Composites of TiO2 Nanoparticles Deposited on

Journal of The Electrochemical Society, 163 (5) A785-A791 (2016) 0013-4651/2016/163(5)/A785/7/$33.00 © The Electrochemical Society

A785

Composites of TiO2 Nanoparticles Deposited on Ti3 C2 MXene Nanosheets with Enhanced Electrochemical Performance JianFeng Zhu,a,=,z Yi Tang,a,=,z ChenHui Yang,b Fen Wang,a and MinJuan Caoa a Key

Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science & Technology, Xi’an 710021, People’s Republic of China b Electronic Materials Research Laboratory, School of Electronic and Information Engineering, and International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049 Shaanxi, People’s Republic of China MXene-based materials are promising electrode materials for electrochemical capacitors (ECs) due to their unique two-dimensional layered structure, high surface area, remarkable chemical stability, and electrical conductivity. TiO2 nanoparticles decorated Ti3 C2 MXene were synthesized through a simple in situ hydrolysis and heat-treatment process and subsequently fabricated as an electrode for ECs. The as-prepared Ti3 C2 , TiO2 , and TiO2 -Ti3 C2 were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results indicated that TiO2 nanoparticles with a diameter of less than 30 nm were decorated onto the Ti3 C2 MXene nanosheets. The resulting composites exhibited significantly higher specific capacitance of 143 F g−1 at 5 mV s−1 , which was 1.5 times that of pure Ti3 C2 (93 F g−1 ). Moreover, TiO2 -Ti3 C2 showed excellent cycling stability, retaining ∼92% of its initial capacitance after 6000 cycles. These results suggest that TiO2 -Ti3 C2 nanocomposite has the potential as an electrode material for high-performance energy storage devices. © 2016 The Electrochemical Society. [DOI: 10.1149/2.0981605jes] All rights reserved. Manuscript submitted September 30, 2015; revised manuscript received January 26, 2016. Published February 19, 2016.

Electrochemical capacitors (ECs), also called ultracapacitors or electrochemical double layer capacitors,1 have attracted a tremendous amount of attention as energy-storage devices due to their high power density, fast charge–discharge ability, excellent reversibility, and long cycling life.2–5 Due to these advantages, supercapacitors make up many markets ranging from electronics to transportation and stationary applications.6 There are two types of ECs, which differ by the charge storage mechanism: the first, known as electrical double-layer capacitors (EDLCs), in which the capacity is due to the electrosorption of ions on porous carbon electrodes, have limited energy density.6,7 Typical EDLC materials have a high surface area, such as activated carbon, carbon nanotubes, and graphene-based active materials.3,5,8 The second, known as pseudocapacitors, in which the capacity is due to redox reactions, provide higher energy densities but usually suffer from shorter cyclic lifetimes.4,5,9 The typical pseudocapacitor materials have transition metal oxides and conductive polymers.2,4,10 To overcome these obstacles, research has been carried out on the development of new materials,11 hybrid structure,12,13 and surface modification,14 for EC eletrodes. Recently, MXenes (of the formula Mn+1 Xn Tx , where M is a transition metal, X is C and/or N, and Tx denotes -OH, -F, and =O surface groups), are a novel family of two-dimensional (2D) metal carbides, which can be produced by the selective etching of the A-group (generally group IIIA and IVA elements) layers from the MAX phases.15–20 MXenes have already demonstrated their potential as promising electrode materials for Li-ion batteries,21–24 supercapacitors,6,7,12,13 and sensors,19,25,26 because of their high electrical conductivity, large surface area, layered structure, remarkable chemical stability, and environmentally friendly characteristics.7,12,18,27,28 Particularly, Ti3 C2 is one of the most widely studied and most promising members of this family,7,15,17,19,25,26 which is frequently applied in ECs. Ti3 C2 MXene is one of very few materials which exhibits “true” pseudocapacitive behavior. It presents a continuous change in the titanium oxidation state during charge/discharge, producing rectangular-shaped CVs. Such behavior can be attributed to the 2D nature of Ti3 C2 MXene: spontaneous ion intercalation naturally provides access to electrochemically active transition metal oxide surfaces and the conductive carbide layer ensures rapid charge transfer.11 To further enhance electrochemical performances of the 2D layerd Ti3 C2 electrode material, one straightforward strategy which has been extensively investigated is the introduction of interlayer spacers.12 For the typical 2D graphene, the incorporation of nanoparticles (NPs),29,30

