Constructing expanded ion transport channels in flexible MXene film for pseudocapacitive energy storage

Applied Surface Science 511 (2020) 145627

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Constructing expanded ion transport channels in flexible MXene film for pseudocapacitive energy storage Feitian Rana,1, Tianlin Wanga,1, Siyu Chenb, Yuyan Liua, Lu Shaoa,

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MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, PR China b Engineering Department, Lancaster University, Lancaster LA1 4YW, UK

A R T I C LE I N FO

A B S T R A C T

Keywords: MXene film Freeze-drying Specific capacitance Symmetric supercapacitor

2D transition metal carbide (MXene) based flexible films have received increasing attention in the application of energy storage for portable and wearable electronic devices. However, the self-restacking caused by Van der Waals interactions between the layers lead to insufficient access to allow the electrolyte ions to contact the active material. Herein, we rationally utilized the freeze-drying technique to prepare MXene film electrode for alleviating its self-restacking and improving the electrochemical performances. Through freeze-drying treatment, the frozen solvent molecules were removed by sublimation, and alleviating the adverse effect of van der Waals forces and contributing to the enlarged layer space. The constructed freeze-dried MXene (f-MXene) films generate unique porous architecture, which can provide highly efficient ion diffusion and transport channels. Consequently, the typical f-MXene-10 film exhibit good specific capacitance (341.5 F g−1/922.1 F cm−3 at 1 A g−1) and desired capacitance retention of 60.4% from 1 to 10 A g−1. Moreover, the assembled asymmetric device delivered a maximal gravimetric energy density of 6.1 Wh Kg−1, volumetric energy density of 16.4 Wh L−1 and a good capacitance retention of 89.3% after 10,000 cycles. The rationally designed synthesizing route has attractive promise to promote MXene materials for diverse energy and environmental applications.

1. Introduction Continuous growing demands of rapid electrical energy storage and delivery for mobile electronics and electric vehicles have stimulated huge interest in design and fabrication of state-of-the-art electrochemical energy storage devices [1–5]. As a typical representative, supercapacitor has developed as an extremely important energy storage devices due to their fast charge-discharge capability, outstanding power density and cycling stability [6–9]. Nevertheless, the disadvantage of lower energy density greatly limits its application in energy storage [10–12]. To overcome this drawback, considerable efforts have been focused on the exploitation of electrode materials. Among them, two dimensional (2D) nanomaterials have become exceptionally promising candidate materials for manufacturing supercapacitors [13,14], especially flexible device, owing to their inherent flexibility and favorable electrochemical activities. Recently, MXenes, an emerging family of 2D transition metal carbides and nitrides, have shown attractive prospect for the application of

energy storage field on account of their high conductivity and reversible surface redox reactions [15–17]. Generally, MXenes have been prepared by selectively etching the layer of A from corresponding MAX phase, where A is mainly group-13 or group-14 elements [18,19]. Traditional MXene can be expressed by the formula denoted as Mn +1XnTx (n = 1–3), where M represents an early transition metal (such as Sc, Ti, Zr, V, etc.), X represents carbon and/or nitrogen, and T generally represents the terminal groups (O, OH, and F) deriving from the HF in situ etching [20,21]. Recently, numerous strategies have been developed to enrich the family of MXene. For instance, Zhou et al. employed layered ternary Zr3Al3C5, quaternary Hf3[Al(Si)]4C6 and ternary ScAl3C3, which are non-MAX-phase precursors, to synthesize several novel MXene (Zr3C2Tz, Hf3C2Tz and ScCxOH) through selective etching approaches. [22–24] Li et al. synthesized Cl-terminated MXenes (Ti3C2Cl2 and Ti2CCl2) by A-site element replacement in between the MAX phase and ZnCl2 molten salts. [25]. Owing to 2D layered structure of MXenes offered abundant electrochemical active sites, MXenes have been regarded ideal electrode materials. Moreover, MXene-based film

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Corresponding author. E-mail address: [email protected] (L. Shao). 1 Feitian Ran and Tianlin Wang contributed equally to this work. https://doi.org/10.1016/j.apsusc.2020.145627 Received 7 April 2019; Received in revised form 20 December 2019; Accepted 2 February 2020 Available online 03 February 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic illustration for fabrication of f-MXene and v-MXene films.

