High-Capacitance Mechanism for Ti3C2Tx MXene by in Situ Electrochemical Raman Spectroscopy Investigation Minmin Hu,†,§ Zhaojin Li,†,‡ Tao Hu,†,‡ Shihao Zhu,†,‡ Chao Zhang,† 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, Hefei 230026, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: MXenes represent an emerging family of conductive two-dimensional materials. Their representative, Ti3C2Tx, has been recognized as an outstanding member in the field of electrochemical energy storage. However, an in-depth understanding of fundamental processes responsible for the superior capacitance of Ti3C2Tx MXene in acidic electrolytes is lacking. Here, to understand the mechanism of capacitance in Ti3C2Tx MXene, we studied electrochemically the charge/ discharge processes of Ti3C2Tx electrodes in sulfate ioncontaining aqueous electrolytes with three different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the H2SO4 electrolyte bonds with the terminal O in the negative electrode upon discharging while debonding occurs upon charging. Correspondingly, the reversible bonding/debonding changes the valence state of Ti element in the MXene, giving rise to the pseudocapacitance in the acidic electrolyte. In stark contrast, only electric double layer capacitance is recognized in the other electrolytes of (NH4)2SO4 or MgSO4. The charge storage ways also differ: ion exchange dominates in H2SO4, while counterion adsorption in the rest. Hydronium that is characterized by smaller hydration radius and less charge is the most mobile among the three cations, facilitating it more kinetically accommodated on the deep adsorption sites between the MXene layers. The two key factors, i.e., surface functional group-involved bonding/debonding-induced pseudocapacitance, and ion exchange-featured charge storage, simultaneously contribute to the superior capacitance of Ti3C2Tx MXene in acidic electrolytes. KEYWORDS: MXene, two-dimensional materials, in situ Raman spectroscopy, charging mechanism, pseudocapacitance, supercapacitor, electrochemical capacitor
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chemistry of carbon, does not tap into metal redox reactions as in ruthenium oxide (RuO2), and its electrical conductivity is substantially decreased by the addition of redox-active functional groups.6 In search for alternatives, MXenes, a recently discovered 2D materials family of early transition metal carbides and carbonitrides, have shown much promise over 2D carbonbased materials in EC electrodes. The most widely studied MXene to date has been Ti3C2Tx. It was obtained by selectively etching off the Al element from Ti3AlC2, a layered ternary carbide among a family referred to as MAX phases.7 The surfaces of the etched Ti3AlC2 are typically terminated by O,
limate change and the decreasing availability of fossil fuels urgently require society to move toward sustainable and renewable resources. To achieve widespread usage of renewable energies, efficient energy storage and conversion technologies are demanded. Electrochemical capacitor (EC) or supercapacitor with excellent rate performance and cyclability plays an important role in the efficient energy storage methodologies. The performance of ECs strongly depends on the electrochemical behavior of their electrode materials.1,2 In the past decade, substantial efforts have been devoted to the development of two-dimensional (2D) materials for electrochemical energy storage applications owing to their outstanding electrochemical performance.3 The most widely used active electrodes are generally made from carbon-based materials, like graphene.4,5 However, graphene, having a limited © 2016 American Chemical Society
Received: September 30, 2016 Accepted: November 12, 2016 Published: November 12, 2016 11344
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Figure 1. Capacitive performances of Ti3C2Tx thin films in 1 mol/L solutions of H2SO4, (NH4)2SO4, and MgSO4 at room temperature. (a) Schematic of Ti3C2Tx films. (b) TEM image of Ti3C2Tx flakes after ultrasonic irradiation. (c) CV curves obtained at a scan rate of 20 mV/s. (d) Gravimetric capacitances at different scan rates.
