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Synthesis and Applications of Two-Dimensional V2C MXene

Song Jiang , Jiapei Wang , Hui Zhu , Qin Zhang , Ye Cong , Xuanke Li

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (5) : 740 -751.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (5) : 740-751 DOI: 10.7536/PC200625
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Synthesis and Applications of Two-Dimensional V2C MXene
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Abstract

MXenes are general term for two-dimensional transition metal carbides, nitrides and carbonitrides, which have been widely used in energy storage, catalysis, electromagnetics and other fields due to their unique physical and chemical properties. As an important member of MXenes, V2C MXene has high conductivity, low transport barrier and pseudocapacitve performance ascribed to the multiple oxidation states of vanadium. Consequently, it has highlighted performances in many aspects, especially in electrochemical energy storage. However, the difficulty of synthesis caused by its high formation energy and the structural instability of V2C MXene have restricted its development. This article reviews the progress of V2C MXene in synthesis, structure, properties and applications, focusing on synthesis methods and applications in electrochemical energy storage and electrocatalytic hydrogen evolution reaction. Meanwhile, the challenges and future perspective in the application of V2C MXene are outlined.

Contents

1 Introduction

2 Synthesis

2.1 Synthesis of V2AlC

2.2 Synthesis of V2C MXene

2.3 Delamination of V2C MXene

3 Structure and properties

3.1 Structure

3.2 Stability

3.3 Other properties

4 Applications

4.1 Supercapacitors

4.2 Secondary batteries

4.3 Electrocatalytic hydrogen evolution

4.4 Other applications

5 Conclusion and perspective

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Keywords

V2C MXene / synthesis / stability / electrochemical performance / application

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Song Jiang, Jiapei Wang, Hui Zhu, Qin Zhang, Ye Cong, Xuanke Li. Synthesis and Applications of Two-Dimensional V2C MXene. Progress in Chemistry, 2021, 33(5): 740-751 DOI:10.7536/PC200625

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1 Introduction

Two-dimensional transition metal carbides, nitrides, and carbonitrides (collectively referred to as MXenes) are new members of the two-dimensional material family, which can be obtained from the precursor MAX phase by a top-down preparation method[1,2]. The general formula for MXene is M n + 1 XnTx (usually n = 1-3), where M represents an early transition metal element, X is C and/or T in the N,Tx represents a surface functional group (= O, — OH, — F and/or — Cl) attached to the outer layer M, and X represents the number of surface functional groups[3⇓~5]. The unique composition and structure of MXenes make it have excellent mechanical properties, conductivity, hydrophilicity and surface structure[4]. Since the discovery of the first MXene Ti3C2Tx in 2011, MXenes and its derivatives have shown great application prospects in energy storage, catalysis, electromagnetism, biomedicine, sensing, environment and other fields[6][4,7].
So far, most of the research on MXenes has focused on the T x of Ti3C2, mainly because the preparation and exfoliation process of Ti3C2Tx has been quite mature[4]. As another important member of the MXenes family, V2CTx has a lower ion transport barrier and a variety of oxidation States of vanadium compared with common Ti3C2Tx, and shows excellent conductivity[8⇓~10]. Theoretical studies have shown that V2CTx and its modified materials will have excellent performance in many fields. For example, single-layer V2C without functional groups on the surface can be used as anode materials for lithium-ion batteries, and its theoretical specific capacity can reach 940 mAh·g-1, which is superior to other MXenes[11]. The V2C(OH)2 has strong adsorption to uranyl ion, and has great potential in the treatment of radioactive waste in solution[12]. Experimental studies have also proved the superiority of V2CTx in electrochemical energy storage, and flexible films of d-V2CTx (layered V2CTx) have been used as electrode materials for supercapacitors.The specific capacitance can reach the 1315 F·cm-3, which is superior to the reported d-Ti3C2Tx (layered Ti3C2Tx) film[8].
Although V2CTx is increasingly showing its outstanding competitiveness in the MXene family, it shows good application prospects in lithium/sodium/zinc ion batteries, lithium-sulfur batteries, supercapacitors and other energy storage devices[11,13⇓⇓ ~16]. However, compared with the Ti3C2Tx, the research on the V2CTx is still very little. The main reason is that the formation energy of :V2CTx (2.98 eV) is higher than that of Ti3C2Tx (2.72 eV), so it is more difficult to prepare V2CTx, especially high-purity V2CTx[17]. Secondly, at present, the preparation of V2CTxstill mainly uses high-concentration hydrofluoric acid as an etchant, which is highly toxic and corrosive, and has great harm to human body and the environment[18]. Moreover, the product prepared by using hydrofluoric acid as an etching agent has more residual V2AlC of the precursor and lower purity of the V2CTx[19]. In addition, the current methods of preparing d-V2CTx are very limited, and V2CTx show poor chemical and electrochemical stability, which limits the research of V2CTx and its composites. Fortunately, some methods of preparing V2CTx with high yield have been reported recently, for example, the yield of V2CTx can reach more than 90% (mass fraction) by using the mixture of lithium fluoride and hydrochloric acid[18].
At present, there is little summary of the research work on V2C MXene, but the development of V2C MXene is rapid, and some new preparation methods and a large number of applications have been reported[20]. In this paper, the latest research progress in the preparation, structure, properties and applications of V2C MXene is reviewed, focusing on the preparation methods and the applications in electrochemical energy storage and electrocatalytic hydrogen evolution. The problems that need to be paid attention to are summarized and the application prospects are prospected, in order to provide some theoretical and practical reference for the research of V2C MXene.

