image
Home Journals Progress in Chemistry
Progress in Chemistry

Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

About  /  Aim & scope  /  Editorial board  /  Indexed  /  Contact  / 
Online first  /  Current issue  /  All issues  /  Top access  /  RSS

Progress in Chemistry >

2023 , Vol. 35 >Issue 7: 1005 - 1017

DOI: https://doi.org/10.7536/PC220811

Review

Preparation of Heteroatom Doped Graphene and Its Application as Electrode Materials for Supercapacitors

  • Yunpeng Wu 1, 2 ,
  • Xiaofeng Wang 1 ,
  • Benxian Li 1 ,
  • Xudong Zhao , 1, * ,
  • Xiaoyang Liu , 1, *
Expand
  • 1 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University,Changchun 130012, China
  • 2 School of Chemistry and Environmental Engineering, Changchun University of Science and Technology,Changchun 130022, China
* Corresponding author e-mail: (Xiaoyang Liu);
(Xudong Zhao)

Received date: 2022-08-15

  Revised date: 2023-02-15

  Online published: 2023-06-15

Supported by

National Natural Science Foundation of China(22171101)

Fold

Abstract

Owing to its vast surface area and remarkable electrical conductivity, graphene has attracted extensive attention in the realm of electrochemical energy storage. Nevertheless, its volumetric energy density as an electrode material is quite low, thus presenting certain difficulties in its application as an electrode material. Heteroatom doping is a viable approach to enhance the electrochemical properties of graphene, thereby augmenting the energy storage capability of graphene as an electrode material. This paper provides a summary of the preparation of heteroatom-doped graphene, examines how heteroatom doping affects graphene’s electrochemical properties, explores the application of graphene in supercapacitors, and finally looks ahead to the future development course of this research domain.

Contents

1 Introduction

2 Preparation of heteroatom doped graphene

2.1 Chemical vapor deposition(CVD)

2.2 Chemical synthesis

2.3 Mechanical ball milling

2.4 Hydrothermal

2.5 Other methods

3 Application of heteroatom doped graphene as electrode material for supercapacitor

3.1 Nitrogen doping

3.2 Boron doping

3.3 Phosphorus doping

3.4 Sulfur doping

3.5 Other heteroatoms doping

3.6 Co-doping

4 Conclusion and outlook

Key words： graphene; heteroatoms-doping; supercapacitors; energy storage

Cite this article

Yunpeng Wu , Xiaofeng Wang , Benxian Li , Xudong Zhao , Xiaoyang Liu . Preparation of Heteroatom Doped Graphene and Its Application as Electrode Materials for Supercapacitors[J]. Progress in Chemistry, 2023 , 35(7) : 1005 -1017 . DOI: 10.7536/PC220811

1 Introduction

Graphene is a two-dimensional carbon material, which has attracted wide attention from the scientific community since its discovery[1]. Its special crystal structure is composed of a single layer of carbon atoms in the form of sp2 hybrid, each carbon atom is connected to three surrounding carbon atoms with a bond angle of 120 ° and a bond length of 1.4242 Å[2,3]. Graphene has excellent physical and chemical properties due to its unique two-dimensional morphological characteristics and honeycomb lattice structure. However, its zero-gap band structure and lack of catalytic ability limit its application in many fields[4,5]. In order to broaden the application field of graphene, researchers have modified graphene by various methods. For example, people have successfully adjusted the morphology of graphene from two-dimensional structure to one-dimensional nanoribbons, zero-dimensional quantum dots, three-dimensional structure and so on, thus bringing new properties and application scenarios for graphene[6,7][8⇓~10][11⇓~13].
The physicochemical and electronic properties of graphene can also be significantly modified by heteroatom doping. Heteroatom doping technology refers to the replacement of carbon atoms in graphene with other elements, such as nitrogen (N), phosphorus (P), sulfur (S), etc. This technique allows heteroatoms to enter the sp2 carbon skeleton of graphene through various experimental means. Heteroatom doping can affect the original structure of graphene and produce new graphene-like structure to some extent[14]. Since the discovery of graphene, the research on heteroatom doped graphene has been increasing year by year. Figure 1 shows the change in the number of "doped graphene" related documents retrieved from the Web of Science database on February 2, 2023. Starting from 2019, more than 7000 articles on heteroatom-doped graphene will be published worldwide every year. It can be seen that heteroatom doped graphene is one of the most popular research topics in the field of inorganic chemistry and materials science.
图1 杂原子掺杂石墨烯的文章发表数量变化趋势

Fig.1 Trends in the amount of articles published about heteroatom-doped graphene

Graphene has the characteristics of high specific surface area, great mechanical strength, strong electronic conductivity, good flexibility and easy chemical processing[15]. Therefore, graphene has great application potential in many fields such as electricity, catalysis, optics and sensors[16⇓⇓⇓~20]. The application of graphene in electrochemical energy storage is particularly representative. In order to cope with the rising world population and the accelerating growth of global energy consumption, the development of new electrochemical energy storage devices is particularly critical[21]. Among many electrochemical energy storage devices, supercapacitors have a wide range of applications. As a new type of capacitor, supercapacitor has the advantages of high capacity, high power and long cycle life, and is widely used in electric vehicles, energy storage systems and other fields. However, the traditional electrode materials usually have the defects of low conductivity and low capacity efficiency. Therefore, it is a hot research topic to find a new type of electrode material with high efficiency[22]. Heteroatom-doped graphene, as a new electrode material, has excellent electrical conductivity, high capacity efficiency, high thermal stability and other characteristics, which provides a new idea for the development of supercapacitors. Heteroatom doping can effectively enhance the electrochemical performance of graphene electrode materials, which is mainly reflected in the following aspects: 1) heteroatom doping can effectively adjust the electronic structure and intrinsic properties of graphene.The electrochemical reaction at the interface between the electrode and the electrolyte is significantly affected, and active sites are provided for the graphene, so that the doped graphene has excellent electrochemical performance; 2) the heteroatom introduces a new electronic energy level or changes the distribution of the original electronic energy level to improve the carrier concentration and mobility of the graphene, thereby enhancing the conductivity of the graphene; 3) The defects caused by heteroatom doping make the graphene structure porous, which further promotes the rapid contact between electrolyte ions and electrode materials[23]. Therefore, heteroatom doping can be used as one of the effective means to improve the electrochemical energy storage properties of graphene-based supercapacitor electrode materials.
In this paper, the research and development of heteroatom doped graphene in recent years will be comprehensively reviewed, and the application prospects and development trends of heteroatom doped graphene materials in the field of electrochemical energy storage will be analyzed from a macro perspective.The preparation methods of heteroatom doped graphene materials, including different doping methods and the choice of heteroatoms, as well as the influence of heteroatoms on the electrochemical properties of the materials, are emphatically introduced. Finally, this paper will look forward to the future development direction in the field of heteroatom doped graphene, and propose the direction and goal of future research, which will provide useful guidance for researchers in the field of heteroatom doped graphene, and provide valuable reference for the research and development of electrochemical energy storage in heteroatom doped grapheme.

2 Preparation of heteroatom-doped graphene.

The preparation method of heteroatom-doped graphene involves a number of key technologies, including dopant selection, doping mode, doping ratio and preparation conditions. In different preparation methods, the selection and adjustment of various factors have an important impact on the final properties. The preparation methods of heteroatom-doped graphene can be roughly divided into two categories: one is the direct synthesis of heteroatom-doped graphene by using gases or small organic molecules as precursors through assembly, and the representative methods are chemical vapor deposition (CVD) and chemical synthesis; The other is graphite, graphene or graphene oxide (GO) as the base material, which is substituted by doping with dopants, and the representative methods are ball milling and hydrothermal methods. In this paper, the synthesis methods of the above typical heteroatom-doped graphene will be introduced in the following content.

