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Review

Preparation and Application of Graphene and Derived Carbon Materials in Supercapacitors: A Review

by
Haiqiu Fang
1,
Dongfang Yang
2,
Zizhen Su
3,
Xinwei Sun
4,
Jiahui Ren
4,
Liwei Li
5,* and
Kai Wang
4,*
1
College of Chemistry and Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China
2
Xi’an Traffic Engineering Institute, Xi’an 710300, China
3
School of Materials Science and Engineering, Beihang University, Beijing 100190, China
4
College of Electrical Engineering, Weihai Innovation Research Institute, Qingdao University, Qingdao 266071, China
5
School of Control Science and Engineering, Shandong Unverisity, Jinan 250100, China
*
Authors to whom correspondence should be addressed.
Coatings 2022, 12(9), 1312; https://doi.org/10.3390/coatings12091312
Submission received: 11 July 2022 / Revised: 17 August 2022 / Accepted: 1 September 2022 / Published: 8 September 2022
(This article belongs to the Special Issue Advanced Technology in Environmental Remediation and Resource Utilization)
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Abstract

:
Graphene has recently attracted a wide range of research interests due to its rigorous two-dimensional structure and extraordinary electrical, thermal and mechanical properties. As a conductive agent, an activated carbon supercapacitor can obtain better performance. This paper summarizes the latest research progress, mainly from two aspects: (1) the preparation of an activated carbon base for a supercapacitor based on waste sugar solution and the relationship between pore structure and activation parameters, and (2) the application of the two-dimensional materials graphene and its composite materials in electric double-layer capacitors, graphene–polymer composite tantalum capacitors, graphene–transition metal oxide composite tantalum capacitors, and asymmetric super capacitors. The studies found that graphene and its composite materials have obvious advantages in improving the cycle efficiency, conversion rate, and energy density of supercapacitors, the overall energy efficiency of mechanical systems, and the chemical properties of nanoelectronics. Therefore, it is urgent to summarize these works in order to promote the next development. Graphene is expected to be effectively and environmentally quantified in the near future, and its application in supercapacitors will be further expanded and matured.

1. Introduction

In the 21st century, one of the biggest challenges in the world is energy consumption. However, the energy stored in nature is decreasing. Therefore, it is important to develop new energy storage and conversion systems. Under this premise, an electrode material with high conductivity, a large surface area, easy functionalization, and excellent electrochemical performance is required. Among these, carbon nanomaterials (for example, carbon nanotubes (CNTs), activated carbon, and graphene) have been widely studied for electrochemical energy storage systems, especially supercapacitors, due to their special physicochemical structures and excellent electrochemical properties [1,2,3,4,5,6].
Since 1985, when Kroto and his colleagues discovered fullerenes (C60), several interesting carbon nanomaterials have been isolated. In 1991, Iijima discovered carbon nanotubes (CNTs), and Novoselov et al. isolated two-dimensional graphite graphenes in 2004 [7,8,9]. Due to its unique physical properties, graphene has become an interesting super material. This new type of two-dimensional carbon nanostructure has attracted great attention in research in physics and chemistry, as well as in materials science. Today, graphene is the most attractive nanomaterial, not only because it is the thinnest known material in the universe but also because it is the strongest material to date, due to its excellent electrical, thermal, mechatronic and optical properties. It has high specific surface area, high chemical stability, high light transmittance, high elasticity, high porosity, biocompatibility, an adjustable band gap, and easy chemical functionalization. These extraordinary properties of graphene can be used to fabricate many novel electronic devices such as field effect transistors (FETs), sensors, and supercapacitors [10,11,12,13,14,15]. Although the specific capacitance of graphene is much higher than that of activated carbon, activated carbon is much cheaper to make than graphene. Therefore, activated carbon has an advantage when the ratio capacitance is within an acceptable range. Therefore, the preparation and porosity of activated carbon and graphene and their composites have become the focus of research in recent years [16,17,18,19,20,21]. In addition, the method of constructing asymmetric supercapacitors to improve their performance has also been widely researched. The main research results are shown in Table 1.
Specifically, the content is divided into five sections. In Section 2, we briefly introduce the double-layer capacitor, point out the relationship between its performance and the porous carbon structure, and analyze the relationship between the pore structure and the activation parameters. Section 3 introduces a graphene–polymer composite pseudo-capacitor. Section 4 introduces the graphene–transition metal oxide composite pseudo-capacitor. Asymmetrical supercapacitors are introduced in Section 5. Finally, a summary and prospects are proposed.

2. Electric Double-Layer Capacitor

The electric double-layer capacitor (EDLC) is a new type of energy storage device with the advantages of stable electrochemical performance, environmental protection, high power density, and a long cycle life [22,23]. The EDLC is a new capacitor based on the theory of the interfacial double-layer proposed by German physicist Helmholtz. As we all know, excess charges of opposite signs will appear on the surface of metal electrodes inserted into an electrolyte solution and on both sides of the liquid surface, resulting in a potential difference between phases. Then, if two electrodes are inserted into the electrolyte at the same time, and a voltage less than the decomposition voltage of the electrolyte solution is applied between them, then the positive and negative ions in the electrolyte will move rapidly to the poles under the action of the electric field, and form a tight charge layer on the surface of the two electrodes, namely the double electric layer. It is formed by the electric double layer and the traditional capacitor of the dielectric in the electric field under the action of the similar polarization charge, resulting in a capacitance effect which is closely similar to the electric double-layer plate condenser. However, due to the tighter-than-ordinary capacitor charge layer between the charge layer, the spacing distance is much smaller, and thus has greater capacity than the general capacitor [24,25,26]. The basic principle is as follows: when charging the electrode, the surface charge of the electrode in the ideal polarized electrode state will attract the opposite ions in the surrounding electrolyte solution, such that these ions will attach to the electrode surface to form a double charge layer, forming an electric double-layer capacitance. Because the distance between the two charge layers is very small (generally less than 0.5 nm), coupled with the special electrode structure, the surface area of the electrode is increased by tens of thousands of times, resulting in a great capacitance.
EDLC has been widely and successfully implemented in many areas, such as energy management/protection applications and day and night storage. The energy storage mechanism of EDLC is the electrostatic interaction at the electrode/electrolyte interface. Carbon-based electrode materials, especially graphene, are considered to be suitable electrode materials due to their high specific surface area (SSA), excellent electrical conductivity, good electrochemical stability, and inexpensive source of raw materials. SSA refers to the surface area per unit of mass of porous solid material. Because the outer surface area of a solid material is so small compared to the inner surface area that it can be ignored, this surface area usually refers to the inner surface area. The common unit is square meters per gram (m3/g). It is essential to determine the type and properties of the material. In addition, the specific capacitance of the EDLC is primarily determined by the effective specific surface area (SSA), which is accessible by the electrolyte ions. The effective SSA depends mainly on the total SSA and pore size distribution (micro, meso and macropores [27,28]).