2 Preparation

The preparation method of the V2C MXene is mainly to remove the Al layer in the V2AlC by destroying the weak metallic bonding force between V and Al and retaining the covalent bonding force between C and V[21]. Generally, the first result is a multilayer structure composed of V2CTx nanosheets, which are connected by Van der Waals forces. It has been shown that MXenes can be intercalated by a variety of metal cations and organic molecules to increase the interlayer spacing, and then exfoliated to obtain monolayer or few-layer MXenes, which are collectively referred to as d-MXenes (delaminated MXenes)[22]. The following is a summary of the preparation process from three aspects.

2.1 Preparation of V2AlC

The preparation of the V2CTx firstly needs to synthesize the MAX phase ceramic V2AlC. The traditional methods of preparing V2AlC include pressureless sintering, hot pressing sintering, spark plasma sintering and so on. Table 1 provides experimental parameters for the preparation of V2AlC using different synthetic routes. Pure V2AlC can be prepared at 1 500 ℃ by pressureless sintering, while high purity V2AlC can be prepared at 1 400 ℃ by hot pressing under higher pressure[19][23]. As more advanced processes, plasma sintering and microwave sintering can reduce the preparation temperature of V2AlC to 1350 ℃ and 1300 ℃, while the combination of the two processes can control the preparation temperature to 1000 ℃[24,25][26]. Although high purity V2AlC can be prepared by these methods, the equipment required for synthesis and the high preparation temperature lead to huge investment and production costs.
Molten salt method promotes the contact and reaction of raw materials through the chemically inert liquid medium formed by molten salt, thus reducing the reaction temperature and shortening the reaction completion time, and the impurities in the reaction products are easy to remove, which can be used to prepare high-purity crystal materials[27]. In recent years, the preparation of V2AlC by molten salt method has attracted the attention of researchers. Wang et al. Found that in the presence of NaCl, V2AlC was formed at 1050 ℃, and pure V2AlC was obtained at 1400 ℃[28]. Recent studies have shown that pure V2AlC can be synthesized at 1100 ℃ by choosing a mixture of NaCl and KCl with a low melting point (600 ℃) as the molten salt, which greatly reduces the reaction temperature[27,29]. To sum up, among various preparation methods, the molten salt method for preparing the V2AlC shows great advantages due to shorter reaction time and lower preparation cost, and will play an important role in the research of the V2AlC and the V2CTx.

2.2 Preparation of V2C MXene

Etching with high concentration of hydrofluoric acid (HF) solution is the most common method to prepare V2CTx. As shown in Figure 1, after HF etching, the layered structure of the V2AlC is opened, the (002) diffraction peak shifts to a smaller angle under the action of surface functional groups and interlayer water molecules, and V exposed in the solution will combine with different functional groups (= O, — OH and — F). However, the etched samples usually still have strong V2AlC diffraction peaks, indicating that there are still unreacted V2AlC residues in the samples[19,30]. Table 2 lists the experimental parameters for the preparation of V2C MXene by HF etching under different conditions. The etching conditions will affect the purity and interlayer spacing of the V2CTx, and the reaction rate will be accelerated by appropriately increasing the acid concentration, reaction temperature and reaction time. In addition, the study shows that the particle size parameters of the V2AlC can also affect the reaction rate and the quality of the V2CTx[19].
Considering the harm of high concentration hydrofluoric acid to people and the environment, it is very important to develop a more mild and environmentally friendly method to prepare V2CTx[18]. High-yield V2CTx, including lithium fluoride, sodium fluoride and potassium fluoride mixed with hydrochloric acid, can be prepared by hydrothermal etching of the mixed solution of fluoride salt and hydrochloric acid, and the yield can reach more than 90% (mass fraction)[18,31⇓ ~33]. Under the same experimental conditions, the mixture of sodium fluoride and hydrochloric acid has a better ability to prepare V2CTx than other fluoride salts[32,33]. At the same time, the experimental results show that the etching time and temperature have an important influence on the structure and morphology of the V2CTx[18,31]. Due to the mild reaction conditions, the prepared V2CTx nanosheets exhibit large lateral dimensions and fewer defects. In the reaction process, metal cations and water molecules will be inserted between the layers, so that the prepared V2CTx has a larger interlayer spacing[18]. However, the V2CTx prepared by (NH4)HF2 etching shows a larger interlayer spacing, which may be due to the intercalation of NH3 and N H 4 + during the reaction[18,31][34]. Table 3 provides experimental parameters for the preparation of V2C MXene by other methods.
In addition to hydrofluoric acid and the mixture of fluoride salt and hydrochloric acid as etchant, some new methods for preparing V2CTx have been reported recently. Ti alloying in the V2AlC can significantly weaken the interlayer force between the MXene layer and the Al layer in the ( V 1 - x) Tix)2AlC, and promote the transformation from MAX phase to MXene phase[35,36]. The flower-like V2C T x can be prepared on a three-dimensional composite electrode which is composed of carbon black (CB) as an additive and carbon fiber cloth (CFC) as a porous substrate by a thermal-assisted electrochemical etching method by using dilute hydrochloric acid as an etching agent[37]; V2C nanosheets were prepared by the intercalation of algal extracts with a yield of 90% (mass fraction)[38]. In addition, Li et al. Reported a general strategy for preparing MXenes by etching MAX phase with Lewis acid molten salt, which provides a new way for preparing V2CTx[39].

