Few-layer Ti3C2Tx MXene delaminated via flash freezing for high-rate electrochemical capacitive energy storage

Journal Pre-proof

Few-layer Ti3 C2 Tx MXene delaminated via flash freezing for high-rate electrochemical capacitive energy storage Xianli Wang , Liubing Dong , Wenbao Liu , Yongfeng Huang , Xuechao Pu , Jinjie Wang , Feiyu Kang , Jia Li , Chengjun Xu PII: DOI: Reference:

S2095-4956(20)30021-8 https://doi.org/10.1016/j.jechem.2020.01.006 JECHEM 1059

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

3 December 2019 7 January 2020 8 January 2020

Please cite this article as: Xianli Wang , Liubing Dong , Wenbao Liu , Yongfeng Huang , Xuechao Pu , Jinjie Wang , Feiyu Kang , Jia Li , Chengjun Xu , Few-layer Ti3 C2 Tx MXene delaminated via flash freezing for high-rate electrochemical capacitive energy storage, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.01.006

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences

Few-layer Ti3C2Tx MXene delaminated via flash freezing for high-rate electrochemical capacitive energy storage Xianli Wanga, Liubing Donga,*, Wenbao Liua,b, Yongfeng Huanga, Xuechao Pua,b, Jinjie Wanga, Feiyu Kanga,b, Jia Lia, Chengjun Xua,* a

Shenzhen Geim Graphene Center, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, Guangdong, China b State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China *

Corresponding authors. E-mail address: [email protected] (L. Dong), [email protected] (C. Xu).

Abstract Few-layer Ti3C2Tx MXene is synthesized from multi-layered Ti3C2Tx via a flash freezing-assisted delamination process. During the flash freezing process, the water molecules in the interlayers of multi-layered MXene are induced to rearrange and produce volume expansion, thus notably expand the MXenes’ interlayer distance to form few-layer MXene. The synthesized few-layer Ti3C2Tx MXene nanosheets display a very small thickness (less than 5 Ti3C2 atom-layers) and expanded interlayer spacing. Consequently, the few-layer Ti3C2Tx exhibits enhanced capacitance (255 F g−1 vs. 177 F g−1 for the multi-layered Ti3C2Tx) and significantly optimized rate capability (150 F g−1 at 200 mV s−1 vs. 25 F g−1 for the multi-layered Ti3C2Tx), because redox-active sites in the few-layer MXene are easily accessible to electrolyte ions. Moreover, an asymmetric supercapacitor is constructed using the few-layer Ti3C2Tx negative electrode and an activated carbon fiber positive electrode. The asymmetric supercapacitor presents a high energy density of 17.9 Wh kg−1 and a high power density of 14 kW kg−1, which is inseparable from its wide voltage window of

1.4 V and the good rate performance of the few-layer Ti3C2Tx MXene electrode. Overall, the flash freezing-assist delamination provides an effective and environmental-friendly strategy to synthesize few-layer MXene materials for high-rate electrochemical energy storage. Keywords: Ti3C2Tx MXene; Pseudocapacitance; Activated carbon fiber; Asymmetric supercapacitor; Flash freezing

1. Introduction Supercapacitors, which exhibit good rate capability, high power density and superior cycling stability, have been occupying a pivotal position in electrochemical energy storage instruments all the time [1–4]. Hunting for high-performance electrode materials is always vital to supercapacitors. Typically, electrode materials used in supercapacitors are classified into electric double-layer capacitive materials and pseudocapacitive materials according to different charge storage mechanisms. The former ones mainly include carbon materials that store charge through electrostatic adsorption, and their energy density is relatively low [5–7]. Pseudocapacitive materials, which store energy through reversible surface or near-surface redox reactions, seem to make up for the above limitation. Conducting polymers and metal oxides/hydroxides are exploited as typical pseudocapacitive materials [8–11]. Although they can provide high gravimetric capacitance and energy density, they generally suffer from poor cycling stability and low power density [12–14]. MXenes, a group of newly emerging two-dimensional layered materials, offer new options to realize high-performance pseudocapacitors [15–18,57]. Ti3C2Tx is the most commonly studied one among the MXenes’ family. Ti3C2Tx MXenes usually exhibit superb conductivity of ~104 S cm−1, which is comparable with carbon-based materials, favoring fast electron transportation [15,16]. Meanwhile, they have

