Graphene based 2D-materials for supercapacitors

Ever-increasing energy demands are compelling humanity toward the development of suitable electrochemical energy conversion and storage devices with high energy density and power density to provide clean energy with zero emissions. Currently, many electrochemical devices have been established as energy conversion and storage devices, including supercapacitors, lithium-ion batteries (LIBs) [1], metal–air cells [2], fuel cells [3] and solar cells [4]. Among these, energy storage devices, in particular supercapacitors, have drawn a great deal of attention due to their high charge–discharge characteristics and high power density with cycling ability. Based on their storage mechanism, supercapacitors are categorized into two types: electrochemical double layer capacitors (EDLCs) and pseudo-capacitors. EDLCs can store energy by reversible ion adsorption at the electrode–electrolyte interface, whereas pseudo-capacitors store energy through a redox reaction in the vicinity of the electrode surface. In fact, all carbon-based materials with a high surface area, including activated carbon (ac), carbon nanotubes (CNTs), graphene, etc, are employed as electrode materials for EDLCs, while transition metal oxides (e.g., RuO₂, MnO₂, CoO_x, NiO, Fe₂O₃, etc) and conducting polymers (e.g., polypyrrole (PPY), polyaniline (PANI), poly(3, 4-ethylenedioxythiophene) (PEDOT), etc) are employed as electrode materials for pseudo-capacitors. LIBs are regarded as a potential competitor for supercapacitors due to their high energy density. In a few cases, supercapacitors were unable to meet the excessive energy required in several energy devices due to their lower energy density compared LIBs. Hence recently, supercapacitors have been hybridized with LIBs to provide the excessive power required in such devices. However, their high power density with longer cycle lives still make supercapacitors superior to LIBs in some cases.

In both energy conversion and storage devices, nanomaterials play a vital role in bringing these devices toward practical applications. Recently, much attention has been paid to the development of nanomaterials, which are expected to suggest the design and fabrication of effective electrodes for the above-mentioned major energy conversion and storage (electrochemical capacitor (EC)) devices. However, detailed studies in this field have clearly revealed that the intrinsic low surface area associated with the low electrical conductivity of the nanomaterials limits their performance. Hence, suitable nanoscale engineered materials with unique morphologies and higher conductivities are necessary to tune the performance characteristics. Carbonaceous materials such as carbon black (CB), CNTs, carbon nanofibers, two-dimensional (2D) graphene, etc, have been employed as good support materials for various metal nanoparticles, which should enhance the catalytic activity. The scientific community has been strongly motivated to improve the energy and power densities of these energy storage (i.e. EC) devices by developing novel electrode materials with low cost and environmentally benign production, electrolytes and inventive device designs. To accomplish the desired advancements in these storage devices, a strong fundamental understanding is needed of the charge storage mechanisms, the transport pathways of electrons and ions, and the electrochemically active sites. The revolution in the discovery of nanomaterials, potential candidates for facile ion adsorption and faster surface reactions, has encouraged extensive research efforts toward the enhancement of electrochemically active sites for charge transfer, and controlled ionic and electronic transport at small diffusion length scales.

Undoubtedly, the discovery of graphene has enabled great advances in the field of materials chemistry. Graphene's inherent high surface area [5], high electrical conductivity, mechanical properties, chemical stability, high thermal conductivity (5000 W m⁻¹ K⁻¹) [6], high optical transmittance (97.7%) [7] and intrinsic carrier mobility (200 000 cm² V⁻¹ s⁻¹) [8] are among the key prerequisites for energy storage devices. Due to these remarkable properties, graphene-based composites have been endorsed for high-power and high-energy supercapacitors to overcome the limitations facing current supercapacitors due to the electrode kinetics associated with poor ion transport in the confined ultramicropores of carbon nanomaterials. Being similar to 2D graphene, other 2D materials including transition metal double layer hydroxides (DLHs), chalcogenides and carbides have been identified as potential materials for supercapacitor applications. Of course, several reviews have covered the same context [9], but this review focuses on a few different important aspects, including the use of different dopants in graphene, the activation of graphene, the composition of flexible devices with other hybrids and the use of graphene with other 2D analogues for supercapacitors. Herein, we have summarized the recent progress in graphene-based composites for supercapacitors including reduced graphene oxide (RGO), activated graphene, doped graphene, graphene/metal oxide composites and graphene/polymer composites. In addition, we have briefly addressed a few aspects in the development of high volumetric supercapacitors and next-generation 2D layered materials as supercapacitor electrodes, while their advantages and limitations are outlined according to recent reports in the literature.

As discussed above, graphene has been effectively utilized as an electrode material due to its unique physical and chemical properties. Rouff et al [10] initiated the direct exploitation of graphite into graphene oxide (GO), with some chemical modifications, for supercapacitors. The resulting specific capacitances of graphene were 135 and 99 F g⁻¹ in aqueous and organic electrolytes, respectively. The authors suggested that their chemically modified graphene, with good electrical conductivity and a high surface area, would be a promising candidate for EDLC ultracapacitors. Ultracapacitors based on their graphene could have a cost and performance that would allow their adoption in a wide range of energy storage applications. Since then, several research groups have explored the direct exploitation of graphene and its composites in supercapacitors. Rao et al employed graphene as a potential electrode for supercapacitors [11]. They prepared graphene using three independent methods and the capacitive nature of the graphene was correlated using laboratory-made single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs). The graphene derived from the exfoliation of graphite using an acid mixture followed by thermal reduction exhibited an electrical double layer capacitance of 117 F g⁻¹. The capacitance exhibited by the exfoliated graphene was relatively higher than that for the other graphenes derived from nanodiamond (35 F g⁻¹) and camphor (6 F g⁻¹). Their study was extended by utilizing an ionic liquid (i.e., N-butyl-N-methylpyrrolidinium bis (trifluoromethanesulfonyl)imide) as a electrolyte to increase the potential window. In the ionic liquid, the active material (exfoliated graphite) exhibited a specific capacitance of 75 F g⁻¹ with an energy density of 39 W h kg⁻¹. Their study revealed that the supercapacitive nature of the graphene sheets is directly related to the quality of the graphene sheets with a higher surface area. Although the specific capacitance value was low in their study, it could be increased by tuning or improving the quality of the graphene sheets.

In the same context, an environmentally friendly approach for the reduction of GO using urea was introduced and the resulting RGO was employed as an efficient electrode for supercapacitors (figure 1). In this case, urea was recognized as a reducing agent which extensively reduces the oxygen functional groups on the basal plane and at the edges of the GO. The resulting RGO exhibited specific capacitances of 255 and 100 F g⁻¹ at current densities of 0.5 and 30 A g⁻¹, respectively, with a noticeable retention in capacity of 93% [12].

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Moreover, the specific capacitance they obtained was relatively high compared to the RGO obtained through hydrazine reduction and hydrothermal processes, owing to a porous network and a partially restored π-conjugation structure. On the other hand, thermally RGO exhibited the maximum capacitance (260.5 F g⁻¹), which varied with respect to the reduction temperature. The maximum capacitance was displayed by the sample reduced at 200 °C, demonstrating that higher capacitance has a direct relation with the oxygen functional groups present on the graphene sheets [13]. Hence, it is clear that oxygen functional groups play a definite role in enhancing specific capacitances. The active graphene sheets were then prepared by subjecting the graphene to chemical activation to enrich the oxygen functional groups and defects, thereby increasing the surface area of the graphene sheets. Accordingly, Rouff et al [14] prepared a high surface area for graphene by simple chemical activation of RGO. The resulting porous graphene exhibited a Brunauer–Emmett–Teller (BET) surface area of 3100 m² g⁻¹ with high electrical conductivity and a low content of oxygen and hydrogen. Consequently, an energy density of 70 W h kg⁻¹ was achieved at a current density of 5.7 A g⁻¹ and a working voltage of 3.5 V using an organic electrolyte. Similarly, another RGO enriched by KOH activation lead to a high surface area and exhibited a higher capacitance up to 180 F g⁻¹ with a wide electrochemical window up to 3.5 V over the temperature range from –50 to 80 °C [15]. In a few cases, electrochemical techniques were employed to prepare graphene sheets with controllable size and thickness [16]. The resulting graphene films showed a specific capacitance of 128 F g⁻¹ with an energy density of 17.8 W h kg⁻¹ operating within a potential window of 1.0 V in 1.0 M NaNO₃. In this study, the graphene-based supercapacitor was shown to be stable as it retained 86% of the original specific capacitance after 3500 charge–discharge cycles.