2.1 Chemical vapor deposition (CVD)

CVD is a common method for the preparation of nanomaterials, which can be applied to the preparation of graphene. In this method, volatile gaseous substances are deposited on metal substrates such as nickel and copper, and chemical reactions occur to grow graphene[24⇓~26]. Although CVD has the disadvantages of low deposition rate and flammable deposition gas, which is not conducive to the large-scale preparation of graphene, it can prepare large area and uniform graphene film with high quality, and can control the type of doping atoms in graphene by changing the type of deposition gas[27⇓~29]. Therefore, the CVD method has special advantages in the preparation of heteroatom-doped graphene.
Among many doping elements, the atomic radius and valence electron number of boron and nitrogen are closer to those of carbon, so it is easier to form covalent bonds with graphene lattice during chemical vapor deposition, thus forming a more stable structure. For example, Zhai et al. Used CVD to prepare boron-doped graphene with diborane (B2H6) and acetylene (C2H2) as gas sources[30]. The main body of the synthesized boron-doped graphene is nano-graphene particles, and has the characteristics of uniform doping and large boron-doped content (5.3 at%). Compared with ordinary graphene, this boron-doped graphene shows excellent conductive properties. In addition, Arkhipova et al. Prepared nitrogen-doped graphene nanosheets with mesoporous structure using magnesium oxide as the growth template and acetonitrile and benzene as the precursors, and the nitrogen content was as high as 10.7 at%[31]. The experimental results show that the mesoporous structure of N-doped graphene promotes the charge transfer of electrolyte ions, which greatly enhances the electrochemical performance of the material as a supercapacitor electrode material.

2.2 Chemical synthesis

Chemical synthesis is a method of organic synthesis, which makes some specific organic precursors react to prepare graphene[32]. Polycyclic aromatic hydrocarbons are generally used as precursor molecules, and their structures are similar to those of graphene. These polycyclic aromatic hydrocarbons undergo coupling, polymerization, or cyclization reactions, resulting in a gradual increase in molecular size, and when the molecular size reaches about 5 nm, graphene molecules can be formed. The advantage of preparing graphene by chemical synthesis is the controllability. By adjusting the precursor molecules and reaction conditions, the type, content and position of the doped atoms of the prepared graphene can be controlled, and the structure and size of the graphene molecules can be precisely regulated. However, it is still very difficult to prepare graphene on a large scale by this method. Amaro-Gahete et al. Used nitrogen-containing organic compounds such as cyclohexylamine, pyridine, dipropylamine and triethylamine as precursors, and prepared nitrogen-doped nano-oligolayer graphene by plasma treatment of the initial raw materials under atmospheric pressure. The number of layers is 8 to 22, and the particle size is between 10 and 100 nm[33]. This approach provides a new bottom-up strategy for the synthesis of heteroatom-doped graphene materials with strong versatility.

2.3 Mechanical ball milling

Mechanical ball milling is a commonly used powder sample preparation technique, which has the advantages of low cost, simple operation and high efficiency. Theoretically, graphene can be exfoliated from bulk graphite by mechanical ball milling. During the ball milling process, the van der Waals attraction and intermolecular force between adjacent graphene layers are the resistance to be overcome, while the exfoliation of graphene is achieved by the friction and shear stress between the sphere and graphite. This method can prepare graphene with uniform size, and the distance between graphene sheets can be adjusted by controlling the parameters of mechanical ball milling, so as to control the structure of graphene[34⇓~36]. In order to exfoliate graphite into graphene more effectively, it is often necessary to add an intercalation agent with a surface energy close to that of graphite to assist in the experiment, so as to obtain a better exfoliation effect[37]. During the ball milling process, the ball milling tank rotates at high speed, and the intercalation agent molecules collide with the graphite at high energy, so that the graphene is finally exfoliated from the graphite. At the same time, the heteroatoms in the intercalation agent molecules can be introduced into the graphene sheet and replace the carbon atoms in the graphene, thus generating heteroatom-doped graphene. Graphene materials can be produced on a large scale by ball milling, but in reality, the prepared graphene often has many defects.
From 2019 to 2021, our research team has continuously published the research work on heteroatom-doped graphene by mechanical ball milling. The specific method comprises the following steps of: mixing melamine and expandable graphite, heating and expanding, and then putting into a planetary ball mill for high-energy ball milling to finally prepare the oligolayer nitrogen-doped graphene. The innovation is that melamine can be used as nitrogen source, intercalation agent and reducing agent in the whole reaction process. Through characterization, we found that the prepared N-doped graphene not only has the morphology of oligolayer, but also has a nitrogen content of more than 6 at%. The prepared nitrogen-doped graphene not only has ferromagnetism, but also can be used as an electrode material of a supercapacitor[38,39]. In the subsequent work, we further improved the method, that is, graphite powder, Ni(NO3)2·6H2O and melamine were used as raw materials to prepare nickel, nitrogen and oxygen co-doped graphene (NiNOG) by one-step ball milling method, and the specific process is shown in Figure 2A. Through the observation of its scanning electron microscope (SEM) image, it can be found that the graphite can be exfoliated into oligolayer graphene during the ball milling process (Figure 2B). According to the X-ray photoelectron spectroscopy (XPS) characterization results of NiNOG (Figure 2C), it can be found that NiNOG contains four elements of Ni, C, N and O, and it can be speculated that melamine and transition metal salts contribute the nitrogen-containing and oxygen-containing groups required for doping graphene. At the same time, oxygen and nitrogen atoms can also act as coordination atoms to form coordination bonds with Ni2+, thus ensuring that nickel ions are tightly adsorbed on the surface of graphene. The asymmetric supercapacitor assembled with NiNOG as the positive electrode and activated carbon as the negative electrode shows excellent application potential. In this work, the electrochemical energy storage characteristics of graphene co-doped with non-metallic atoms and metal ions were systematically studied for the first time, which provides a new idea for large-scale preparation of metal ion-doped graphene and its application in energy storage devices[40].
图2 NiNOG的(a)制备过程示意图;(b)SEM图像;(c)XPS光谱[40]

Fig.2 (a) Schematic illustration of the preparation process of NiNOG; (b) SEM image of NiNOG; (c) XPS spectra of NiNOG[40]. Copyright 2022, Elsevier

2.4 Hydrothermal method

Hydrothermal method is the most popular method to prepare graphene materials in recent years. This is mainly because the traditional solid phase synthesis method has the limitations of harsh reaction conditions, low yield and high cost, while the hydrothermal method can produce graphene materials on a large scale under relatively mild conditions[41]. Generally speaking, the starting material of heteroatom-doped graphene prepared by hydrothermal method is often graphene oxide, not graphite used in solid phase synthesis. This is because graphene oxide has a higher surface activity than graphene. Graphene oxide is obtained by oxidation of graphite and subsequent exfoliation, and its surface is grafted with a large number of oxygen-containing groups, such as carbonyl, hydroxyl and epoxy groups. These oxygen-containing groups will destroy the van der Waals force and intermolecular force between the sheets, and increase the distance between the graphite sheets, thus realizing the chemical exfoliation of graphene[42]. Among them, the Hummer method uses H2SO4, NaNO3 and KMnO4 as oxidants, which is the most common method to prepare graphene oxide at present[43].
Graphene oxide is more hydrophilic than graphite, can be effectively dispersed in water, and does not repack to form bulk graphite. The oxygen-containing functional groups on the surface of graphene oxide contain very strong chemical reactivity, which enables it to provide active bond sites for heteroatom doping. In addition, another advantage of doping graphene oxide by hydrothermal method is that it tends to produce aerogel with three-dimensional structure. The three-dimensional graphene network can be used as a conductive supporting substrate of some nanomaterials (black phosphorus, carbon nanotubes, gold nanoparticles or g-C3N4, etc.), and the extremely high specific surface area of the three-dimensional graphene network not only can adsorb a large number of electrolyte ions,It can also be compounded with other nanomaterials to enhance the overall conductivity of the composite material and achieve excellent structural stability, thus achieving the effect of "1 + 1 > 2"[44⇓⇓~47]. The cost of hydrothermal method is relatively low, and mass production can be realized. However, in the process of oxidation of graphite, the oxygen-containing groups destroy the structure of graphene and reduce its conductivity, and the incomplete reduction leads to more defects in the obtained graphene, which is not conducive to the application of graphene materials in devices with high performance.
In the earliest work on the preparation of heteroatom-doped graphene by hydrothermal method, the experimenter mixed graphene oxide solution with dopants (hydrazine and ammonia, etc.), and then reacted under hydrothermal conditions at 160 ℃ to obtain heteroatom-doped graphene[48]. With the development of this kind of research work in the past decade, researchers have fully improved the relevant experimental conditions to obtain heteroatom-doped graphene with better properties. For example, Ge et al. Reported a hydrothermal method using melamine sponge (MS) as a template to prepare nitrogen-doped graphene nanostructured aerogel (Figure 3)[49]. The innovation of this synthesis method is to design and synthesize different nitrogen-doped graphene nanostructures through different solution treatments and different reaction conditions under the premise of using MS template. This templated hydrothermal method will open up a completely new path for fundamental research on three-dimensional nitrogen-doped graphene with highly complex nanostructures, and is expected to be applied in the field of energy storage.
图3 不同环境下氧化石墨烯包覆MS的水热处理示意图[49]

