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. 2024 Feb 14;9(8):8763–8772. doi: 10.1021/acsomega.3c04932

Enhanced Electrochemical Performance of MWCNT-Assisted Molybdenum–Titanium Carbide MXene as a Potential Electrode Material for Energy Storage Application

M Waqas Hakim †, Irfan Ali †, Sabeen Fatima †, Hu Li ‡,§, Syed Hassan Mujtaba Jafri ∥, Syed Rizwan †,*
PMCID: PMC10905737  PMID: 38434870

Abstract

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Two-dimensional (2D) materials such as MXenes have attracted considerable attention owing to their enormous potential for structural flexibility. Here, we prepared a Mo2TiC2Tx-layered structure from parent Mo2TiAlC2Tx MAX by chemically selective etching of the aluminum layer. The prepared MXene was employed in composite formation with CTAB-grafted multiwalled carbon nanotubes (MWCNTs) to have a structure with improved electrochemical performance. The samples were characterized to analyze the structure, morphology, elemental detection, vibrational modes, and surface chemistry, followed by an electrochemical performance of the Mo2TiC2Tx MXene and MWCNTs@Mo2TiC2Tx composite using the GAMRAY Potentiostat under a 1 M KOH electrolyte. The specific capacitance of pristine Mo2TiC2Tx was 425 F g–1, which was enhanced to 1740 F g–1 (almost 4 times) at 5 mV s–1 due to the increase in active surface area and conductive paths between the MXene sheets. The charge storage mechanism was studied by further resolving the cyclic voltammograms. MWCNTs@Mo2TiC2Tx showed much improved electrochemical performance and reaction kinetics, making it an ideal material candidate for supercapacitor applications.

Introduction

With the growing world needs, energy consumption is also increasing, and with the emerging advanced electronics, the storage of energy is becoming critically important.1 For this, researchers are working to find improved and efficient methods for higher energy storage. Supercapacitors have been a vital candidate for energy storage for many decades.2 Scientists are working on various ways to enhance their storage capacity, lifetime, and recyclability. In the modern age, flexible and wearable electronics are in demand due to which it is important to work on the materials that could play their role in creating compact and foldable supercapacitor technology.3 The storage capacity usually depends on the efficient adsorption of electrolytic ions over the electrode surface.4,5 So, if the electrode has enough surface with active sites to interact with the electrolytic ions, the storage capacity will be increased. Two-dimensional (2D) materials have been playing their role in providing enough surface area to fabricate high-storage supercapacitors for many decades.6−9 Also, their high flexibility paved a pathway toward flexible energy storage applications.10−12

Recently introduced MXene is a 2D water-attracting nanomaterial characterized by the general formula Mn+1XnTx. It is composed of n + 1 layers of early transition metal elements from groups 3–6, separated by n layers of carbon or nitrogen atoms. The surface terminations, such as –F, –OH, and –O, are indicated by Tx. Ti3C2Tx, the pioneering member of the MXene family, has been explored in diverse fields and for an array of applications.13−16 Their metallic nature along with increased package density as well as pseudocapacitive behavior helps in enhancing the gravimetric as well as the volumetric capacitance of the devices.17−19 Molybdenum-based MXene and other types of single- and double-transition metal carbide MXene phases have been studied for a variety of applications including electrocatalysts for water splitting application,20 electromagnetic interference (EMI) shielding,21 and electrochemical energy storage22−25 and show relatively stable properties as compared to other MXene phases. Even though MXenes are a relatively newer class of materials, they still hold remarkable properties in many practical applications. The loss of the properties of MXenes is mainly due to the loss of their two-dimensional nature. MXene in its pristine form is subjected to the phenomenon of restacking which is intrinsic to the nature of MXenes; as the sheets of MXene are separated by the intercalation of water molecules in between their sheets which holds them apart, drying or evaporation of water molecules are the main causes of restacking.26−30 As a result, MXene in its bare form is unable to utilize its full electrochemical active surface for energy storage.31 To overcome this hurdle, researchers are introducing various methods of intercalation in between the MXene sheets as well as their hybrid or composite structure formation to forbid the process of restacking so that they could utilize it completely for charge storage.23,32−36