film, the freeze-dried MXene (f-MXene) film appeared porous structure, which could enhance electrolyte ions shuttling and thus improving the electrochemical performance. Consequently, the f-MXene film electrode exhibits the highest specific capacitance of 341.5 F g−1 at 1 A g−1 and 206.2 F g−1 when the current density is high as 10 A g−1, which is superior to the v-MXene film electrode. Furthermore, we investigated the MXene film electrode with various mass loading under different drying techniques. The results revealed that the f-MXene films present a superior capacitance and rate performance compared to v-MXene films with identical mass loading. Additionally, a symmetric supercapacitor based f-MXene films was also fabricated to verify its practicability.

electrodes possess high volumetric capacitance benefited from their large mass density [26]. Thus, these above-mentioned capabilities make MXenes have huge prospect in energy storage used for portable and wearable devices. Typically, Ti3C2Tx, as the most extensively studied MXene, has been prepared by selectively etching aluminum layers from Ti3AlC2 and investigated in various energy storage devices [27–30]. The delaminated Ti3C2Tx can be employed to assembly free-standing electrodes under the circumstance of without using additional conductive agents, polymeric binders and current collectors [31]. The MXene-based film combines good capacitive properties, favorable metallic conductivity and flexibility, which is essential for portable and wearable electronic devices [32]. However, similar to other 2D materials such as transition metal oxides/hydroxides [33], transition metal dichalcogenides [34] and graphene [35]. Usually, the delaminated MXene nanosheet have strong self-restacking tendency due to the van der Waals interaction between the layers during drying and film electrode production process [36,37]. As a result, the self-restacking MXene film cannot supply sufficient access to allow the electrolyte ions to contact the active material, resulting in a torpid redox reaction rate and poor rate performance. Furthermore, the stacking between the layers would also cause additional resistance to deteriorate ionic dynamical diffusion. To alleviate this problem, numerous strategies have been developed to restrain the self-restacking [38,39]. For example, Zhao et al. introduced carbon nanotubes into MXene layers to establish a free-standing MXene/CNT film, which have a high volume of around 350 F cm−3 at 5 A g−1 and no attenuation after 10,000 cycles [40]. Fan et al. prepared Fe(OH)3 nanoparticles and introduced them into MXene to construct film with larger layer spacing. The as-prepared porous film shows a high volumetric capacitance of 749 F cm−3, even with a high mass loading of 11.2 mg cm−2 [41]. Besides, Ag nanoparticles, MnO2, and polypyrrole are also investigated as interlayer spacers [42]. Although these intercalation or template approaches have improved the specific capacitance of MXene film electrodes in a certain degree, the tedious spacer preparation and template removal unavoidably complicated manufacturing procedures. Moreover, the intercalation of interlayer spacers decreased the film density thus leading to lower volumetric capacitance, as well as weakened mechanical strength. Accordingly, to realize lightweight and flexible require of wearable and portable electronic devices, the rational design strategies for constructing effective channels to facilitate ions transport in MXene films is urgently desirable. Herein, we demonstrate free-standing and flexible MXene films constructed through vacuum filtration combined with freeze-drying approach. Through freeze-drying treatment, the frozen solvent could be removed by sublimation to construct efficient ion transport channels. Compared with the dense stacking of vacuum-heated MXene (v-MXene)

