Synthesis of a MoO₃/Ti₃C₂T_x composite with enhanced capacitive performance for supercapacitors

Abstract

In this work, a composite of molybdenum trioxide (MoO₃) nanoparticles anchored on two-dimensional layers titanium carbide (Ti₃C₂T_x) has been successfully synthesized via a facile hydrothermal process. MoO₃ was introduced to improve the contact region between electrode and electrolyte to shorten the paths of ion migration and intercalation. Ti₃C₂T_x has been integrated to act as a supporter to prevent the self-aggregation of MoO₃ combined with the charge transport channel and high conductivity donor to the composite. The structure and morphology of the MoO₃/Ti₃C₂T_x composite were characterized by X-ray diffraction, nitrogen adsorption–desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cyclic voltammetry tests showed that the MoO₃/Ti₃C₂T_x (III) composite exhibited a maximum specific capacitance of 151 F g⁻¹ at a scan rate of 2 mV s⁻¹, which is higher than that of pure Ti₃C₂T_x (103 F g⁻¹). Furthermore, the composite displays an excellent cycling stability with 93.7% specific capacitance retention after 8000 cycles at 1 A g⁻¹.

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1. Introduction

Electrochemical capacitors, also called supercapacitors, have drawn considerable attention in the energy storage field due to their high power density, long cycle stability and high rate of charge and discharge.^1,2 As a matter of fact, the performance of the electrode materials plays a dominant role in supercapacitors.^3,4 Generally, supercapacitors can be classified in two types, electrical double-layer capacitors (EDLCs) and pseudocapacitors, according to the energy storage mechanism. The former stores energy by charging electrical double layers through highly reversible ion adsorption in a fast manner on the surface of high surface-area electrodes and the latter depends on reversible redox reactions occurring at or near the surface of an appropriate electrode material.^5,6 Up to now, three main categories of materials used as electrodes have been investigated as electrodes in supercapacitors, including transition metal oxides,^7,8 electronically conducting polymers,^9–11 and carbon-based materials.^3,12

During the last decades, two-dimensional (2D) titanium carbide, Ti₃C₂T_x (T represent terminations of –OH/=O, and –F), belonging to family of MXene,^13,14 has shown its potential application in Li-ion batteries,¹⁵ supercapacitors,³ absorption materials¹⁶ and photocatalysis.¹⁷ As for electrochemical properties, the short path of ion transportation is established by the layered structure and decreases internal resistance through a fast charge/discharge rate.¹⁵ M. Naguib et al. demonstrated that plenty of oxygen-containing functional groups (such as –OH/=O) and some fluorine (–F) make MXene show a promising pseudocapacitance performance, which is introduced by etching with aqueous hydrofluoric acid.^18,19 The electrochemical properties of Ti₃C₂T_x and delaminated Ti₃C₂T_x had been investigated in alkaline and acidic electrolyte.^3,6 However, Ti₃C₂T_x was required to obtain high capacitance to meet the requirements of devices, even though its large specific surface area and good electron conductivity were beneficial for improving capacitance.

In order to enhance electrochemical performance, hybrid electrodes have been developed, in which Ti₃C₂T_x could modified with pseudo-faradic materials for high performance.^4,8 Transition metal oxides, with high capacitance based on redox mechanism, could incorporate onto MXene to enhance capacitance. Typical active transition metal oxides, such as RuO₂,²⁰ CoO,²¹ NiO,²² and MoO₃,²³ were well investigated for electrode materials. As a candidate, molybdenum trioxide (MoO₃) is one of the most widely investigated electrode materials, owing to its low cost, high electrochemical activity, and nontoxicity.²³ Normally, MoO₃ has orthorhombic, monoclinic and hexagonal phase.²⁴ Micro-sized hexagonal-phase MoO₃ crystals were regarded as a promising material because of various intercalation sites for collecting electrolyte ions. Pooi See Lee et al. revealed that the charge storage of h-MoO₃ can be ascribed to its abundant active sites.²⁵ It is an efficient way to load MoO₃ nanoparticles on Ti₃C₂T_x sheets through increasing active sites and specific surface area to enhance electrochemical performance.

