Different carbon nanostructures used as electrode materials for EDLCs with onion-like carbon, carbon nanotubes, graphene, activated carbon, carbide-derived carbon, and templated carbon .
Lightweight, flexible, wearable, and portable electronic gadgets have drawn significant attention in modern electronics industry. To power these gadgets, great efforts have been made to develop highly efficient energy-storage equipment. Among various power sources, a supercapacitor, acting as a bridge between the conventional battery and electrolytic capacitor, has been considered a promising portable energy storage device because of its high power density, fast charge/discharge rate, adequate operational safety, and excellent working lifetime. Hybrid supercapacitors, which combine redox materials with carbon-based materials, exhibit tremendous potential to fulfill the requirement of practical applications. In this chapter, we will review recent reports focusing on composite materials (i.e. metal oxide, metal hydroxide, and metal dichalcogenide composited with carbon materials) for the application in supercapacitors. The conclusion and futuristic prospects and challenges of highly efficient supercapacitors are briefly discussed.
- energy storage
- metal oxides
- metal hydroxides
- transition metal dichalcogenides
There is sharply increasing demand for energy with the rapid growth of the global economy. The energy generation from sustainable sources, such as wind and solar, plays an important role in power supply. However, the intermittent nature and imbalanced regional distribution of the sustainable energy make them unable to stably supply the power . The development of energy storage systems is an urgent requirement to meet the sufficient and stable power supply for industrial and residential usage. Although rechargeable lithium-ion batteries, dominant energy sources in each field, as high energy density providers have filled their position , lithium-ion batteries still have the limitations of poor cycle life and low power performance . Supercapacitors (SCs), also known as ultracapacitor and electrochemical capacitors, are an emerging class of energy storage device, which possess high power density and tens of thousands of charge/discharge cycles [4, 5]. Figure 1 shows the Ragone plot of different energy conversion and storage devices. SCs have a unique position to bridge the gap between conventional capacitors and batteries. Compared with conventional capacitor, SCs possess higher specific energy density in several orders of magnitude. Moreover, SCs provide higher specific power density than batteries due to its unique charge storage mechanism.
2. Theoretical background for supercapacitors
2.1 Principle and mechanism of supercapacitors
Based on different charge storage mechanisms, SCs are mainly divided into two categories, electrical double layer capacitors (EDLCs) and pseudocapacitors, as shown in Figure 2. EDLCs store the electrical charge by electrostatic force at the electrode-electrolyte interface, which is a physical process without involving electrochemical reactions on the electrode surface. In order to increase the capacitance and energy density of SCs, some electrochemically active materials, such as transition metal oxide and conducting polymers, have been explored as electrode materials for pseudocapacitors. The energy storage in pseudocapacitors originates from reversible surface faradaic redox reactions at the interface of electrolyte and electroactive materials.
2.2 Factors affecting the performance of Supercapacitors
The capacitance of EDLCs is strongly dependent on effective surface area and the pore size distribution of the electrode [7, 8]. Typically, the carbon-based materials and their derivatives, including activated carbon, carbon nanotubes (CNTs) and graphene, with high conductivity, chemically-stability, and large surface area are widely utilized in EDLCs. Although the EDLCs possess high power density and excellent charge/discharge cycling stability, they suffer from low energy density owing to the relatively low capacitance of carbon-based materials. Pseudocapacitors can achieve significantly higher energy density, as compared to EDLCs, because they have a variety of oxidation states for redox charge transfer reactions. However, relatively low electrical conductivity and poor rate capability and cycle stability of pseudocapacitive materials limit their widespread commercial applications . Therefore, carbon-based materials with high conductivity and distinct structures can be combined with pesudocapacitive materials to exhibit synergistic effects for supercapacitive performance, known as hybrid SCs.
3. Carbon based composite electrode materials
Carbon material is EDLCs type for supercapacitor. In section 2.1, EDLCs has introduced their property, which store the electrical charge by electrostatic force at the electrode-electrolyte interface, as shown in Figure 2. It is not involving electrochemical reactions on the electrode surface. There are different types of carbon nanostructured materials, which can be used as single electrode materials due to their unique structural, mechanical, and electrical properties.
3.1 Zero-dimensional (0-D) carbon nanoparticles
They are round-shaped particles such as ultrafine activated carbon (AC), mesoporous carbon, carbon nanosphere, and carbon quantum dot, with a high specific area (AC: ⁓3000 m2 g−1) and an aspect ratio of nearly . In addition, by tuning the pore size distribution and pore content, they can use as suitable supporting materials for composite electrodes.
3.2 One-dimensional (1-D) carbon nanostructures
These are the high aspect ratio materials with fiber shaped and good electronic properties e.g. carbon nanotubes (CNT), carbon nanocoils, and carbon nanofibers (CNF), which facilitates the electrochemical reaction kinetics by 1-D charge transfer pathway.
3.3 Two-dimensional (2-D) nanosheets
They are sheet like structures with high aspect ratio such as graphene, graphene oxide (GO) or reduced graphene oxide (rGO). In addition, they have high specific surface area, good mechanical strength, and excellent electrotonic conductivity, which helps them as promising electrode materials for SCs. For an example, single layered graphene has theoretical surface area of 2756 m2 g−1 and charge mobility of 200000 cm2 V−1 s−1 .
