Surface Functional Groups and Interlayer Water ...

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Surface Functional Groups and Interlayer Water Determine the Electrochemical Capacitance of Ti3C2Tx MXene

Minmin Hu,†,‡ Tao Hu,†,§ Zhaojin Li,†,§ Yi Yang,⊥ Renfei Cheng,†,‡ Jinxing Yang,†,‡ Cong Cui,†,‡ and Xiaohui Wang*,† †

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China § University of Chinese Academy of Sciences, Beijing 100049, China ⊥ Suzhou Niumag Analytical Instrument Corporation, Suzhou 215163, China ‡

S Supporting Information *

ABSTRACT: MXenes, an emerging class of conductive two-dimensional materials, have been regarded as promising candidates in the field of electrochemical energy storage. The electrochemical performance of their representative Ti3C2Tx, where T represents the surface termination group of F, O, or OH, strongly relies on terminationmediated surface functionalization, but an in-depth understanding of the relationship between them remains unresolved. Here, we studied comprehensively the structural feature and electrochemical performance of two kinds of Ti3C2Tx MXenes obtained by etching the Ti3AlC2 precursor in aqueous HF solution at low concentration (6 mol/L) and high concentration of (15 mol/L). A significantly higher capacitance was recognized in a low-concentration HF-etched MXene (Ti3C2Tx−6M) electrode. In situ Raman spectroscopy and X-ray photoelectron spectroscopy demonstrate that Ti3C2Tx−6M has more components of the −O functional group. In combination with X-ray diffraction analysis, low-field 1H nuclear magnetic resonance spectroscopy in terms of relaxation time unambiguously underlines that Ti3C2Tx−6M is capable of accommodating more high-mobility H2O molecules between the Ti3C2Tx interlayers, enabling more hydrogen ions to be more readily accessible to the active sites of Ti3C2Tx−6M. The two main key factors (i.e., high content of −O functional groups that are involved bonding/ debonding-induced pseudocapacitance and more high-mobility water intercalated between the MXene interlayers) simultaneously account for the superior capacitance of the Ti3C2Tx−6M electrode. This study provides a guideline for the rational design and construction of high-capacitance MXene and MXene-based hybrid electrodes in aqueous electrolytes. KEYWORDS: MXene, two-dimensional materials, functional groups, interlayer water, supercapacitor

T

Recently, MXenes, an emerging two-dimensional (2D) materials group of early transition metal carbides and/or carbonitrides, have proven to be a pseudocapacitive electrode material with high capacitance.14−23 MXenes can be prepared by selectively removing the A element from MAX phases, a family of layered solids with a chemical formula of Mn+1AXn, where M is a transition metal, A is an A-group element, and X is C and/or N.24,25 After etching of the A layers, the MX layers left in the etchants are spontaneously terminated with O, F, or OH groups, giving a general formula Mn+1XnTx, where Tx represents a general surface termination.26 Ti3C2Tx is the most intensively studied to

he growth of energy demands and more attention to global environmental issues call for urgent exploration for clean energies and advanced energy storage systems.1,2 Electrochemical capacitors, known as supercapacitors, are the ideal candidates among the efficient energy storage methodologies, owing to the high power density, excellent rate performance, long cycle life, and so on.3,4 Pseudocapacitors that can provide energy densities higher than those of electrical doublelayer capacitors, making use of reversible and fast surface redox reactions, have attracted much attention.5−7 The most studied materials for pseudocapacitors are RuO2,8 MnO2,9 Nb2O5,10 NiO,11 and so on,12 but the high electrode resistance resulting from the limited electronic conductivity of most pseudocapacitive oxides always leads to a lower rate performance.13 Therefore, it is urgent to explore promising active materials with excellent electrochemical performance. © XXXX American Chemical Society

Received: January 25, 2018 Accepted: April 2, 2018 Published: April 2, 2018 A

DOI: 10.1021/acsnano.8b00676 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Capacitive performances of Ti3C2Tx−6M and Ti3C2Tx−15M electrodes in 1 mol/L H2SO4 electrolyte. (a) CV curves collected at a scan rate of 20 mV/s. (b) Gravimetric capacitances at different scan rates. Note that the capacitance of Ti3C2Tx−6M is significantly greater than that of Ti3C2Tx−15M at each scan rate.

