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

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Progress in Chemistry >

2024 , Vol. 36 >Issue 7: 961 - 974

DOI: https://doi.org/10.7536/PC231016

Review

Enhanced Mechanism of Supercapacitance by Regulating the Surface Interface of Transition Metal Compounds

  • Xing Chen 1, 2, 3 ,
  • Demin Jiang 1, 2 ,
  • Kun Xie , 1, * ,
  • Lijun Liu 1 ,
  • Yin Wang 1 ,
  • Yuqiao Wang , 2, *
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  • 1 School of Environmental and Chemical Engineering, Chongqing Three Gorges University, Chongqing 404100, China
  • 2 Research Center for Nano Photoelectrochemistry and Devices, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China
  • 3 Beijing Institute of Technology Chongqing Innovation Center, Chongqing 401120, China
* e-mail: (Kun Xie);
(Yuqiao Wang)

Received date: 2023-10-30

  Revised date: 2024-04-01

  Online published: 2024-07-01

Supported by

National Natural Science Foundation of China(61774033)

Science and Technology Research Program of Chongqing Municipal Education Commission(KJZD-K202201206)

Science and Technology Research Program of Chongqing Municipal Education Commission(KJQN202101238)

Natural Science Foundation of Chongqing(cstc2021jcyj-msxmX1016)

Science and Technology Innovation Program of Wanzhou(wzstc20220302)

National Undergraduate Training Program of Innovation and Entrepreneurship(202310643004)

National Undergraduate Training Program of Innovation and Entrepreneurship(202310643009)

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Abstract

the development of supercapacitors with high efficiency and good stability is of great significance in alleviating the energy crisis and environmental pollution issues.transition metal compounds store charge through Faraday redox reactions,leading to higher specific capacities.However,transition metal compounds suffer from poor electrical conductivity,slow reaction kinetics,and few exposed electroactive sites,thus leading to a definite difficulty in practical applications.in This paper,We have summarized the research progress of surface-interface modulation strategies in enhancing the electrochemical performance of supercapacitors to address the problems of transition metal compounds,such as morphology modulation,heterojunction,elemental doping,and vacancy engineering.this paper focuses on the mechanism of the above-mentioned methods from the perspective of morphological and electronic structure modulation on the physical and chemical properties of active materials.we aim to clarify the performance enhancement mechanism of supercapacitors and provide an important theoretical basis for developing high-performance and high-stability supercapacitors.Finally,the reasons for structural design and electronic modulation to improve the performance of supercapacitors are summarized,and the challenges faced by structural design and electronic modulation in constructing high-performance supercapacitors are outlined。

Contents

1 Introduction

2 Surface-interface modulation strategies for transition metal compound electrode materials

2.1 Structural regulation

2.2 Heterostructure

2.3 Elemental doping

2.4 Vacancy engineering

3 Conclusion and outlook

Key words： surface interface; supercapacitance; electronic structure; morphology; mechanism

Cite this article

Xing Chen , Demin Jiang , Kun Xie , Lijun Liu , Yin Wang , Yuqiao Wang . Enhanced Mechanism of Supercapacitance by Regulating the Surface Interface of Transition Metal Compounds[J]. Progress in Chemistry, 2024 , 36(7) : 961 -974 . DOI: 10.7536/PC231016

1 Introduction

With the depletion of fossil fuels and the increasingly serious environmental pollution,the demand for the development of renewable energy is increasing[1]。 the development of efficient energy storage and conversion devices is the key to the large-scale practical application of renewable energy.Supercapacitor is an important electrochemical energy storage device,which has the characteristics of high power density,fast charge and discharge rate,and long cycle life.It is widely used in high-power applications such as electric vehicles,lifting equipment,portable electronic equipment and weapon launching systems[2]。 However,the relatively low energy density of supercapacitors hinders their large-scale commercial application.Therefore,improving the charge storage capacity and energy density of supercapacitors without sacrificing the power density and cycle life is an important problem to be solved。
the energy density of supercapacitors is closely related to The physical and chemical characteristics of electrode materials[3]。 So far,a series of electrode materials with excellent properties have been developed,including nano-carbon materials,two-dimensional materials,conducting polymers,and transition metal oxides,hydroxides,nitrides,phosphides,and sulfides,which effectively promote the development of supercapacitors[4,5]。 transition metal compounds belong to pseudocapacitive electrode materials and battery-type electrode materials,which store charge through Faradaic redox reaction and have higher specific capacity,so they have attracted wide attention.However,Transition metal compounds have some problems,such as poor conductivity,slow reaction kinetics and few exposed electroactive sites,which lead to their unsatisfactory specific capacity,rate capability and stability during charge-discharge process,and hinder their wide application in practice[6]。 According to the charge storage mechanism,the conductivity of electrode materials,the rate of electron transfer,the diffusion rate of electrolyte ions,the adsorption energy of OH−and the number of electrochemical active sites are the key factors restricting the electrochemical performance of active materials[6,7]。 Therefore,it is an important means to improve the electrochemical performance of electrode materials by designing excellent morphological structures and regulating the intrinsic characteristics of active materials。
surface and interface are important sites for electrochemical reactions.In electrochemical reactions,the micro-nano structure of the surface of active materials and the electronic structure of the interface have an important impact on the rate of electron transfer,the number of electrochemical active sites,the conductivity of electrode materials and the diffusion rate of electrolyte ions.Regulating the micro-nano structure of the electrode surface is an effective means to improve the electrochemical performance of electrode materials.Excellent micro-nano structure can increase the contact area between the electrode and the electrolyte,expose more electrochemical active sites,and shorten the distance of electron and ion transfer or diffusion,thereby improving the electrochemical performance of electrode materials[8,9]。 In addition,the electronic structure on the surface and interface of the active material can regulate the energy band structure of the material,reduce the band gap energy of the material,enhance the charge density near the Fermi level,and improve the conductivity of the material.at the same time,the coupling and redistribution of electrons At the surface and interface can accelerate the transfer of electrons,enhance the adsorption capacity of the interface for electrolyte ions,and improve the electrochemical reaction kinetics[10⇓~12]。 Therefore,the regulation of the surface and interface electronic structure of the electrode material can effectively improve the conductivity of the material,increase the electron transmission rate,improve the electrochemical reaction kinetics,and obtain excellent electrochemical performance。
in this paper,the research and development of surface and interface regulation of transition metal compounds in recent years are reviewed,and the research progress of surface and interface regulation strategies such as morphology regulation,heterostructure,element doping and vacancy engineering in improving the electrochemical performance of supercapacitors is summarized.the effects of the above strategies on the electrochemical properties of the active materials were analyzed from the perspective of the micro-nano structure and electronic structure of the surface and interface.Finally,this paper summarizes the reasons for the improvement of supercapacitor performance by surface and interface regulation,and looks forward to the challenges faced by surface and interface regulation in improving supercapacitor performance,which provides an important reference for the development of supercapacitors with high performance and high stability。

2 Surface and Interface Control Strategy of Transition Metal Compound Electrode Materials

surface and interface are important sites for faradaic redox reaction of transition metal compounds.the electrochemical performance of transition metal compounds can be effectively improved by controlling the micro-nano structure and electronic structure of the surface and interface of transition metal compounds through morphology control,heterostructure,element doping,vacancy engineering and other strategies.Understanding and mastering the mechanism of surface and interface control strategies on the electrochemical performance of transition metal compound electrode materials is conducive to providing a theoretical basis for the preparation of high-performance and high-stability supercapacitor electrode materials。