abundant hydrophilic functional groups on their nanosheet surfaces [19,20], which reduce the interface contact impedance between electrode materials and aqueous electrolytes. Besides, the relatively large interlayer distance of MXenes benefits the fast insertion/desertion of electrolyte ions [21–26]. The above features theoretically endow Ti3C2Tx MXenes with enhanced volumetric capacitance, rate performance and cycling stability when compared with many other common pseudocapacitive materials such as MnO2 and polyaniline [27–29]. In practical researches, however, Ti3C2Tx MXenes still face some challenges for electrochemical capacitive energy storage. One of the key challenges is how to effectively delaminate MXenes to produce few-layer Ti3C2Tx nanosheets because multi-layered MXenes go against the accessibility of ions to redox-active sites and thus lead to unsatisfactory pseudocapacitive performance especially poor rate capability. The commonly used method at present to delaminate multi-layered Ti3C2Tx MXene is sonicating the suspension containing multi-layered Ti3C2Tx for several hours, followed by centrifugation to get supernatant with few-layer Ti3C2Tx MXenes [19,30]. However, the concentration of the few-layer Ti3C2Tx MXenes in the supernatant is low [30], suggesting a low yield. Herein, few-layer Ti3C2Tx MXene was synthesized in high yield with the assist of a flash freezing-drying method and used for high-rate electrochemical capacitive energy storage. Multi-layered Ti3C2Tx obtained through etching Ti3AlC2 powder was dispersed in aqueous suspension and then experienced a flash freezing-drying process (flash froze the suspension at −196 °C and then dried it in a freeze-dryer), as illustrated in Fig. 1. Researches revealed that for Ti3C2Tx MXenes, a large number of water molecules are trapped into interlayers because of their hydrophilic surfaces and relatively weak interaction between MXene layers [31–35]. During the flash freezing

process, water molecules in the interlayers of multi-layered MXene were rearranged to cause a large volume expansion, which would notably expand the interlayer distance of Ti3C2Tx MXene nanosheets and realize the delamination of MXene. The yield of few-layer Ti3C2Tx MXene in such a process was ~100%. The synthesized few-layer Ti3C2Tx MXene was comprehensively characterized from the aspects of micro-morphology, composition and surface chemistry, and they exhibited remarkably optimized capacitance and rate capability. Asymmetric supercapacitors were further constructed by coupling the few-layer Ti3C2Tx MXene negative electrode with an activated carbon fiber (ACF) positive electrode to achieve high power density as well as high energy density.

Fig. 1. Schematics for the synthesis of few-layer MXene: step I, etching process; step II, flash freezing-assisted delamination.

2. Experimental 2.1. Synthesis of multi-layered Ti3C2Tx MXenes Ti3AlC2 powder purchased from Laizhou Kai Kai Ceramic Materials Co., Ltd., was sieved from a 400-mesh sieve. 30 mL of 6 M HCl solution and 1.98 g of LiF

were mixed in a polytetrafluoroethylene container. After magnetic stirring for 5 min, 3 g of Ti3AlC2 powder was slowly added into the container. The whole appliance was put into a water bath. The reaction was lasting 45 h with stirring at 40 °C. The mixture was washed with deionized water by centrifugation until the pH reached 6. The bottom sediment which was got from the last centrifugation cycle was frozen in a refrigerator (the freezing temperature was about −20 °C) for one night. Then it was transformed into a freeze dryer (Scientz-10N) to get multi-layered Ti3C2Tx MXene powder, which was denoted as R-Ti3C2Tx. 2.2. Synthesis of few-layer Ti3C2Tx MXene 200 mg of R-Ti3C2Tx powder was dispersed into 50 mL of deionized water in a plastic pipe and sonicated for 2 h. The pipe was quickly immersed into liquid nitrogen (−196 °C) to flash freeze for 8 h and then fully dried in a freeze dryer to obtain few-layer Ti3C2Tx MXene powder. The powder was named as LN-Ti3C2Tx. For comparison, we also sonicated 200 mg multi-layered Ti3C2Tx powder into 50 mL of deionized water, and then froze at −20 °C for 8 h in a refrigerator, followed by drying in a freeze dryer. The obtained sample was marked as F-Ti3C2Tx. 2.3. Characterization of Ti3C2Tx MXene A field emission scanning electron microscopy (SEM, Zeiss supra55) and a transmission electron microscopy (TEM, FEI Tecnai G2 F30) were used to observe the micro-morphology. Atomic force microscope (AFM, Bruker Dimension Icon) was operated to obtain the thickness of Ti3C2Tx flakes. Crystallographic and structure data were characterized by X-ray diffraction instrument (XRD, Rigaku 2500, Cu Kα radiation, λ=0.154056 nm) with a scan rate of 5° min−1 with 2-theta changing from 5° to 80°. The powder was pressed on the sample holder for XRD test. Energy dispersive X-ray spectroscopy was obtained by SEM. X-ray photoelectron spectroscopy (XPS)

analysis

was

performed

through

PHI

5000

VersaProbe

II.

Nitrogen

adsorption-desorption isotherms were gained by ASAP 2020M+C. Note that the samples will not be oxidized during degassing process at 200 °C because the samples were in vacuum atmosphere. 2.4. Electrochemical measurements of Ti3C2Tx MXene Electrochemical properties of R-Ti3C2Tx, LN-Ti3C2Tx and F-Ti3C2Tx powder samples were tested by constructing symmetric supercapacitors. To prepare the electrodes,

we

mixed

active

materials,

conductive

acetylene

black

and

poly(vinylidene fluoride) binder with the ratio of 8:1:1 in N-methyl pyrrolidone to form a slurry and coated the slurry onto a stainless-steel foil, followed by drying at 60 °C for 12 h in a vacuum oven. In the assembled symmetric supercapacitors, 1 M H2SO4 aqueous solution was used as electrolyte and air-laid paper was used as separator. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical

impedance

spectroscopy

(EIS)