3.4 Three-dimensional (3-D) porous nanostructures
These are the low dimensional building blocks such as carbon nanofoams or sponges with hierarchical porous channels, rich pore structures, higher electrical conductivity and better structural mechanical stability, which are extensively used in composite electrode materials for SCs. For an example, foam has high specific surface area with continuous electron transport path and large area of electrolyte-electrode interface.
Table 1 shows some examples of different carbon nanostructured materials such as carbon onions, carbon nanotubes, graphene, and templated carbon, which are used as electrode materials for EDLCs. Each carbon nanostructured materials have its advantages and disadvantages. For example, carbon onions have high power performance due to excellent conductivity with high accessible ion adsorption capacity but low capacitance of ⁓30 F g−1 . On the other hand, CNTs have high energy density due to superior electrical properties and unique tubular structures for fast charge transportation but due to the high cost, their widespread applications are limited . Recently, graphene has been attracted much attention as electrode materials for EDLC applications due to unique properties, like as ultrahigh specific surface area, unique conductivity, and exceptionally high mechanical strength . However, the aggregation of sheets during electrode preparation limits the aspect of application. More recently, 3D porous carbon nanostructured materials are widely used for EDLCs because of rich pore structures and high surface areas but due to relative low conductivity and presence of micropores specific capacitance is insufficient at a high current density . Therefore, it is necessary to construct composite materials by coupling the advantages of different types of carbon nanostructured materials and high energy electrode materials such as transition metal oxides, metal hydroxides and metal dichalcogenides (TMDs) to enhance the energy density without the compromise of power density and also meet the requirement for fabrication of high energy storage devices. In the composite electrode material, different types of carbon nanostructured materials not only contribute to high capacitance but also provide an easy conductive path for charge transportation due to conductive nature.
4. Carbon-metal oxide composite electrode materials
Many metal oxide such as RuO2, MnO2, Fe3O4, V2O5, NiO, Co3O4, and TiO2, has been received significant attention and extensive studied as SC electrode materials due to Pseudo capacitance nature, which depends on the fast reversible redox reaction of electroactive species directly as well as in the vicinity of electrode surface [17, 18, 19, 20]. The redox behavior is due to the multivalent property of the above oxides which changes their oxidation states by interaction with protons or hydroxide ions reversibly. In spite of their excellent specific capacitance, they still suffer from low conductivity, low rate capability, poor stability and durability during the process of charge/discharge. In contrast carbon materials shows excellent performance in these regards but suffer from comparatively limited specific capacitance. Hence, the synergic integration of metal oxides with conducting carbon supports may form high potential carbon-metal oxide composite electrodes materials for SCs and hybrid devices because of their enhanced electrochemical performance through the combined effect of pseudocapacitive/faradaic charge storage and electrical double layer capacitance mechanisms [21, 22, 23].
4.1 Carbon-ruthenium oxide (RuO2)-based composite electrode materials
Among the metal oxides, ruthenium oxide (RuO2) has been considered as very common electrode materials for SCs in acidic medium due to their excellent pseudocapacity which is arising from high conductivity, good thermal stability, highly reversible redox reactions, three different oxidation states within 1.2 V, and high specific capacitance natures. The pseudocapacitance mechanism of RuO2 for SC electrodes can be described as equation :
However, its scarcity and high cost limits the fabrication of RuO2 based electrodes for potential applications. But, smartly use of composite materials by synergic integration of pseudocapacitive RuO2 materials with conductive carbonaceous substrates not only improves the capacitance but also reduces the cost of the electrode. Recent studies are more focus about the selecting the best carbonaceous substrate and the synthesis procedures to fabricate ruthenium oxide (RuO2)-coated on the porous carbonaceous substrates.
RuO2-CNT composite has been prepared by uniformly coating of RuO2 on the vertically aligned porous carbon nanotubes porous through atomic layer deposition (ALD) technique and further activation by voltammetry potential coulometry (Figure 3(a-c)) . This ALD technique has many advantages such as deposition on large surface area, accurate thickness and exceptional uniformity for electrode designing in energy storage devises. The as-prepared RuO2-CNT composite shows excellent electrochemical performance as an electrode material for SC in respect of capacitance, power density and stability. Several publications have been reported the specific capacitance and power density of RuO2-CNT composite, which are around 650 F g−1 and 17 kW kg−1, respectively. Kaner et.al recently demonstrated the synthesis and processing of 3D porous RuO2/laser-scribed graphene (LSG) composite electrode for miniaturized and interdigitated SC that exhibit ultrahigh energy and power density (Figure 3(d)) . The high-resolution TEM (HRTEM) image of 3D porous RuO2/LSG composite in Figure 3(e) shows that multiple layers of the graphene sheets wrap around each RuO2 nanoparticle. 3D porous RuO2/LSG composite electrode showed an ultrahigh specific capacitance of 1139 Fg−1 with outstanding rate capability and the asymmetric supercapacitor (ASC) made of 3D porous RuO2/LSG composite electrode as positive electrode exhibited an extremely high energy density of 55 W h kg−1 at a power density of 12 kW kg−1 (Figure 3(f)). Other interesting composite of RuO2 made of RuO2 decorated nitrogen-doped reduced graphene oxide aerogel (NGA) are used as high-performance transparent solid-state supercapacitors. RuO2/NGA composite with finely tuned mass loading of 16.3 μg cm−2 and transmittance of 34.1% (λ = 550 nm) demonstrated maximum areal energy of 0.074 μW h cm−2 and power of 64 μW cm−2 with cyclic stability of 100% over 2000 cycles . This RuO2/NGA based high transparent SC can be practically used in many advanced transparent electrical devices.