2.3 Stratification of V2C MXene.

Layering two-dimensional materials is a necessary step to study their properties in the two-dimensional state, and the specific surface area of two-dimensional materials is greatly increased after layering, which will show excellent performance in energy storage, catalysis and sensing, so the preparation of two-dimensional material dispersions by layering has attracted a lot of attention in recent years[22,40]. The delamination of MXenes mainly includes two processes: intercalation and exfoliation. At present, the most studied process is Ti3C2Tx. The specific preparation methods include intercalation by DMSO and then exfoliation by ultrasound in water. Etching with mixed solution of lithium fluoride and hydrochloric acid, and ultrasonic stripping. However, the effectiveness of these methods to stratify the V2CTx is limited[41].
Cation intercalation is an important characteristic of MXenes, and it has been reported that cation intercalation of V2CTx is mainly achieved by alkalization treatment. The bases used include LiOH, KOH, TMAOH (tetramethylammonium hydroxide), TPAOH (tetrapropylammonium hydroxide) and TBAOH (tetrabutylammonium hydroxide), etc[42][43][15,44,45][46][8,9,30]. The existence of a large number of functional groups on the surface of MXene makes its sheet negatively charged. In order to achieve charge balance, H+ will enter the interlayer. When treated with alkali, the cation in the alkali will replace the H+ and insert into the interlayer[47]. After cation intercalation, the V2CTx prepared by hydrofluoric acid etching expands along the c-axis direction, and the c-lattice parameter increases[42,44]. At the same time, the interaction between vanadium atoms and fluorine functional groups is unstable under alkaline conditions, and the fluorine functional groups will be gradually replaced by hydroxyl functional groups[41,48]. In addition, Song et al. Showed that the V2CTx was first alkalized by KOH to intercalate the K+, and then stirred in the salt solution.Through cation exchange and electrostatic interaction, cations (including Co2+, Mn2+ and Sn4+) in salt can enter the interlayer of V2CTx and increase the interlayer spacing[13,48,49].
After treatment with organic bases such as tetrabutylammonium hydroxide, tetramethylammonium hydroxide, choline hydroxide and n-butylamine, the intercalated cations can weaken the interaction between the layers of the V2CTx, and then the V2CTx can be layered by simple stripping to obtain a uniformly dispersed layered V2CTx(d-V2CTx) dispersion[8,9,41][45][41]. However, it is worth noting that in the current reports related to the preparation of d-V2CTx, the (multilayer) V2CTx prepared by hydrofluoric acid etching is uniformly layered, while the (multilayer) V2CTx prepared by using a mixture of fluoride salt and hydrochloric acid as an etchant has not been reported. Moreover, the specific surface area of the V2CTx can be increased through the layering, so that the performance of the V2CTx is close to the theoretical value and the application range of the V2CTx is expanded. However, compared with Ti3C2Tx, the organic bases used in intercalation are expensive and harmful to human beings, so more suitable methods of delamination should be explored.

3 Structure and performance

3.1 Structure

The V2CTx inherits the hexagonal structure of the P63/mmc space group from the precursor V2AlC, where C is located in the octahedral interstice[26,50]. As shown in Figure 2, a rhombic unit (shown in dotted lines) can be seen from top to bottom. Viewed from the side, the monolayer V2CTx consists of three layers of atoms, with a C layer sandwiched between two V layers. Fig. 2 provides top and side views of two models of the structure of the V2CT2, with two functional groups on top of the midpoints A and B, respectively[51,52].
In the preparation process, the V2C is easy to form surface functional groups. Density functional theory (DFT) reveals the strong interactions between transition metal atoms and surface functional groups[53]. Because that formation energy of the surface functional group of MXenes is negative, MXenes with surface functional groups are thermodynamically stable, and the formation of MXenes with surface completely cover by functional groups is thermodynamically more favorable than that of partially covered. Eventually, the MXenes surface will all form functional groups[52,54].

