Synthesis of High-Quality Two-Dimensional V₂C MXene for Supercapacitor Application ^†

Abstract

Two-dimensional (2D) V₂C MXene has fascinating potential for use as electrodes in high-energy-density supercapacitors because of its excellent electrical conductivity and large specific surface area. However, it is not feasible to synthesize V₂C by etching vanadium carbon aluminide (V₂AlC) with hydrofluoric acid, which is commonly used for preparing other MXenes. In this work, a modified method is developed for synthesizing high-quality 2D V₂C. A mixture of sodium fluoride (NaF) and hydrochloric acid (HCl) was used as the etching agent, where V₂AlC can be gently etched by a hydrothermal reactor-assisted method. As electrode materials for supercapacitors, V₂C shows the characteristics of electric double layer capacitance. The electrochemical results show high specific capacitance (223.5 F/g in 1 M Na₂SO₄ at a current density of 100 mA/g) and good cycling stability (the capacitance retention rate can be maintained at 94.7% after 5000 cycles). This work provides a new method for the synthesis of high-quality V₂C for application in related fields.

1. Introduction

Supercapacitors are an internationally recognized, new energy storage device between secondary batteries and ordinary capacitors, because they have higher specific power, longer cycle life, and faster response than secondary batteries [1,2,3]. Thus, supercapacitors have attracted extensive consideration for their superior energy storage efficiency. Depending on the different energy storage mechanisms, capacitors generally can be classified into two types: electric double-layer capacitors (EDLCs) and pseudocapacitors [4,5]. Porous carbon materials and most transition metal oxides/sulfides are common materials for the capacitors, which show the electric double-layer capacitance and pseudocapacitance [6,7,8,9,10,11], respectively. Although carbon materials have a large specific surface area (about 1000–2000 m²/g), their conductivity is discounted because of highly porous structures that affect their performance as EDLCs electrode materials [12]. Transition metal oxides/sulfides have much higher capacitance than carbon materials, but the disadvantages of narrow voltage window, poor cycle performance, and low electrical conductivity are still large challenges [13]. Therefore, development of new electrode materials to meet typical merits of the supercapacitors is necessary.

In recent years, MXenes, as a new family of two-dimensional layered materials, have been widely studied as electrode materials for the supercapacitors. MXenes have been highly anticipated in the energy storage field since they were first reported by Barsoum and Gogotsi in 2011 [14]. As reported elsewhere, wet synthesis is the main preparation method for MXenes. The common method employed is hydrofluoric acid etching, and the A phase is selectively etched away from the M_n+1AX_n compound. In the process of etching, the surface of MXenes is generally covered with oxygen (-O), hydroxyl (-OH), and fluorine (-F) groups; MXenes are also named M_n+1X_nT_x, while T_x represents the surface chemical groups (-O, -OH, and -F) on MXenes. Gogotsi’s team found that MXenes have high electrical conductivity, tunable surface (functional group) structure, surface hydrophilicity, and great stability [15]. Many research reports have demonstrated the promising application prospects of MXenes as candidate materials for the preparation of secondary batteries and supercapacitor electrodes [16,17,18]. However, Ti₃C₂ has been mostly studied in the MXenes family. There are only a few studies on V₄C₃ [19], Ti₂C [20], Nb₂C [21], Ta₃C₂ [22], Ta₂C [23], Ti₃C₂ [24], Hf₃C₂ [25], Zr₂C [26], Cr₂C [27], etc. Only limited research on V₂C has been performed recently [28,29,30] because the traditional method of using fluoride salt and hydrochloric acid as a wet-chemical etching solution cannot produce effective etching of V₂AlC, which also leads to the generation of impurity. Therefore, the more effective method should be developed to synthesize high-quality V₂C. The two different etching methods on the preparation of V₂C were investigated, and the electrochemical performance of V₂C prepared without hydrofluoric acid was also explored.