= z

These authors contributed equally to this work. E-mail: [email protected]; [email protected]

nanotubes/nanowires,31–35 and nanosheets36–39 between the graphene layers has been shown to prevent the restacking of the latter, leading to greatly improved performance in supercapacitors, Li-ion, and Li–S batteries.12 Notably, one of the most effective ways for doing this is employing various transition metal oxides to insert among the nanosheets.5,40–42 Among the transition metal oxides, titanium dioxide (TiO2 ) is considered as one of the most promising candidates to be applied in Liion capacitor43 and supercapacitor,44 due to its low cost, nontoxicity, eco-friendliness, abundant availability high surface area, and ease of preparation in defined nanoscale dimensions.40,45 Hence, nano-scale TiO2 with different morphologies is a practical candidate for low-cost electrode material in supercapacitor applications. For example, Rakhi et al.14 stated that the Ti2 C MXene nanosheets annealed in air consisted of nanosheets and numerous TiO2 nanocrystals on thin graphitic nanosheets, which were similar to the ones reported by Naguib et al.46 However, the oxidized Ti2 C MXene nanosheets used in ECs had poor capacitive performance (specific capacitance value less than 5 F/g at 5 mV/s, and rate performance 35%). Naguib et al.46 showed a similar one-step synthesis by heating 2D Ti3 C2 in air at 1150◦ C for 30 s. The resulting TiO2 nanocrystals were enmeshed in thin sheets of disordered graphitic carbon structures which could handle extremely high cycling rates when tested as anodes in lithium-ion batteries (LIBs). But the method was limited by the uncontrolled synthesis condition. It should be mentioned that the hybrid TiO2 -C phase was different from the TiO2 -Ti3 C2 phase. Recently, Gao et al.47 reported that TiO2 /Ti3 C2 nanocomposites could be fabricated by a hydrothermal process in which titanium sulfate (TiSO4 ) and Ti3 C2 were mixed and autoclaved. The resulted hybrid structure showed good photocatalytic performance to methyl orange (MO) under ultraviolet light illumination. Our previous work revealed that TiO2 /Ti3 C2 nanocomposite could be synthesized by hydrolysis of tetrabutyl titanate (TBOT) and an microwave hydrothermal system.25 The prepared TiO2 /Ti3 C2 nanocomposite exhibited excellent electrochemical performance for direct electrochemical biosensor. In this work, multilayered TiO2 -Ti3 C2 nanocomposites were synthesized by a simple in situ hydrolysis of TBOT followed by a heattreatment process in vacuum, and subsequently used as a novel electrode material for supercapacitor. The schematic in Fig. 1 shows numerous TiO2 nanoparticles were deposited on the multilayered Ti3 C2 nanosheets substrate, which made it possible for the nanocomposite to offer a number of advantages when it was used as an electrode material for supercapacitor energy storage. The synergetic effects of Ti3 C2 and TiO2 endowed TiO2 -Ti3 C2 nanocomposites with excellent properties and improved functionalities. Ti3 C2 and TiO2 nanoparticles offered excellent connection between them and facilitated electron exchange.

Downloaded on 2016-02-23 to IP 113.200.58.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

A786

Journal of The Electrochemical Society, 163 (5) A785-A791 (2016)

Figure 1. Schematic showing the procedure used herein to prepare TiO2 -Ti3 C2 nanocomposite electrodes.

Furthermore, exposed TiO2 on the surface of the Ti3 C2 nanolayers was beneficial for improving the contact with electrolyte and insertion/extraction of cations, which resulted in higher capacitance and presented new avenues for research in energy conversion and storage. Herein, compared to TiO2 and Ti3 C2 , the synthesized TiO2 -Ti3 C2 nanocomposite with advantageous synergistic nano-sized effects exhibited excellent properties in a supercapacitor. The higher specific surface area of TiO2 -Ti3 C2 and greater ion diffusion process between TiO2 -Ti3 C2 and the electrolyte could enhance specific capacitance, the rate capability, and exceptional durability. These results indicated that TiO2 -Ti3 C2 nanocomposite has potential as an electrode material for high-performance ECs. Experimental PTFE (60 wt%) was purchased from Sigma. Unless otherwise noted, all other chemicals (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. In all experiments deionized water was used. Synthesis of Ti3 C2 MXene.—Ti3 C2 was successfully prepared by etching Al from Ti3 AlC2 in HF at room temperature.17,19 First, 3.0 g as-prepared Ti3 AlC2 powders were immersed in 60 mL 40% HF solution under magnetic stirring at room temperature for 24 h. Then, the resulting MXene suspension was washed six times using deionized water and centrifuged to separate the powder at 4000 rpm until the pH value of the liquid reached ∼6. After decantation, the resulting powder was washed three times with absolute ethanol and centrifuged to separate the powder, and left to dry in air at RT for 3 days. Finally, the powder was dried in the vacuum oven (3 h before the test. The specific surface areas of the materials were calculated by the Brunauere Emmette Teller (BET) method. The pore size distributions were derived from the adsorption branch using the Barrette JoynereHalenda (BJH) model. The total pore volume was determined from the amount of N2 uptake at P/P0 = 0.99. Fabrication of electrodes.—To investigate the electrochemical behavior of TiO2 -Ti3 C2 , a three-electrode experimental cell was assembled. A mixture of 80 wt% the as-perpared TiO2 -Ti3 C2 nanocomposite, 15 wt% acetylene black, and 5 wt% polytetrafluoroethylene (PTFE) binder was fabricated using ethanol as a solvent and mixed in an agate mortar. At this stage, the resultant slurries exhibited



A787

Figure 2. SEM image of (a) typical as-fabricated Ti3 C2 ; (b)-(c) TiO2 -Ti3 C2 nanocomposite; TEM image of (d) typical as-fabricated Ti3 C2 ; (d)-(f) TiO2 -Ti3 C2 nanocomposite.