3.2 Stability

3.2.1 Thermal Stability

The surface of MXenes is thermodynamically metastable, which is easily oxidized at room temperature, resulting in the change of electronic properties and irreversible loss of surface activity. Therefore, it is very important to study the stability of V2CTx under different conditions for their preservation and application[55]. However, there are few literatures related to the stability of V2CTx. Wu et al found that V2CTx is stable below 375 ℃ in Ar, and it will be oxidized to form V2O3 if the temperature continues to rise[33]. In air, the V2CTx maintains a stable two-dimensional structure at 150 ℃, and will be oxidized if the temperature continues to rise, and the final product is a rectangular blocky V2O5 crystal.
Thakur et al. analyzed the stability of V2CTx heat treated to 600 ° C in N2, air, CO2, and H2[56]. The results show that in the N2, the layered structure of V2CTx remains stable below 600 ℃, but the surface of V2CTx is slightly oxidized by interlayer water, and the interlayer spacing decreases. In the CO2, the V2CTx is oxidized above 300 ℃ to form a VOx mixture. In air, the V2CTx completely loses its layered structure and transforms into a V2O5 above 300 ℃. In the H2, the (002) diffraction peak of the V2CTx disappears due to the removal of water molecules, and the surface functional group is removed above 300.d egree. C.
From the above experimental results, it can be seen that, compared with other MXenes, V2CTx is very fragile and shows poor chemical stability in air or inert atmosphere at high temperature. This may be due to the fact that V2CTx belongs to MXenes of M2X type, which is not as stable as MXenes of M3X2 and M4X3 type, and V has multiple valences[57]. However, it is also possible to use this property to prepare V2O5@C and VxOyCz catalysts using V2CTx as a precursor[56]. At the same time, it should be noted that there are cations and water molecules intercalated in the V2CTx prepared by the mixture of fluoride salt and hydrochloric acid. The water molecules between these layers may accelerate the transport of electrolyte ions, but when heated, the removal of water molecules will lead to the contraction and oxidation of the layers of the V2CTx[58][59].

3.2.2 Aqueous solution stability

The stability of d-V2CTx in aqueous solution is very important for its practical application. Studies have shown that d-V2CTx has poor stability in aqueous environment. For example, Naguib et al. Put the dispersion of d-V2CTx in the air, and the V in the nanosheet was oxidized to + 5 ( VO 2 + or VO 3 -) by the dissolved oxygen in water 48 h,V2CTx, which made the solution change from black to yellow[41]. Vahid Mohammadi et al. Exposed the aqueous solution of d-V2CTx to air, it was oxidized 6 h,d-V2CTx, and the solution changed from green to light yellow[8].
Improving the stability of d-V2CTx in aqueous dispersions is a challenging task. MXene dispersions can usually be stored at low temperatures in the absence of oxygen and in the dark to extend their storage time, but this is very limited for d-V2CTx dispersions, and more efficient methods need to be explored[22,60]. The oxidation process of MXene starts from the edge and extends to the inside of the whole sheet[22]. Natu et al. Found that polyphosphate can seal the edges of positively charged V2CTx nanosheets, preventing water molecules and dissolved oxygen in water from attacking V2CTx[60]. It was found that the addition of a small amount of polyphosphate could keep the d-V2CTx dispersion stable for at least 3 weeks in air at room temperature, significantly inhibiting its oxidation. Based on a similar principle, Zhao et al. Showed that anti-L-ascorbate anions can be adsorbed on the edge of Ti3C2Tx nanosheets to prevent water molecules from acting on them to inhibit oxidation, which provides a reference for improving the stability of V2CTx in aqueous environments[61]. It is worth mentioning that Vahid Mohammadi et al. Proposed a cation-driven assembly method, in which alkali metal chloride was added to the dispersion of d-V2CTx, and highly ordered flexible films could be prepared under the electrostatic action of metal cations and V2CTx nanosheets[8]. The film has excellent chemical stability, and the XRD pattern does not change after being stored in air for one month.
Compared with multi-layer V2CTx, d-V2CTx nanosheets floating in solution are easily oxidized and have a short storage time, which limits the research and application of V2CTx. Therefore, it is extremely important to avoid or slow down the oxidation of V2CTx by improving the storage methods.

3.3 Other properties

At present, researchers have predicted the mechanical, electrical, magnetic and optical properties of V2CTx by using the first principles. Kurtoglu et al. Found that MXenes materials such as V2CTx have a high elastic modulus when stretched along the basal plane[62]. Theoretical calculations show that the monolayer V2C without surface functional groups exhibits metallic conductivity. However, the surface functionalization of the V2C during the preparation process makes it exhibit semiconducting properties. V2C(OH)2 and V2CF2 are semiconductors with narrow band gaps[9,51][11]. These conclusions have also been verified by experiments. Urbankowski et al found that the resistivity of V2CTx at room temperature is 2.6ⅹ10-3Ωcm, showing semiconductor properties[63]. However, heat treatment can make the V2CTx show metallic characteristics. Ying et al. Heat treated the V2CTx film in vacuum to make its conductivity reach the 3300 S·cm-1[9]. The V2CTx also has magnetic properties, and the density functional theory calculation predicts that the V2CTx has antiferromagnetism[64,65]. The absorption coefficient of the V2CTx[9]. The above research shows that the V2CTx has potential mechanical, magnetic and photoelectric properties, and has a very broad application prospect.