Fig.3 Schematic illustration of the hydrothermal treatment of GO coated MS in different conditions[49]. Copyright 2020, Chinese Chemical Society

In addition, the idea that heteroatom-doped graphene prepared by hydrothermal method can be used as a conductive supporting substrate has also been certified by several experimental groups in recent years[50⇓~52]. Sun et al. Used graphene oxide, nickel sulfate hexahydrate, cobalt sulfate heptahydrate and ammonium phosphate as raw materials to prepare nitrogen-doped graphene-supported cobalt nickel pyrophosphate (Co0.4Ni1.6P2O7/NG) under hydrothermal conditions[53]. Graphene with nitrogen doping up to 9.1 at% provides a good conductive mesoporous structure for cobalt nickel pyrophosphate. The morphology of Co0.4Ni1.6P2O7/NG also evolved from the original flaky agglomerates of cobalt nickel pyrophosphate to flower-like clusters, which greatly improved its electrochemical energy storage performance as an electrode material. As a cathode material for asymmetric supercapacitors, Co0.4Ni1.6P2O7/NG exhibits high energy density and very long cycle life. The specific capacitance of the material electrode is as high as 1473 F/G at a current density of 1 A/G, and 70% of the initial specific capacitance is maintained after 5000 cycles of charge-discharge, indicating that the material has great potential as an electrode material for supercapacitors.

2.5 Other synthetic methods

Yu et al. Prepared sulfur and phosphorus co-doped porous graphene film (s-SPG) using a vapor activation method[54]. Specifically, the method uses liquid nitrogen to freeze the graphene film and freeze-dries it under vacuum for 3 days to obtain a porous structure. The film was then activated in sulfur and phosphorus vapors at 900 ° C. The s-SPG thin film prepared by the method can be directly used for electrochemical test and shows good electrochemical energy storage property. Arvas et al. Prepared graphene doped with different heteroatoms by electrochemical doping[55]. In this method, the graphene oxide electrode was prepared firstly, and then the electrode was put into sulfuric acid, nitric acid and perchloric acid solution, respectively, and the sulfur-doped graphene (S-GO), nitrogen-doped graphene (N-GO) and chlorine-doped graphene (Cl-GO) were prepared by chronoamperometry at a constant potential of 1. 9 V. Through various characterization methods, it can be found that Cl-GO has the largest doping amount among the three, and the mass ratio of chlorine is about 18.26%, and Cl-GO also has the highest specific capacitance (1098 mF/cm2) as a supercapacitor electrode material. Since Cl-GO electrodes are prepared in one step at an extremely fast rate, they are suitable for industrial mass production. Tang's group used an experimental method of hyperdoping to prepare graphene with extremely high doping content[56,57]. In this method, graphite is first fluorinated to generate fluorinated graphene. Due to the strong corrosiveness of fluorine atoms, a large number of vacancies are generated inside the graphene, and in the subsequent heteroatom doping step, there will be a large number of reactive areas for heteroatom doping reaction (Figure 4). Nitrogen-doped graphene, sulfur-doped graphene, boron-doped graphene and phosphorus-doped graphene prepared by this method contain as high as 29.82, 17.55, 10.79 and 6.4 at% of doping atoms, respectively, which are much higher than other related work. This research method provides scientists with valuable experience in studying graphene doped with high content of heteroatoms.
图4 超掺杂法制备(a) 氮掺杂石墨烯, (b) 磷掺杂石墨烯示意图[56,57]

Fig.4 Schematic of the preparation of (a) nitrogen doping graphene, and (b) phosphorus doping graphene by elemental superdoping[56,57] Copyright 2019, American Chemical Society; 2016, Springer

3 Heteroatom-Doped Graphene for Supercapacitor Electrode Materials

Supercapacitor is an efficient electric energy storage device, which has the characteristics of environmental protection, simple structure, high power density, fast charge and discharge, and superior cycle stability. It is suitable for flexible wearable equipment, hybrid electric vehicles, aerospace electronic equipment and other fields as an efficient energy storage device[58]. As shown in Figure 5A, the supercapacitor consists of a current collector, positive and negative electrodes, an electrolyte, and a separator. Generally speaking, supercapacitors can be divided into two categories according to the charge storage mechanism: one is the electric double layer capacitor (EDLC) with electrostatic energy storage; The other is a pseudocapacitor (PC) that stores energy through redox reactions[22]. In the electric double layer capacitor, the capacitance is generated by the charge separation at the interface between the electrode and the electrolyte, which is a purely physical phenomenon, and its principle is shown in Figure 5B. Electric double layer capacitor materials tend to have a large specific surface area, which can adsorb a large number of charges on their surface, and have good conductivity. Carbon-based materials such as activated carbon, graphene, carbon nanotubes, and carbon fibers are the main electrode materials for electric double-layer capacitor electrodes. In the pseudocapacitor, the storage of electrical energy is accomplished by reversible Faradaic reactions and electron transfer at the electrode/electrolyte interface, the principle of which is shown in Figure 5C. Pseudocapacitive materials usually include metal oxides and conductive polymers, which can undergo redox reactions on the surface of electrodes and have higher specific capacitance than electric double layer capacitors.However, the disadvantage is that the speed of redox reaction is slow, and the cycle stability is much worse than that of electric double layer capacitor materials which only rely on physical adsorption[38].
图5 (a) 超级电容器的组成;(b, c) EDLC和PC充电时的工作原理

Fig.5 (a) Composition of a supercapacitor; (b, c) schematic of the charge storage mechanism of EDLC and PC

Graphene has attracted much attention because of its high specific surface area and high conductivity, and is a promising electrode material for supercapacitors. Unmodified graphene is a kind of electric double layer capacitance material, because graphene can not undergo redox reaction, and can only achieve capacitance effect by surface adsorption of charges. Although the theoretical value of the mass specific capacitance of graphene as an electrode is as high as 550 F/G, the experimental results are often only in the range of 100 ~ 200 F/G[59]. Heteroatom doping of graphene can effectively improve the electronic properties and chemical activity of graphene, and significantly improve the electrochemical and conductive properties of materials. The electronegativity difference between heteroatoms and carbon atoms, as well as the degree of disorder introduced by heteroatoms for graphene, can increase the specific capacitance and power characteristics of graphene materials. The introduction of heteroatoms will also form specific structures with reactivity on the surface of graphene, which can undergo redox reactions during charging and discharging, thus introducing a certain amount of additional pseudocapacitance[60⇓~62]. This mechanism not only improves the overall electrochemical performance of graphene-based materials, but also provides a new way to combine double layer capacitance and pseudocapacitance. In the following part of this paper, the effects of different doping elements (N, S, B, and P, etc.) on graphene materials and their performance differences in supercapacitor applications will be summarized.