OH, and/or F with a formula Ti3C2Tx, where Tx stands for a general surface termination.8 Ti3C2Tx is intrinsically of electronic conductivity,9 which is crucial for electrochemical energy storage. More recent results have shown that Ti3C2Tx electrodes have high capacitances and perform well at high rates, no matter Ti3C2Tx free-standing film,6 self-assembled Ti3C2Tx films with nickel foam,10 sandwich-like MXene/CNT papers,11 or PPy/Ti3C2Tx composite12 and so on.13,14 As for its amazing electrochemical performance, attempts have been made to explain it. For example, X-ray diffraction (XRD) revealed the spontaneous intercalation of cations from aqueous salt solutions between 2D Ti3C2Tx MXene layers.15 The presence of both shallow and deep adsorption sites in the interlayer gap of the Ti3C2Tx layers is suggested to account for the excellent rate performance of Ti3C2Tx.16 Moreover, it was found that the electrochemical behavior of Ti3C2Tx in sulfuric acid is predominantly pseudocapacitive.17 It is interesting to note that an acidic solution was frequently chosen as the electrolyte in many reports, wherein remarkably higher capacitances are all obtained. However, an in-depth understanding of fundamental processes responsible for the superior performance of Ti3C2Tx electrode in acidic electrolyte is lacking. Herein, to understand the mechanism of capacitance in Ti3C2Tx MXene, we studied electrochemically the charge/ discharge processes of Ti3C2Tx electrodes in sulfate ioncontaining aqueous electrolytes with three different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the H2SO4 electrolyte is involved in bonding with the terminal O in the Ti3C2Tx negative electrode upon discharging, while debonding occurs upon charging. As a result, the reversible surface functional group-involved bonding/ debonding changes the valence state of the Ti element, giving rise to the pseudocapacitance in the acidic electrolyte. Whereas only an electric double layer capacitance is observed in
(NH4)2SO4 or MgSO4 electrolyte. The charge storage ways also differ: ion exchange in H2SO4 and counterion adsorption in (NH4)2SO4 and MgSO4. Kinetically, hydronium with smaller hydration radius and less charge is the most mobile among the three cations in this study, facilitating it more accommodated on the deep adsorption sites between the Ti3C2Tx layers. The redox process is therefore not diffusion controlled but rather due to surface capacitive effects in the case of H2SO4. The two key factors, i.e., surface functional group-involved bonding/ debonding-induced pseudocapacitance and ion exchangefeatured charge storage, simultaneously contribute to the superior capacitance of Ti3C2Tx MXene in acidic electrolyte over the other two aqueous electrolytes.
RESULTS AND DISCUSSION The preparation of Ti3C2Tx film electrodes followed the method10 reported previously (Figures 1a,b and S1−3). Figure 1c presents the cyclic voltammetry (CV) curves as specific gravimetric capacitances vs potential of the Ti3C2Tx film electrodes in different electrolytes. Interestingly, for the identical electrodes, a much higher capacitance was recorded in H2SO4 electrolyte than those in its counterparts of (NH4)2SO4 and MgSO4. Noteworthily, the Ti3C2Tx film electrodes exhibit excellent rate performance in the three electrolytes, as shown in Figure 1d. The dramatic difference in capacitance for the three different electrolytes with the same identity of anion witnesses that the cation plays a critical role in charge storage. There are two kinds of cations in (NH4)2SO4 electrolyte, H+ and NH4+. To figure out which kind of cations (H+ or NH4+) dominates the capacitance, we performed CV scans in (NH4)2SO4 solution (1 mol/L, pH = 5.22) and a diluted H2SO4 solution with the same pH value as the ammonium sulfate solution, respectively (Figure S4). Clearly, the negligible capacitance in the diluted H2SO4 solution demonstrates that NH4+ instead of H+ dominates in electro11345
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Figure 2. In situ Raman spectra of Ti3C2Tx MXene recorded on: (a) positive electrode in H2SO4 electrolyte, (b) negative electrode in H2SO4 electrolyte, (c) positive electrode in (NH4)2SO4 electrolyte, and (d) negative electrode in (NH4)2SO4 electrolyte. Significant voltagedependent changes in Raman bands are only recognized in (b). The band at 726 cm−1 red shifts continuously down to 708 cm−1 when the potential sweeps from 0 V to −0.4 V and shifts back completely when scanned reversely.