2.1. Research Progress of Activated Carbon Based on Waste Sugar Solution

Among the many active electrode materials, graphene shows the best performance. However, compared with activated carbon materials, the preparation cost of graphene is relatively high. Improving the production process of graphene and reducing the production cost has become one of the most important research topics. In many pieces of research, the production method using waste sugar solution (WSS) as a raw material has been widely investigated. In the current processing of many sugar products (xylose, glucose, galactose), the production of waste sugar solution is inevitable. It contains all kinds of substances that are harmful to the environment. Therefore, recycling waste sugar solution can not only effectively protect the environment but also greatly reduce the wastage of resources. Therefore, the experiment of using WSS as the material for the preparation of activated carbon is most often introduced.
The abundant O-containing functional groups (-COOH, -OH and C=O) of WSS are the basis for the preparation of high-performance supercapacitor materials. Hao et al. prepared porous carbon structures using dried WSS as precursors using conventional methods. However, its performance is not satisfactory when it is applied to supercapacitors [29]. In order to enhance the SSA and optimize the pore structure, Wu et al. successfully fabricated a porous carbon mechanism with a relatively high specific capacity (342 F/g) by the direct high-temperature decomposition of WSS using the nitrogen atom doping method [30]. However, under the condition of a high temperature, the utilization of nitrogen atoms is not high. In order to solve this problem, Lin et al. and Hao et al. improved the production process, changing the high-temperature production method to a hydrothermal method, greatly improving the availability of nitrogen atoms [31]. The relationship between the pore structure and the activation parameters was investigated by using KOH as an activator to increase the SSA of carbon spheres (CS). At the same time, the oxygen-containing groups on the surface of the porous carbon spheres (PCS) generated during the activation of the Heat Transfer Compound (HTC) and KOH can enhance the specific capacitance by improving the wettability of the electrodes, and in the form of a tantalum capacitor. In summary, it is meaningful and important to prepare carbon electrode materials by recycling WSS for EDLC. The current studies on the preparation of porous carbon structures by WSS are shown in Table 2.

2.2. Research Progress of Hollow Carbon Spheres

Among various carbon materials, hollow carbon spheres have attracted more and more attention due to their unique structure and potential applications in supercapacitors. Hollow carbon spheres differ from active carbon in that they can control not only shell thickness but also surface characteristics. Therefore, they are more flexible than active carbon [32,33,34]. At present, there are two ways to prepare hollow carbon spheres: the hard-templating and soft-templatingg routes. The hard-templating method mainly includes the coating of the template with the carbon precursor, pyrolysis, and the removal of the template. Soft-templating routes are very attractive because of the easier removal of the templates. This method is usually adopted by the template for the colloidal system (emulsion, droplets, micelles and vesicles, and gas bubbles) [35].
The hollow carbon-sphere structure can act through “ion-buffering desorption”, such that the distance between the ions in the electrolyte and the pores in the shell is greatly reduced. Therefore, use the hollow carbon sphere as the electrode material of the supercapacitor shows high specific capacitance, according to Portet et al. and Murali et al. The hollow carbon structures proposed by Bhattacharjya et al. are 146 F/g and 122 F/g, respectively [36,37]. The paper presented a kind of core mesoporous shell carbon capsules, the specific capacity of which was 162 F/g. Under the same conditions, their performance is better than that of activated carbon. Similarly to the activated carbon material, the nitrogen doping method is also suitable for hollow carbon structures. It can not only make it have higher surface wetness but also increase SSA.

2.3. Research Progress of Graphene

Graphene is a well-known one-atom-thick sheet; it has 2-D monolayers composed of all-sp2 hybridized carbon atoms in a polyaromatic crystal lattice with a honeycomb structure. It has the advantages of active carbon and hollow carbon spheres; as such, it has become the first choice for energy storage devices such as supercapacitors [38,39]. In the traditional method of obtaining graphene, first, graphite is oxidized to produce graphite oxide; second, the graphite oxide is completely stripped by simple ultrasonic treatment; graphene is obtained by the reducing agent [40,41]. A graphene-based supercapacitor with ionic liquid electrolytes has an energy density value of 31.9 Wh/kg and a specific capacity of 135 F/g.
The effective capacity of graphene depends on the number of layers, and the problem of restacking will inevitably occur during use. In order to avoid this phenomenon, combining graphene with porous carbon, carbon nanotubes and carbon nanofibers has become a good solution [42]. In many current studies, different electrolytes have been used to improve the performance of graphene-based supercapacitors. For ionic liquid electrolytes, the specific capacity can reach 75 F/g [43]. In aqueous electrolytes, the specific capacitance can reach 135 F/g [44]. This can be seen for 2D and 3D macroscopic structures after peeling the graphene using different materials for the organic electrolyte to reach a specific capacity of 205 F/g [45]. A higher surface area can be obtained. Doping graphene with chemicals with electron donors and acceptors is also an effective measure to improve the electrochemical performance of graphene electrodes [46].