were

tested

at

a

VMP3

electrochemical workstation (Biologic, France). The scan rates ranged from 2 to 200 mV s−1 in the CV testing. GCD was performed between 0 and 0.8 V under charge/discharge current densities of 1, 2, 5, 10 and 20 A g−1. During EIS measurement, the frequency was 200 kHz-10 mHz and the amplitude was set to 10 mV. Asymmetric supercapacitors were constructed using an ACF positive electrode (the ACF was washed several times by deionized water and ethanol before used), the LN-Ti3C2Tx negative electrode, 1 M H2SO4 aqueous electrolyte and air-laid paper separator. Besides, three-electrode system was utilized to determine the appropriate working potential of ACF electrode and LN-Ti3C2Tx electrode in 1 M H2SO4 electrolyte. In the three-electrode system, a platinum plate and a saturated calomel

electrode were chosen as counter electrode and reference electrode respectively, while the ACF electrode or LN-Ti3C2Tx electrode was used as working electrode. For the assembled ACF//LN-Ti3C2Tx asymmetric supercapacitors, CV tests were carried out in the voltage range of 0–1.4 V at scan rates of 2–200 mV s−1, GCD curves were gained at 1–20 A g−1 and EIS was examined at 200 kHz–10 mHz under the amplitude of 10 mV.

3. Results and discussion 3.1. Morphology characterization and phase analysis of Ti3C2Tx samples SEM and TEM were used to observe the micro-morphologies of Ti3AlC2 and Ti3C2Tx MXene samples. Ti3AlC2 has a densely layered structure as shown in Figs. 2(a) and S1. Through etching in LiF/HCl mixture solution, accordion-like morphology of multi-layered Ti3C2Tx MXene (noted as R-Ti3C2Tx) emerges (Figs. 2b and S2). Note that we chose the LiF/HCl mixture solution as etching agent instead of HF solution because the former tends to create MXenes with a larger interlayer distance and less defects. Note that both Li+ and water molecules will intercalate into the interlayers after etching [1,20]. The multi-layered R-Ti3C2Tx powder was dispersed in deionized water, flash-frozen at −196 °C and then dried in a freeze dryer (details can be found in Experimental section). After the flash freezing-drying process, the obtained Ti3C2Tx MXene (denoted as LN-Ti3C2Tx) flakes become much thinner than the R-Ti3C2Tx flakes, although accordion-like morphology is retained (Fig. 2c, d). High-resolution TEM and AFM were further utilized to analyze these MXenes’ layer structure. For the R-Ti3C2Tx sample, its nanosheets are too thick to be penetrated by electrons for TEM observations (Fig. 2e). AFM image (Fig. 2f) shows that the nanosheet thickness is about 75.0 nm, indicating a multi-layered MXene structure [16]. By contrast, the MXene nanosheets in the LN-Ti3C2Tx sample are very thin,

which makes them translucent in the TEM view (Fig. 2g). The high-resolution TEM image in Fig. 2(h) visually shows that the LN-Ti3C2Tx is few-layer MXene nanosheets with a layer number of less than 5. Consistently, the thickness of the MXene nanosheets in the LN-Ti3C2Tx sample is measured to be only about 5.2 nm (Fig. 2i). These phenomena indicate the successful delamination from multi-layered R-Ti3C2Tx to few-layer LN-Ti3C2Tx MXene.

Fig. 2. SEM images of the (a) Ti3AlC2, (b) R-Ti3C2Tx and (c, d) LN-Ti3C2Tx samples. (e) TEM and (f) AFM images of the R-Ti3C2Tx. (g) TEM, (h) high-resolution TEM and (i) AFM images of the LN-Ti3C2Tx. As is known to all, flash freezing will cause the rearrangement of water molecules in liquid water to form ices, accompanying with a volume expansion (Fig. S3) [36]. Given that a considerable amount of water molecules always exist in the interlayer space of MXene [15,19], it is reasonable to believe that during the flash freezing process, the rearrangement of these water molecules may shove two Ti3C2Tx

layers away from each other (as illustrated in Fig. 1), and therefore few-layer Ti3C2Tx MXene can be obtained. At the same time, the flash freezing process also causes a reduced lateral size of the MXene flakes (Fig. S4), which has a negative effect on electrical conductivity of MXene flakes and MXene electrodes.

Fig. 3. (a) XRD patterns, (b) XPS survey spectra, (c) high-resolution Al 2p XPS spectra and (d) Raman spectra of the Ti3AlC2, R-Ti3C2Tx and LN-Ti3C2Tx samples. To investigate the phase transformation during the etching process and delamination process, X-ray diffraction instrument (XRD) tests were carried out in Fig. 3(a). The diffraction peak of (002) plane shifts from 9.5° for Ti3AlC2 to 6.5° for R-Ti3C2Tx after the etching process, which means that the interlayer spacing of (002) plane significantly enlarges. Other characteristic peaks of Ti3AlC2 almost disappear after etching, indicating the successful transformation from Ti3AlC2 to Ti3C2Tx. After flash freezing-assisted delamination, the peak position of (002) plane further shifts from 6.5° for R-Ti3C2Tx to 6.2° for LN-Ti3C2Tx, indicating the interlayer distance of

