Molecular-Level Heterostructures Assembled from Titanium Carbide

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Letter

Molecular-Level Heterostructures Assembled from Titanium Carbide MXene and Ni-Co-Al Layered Double Hydroxide Nanosheets for All-solid-state Flexible Asymmetric High-Energy Supercapacitors Ruizheng Zhao, Mengqiao Wang, Danyang Zhao, Hui Li, Chengxiang Wang, and Longwei Yin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01063 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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ACS Energy Letters

Molecular-Level Heterostructures Assembled from Titanium Carbide MXene and Ni-Co-Al Layered Double Hydroxide Nanosheets for All-solid-state Flexible Asymmetric High-Energy Supercapacitors Ruizheng Zhao, Mengqiao Wang, Danyang Zhao, Hui Li, Chengxiang Wang*, Longwei Yin* Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan 250061, P. R. China Corresponding Authors: * Email: [email protected]. Tel.: + 86 531 88396970. Fax: + 86 531 88396970 (L. Yin). * Email: [email protected]. Tel.: + 86 531 88396970. Fax: + 86 531 88396970. (C. Wang).

Abstract Unique layered Ti3C2/Ni-Co-Al-LDH heterostructures alternatively stacked with molecular-level nanosheets are for the first time synthesized by facile liquid phase co-feeding and electrostatic attraction hetero-assemble strategy between negatively charged Ti3C2 and positively charged Ni-Co-Al-LDH nanosheets. The molecular-level Ti3C2/Ni-Co-Al-LDH heterostructures taking both merits of conductive and pseudocapacitive components, can show greatly enhanced dynamic behavior in Faradic reaction, which is significant to get a high powder density. Electrons penetrate in Ti3C2 layers, while ions diffuse rapidly along two-dimensional galleries, displaying the shortest diffusion pathway and highest efficiency for charge transfer. The Ti3C2/Ni-Co-Al-LDH heterostructure exhibits specific capacitance of 748.2 F g-1 at current density of 1 A g-1, showing an enhanced rate capacity. Importantly, a maximum energy density of 45.8 Wh· kg-1 is obtained when Ti3C2/Ni-Co-Al-LDH acted as positive electrode for an all-solid-state flexible asymmetric supercapacitor. The results indicate that molecular-level heterotructure is a promising candidate for future high-energy supercapacitors.

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Due to increased requirement for portable clean energy, tremendous efforts have been committed to develop various energy storage devices.1-5 Supercapacitor is one of the most important next-generation promising energy storage devices owing to its higher power density, excellent charge/discharge property and long cycling stability.6-8 However, the main obstacle for supercapacitor is its low energy density. An efficient route to solve this problem is to rationally design composites combining advantages of conductive and pseudocapacitve materials together.9, 10 Nevertheless, it is difficult to fully make use of electrochemical active sites because it cannot get a sufficient contact between both the components. A unique molecular-level heterostructure provides a strong hint to solve this problem. For example, Ma et al. reported a layered double hydroxides (LDH)/rGO heterostructure substantially improved charge transfer efficiency.11 As usual, graphene or reduced graphene oxide is the most widely-used conductive material with molecule thickness, but the specific capacitance is very low due to the absence of redox reactions, which dilutes the energy density of pseudocapacitive materials.12, 13 In this regard, 2D transition metal carbide and nitride (MXenes) nanosheets are favorable candidates due to their excellent pseudocapacitive properties,14-21 high metallic conductivity and surface hydrophilicity.22, 23 MXenes with a general composition of Mn+1XnTx, (where M is an early transition metal, X is C or N, n = 1 to 3, and T represents O, OH, F surface functional groups) can be produced by selectively etching out the A layer (A is mainly a group IIIA or IVA element) from the MAX phases.24-27 Pristine Ti3C2Tx MXene film is reported to exhibit a high gravimetric capacitance of 245 F g-1 and a high conductivity of 150,000 S m-1,14 much higher than commonly-used carbon materials. For MXene-based electrodes for energy storage devices,