2. Experimental section 2.1. Preparation of Ti3C2Tx (MXene) suspension Ti3C2Tx was prepared by selectively etching Al layer from Ti3AlC2 phase as previously reported [43]. Typically, 1.0 g LiF (Shanghai Aladdin Biochemical Technology Co. Ltd.) was dispersed in 20 mL 9 M HCl solution under magnetic stirring in a Teflon beaker for 5 min. Subsequently, 1.0 g Ti3AlC2 (Laizhou Kaikai Ceramic Materials Co. Ltd.) was slowly added to above mixture solution, followed by stirring at 35 °C for 24 h. Afterwards, the product was centrifuged and washed with deionized water till the pH value was greater than 6. Then, the clay-like precipitate was re-dispersed in the 100 mL deionized water and sonicated for 30 min under low-temperature. The obtained dispersion was centrifuged for 60 min at 3500 rpm to separate the sediment. Finally, Ti3C2Tx suspension was collected and stored at 5 °C to avoid oxidation. 2.2. Preparation of Ti3C2Tx film electrode The fabrication of the Ti3C2Tx film electrode is illustrated in Fig. 1. Briefly, 10 mL Ti3C2Tx suspension was vacuum-filtered using polyethersulfone (PES) membrane, then freeze-dried or vacuum heated at 70 °C. After completely dried, the flexible and freestanding film electrode can be obtained by removing the filter membrane. The as-prepared Ti3C2Tx films were labelled as f-MXene film and v-MXene film, which represent dried via freeze-drying or vacuum heated, respectively. For comparison, different volumes (15, 20, 25 mL) of Ti3C2Tx suspension were vacuum-filtered to prepare a series of film electrodes with various thickness. 2.3. Materials characterization Field-emission scanning electron microscopy (SEM, MERLIN 2

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surface of film, which may be due to the shrinkage during drying process. In order to probe the effect of different drying methods on microstructure, meanwhile, we also prepared v-MXene-10 through traditional vacuum heated drying. Obviously, relatively more wrinkles could be observed in the surface of v-MXene-10 (Fig. 2f) compared to fMXene-10, indicating the vacuum heated may accelerate the shrinkage and thus forming compact stacking of film. To verify this speculation, the cross-section SEM images of f-MXene-10 and v-MXene-10 were characterized as shown in Fig. 2g, h. It can be clearly seen that f-MXene10 show a fluffy and undulating morphology with thickness of ca. 7 μm, while the v-MXene-10 displays a dense-stacked morphology. Such obvious morphological difference may be ascribed to the fact that the ice crystal is in-situ removed by sublimation when freeze-drying treated, thus constructing porous channels for ion transportation. Nevertheless, the conventional vacuum heated drying made the water in inter-layers rapid evaporate and formed a layer-by-layer stacking microstructure. Generally, the capacitive performance of MXene film electrode is mainly depended the access of electrolyte ion to active materials. Hence, the f-MXene-10 film was expected to provide a larger space for electrolyte ions to diffuse and transport efficiently compared to the dense-stacked v-MXene-10 film with limited ions transfer channel. Also, the cross-section SEM images (Fig. S1) of v-MXene and f-MXene films with different thickness evidenced that freeze-drying treatment could indeed construct fluffy structure for ion diffusion. Furthermore, the digital images of f-MXene-10 and v-MXene-10 films were also presented in Fig. 2i, j. Notably, the as prepared MXene films could be easily bent at a large angle without damage indicating good flexibility whether it was freeze dried or vacuum heated. To further illustrate structural characteristics of samples, the typical XRD patterns of f-MXene-10, v-MXene-10 and corresponding Ti3AlC2 MAX phase are presented in Fig. 3a. After in situ etching, the (0 0 2) diffraction peak of Ti3AlC2 is shifted from 9.7° to 7.0° in films assembled by MXene flakes, which is in accordance with previous reports. A change of layer spacing from 9.1 Å to 12.6 Å is also detected, which is ascribed to the fact that etching of Al layers and formation of oxygen and fluorine-containing functional groups [43]. The most intense peak (2θ = 39°) of Ti3AlC2 corresponding to (1 0 4) plane disappears, indicating that Al has been successfully etched out. Remarkably, the obvious appearance of (0 0 4), (0 0 6), and (0 0 8) diffraction peaks in vMXene-10 reveal a more ordered structure compared to f-MXene-10. XPS was applied to further examine the surface elemental composition and electronic states of f-MXene-10 and v-MXene-10. The chemical composition of MXene mainly includes Ti, C, F and O, as shown in Fig. 3b. The F was introduced by the formation of fluorine-containing groups during the etching process, in which the mixture of LiF and HCl was used as etching agent [47]. The high resolution Ti 2p spectrum can be fitted well with four doublets (Ti 2p3/2 and Ti 2p1/2) for Ti-C, Ti2+, Ti3+ and Ti-O, respectively [48,49], as shown in Fig. 3c and d. Regarding the C 1s spectrum, five components correspond to CeTi, TieCeO, CeC, CeO and O]CeO bonds [49,50], as shown in Fig. 3e, f. The detailed fitting results of each component have been provided in the Table S1 and Table S2 of Supplementary materials, respectively. By comparing the C 1s spectra of f-MXene-10 and v-MXene-10, the proportion of CeO of v-MXene-10 trends to increase, while the proportion of C-Ti decreases, which may be due to the Ti oxidation caused by high temperature in the vacuum drying process. The pseudocapacitance of MXene is partly derived from the change of the valence state of Ti, thus, the oxidation of Ti would give rise to a lower capacitive performance. Based above description, the freeze-drying treatment could weaken the restacking of MXene, and thus constructing efficient ion transport channels, which could be expected to enhance electrolyte ions diffusion and improve the electrochemical performance. Consequently, the MXene film was measured in 3 M H2SO4 aqueous in a three-electrode system. The CV curves of f-MXene-10 and v-MXene-10 at a scan rate of 20 mV s−1 are represented in Fig. 4a. The slightly larger integral area indicates a superior capacitive performance of f-MXene-10 due to its