In this work, we report a MoO₃/Ti₃C₂T_x composite that was successfully fabricated by incorporating certain amount of molybdenum trioxide into Ti₃C₂T_x via a hydrothermal method employing hydrochloric acid (HCl) to adjust pH value of the solution to control the crystal oriented growth. The preparation procedure was illustrated in Scheme 1. The electrochemical tests indicated that the obtained MoO₃/Ti₃C₂T_x composite showed a significant increase in capacitance and excellent cycling stability as an electrode material for supercapacitors in 1 M KOH electrolyte due to its low internal resistance and charge-transfer resistance.

Scheme 1 Schematic illustration of the fabrication process of MoO₃/Ti₃C₂T_x composites.

2. Experiment

2.1 Synthesis of MoO₃/Ti₃C₂T_x composite

Typically, Ti₃C₂T_x used in this study was prepared by a previously reported method.¹⁵ In detail, Ti₃AlC₂ powder was immersed into 40 wt% HF and magnetically stirred for 48 h at room temperature, and then the suspension was centrifuged to separate the powder from liquid. Successively, the powder was rinsed and centrifuged with deionized water and ethyl alcohol several times to rid of residual HF and dried in vacuum oven at 80 °C. The MoO₃/Ti₃C₂T_x composite was synthesized via a hydrothermal method. In a typical procedure, 0.05 g ammonium heptamolybdate tetrahydrate (AHM) was dissolved into 20 mL distilled water. Then 6 mol L⁻¹ HCl was added into this solution to adjust pH values (pH = 0.5) under vigorously stirring for an hour at room temperature. Then 30 mL distilled water and 0.15 g Ti₃C₂T_x was added and the mixture was continuously stirred for another 2 hours. For different samples, the feeding weight ratios of AHM (Mo source) to Ti₃C₂T_x were 1 : 15, 3 : 15, 5 : 15 and 6 : 15, which labeled as I, II, III and IV, respectively. Afterward, the suspension was sealed in a 100 mL Teflon lined stainless steel autoclave for hydrothermal reaction at 180 °C for 24 h. After cooling down naturally, the MoO₃/Ti₃C₂T_x composite was collected by rinse-centrifugation cycles and fully dried at 80 °C.

2.2 Materials characterization

The powder X-ray diffraction (XRD, Rigaku Japan; CuKa) was used to determine the phase composition of the samples radiation (λ = 0.15406 nm). The sample surface morphology and structure were characterized using field-emission scanning electron microscopy (FESEM, Hitachi S-4800) with an operating voltage of 10 kV. Furthermore, elemental analysis was conducted via an energy-dispersive X-ray spectrometry (EDS). Transmission electron microscopy (TEM) was conducted to investigate the details of the surface. Finally, the physical surface area and pore size distribution were measured using N₂ adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) (ASAP 2460, Micromeritics Instrument Corporation, Norcross (Atlanta), Georgia, USA).

2.3 Preparation of electrodes and electrochemical measurements

The working electrodes for electrochemical measurements were fabricated by dispersing the mixture consisting of MoO₃/Ti₃C₂T_x powder, acetylene black and polyvinylidene difluoride (PVDF) at weight ratio of 8 : 1 : 1, and then N-methylpyrrolidone solution was added to prepare the electrode slurry. Finally, the slurry was coated onto nickel foam, and then dried in an oven at 80 °C for 8 h.

In this work, a typical three-electrode test cell was used for electrochemical measurements of the electrode materials. Pt plate and Ag–AgCl electrodes were used as the counter electrode and reference electrode, respectively. All of the measurements were carried out in a 1 M KOH aqueous electrolyte solution at room temperature. Cyclic voltammograms (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) were measured by CHI660D (Chenhua, Shanghai, China) electrochemical working station. CV curves were recorded at different scan rates from 2 to 100 mV s⁻¹ and galvanostatic charge–discharge curves were performed at different current densities within the potential range from −1.0 to −0.3 V, respectively. EIS measurements were carried out from 0.1 Hz to 100 kHz with amplitude of 5 mV. Cycling stability of the electrode was detected by continuously galvanostatic charge–discharge at 1 A g⁻¹ using this working station.