date among the discovered MXenes. The Ti3C2Tx interlayers have interactions like graphite or MoS227 and have intrinsic electronic conductivity.28 Ti3C2Tx MXene stores charge through pseudocapacitance induced by surface functional group bonding/debonding in the H2SO4 electrolyte.17,18,29 Termination plays important roles in this charge storage process. Many efforts have been devoted to tune the surface groups to modulate the electrochemical properties of MXene. For example, MXenes, after cation intercalation30,31 and/or annealing treatment,32 show an improvement in the electrochemical performance, which is largely attributed to the decrease of the content of −F groups. So far, in-depth and comprehensive research about the relationship between structural features and fundamental electrochemical properties of Ti3C2Tx MXene remains lacking, which critically impedes the process of exploring MXene and MXene-based hybrid electrodes for electrochemical applications. In order to figure out the correlation between termination and electrochemical performance, we should first obtain the MXenes with different termination species and proportions. It is worth noting that the termination species and proportions are dependent on the treatment conditions.33−37 Lower HF concentrations always result in a larger −O group to −F group ratio.33 NMR spectra of Ti3C2Tx MXene demonstrated that HF synthesized material has almost 4 times as much −F termination as the LiF−HCl synthesized material.36 Importantly, the capacitive performance is highly dependent on the synthesis methods by means of selectively etching off Al atomistic layers from the laminated Ti3AlC2 precursor (Table S1). It is thus of scientific importance to have insight into the huge discrepancy in electrochemical capacitance. Importantly, etching is the first step to prepare MXene electrodes. Once the relationship between the structure and electrochemical performance is confirmed, one can directly optimize the etching environment to achieve higher capacitance instead of conducting further subsequent treatments such as annealing in a variety of environments32 or intercalations.30,31,38 Herein, we comprehensively studied the correlation between the structure and electrochemical performance of two kinds of Ti3C2Tx MXenes obtained by etching the Ti3AlC2 precursor in two aqueous HF solutions with different concentrations (6 and 15 mol/L), followed by ultrasonication. Even though the acidic etchant and the sonication method are identical, the capacitance recorded in the 6 mol/L HF-etched Ti3C2Tx MXene (denoted as Ti3C2Tx−6M) electrode is nearly 2 times that of the 15 mol/L HF-etched Ti3C2Tx MXene (Ti3C2Tx−15M) electrode. Such a huge difference in capacitance is attributed to the distinction in −O functional group content confirmed by in situ Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, more high-mobility H2O molecules between the

Ti3C2Tx layers facilitate more hydrogen ions to have access to the active sites of Ti3C2Tx−6M, giving a dramatic increase in gravimetric performance.

RESULTS AND DISCUSSION The preparation of Ti3C2Tx−6M and Ti3C2Tx−15M followed the method described in the Experimental Section. X-ray diffraction (XRD) patterns demonstrated the extraction of Al from the laminated Ti3AlC2 precursor for the two samples investigated in this study (Figure S1). In order to figure out whether the two etching conditions would affect the electrochemical performance of the samples, we carefully examined the capacitive performance of the MXene electrodes prepared from Ti3C2Tx−6M and Ti3C2Tx−15M. The fabrication of Ti3C2Tx electrodes followed the method reported previously (Figure S2).18,39 Figure 1a plots the cyclic voltammetry (CV) curves of the Ti3C2Tx−6M and Ti3C2Tx−15M electrodes in H2SO4 electrolyte. The more rectangular and symmetric shape of the CV curve indicates a superior capacitive performance of the Ti3C2Tx−6M electrode. Additionally, a much higher capacitance was obtained in the Ti3C2Tx−6M electrode than in the Ti3C2Tx−15M electrode. The difference in capacitance between the two electrodes is as high as 192 F/g (Figure 1b). First, we examined the specific surface areas of the two samples. As shown in Figure 2, the Brunauer−Emmett−Teller (BET) specific surface areas of the Ti3C2Tx particulates obtained in 6 and 15 mol/L HF are determined to be 3.5 and 40.5 m2/g by nitrogen adsorption, respectively. Notably, the accordion-like MXene particulates with more microscopically visible slits can adsorb more N2 gas. For the purpose of film electrode preparation, the MXene particulates were delaminated by ultrasonic treatment. With the ultrasonication, the specific surface area of Ti3C2Tx−15M (25.6 m2/g) is still larger than that of Ti3C2Tx−6M (16.2 m2/g). Large specific surface area typically leads to large capacitances for carbon-based materials in which electrical double-layer capacitance dominates the capacitances.19 In contrast, Ti3C2Tx−6M with smaller BET specific surface area exhibited much higher capacitance. Thus, pseudocapacitance predominates in capacitance for the Ti3C2Tx−6M, which is relevant to the surface functional groups on the MXene. In order to figure out whether the large difference in electrochemical performance comes from the surface functional groups, we conducted in situ electrochemical Raman spectroscopy measurements to monitor the electrochemical processes of Ti3C2Tx−6M and Ti3C2Tx−15M electrodes in H2SO4. Herein, a two-electrode open device was adopted for the measurements. Figure 3 shows that Raman band on the negatively charged electrode varies orderly and reversibly with the change of voltage B