4, 15, 24, 28

hybrid films of Ti3C2Tx MXene/transition metal oxides (TMOs)

have also been reported to exhibit enhanced Li-ion storage performance, based on excellent conductivity of MXene and pseudocapacitance of TMOs.4 Moreover, the PPy/Ti3C2Tx film heterostructure due to the redox contribution and higher conductivity of PPy and Ti3C2Tx reveals an outstanding specific capacitance of about 416 F g−1. 28 However, most MXene-based supercapacitors still lack the careful electrode structure design although they indeed enhance the capacitive performance. For example, Wang et al. reported that a composite based on the arbitrarily grown arrays of Ni-Al-LDH platelets on the e-MXene sheets substrate, which is conducive to expose the active sites of LDH and facilitate the penetration of electrolyte, further alleviate the volume change of LDH. 29 Nevertheless, this composite is based on the arbitrarily grown arrays of layered double hydroxide (LDH) on Ti3C2, the real heterostructure between Ti3C2 and LDH and a sufficient contact in this composite is are not realized, furthermore, the wall thickness of LDH is still relatively large. Till now, the aformentioned molecular-scale heterostructures based on MXene and pseudocapacitive materials is still limited. Among pseudocapacitive electrode materials,

11, 28, 30-32

layered double hydroxides (LDHs) bearing positive charge and high theoretical

specific capacity are considered as an ideal candidate to combine with MXene to make a heterostructure. 11, 33 It is of great challenge and fundamentally importance to rationally design molecular-scale of Ti3C2 MXene and Ni-Co-Al-LDH nanosheets to heterostructure for all-solid-state flexible asymmetric high-energy supercapacitors. This molecule-level heterostructure would sufficiently make use of high conductivity of Ti3C2 and large capacitance of LDH because of the face-to-face contact of both components. And the parallel 2D interlayer spacing provides suitable diffusion channels for ions. Both are important for high-energy electrodes design for supercapacitor, which pursuit higher energy density and power density. Herein, unique layered Ti3C2/Ni-Co-Al-LDH heterostructures alternatively stacked with molecular-level 2


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ACS Energy Letters

nanosheets are for the first time synthesized via a facile liquid phase co-feeding and electrostatic attraction hetero-assemble strategy between negatively charged Ti3C2 and positively charged Ni-Co-Al-LDH nanosheets. The Ti3C2/Ni-Co-Al-LDH heterostructure exhibits an outstanding specific capacitance of 748.2 F g-1 at current density of 1 A g-1, showing an excellent rate performance. Furthermore, a maximum energy density of 45.8 Wh· kg-1 is obtained when acted as positive electrode and active carbon (AC) as the negative electrode of an all-solid-state flexible asymmetric supercapacitor. The negatively charged Ti3C2 MXene nanosheets (Fig. S1) were prepared via a controllable HF etching, Tetramethylammonium (TMAOH) intercalation and N, N-Dimethylformamide (DMF) delamination process. 17, 32, 34 (The Ti3AlC2 was selectively etched to remove Al layers by HF labeled as et-Ti3C2Tx. The et-Ti3C2Tx intercalated with TMAOH solution was labeled as in-Ti3C2. The in-Ti3C2 exfoliated with DMF was labeled as ex-Ti3C2.) The ex-Ti3C2 nanosheets are successfully synthesized (Fig. S2-S3, Fig. S4a-S4c, Fig. S5-S9) and its colloidal suspension shows a clear Tyndall effect (Inset in Fig. S4c).35 The thickness of ex-Ti3C2 is about 1.30 nm as shown by AFM image (Fig. S4g-4h), which is close to that of a single-layer of Ti3C2 nanosheet based on the previous reports.14, 35-37 On the other hand, the positively charged Ni-Co-Al-LDH nanosheets were obtained by a facile two-step anion exchange and formamide delamination process (Fig. S1, Fig. S4d-S4f, Fig. S10-S14, Table S1) (The Ni-Co-Al-LDH-NO3- exfoliated with formamide was labeled as ex-Ni-Co-Al-LDH.), exhibiting an evident Tyndall effect (Inset in Fig. S4f), and a thickness of ~0.8 nm (Fig. S4i-4j) is in agreement with that of single-layer LDH nanosheets in previous reports.11, 33 The Ti3C2/Ni-Co-Al LDH heterostructures (Fig. 1a, inset) (The ex-Ti3C2/ex-Ni-Co-Al-LDH heterostructure composite was labeled as Ti3C2/Ni-Co-Al LDH.) with a zeta potential of -10.5 mV were prepared through the flocculation of Ti3C2 and LDH nanosheets because they are oppositely-charged. According to the calculation result (Table S2), the ideal feeding rate of ex-Ti3C2 and ex-Ni-Co-Al-LDH nanosheet suspension was determined to be 2:1. TEM image in Fig. 1a reveals that ultrathin hexagonal nanosheets homogeneously distribute in transparent Ti3C2 nanosheets. A typical HRTEM image of Ti3C2/Ni-Co-Al-LDH composite through freeze drying is shown in Fig. 1b. It shows lamellar lattice fringes with two alternating layers of ~0.7 and ~0.4 nm, which maybe roughly correspond to the thickness of ex-Ti3C2 and ex-Ni-Co-Al-LDH nanosheet, respectively. This is somewhat smaller than the AFM thickness of ex-Ti3C2 (1.30 nm) and ex-Ni-Co-Al-LDH nanosheet (0.82 nm) due to the presence of stress from electrostatic attraction and the AFM thickness from the water molecular and surface groups. It demonstrates that the interlayered water molecular is removed by freeze drying. Electron diffraction pattern shown in Fig. 1c is indexed to be that of ex-Ni-Co-Al-LDH (L100, L110) and ex-Ti3C2 nanosheets (T100, T110), respectively, indicating that two materials are closely assembled on microscopic scale. XRD diffraction peaks of flocculated Ti3C2/Ni-Co-Al-LDH material (Fig. 1d) at about 5.2°and 10.4° can be indexed to (001) and (002) planes of the composite materials, which demonstrates the characteristic of layered structure. According to Bragg equation, the (001) plane at 5.2° reveals a basal spacing of ~1.71 nm, which is similar to the previous report.38 This value is very close to the basal spacing of the configuration (~1.77 nm) shown in the inset of Fig. 1d. This microscopic resolution indicates the formation of Ti3C2/Ni-Co-Al-LDH heterostructure, which is also in accordance with HRTEM results. A typical energy dispersive X-ray spectroscopy (EDS) spectrum (Fig. 1e) and elemental mapping images (Fig. 1f) of as-prepared Ti3C2/Ni-Co-Al-LDH composite reveal a relatively uniform distribution of Ti, C, O, F, Ni, Co, Al elements over the material, suggesting that heterostructure is uniform in lateral dimension. Other structure characterizations (Fig. S15-S17, Table S3) show there are no redox reactions happened and the presence of surface groups in the flocculation process.31, 35, 39, 40 Moreover, the specific surface 3