3.1 Nitrogen doping

Nitrogen doping is an important method to modify graphene, which has low cost and high controllability, so it has a very broad application prospect. The atomic radius of carbon is slightly larger than that of nitrogen, but it is very close, which leads to the stability of the products formed by nitrogen-doped graphene. Because the electronegativity of nitrogen (χ = 3. 04) is higher than that of carbon (χ = 2. 55), the doping of nitrogen will produce polarization in the sp2 hybrid lattice of graphene, which will affect the physical and chemical properties of graphene, accelerate the rate of electron transport, and then improve the power density and rate performance of supercapacitors[23].
As shown in fig. 6, after nitrogen doping, three main nitrogen-containing structures appear in graphene: pyridine nitrogen (N-6), pyrrole nitrogen (N-5), and graphite nitrogen (N-Q). N-6 and N-5 generally appear at the edge defect or vacancy sites of graphene, replacing the carbon atoms in the six-membered ring and five-membered ring, respectively, while N-Q replaces the carbon atoms in the intrinsic structure of the six-membered ring in graphene. Among them, N-Q located in the center of graphene can form bonds with three carbon atoms hybridized with sp2, contributing two electrons to the Π electron network, so it is the main source of graphene conductivity enhancement, and N-Q also promotes the interaction between graphene and electrolyte ions, which helps to improve the double layer capacitance[63,64]. N-5 and N-6 can participate in redox reactions and thus can contribute a certain additional pseudocapacitance[65].
图6 N-6、N-5和N-Q的结构示意图

Fig.6 Schematic structures of N-6, N-5 and N-Q

Nitrogen-containing substances such as urea, nitric acid, melamine and ammonia are often used as dopants for the preparation of nitrogen-doped graphene. Nitrogen-containing components can be introduced into the graphene surface and doped by hydrothermal or solid phase heating treatment. Su et al. Prepared one-dimensional porous nitrogen-doped graphene (NHGNSs) by annealing graphene oxide in ammonia atmosphere using a simple method[66]. Benefiting from the synergistic effect of one-dimensional tubular structure, porous nature and nitrogen doping, the specific capacitance of NHGNs in electrolyte reaches 126 F/G. This unique structure makes NHGNs promising as electrode materials in supercapacitors and even other electrochemical energy storage devices. Mishra et al. Mixed urea and graphene oxide, prepared nitrogen-doped graphene by hydrothermal reaction at 160 ℃, and used it to assemble symmetric supercapacitor (RGO/N-RGO)[67]. The capacitor is small and light, with a mass of only 2.32 mg. According to the results of cyclic voltammetry (CV) curve and galvanostatic charge-discharge (GCD) curve, RGO/N-RGO has a wide voltage window (2. 2 V), high energy density (106. 3 Wh/kg) and extremely high power density (15184. 8 W/kg), and can still maintain 95. 5% of the initial capacitance after 10 000 cycles of galvanostatic charge-discharge. The capacitor device can be used to light a small LED bulb. Therefore, RGO/N-RGO has the potential to be used in portable electronic devices in the future.
The simultaneous nitrogen doping and porous treatment of graphene can produce a synergistic effect, and the prepared porous nitrogen-doped graphene can be used as an electrode material to enable a supercapacitor to have strong electrochemical energy storage properties. He et al. Reported that porous nitrogen-doped graphene (PG-Ni) was prepared by annealing with reduced metal salt as etchant and nitrogen as dopant[68]. It is found that the pore structure (pore size and density), nitrogen-doped structure and doping amount of graphene can be controlled by changing the type and content of nitrogen source and metal salt, which enables us to study the synergistic effect of pore structure and nitrogen-doped structure of porous graphene on its electrochemical energy storage properties. The experimental results show that it is easier to form N-doped graphene with N-5 as the main component when urea is used as the nitrogen source, while N-6 is the main component when nitrogen is used as the nitrogen source. Although the nitrogen-doped content of PG-Ni is lower than that of N-doped graphene generated by urea as a nitrogen source, N-6 can contribute more pseudocapacitance than N-5, so PG-Ni as an electrode material has higher specific capacitance and better electrochemical properties. The specific capacitance of PG-Ni electrode is as high as 575 F/G. This work provides a new way to develop high-performance supercapacitors based on graphene materials.

3.2 Boron doping

The atomic radius of boron is slightly larger than that of carbon in graphene, so the introduction of boron into the graphene lattice can promote the charge transfer between its adjacent carbon atoms. Since boron has three valence electrons, it can act as an electron acceptor and make the charge distribution of the graphene lattice inhomogeneous, thus improving its electrochemical performance. In general, when boron is doped into graphene, oxygen atoms are also introduced, and three different structures of BC3, BC2O and BCO2 are generated (Fig. 7), while vacancies are introduced on the surface of graphene. The functional groups in the boron-oxygen structure can make graphene more hydrophilic, and can also serve as active sites for electrochemical reactions to carry out redox reactions and introduce additional pseudocapacitance. Therefore, the boron-oxygen structure is an important content in the analysis of the structure and properties of doped graphene.
Fig.7 BC3、BC2O和BCO2的结构示意图
Schematic structures of BC<sub>3</sub>、BC<sub>2</sub>O和BCO<sub>2</sub>
Boron doping is more difficult than nitrogen doping. Therefore, there are not many articles on the application of boron-doped graphene in supercapacitor electrode materials. The most commonly used dopant for boron doping is boric acid (H3BO3). Balaji et al. Prepared boron-doped graphene (SCBAGO) by supercritical fluid processing[70]. They mixed graphene oxide solution with boric acid in a stainless steel reactor, then reacted at 400 ℃ for 1 H, and then cleaned and dried the sample to obtain the final product. The boron atom content of SCBAGO was found to be about 8.9 at% by characterization. The SCBAGO electrode has a high mass specific capacitance of 286 F/G at a current density of 1 A/G, and retains 96% of its initial specific capacitance after 10,000 cycles at a current density of 20 A/G. Thirumal et al. Also prepared boron-doped graphene nanosheets (HB-GNS) using boric acid, ethanol and reduced graphene oxide as raw materials in a hydrothermal environment at 150 ° C[71]. The boron content of HB-GNS was found to be about 2.56 at% by characterization, and its mass specific capacitance was 113 F/G at a current density of 1 A/G. According to the AC impedance test of HB-GNS, the boron doping of graphene reduces the resistance (Rct) of HB-GNS, thus enhancing the conductivity of HB-GNS materials, which is the main reason for the improvement of specific capacitance of HB-GNS.

3.3 Phosphorus doping

Phosphorus and nitrogen have the same number of valence electrons, and they have similar electronic structure characteristics. However, the atomic size of phosphorus is larger, which causes phosphorus atoms to extend out of the plane of graphene, thus inducing distortion of the hexagonal carbon framework structure. Phosphorus is less electronegative (χ = 2.19) than both carbon (χ = 2.55) and nitrogen (χ = 3.04)[23]. Multiple bonding modes play an important role in the electrochemical performance of phosphorus-doped graphene materials, because the appearance of C-P bonds in graphene (Figure 8 a) can change the charge and spin density distribution of carbon, adjust the band structure of graphene, and thus enhance its electrochemical activity. Phosphorus doping at the edge of graphene can induce oxygen-containing groups such as C — O — P bonds, C — P — O bonds and C — P = O bonds (Fig. 8B – d), which can not only enhance the hydrophilicity of graphene materials, but also introduce additional pseudocapacitance to graphene[61].
图8 磷掺杂石墨烯中含磷结构的示意图

Fig.8 Schematic diagram of phosphorus-containing structure in phosphorus-doped graphene