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Figure 2. Nitrogen adsorption/desorption isotherms of (a,b) as-prepared Ti3C2Tx particulates and (c,d) those treated by ultrasonication. The Ti3C2Tx MXenes are prepared in (a,c) 6 mol/L HF and (b,d) 15 mol/L HF. Insets show their typical scanning electron microscopy images of as-prepared Ti3C2Tx particulates.

Figure 3. In situ Raman spectra recorded on the negative electrode of Ti3C2Tx−6M and Ti3C2Tx−15M in the H2SO4 electrolyte. Note that the Raman bands are voltage-dependent in a reversible manner.

Ti3C2O2 gradually weaken, whereas the mode at 708 cm−1 of Ti3C2O(OH) strengthens, and the mode (630 and 672 cm−1) in Ti3C2(OH)2 almost remains intact, which indicates that both electrodes undergo the transformation from Ti3C2O2 to Ti3C2O(OH) with applied voltage. An obvious distinction is that this transformation is more dramatic in Ti3C2Tx−6M, which is because Ti3C2Tx−6M possesses more −O terminations that involve bonding/debonding and induce pseudocapacitance. For the bands in the range of 150−300 cm−1, a similar mode evolution trend was observed (Figure S3). Hence, the increase of capacitance for Ti3C2Tx−6M electrode is attributed to the higher −O group content to a large extent. To shed light on the relationship between surface functional groups and capacitances quantitatively, we characterized the two samples by XPS in terms of surface chemical composition and

in H2SO4 electrolyte. To observe the change of Raman peaks explicitly and directly, corresponding Lorentzian peak fitting of representative bands in the range of 530−770 cm−1 is performed through PeakFit software (Figure 4). Based on the result previously reported,18,40 and as summarized in Table S2, the mode (590 and 726 cm−1) comes from the vibrations of atoms in Ti3C2O2; the band at 708 cm−1 is assigned to out-of-plane vibrations of C atoms in Ti3C2O(OH), and the bands at 630 and 672 cm−1 belong to the vibrations of atoms in Ti3C2(OH)2. Prior to applying voltage (at 0 V), the Raman peak intensity of Ti3C2O2 for Ti3C2Tx−6M electrode is higher than that of Ti3C2Tx−15M electrode, which implies that −O terminations in Ti3C2Tx−6M are more than those in Ti3C2Tx−15M. When scanned from 0 to −0.4 V, whether for Ti3C2Tx−6M electrode or for Ti3C2Tx−15M electrode, the peaks at 590 and 726 cm−1 in C


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Figure 4. Selected Raman shift window and Lorentzian fits of the Raman bands centered at 590, 630, 672, 708, and 726 cm−1 for (a) Ti3C2Tx−6M and (b) Ti3C2Tx−15M. Note that the band at 726 cm−1 weakens, whereas the mode at 708 cm−1 strengthens dramatically when the potential sweeps from 0 to −0.4 V for the Ti3C2Tx−6M electrode compared with Ti3C2Tx−15M.

