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MXene hybrid fiber-like electrode with high volumetric capacitance
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Facile fabrication of flexible rGO/MXene hybrid fiber-like electrode with high volumetric capacitance Zhe Wang a, Yaoyan Chen a, Mengyao Yao a, Jie Dong a, Qinghua Zhang a, Lili Zhang b, **, Xin Zhao a, * a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island, 627833, Singapore
b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A facile confined hydrothermal strategy is developed to fabricate rGO/MXene hybrids. � rGO/MXene hybrids show robust gra phene skeleton with MXene filling the microvoids. � Hybrid with low MXene content exhibits a large volumetric capacitance of 345 F cm 3. � The rGO flakes could well suppress the oxidation process of MXene.
A R T I C L E I N F O
A B S T R A C T
Keywords: rGO MXene Fiber Flexible Supercapacitors
Graphene fibers or fiber-like graphene materials have been successfully adopted as electrodes for flexible and wearable supercapacitors. However, they normally exhibit relatively low volumetric capacitance, which will seriously hinder their wide applications in smart textile as power supply. Here, a facile confined hydrothermal strategy to fabricate fiber-like reduced graphene oxide (rGO)/MXene hybrids is designed through ingenious assemble of two functional two dimensional materials of graphene and MXene nanosheets. Owing to the syn ergistic interactions between two layers, the hybrid rGO/M 5 exhibits a large volumetric capacitance of 345 F cm 3 with the gravimetric capacitance of 195 F g 1 at 0.1 A g 1 with only 5 wt% MXene layers. A volu metric energy density of 30.7 mWh cm 3 with power density of 70.7 mW cm 3 could be delivered. After 7500 cycles at a current density of 0.5 A g 1, about 125% of its initial capacitance value is retained, demonstrating excellent electrochemical stability. More importantly, due to the suppressed oxidation process of MXene sheets in the hybrid, rGO/MXene could retain 95% of its original capacitance after one month.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Zhang), [email protected] (X. Zhao). https://doi.org/10.1016/j.jpowsour.2019.227398 Received 17 August 2019; Received in revised form 28 October 2019; Accepted 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhe Wang, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227398
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1. Introduction
nanosheets were simultaneously achieved during the hydrothermal process, forming hybrid materials with a robust skeleton of large gra phene sheets and small MXene sheets filling the spaces and microvoids. With lower loading of MXene about 5 wt%~15 wt%, the fiber-like hybrid electrodes could achieve a high electrical conductivity of 1339 S m 1 and a large volumetric capacitance of 345.2 F cm 3 with the gravimetric value of 195 F g 1. Moreover, there was still 125% capaci tance retention after 7500 cycles at a current density of 0.5 A g 1, illustrating its great potential as electrode for flexible supercapacitors. The investigation also revealed that the graphene nanosheets could act as barrier or protection layers to prevent the oxidation of MXene in the hybrids.
Recent advantages of flexible and wearable supercapacitors have motivated the increasing demands for compatible energy resources that can be integrated into fabric for powering electronic devices [1–3]. Due to the merits of potential compatibility with textile processing, i.e. weaving or knitting, fiber-like electrode materials have gained tremen dous attentions as alternatives to traditional forms of bulky or rigid materials [4,5]. To date, carbonaceous fibers, such as carbon fibers [6], CNTs [7], graphene [8], have been favorably adopted as fiber-like electrode ma terials for wearable devices. Notably, graphene-based fibers, which have been successfully fabricated via various spinning and assembly tech niques from graphene oxide liquid crystalline [9–13], have shown great promise owing to their remarkable mechanical, electrical and electro chemical properties. However, the relatively low conductivity and strong π-π interactions negatively affect the charge transport and elec trolyte access. The limited electric double layer capacitance of pure graphene-based material hinders the energy density of the device. As a result, graphene-based fibers normally exhibit moderate energy storage performance with low volumetric energy density and power density [14]. To achieve high volumetric energy storage performance, many efforts have been made including the integration of graphene and pseudocapacitive materials with high electrochemical activity into fiber shape, such as such as CNTs [15], transition metal oxides [16] and conducting polymers [17], or the decoration of electrochemically active materials on the outer surface of graphene fibers [18]. Although graphene-based fiber-like electrodes has been explored with great progress, a favorable assembly method in a well-controlled way and the optimal combination with guest nanomaterials still need to be further pursued. As a new member of two-dimensional materials, transition metal carbides and nitrides (MXene) have shown outstanding performances in electrochemical energy storage [19,20] and many other applications [21–24], due to its excellent electrochemical properties, metallic con ductivity, high density and surface hydrophilicity [25,26]. With a gen eral formula of Mnþ1XnTx, where M is an early transition metal, X represents C and/or N, Tx denotes surface functional groups, and n ¼ 1, 2, or 3, MXene are generally synthesized by selectively etching of the “A” element layer (group III A or IV A) from their parent MXA phase, following by an intercalation-assisted delamination process to yield single or few layer nanosheets [27–29]. With unique metallic conduc tivity as high as 105 S cm 1 [30] and excellent high volumetric capaci tance of 1500 F cm 3 [31], MXene nanosheets have exhibited great potential as good fillers to improve the volumetric performance of film and fiber-like graphene electrodes. For example, Li H et al. [32] ob tained MXene/graphene flexible film by vacuum filtration, and assem bled a flexible all-solid supercapacitor with a volumetric capacitance of 216 F cm 3. Yan J et al. [33] prepared composite films by electrostatic self-assembly of rGO and MXene, which not only achieved good elec trical conductivity (2261 S cm 1), but also achieved volumetric capaci tance of 1040 F cm 3. Gao et al. [34] and Razai et al. [35] fabricated graphene/MXene composite fiber by wet spinning from graphene oxide (GO)/MXene solutions. After the reduction of GO, hybrid fibers with the highest MXene mass ratio about 80%–95% was obtained which achieved good performance for flexible supercapacitors. Although hybrid fibers with large loadings of MXene or MXene as the host material could indeed benefit the volumetric energy storage performance, the inherent chemical instability of MXene requires a very stringent environment during the fabrication and applications of such hybrids [36]. Therefore, an effective approach to fabricate graphene/MXene hybrid fibers by using low loading of MXene to achieve high volumetric performance is still challenging. Herein, we designed a facile method to fabricate fiber-like rGO/ MXene hybrid electrodes by one-step hydrothermal strategy. The reduction of GO and the ingenious macro-assembly of GO with MXene
2. Experimental 2.1. Materials Lithium fluoride (LiF) was purchased from Alfa Aesar (97%); hy drochloric acid (HCl, 36%~38%) and vitamin C (VC, 99%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Ti3AlC2 (75–79 wt%) was purchased from Carbon-Ukraine, Ltd. Multilayer graphene oxide (GO, 95%) was obtained from Suzhou Carbon Technology Co., Ltd. All other reagents were used as received without any purification. 2.2. Fabrication of Ti3C2Txnanosheets The preparation of Ti3C2Tx nanosheets were synthesized similarly to a previous report [37]. Typically, 0.8 g LiF was completely dissolved in 10 mL 9 M HCl solution, followed by the slow addition of 0.5 g Ti3AlC2 powders with the lateral size less than 38 μm. The reaction was kept at 35 � C for about 24 h under magnetic stirring. After the etching reaction, the sediments were washed with deionized water until the PH value of supernatant higher than 6. Then the products were freeze dried to obtain multilayer Ti3C2Tx powders. Next, the multilayer Ti3C2Tx powders were again mixed with 160 mL deionized water under the protection of argon and then treated by sonication in an ice water bath for 1 h. Afterwards, the dispersion was centrifuged at 3500 r min 1 for 1 h to get a stable colloidal solution composed of few layer of Ti3C2Tx. In order to prevent the rapid oxidation of Ti3C2Tx nanoflakes, a certain amount of the dispersion was vacuum filtered to obtain MXene films for storage. 2.3. Preparation of GO dispersion 45 mg multilayer GO powder was dispersed in 30 mL deionized water and the monolayer GO dispersion was obtained by sonication for 0.5 h, 1 h, 1.5 h and 2 h, respectively, and the obtained samples were denoted as GO-0.5, GO-1, GO-1.5 and GO-2. 2.4. Fabrication of rGO/MXene fiber-like hybrids According to the previous method for preparing rGO fiber-like ma terial [38], a certain amount of MXene film was added into GO disper sion to get homogeneous mixed suspension under sonication. Then VC powder was added as reduction agents with the mass ratio with GO of 5:1. After fully stirring, the mixture was injected into a polypropylene (PP) tube with a length of 20 cm and a diameter of 8 mm. With sealing the two ends, the tube was placed in water bath at 80 � C for 1 h. Then rGO/MXene hybrid fiber could be obtained by opening both ends of the tube and drying at 60 � C for 12 h in a vacuum oven. The feeding ratio of MXene in hybrid fiber was 5 wt%, 10 wt% and 15 wt% and the obtained samples were designated as rGO/M 5, rGO/M 10 and rGO/M 15, respectively. 2.5. General characterizations The surface morphologies and nanostructures of as-prepared samples 2
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Fig. 1. Schematic of the fabrication procedure of rGO/MXene fiber-like hybrids.