Carbon nanotubes (CNTs), an allotrope of carbon, have properties that enable them to be used inside energy conversion systems by enhancing the overall conductivity of the material by providing conductive channels throughout the material.37 Their unique features like conductivity as well as tensile strength support their role inside the fields of sensors,38 fuel cells,39 metal-ion batteries,40,41 and supercapacitors.42 CNTs have also been used for synthesizing composite structures with other materials to enhance their storage capacity as electrodes inside supercapacitors.43 By combining MXene with the CNTs, extraordinary supercapacitive properties can be achieved toward flexible energy storage applications. Many studies have been published depicting that CNTs in combination with MXene provide efficient electrodes for enhanced charge storage.33,35 Recently, single-wall CNT/MXene (Ti3C2Tx) composites have been reported with a high volumetric capacitance of 2400 F g–1 at a scan rate of 2 mV s–1.44 Also, the V2CTx/CNTs composite showed a higher storage capacity of 1842 F g–1 at a scan rate of 2 mV s–1.45 The Mo2TiC2 MXene phase has been rarely explored in the field of energy storage, and according to a report on Li-ion batteries, it achieved 1.7 times higher capacity than other multilayered MXene forms.46

Here in this report, we used the double-transition metal MXene (Mo2TiC2Tx) and then synthesized its composites with multiwall carbon nanotubes (MWCNTs) to see their role as electrode material inside the supercapacitors, which is to our knowledge the very first report of energy storage for this MXene phase and MWCNTs. The structural and morphological properties are also explored to see the crystallinity and uniformity of the structures. To observe the morphology of the prepared samples, the material is characterized using SEM and HRTEM. Further different spectral modes and the effect of composite formation of functional groups have been evaluated using RAMAN and XPS analyses. The composites provided ultrahigh charge storage for the fabricated supercapacitor electrode.

Results and Discussion

For the structural and crystallographic analyses of the synthesized material, we have performed X-ray diffraction (XRD) analysis of the Mo2TiAlC2 MAX phase, Mo2TiC2-etched MXene, MWCNTs, and the synthesized composites. Figure 1 shows the XRD plots of the synthesized materials. MAX to MXene transition after the 50% hydrofluoric acid (HF) etching shows the shifting of the (002) peak; this shifting from 9.4° 2θ to 6.55° 2θ corresponds to the change of the c-lattice parameter (c-Lp) from 18.6 to 26.73 Å. This shift of peak to lower angle corresponds to good etching and agrees with the already reported data for this specific MXene.14,47,48 The increase in the intensity and back-shifting of the (002) peak represent the opening in the hexagonal structure of MXene, and the removal of the aluminum layer and the sharpness of the (002) peak represent the increase in the overall crystallinity and reduction of defects in the MXene structure. MWCNTs show prominent peaks at 25.75 and 42.5° 2θ, respectively, which correspond to (002) and (001) peaks of the hexagonal plane of graphite in carbon nanotube’s structure (JCPDS no. 41-1487).49 The XRD MWCNTs@Mo2TiC2Tx composites show the (002) peak of Mo2TiC2Tx MXene at a similar angle of 6.55° 2θ, and the downshifting of the (002) MWCNT peak to 20.8° 2θ corresponds to the successful formation of the composite. The hump-like presence of the (002) peak at 20.8° 2θ represents the presence of the MWCNT structure in a hybrid structure.

Figure 1.

Figure 1

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XRD pattern of (a) Mo2TiAlC2 MAX, (b) Mo2TiC2Tx MXene etching using HF, (c) MWCNTs, and (d) S2 (MWCNTs@Mo2TiC2Tx).