The difference in −O functional group content between them is 0.28. According to the result of in situ electrochemical Raman spectroscopy measurements, the electrochemical reaction upon discharging follows the equation

chemical state.41 Among the three functional groups of −F, −OH, and −O, the functional groups of −O are involved in the electrochemical reaction to store energy according to the abovediscussed and previously reported18 in situ electrochemical Raman spectroscopy analysis. So, we focus on the discrepancy of the proportion of −O groups between Ti3C2Tx−6M and Ti3C2Tx−15M. To avoid the influence of any contamination on the surface of samples from the ambient environment, we recorded the XPS spectra of Ti3C2Tx−6M and Ti3C2Tx−15M after sputtering (Figure S4). Figure 5 shows the deconvolution of Ti 2p and O 1s XPS spectra for Ti3C2Tx−6M and Ti3C2Tx−15M, respectively. The Ti 2p spectra are deconvolved with three doublets (Ti 2p 2p3/2−Ti 2p1/2) with a set area ratio equal to 2:1. The confirmation of peak position of moieties is shown in Figure S5. According to the Ti 2p and O 1s spectra of both samples, the proportion of the moieties containing the −O group in the whole spectrum for Ti3C2Tx−6M is higher than those for Ti3C2Tx−15M (Table 1), which is consistent with the result of Raman spectroscopy analysis (Figure 4). Combining the O 1s spectra with the percentage of O atoms in the Ti3C2Tx unit, we can obtain the ratio of the −O group in the Ti3C2Tx unit (Table S3). The proportion determined from C−Ti−O in the O 1s spectrum for Ti3C2Tx−6M is 49%, which is 6% higher than that for Ti3C2Tx−15M. Additionally, one Ti3C2Tx unit possesses 1.9 O atoms for Ti3C2Tx−6M, whereas one Ti3C2Tx unit has 1.5 O atoms for Ti3C2Tx−15M. Based on the above results, we can deduce that the quantity of −O groups in one Ti3C2Tx unit is 0.93 for Ti3C2Tx−6M and is 0.65 for Ti3C2Tx−15M.

(M−Ox ) +

1 − 1 + xe + x H → M−O1/2x (OH)1/2x 2 2

(1)

In the case where x is 0.28, the specific capacitance value is 167 F/g. This means that the Ti3C2Tx−6M electrode would give rise to the capacitance of 167 F/g, which is more than that for the Ti3C2Tx−15M electrode in H2SO4 solution. The value is very close to the difference in specific capacitances experimentally measured herein (192 F/g). In the above discussion, we do not take into account the −F functional group due to its low content and unstable chemical nature. Once Ti3C2Tx MXene is ideally terminated with −O functional groups (where x is 2), after discharging, an ultrahigh capacitance of 1190 F/g is predicted. By means of carefully tuning the surface chemistry of Ti3C2Tx MXene, this huge theoretical capacitance is reasonably expected. In addition to the difference in −O functional group content, there exists another siginificant difference of proportion of H2O molecules between the Ti3C2Tx−6M and Ti3C2Tx−15M interlayers, as shown in Figure 5c,d and Table 1. We know that the incorporation of confined fluid molecules into the layered and 2D materials interlayer can improve ion transport and electrochemical activity.42 To further shed light on the reason for the difference in electrochemical performance between the two samples, it is necessary to distinguish the difference of the amount D


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Figure 5. Component peak fitting of XPS spectra of Ti3C2Tx−6M and Ti3C2Tx−15M in the (a,b) Ti 2p region and (c,d) O 1s region. The proportion of the moieties containing the −O functional group in the Ti 2p spectrum or O 1s spectrum for Ti3C2Tx−6M is higher than those for Ti3C2Tx−15M.

Table 1. Fractions of the Fitted Moieties in Ti 2p and O 1s XPS Spectra for Ti3C2Tx−6M and Ti3C2Tx−15M Ti 2p

O 1s

fraction (%)

Ti−C (MXene)