Compact, Carl Zeiss, Germany) and transmission electron microscope (TEM, Tecnai G2 F30, Netherlands) were conducted to observe the morphologies and microstructure. Prior to the SEM and TEM characterization, the ethanol suspension of MAX and MXene was dropped onto the cleaned Si substrate and holey carbon-coated carbon support copper grids, respectively. The MXene films were directly fixed onto conductive adhesive. The crystal structures were measured by X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer operating at 40 kV and 30 mA with Cu Kα radiation, λ = 1.540598 Å) under 2θ range of 5°–80°. Before XRD measurements, the powder samples were grinded and film samples were directly tested without pretreatment [44]. To analyze the elemental composition and electronic states, X-ray photoelectron spectroscopy (XPS) were conducted in a Shimadzu AXIS Ultra DLD spectrometer with an Al-Kα X-ray source, and the photoelectron take-off angle were 90° in regard to the specimen surface. The binding energy scale of all XPS spectra was referenced to the Fermiedge (Ef), which was set to a binding energy of zero eV. The data processing was performed using XPSPEAK software. The peak fitting was conducted using a Shirley background with 20% constant Lorentzian-Gaussian ratio [45,46]. 2.4. Electrochemical measurements Electrochemical measurements were carried out by CHI660E electrochemical workstation. In the three-electrode system, the working electrode was MXene film, the counter electrode was platinum plate, the reference electrode was Ag/AgCl electrode and the electrolyte was 3 M H2SO4. The cyclic voltammograms (CV) had a voltage interval from −0.4 to 0.2 V at scanning rate range from 5 mV s−1 to 100 mV s−1. In galvanostatic charge–discharge (GCD) measurements, the current density from 0.5 to 10 A g−1 and potential interval from −0.4 to 0.2 V were used. And a frequency range from 0.01 to 100000 Hz was applied in electrochemical impedance spectroscopy (EIS). For two electrode system, two identical v-MXene-10 film electrodes and 3 M H2SO4 aqueous solution were employed to construct the symmetrical cell. The CV curves operated at a potential interval from 0 to 0.7 V from 5 mV s−1 to 100 mV s−1. In GCD measurements, a current density from 0.5 to 10 A g−1 and voltage range from 0 to 0.7 V were used. The galvanostatic cycling test were performed in LAND-BTS battery test system. The formulas used in this study are supplied in the supplementary material. 3. Results and discussion The fabrication process of flexible and free-standing f-MXene and vMXene films is schematically illustrated in Fig. 1. Firstly, by selectively etching Al layers of Ti3AlC2 MAX phase and delamination of multilayered Ti3C2Tx, the single/few layered MXene could be obtained. As observed from scanning SEM image in Fig. 2a, the original Ti3AlC2 presents bulk morphology. Through in situ etching, the Al layers was removed by the etching agent of HCl/LiF, as shown in Fig. 2b, the bulk Ti3AlC2 transform to accordion-like Ti3C2Tx. With the assistance of sonication, the layer structured Ti3C2Tx was delaminated to form Ti3C2Tx suspension. The Ti3AlC2 colloidal suspension appears in greenblack color with an obvious Tyndall scattering effect when a beam is incident from the side to the dispersion, as observed in Fig. 2c, which give an evidence of delaminated Ti3AlC2 nanosheet crystallites. To more intuitively observe the Ti3C2Tx flakes, it is further elucidated by TEM image. In Fig. 2d, it could be clearly observed thin flakes with size of several micrometer, furtherly indicating the successful delamination. Afterwards, the as-obtained colloidal solution of Ti3C2Tx flakes were vacuum filtered to form MXene film. To generate ion transport channels in the MXene film, the freeze-drying approach was employed in the following drying process. Typically, the MXene-10 films were used to investigate the microstructure evolution. As observed from top-view images of the f-MXene-10 (Fig. 2e), some wrinkles generated on the 3