High-quality V₂C was successfully prepared by a modified methodology and is reported in this paper. The Al layer in the V₂AlC phase was stripped off by NaF/HCl solution in a hydrothermal reaction at high pressure and high temperature. The Na₅Al₃F₁₄ impurity generated in the reaction was removed in sequence by a rinse with Na₂CO₃ solution and diluted sulfuric acid. The synthesis conditions of V₂C were optimized and the electrochemical performance of V₂C as a supercapacitor electrode was studied. The as-prepared material shows the best performance with a specific capacitance of 556.7 F/g in 1 M Na₂SO₄ at a scan rate of 2 mV/s, which is much better than Ti₃C₂ (~100 F/g) [31]. The specific capacitance was 223.5 F/g at the current density of 100 mA/g, and the capacitance retention rate was 94.7% after 5000 cycles, indicating good cycling stability.

2. Materials and Preparation Methods

2.1. Materials

The chemical materials are shown in

Table 1. Chemical materials.

Materials	Chemical Formula	Molecular Mass
Vanadium Carbide Aluminide	V2AlC	140.8
Hydrochloric Acid	HCl	36.46
Sulfuric Acid	H2SO4	98.08
Sodium Fluoride	NaF	41.99
Potassium Hydroxide	KOH	56
Sodium Carbonate	Na2CO3	105.99
Polyvinylidene Fluoride	-(C2H2F2)n-	----
N-Methylpyrrolidone	C5H9NO	99.13

.

2.2. Synthesis of V₂C

V₂C was synthesized by using a mixture of HCl (Hangzhou ShuangLin Chemical Reagent Co., Ltd., Hangzhou, China) and NaF (Sinopharm Chemical Reagent Co., Ltd., Beijing, China) and selective etching of Al atoms from V₂AlC (11 Technology Co., Ltd., Jilin, China). The fabrication process is shown in Figure 1. In detail, 1 g of V₂AlC was placed in an etching solution consisting of 50 mL hydrochloric acid and 2 g sodium fluoride. This mixture was transferred to a 100 mL Teflon stainless-steel autoclave after 30 min, and then cooled to room temperature after 96 h [32,33,34]. The sediment in the stainless-steel autoclave was collected after centrifuging for 5 min (3500 rpm). The products were then washed with ample deionized water (until pH ≥ 6) and dried at 60 °C overnight in a vacuum oven. This product was recorded as V₂C-96. The product was collected and treated with Na₂CO₃ at 90 °C for 12 h, and washed with diluted sulfuric acid for 6 h to obtain multilayered V₂C. For comparison, the samples with 24, 48, 72, and 120 h reaction times were prepared by the same method, recorded separately as V₂C-24, V₂C-48, V₂C-72, and V₂C-120. Similarly, the samples were also prepared at different reaction temperatures of 50 and 70 °C, recorded separately as V₂C-50° and V₂C-70°.

2.3. Preparation of the V₂C Flexible Electrodes

Carbon cloth was used as the current collector for the fabricated flexible electrodes. A certain mass of Polyvinylidene Fluoride (PVDF) was dispersed in 2 mL of N-Methylpyrrolidone (NMP) and stirred for 12 h. A certain mass of MXene and acetylene black was weighed in proportion and ground in a mortar for 10 to 15 min to obtain a well-mixed powder. The mass ratio of V₂C:PVDF:acetylene black is 8:1:1 [32]. Then, the well-mixed powder was put into the prepared PVDF solution and stirred for more than 12 h to make a homogeneous slurry, which was uniformly painted on a piece of carbon cloth (8 mm in radius, ~25 mg) with a mass loading of 1 mg/cm². The electrode sheet coated with the active material was placed in a vacuum oven at 60 °C to dry overnight.

2.4. Characterization and Electrochemical Analysis

Scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, Talos-F200X) combined with energy dispersive X-ray (EDX) were used to analyze the surface morphologies, microstructures, and elements of the prepared samples in this work. The crystal structure was analyzed using X-ray diffraction (XRD D8 Advance, Bruker) conducted at 40 mA and 45 kV with Cu Kα radiation (λ = 0.15418 nm, 5° min⁻¹). X-ray photoelectron spectra (XPS) were acquired on a Scanning Auger XPS Thermo Scientific K-Alpha. The fitting of the XPS spectra was carried out using Avantage software.