Structural, crystallinity, and phase transformational information on material was characterized using Raman spectroscopy analysis. Figure 2 shows the Raman spectrum for the prepared samples. Figure 2a represents the Raman spectra of Mo2TiAlC2 MAX which shows vibration corresponding to the structure and surface of flakes. The Mo2TiAlC2 bulk MAX phase shows a Raman peak at 160.47 cm–1, which is assigned to the E2g (in-plane) small displacement of Al atom. Similarly, peaks observed at 239.31, 299.6, and 408.45 cm–1 are all assigned to A1g out-of-plane vibration and displacement of Al atom attached to Mo atom in the MAX phase.50,51 In addition, peaks at 598.56 and 834.25 cm–1 are related to the A2g out-of-plane vibration of Al atom in Mo–Al–Mo bonding.52 After HF treatment, the Raman spectra of MXene in Figure 2b show the sharpening of Raman peaks, which shows the increase in crystallinity of the synthesized material.53 The peak at 147.46 cm–1 is attributed to Eg in-plane vibration of Mo atom in the MXene flake. Peak at 279.4 cm–1 appears due to the B2g and B3g vibrations related to the O atom attached as a termination group to the MXene flake. Other vibration modes at 331.81, 376.34, and 605.47 cm–1 belong to the Tx-region of etched MXene and are assigned to Eg (in-plane) vibrations of Mo–(O)2, Mo–(OH)2, and A1g (out-of-plane) vibrations of Mo–Tx, respectively. Raman peaks at 661.82, 818.24, and 989.71 are due to the presence of oxygen functionalization and are assigned to A1g, B3g, B1g, and A1g, respectively.54

Figure 2.

Figure 2

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Raman spectrum of (a) Mo2TIAlC2 (312) MAX phase, (b) Mo2TiC2Tx power MXene, (c) MWCNTs, and (d) MWCNTs@Mo2TiC2Tx composite with the inset showing the Raman shift region at 1200–1600 cm–1.

Figure 2c shows the Raman spectra of MWCNTs; characteristics Raman bands were observed for D, G, D′, and 2D or D′ bands at 1334.46, 1564.51, and 1574.16 cm–1, respectively.55 The MWCNT@Mo2TiC2Tx Raman spectrum is shown in Figure 2d; shifting peaks were observed when comparing it with pristine MXene spectra in Figure 2b. The peak at 147.46 downshifted to 144.9 cm–1 after composite formation, which is related to the Eg (in-plane) vibration of Mo atom. Downshifting of the Raman peak is associated with the material subjected to tensile stress, which can be due to the pressure caused by the presence of MWCNTs on the surface of MXene flakes. Another peak shift was observed from 331.81 to 336.77 cm–1 related to the Eg (in-plane) vibration of the Mo atom attached to the –OH functional group. This slight upshifting can be related to an increase in the bond length of change in the functional group. A peak at 605.47 cm–1 was downshifted to 600.74 cm–1 in MWCNTs@Mo2TiC2Tx related to the A1g (out-of-plane) vibration of the attached functional groups. Peak-related B1g (Mo–O) upshifted from 818.24 to 820.51 cm–1. Figure 2d represents the Raman spectra of the MWCNTs@Mo2TC2Tc composite containing vibrational modes for both MXene and MWCNTs in the data. Raman peaks at lower wavenumbers are present due to vibrations caused by the Raman laser interaction with the structure of MXene. A D and G bands were observed at wavenumbers 1335.99 and 1558.05 cm–1, respectively, because of the graphitic MWCNT structure along with the functional groups present on the surface of Mo2TiC2Tx. Furthermore, Inline graphic ratio for MWCNTs and prepared MWCNTs@Mo2TiC2Tx composite was calculated, and the value decreased from 1.55 to 1.10, which shows the increase in the structural stability and decrease in the defects of the overall MWCNT structure.60

For the detailed elemental composition and surface study of the material, X-ray photoelectron spectroscopy (XPS) was conducted. Figure 3 shows the overall data for the XPS analysis. Figure 3a displays a comparison plot for both pristine Mo2TiC2Tx and MWCNTs@Mo2TiC2Tx. Composite data shows the increase in the peak intensity in the region C 1s, which reveals the presence of MWCNTs on the surface of Mo2TiC2Tx MXene. Data shows the possible termination groups –F, –OH, and –O as (Mo2TiC2(OH)2), (Mo2TiC2F2), or (Mo2TiC2O2).18Figure 3b demonstrates the high-resolution spectra in the region of O 1s of Mo2TiC2Tx; the data show the O–Mo bonding of the MXene surface atom and –OH bonding with the Mo atom at 530.17 and 532.37 eV, respectively.56,57 Three XPS fitting peaks in the region of C 1s are presented in Figure 3c for Mo2TiC2Tx. The first peak at 282.89 eV is attributed to Mo–C bonding from within the structure of MXene, the second peak at 284.77 eV is associated with C–C bonding, and the peak at 286.34 is attributed to C–O.58Figure 3d shows that the high-resolution XPS peak fitting in the region of O 1s for MWCNTs@Mo2TiC2Tx data shows only one peak associated with Mo–O at 530.03 eV.21 Fitting in the region of C 1s for MWCNTs@Mo2TiC2Tx is shown in Figure 3e with three similar peaks observed in the data. Mo–C and C–O were observed at 282.52 and 286.03 eV, respectively. The C–C peak was observed at 284.83 eV. The increase in the intensity of the C–C peak in the data corresponds to the presence of MWCNTs on the surface of MXene.59 Also, O 1s of MXene and MWCNTs@Mo2TiC2Tx, Figure 3b,d, show the absence of –OH functional group on the composite material and the attachment of MWCNTs with the oxygen functional group replacing the H atom, and the increase in the intensity of the C–C peak suggests the successful formation of Mo2TiC2Tx and MWCNT composite.