3. Graphene–Polymer Composite Pseudo-Capacitor

Recent developments in graphene and polymer composites have shown the very promising features of these composites for supercapacitor applications. Graphene can be divided into three types: graphene, graphene oxide, and reduced graphene oxide. Pseudo-capacitors are a type of supercapacitor that can mimic the behavior of EDLC for energy storage and release through multiple rapid and highly reversible processes of Faradaic redox reactions on the electrode surface [47,48]. The capacitance properties depend on the porosity of the material and the insertion and exit of ions at the electrode/electrolyte interface. Compared with EDLC, the specific capacitance of a pseudo-capacitor is higher. However, the cycle stability is low due to volume changes during charging and discharging. Conductive polymers (CPs), metal sulfides, metal oxides, metal carbides and metal nitrides are the most common pseudo-capacitor materials, and CPs are the representative electrode material of a pseudo-capacitor. Among them, CPs are probably the most representative, showing excellent pseudo-capacitive performance through rapid, reversible redox reactions. However, there are some limitations to this type of SC. For example, the most significant disadvantage is the rapid decay cycle stability during charging and discharging. This is mainly due to considerable mechanical degradation (such as expansion and contraction) and irreversible structural changes. At the same time, because of its compact structure, the power density of CPs that contact the electrolyte only with limited properties is very low [49].
In order to overcome the shortcomings of pseudo-capacitors, it has become a research hotspot to combine carbon materials with pseudo-capacitors to make new hybrid supercapacitors [50,51]. The combination of CP/graphene to form new composites has been widely studied for its advantages of containing both CP and graphene. Due to its excellent electrochemical performance, environmental stability, and rapid Faraday reaction, the method of combining polyaniline with graphene has been widely studied in recent decades. Zhao et al. proposed a synthesis using in situ high-gravity chemical polymerization means in a rolenating-bed (RPB) polyaniline/graphene method. In the process of preparation, it was pointed out that the ammonium persulfate/aniline mole ratio, graphene dosage, reactor type, and aniline concentration play a crucial role in the performance of the synthesized electrode materials [52]. Polyaniline is another material with high specific capacity, which is widely used in the manufacture of flexible electronic equipment due to its flexibility. However, polyaniline has poor cyclic stability. In order to alleviate this defect, Jia et al. synthesized polyaniline/graphene materials by electrochemical synthesis. The experiment showed that the specific capacity of the improved composite material can reach 202 F/g [53]. The current research on binary composites is shown in Table 3.
Binary composites synthesized from conductive polymers and graphene have been widely proven to have good electrochemical properties. Inspired by this, binary composites are combined with other materials (metal sulfide, metal oxide and non-metal oxide) to form ternary composites. Finally, the overall electrochemical performance can be maximized. Wang et al. proposed a material that combines SiO2, polyaniline and graphene simultaneously. The introduction of SiO2 can inhibit the accumulation of graphene’s hierarchical structure to a certain extent, and can effectively improve the ion exchange and interaction at the electrolyte/electrode interface. Moreover, the specific capacity reached an astonishing 727 F/g [59]. The current research on ternary composites is shown in Table 4.
It is well known that polymeric binders are a very important part of forming supercapacitor electrodes. However, one disadvantage of using polymeric binders is that they are generally not electrically conductive, and they may reduce the energy density of the supercapacitors. In combination with the binder polymer, graphene will compensate for the undesirable characteristics of the insulating polymer (e.g., its insulating properties, low surface area, and low specific capacitance). On the other hand, when graphene is made of a composite material with a conductive polymer, it will greatly improve the cycle performance and specific capacitance. In addition, the flexible nature of the graphene/polymer film enables flexible, wearable, conformal energy storage devices [67,68]. Whilst graphene/polymer supercapacitor devices exhibit innovative concepts and technologies that are unique to current state-of-the-art technologies, graphene/polymer composite-based supercapacitors still present many challenges to their full potential. One of the main challenges is to find a viable method for the low-cost mass production of graphene/polymer supercapacitor electrodes without compromising the micro/nanostructure of graphene due to stacking or aggregation. In addition, the integration of graphene–polymer supercapacitors with other electronic devices (e.g., solar cells, batteries) remains a practical challenge. In summary, graphene–polymer composites have great potential for supercapacitor applications.

4. Graphene–Transition Metal Oxide Composite Pseudo-Capacitor

Among the many materials used in supercapacitors, metal oxides have been widely used because of their superior physical and chemical properties. Compared to metal sulfides, metal oxides are easy to prepare (available for mass production), chemically stable, environmentally friendly, and compatible with a wide range of electrolytes. These advantages are essential for the production of durable and cost-effective supercapacitors. In addition, metal oxides have a higher theoretical surface area due to the combined effect of a high electrode surface area and various oxidation states. Metal oxide is based on Faraday’s interesting electrochemical redox reaction performance, but because of the restriction, it may not be in the form of the original as a supercapacitor electrode: (1) most of the metal oxides’ conductivities are very poor, which leads to the slice resistance of the electrode and the charge transfer resistance increases, which in turn leads to the attenuation rate capacity and power density of supercapacitor devices; (2) during the cycle of metal-oxide-based supercapacitors, strain will be generated on the surface of metal oxides, resulting in the structural degradation of electrode materials and a poor cycle life, and adjusting some of the properties of metal oxides is tricky. Therefore, it is important to develop composite and/or hybrid electrode materials by combining metal oxides with carbonaceous materials or conductive polymers.
Combining metal oxides with graphene has great potential. (1) Due to metal oxide/graphene’s heterozygous structure due to the injection of electrons from the graphene layer in the metal oxide, the hole concentration in graphene is increased, which can lead to high conductivity. (2) The high surface area of the metal oxide/graphene hybrid enhances the interaction between the electroactive material and the electrolyte, thereby improving the electrochemical performance. (3) In the metal oxide/graphene hybrid, the metal oxide nanosheets are sandwiched between the graphene nanosheets, which can effectively avoid the re-stacking of graphene layers. Zhang et al.’s MnO2/graphene hybrid was prepared by electrostatic precipitation with a specific capacity of 188 F/g. Due to electrostatic interactions, MnO2 nanosheets are dispersed on the surface of the graphene substrate. Compared with the original MnO2 and graphene, the 2D MnO2/graphene hybrid exhibits better electrochemical performance due to the simultaneous contribution of the excellent conductivity in graphene and the pseudo-capacitance properties of the MnO2 nanosheets [69]. The current studies on the heterozygous structure of metal oxides and graphene are shown in Table 5.