were characterized by field-emission scanning electron microscope (FESEM, HITACHI S-4800) and transmission electron microscopic (TEM, JEM 2100F). The size distribution of Ti3C2 sheet was analyzed by Nano ZS nanoparticle size analyzer. The structures of obtained produces were studied by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscope (XPS). XRD patterns were carried out on a RIGAKU D/Max2550 diffractometer (Copper Kα radiation with λ ¼ 0.1514 nm). Raman spectroscopy was conducted on a HORIBA JobinYvon LabRarn-1B (He–Ne laser with k ¼ 632.8 nm). And X-ray photoelectron spectro scope (XPS) were applied on Escalab 250xi X-ray photoelectron spec trometer with Al Kα radiation.
2.6. Electrochemical measurements Three-electrode system was employed to measure the electro chemical performances of rGO/MXene fiber-like hybrids. The electrode was tested in 1 M H2SO4 aqueous solution with the platinum mesh as the counter electrode and Ag/AgCl electrode as the reference electrode, respectively. The cyclic voltammetry (CV) and galvanostatic chargedischarge curves were tested on an Auto-lab PGSTAT302 N electro chemical working station, with an applied potential window ranging from 0.2 to 0.6 V. Electrochemical impedance spectroscopy (EIS) was tested in the frequencies between 0.01 Hz and 100 kHz, with a pertur bation amplitude of 10 mV. The cyclic stability test was carried out on LAND test system. The gravimetric value can be calculated from the
Fig. 2. (a) SEM image of multi-layerTi3C2Tx; (b) TEM image of monolayer Ti3C2Tx; (c–d) XRD pattern and Raman spectra of Ti3C2Tx; (e) SEM image of GO nanosheet obtained by sonication for 2 h; (f) SEM image of the surface for pure rGOspecimen; (g) Digital image of rGO specimen forming in PP tube; (h) Digital image of rGO specimen woven in knitted glove. 3
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Fig. 3. SEM images of the surface for (a) rGO/M C, Ti and F.
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5, (b)rGO/M
10 and (c)rGO/M-15; (d) SEM image of the cross section for rGO/M
galvanostatic charge-discharge curves by using the followed equation: Cm¼I⋅Δt/(m⋅ΔV), where Cm is the mass-specific capacitance (F g 1), I is the current (A), Δt is the discharge time, m is the mass (g) of the active materials, and ΔV is the potential window (V). The volumetric value can be calculated from the galvanostatic charge-discharge curves by using the followed equation: Cv¼I⋅Δt/(V⋅ΔV), where Cv is the volumetric capacitance (F cm 3), I is the current (A), Δt is the discharge time, and V is the volume (cm 3) of the active materials, and ΔV is the potential window (V). The volumetric energy density and power density of the rGO/M-X (X ¼ 5, 10 and 15) could be calculated by the follow equations: Ev¼1/2⋅Cv⋅ΔV2 and Pv¼Ev/Δt, respectively, where Ev is the volumetric energy density (mWh cm 3), Cv is the volumetric capacitance (F cm 3), ΔV is the potential window (V), Pv is the volumetric power density (mW cm 3), andΔt is the discharge time.
5 with the EDS mapping of
layers exfoliated from each other. Transmission electron microscopy (TEM) image of the colloidal suspension after sonication (Fig. 2(b)) confirms the synthesis of delaminated MXene nanosheets. It displays an ultrathin sheet with the size about 0.5–1 μm and clear lattice morphology could be noted from the image with high resolution. Further analyses of the crystal structure are performed by X-ray diffraction (XRD) of pristine Ti3AlC2 and Ti3C2Tx (Fig. 2(c)). It could be seen that the diffraction peak at 39� for the (104) planes of Ti3AlC2 is absent in the XRD pattern of Ti3C2Tx, suggesting the near complete removal of Al layers by etching. In addition, the (002) peak of Ti3C2Tx has shifted to 5.7� with the interplanar spacing of 15.5 Å (9.6� for Ti3AlC2), indicating an enlarged interlayer spacing of ~0.7 nm (~0.5 to for Ti3AlC2). According to Raman spectrum (Fig. 2(d)), the character istic signals of A1g and Eg appear for few-layer Ti3C2Tx nanosheets, which could be ascribed to the symmetric out-of-plane vibration of Ti, C atoms and the in-plane shear vibration of Ti, C and terminal atoms, respectively [35]. Moreover, the size of the monolayer Ti3C2Tx ranges from 600 nm to 1.5 μm from the nanoparticle size analyzer (Fig. S1c), which is agree with the observation from TEM images. The chemical composition and oxidization states of the as-prepared Ti3C2Tx nanp sheets are also studied by X-ray photoelectron spectroscopy (XPS) (Fig. S1d), which suggests the presence of surface-anchored -(OH)x and -Fx groups, rendering the Ti3C2Tx nanosheets highly hydrophilic. Full characterization data confirm the successful etching and exfoliation of the MXA phase to MXene nanosheets. As the neat exfoliated Ti3C2Tx nanosheets can not be readily assembled into continuous fibers due to the weak interlaminar in teractions between MXene flakes with relatively small lateral sizes, large GO sheets were introduced as the scaffold to aid the formation of MXenebased fibers. However, the one-dimensional hybrid materials with higher MXene loading than 15 wt% is hard to be realized, mainly due to the lack of shearing forces during the static hydrothermal process, which is different from the extruding produre of wet spinning technique. Before the combination with MXene, the self-assmeble behaviours of exfoliated monolayer GO nanosheets have been investigated. SEM im ages of different samples with various sonication durations of GO showed that an appropriate treatment time is beneficial for the exfoli ation of large GO layers in solution and could get much thinner nano sheets with more local wrinkles (Fig. S2). As a result, integral fiber-like samples of GO-0.5 and GO-1 are difficult to be formed due to weak electrostatic interactions between unexfoliated multilayer GO sheets
3. Results and discussion The well-explored Ti3C2Tx was used here as a representative example of 2D layered MXene because it has been found to possess the highest electrical conductivity and volumetric capacitance among the MXene types synthesized to date [39]. The rGO/MXene hybrids were fabricated by a one-pot facile hydrothermal process as illustrated in Fig. 1. Spe cifically, multilayer Ti3C2Tx powders were first prepared by selective etching of the Al layers in bulk Ti3AlC2 MAX phase with moderate re gents of LiF and HCl, which were then exfoliated by sonication to give a colloidal suspension of isolated MXene nanoflakes. Then homogeneous MXene/GO mixture with VC as reducing agent was injected into poly propylene (PP) tube and kept under a mild condition in water bath at 80 � C for 1 h. Owing to the similar structural characteristics which could make it facile to match and fuse with each other to form strong in teractions, the continuous fiber-like rGO/MXene hybrids were obtained via self-assembly of ultrathin nanosheets with simultaneous reduction of GO. This process can be scaled up by simply increasing the length of the reaction vessel. To confirm the successful fabrication of delaminated MXene nano sheets, the morphologies and structures of Ti3C2Tx products during etching and exfoliating process are investigated. It is well known that the pristine MAX phase of Ti3AlC2 demonstrates a closely overlapped lamellar layer structure with a reminiscent of graphite (Figs. S1a–b). After etching with LiF and HCl, the scanning electron microscopy (SEM) image (Fig. 2(a)) shows loosely packed multilayer Ti3C2Tx with the 4
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Fig. 4. (a) XRD patterns of rGO, Ti3C2Tx film and rGO/MXene hybrids with different Ti3C2Tx content; (b) Raman spectra of rGO fiber-like sample, Ti3C2Tx film and various rGO/MXene hybrids; (c–d) C 1s XPS spectra of Ti3C2Tx film and rGO/M-10; (e–f) Ti 2p XPS spectra of Ti3C2Tx film and rGO/M-10.
