New‐Generation Graphene from Electrochemical Approaches: Production and Applications

Extensive research suggests a bright future for the graphene market. However, for a long time there has been a huge gap between laboratory‐scale research and commercial application due to the challenging task of reproducible bulk production of high‐quality graphene at low cost. Electrochemical exfoliation of graphite has emerged as a promising wet chemical method with advantages such as upscalability, solution processability and eco‐friendliness. Recent progress in the electrochemical exfoliation of graphite and prospects for the application of exfoliated graphene, mainly in the fields of composites, electronics, energy storage and conversion are discussed.


Introduction
Graphene, a monolayer of tightly packed carbon atoms arranged in a honeycomb lattice, has led to widespread enthusiasm both in academia and industry, owing to its remarkable mechanical, electrical, thermal, and optical properties. Despite the advantages of this exciting material, it has not yet found its way into everyday life products with the major hurdles in commercialization lying in high production costs and limited scalability. [ 1 ] Various methodologies such as scotch-tape isolation, [ 2 ] epitaxial growth, [ 3 ] bottom-up synthesis from aromatic precursors, [ 4 ] and chemical vapor deposition of gaseous reagents [ 5,6 ] do not seem readily scalable because of high cost, process complexity and/or low yield. Beyond that, wet chemical approaches, including reduction of graphene oxide (GO) [ 7 ] and liquid-phase exfoliation of graphite, [ 8,9 ] may present plausible alternatives for manufacturing graphene on a large scale. However, GO is a highly oxidized and disordered form of graphene, produced by treating graphite with strong oxidizers. Subsequent reduction processes (i.e., by means of chemical, thermal or electrochemical routes) [ 7,10,11 ] that are essential for conversion into reduced graphene oxide (rGO) in order to restore the unique properties RESEARCH NEWS and the physical expansion/exfoliation towards thin fl akes have been long known, but did not attract signifi cant attention until the fi rst successful isolation of pristine graphene by micromechanical cleavage in 2004. [ 2 ] In the 1980s, electrochemical techniques were investigated to prepare GICs by the intercalation of pure sulfuric acid into graphite interlayers, [ 17 ] which led to the formation of thin GO-sheets with abundant defects as a result of intense oxidation. Accordingly, to preserve the quality of graphene materials, dilute sulfuric acid has become a better choice as it is more effective but less disruptive. [ 18,19 ] In 2011, highly ordered pyrolytic graphite (HOPG) was exfoliated in a H 2 SO 4 and KOH mixed electrolyte, leading to thin graphene layers with good quality. [ 18 ] The addition of alkaline, in order to neutralize acids, was believed to suppress the strong oxidation of the fi nal products. The thickness of the exfoliated graphene (EG) sheets was less than 3 nm (more than 65% of the sheets were thinner than 2 nm). Unfortunately, the yield of the exfoliated graphene was quite limited (5-8 wt%) and only low concentrations (0.085 mg mL −1 ) in DMF could be realized. In 2013, a higher yield of EG (≈60 wt%) with concentrations up to 1 mg mL −1 in DMF was achieved based on the variation of H 2 SO 4 concentration and/or working bias during the electrochemical exfoliation. [ 19 ] When the concentration of H 2 SO 4 was below 0.1 M , the yield of EG was signifi cantly lower. EG fl akes made from 0.1 M H 2 SO 4 exhibited a high fraction (i.e., 80%) of 1 to 3 layer graphene with a C/O ratio of 12.3. Moreover, fi eld-effect transistor (FET) device based on a typical bilayer EG was fabricated, giving a mobility of up to 233 cm 2 V −1 s −1 . More recently, by adding glycine into dilute sulfuric acid, a glycinebisulfate ionic complex was employed for anodic graphite exfoliation in which the formation of surface molecule nuclei by the polymerization of intercalated monomeric HSO 4 − and SO 4 2− ions played a key role, producing few-layer EG (2-5 layer) with a C/O ratio of 8.1. [ 20 ] The use of melamine additives in sulfuric acid generates graphene with high C/O ratio (26.2), good uniformity (over 80% are less than 3 layers), and low defect density (Id/Ig< 0.45). [ 21 ] The interplay between melamine and the basal plane of graphene facilitates the exfoliation process and protects graphene fl ake against excess oxidation.
Apparently, sulfuric acid is a suitable media for electrochemical intercalation as well as the exfoliation of graphite precursors because: 1) the ionic size of sulfate ion (0.46 nm) is similar to the graphite interlayer spacing (0.335 nm), [ 22 ] which is favorable for the intercalation process; 2) the electrolysis of sulfate ions and the co-intercalated water lead to the generation of gaseous species such as SO 2 , O 2 and H 2 . The effi cient intercalation as well as gas eruption promote the separation of graphene sheets from neighboring graphene layers. [ 23 ] Moreover, water is crucial for the overall exfoliation process, as it not only serves as a solvent to decrease the viscosity of the solution, but also produces reactive oxygen-containing species from anodic oxidation, which in turn corrode the graphite anode at grain boundaries or edges, facilitating SO 4 2− intercalation. [ 24 ] However, in acidic electrolytes, the ion intercalation is so fast that it occurs simultaneously with exfoliation at all graphite edges, in contrast to the ideal model, where graphene layers peel off layer by layer individually. In reality, most of the expanded graphite particles leave the graphite anode without the completion of full intercalation. In addition, though the oxidation extent is much less intensive than that in chemical oxidation routes, the interplay between hydrogen ions (H + ) and sulfate ions (SO 4 2− ) results in the excess oxidation of graphene at low pH. Therefore, inorganic sulfate salts are rational alternatives aiming at high-quality graphene. Several sulfate salts, including ammonium sulfate, sodium sulfate, and potassium sulfate, have been examined for the electrochemical exfoliation process. Among them, ammonium sulfate represents the best performance. Over 85% thin graphene fl akes consisting of 1-3 layers could be obtained after graphite exfoliation in 0.1 M (NH 4 ) 2 SO 4 , yielding concentration of 2.5 mg mL −1 in DMF ( Figure 1 ). The fl akes demonstrated average lateral size of 5 µm and outstanding C/O ratio of 17.2. [ 24 ] The Raman spectra further revealed that the EG