Xu et al [17] also constructed graphene self-assembled macrostructures with three-dimensional (3D) networks using a hydrothermal method. The self-assembled graphene hydrogel (GH) exhibited high electrical conductivity and mechanical stability. Interestingly, the self-assembled GH showed specific capacitances of 175 and 152 F g⁻¹ at potential scan rates of 10 and 20 mV s⁻¹, respectively, which were 50% superior to those of supercapacitors based on RGO agglomerate particles tested under similar experimental conditions (100 F g⁻¹) [18]. The methodology adopted here afforded a deeper understanding of the self-assembly behavior of functionalized graphene as a 2D molecular building block and stimulated the fabrication of similar novel electrodes using hierarchical and functional materials based on graphene. In an extension, Dai et al [19] prepared polymer modified graphene sheets via in situ reduction of exfoliated GO in the presence of cationic poly(ethyleneimine) (PEI). Subsequently, the resulting PEI-modified graphene sheets were used for sequential self-assembly with acid-oxidized MWCNTs resulting in hybrid carbon films. These hybrid films exhibited an average specific capacitance of 120 F g⁻¹ with a scan rate of 1 V s⁻¹.

In many cases, pristine graphene appears to be inappropriate for use as an efficient supercapacitor electrode due to the inaccessibility of ions on the active surface due to the random orientation, large aggregation and hydrophobicity of graphene sheets. Unfortunately, the intrinsic specific surface area of graphene sheets has been minimized largely due to the restacking of graphene sheets by π–π interactions, which causes a reduction in overall efficiency. Thus, researchers have aimed to inhibit the restacking of graphene sheets and extend their wider applications for supercapacitors. In fact, these limitations can be tackled by separating graphene sheets with the intercalation of foreign moieties, which enlarge the electrochemical performance of the graphene sheets by increasing their surface area and also opening new channels for facile ion transport [20, 21]. Accordingly, Zhao et al [22] demonstrated a unique way of preparing a 3D hierarchical carbon nanostructure with RGO sheets intercalated by mesoporous carbon spheres. Interestingly, the intercalation of mesoporous carbon spheres reduced the ion diffusion resistance with significant enhancement of the electrical conductivity, leading to high-rate performance of the supercapacitor with 94% capacitance retention even after 1000 charge–discharge cycles. Similarly, Zhang et al [23] studied the effect of surfactant-stabilized graphene materials in supercapacitors by intercalation of GO with different surfactants, including tetrabutylammonium hydroxide (TBAOH), cetyltrimethylammonium bromide and sodium dodecylbenzene sulfonate, followed by reduction using hydrazine. The surfactants stabilized the morphology of the graphene sheets with significant enhancement of the hydrophilicity of the graphene, and thus enhanced their performance as supercapacitor electrodes. This was evidenced by the highest specific capacitance of 194 F g⁻¹ attributed to the TBAOH-stabilized graphene at a specific current density of 1 A g⁻¹ in 2 M H₂SO₄ electrolyte. Similarly, nanostructures of 3D CNTs sandwiched between graphene sheets were effectively constructed as active electrodes for supercapacitors. The active sandwich-based composites exhibited a specific capacitance of 385 F g⁻¹ at 10 mV s⁻¹ and enhancement in the capacitance was observed after 2000 cycles due to the increased effective interfacial area.

As mentioned previously, the restacking of graphene sheets is a primary limiting factor which reduces storage capacitance. To tackle this problem, much attention has been paid to the design of high-porosity graphene electrodes, which are achieved by tuning the morphology of an individual graphene sheet into a 'crumpled' or 'curved' shape, thereby creating artificial porosity around each layer [24]. Other cases are the use of spacers such as CNTs in graphene films to control the graphene–graphene interlayer distance [25] and the preparation of 3D porous structures [26], etc. Although the above methods result in a high capacitance per weight due to the improved accessibility of electrolytes and ions to the graphene surface, these porous electrodes have a low mass density (F in g L⁻¹), which results in a low capacitance per volume (C_volume = C_weightXρ) at the electrode level. It is well known that pristine graphene is not permeable to gas species and metal ions, leading to infinite ion transport across the graphene plane which significantly hampers the ion accessibility for high-performance ultracapacitors. Recently, holey graphene-based composites have been demonstrated as efficient electrodes for supercapacitors. These holey graphene sheets with defined 2D nanoholes provide channels for easy ion transport across the graphene plane and ultimately result in the accessibility of ions to the inner electrode surface. Therefore, a higher capacitance could be achieved for ultrasupercapacitors. In this context, Duan et al [27] fabricated a 3D holey graphene framework with a hierarchical porous structure as a high-performance binder-free supercapacitor electrode (figure 2). Due to the large ion-accessible surface area, the efficient electron and ion transport pathways, and high packing density, the holey graphene framework electrode provided a gravimetric capacitance of 298 F g⁻¹ and a volumetric capacitance of 212 F cm⁻³ in an organic electrolyte. The higher capacitance was attributed to the easy access of ions with enhanced ion transport through the porous network structure of the 2D graphene surface.

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Similarly, another holey graphene was constructed using rapid heating (<500 °C) and cooling of graphene under an air atmosphere [28]. The ultracapacitor electrodes based on this holey graphene displayed an improved volumetric capacitance (54 F cm⁻³) compared to that of regular graphene electrodes with 98% retention of capacity even after 10 000 cycles. Chang et al also demonstrated a simple method for the preparation of holey graphene nanosheets (GNSs) using ultrarapid heating during the process of thermal reduction/exfoliation of graphite oxide [29]. They found that the density of the holes increases with respect to heating rate. The holey graphene-based electrodes displayed a large capacitance of 170 F g⁻¹ at 50 A g⁻¹ in a 6 M KOH aqueous electrolyte. Furthermore, in the low-rate limit, the holey graphene electrodes exhibited a large specific capacitance of 350 F g⁻¹ at 0.1 A g⁻¹ due to the shorter cross-plane ion transport paths in the graphene stack through the large number of holes in the graphene sheets.

Although pristine graphene and its nanocomposites exhibit remarkable specific capacitances, their performance still needs to be increased to realize practical applications. Recently, the effects of heteroatom doping on graphene (boron (B), nitrogen (N), phosphorus (P) and sulfur (S)) have been studied in several applications such as supercapacitors, LIBs, solar cells and fuel cells. Among these, N-doping on carbon frameworks has gained a great deal of attention for various positive reasons. It is a well-known fact that foreign N atoms in carbon-based materials can tune the basic electronic properties, mechanical stability and hydrophilicity, therefore N-doped carbon-based materials can be utilized as potential candidates in the aforementioned applications. Thus, several research groups have focused on the direct exfoliation of N-doped graphene and its composites for potential application in supercapacitors [30] and LIBs. In 2011, Choi et al prepared N-doped graphene using a simple plasma process and exploited the resultant graphene as an efficient ultracapacitor (figure 3) [31]. The N-doped graphene exhibited a capacitance of ∼280 F g⁻¹, which was nearly four times higher than their pristine graphene without compromising other essential properties including a higher cycle life (>200 000), high power capability and compatibility with flexible substrates. It was determined that N-doped sites at the edges and the basal plane were solely responsible for the improved capacitance and enhanced cycle life.