The main dopants for phosphorus doping include phosphoric acid (H3PO4), phytic acid (C6H18O24P6), and triphenylphosphine (C18H15P). Wen et al. prepared phosphorus-doped graphene (P-TRG) by a phosphoric acid activation method, in which the mixed solution of graphene oxide and phosphoric acid was heated and activated at 800 ℃, and then the product was reduced to obtain the final product[72]. The phosphorus doping ratio of P-TRG was found to be about 1.3 at% by XPS characterization. Phosphorus doping can not only improve the specific capacitance of electrode materials, but also broaden the working voltage window of electrode materials, thus improving the overall energy density of capacitors. Nie et al. Used a simple hydrothermal synthesis method to prepare gel-state phosphorus-doped graphene (PGA) by heating a mixed solution of phytic acid, ethylene glycol, and graphene oxide in a hydrothermal environment at 150 ° C[73]. Through the morphology observation of the scanning electron microscope (SEM) image, it is found that the PGA in the gel state has a three-dimensional graphene network (Fig. 9a). PGA as an electrode material has a specific capacitance of 225. 3 F/G at a current density of 1 A/G and excellent cycle stability. After 10,000 charge-discharge cycles, PGA still maintained 95% of its initial specific capacity (Fig. 9b). Therefore, PGA can be used as an ideal electrode material for supercapacitors.
图9 (a)PGA的SEM图像;(b)1 A/g电流密度下,PGA电极的循环稳定性(插图为第1次和第10 000次循环的CV曲线)[73]

Fig.9 (a) SEM image of PGA; (b) cycle stability of PGA electrode at 1 A/g (Inset: CV profile for 1st and 10 000th cycle)[73]. Copyright 2020, ESG

3.4 Sulfur doping

Similar to other heteroatom-doped graphene, sulfur doping can also improve the electrochemical properties of graphene electrodes. However, its application has been hindered due to the complex route of sulfur doping graphene and the weak polarization between C-S bonds[60]. Sulfur doping is an effective way to improve the hydrophilicity of graphene materials, and it can also reduce the resistance of active materials, thus improving the energy density of graphene materials. However, its low power density and low cycle stability are the main obstacles to its application.
As shown in Fig. 10, the sulfur-containing structures introduced by sulfur doping are divided into two categories. One is substitutional doping, resulting in a thiophene-like structure (-C-S-C-); The other is the doping of adsorption, which generates functional groups containing sulfur-oxygen structure (-SOX-)[61].
图10 硫掺杂石墨烯中含硫结构的示意图

Fig.10 Schematic diagram of Sulfur-containing structure in sulfur-doped graphene

Arvas et al. Reported an electrochemical route to prepare sulfur-doped graphene (S-GEs) by cyclic voltammetry of graphene electrodes in concentrated sulfuric acid solution for 50 times in a three-electrode system[74]. In this work, Arvas et al. Found that when the scanning voltage window for electrochemical synthesis became smaller, the prepared S-GEs had higher -SOX- content, while the content of -C-S-C- tended to decrease. When the content of -SOX- is higher, the S-GEs electrode tends to be more pseudocapacitive. This result proves that the redox reaction occurs during the charge and discharge of the -SOX-, which introduces additional pseudocapacitance and enhances the electrochemical properties of the material. At a current density of 10 mA/cm2, the specific capacitance of the S-GEs electrode is 1833 mF/cm2, which is 25 times higher than that of the electrode obtained by electrochemical synthesis with only 10 cycles of cyclic voltammetry. In addition, Rosli et al. Prepared sulfur-doped graphene (S-rGO) by microwave heating the mixed solution of graphene oxide and Na2S, and the content of sulfur atoms in the material was 1.09 at%.The specific capacitance of the material electrode is 237.6 F/G at a current density of 0.1 A/G, and the specific capacitance does not change much after 5000 charge-discharge cycles, which further indicates that the material electrode has very good electrochemical performance[75].

3.5 Other heteroatom doping

The heteroatoms doped in graphene are usually dominated by non-metallic elements. The above content details the effects of nitrogen, boron, phosphorus and sulfur on the electrochemical properties of graphene doped supercapacitor electrode materials. In particular, it should be pointed out that halogen elements have higher chemical reactivity than IIIA-VIA elements. When graphene is doped with halogen elements, some carbon atoms in graphene change from sp2 hybridization to sp3 hybridization, which causes significant changes in the geometric structure and electronic structure of graphene, thus affecting its electrochemical performance[76]. Jiang et al. Prepared high-performance chlorine-doped reduced graphene oxide films (Cl-RGOFs) by a simple hydrochloric acid solvothermal method, that is, the graphene oxide film was hydrothermally reacted with hydrochloric acid at 180 ℃ for 3 H[77]. Graphene oxide was reduced by HCl in the hydrothermal process, and the electron-withdrawing effect was induced by the doping of chlorine atoms, so the conductivity of Cl-RGOF was 2. 5 times higher than that of undoped graphene oxide film, which effectively reduced its internal resistance and ensured the good electrochemical performance of the electrode material. In addition, the specific capacitance of Cl-RGOF increases nearly linearly with the increase of the content of doped chlorine atoms. When the chlorine doping amount reaches the maximum (1. 4 at%), the specific capacitance of Cl-RGOF is 210 F/G at the current density of 1 A/G. The Cl-RGOFs film shows good flexibility and stability, and the symmetrical supercapacitor assembled by the Cl-RGOFs electrode can drive the rotation of a small electric fan even under strong bending. Therefore, the material is expected to be used in the field of flexible wearable electronic devices.
Metal atom doping of graphene has rarely been reported. The bonding ability between metal atoms and graphene is much lower than the bonding strength of their own structure. Therefore, the graphene doped with metal atoms often presents a cluster structure, rather than being uniformly doped into the graphene surface. Through density functional theory calculations, Wang et al. Speculated that transition metals doped in the plane of graphene could form symmetric structures[78]. When nitrogen and metal are co-doped in graphene, vacancy graphene compounds will be formed. If a suitable synthesis method is developed in the future to prepare transition metal doped graphene or transition metal/nitrogen co-doped graphene, both of them can be used as anode materials for asymmetric supercapacitors.

3.6 Multi-atom co-doping

The doping of graphene is not only limited to single atom doping, but also can improve its electrochemical performance as an electrode material by co-doping multiple atoms. The synergistic effect of multi-element doping can significantly improve the overall performance of electrochemical memory devices. With the further study of heteroatom-doped graphene, the study of multi-atom co-doping has become a hot frontier direction. The effect of multi-atom doping on the electrochemical properties of graphene is quite different from that of single-atom doping, which is mainly due to the interaction between multiple atoms.Different atomic combinations will also produce different synergistic mechanisms, which will help to understand the improvement mechanism of their electrochemical properties, and also provide strong support for the design and synthesis of new electrochemical materials.
At present, there have been many studies on sulfur/nitrogen, sulfur/phosphorus, nitrogen/sulfur, nitrogen/boron and even three to four elements co-doped graphene, which have been applied to electrode materials for supercapacitors[79][80][81][82]. For the selection of the dopant, a plurality of substances containing a single element component may be used, or a single substance containing a plurality of element components may be used. For example, Pham et al. Used ammonia borane (BNH6) as a dopant to prepare nitrogen/boron co-doped graphene (ABRGO)[83]. In this work, tetrahydrofuran (THF) was used as solvent, and BNH6 could reduce graphene oxide and dope graphene at 80 ℃. Through a series of electrochemical tests, it can be found that the ABRGOs electrode has a high specific capacitance (130 F/G at a current density of 8 A/G) and excellent cycling stability (95.4% of the initial specific capacitance is maintained after 1000 cycles), showing great potential in high-performance supercapacitor applications. Rotte et al. First prepared sulfur/nitrogen co-doped oligolayer graphene by treating graphite flakes with sulfuric acid and nitric acid, and microwave treating the treated graphite flakes[84]. The experiment shows that the graphite is successfully exfoliated into 15 layers and below of oligolayer graphene, and the content of doped nitrogen atoms and sulfur atoms is 5.22 at% and 3.84 at%, respectively. The specific capacitance of the electrode material is as high as 298 F/G at a current density of 1 A/G. The co-doping effect of nitrogen and sulfur on graphene is the main reason for the excellent electrochemical performance of the material.
Doping graphene with more than three heteroatoms can not only synergistically enhance the electrochemical performance of the material, but also possibly change the pore size distribution of the material. Liu et al. Prepared nitrogen, phosphorus and sulfur co-doped porous graphene oxide (N, S, P-HHGO) by hydrothermal etching using ammonium dihydrogen phosphate and L-cysteine as dopants[85]. The excellent electrochemical energy storage performance comes from the synergistic effect of nitrogen, sulfur and phosphorus heteroatoms and the mesoporous structure generated during the doping process.
Fig. 11 summarizes several common types of graphene doping atoms and the corresponding main characteristics of the doped graphene. By understanding the influence of different doping elements on the properties of graphene, readers can discover and design new functions of graphene, thus broadening the application fields of doped graphene.
图11 掺杂原子种类及其特性