range of 530−770 cm−1 and corresponding Lorentzian peak fits (Figure 3) demonstrate that as the intercalation goes further, the peak at 726 cm−1 shifts toward lower wavenumber and strengthens simultaneously. According to our previous work19 in Table S1, the mode at 726 cm−1 is assigned to out-of-plane vibrations of C atoms in Ti3C2O2, and 708 cm−1 belongs to out-of-plane vibrations of C atoms in Ti3C2O(OH). Upon discharging, hydronium ions are capable of intercalating into Ti3C2Tx layers in H2SO4 solution. Consequently, the gradually softened band evolution, as shown in Figures 2b and 3, is reasonably ascribed to the transformation from Ti3C2O2 to Ti3C2O(OH). This transformation can be proven by the change of other Raman bands. The 590 cm−1 band in Ti3C2O2 gradually weakens, while the modes at 630 and 672 cm−1 in Ti3C2(OH)2 almost remain unchanged during discharging, indicating that the quantity of Ti3C2O2 unit in the electrode decreases and that of Ti3C2(OH)2 is invariable. Similar change trend was observed for the bands in the range of 150−300 cm−1 (Figure S7). This result implies that Ti3C2O2 transformed into Ti3C2O(OH) rather than Ti3C2(OH)2, in other words, only half of −O in Ti3C2O2 transformed into −OH. Generally, the tilting of samples may also influence the Raman peak intensity in some anisotropic systems. We have carefully excluded this possibility (Figure S8). Based on the
chemical energy storage in the (NH4)2SO4 electrolyte. Similarly, for the MgSO4 electrolyte, divalent Mg2+ dominates. Raman spectroscopy allows for a detailed and time-resolved investigation of the kinetics of complex physical or chemical processes in a nondestructive manner.18 To shed light on the mechanism hidden in the electrochemical performance, we employed in situ electrochemical Raman spectroscopy measurements to accurately monitor the processes of Ti3C2Tx film electrodes in H2SO4, (NH4)2SO4, and MgSO4 electrolytes. We used a two-electrode open system for the measurements. The schematic of in situ measurement setup is illustrated in Figure S5. Very interestingly, significant voltage-dependent changes in Raman bands assigned to Ti3C2Tx are only recognized in such a condition that the electrode is negatively charged in the H2SO4 electrolyte (Figure 2). In contrast, the Raman bands almost remain intact when the electrodes are negatively charged in the others electrolytes investigated in this work (Figures 2d and S6a). Figure 2b shows the voltage-dependent Raman bands evolution when the electrode is negatively charged in H2SO4 electrolyte. There is a striking feature that the evolution in Raman bands is reversible. When scanned from 0 to −0.4 V, some specific Raman peaks shift dramatically, especially the mode at 726 cm−1. When scanned backward from −0.4 to 0 V, these peaks reversely shift back. Representative bands in the 11346
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H2SO4, while only electric double layer capacitance is observed in (NH4)2SO4 or MgSO4 solution. When testing the positive electrode of the two-electrode system in the three different electrolytes respectively, the spectra all remain unchanged (Figures 2a,c and S6b). It can be explained by the counterions (SO42−) only adsorb on the positively charged electrode and have little effect on the material. We should notice that the active nickel foam might affect the mechanism of the charge storage. To exclude the possible influence, we performed the same in situ measurement by using a carbon fiber-based current collector (Figure S9). As shown in Figure S9, the significant voltage-dependent changes in Raman bands assigned to Ti3C2Tx are also recognized, which indicates that the evolution in Raman bands of Ti3C2Tx with the applied potential is intrinsically featured rather than from the current collector. Accompanying the shift in Raman bands of the Ti3C2Tx MXene, another very interesting phenomenon is that the