2.1 Morphology control

The microstructure of electrode materials has an important influence on the accessibility,diffusion path and structural stability of electrolyte ions,and then on the capacity,rate capability and stability of electrode materials.Traditional nanoparticles usually have high surface energy and are prone to agglomeration,which reduces the specific surface area and stability of the material.the electrochemical performance of electrode materials can be improved by designing and constructing special nanostructures,which can effectively increase the specific surface area,increase the number of active sites,increase the structural stability of electrode materials and shorten the diffusion distance of electrolyte ions.the common electrode structures include one-dimensional nanowire structure,two-dimensional nanosheet structure,three-dimensional porous structure and three-dimensional hollow structure。
One-dimensional nanostructures can provide fast and efficient electron transport channels,while shortening the diffusion path of electrolyte ions,thereby improving the rate performance of electrode materials,such as ZnCo2O4nanorods,MnCo2O4nanowires[13][14]。 Shen et al.Prepared the NiCo2O4nanowire/carbon fiber composite by a simple surfactant-assisted hydrothermal method and calcination,and the NiCo2O4nanowire was composed of a large number of well-crystallized nanoparticles,forming a mesoporous structure,as shown in Fig.1[15]。 Mesoporous nanowires can provide a fast channel for the transmission of electrons and the diffusion of ions,and promote each nanowire to participate in rapid electrochemical reactions.The specific capacitance at 1 A·g-1is 1283 F·g-1,and the capacitance retention is still as high as 78.7%when the current density increases to 20 A·g-1,indicating that the nanowire structure can promote the transmission of electrons and improve the capacity and rate performance of the material 。
图1 NiCo2O4纳米线/碳纤维复合材料的制备流程[15]

Fig. 1 Preparation process of NiCo2O4 nanowire/carbon fiber composite[15]. Copyright 2014, Wiley

One-dimensional/two-dimensional composite nanostructures can fully integrate the advantages of their respective structures,which can not only provide effective electron and ion transmission paths,but also provide more edge active sites,which is conducive to improving the kinetics of electrochemical redox.Liu et al.Prepared excellent NiCo2O4nanowire/Ni3S2nanosheet core-shell hierarchical material（NiCo2O4@Ni3S2）through simple hydrothermal,chemical deposition and sulfurization processes[16]; The NiCo2O4nanowires are cross-connected to form a network structure,which provides a high-speed channel for the transmission of ions and electrons,and the nanowire network structure provides a growth framework for Ni3S2.The Ni3S2nanosheets can expose more edge active sites while providing effective protection for NiCo2O4,and the specific capacity and cycling stability of the NiCo2O4@Ni3S2electrode are improved compared with those of the single electrode material(the specific capacity can reach 3.0 F·cm-2at a current density of 5 mA·cm-2,and the capacitance retention is as high as 93.3%after 10 000 cycles).Chen et al.Used hydrogen peroxide as an inducer to prepare Ni3S2rod-sheet structures on nickel foam in situ by a one-step hydrothermal method,as shown in Fig.2[17]。 In the reaction,sodium thiosulfate is hydrolyzed to form sulfide ions,which react with nickel to form Ni3S2.The hydrolysis speed of the sodium thiosulfate is adjusted and controlled by an additive to control the generation speed of the Ni3S2,and finally the purpose of accurately adjusting and controlling the structure is achieved.In the Ni3S2rod-sheet structure,the one-dimensional nanorod framework can provide a channel for high-speed electron transfer,and the two-dimensional nanosheet can provide abundant electrochemical edge active sites and protect the framework from electrochemical corrosion.Benefiting from the above advantages,the specific capacity and cycle stability of the Ni3S2rod electrode are improved(at a current density of 5 mA·cm-2,the specific capacity can reach 6.2 F·cm-2;The capacitance retention was as high as 85.7%after 3000 cycles at a current density of 15 mA·cm-2 )。
图2 泡沫镍上原位生长Ni3S2棒片结构的制备示意图[17]

Fig. 2 Schematic diagram of the preparation of NiCo2O4@Ni3S2 core-shell structure[17]. Copyright 2020, Elsevier

Three-dimensional micro-nano structures are composed of different types of nanostructures,while maintaining the unique functions of different nanostructures,resulting in different electrochemical properties.Chen et al.Synthesized porous NiCo2O4nanoflowers,and the scanning electron microscope(SEM)images showed that the flower-like structure was composed of different nano-petals,and the petals extended outward from the center of the flower(Fig.3A,B )[18]; Transmission electron microscopy(TEM)characterization shows that there are a large number of porous structures on the petals,which can provide more electroactive area and facilitate the diffusion of electrolyte ions,thereby increasing the rate performance of the electrode material(Fig.3C);At 1 A·g-1,the specific capacitance can reach 658 F·g-1,and the capacitance retention rate is as high as 78%when the current density increases to 20 A·g-1;The capacitance retention is nearly 100%after 10 000 cycles at 5 A·g-1.Xiao et al.Controlled the preparation of Ni/Co-layered double hydroxide(LDH)microspheres by alkali etching using Ni/Co-MOF microspheres as templates(Fig.3D~f )[19]。 The prepared Ni/Co-LDH microspheres are composed of ultrathin nanosheets,and the three-dimensional structure has a large contactable surface area and a hierarchical porous structure with a specific surface area up to 201.0 m2·g−1,which can provide more active sites and mesoporous channels,and is beneficial to the transmission of electrolyte ions and electrons;The specific capacity,rate capability and cycle stability of the electrode are improved,and the specific capacity can reach 1652 F·g-1at 1 A·g-1,and the capacitance retention rate is as high as 78.7%when the current density increases to 25 A·g-1;The capacitance retention was nearly 100%after 2000 cycles at 5 A·g-1 。
图3 多孔NiCo2O4纳米花的（a, b）SEM和（c）TEM表征图[18]；Ni/Co-层状双氢氧化物微球的（d, e）SEM和（f）TEM表征图[19]

Fig. 3 (a, b) SEM and (c) TEM images of porous NiCo2O4 nanoflowers[18]. Copyright 2014, Elsevier. (d, e) SEM and (f) TEM images of Ni/Co-layered double hydroxide microspheres[19]. Copyright 2019, American Chemical Society

In recent years,hollow structure has been widely used in the morphology design of transition metal-based electrode materials.The main reasons are as follows:the surface of hollow structure materials can expose more atoms,which is conducive to the contact between electrode materials and electrolyte,and shortens the diffusion distance of electrolyte ions;Secondly,the hollow structure can protect the inner layer material and alleviate the volume change of the material during charging and discharging.The hierarchical tetrahedral NiCo2O4hollow microtubes prepared by Ma et al.Exhibited excellent electrochemical performance[20]。 The tetrahedral NiCo2O4hollow microtube is composed of ultrathin mesoporous NiCo2O4nanosheets,has a large specific surface area,can greatly reduce the diffusion distance of electrolyte ions and electrons,More electroactive sites are exposed in the electrochemical redox reaction,and the ultrathin nanosheet can ensure that most of the electroactive materials participate in the charge storage process,thereby effectively improving the electrochemical performance of the electrode material;At 1 A·g-1,the specific capacitance can reach 1387.9 F·g-1,and the capacitance retention rate is as high as 62.2%when the current density increases to 30 A·g-1;After 12 000 cycles at 10 A·g-1,the capacitance retention was as high as 89.4%.In addition,the multi-shell hollow structure can further play the unique advantages of the hollow structure.Compared with the single-layer hollow structure,the multi-layer hollow structure can expose more internal active sites,and has higher packing density and structural stability,for example,the multi-wall Ni-Co oxide and NiCo2S4onion-like structure reported by Lou et al[21,22]。 Therefore,the excellent morphology structure can improve the capacity,rate performance and stability of the material,and play an important role in the preparation of high-performance electrode materials。