In this work, the electrochemical testing of the V₂C as the supercapacitor electrodes were performed with a three-electrode system in CHI660E. For the three-electrode tests, the V₂C electrodes were employed as the working electrode, Ag/AgCl as the reference electrode, and a carbon rod as the counter electrode. The tests were carried out in 1 M Na₂SO₄, H₂SO₄, and KOH electrolytes. The supercapacitor was constructed by two MXene electrodes and a filter paper separator. The cyclic voltammetry (CV) tests were carried out with the potential window −0.4~0.4 V in the different electrolytes, 1 M Na₂SO₄, 1 M KOH, and 1 M H₂SO₄. The electrochemical tests, including CV, electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge (GCD), were performed at room temperature (~25 °C). For the supercapacitor device that was prepared, the V₂C electrodes and separator (i.e., the cellulose paper) were soaked in 1 M Na₂SO₄ for 6 h, and then the symmetric cell was assembled in sequence with a V₂C electrode, a separator, and a V₂C electrode, which was then packaged and sealed with parafilm. The cycling stability testing was performed on a multichannel battery testing system (LAND CT2001A) at room temperature. According to the result of the CV curves, the gravimetric capacitance of the electrode was calculated by using Formula (1) [11]:

$$C=\frac{\int IdV}{vm∆V}$$

(1)

where ∆V is the potential window, I is the applied current, v is the scan rate, and m is the mass of the active materials.

For the three-electrode system, the specific capacitance ($C_A$, F/g) was calculated using Formula (2), according to the GCD curves [35]:

$$C_A=\left(I\times ∆t\right)/\left(m\times ∆V\right)$$

(2)

where ∆V is the potential window, I is the discharge current, m is the mass of the active materials, and $∆t$ is the discharge time.

The two-electrode device was assembled for energy density (E, Wh/kg), evaluated using Formula (3), while the specific capacitance for the two-electrode device was calculated by using Formula (4) [36,37].

$$E=1/2\left(C_{device}\times ∆V^2\right)$$

(3)

$$C_{device}=4\left(I\times ∆t\right)/\left(M\times ∆V\right)$$

(4)

where ∆V is the potential window of the discharge process excluding the potential drop, $∆t$ is the discharge time, I is the discharge current, M is the total mass of the two electrodes.

3. Results and Discussion

3.1. Material Characterization

As shown in Figure 2a, the original XRD spectra of V₂AlC shows typical characteristic peaks at 2θ ≈ 13.5° and 41.3°, which belong to (002) and (103) crystal phases, respectively. It has been reported that the hydrofluoric acid etching is effective for the preparation of Ti₃AlC₂ but not for V₂AlC [14,24,38]. The sodium fluoride (NaF) and hydrochloric mixture is often used for V₂C preparation [39,40]. In order to obtain highly delaminated V₂C, a hydrothermal reactor was used to assist the etching reaction in this work, which can provide a high-pressure and temperature environment. V₂C was prepared with different reaction time (Figure 2b) and temperatures (Figure 2c) in the etching reactions, by which the temperature and time for the etching reactions were optimized. The XRD results are shown in Figure 2b,c. When the etching was performed at 90 °C, the typical (002) peak of V₂C shifted to 7.41° from 13.45° with increase in the reaction time. During the etching process, the interlayer space increased significantly; the intensity of (103) peak for V₂AlC greatly decreased, which implies that the Al layer in the MAX phase was etched away by the mixture of the fluorine salt and hydrochloric acid.

Therefore, 96 h are considered enough for the etching reactions. It was noticed that the byproducts of Na₅Al₃F₁₄ could be removed by Na₂CO₃ and the diluted sulfuric acid, as shown in Figure 2d. The etching experiments were also conducted under much lower temperatures, 50 and 70 °C. The XRD results show that the two characteristic peaks of the MAX phase for (002) and (103) did not change significantly at 50 °C. The characteristic diffraction peaks of the MAX phase decreased greatly when the temperature increased to 70 °C. If the temperature rises to 90 °C, the (002) peak shifts from 13.45° to 7.41°, and the (103) peak disappears, which indicates that 90 °C is considered as the optimal temperature (Figure 2c).

