan, (b) Au NPs/chitosan, (c) graphene/ chitosan, and (d) chitosan-modified electrodes in N 2 saturated PBS (0.05 M, ph 7.4) containing 2.5 mm H 2 O 2, and graphene/aunps/chitosan-modified electrode (e) in N 2 saturated PBS, scan rate 0.05 V s 1. The inset is the CV of graphene/chitosan (blue) and graphene/au NPs/ chitosan-modified electrodes (red) in PBS saturated with oxygen and graphene/au NPS/ chitosan-modified electrodes in PBS saturated with N 2 (black) (reprinted with permission from ref. 82). (Fig. 21A). After combing the gold-rgo with either a chitosan solution or with a chitosan-go x solution, the suspension of the composite materials was coated onto a supporting gold electrode. This interface generated larger oxidation and reduction currents for hydrogen peroxide at lower potentials than the components individually or in pairs (Fig. 21B). 6 Graphene in biofuel cells In addition to the tremendous impact of graphene in the field of sensing and biosensing, graphene has generated a huge interest for the development of biofuel cells. The interest in enzymatic biofuel cells is due to their use as an in vivo power source for implantable medical devices such as pacemakers using glucose or other carbohydrates present in the body as fuel. The current major challenges are low power densities as the active site of the enzyme is buried deeply under the protein shell and the poor stability of the enzymatic biofuel cell in comparison to conventional inorganic fuel cells. Li and co-workers advanced considerably this field by reporting on a membrane-less enzymatic biofuel cell based on graphene nanosheets. 83 Reduced graphene oxide prepared chemically from GO was employed to fabricate anode and cathode of the biofuel cell (Fig. 22A). The anode consists of a gold electrode onto which rgo with glucose oxidase (GO x ) was immobilized using silica sol-gel matrix. The cathode was constructed in the same manner except that bilirubin oxidase (BOD) was used as the enzyme instead of GO x. A maximum power density of mw at 0.38 V was obtained, being nearly twice than that of single walled carbon nanotubes based enzymatic biofuel cells, with a current density of ma cm 2 (Fig. 22B). The power output decayed only slowly and the performance of the graphene biofuel cell lasted for 7 days, which outperforms other enzyme based biofuel cells (Fig. 22B). The authors stated that the enhanced performance was due to the larger surface area of graphene (2630 m 2 g 1 )in comparison to carbon nanotubes (1315 m 2 g 1 ), 84 the greater sp 2 character Electrochemistry, 2013,12,

24 Fig. 22 (A) Graphene based membrane-less enzymatic biofuel cell components. (B) (a) Current voltage behaviours of ( ) graphene based EBFC and (7) SWCNT based EBFC with different external loads in 100 mm glucose solution, (b) power densities at different cell voltage for ( ) graphene based EBFC and (7) SWCNT based EBFC in 100 mm glucose solution, (c) stability of the assembled graphene based EBFC as a function of time. The external load in the test was 15 ko. Other conditions are the same as those in (a) and (b) (reprint permission from ref. 83). responsible for shuttling the electrons and the larger number of dislocations and electroactive functional groups present on graphene. Indeed, graphene could bind more GO x and BOD thereby catalysing the redox reaction more efficiently. 7 Graphene as energy storage devices The investigation of novel, low-cost, environmentally friendly, and highperformance energy storage systems has been under an ever increasing demand as a result of the needs of modern society and emerging ecological concerns. 85 Supercapacitors are promising for alternative energy storage devices for portable electronics (e.g. mobile phone) and hybrid cars as a result of their high pulse power supply, high amount of energy density that can be stored, long cycle life, low maintenance cost and simple principle. Actually, based on the different energy storage mechanisms, supercapacitors can be divided into two classes: 86 i) electrochemical double layer capacitors (EDLC), storing energy using the adsorption of both anions and cations ii) pseudo-capacitors, storing energy through fast surface redox reactions EDLCs are non-faradaic ultracapacitors deriving their performance from the so-called double-layer capacitance. 87 The capacitance is stored as a build-up of charge in the layers of the electrical double-layer formed at the 234 Electrochemistry, 2013, 12,