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sheets contained a low defect density (I D /I G ratio of 0.25) and a high FET mobility of 310 cm 2 V −1 s −1 was measured on a single EG sheet. More importantly, this exfoliation process could be readily scaled up, and over 16 g of high-quality EG could be produced in less than one hour.
Additionally, anionic surfactants bearing sulfate or sulfonate groups as their hydrophilic terminals were applied for graphite exfoliation. They are not only used as intercalating agents but also as stabilizers to prevent exfoliated graphene from restacking in the solution, yielding stable graphene suspensions. For instance, sodium dodecyl sulfate (SDS) anions were driven into graphite interlayer via applying an anodic potential of 1.6 V for 12 h, followed by a cathodic potential of −1 V for 2 h to achieve complete exfoliation. The produced graphene fl akes exhibited an average size of 500 nm and a thickness of 1 nm, but the strongly absorbed SDS molecules were hard to be entirely cleaned from the surface even after multiple washing steps. [ 25 ] Later on, a mixed solution containing 0.01 M SDS and 0.15 M sodium sulfate was selected for electrochemical exfoliation. In this method, ultrasound assistance was necessary to realize high-yield (54%) bilayer graphene, whereas without sonication, four-layer graphene sheets were the major product (52%). [ 26 ] Poly(sodium-4-styrenesulfonate) (PSS) is another possible candidate electrolyte. 1 mM PSS aqueous electrolyte resulted in the surface modifi cation of fl exible graphite foils with "wavy structures" instead of full exfoliation at a constant current of 300 mA for 2 h. [ 27 ] By using a constant voltage of 5 V for 4 h, successful graphite exfoliation was achieved in this electrolyte. The total yield was about 15% and monolayer graphene sheets (0.8 nm) were detected in the product. [ 28 ] Although PSS surfactant did not covalently bond to graphene, a certain amount of PSS remained after washing, which would presumably alter the electronic quality associated with pristine graphene.