3. Results and discussion

3.1 Composition and morphological characterization

The crystal structure and chemical composition of samples were investigated by XRD, shown in Fig. 1. Both Ti₃C₂T_x and MoO₃/Ti₃C₂T_x patterns show two diffraction peaks appeared at 2θ = 35.9°, 41.7°, can be indexed to impurities TiC. As shown in curve a, as-prepared Ti₃C₂T_x exhibits the major peaks centered at 2θ ≈ 9.8°, 18.5°, 27.9°, 35.1°, 42° and 60.8°, corresponding to (002), (004), (006), (008), (0010) and (110) diffractions respectively. Moreover, both of these peaks exhibit lower 2θ values and intensity than those of Ti₃AlC₂.¹⁹ As for MoO₃/Ti₃C₂T_x composite (curve b), it exhibits a similar XRD pattern to the as-prepared Ti₃C₂T_x. While, two apparent new reflection peaks at 9.7°, 25.8° correspond to the (100) and (210) planes of hexagonal MoO₃ crystalline structure (according to JCPDS card no. 21-0569), respectively, indicating MoO₃/Ti₃C₂T_x composite was successfully synthesized. Additionally, very few TiO₂ peaks can be detected in curve b, which was fabricated by oxidation of active Ti atom during the hydrothermal process.²⁶

Fig. 1 XRD patterns of as-prepared Ti₃C₂T_x (a) and MoO₃/Ti₃C₂T_x composite (b).

SEM micrographs vividly depict the layered morphology of the as-etched Ti₃C₂T_x and MoO₃/Ti₃C₂T_x composites, as shown in Fig. 2. Fig. 2a shows the microstructure of Ti₃C₂T_x. As can be seen in the picture, Ti₃C₂T_x has a multilayered morphology like graphene, in which the layers separated from each other. From the SEM images of MoO₃/Ti₃C₂T_x composite (Fig. 2b), it could be clearly seen that MoO₃ nanoparticles were successfully loaded on the surfaces and edges of the Ti₃C₂T_x sheets, indicating the heterogeneous nucleation and crystal growth of MoO₃ nanoparticles on the surface of Ti₃C₂T_x sheets during hydrothermal process. The distance between Ti₃C₂T_x layers is not enough so that the size of MoO₃ nanoparticles is limited. It is apparently that the diameter of MoO₃ nanoparticles were influenced by the laminated structure of Ti₃C₂T_x, and the MoO₃ nanoparticles positioned at edges were larger than those between layers. In addition, it is also found that the surfaces of Ti₃C₂T_x sheets were densely covered by MoO₃ nanoparticles and no big conglomeration of MoO₃ nanoparticles was observed. As shown in Fig. 2c, EDS result further reveals that the MoO₃/Ti₃C₂T_x composite is mainly composed of Ti, C, Mo, and O. The presence of a small amount of F in composite is attributed to the replacement of Al layers with OH and/or F.²⁷

Fig. 2 SEM images of (a) as-prepared Ti₃C₂T_x powder and MoO₃/Ti₃C₂T_x composite (b) (inset is high magnification image of (b)); (c) EDS image of MoO₃/Ti₃C₂T_x composite.

Transmission electron microscopy (TEM) was used to provide more morphological details of MoO₃/Ti₃C₂T_x composites, shown in Fig. 3a and b. The morphology of the samples is similar to SEM images. In addition, the size of MoO₃ nanoparticles in the range from 40 to 70 nm could be clearly observed. The special layered structure could prevent the MoO₃ nanoparticles from aggregating and be beneficial for exposing more active sites.²⁸ At the same time, particles with different size could make sure absolutely contact between the electrode and electrolyte, and it would be an advantage for ion migration and intercalation. These mentioned above are important for high-rate performance and improving the capacitance. Fig. 3c and d show the HRTEM micrographs of MoO₃/Ti₃C₂T_x composite, both of which are coincide with SEM results. The lattice fringe spacing of 0.18 nm and 0.21 nm displayed in the HRTEM images are quite consistent with the lattice spacing of (002) and (221) planes of hexagonal molybdenum trioxide (h-MoO₃),²⁵ respectively, which is in good agreement with XRD results.