25 interface between a high-surface area electrode and an electrolyte. Porous carbon materials such as activated carbon or mesoporous carbon are generally used for EDLCs as they show high specific surface areas. 88 The low conductivity of porous carbon materials restricts however their application in high power density supercapacitors. 89 In contrast to EDLCs, pseudocapacitors store energy through a Faradaic process, involving fast and reversible redox reactions between the electrode and the electrolyte. Here, the most widely used material includes transition metal oxides and hydroxides, and conducting polymers. The problems encountered are related to the low power density that arises from the poor electrical conductivity restricting fast electron transport and the lack of material stability during the redox cycling process. Given the many exceptional properties of graphene such as high electrical conductivity, large surface area and profuse interlayer structure, graphene is considered as one of the most suitable substrate material for preparing super-capacitors. Ruoff and co-workers first exploited chemically reduced GO and achieved specific capacitances of F/g in organic and aqueous electrolytes. 90 Using hydrobromic acid rather than hydrazine for reducing GO, results in rgo with more oxygen functional groups on rgo. These groups not only promote the wettability of the rgo and thus the penetration of the aqueous electrolyte, they also introduce pseudo-capacitance effects with a maximum specific capacitance of 348 F/g in H 2 SO 4 (1 M) (Fig. 23). 91 The improvement of the specific capacity was one of the main research goals in the last years. Research was mainly focused on the development of different graphene-based electrode materials such as graphene-based hydrogels, 92,93 electrochemically 94 and chemically activated rgo, 95 or intercalated-graphene nanosheets with specific capacitance values varying between F/g depending on the method used. A highly performance electrode material based upon fibrillar polyaniline (PANI) doped with graphene oxide sheets has been reported by Wang et al. (Fig. 24A). 99 Its specific capacitance was up to 531 F/g, much higher than pure PANI (216 F/g), indicating the synergistic effect between GO and PANI. The composites are proposed to be combined through an electrostatic interaction (doping process), hydrogen bonding and p p stacking View Online Fig. 23 (A) CVs of rgo electrode at 1, 10, 50 and 100 mv s 1 in H 2 SO 4 (1 M). (B) Charge/ discharge curves of rgo electrode at current densities of 1, 0.5 and 0.2 A g 1 (reprint with permission from ref. 91). Electrochemistry, 2013,12,

26 Fig. 24 (A) Possible model proposed for GO/PANI composite. (B) Initial specific capacitance of the composites at different mass ratios (graphene oxide/aniline) (reprinted with permission from ref. 99). interactions (Fig. 24A). The morphology was found to influence dramatically. Furthermore, Hao s group investigated the effect of raw graphite material sizes and feeding ratios on the electrochemical properties of the GO-PANI composites and found that the morphology is dramatically 236 Electrochemistry, 2013, 12,

27 influenced by the different mass ratios. A specific capacitance of 746 F/g corresponding to a mass ratio of 1/200 (GO/aniline) could be obtained this way (Fig. 24B). Due to the electrochemical instability of GO, GO-PANI composites cannot take advantage of the good properties of GO. Reduced GO would be more favorable for the preparation of PANI composites. Zhao et al. fabricated graphene/pani nanofibre composites through an in situ polymerization of aniline monomer in the presence of GO under acid conditions, followed by the reduction of GO to graphene using hydrazine. 100 The composite that contained 80 wt% rgo showed the highest specific capacitance of 480 F/g at a current density of 0.1 A/g. Further work was conducted by Wang and co-workers 101 by investigating for the first time a simple three-step synthesis method of graphene/pani composite as a supercapacitance electrode with 1126 F/g specific capacitance with a retention life of 84% after 1000 cycles. The preparation process of the hybrid material consisted on in-situ polymerization/reductiondedoping/redoping process (Fig. 25). It was observed that the reduced graphene sheets are covered by nanostructured PANI granules, and this perfect surface coverage of PANI on graphene is favorable for the enhancement of the electrochemical properties of the composite material such as high specific capacitance and high retention life. In another report, using graphene-nanosheet support materials to provide active sites for the nucleation of PANI and for excellent electron transfer, Yan et al. 102 synthesized a graphene-pani composite by in situ polymerization, where graphene not only serves as a high conductive supporting View Online Fig. 25 Schematic illustration of the process for the preparation of graphene/pani hybrid material: it consists of exfoliating GO by ultrasonication forming exfoliated GO (GEO), which reacts with aniline:hcl:ammonium persulfate (APS) in 1:1:1 ratio with GEO forming product GEOP-1. The GO in GEOP-1 is chemically reduced with NaOH and at the same time undoped to form GEOP-2, which is then re-doped by HCl forming GEOP-3 (reproduced with permission from ref. 101). Electrochemistry, 2013,12,