Non-Aqueous Electrolytes
Non-aqueous electrolytes have caused intense interest as they avoid the presence of strong oxidizing species otherwise coming from the electrolysis of water, simplifying the feasibility to produce high-quality graphene. Graphite is generally utilized as the cathode; in consequence, the cationic reduction prevents the pristine graphitic structures from excess oxidation. Inspired by the destructive behavior of propylene carbonate (PC) in the cycling process of lithium-ion batteries, in 2011, a graphite cathode was exfoliated into few-layer graphene fl akes through the intercalation of Li + /PC complexes and subsequent sonication of the intercalated compound. Over 70% of the exfoliated graphene were less than 5 layers, but due to intense sonication more than 80% of fl akes were smaller than 2 µm. [ 31 ] Later, in 2012, a modifi ed two-stage process, which initially intercalates Li + /PC complexes followed by tetra-butyl-ammonium (TBA) cations has been invented where only mild sonication is required to obtain a graphene dispersion. Graphene sheets were tens of micrometers, and the yield ranged from 30% to 40%. Additionally, this protocol allowed for in situ electrochemical diazonium functionalization on graphene basal planes. Nevertheless, the time-consuming intercalation of TBA cations (24 h) limited the scale-up potential of this method. [ 32 ] Similarly, dimethylsulfoxide (DMSO), acetonitrile, and 1-methyl-2-pyrrolidone (NMP) have been explored as effective media to exfoliate graphite. DMSO offers a wide electrochemical window and its surface energy (43.5 mJ m −2 ) is close to that of graphite (53 mJ m −2 ), [ 33 ] which weakens the van der Waals interactions between graphene sheets and prevents the aggregation of detached graphene. Li + -DMSO solvated ions are able to intercalate into graphite layers and subsequently break down to gases (such as SO 2 ). Cathodic exfoliation in 1 M LiCl-DMSO electrolyte produces graphene with various fl ake sizes ranging from 1 to 20 µm and thickness lower than 5 nm (5% less than 0.9 nm). [ 34 ] Furthermore, the intercalation behavior of tetraalkylammonium cations with various alkyl chains has been studied in NMP. This process yields few-layer graphene (2-5 layers via tetramethyl-ammonium cation intercalation) with only 3% increase of oxygen content compared to graphite precursor. However, an additional sonication step is necessary to reach thorough exfoliation. [ 35 ] Notably, the aforementioned methods are limited to the use of high-boiling solvents, which are hard to remove in order to obtain "clean" graphene. Consequently, low-boiling electrolytes, such as acetonitrile, have become popular alternatives. By electrochemical co-insertion of perchlorate anions and acetonitrile molecules, graphite was intercalated and partially expanded at a voltage of +5 V for 30 min, then microwave irradiation was applied to complete the exfoliation process ( Figure 2 ), yielding 61% graphene by recycling the sediment. Notably, 69% of the graphene fl akes were bilayers and 28% of them were single layers with mean lateral dimensions of 1-2 µm. [ 36 ] Ionic liquids (ILs) have been proposed as the alternatives to conventional organic electrolytes. Previous studies have revealed that a small amount of water in imidazolium-based ionic liquids is crucial for effi cient exfoliation. The products can be tuned from IL-functionalized graphene to oxidized carbon materials by varying water contents. [ 37,38 ] In contrast to traditional water-based systems, ILs/acetonitrile electrolyte (1:50 IL/acetonitrile volume ratio) exhibits much higher efficiency (86% in 4 h) towards few-layered graphene. [ 39 ] However, the electrochemical reactions of IL molecules on the graphite surface will alter the properties of graphene, and post-treatment techniques are required to remove the functional groups.
In general, the intercalation process in non-aqueous systems requires several hours and the exfoliated materials are mainly Adv. Mater. 2016, 28, 6213-6221 www.advmat.de www.MaterialsViews.com Figure 2. Multiple steps for graphite exfoliation (from left to right: electrochemical intercalation of perchorate anions, gas release by microwave irradiation, electrochemical reduction by negative potential). Reproduced with permission. [ 36 ] Copyright 2014, Wiley-VCH.