(002) diffraction planes increases by ~0.38 Å. Besides, compared with the R-Ti3C2Tx sample, there are no new diffraction peaks in the XRD pattern of the LN-Ti3C2Tx. In a word, the flash freezing-assisted delamination would broaden the interlayer spaces of Ti3C2Tx without new phase generating. XPS further confirms the above discussions about phase transformation. From Ti3AlC2 to R-Ti3C2Tx, the signal of Al element in their XPS survey spectra (Fig. 3b) almost disappears, while the orbits of F element emerge at ~687 eV after etching. Even based on the energy dispersive X-ray spectroscopy data in Fig. S5, only 0.4 at% of Al element is detected in the R-Ti3C2Tx powder (atom fraction of Al element in the as-received Ti3AlC2 powder is 16.7%), while F element accounts for 9.8 at%. These prove the successful etching of Ti3AlC2. Corresponding etching reactions have been put forward by Naguib et al. [15], as described in Eqs. (1–3), Ti3 AlC2 +3HF→AlF3 +3/2H2 +Ti3 C2

(1)

Ti3 C2 +2H2 O→Ti3 C2 (OH)2 +H2

(2)

Ti3 C2 +2HF→Ti3 C2 F2 +H2

(3)

Herein, the HF was synthesized by mixing LiF and HCl. Although almost no Al element remains in the R-Ti3C2Tx sample based on XPS analysis, the Al element appears in the LN-Ti3C2Tx (Fig. 3b). Fig. 3(c) depicts high-resolution XPS spectra of Al 2p in the Ti3AlC2, R-Ti3C2Tx and LN-Ti3C2Tx samples. Compared with the Ti3AlC2, a shift toward higher binding energy is observed for the Al 2p peak in the LN-Ti3C2Tx. The peak position of Al 2p (76.3 eV) in the LN-Ti3C2Tx is consistent with aluminum fluoride (AlF3), a byproduct produced in reaction (1) mentioned above [37]. Generally, AlF3 can be removed through washing for 5–6 times in the centrifugation process after etching reactions. Therefore, it seems that Al element

(from AlF3) should not appear in the R-Ti3C2Tx and LN-Ti3C2Tx. As mentioned above, the Al element is indeed not detected in the R-Ti3C2Tx. But it is detected in LN-Ti3C2Tx (Fig. 3b, c). The reasonable explanation is that XPS technology can only collect the signals within a few nanometers of material surfaces. After etching, there is still some Al element remained in the inner surfaces in the R-Ti3C2Tx but cannot be detected by XPS. However, the lyophilization process turned multi-layered Ti3C2Tx MXenes into few-layer ones and some inner surfaces containing Al elements exposed outside. Accordingly, the specific surface area of the MXene increases from 1 m2 g-1 for the R-Ti3C2Tx to 6 m2 g−1 for the LN-Ti3C2Tx (Fig. S6). Furthermore, the characteristic peaks of Ti 2p and C 1s in corresponding XPS spectra (Fig. S7) have no shifts after the flash freezing-assisted delamination process. They originate from Ti−O and Ti−C bonds. From the fitting results of Ti 2p, the majority singals are consistent with Ti element at different state (Ti, Ti2+, Ti3+). The peak at 458.6, 459.3 and 460.2 eV respresent TiO2, TiO2-xFx and C−Ti−Fx respectively.[58] Raman spectra of the Ti3AlC2, R-Ti3C2Tx and LN-Ti3C2Tx are shown in Fig. 3(d). The spectra of the R-Ti3C2Tx and LN-Ti3C2Tx are similar, and they have a great difference with that of the Ti3AlC2. In view that terminal groups at the interlayer could affect the out-of-plane motion of Ti atoms and vibration of C atoms, the peaks at around 208, 280, 380, 627 and 727 cm−1 in the Raman spectra of the R-Ti3C2Tx and LN-Ti3C2Tx samples represent typical surface functional groups (−F, −OH and −O) of Ti3C2Tx MXenes [25,38,55]. This also implies that after the flash freezing-assisted delamination process, abundant functional groups still exist on the MXene nanosheet

surfaces in the LN-Ti3C2Tx. 3.2. Electrochemical performance of Ti3C2Tx samples Electrochemical properties of the R-Ti3C2Tx and LN-Ti3C2Tx were evaluated in symmetric supercapacitors with 1 M H2SO4 electrolyte. Fig. 4(a, b) presents cyclic voltammetry (CV) curves of the R-Ti3C2Tx electrode and LN-Ti3C2Tx electrode-based symmetric supercapacitors. The supercapacitors are capable of working in a voltage range of 0–0.8 V, which has also been observed in previous researches [39,40]. Specific capacitance of the R-Ti3C2Tx electrode and LN-Ti3C2Tx electrode was calculated on the basis of the CV results. Mass loading of both R-Ti3C2Tx electrodes and LN-Ti3C2Tx electrodes is about 1.1–1.2 mg cm−2. As summarized in Fig. 4(c), the specific capacitance of 175 F g−1 is obtained for the R-Ti3C2Tx electrode at the scan rate of 2 mV s−1. However, the specific capacitance dramatically drops to 25 F g−1 at 200 mV s−1, which demonstrates a poor rate capability of the multi-layered R-Ti3C2Tx MXene. For the few-layer LN-Ti3C2Tx MXene, its specific capacitance is higher than R-Ti3C2Tx at various scan rates (Fig. 4a–c). For instance, the maximum specific capacitance of the LN-Ti3C2Tx is enhanced to 255 F g−1 at 2 mV s−1.