(Fig. S3). Although GO-1.5 could be shaped, it is fragile and hard to be handled (Fig. S4). An excessive sonication force is also unfavorable to obtain exfoliated GO nanosheets with large size, thereby affecting the self-assembly during hydrothermal process (Fig. S5). Here, an appro priate sonication time of 2 h for GO sheets is optimized. Under this condition, the obtained exfoliated GO nanosheets display very thin layers with abundant wrinkles on the surface and very large size of 10–15 μm (Fig. 2(e)). The pure rGO fiber-like product exhibits a crumple surface with the folds along the axial array (Fig. 2(f)). The cross section (Fig. S6) reveals loosely packed layers with many macro/micro voids among them. During the hydrothermal treatment, the graphene sheet is shrink-assembled by the π-π interactions between the layers along with the reduction of GO, and then completely separated from the aqueous phase to form fiber-like specimen (Fig. 2(g)). Although the aspect ratio of the product is much lower than the commercial fiber, it is expected to be improved by simply increasing the length and decreasing the diam eter of the pipe. The obtained pure rGO sample is robust with good flexibility and could also be knitted into knitted gloves (Fig. 2(h)), further demonstrating its fiber-like features. Subsequently, the rGO/ MXene hybrid fiber-like hybrids were prepared by combining Ti3C2Tx with GO solution obtained by sonication for 2 h and treated under the
same conditions. As expected, the rGO/MXene fiber-like hybrids containing 5%, 10% and 15% of Ti3C2Tx still possess good flexibility similar to that of the pristine rGO sample (Fig. S7). The fixed solution of GO nanosheets could lead to a relatively stable GO skeleton and the similar lamellar features of GO and MXene are favorable to assemble with each other via strong π-π interactions. The surface morphologies of obtained hybrids show that (Fig. 3(a–c)) the hybrids display much rougher surface with disor dered wrinkles than that of pure rGO, indicating the incorporation of Ti3C2Tx layers. Meanwhile, the surface roughness of rGO/MXene hy brids increases with the increased mass of Ti3C2Tx, possibly due to the large size discrepancies between Ti3C2Tx and GO layers. Besides, the cross section of several hybrids demonstrate more compact features with smaller interlaminar gap (Fig. S8), indicating the interlayer interactions of two different sheets with more Ti3C2Tx effectively filling the voids between the graphene sheets. From the EDS-mapping of rGO/MXene hybrids (Fig. 3(d)), the uniform distribution of Ti and F elements further confirms the presence of Ti3C2Tx layers which are wrapped in graphene nanosheets. X-ray diffraction (XRD) and Raman spectra are further conducted to explore the structure of the as-prepared rGO/MXene hybrids. The XRD 5
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Fig. 5. (a) CV curves at a scan rate of 10 mV s 1and (b)galvanostatic charge-discharge curves of a current density of 0.1 A g 1for several samples; (c) CV curves at various scan rates and (d) galvanostatic charge-discharge curvesat various current densities for rGO/M-5; (e) Nyquist plot of vairous samples in the frequency range of 100 to 0.01 kHz; (f) Rate properties of rGO/MXene hybrids; (g) Schematic diagram of Hþ flow during electrochemical reaction.
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curves show (Fig. 4(a)) that the addition of Ti3C2Tx enlarges the halfpeak width of rGO around 2θ ¼ 10.94� , and the interlayer spaces of 8.08 Å for rGO sample change to 8.60 Å, 8.60 Å, and 8.53 Å for rGO/ M 5, rGO/M 10 and rGO/M 15, respectively. The (002) crystal plane peak of Ti3C2Tx at 2θ ¼ 7.10� also are found from the XRD curve of rGO/M 15, indicating the presence of MXene layers. From the Raman spectra (Fig. 4(b)), the characteristic peaks of D and G bands for gra phene could be observed in composite fibers. With the addition of Ti3C2Tx, the G peak shifts slightly from 1584 cm 1 for rGO to 1579 cm 1, 1578 cm 1 and 1582 cm 1 for rGO/M 5, rGO/M 10 and rGO/M 15, respectively, which might be due to the interaction/reac tion of MXene on GO sheets. With the increasing content of Ti3C2Tx, the characteristic peaks of Ti3C2Tx at 151 cm 1 could be observed, which is attributed to the vibration of Ti and C atoms, respectively, affirming the incorporation of MXene nanosheets in the hybrids. Furthermore, the XPS surveys of purer GO and rGO/MXene hybrids confirm the presence of C, Ti, O and F elements (Fig. S9). With the increasing feedings of Ti3C2Tx, the atom content of Ti gradually in creases from 0.92% to 6.05% and 9.83% for rGO/M 5, rGO/M 10 and rGO/M 15, respectively. From the C 1s spectra of pure Ti3C2Tx film and rGO/M 10 (Fig. 4(c and d)), it is noticed that the intensities of C–C and C–O bonds increase in the hybrids due to the main component of graphene. The C–Ti-Tx signal for the rGO/M 10 further proves the successful incorporation of MXene and graphene. Ti 2p spectra of pure Ti3C2Tx film and rGO/M 10 are also studied (Fig. 4(e and f)). The weak intensity of TiO2 in pure Ti3C2Tx indicates little degree of oxidation of the pristine nanosheets. However, the intensity of TiO2 obviously in creases in rGO/MXene hybrids with the substantial intensity decrease of Ti suboxide and/or hydroxide. This implies the simultaneous oxidation process of partial MXene sheets with the functional groups of GO layers, i.e., the reduction of GO layer in the presence of MXene. Such an interaction between GO and MXene layers is believed to be essential for the continuous formation of fiber-like hybrids. Benefiting from the synergistic contribution from two types of nanosheets and the evenly distributed metallic conductivity of MXene sheets, the fiber-like rGO/MXene hybrids possess higher electrical con ductivity than that of pure rGO (Fig. S10(a)). The electrical conductivity of rGO/M 15 could reach a high value of 1339 S m 1, which is com parable to that of the reported rGO/MXene fibers being fabricated via wet-spinning with high loading of MXene sheets (>80 wt%). The in situ reduction of GO during the macro-assemble formation of rGO/MXene hybrid could not only facilitate the full recovery of graphene structures but also favor the good contact between rGO and MXene nanosheets, thus leading to good electric properties. In addition, 330% improvement of the tensile strength was obtained for rGO/M 5 (Fig. S11a), further reflecting the synergistic contribution from GO and MXene sheets. We also compare the specific capacitance and ultimate strength of rGO/M hybrid with the work reported by others (Fig. S11b), and it could be seen that the comprehensive performance of rGO/M hybrid is far higher than that of the carbon derived carbon (CDC) MWCNT composite fiber [40], CNF-PANI/NR elastomer electrode [41] and BC/Fe3O4@PPy film [42], and comparable to CNCs-CNTs/PVA fiber [43] and other graphene composites [44–46]. Cyclic voltammetry (CV) test, galvanostatic charge-discharge and frequency response measurements were carried out by a three-electrode system in 1 M H2SO4 aqueous solutions. The CV curves for various fibers at a scan rate of 10 mV s 1 (Fig. 5(a)) show relatively rectangular shape, suggesting a good electrochemical feature. The redox peak around 0.3 V might be attributed to the pseudo-active groups of O, F, Ti, et al., on the surface of graphene and MXene. The slightly higher integrated CV area of rGO/MXene than pure rGO implies the better electrochemical prop erties of hybrid fibers. The galvanostatic charge-discharge curves of obtained fiber-like hybrids with the current density of 0.1 A g 1 display relatively symmetrical triangles. Among them, rGO/M 5 demonstrates the largest gravimetric specific capacitance with the longest discharge time (Fig. 5(b)). The CV curves and charge-discharge lines of rGO/M 5
