Porous and free-standing Ti3C2Tx-RGO film with ultrahigh gravimetric capacitance for supercapacitors

Porous and free-standing Ti₃C₂T_x-RGO film with ultrahigh gravimetric capacitance for supercapacitors

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Yongzheng Fang^a^,¹, Bowen Yang^a^,¹, Dongtong He^a, Huipeng Li^a, Kai Zhu^a^,^c^,^*, Lin Wu^b^,^**, Ke Ye^a, Kui Cheng^a, Jun Yan^a, Guiling Wang^a, Dianxue Cao^a^,^*

Chinese Chemical Letters | 2020, 31(4) : 1004 - 1008

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Chinese Chemical Letters | 2020, 31(4): 1004-1008

• Communication •

Porous and free-standing Ti₃C₂T_x-RGO film with ultrahigh gravimetric capacitance for supercapacitors

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Yongzheng Fang^a^,¹, Bowen Yang^a^,¹, Dongtong He^a, Huipeng Li^a, Kai Zhu^a^,^c^,^*, Lin Wu^b^,^**, Ke Ye^a, Kui Cheng^a, Jun Yan^a, Guiling Wang^a, Dianxue Cao^a^,^*

Affiliations

^a Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

^b Teachers Education College of Harbin Normal University, Harbin 150001, China

^c Key Laboratory of Advanced Chemical Power Sources, Guizhou Meiling Power Sources Co., Ltd., Zunyi 563003, China

Published: 2020-04-15 doi: 10.1016/j.cclet.2019.08.043

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Abstract

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MXene-based electrode materials exhibit favorable supercapacitor performance in sulfuric acid due to praised pseudocapacitance charge storage mechanism. However, self-stacking of conventional MXene electrodes severely restricts their electrochemical performance, especially at high loading. Herein, a flexible cross-linked porous Ti₃C₂T_x-MXene-reduced graphene oxide (Ti₃C₂T_x-RGO) film is skillfully designed and synthesized by microscopic explosion of graphene oxide (GO) at sudden high temperature. The generated chamber structure between layers could hold a few of electrolyte, leading to a close-fitting reaction at interlayer and avoiding complex ions transmission paths. The Ti₃C₂T_x-RGO film displayed a preferable rate performance than that of pure Ti₃C₂T_x film and a high capacitance of 505 F/g at 2 mV/s. Furthermore, the uniform intralayer structure and unique energy storage process lead to thickness-independenct electrochemical performances. This work provides a simple and feasible improvement approach for the design of MXene-based electrodes, which can be spread other electrochemical systems limited by ions transport, such as metal ions batteries and catalysis.

Key words

MXene / RGO / Cross-linked porous / Supercapacitors / High capacitance / Thickness independent

Cite this Article

Yongzheng Fang, Bowen Yang, Dongtong He, Huipeng Li, Kai Zhu, Lin Wu, Ke Ye, Kui Cheng, Jun Yan, Guiling Wang, Dianxue Cao. Porous and free-standing Ti₃C₂T_x-RGO film with ultrahigh gravimetric capacitance for supercapacitors[J]. Chinese Chemical Letters, 2020 , 31 (4) : 1004 -1008 . DOI: 10.1016/j.cclet.2019.08.043

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Supercapacitors have been regarded as one of promising representative energy storage devices with high-power density due to their unique advantages such as a long cycle life and environmental friendliness [1]. Usually, supercapacitors are classified into two categories based on charge storage mechanism of electrode materials. One is typical electrical double-layer capacitors (EDLCs), which storages charges through adsorbing/ desorbing process at the electrode/electrolyte interface [2]. Although EDLCs provide a stable cycling performance, their low capacitance

cannot meet increasing requirement of high-energy devices. Pseudocapacitors, as another type supercapacitor, permit fast redox reaction on materials surface layer, engendering larger energy density due to more charges transfer [3]. Thus, pseudo-capacitors become a promising candidate that can offer not only high power density but also capable energy density for power equipment. However, traditional pseudocapacitors electrode materials such as RuO₂ [4], MnO₂ [5-9], V₂O₅ [10, 11] are confronted with many problems such as high price, low electrical conductivity, and poor cycling performance [12]. Thus, it is necessary to explore capable electrode materials for pseudocapa-citors.