C−Ti−OH

C−Ti−O

Ti−O−H

Ti−O

H2O

Ti3C2Tx−6M Ti3C2Tx−15M

31 33

26 39

43 28

33 45

49 43

18 12

can be excluded by the simulation of XRD patterns for Ti3C2Tx with different terminations (Figure S6). More −O terminations alone in Ti3C2Tx do not increase interlayer spacing. So, the increased interlayer spacing may originate from the presence of water intercalated between the MXene layers. Upon drying at elevated temperature, the (0002) reflections significantly shift to similarly high angles for both MXenes, indicating that the interlayer distances of two samples are comparable after drying at 120 °C. As discussed above, the drying below 200 °C does not change the surface termination, and the amount of water is sensitive to the drying temperature. Consequently, the change in interlayer spacing is ascribed to the volatility of the water molecules intercalated between Ti3C2Tx interlayers. The interlayer spacing difference between room temperature drying and elevated temperature (120 °C) drying for Ti3C2Tx−6M is 3.2 Å, whereas that for Ti3C2Tx−15M is only 1.1 Å. This distinction for these two samples indicates that more water molecules are accommodated between the Ti3C2Tx−6M MXene interlayers. Note that the weight loss difference from ambient drying to drying at 120 °C for Ti3C2Tx−6M is, however, comparable to Ti3C2Tx−15M (4.8% for Ti3C2Tx−6M versus 3.6% for Ti3C2Tx−15M). This discrepancy probably results from the microscopically visible slits, as seen in Figure 2, that accommodate an non-negligible amount of water for the Ti3C2Tx−15M. Having firmly established the amount of water molecules intercalated between Ti3C2Tx layers, to gain insight into the

and state of confined water between both MXene layers. The amount of water is sensitive to the degree of drying.43,44 Hence, the analysis of both MXenes at different moisture levels can help us to further understand the hydration effect between Ti3C2Tx layers. As shown in Figure 6a, with the increase of drying temperature, the weights decrease above 3% whether for Ti3C2Tx−6M or for Ti3C2Tx−15M. Generally, the drying below 200 °C does not lead to the change of the surface termination.36 In addition, the weight of MXene should remain fairly unchanged even though the surface functional groups are converted to one another owing to the close molar weights for O (16), OH (17), and F (19). Therefore, the weight decrease upon drying is reasonably ascribed to the evaporation of interlayer water. The loss of water always gives rise to the different interlayer spacing, which is confirmed by XRD. As shown in Figure 6b, when the samples are dried at room temperature (RT), the distance between MXene layers of the Ti3C2Tx−6M is significantly larger than that of the Ti3C2Tx−15M, indicating that the unit sheets of Ti3C2Tx−6M MXene are more separated. This is consistent with the fact that etching in more concentrated HF (22.6 mol/L) gives rise to decreased interlayer spacing.45 For film electrode preparation, the Ti3C2Tx−6M and Ti3C2Tx−15M particulates were delaminated by ultrasonication. We note that this treatment, however, does not lead to the increase in the interlayer distance (Figure S1). One possibility for the increase of interlayer spacing comes from the difference in surface functional groups between both samples. This possibility E


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Figure 6. (a) Normalized weight (W/WRT, where WRT is the weight of the sample dried at 25 °C) of Ti3C2Tx−6M and Ti3C2Tx−15M dried at different temperatures. Insets show the schematics of Ti3C2Tx−6M and Ti3C2Tx−15M dried at RT. (b) (0002) peaks of XRD patterns show that the interlayer spacing shrinks upon drying at 120 °C for Ti3C2Tx−6M and Ti3C2Tx−15M, whereas the shringkage extent is more obvious for Ti3C2Tx−6M. (c) 1H time-domain nuclear magnetic resonance spectra of as-synthesized Ti3C2Tx MXenes and those dried at 120 °C overnight.

hydrogen ion access to the active sites of MXenes. As a result, an improvement in gravimetric performance for the Ti3C2Tx−6M electrode was observed as its wide interlayer slit can accommodate more water molecules. The excellent capacitance of the Ti3C2Tx−6M electrode is also understood from a kinetic point of view. In general, the capacitance of an electrode may come from the contribution of a diffusion-limited process and that of a surface-capacitive effectlimited process. These effects can be characterized by analyzing the CV data. To shed light on the charge storage kinetics, we analyzed the relationship between current and scan rates. Assuming power-law dependence of the current i on sweep rate ν gives rise to