Fig.11 Species and characteristics of doped atoms

Table 1 details the latest research on heteroatom-doped graphene in recent years, and summarizes all relevant factors (types of doping atoms, experimental methods, dopants, carbon sources, reaction conditions and electrochemical properties of heteroatom-doped graphene in supercapacitor electrode materials).It provides readers with a systematic and comprehensive overview to clarify the practical application effect of heteroatom doped graphene in supercapacitor electrode materials.
表1 杂原子掺杂石墨烯作为超级电容器电极材料的性能比较

Table 1 Performance of heteroatom-doped graphene as electrode materials for supercapacitors

Material Atom(s) Synthesis method/ React condition Dopant Carbon source Performance Ref
1 N-HtrGO N Hydrothermal/150℃, 12 h Urea GO 244 F/g at 50 mV/s, 105% at 2000 cycles 86
2 NHGNSs N Thermally annealed/ 360℃, 5 h NH3 GO 126 F/g at 1 A/g, 91% at 2000 cycles 66
3 PG-Ni N Thermally annealed/ 800℃, 2 h N2 GO 575 F/g at 0.5 A/g, 89.5% at 10 000 cycles 68
4 FNG N Ball milling/500 rpm, 24 h Melamine Expanded graphite 83.8 mF/cm2 at 0.6 mA/cm2, 93.8% at 5000 cycles 38
5 NG-DWCNT N CVD/ 1300℃ under Ar Urea Ethanol 563 F/g at 50 A/g, 94.35% at 5000 cycles 27
6 NGH N Hydrothermal/ 90℃, 4 h Carbamide GO 199.8 F/g at 2 A/g, 97% at 20000 cycles 87
7 NG N Hydrogel strategy Pyrrole GO 455.4 F/g at 1 A/g, 97.4% at 5000 cycles 88
8 BMG B Hydrothermal/180℃, 4 h Boric acid GO 336 F/g at 0.1 A/g, 98% at 5000 cycles 89
9 HTBAGO B Supercritical fluid processing/400℃, 1 h Boric acid GO 286 F/g at 1 A/g, 96% at 10 000 cycles 70
10 B-rGO B Electrochemical synthesis Boric acid GO 446 F/g at 0.1 A/g, 95.6% at 2000 cycles 90
11 BGNS B Solvothermal/150℃, 12 h Boric acid GO 125 F/g at 1 A/g, 83% at 2000 cycles 91
12 P-TRG P Thermal annealing/ 800℃, 30 min H3PO4 GO 115 F/g at 0.05 A/g, 97% at 5000 cycles 72
13 PO-graphene P Electrochemical synthesis (NH4)3PO4 Graphite rod 1634.2 F/g at 3.5 mA/cm2, 67% at 500 cycles 92
14 PGA P Solvothermal/150℃, overnight Phytic Acid GO 225.3 F/g at 1 A/g, 95% at 10 000 cycles 73
15 PGO P Supercritical fluid processing/400℃, 1 h Na3PO4 GO 518 F/g at 1 A/g, 98% at 5000 cycles 93
16 S-GEs S Electrochemical synthesis H2SO4 Pencil graphite 1833 mF/cm2 at 10 mA/cm2, 95% at 1000 cycles 74
17 S@G S Heat treatment/155℃, 8 h S Nanomesh graphene 257 F/g at 0.25 A/g, 87% at 10 000 cycles 94
18 S-rGO S Microwave-assisted synthesis/140℃, 30 min Na2S GO 237.6 F/g at 0.1 A/g, 113% at 5000 cycles 75
19 L-P LIG S Laser direct writing Polyethersulfone Lignin 22 mF/cm2 at 0.05 mA/cm2, 89.8% at 9000 cycles 95
20 Cl-RGOFs Cl Hydrothermal/180℃, 3 h HCl GO 210 F/g at 1 A/g, 94.3% at 5000 cycles 77
21 FGA F Hydrothermal/150℃, 12 h HF GO 279.8 F/g at 0.5 A/g, 94.3% at 5000 cycles 96
22 NiNOG Ni, N, O Ball milling/ 400 rpm, 10 h Ni(NO3)2·6H2O Melamine Graphite 532 F/g at 1 A/g, 87.5% at 10 000 cycles 40
23 NP-rGO N, P Supramolecular
polymerization		 Melamine
Phytic acid		 GO 416 F/g at 1 A/g, 94.63% at 10 000 cycles 97
24 s-SPG S, P Thermal activation/ 900℃, 1 h Phytic acid
Thioglycolic acid		 GO 168 F/g at 1 A/g, 91.7% at 2000 cycles 54
25 N, S, PHHGO N, S, P Hydrothermal/ 140℃, 2 h NH4H2PO4
L-cysteine		 GO 295 F/g at 1 A/g, 93.5% at 10 000 cycles 85
26 S, N-FLG N, S Microwave irradiation/900 W and 2.45 GHz for a few seconds H2SO4 HNO3 Graphite 298 F/g at 1 A/g, 95% at 10 000 cycles 84

4 Summary and Prospect

In this paper, the research progress of heteroatom doped graphene materials in recent years is summarized, and the application effects of different kinds of heteroatom doped graphene in supercapacitor electrode materials are introduced. Through heteroatom doping, the conductivity and electrochemical activity of graphene materials can be improved, and then the energy density of supercapacitors can be improved. Therefore, as a promising electrode material, the study of heteroatom-doped graphene materials is of great significance to promote the development of graphene-based electrode materials. At present, human beings are facing severe environmental problems such as insufficient energy, excessive carbon emissions, and rapid population growth.Scientists can produce supercapacitor devices with larger specific capacitance, higher energy density and power density, and better charge-discharge cycle stability, and through continuous optimization of resource allocation, it is expected to solve the above problems. However, there are still many problems and challenges in heteroatom-doped graphene electrode materials, and the current industrial promotion has not yet been formed, and the main challenges in research and development and application are as follows:
(1) Although fundamental research on heteroatom-doped graphene has been conducted for many years, its practical applications have complexities that cannot be covered by laboratory testing environments, so large-scale industrial production still faces challenges. In the future, more in-depth application research is needed in order to make heteroatom-doped graphene practical application as soon as possible.
(2) In the current experimental stage, the content of heteroatoms in heteroatom-doped graphene is still not high, in which the proportion of atoms in nitrogen-doped graphene is generally less than 10 at%, while the doping of sulfur and phosphorus can only reach 5 at%. Although there are experimental methods of hyperdoping, the experimental operation is extremely difficult. Therefore, it is necessary to explore the experimental methods to prepare products with higher content.
(3) A variety of heteroatom co-doped graphene has opened up new research directions and become a hot topic in recent years. However, the specific synergistic mechanism remains unrevealed. The results show that the doping of graphene with more than two heteroatoms may produce complementary synergistic effects, thus significantly improving the electrochemical performance of the material, and the reasons still need further study.
(4) The electrochemical performance of heteroatom-doped graphene can also be further improved by controlling the morphology and pore size, such as preparing heteroatom-doped graphene with three-dimensional structure or controlling its pore structure. In the preparation process, how to use a variety of modification methods to enhance the electrochemical energy storage performance of graphene is the research focus of future researchers.

References

Publishing order | Descend order by publishing year | Descend order by cited within
[1]
Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V, Firsov A A. Science, 2004, 306(5696): 666.

[2]
Pan X R, Ji J H, Zhang N N, Xing M Y. Chin. Chemical Lett., 2020, 31(6): 1462.

[3]
Ares P, Novoselov K S. Nano Mater. Sci., 2022, 4(1): 3.

[4]
Han J H, Huang G, Wang Z L, Lu Z, Du J, Kashani H, Chen M W. Adv. Mater., 2018, 30(38): 1803588.