4.2 Carbon-manganese oxides (MnO2)-based composite electrode materials
MnO2 has been considered as a promising pseudocapacitive electrode materials for energy storage applications due to low price, abundant reserve, high specific capacitance, and environmental environment benign nature and low toxicity in comparison to other transition-metal oxides. In general, the charge storage mechanism of MnO2 involves change in manganese oxidation state from +3 to +4 and the contribution of protons or alkali cations, which can be shown in the following equation .
Where C+ represents protons or alkali cations (Li+, Na+, K+).
However, MnO2 based electrodes limits the capacity and power density due to their low surface area and poor electronic/ionic conductivity. Therefore, the composite of MnO2 with high-surface area and conducting carbonaceous materials may improve the electrochemical performance in terms of specific capacity, energy and power densities by providing the larger interfacial area between the MnO2 particles and the electrolyte solution .
Gao et al. fabricated a MnO2/activated carbon (AC) based hybrid SC, where AC not only acted as a conducting support but also increase the capacitance as well as energy and power densities . In addition, engineering the morphology of MnO2 into different nanostructures is considered to be a practical approach to increase its electrochemical performance. It is reported that the pore sizes of the mesoporous-MnO2/AC are greatly affected the specific capacitance and the rate capability of the SCs. Huang et al. demonstrated the influence of CNT on the electrochemical properties of MnO2-CNT composite electrode by controlling the growth of MnO2 nanostructures on CNTs through a facile redox approach (Figure 4(a-c)) . The as-prepared MnO2-CNT composite electrode showed a maximum specific capacitance of 247.9 F g−1 with outstanding cyclic stability of 92.8% after 5000 cycles. In addition, it has been noticed that the aligned CNTs are more favoured as SC electrodes over nonaligned CNTs due to their large specific surface area, low contact resistance, and fast electron-transfer kinetics. Graphene is being used as a supporting material for MnO2 nanostructures due to its large surface area, high conductivity, and high stability nature. For example, microwave irradiation synthesised MnO2-graphene composites exhibited the maximum capacitance of 310 F g−1, which is much higher than the bare graphene and MnO2 (110 F g−1) . Beside their high capacitance, MnO2-graphene composites have better cyclic stability of 95% over 15000 cycles. The excellent electrochemical performance of MnO2-graphene composites is due to large surface area and high conductivity of graphene network. Recently, Zhang et al. reported highly flexible ASCs based on graphene hydrogel (GH)/copper wire (CW) as the negative electrode and hierarchical MnO2/graphene/carbon fiber (CF) as the positive electrode, which exhibited excellent areal energy density of 18.1 μW h cm−2 and operated reversibly at potential window of 0-1.6 V . 3D porous carbon nanostructures can also be used as MnO2 support for supercapacitor (SC) electrodes as they provided large surface area, well-defined pathways to electrolyte access, and better mechanical stability. Fang et al. demonstrated a novel solid-state symmetric supercapacitor (SSC) based on 3D rGO@MnO2 foam electrode and Polyacrylic Acid (PAA)-Portland cement-KOH electrolyte, which showed a very high areal capacity of 1.84 F cm−2 at current density of 0.5 mA cm−2 and excellent capacitance retention of 61% at a current density of 40 mA cm−2 .