nature of water, we further characterized the state of water by means of proton NMR. It had been widely accepted that 1 H NMR relaxation times are essential parameters to monitor the state of water in a variety of material systems.46,47 In this study, the 1H NMR time-domain spectra were recorded to distinguish the identities of H-containing species in Ti3C2Tx−6M and Ti3C2Tx−15M. Figure 6c depicts the continuous distribution of spin−spin relaxation time, T2, for the two samples. Generally, there are three recognizable states of water molecular in materials, that is, bonded water, confined water, and free water, which are distinguishable by the time-domain parameter.48,49 For Ti3C2Tx−6M, it shows a broad population in the time-domain region of 1.0−3.0 ms. A similar population in the range of 0.5−2 ms can be seen for Ti3C2Tx−15M. The populations in these ranges are less mobile, coming from −OH groups with variable degrees of hydrogen bonding. The two 1H spectra show a more mobile population in the region of 7−200 ms, ascribed to H2O as their intensities are highly sensitive to the drying conditions (RT or 120 °C). The signal in the intermediate time-domain region of 7−200 ms is reasonably assigned to confined water (in the present case, it is referred to as interlayer water). Here, we should note that the interlayer water in Ti3C2Tx−6M MXene with a time-domain range of 25−200 ms is more easily relaxed than that in Ti3C2Tx−15M MXene whose time-domain range is in the range of 7−75 ms. This unambiguously demonstrates that it is easier to diffuse between MXene interlayers for hydrogen ions in Ti3C2Tx−6M than in Ti3C2Tx−15M. In order to figure out whether a larger width of interlayer slit, in which more layers of water with high mobility intercalated can store more energy, we comprehensively investigated the electrochemical performance of the Ti3C2Tx−6M electrode at different moisture levels. As shown in Figure S7, the electrode dried at low temperature exhibits a higher capacitance. We know that hydrogen ion transport is assisted by H2O molecules. More H2O lying between the Ti3C2Tx interlayers reasonably facilitates

i = aν b

(2)

where a and b are adjustable values. In particular, a b value of 0.5 is an indication of the diffusion-controlled process, whereas a value of 1 represents a surface-capacitive storage.50,51 Herein, for the Ti3C2Tx−6M electrode, the b values are about 1.0 at the measured potential, demonstrating that the current comes primarily from the surface-capacitive effect-controlled process, whereas the b values for the Ti3C2Tx−15M electrode are around 0.8, which suggests a diffusion and surface-capacitive effect jointly limited behavior as discussed previously (Figure S8a). Due to the fact that water-richer interlayers minimize ion transport limitations, surface-capacitive effect was dominated in the Ti3C2Tx−6M electrode, which is beneficial for the rate performance. Electrochemical impedance spectroscopy (EIS) data also can support this argument. In Figure S8b, the impedance spectra include a high-frequency region with a semicircle arc and a low-frequency region with a straight line. The value of charge transfer resistance (Rct) determined from the diameter of the semicircle in the high-frequency range52 of the Ti3C2Tx−6M electrode is lower than that of the Ti3C2Tx−15M electrode (Figure S9 and Tables S4 and S5). Generally, Rct is significantly negatively correlated with the electrolyte accessible area.32 The lower value of Rct for the Ti3C2Tx−6M electrode means F


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The sample for BET specific surface area measurement was prepared by being dried at 60 °C overnight. XPS was performed by a surface analysis system (ESCALAB250, Thermo VG company, USA) using monochromatic Al K (1486.6 eV) radiation. The data were collected before and after Ar+ beam (3 kV) sputtering for 60 s. Component peak fitting of XPS spectra was performed by using XPS curve-fitting software. NMR experiments were performed under 0.5 T applied magnetic field using a low-field nuclear magnetic resonance spectrometer (MesoMR, Niumag Corporation, Shanghai, China). A corresponding resonance frequency for 1H of 23.311 MHz was used for the measurement. The volume of the sample is around 2 mL. The spin−spin relaxation time, T2, was collected through Carr−Purcell−Meiboom−Gill sequences. The pulse width was 3 μs, sequence repetition time was 5 s, and the dwell time between data was 5 μs. The NMR data were analyzed using the Niumag NMR inverse software. The in situ Raman spectroscopy measurements followed the previously reported method.18 Computation Method. The Cambridge sequential total energy package55 was used to carry out the density functional theory calculations. The calculations include a complete geometry optimization40 and a subsequent core level spectra calculation. Core level spectra were calculated based on the as-obtained ground-state structures. In the core level spectra calculation, the electronic exchange energy was treated as GGA-PBE.56 In the irreducible Brillouin zone, the Monkhorst−Pack scheme57 with 9 × 9 × 1 k point meshes were used, and the individual spacing was less than 0.05 Å−1. On-the-fly generated pseudopotentials58 were used to calculate the core level spectra. The cutoff energy was set to 610 eV; the convergence for energy was assigned to be 1.0 × 10−10 eV/atom, and Fermi level was smeared by 0.1 eV.

it has a larger electroactive surface area to make contact with hydronium ion. Additionally, the nearly vertical line in the lowfrequency region and a constant-phase element with a fractional exponent α = 0.96 (Table S4) suggest that the Ti3C2Tx−6M electrode behaves more closely as an ideal capacitor.