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area of Ti3C2/Ni-Co-Al-LDH composite (Fig. S18, Table S4) is much higher than those of in-Ti3C2 and Ni-Co-Al-LDH-NO3- materials (Fig. S14a). The cyclic voltammograms (CVs) curves of in-Ti3C2 electrode (Fig. S19a) exhibit a pair of redox peaks corresponding to a pseudocapacitve behavior of the redox reactions of Ti in 1.0 M KOH aqueous electrolyte, corresponding to reaction (1).41 Ni-Co-Al-LDH-NO3- electrode also reveals obvious oxidation and reduction peaks (Fig. S19b), assigning to a typical pseudocapacitve behavior of reversible conversion between Co2+/Co3+ and Ni2+/Ni3+ shown by reaction (2).39 CV curves of the Ti3C2/Ni-Co-Al-LDH electrode (Fig. 2a) show a small difference in comparison to those of in-Ti3C2 and Ni-Co-Al-LDH-NO3- at anodic and cathodic peaks owing to the typical pseudocapacitive behavior of reversible redox reactions between Co2+/Co3+, Ni2+/Ni3+ and Ti. The reversible redox reactions can be described as follows: Ti3C2Tx (T= O, OH, F) + y K+ + y e- ↔ Ti3C2TxKy -

(1)

-

M (OH) 2 + OH ↔ MOOH + H2O + e (M represents Ni or Co)

(2)