(Fig. 5(c and d)) could keep the rectangular and symmetrical triangular shapes with increasing scan rates and current densities, indicating its good capacitance behavior. The calculated gravimetric capacitance from charge-discharge curves at various current densities demonstrate that rGO/MXene hybrids show higher capacitance than that of pure rGO sample and rGO/M 5 could deliver a value of 195 F g 1 (Fig. S10(b)). As the evaluated density of each fiber is about 1.06, 1.77, 1.90, 2.04 g cm 3 for rGO, rGO/M 5, rGO/M 10, rGO/M 15, respec tively, the volumetric capacitances of various samples (Fig. 5(f)) show that rGO/M 5 could deliver the largest capacitance value of 345.2 F cm 3 at 0.1 A g 1, which is higher than the reported rGO or CNTs-based hybrid fibers [34,47–53] and comparable to that of rGO/MXene fiber fabricated by other methods [34,35] (Table S1). The impressive electrochemical performance of rGO/MXene could be attributed to the synergistic interactions of effective connections be tween two layers, which could alleviate the self-restacking phenomenon of each nanosheets, thus leading to a full utilization of two components. The smaller capacitance of rGO/M 15 fiber might be attributed to the restacking of MXene layers. In addition, no hybrid fiber could be formed when MXene loading is higher than 15 wt%, possibly due to the strong self-restacking force with small lamella size flakes for MXene and the lack of shearing forces in our system. Furthermore, a frequency response analysis in the frequency range from 100 to 0.01 kHz yields the Nyquist plots for rGO and rGO/MXene fiber-like hybrids (Fig. 5(e)) with an expanded view of high-frequency region. The curves are typically composed of a semi-circular arc in the high frequency region and a straight line in the low frequency region. It is shown that with the addition of Ti3C2Tx, the diameter of the semi-circular arc in the high frequency region decreases, suggesting the smaller charge transfer resistance, which will facilitate the internal charge transfer in the elec trode. The more vertical the line in the low frequency region with more Ti3C2Tx loadings indicates the smaller the diffusion resistance. This result is attributed to the high conductivity of Ti3C2Tx, which could be further confirmed by the improved conductivity of rGO/MXene hybrid fiber (Fig. S10(a)). The improved specific capacitance shows that appropriate amount of MXene sheets with homogenous dispersion between graphene layers could efficiently improve the electrochemical properties of fiber-like hybrids due to their synergistic effects, that is, GO guides the fiber for mation and the well dispersed MXene contributions to the electrical conductivity and the volumetric electrochemical performance of rGO/ MXene composites. Several unique characteristic of the rGO/MXene hybrids which make them a promising candidate as high-performance one-dimensional electrode materials are (Fig. 5(g)): (i) the incorpora tion of MXene thin layers within graphene fibers could prevent the restacking of graphene sheets, which increases the contact interface between the electrode and the electrolyte, thus greatly shortening the ion-diffusion pathway; (ii) the high conductivity of MXene sheets being embedded could form a long-term conductive and robust network, which highly facilitates the electron transfer inside the electrode.; (iii) the intrinsic high density of MXene could further improve the volu metric capacitance of graphene-based electrodes. However, with the excessive feedings of Ti3C2Tx sheets, unavoidable aggregation of Ti3C2Tx sheets leads to declined electrochemical properties. It is well known that the applications of Ti3C2Tx are largely hindered by its unstable chemical property (easily being oxidized), which leads to reduced conductivity and electrochemical performance. The incorpo ration of graphene and MXene sheets, on the other hand, is favorable to prevent the oxidization of MXene with the protection of graphene layers, especially for those with lower loadings of MXene sheets. In our work, the freshly made pure Ti3C2Tx film could reach a high gravimetic spe cific capacitance of 287.9 F g 1 with the volumetric specific capacitance of 575.8 F cm 3 at the current density of 0.5 A g 1 (Fig. S12). However, the value decreases by 18.5% after one month (Fig. S13a) due to the oxidization of Ti elements in MXene layers. On the other hand, the specific capacitance of rGO/M 10 only reduces by 5.0% at various 7
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Fig. 6. (a) Cycling life of rGO/M-5; (b) SEM image of the cross section for rGO/M after cycling test.
5 after cycling test; (c) Raman spectra and (d) C 1s XPS spectra of rGO/M
5
Fig. 7. (a) The gravimetric and volumetric capacitances of rGO/MXene fiber-like hybirds compared with other reported values; (b) the gravimetric energy density vs capacitance retention of rGO/M 5 compared with other reported values.
current densities (Fig. S13b), showing more stable feature than that of pure MXene. Therefore, the rGO/MXene hybrid can effectively prevent Ti3C2Tx from being completely exposed to air, thus slowing a much slower degradation rate and better electrochemical stability of the electrodes. Moreover, the cyclic stability of rGO/M 5 hybrid is further explored (Fig. 6(a)). After 7500 cycles at 0.5 A g 1, its capacitance reaches about 124.8% of the initial value and no deformation occurs, indicating that the as-prepared fiber-like hybrid has a good cyclic sta bility. With the increase of cycles, the wettability of the sample is improved and the migration rate of Hþ to the surface of electrode is enhanced. This ‘activation’ process results in the higher-than-initial capacitance value. The morphology of the sample after cycling test did not change, further confirming the structural stability of rGO/MXene hybrids (Fig. 6(b)). Raman spectra (Fig. 6(c)) also demonstrates similar
characteristic peak for Ti3C2Tx at low wave number after cyclic test. The C 1s spectrum (Fig. 6(d)) shows an increased intensity of C–Ti bond after cycling. This indicates that the two layer structures are well integrated with strong interactions, and C–Ti-Tx positively contributes to the cyclic stability of the samples. To coordinate the trade-off between gravimetric and volumetric capacitance of electrode materials is significant for practical applica tions, especially for carbon-based materials. It is exhibited that (Fig. 7 (a)).rGO/M 5 achieves superior performance than other graphene or CNTs based electrodes [33,49,53–57], nearly rivaling those of rGO/M Xene fiber electrode with large loading of MXene (~95%) by wet spin ning technique [34,35]. The calculated energy density and power density of rGO/M 5 is calculated as 30.7 mWh cm 3 and 70.7 mW cm 3, respectively, at a current density of 0.1 A g 1. In addi tion, compared to the literature reports, rGO/M 5 demonstrates a 8
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favorable energy density and excellent cycling stability. All the above results suggest that our fiber-like hybrids could be promising flexible electrode materials for supercapacitors.
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4. Conclusions In summary, a facile one-step self-assembled hydrothermal strategy was designed to fabricate flexible and fiber-like rGO/MXene hybrid electrodes for supercapacitors. Thanks to the synergistic interactions between graphene and MXene nanosheets, the intercalated nano structures could effectively combine the high electrical conductivity and the pseudocapaitance from MXene, thus greatly improving the electro chemical properties of the electrodes. With only 5 wt% loading of MXene, the fiber-like hybrid electrode could reach a volumetric capac itance of 345.2 F cm 3 with the gravimetric value of 195 F g 1at 0.1A g 1,higher than that of other reported graphene or CNTs based elec trodes and comparable to those of rGO/MXene fiber electrode with large loading of MXene. It also shows a favorable cycling stability of the capacitance retention of 124.8%after 7500 cycles at a current density of 0.5A g 1.Moreover, the oxidation process of MXene could be suppressed by the protection of graphene in the hybrids. This work represents a general and effective strategy for the development of high-performance hybrid electrode matterials for supercapacitors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the Program of Shanghai Academic Research Leader (18XD1400100), Natural Science Founda tion of Shanghai (18ZR1400600), the National Natural Science Foun dation of China (No. 21774019), DHU Distinguished Young Professor Program, the Special Excellent PhD International Visit Program and Graduate Student Innovation Fund of Donghua University (CUSF-DH-D2018008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227398. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile fabrication of flexible rGO/MXene hybrid fiber-like electrode with high volumetric capacitance Zhe Wang a, Yaoyan Chen a, Mengyao Yao a, Jie Dong a, Qinghua Zhang a, Lili Zhang b, **, Xin Zhao a, * a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island, 627833, Singapore
b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A facile confined hydrothermal strategy is developed to fabricate rGO/MXene hybrids. � rGO/MXene hybrids show robust gra phene skeleton with MXene filling the microvoids. � Hybrid with low MXene content exhibits a large volumetric capacitance of 345 F cm 3. � The rGO flakes could well suppress the oxidation process of MXene.