Figure 3.

Figure 3

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XPS analysis of Mo2TIC2Tx and MWCNTs@Mo2TiC2Tx MXene composite: (a) comparison plot for both samples; (b,c) high-resolution XPS spectrum in the region of O 1s and C 1s for Mo2TiC2Tx; and (d,e) O 1s and C 1s spectra for MWCNTs@Mo2TiC2Tx.

Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) was used for the morphological study and elemental detection with percentage concentration analysis. Figure 4 shows the SEM images, HRTEM images, and EDX spectra for the synthesized material. Bulk-sized Mo2TiAlC2 particle phase of the MAX SEM image is shown in Figure 4a after HF etching, and removal of A-layer Mo2TiC2Tx shows the sheet nature of the material as shown in Figure 2b. The SEM image of the composite MWCNTs@Mo2TiC2Tx prepared sample in Figure 4c shows the attached MWCNTs as a web which covers the whole structure of MXene. This combination of MXene and MWCNTs provides an improved conductive medium, which results in superior electrochemical performance.40,61,62 High-resolution transmission electron microscopic image of MWCNTs@Mo2TiC2Tx is shown in Figure 4d. The image shows a web of MWCNTs on the surface of MXene. The selective area electron diffraction (SAED) pattern of the composite material is shown in Figure 4e, showing the polycrystalline nature of the composite, where the innermost ring corresponds to the (002) plane of MXene. Composition and elemental concentration were analyzed using EDX analysis. Figure 4f shows the EDX spectra for Mo2TiC2Tx MXene. The small concentration of aluminum in the MXene sample shows the effective removal of the A-layer from the MAX phase.63 The presence of fluorine and oxygen shows that most of the functional groups attached are –F, –OH, and –O as already discussed in the XPS analysis. Figure 4g is the EDX spectrum of the prepared sample S2 (MWCNTs@Mo2TiC2Tx). The rise in the concentration of carbon in the sample shows the presence of MWCNTs in the prepared sample. The presence of MWCNTs on the surface of MXene sheets acts as a conductive channel which results in enhanced charge storage capabilities and charge transport behavior.45

Figure 4.

Figure 4

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Electron microscopic images of prepared samples: (a) Mo2TiAlC2 MAX, (b) Mo2TiC2Tx MXene, and (c) MWCNTs@Mo2TiC2Tx composite; (d) HRTEM image of MWCNTs@Mo2TiC2Tx composite; (e) selected area electron diffraction (SAED) pattern of MWCNTs@Mo2TiC2Tx composite; (f) EDX of pristine MXene; and (g) EDX of MWCNTs@Mo2TiC2Tx composite.