4.1. Preparation of the PGO-Ni Electrode and the Application of Graphene

This paper reviews a method for obtaining vibration energy by using a hybrid system containing a porous graphene nickel oxide (PGO-Ni) electrode and potassium chloride (KCl) solution. The results of this work are of great significance to improve the overall energy efficiency of the mechanical system
The electromechanical energy conversion system is composed of KCl solution flowing through the PGO-Ni electrode. Here, firstly, the foam nickel is ultrasonically cleaned in ethanol and acetone solution for 15 min, then 3 mol−1 hydrochloric acid is used to remove the surface oxide layer, and the sample is cleaned with deionized water to remove the surface oxide layer of foam nickel. After that, the foam nickel was immersed in 3 mg mL−1 graphene oxide suspension to adsorb some graphene oxide sheets on the surface. The graphene oxide suspension was prepared by a modified Hummers method. Then, the foam nickel with graphene oxide on the surface was dried at 60 °C in a blast dryer. After three repetitions of soaking and drying, the platinum group metal nickel electrode was prepared, and some graphene oxide sheets were adsorbed on the electrode surface. The platinum group metal nickel electrode was prepared by this method [84,85,86].
The SEM image of the PGO-Ni electrode is shown in Figure 1b. Compared with the typical SEM images of foam nickel, the surface of the foam nickel was covered by some graphene oxide sheets after impregnation and drying. In addition, the colors of foam nickel and PGO-Ni are also different in the optical image, as shown in the illustrations of Figure 1a,b. The XRD (performed using a Rigaku X-ray diffractometer) and EDS (performed using the energy spectrum analyzer of a JSM-6390A) of the PGO-Ni electrode are shown in Figure 1c,d. There is a peak at 12° in the XRD pattern. The chemical elements of carbon and oxygen are found in the EDS pattern, which further confirms the existence of graphene oxide.
In order to further enhance its performance, transition metal oxide nanoparticles (TMO NPs) are hybridized with graphene. Some of the disadvantages of nanoparticles (NPs) can be offset by graphene. Common disadvantages of semiconductor nanoparticles (NPs) include their relatively low conductivity and high recombination of photo generated electron–hole pairs. Additionally, during Li insertion–extraction processes in LIB applications, NPs cause large-volume expansions, which hinder their use in applications such as energy storage, sensing, advanced catalysis, solar cells, diodes, and biometrics. Therefore, the strongly coupled graphene–NP hybrid system appears to be promising to overcome these problems.
Among various graphene-based materials, graphene–transition metal oxide nanoparticles have recently attracted great research interest due to their unique structural advantages. In reference [84], the paper mainly introduces the influence of pH on the structural, optical and electrical properties of graphene–vanadium oxide nanoparticle (rGO/VO-NPs) nanocomposites. This study reveals the high crystallinity of rGO/VO-NPs nanocomposites and the graphene layer formed on the surface of vanadium oxide nanoparticles, ensuring good electrical contact and higher conductivity. In addition, UV-Vis (absorbance) confirmed an improvement in optical properties. Therefore, the growth conditions have a great influence on the synthesis of the final material. The use of these nanocomposites is a promising way to develop technology applications, particularly for energy storage devices that use lithium-ion batteries.

4.2. The Effect of Vanadium Oxide on Graphene

The experimental results demonstrate the pH effect of vanadium oxide growth on the graphene layer. FTIR spectroscopy revealed the formation of new bonds which, due to the functionalization of oxygen functional groups on the rGO surface, indicate the reoxidation of rGO nanosheets after coating with vanadium oxide nanoparticles. The control of the NaOH concentration has an important influence on the formation of VO-NPs in the composite. The optical properties indicate that the absorbance intensity of rGO/VO-NPs nanocomposites increases in the acidic range and gradually decreases in the alkaline range, which may be due to the stoichiometry and crystallinity of vanadium oxide nanoparticles. In addition, the mobility of charge carriers increases with the increasing pH, indicating that a large number of charge carriers can be used for conduction. Thus, the uniform coating of the graphene layer around the surface of the vanadium oxide nanoparticles ensures good electrical contact and exhibits higher conductivity under the effect of pH. In general, this is a new insight into controlling the preparation of such mixtures and tailoring such material properties to improve their efficiency and performance in many applications, particularly as storage devices.