A R T I C L E I N F O
A B S T R A C T
Keywords: rGO MXene Fiber Flexible Supercapacitors
Graphene fibers or fiber-like graphene materials have been successfully adopted as electrodes for flexible and wearable supercapacitors. However, they normally exhibit relatively low volumetric capacitance, which will seriously hinder their wide applications in smart textile as power supply. Here, a facile confined hydrothermal strategy to fabricate fiber-like reduced graphene oxide (rGO)/MXene hybrids is designed through ingenious assemble of two functional two dimensional materials of graphene and MXene nanosheets. Owing to the syn ergistic interactions between two layers, the hybrid rGO/M 5 exhibits a large volumetric capacitance of 345 F cm 3 with the gravimetric capacitance of 195 F g 1 at 0.1 A g 1 with only 5 wt% MXene layers. A volu metric energy density of 30.7 mWh cm 3 with power density of 70.7 mW cm 3 could be delivered. After 7500 cycles at a current density of 0.5 A g 1, about 125% of its initial capacitance value is retained, demonstrating excellent electrochemical stability. More importantly, due to the suppressed oxidation process of MXene sheets in the hybrid, rGO/MXene could retain 95% of its original capacitance after one month.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Zhang), [email protected] (X. Zhao). https://doi.org/10.1016/j.jpowsour.2019.227398 Received 17 August 2019; Received in revised form 28 October 2019; Accepted 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhe Wang, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227398
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1. Introduction
nanosheets were simultaneously achieved during the hydrothermal process, forming hybrid materials with a robust skeleton of large gra phene sheets and small MXene sheets filling the spaces and microvoids. With lower loading of MXene about 5 wt%~15 wt%, the fiber-like hybrid electrodes could achieve a high electrical conductivity of 1339 S m 1 and a large volumetric capacitance of 345.2 F cm 3 with the gravimetric value of 195 F g 1. Moreover, there was still 125% capaci tance retention after 7500 cycles at a current density of 0.5 A g 1, illustrating its great potential as electrode for flexible supercapacitors. The investigation also revealed that the graphene nanosheets could act as barrier or protection layers to prevent the oxidation of MXene in the hybrids.
Recent advantages of flexible and wearable supercapacitors have motivated the increasing demands for compatible energy resources that can be integrated into fabric for powering electronic devices [1–3]. Due to the merits of potential compatibility with textile processing, i.e. weaving or knitting, fiber-like electrode materials have gained tremen dous attentions as alternatives to traditional forms of bulky or rigid materials [4,5]. To date, carbonaceous fibers, such as carbon fibers [6], CNTs [7], graphene [8], have been favorably adopted as fiber-like electrode ma terials for wearable devices. Notably, graphene-based fibers, which have been successfully fabricated via various spinning and assembly tech niques from graphene oxide liquid crystalline [9–13], have shown great promise owing to their remarkable mechanical, electrical and electro chemical properties. However, the relatively low conductivity and strong π-π interactions negatively affect the charge transport and elec trolyte access. The limited electric double layer capacitance of pure graphene-based material hinders the energy density of the device. As a result, graphene-based fibers normally exhibit moderate energy storage performance with low volumetric energy density and power density [14]. To achieve high volumetric energy storage performance, many efforts have been made including the integration of graphene and pseudocapacitive materials with high electrochemical activity into fiber shape, such as such as CNTs [15], transition metal oxides [16] and conducting polymers [17], or the decoration of electrochemically active materials on the outer surface of graphene fibers [18]. Although graphene-based fiber-like electrodes has been explored with great progress, a favorable assembly method in a well-controlled way and the optimal combination with guest nanomaterials still need to be further pursued. As a new member of two-dimensional materials, transition metal carbides and nitrides (MXene) have shown outstanding performances in electrochemical energy storage [19,20] and many other applications [21–24], due to its excellent electrochemical properties, metallic con ductivity, high density and surface hydrophilicity [25,26]. With a gen eral formula of Mnþ1XnTx, where M is an early transition metal, X represents C and/or N, Tx denotes surface functional groups, and n ¼ 1, 2, or 3, MXene are generally synthesized by selectively etching of the “A” element layer (group III A or IV A) from their parent MXA phase, following by an intercalation-assisted delamination process to yield single or few layer nanosheets [27–29]. With unique metallic conduc tivity as high as 105 S cm 1 [30] and excellent high volumetric capaci tance of 1500 F cm 3 [31], MXene nanosheets have exhibited great potential as good fillers to improve the volumetric performance of film and fiber-like graphene electrodes. For example, Li H et al. [32] ob tained MXene/graphene flexible film by vacuum filtration, and assem bled a flexible all-solid supercapacitor with a volumetric capacitance of 216 F cm 3. Yan J et al. [33] prepared composite films by electrostatic self-assembly of rGO and MXene, which not only achieved good elec trical conductivity (2261 S cm 1), but also achieved volumetric capaci tance of 1040 F cm 3. Gao et al. [34] and Razai et al. [35] fabricated graphene/MXene composite fiber by wet spinning from graphene oxide (GO)/MXene solutions. After the reduction of GO, hybrid fibers with the highest MXene mass ratio about 80%–95% was obtained which achieved good performance for flexible supercapacitors. Although hybrid fibers with large loadings of MXene or MXene as the host material could indeed benefit the volumetric energy storage performance, the inherent chemical instability of MXene requires a very stringent environment during the fabrication and applications of such hybrids [36]. Therefore, an effective approach to fabricate graphene/MXene hybrid fibers by using low loading of MXene to achieve high volumetric performance is still challenging. Herein, we designed a facile method to fabricate fiber-like rGO/ MXene hybrid electrodes by one-step hydrothermal strategy. The reduction of GO and the ingenious macro-assembly of GO with MXene
2. Experimental 2.1. Materials Lithium fluoride (LiF) was purchased from Alfa Aesar (97%); hy drochloric acid (HCl, 36%~38%) and vitamin C (VC, 99%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Ti3AlC2 (75–79 wt%) was purchased from Carbon-Ukraine, Ltd. Multilayer graphene oxide (GO, 95%) was obtained from Suzhou Carbon Technology Co., Ltd. All other reagents were used as received without any purification. 2.2. Fabrication of Ti3C2Txnanosheets The preparation of Ti3C2Tx nanosheets were synthesized similarly to a previous report [37]. Typically, 0.8 g LiF was completely dissolved in 10 mL 9 M HCl solution, followed by the slow addition of 0.5 g Ti3AlC2 powders with the lateral size less than 38 μm. The reaction was kept at 35 � C for about 24 h under magnetic stirring. After the etching reaction, the sediments were washed with deionized water until the PH value of supernatant higher than 6. Then the products were freeze dried to obtain multilayer Ti3C2Tx powders. Next, the multilayer Ti3C2Tx powders were again mixed with 160 mL deionized water under the protection of argon and then treated by sonication in an ice water bath for 1 h. Afterwards, the dispersion was centrifuged at 3500 r min 1 for 1 h to get a stable colloidal solution composed of few layer of Ti3C2Tx. In order to prevent the rapid oxidation of Ti3C2Tx nanoflakes, a certain amount of the dispersion was vacuum filtered to obtain MXene films for storage. 2.3. Preparation of GO dispersion 45 mg multilayer GO powder was dispersed in 30 mL deionized water and the monolayer GO dispersion was obtained by sonication for 0.5 h, 1 h, 1.5 h and 2 h, respectively, and the obtained samples were denoted as GO-0.5, GO-1, GO-1.5 and GO-2. 2.4. Fabrication of rGO/MXene fiber-like hybrids According to the previous method for preparing rGO fiber-like ma terial [38], a certain amount of MXene film was added into GO disper sion to get homogeneous mixed suspension under sonication. Then VC powder was added as reduction agents with the mass ratio with GO of 5:1. After fully stirring, the mixture was injected into a polypropylene (PP) tube with a length of 20 cm and a diameter of 8 mm. With sealing the two ends, the tube was placed in water bath at 80 � C for 1 h. Then rGO/MXene hybrid fiber could be obtained by opening both ends of the tube and drying at 60 � C for 12 h in a vacuum oven. The feeding ratio of MXene in hybrid fiber was 5 wt%, 10 wt% and 15 wt% and the obtained samples were designated as rGO/M 5, rGO/M 10 and rGO/M 15, respectively. 2.5. General characterizations The surface morphologies and nanostructures of as-prepared samples 2
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Fig. 1. Schematic of the fabrication procedure of rGO/MXene fiber-like hybrids.