[5]
Nandee R, Chowdhury M A, Shahid A, Hossain N, Rana M. Results Eng., 2022, 15: 100474.

[6]
Sung Y Y, Vejayan H, Baddeley C J, Richardson N V, Grillo F, Schaub R. ACS Nano, 2022, 16(7): 10281.

[7]
Luo H, Yu G. Chem. Mater., 2022, 34(8): 3588.

[8]
Zhu J Y, Wang L X, Gan X M, Tang T T, Qin F W, Luo W X, Li Q Q, Guo N N, Zhang S, Jia D Z, Song H H. Energy Storage Mater., 2022, 47: 158.

[9]
Mondal S, Das S R, Sahoo L, Dutta S, Gautam U K. J. Am. Chem. Soc., 2022, 144(6): 2580.

[10]
Guo T H, Wang H Y, Han W H, Zhang J, Wang C L, Ma T S, Zhang Z Q, Deng Z Q, Chen D, Xu W W, Liu X H, Huang L K, Hu Z Y, Zhu Y J. Nano Energy, 2022, 98: 107298.

[11]
Chen W Q, Xiao P S, Chen H H, Zhang H T, Zhang Q C, Chen Y S. Adv. Mater., 2019, 31(9): 1802403.

[12]
Lin X R, Hu Y X, Hu K L, Lin X, Xie G Q, Liu X J, Reddy K M, Qiu H J. ACS Mater. Lett., 2022, 4(5): 978.

[13]
Yoshii T, Chida K, Nishihara H, Tani F. Chem. Commun., 2022, 58(22): 3578.

[14]
Wang X W, Sun G Z, Routh P, Kim D H, Huang W, Chen P. Chem. Soc. Rev., 2014, 43(20): 7067.

[15]
Tiwari S K, Mishra R K, Ha S K, Huczko A. ChemNanoMat, 2018, 4(7): 598.

[16]
Zhan D, Yan J X, Lai L F, Ni Z H, Liu L, Shen Z X. Adv. Mater., 2012, 24(30): 4055.

[17]
Yang Y L, Wu M G, Zhu X W, Xu H, Ma S, Zhi Y F, Xia H, Liu X M, Pan J, Tang J Y, Chai S P, Palmisano L, Parrino F, Liu J L, Ma J Z, Wang Z L, Tan L, Zhao Y F, Song Y F, Singh P, Raizada P, Jiang D L, Li D, Geioushy R A, Ma J Z, Zhang J T, Hu S, Feng R J, Liu G, Liu M H, Li Z H, Shao M F, Li N, Peng J H, Ong W J, Kornienko N, Xing Z Y, Fan X J, Ma J M. Chin. Chemical Lett., 2019, 30(12): 2065.

[18]
Shao G F, Ovsianytskyi O, Bekheet M F, Gurlo A. Chem. Commun., 2020, 56(3): 450.

[19]
Vangelidis I, Bellas D V, Suckow S, Dabos G, Castilla S, Koppens F H L, Ferrari A C, Pleros N, Lidorikis E. ACS Photonics, 2022, 9(6): 1992.

[20]
Ye X H, Qi M, Yang H Y, Mediko F S, Qiang H, Yang Y L, He C Z. Chem. Eng. Sci., 2022, 247: 117017.

[21]
Chen K N, Wang Q R, Niu Z Q, Chen J. J. Energy Chem., 2018, 27(1): 12.

[22]
Wang Y M, Wu X L, Han Y Q, Li T X. J. Energy Storage, 2021, 42: 103053.

[23]
Kumar R, Sahoo S, Joanni E, Singh R K, Maegawa K, Tan W K, Kawamura G, Kar K K, Matsuda A. Mater. Today, 2020, 39: 47.

[24]
Rösicke F, Gluba M A, Shaykhutdinov T, Sun G G, Kratz C, Rappich J, Hinrichs K, Nickel N H. Chem. Commun., 2017, 53(67): 9308.

[25]
Sun X L, Zhang J, Wang X N, Zhang C Y, Hu P G, Mu Y B, Wan X B, Guo Z X, Lei S B. Chem. Commun., 2013, 49(87): 10317.

[26]
Sealy C. Nano Today, 2022, 43: 101442.

[27]
Muangrat W, Obata M, Htay M T, Fujishige M, Dulyaseree P, Wongwiriyapan W, Hashimoto Y. FlatChem, 2021, 29: 100292.

[28]
Ion-Ebrşu D, Dorin Andrei R, Enache S, Căprărescu S, Negrilă C C, Jianu C, Enache A, Boeraşu I, Carcadea E, Varlam M, Varlam B Ş, Ren J W. Materials, 2021, 14(17): 4952.

[29]
Ullah S, Liu Y, Hasan M, Zeng W W, Shi Q T, Yang X Q, Fu L, Ta H Q, Lian X Y, Sun J Y, Yang R Z, Liu L J, Rümmeli M H. Nano Res., 2022, 15(2): 1310.

[30]
Zhai Z H, Shen H L, Chen J Y, Li X M, Li Y F. ACS Appl. Mater. Interfaces, 2020, 12(2): 2805.

[31]
Arkhipova E A, Ivanov A S, Maslakov K I, Savilov S V. Electrochimica Acta, 2020, 353: 136463.

[32]
Koehler F M, Stark W J. Acc. Chem. Res., 2013, 46(10): 2297.

[33]
Amaro-Gahete J, Mora M, GutiÉrrez P, Cosano D, Esquivel D, Ruiz J R, JimÉnez-Sanchidrián C, García M C, Romero-Salguero F J. J. Phys. D: Appl. Phys., 2020, 53(43): 435202.

[34]
Rio-Castillo A E, Merino C, Díez-Barra E, Vázquez E. Nano Res., 2014, 7(7): 963.

[35]
Yi M, Shen Z G. J. Mater. Chem. A, 2015, 3(22): 11700.

[36]
Li H N, Zhang H M, Huang K K, Liang D, Zhao D D, Jiang Z Y. Ceram. Int., 2022, 48(12): 17171.

[37]
LeÓn V, Rodriguez A M, Prieto P, Prato M, Vázquez E. ACS Nano, 2014, 8(1): 563.

[38]
Wu Y P, Liu X Y, Xia D D, Sun Q S, Yu D Y, Sun S G, Liu X L, Teng Y F, Zhang W G, Zhao X D. Chin. Chemical Lett., 2020, 31(2): 559.

[39]
Wu Y P, Yu D Y, Feng Y, Han L Y, Liu X L, Zhao X D, Liu X Y. Chin. Chemical Lett., 2021, 32(12): 3841.

[40]
Liu L Y, Xie Z J, Du X M, Yu D Y, Yang B, Li B, Liu X Y. Chem. Eng. J., 2022, 430: 132815.

[41]
Tung V C, Allen M J, Yang Y, Kaner R B. Nat. Nanotechnol., 2009, 4(1): 25.

[42]
Wu Y P, Feng Y, He Z Y, Yu D Y, Xue Y, Liu X L, Han L Y, Zhao X D, Liu X Y. Chin. Chemical Lett., 2021, 32(11): 3596.

[43]
Ang P K, Wang S, Bao Q L, Thong J T L, Loh K P. ACS Nano, 2009, 3(11): 3587.

[44]
Wu Y P, Sun Q S, Yu D Y, Teng Y F, Liu X L, Feng Y, Han L Y, Zhao X D, Liu X Y. Chem. Commun., 2020, 56(13): 2016.

[45]
Jones S, Pramanik A, Kanchanapally R, Viraka Nellore B P, Begum S, Sweet C, Ray P C. ACS Sustainable Chem. Eng., 2017, 5(8): 7175.

[46]
He H J, Huang L H, Zhong Z J, Tan S Z. Appl. Surf. Sci., 2018, 441: 285.

[47]
Zakaria M R, Omar M F, Zainol Abidin M S, Md Akil H, Al Bakri Abdullah M M. Compos. A Appl. Sci. Manuf., 2022, 154: 106756.

[48]
Long D H, Li W, Ling L C, Miyawaki J, Mochida I, Yoon S H. Langmuir, 2010, 26(20): 16096.