Fig. 3 TEM images of (a) MoO₃/Ti₃C₂T_x composite, (b) high magnification image of (a), and (c), (d) HRTEM images of MoO₃ loaded on the Ti₃C₂T_x sheets.

N₂ adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution plots are obtained to check the surface areas of these materials, as shown in Fig. 4. In Fig. 4a, it can be clearly seen that the typical N₂ adsorption–desorption isotherms belong to type-IV species according to IUPAC classification. Additionally, the obvious hysteresis loop between the relative pressure scope of 0.43–1.0 indicates the existence of mesopores and/or macropores within MoO₃/Ti₃C₂T_x composite. The BET surface areas of Ti₃C₂T_x and MoO₃/Ti₃C₂T_x composite are calculated to be 5.33 and 37.28 m² g⁻¹, respectively. This tremendous increase of specific surface area and higher adsorption capacity can be attributed to the introduction of MoO₃ nanoparticles. Fig. 4b demonstrates the cumulative pore volume and pore size distribution curve of MoO₃/Ti₃C₂T_x composite, which exhibits an evident peak in the pores size of around 4 nm. The large specific surface area and mesoporous structure are beneficial for increasing contact area and shortening ion diffusion paths between electrode and electrolyte, leading to a fast rate of charge transfer. These mentioned above are important for improving the electrochemical properties.

Fig. 4 Nitrogen adsorption and desorption isotherms of as-prepared Ti₃C₂T_x and MoO₃/Ti₃C₂T_x composite (a), and pore size distributions of MoO₃/Ti₃C₂T_x composite (b).

3.2 Electrochemical properties

The electrochemical performance of the MoO₃/Ti₃C₂T_x composite is evaluated with a three-electrode system in 1 M KOH electrolyte. The cyclic voltammograms (CVs) for as-prepared Ti₃C₂T_x powders, pristine MoO₃ and MoO₃/Ti₃C₂T_x (III) composite at a scan rate of 100 mV s⁻¹ are presented in Fig. 5a. Both Ti₃C₂T_x and MoO₃/Ti₃C₂T_x (III) are identified as the nearly rectangle-like shapes of CV curves, indicating a typical electrical double-layer capacitance feature. In addition, pure MoO₃ does not exhibit apparent redox peaks within the potential window of −0.3 to −1 V (vs. Ag/AgCl) in 1 M KOH alkaline electrolyte. It is apparent that a large difference can be observed in the CV loops area among these three electrodes. The increased capacitance of MoO₃/Ti₃C₂T_x (III) composite is result from the layered structure and synergistic effect between components. Lots of K⁺ cations can readily intercalate between the Ti₃C₂T_x layers in aqueous solution under the applied voltage.²⁹ Certainly, the large specific surface area by loading MoO₃ nanoparticles is another advantage for MoO₃/Ti₃C₂T_x composite to get better electrochemical properties.

Fig. 5 CV curves of (a) the as-prepared Ti₃C₂T_x powders, pristine MoO₃ and MoO₃/Ti₃C₂T_x (III) composite at 100 mV s⁻¹ and (b) MoO₃/Ti₃C₂T_x (III) composite at different scan rates of 2, 10, 20, 50, 100 mV s⁻¹ in 1 M KOH solution. The arrow indicates the increase of the scan rates from 2 to 100 mV s⁻¹.

To research the properties of MoO₃/Ti₃C₂T_x composite electrode further, CV curves of MoO₃/Ti₃C₂T_x (III) composite at different scan rates from 2 to 100 mV s⁻¹ are shown in Fig. 5b. These CV curves exhibit approximately rectangular shapes and have not significant change as the scan rate increasing, indicating that MoO₃/Ti₃C₂T_x (III) has an ideal capacitive behavior at a large range of different scan rates and a good rate capability. Obviously, the specific capacitance increases with the decrease of scan rates from 100 to 2 mV s⁻¹, owing to the enhanced diffusion rate of electrolyte and full utilization of electrode.²⁴ At the same time, MoO₃/Ti₃C₂T_x (III) electrode exhibits good charge collection performance as well as facilitates diffusion of K⁺ in the electrode.