clay-like properties and could be directly processed into freestanding films by rolling. Then the freestanding film (1 × 2 cm) was subsequently pressed onto nickel foam under a pressure of 20 MPa for 1 min. The prepared electrode was placed in a vacuum drying oven at 80◦ C for 24 h. The as-prepared electrodes were then obtained. For comparison, TiO2 and Ti3 C2 were used for the fabrication of electrode with similar procedures as described above. Electrochemical setup.—All electrochemical experiments were performed with a CHI 660E electrochemical workstation (CH Instruments, Shanghai, China). A conventional three-electrode system, which consisted of a platinum (1 × 2 × 0.1 cm) as the counter electrode, an Ag/AgCl/3 M KCl as the reference electrode and the as-prepared TiO2 , Ti3 C2 , and TiO2 -Ti3 C2 as the working electrode, was used in all electrochemical experiments. All experiments were conducted in 6 M KOH as the electrolyte solution. Electrochemical measurements.—Cyclic voltammetry, electrochemical impedance spectroscopy, galvanostatic charge–discharge, and galvanostatic cycling were performed. Cyclic voltammetry of the working electrode were measured at different scan rates of 5, 10, 20, 50, 100, and 200 mV s−1 from −1 V to −0.35 V versus Ag/AgCl. Electrochemical impedance spectroscopy was performed at open-circuit potential, with a 5 mV amplitude, and frequencies that ranged from 10 mHz to 100 kHz. Galvanostatic charge–discharge was performed at different current densities of 0.5, 1, 1.5, 2, and 3 A g−1 between the potential limits of −1 V to −0.35 V versus Ag/AgCl. Galvanostatic cycling was performed at 1 A g−1 from −1 V to −0.35 V versus Ag/AgCl. Results and Discussion Characterization of TiO2 -Ti3 C2 .—The morphology of asfabricated Ti3 C2 (a) and TiO2 -Ti3 C2 nanocomposite (b)–(c) were studied by using SEM, as shown in Fig. 2. In Fig. 2a, the as-synthesized Ti3 C2 sample possessed a layered morphology resembling exfoliated graphite,48 where the nanolayers were clearly separated from each other compared to the unreacted powders (Fig. S3(a)). As Figs. 2b–2c shows, after deposition, numerous TiO2 nanoparticles (less than 30 nm in size) could be easily loaded on the Ti3 C2 nanosheets. What’s

more, the introduction of TiO2 nanoparticles into the inner surface of the Ti3 C2 nanolayers could effectively impede the stacking of individual Ti3 C2 nanosheets and enlarge the distance between Ti3 C2 nanosheets.13 Note that deposition of numerous TiO2 nanoparticles on Ti3 C2 nanolayers have larger surface area compared to pure Ti3 C2 . Figure 2d shows a TEM image indicating that the Ti3 C2 had a perfect layered structure. Meanwhile, numerous small TiO2 nanoparticles were found to evenly attach on the Ti3 C2 layers, as shown in Figs. 2e–2f. TEM observations further demonstrated that TiO2 nanoparticles were deposited on Ti3 C2 nanosheets surfaces and TiO2 Ti3 C2 nanocomposites were synthesized. Hence, the incorporation of TiO2 nanoparticles between the Ti3 C2 nanolayers not only increased surface area of TiO2 -Ti3 C2 and enlarged interlayer space between the Ti3 C2 flakes for cation intercalation, but also provided additional diffusion paths for electrolyte ions.12 Therefore, the unique multilayered TiO2 -Ti3 C2 was expected to exhibit improved supercapacitive performances, when employed as supercapacitor electrodes. Energy-dispersive X-ray analysis (EDAX) was employed to study the distribution of the elements Ti, Al, C, O, and F in as-prepared Ti3 C2 and TiO2 -Ti3 C2 samples (Fig. S5). The relative intensity of the F element peak of TiO2 -Ti3 C2 nanocomposite became less intense than that of the spectrum of Ti3 C2 . Note that surface chemistry had a significant effect on the capacitive response: reduction of Fterminations, relative to those of O or OH, resulted in a significant increase in specific capacitance.14,49 Meanwhile, after deposition, the increase relative intensity of C element showed the formation of a small amount of graphite carbon after heat-treatment. In high vacuum (less than 4.0 × 10−2 Pa), this was possible within oxidation of a little Ti-OH/Ti = O on the Ti3 C2 surface, which could lead to the formation of a little graphite carbon. The presence of a graphite carbon indicated that the conductive carbide layer ensured rapid charge transfer,11 which improved supercapacitive performance. The crystal structure and orientation of the as-prepared samples were studied using XRD. The diffractogram of pure TiO2 was observed in Fig. 3a (curve a), the peak characteristics at 25.28◦ , 37.80◦ , 48.05◦ , 53.89◦ , and 55.06◦ on the 2θ scale corresponded to the (101), (004), (200), (105), and (211) planes of the anatase phase. The observed data were in good agreement with the standard anatase phase of TiO2 (JCPDS card number 21-1272). The diffractogram of the


A788


Figure 3. Characterization of TiO2 -Ti3 C2 nanocomposite. (a) XRD plots of TiO2 -Ti3 C2 , Ti3 C2 and TiO2 . (b) Raman spectroscopy of TiO2 -Ti3 C2 , Ti3 C2 and TiO2 .