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few-layered graphene ( Table 2 ). Note that, although exfoliation in non-aqueous electrolytes could avoid excess oxidation, the electronic properties of such graphene have not been studied, possibly owing to the multilayer structure and/or the limited lateral dimensions of fl akes.

The Application of Exfoliated Graphene
As explained above, the electrochemical exfoliation approach allows for the production of graphene with variable properties. [ 15 ] By changing basic parameters such as the graphite sources [ 40 ] or the intercalation-and exfoliation-potentials and durations, [ 41 ] the quality and properties of the produced graphene can be tailored to meet the demands of various applications.
Besides the use of graphene in sensors and composites, the most popular area in the literature so far has been energy storage and conversion ( Figure 3 ). In particular, the use of electrochemically exfoliated graphene (EEG) in supercapacitors has been widely explored (Figure 3 a, Figure 3 b). [ 24,26,42 ] However, the surface area of graphene decreases rapidly with an increasing number of layers and therefore the combination of EEG sheets with other electrochemically active materials like sulfonated polyether ether ketone (PEEK), [ 43 ] polyaniline, [ 44 ] poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), [ 45 ] carbon nanotubes, [ 46 ] or MnO 2 [ 47 ] have been demonstrated recently that can increase performance and/ or avoid restacking of graphene sheets. As an alternative to these approaches pristine graphene has been used to create 3D porous structures such as aerogels [ 48 ] and foams. [ 36,49 ] Furthermore, electrochemical exfoliation has been facilitated as a surface treatment for graphitic fi brous networks and graphite foils; here the EEG sheets are only partially exfoliated whereas the graphene source itself remains structurally intact, providing support for the exfoliated material. [ 27,50 ] Another promising fi eld is the use of graphene as an anode material in lithium ion batteries. Generally, a trend of combining graphene with other active materials is seen here too, which are typically deposited onto the EEG surface in situ during the electrochemical exfoliation process. It was shown that adding an ionic liquid to the aqueous exfoliation bath produced a functionalized EEG, which ultimately led to a threefold increase in the specifi c capacity of the graphene based anode, compared to pristine graphite. [ 51 ] Electrolysis has also been used to intercalate and polymerize acrylonitrile molecules, forming polyacrylonitrile (PAN)/EEG composite anode materials that showed initial discharge capacities above 2000 mAh g −1 and reversible capacities of around 300 mAh g −1 . [ 52 ] A one-step electrochemical preparation of graphene-based nanocomposites with Fe, Co and V oxides for lithium storage was presented and specifi c capacitances as high as 894 mAh g −1 for an Fe 2 O 3 -loaded anode have been achieved. [ 53 ] Recently, we investigated nanohybrids based on graphene/polyaniline sandwichstructures with different inorganic nanoparticles and were able to demonstrate stable reversible capacities of more than 1300 mAh g −1 in combination with Si-NPs (Figure 3 c). [ 54 ] For use as a cathode material sulfur has been deposited onto graphene in situ during electrochemical exfoliation to generate a material with a capacity of 900 mAh g −1 after 60 cycles. [ 55 ] EEG has also proven to be useful as an additive to LiFePO 4based cathodes, boosting its capacity to about 22% above the theoretical value. [ 56 ] In addition to energy storage applications, EEG has been used in the fi eld of energy conversion and it was shown here that graphene can be employed as a catalyst carrier for highly active silver nanoparticles with an onset potential of Adv. Mater. 2016, 28, 6213-6221 www.advmat.de www.MaterialsViews.com