4.3. The Effect of Polyoxometalate-Based Complexes on Graphene

Polyoxometalate has emerged as a promising candidate for the design of nanocomposites with unique properties and improved functionality. Polyoxometalate is an inorganic cluster composed of oxygen and early transition-metal atoms, usually in the highest oxidation state, with d0 or d1 electron configurations. Polyoxometalate is typically an extensive library of electrons that can easily perform rapid multielectron redox reactions and transfer multiple electrons per molecule while maintaining their intrinsic stability. Therefore, they are considered to be promising electroactive materials for energy storage applications. However, the inherent low conductivity and high solubility of these metal oxides in many solvents seriously hinder their direct use as electrode materials in supercapacitors. Polyoxometalate clusters are usually immobilized on chemically stable, high-surface-area substrates. Therefore, anchoring paraformaldehyde, which exhibits high redox activity, into capacitive carbon electrodes (e.g., GO, CNTS, AC) can increase the net specific energy and operating voltage range of storage devices. Various properties of polyoxometalate-based hybrid materials have been studied, including electrochemical adsorption and catalytic capacity. Sukanya Maity et al. doped different masses of K2H5[NiV14O40](NiV14) into porous activated carbon, and found that the resulting complex had the electrochemical enhancement function. When applied to supercapacitors, the specific capacitance reaches 375 F/g [87]. In order to further improve its electrochemical performance, two metaloxates ([PVMo11O40]4− and [PV2Mo10O40]5−) were integrated with AC in a new way. The composite shows a high specific capacitance of 450 F/g. In addition, metaloxate not only plays an important role in improving the electrochemical performance of AC but also improves the electrochemical performance to a large extent when the metaloxate is applied to graphene materials [88]. Decavanadate–graphene oxide nanocomposite is an electrode material for electrochemical capacitors. It uses Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The electrochemical behavior of the composite electrode was studied using a neutral 1M sodium sulfate (Na2SO4) solution in a three-electrode cyclic voltammetry (CV) system, which showed a specific capacitance of 306 F/g at a scan rate of 5 mV/s [89]. In addition, MnV14O40 is impregnated on a high-surface-area substrate of GO; the synergistic effect between GO and MnV14 provides a better route for ion transport to the interface, thus improving the conductivity and diffusion ability of the nanohybrid. Oxygen-containing functional groups in GO act as active sites anchored to the MnV14 cluster. Therefore, the surface modification of the MnV14 cluster improves the specific capacitance of the nanohybrid material, and has significant electrical and mechanical stability [90].

5. Asymmetric Super Capacitors Based on Graphene

The energy density of supercapacitors is much lower than the energy density of batteries, thus limiting their practical application. The method by which to construct high-energy-density supercapacitors is not particularly important without sacrificing power density and cycle life. Currently, two effective strategies are widely used to increase the energy density of supercapacitors. One is to enhance the specific capacitance of the electrode material; the other is to increase the operating voltage window by developing an asymmetric supercapacitor device with a battery-type Faraday electrode and a capacitor-type electrode. This paper summarizes the second method; the hollow NiCo2O4 material with a high surface area is introduced, and its influence on the energy density of asymmetric supercapacitor is analyzed [91,92,93,94]. An asymmetric supercapacitor was assembled by using the hollow NiCo2O4 nanosphere electrode as the positive and the AC electrode as the negative, which manifests high energy density with excellent cycling stability.
Figure 2a shows a scanning electron microscope (SEM) image of SiO2 nanospheres. SiO2 nanospheres with a smooth surface and a highly uniform diameter of about 140–170 nm were obtained by the improved St ö BER method. Subsequently, the prepared SiO2 nanospheres were used as hard templates, and hollow NiCo2O4 nanospheres were prepared by simple hydrothermal process and post-annealing treatment. Figure 2b shows a hollow NiCo2O4 nanosphere with a diameter of 200–220 nm assembled from a number of NiCo2O4 nanosheets. The hollow nanostructures are clearly visible from the figure. This hollow nanostructure can provide more active sites for ion access and transport, and can thus shorten the diffusion distance of ions and electrons, which is very important for tantalum capacitors. For comparison, NiCo2O4 nanospheres with a diameter of 4.7 μm were also prepared under similar synthesis conditions, as shown in Figure 2c. The crystal phases and structures of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres were characterized by X-ray diffraction (XRD), as shown in Figure 2d. Through the experiment, no residue or contaminant was detected, indicating that the purity of the sample was high.
The Brunauer-Emmett-Teller (BET) surface area of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres was further investigated by nitrogen adsorption–desorption measurements. The increase in the BET surface area of hollow NiCo2O4 nanospheres can be explained by their fractional hollow nanostructures. The pore size distribution curve is shown in the inset of Figure 3, in which the average pore diameters of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres were 3.8 nm and 4.3 nm, respectively (calculated by the Barrett-Joyner-Halenda ((BJH) method). The larger BET surface area of the hollow NiCo2O4 nanospheres can provide more electrochemical reaction sites for efficient electrolyte ion transport, thereby improving the supercapacitor’s performance.
The electrochemical properties of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres were measured using a three-electrode configuration with the Pt plate counter electrode and the SCE reference electrode in a 1 M KOH aqueous electrolyte. Figure 4a shows cyclic voltammetry (CV) curves for hollow NiCo2O4 nanospheres at various scan rates in the range of 10–50 mV/s. The CV curve shows a well-defined redox peak indicating the tantalum capacitance of the electrode during charge storage.
It is worth noting that as the scanning rate increases, no significant changes are observed in the shape and position of the oxidation and reduction peaks, indicating that the hollow NiCo2O4 nanosphere electrode has good capacitance characteristics and a high rate of performance. Figure 4b compares the CV curves of a hollow NiCo2O4 nanosphere electrode and a NiCo2O4 microsphere electrode at a scan rate of 50 mV/s. It can be seen that the CV integral area of the hollow NiCo2O4 nanosphere electrode is significantly larger than the CV integral area of the NiCo2O4 microsphere electrode, indicating that the electrochemical capacitance of the hollow NiCo2O4 nanosphere electrode is significantly improved. Figure 4c shows the constant current charge and discharge (CD) curves for hollow NiCo2O4 nanosphere electrodes between 0 and 0.45 V at different current densities. The charging and discharging platforms can be clearly seen in the CD curve, indicating the presence of the Faraday process. In addition, these CD curves are approximately symmetrical, indicating that the hollow NiCo2O4 nanosphere electrode has good electrochemical reversibility. Figure 4d shows a comparison of the CD curves for hollow NiCo2O4 nanosphere electrodes and NiCo2O4 microsphere electrodes at the same current density of 1 A/g. Obviously, the hollow NiCo2O4 nanosphere electrode has a longer discharge time and a higher capacity than the NiCo2O4 microsphere electrode. As shown in Figure 4e, the specific capacitance of the hollow NiCo2O4 nanosphere electrode and the NiCo2O4 microsphere electrode was calculated. The hollow NiCo2O4 nanosphere electrode produced a significant specific capacity of 1229 F/g at 1 A/g, which is much larger than that of the NiCo2O4 microsphere electrode (869 F/g). In addition, the hollow NiCo2O4 nanosphere electrode showed a significantly better rate of performance than the NiCo2O4 microsphere electrode. When the capacitance density of the hollow NiCo2O4 nanosphere electrode increased from 1 to 25 A/g, 83.6% of the capacitance was retained. The cyclic stability of the hollow NiCo2O4 nanosphere electrode and the NiCo2O4 microsphere electrode was 3000 cycles at 50 mV/s (Figure 4f). After 3000 cycles, the total permittivity retention of the hollow NiCo2O4 nanosphere electrode was 86.3%, while the permittivity retention of the NiCo2O4 microsphere electrode was 83.7%. The remarkable electrochemical properties of hollow NiCo2O4 nanospheres can be attributed to their uniform fractionated hollow nanostructures with high specific surface areas, which can provide more electroactive sites for rapid ion implantation in the entire electrode material, thereby improving the electrode material’s utilization rate [17,18,19,95,96,97].
In order to further evaluate the practical application effect of the hollow NiCo2O4 nanosphere electrode, a hollow NiCo2O4 nanosphere electrode was used as the positive electrode, the AC electrode was used as the negative electrode, and the asymmetric supercapacitor was assembled by the separator. Figure 5a shows the CV curve of a NiCo2O4//AC-ASC device collected at a scan rate of 50 mV/s in different voltage windows. As expected, the stable potential window of the fabricated device can be extended to 1.5 V. A typical CV curve for a NiCo2O4//AC-ASC device is shown in Figure 5b. These CV curves exhibit similar shapes, indicating that the asymmetric supercapacitor device has excellent capacitive performance. Based on the total mass of the active material of the two electrodes, the specific capacitance of the NiCo2O4//AC-ASC device is shown in Figure 5c. Figure 5d shows the energy and power density of a NiCo2O4//AC-ASC device. It is worth noting that these results are superior to other previously reported asymmetric supercapacitors. Figure 5e shows the Nyquist impedance spectrum of the fabricated NiCo2O4//AC-ASC device. It is worth noting that the equivalent series resistance (ESR, the intercept on the real axis) is estimated to be 0.6 Ω, indicating that the internal resistance of the NiCo2O4//AC-ASC device is low. The cycle performance of the fabricated asymmetric supercapacitor device was evaluated by repeating the CD test for 2000 cycles at a large current density of 5 A/g. As shown in Figure 5f, the total specific capacitance can remain approximately 87.8% even after 2000 cycles. In addition, it maintains significant coulombic efficiency close to 100% in continuous cycles.
In summary, hollow NiCo2O4 nanospheres exhibit excellent electrochemical performance. The specific capacitance and rate of performance of the hollow NiCo2O4 nanosphere electrode are much better than those of the NiCo2O4 microsphere electrode. In addition, the asymmetric supercapacitor is assembled with hollow NiCo2O4 nanosphere electrodes and AC electrodes, and the stable potential window can be extended to 1.5V. Importantly, the device has a high energy density of 21.5 Wh/kg, an significant cycle performance and coulombic efficiency.