HF-etched Ti3 C2 nanosheets (curve b) in Fig. 3a, in its vacuum-dried multilayered state, showed the (00l) peaks, such as the (002), (004), and (0010), broadened, lost intensity, and shifted to lower angles compared to their location before treatment (Fig. S2). Because the experiments were conducted in HF aqueous solution environment, -OH, -F and/or =O were the probable ligands. The EDAX pattern of HF treated sample (Fig. S5) further confirmed the removal of Al and the presence of OH, F and O groups, indicating the possible surface termination in the exfoliated nanosheets with F, OH, or O groups. And the experimental results provided strong evidence of the formation of Ti3 C2 (OH)x Fy Oz ,17,28 labeled Ti3 C2 for short. Curve c exhibited XRD pattern of the as-prepared TiO2 -Ti3 C2 hybrid nanostructure, all the diffraction peaks were good agreement with that of anatase TiO2 (curve a) and Ti3 C2 (curve b), which confirmed the co-existence of anatase TiO2 and Ti3 C2 in the nanocomposites. Figure 3b (curve b) shows Raman spectroscopy of the as-prepared Ti3 C2 ; all the diffraction peaks were consistent with that the reported data of Ti3 C2 .46 Raman spectroscopy of TiO2 -Ti3 C2 nanocomposites (Fig. 3b curve c) showed a strong peak at 144 cm−1 , together with three other peaks at 394, 513, and 635 cm−1 . These peaks could be assigned to the following anatase vibrational modes (curve a).50 The two broad peaks between 1000 and 1800 cm−1 were characteristic for the D- and G- modes of graphitic carbon. After heat-treatment, D- and Gband intensity increased in the Raman spectra of the sample, which might show the formation of a little graphite carbon. The presence of graphite carbon indicated that the conductive carbide layer ensured rapid charge transfer,11 which improved supercapacitive performances. After heat-treatment, the as-prepared nanocomposite mainly consisted of Ti3 C2 and TiO2 from TBOT (Fig. 3a (curve c)). There-

fore, the results suggested that TiO2 -Ti3 C2 nanocomposite had been successfully synthesized. Nitrogen adsorption–desorption isotherms as shown in Fig. 4a were measured to evaluate the BET surface area and the pore size distribution of Ti3 C2 and TiO2 -Ti3 C2 samples. The N2 adsorption– desorption isotherms at –195.8◦ C showed an increase in the specific surface area (SSA) after deposition of TiO2 . The SSA estimated using the BET equation51 for the deposited sample was found to be 31.5679 ± 0.6176 m2 g−1 , a factor of 4 greater than that of the as-synthesized Ti3 C2 (8.1792 m2 g−1 ). This increase in SSA could be explained by the formation of numerous nanometer-sized TiO2 particles, and enlarging interlayer space between the Ti3 C2 flakes after heat-treatment. The obvious hysteresis loop between adsorption and desorption branches could be observed at relative pressure scope of 0.45–1.0 for two samples in Fig. 4a, which demonstrated the existence of mesopores. The almost vertical tails at the relative pressure near to 1.0 pointed to the presence of macropores. And, the ratio of micropores was also low for two samples. Pore size distributions of two samples calculated from the nitrogen desorption branches were observed in Fig. 4b, which displayed very close pore size distribution with a peak centering at ca. 15.0 nm and more (nearly 2 times) mesopores of TiO2 -Ti3 C than that of Ti3 C2 . This was in agreement with the N2 adsorption–desorption isotherms. Therefore, the incorporation of TiO2 nanoparticles into the Ti3 C2 nanolayers provided larger SSA and more mesopores for cation intercalation. The unique multilayered TiO2 -Ti3 C2 could enhance supercapacitive performance when employed as supercapacitor electrodes. X-ray photoelectron spectroscopy (XPS), as shown in Fig. 5, was acquired to determine the nature of chemical bonding in the Ti 2p

Figure 4. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions calculated from N2 desorption isothermals for Ti3 C2 and TiO2 -Ti3 C2 samples. The closed and open symbols refer to adsorption and desorption branches, respectively. Downloaded on 2016-02-23 to IP 113.200.58.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).


A789

Figure 5. High-resolution XPS in the Ti 2p region for Ti3 C2 Tx , (a) before TiO2 deposition, (b) after TiO2 deposition. The fits are color-coded; the key is shown on this figures. An increase in the Ti 2p3/2 component corresponding to TiO2 (colored magenta) can be observed at 459.2 eV.