intensities of Raman bands assigned to SO42− (980 cm−1) and HSO4− (1040 cm−1)22 increased with stepwise potential from 0 V to −0.4 V and decreased when the potential scanned reversely in the H2SO4 electrolyte (Figure 2b), while in the other two electrolytes of (NH4)2SO4 and MgSO4, the observed SO42− Raman bands remain almost intact (Figures 2c,d and S6). The change in peak intensity is consistent with the number change of SO42− or HSO4−. According to the state-of-the-art perspective on the supercapacitor charging mechanism proposed very recently by Clare P. Gray,23 the charge storage ways can be categorized into three distinct situations, given the large number of ions inside the material at 0 V. First, charge may be balanced by the adsorption of counterions, which is the traditional view of charging. Second, counterion adsorption is accompanied by simultaneous co-ion desorption from the materials, which was referred to as ion exchange (where co-ions are defined as having charge with the same sign as the electrode). Third, charging is driven purely by the desorption of co-ions. At 0 V, the signals of anions are observable in Raman spectra−the mode of SO42− (980 cm−1) and/or of HSO4− (1040 cm−1), indicating that the material is filled with ions initially. The increase and decrease in the number of SO42− and HSO4− as recognized by Raman spectra with changing the applied potentials on negative electrode in H2SO4 electrolyte may result from ion exchange occurring during discharging (Figure 4a). Under other circumstances, as in (NH4)2SO4 or MgSO4 electrolyte, counterion adsorption (Figure 4b) is evidenced by the invariability of the number of anions. Ion exchange is more favorable in the mobility of ions between MXene layers. The excellent capacitance of Ti3C2Tx film electrodes in H2SO4 electrolyte is also understood from a kinetic point of view. For a capacitive energy storage device, the stored charge of an electrode comes from two components: the contribution of diffusion-controlled process and that of surface capacitive effect-controlled process.24 These effects can be characterized by analyzing the CV data. Assuming that the current obeys a power-law relationship with the sweep rate, ν, leads to25
Figure 3. Selected spectra and Lorentzian fits of bands at 590, 630, 672, 708, 721, and 726 cm−1 of the negative electrode in H2SO4 electrolyte, and the change in Raman vibration modes during charging and discharging. Note that there is a reversible electrochemical transformation between MO and M−OH in the presence of hydronium. Green balls: Ti atoms, black: C atoms, red: O atoms, and white: H atoms.
above discussion, we can deduce an electrochemical reaction formula upon discharging: (MOx ) + 1/2x e− + 1/2x H+ → M−O(1/2)x (OH)(1/2)x (1)
where 1/2x is the number of electrons participating in electrochemical reaction, and M represents transition metal (herein, M = Ti). The transformation of O groups to OH supports Gogotsi’s work by X-ray adsorption spectroscopy.17 The process is analogue to the quinone−hydroquinone reaction in the case of SWCNTs with oxygen-containing surface functionalities.20 Assuming that the O:OH ratio is 0.54:0.12 (ref 21) and only half of O functional groups transformed into OH, we can get a specific capacitance value of 322 F/g. The value is in a good agreement with the experimental specific capacitances measured herein (around 400 F/g). When the Raman spectra were collected in (NH4)2SO4 electrolyte (Figure 2c,d), all Raman bands assigned to Ti3C2Tx MXene remain intact. In the case of MgSO4 electrolyte, the Raman spectra are similar to those in (NH4)2SO4 electrolyte (Figure S6). Based on these experimental results, it is reasonable to attribute the distinction to the fact that the hydronium in the H2SO4 electrolyte is involved in bonding with the termination of O in the Ti3C2Tx negative electrode under the drive of applied potential. The bonding changes the state of Ti and transforms Ti3C2O2 into Ti3C2O(OH), while NH4+ or Mg2+, the major cation, just adsorbs on the surface of the Ti3C2Tx film. As a result, reversible redox reactions occur at or near the surface of Ti3C2Tx MXene when hydronium ions contact with it in