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Similarly, Feng et al [32] also found a unique strategy for fabricating highly crumpled N-doped GNSs (C-NGNSs) with a pore volume of 3.42 cm³ g⁻¹ as an efficient electrode for supercapacitors. The C-NGNSs showed a remarkable enhancement in terms of high specific capacity, excellent rate capability and cycle performance owing to their wrinkled structures, high pore volume, N-doping and improved electrical conductivity. The resultant specific capacitance was 245.9 F g⁻¹ at a current density of 1.0 A g⁻¹. Interestingly, the supercapacitors with C-NGNSs-900 maintained capacities of 96.1%, after 5000 cycles compared to their initial specific capacity, whereas their counterpart (i.e. thermally reduced graphene sheets (TRGs)) showed a capacity retention of 87.3%. This highlighted that the C-NGNSs had excellent electrochemical stability and a high degree of reversibility. A computational study of the same topic also showed that N-doping can lead to potential enhancement in electrode capacitance as a result of electronic structure modifications, while there is virtually no change in the double layer capacitance [33]. In a few cases, N has been co-doped with other heteroatoms which has produced efficient capacitors. Mullen et al [34] demonstrated a simplified prototype device for a high-performance all-solid-state supercapacitor (ASSS) based on 3D B and N co-doped monolithic graphene aerogels (BN/GAs). The resulting BN/GA-based ASSS not only showed minimized device thickness, but also showed a high specific capacitance (∼62 F g⁻¹), a good rate capability and an enhanced energy density (∼8.65 W h kg⁻¹) with a power density (∼1600 W kg⁻¹) related to un-doped, N-doped, or B-doped GAs, or layer-structured graphene paper (GP). The effect of co-doping was demonstrated in the enhanced electrochemical activity and pseudo-capacitive behavior of the graphene materials which mainly arise due to the synergistic effect between the doped N and B in the graphene sheets. Inspired by N-doped graphene, other heteroatom (B, S or P)-doped graphenes have been investigated as electrodes for supercapacitors. Interestingly, these heteroatom-doped graphene electrodes also exhibited improved capacitive behavior, similarly to N-doped graphene. To obtain further insight, the capacitive behaviors of different heteroatom-doped graphene electrodes with their energy densities and power densities have been summarized in table 1.

To date, a number of successful attempts have been undertaken in the development of efficient electrodes for supercapacitors, mainly using three different types of electrode materials, including: (1) different carbon nanostructures with a high surface area, (2) transition metal oxides and (3) conducting polymers (e.g., PANI and PPY). Among these, transition metal oxides, in particular RuO₂ and conducting polymers, have been recognized as promising potential candidates owing to their electrical double layer and redox capacitive nature. However, conducting polymers have received much more attention than RuO₂ due to the high cost and scarcity of RuO₂. The pseudo-capacitive nature of conducting polymers has been demonstrated by the fast and reversible oxidation and reduction processes related to π-conjugated polymer chains, while their electronic instability has limited their wider practical application. The poor stability of these polymers arises due to the rapid aging of the polymer during electrochemical cycling. To tackle this problem, several conducting polymer-based hybrids with different carbon nanostructures, in particular using 2D graphene, have recently been constructed as efficient supercapacitors. Interestingly, conducting polymers have a characteristic structure with aromatic rings. In the composite materials, the conducting polymers have a strong covalent interaction with carbon moieties, thus increasing the electrical conductivity and stability of the composite. Recently, several composite materials using different light-weight polymers, such as PANI, PPY, PEDOT, etc, have been widely investigated with different carbon nanostructures. Among these, conducting polymer-based composites with graphene have received a great deal of attention due to their large surface area and strong covalent bond interactions [57].

Accordingly, Manthiram et al [58] opened a distinctive way for the preparation of GNSs through the microwave-assisted solvothermal reduction of exfoliated GO with nontoxic solvents within a short reaction time of 5–15 min at a low temperatures (180–300 °C). The nanocomposites derived using these GNS with PANI (GNS/PANI) provide a specific capacity of ∼408 F g⁻¹, which is nearly four times higher than that of pristine graphene (100 F g⁻¹). The significant improvement in capacity retention on decoration with a small amount (10 wt.%) of PANI could be due to the modification of the graphene surface and pore structure by PANI (figure 4). The adapted methodology offers the possibility of combining graphene with other redox pseudo-capacitive materials such as polythiophenes, MnO₂ and RuO₂ to enhance the energy density of ECs.

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Later, Wu et al [59] investigated the preparation of chemically modified graphene by in situ polymerization of an aniline monomer with GO under acidic conditions. The resultant GO/PANI composite was further reduced using hydrazine followed by reoxidation and reprotonation of the reduced PANI to produce the desired graphene/PANI nanocomposites. The active composites exhibited a specific capacitance of 480 F g⁻¹ at a current density of 0.1 A g⁻¹. The results revealed that a high specific capacitance and good cycling stability could be achieved either by doping the chemically modified graphene with PANI or by doping the bulky PANIs with graphene/GO. Similarly, Song et al [60] also demonstrated the preparation of hierarchical nanocomposites of PANI nanowire (NW) arrays and functionalized RGO (PANI/FRGO), where the PANI NWs were covalently bonded with RGO for supercapacitors. The resultant PANI/FRGO nanocomposites displayed a high capacitance of 590 F g⁻¹ at 0.1 A g⁻¹ and demonstrated no loss of capacitance even after 200 cycles at 2 A g⁻¹. In this study, the strong covalent interaction between the FRGO and the vertically aligned PANI NW arrays was the primary reason for the improved capacitance with a remarkable cycle life compared to other graphene/PANI nanocomposites. Previously, hierarchical nanocomposites combining one-dimensional (1D)) conducting PANI NWs with 2D GO nanosheets (NSs) were demonstrated [61]. The resultant vertically aligned PANI NW arrays on GO exhibited capacitances of 555 and 227 F g⁻¹ at current densities of 0.2 and 2 A g⁻¹, respectively. Park et al [62] prepared carbon–carbon nanocomposites using the chemical reduction of graphite oxide and acid treated MWCNTs as a function of the MWCNT/graphite oxide ratio. The MWCNT-graphene/PANI nanocomposites were further prepared using oxidative polymerization. The resultant composite showed increased electrical conductivity due to π–π interactions, consequently resulting in a maximum specific capacitance of ∼1118 F g⁻¹ at a current density of 0.1 A g⁻¹, which was significantly higher than that for graphene/PANI alone. These results were attributed to the highly conductive pathway and easy diffusion of the electrolyte ions in the composites using the MWCNT nanochannel formed on the graphene surface. Similarly, graphene has also been composited with other conducting polymers for use as supercapacitors. For example, hierarchical GNS/PPY composites were fabricated using an in situ chemical oxidation polymerization method [63]. The sheet-like PPY hierarchical composite exhibited a large electrochemical capacitance (318.6 F g⁻¹) at a scan rate of 2 mV s⁻¹ with superior electrochemical rate capability and cycling stability. In 2013, Zhu et al [64] prepared a PEDOT/graphene hybrid using in situ polymerization of EDOT in the presence of GO under microwave conditions. The resulting composite affords a high specific capacitance (270 F g⁻¹ at a current density of 1 A g⁻¹) and good cycling stability (93% retention after 10 000 cycles at a high current density of 5 A g⁻¹) during the charge–discharge process.

Notably, Zhao et al [65] effectively compared the electrochemical capacitance of conducting polymers with graphene-based composites under identical conditions (figure 5). The conducting polymers PEDOT, PANI and PPY were coated on the surface of RGO sheets via an in situ polymerization process with different loadings of the conducting polymers. Among these, RGO/PANI was recognized as a potential electrode as it exhibited a specific capacitance of 361 F g⁻¹ at a current density of 0.3 A g⁻¹, which is relatively high compared to RGO/PPY (248 F g⁻¹) and RGO/PEDOT (108 F g⁻¹) at the same current density.

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As they are similar to graphene, metal oxides have also received a great deal of attention as potential effective energy storage materials. The storage mechanism of these metal oxides arises from Faradaic reactions, which help to realize their large pseudo-capacitance. Their wider application has been limited by their low conductivity and stability, which require a strong conducting medium and a stabilizing agent. The recognition of the remarkable nature of 2D graphene, particularly its high theoretical surface area and conductivity, was used to create hybrids using different metal oxides. Accordingly, several composites have been reported in the literature for potential application in graphene-based metal oxide hybrids for supercapacitors. In the resulting composites, 2D graphene plays a key role by increasing the conductivity and stability of these metal oxides. In a few cases, graphene has been doped with heteroatoms, in particular N, where the doped foreign atoms act as anchoring sites for metal oxides, thus increasing the electrochemical capacity and stability. In addition, the doped N affords more active sites and nucleation sites, which facilitate the control of the morphology and particle size of the hybrid and strengthen the metal oxide–graphene interaction [66]. Among the many metal oxides, MnO₂ and RuO₂ have been recognized as promising candidates for supercapacitors due to their high theoretical specific capacitance, low cost, environmental benignity and natural abundance. As usual, MnO₂ electrodes also suffer from poor electronic and ionic conductivities, resulting in their limited performance in terms of power density and cycling stability. To tackle these problems, Bao et al [67] constructed a 3D composite with 2D graphene and MnO₂ using a 'conductive wrapping' method to greatly improve the supercapacitor performance of graphene/MnO₂-based nanostructured electrodes (figure 6). These ternary composite electrodes exhibited a specific capacitance of ∼380 F g⁻¹ and an excellent cycling performance with >95% capacitance retention over 3000 cycles. This 3D conductive wrapping approach opens a new direction for enhancing the device performance of metal-oxide-based electrochemical supercapacitors and can be generalized for the design of next-generation high-performance energy storage devices.