region for Ti3 C2 , (a) before TiO2 deposition, (b) after deposition. High-resolution XPS spectra in the Ti 2p region of the powder revealed that peaks could be deconvoluted into components corresponding to Ti bound to C, Ti(II), Ti(III) and Ti(IV) peaks (Fig. 5(a)). Ti-C 2p3/2, Ti(II) 2p3/2, Ti(III) 2p3/2, and Ti(IV) 2p3/2 peaks were detected at binding energies of 455.3, 456.1, 457.2, and 458.8 eV, respectively. It could be seen that intensity of Ti-C peak was the strongest, Ti(IV) was the weakest, together with Ti-C signals arising from Ti atoms in the interior of the Ti3 C2 MXene nanolayers,28,52,53 which indicated that this spectrum was typical of Ti3 C2 . After TiO2 deposition, Ti(IV) peak corresponding to TiO2 became quite clear and the strongest (Fig. 5b). The relative intensities of Ti-C, Ti(II) and Ti(III) peaks were less intense than that of the spectrum of Ti3 C2 . Concomitant with the increase in the TiO2 signal, there was a decrease in the Ti3 C2 signal. The observation might show the formation of a small amount of graphite carbon and agree with EDAX and Raman spectroscopy results of TiO2 -Ti3 C2 nanocomposite. The slight shift to lower binding energy for the Ti-C component and the slight shift to higher binding energy for the TiO2 component indicated that the original stages of the synthesis of these phases as the local effect of removing oxygen from amorphous TiO2 into anatase TiO2 could cause a shift in electron density. All of these observations indicated that TiO2 was deposited on the surface of multilayered Ti3 C2 . The introduction of TiO2 on the Ti3 C2 surface could lead to an increase in the accessible surface area, which could explain supercapacitive performances of TiO2 -Ti3 C2 was better than that of single Ti3 C2 phase. Electrochemical performance of the as-prepared Ti3 C2 , TiO2 , and TiO2 -Ti3 C2 electrode.—Figure 6a showed the cyclic voltammetric (CV) curves of the TiO2 -Ti3 C2 , Ti3 C2 and TiO2 electrode at a scan rate of 20 mV s−1 . The three CV curves approximated symmetrical rectangular shapes. Ti3 C2 exhibited “true” pseudocapacitive behavior: it presented a continuous change in the titanium oxidation state during charge/discharge, producing rectangular-shaped CVs. And the pseudocapacitive behavior of the nanocrystalline TiO2 particles14 was poor. In comparison to the Ti3 C2 and TiO2 , the TiO2 -Ti3 C2 showed substantially greater area and more symetrical shape, suggesting its higher electrochemical activity. The improvement of TiO2 -Ti3 C2 could be attributed to introduction of TiO2 for larger SSA (about 4 times) and more mesopores, and the formation of a little graphite carbon for better electronic conductivity,11 synergistically. Figure 6b and Fig. S6 displayed the CV curves of TiO2 -Ti3 C2 electrode at scan rates from 5 to 200 mVs−1 . These results showed that the areas defined by the CV curves increased with scan rate increasing and exhibited quasi-rectangular shape without obvious distortion even at scan rates of up to 200 mV s−1 , indicating excellent capacitive behavior and high rate capability. The excellent capacitance behavior of TiO2 -Ti3 C2 composites was mainly provided by higher surface area

interface and more ion diffusion process between TiO2 -Ti3 C2 and the electrolyte. Specific capacitances (Cs) of the TiO2 -Ti3 C2 electrode were calculated by Eq. 1 at various scan rates. A capacitance of 143 F g−1 was achieved for the TiO2 -Ti3 C2 electrode at a scan rate of 5 mV s−1 , which was almost 1.5 times as great as that of Ti3 C2 electrode at the same scan rate. Even at 200 mV s−1 , a capacitance of 117 F g−1 was measured, 82% capacitance retention as the scan rate increases from 5 to 200 mV s−1 . The Cs values of an electrode can be calculated from the CV curve using18 Cs = I dV / (msV ) [1] where Cs is the specific capacitance of the electrode (F g−1 ), I is the response current under the integerated area of the CV curves (A), m is the mass of the electrode material (g), s is the scan rate (mV s−1 ), and V is the potential window (V). Figure 6c shows the variation in specific capacitance as a function of scan rate. These results indicate that specific capacitances decreased with scan rate increasing. The decreasing trend in capacitance indicated that parts of the electrode surface were inaccessible at high charging–discharging rates. Furthermore, TiO2 -Ti3 C2 electrode consistently demonstrated that specific capacitances were much higher than those of Ti3 C2 and TiO2 electrodes at the same rate (some statistical experiment data of Ti3 C2 and TiO2 electrodes were placed in Figs. S7 and S8). The maximum Cs of Ti3 C2 and TiO2 electrodes were 93 F g−1 and 6.4 F g−1 at 5 mV s−1 , respectively. The Cs values of Ti3 C2 and TiO2 electrodes were less than that of TiO2 -Ti3 C2 electrode. Electrochemical impedance spectroscopy (EIS) was also performed to investigate the internal resistance and capacity of the electrode material. Figure 6d revealed the Nyquist plots of Ti3 C2 and TiO2 -Ti3 C2 electrodes. The Nyquist plots for both types of electrodes consisted of a semicircle arc in the high-frequency region and a straight line in the low-frequency region. In general, the semicircle arc in the high-frequency region was related to electronic resistance, and the vertical line in the low-frequency region indicated pure capacitive behavior.54 Equivalent series resistance (ESR) was determined by the interception of the high-frequency arc with the real axis.14 From the inset(a) of Fig. 6d, ESR of TiO2 -Ti3 C2 and Ti3 C2 electrodes were found to be 0.46 and 0.52 , respectively. The lower value indicates consistent interfacial contact between active materials and substrate. The semicircle was associated with electrode surface properties and corresponded to the charge transfer resistance (Rct).14 Such small resistance of TiO2 -Ti3 C2 electrode indicated larger electroactive surface area and lower charge transfer resistance than Ti3 C2 electrode. The inset (b) of Fig. 6d shows the Nyquist plot of the TiO2 electrode. Note that the TiO2 Nyquist plot did not contain a semicircle region.