2.2 Heterostructure

transition metal compounds are widely used as electrode materials for supercapacitors,which store charge through redox reactions on the electrode surface during charge/discharge,and belong to the pseudocapacitive or battery-type storage mechanism.However,the poor conductivity and stability of pure transition metal compounds limit their practical applications.heterostructure is an effective strategy to improve the electrochemical properties of transition metal compounds.heterostructures are mainly combined by van der Waals attraction and chemical bonding,showing complex geometries and fuzzy connection interfaces,forming heterostructures.Heterostructures not only contain the complementary advantages of many materials,but also affect the intrinsic properties of the whole composite,such as band structure,semiconductor properties and Fermi level difference.the different materials of the Heterostructure electrode will produce a certain synergistic effect,which will provide a large number of electrochemical active sites and accelerate charge transport.transition metal compound heterostructures mainly include transition metal compound/carbon-based heterostructures,transition metal compound/transition metal compound heterostructures,and MXene/transition metal compounds heterostructures。

2.2.1 Transition metal compound/carbon-based heterostructure.

Compositing pseudocapacitive materials with carbon-based nanomaterials can improve the conductivity,specific surface area,mechanical/electrochemical stability and other physical and chemical properties of pseudocapacitive materials[23,24]。 Jiang et al.Uniformly immobilized Ni-S-P nanoparticles on the surface of graphene nanosheets by hydrothermal and phosphating methods to obtain graphene/Ni-S-P heterostructure(G/Ni-S-P)(Fig.4A,B)[25]。 Electrochemical impedance analysis shows that the internal resistance and charge transfer resistance of the G/Ni-S-P electrode are lower than those of the pure Ni-S-P electrode.In addition,the OH−diffusion coefficient of the G/Ni-S-P electrode is significantly larger than that of the Ni-S-P electrode(Fig.4 C,d),and finally,the G/Ni-S-P-electrode shows superior supercapacitor performance,with a specific capacity of 1406 C·g-1at 1 A·g-1and a capacitance retention of up to 60.2%when the current increases to 120 A·g-1(Fig.4E).The assembled G/Ni-S-P//graphene/FeOOH asymmetric supercapacitor exhibited excellent cycling stability with an energy density up to 58.1 Wh·kg-1,corresponding to a power density of 4.7 kW·kg-1,and a capacitance decay of only 4.9%after 30 000 cycles.By compounding with carbon-based materials and making full use of the excellent conductivity of carbon-based materials,the specific capacity and rate performance of electrode materials can be significantly improved 。
图4 G/Ni-S-P复合物：（a）制备示意图；（b）透射电镜图；（c）电化学阻抗图；（d）低频区Z′和ω−1/2关系图；（e）不同电流密度下材料的质量和体积比电容[25]

Fig. 4 G/Ni-S-P composite: (a) Schematic diagram of the preparation, (b) HRTEM image, (c) Nyquist plots, (d) linear relationship between Z′ and ω−1/2 in the low-frequency region, (e) gravimetric and volumetric specific capacities at different current densities[25]. Copyright 2019, Royal Society of Chemistry

g-C3N4is a kind of carbon-based material rich in nitrogen.Wang et al.Constructed a new p-n heterostructure CoNixSy/g-C3N4material by combining p-type semiconductor CoNixSyand n-type semiconductor g-C3N4,and formed a built-in electric field in the material,which is conducive to the diffusion of electrons from n-type semiconductor to p-type semiconductor and holes from p-type semiconductor to n-type semiconductor[26]。 A space charge region is formed at the interface of the two semiconductor materials,providing a favorable driving force for the transfer of ions/electrons and the electrochemical reaction kinetics.The specific capacity of the CoNixSy/g-C3N4electrode is up to 1029 C·g-1at 1 A·g-1,and the energy density of the assembled CoNixSy/g-C3N4//AC asymmetric supercapacitor is up to 71.9 Wh·kg-1.The corresponding power density is 0.23 kW·kg-1,and the capacitance retention is as high as 72.2%after 5000 cycles.Different types of semiconductors are combined to form a heterojunction,which is beneficial to improving the transmission rate of interface electrons 。

2.2.2 Transition metal compound/transition metal compound heterostructure

Transition metal compound/transition metal compound heterostructure(TMC/TMC)with excellent morphology can exert a synergistic effect between different materials,providing a large number of electrochemical active sites and accelerating charge transport.Core-shell structure is one of the most commonly used configurations to construct TMC/TMC heterostructures.Among many core-shell structures,1D@2D core-shell structure is widely used in supercapacitors.The main reason is that the one-dimensional nanowire/rod core material can shorten the diffusion path of ions,and the two-dimensional nanosheet shell material can increase the specific surface area of the material and expose more electrochemical active sites.The reported 1D/@2D core/shell heterostructures are NiCo2O4@NiMoO4,Co3S4@Ni3S4,NiCo-LDH@NiOOH,NiMn-LDH@CuO/CF,etc[27][28][29][30]。 Yun et al.Prepared heterostructured NiSe2/CoSe2hollow microspheres by one-pot hydrothermal method,and studied the influence of the behavior of heterostructured interface charges on the reaction kinetics by density functional theory(DFT)calculations[31]。 The DOS results show that the intensity of NiSe2/CoSe2near the Fermi level increases,indicating the enhanced conductivity of NiSe2/CoSe2(Figure 5A).The difference charge density map of NiSe2/CoSe2clearly shows the accumulation(yellow ball)and depletion(blue ball)of electrons at the phase interface,and Bader charge analysis confirms that 0.79 eV electrons are accumulated near NiSe2,indicating that electrons will be transferred from NiSe2to the CoSe2side(Figure 5 B).The strong signal appearing near the heterointerface in the charge density difference map indicates the occurrence of electron gain and loss(red line),and the in-plane average electrostatic potential of NiSe2(−33.87 eV)is much lower than that of CoSe2(-27.46 eV),further illustrating the electron transfer from NiSe2to the CoSe2side(Figure 5C).Therefore,the surface of CoSe2will accumulate electrons,while the surface of NiSe2will lose electrons,which will eventually redistribute the interfacial charge.At the same time,the electron loss characteristics of the NiSe2surface will make it have a strong ability to adsorb OH−ions at the phase interface.By calculating the adsorption energy of the OH−,It is found that the adsorption energy of NiSe2/CoSe2heterointerface for OH−(-3.33 eV)is much lower than that of NiSe2（-1.61 eV）and CoSe2（-1.67 eV）for OH−,It shows that the heterogeneous interface of NiSe2/CoSe2is beneficial to the adsorption of OH-and improves the kinetics of redox reaction(Figure 5D).Finally,the NiSe2/CoSe2electrode obtained a high specific capacitance of 171.5 mAh·g-1(1 A·g-1),with a capacitance retention of up to 40.9%at 100 A·g-1and 109.8%after 5000 cycles at 10 A·g-1,showing excellent rate and cycling performance.This work reveals the charge redistribution law of selenide heterojunction,and provides a reference for the construction of metal selenide heterostructure with rich phase interface 。
图5 （a）NiSe2、CoSe2和NiSe2/CoSe2异质结的总态密度和分波态密度；（b）NiSe2和CoSe2异质结界面差分电荷密度；（c）平面和宏观平均静电势；（d）NiSe2、CoSe2和NiSe2/CoSe2异质结对OH-的吸附能[31]