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In 2014, a MnO₂/GA composite produced using electrochemical deposition was reported and the composite exhibited a high specific capacitance of 410 F g⁻¹ at 2 mV s⁻¹, which is two times higher than that for the samples prepared in the absence of the GA support [68]. The success in achieving a high capacitance for this MnO₂/graphene composite was attributed to the structural advantages of the high specific surface area, high pore volumes, large pore sizes and the 3D well-connected network of the GA support. In another work, Ren and co-workers [69] developed hydrous RuO₂/graphene composites with variation in the loadings of Ru. The active composites exhibited a capacitance of 570 F g⁻¹ at a 38.3 wt.% Ru loading with enhanced rate capability, excellent electrochemical stability (97.9% retention after 1000 cycles) and a high energy density (20.1 W h kg⁻¹). The composite electrode performed electrocapacitively much better than the counterpart RuO₂ and pristine graphene electrodes. Similar to the aforementioned metal oxides, Fe₃O₄ is also regarded as a promising electrode material due to its low cost and low environmental impact. Yan et al [70] reported a supercapacitor consisting of a Fe₃O₄/RGO composite produced using a solvothermal process which combines the growth of Fe₃O₄ nanoparticles and the reduction of GO in a single step. The resulting hybrid Fe₃O₄/RGO-nanocomposite-based thin-film supercapacitor electrodes showed a high specific capacitance of 480 F g⁻¹ at a discharge current density of 5 A g⁻¹, which was higher than those for the RGO and Fe₃O₄. Interestingly, these Fe₃O₄/RGO nanocomposites also showed stable cycling performance without any decrease in the specific capacitance after 1000 charge–discharge cycles. Chen et al [71] reported the development of 3D porous graphene/cobalt oxide electrodes for a high-performance supercapacitor. These hybrid electrodes displayed a high specific capacitance of 768 F g⁻¹ at a current density of 10 A g⁻¹. On the other hand, Mai et al [72] reported a facile methodology for the preparation of composite of nitrogen-doped reduced graphene oxide (NRGO)/Fe₂O₃ for supercapacitors. The composite delivered an excellent capacitance of 268.4 F g⁻¹ at a current density of 2 A g⁻¹. In addition to high capacitance, the composite showed an excellent rate capability and good cycle life, which were mainly due to a positive synergetic effect between the NRGO and α-Fe₂O₃. Zhao et al [73] developed a 3D nanostructure based on nickel hydroxide/graphene/CNT composites for supercapacitors, in which nickel hydroxide was embedded in a CNT acting as a pillar for graphene sheets. The charge–discharge profile for this 3D nanostructure showed a high specific capacitance of 1235 F g⁻¹ at 1 A g⁻¹, while the capacitance fade was observed to be 780 F g⁻¹ with an increase in current density (up to 20 A g⁻¹). In this study, the mass ratio of nickel to graphene, the morphology and the microstructure were identified as the crucial parameters for the performance enhancement of the resultant nanocomposites. Similarly, other transition metal oxides [74] and hydroxides [75] have also been used in the production of effective electrodes for supercapacitors. In all these cases, 2D graphene was identified as a vital component, due to its potential contribution toward enhanced electrical conductivity and stability.

Similarly to other pseudo-capacitive materials, ternary metal oxides (A_xB_yO_z) have also received a great deal of attention in the fabrication of potential electrodes for supercapacitors. Ternary metal oxides can be classified into several types based on the molecular formulas AB₂O₄, ABO₄, and A₃B₂O₈, etc, where A and B are two different transition metals which are different from their oxidation states, which allows multiple redox reactions during electrochemical reactions. Thus, these ternary metal oxides provide higher capacitances than single component metal oxides. Several spinal structured metal oxides with the general formula AB₂O₄ (e.g., Mn₃O₄ [103], Co₃O₄ [104], Fe₃O₄ [105], etc) have been directly exploited as energy storage materials for supercapacitors. Among them, NiCoO₄ has been observed to be more promising for high energy storage behavior [106]. Indeed, NiCoO₄ exhibits strong electrochemical behavior with better electrical conductivity compared to monometallic nickel oxide and cobalt oxide_. Similarly, other ternary metal oxides with different heterostructures (1D, 2D and 3D) have also been recognized as possible candidates for supercapacitors [107]. However, their practical application has been hampered mainly due to their low electrical conductivity and poor stability. To tackle these issues, the ternary metal oxides are composited with carbon allotropes, in particular graphene, to obtain improved capacitance with long-term stability. Thus, the inherent properties of graphene with ternary metal oxides are expected to produce a synergistic effect to overcome their performance related drawbacks such as limited capacitance, poor electrical conductivity and decreased cycle stability.

Accordingly, Li et al [108] prepared a composite based on RGO/nickel cobaltite (NiCo₂O₄) nanoflakes for supercapacitor applications. The resultant composite displayed superior capacitance (1693 F g⁻¹ at 1 A g⁻¹) with good a rate capability (67.6% capacity retention at 16 A g⁻¹) and long-term stability (89.8% retention). In their study, the complemental effect of the binary redox couples of Ni²⁺/Ni³⁺and Co²⁺/Co³⁺, the ultrathin nanoflake structure and the high electrical conductivity of the RGO in achieving higher electrochemical performances was realized. Similarly, another ternary-metal-oxide-based composite (CoMoO₄/graphene) was prepared by Wang and co-workers [109] using a simple hydrothermal method for supercapacitor applications. During the hydrothermal reaction, Co²⁺ ions interacted electrostatically with functional groups present on the GO, thereby CoMoO₄ particles were grown on the graphene surface with the subsequent addition of (NH₄)₆Mo₇O₂₄. The CoMoO₄/graphene composite delivers a specific capacitance of about 394.5 F g⁻¹ with a specific energy density of 54.8 W h kg⁻¹. The obtained capacitance value was significantly higher than that of unsupported CoMoO₄ (72.0 F g⁻¹, 10.0 W h kg⁻¹), which clearly reveals that graphene has played a beneficial role in achieving improved performance with higher stability. Prior to the above study, Mitlin et al [110] constructed an asymmetric supercapacitor (ASC) using a graphene/nickel cobaltite nanocomposite (GNCC) as the positive electrode and commercial ac as the negative electrode. The active material displayed a specific capacitance of 618 F g⁻¹ (at 5 mV s⁻¹) which was higher than that for the other composites (graphene/Co₃O₄: 340 F g⁻¹; graphene/NiO: 375 F g⁻¹), which were prepared under identical conditions. The combination of the Faradaic reactions on NiCo₂O₄ with the electrical double layer capacitance of ac resulted in an improved capacitance compared to other composites. The cycling performance of the GNCC/ac asymmetric capacitor initially showed a fade-in capacitance (after 100 cycles), due to the pulverization effect. However, the capacitance increased up to 1600 cycles due to the activation of the electrode. Furthermore, the same capacitance (102%) was maintained up to 10 000 cycles which revealed the no capacity fade occurred during the cycling test.

In the same context, a few more ternary metal oxide/graphene-based composites with unique structures have been investigated to obtain enhanced storage capacitance with high energy density and power density [111]. In all cases, the RGO was identified as the crucial component for delivering high specific capacitance with enhanced cycling durability.