MXene, an emerging group of two-dimensional (2D) materials, have present their potential as electrochemical energy storage materials with high energy and power densities due to the large surface to volume ratios, high conductivity, rich exposed active site, and lack of bulk phase diffusion [13, 14]. Usually, MXene is prepared by selective etching of the most lively "A" atoms layer from the layered transition metal carbides (MAX) by using the synthetic hydrofluoric acid solution (LiF and HCl), which results in abundant of surface functional groups (—OH, —F and —O) [15, 16]. The surface functional groups provide hydrophilicity and a large number of active sites [17]. Recently, Gogotsi's group demonstrated the pseudocapacitance charge storage mechanism of MXene in H₂SO₄ electrolyte. The Ti oxidation state and deprotonation/ protonation of —O function groups change with the stripping/ plating processes of H⁺, leading to a large capacitance [2, 18]. However, similar to other 2D materials, when MXene nanosheets are fabricated into electrodes, they suffer from the self-restacking problem due to van der Waals force between layers, resulting in a covered reaction sites and tortuous ions transmission path [15]. Recently, constructing a 3D electrode becomes an effective strategy to enhance the electrochemical performance [19]. R. Lukatskaya et al. synthesized a macroporous MXene film by using polymethyl methacrylate as a template to construct a 3D film electrode, which deliver up to 310 F/g [2]. Xia et al. realized the vertical alignment of MXene by mechanically shearing of a discotic lamellar liquid-crystal phase of Ti₃C₂T_x and a capacitance of 275 F/g was obtained [13]. These strategies realized short vertical transmission paths, resulting in a favorable rate performance. Unfortunately, the complex and demanding preparation processes limit their large-scale production and application.

Herein, we designed and prepared a porous MXene-RGO flexible film by a self-puffing reaction of MXene-GO film. The sparking reaction of GO created a large amount of cross-linked porous structure between the MXene layers. As-prepared MXeneRGO film was employed as electrode directly without addition of heavy metal current collector and inactive binder. RGO successfully prevents the self-restacking of MXene and the chamber structure between layers can accommodate a little of electrolyte, which causes a close-fitting reaction, avoiding complex ions transmission paths and promising fast ion storage behaviors. Therefore, the MXene-RGO films displayed a large capacitance, remarkable rate ability and stable cycling performance. Moreover, such a unique chamber structure made the capacitance was less affected by thickness of electrode and loading of active materials.

Fig. 1a displays schematic illustration of fabrication process of the Ti₃C₂T_x-RGO film with an interlayer link-cross porous structure. Firstly, the uniform mixture of few-layered Ti₃C₂T_x and GO nanosheets was transformed into the compact Ti₃C₂T_x-GO film through vacuum filtration. Then, Ti₃C₂T_x-GO film touched a hottable and a micro-explosion reaction occurred. Due to abrupt and larger heat, GO was reduced into RGO. During oxygen-containing groups removal process, generated gas enlarged the interlayer spacing and created a cross-linked porous structure [20]. Finally, Ti₃C₂T_x-RGO flexible film was obtained. In such a porous structure, H₂SO₄ electrolyte will infuse the nano-chambers among layers due to the hydrophilic surface of Ti₃C₂T_x as shown in Fig. 1b [21]. During the discharging and charging electrochemical reaction, the H⁺ will move toward Ti₃C₂T_x surface/interlayer, and back into the chamber, respectively. Such a close-fitting reaction can accelerate the ions transport. Different with the lengthy and tortuous H⁺ transmission path in traditional multilayer Ti₃C₂T_x films, such reaction process achieves ultra-fast ions storage behavior. In addition, the porous structure prepared by micro-explosion makes Ti₃C₂T_x separate from each other, which enables single layer or a few layers of Ti₃C₂T_x participate in the reaction at each nanoelectrode between the layers, increasing electrochemical utilization. Fig. 1c shows the images of Ti₃C₂T_x-GO and Ti₃C₂T_x-RGO films. Ti₃C₂T_x-GO film shows a black and smooth surface. After micro-explosion, the Ti₃C₂T_x-RGO film presented gray and pleated surface due to reducing process and increased interlayer spacing. Meanwhile, Ti₃C₂T_x-RGO film displayed a good flexibility.