were characterized by field-emission scanning electron microscope (FESEM, HITACHI S-4800) and transmission electron microscopic (TEM, JEM 2100F). The size distribution of Ti3C2 sheet was analyzed by Nano ZS nanoparticle size analyzer. The structures of obtained produces were studied by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscope (XPS). XRD patterns were carried out on a RIGAKU D/Max2550 diffractometer (Copper Kα radiation with λ ¼ 0.1514 nm). Raman spectroscopy was conducted on a HORIBA JobinYvon LabRarn-1B (He–Ne laser with k ¼ 632.8 nm). And X-ray photoelectron spectro scope (XPS) were applied on Escalab 250xi X-ray photoelectron spec trometer with Al Kα radiation.
2.6. Electrochemical measurements Three-electrode system was employed to measure the electro chemical performances of rGO/MXene fiber-like hybrids. The electrode was tested in 1 M H2SO4 aqueous solution with the platinum mesh as the counter electrode and Ag/AgCl electrode as the reference electrode, respectively. The cyclic voltammetry (CV) and galvanostatic chargedischarge curves were tested on an Auto-lab PGSTAT302 N electro chemical working station, with an applied potential window ranging from 0.2 to 0.6 V. Electrochemical impedance spectroscopy (EIS) was tested in the frequencies between 0.01 Hz and 100 kHz, with a pertur bation amplitude of 10 mV. The cyclic stability test was carried out on LAND test system. The gravimetric value can be calculated from the
Fig. 2. (a) SEM image of multi-layerTi3C2Tx; (b) TEM image of monolayer Ti3C2Tx; (c–d) XRD pattern and Raman spectra of Ti3C2Tx; (e) SEM image of GO nanosheet obtained by sonication for 2 h; (f) SEM image of the surface for pure rGOspecimen; (g) Digital image of rGO specimen forming in PP tube; (h) Digital image of rGO specimen woven in knitted glove. 3
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Fig. 3. SEM images of the surface for (a) rGO/M C, Ti and F.
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5, (b)rGO/M
10 and (c)rGO/M-15; (d) SEM image of the cross section for rGO/M
galvanostatic charge-discharge curves by using the followed equation: Cm¼I⋅Δt/(m⋅ΔV), where Cm is the mass-specific capacitance (F g 1), I is the current (A), Δt is the discharge time, m is the mass (g) of the active materials, and ΔV is the potential window (V). The volumetric value can be calculated from the galvanostatic charge-discharge curves by using the followed equation: Cv¼I⋅Δt/(V⋅ΔV), where Cv is the volumetric capacitance (F cm 3), I is the current (A), Δt is the discharge time, and V is the volume (cm 3) of the active materials, and ΔV is the potential window (V). The volumetric energy density and power density of the rGO/M-X (X ¼ 5, 10 and 15) could be calculated by the follow equations: Ev¼1/2⋅Cv⋅ΔV2 and Pv¼Ev/Δt, respectively, where Ev is the volumetric energy density (mWh cm 3), Cv is the volumetric capacitance (F cm 3), ΔV is the potential window (V), Pv is the volumetric power density (mW cm 3), andΔt is the discharge time.
5 with the EDS mapping of
layers exfoliated from each other. Transmission electron microscopy (TEM) image of the colloidal suspension after sonication (Fig. 2(b)) confirms the synthesis of delaminated MXene nanosheets. It displays an ultrathin sheet with the size about 0.5–1 μm and clear lattice morphology could be noted from the image with high resolution. Further analyses of the crystal structure are performed by X-ray diffraction (XRD) of pristine Ti3AlC2 and Ti3C2Tx (Fig. 2(c)). It could be seen that the diffraction peak at 39� for the (104) planes of Ti3AlC2 is absent in the XRD pattern of Ti3C2Tx, suggesting the near complete removal of Al layers by etching. In addition, the (002) peak of Ti3C2Tx has shifted to 5.7� with the interplanar spacing of 15.5 Å (9.6� for Ti3AlC2), indicating an enlarged interlayer spacing of ~0.7 nm (~0.5 to for Ti3AlC2). According to Raman spectrum (Fig. 2(d)), the character istic signals of A1g and Eg appear for few-layer Ti3C2Tx nanosheets, which could be ascribed to the symmetric out-of-plane vibration of Ti, C atoms and the in-plane shear vibration of Ti, C and terminal atoms, respectively [35]. Moreover, the size of the monolayer Ti3C2Tx ranges from 600 nm to 1.5 μm from the nanoparticle size analyzer (Fig. S1c), which is agree with the observation from TEM images. The chemical composition and oxidization states of the as-prepared Ti3C2Tx nanp sheets are also studied by X-ray photoelectron spectroscopy (XPS) (Fig. S1d), which suggests the presence of surface-anchored -(OH)x and -Fx groups, rendering the Ti3C2Tx nanosheets highly hydrophilic. Full characterization data confirm the successful etching and exfoliation of the MXA phase to MXene nanosheets. As the neat exfoliated Ti3C2Tx nanosheets can not be readily assembled into continuous fibers due to the weak interlaminar in teractions between MXene flakes with relatively small lateral sizes, large GO sheets were introduced as the scaffold to aid the formation of MXenebased fibers. However, the one-dimensional hybrid materials with higher MXene loading than 15 wt% is hard to be realized, mainly due to the lack of shearing forces during the static hydrothermal process, which is different from the extruding produre of wet spinning technique. Before the combination with MXene, the self-assmeble behaviours of exfoliated monolayer GO nanosheets have been investigated. SEM im ages of different samples with various sonication durations of GO showed that an appropriate treatment time is beneficial for the exfoli ation of large GO layers in solution and could get much thinner nano sheets with more local wrinkles (Fig. S2). As a result, integral fiber-like samples of GO-0.5 and GO-1 are difficult to be formed due to weak electrostatic interactions between unexfoliated multilayer GO sheets
3. Results and discussion The well-explored Ti3C2Tx was used here as a representative example of 2D layered MXene because it has been found to possess the highest electrical conductivity and volumetric capacitance among the MXene types synthesized to date [39]. The rGO/MXene hybrids were fabricated by a one-pot facile hydrothermal process as illustrated in Fig. 1. Spe cifically, multilayer Ti3C2Tx powders were first prepared by selective etching of the Al layers in bulk Ti3AlC2 MAX phase with moderate re gents of LiF and HCl, which were then exfoliated by sonication to give a colloidal suspension of isolated MXene nanoflakes. Then homogeneous MXene/GO mixture with VC as reducing agent was injected into poly propylene (PP) tube and kept under a mild condition in water bath at 80 � C for 1 h. Owing to the similar structural characteristics which could make it facile to match and fuse with each other to form strong in teractions, the continuous fiber-like rGO/MXene hybrids were obtained via self-assembly of ultrathin nanosheets with simultaneous reduction of GO. This process can be scaled up by simply increasing the length of the reaction vessel. To confirm the successful fabrication of delaminated MXene nano sheets, the morphologies and structures of Ti3C2Tx products during etching and exfoliating process are investigated. It is well known that the pristine MAX phase of Ti3AlC2 demonstrates a closely overlapped lamellar layer structure with a reminiscent of graphite (Figs. S1a–b). After etching with LiF and HCl, the scanning electron microscopy (SEM) image (Fig. 2(a)) shows loosely packed multilayer Ti3C2Tx with the 4
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Fig. 4. (a) XRD patterns of rGO, Ti3C2Tx film and rGO/MXene hybrids with different Ti3C2Tx content; (b) Raman spectra of rGO fiber-like sample, Ti3C2Tx film and various rGO/MXene hybrids; (c–d) C 1s XPS spectra of Ti3C2Tx film and rGO/M-10; (e–f) Ti 2p XPS spectra of Ti3C2Tx film and rGO/M-10.