Similarly to planar ECs, flexible supercapacitors have also attracted a great deal of interest owing to their high storage capability, power output and high malleability. The primary prerequisite for making flexible ECs is that the active material must possess good electrochemical properties accompanied by high mechanical integrity upon bending or folding, have a compact design and be light in weight. Previously, a few 1D nanomaterials, including CNTs, polymer NWs, transition metal oxide NWs and core–shell hybrid NWs have been investigated as electrodes to meet the demands of flexible supercapacitors [112, 113]. However, these 1D nanomaterials are limited as flexible electrodes due to their smaller contact area with the matrix, which results in limited exposure for the storage of charge on their surface. Moreover, the flexibility of these 1D nanomaterial-based supercapacitors is possible due to their flexible matrix, but degradation of the electrode might result from the repeated bending/folding of these electrodes. 3D nanomaterials have also been investigated as flexible electrodes for supercapacitors. Interestingly, the porous-and-loose features of the 3D building blocks of these nanomaterials afford more benefits as efficient supercapacitors due to their 3D active sites. However their spatial disadvantage makes them unsuitable for compression in making thinner and lighter structures as, when the 3D nanostructures are subjected to a compressive force, they tend to break their 3D nanostructures. Thus, a compressive force would spoil the adsorption/desorption pathway of their electrolyte ions which would further decrease their storage capacity. The aforementioned issues make it clear that electrode materials with a unique morphology and stability play a crucial role in determining the performance of flexible supercapacitors.

After the pioneering work of Geim and Novoselov [8], 2D graphene has been identified as a potential candidate which can meet the all the requirements for flexible supercapacitors owing to its large surface area, high electrical conductivity and mechanical stability. In addition to graphene, other inorganic graphene-like materials have also gained significant attention due to their high specific surface area, high mechanical flexibility and additional electrochemical active sites, which offer a platform for the fabrication of electrodes for flexible supercapacitors. In 2010, Liu et al [114] fabricated a paper-like solid-state supercapacitor using two slightly separated PANI/CNT composites, which were solidified in an H₂SO₄/polyvinyl alcohol (PVA) gel electrolyte. The resultant polymer-based integrated device showed a thickness comparable to A4-sized printing paper. The active flexible device (with twisting) delivered a specific capacitance of 350 F g⁻¹ with a high retention of capacity even after 1000 cycles. The discovery of this flexible all-solid-state capacitor gave the inspiration for the design of new flexible electrodes containing graphene for energy storage devices. Accordingly, graphene [115] and its hybrids have been widely investigated as flexible electrodes in making active flexible supercapacitor devices [116]. For example, Ruoff et al fabricated a free-standing porous graphene electrode for supercapacitors using chemical activation of GO (figure 7). The resulting porous RGO showed a BET surface area of 2400 m² g⁻¹, consequently delivering a specific capacitance of up to 120 F g⁻¹ with an energy density of 26 W h kg⁻¹ [117]. However, in many cases, graphene-based free-standing solid-state supercapacitors exhibit values in the range of 80–120 F g⁻¹, which is far lower than the theoretical values [118]. The restacking of graphene sheets has been identified as the primary reason for the reduction of their active surface area and thus the reduction of the ion transport/diffusion within the active materials.

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To tackle this issue, Lian and co-workers [119] introduced a unique flexible electrode by using the intercalation of inexpensive CB nanoparticles as spacers into the layered graphene assembly to prevent the restacking of graphene sheets and thus improved the specific capacitance of the graphene sheets. The resultant flexible pillared GP electrode with open structures demonstrated excellent potential for electrochemical energy storage (figure 8). As a consequence of superior ionic and electronic transport/diffusion, the pillared GP electrodes displayed an over 700% improvement compared to the original GP electrode at a fast scan rate of 500 mV s⁻¹. Specific capacitances for the pillared GP of 138 and 83.2 F g⁻¹ were achieved in aqueous and organic electrolytes at room temperature, respectively, exhibiting negligible (3.85 and 4.35%) degradation to the specific capacitance after 2000 cycles. Indeed, the method developed for assembling non-stacking graphene sheets provides the possibility of designing high-performance GP-based electrodes for supercapacitor applications. Interestingly, no binder was required for assembling these supercapacitor cells and the resultant pillared composite itself acted as a current collector, due to its intrinsic high electrical conductivity. This approach provided new scope for the development of promising flexible and ultralight-weight supercapacitors.

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In an earlier study, a unique laser reduction approach was used by Kaner and co-workers [120] to produce a porous RGO thin-film electrode for solid-state supercapacitors. They presented a simple method of irradiation of GO film with an infrared laser to prevent the restacking of graphene sheets and the exfoliation of graphitic layers was confirmed with experimental evidence. Their as-obtained exfoliated graphene supercapacitors presented a highest specific capacitance of ∼204 F g⁻¹, which was significantly higher than other solid-state ECs. Duan et al [121] fabricated 3D-GH-based solid-state flexible supercapacitors with a highly interconnected 3D network structure. The GH exhibited a high gravimetric specific capacitance of 186 F g⁻¹ (up to 196 F g⁻¹ for a 42 μm thick electrode), an unprecedented areal specific capacitance of 372 mF cm⁻² (up to 402 mF cm⁻² for a 185 μm thick electrode) with excellent cycling stability and mechanical flexibility. Their study revealed the potential application of these 3D graphene-based macrostructures for high-performance flexible energy storage devices.

It is clear from the aforementioned reports that the unique electrical properties and mechanical integrity of the electrodes are the prerequisites for making ideal flexible supercapacitors. However, to realize their practical utilization, it is highly desirable to improve their capacitance, which can be achieved by optimization of the ion transport of the electrolyte. To address this issue, Choi and co-workers [122] fabricated a graphene-based flexible supercapacitor using FRGO thin films with solvent-cast Nafion electrolyte membranes. The resulting composite showed an enhanced electrochemical performance due to the improved ion and charge transport at the interface from the high ionic conductivity of Nafion. Furthermore, the Nafion polymer acted as an electrochemical binder for the adhesion of the Nafion-coated electrode and Nafion membrane in the process of assembly into ASSSs. As expected, positive feedback was observed in the decreased charge transfer resistance (R_CT, unit Ω) for the FRGO compared the RGO, due to the close contact between the electrode electrolyte interfaces. Consequently, the FRGO-based supercapacitors displayed a specific capacitance (118.5 F g⁻¹ at 1 A g⁻¹) and a rate capability (retention rate 90% at 30 A g⁻¹) which were nearly two times higher than those of all-solid-state graphene supercapacitors (62.3 F g⁻¹ at 1 A g⁻¹ and 48% retention at 30 A g⁻¹). In 2011, an interesting new graphene-based flexible electrode was reported by Cheng and co-workers [123], using a simple and scalable method for fabricating a graphene–cellulose-paper-based flexible device for supercapacitors. In the active material, graphene sheets were strongly bound with cellulose paper due to the strong interaction of graphene with the abundant functional groups present on the cellulose paper. Consequently, a conductive interwoven network resulted from the uniform distribution of graphene sheets on the surface of the cellulose paper. Due to the complemental effect between the graphene and cellulose paper, the active electrode showed a capacitance per geometric area of 81 mF cm⁻², which was equivalent to a gravimetric capacitance of 120 F g⁻¹ for pristine graphene. Moreover, the same electrode showed a 99% retention in capacitance over 5000 cycles at 50 mV s⁻¹.

From the above studies, three different possible ways can be identified for enhancing the performance of graphene-based flexible supercapacitors including: tuning the interlayer distance between the stacked graphene layers, tailoring the ion transportation and constructing 3D porous network structures. However, the limited structural disadvantages of pure graphene encourage its use in hybrids with other carbon allotropes and inorganic materials [124] to enhance the electrochemical performance of flexible supercapacitors.

Accordingly, several successful attempts have been made by researchers in this context to improve the performance of flexible supercapacitors by creating composites with metal oxides, [125] DLHs [126] and conducting polymers [127]. In a few cases, graphene has been composited with other carbon allotropes with metal oxides. Accordingly, Cheng et al [128] prepared free-standing CNT/MnO₂/graphene composite films with excellent mechanical properties using a vacuum filtering method and demonstrated their effectiveness as flexible electrodes in supercapacitors (figure 9). Due to the synergistic effects of graphene, CNT and MnO₂, the active composite electrode delivered a specific capacitance of up to 372 F g⁻¹ with a good rate capability without the need for current collectors and binders.