The microscopic morphologies of the Ti₃C₂T_x-GO and Ti₃C₂T_xRGO films were observed by a scanning electron microscopy (SEM).Figs. 2a and b exhibit a tightly stacked structure for Ti₃C₂T_x-GO film, and Figs. 2c and d display a cross-linked and porous structure in the Ti₃C₂T_x-RGO interlayer after micro-explosion. It is noteworthy that the layer spacing is enlarged and the chamber height ranges from 100 nm to 1 μm, implying that a few of electrolyte can be stored between layers. X-ray diffractometer (XRD) was employed to investigate crystal structure changes from Ti₃AlC₂ to Ti₃C₂T_x and from Ti₃C₂T_x-GO to Ti₃C₂T_x-RGO. As shown in Fig. S1 (Supporting information), the highest peak at 39.5° of Ti₃AlC₂ was completely obliterated and the (002) characteristic peak of Ti₃C₂T_x before 10° occurred after etching and ultrasound process, suggesting successful synthesis of Ti₃C₂T_x MXene [3]. For the Ti₃C₂T_x-GO film, the (002) peak strength weakened and moved 0.6° to the left due to the introduction of GO and increased interlayer spacing, which is similar to Yan's report [15]. After the reduction process, (002) peak of the Ti₃C₂T_x-RGO film became very wide and flat, which can be attributed to enlarged layer spacings and different orientations for Ti₃C₂T_x nanosheets (Fig. 2e) [22]. In order to investigate the functional groups on the film surface before and after the micro-explosion, X-ray photoemission spectroscopy (XPS) was performed. The C 1s spectra as shown in Fig. 2f indicated the existence of lots of oxygen-containing groups on Ti₃C₂T_x-GO film, with the peaks at 284.0, 284.6, 285.6, 286.7 and 288.2 eV correspond to C = C, C——C, C——OH, C–O and C=O bonds, respectively [23]. After the micro-explosion, a large proportion of function groups were removed, which can be attributed to the reduction process from GO to RGO (Fig. 2g). In contrast, from Ti 2p (Fig. S2 in Supporting information) and F 1s (Fig. S3 in Supporting information) spectra, Ti₃C₂T_x surface functional groups had almost no change, suggesting stable functional groups and stronger binding energy between Ti₃C₂ and T_x. The generated RGO enhance electronic conductivity and remained functional groups ensure the hydrophilicity, promising the fast penetration of electrolyte.

Electrochemical performance of Ti₃C₂T_x-RGO freestanding electrode was evaluated in a three-electrode system with carbon-point as a counter electrode and Ag/AgCl as a reference electrode in 3 mol/L H₂SO₄ electrolyte. The detailed experimental method is provided in supporting information. To investigate the synergy effect between Ti₃C₂T_x and RGO, and their optimized mass ratio, five samples were designed and prepared with the same quality (Table 1).