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Fig. 2. SEM images of (a) Ti3AlC2 particles and (b) Ti3C2Tx. (c) Tyndall scattering effect and (d) TEM image of Ti3C2Tx suspension. Top-view SEM images of (e) fMXene-10 and (f) v-MXene-10. Cross-section SEM images of (g) f-MXene-10 and (h) v-MXene-10. Digital images of (i) f-MXene-10 and (j) v-MXene-10 films.

of 2 A g−1 are plotted in Fig. 4b. It is clear that f-MXene-10 has longer discharged curves, suggesting a larger specific capacitance. Fig. 4c plots the CV curves of f-MXene-10 at different scan rates from 5 mV s−1 to 100 mV s−1. The curve maintain similar shape to the lower scan rate one when the scan rate is high up to 100 mV s−1, confirming an good rate capability. GCD curves of f-MXene-10 at various current density

well-organized ion transport channels [51]. The charge storage mechanism of Ti3C2Tx film mainly depend on variations of titanium oxidation state, which could be described the electrochemical reaction as follows [52]: Ti3C2Ox(OH)yFz + δ‾e + H+ → Ti3C2Ox-δ(OH)y+δFz The GCD curves of f-MXene-10 and v-MXene-10 at a current density

Fig. 3. (a) XRD patterns of f-MXene-10, v-MXene-10 and Ti3AlC2. (b) XPS patterns of f-MXene-10, v-MXene-10 and Ti3AlC2. Ti 2p XPS spectra of (c) f-MXene-10 and (d) v-MXene-10. C 1s XPS spectra of (e) f-MXene-10 and (f) v-MXene-10. 4

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Fig. 4. (a) CV curves of f-MXene-10 and v-MXene-10 at a scan rate of 20 mV s−1. (b) GCD curves of f-MXene-10 and v-MXene-10 at a current density of 2 A g−1. (c) CV curves of f-MXene-10 film at different scan rates from 5 mV s−1 to 100 mV s−1 and (d) GCD curves of f-MXene-10 at various current density from 0.5 A g−1 to 10 A g−1. (e) Specific capacitance of f-MXene-10 and v-MXene-10 at different current density (f) Nyquist plots of f-MXene-10 and v-MXene-10 (Inset magnifies the high-frequency region).