Figure 3 shows the SEM images of the samples synthesized with different reaction time and temperatures. It can be seen that V₂AlC has a dense structure without any delamination, as shown in Figure 3a. Figure 3b–f shows the samples for different reaction times of 24, 48, 72, 96, and 120 h, respectively. Significantly, increasingly more MAX can be fully etched with prolongation of the etching time. The byproduct of Na₅Al₃F₁₄, also shown in Figure 3i, could be removed by sodium carbonate and dilute sulfuric acid. Note that the Al layer in V₂AlC was almost fully removed after 96 h, and a typical 2D layered “accordion” morphology is observed. As shown in Figure 3g,h, the V₂AlC could not be etched at 50 °C, while a large number of the V₂C nanosheets were observed at 70 °C. The Al atoms were almost completely etched out when the temperature rose to 90 °C. The SEM images and XRD results indicate that 90 °C and 96 h are the optimal reaction conditions for V₂C preparation. Therefore, the V₂C-96 samples after purification were used for the following characterizations and electrochemical performance evaluation.

Figure 4 shows the TEM and EDX image of V₂CT_x. It can be found that the d-spacing value is 0.95 nm from the pattern obtained (Figure 4a), which is consistent with that in the previous study [41,42]. Figure 4b shows that a large number of V₂C nanosheets are connected together, which indicates that the A layer in the MAX phase is basically etched. The EDX mapping images clearly show the V, C, Al, and F elements, in which Al may come from a small amount of MAX residue. The contents of V, Al, C, and F elements are shown in

Table 2. The element contents in the material (area 1).

Element	Atomic Fraction (%)	Mass Fraction (%)
C	29.82	10.09
F	11.24	6.01
Al	1.01	0.77
V	57.93	83.13

; it was found that the atomic ratio of V:Al is 57.93:1.01, which indicates that more than 95% of V₂AlC was converted into V₂C.

The elemental composition and termination species of the MAX phase V₂AlC and the corresponding MXene V₂C were measured by the X-ray photoelectron spectroscopy (XPS). Almost complete disappearance of the Al 2p and Al 2s peaks in the XPS spectrum proves that the Al layer in the MAX phase was successfully etched away (see Figure 5a) [19]. As shown in Figure 5b, the peaks at 513.6 and 521.2 eV correspond to the V-C bond, and the peaks at 516.5 and 524.1 eV correspond to the V-O bond in V₂C. The spectrum in the O 1s region (Figure 5c) for the V₂CT_x is well fitted by V-O, O-H, and the adsorbed H₂O, while the corresponding binding energy peaks are located at 530.3, 531.8, and 533.6 eV [43], respectively. From the peak fitting of C 1s (Figure 5d), the peaks at 282.2, 284.6, 286.2, and 288.9 eV can be assigned to the C-V, C-C, C-O, and O-C=O bonds, respectively, which are consistent with those reported elsewhere [18]. From the XPS spectra, it is confirmed that the V₂C prepared by the fluoride salt method, similar to other MXenes, also has functional groups such as -F, -O, and -OH. There is also H₂O adsorbed on the surface of V₂C [43,44].