Fig. 5 (a) Total DOS and PDOS of NiSe2, CoSe2 and heterostructured NiSe2/CoSe2. (b) The computed differential charge density between NiSe2 and CoSe2 in heterogeneous phase interfaces. (c) Planar and macroscopic averaged electrostatic potential. (d) Calculations of the OH− ions adsorption energy of NiSe2, CoSe2 and heterostructured NiSe2/CoSe2[31]. Copyright 2021, Elsevier

from the above discussion,it can be seen that the heterostructure electrode composed of transition metal compounds shows excellent supercapacitive performance,and the unique properties of the heterostructure are derived From the synergistic effect between different components,which improves the conductivity of the electrode and the electron transfer kinetics during the electrochemical reaction.However,the synergistic mechanism between the materials of the heterostructure still needs to be further explored in order to further grasp the law of the heterostructure to improve the performance of supercapacitors。

2.2.3 MXene/transition metal compound heterostructure

the construction of MXene/transition metal compound heterostructure can effectively utilize The high conductivity,excellent mechanical stability and large specific capacitance of MXene,and then obtain better electrochemical performance.Wang et al.grew Ni-Mn LDH nanosheets on MXene by hydrothermal method to obtain sandwich-like Ni-Mn LDH-MXene-Ni-Mn LDH(LDH-MXene-LDH)(Fig.6)[32]; LDH is uniformly distributed on the surface of MXene,which can reduce the agglomeration of LDH,and the specific surface area reaches 103.0 m2·g-1.The large specific surface area is conducive to increasing the number of active sites and improving the specific capacitance.The density of States calculation shows that MXene@Ni-Mn LDH has the maximum intensity near the Fermi level,indicating that MXene@Ni-Mn LDH has good electron transfer characteristics.Compared with the single Ni-Mn LDH,the band of MXene@Ni-Mn LDH becomes curved,indicating that the electron transport rate inside the material is enhanced,which is consistent with the density of States results.The binding energy of MXene@Ni-Mn LDH is about 0.24 eV,indicating that the recombination of the material is an endothermic process,which is beneficial to improve the stability of the material.The above analysis shows that the electronic coupling between NiMn-LDH and MXene can improve the electron transfer rate and ensure the structural stability during the charge-discharge cycle.The specific capacitance of LDH-MXene-LDH electrode at 1 A·g-1was 179 mAh·g-1,and the capacitance retention at 10 A·g-1was 62.6%,which was as high as 79.1%after 5000 cycles at 6 A·g-1,showing excellent electrochemical performance.Lu et al.Grew Ni2Co-LDHs nanoarrays on Ti3C2MXene of 1–3 atomic layers to obtain honeycomb-like sandwich-type Ni2Co-LDHs@AL-Ti3C2MXene composites[33]。 It benefits from the strong interfacial interaction between Ni2Co-LDHs and MXene and the enhanced conductivity.The Ni2Co-LDHs@AL-Ti3C2MXene composite exhibited ultrahigh rate capability and excellent cycling stability.The specific capacitance at 1 A·g-1was 227 mAh·g-1,and the capacitance retention was still as high as 55.5%at an ultra-high current density of 150 A·g-1,indicating the excellent conductivity of the electrode material.After 10 000 cycles at 10 A·g-1,the capacitance retention was as high as 90.0%.The assembled Ni2Co-LDHs@AL-Ti3C2MXene//GO hybrid supercapacitor has an energy density up to 68.0 Wh·kg-1,corresponding to a power density of 388 W·kg-1,and a capacitance retention of up to 90.0%after 10 000 cycles at a current density of 5 A·g-1.The above results show that MXene can control the band structure,electronic configuration and charge distribution near the Fermi level of the material,promote the electron transport in the material,and improve the specific capacity and rate performance of the material 。
图6 MXene@Ni-Mn LDH电极电化学性能增强机理图[32]

Fig. 6 Schematic diagram of the enhanced electrochemical performance of the MXene@Ni-Mn LDH electrode[32]. Copyright 2020, Elsevier

2.3 Element doping

the physical and chemical properties of electrode materials are closely related to their electronic structure,and it is helpful to obtain better electrochemical performance by adjusting the electronic structure of electrode materials.Element doping is an important means to construct defects,which can control the electronic structure of active materials and the surface properties of electrode materials,and effectively improve the electrochemical performance of electrode materials[34]。 Due to the different atomic radius and electronegativity introduced into the lattice,it will cause slight distortion of the lattice and redistribution of electrons,so as to achieve the purpose of regulating the electronic structure of active materials and improving their inherent conductivity.Up to now,element doping has been widely used to improve the electrochemical performance of electrode materials,which mainly includes metal element doping and non-metal element doping。

2.3.1 Metal element doping

Chen et al.Doped Au with free electrons into the MnO2lattice to improve the conductivity and specific capacitance of MnO2[34]。 The results show that Au is uniformly distributed in the MnO2lattice in the form of individual atoms,and the valence state of Mn changes after Au doping,indicating a strong chemical interaction between Au and MnO2(Fig.7).First-principles calculations confirm that Au doping can significantly reduce the band gap energy of MnO2and improve the conductivity of MnO2.Compared with the pure MnO2film,the Au-doped MnO2film achieved a specific capacity of 626 F·g-1as well as excellent cycling stability(7%increase in specific capacity after 15 000 cycles).However,the specific capacity of the pure MnO2film decreased by 34%after 15 000 cycles,which fully demonstrates that Au doping can improve the specific capacity and cycle stability of MnO2.Guo et al.Introduced Cu element into NiCo-LDH to prepare Cu-doped NiCo-LDH nanowires[35]。 UV photoelectron spectroscopy characterization shows that the work function of NiCo-LDH decreases from 5.24 eV to 4.84 eV after Cu doping,which indicates that the electronic structure of NiCo-LDH changes after Cu doping,resulting in the decrease of work function and the increase of conductivity.The as-prepared CCCH@NiCo-LDH NWAs@Au-CuO/Cu fibers obtained a maximum specific capacitance of 1237 F·g-1,and the capacitance retention was as high as 90.8%after 30 000 cycles at a current density of 33.3 A·g-1 。
图7 Au-doped MnO2薄膜电极制备过程及扫描透射电子显微镜图[34]

Fig. 7 Fabrication process and STEM images of the Au-doped MnO2 electrodes[34]. Copyright 2013, Wiley