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In the case of composites with conducting polymers, the graphene sheets provide the additional conductivity and mechanical stability to improve the electrochemical performance of the final material [129]. Shi et al [130] fabricated a flexible PANI/graphene sandwich electrode using vacuum filtration of mixed dispersions of both components. The layered PANI/nickel foam (NF)/graphene composite film exhibited a high mechanical stability and high flexibility. Thus, it could be bent into large angles or be shaped into various desired structures. The value of the conductivity of the composite film was 5.5 × 10² S m⁻¹, which is ten times higher than that of PANI/NF film alone. The EC devices based on this conductive flexible composite film displayed a large electrochemical capacitance (210 F g⁻¹) at a discharge rate of 0.3 A g⁻¹ with improved electrochemical stability and rate performance.

Recently, micro-supercapacitors, newly developed miniaturized electrochemical energy storage devices, have received a great deal of attention as promising energy storage devices due to their potential ability to complement or replace batteries in miniaturized portable electronics and microelectromechanical systems [131, 132]. Micro-supercapacitors offer high power densities, which are several times higher than those of conventional batteries and supercapacitors due to their short ion diffusion length. In this regard, different nanostructured carbon materials, including ac [133], onion-like carbon [134], carbide derived carbon [135], CNTs [136] and graphene dots [137], have been directly exploited as the electrodes for micro-supercapacitors. In addition, other inorganic materials such as RuO₂ [138] and MnO₂ [139] and conducting polymers such as PANI [140] and PPY [141] have also been investigated as electrodes for micro-supercapacitors. In the same context, 2D graphene and its composites have also been studied extensively as electrodes owing to their inherent electrical and mechanical properties. Accordingly, Ajayan and co-workers demonstrated the fabrication of a monolithic supercapacitor using laser reduction and patterning of graphite oxide films [142]. The microelectrode was fabricated by sandwiching graphite oxide between reduced graphite oxide, in which the graphite oxide acted as a solid electrolyte (figure 10). The notable amounts of trapped water in the graphite oxide makes it a good ionic conductor as well as an electrical insulator, allowing it to serve as both an electrolyte and an electrode separator with ion transport characteristics similar to Nafion membranes. However, the poor frequency response and the large internal resistance (6.5 kΩ) of the fabricated devices limit their practical applications.

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Wang et al [143] fabricated binder-free RGO/CNT composite micro-supercapacitors using a combination of micro-fabrication techniques and electrostatic spray deposition. Their electrochemical studies indicate that the in-plane interdigital design of the microelectrodes is promising due to the increased accessibility of the electrolyte ions between stacked RGO sheets through an electro-activation process. The resultant composite micro-supercapacitor exhibited a high volumetric capacitance (5.0 F cm⁻³ at a scan rate of 1 V s⁻¹) with a resistance capacitance (RC) time constant of 4.8 ms, which is notably lower than for any other reported micro-supercapacitor. In all these cases, the fabrication of micro-supercapacitor electrodes involves conventional lithographic techniques or employs masks for the definition of patterns on substrates. Hence, these methods are unsuitable for making cost-effective devices for practical applications, as additional processing or sophisticated operation are still required to make high-performance micro-devices.

In considering the above issues, Kaner et al [144] demonstrated the scalable fabrication of graphene micro-supercapacitors over large areas by direct laser writing on graphite oxide films using a standard LightScribe DVD burner (figure 11). These micro-supercapacitor devices developed on flexible substrates can be used for flexible electronics and on-chip uses and can be integrated with MEMS or CMOS in a single chip. The developed micro-supercapacitors exhibited a power density of ∼200 W cm⁻³. This value is remarkably high compared to any other supercapacitor reported so far. Similarly, Mullen et al developed a class of all-solid-state graphene-based in-plane interdigital micro-supercapacitors on both rigid and flexible substrates through micropatterning of graphene films [145]. The resultant solid-state micro-supercapacitors delivered a maximum area capacitance of 80.7 mF cm⁻² and a power density of 495 W cm⁻³, which are notably higher than those for other reported supercapacitors. Furthermore, the same system delivered an energy density of 2.5 mW h cm⁻³, which is nearly comparable to that of lithium thin-film batteries, and showed a superior cycling stability (>98.3% capacitance retention after 100 000 cycles). The superiority of the maximum areal capacitance and high energy density of these graphene-based thin-film micro-supercapacitors arise from the cooperative effects of the high conductivity of graphene materials and the in-plane geometry of the devices. Another carbon microfiber-based micro-supercapacitor was also reported, in which the scalable synthesis of a hierarchically structured carbon microfiber made of an SWCNT/N-doped RGO sheet interconnected network architecture was demonstrated [146]. The fiber electrode, with high electrical conductivity (102 S cm⁻¹) and high packing density, provided a large accessible surface area for the ions, thereby delivering a volumetric energy density of ∼6.3 mW h cm⁻³ without compromising cyclability. The delivered energy density is comparable to 4 V/5 00 mA h thin-film lithium batteries. High energy density is highly desirable for making successful high-volumetric-performance micro-supercapacitors.

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In addition to micro-supercapacitors, yarn supercapacitors have also received a lot of attention in portable and wearable electronics due to their tiny volume, flexibility and weavability [147]. However, their low energy density still limits their development in the area of wearable high energy density devices, although considerable efforts are under way to increase the energy density of yarn supercapacitors while retaining their high power density [148]. Gao et al [149] proposed a coaxial wet-spinning assembly approach for continuously spun polyelectrolyte-wrapped graphene/CNT core–sheath fibers, which were used directly as safe electrodes in assembling two-ply yarn supercapacitors (figure 12). The resultant yarn supercapacitors exhibited ultrahigh capacitances of 269 and 177 mF cm⁻² and energy densities of 5.91 and 3.84 μW h cm⁻². In their study, a cloth supercapacitor, which was superior to commercial capacitors, was further interwoven from two individual 40 cm long coaxial fibers. This unique method for the construction of yarn supercapacitor electrodes with a combination of scalable coaxial wet-spinning technology and ultrahigh electrochemical performance opens a new route to safe, wearable electronics.

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Generally, supercapacitors exhibit a higher power density than fuel cells and batteries, but their energy density remains lower than the above-mentioned energy devices. Hence, the ultimate aim of current research is the development of a supercapacitor which has both a high energy density and a high power density. The replacement of aqueous electrolytes with organic electrolytes and the construction of hybrid supercapacitors have been identified as possible ways to accomplish this goal. As mentioned, ASCs contain two electrodes (positive and negative) in which Faradaic reactions are taking place in the positive electrode and electrical double layer capacitance due to adsorption–desorption reactions is taking place in the negative electrode [150]. In ASCs, by utilizing two different electrodes, an improved operation voltage can be realized, thereby the improvement of specific capacitance values with a high energy density can be achieved. In general, carbon-based nanostructures have been employed as the negative electrode [151], due to the adsorption–desorption processes on their large surface areas. Inorganic materials such as transition metal oxides and metal hydroxides have been employed as the positive electrode [152]. Of course, in a few cases, carbon nanostructures have also been employed as the positive electrode. In recent reports, graphene was extensively utilized in constructing ASCs due to its outstanding characteristics of large surface area and conductivity. For example, Cheng et al [153] fabricated ASCs using graphene as the negative electrode and a MnO₂ NW/graphene composite (MGC) as the positive electrode in an aqueous Na₂SO₄ solution (figure 13).