i = avb
(2)
where a and b are adjustable values. In particular, a b-value of 0.5 indicates a totally diffusion-controlled process, whereas a value of 1 represents a surface capacitive process. By plotting log i vs log ν, b-values determined as the slopes as a function of potential are about 1.0 in H2SO4, indicating that the current comes primarily from the surface capacitive effect-controlled 11347
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It is well-known that there exists shallow adsorption sites near the edges of the film electrodes that are water-rich, and deep adsorption sites with higher activation energies for ion adsorption in the film’s interior.16 Due to that in H2SO4 electrolyte, the cation H+ with a smaller hydration ion radius is easily intercalated into deep-adsorption sites, while NH4+ or Mg 2+ with larger hydration ion radius is difficult to accommodate on the deep-adsorption sites, the surface capacitive effect was dominated in the H2SO4 electrolyte. This argument can be supplemented by electrochemical impedance spectroscopy (EIS). In Figure 5b, the impedance spectra can be divided into two regions by the so-called knee frequency, with a semicircle arc in the high-frequency region and a straight line in the low-frequency region. The diameter of the semicircle in the high frequency range determines the value of charge transfer resistance (Rct), which originates from ion transfer across the entire interface of Ti3C2Tx flakes in contact with the electrolyte solution. It is a combination of electrolyte accessible area and electrical conductivity of the electrode material. The larger the electroactive surface area, the lower the Rct. The electrode material in these three electrolytes is identical, so the difference in Rct values (Table S3) results from the electroactive surface area. The equivalent circuit adopted in the simulation of EIS spectra is illustrated in Figure S12. The lower Rct value of the film electrode in H2SO4 electrolyte means the larger the electroactive surface area that hydronium ion contacted with. Besides, the Ti3C2Tx electrode in H2SO4 electrolyte behaves more closely as an ideal capacitor, which can be presented by the nearly vertical line in the low-frequency region and a constant-phase element (CPE) with a fractional exponent α = 0.96 (Table S3). Figure S13 also describes the effect of the presence of both shallow and deep adsorption sites on the CV curves at different scan rates.
Figure 4. Schematics showing charge storage mechanisms of Ti3C2Tx negative electrode. For the identical electrodes, the charging mechanism differs depending on the electrolyte: (a) ion exchange in H2SO4 solution and (b) counterion adsorption in (NH4)2SO4 solution.
process, while the b-values in (NH 4 ) 2 SO 4 and MgSO 4 electrolytes are around 0.8, exhibiting a diffusion-limited and surface capacitive effect-limited behavior as discussed above (Figure S10). To further quantify the surface capacitive effect and diffusion-limited contributions to the overall capacitance, we followed the approach described in the literature.26 The results are summarized in Figures 5a and S11 and Table S2. As shown in Figure 5a, it is evident that far greater surface capacitive effect-controlled currents were observed for the electrode in H2SO4 electrolyte compared to in (NH4)2SO4 and MgSO4 electrolytes. For example, at a scan rate of 20 mV/s, 87.3% of the total current, namely the surface capacity, is obtained in H2SO4 while 69.1% in MgSO4. These results are consistent with those in Figure S10 in that the surface capacitive effect-controlled currents are dominant in H2SO4 (b = 1).