On the other hand,metal doping can not only control the electronic structure of materials,but also change the morphology of materials.Li et al.Found that metal element doping can also promote the structural transformation,doped Cu element into Ni3S2,prepared Cu-doped Ni3S2nanosheet/rod nanoarrays by one-step hydrothermal method,and studied the effect of Cu doping on the morphology,structure and properties of Ni3S2[36]。 The results show that Cu doping can increase the number of free carriers near the Fermi level,which is beneficial to improve the electron transport efficiency and the rate capability of the electrode.In addition,Cu doping can induce the formation of nanosheet rod structures,providing more ion and electron transport pathways and exposing more electroactive sites.Compared with the pure Ni3S2nanowire,the specific capacity of the Cu-doped Ni3S2nanosheet/rod electrode is three times higher at a current density of 10 A·g-1,and the capacitance retention is as high as 94.0%after 5000 cycles at 5 A·g-1 。
Subsequently,Chen et Al.Designed and prepared Al-doped NiCo2O4nanosheet-wire arrays by using the strategy of Al-doping to control the electronic structure and morphology of materials（Al-NiCo2O4NSW）（Fig.8 )[37]。 The nanosheet-wire hybrid structures are interconnected to form an efficient transport network,which facilitates electron/ion transport and exposes more active sites.Experimental analysis and theoretical calculation show that Al doping can control the electronic structure of materials and improve the conductivity of materials.DFT confirmed that Al-NiCo2O4has a stronger OH−adsorption capacity than pure NiCo2O4.The constructed Al-NiCo2O4NSW electrode exhibited a specific capacity of up to 1441 C·g-1at 1 A·g-1and a capacitance retention of 87.6%after 5000 cycles at 10 A·g-1,demonstrating superior stability.The assembled Al-NiCo2O4NSW//AC hybrid supercapacitor has an energy density up to 46.2 Wh·kg-1at a power density of 800 W·kg-1and a capacitance retention of up to 90.9%after 15 000 cycles at 10 A·g-1.Metal doping can not only control the electronic structure and conductivity of electrode materials,but also control the morphology of electrode materials.Under certain conditions,different types of composite structures can be obtained,which can effectively improve the electrochemical performance of electrode materials 。
图8 Al-NiCo2O4 NSW的结构和性能示意图[37]

Fig. 8 Structure and properties of Al-NiCo2O4 NSW[37]. Copyright 2022, Elsevier

2.3.2 Non-metallic element doping

Non-metallic element doping also plays an important role in improving the electrochemical performance of electrode materials.The common non-metallic elements are mainly boron(B),phosphorus(P),sulfur(S)and so on.The B atom has a small atomic radius,and there are usually two forms of substitution doping（Bsub）and interstitial doping（Bint）when doping.Li et al.Studied the effect of B doping on the electrochemical performance（B-NiCo2S4）of NiCo2S4by theoretical prediction and experiment[38]。 Theoretical calculations show that the Bsubcan increase the density of States of the B-NiCo2S4near the Fermi level and increase the conductivity of the material(Fig.9a,B);The partial wave density of States shows that B plays an important role in regulating the electronic structure.From the Bsuband interstitial Bintpartial DOS plots,we know that Bsubhas a higher DOS near the Fermi level(Fig.9 C,d).Bintredistributes the surface charge,which is beneficial to improve the adsorption capacity of OH−and the kinetic process of Faraday oxidation reaction(Fig.9e,f).The OH−adsorption energy calculation yields that the Bintsite has the minimum adsorption energy,indicating that Binthas a strong OH-adsorption capacity(Fig.9 G,H).The optimized B-NiCo2S4achieved a high specific capacity of 738.0 C·g-1at 1 A·g-1and exhibited excellent cycling performance with a capacity retention of up to 90.2%after 8000 cycles at 10 A·g-1 。
图9 （a）能带结构图；（b）NiCo2S4和B-NiCo2S4的总态密度图；（c）B-NiCo2S4的分波态密度图；（d）Bint和Bsub掺杂的态密度图；（e, f）NiCo2S4和B-NiCo2S4的电荷密度分布；（g）NiCo2S4和(h) B-NiCo2S4对OH−的吸附能[38]

Fig. 9 (a) Band structures and (b) TDOS of NiCo2S4 and B-NiCo2S4. (c) PDOS of B-NiCo2S4. (d) DOS of Bint and Bsub. Charge density distributions of (e) NiCo2S4 and (f) B-NiCo2S4. Calculated OH− adsorption energies of (g) NiCo2S4 and (h) B-NiCo2S4[38]. Copyright 2022, American Chemical Society

Sulfur element has strong electronegativity,which can regulate the conductivity,ion diffusion rate and other parameters of active materials,thus improving the electrochemical performance of transition metal oxide and phosphide electrode materials[39]。 Gao et al.Prepared partially S-doped NiFe2O4nanoparticles(NFO-S)by a simple hydrothermal method[40]。 HRTEM analysis shows that the interplanar distance of(222)crystal plane of S-doped NiFe2O4increases from 2.4040 to 2.4545;The increase of interplanar distance is beneficial to the intercalation of OH−ions in the electrochemical reaction process and improves the electrochemical performance of electrode materials.The theoretical calculation also confirms that the size of the crystal becomes larger after S doping.NFO-S achieved a high specific capacity of 284.0 F·g-1at a current density of 1 A·g-1.Liu et al.Doped S element into Nb2O5microspheres to form Nb4+/oxygen vacancies in Nb2O5crystals,which effectively improved the conductivity of Nb2O5[41]。 S-Nb2O5HNS@S-rGO was used as an electrode for sodium-ion batteries,achieving excellent cycling performance and rate capability 。
In terms of improving the performance of phosphide,Elshahawy et al.doped S element into CoP to obtain S-Doped CoP nanotube arrays[42]。 It is found that after the introduction of S element,S atom and Co atom form a compound of cobalt phosphide and cobalt sulfide,and the presence of sulfur reduces the formation of CoP passivation layer,which is conducive to improving the conductivity of electrode materials.Compared with the CoP electrode,the specific capacitance of the Co(P,S)electrode is increased by about 78%,and the capacitance retention rate is still as high as 99%after 10 000 cycles,showing ultra-high cycle stability.The as-prepared Co(P,S)//porous carbon-rGO asymmetric supercapacitor achieved a high energy density of 39 Wh·kg-1at a power density of 0.8 kW·kg-1 。
Compared with S atom,P atom has lower electronegativity and stronger electron-donating ability.the introduction of P atoms into electrode materials can control the electronic structure of electrode materials,improve the electron transport rate and electrochemical reaction kinetics,and thus improve the electrochemical performance of electrode materials[43]。 P atom doping is widely used in water splitting reactions and supercapacitors[44,45][46,47]。 Chu et al.Prepared P-doped NiCo2O4（P-NiCo2O4）by phosphating method.The experiment results showed that after introducing P atoms,the oxygen atoms in the NiCo2O4lattice were adsorbed and a large number of oxygen vacancies were produced,which improved the conductivity of the P-NiCo2O4,reduced the band gap energy and accelerated the charge transfer rate[48]。 In addition,due to the smaller electronegativity of P atom,the introduction of P atom reduces the attraction of electrons in the process of electron gain/loss,which not only promotes the redox reaction and further improves the specific capacitance,but also improves the electrocatalytic activity of hydrogen evolution and oxygen evolution.Through optimization,the P-NiCo2O4obtained an ultra-high specific capacitance of 2747.8 F·g-1at 1 A·g-1with excellent rate capability(50%capacitance retention at 100 A·g−1).The energy density of the assembled p-NCO NWs/NF//RGO asymmetric supercapacitor was 28.2 Wh·kg-1,corresponding to a power density of 7750.6 W·kg-1.Subsequently,Miao et al.Doped P atoms into the CoWO4@NiWO4nanocomposite,and also found that P doping could induce the generation of abundant oxygen vacancies,and the specific capacitance of P-NiWO4@CoWO4was 2.5 times higher than that of the original NiWO4@CoWO4,and both the specific capacitance and rate capability were improved[49]。 Doping P element into transition metal oxide can produce a certain amount of oxygen vacancies at the same time,which can improve the conductivity of the material。
Li et al.Prepared heteroatom-doped carbon-shell-coated phosphorus-doped zinc-cobalt-sulfur composite(P-ZCS/HC)by sulfidation and phosphation using ZnCo-MOF as a template(Fig.10a)[50]。 DFT theoretical calculations confirm that P-doped ZCS have a higher density of States at the Fermi level,indicating that P-ZCS have higher charge occupation States and enhanced electron mobility.At the same time,the results also show that the metallic characteristics of the electrode are more obvious after P doping,and the conductivity is improved(Fig.10B,C).Secondly,the adsorption energy of OH−on P-ZCS(200)plane and ZCS(200)plane Co site calculated by DFT is−3.30 and−2.95 eV,respectively,indicating that P doping is beneficial to improve the adsorption capacity of electrode materials for OH−(Fig.10 d,e).The results of DFT theoretical calculation are in good agreement with the experimental results of XPS analysis,such as the strengthening of bonds between metal ions and anions,and the decrease of electrode volume resistance in electrochemical impedance spectroscopy.Finally,the P-ZCS/HC achieved a high specific capacity of 1080 C·g-1at 1 A·g-1with excellent cycle performance.The assembled hybrid supercapacitor has a high energy density of 62.9 Wh·kg-1,corresponding to a power density of 16 kW·kg-1,and a high capacitance retention of 92.0%after 10 000 cycles.Therefore,doping P atoms into the lattice of transition metal sulfides changes the electronic structure,creating more accessible active sites and lowering the adsorption energy for OH− 。
图10 （a）两个视角下的P-doped ZCS晶体结构；（b）ZCS和（c）P-doped ZCS的态密度图；（d）OH-在P-doped ZCS（200）面Co位点上的吸附示意图；（e）OH-在ZCS和P-doped ZCS不同位点上的吸附能[50]