4.3 Carbon-cobalt oxides (CoO/Co3O4)-based composite electrode materials
Cobalt oxides has been received considerable attention as highly promising SC electrode materials due to their non-toxic, low cost, easy synthesis, environmentally friendly, and more importantly high theoretical capacitance (CoO: 4292 F g−1, Co3O4: 3560 F g−1) . In addition, cobalt oxides exhibits outstanding electrochemical behaviour in alkaline as well as organic electrolyte, which is possible due to their ability to interact with the ions at the electrolyte surface as well as through the bulk of the material. The pseudocapacitance of cobalt oxides (CoO/Co3O4) are originates from the following redox reaction: 
However, the low electrical/ionic conductivity of cobalt oxides hinders their practical performance as SC electrodes. Most efficient way to improve their electrochemical performance is to form composites of cobalt oxides by incorporation into a carbon-based conducting supports. A Co3O4/AC composite SC electrode was reported by Iqbal et al.. The electrode exhibited maximum achievable specific capacitance 567 F g−1 and maximum energy density of 63 W h kg−1 at 0.7 A g−1. In addition to the high specific capacitance, Co3O4/AC composite of capacitive retentivity is 82% after 6000 charge/discharge cycles and safe to handle due to no leakage. The specific capacitance of the cobalt oxide strongly depends on the microstructure and morphology of the materials, which facilitate the electrolyte ion transport through the material more effectively. Sun et al. demonstrated a simple and effective approach to grow well-aligned 3D cobalt oxide nanowire arrays (Co3O4 NWAs) directly on carbon nanotube fibers (CNTFs) through CVD process . The Co3O4 NWAs/CNFs showed a specific capacitance of 734.25 F cm−3 (2210 mF cm−2) at 1.0 A cm−3 and a high energy density of 13.2 mW h cm−3 at a current density of 1.0 A cm−3. Graphene along with cobalt oxides can be used as a composite material for SCs because of its high conductivity, high surface area, high carrier mobility, and excellent mechanical strength. For example, an in situ synthesised Co3O4/graphene@NF hybrid composite electrode with a thickness of 13 nm exhibited a high specific capacitance of 1.75 F cm−2 at 1 mA cm−2 and a capacitance increase of 12.2% after 5000 cycles at 10 mA cm−2 (Figure 4(d-f)) . Tseng et al. demonstrate a binder-free and flexible SC based on CoO/graphene hollow nanoballs (GHBs) composite electrode . The as fabricated CoO/GHBs composite electrode exhibits high specific capacitance of 2238 F g−1 at a current density of 1 A g−1 and good rate capability of 1170 F g−1 at a current density of 15 A g−1. The excellent capacitive performance and high rate capability were accomplished by the synergistic combination of conductive GHBs with large surface areas and highly pseudocapacitive CoO. In addition, as fabricated SSC demonstrated a very high power density (6000 W kg−1 at 8.2 W h kg−1), high energy density (16 W h kg−1 at 800 W kg−1), good cycling stability (∼100% capacitance retention after 5000 cycles), and excellent mechanical flexibility at various bending positions. Recently, 3D-carbon aerogels (3D-CA) with appropriate electrical conductivity, high specific surface area and rich dielectric electrochemical stability when combined with the porous cobalt oxides can enabled the fabrication of an composite electrode with outstanding electrochemical performance.Co3O4/CA composite electrode which was synthesized through in situ growth method showed a specific capacitance of 350 F g−1 at 1 A g−1 and Energy density of 23.82 kW kg−1 at a power density of 95.96 W kg−1 (Figure 4(g-i)) . The as-prepared ASC device could be cycled reversibly in a potential range of 0.0 to 1 V at 1 A g−1 and showed a capacity retention of 210% over 6000 cycles. Zhu et al. adopted a facile hydrothermal method to synthesize self-assembled cobalt oxide (CoO) nanorod cluster on 3D-graphene foam (CoO-3DGF) which exhibits a very high performance compared with CoO nanorod clusters grown on Ni foam (680 F g−1) in terms of specific capacitance 980 F g−1 at 1 A g−1 and cycling stability of 103% over 10,000 cycles .
4.4 Carbon-binary metal oxide based composite electrode materials
Recently, binary metal oxides such as NiCo2O4, NiFe2O4, CoFe2O4, ZnMnO4, and ZnCo2O4 have attracted much attention due to higher electrical conductivity than individual metal oxide and provide higher capacitance due to more affluent redox reaction than individual components . Even though binary meal oxides possess better electrochemical performance than individual metal oxide extremely, they still suffer from inferior rate performance, low utilization rate and poor cycle stability. However, by incorporating carbon based materials improve their conductivity as well as power density due to high surface area, high conductivity and stable chemical properties of carbon based materials . Kumar et al. fabricated Carbon black (CB) decorated Ni/Co oxide composite electrode through by using the successive ionic layer adsorption and reaction (SILAR) method . Carbon black (CB) decorated Ni/Co oxide composite electrode with 7% weight percentage of CB exhibited a high specific capacitance of 1811 F g−1 at 0.5 mA cm−2 with excellent cyclic retention of 92% over 8000 cycles and delivered an impressive high energy density of 91 W h Kg−1 at a power density of 151 W Kg−1, which is significantly higher than pure Ni/Co oxide composite electrode as well as other carbon embedded composites. Veerasubramani et al. have adopted a novel approach to fabricate CNT-deposited CoMoO4/Ni foam through a hydrothermal method followed by dry reforming reaction (DRR) of propane and CO2 [46, 47]. The as fabricated CNT-deposited CoMoO4/Ni foam electrode achieved a maximum areal capacity of 160 μAh cm−2 at 1 mA cm−2 with excellent cyclic stability of ~105% over 3000 cycles and showed 22-fold higher performance than the heat-treated CoMoO4/Ni foam. The high electrochemical performance is due to the presence of CNTs on the surface of CoMoO4/Ni foam electrode, which increases the conductivity of the electrode and enhances the ion transport kinetics. Further as fabricated ASC device, consists of CNT-deposited CoMoO4/Ni foam as the positive electrode and reduced graphene oxide (rGO)-coated carbon cloth (CC) as the negative electrode stored a maximum areal energy density of 122 μWh cm−2 (29.04 Wh kg−1) at 2 mA cm−2 and delivered a high power density of 7,727 μWcm−2 (1835 W kg−1) 10 mA cm−2 with excellent capacitance retention of more than 95% of its initial capacitance over 1500 cycles. Soam et al. synthesized porous type of NiFe2O4/graphene nanocomposite electrode by a solution based process for supercapacitor application . The as-prepared NiFe2O4/graphene nanocomposite electrode exhibited a maximum specific capacitance of 207 Fg−1 at a scan rate of 5 mV/sec, which is almost 4 times larger than pure NiFe2O4 (60 Fg−1) and showed the capacitance retention of 95% over 1000 cycles. The significantly enhanced specific capacitance of the NiFe2O4/graphene nanocomposite electrode material is due to the synergic effect of high porous graphene sheets and NiFe2O4 particles, which are strongly interconnected together leading to a good electric/ionic conduction on the electrode and better contact of ions with the electrode materials. Zhou et al. reported a novel and green Cu2O template-assisted route based on “coordinating etching and precipitating” process for the synthesis of 3D porous reduced graphene (rGN)/NiCo2O4 film . The as-synthesized 3D rGN/NiCo2O4 film exhibited high specific capacitance of 708.36 F g−1 at a current density of 1 A g−1 with a rate retention of 82.2% as current density ranges from 1 to 16 Ag−1, and remarkable capacitance retention of 94.3% after 6000 cycles at a high current density of 10 A g−1.