6. Summary and Outlook

Carbon material, metal oxide, and chlorinated paraffin are three commonly used electrode materials for supercapacitors. Metal oxides have higher specific capacity, but their low conductivity, high cost and environmental pollution limit their application in supercapacitors. Graphene is one of the ideal supercapacitor materials due to its superior electrochemical properties (such as high conductivity) and high specific surface area.
It is expected that the remarkable electronic properties of graphene will bring a new era to nanoelectronics. The excellent electronic properties of graphene are critical for many device applications. Graphene—with its good electron conductivity, large specific surface area, and suitable porosity—is an ideal supercapacitor electrode material. It can effectively improve the power and energy density of supercapacitors: on the one hand, it can effectively avoid the agglomeration problem of graphene; on the other hand, it can improve the shortcomings of the poor conductivity, low specific capacitance and poor power characteristics of traditional materials. The most direct application of graphene may be in the design of tough, lightweight materials by spreading a small amount of graphene in the polymer. The composite is electrically conductive and can withstand much higher temperatures than polymers.
At present, supercapacitors have high power density but still have the problem of low energy density. The low ionic conductivity (especially for the solid electrolytes used in flexible supercapacitors and organic electrolytes) and narrow potential windows of electrolytes limit the performance of supercapacitors in terms of power density and energy density. This problem can be solved by using novel electrolytes with high ionic conductivity and a large electrochemical potential window. Recently, there have been new efforts to manufacture hybrid energy storage devices, such as lithium/sodium ion hybrid capacitors, which consist of both battery-type and capacitor-type electrodes, and can have both high energy and power density. Graphene and its composite materials have shown great advantages as electrodes for supercapacitors. However, their electrochemical properties largely depend on how the two components interact and the resulting structure/morphology. In this regard, the structure and interface of electrode materials must be optimized in order to provide more electroactive sites and greatly improve ion transport kinetics. One of the main goals in the design of new electrode materials is to improve their rate performance at high current densities, which requires the electrode materials to have high conductivity.
However, regarding how to give full play to the excellent electrical properties of graphene and accelerate its industrialization process, this paper believes that we should also pay attention to the following aspects:
(1)
Improving the uniformity of the distribution of nanoparticles on the surface of graphene, controlling the morphology and structure of the nanoparticles, and increasing the specific capacitance need further study.
(2)
We should improve the synergistic effect of graphene composites, prevent graphene agglomeration, and improve the wettability of composite electrode materials and electrolytes without affecting the conductivity of graphene.
(3)
We should simplify the electrode material synthesis process, improve its production efficiency, and reduce its production costs.
(4)
The general graphene-based composite electrode materials only discuss mass-to-capacitance capacitance, and there is very little research on volume-to-capacitance.
In summary, graphene supercapacitors also need to improve relevant theoretical research and develop new synthetic processes. In the near future, it is foreseeable that with the efficient and environmentally-friendly production of graphene, its application in supercapacitors will mature.