CONCLUSIONS In summary, we have studied electrochemically the charge/ discharge processes of Ti3C2Tx electrodes in sulfate ioncontaining aqueous electrolytes with different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the H2SO4 electrolyte is involved in bonding with the termination of O in the Ti3C2Tx negative electrode upon discharging, while debonding occurs upon charging. Consequently, the surface functional group-involved reversible bonding/debonding changes the valence state of Ti, accounting for the pseudocapacitance in the acidic electrolyte. In contrast,
Figure 5. (a) Contribution ratio of the surface capacitive effect-controlled charge at various scan rates in H2SO4, (NH4)2SO4, and MgSO4 electrolytes. (b) Nyquist plots of Ti3C2Tx thin film in various solutions investigated. 11348
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Fabrication of Ti3C2Tx/Carbon Fibers Electrodes. Copper foam (thickness: 2 mm, areal pore density: 480 ppi) was first rolled into a 0.4 mm-thick foam sheet and then cut into the required size (1.5 cm × 4 cm) using scissors. A hole of 0.8 cm × 0.8 cm was made in the front of the copper sheet. The edges of two sides of the hole were glued to the conductive paste. Carbon fiber bundles were fixed on the conductive paste hanging above the hole. To prevent the acid electrolyte from diffusing into the copper foam, the room temperature curable silicone rubber was filled in the pores of the foil. Prior to MXene deposition, the carbon fibers were exposed to oxygen plasma for 3 min to make them hydrophilic. The process of coating Ti3C2Tx on carbon fibers is the same as the fabrication of Ti3C2Tx on nickel foam. Assembly of Three-Electrode Supercapacitor. All electrochemical measurements were performed in three-electrode cells unless specified otherwise, in which Ti3C2Tx/nickel foam served as the working electrode, platinum was used as the counter electrode, and Ag/AgCl in saturated KCl was the reference electrode.10 A porous filter membrane (pore size: 0.22 μm, thickness: 140 μm, Shanghai Xinya Purification Device Factory, China) was used as separator. The electrolytes were H2SO4, (NH4)2SO4, and MgSO4. Their concentrations are all 1 mol/L. Electrochemical Measurements. The electrochemical performance of the Ti3C2Tx material on nickel foam was evaluated using a three-electrode test cell at room temperature.10 CV and EIS measurements were all performed on an electrochemical workstation (PARSTAT 2273, Princeton Applied Research). The voltage ramp rates ranged from 2 to 100 mV/s. The gravimetric capacitance was calculated from the CV curve according to Cg = (∫ idV)/(vmV), where i is the current, v the voltage scan rate, m the mass of Ti3C2Tx in working electrodes, and V the voltage window. EIS spectra were recorded from 10 mHz to 200 kHz at 0 V with an amplitude of 10 mV. In Situ Electrochemical Raman Spectroscopy Measurements. For the in situ Raman spectroscopy measurements, we used a two-electrode open system. Two pieces of the prepared Ti3C2Tx/ nickel foam or Ti3C2Tx/carbon fibers composite electrodes were assembled face-to-face, between which a porous filter membrane (pore size: 0.22 μm, thickness: 140 μm) was placed. The electrolytes investigated in this study were H2SO4, (NH4)2SO4, and MgSO4. Their concentrations are all 1 mol/L. The unpolarized Raman spectra were collected on a LabRAM HR800 (Horiba Jobin-Yvon, France) equipped with an air-cooled CCD array detector in the backscattering configuration. A He−Ne laser (632.82 nm) with an incident power of around 8 mW was used as the excitation source. A 50 long workingdistance objective with numerical aperture of 0.50 was used. The spot size of the laser was focused to 2 μm. The spectral resolution was 0.65 cm−1. The schematic diagram of the measurement device is illustrated in Figure S5. The in situ Raman spectra of the two electrodes were collected, respectively, when they were potentiostatically charged stepwise using a DC power supply (Agilent E3641A) from 0 to 0.4 V with a step of 0.1 V and then reversibly from 0.4 to 0 V.
only electric double layer capacitance is recognized in the (NH4)2SO4 or MgSO4 electrolyte. In addition, for the identical electrode, the charge storage ways also differ depending on the electrolyte: ion exchange in H2SO4 and counterion adsorption in (NH4)2SO4 and MgSO4. Hydronium with a smaller hydration radius and less charge is the most mobile among the three kinds of cations in this study, facilitating it more kinetically accommodated on the deep adsorption sites between the Ti3C2Tx layers. In H2SO4, the redox process is therefore not diffusion-controlled but rather due to surface capacitive effects. The two key factors, i.e., surface functional group-involved bonding/debonding-induced pseudocapacitance and ion exchange-featured charge storage, simultaneously contribute to the superior performance of Ti3C2Tx electrode in acidic electrolyte over the other two aqueous electrolytes. Our understanding here provides guidelines to design MXenebased materials with higher capacitance in the near future.