5. Carbon-metal hydroxide composites electrode materials
Among the active materials, metal hydroxides have also been considered promising electrode materials for electrochemical SCs because of extremely high specific capacitance. Metal hydroxide in several forms such as Ni(OH)2, Co(OH)2, NiCo(OH)2, Cu(OH)2, FeOOH have been investigated as electrodes for SC [50, 51, 52]. These materials have large internal spaces for fast insertion and desertion of electrolyte ions. Moreover, these metal hydroxides can be synthesized using simple synthetic approaches. Metal hydroxide consists of stacked layers intercalated having interlayer space to occupy more ions hence larger capacitance.
5.1 Carbon-nickel hydroxide (Ni(OH)2) composite electrode materials
Ni(OH)2 is being considered as an attractive candidate as electrode in SCs because of its high theoretical capacitance (2358 F g−1). It can be prepared by a simple and low cost process. It has demonstrated good stability in alkaline electrolytes. Its low electrical conductivity is a barrier to achieve higher capacitance. Therefore, a thin region near the surface of nickel hydroxide contributes to the charge storage process due to diffusion-limited redox reactions. To obtain larger capacitance, it has to be utilized completely in the charge storage process. In this regard, researchers have generally adopted conductive additives to effectively improve utilization of active materials and result in larger capacitance. Kang et al. have used the same concept and deposited an ultrathin nickel hydroxide film on carbon-coated 3D porous copper structure in order to prepare binder-free conductive electrode (Figure 5(a-b)) . This electrode has short electron path distances and large electrochemical active sites, which improved structural stability for high performance SCs. A carbon coating was used to improve the electron transport behavior and to prevent the oxidation of Cu. Nickel hydroxide supported on mesoporous hollow dendritic three-dimensional-nickel exhibited a specific capacitance of 1860 F g−1 at a current density of 1 A g−1 (Figure 5(c)). It could retain 86.5% capacitance over 10,000 cycles. Tang et al. have prepared an additive-free, nano-architectured nickel hydroxide/carbon nanotube (Ni(OH)2/CNT) electrode for high performance SCs . This Ni(OH)2/CNT electrode was fabricated by depositing Ni(OH)2 nano-flakes on CNT bundles which were directly grown on Ni foams. The above electrode exhibited the specific capacitance of 3300 F g−1 and an aerial capacitance of 16 Fcm−2. Ma et al. have synthesized electrode of Ni(OH)2 nanosheet/3D GF framework using two methods, CVD and hydrothermal . They have compared the capacitive properties of Ni(OH)2 electrode/graphene fiber with Ni(OH)2/Ni foam and Ni(OH)2 nanosheet/carbon fiber cloth electrodes. Ni(OH)2 electrode with graphene fiber exhibited better performance in terms of specific capacitance and rate capability. The Ni(OH)2 nanosheet/graphene fiber electrode exhibited electrochemical capacitance as high as 2860 F g−1 at a current density of 2 A g−1, and maintains 1791 F g−1 at 30 A g−1.
5.2 Carbon-cobalt hydroxide (Co(OH)2) composite electrode materials
Co(OH)2 has recently received increasing attention as electrode for SC application because of its low cost and high capacitance.
Two possible reactions are suggested for the electrochemical reactions of Co(OH)2 in KOH electrolyte :
Co(OH)2 nano-sheet-decorated graphene-CNT composite structure has been designed for SC application . Suspensions method was used to prepare graphene-CNT composite by sonication and vacuum filtration. The graphene-CNT composite may offer high porosity with high conductivity, chemical stability and a three-dimensional structure. The vertically aligned Co(OH)2 nano-sheets were then deposited on 3D graphene-CNT composite by solution based process. The ASC of Co(OH)2 with graphene-CNT has shown a specific capacitance of 310 F g−1. The electrode exhibited an energy density of 172 W h kg−1 and maximum power density of 198 kW kg−1 in ionic liquid electrolyte 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfone)imide (EMI-TFSI). Zhang et al. have deposited Co(OH)2 on multi-walled CNT which were grown on the carbon paper substrate . The composite electrode showed the specific capacitance of 1083 F g−1 determined at a current density of 0.83 A g−1 in aqueous electrolyte. CNTs were added to Co(OH)2 in order to improve the electrical conductivity of the electrode. The interconnected nanosheets of the Co(OH)2 would help to facilitate the contact of the electrolyte with active materials, exhibiting good cycling stability and lifetime.