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+0.05 V in the oxygen reduction reaction (ORR). [ 57 ] If the exfoliation conditions are adjusted properly nitrogen doping can be introduced into the EEG, which then can act as an ORR-catalyst and the onset potential of -0.2 V has been achieved with this method. [ 58 ] Other examples such as methanol oxidation on graphene/Pt catalysts [ 59 ] and benzaldehyde reduction on Au-loaded N-doped EEG [ 60 ] have also been presented to demonstrate the versatility of the EG materials. The outstanding electrical conductivity is one of the most important features of graphene that caught the particular attention of the scientifi c community. In this respect, EEG has been applied in various related applications such as easyto-prepare graphene papers [ 49,61,62 ] as well as the more sophisticated transparent conductive (TC) fi lms. In 2013 a simple spray-coating process was used to fabricate transparent electrodes with a sheet resistance of around 10 kΩ ᮀ −1 and 60% transmittance [ 63 ] and just recently a self-assembled graphene fi lm with a resistance of 21.8 kΩ ᮀ −1 at over 95% transparency has been reported; this value was further improved to 13.5 kΩ ᮀ −1 by doping with AuCl 3 . [ 21 ] In another work sheet resistances down to 210 Ω ᮀ −1 have been achieved after thermal annealing at 450 °C and simple treatment in HNO 3 with transparencies of 96%. [ 18 ] With a treatment at only 200 °C, already suitable for some polymer substrates, we yielded conductive fi lms on PET substrate with 85% transparency and sheet resistances of 500 Ω ᮀ −1 that have been successfully patterned and shown their usability in a fl exible OFET-device. [ 19 ] For large area fabrication the combination of graphene and conductive polymers like PEDOT:PSS is a promising approach. By spray coating such a hybrid ink we were able to fabricate electrodes with R s of 1200 and 500 Ω ᮀ −1 with 90 and 80% transmittance, respectively. [ 64 ] Taking advantage of their excellent mechanical properties, these fi lms have been successfully integrated into ultrathin organic photodetectors that performed comparably to state-of-the-art Si-based inorganic photodetectors (Figure 3 d). [ 64 ] On the other hand, EEG with a comparably high oxygenated degree may not be the perfect choice for transparent conductive fi lms, but deposited onto ITO substrates they have shown promising performance as sensors for nucleic acids with low detection limits and high sensitivity, reusability, and stability. [ 65 ] Apart from the application of EEG in conductive fi lms, EEG has been recently demonstrated as a stable saturable absorber and mode-locker for fi ber lasers. [ 66 ] Except from thin fi lm applications, graphene can also be incorporated into three-dimensional composites. In 2008 Liu et al. presented the integration of EEG into a bulk polystyrene matrix, proving a low percolation threshold of about 0.1 vol% and a rapid increase in the conductivity of up to 5.77 S m −1 at only 0.38 vol% loading. [ 38 ] EEG-based nanocomposite with polyvinyl butyral (PVB) has been reported exhibiting an electrical conductivity of 3.3 x10 −3 S m −1 for the graphene loading fraction of 0.46 vol%. [ 21 ] Recently Ryu et al. showed that conductivities of up to about 420 S m −1 are possible in poly(methyl methacrylate) (PMMA) with graphene loadings as low as 5.8 vol%. [ 67 ] EEG has also proven itself a noteworthy additive to non-polymer matrices. As a fi ller in Sn-based anticorrosion coatings it was able to signifi cantly increase the corrosion resistance by reduction of corrosion current and corrosion rate, and increase in polarization resistance. [ 68 ] Another example is a composite of EEG and Y 2 O 3 quantum dots that showed good carbon dioxide Adv. Mater. 2016, 28, 6213-6221 www.advmat.de www.MaterialsViews.com Figure 3. EEG has served as platform for various functional devices. a) Reproduced with permission. [ 24 ] Copyright 2014, the American Chemical Society. b) Reproduced with permission. [ 44 ] Copyright 2015, Wiley-VCH. c) Reproduced with permission. [ 54 ] Copyright 2015, the American Chemical Society. d) Reproduced with permission. [ 64 ] Copyright 2015, Wiley-VCH.