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The resultant ASC was cycled reversibly in the high potential range of 0–2.0 V. The obtained energy density with the graphene-based ASC was 30.4 W h kg⁻¹, which was significantly higher than those of the symmetric ECs based on graphene/graphene (2.8 W h kg⁻¹) and MGC/MGC (5.2 W h kg⁻¹), and other MnO₂-based asymmetric ASCs [154]. The advantage of coupling metal oxide with graphene was reflected in the strong electrochemical behavior. Similarly, Due et al [155] constructed an ASC with high energy and power densities using GH with 3D interconnected pores as the negative electrode and vertically aligned MnO₂ nanoplates on NF (MnO₂/NF) as the positive electrode in an aqueous Na₂SO₄ electrolyte. The resultant ASC delivered an energy density of 23.2 W h kg⁻¹ with a power density of 1.0 kW kg⁻¹ and cycled between a large potential window (2.0 V). The obtained energy density was notably higher than that for the symmetric supercapacitors based on GH (5.5 W h kg⁻¹) and MnO₂/NF (6.7 W h kg⁻¹). The desirable high energy density in the resultant ASCs was obtained due to the influence of the porous structure, the high specific capacitance of the GH/MnO₂/NF composite, the complementary potential window of the two electrodes and the elimination of polymer binders. Guo et al [156] also reported a high voltage ASC using amorphous MnO₂ nanoparticles and graphene as the positive and negative electrodes, respectively. As expected, the resultant ASC operated reversibly at a high cell voltage of 2.0 V and displayed an energy density of 25.2 W h kg⁻¹ with a power density of 100 W kg⁻¹, which were much higher than those of symmetric supercapacitors based on MnO₂/MnO₂ (4.9 W h kg⁻¹) and graphene/graphene (3.6 W h kg⁻¹). In addition, the laboratory-made ASC showed a capacity retention of 96% even after 500 cycles. In the same vein, another ASC with high energy and power densities was fabricated using MnO₂/RGO hydrogel as the positive electrode and a pure RGO hydrogel as the negative electrode in an aqueous Na₂SO₄ electrolyte. The resultant ASC delivered high performance with a high energy density (21.2 W h kg⁻¹) and power density (0.82 kW kg⁻¹) [157]. In a few cases, graphene has been modified with a metal oxide and exploited as the negative electrode for ASCs. For example, Zhao et al [158] fabricated an ASC using RGO sheets modified with RuO₂ (RGO/RuO₂) and PANI (RGO/PANI) as the anode and cathode, respectively. Similarly to the above observations, the resultant ASC exhibited an improved capacitance compared to the symmetric supercapacitors fabricated using RGO/RuO₂ and RGO/PANI electrodes (figure 14). This is attributed to the broadening of the potential window, which leads to a high energy density (26.3 W h kg⁻¹). This value is nearly two times higher those that of the symmetrical supercapacitors based on RGO/RuO₂ (12.4 W h kg⁻¹) and RGO/PANI (13.9 W h kg⁻¹) electrodes. In an earlier study, Dai et al [159] fabricated a novel high energy density ASC by pairing metal oxide (RuO₂)/graphene and metal hydroxide (Ni(OH)₂)/graphene hybrids as efficient ASC electrodes. The as-prepared ASC electrode exhibited an energy density of ∼48 W h kg⁻¹ at a power density of ∼0.23 kW kg⁻¹ and a high power density of ∼21 kW kg⁻¹ at an energy density of ∼14 W h kg⁻¹.

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Significant advancement has been made with high energy density ASCs and flexible devices with improved performance are anticipated to realize wearable and bendable consumer electronics for practical applications. To accomplish these requirements, many carbon-based materials are used directly for flexible devices. Many pseudo-capacitive materials are only used for the fabrication of flexible electrodes with carbon support. From this perspective, a great deal of attention has been paid to MoS₂ which exhibits promising activity toward energy storage and conversion reactions, particularly when coupled with conducting materials. Keeping this mind, Kang and co-workers fabricated a flexible ASC, using 2D graphene and its analogue of 2D MoS₂ [160]. Their flexible ASC, based on ultrathin 2D MoS₂ and 2D graphene in an aqueous Ca(NO₃)₂–SiO₂ gel electrolyte, delivered excellent electrochemical performance in terms of energy density up to 97.2 W h kg⁻¹, which is notably higher than traditional MnO₂-based supercapacitors. Moreover, the same ASC exhibits a 3% loss in capacitance after 10 000 cycles. The method used for the fabrication of these flexible devices offers a versatile tool in the design and fabrication of supercapacitors with a certain shape using a simple process. Similarly, Chen and co-workers, reported a fiber-based ASC using MoS₂/RGO/MWCNT and RGO/MWCNT as the anode and cathode, respectively [161]. Their fiber-based asymmetric device displayed a high energy density with cycling stability, due to the complemental effect of MoS₂ with well aligned MWCNT fibers and RGO. Another flexible ASC was constructed using 2D graphene foam (GF) with CNT hybrid film as the support for the electrochemically active materials of MnO₂ and PPY. The desired electrode was constructed using hydrothermally deposited MnO₂ on GF/CNT film with chemically polymerized PPY on GF/CNT film. The active composite exhibited high energy and power densities of 22.8 W h kg⁻¹ at 860 W kg⁻¹ and 2.7 kW kg⁻¹ at 6.2 W h kg⁻¹, respectively [162]. Similarly, other ASCs were also reported based on graphene and its hybrids as high energy density supercapacitors to widen their potential practical applications [94].

The recent endeavors in the development of materials for supercapacitors have been aimed at producing devices with a high energy density and power density. Most recently, much attention has been paid to the development of devices with a higher volumetric capacitance than gravimetric capacitance, which is most the important criterion to deliver maximum energy with limited space [163]. Of course, carbon and its allotropes are the materials of choice for the cheapest ECs. However, their potential applications are limited due to their low energy density. Hence, significant efforts have been made to improve their energy density by hybridizing carbon and its allotropes with other pseudo-capacitive materials. In fact, these carbon-based materials possess high gravimetric capacitance while their volumetric capacitance is far lower, due to their low packing density. In general, this low packing density causes more empty space leading to flooding of the electrolyte with a limited storage capacity for ions. Consequently, the weight of the devices would increase without any improvement in the capacitance, which makes them unsuitable for real-life applications. Hence, novel carbon materials which have a high volumetric capacitance are highly desirable to meet the energy demands of compact and portable energy storage systems. Among the different carbon nanostructures, graphene has gained a great deal of attention in energy storage devices. Similarly to other carbon allotropes, graphene also has a low packing density with abundant empty space and is thus unsuitable for making devices with a high volumetric capacitance. Hence, researchers have focused on finding a way to achieve a highly compact graphene assembly to produce high volumetric capacitance electrodes, without losing graphene's unique properties.

Accordingly, Tao et al [164] reported a high density porous carbon-based macro-assembly resulting from the compact interlinking of two to four layered GNSs for a high volumetric supercapacitor. The resultant composite balanced the two opposing characteristics of high surface area (370 m² g⁻¹) and large density (1.58 g cm⁻³), thereby exhibiting a high volumetric capacitance of 376 F cm⁻³ in an aqueous medium. Moreover, the resultant device delivered an energy density of 13.1 W h L⁻¹ at a power density of 39.5 W L⁻¹. The power density was up to 5.9 kW L⁻¹ at an energy density of 9.1 W h L⁻¹. Due to the easy shaping of the device along with its acceptable conductivity, the active composite can be directly exploited as an electrode for high energy density ECs.

Li et al [165] prepared a graphene-based high volumetric supercapacitor by taking advantage of the microcorrugated 2D configuration and self-assembly behavior of graphene. Moreover, the authors proposed that such materials could be achieved easily by capillary compression of graphene gel in the presence of a nonvolatile liquid electrolyte. Accordingly, the chemically converted GH was compressed by capillary pressure to increase the packing density through controlled removal of the volatile solvent trapped in the hydrogel. Consequently, the graphene sheets were stacked together which further increased the packing density up to 1.33 g cm⁻³. The resultant packing density was two times higher than that of ac (∼0.7 g cm⁻³). The method used increased the packing density of graphene extensively while providing channels for facile ion transport to obtain a high volumetric capacitance. Consequently, the resulting film-based capacitor delivered a volumetric capacitance of 255.5 F cm⁻³ in an aqueous electrolyte and 261.3 F cm⁻³ in an organic electrolyte at 0.1 A g⁻¹. Moreover, the active film-based capacitor delivered a power density of (∼75 kW L⁻¹), which was significantly higher than that of chemically converted graphene (∼8.6 kW L⁻¹) which suggested that the chemically converted graphene (CCG)-based ECs could be employed as potential electrodes for high volumetric supercapacitors.