Fig. 4. CV curves of the (a) R-Ti3C2Tx and (b) LN-Ti3C2Tx electrode based symmetric supercapacitors. (c) Electrode capacitance values at various scan rate. (d) Nyquist

plots of the above two supercapacitors; (e) GCD curves of the LN-Ti3C2Tx electrode based symmetric supercapacitor. (f) Schematic for the electrochemical capacitive energy storage of the few-layer LN-Ti3C2Tx MXene material. More importantly, at a high scan rate of 200 mV s−1, the specific capacitance of LN-Ti3C2Tx is 150 F g−1, 6 times higher than that of the R-Ti3C2Tx. These suggest that rate capability and specific capacitance of the MXene materials are remarkably improved after the flash freezing-assisted delamination. As summarized in Table S1, electrochemical performance of the few-layer LN-Ti3C2Tx is also better than that of many previously reported MXene materials. Besides, electrochemical impedance spectroscopy (EIS) spectra in Fig. 4(d) show that the LN-Ti3C2Tx electrode-based symmetric supercapacitors possess smaller charge-transfer resistance, demonstrating fast kinetics of corresponding electrochemical reactions. The slope of Warburg-type line in low-frequency zone for the LN-Ti3C2Tx symmetric supercapacitors is much higher than the other one, indicating H+ can move faster in the LN-Ti3C2Tx MXene electrode [41]. Galvanostatic charge-discharge (GCD) curves of the LN-Ti3C2Tx in Fig. 4(e) also show very small IR drops. The electrochemical capability of Ti3C2Tx MXenes in aqueous H2SO4 electrolyte has been proven to originate from a fast pseudocapacitive reaction [27,42,43]. Multi-layered structure of R-Ti3C2Tx impedes the accessibility and fast diffusion of H+, thus leading to a lower capacitance and worse rate capability. While as illustrated in Fig. 4(f), for the LN-Ti3C2Tx with smaller thickness, larger interlayer distance and higher specific surface area, more redox-active sites are exposed to participate electrochemical reactions and the diffusion distance of electrolyte ions are shortened. Naturally, higher capacitance and better rate performance can be achieved [27,44].

To further confirm the positive effect of the flash freezing process on electrochemical performance of Ti3C2Tx MXene, we also synthesized another MXene when a freezing temperature of −20 °C was applied: the multi-layered R-Ti3C2Tx powder/deionized water suspension was frozen at −20 °C and then dried in a freeze dryer. The obtained MXene sample is noted as F-Ti3C2Tx. Fig. S8 provides the electrochemical behaviors of the F-Ti3C2Tx. The electrochemical performance including specific capacitance and rate performance of the F-Ti3C2Tx is better than that of the R-Ti3C2Tx but is worse than that of the LN-Ti3C2Tx. We can see from the SEM images in Fig. S9 that the F-Ti3C2Tx possesses thicker MXene flakes than LN-Ti3C2Tx. Furthermore, the XRD patterns of F-Ti3C2Tx and LN-Ti3C2Tx are given in Figure S16(a), from which we can see that the diffraction peak of (002) plane of LN-Ti3C2Tx and F-Ti3C2Tx are 6.2°, 6.4° respectively. This means that LN-Ti3C2Tx sample synthesized based on flash freezing possesses a relatively larger interlayer spacing. The above evidences indicate that flash freezing (at −196 °C) is more effective for the delamination of multi-layered Ti3C2Tx. Previous research pointed out that flash freezing could facilitate the rapid nucleation of ices, and the formed ice crystals compete with each other to grow and cause a relatively large thermal stress [45,46]. On the contrary, slow freezing (i.e., freezing at −20 °C herein) cannot cause the rapid rearrangement of the water molecules in the interlayer space of MXene, thus is unable to effectively delaminate multi-layered MXenes. 3.3. Construction of few-layer Ti3C2Tx based asymmetric supercapacitors To achieve higher energy density, an asymmetric supercapacitor was constructed using the few-layer LN-Ti3C2Tx MXene as negative electrode. Free-standing ACF cloth was chosen as the positive electrode for the asymmetric supercapacitor because it has positive working potential, high gravimetric capacitance

and good rate performance as supercapacitor electrodes [47–50,56]. Typical micro-morphology and pore structure of the ACF electrode are displayed in Fig. S10. The ACF has a large specific surface area of 962 m2 g−1, which contributes to its good electrochemical energy storage ability (Fig. S11). Stable working potential windows of the LN-Ti3C2Tx electrode and ACF electrode in 1 M H2SO4 electrolyte were first determined in a three-electrode system at the scan rate of 20 mV s−1. As shown in Fig. 5(a), the working potential of the LN-Ti3C2Tx electrode and the ACF electrode is −0.4–0.3 V vs. saturated calomel electrode (SCE) and 0–1.0 V vs. SCE, respectively. As a result, the constructed ACF//LN-Ti3C2Tx asymmetric supercapacitors can work in a wide voltage range of 0–1.4 V. Note that in the asymmetric supercapacitors, the mass ratio of the ACF to the LN-Ti3C2Tx MXene is 0.82 to balance the charges stored at these two electrodes. The mass ratio was calculated based on the following formula,