A790


Figure 6. (a) Cyclic voltammetric (CV) curves of TiO2 -Ti3 C2 ,Ti3 C2 and TiO2 electrode at a scan rate of 20 mV s−1 ; (b) CV curves for TiO2 -Ti3 C2 electrode in the potential window of −1 V to −0.35 V at various scan rates (5–200 mV s−1 ); (c) The plots of the Cs of TiO2 , Ti3 C2 , and TiO2 -Ti3 C2 nanocomposite electrodes as a function of scan rate; (d) Nyquist plots of Ti3 C2 , TiO2 Ti3 C2 composites and TiO2 electrodes. All tests were conducted in 6 M KOH electrolyte solution.

This was probably due to the low faradaic resistances of the electrode and the high electrical conductivity between TiO2 nanoparticles and the current collector.54 These results demonstrate that the introduction of TiO2 enhanced the electrochemical performance of the TiO2 -Ti3 C2 hybrid material, indicating a synergistic effect between TiO2 nanoparticles and Ti3 C2 nanosheets. Figure 7a shows the typical galvanostatic charge–discharge (GCD) curves of the TiO2 -Ti3 C2 electrode for current densities of 0.5, 1, 1.5, 2, and 3 A g−1 . It can be seen that all of the curves were linear and exhibited typical triangular shape, which indicated that the good electrochemical capacitive behavior was achieved by the TiO2 -Ti3 C2 electrode. Based on the charge–discharge curve, the Cs of the electrode can be calculated using18 Cs = I t/ (mV )

[2]

where I is the discharge current, t is the discharge time, m is the mass of the electro-active material, and V is the potential window (V). The calculated Cs values of the TiO2 -Ti3 C2 electrode were 39, 35, 33, 33, and 33 F g−1 at 0.5, 1, 1.5, 2, and 3 A g−1 , respectively. These results are mainly consistent with the order indicated by the CV curves. Electrochemical stability was one of the most important factors determining the usefulness of supercapacitors in commercial applications. Figure 7b shows the typical galvanostatic cycling curve of TiO2 -Ti3 C2 electrode at current density of 1 A g−1 . The results suggest that the TiO2 -Ti3 C2 electrode showed little capacitance losses even after 6000 cycles (92% initial capacitance retention after 6000 cycles), confirming good cycle stability. The performances of various electrode materials are listed in Table I. It was clear that the performance of TiO2 -Ti3 C2 electrode in this work was much higher than similar structure TiO2 -Ti2 C “paper” in Ref. 14 and TiO2 /GO composites in Refs. 54 and 5. Meanwhile, the performance of TiO2 -Ti3 C2 electrode was slightly lower than that of d-Ti3 C2 /CNT in Ref. 12, the enhanced performance was mainly attributed to higher SSA of d-Ti3 C2 and good conductivity provided by introduction of CNT and absence of binder. In addition, electrochemical performance of d-Ti3 C2 “paper” in Ref. 18 was slightly higher than that of the TiO2 -Ti3 C2 electrode, the improved performance was not only owing to higher SSA of delaminated flakes and lack of binder, but also ascribed to smaller intercalating cation of H+ provided by

Figure 7. (a) The typical galvanostatic charge–discharge (GCD) curves of the TiO2 -Ti3 C2 electrode for current densities of 0.5, 1, 1.5, 2 and 3 A g−1 ; (b) the galvanostatic cycling curve of TiO2 -Ti3 C2 electrode (6000 GCD cycles), the inset of (b), the galvanostatic cycling data collected at 1 A g−1 .



A791

Table I. Comparison of the performances of the various electrochemical supercapacitors. Electrode material

Binder mass

TiO2 -Ti3 C2 TiO2 -Ti2 C d-Ti3 C2 a d-Ti3 C2 /CNT TiO2 nanorod/GO rGO/TiO2 nanorod/rGO a d- Ti C -delaminated 3 2 b NR-not reported.12,18

5 wt.% 5 wt.% 0 0 NR NR

Electrolyte KOH KOH H2 SO4 MgSO4 Na2 SO4 Na2 SO4

Capacitance (Cs/scan rate) g−1 /5

s−1

143 F mV ∼5 F g−1 /5 mV s−1 150 F g−1 /5 mV s−1 150 F g−1 /2 mV s−1 100 F g−1 /5 mV s−1 114.5 F g−1 /5 mV s−1

Stability (retention/cycle number) 96%/3000 86%/6000 NRb /10000 NR/10000 80%/3000 85%/3000