5.3 Carbon-iron oxy hydroxide (FeOOH) composite electrode materials
FeOOH has been recognized is an attractive electrode material for SC due to low cost, high theoretical specific capacitance, and broad potential window. In addition, the unique tunnel structure of FeOOH with open permeable channels are beneficial for ion transportation and shorten the diffusion path for electrolyte ion diffusion . However, the poor electrical conductivity and low specific surface area limited the use of FeOOH as a potential electrode for SC, which limited specific capacitance and rate capability . Alternatively, composite system by assembling FeOOH on the carbon based supporting materials (AC, carbon black, graphene, etc.) can be enhance the capacitive performance. Shen et al. synthesized radiating γ-FeOOH Nanosheets on CC substrate (γ-FeOOH NSs/CC) by a simple one-step electrodeposition method and investigated its pseudocapacitive behaviour in a typical ionic liquid [1-ethyl-3-methylimidazolium bis imide (EMIM-NTF2)] through electrochemical quartz crystal microbalance (EQCM). The charge storage is mainly due to the insertion and extraction of [EMIM]+ cations through the transport pathways offered by the crystalline network of γ-FeOOH during charging-discharging process. γ-FeOOH NSs/CC exhibited a good areal capacitance of 210 mF cm−2 at a current density of 1 mA cm−2 and the ASC device made of γ-FeOOH||APDC (activated polyaniline-derived carbon nanorods) solid-state flexible SCs acquired a high energy density of 1.44 mW h cm−3 at a current density of 3 A g−1 with a cycling stability of 80.5% retention over 2000 cycles (Figure 6(a-c)) . An amorphous FeOOH nanoflowers@multi-walled CNT (FeOOH NFs@ MWCNTs) composite was prepared by Sun et al. . The as-prepared composite electrode displays a high specific capacitance of 345 F g−1 at 1 A g−1 current density and outstanding rate performance (167 F g−1 at 11.4 A g−1) with good cycling stability of 76.4% over 5000 cycles. The outstanding electrochemical performance of the composite electrode is due to the mesoporous structure and high surface area of the electrode materials as well as fast ion/electronic transport and easy accessibility of the active materials to electrolytes. Liu et al. demonstrated FeOOH quantum dots (QDs)/graphene hybrid nanosheets, which exhibited a high specific capacitance of 365 F g−1 at a current density of 1 A g−1 with excellent capacitance retention of 89.7% of initial capacitance over 20000 cycles as well as a great rate capability (189 F g−1 at a high current density of 128 A g−1) (Figure 6(d-f)) . In addition, specific capacitance of the SC increased to 1243 F g−1 at 5 mV s−1 while the voltage window was extended from -0.8 to 0 V to -1.25 to 0 V but the cycling performance declined sharply. Wei et al. synthesized ultrathin α-FeOOH nanorods/graphene oxide (GO) composite by hydrothermal method, which exhibited high specific capacitance of 127 F g−1 at a current density of 10 A g−1, good cyclic performance of 85% capacitance retention over 2000 cycles, and excellent rate capability (100 F g−1 at 20 A g−1) as compared to than bare α-FeOOH nanorods . The outstanding electrochemical performance of α-FeOOH nanorods/GO composite is due to its unique structure, which provides fast electron/ions transport and high charging/discharging rate. 3D FeOOH/reduced graphene oxide/Ni foam (FeOOH/rGO/NF) based hybrid electrodes fabricated by the electrodeposition of FeOOH nanosheets on the rGO/Ni foam surface exhibited an exception high areal capacitance of 406.5 mF cm−2 at a scan rate of 10 mV s−1, which is 10-fold higher than the bare FeOOH/NF electrode (Figure 6(g-i)) . This high areal capacitance of FeOOH/rGO/NF is due to the improved conductivity and increased surface area, which not only provide a superior pathway for electron transfer, but also offer more active sites for energy storage. In addition, an ASC device made of 3D FeOOH/rGO/NF electrode as anode and MnO2@TiN electrode as cathode attained a remarkable maximum power density of 0.19 W cm−3 with maximum energy density of 0.48 mW h cm−3.