m C V  m C V

(4)

where m+/m− means the mass ratio of the ACF to the LN-Ti3C2Tx MXene, C+ (or C−) is the specific capacitance of ACF (or LN-Ti3C2Tx), ΔV+ (or ΔV−) refers to the voltage interval of the ACF positive electrode (or the LN-Ti3C2Tx negative electrode) [51]. According to the CV curves in Fig. 5(a), the specific capacitance of the ACF and the LN-Ti3C2Tx is 177.5 and 208.4 F g−1 respectively at the scan rate of 20 mV s−1. To evaluate the electrochemical performance of the ACF//LN-Ti3C2Tx asymmetric supercapacitors, CV was tested with the voltage window ranging from 0 to 1.4 V. The curves behave in similar shape at scan rates of 2–100 mV s−1 as shown in Fig. 5(b), which indicates a relatively good rate capability of the asymmetric supercapacitors. GCD curves at 1–10 A g−1 in Fig. 5(c) present a nearly symmetric triangle with small

IR drops, meaning a small internal resistance of the whole cells. This is quantitatively confirmed by EIS analysis (Figs 5d and S12): as fitted in high frequencies, the intercept on the real axis (i.e., Rs) is 0.5 Ω, which is equal to the sum of ionic resistance of electrolyte and contact resistance between the current collector and electrode material [48,52,53]. The charge-transfer resistance (i.e., Rct) is only about 8.0 Ω. While in low frequency, the slope towards 90° suggests the diffusion resistance of the electrolyte ions is very small. These features endow the ACF//LN-Ti3C2Tx asymmetric supercapacitors with good rate performance (Figs. 5c and S13): when charge/discharge current increases from 0.5 to 10 A g−1, the capacitance of the supercapacitors remains about 70%. In addition, the asymmetric supercapacitors exhibit a relatively stable cycling behavior, with 93% capacitance retention over 2000 charge/discharge cycles at 10 A g−1. Capacitance increase in the initial charge/discharge cycles is mainly due to the activation of ACF positive electrode [50,54]. More importantly, as depicted by the Ragone plots in Fig. 5(f), the ACF//LN-Ti3C2Tx asymmetric supercapacitors possess both high power density and high

energy

supercapacitors

density, and

compared ACF//ACF

with

LN-Ti3C2Tx//LN-Ti3C2Tx

symmetric

supercapacitors.

The

symmetric superior

electrochemical performance of the ACF//LN-Ti3C2Tx asymmetric supercapacitor is strongly associated with its wide voltage window and the high-rate few-layer LN-Ti3C2Tx electrode.

Fig. 5. (a) CV curves at 20 mV s−1 of the LN-Ti3C2Tx electrode and the ACF electrode. Electrochemical performance of the ACF//LN-Ti3C2Tx asymmetric supercapacitor: (b) CV curves, (c) GCD curves, (d) Nyquist plot (inset: enlarged high-frequency region), (e) cycling behavior at 10 A g−1 and (f) Ragone plots in comparison with the ACF and the LN-Ti3C2Tx electrode based symmetric supercapacitors.

4. Conclusions In summary, few-layer Ti3C2Tx MXene was synthesized via a flash freezing method assisted delamination process, i.e., multi-layered Ti3C2Tx obtained through etching Ti3AlC2 powder was dispersed in aqueous suspension and then experienced a flash freezing-drying process. The flash freezing process induced the rearrangement and volume expansion of the water molecules in the interlayers of multi-layered MXene, thus notably expanded the interlayer distance of Ti3C2Tx MXene nanosheets to realize the delamination of MXene. Consequently, the synthesized few-layer Ti3C2Tx MXene nanosheets displayed a very small thickness (corresponding to less than 5 Ti3C2 atom-layers) and expanded interlayer spacing. Besides, abundant functional groups were found on the surface of the few-layer Ti3C2Tx. Compared with the multi-layered Ti3C2Tx, the few-layer Ti3C2Tx exhibited superior electrochemical

performance including significantly enhanced capacitance and superior rate performance, because redox-active sites in few-layer MXene are easily accessible to electrolyte ions even at high scan rates. Moreover, the few-layer Ti3C2Tx negative electrode was coupled with ACF positive electrode to construct an asymmetric supercapacitor. The asymmetric supercapacitor presented both high energy density and high power density, which is inseparable from its wide voltage window of 1.4 V and the good rate performance of the few-layer Ti3C2Tx MXene electrode. All in all, the

flash

freezing-assist

delamination

provides

an

effective

and

environmental-friendly strategy to synthesize few-layer MXene materials for high-rate electrochemical energy storage.

Declaration of interests 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.

Acknowledgments The authors appreciate the financial supports from Shenzhen Technical Plan Project (No.

JCYJ20160301154114273;

No. JCYJ20170412171430026),

International

Science & Technology Cooperation Program of China (No. 2016YFE0102200), National Key Basic Research (973) Program of China (No. 2014CB932400), and Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N111).