[49]
Ge J, Zhu H W, Yang Y, Xie Y F, Wang G, Huang J, Shi L A, Schmidt O G, Yu S H. CCS Chem., 2020, 2(2): 1.

[50]
Liu H, Zhang M M, Ma T J, Wang Y, Song Z X, Wang A, Huang Z Y. Chem. Eng. Sci., 2021, 238: 116613.

[51]
Zhao Z F, Liu Y, Wan F, Wang S, Zhang N N, Liu L L, Cao A Y, Niu Z Q. Chin. Chemical Lett., 2021, 32(2): 594.

[52]
Tao S, Wu D J, Chen S M, Qian B, Chu W S, Song L. Chem. Commun., 2018, 54(60): 8379.

[53]
Sun P P, Li Z H, Zhang L, Dong C, Li Z J, Yao H C, Wang J S, Li G H. J. Alloys Compd., 2018, 750: 607.

[54]
Yu X, Pei C G, Feng L G. Chin. Chemical Lett., 2019, 30(5): 1121.

[55]
Arvas M B, Gürsu H, Gencten M, Sahin Y. J. Energy Storage, 2021, 35: 102328.

[56]
Lin L H, Fu L, Zhang K Y, Chen J, Zhang W L, Tang S L, Du Y W, Tang N J. ACS Appl. Mater. Interfaces, 2019, 11(42): 39062.

[57]
Liu Y, Shen Y T, Sun L T, Li J C, Liu C, Ren W C, Li F, Gao L B, Chen J, Liu F C, Sun Y Y, Tang N J, Cheng H M, Du Y W. Nat. Commun., 2016, 7: 10921.

[58]
Şahin M, Blaabjerg F, Sangwongwanich A. Energies, 2022, 15(3): 674.

[59]
Nithya V D. J. Energy Storage, 2021, 44: 103380.

[60]
Abbas Q, Raza R, Shabbir I, Olabi A G. J. Sci. Adv. Mater. Devices, 2019, 4(3): 341.

[61]
Li Z S, Lin J P, Li B L, Yu C L, Wang H Q, Li Q Y. J. Energy Storage, 2021, 44: 103437.

[62]
Cui L L, Xu H P, An Y R, Xu M C, Lei Z J, Jin X J. Adv. Powder Technol., 2022, 33(6): 103571.

[63]
Zhang Y, Lu J C, Li Y, Li B J, Ruan Z L, Zhang H, Hao Z L, Sun S J, Xiong W, Gao L, Chen L, Cai J M. Angewandte Chemie Int. Ed., 2022, 61(28): e202204736.

[64]
Cai J M, Pignedoli C A, Talirz L, Ruffieux P, Söde H, Liang L B, Meunier V, Berger R, Li R J, Feng X L, Müllen K, Fasel R. Nat. Nanotechnol., 2014, 9(11): 896.

[65]
Zhang Y, Yu Y, Xiao R, Du C, Wan L, Ye H, Chen J, Wang T L, Xie M J. J. Energy Storage, 2022, 54: 105299.

[66]
Su F, Zheng S H, Liu F Y, Zhang X, Su F Y, Wu Z S. Chin. Chemical Lett., 2021, 32(2): 914.

[67]
Mishra R K, Choi G J, Sohn Y, Lee S H, Gwag J S. Chem. Commun., 2020, 56(19): 2893.

[68]
He X X, Tang Z, Gao L L, Wang F Y, Zhao J P, Miao Z C, Wu X Z, Zhou J, Su Y, Zhuo S P. J. Electroanal. Chem., 2020, 871: 114311.

[69]
Agnoli S, Favaro M. J. Mater. Chem. A, 2016, 4(14): 5002.

[70]
Balaji S S, Karnan M, Anandhaganesh P, Tauquir S M, Sathish M. Appl. Surf. Sci., 2019, 491: 560.

[71]
Thirumal V, Pandurangan A, Jayavel R, Ilangovan R. Synth. Met., 2016, 220: 524.

[72]
Wen Y Y, Wang B, Huang C C, Wang L Z, Hulicova-Jurcakova D. Chem. Eur. J., 2015, 21(1): 80.

[73]
Nie G S, Deng H C, Huang J, Wang C Y. Int. J. Electrochem. Sci., 2020, 15:12578.

[74]
Arvas M B, Gençten M, Sahin Y. Int. J. Energy Res., 2020, 44(3): 1624.

[75]
Rosli N H A, Lau K S, Winie T, Chin S X, Chia C H. Diam. Relat. Mater., 2021, 120: 108696.

[76]
Wang H D, Narasaki M, Zhang Z W, Takahashi K, Chen J, Zhang X. Sci. Rep., 2020, 10: 17562.

[77]
Jiang H D, Ye X K, Zhu Y C, Yue Z Y, Wang L H, Xie J L, Wan Z Q, Jia C Y. ACS Sustainable Chem. Eng., 2019, 7(23): 18844.

[78]
Wang M, Chen L L, Zhou J Q, Xu L R, Li X Y, Li L J, Li X. J. Mater. Sci., 2019, 54(1): 483.

[79]
Cheng L L, Hu Y Y, Qiao D D, Zhu Y, Wang H, Jiao Z. Electrochimica Acta, 2018, 259: 587.

[80]
Jing C, Guo X L, Xia L H, Chen Y X, Wang X, Liu X Y, Dong B Q, Dong F, Li S C, Zhang Y X. Chem. Eng. J., 2020, 379: 122305.

[81]
Zhang L L, Chen H X, Lu X Y, Wang Y, Tan L L, Sui D P, Qi W. Appl. Surf. Sci., 2020, 529: 147022.

[82]
Khandelwal M, Van Tran C, Lee J, Bin In J. Chem. Eng. J., 2022, 428: 131119.

[83]
Pham V H, Hur S H, Kim E J, Kim B S, Chung J S. Chem. Commun., 2013, 49(59): 6665.

[84]
Rotte N K, Naresh V, Muduli S, Reddy V, Srikanth V V S, Martha S K. Electrochimica Acta, 2020, 363: 137209.

[85]
Liu J L, Zhu Y R, Chen X H, Yi W J. J. Alloys Compd., 2020, 815: 152328.

[86]
Lee H J, Abdellah A, Ismail F M, Gumeci C, Dale N, Parrondo J, Higgins D C. Electrochimica Acta, 2021, 397: 139241.

[87]
Su W F, Gao D M, Zheng W C, Lu F, Liu J Y, Chen X C, Sha O, Chen L. Mater. Technol., 2020, 35(4): 195.

[88]
Jin Y N, Meng Y N, Fan W, Lu H Y, Liu T X, Wu S X. Electrochimica Acta, 2019, 318: 865.

[89]
Nankya R, Lee J, Opar D O, Jung H. Appl. Surf. Sci., 2019, 489: 552.

[90]
Muthu R N, Tatiparti S S V. Energy Storage, 2020, 2(4): e134.

[91]
Naresh V, Bhattacharjee U, Martha S K. J. Alloys Compd., 2019, 797: 595.

[92]
Zu L, Gao X, Lian H Q, Cai X M, Li C, Zhong Y, Hao Y C, Zhang Y F, Gong Z, Liu Y, Wang X D, Cui X G. Nanomaterials, 2018, 8(6): 417.

[93]
Suresh Balaji S, Sandhiya M, Sathish M. J. Energy Storage, 2021, 33: 102085.

[94]
Kan Y F, Ning G Q, Ma X L. Chin. Chemical Lett., 2017, 28(12): 2277.

[95]
Sun X Z, Liu X J, Li F. Appl. Surf. Sci., 2021, 551: 149438.

[96]
Jin T X, Chen J F, Wang C Y, Qian Y, Lu L M. J. Mater. Sci., 2020, 55(26): 12103.

[97]
Cheng H H, Yi F Y, Gao A M, Liang H F, Shu D, Zhou X P, He C, Zhu Z H. ACS Appl. Energy Mater., 2019, 2(6): 4084.

Options
Outlines

/

〈 〉