The CVs demonstrates that Ti3C2 also contributes to the pseudocapacitive performance for Ti3C2/Ni-Co-Al-LDH electrode. More importantly, the current density of Ti3C2/Ni-Co-Al-LDH heterostructure is evidently larger than that of single Ni-Co-Al-LDH-NO3- (Fig. 2b), although the latter should have a larger current density than the former according to the theoretical capacity. This indicates that introduction of Ti3C2 indeed improves the utilization of Ni-Co-Al-LDH. The nonlinear variation of potential with cycle time of the Ti3C2/Ni-Co-Al-LDH electrode in charge/discharge curves (Fig. 2c) exhibits typical pseudocapacitive behavior, corresponding to CV results. Furthermore, the charge/discharge curves are well symmetrical, which proves the faradic redox reaction occurs in a well reversible manner. The in-Ti3C2 and Ni-Co-Al-LDH-NO3- electrodes (Fig. S20) show the similar galvanostatic charge/discharge curves at various current densities, but the Ti3C2/Ni-Co-Al-LDH electrode reveals much longer discharge time than others at the same current density, suggesting the best electrochemical performance. For a comparison, the galvanostatic charge/discharge curves (Fig. 2d) of in-Ti3C2, Ni-Co-Al-LDH-NO3- and Ti3C2/Ni-Co-Al-LDH electrodes at the same current density of 3 A g-1 show different discharge time of 81.7, 111.7 and 144.0 s, corresponding to the discharging capacity of 408.5, 558.5 and 720.2 F g-1, respectively. These results also prove the evident improvement of the heterostructure in terms of specific capacitance, as shown in CV profiles. Usually, the specific capacitance decreases as the current density increases because large current density requires a quick charge/discharge reaction, which happens on or near to the surface. This is much related to the electron and ion transport speed, so it is dependent on the conductivity and diffusion channel of the electrode materials. As shown in Fig. 2e, although Ni-Co-Al-LDH-NO3- exhibits a much larger specific capacitance of 756.7 F g-1 than 440.2 F g-1 of in-Ti3C2 at 1 A g-1, it quickly drops to 252.5 F g-1 at 15 A g-1, while in-Ti3C2 still maintains at 295 F g-1. It evidently reveals the significance of conductivity for the sufficient use of electrode material, especially at a high-power level. Interestingly, when Ni-Co-Al-LDH is combined with Ti3C2 in a heterostructure configuration, it exhibits a comparable stability to Ti3C2 electrodes at large current density more than 15 A g-1. This means the molecular-level heterostructure probably is comparable with the conductivity layer itself in terms of conductivity. Also, the 2D layered structure provides suitable channels for rapid ion diffusion. 4


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ACS Energy Letters

Therefore, the Ti3C2/Ni-Co-Al-LDH electrode exhibits an impressive specific capacitance of 748.2 F·g-1 at a current density of 1 A·g-1. Even upon 20 A·g-1, an excellent capacity of 569.8 F·g-1 is still retained, showing an excellent rate capacity. The results indicate this strategy can significantly improve the specific capacitance, particularly at a high current density.14, 17, 42 Fig. 2f shows cycling stability at a current density of 2 A g-1 within 10000 cycles. Although Ni-Co-Al-LDH-NO3- exhibits a much larger specific capacitance of 617.25 F g-1 than 351.4 F g-1 of in-Ti3C2 at the first cycle, it rapidly drops to 244.53 F g-1 after 5600 cycles while in-Ti3C2 still maintains 263.77 F g-1, which may be attributed to the structure collapse of Ni-Co-Al-LDH-NO3-. After combining with Ti3C2 nanosheets to form the Ti3C2/Ni-Co-Al-LDH heterostructure, the cycle stability can be further improved. After 10000 cycle, the Ti3C2/Ni-Co-Al-LDH electrode still maintains an outstanding specific capacitance of 589.4 F·g-1, while in-Ti3C2 and Ni-Co-Al-LDH-NO3- electrode encounters a severe degradation to 253.9 and 201.35 F·g-1, exhibiting excellent cycling stability. It evidently reveals that 2D Ti3C2/Ni-Co-Al-LDH heterostructures electrode intrinsically has a better ability to accommodate and survive large stresses and strains during ion intercalation/deintercalation, because of weak bonding between their layers and large anisotropy of properties, high in-plane strength and low strength in the out-of-plane directions where dimensional change occurs, which was supported by the SEM images after 10000 cycles (Fig. S21). As the Ragone plots shown in Fig. 3a, it is conspicuous that the Ti3C2/Ni-Co-Al-LDH electrode presents much higher energy density than in-Ti3C2 and Ni-Co-Al-LDH-NO3- electrodes under the same power density. The Ti3C2/Ni-Co-Al-LDH supercapacitor can obtain a maximum energy density of 37.41 Wh kg-1 at the power density of 0.35 kW kg-1. Even at a high power density of 9.42 kW kg−1, the energy density can still retain 28.49 W h kg−1, which is higher than other previously reported MXene-based supercapacitors (Table S5). It indicates that 2D molecular-level Ti3C2/Ni-Co-Al-LDH heterostructures could provide multiple sites for ions and enable their dense packing between the layers, thus fully utilizing all interlayer volume. Furthermore, it could increase the interlayer distance, open a way not only to accommodate larger ions and decrease energy barrier to ion movement, thus accelerating diffusion, but also to incorporate larger amounts of ions leading to higher specific capacity and energy density. The Nyquist plot of Ti3C2/Ni-Co-Al-LDH electrode (Fig. 3b) shows almost the same juncture on the real axis as Ni-Co-Al-LDH-NO3-, suggesting them with nearly equal electrode resistance. As for in-Ti3C2 electrode, the juncture on the real axis is the smallest compared to Ni-Co-Al-LDH-NO3- or Ti3C2/Ni-Co-Al-LDH electrodes, exhibiting the smallest charge transfer resistance among three electrodes. However, in low frequency region, Ti3C2/Ni-Co-Al-LDH electrode shows the largest slope, which probably results from that the Ni-Co-Al-LDH nanosheets anchored Ti3C2 nanosheets effectively prevents nanosheets stacking and making a sufficiently contact between both kinds of nanosheets, indicating enhanced ion transfer ability than others in the electrolyte. Furthermore, Bode plots in Fig. 3c show that the knee frequency f Φ =–45°of in-Ti3C2 is about ca. 27 Hz, suggesting energy storage/output is accessible to this frequency, which also confirms excellent rate performance inherent to MXene-based pseudocapacitor. While the knee frequency of Ni-Co-Al-LDH-NO3- is about 0.5 Hz, indicating that Ni-Co-Al-LDH-NO3- nanosheets alone cannot be potential for developing high-power 5