Ref. In this work 14 18 12 54 5

Ti3 C2 .12,18

1 M H2 SO4 electrolyte solution. Therefore, the electrochemical performances of TiO2 -Ti3 C2 nanocomposites could be highly enhanced by depositing TiO2 nanoparticles on the Ti3 C2 nanosheets. Conclusions TiO2 -Ti3 C2 nanocomposite was synthesized through a simple synthetic route and subsequently fabricated as an electrode material in electrochemical supercapacitors. The results indicat that the TiO2 Ti3 C2 electrode exhibited high specific capacitance (143 F g−1 at 5 mV s−1 ), superior rate capability (82% of its initial capacitance retention as the scan rate increased from 5 to 200 mV s−1 ), and excellent cycling stability (92% initial capacitance retention after 6000 cycles) in 6 M KOH aqueous electrolyte. The incorporation of TiO2 nanoparticles into Ti3 C2 layers could significantly enhance the electrochemical performance by providing larger SSA, impeding the stacking of Ti3 C2 and enlarging the distance between Ti3 C2 nanosheets for cation intercalation. More importantly, numerous TiO2 nanoparticles provided additional diffusion paths for electrolyte ions. These results suggest that TiO2 -Ti3 C2 electrode was a highly suitable, promising electrode material for new generation high performance supercapacitors. Acknowledgments This work was supported by the National Natural Science Foundation of China (51572158), and the Graduate Innovation Fund of Shaanxi University of Science and Technology. References 1. Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya, and L.-C. Qin, Carbon, 49, 2917 (2011). 2. P. Simon and Y. Gogotsi, Nature Materials, 7, 845 (2008). 3. L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 38, 2520 (2009). 4. Q. Zhang, E. Uchaker, S. L. Candelaria, and G. Cao, Chem. Soc. Rev., 42, 3127 (2013). 5. A. Ramadoss, G.-S. Kim, and S. J. Kim, CrystEngComm, 15, 10222 (2013). 6. X. Wang, S. Kajiyama, H. Iinuma, E. Hosono, S. Oro, I. Moriguchi, M. Okubo, and A. Yamada, Nat. Commun., 6, 6544 (2015). 7. M. R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall’Agnese, P. Rozier, P. L. Taberna, M. Naguib, P. Simon, M. W. Barsoum, and Y. Gogotsi, Science, 341, 1502 (2013). 8. G. Lota, K. Fic, and E. Frackowiak, Energy Environ. Sci., 4, 1592 (2011). 9. W. Wei, X. Cui, W. Chen, and D. G. Ivey, Chem. Soc. Rev., 40, 1697 (2011). 10. G. Wang, L. Zhang, and J. Zhang, Chem. Soc. Rev., 41, 797 (2012). 11. M. R. Lukatskaya, S.-M. Bak, X. Yu, X.-Q. Yang, M. W. Barsoum, and Y. Gogotsi, Adv. Energy. Mater., 5 (2015). 12. M. Zhao, C. E. Ren, Z. Ling, M. R. Lukatskaya, C. Zhang, K. L. Van Aken, M. W. Barsoum, and Y. Gogotsi, Adv. Mater., 27, 339 (2015). 13. P. Yan, R. Zhang, J. Jia, C. Wu, A. Zhou, J. Xu, and X. Zhang, J. Power Sources, 284, 38 (2015). 14. R. B. Rakhi, B. Ahmed, M. N. Hedhili, D. H. Anjum, and H. N. Alshareef, Chem. Mater., 27, 5314 (2015). 15. M. Naguib, V. N. Mochalin, M. W. Barsoum, and Y. Gogotsi, Adv. Mater., 26, 992 (2014). 16. M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi, and M. W. Barsoum, ACS Nano, 6, 1322 (2012). 17. M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, and M. W. Barsoum, Adv. Mater., 23, 4248 (2011).