Funding

This paper has been supported by the project of Industry University Cooperation and Collaborative Education of the Ministry of Education (No. 202102152006).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Characteristics of PGO-Ni electrodes. Reprinted with permission (a) SEM observations of the microstructure of nickel foam; (b) SEM observations of the micro-structure of PGO-Ni electrode; (c) XRD patterns of PGO-Ni electrode showing the peak of graphene oxide at 12° and the peaks of nickel at 44°, 52°, 76°. (d) EDS patterns of PGO-Ni electrode showing the elements C, O, Ni. from Ref. [84]. Copyright 2017, Elsevier B.V.
Figure 1. Characteristics of PGO-Ni electrodes. Reprinted with permission (a) SEM observations of the microstructure of nickel foam; (b) SEM observations of the micro-structure of PGO-Ni electrode; (c) XRD patterns of PGO-Ni electrode showing the peak of graphene oxide at 12° and the peaks of nickel at 44°, 52°, 76°. (d) EDS patterns of PGO-Ni electrode showing the elements C, O, Ni. from Ref. [84]. Copyright 2017, Elsevier B.V.
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Figure 2. SiO2 nanospheres and NiCo2O4 nanospheres. SEM images of (a) SiO2 nanospheres, (b) hollow NiCo2O4 nanospheres, (c) NiCo2O4 microspheres, (d) XRD patterns of hollow NiCo2O4 nanospheres and NiCo2O4 microspheres. Reprinted with permission from Ref. [86]. Copyright 2017, Elsevier Inc.
Figure 2. SiO2 nanospheres and NiCo2O4 nanospheres. SEM images of (a) SiO2 nanospheres, (b) hollow NiCo2O4 nanospheres, (c) NiCo2O4 microspheres, (d) XRD patterns of hollow NiCo2O4 nanospheres and NiCo2O4 microspheres. Reprinted with permission from Ref. [86]. Copyright 2017, Elsevier Inc.
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Figure 3. Nitrogen adsorption–desorption isotherms of hollow NiCo2O4 nanospheres and NiCo2O4 microspheres. Reprinted with permission from Ref. [86]. Copyright 2017, Elsevier Inc.
Figure 3. Nitrogen adsorption–desorption isotherms of hollow NiCo2O4 nanospheres and NiCo2O4 microspheres. Reprinted with permission from Ref. [86]. Copyright 2017, Elsevier Inc.
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Figure 4. Hollow NiCo2O4 nanospheres’ characteristic curve. (a) CV curves of the hollow NiCo2O4 nanospheres at varied scan rates, (b) CV curves of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres at a scan rate of 50 mV/s, (c) CD curves of the hollow NiCo2O4 nanospheres at different current densities, (d) CD curves of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres at a current density of 1 A/g, (e) The specific capacitances and capacitance retentions of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres, (f) Cycle performance of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres at 50 mV/s for 3000 cycles. Inset is schematic of the charge storage advantage of the hollow NiCo2O4 nano-spheres. Reprinted with permission from Ref. [86]. Copyright 2017, Elsevier Inc.
Figure 4. Hollow NiCo2O4 nanospheres’ characteristic curve. (a) CV curves of the hollow NiCo2O4 nanospheres at varied scan rates, (b) CV curves of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres at a scan rate of 50 mV/s, (c) CD curves of the hollow NiCo2O4 nanospheres at different current densities, (d) CD curves of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres at a current density of 1 A/g, (e) The specific capacitances and capacitance retentions of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres, (f) Cycle performance of the hollow NiCo2O4 nanospheres and NiCo2O4 microspheres at 50 mV/s for 3000 cycles. Inset is schematic of the charge storage advantage of the hollow NiCo2O4 nano-spheres. Reprinted with permission from Ref. [86]. Copyright 2017, Elsevier Inc.
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Figure 5. NiCo2O4//AC-ASC characteristic chart. (a) CV curves of NiCo2O4//AC-ASC at different scan voltage windows, (b) CV curves, (c) The specific capacitances (based on the total mass of active materials), (d) Ragone plots, (e) Nyquist plots, (f) Cycle performance and Coulombic efficiency of NiCo2O4//AC-ASC device. Inset in (c,f) is CD curves of NiCo2O4//AC-ASC at different current densities and the first 10th CD curves of NiCo2O4//AC-ASC device, respectively. Reprinted with permission from Ref. [86]. Copyright 2017, Elsevier Inc.
Figure 5. NiCo2O4//AC-ASC characteristic chart. (a) CV curves of NiCo2O4//AC-ASC at different scan voltage windows, (b) CV curves, (c) The specific capacitances (based on the total mass of active materials), (d) Ragone plots, (e) Nyquist plots, (f) Cycle performance and Coulombic efficiency of NiCo2O4//AC-ASC device. Inset in (c,f) is CD curves of NiCo2O4//AC-ASC at different current densities and the first 10th CD curves of NiCo2O4//AC-ASC device, respectively. Reprinted with permission from Ref. [86]. Copyright 2017, Elsevier Inc.
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Table
Table 1. Main research results on the properties of graphene supercapacitors and porous activated carbon.
Table 1. Main research results on the properties of graphene supercapacitors and porous activated carbon.
ResearchersResearch TechniqueResearch Results
Zhu C et al. [2]Three-Dimensional Hierarchical Graphene Aerogels with Periodic MacroporesPower densities (>4 kW kg−1)