6. Carbon-transition metal dichalcogenides (TMDs) composite electrode materials
TMDs are layered inorganic materials with a chemical configuration of MX2, in which M is a transition metal element (M: Ti, Mo, V, W, Re, Ta), and X can be any chalcogenide element (X: S, Se, Te) (Figure 7(a)). Each MX2 unit cell is stacked together through Vander Waals force in such a way that transition metal layer is present in between the two chalcogen sheets . On the basis of crystal structure, there are two types of phases of TMDs, which are metallic 1T phase with an octahedral structure and semiconducting 2H phase with a trigonal structure. Recently, TMDs have been attracted great attention as SC electrode materials due to their large surface area, low cost, variable oxidation states, high mechanical properties, high chemical stability and easy synthesis . The variable oxidation states, large surface area, and active edges of TMDs allow electrical double layer and fast/reversible redox charge storage mechanisms and offer high energy storage capability in SCs. However, due to the inherently low conductivity, poor cycle life, large volume change during cycling and restacking limits their electrochemical performance as SC electrodes . For example, Soon et al. has synthesized sheet-like morphology of MoS2 by chemical vapor deposition method, which has a very large surface area favorable for double layer storage. But due to its poor electrical conductivity, it showed low specific capacitance of ∼100 F g−1 at a scan rate of 1 mV s−1 . Therefore, in order to improve the electrochemical performance of TMDs, they have been compositing with highly conducting/electroactive carbon based supporting materials by various top-down/bottom-up and both synthetic approaches. The synergic effect of carbon-TMDs based composite materials such as carbon offers conductive channels and increasing the interfacial contact, whereas TMDs provide a short ion diffusion path and followed by short electron transport path enhances the overall electrochemical performance of the SC.
6.1 Carbon-MoS2 composite electrode materials
MoS2/MWCNT nanocomposite synthesized by a hydrothermal method exhibited a large surface area and fast ionic transport properties and showed a high specific capacitance of 452.7 F g−1 with good cycling stability (95.8% retention after 1000 cycles), which is almost three times larger than the bare MoS2 (149.6 to 452.7 F g−1) . Ali et al. fabricated MoS2/graphene composite from bulk MoS2 and graphite rod through facile electrochemical exfoliation method and exhibited high specific capacitance of 227 F g−1 as compared with the exfoliated MoS2 (70 F g−1) and exfoliated graphene (85 F g−1) at a current density of 0.1 A g−1 . The high specific capacitance of MoS2/graphene composite is due to the synergistic effect between MoS2 and graphene. Ali et al. demonstrated the electrochemical performance of MoS2/CNT/GNF composite and compared the performance with MoS2/CNTs, MoS2/graphene nanoflakes . It has been noticed that the electrochemical charge storage performance has been improved by incorporation of the carbon materials into the composite and the composite showed a maximum specific capacitance of 104 F g−1 at a current density of 0.5 A g−1 with capacitance retention of 75% after the 1000 cycle at a scan rate of 10 mV/s. Another interesting MoS2-rGO/MWCNT fiber electrode was fabricated by incorporating rGO nanosheets and MoS2 into aligned MWCNT, which operated at a stable potential window of 1.4 V and exhibited high coulombic efficiency of 100% over 7000 cycles in the bending state (Figure 7(b-f)) . Zhang et al. reported an agarose induced technique to synthesize MoS2/carbon composite aerogel, which showed a high specific capacitance of 712.6 F g−1 at a current density of 1 A g−1 with cyclic stability of 97.3% over 13000 charge-discharge cycles (Figure 7(g-j)) . The high specific capacitance of MoS2/carbon composite aerogel is because of 3D intercalated network with hierarchal porous and interlayer MoS2 expanded structures, which were beneficial for easy ion transportation. 3D graphene/MoS2 composite electrode material has been synthesized by Sun et al and co-workers through a simple and facile one-step hydrothermal process . The as-synthesized composite electrode exhibited gravimetric capacitance of 410 F g−1 at a current density of 1 A g−1 and an excellent cycling stability of 80.3% over 10,000 continuous charge-discharge cycles at 2 A g−1 current density. The outstanding electrochemical performance of 3D graphene/MoS2 composite electrode is due to the 3D architecture of conducting network graphene and flower-like structure of MoS2, which enhances the electrolyte ions diffusion process.