Fig. 6a shows the galvanostatic charge–discharge curves of MoO₃, Ti₃C₂T_x and MoO₃/Ti₃C₂T_x (III) composite at a current density of 1 A g⁻¹. The longer discharge time of the MoO₃/Ti₃C₂T_x (III) indicates its higher capacitance than both MoO₃ and Ti₃C₂T_x. Therefore, the charge storage capacity in galvanostatic charge–discharge tests is consistent with the result of previous CV tests. Note that MoO₃/Ti₃C₂T_x (III) electrode obviously exhibits a lower IR drop than MoO₃, suggesting that the introducing of Ti₃C₂T_x could greatly reduce the internal resistance of MoO₃/Ti₃C₂T_x composite. However, the IR drop is slightly increased after loading MoO₃ nanoparticles on Ti₃C₂T_x sheets. Thus, the increased specific capacitance of MoO₃/Ti₃C₂T_x composite can be ascribed to the synergistic effect of superior electrical conductivity of Ti₃C₂T_x and pseudocapacitive performance of MoO₃. Further investigation of MoO₃/Ti₃C₂T_x (III) electrode at different current density presents in Fig. 6b, the shapes of charge curves were almost linear, indicating its good electrochemical capacitance performance.

Fig. 6 GCD curves of (a) pristine MoO₃, as-prepared Ti₃C₂T_x powders and MoO₃/Ti₃C₂T_x (III) composite at 1 A g⁻¹ and (b) MoO₃/Ti₃C₂T_x (III) composite at different current densities.

To elucidate the effect of various MoO₃ content on the electrochemical properties of MoO₃/Ti₃C₂T_x composite, different feeding weight ratios of AHM (Mo source) to Ti₃C₂T_x (1 : 15, 3 : 15, 5 : 15, 6 : 15 labeled as I, II, III, IV) composite were synthesized and studied by CV curves and GCD curves (Fig. 7a and b). CV curves area increase with the increase of MoO₃ content, indicating that MoO₃ nanoparticles contribute to ion adsorption to increase the specific capacitance. Apparently, the MoO₃/Ti₃C₂T_x (III) composite with feeding weight ratio of AHM to Ti₃C₂T_x is 5 : 15 exhibits largest CV loop area (Fig. 7a) and discharge time (Fig. 7b), indicating its best capacitance properties among these electrodes. However, as the feeding weight ratio of AHM to Ti₃C₂T_x increase to 6 : 15 (IV), the specific capacitance of this sample is reduced. It may be due to the aggregation of MoO₃ nanoparticles hinder the electron and ion transportation.

Fig. 7 CV curves (a), GCD curves (b) and different specific capacitance (c) of MoO₃/Ti₃C₂T_x (I), MoO₃/Ti₃C₂T_x (II), MoO₃/Ti₃C₂T_x (III), MoO₃/Ti₃C₂T_x (IV) and Ti₃C₂T_x.

Fig. 7c shows the specific capacitance of Ti₃C₂T_x and different MoO₃/Ti₃C₂T_x composite electrodes at different scan rates from 2 to 100 mV s⁻¹. The specific capacitance was calculated from the CV curves according to the following equation:

where I (A) is the response current, ν (V s⁻¹) is the scan rate, ΔV (V) is the potential window, and m (g) is the mass of active electrode material. Strikingly, MoO₃/Ti₃C₂T_x (III) composite displays a higher specific capacitance than other composites and as-prepared Ti₃C₂T_x. As shown in Fig. 7c, the corresponding specific capacitances of MoO₃/Ti₃C₂T_x (I), MoO₃/Ti₃C₂T_x (II), MoO₃/Ti₃C₂T_x (III) and MoO₃/Ti₃C₂T_x (IV) electrodes were calculated from the CV curves at 2 mV s⁻¹ are 95.3, 103.2, 122.7, 150.7, 130 F g⁻¹, respectively. As we know, the electrical conductivity of the electrode plays an important role on the capacitive performance. So, the lower capacitance of MoO₃/Ti₃C₂T_x (IV) composite than MoO₃/Ti₃C₂T_x (III) may be due to excessive MoO₃ nanoparticles decreases the contribution of good electron conductive materials of Ti₃C₂T_x.