For the electrochemical testing of the material, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are performed by using a three-electrode system. The working electrode was made by depositing the material under testing onto nickel foam; Ag/AgCl was used as a reference electrode, and a platinum wire was used as a counter electrode using 1 M KOH solution as an electrolyte because of its ionic conductivity and economic price, low toxicity, and nondamaging behavior toward the electrode material.64,65CV measurements were carried out for all the electrodes from the scan rate of 200 to 5 mV s–1. Figure 5 shows the electrochemical data for the MXene and composite prepared. CV curves for pristine MXene powder were taken over the potential window of 0.05–0.55 V and are shown in Figure 5a. Improved electrochemical performance of an electrode was observed for MWCNTs@Mo2TiC2Tx due to the addition of MWCNTs, which results in enhanced performance compared to that of pristine MXene. The addition of MWCNTs results in an increase in the available surface area, and the overall conductivity of the material CV data for the MWCNTs@Mo2TiC2Tx composite (4:20) ratio is given in Figure 5b. A comparison of the CV curves for all the prepared samples is reported in Figure 5c and shows the change in the electrochemical behavior of the electrode with a change of ratio for the composite formation. At higher composite ratios, the surface of the MXene particle is fully covered with the web of MWCNTs resulting in the reduction of the energy storage property of the material. In Figure 5d, comparison of specific capacitance for each sample prepared is reported; at 5 mV s–1, the specific capacitance of MXene was observed to be at 425 F g–1; with the increase in the concentration of the MWCNTs, the specific capacitance of the electrode rises to a value of 1740 F g–1 for 2:10 (S2) MWCNTs@Mo2TiC2Tx and then with the further increase in concentration-specific capacitance reduced to 736 F g–1 at a ratio of 1:2 for (S4) MWCNTs@Mo2TiC2Tx. Table 1 shows a generic comparison of the prepared electrode material (MWCNTs@MXene) with already published reports in the field of MXene supported by carbon-based structure in two- or three-electrode systems, and the data show the importance and high performance of the MWCNTs@Mo2TiC2Tx electrode material in the field of supercapacitor applications.

Figure 5.

Figure 5

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Electrochemical analysis of the prepared samples: (a) CV profile of Mo2TiC2Tx pristine MXene; (b) CV profile of S2 (MWCNTs@Mo2TiC2Tx) composite; (c) CV curve comparison of all the samples prepared at 100 mV s–1; (d) capacitance value comparison at 5 mV s–1.

Table 1. Electrochemical Performance Comparison of MXene/CNT Structure and Similar Fabricated Electrode Materials.

electrode material configuration electrolyte findings year refs
MWCNTs-Ti3C2Tx@carbon cloth 3-electrode system Ni(NO3)2·6H2O and Al(NO3)3·9H2O aqueous mixed electrolyte 114.58 mF cm–2@1 mA cm–2 2020 (66)
Nb2CTx/CNT/activated carbon 2-electrode system 1 M H2SO4 462.0 mF cm–2@2 mV s–1 2020 (67)
Ti3C2Tx@/CNTs 3-electrode system 1 M H2SO4 423.4 F g–1@1 A g–1 2020 (68)
Ti3C2Tx/carbon nanofibers 3-electrode system 1 M H2SO4 120 F g–1@2 mV s–1 2021 (69)
MWCNTs/V2CTx 3-electrode system 1 M KOH 1842 F g–1@2 mV s–1 2022 (70)
Ti3C2Tx/CNTs/sodium hyaluronate 3-electrode System 1 M NaOH 13.95 ± 2.45 F cm–3 2023 (71)
Ti3C2Tx/RGO/CNTs nanofilm 2-electrode system 1 M PVA/H2SO4 463.5 F g–1@1 A g–1 2023 (72)
MWCNTs@Mo2TiC2Tx 3-electrode system 1 M KOH 1740 F g–1@5 mV s–1 2023 this work
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To study the kinetics of the electrode material, cyclic voltammogram curves are further resolved to get the nature of the types of contributions taking part in the capacitance with the change in scan rates. Generally, there are two types of charge storage contributions of an electrode following a faradaic charge storage mechanism: one is diffusion-controlled faradaic redox reaction and the other is a faradaic surface-controlled redox reaction (pseudocapacitive). From the CV profile, these two mechanisms can be separated to get a better understanding of the kinetics happening at the electrode surface.