References [1] P. Simon, Y. Gogotsi, B. Dunn, Science 343 (2014) 1210-1211. [2] L. Zhang, X. Zhao, Chem. Soc. Rev. 38 (2009) 2520-2531. [3] W. Liu, M. S. Song, B. Kong, Y. Cui, Adv. Mater. 29 (2017) 1603436. [4] L. Dong, C. Xu,Y. Li, Z. Huang, F. Kang, Q. Yang, X. Zhao, J. Mater. Chem. A 4 (2016) 4659-4685. [5] X. Li, B. Wei, Nano Energy 2 (2013) 159-173. [6] L. Hao, X. Li, L. Zhi, Adv. Mater. 25 (2013) 3899-3904. [7] A. Borenstein, O. Hanna, R. Attias, S. Luski, T. Brousse, D. Aurbach, J. Mater. Chem. A 5 (2017) 12653-12672. [8] Q. Meng, K. Cai, Y. Chen, L. Chen, Nano Energy 36 (2017) 268-285. [9] Q. Li, S. Zheng, Y. Xu, H. Xue, H. Pang, Chem. Eng. J. 333 (2018) 505-518. [10] W. Guo, C. Yu, S. Li, Z. Wang, J. Yu, H. Huang, J. Qiu, Nano Energy 57 (2019) 459-472. [11] L. Dong, G. Liang, C. Xu, D. Ren, J. Wang, Z. Pan, B. Li, F. Kang, Q. Yang, J. Mater. Chem. A 5 (2017) 19934-19942. [12] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, ACS Nano 4 (2010) 1963-1970. [13] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Adv. Funct. Mater. 21 (2011) 2366-2375. [14] Y. Hu, C. Guan, G. Feng, Q. Ke, X. Huang, J. Wang, Adv. Funct. Mater. 25 (2015) 7291-7299. [15] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, M. Heon, L. Hultman, Y. Gogotsi, M. Barsoum, Adv. Mater. 23 (2011) 4248-4253. [16] M. Naguib, V.N. Mochalin, M.W. Barsoum, Y. Gogotsi, Adv. Mater. 26 (2014) 992-1005.

[17] J. Nan, X. Guo, J. Xiao, X. Li, W. Chen, W. Wu, H. Liu, Y. Wang, M. Wu, G. Wang, Small (2019) 1902085. [18] X. Guo, J. Zhang, J. Song, W. Wu, H. Liu, G. Wang, Energy Storage Materials 14 (2018) 306-313. [19] B. Anasori, M.R. Lukatskaya, Y. Gogotsi, Nat. Rev. Mater. 2 (2017) 16098. [20] M. Ghidiu, M.R. Lukatskaya, M. Zhao, Y. Gogotsi, M.W. Barsoum, Nature 516 (2014) 78-81. [21] S. Lin, X. Zhang, J. Power Sources 294 (2015) 354-359. [22] Y. Peng, B. Akuzum, N. Kurra, M. Zhao, M. Alhabeb, B. Anasori, E. Kumbur, H.N. Alshareef, M.-D. Ger, Y. Gogotsi, Energy Environ. Sci. 9 (2016) 2847-2854. [23] S. Xu, Y. Dall’Agnese, G. Wei, C. Zhang, Y. Gogotsi, W. Han, Nano Energy 50 (2018) 479-488. [24] C. Zhang, M.P. Kremer, A. Seral-Ascaso, S.H. Park, N. McEvoy, B. Anasori, Y. Gogotsi, V. Nicolosi, Adv. Funct. Mater. 28 (2018) 1705506. [25] W. Bao, L. Liu, C. Wang, S. Choi, D. Wang, G. Wang, Adv. Energy Mater. 8 (2018) 1702485. [26] M. Lu, W. Han, H. Li, W. Shi, J. Wang, B. Zhang, Y. Zhou, H. Li, W. Zhang, W. Zheng, Energy Storage Materials 16 (2019) 163-168. [27] M.R. Lukatskaya, S. Kota, Z. Lin, M. Zhao, N. Shpigel, M.D. Levi, J. Halim, P.L. Taberna, M.W. Barsoum, P. Simon, Y. Gogotsi, Nat. Energy 2 (2017) 17105. [28] S. Niu, Z. Wang, M. Yu, L. Xiu, S. Wang, X. Wu, J. Qiu, ACS Nano 12 (2018) 3928-3937. [29] X. Xie, M. Zhao, B. Anasori, K. Maleski, C.E. Ren, J. Li, B.W. Byles, E. Pomerantseva, G. Wang, Y. Gogotsi, Nano Energy 26 (2016) 513-523. [30] M. Alhabeb, K. Maleski, B. Anasori, P. Lelyukh, L. Clark, S. Sin, Y. Gogotsi,