from 0.5 A g−1 to 10 A g−1 are shown in Fig. 4d. All lines are almost triangular but not completely linear, illustrating the pseudocapacitive nature of MXene. The high symmetry of the curves also indicates the favorable electrochemical reversibility. And the values of specific capacitance calculated from GCD curves is exhibited in Fig. 4e. The fMXene-10 possesses a superior specific capacitance of 341.5 F g−1 at 1 A g−1 and 206.2 F g−1 at 10 A g−1, while that of v-MXene-10 is lower, which is 306.2 F g−1 at 1 A g−1 and 163.8 F g−1 at 10 A g−1. Remarkably, with the current density increasing, the boosted tendency of specific capacitance for f-MXene-10 appear more pronounced; the retention of f-MXene-10 can reach 60.4% at a high current density of 10 A g−1, while that of v-MXene-10 is 53.5%, verifying a better rate capability of f-MXene-10. In addition, based on the density of MXene films (2.7 mg cm−3 for f-MXene-10 and 3.0 mg cm−3 for v-MXene-10), the volumetric capacitances were calculated as given in Fig. S2. The volumetric capacitance of 922.1 F cm−3 at 1 A g−1for f-MXene-10 can be obtained. And a volumetric capacitance value of 556.7 F cm−3 could still be remained even the current density is high up to 10 A g−1, which exceed that of v-MXene-10 for 494.1 F cm−3, demonstrating its ideal volumetric capacitances and rate performances. Compared with some reported works (Table S3), it can be also found that the superiority of freeze-drying treatment. [53,54] Notably, the superiority of volumetric capacitance of f-MXene-10 become more obvious under the larger current density, suggesting its high-efficiency reaction kinetics. Such enhancement of f-MXene-10 could be attributed to the fact that ion transport channels constructed by freeze-drying could efficiently enhance the diffusion and transfer of electrolyte ions in electrodes. To deeply analyze the ion and electron transport behaviors, the electrochemical impedance spectroscopy (EIS) of f-MXene-10 and vMXene-10 were conducted. As illustrated in Fig. 4f, in the Nyquist plots, the small intersection of real axis provide a compelling evidence for their small bulk resistance (Rs) of MXene films. Nevertheless, compared to v-MXene-10, a smaller semicircle corresponding to charge-transfer resistance (Rct) of f-MXene-10 could be obviously observed, indicating its superior ion conductivity. The larger slope of straight line for fMXene-10 in low-frequency region represent its low diffusion

resistance, which is closely related to rapid transport of ions in the channels constructed by freeze-drying approach. Besides, the Bode plots of f-MXene-10 and v-MXene-10 display a phase angle of 45° at 0.083 and 0.056 Hz (Fig. S3), corresponding to relaxation time constant τ0 (τ0 = 1/f0) of 12.05 and 17.86 s, respectively. Apparently, the smaller τ0 value of f-MXene-10 indicates its quicker charge/discharge rate. In practical application, it is important to explore the electrochemical performance of electrodes with high mass loading active materials [55–57]. Therefore, the electrochemical performance of fMXene and v-MXene films with different mass loading (f-MXene-15, fMXene-20, f-MXene-25, v-MXene-15, v-MXene-20 and v-MXene-25) were also fabricated to verify the influence of freeze-drying on their capacitive performances. The GCD curves of f-MXene-15, f-MXene-20 and f-MXene-25films are exhibited in Fig. 5a-c, respectively. It is clear that all of them display triangular shapes with a minor distortion caused by redox reactions from MXene. The specific capacitances at various current densities calculated by GCD are exhibited in Fig. 5d. Noticeably, the specific capacitance of f-MXene-15, f-MXene-20 and f-MXene-25 can reach to 335.7, 299.2 and 239.3 F g−1 at 1 A g−1, respectively. When the current density is up to 10 A g−1, the specific capacitance can still reach 186.7, 168.5 and 118.4 F g−1, with capacitance retention of 55.6%, 56.3% and 49.5%, respectively, indicating a good rate performance of f-MXene films even with high mass loading. However, the vMXene-15, v-MXene-20 and v-MXene-25 only delivered capacitance retentions of 49.4%, 40.6% and 33.7%, respectively. Obviously, the specific capacitance for f-MXene films are superior to that of v-MXene films (Fig. S4) with various mass loading under coincident current density, confirming the efficiency of ion channel constructed by freezedrying. The corresponding volumetric capacitance of f-Mxene and vMXene films at different current density are presented in Fig. S5, S6. The volumetric capacitance of f-MXene-15, f-MXene-20 and f-MXene-25 can reach to 906.4, 807.8 and 646.1 F cm−3 at 1 A g−1, respectively. When the current density is up to 10 A g−1, the volumetric capacitance of them can still reach 504.1, 455.0 and 319.5 F cm−3, respectively. Obviously, compared with the v-MXene films, the f-MXene films demonstrate the superiorities in the both of volumetric capacitance and

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Fig. 5. GCD curves at different current density from 0.5 A g−1 to 10 A g−1 of (a) f-MXene-15, (b) f-MXene-20 and (c) f-MXene-25. (d) Specific capacitance of f-MXene films at various current density.