4 Application

4.1 Upercapacitor

Supercapacitor is one of the most important electrochemical energy storage devices, which has attracted much attention because of its high power, fast charging and long cycle life[66]. The charge storage mechanism of supercapacitor capacitive electrode materials can be divided into electric double layer capacitance and pseudocapacitance. Electric double layer capacitance is that ions store charge through electrostatic adsorption at the interface between electrode and electrolyte.Pseudocapacitors store charge by rapid and reversible redox reactions on or near the surface of active materials, and pseudocapacitors have attracted much attention because they can provide higher energy density than electric double layer capacitors. Finding suitable electrode materials is one of the core contents of supercapacitor research. High conductivity, spontaneous intercalation of polar organic molecules and metal ions, and rapid redox reactions on the surface make MXenes an ideal pseudocapacitance charge storage material[8,22].
Theoretical calculations show that V2CTx has better electrochemical performance than other MXenes, and some characteristics make it a promising electrode material for supercapacitors, including that V is a lighter transition metal atom; The thin atomic layer thickness of the V2CTx makes the ion diffusion speed faster; The multiple oxidation States of V make it possible for the V2CTx to have pseudocapacitance properties[8,10]. Table 4 lists the performance of the V2CTx as a supercapacitor electrode material.

4.1.1 Pseudocapacitor

It is found that the specific capacitance of the V2CTx in seawater is 317.8 F·cm-3 at the scan rate of 0.2 A·g-1.The specific capacitance is 164 F·g-1 at the scan rate of 1 M Na2SO4 and 2 mV·s-1, and the performance is better than that of other MXenes under the same conditions[10,18]. Since no binder and current collector are added, the V2CTx film electrode will show more excellent performance[67]. Shan et al. Made a thin film of TMAOH-treated clay-like V2CTx, which had a specific capacitance of 487 F·g-1 in a 1 M H2SO4 at a scan rate of 2 mV·s-1 and a capacitance retention of 83% after 10,000 cycles at a high current density of 10 A·g-1[44]. However, in the 1 M Mg2SO4 under the same test conditions, the capacitance retention rate can reach 99%. It can be seen that the electrochemical stability of V2CTx in acidic electrolyte is far less than that in neutral electrolyte, which is due to the oxidation of V2CTx in acidic environment accompanied by the dissolution of V. The specific capacitance of the d-V2CTx flexible film prepared by Vahid Mohammadi et al. Using the cation-driven method is 1315 F·cm-3 in the 3 M H2SO4, which is superior to the reported d-Ti3C2Tx film (900 F·cm-3).Its excellent performance is attributed to the fact that hydrogen ions, as the smallest cations, can contact a large number of electrochemically active sites and the charge storage mechanism of intercalation pseudocapacitance similar to Ti3C2Tx[8][68]. After 50 000 cycles at 100 A·g-1 current density, 84% of the capacitance was retained, showing excellent electrochemical stability. In fact, in the process of driving film formation, the electrostatic interaction between metal cations and nanosheets will reduce the interlayer distance. If the electrochemical stability of the film can be guaranteed without affecting the interlayer distance, the flexible film of d-V2CTx will have better performance.

4.1.2 Hybrid capacitor

As a member of supercapacitors, hybrid capacitors use battery-type and capacitive-type materials as electrodes, respectively, so they can give consideration to both the high energy density of batteries and the high power density of supercapacitors. In recent years, the application of V2CTx in hybrid capacitors has attracted wide attention[66,69].
Increasing the specific surface area and introducing the pore structure can expose the electrochemical active sites and reduce the ion diffusion resistance to enhance the electrochemical performance. Ming et al. prepared a three-dimensional porous K-V2C by treating V2CTx with potassium hydroxide. After alkalization,The V2CTx nanosheets become thinner, while many pores and cracks appear in the V2CTx, and the specific surface area increases from 14.26 m2·g-1 to 28.4 m2·g-1[43]. The potassium ion storage capacity of the V2CTx increases from 72 mAh·g-1 to 152 mAh·g-1 at 100 mA·g-1 current density. The energy density of the potassium ion capacitor with K-V2C as the negative electrode material and KxMnFe(CN)6 as the positive electrode material reaches 145 Wh·kg-1 under the power density of 112.6 W·kg-1.
Opening the interlayer by layers can promote the diffusion of electrolyte ions, and introducing low-dimensional carbon such as carbon nanotubes (CNT) into the interlayer can inhibit the accumulation of V2CTx nanosheets and construct a three-dimensional conductive network, and can also enhance the stability of V2CTx during electrochemical cycling. Wang et al. First used TMAOH and NH3·H2O to delaminate and etch the V2CTx, then introduced CNT between the layers, and used the prepared DV2C@CNT(D (delamination) self-supporting film as the positive electrode, metal zinc as the negative electrode, and ZnSO4 aqueous solution as the electrolyte to form an aqueous zinc ion hybrid capacitor.The specific capacitance of the material reaches a 190.2 F·g-1 at a voltage of 1.0 V and a current density of 0.5 A·g-1, and the material has good cycle stability[45].
The interlayer structure of the V2CTx can be adjusted by cation intercalation. The larger interlayer spacing accelerates the diffusion of electrolyte ions, and these electrochemically active cations can also contribute to charge storage after entering the interlayer. Song et al. prepared the V2C@Co,Co2+ by alkalization and liquid immersion, and the interlayer spacing increased from 0. 735 nm to 0. 952 nm after entering the V2CTx, and the V-O-Co bonds formed between the layers could keep the V2C@Co structure stable[48]. The specific capacity of the V2C@Co is 1117.3 mAh·g-1 at the current density of 0.1 A·g-1, and there is almost no capacity degradation after 15 000 cycles at the current density of 8 A·g-1. It is worth noting that the electrochemical test data show that the charge storage is mainly pseudocapacitance, and there is no diffusion limitation at least below the 20 mV·s-1 scan rate, which means that both V2CTx and V2C@Co are capacitive electrode materials. A lithium ion capacitor is composed of a V2C@Co as a capacitor type positive electrode material and a lithiated graphene as a battery type negative electrode material, and the stable reversible specific capacity of the lithium ion capacitor is 80 mAh·g-1 at 0.05 A·g-1 current density. In addition, Simon et al. Made a sodium-ion capacitor with V2CTx as the positive electrode and hard carbon as the negative electrode, and the specific capacity reached 50 mAh·g-1 at a high voltage of 3.5 V[70]. These results indicate that V2CTx is a promising electrode material for hybrid capacitors.
To sum up, although the research of V2CTx in supercapacitor electrode materials is not much, the prospect is very broad. Attention should be paid to the following aspects: At present, the experimental conditions for the study of V2CTx are relatively mild, and compared with Ti3C2Tx, the study of V2CTx in aqueous electrolyte is still less, mainly due to its poor chemical and electrochemical stability, so it is necessary to design a stable structure and optimize the experimental conditions to inhibit the oxidation and dissolution of V2CTx. In addition, MXenes generally show excellent performance by electrochemical reaction at negative potential, while irreversible oxidation is prone to occur at higher anodic potential, resulting in resistance increase and capacitance loss, such as the -0.9~-0.3 V(vs Hg/Hg2SO4) of V2CTx in H2SO4. Therefore, the cathode materials with high stable capacitance (such as RuO2/graphene in acid) should be explored to assemble asymmetric supercapacitors.