Chem. Mater. 29 (2017) 7633-7644. [31] M.A. Hope, A.C. Forse, K.J. Griffith, M.R. Lukatskaya, M. Ghidiu, Y. Gogotsi, C.P. Grey, Phys. Chem. Chem. Phys. 18 (2016) 5099-5102. [32] M. Naguib, J. Halim, J. Lu, K.M. Cook, L. Hultman, Y. Gogotsi, M.E. Barsoum, J. Am. Chem. Soc. 135 (2013) 15966-15969. [33] N.C. Osti, M. Naguib, A. Ostadhossein, Y. Xie, P.R.C. Kent, B. Dyatkin, G. Rother, W.T. Heller, A.C.T. van Duin, Y. Gogotsi, E. Mamontov, ACS Appl. Mater. Interfaces 8 (2016) 8859-8863. [34] C. Chen, M. Boota, P. Urbankowski, B. Anasori, L. Miao, J. Jiang, Y. Gogotsi, J. Mater. Chem. A 6 (2018) 4617-4622. [35] M. Hu, T. Hu, Z. Li, Y. Yang, R. Cheng, J. Yang, C. Cui, X. Wang, ACS Nano 12 (2018) 3578-3586. [36] B. Santra, J. Klimeš, D. Alfè, A. Tkatchenko, B. Slater, A. Michaelides, R. Car, M. Scheffler, Phys. Rev. Lett. 107 (2011) 185701. [37] R. Ko¨nig, G. Scholz, K. Scheurell, D. Heidemann, I. Buchem, W.E.S. Unger, E. Kemnitz, J. Fluorine Chem. 131 (2010) 91-97. [38] Q. Jiang, N. Kurra, M. Alhabeb, Y. Gogotsi, H.N. Alshareef, Adv. Energy Mater. 8 (2018) 1703043. [39] H. Li, Y. Hou, F. Wang, M. Lohe, X. Zhuang, L. Niu, X. Feng, Adv. Energy Mater. 7 (2017) 1601847. [40] N. Kurra, B. Ahmed, Y. Gogotsi, H.N. Alshareef, Adv. Energy Mater. 6 (2016) 1601372. [41] Z. Li, L. Zhang, B.S. Amirkhiz, X. Tan, Z. Xu, H. Wang, B.C. Olsen, C.M.B. Holt, D. Mitlin, Adv. Energy Mater. 2 (2012) 431-437. [42] M.R. Lukatskaya, S.-M. Bak, X. Yu, X. Yang, M.W. Barsoum, Y. Gogotsi, Adv.

Energy Mater. 5 (2015) 1500589. [43] M. Okubo, A. Sugahara, S. Kajiyama, A. Yamada, Acc. Chem. Res. 51 (2018) 591-599. [44] O. Mashtalir, M. Naguib, V.N. Mochalin, Y. Dall’Agnese, M. Heon, M.W. Barsoum, Y. Gogotsi, Nat. Commun. 4 (2013) 1716. [45] L. Dong, Y. Li, L. Wang, S. Xu, F. Hou, Mater. Lett. 130 (2014) 180-183. [46] Z. Pan, W. Lv, Y. He, Y. Zhao, G. Zhou, L. Dong, S. Niu, C. Zhang, R. Lyu, C. Wang, H. Shi, W. Zhang, F. Kang, H. Nishihara, Q. Yang, Adv. Sci. 5 (2018) 1800384. [47] S. Hu, S. Zhang, N. Pan, Y.-L. Hsieh, J. Power Sources 270 (2014) 106-112. [48] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, F. Wei, Adv. Funct. Mater. 21 (2011) 2366-2375. [49] L. Dong, C. Xu, Y. Li, C. Wu, B. Jiang, Q. Yang, E. Zhou, F. Kang, Q. Yang, Adv. Mater. 28 (2016) 1675-1681. [50] L. Dong, G. Liang, C. Xu, W. Liu, Z. Pan, E. Zhou, F. Kang, Q. Yang, Nano Energy 34 (2017) 242-248. [51] V. Khomenko, E. Raymundo-Piñero, F. Béguin, J. Power Sources 153 (2006) 183-190. [52] A. Di Fabio, A. Giorgi, M. Mastragostino, F. Soavi, J. Electrochem. Soc. 148 (2001) A845-A850. [53] Y. Li, C. Chen, J. Mater. Sci. 52 (2017) 12348-12357. [54] L. Dong, C. Xu, Q. Yang, J. Fang, Y. Li, F. Kang, J. Mater. Chem. A 3 (2015) 4729-4737. [55] T. Hu, J. Wang, H. Zhang, Z. Li, M. Hu, X. Wang, Phys. Chem. Chem. Phys. 17 (2015) 9997-10003.

[56] J. Yang, X. Xiao, P. Chen, K. Zhu, K. Cheng, K. Ye, G. Wang, D. Cao, J. Yan, Nano Energy 58 (2019) 455-465. [57] H. Niu, X. Yang, Q. Wang, X. Jing, K. Cheng, K. Zhu, K. Ye, G. Wang, D. Cao, J. Yan, J. Energy Chem. 46 (2020) 105-113. [58] J. Halim, K.M. Cook, M. Naguib, P. Eklund, Y. Gogotsi, J. Rosén, M. Barsoum, Applied Surface Science 362 (2016) 406-417.

Graphical abstract Few-layer Ti3C2Tx MXene was synthesized from multi-layered Ti3C2Tx via a flash freezing-assisted delamination process, during which the water molecules in the interlayers of multi-layered MXene are induced to rearrange and produce volume expansion.