Fig. 3a shows the cyclic voltammetry (CV) curves of pure Ti₃C₂T_x and Ti₃C₂T_x-RGO films at the scan rate of 20 mV/s. When the content of GO is little, Ti₃C₂T_x-RGO-0.1 films exhibit smaller integral area, which can be attributed to a smaller layer spacing variation. When the content of GO is nimiety, the smallest area was obtained for Ti₃C₂T_x-RGO-0.4 without redox peaks, due to low capacitance and conductivity of RGO. In contrast, Ti₃C₂T_x-RGO-0.2 and Ti₃C₂T_x-RGO-0.3 films display larger area than the pure Ti₃C₂T_x, and a couple of redox peaks locate at —0.3 V and —0.45 V (vs. Ag/AgCl) represent a reversible pseudocapacitance reaction associated with the deprotonation/protonation process of functional groups and the change of Ti oxidation state. The CV curves of these at different scan rates are shown in Fig. 3c and Fig. S4 (Supporting information). Ti₃C₂T_x-RGO-0.3 film kept the shape best, suggesting the outstanding rate ability. The specific capacitance was calculated from the CV curves as shown in Fig. 3b. At 2 mV/s, the Ti₃C₂T_x-RGO-0.3 showed the highest capacity of 505 F/g (808 F/cm²), among Ti₃C₂T_x-RGO-0.2 (452 F/g), Ti₃C₂T_x(436 F/g), Ti₃C₂T_x-RGO-0.1 (433 F/g) and Ti₃C₂T_x-RGO-0.4 (373 F/g) samples. Even at 100 mV/s, Ti₃C₂T_x-RGO-0.3 still maintained a high capacity of 353 F/g (564.8 F/cm²). It suggests that the optimal ratio of Ti₃C₂T_x:GO is 7:3. It should be noticed that the gravimetric capacitance of 505 F/g (561 C/g in the voltage window of 0.9 V) is closed to its theoretical value (615 C/g) reported by Yury's group [2]. Such a result is competitive with previous excellent reports is shown in Table S1 (Supporting information). Ti₃C₂T_x-RGO-0.3 film exhibits the nonlinear galvanostatic charge/discharge curves with a couple of charge/discharge platforms corresponding to redox peaks in the CV curves (Fig. 3d). The discharge specific capacitance was calculated to be up to 612 F/g at 0.5 A/g, followed by 505, 386, 385 and 371 F/g at 1.0, 2.0, 5.0 and 10 A/g, respectively. In addition, Ti₃C₂T_x-RGO-0.3 presents an excellent cycling performance at 1.0 A/g as shown in Fig. 3e, corresponding to the decay of only 0.0071% per cycle. In order to certify the practical application value, an asymmetric supercapacitor was assembled with the Ti₃C₂T_x-RGO- 0.3 as a cathode and active carbon (AC) as an anode. The electrochemical performance was performed in 0~ 1.0 V as shown the Fig. S5 (Supporting information). Stable CV curves indicate a good dynamic match for cathode and anode. A pair of slight redox peaks (0.6 V and 0.5 V) can be observed and classified as a Faraday reaction of Ti₃C₂T_x.

To further understand electrochemical dynamics of Ti₃C₂T_x-RGO-0.3 film, the mechanism of electrochemical process was investigated. In the CV curves, the peak current (i) and scan rate (v) obey the relationship as follow:

(1)

in which the a and b are the constant and b-value is between 0.5 to 1.0. When the electrochemical process controlled by diffusion behavior, the b-value is close to 0.5, while the b-value of 1.0 represents a fast reaction without diffusion-controlled [24, 25]. As shown in Fig. 4a, the b-values of Ti₃C₂T_x-RGO-0.3 electrode are 0.86 and 0.93 for cathodic and anodic peaks, respectively. In contrast, the lower b-values of 0.84 and 0.89 of pure Ti₃C₂T_x film as shown in Fig. 4d indicate a slower reaction rate due to the longer transmission path of H⁺ in the tightly stacked layers. While the modified electrode structure makes a close-fitting electrochemical reaction, that is, interlayer MXene reacts directly with H⁺ in the interlayer pores, avoiding longer transport. In order to further certify that, the impedance spectroscopy of they were studied through a complex model capacitance as follow:

(2)