EXPERIMENTAL METHODS Preparation of Porous Ti3AlC2 Monolith. The porous Ti3AlC2 monolith (porosity, ∼40%) used in this work was prepared in the authors’ laboratory by the solid−liquid reaction synthesis method27 using elemental powders of Ti (99%, −300 mesh), Al (99%, D50 = 10 μm) and graphite (99%, D90 = 6.5 μm) in a molar ratio of 3:1.1:1.88. Briefly, the powders were mixed for 12 h with agate balls and absolute alcohol in an agate jar, followed by drying at 70 °C for 8 h in air. The homogenized mixture was then uniaxially cold pressed into a green compact in a graphite mold. Subsequently, the green compact together with the mold was heated in a furnace up to 1550 °C and held at this temperature for 2 h in a flowing Ar atmosphere. Finally, the sample was naturally cooled down to room temperature. The prepared porous monolith is phase pure as determined by XRD examination (see Figure S1). Synthesis of Ti3C2Tx MXene. A piece of the prepared Ti3AlC2 monolith (1 g) was immersed in 6 mol/L HF solution for 72 h at room temperature.10 With this treatment, the monolith turned into particulate-like sediment (see Figure S2a). The resulting particulates were separated by vacuum filtration with a porous membrane filter (0.22 μm pore size) and washed with deionized water until the pH of the supernatant reached approximately 4.0. The separated wet sediment was immersed in deionized water, after which the mixture was sonicated in a pulse mode for 1 h by using an ultrasonic homogenizer (JY96-IIN, Scientz) to obtain a suspension. The suspension was then centrifuged for 30 min at 2000 rpm to remove the large particulates. Finally, a bottle of black Ti3C2Tx MXene colloidal supernatant was obtained after decantation (Figure S2b). The prepared Ti3C2Tx MXene and Ti3C2Tx MXene colloidal supernatant are determined by XRD examination (see Figure S1). Fabrication of Ti3C2Tx/Nickel Foam Electrodes. Nickel foam (thickness: 1.65 mm, areal pore density: 480 ppi, Heze Tianyu Technology Development Co., LTD, China) was first rolled into a 0.32 mm-thick foam sheet and then cut into the required size (1 cm × 1 cm) using scissors, followed by rinsing in alcohol to clean the foam surface.10 The Ti3C2Tx colloidal suspension was loaded dropwise with a pipet on the pretreated nickel foam sheet that was placed on a hot plate kept at 50 °C. To prevent the hot plate surface from contamination, a hydrophobic polytetrafluoroethylene (PTFE) film was placed between the nickel foam and the hot plate. The Ti3C2Tx content of the whole electrode can be easily controlled by adjusting the volume of the suspension. Upon mild baking, water in the suspension evaporated, and the remaining flakes self-assembled into a coating on the skeleton of the nickel foam. The mass of the Ti3C2Tx loaded on the nickel foam was determined by measuring the weight change after the mild baking. An electronic balance (BP 211D, SARTORIUS) used for the weight measurement has a resolution of 0.01 mg. The cross-section image of the coating film is shown in Figure S3.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06597. Experimental details and data (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaohui Wang: 0000-0001-7271-2662 Notes
The authors declare no competing financial interest. 11349
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DOI: 10.1021/acsnano.6b06597 ACS Nano 2016, 10, 11344−11350