CONCLUSION In summary, we have comprehensively compared the structure and electrochemical performance of two electrodes in which a capacitance was recorded in the Ti3C2Tx−6M electrode that was much higher than that in the Ti3C2Tx−15M electrode. Using a combination of spectroscopic, structural, and electrochemical characterization tools, we demonstrated that the huge difference in capacitance between the two Ti3C2Tx MXenes came from two aspects: first, the increased content of −O functional groups in the Ti3C2Tx−6M electrode; second, the more H2O molecules intercalated between the Ti3C2Tx interlayers enable more hydrogen ions to have access to active sites of the Ti3C2Tx−6M electrode, giving a dramatic increase in gravimetric performance. The knowledge established here not only reasonably elucidates the long-standing discrepancy in gravimetric capacitance for Ti3C2Tx MXene13,16,23,30,31,38 but also more importantly offers a practical guideline for further improvement of the capacitance. For instance, one can tune the surface chemistry through etching in lower concentrations of HF or other milder systhesis methods and/or expand the interlayer spacing by accommodating more H2O molecules between the MXene interlayers. The MXene with larger interspaces and a greater amount of water can achieve not only high capacitance but also excellent cycling performance.53

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b00676. Additional supporting figures and tables (PDF)

EXPERIMENTAL SECTION Synthesis of Ti3C2Tx MXene. The porous Ti3AlC2 monolith was prepared by means of a solid−liquid reaction.54 In a typical synthesis, two pieces of the monolith with a total weight of roughly 2 g were immersed in 10 mL of 6 and 15 mol/L HF solution (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) at RT for 72 and 48 h, respectively. The resulting powders were vacuum filtered and washed several times with deionized water. Then, deionized water was added into the as-separated wet sediment, followed by sonication in a pulse mode for 1 h, and centrifuged at 2000 rpm for 30 min to remove the large particulates. After decantation, the black Ti3C2Tx MXene colloidal supernatants were obtained for further use. Electrochemical Measurements. The electrode fabrication followed the dropping/mild baking method.18,39 The Ni foam was employed as a current collector, and the Ti3C2Tx MXene film was uniformly coated on the skeleton of the foam as the active material. The mass loading of MXene is about 1.2 mg/cm2, and the thickness of the Ti3C2Tx film is statistically measured to be 1.0 ± 0.3 μm for both samples (Figure S2). All electrochemical measurements were performed in three-electrode cells, where Ti3C2Tx−6M and Ti3C2Tx−15M served as the working electrodes; platinum was used as the counter electrode, and Ag/AgCl in saturated KCl was a reference in order to precisely control electrochemical potentials. The electrolyte was 1 mol/L H2SO4. CV and EIS measurements were performed using an electrochemical workstation (PARSTAT 2273, Princeton Applied Research). The CV curves were recorded using scan rates from 2 to 100 mV/s. EIS investigations were performed at 0 V with a 10 mV amplitude between 10 mHz and 100 kHz. Characterization of Ti3C2Tx MXenes. Microstructural morphology studies of Ti3C2Tx paticulates were conducted by scanning electron microscope (LEO Supra35, Zeiss). XRD patterns were collected on a powder diffractometer (D/max-2400, Rigaku) using Cu Kα radiation (λ = 1.5406 Å) in the range of 2θ = 5−80° with a step of 0.02°. BET specific surface areas were characterized with N2 as adsorption gas at 77 K on a physisorption analyzer (ASAP 2020, Micromeritics).

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xiaohui Wang: 0000-0001-7271-2662 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Youth Innovation Promotion Association, Chinese Academy of Sciences (CAS) under Grant No. 2011152, and Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, and by Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501. REFERENCES (1) Gogotsi, Y.; Simon, P. True Performance Metrics in Electrochemical Energy Storage. Science 2011, 334, 917−918. (2) Yang, Z. G.; Zhang, J. L.; Kintner-Meyer, M. C. W.; Lu, X. C.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577−3613. (3) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854. (4) Zhai, Y. P.; Dou, Y. Q.; Zhao, D. Y.; Fulvio, P. F.; Mayes, R. T.; Dai, S. Carbon Materials for Chemical Capacitive Energy Storage. Adv. Mater. 2011, 23, 4828−4850. G


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