(3)

in which, the z'(ω), z"(ω), and ω represent real, imaginary parts, and angular frequency of the complex impedance, respectively; while the C'(ω) and C"(ω) are the real and imaginary parts for the complex capacitances [24]. As shown in Figs. 4b and e, the C'(ω) decreases slower with the frequency for Ti₃C₂T_x-RGO-0.3 electrode, suggesting faster diffusion and reaction rate of ions. In addition, the minimal characteristic relaxation time constant t0 (the minimum time required to release all energy when the efficiency is greater than 50%) can be calculated by the characteristic frequency f₀, corresponding to the half value of C' and the highest value of C" [15]. Half of the time (3.3 s) of MXeneRGO-0.3 electrodes was obtained compared to the pure MXene (6.6 s). The above results demonstrate the faster reaction dynamics for the porous MXene-RGO-0.3 film. Moreover, on account of the uniform intralayer structure, Ti₃C₂T_x-RGO film will get rid of the surface loading limit, which is an affliction in traditional tight stacked film. As shown in Fig. 4f, the specific capacitance of pure Ti₃C₂T_x film decreases sharply with increasing surface loading. In contrast, Ti₃C₂T_x-RGO-0.3 presents a good rate performance independent of the mass loading (Fig. 4c), which is very industrially significant for obtaining high-energy electrochemical devices.

In summary, a flexible cross-linked porous Ti₃C₂T_x-RGO film was prepared through a micro-explosion reaction, which prevented the self-restacking of MXene, increased interlayer spacing obviously and exposed a large number of electrochemical active sites. Furthermore, the chamber structure between layer could accommodate a few of electrolyte, achieving a close-fitting reaction and avoiding lengthy transmission paths of active ions. As-prepared Ti₃C₂T_x-RGO-0.3 film exhibited a high rate performance and ultra-high gravimetric capacitance of 505 F/g at 2 mV/s and 353 F/g at 100 mV/s in the 3 mol/L H₂SO₄ electrolyte. Meanwhile, it also displayed excellent cycling stability. In addition, the unique close-fitting reaction and uniform intralayer structure promised the electrochemical performance of Ti₃C₂T_x-RGO film is less effect by mass loading than pure Ti₃C₂T_x film. This work paved a way for the design and development of capable electrochemical energy storage devices with high mass loading of active materials.

Acknowledgments

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This work was supported by the National Natural Science Foundation of China (Nos. 51702063, 51672056), Natural Science Foundation of Heilongjiang Province (No. LC2018004), China Postdoctoral Science Foundation (Nos. 2018M630340, 2019T120254) and the Fundamental Research Funds for the Central University (No. 3072019CF1006). The authors also thank the support from the Starting Research Fund from Harbin Normal University (No. XKB201420).

Appendix A. Supplementary data

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Supplementary material related tothis article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.08.043.

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Year 2020 volume 31 Issue 4

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doi: 10.1016/j.cclet.2019.08.043

Receive Date：2019-07-29
Online Date：2026-01-31
Published：2020-04-15

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Received：2019-07-29
Revised：2019-08-15
Accepted：2019-08-19

Affiliations

^a Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

^b Teachers Education College of Harbin Normal University, Harbin 150001, China

^c Key Laboratory of Advanced Chemical Power Sources, Guizhou Meiling Power Sources Co., Ltd., Zunyi 563003, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Fig. 1 (a) Synthetic schematic for Ti₃C₂T_x-RGO film; (b) Reaction mechanism for Ti₃C₂T_x-RGO in H₂SO₄ electrolyte; (c) Physical pictures for Ti₃C₂T_x-GO and Ti₃C₂T_x-RGO films.

Fig. 2 (a–d) Microscopic morphology and (e) XRD before and after micro-explosion reaction of Ti₃C₂T_x-GO films; XPS for (f) Ti₃C₂T_x-GO and (g) Ti₃C₂T_x-RGO.

Fig. 3 Electrochemical performance for Ti₃C₂T_x-RGO films in 3 mol/L H₂SO₄ electrolyte. (a) CV curves at 20 mV/s and (b) gravimetric capacitances at different scan rates of different Ti₃C₂T_x-RGO films; (c) CV curves, (d) charge-discharge curves and (e) cycling performance of Ti₃C₂T_x-RGO-0.3 film.

Fig. 4 Performance comparison between Ti₃C₂T_x-RGO-0.3 (up) and pure Ti₃C₂T_x films (below). (a, d) The relationship between peak currents and scan rates from 2 mV/s to 50 mV/s; (b, e) Normalized real and imaginary capacitances; (c, f) Gravimetric capacitances of different surface loads in at different scan rates.

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