3.2. Electrochemical Performance of Flexible V₂C Electrodes

The flexible V₂C electrode sheets were prepared for the electrochemical performance evaluated in 1 M H₂SO₄, Na₂SO₄, and KOH solutions using a three-electrode system on CHI660E. Figure 6a–c shows the characteristic CV curves for different scan rates of 2, 5, 10, 20, 50, and 100 mV/s with the potential window of −0.4~0.4 V in 1 M H₂SO₄, KOH, and Na₂SO₄, respectively. At the scan rate of 2 mV/s, the specific capacitance is 113.4 F/g, 96.4 F/g, and 556.7 F/g for the acid electrolyte, alkaline electrolyte, and neutral electrolyte, respectively. At a higher scan rate of 100 mV/s, the specific capacitance of V₂C can still be maintained at 194.6 F/g in 1 M Na₂SO₄ electrolyte. The specific capacitances of the V₂C electrodes decrease gradually with increasing scan rates (Figure 6d). Interestingly, the V₂C materials show much higher specific capacitance in Na₂SO₄ than in H₂SO₄ or KOH electrolytes. Compared with K⁺, Na⁺ has a much smaller hydrated ionic radius, which can be inserted more easily into the interlamination of V₂C layers. Moreover, the cation concentration in Na₂SO₄ is much better than that in KOH, and thus the former has higher ionic conductivity. The reason is because a large number of F terminal groups on the surface of V₂C may form hydrogen bonds with H in H₂SO₄, which greatly restricts the movement of H⁺ and leads to the worse performance. Moreover, note that the as-prepared electrodes show much higher specific capacitance in Na₂SO₄ electrolyte compared with that of the V₂C samples synthesized by other methods (

Table 3. The specific capacitance of different V₂C electrodes.

Electrode Materials	Specific Capacitance (F/g)	Scan Rates (mV/s)	Electrolyte	References
V2C (NaF + HCl)	556.7	2	Na2SO4	This work
V2C (NaF + HCl)	113.4	2	H2SO4	This work
V2C (NaF + HCl)	96.4	2	KOH	This work
V2C (NaF + HCl)	361	10	Na2SO4	This work
V2C (KF + HCl)	164	2	Na2SO4	[42]
V2C (49 wt% HF)	225	2	Mg2SO4	[45]
V2C (49 wt% HF)	487	2	H2SO4	[45]
V2C (50 wt% HF)	100	0.2	NaPF6	[46]
V2C (50 wt% HF)	120	10	Na2SO4	[47]

), indicating an improved performance is obtained for the as-synthesized V₂C samples.

Figure 7a,b shows the GCD curves and specific capacitances of V₂C electrodes at different current densities in 1 M Na₂SO₄ electrolyte. The voltage drops of these electrodes are not observed, indicating that the prepared electrodes have good electrical conductivity and less polarization. The single electrode specific capacitance of the supercapacitor is as high as 223.5 F/g, achieved at a current density of 100 mA/g. Figure 7c shows the impedance of V₂C and carbon cloth electrodes in 1 M Na₂SO₄ electrolyte. It can be seen that the solution resistance (R_s) of 1.982 ohm (including the resistances of the electrode components) for the carbon cloth electrode is higher than that of the V₂C electrode (1.175 ohm), as shown in the magnified area in Figure 7d, which may result from the high hydrophobic surface area of the carbon cloth in the aqueous electrolyte. However, also note that the V₂C electrode shows a very small arc in the high frequency part, indicating that the charge transfer resistance is small. Therefore, the EIS results further confirm that the as-prepared V₂C electrodes have good electrical conductivity. Moreover, the cycling performance of the electrodes was also studied. Note that the capacitance decreased quickly during the initial 100 cycles and then became steady, which may ascribe to a small number of V₂C nanosheets falling off from the carbon cloth (Figure 7e). After the performance testing of 5000 cycles, a high capacitance retention rate up to 94.7% and an energy density of 9.3 Wh/kg for the device were obtained, indicating that excellent cycling stability was achieved for the flexible electrodes, which is comparable to that of Ti₃C₂T_x MXene and Ti₃C₂T_x/RGO reported elsewhere [48].

4. Conclusions

The new method for synthesizing 2D V₂C MXene was developed to synthesize high-quality V₂C with good electrochemical performance as supercapacitor electrodes. The V₂C obtained was synthesized by a hydrothermal reactor-assisted etching method followed by an impurity removal process, and the synthesis conditions were optimized. The maximum specific capacitance of 223.5 F/g was obtained in 1 M Na₂SO₄. At a current density of 100 mA/g, the capacitance retention rate of 94.7% was obtained after 5000-cycle testing. The excellent electrochemical performance of the as-prepared V₂C MXene is attributed to its high conductivity and 2D nanosheet structure with a high specific surface area, which offers additional options for the development of high-capacity and high-stability MXene-based supercapacitors.