The current at the electrode when a constant scan rate is used is controlled by the equation i = avb, where i is the current, v is the scan rate, and a and b are the constants. The value of b is used to determine the nature of the charge storage mechanism. b value of 0.5 corresponds to the diffusion-controlled processes, and b value of 1 corresponds to the capacitive charge storage mechanism.73 From Figure 5a,b, it can be seen that the CV curves of pristine MXene have dominated redox peaks, showing an increase in current with an increase in scan rate, which shows that the electrode has a more capacitive-controlled current contribution in the overall capacitance. The CV plots of the composite material show slightly more diffusive trends than the capacitive ones. Further, if we take the logarithm of the equation, we can write the equation as log i = b log v + log a. The equation gets the form of a straight line equation, where b is the slope and can be calculated as shown in Figure 6a. In this work, the obtained b value for prestige Mo2TiC2Tx was 0.62, and for the composite, we got the value of 0.77. This shifting of value toward 1 signifies that the charge storage mechanism for the composite is mainly due to the capacitive or surface-controlled reaction. The nature of the charge storage mechanism with changing scan rates for MXene and MWCNTs@Mo2TiC2Tx is shown in Figure 6b,c. From the data, a trend can be seen, as for pristine MXene at higher scan rates, capacitive and diffusion-controlled processes are contributing side by side, but for the slower scan rate, diffusion-controlled processes are more dominant. Similarly, for MWCNTs@Mo2TiC2Tx at higher scan rates, the main contribution is capacitive. At a slower scan rate, the composite electrode shows more contribution of diffusive-controlled processes.

Figure 6.

Figure 6

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Charge storage mechanism and EIS analysis of Mo2TiC2Tx and MWCNTs@Mo2TiC2Tx. (a) Log of peak current vs log of scan rate plot for both pristine Mo2TiC2Tx MXene and MWCNTs@Mo2TiC2Tx composite. (b) Capacitive-controlled and diffusive-controlled current contribution for pristine MXene. (c) Capacitive-controlled and diffusive-controlled current contribution for MWCNTs@Mo2TiC2Tx composite electrode. (d) EIS spectrum of the materials with the inset plot for the demonstration of sigma value from the EIS data.

Electrochemical impedance spectroscopy (EIS) analysis (Figure 6d) demonstrates the Nyquist plot to analyze the electrochemical response of both samples using nickel foam as a current collector in 1 M KOH solution as an electrolyte. The inset shows that the higher frequency region of EIS Nyquist data shows that the solution resistance Rs is the same for both pristine Mo2TiC2Tx and MWCNTs@Mo2TiC2Tx composite. The composite material shows lower electrode resistance as compared to the pristine MXene electrode. Figure 6d shows an inset plot between real impedance and inverse sqrt of frequency, a plot drawn to evaluate the value of σ, and a slope of the plotted data that belongs to the Warburg factor. The coefficient of diffusion for K+ ions at a lower frequency can be calculated through σ value using eq 1.35

graphic file with name ao3c04932_m002.jpg 1

where every term is constant except D, which has an inverse relation to the Warburg coefficient (σ). The value of σ changes from 32.91 to 5.23 for Mo2TiC2Tx and MWCNTs@Mo2TiC2Tx electrodes, respectively. This shows that the diffusion of electrolytic ions is more dominant in the composite material, which increases the electrochemical performance of the electrode material.74,75

Conclusions

To summarize, we have exhibited a straightforward method for the synthesis of 2D Mo2TiC2Tx MXene with a high specific capacitance of 425 F g–1 at 5 mV s–1. MWCNTs were used to further enhance the storage capacity of Mo2Ti2Tx MXene by forming a weblike structure around MXene particles. A successful synthesis of the material is supported by XRD, XPS, and Raman spectroscopy. The composite material showed a high value of 1740 F g–1 specific capacitance at 5 mV s–1. From the cyclic voltammetry plots, the b-value showed that pristine MXene shows the dominance of diffusion-controlled processes at lower scan rates after composite formation material shows a charge storage mechanism with the contribution of both capacitive and diffusion-controlled processes. Similarly, from the EIS data, the calculated σ value supports the CV data as the composite shows a higher value of diffusion coefficient compared to pristine Mo2TiC2Tx MXene. The synthesized composite MWCNTs@Mo2TiC2Tx shows a high capacitance and hence is a candidate for energy storage devices.

Experimental Section

Synthesis of Mo2TiC2 MXene Powder

Mo2TiAlC2, MAX powder was etched using HF as an etchant. 1 g of Mo2TiAlC2 was added to 10 mL of HF using an ice bath over the time scale of 10 min to minimize the localized heating effect. After the addition of MAX into the Teflon beaker with HF, the beaker was transferred onto the oil bath kept at 55 °C and kept there for 72 h with continuous stirring at 250 rpm using a Teflon-coated magnetic stirring bar. The mixture was then washed five times until the overall pH of the solution was above 5. For each cycle, 50 mL of DI water was used to centrifuge out the products from the etching reaction. Centrifugation was carried out at 3500 rpm, decanting supernatant every time. After the last centrifugation, sedimented MXene was mixed in 500 mL of DI water, then filtered on a nylon membrane filter of 0.22 μm pore size, and then vacuum-dried at 60 °C overnight. Schematics for the synthesis procedure are listed in Figure 7a.