4.2 Secondary Battery

Compared with other two-dimensional materials, MXenes has more chemical and structural diversity, and is a potential electrode material for secondary batteries[52]. In the structure of MXene, M-X is covalent bond and ionic bond, and the bond strength is high, so it is difficult for lithium ions to insert between the layers of M and X. Therefore, lithium ions are mainly intercalated and deintercalated between the layers of MXene with Mn+1Xn as the structural unit. Therefore, MXenes of M2X type, such as V2CTx, which are generally lighter in weight, have a larger amount of charge storage per unit mass than heavier MXenes (such as M3X2, M4X3 type) due to the absence of an inactive MX layer[22]. At present, the research of V2CTx in secondary batteries is still in the exploratory stage. Table 5 lists the performance of V2C MXene and its derivative materials as secondary battery electrodes.

4.2.1 Lithium-ion battery

Lithium-ion batteries are widely used in consumer electronics due to their high energy density (close to 180 Wh·kg-1)[66]. First-principles calculations show that the theoretical lithium ion storage capacity of the monolayer V2C without surface functional groups is as high as 940 mAh·g-1,V2CO2, and the theoretical lithium ion storage capacity is 735 mAh·g-1, which is better than that of graphite and other MXenes[11,71]. Therefore, V2CTx is expected to become a new generation of high-performance anode materials for lithium-ion batteries. Naguib et al. Showed that the specific capacity of the V2CTx was 260 mAh·g-1;Liu after 150 cycles at 1 C, and others found that the specific capacity of the V2CTx was 243 mAh·g-1 after 500 cycles at a current density of 500 mA·g-1[19][32]. Although these performances are better than other MXenes, there is still a big gap compared with the theoretical capacity of V2C MXene. This is mainly due to the easy collapse and stacking of V2CTx layers, which leads to the decrease of surface area, the decrease of layer spacing and the increase of interlayer resistivity[72]. At the same time, the coulombic efficiency of the V2CTx in the first cycle is low. Based on this, Liu et al. Proposed a strategy of pre-lithiation, which can improve the first efficiency of V2CTx to 80%[73]. In addition, the intercalation and deintercalation of lithium ions correspond to the expansion and contraction of the V2CTx, and the lithium storage potential of the V2CTx can be further released by expanding the interlayer spacing through ion intercalation. Song et al. prepared the V2C@Sn by a method similar to the method for preparing the V2C@Co, and the function of the V2C@Sn is similar to that of Co, so that the interlayer space can be increased to rapidly diffuse lithium ions, and the V-O-Sn bond formed between the layers can keep the structure of the V2CTx stable during electrochemical cycling[13]. After 90 cycles at 100 mA·g-1 current density, the specific capacity of the V2C@Sn is 1262.9 mAh·g-1, and the stable specific capacity can also reach 205.2 mAh·g-1,Sn at high current density of 20 A·g-1. The reversible electrochemical activity of V contributes to the excellent performance. It is worth mentioning that, unlike the V2C@Co, the electrochemical test data of the V2C@Sn show that the redox peak disappears above the scan rate of the 8 mV·s-1, and the charge storage is mainly diffusion-controlled.
The surface functional groups of MXene play an important role in its electrochemical performance. First-principles calculations show that the hydroxyl and fluorine functional groups on the surface of V2CTx can hinder the diffusion of lithium ions between layers, thus greatly reducing the specific capacity of lithium storage[11]. The oxygen functional groups on the surface of the V2CTx can promote the diffusion of lithium ions, and can adsorb multi-layer lithium, so that the theoretical lithium storage capacity of the V2CTx is higher than that of hydroxyl and fluorine functional groups on the surface, and the oxygen functional groups can effectively inhibit the formation of lithium dendrites[59,71]. In addition, when there are S, P or Si functional groups on the surface of the V2CTx, the diffusion barrier of lithium ions will be further reduced, and the theoretical lithium storage capacity of the corresponding V2CP2 and V2CSi2 can reach 1220 and 1592 mAh·g-1[74,75], respectively. Therefore, the reasonable regulation of surface functional groups to reduce the diffusion barrier of lithium ions on the V2CTx and increase the adsorption capacity of lithium ions on the V2CTx will make the V2C MXene have better performance as anode materials for lithium ion batteries.