(Fig. S3). Although GO-1.5 could be shaped, it is fragile and hard to be handled (Fig. S4). An excessive sonication force is also unfavorable to obtain exfoliated GO nanosheets with large size, thereby affecting the self-assembly during hydrothermal process (Fig. S5). Here, an appro priate sonication time of 2 h for GO sheets is optimized. Under this condition, the obtained exfoliated GO nanosheets display very thin layers with abundant wrinkles on the surface and very large size of 10–15 μm (Fig. 2(e)). The pure rGO fiber-like product exhibits a crumple surface with the folds along the axial array (Fig. 2(f)). The cross section (Fig. S6) reveals loosely packed layers with many macro/micro voids among them. During the hydrothermal treatment, the graphene sheet is shrink-assembled by the π-π interactions between the layers along with the reduction of GO, and then completely separated from the aqueous phase to form fiber-like specimen (Fig. 2(g)). Although the aspect ratio of the product is much lower than the commercial fiber, it is expected to be improved by simply increasing the length and decreasing the diam eter of the pipe. The obtained pure rGO sample is robust with good flexibility and could also be knitted into knitted gloves (Fig. 2(h)), further demonstrating its fiber-like features. Subsequently, the rGO/ MXene hybrid fiber-like hybrids were prepared by combining Ti3C2Tx with GO solution obtained by sonication for 2 h and treated under the
same conditions. As expected, the rGO/MXene fiber-like hybrids containing 5%, 10% and 15% of Ti3C2Tx still possess good flexibility similar to that of the pristine rGO sample (Fig. S7). The fixed solution of GO nanosheets could lead to a relatively stable GO skeleton and the similar lamellar features of GO and MXene are favorable to assemble with each other via strong π-π interactions. The surface morphologies of obtained hybrids show that (Fig. 3(a–c)) the hybrids display much rougher surface with disor dered wrinkles than that of pure rGO, indicating the incorporation of Ti3C2Tx layers. Meanwhile, the surface roughness of rGO/MXene hy brids increases with the increased mass of Ti3C2Tx, possibly due to the large size discrepancies between Ti3C2Tx and GO layers. Besides, the cross section of several hybrids demonstrate more compact features with smaller interlaminar gap (Fig. S8), indicating the interlayer interactions of two different sheets with more Ti3C2Tx effectively filling the voids between the graphene sheets. From the EDS-mapping of rGO/MXene hybrids (Fig. 3(d)), the uniform distribution of Ti and F elements further confirms the presence of Ti3C2Tx layers which are wrapped in graphene nanosheets. X-ray diffraction (XRD) and Raman spectra are further conducted to explore the structure of the as-prepared rGO/MXene hybrids. The XRD 5
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Fig. 5. (a) CV curves at a scan rate of 10 mV s 1and (b)galvanostatic charge-discharge curves of a current density of 0.1 A g 1for several samples; (c) CV curves at various scan rates and (d) galvanostatic charge-discharge curvesat various current densities for rGO/M-5; (e) Nyquist plot of vairous samples in the frequency range of 100 to 0.01 kHz; (f) Rate properties of rGO/MXene hybrids; (g) Schematic diagram of Hþ flow during electrochemical reaction.
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curves show (Fig. 4(a)) that the addition of Ti3C2Tx enlarges the halfpeak width of rGO around 2θ ¼ 10.94� , and the interlayer spaces of 8.08 Å for rGO sample change to 8.60 Å, 8.60 Å, and 8.53 Å for rGO/ M 5, rGO/M 10 and rGO/M 15, respectively. The (002) crystal plane peak of Ti3C2Tx at 2θ ¼ 7.10� also are found from the XRD curve of rGO/M 15, indicating the presence of MXene layers. From the Raman spectra (Fig. 4(b)), the characteristic peaks of D and G bands for gra phene could be observed in composite fibers. With the addition of Ti3C2Tx, the G peak shifts slightly from 1584 cm 1 for rGO to 1579 cm 1, 1578 cm 1 and 1582 cm 1 for rGO/M 5, rGO/M 10 and rGO/M 15, respectively, which might be due to the interaction/reac tion of MXene on GO sheets. With the increasing content of Ti3C2Tx, the characteristic peaks of Ti3C2Tx at 151 cm 1 could be observed, which is attributed to the vibration of Ti and C atoms, respectively, affirming the incorporation of MXene nanosheets in the hybrids. Furthermore, the XPS surveys of purer GO and rGO/MXene hybrids confirm the presence of C, Ti, O and F elements (Fig. S9). With the increasing feedings of Ti3C2Tx, the atom content of Ti gradually in creases from 0.92% to 6.05% and 9.83% for rGO/M 5, rGO/M 10 and rGO/M 15, respectively. From the C 1s spectra of pure Ti3C2Tx film and rGO/M 10 (Fig. 4(c and d)), it is noticed that the intensities of C–C and C–O bonds increase in the hybrids due to the main component of graphene. The C–Ti-Tx signal for the rGO/M 10 further proves the successful incorporation of MXene and graphene. Ti 2p spectra of pure Ti3C2Tx film and rGO/M 10 are also studied (Fig. 4(e and f)). The weak intensity of TiO2 in pure Ti3C2Tx indicates little degree of oxidation of the pristine nanosheets. However, the intensity of TiO2 obviously in creases in rGO/MXene hybrids with the substantial intensity decrease of Ti suboxide and/or hydroxide. This implies the simultaneous oxidation process of partial MXene sheets with the functional groups of GO layers, i.e., the reduction of GO layer in the presence of MXene. Such an interaction between GO and MXene layers is believed to be essential for the continuous formation of fiber-like hybrids. Benefiting from the synergistic contribution from two types of nanosheets and the evenly distributed metallic conductivity of MXene sheets, the fiber-like rGO/MXene hybrids possess higher electrical con ductivity than that of pure rGO (Fig. S10(a)). The electrical conductivity of rGO/M 15 could reach a high value of 1339 S m 1, which is com parable to that of the reported rGO/MXene fibers being fabricated via wet-spinning with high loading of MXene sheets (>80 wt%). The in situ reduction of GO during the macro-assemble formation of rGO/MXene hybrid could not only facilitate the full recovery of graphene structures but also favor the good contact between rGO and MXene nanosheets, thus leading to good electric properties. In addition, 330% improvement of the tensile strength was obtained for rGO/M 5 (Fig. S11a), further reflecting the synergistic contribution from GO and MXene sheets. We also compare the specific capacitance and ultimate strength of rGO/M hybrid with the work reported by others (Fig. S11b), and it could be seen that the comprehensive performance of rGO/M hybrid is far higher than that of the carbon derived carbon (CDC) MWCNT composite fiber [40], CNF-PANI/NR elastomer electrode [41] and BC/Fe3O4@PPy film [42], and comparable to CNCs-CNTs/PVA fiber [43] and other graphene composites [44–46]. Cyclic voltammetry (CV) test, galvanostatic charge-discharge and frequency response measurements were carried out by a three-electrode system in 1 M H2SO4 aqueous solutions. The CV curves for various fibers at a scan rate of 10 mV s 1 (Fig. 5(a)) show relatively rectangular shape, suggesting a good electrochemical feature. The redox peak around 0.3 V might be attributed to the pseudo-active groups of O, F, Ti, et al., on the surface of graphene and MXene. The slightly higher integrated CV area of rGO/MXene than pure rGO implies the better electrochemical prop erties of hybrid fibers. The galvanostatic charge-discharge curves of obtained fiber-like hybrids with the current density of 0.1 A g 1 display relatively symmetrical triangles. Among them, rGO/M 5 demonstrates the largest gravimetric specific capacitance with the longest discharge time (Fig. 5(b)). The CV curves and charge-discharge lines of rGO/M 5