Figure 7.

Figure 7

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Schematic representation of (a) etching procedure for the synthesis of Mo2TiC2Tx MXene. (b) MWCNTs@MXene composite synthesis.

Synthesis of MWCNTs@Mo2TiC2Tx

The composite was formed by making the dispersion of MWCNTs in DI water. The dispersion was obtained by grafting MWCNTs using cetyl-trimethyl ammonium bromide (CTAB). Initially, a 2 mM solution of CTAB in DI water was obtained. MWCNTs and CTAB solution were probe-sonicated for 1 h with a concentration of 5 mg of MWCNTs per 10 mL of CTAB solution. CTAB-grafted MWCNTs were added dropwise to the already prepared Mo2TiC2 suspension. The weight of Mo2TiC2 MXene was fixed to 100 mg for each sample prepared to keep the concentration of the dispersion at 1 mg per mL. Figure 7b shows a schematic diagram for the composite synthesis. The formed sample was then filtered using a nylon membrane and dried in an oven at 60 °C for 24 h. By following a similar protocol, samples with different ratios (1:20, 4:20, 7:20, and 10:20) were prepared and labeled S1, S2, S3, and S4, respectively.

Electrode Fabrication for Electrochemical Testing

The electrode for electrochemical characterization was made using a 1 × 1.5 cm piece of Ni foam. The cleaning process was performed on Ni foam by processing it with 1 M HCl and bath-sonicated for 15 min to remove the oxide layer if present, followed by 10 min bath sonication using ethanol and then DI water. The cleaned electrode is then dried at 60 °C until water is completely evaporated. The slurry for the deposition is prepared in two drops of (n-methyl-2-pyrrolidone) NMP as a solvent with the active material, carbon black, and (polyvinylidene fluoride or polyvinylidene difluoride) PVDF binder in the ratio of 80:10:10, respectively. The mixture is sonicated for 15 min to obtain a slurry with continuous homogeneity. The obtained slurry was drop cast onto 1 × 1 cm of Ni foam, followed by 12 h of vacuum-assisted drying at 60 °C.

Materials Characterization

The crystal structure and identification of phase of the prepared material was done by using powder X-ray diffraction (XRD). Scanning electron microscopy (SEM and EDX) data were taken by using a JEOL JSM-6490A. Raman spectra are obtained using a NIKON 716671 spectrometer. XPS results are obtained using a PHI Quantera SXM, and for the electrochemical analysis, a GAMRY (1010B) Potentiostat is used.

Materials Used

The MAX phase was bought from sLuoyang Tongrun Info Technology, and the phase purity was tested from XRD analysis. To remove any organic impurities of Mo2TiAlC2, the MAX phase was treated with HCL and dried before HF etching. MWCNTs (battery-graded) were bought from Linyi Gelon libco. Ltd. (Shandong China). CTAB was provided by Sigma-Aldrich in powder form with purity mentioned greater than 99%. HF used for etching purposes was also bought from Sigma-Aldrich with a concentration of 48%. DI water was provided by a local vendor with Ph maintained on 7 at 25 °C.

Acknowledgments

The authors thank the Higher Education Commission (HEC) of Pakistan for providing research funding under project no.: 20-14784/NRPU/R&D/HEC/2021. The authors also acknowledge the financial support from the Shandong Provincial Natural Science Foundation (grant no.: ZR2021QE148) and the Shandong Provincial Natural Science Foundation for Excellent Young Scientists Fund Program (Overseas) (grant no.: 2022HWYQ-060).

Data Availability Statement

The data will be available on demand.

Author Contributions

M. Waqas Hakim performed the experimentation and testing and wrote the manuscript, Irfan Ali and Sabeen Fatima helped in understanding and reviewing, Hu Li and S. H. M. Jafri performed XPS analysis, and Syed Rizwan supervised and helped in completing the project.

The authors declare no competing financial interest.

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