4.2.2 Sodium-ion battery

Sodium-ion batteries have become one of the most attractive alternatives to lithium-ion batteries because of their abundant sodium resources, low cost and similar energy storage mechanism. Because the radius of sodium ion is larger than that of lithium ion, the development of lithium ion batteries is restricted by the lack of suitable anode materials. MXenes is expected to be a new anode material for sodium-ion batteries due to its unique properties and structure[52]. Bak et al. Found that when sodium ions are intercalated and deintercalated between layers, the V2CTxstores charge through redox reactions on V[76]. Song et al. Obtained the V2C@Mn by a method similar to the preparation of the V2C@Co, the specific capacity of which was 425 mAh·g-1 at a current density of 0.05 A·g-1, and 70% of the initial capacity was retained after 1600 cycles at a current density of 1 A·g-1[49]. Wei et al. Ammoniated V2C MXene to prepare layered VN supported by carbon coating and aluminum atoms. The specific capacity was 372 mAh·g-1 at a current density of 50 mA·g-1, and after 7500 cycles at a current density of 500 mA·g-1, the specific capacity was 115 mAh·g-1 without capacity fading. Its excellent performance was attributed to the excellent conductivity of VN and the charge storage mechanism of intercalation pseudocapacitance[14].

4.2.3 Lithium-sulfur battery

Lithium-sulfur batteries are considered to be the best alternative to lithium-ion batteries because of their high theoretical specific capacity, energy density and environmental friendliness. However, the widespread problems of shuttle effect, volume change during charge and discharge, and poor electronic/ionic conductivity have greatly hindered the application of lithium-sulfur batteries[42]. The unique two-dimensional structure and excellent conductivity of V2CTx make it an effective carrier of sulfur in lithium-sulfur batteries. And the V2CTx can immobilize the soluble polysulfide through adsorption to inhibit the shuttle effect. For example, Chen et al designed and synthesized a S@V2C-Li/C electrode, which has a specific capacity of 970 mAh·g-1,0.5 C at 0.2 C and a specific capacity of 600 mAh·g-1 after 500 cycles[42]. Wang et al prepared VO2(p)-V2C/S,VO2 with V2CTx as the matrix to enhance the conductivity of the three-dimensional structure, and catalyzed the conversion of lithium polysulfide (LIPSs) to Li2S2 and Li2S to inhibit the shuttle effect[15]. The initial discharge specific capacity was 1120 mAh·g-1,2 C at 0. 2 C, and the specific capacity was 855 mAh·g-1 after 500 cycles. In addition, the calculation shows that V2CS2 and Li2S@V2CTx are excellent anchor materials and cathode materials for lithium-sulfur batteries, and the application of V2CTx in lithium-sulfur batteries has a bright future[77,78].
In addition to lithium-ion batteries, sodium-ion batteries and lithium-sulfur batteries, V2CTx is also used as the anode of aluminum batteries, zinc-ion batteries (ZIBs) and magnesium/lithium hybrid ion batteries (MLIBs), showing good application prospects[30][16,79][73].