Fig. 10 (a) Crystalline structure of P-doped ZCS from two perspectives. DOS of (b) ZCS and (c) P-doped ZCS. (d) Diagrammatic sketch of the OH− adsorption on Co sites of the (200) facet of P-doped ZCS. (e) OH− adsorption energies of different sites in ZCS and P-doped ZCS[50]. Copyright 2021, Springer

2.4 Vacancy engineering

vacancy engineering is the formation of atomic vacancy defects on the surface of materials by special methods,such as O vacancy and S vacancy.the formed vacancies can control the surface properties and electronic structure of the nanomaterials,generate more electroactive sites,improve the conductivity of the active materials,and improve the energy storage capacity of the electrode materials[51⇓~53]。

2.4.1 Single Vacancy Control

Yang et al.Prepared the Co3O4/graphene composite by introducing abundant oxygen vacancies on the surface of Co3O4through one-step laser irradiation[54]。 First-principles calculations show that surface oxygen vacancies can promote charge transfer in electrochemical reactions by creating mid-gap electronic States.When it is used as a supercapacitor electrode material,the specific capacitance is as high as 978.1 F·g-1at a current density of 1 A·g-1;The rate capability can reach 93.7%at a high current density of 10 A·g-1;In addition,after 20 000 charge-discharge cycles,the capacitance retention rate is as high as 99.3%,showing ultra-high cycle stability 。
Tan et al.Used room temperature NaBH4reduction to prepare Co3S4hierarchical hollow nanosheets（VS-Co3S4/rGO/NF）with abundant sulfur vacancies[55]。 It can be seen from Fig.11 that the discontinuous lattice fringes indicate the existence of vacancies,and the electron paramagnetic resonance signal at G=2.002 indicates that there are abundant sulfur vacancies VS-Co3S4/rGO/NF.Sulfur vacancies can significantly increase the conductivity of electrode materials and expose more active sites.At 1 A·g-1,the specific capacitance of VS-Co3S4/rGO/NF can reach 2615 F·g-1,and at 30 A·g-1,the capacitance retention is 70.2%after 5000 cycles,showing excellent electrochemical performance.The maximum energy density of the VS-Co3S4/rGO/NF//AC asymmetric supercapacitor constructed by using it as the positive electrode and activated carbon as the negative electrode is 43.8 Wh·kg-1,and the corresponding power density is 850 W·kg-1 。
图11 （a）VS-Co3S4/rGO/NF样品的高分辨透射电镜图；（b）VS-Co3S4/rGO/NF和Co3S4/rGO/NF的电子顺磁共振谱图[55]

Fig. 11 High-resolution transmission electron microscope image of VS-Co3S4/rGO/NF sample. (b) Electron paramagnetic resonance spectra of VS-Co3S4/rGO/NF and Co3S4/rGO/NF[55]. Copyright 2022, Royal Society of Chemistry

With the development of vacancy engineering,how to simplify the preparation steps of vacancy and optimize the experimental process has become a hot research topic.Zhang et al.Prepared sulfur vacancy-rich Ni3S4−xhollow microspheres（Ni3S4−xHMs）（(Fig.12)directly by liquid-phase ion exchange method using Ni-MOF as precursor without reducing agent treatment[56]。 The method directly obtains the Ni3S4−xwith rich sulfur vacancies by controlling the time of the ion exchange reaction,avoids the use of a reducing agent for treatment,and simplifies the experimental process.The experimental and theoretical analysis showed that the introduction of sulfur vacancies could effectively improve the adsorption capacity of nickel sulfide for anions,and the specific capacity and cycle performance of the Ni3S4−xHMs electrode were improved.The specific capacitance at 2 A·g-1was 1884 F·g-1,and the capacitance retention rate was as high as 97.9%after 10 000 cycles at 10 A·g-1.This work can provide a technical reference for the preparation of binary or ternary metal sulfide hollow microspheres and hollow nanosheets rich in sulfur vacancies 。
图12 （a）Ni3S4−x HMs的制备示意图；密度泛函理论计算OH−在Ni3S4−x的（b）（311）面和（c）（111）面上的吸附能[56]

Fig. 12 (a) Schematic diagram of the preparation of Ni3S4−x HMs. The adsorption energy of OH− (b) in (311) plane and (c) in (111) plane of Ni3S4−x from density functional theory calculations[56]. Copyright 2022, Wiley