restacking, thus improving the electrochemical performance. Through this facile strategy, without any extra addition, the effective ion channel can be constructed in the as-fabricated f-MXene films. The f-MXene-10 delivered a good gravimetric capacitance of 341.5 F g−1 at 1 A g−1 and 206.2 F g−1 even the current density up to 10 A g−1 with a high capacitance retention of 60.4%, indicating an outstanding rate capability. And a high volumetric capacitance of 922.1 F cm−3 can be reached at 1 A g−1. Furthermore, the specific capacitance for f-MXene films all exceeded that of v-MXene films with various mass loading under coincident current density, giving a compelling evidence for the efficiency of ion channel constructed by freeze-drying. The symmetric device based on f-MXene-10 shows an exceptional cycling stability (89.3% capacitance could still be maintained after 10,000 charge–discharge cycles), a desired energy density of 6.1 Wh Kg−1 while the power density is 175.0 W Kg−1, and a volumetric energy density of 16.4 Wh L−1 at a volumetric power density of 472.5 W L−1. Compared to v-MXene films, the superior performance of f-MXene films are ascribed to the constructed ion channel and low charge transfer resistance. We believe this strategy not only can effectively retard the selfrestacking of MXene, but also can give the inspiring and learning to the assembling of other 2D materials.

rate performance. It can be seen that the volumetric capacitance gap is largely enlarged for them at higher current density, implying that the fMXene films has small ion transport resistance compared to v-MXene films. In order to test the applicability of as-fabricated f-MXene films, a symmetric device based on f-MXene-10 was assembled. The CV curves of symmetric supercapacitor (SC) under different voltage windows are shown in Fig. S7. It can be obviously observed that CV curves in the range of 0–0.7 V can maintain a good quasi-rectangular shape without deformation. Thus, the voltage window was conducted as 0–0.7 V. The CV curves of SC at scan rate from 5 mV s−1 to 100 mV s−1 are represented in Fig. 6a. The profile is almost rectangular, without significant distortion even the scan rate is as high as 100 mV s−1, demonstrating good rate performance and rapid current response. All GCD curves in Fig. 6b are almost mirror-symmetric triangles, which also demonstrates the good electrochemical reversibility of f-MXene-10. The specific capacitance of SC can reach to 83.0 F g−1 at 1 A g−1 and 57.0 F g−1 can be retained even at a high current density of 10 A g−1, as shown in Fig. 6c. Fig. 6d exhibits the gravimetric energy densities and power densities of symmetric SC, verifying the promise of real application. An energy density of 6.1 Wh Kg−1 could be obtained while the power density is 175.0 W Kg−1. And a volumetric energy density of 16.4 Wh L−1 can be attained at a volumetric power density of 472.5 W L−1, as shown in Fig. 6e. Additionally, as shown in Fig. 6f, the symmetric device based on f-MXene-10films shows a good cycling stability (89.3% capacitance could still be maintained after 10,000 charge-discharge cycles) and high Coulombic efficiency, which indicated the surface oxidation-reduction reactions and insertion/extraction of electrolyte cations are highly reversible. The capacitance loss can be attributed to the gradual mechanical deformation caused by insertion/ extraction of ions in the electrode materials during cycling.

CRediT authorship contribution statement Feitian Ran: Conceptualization, Methodology, Writing - review & editing. Tianlin Wang: Validation, Formal analysis, Data curation, Writing - review & editing. Siyu C hen: Validation, Formal analysis, Data curation, Writing - review & editing. Yuyan Liu: Writing - review & editing. Lu Shao: Conceptualization, Methodology, Writing - review & editing. Declaration of Competing Interest

4. Conclusions 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.

In summary, the freeze-drying technique was rationally introduced into the prepared process of MXene film electrode to alleviate the self6

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Fig. 6. (a) CV curves at scan rate from 5 mV s−1 to 100 mV s−1, (b) GCD curves at current density from 0.5 A g−1 to 10 A g−1, (c) Specific capacitance, (d) Gravimetric energy and power densities, (e) Volumetric energy and power densities and (f) Cycling stability of f-MXene-10 based symmetric supercapacitor.

Acknowledgment

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