Another method in the same context was reported by Lee et al [166], where a highly dense and vertically aligned RGO electrode was fabricated using hand-rolling and cutting processes. Due to the vertically aligned and opened-edged graphene structure, the resultant vertically aligned RGO electrodes showed a high packing density of 1.18 g cm⁻³. Furthermore, the active electrode delivered a volumetric capacitance of 171 F cm⁻³ which was notably higher than that of RGO powder (62 F cm⁻³). The method they adopted for the fabrication of graphene-based materials by tuning their orientation in a controlled manner offers a unique way to facilitate ion transport to obtain enhanced volumetric capacitance. From all the studies described, it can be seen that the self-assembly of graphene into densely packed carbon has significantly improved volumetric performance. Similarly, the preparation of graphene-based composites with other, non-carbon materials is expected to tune their performance toward volumetric capacitance.

Accordingly, a few methods have been demonstrated to prepare graphene-based composites for high volumetric supercapacitors [167]. In particular, Ruoff et al [168] fabricated a graphene-based composite with MnO₂ (activated microwave expanded graphite oxide (aMEGO)/MnO₂) which delivered a gravimetric capacitance of 256 F g⁻¹ and a volumetric capacitance up to 640 F cm⁻³. The resultant gravimetric capacitance and volumetric capacitance were significantly higher than for self-assembled aMEGO. It was identified that the pseudo-capacitive nature of MnO₂ significantly improves the volumetric capacitance without increasing the volume of the products. In a few cases, non-carbon 2D materials have also delivered a notable volumetric capacitance compared to graphene-based composites. For example, Gogotsi et al [169] reported a novel method for the fabrication of 2D-Ti₃C₂ (MXene) for high volumetric supercapacitors. Interestingly, the obtained hydrophilic material swelled in volume when hydrated and could be molded into any form, like clay. Accordingly, the Ti₃C₂ material was molded into different shapes, from a conductive solid to a rolled thin-film material. The thin-film-like material was found to show a volumetric capacitance of up to 900 F cm⁻³ when it was employed as the electrode for supercapacitors. This value is enormous compared to other graphene-based composites which opens a new way for the potential utilization of non-carbon 2D materials for volumetric capacitors.

A highly porous yet densely packed nature, a high ion-accessible surface area and open channels for easy diffusion of ions are believed to be the primary prerequisites for the development of active electrodes for high volumetric supercapacitors. Although graphene and its composites exhibit remarkable capacitance, these values must be improved to meet the energy required for consumer electronics and hybrid vehicles. Tuning the structure of graphene could be a possible way to achieve easy access of ions with improved capacitance. For example, the introduction of nanosized holes in 3D graphene (i.e. 3D holey graphene) would result in 3D graphene with macrosized pores and nanosized holes. The nanosized holes are, of course, expected to improve the facile transport of ions, while the macrosized holes are expected to provide a larger surface area for larger electrical double layer capacitance. Holey graphene-based composites with other 2D inorganic materials are also recommended to obtain improved gravimetric capacitance with easy diffusion of ions for high volumetric supercapacitors.

As discussed previously, inorganic nanomaterials show promising potential as flexible supercapacitor electrodes, but they are limited by poor electrical conductivity and poor cyclic performance due to strain induced cracking of the electrode during the charge–discharge process, and are difficult to access due to diverse coordination styles. In many cases, the active materials do not have layered structures, which hampers their exfoliation into ultrathin NSs with atomic thickness. However, recently, the exfoliation of inorganic bulk material into 2D layered NSs has been achieved [170], which supports a new way for the preparation of 2D layered composites for energy storage applications [171]. Recently, some other inorganic graphene analogues. i.e., exfoliated 2D transition metal chalcogenides (TMC) such as MoS₂, TiS₂, WS₂ and VS₂, and DLHs have been studied extensively for both energy storage and conversion devices due to their versatile properties [172]. However, these TMC and DLH NSs are rarely applied to the electrode material of supercapacitors due to their intrinsically low electrical conductivity, although in a few cases they have been used as supercapacitor electrodes with graphene sheets [173]. In the resultant composite, the inorganic lattices provide an additional contribution toward higher capacitance with abundant active pseudo-capacitive centers and unique physicochemical properties. In fact, the improved electrochemical performance arises from their high specific surface area and the chemical activity of the lattice framework.

In this context, Jin and co-workers fabricated layer-by-layer (LBL) assembled multilayer films of Co–Al DLH NSs and GO electrodes as efficient supercapacitors (figure 15) [174]. In their study, a positively charged Co–Al DLH NS with a thickness of 1.0 nm was successfully exfoliated from their bulk Co–Al DLH. The resultant composite film exhibited a high specific capacitance of 880 F g⁻¹ and an area capacitance of 70 F m⁻² under a scan rate of 5 mV s⁻¹, and the film exhibited no noticeable decrease in capacitance over 2000 cycles. It is well-known that high electrical conductivity accompanied by pseudo-capacitance is the primary factor for high-performance pseudo-capacitors. Furthermore, it has been reported that a high surface area or a thin layer of active surface play a key role during pseudo-capacitive processes. Favorably, the charge matching of the RGO and Co–Al DLH NS in the LBL assembly has, through electrostatic interaction, helped to fabricate a potential electrode with a high surface area and the desired pseudo-capacitive nature of Co–Al DLH NS.

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Inspired by the above report, Ma and co-workers [175] fabricated a sandwich-based composite with (Co–Al, Co–Ni) DLH NSs and graphene (oxide) NSs (figure 16). As a result of the synergistic effect between the layered materials, a high capacity and high power rate were achieved. The overall capacitance of the composite was significantly enhanced by the hybridizing pseudo-capacitance derived from redoxable DLH NSs, resulting a high capacity up to about 650 F g⁻¹, which is six times higher than that of pure GNSs. The conductive graphene adjacent to the insulating DLH NSs amended the charge transfer efficiency, which resulted in significant enhancement of the pseudo-capacitance of the hybrid material. However, the Faradaic reactions occurring on the surface of the layered NSs exhibited potential windows of only about 0.5 V, which were inferior to the electrochemical potential windows of the aqueous solution (1.23 V).

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Thus, it is desirable to find a new composite, which is graphene-layered and pseudo-capacitive, with a higher redox potential accompanied by a high power density and energy density, to achieve practical applications of power supply. WS₂ with RGO was fabricated using a hydrothermal method as an energy storage material [176]. The WS₂/RGO hybrids exhibited an enhanced specific capacitance of 350 F g⁻¹, due to their unique microstructure with the combination of two layered materials. WS₂/RGO hybrids have emerged as a promising supercapacitor electrode material with high specific capacitance, high energy density and excellent cycling stability.

In this review, the recent progress in 2D materials, in particular graphene and its composites, was summarized in terms of their use as electrode materials for supercapacitors. The importance of the intrinsic high surface area, high electrical conductivity and mechanical stability of graphene have been recognized for its potential in producing planar and flexible electrodes for supercapacitors. Several research groups have achieved remarkable breakthroughs over the past few years in using graphene as an energy storage material, although a few challenges still remain. For example, the higher EDLC of graphene proves its ability to compete with other energy storage materials. However, the exhibited capacitance values are notably lower than the expected values for several reasons, including restacking of graphitic layers, poor ion transport, hydrophobicity, etc. Hence, the aforementioned limitations must be tackled before this material can be practically used in electrochemical energy storage devices. In several cases the pseudo-capacitive electrode materials are insulators, which do not meet the requirements for electron and ion transport during the electrochemical processes. A possible way to obtain improved energy storage performance is the hybridization of 2D graphene with 2D layered pseudo-capacitors. Although these approaches have received extensive attention, heterostructural interface problems still exist due to the weak interactions in the hybrid material. Hence, a unique way to introduce additional electrochemical active sites is highly desirable to stimulate the electrochemical behavior of such composites under high rate conditions. Furthermore, substantial advancements have been made in graphene-based flexible supercapacitors, but the chemical and physical stabilities still need to be improved to realize higher flexibility and better performance for these preferred 2D nanomaterials. Most importantly, it is highly desirable to develop efficient materials by using cost-effective and environmentally benign methods to realize excellent performance. Thus, it would be very valuable if future research were to be centered toward the development of potential 2D electrode materials with a high charge capacity and high stability at minimum cost.

This research was supported by the Creative Research Initiative (CRI), Mid-Career Researcher (MCR), BK21 Plus, Basic Research Laboratory (BRL) and Basic Science Research programs through the National Research Foundation (NRF) of Korea, and the US Air Force Office of Scientific Research through Asian Office of Aerospace R&D (AFOSR-AOARD).