2.4.2 Element doping and vacancy control

Compared with a single defect,the construction of multiple defects can achieve the synergistic effect between different defects,which is conducive to obtaining better electrochemical performance.Kang et al.Constructed a double-defect bimetallic sulfide nanotube active material（P-NiCo2S4−x）by P doping and reduction to form sulfur vacancies[57]。 After introducing P element and S vacancy into the crystal,new orbitals are created in the forbidden band region,which act as electron recombination centers and improve the conductivity of the material.The double defect can also promote the generation of Ni and Co in low oxidation States,stimulating charge transfer kinetic behavior and providing abundant faradaic redox reactions.The specific capacitance of P-NiCo2S4−xas an electrode material for supercapacitor is as high as 1806.4 C·g-1at a current density of 1 A·g-1;The capacitance retention was as high as 95.5%after 5000 charge-discharge cycles at a high current density of 30 A·g-1,showing excellent cycling stability 。
The construction of electrode materials with double defects by simple and efficient methods is the key to their practical application.Liu et al.Used a multi-metal organometallic framework(Fe-NiCo-ZIF)as a precursor,introduced Fe directly into the preparation process,optimized the parameters of the sulfurization process,simplified the experimental process,and successfully prepared Fe-CoNi2S4−xactive materials with double defects of Fe-doping and sulfur vacancy[58]。 DFT calculations show that Fe doping and sulfur vacancy can increase the number of electrons near the Fermi level,which is beneficial to the electron transfer in the electrochemical reaction process.In addition,the total density of States of Fe-CoNi2S4−xbegins to delocalize,favoring the transfer of electrons.The calculation of the adsorption of Fe-CoNi2S4−xon OH−shows that the bonding strength between the surface of CoNi2S4−xand OH−is the largest.Meanwhile,the prepared Fe-NiCo-S material has large specific surface area and rich pore structure,and has ultrahigh specific capacitance 2779.6 F·g-1when used as a supercapacitor electrode material at 1 A·g-1.The assembled Fe-NiCo-S//rGO hybrid supercapacitor achieved a high energy density of 56.0 Wh·kg-1at a power density of 847.1 W·kg-1 。
It is found that the N atoms embedded in the metal oxide lattice will replace part of the inherent O atoms,resulting in a large number of oxygen Defects.Defects will interfere with the surrounding atoms to a certain extent,causing lattice distortion of crystal materials,thus effectively regulating the electronic structure,chemical properties and conductivity of materials[59]。 Wei et al.Employed N2/Ar plasma to directly etch NiCo2O4to generate N-doped and O-vacancy rich NiCo2O4mesoporous nanograss（N-Ov-NCO MiNG）（Fig.13 )[60]。 According to the method,an N element is introduced,and an O vacancy is spontaneously generated in the material,so that the preparation efficiency of the material is improved,and the production cost is saved.N doping and O vacancies are able to tune the electronic structure and improve the availability of N-Ov-NCO MiNG active sites,thereby significantly improving the electron mobility and facilitating the reaction kinetics.By optimizing the reaction time of plasma,the prepared N-Ov-NCO MiNG-15 electrode achieved an ultra-high specific capacity of 2986.3 F·g-1at 1 mA·cm-2,and exhibited excellent rate capability and cycle performance(Fig.10 B,C).The assembled N-Ov-NCO MiNG-15//AC asymmetric supercapacitor obtains a high energy density of 103.2 Wh·kg-1.The cost of electrode preparation can be reduced to a certain extent by introducing both element doping and vacancy through a simple preparation process,which makes the application of electrode materials with double defects possible in practice 。
图13 （a）N-Ov-NCO MiNG-x电极的制备示意图；（b）不同电极材料在不同电流密度下的比电容；（c）样品在20 mA·cm−2下的循环性能[60]

Fig. 13 (a) Schematic illustration for fabrication of the N-Ov-NCO MiNG-x. (b) Specific capacitances of the various electrodes at different current densities. (c) Cycling performance at the current density of 20 mA·cm−2 [60]. Copyright 2022, Elsevier

Chen et al.Used the strategy of element doping to control element vacancy to realize the synergistic improvement of material conductivity and reaction kinetics process by element doping and vacancy engineering[61]。 P-doped sulfur-rich NiCo2S4hollow microspheres were designed and prepared by controlling material element vacancies through P-doping（P-NiCo2S4−xHMSs）（Fig.14 a).Experiments show that P doping changes the electronic structure of the NiCo2S4,enhances the mobility of electrons,increases the number of sulfur vacancies,improves the conductivity of the material and exposes more active sites(Fig.14b).Meanwhile,P-NiCo2S4−xhas a higher OH−diffusion coefficient than NiCo2S4(Figure 14C).Density functional theory calculations confirm that the adsorption energy of P-NiCo2S4−xto OH-is lower than that of NiCo2S4to OH-,which is beneficial to the storage of OH-(Fig.14 d).The specific capacity of the prepared P-NiCo2S4−xHMSs electrode can reach 1146.0 C·g-1at 1 A·g-1;At 30 A·g-1,the capacity retention rate is as high as 61.8%;After 20 000 cycles at 10 A·g-1,the capacity retention was still as high as 79.5% 。
图14 （a）P-NiCo2S4−x HMSs的TEM图；（b）电子顺磁共振谱图；（c）低频区Z´对ω−1/2的线性拟合关系图；（d）密度泛函理论计算OH-在P-NiCo2S4−x和NiCo2S4的（311）和（111）面上的吸附能[61]

Fig. 14 (a) The TEM image of P-NiCo2S4−x HMSs. (b) Electron paramagnetic resonance spectra. (c) Linear relationship between Z′ and ω−1/2 in the low-frequency region. (d) The adsorption energy of OH- in (311) and (111) planes for P-NiCo2S4−x and NiCo2S4 from density functional theory calculation[61]. Copyright 2023, Elsevier

the above analysis shows that the vacancy defect can improve the electron density near the Fermi level,increase the conductivity of the material,accelerate the electron transfer in the chemical reaction process,regulate the charge transfer kinetic behavior,and provide abundant Faradaic redox chemical reactions,and the specific capacity,rate capability,and cycle stability of the material are improved to a certain extent.Secondly,the simultaneous introduction of elemental doping and vacancies into the material by a simple method will lead to unexpected electrochemical performance。
Table 1 shows the electrochemical performance parameters of electrode materials prepared By different regulation strategies.by comparison,it provides readers with a more comprehensive overview of the research on surface and interface regulation in supercapacitor electrode materials,and proves the practical application effect of surface and interface regulation in supercapacitor electrode materials。
表1 Comparison of Electrochemical Performance Parameters of Electrode Materials Prepared by Different Control Strategies

Table 1 Comparison of electrochemical performance of electrode materials prepared by different modulation strategies