4.3 Lectrocatalytic hydrogen evolution

With the emergence of energy crisis and environmental problems caused by the excessive consumption of traditional fossil fuels, electrocatalytic hydrogen evolution reaction (HER) has attracted much attention as an efficient and clean method for hydrogen production. Recent studies have shown that MXene-based catalysts have broad application prospects in the field of HER.The oxygen functional groups on the surface of MXenes are the active sites of HER[55]. Fig. 3 provides a plot of the theoretical exchange current versus the average Gibbs free energy of hydrogen adsorption, from which it is predicted that the high binding strength of the V2CO2 to H results in its HER catalytic performance being weaker than that of the Ti2CO2 and Ti3C2 O 2 [80]. Therefore, when the elements that can provide electrons are introduced into the surface of the V2CTx, the interaction between the adsorbed hydrogen (Hads) and the oxygen functional group will be weakened after the oxygen functional group receives additional electrons, and the surface performance of the V2CTx will be improved. As predicted by Ling et al., the introduction of transition metals Ni, Co and Fe, which can provide electrons, into the surface of V2CTx will greatly improve their HER performance[81]. Yoon et al. adjusted the electronic structure of V2CTx by doping P with low electronegativity, and the results showed that the P-C bond in the P-V2CTx as an active site would promote the desorption of Hads.The overpotential of the P-V2CTx in the 0.5 MH2SO4 at 10 mA·cm-2 current density is 163 mV, and the Tafel slope is 74 mV·dec-1[82].
The unique structure and excellent conductivity of V2CTx make the charge transfer more rapid, so the preparation of three-dimensional structure of V2CTx combined with active materials is an effective way to improve the activity of electrocatalytic hydrogen evolution. Wang et al. Prepared a three-dimensional structure of VS2@V2CTx by synthesizing VS2 nanosheets on the surface of V2CTx, and the charge can be transferred rapidly at the interface between the two phases[83]. The overpotential at 20 mA·cm-2 in the 0.5 M H2SO4 was 138 mV, and the Tafel slope was 37.9 mV·dec-1. Kuang et al. designed a sandwich conductive structure of NiS2/V2CTx with an overpotential of 179 mV and a Tafel slope of 85 mV·dec-1 in 1 M KOH at 10 mA·cm-2 current density[84]. In addition, the calculation shows that the monolayer V2CTx and the two-dimensional heterostructure of N-doped graphene will show significant HER activity, which provides a new idea for the design of Gadolinium-based HER catalysts[85].

4.4 Other application

Previous studies on two-dimensional materials such as graphene have shown that dispersions containing two-dimensional nanosheets or quantum dots can be used in printing electronics, biological imaging agents and photothermal therapy[52]. The same application can also be achieved with V2C QDs (quantum dots), which can be prepared by mixing V2CTx dispersed in water with ammonia, hydrothermal treatment, and separation and purification of the product[46,86]. Cao et al. Found that when used in photothermal therapy, V2C QDs showed strong photothermal effect in the near-infrared II region (1000 ~ 1350 nm), and had good fluorescence imaging, photoacoustic imaging and magnetic resonance imaging capabilities[46]. Huang et al studied the white laser based on V2C QDs, which promoted the application of V2CTx in the field of optics[86].
In addition, the theoretical calculation shows that the V2CTx has a good adsorption capacity for uranyl and CO2, which shows a good application prospect in the field of environment[12,87]. Thakur et al. Used V2CTx as a precursor to generate V2O3-V8C7/m-V2CTx catalysts and applied them to carbon oxide catalysis. The results showed that V2CTx could be used as a catalyst precursor for heterogeneous catalysis and important catalytic reactions at higher temperatures[88]. Chen et al. Found that V2C@PDMAEMA composites can respond to changes in CO2 and temperature, extending the application of V2CTx to the biological field[89].

5 Conclusions and outlook

As a member of the MXenes family, V2CTx has shown more excellent performance than Ti3C2Tx in many aspects, and has shown great potential applications in supercapacitors, secondary batteries, environmental protection, electrocatalytic hydrogen evolution and so on. At present, the research of V2CTx is still in the initial stage, and it also faces many challenges. The author summarizes the research directions that need to be paid attention to.
Firstly, in terms of synthesis, we should develop environmentally friendly and efficient methods to prepare V2CTx with large lateral size, less defects and controllable surface functional groups. Based on the high conductivity and potential excellent electrochemical properties, the layering methods of synthesizing V2C MXene;V2CTx without surface functional groups by chemical vapor deposition or physical methods are still very limited, and appropriate layering methods should be explored to prepare d-V2CTx flexible films.
Secondly, in the aspect of structure design, we should focus on improving the specific surface area and reactivity of V2CTx. It has been shown that the specific surface area of the V2CTx can be increased and the pore structure can be introduced by layering or alkali (KOH, NH3·H2O, etc.) Treatment. Appropriate surface oxygen functional groups are beneficial to electrochemical energy storage, and the content of oxygen functional groups should be increased and the content of fluorine functional groups should be reduced (such as alkali treatment, heat treatment, etc.) To improve the reactivity. Treatment in different atmospheres (CO2, H2S, NH3, etc.) Can incorporate heteroatoms into or synthesize heterostructures to increase the active sites of V2CTx; Introducing low-dimensional carbon, conductive polymers, metal oxides or cations into the interlayer to regulate the interlayer structure will play a positive role in improving the stability of the V2CTx during electrochemical cycling, while the interlayer materials can improve conductivity and contribute to performance. In view of the instability of V2CTx, slowing down its oxidation should be considered in the selection of experimental conditions. Improving the stability of V2CTx nanosheets in aqueous environment by using inert atmosphere, low temperature refrigeration and adding polyphosphate anions will promote their application.
In terms of application, the charge storage of V2CTx in secondary batteries and supercapacitors is mainly based on intercalation pseudocapacitance, and more specific energy storage mechanisms and catalytic mechanisms in applications should be studied to guide the experimental design.
Recently, great progress has been made in the study of V2C MXene, focusing on their preparation and electrochemical applications. With the continuous progress of research, the understanding and application of V2C MXene will be more in-depth, and it will play a more important role in many fields.

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Funding

National Natural Science Foundation of China(51472186)

National Natural Science Foundation of China(51902232)

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