Shao YL et al. [4]Micro-Super capacitors and fiber-type Super capacitorsProved that graphene material in wearable super capacitor application prospect
Yan J et al. [5]Integrating Super capacitors with other applicationsDeveloped multi-functional super capacitor
Liu L et al. [6]Structure-designed fabrication of all-printed flexibleHigher energy density of asymmetric Super capacitors can be achieved (from 0.00177 mWh cm−2 to 0.00687 mWh cm−2)
Zhu S et al. [8]Design and construction of three-dimensional CuO/polyaniline/rGO ternary hierarchical architecturesEnergy density of 126.8 Wh kg−1 with a power density of 114.2 kW kg−1 at a current density of 1.0 A g−1
Sun HT et al. [9]Three-dimensional holey-graphene/niobia composite architectures for ultrahigh-rate energy storageThe highly interconnected graphene network in the 3D architecture provides excellent electron transport properties
Yu DS et al. [10]Scalable synthesis of hierarchically structured carbon nanotube-graphene fibresEnergy density of similar to 6.3 mWh cm−3
Zhang L et al. [12]Freestanding three-dimensional reduced graphene oxide/MnO2 on porous carbon/nickel foam as a designed hierarchical multihole Super capacitor electrodePower density of 13.5 kW kg−1
Wang X et al. [13]Dielectric and optical properties of porous graphenes with uniform pore structuresChemical synthesis for graphenes with uniform pore structures opens a new way for the precise modulation toward the performances of graphene-based materials.
Zhou YA et al. [14]Transition-metal single atoms in nitrogen-doped graphenes as efficient active centersThe results shed light on the potential applications of TM and N co-doped graphenes as efficient single-atom bifunctional catalysts for water splitting
Table
Table 2. Research progress and properties of porous carbon structures prepared by WSS.
Table 2. Research progress and properties of porous carbon structures prepared by WSS.
ResearchersMethodSpecific Capacitance
Hao et al. [29].Typical carbonation/activation method240 F/g
Wu et al. [30].Nitrogen doping method342 F/g
Lin et al. [31].Hydrothermal treatment406 F/g
Hao et al. [29].Hydrothermal treatment296 F/g
Table
Table 3. Electrochemical performance of some of the previously reported ternary composites.
Table 3. Electrochemical performance of some of the previously reported ternary composites.
ResearchersMaterialsSpecific Capacitance
Y. Xu et al. [53].Nano graphene platelet/polyaniline269 F/g
M. Xue et al. [54].Graphene oxide patterns970 F/g
Y. Meng et al. [55].Graphene/polyaniline composite385 F/g
L. Liu et al. [56].Nanostructured graphene composite224 F/g
Z.-S. Wu et al. [57].Ultrathin printable grphene348 F/g
J.W. Park et al. [58].Graphene/polyselephene293 F/g
Table
Table 4. Electrochemical performance of some of the previously reported ternary composites.
Table 4. Electrochemical performance of some of the previously reported ternary composites.
ResearchersMaterialsSpecific Capacitance
Dywili, N.R. et al. [60].Graphene Oxide Decorated Nanometal-Poly227.2 F/g
Xu, Z. et al. [61].zinc sulfide/reduced graphene oxide/conductive polymer722 F/g
Golkhatmi, S.Z. et al. [62].nickel oxide/graphene/Polyaniline hybrid970.85 F/g
Azizi, E. et al. [63].reduced graphene oxide/polyindole/gamma—Al2O3308 F/g
Ramesh, S. et al. [64].Co3O4graphene oxide/polyindole composite680 F/g
Li, S. et al. [65].TiO2@PPy/rGO462.1 F/g
Wang, H. et al. [66].Graphene Hybrids Embedded with Silica727 F/g
Table
Table 5. Study on the heterozygous structure of metal oxides and graphene.
Table 5. Study on the heterozygous structure of metal oxides and graphene.
ResearchersMaterialsSpecific Capacitance
Wang Y et al. [70].CeO2 nanoparticles/graphene18 F/g
Dong X et al. [71].3D Graphene-Cobalt oxide1100 F/g
He G et al. [72].Co3O4@graphene nanocomposite415 F/g
Qu Q et al. [73].2D sandwich-like sheets of iron oxide grown on graphene349 F/g
Wang Z et al. [74].Fe2O3–graphene nano composite226 F/g
Wang Q et al. [75].Fe3O4 nanoparticles grown on graphene220 F/g
Peng L et al. [76].MnO2/Graphene267 F/g
Zhao K et al. [77].Mn3O4@N-doped carbon/graphene456 F/g
Wang C et al. [78].Hierarchical composite electrodes of nickel oxide nanoflake 3D graphene1829 F/g
Wang W et al. [79].Hydrous ruthenium oxide nanoparticles anchored to graphene502.78 F/g
Li F et al. [80].Graphene/SnO243.4 F/g
Zhang Z et al. [81].TiO2—Graphene206.7 F/g
Wang H et al. [82].Three-dimensional Graphene/VO2426 F/g
Perera SD et al. [83].Vanadium oxide nanowire—Graphene80 F/g
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Fang, H.; Yang, D.; Su, Z.; Sun, X.; Ren, J.; Li, L.; Wang, K. Preparation and Application of Graphene and Derived Carbon Materials in Supercapacitors: A Review. Coatings 2022, 12, 1312. https://doi.org/10.3390/coatings12091312

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Fang H, Yang D, Su Z, Sun X, Ren J, Li L, Wang K. Preparation and Application of Graphene and Derived Carbon Materials in Supercapacitors: A Review. Coatings. 2022; 12(9):1312. https://doi.org/10.3390/coatings12091312

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Fang, Haiqiu, Dongfang Yang, Zizhen Su, Xinwei Sun, Jiahui Ren, Liwei Li, and Kai Wang. 2022. "Preparation and Application of Graphene and Derived Carbon Materials in Supercapacitors: A Review" Coatings 12, no. 9: 1312. https://doi.org/10.3390/coatings12091312

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