Materials Capacity
(F·g-1, 1 A·g-1)		 Rate capability Cyclic stability Supercapacitor Power denstity
(Wh·kg-1)		 Energy density
(W·kg-1)		 Ref
NiCo2O4 nanowire 1283 F·g-1 at 1 A·g-1 79.0% at 20·A g-1 72. 7% after 2100 cycles at 1 A·g-1 - - - 15
NiCo2O4@Ni3S2 nanowire 3.0 F·cm-2 at 5 mA·cm-2 - 93.3% after 10000 cycles at 1 A·g-1 NiCo2O4@Ni3S2//AC 5.81 W·cm-3 1.89 mWh·cm-3 16
Ni3S2 nanorod/sheet 6.24 F·cm-2 at 5 mA·cm-2 85.7% at 20 A·g-1 85.7% after 3000 cycles at 15 mA·cm-1 Ni3S2 RS//AC 15.00 W·cm-3 1.16 mWh·cm-3 17
NiCo2O4 nanoflower 658 F·g-1 at 1 A g-1 78.0% at 20 A·g-1 93.5% after 10000 cycles at 5 A·g-1 NiCo2O4//RGO 650 23.9 18
Ni/Co-LDH microspheres 1652 F·g-1 at 1 A·g-1 87% at 25 A·g-1 100% after 2000 cycles at 5 A·g-1 Ni/Co-LDH//AC 74.3 32.9 19
NiCo2O4 microtubes 1387.9 F·g-1 at 1 A·g-1 62.2% at 30 A·g-1 89.4% after 12000 cycles at 10 A·g-1 - - - 20
G/Ni-S-P 1406 C·g-1 at 1 A·g-1 60.2% at 120 A·g-1 89.4% after 12000 cycles at 10 A·g-1 G/Ni-S-P//graphene/FeOOH 4.7 kW·kg-1 58.1 25
NiSe2/CoSe2 171.5 mAh·g-1 at 1 A·g-1 60.8% at 50 A·g-1 109.8% after 5000 cycles at 10 A·g-1 NiSe2/CoSe2//N，S-rGO HSC 26.1 kW·kg-1 53.7 31
LDH-MXene-LDH 179 mAh·g-1 at 1 A·g-1 62.6% at 10 A·g-1 79.1% after 5000 cycles at 6 A·g-1 MXene@Ni-Mn LDH//AC 800 W·kg-1 44.7 32
Ni2Co-LDHs@AL-Ti3C2 MXene 227 mAh·g-1 at 1 A·g-1 55.5% at 150 A·g-1 90.0% after 10000 cycles at 10 A·g-1 Ni2Co-LDHs@AL-Ti3C2 MXene//GO 388 68.0 33
Cu-Ni3S2 847 F·g-1 at 1 A·g-1 66.3% at 10 A·g-1 94.0% after 5000 cycles at 5 A·g-1 Cu-Ni3S2//AC 850 33.7 36
Al-NiCo2O4 NSW 1441 C·g-1 at 1 A·g-1 60.5% at 20 A·g-1 87.6% after 5000 cycles at 10 A·g-1 Al-NiCo2O4 NSW//AC 800 46.2 37
B-NiCo2S4 738.0 C·g-1 at 1 A·g-1 72.9% at 10 A·g-1 98.5% after 5000 cycles at 5 A·g-1 B-NiCo2S4//AC 804 32.9 38
S-NiFe2O4 284.0 F·g-1 at 1 A·g-1 82.0% at 10 A·g-1 - NFO-S//AC 375 21.1 40
P-NiCo2O4 2747.8 F·g-1 at 1 A·g-1 50% at 100 A·g-1 94.6% after 5000 cycles at 15 A·g-1 P-NCO NWs/NF//RGO 7750 28.2 48
P-NiWO4@CoWO4 1683.4 F·g-1 at 1 A·g-1 88% at 10 A·g-1 84.0% after 1000 cycles at 1 mA cm-2 P-NiWO4@CoWO4//AC 825 26.8 49
P-ZCS/HC 1080 C·g-1 at 1 A·g-1 63.3% at 20 A·g-1 93.8% after 10000 cycles at 15 A·g-1 P-ZCS/HC//RGO 16 kW·kg-1 62.9 50
Ov-Co3O4 /graphene 978.1 F·g−1 at 1 A·g−1 93.7% at 10 A·g-1 99.3%% after 20000 cycles at 10 A·g-1 - - - 54
VS-Co3S4/rGO/NF 2615 F·g−1 at 1 A·g−1 51.8% at 20 A·g-1 70.2%% after 5000 cycles at 30 A·g-1 VS-Co3S4/rGO/NF//AC 850 43.8 55
Ni3S4−x HMs 1884 F·g-1 at 2 A·g-1 85.6% at 10 A·g-1 97.9% after 10000 cycles at 10 A·g-1 Ni3S4−x HMs//AC 1.68 kW·kg-1 33.1 56
Fe-CoNi2S4−x 2779.6 F·g-1 at 1 A·g-1 58.5% at 10 A·g-1 78.0% after 5000 cycles at 10 A·g-1 Fe-NiCo-S//GO 847 56.0 58
N-Ov-NCO MiNG 2986.3 F·g-1 at 1 mA·cm-2 53% at 10 A·g-1 96.5% after 12000 cycles at 1 mA·cm-2 N-Ov-NCO MiNG-15//AC 748 103.2 60
P-NiCo2S4−x 1146.0 C·g-1 at 1 A·g-1 61.8% at 30 A·g-1 79.5% after 20000 cycles at 10 A·g-1 P-NiCo2S4−x //AC 867 62.7 61

3 Conclusion and prospect

transition metal compound-based supercapacitors have high energy density and have broad application prospects In energy storage systems.However,the problems of poor conductivity,slow reaction kinetics and few exposed electroactive sites restrict the large-scale application of transition metal compounds in practice.in order to solve the problems of poor conductivity,slow reaction kinetics and few exposed electroactive sites of transition metal compounds,the micro-nano structure and electronic structure of transition metal compounds were controlled by surface and interface control strategy.the key factors such as conductivity,electron transfer rate,electrolyte ion diffusion rate and the number of electrochemical active sites of the material are optimized,and the electrochemical performance of the transition metal compound is effectively improved.the mechanism of enhancing the electrochemical performance of transition metal compound electrodes by surface and interface regulation can be attributed to the following four reasons:(1)micro-nano structure can increase the specific surface area of electrode materials,expose more electroactive sites,increase the structural stability of electrode materials,and shorten the diffusion distance of electrolyte ions.O as to improve the electrochemical performance of the electrode material;(2)Heterostructure regulation can form special micro-nano structures,complement the advantages of various materials,optimize the energy band structure,semiconductor characteristics,carrier concentration and Fermi energy level difference of composite materials,exert the synergistic effect between different materials,produce a large number of electrochemical active sites and accelerate charge transport;(3)element doping can cause slight distortion of lattice and redistribution of electrons,regulate the electronic structure of active materials and improve their inherent conductivity,and improve the electrochemical performance of electrode materials;(4)the Element vacancy formed by vacancy engineering can control the surface properties and electronic structure of nanomaterials,produce more electroactive sites,improve the conductivity of active materials,and improve the charge storage capacity of electrode materials。
Although the surface and interface control strategy has made significant progress in improving the electrochemical performance of transition metal compound electrodes,it still faces many challenges in industrial application:(1)the micro-nano structure of the electrode surface and interface is diverse.There is no unified metric,and the structure-activity relationship between electrode morphology and electrochemical performance is not clear,which can not provide strong guidance for the design of electrode structure.to explore the structure-activity relationship between surface and interface micro-nano structure and electrochemical performance,and to provide an important theoretical basis for the design of high-performance electrode materials;(2)the surface interface is an important place for electrochemical reaction,and improving the stability of the surface interface plays an important role in the development of high-performance supercapacitors.the development of advanced composite preparation techniques to improve the stability of the surface and interface is a focus of future research;(3)the lack of in-depth research on the charge storage mechanism of transition metal compound electrode materials hinders the development of transition metal compound electrode materials.the development and utilization of advanced in situ characterization techniques to study the relationship between electrochemical redox reactions and the physical and chemical properties of materials can provide important theoretical guidance for the preparation of high-performance transition metal compound electrode materials;(4)There are many kinds of transition metal compounds,and the current research is mainly through the design and verification of the electrochemical performance of the materials,so the experimental workload is large.in order to further optimize the performance of materials and select high-performance electrode materials,the electrochemical performance of materials can be theoretically predicted by means of simulation calculation,mathematical model construction,machine learning and other means,so as to narrow the selection range of materials and avoid a large number of experiments;(5)At present,transition metal compound-based supercapacitors are still in the laboratory test stage,facing the problems of complex preparation process and high production cost,which hinder their large-scale application in production practice.in order to meet the needs of large-scale production and commercial applications,the development of electrode preparation methods with simple process and low cost is an important research direction in the future。

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