18. M. Ghidiu, M. R. Lukatskaya, M.-Q. Zhao, Y. Gogotsi, and M. W. Barsoum, Nature, 516, 78 (2014). 19. F. Wang, C. Yang, C. Duan, D. Xiao, Y. Tang, and J. Zhu, J. Electrochem. Soc., 162, B16 (2015). 20. J. Come, M. Naguib, P. Rozier, M. W. Barsoum, Y. Gogotsi, P. L. Taberna, M. Morcrette, and P. Simon, J. Electrochem. Soc., 159, A1368 (2012). 21. D. Sun, M. Wang, Z. Li, G. Fan, L.-Z. Fan, and A. Zhou, Electrochem. Commun., 47, 80 (2014). 22. M. Naguib, J. Come, B. Dyatkin, V. Presser, P.-L. Taberna, P. Simon, M. W. Barsoum, and Y. Gogotsi, Electrochem. Commun., 16, 61 (2012). 23. Q. Tang, Z. Zhou, and P. W. Shen, J. Am. Chem. Soc., 134, 16909 (2012). 24. Y. Xie, M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, X. Yu, K.-W. Nam, X.-Q. Yang, A. I. Kolesnikov, and P. R. C. Kent, J. Am. Chem. Soc., 136, 6385 (2014). 25. F. Wang, C. H. Yang, M. Duan, Y. Tang, and J. F. Zhu, Biosens. Bioelectron., 74, 1022 (2015). 26. H. Liu, C. Duan, C. Yang, W. Shen, F. Wang, and Z. Zhu, Sens. Actuators B: Chem., 218, 60 (2015). 27. Q. Peng, J. Guo, Q. Zhang, J. Xiang, B. Liu, A. Zhou, R. Liu, and Y. Tian, J. Am. Chem. Soc., 136, 4113 (2014). 28. O. Mashtalir, M. Naguib, V. N. Mochalin, Y. Dall’Agnese, M. Heon, M. W. Barsoum, and Y. Gogotsi, Nat. Commun., 516, 78 (2013). 29. Q. Qu, S. Yang, and X. Feng, Adv. Mater., 23, 5574 (2011). 30. S. Liu, J. Xie, C. Fang, G. Cao, T. Zhu, and X. Zhao, J. Mater. Chem., 22, 19738 (2012). 31. R. Lv, E. Cruz-Silva, and M. Terrones, ACS Nano, 8, 4061 (2014). 32. J. Zhu, H. Zhang, and N. A. Kotov, ACS Nano, 7, 4818 (2013). 33. C. Tang, Q. Zhang, M.-Q. Zhao, J.-Q. Huang, X.-B. Cheng, G.-L. Tian, H.-J. Peng, and F. Wei, Adv. Mater., 26, 6100 (2014). 34. Y. Wu, T. Zhang, F. Zhang, Y. Wang, Y. Ma, Y. Huang, Y. Liu, and Y. Chen, Nano Energy, 1, 820 (2012). 35. M.-Q. Zhao, Q. Zhang, J.-Q. Huang, G.-L. Tian, J.-Q. Nie, H.-J. Peng, and F. Wei, Nat. Commun., 5, 3410 (2014). 36. S. Yang, X. Feng, and K. M¨ullen, Adv. Mater., 23, 3575 (2011). 37. G. Zhou, S. Pei, L. Li, D.-W. Wang, S. Wang, K. Huang, L.-C. Yin, F. Li, and H.-M. Cheng, Adv. Mater., 26, 625 (2014). 38. R. Chen, T. Zhao, J. Lu, F. Wu, L. Li, J. Chen, G. Tan, Y. Ye, and K. Amine, Nano Lett., 13, 4642 (2013). 39. Y. Cao, X. Li, I. A. Aksay, J. Lemmon, Z. Nie, Z. Yang, and J. Liu, Phys Chem Chem Phys, 13, 7660 (2011). 40. H. Kim, M.-Y. Cho, M.-H. Kim, K.-Y. Park, H. Gwon, Y. Lee, K. C. Roh, and K. Kang, Adv. Energy. Mater., 3, 1500 (2013). 41. R. Liu, W. Guo, B. Sun, J. Pang, M. Pei, and G. Zhou, Electrochimica Acta, 156, 274 (2015). 42. A. Ramadoss and S. J. Kim, Carbon, 63, 434 (2013). 43. H. Wang, C. Guan, X. Wang, and H. J. Fan, Small, 11, 1470 (2015). 44. M. Sawczak, M. Sobaszek, K. Siuzdak, J. Ryl, R. Bogdanowicz, K. Darowicki, M. Gazda, and A. Cenian, J. Electrochem. Soc., 162, A2085 (2015). 45. L. T. Yan, Y. Xu, M. Zhou, G. Chen, S. G. Deng, S. Smirnov, H. M. Luo, and G. F. Zou, Electrochim. Acta, 169, 73 (2015). 46. M. Naguib, O. Mashtalir, M. R. Lukatskaya, B. Dyatkin, C. Zhang, V. Presser, Y. Gogotsi, and M. W. Barsoum, Chem Commun, 50, 7420 (2014). 47. Y. Gao, L. Wang, A. Zhou, Z. Li, J. Chen, H. Bala, Q. Hu, and X. Cao, Mater. Lett., 150, 62 (2015). 48. X. Li, X. Wang, L. Zhang, S. Lee, and H. Dai, Science, 319, 1229 (2008). 49. Y. Dall’Agnese, M. R. Lukatskaya, K. M. Cook, P.-L. Taberna, Y. Gogotsi, and P. Simon, Electrochem. Commun., 48, 118 (2014). 50. V. Swamy, A. Kuznetsov, L. S. Dubrovinsky, R. A. Caruso, D. G. Shchukin, and B. C. Muddle, Phys. Rev. B, 71, 184302 (2005). 51. S. Brunauer, P. H. Emmett, and E. Teller, J. Am. Chem. Soc., 60, 309 (1938). 52. O. Wilhelmsson, J. P. Palmquist, E. Lewin, J. Emmerlich, P. Eklund, P. O. Å. Persson, H. H¨ogberg, S. Li, R. Ahuja, O. Eriksson, L. Hultman, and U. Jansson, J. Cryst. Growth, 291, 290 (2006). 53. S. Myhra, J. A. A. Crossley, and M. W. Barsoum, J. Phys. Chem. Solids, 62, 811 (2001). 54. R. R. Liu, W. J. Guo, B. Sun, J. L. Pang, M. S. Pei, and G. W. Zhou, Electrochim. Acta, 156, 274 (2015).