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sensing response and stability at room temperature, [ 69 ] whereas a composite with iridium oxide produced via electrolytic codeposition showed an improved effect on the cell viability in neural stimulation electrodes and superior performance compared to pure iridium oxide or GO fi lms. [ 70 ]

Summary and Outlook
As a new generation of graphene materials, exfoliated graphene from electrochemical approaches suggests great prospects towards high-performance devices. Signifi cant efforts have been devoted to the further development of electrochemical exfoliation process, aiming at low production cost, high product purity and high output yield of graphene materials. However, several issues remain be addressed for future studies, as detailed in the following sections.

Design of Electrolytes
It has been demonstrated that the quality of graphene is strongly dependent on the selection of electrolytes. The ideal electrolyte should induce effective intercalation of guest ions and subsequently separate the graphene layers without causing any destruction of the graphitic morphology, and also work as dispersants to stabilize the exfoliated graphene fl akes, avoiding re-aggregation. In reality, the anodic exfoliation in aqueous electrolytes is favorable for thin-layer graphene (1-3 layers) formation, but always causes oxidation of the EEG. Very recently, we reported that the addition of scavengers into an aqueous electrolyte could partially alleviate the serious oxidation caused by water electrolysis. [ 71 ] As an alternative, cationic intercalation in non-aqueous media is also able to avoid excess oxidation, however, in most cases, few-layer graphene is the major product. Therefore an electrolyte, which could introduce gaseous species within graphite interlayers during cationic reduction process, might be useful to achieve non-oxidative thin graphene fl akes.

Understanding of the Exfoliation Mechanism
Owing to the high complexity of electrochemical reactions, the exfoliation process at the graphite interface is not trivial. Essentially, successful exfoliation involves three stages: intercalation of anions/cations, expansion of graphite layers, and exfoliation at the graphite surface. By monitoring the structural deformation at the graphite interface, Palermo et al. have proven that effi cient intercalation and bubble formation inside graphite interlayers are important factors governing fast and complete exfoliation. [ 23 ] In practice, exfoliation begins at any part of graphite and these stages do not occur in order, which is the main reason for the wide distribution of graphene layers. There seems to be a trade-off between the effi ciency of exfoliation and the preservation of graphene quality. Therefore, an in-depth understanding of the fundamental mechanisms is crucial for optimization of exfoliation processes to produce graphene with homogeneous size and thickness distribution.

Optimization of Exfoliation Conditions
In addition to electrolytes, graphite sources also show great infl uence on the oxygen content and defect density of exfoliated graphene. [ 40 ] A wide range of graphite precursors, such as HOPG, expanded graphite, natural graphite fl akes, graphite powder, graphite foil, and graphite intercalated compounds, have been explored. According to the needs in diverse electrochemical cells, rational selection of starting graphite suggests a considerable strategy in pursuit of high-quality nanosheets. Moreover, a recent study reveals that the variation of applied potential on graphite could be another important factor with which to control the exfoliation yield, the numbers of graphene layer, as well as the graphene quality. [ 41 ]

Equipment Engineering
To build a bridge between laboratory-scale tests and industrial production, rational design of electrochemical cells is important. In particular, realizing continuous production and separation of EEG, recycling of the electrolyte, and removal of un-exfoliated impurities are considerable factors for the scaleup process. As an example, Kinloch et al. showed a conceptual cell design for a continuous process in an organic electrolyte, where a consumable graphite cathode was placed in the bottom of the cell, while the reference electrode was inserted from the top. This setup allows for continuous production of few-layer graphene with a rate of 0.5-2 g per hour. [ 34 ] Exfoliated graphene with a tunable quality has already demonstrated applications in hybrid composites, conductive papers, and devices for energy storage and conversion, but also indicates a wide spectrum of bright perspectives, including, but not limited to, integrated circuits, optical modulators as well as high-performance, lightweight electronics. As a consequence, electrochemical exfoliation of graphite is expected to overcome the big gap between materials science and practical applications.