6.2 Carbon-WS2 composite electrode materials
WS2 nanoplates supported on carbon fiber cloth (WS2/CFC) have been synthesized by a facile solvothermal process and used as electrode material for SC . The 3D network of CFC not only prevent the agglomeration of WS2 nanoplates but also enhances the ion transport efficiency due to low charge transfer resistance (Rct) of 0.1 Ω. The as fabricated WS2/CFC electrode exhibited a high specific capacitance of 399 F g−1 at 1 A g−1 current density with cyclic retention of 99% over charge-discharge 500 cycles, which is higher than compared with bare WS2. In addition, developing such composite of WS2 with the carbon fibre helps for fabricating wearable SCs which are in demand for wearable electronics. Yang et al. fabricated WS2@CNT hybrid film electrode by incorporating conducting CNTs into WS2. The WS2@CNT hybrid film with a unique skeleton structure showed a maximum specific area capacitance of 752.53 mF cm−2 at a scan rate 20 mV s−1 with very good cyclic stability by only loss of 1.28% capacitance after 10,000 cycles. In addition, a quasi-solid-state flexible SC made by WS2@CNT hybrid film exhibited excellent bendability under bending to 135 10, 000 times with the loss of 23.12% at scan rate of 100 mV s−1 . Tu et al. have been synthesized WS2/RGO hybrid material by using a simple molten salt process, which showed a high specific capacitance of 2508.07 F g−1 at 1 mV s−1 scan rate with excellent capacitance retention of 98.6% over 5000 cycles, due to synergic effect of highly conducting RGO and large charge-accumulating sites of WS2 networks. Likewise, Xu et al. demonstrated 3D composite of WS2 nanoflakes and quantum dots on N and S co-doped reduced graphene oxide (WS2/N,S-rGO) crumpled nanosheets through a rapid solution combustion synthesis of the precursor and subsequent gas-solid phase sulfurization process, which presented a significant specific capacitance of 1562.5 F g−1 at 1 A g−1 current density, and a rate capability of 780 F g−1 at 40 A g−1 (Figure 8(a-c)) . The high specific capacitance of WS2/N,S-rGO hybrids is because of synergistic effect between WS2 and N,S-rGO, where N,S-rGO provides larger contact surface area, excellent charge transport, and shorter ion diffusion path. Hierarchical MoSe2/C hybrid was successfully fabricated by facile one-step hydrothermal strategy, which composed of few-layered MoSe2 nanosheets and amorphous carbon obtained from the decomposition of the triethylene glycol. As fabricated hierarchical MoSe2/C electrode exhibited high specific capacitance of 878.6 F g−1 in comparison with the bare MoSe2 at current density of 1 A g−1 and maintained 98% of initial capacitance over 2000 cycles without obvious decrease. The superior electrochemical performances of MoSe2/C hybrid can be ascribed to hierarchical structure of MoSe2 and conducting nature of carbon, which help for providing large surface area for electrochemical reactions and enhancing charge carriers transfer at the electrolyte/electrode interface . Liu et al. fabricated VACNTF@MoSe2/NF composite electrode through a combined chemical vapor deposition method and solvothermal methods by growing MoSe2 nanoflakes on the vertically aligned carbon nanotube array film (VACNTF) with binder-free nickel foam as current collector . The as fabricated VACNTF@MoSe2/NF composite electrode exhibited high specific capacitance of 435 F g−1 at a current density of 1 A g−1 with outstanding cycling stability of 92% after 5000 cycles (Figure 8(d-f)). In addition, the VACNTF@MoSe2/NF composite based ASC displays a high energy density with 22 W h kg−1 for a power density of 330 W kg−1. Kirubasankar et al. MoSe2/graphene nanohybrid based electrode prepared by a simple and facile sonochemical route, which showed higher specific capacitance (945 F g−1) as compared to MoSe2 nanosheets (576 F g−1) at 1 A g−1 current density. Further, as fabricated ASC device based on MoSe2/graphene nanohybrid retains 88% of its capacitance over 3000 cycles and delivers an energy density of 26.6 W h kg−1 at a power density of 0.8 kW kg−1 (Figure 8(g,h)) . The high specific capacitance with better rate capability is due to the effective penetration and migration of electrolyte, reduction of the contact resistance and shortness of the diffusion path of ions between the electrode-electrolyte interface, which enhances the redox kinetics and provide maximum utilization of the electroactive area, so providing a high structural stability during charge-discharge processes. Similarly, Huang et al. demonstrated MoSe2/graphene on flexible Ni electrode, which could deliver a specific capacitance of 1422 F g−1and fully retention of initial capacitance over 1500 cycles . Wei et al. first time fabricated free-standing SC anode based on 3D MoSe2 nanoflowers (MoSe2 NFs) and hierarchically porous anisotropic carbonized delignified wood (CDW), which exhibited ultrahigh capacitance of 1043 mF cm−2 at a current density of 1 mA cm−2 and excellent cycling stability less than 5% capacitance loss over 5000 cycles. The ASC device was made by integration of 3D MoSe2 NFs@CDW anode and a common MnO2-based cathode, which exhibited a high capacitance of 415 mF cm−2 at a current density of 2.5 mA cm−2 with high energy density of 147 mW h cm−2 at power density of 2 mW cm−2. These results confirm that 3D MoSe2 NFs@CDW based anode can be used as a potential anode for the development of high-performance SCs .
In the past few decades, SCs have been extensively studied as energy storage devices and more focusing area in the multidisciplinary science over the world. The selection of high performance SC electrode materials based on high specific capacitance, low internal resistance and good stability. In this article, we have reviewed the carbon-based composite materials (i.e., metal oxide, metal hydroxide, TMDs composited with carbon materials) as promising SC electrode materials due to the synergic effect of the composite materials such as high surface area, interconnected porous structure, high electrical conductivity, excellent wettability towards the electrolyte, and presence of electrochemically active surface functionalities of the carbon supports which improves the EDL capacitance while metal oxide or metal hydroxide or TMDs enhances electrochemical performance through pseudocapacitive/faradaic charge-storage process. The carbon-based composite materials demonstrated herein usually possesses high specific capacity, impressive energy density and maintain long term stability with better mechanical flexibility. We also observe the microstructural changes in the carbon-based composite materials would be more favorable for fabrication of high performance supercapacitor. We also explained how the composite materials overcome the traditional obstacles while formulating the standard electrode designs as compare to individual components.
This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under grant numbers 107-2113-M-845-001-MY3.