(Fig. 5(c and d)) could keep the rectangular and symmetrical triangular shapes with increasing scan rates and current densities, indicating its good capacitance behavior. The calculated gravimetric capacitance from charge-discharge curves at various current densities demonstrate that rGO/MXene hybrids show higher capacitance than that of pure rGO sample and rGO/M 5 could deliver a value of 195 F g 1 (Fig. S10(b)). As the evaluated density of each fiber is about 1.06, 1.77, 1.90, 2.04 g cm 3 for rGO, rGO/M 5, rGO/M 10, rGO/M 15, respec tively, the volumetric capacitances of various samples (Fig. 5(f)) show that rGO/M 5 could deliver the largest capacitance value of 345.2 F cm 3 at 0.1 A g 1, which is higher than the reported rGO or CNTs-based hybrid fibers [34,47–53] and comparable to that of rGO/MXene fiber fabricated by other methods [34,35] (Table S1). The impressive electrochemical performance of rGO/MXene could be attributed to the synergistic interactions of effective connections be tween two layers, which could alleviate the self-restacking phenomenon of each nanosheets, thus leading to a full utilization of two components. The smaller capacitance of rGO/M 15 fiber might be attributed to the restacking of MXene layers. In addition, no hybrid fiber could be formed when MXene loading is higher than 15 wt%, possibly due to the strong self-restacking force with small lamella size flakes for MXene and the lack of shearing forces in our system. Furthermore, a frequency response analysis in the frequency range from 100 to 0.01 kHz yields the Nyquist plots for rGO and rGO/MXene fiber-like hybrids (Fig. 5(e)) with an expanded view of high-frequency region. The curves are typically composed of a semi-circular arc in the high frequency region and a straight line in the low frequency region. It is shown that with the addition of Ti3C2Tx, the diameter of the semi-circular arc in the high frequency region decreases, suggesting the smaller charge transfer resistance, which will facilitate the internal charge transfer in the elec trode. The more vertical the line in the low frequency region with more Ti3C2Tx loadings indicates the smaller the diffusion resistance. This result is attributed to the high conductivity of Ti3C2Tx, which could be further confirmed by the improved conductivity of rGO/MXene hybrid fiber (Fig. S10(a)). The improved specific capacitance shows that appropriate amount of MXene sheets with homogenous dispersion between graphene layers could efficiently improve the electrochemical properties of fiber-like hybrids due to their synergistic effects, that is, GO guides the fiber for mation and the well dispersed MXene contributions to the electrical conductivity and the volumetric electrochemical performance of rGO/ MXene composites. Several unique characteristic of the rGO/MXene hybrids which make them a promising candidate as high-performance one-dimensional electrode materials are (Fig. 5(g)): (i) the incorpora tion of MXene thin layers within graphene fibers could prevent the restacking of graphene sheets, which increases the contact interface between the electrode and the electrolyte, thus greatly shortening the ion-diffusion pathway; (ii) the high conductivity of MXene sheets being embedded could form a long-term conductive and robust network, which highly facilitates the electron transfer inside the electrode.; (iii) the intrinsic high density of MXene could further improve the volu metric capacitance of graphene-based electrodes. However, with the excessive feedings of Ti3C2Tx sheets, unavoidable aggregation of Ti3C2Tx sheets leads to declined electrochemical properties. It is well known that the applications of Ti3C2Tx are largely hindered by its unstable chemical property (easily being oxidized), which leads to reduced conductivity and electrochemical performance. The incorpo ration of graphene and MXene sheets, on the other hand, is favorable to prevent the oxidization of MXene with the protection of graphene layers, especially for those with lower loadings of MXene sheets. In our work, the freshly made pure Ti3C2Tx film could reach a high gravimetic spe cific capacitance of 287.9 F g 1 with the volumetric specific capacitance of 575.8 F cm 3 at the current density of 0.5 A g 1 (Fig. S12). However, the value decreases by 18.5% after one month (Fig. S13a) due to the oxidization of Ti elements in MXene layers. On the other hand, the specific capacitance of rGO/M 10 only reduces by 5.0% at various 7
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Fig. 6. (a) Cycling life of rGO/M-5; (b) SEM image of the cross section for rGO/M after cycling test.
5 after cycling test; (c) Raman spectra and (d) C 1s XPS spectra of rGO/M
5
Fig. 7. (a) The gravimetric and volumetric capacitances of rGO/MXene fiber-like hybirds compared with other reported values; (b) the gravimetric energy density vs capacitance retention of rGO/M 5 compared with other reported values.
current densities (Fig. S13b), showing more stable feature than that of pure MXene. Therefore, the rGO/MXene hybrid can effectively prevent Ti3C2Tx from being completely exposed to air, thus slowing a much slower degradation rate and better electrochemical stability of the electrodes. Moreover, the cyclic stability of rGO/M 5 hybrid is further explored (Fig. 6(a)). After 7500 cycles at 0.5 A g 1, its capacitance reaches about 124.8% of the initial value and no deformation occurs, indicating that the as-prepared fiber-like hybrid has a good cyclic sta bility. With the increase of cycles, the wettability of the sample is improved and the migration rate of Hþ to the surface of electrode is enhanced. This ‘activation’ process results in the higher-than-initial capacitance value. The morphology of the sample after cycling test did not change, further confirming the structural stability of rGO/MXene hybrids (Fig. 6(b)). Raman spectra (Fig. 6(c)) also demonstrates similar
characteristic peak for Ti3C2Tx at low wave number after cyclic test. The C 1s spectrum (Fig. 6(d)) shows an increased intensity of C–Ti bond after cycling. This indicates that the two layer structures are well integrated with strong interactions, and C–Ti-Tx positively contributes to the cyclic stability of the samples. To coordinate the trade-off between gravimetric and volumetric capacitance of electrode materials is significant for practical applica tions, especially for carbon-based materials. It is exhibited that (Fig. 7 (a)).rGO/M 5 achieves superior performance than other graphene or CNTs based electrodes [33,49,53–57], nearly rivaling those of rGO/M Xene fiber electrode with large loading of MXene (~95%) by wet spin ning technique [34,35]. The calculated energy density and power density of rGO/M 5 is calculated as 30.7 mWh cm 3 and 70.7 mW cm 3, respectively, at a current density of 0.1 A g 1. In addi tion, compared to the literature reports, rGO/M 5 demonstrates a 8
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favorable energy density and excellent cycling stability. All the above results suggest that our fiber-like hybrids could be promising flexible electrode materials for supercapacitors.
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4. Conclusions In summary, a facile one-step self-assembled hydrothermal strategy was designed to fabricate flexible and fiber-like rGO/MXene hybrid electrodes for supercapacitors. Thanks to the synergistic interactions between graphene and MXene nanosheets, the intercalated nano structures could effectively combine the high electrical conductivity and the pseudocapaitance from MXene, thus greatly improving the electro chemical properties of the electrodes. With only 5 wt% loading of MXene, the fiber-like hybrid electrode could reach a volumetric capac itance of 345.2 F cm 3 with the gravimetric value of 195 F g 1at 0.1A g 1,higher than that of other reported graphene or CNTs based elec trodes and comparable to those of rGO/MXene fiber electrode with large loading of MXene. It also shows a favorable cycling stability of the capacitance retention of 124.8%after 7500 cycles at a current density of 0.5A g 1.Moreover, the oxidation process of MXene could be suppressed by the protection of graphene in the hybrids. This work represents a general and effective strategy for the development of high-performance hybrid electrode matterials for supercapacitors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the Program of Shanghai Academic Research Leader (18XD1400100), Natural Science Founda tion of Shanghai (18ZR1400600), the National Natural Science Foun dation of China (No. 21774019), DHU Distinguished Young Professor Program, the Special Excellent PhD International Visit Program and Graduate Student Innovation Fund of Donghua University (CUSF-DH-D2018008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227398. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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