Electrochemical impedance spectroscopy (EIS) was used to evaluate the charge transfer resistance and ion diffusion resistance of electrodes. Fig. 8 shows the Nyquist plots of Ti₃C₂T_x and MoO₃/Ti₃C₂T_x (III) electrodes, both of which exhibit capacitive-type impedance, contacting with the KOH electrolyte in a quite wide frequency region. As usual, the Nyquist plot is composed of a semicircle in the high frequency range and a near vertical line to the abscissa axis in the low frequency range. As shown in image, the resistance value includes the solution resistance (R_s) and the charge transfer resistance (R_ct) within the electrode materials, which are represented by the intersection point of the semicircle on the real axis and the diameter of the semicircle at high frequency region, respectively. It is apparent that the R_s value of MoO₃/Ti₃C₂T_x (III) electrode is 1.5 Ω, which is slightly higher than that of Ti₃C₂T_x (1.29 Ω), suggesting that the MoO₃/Ti₃C₂T_x (III) composite has a relatively high internal resistance. It is consistent with the result of galvanostatic charge–discharge tests, shows in Fig. 6a. However, the MoO₃/Ti₃C₂T_x (III) composite with a lower ion diffusion/transport resistance between electrolyte and electrode surface is confirmed by the almost vertical line in the low-frequency range.

Fig. 8 The Nyquist plots of Ti₃C₂T_x and MoO₃/Ti₃C₂T_x (III) composite at a large frequency range of 0.1 to 10⁵ Hz.

The cycling stability is another critical requirement to estimate the electrochemical behavior of the electrodes. Continuously galvanostatic charge–discharge tests are conducted to investigate cycling stability of as-prepared Ti₃C₂T_x and MoO₃/Ti₃C₂T_x (III) composite at a current density of 1 A g⁻¹ in a potential range from −1 to −0.3 V (vs. Ag/Ag⁺), as shown in Fig. 9. Cycling stability tests show that the specific capacitance retention of MoO₃/Ti₃C₂T_x (III) is 93.7% after 8000 charge–discharge cycles, slightly less than as-prepared Ti₃C₂T_x (95%). So, loading of MoO₃ makes MoO₃/Ti₃C₂T_x (III) composite exhibit just a slight reduction in cycling stability comparing to Ti₃C₂T_x. As we know, MoO₃ has a bad cycling stability, thus the layered Ti₃C₂T_x acts as a supporter to decrease the volume change of MoO₃ nanoparticles during charge–discharge process to enhance the stability of MoO₃/Ti₃C₂T_x (III) composite.

Fig. 9 Cycle stability of Ti₃C₂T_x and MoO₃/Ti₃C₂T_x (III) composite at a current density of 1 A g⁻¹, the inset is CV curves of the first and after 8000 cycles of MoO₃/Ti₃C₂T_x composite in 1 M KOH solution.

4. Conclusion

In summary, this article reported a convenient method for preparing MoO₃/Ti₃C₂T_x composite for supercapacitors through hydrothermal treatment. The MoO₃/Ti₃C₂T_x composite exhibited a layered morphology with crystalline MoO₃ nanoparticles loaded on the Ti₃C₂T_x sheets. The layered structure is convenient for penetration of electrolyte and diffusion of ions into the electrode material. Moreover, the deposition of MoO₃ contributes to increase contact area and shorten ion diffusion paths between electrode and electrolyte. The electrochemical test reveals a maximum specific capacitance of 151 F g⁻¹, and remarkable capacity retention of 93.7% after 8000 cycles for MoO₃/Ti₃C₂T_x (III) composite in 1 M KOH electrolyte. The excellent electrochemical properties imply this novel composite has significant potential to be exploited in supercapacitors. Note that, two-dimensional titanium carbides and other families of MXene can be potential electrodes through surface modification to improve the electrochemical properties.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51572158), and the Graduate Innovation Fund of Shaanxi University of Science and Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15651g

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