supercapacitors owing to poor Faradaic pseudocapcitance response to frequency. It is noted that the knee frequency of Ti3C2/Ni-Co-Al-LDH electrode is estimated to be about 2 Hz, higher than that of Ni-Co-Al-LDH-NO3- nanosheets, which may be attributed to the introduction of high electrical conductivity and charge transport efficiency for in-Ti3C2 nanosheets.11 According to the HRTEM and XRD results, the molecular-scale heterostructure between Ti3C2 and Ni-Co-Al-LDH nanosheets with full contact is successfully synthesized. The electron penetration and ion diffusion process in one stacking unit of Ti3C2/Ni-Co-Al-LDH heterostructure can be schematically shown in Fig. 3d. This heterostructure displays three plausible advantages: (i) Nanosheets have molecular-level thickness, which expose almost all the active sites on the surface; (ii) Electrons are expected to transport in Ti3C2 layers, and then penetrate to Ni-Co-Al-LDH layers through the contact area. The 2D feature allows both kinds of nanosheets to have a face-to-face contact, and thus ensures the effective electron transport to Ni-Co-Al-LDH nanosheets; (iii) 2D Ti3C2/Ni-Co-Al-LDH heterostructure channel eliminates the limitation of size and charge of electrochemically cycled ions, which is suitable for the rapid ion diffusion dynamics. The latter two aspects can be deduced from the EIS measurements, the equivalent circuit is shown in upper inset in Fig. 3d. Rs stands for the resistance of electrolyte, Cdl is capacitance of electrochemical double layer, Rct is resistance of Faradic charge transfer, and CΦ is Faradic pseudocapacitance. As is well-known, the performance of the heterostructure system is related with the series of Rct and CΦ. By fitting Fig. 3b, Rct of in-Ti3C2, Ni-Co-Al-LDH-NO3-, Ti3C2/Ni-Co-Al-LDH electrodes are 0.66, 0.72 and 0.28 Ω, respectively. The smallest Rct of Ti3C2/Ni-Co-Al-LDH, corresponding to the smallest semicircle region, demonstrates that the heterostructure exhibits the most rapid dynamic behavior in Faradic reactions, which is significant to get a high powder density. It seems that the heterostructure is conducive to charge transfer across the interface, and conductivity of electrodes is not the dominating parameter here. The nearly perpendicular line in high frequency region demonstrates the capacitive feature of the electrodes. All-solid-state flexible asymmetric supercapacitors are potential energy storage devices due to improved safety and good reliability, etc.6, 7 However, the pursuit of all-solid-state flexible asymmetric supercapacitors with high specific capacitances remains a great challenge. The molecular-scale Ti3C2/Ni-Co-Al-LDH heterostructures are expected to possess greatly improved electrolyte accessibility, high specific capacitance, high rate capability and high energy output, rendering them a potential candidate for all-solid-state flexible asymmetric high-energy supercapacitors. To further characterize performance of such unique Ti3C2/Ni-Co-Al-LDH heterostructure electrode, we design an all-solid-state flexible asymmetric supercapacitor device consisting of Ti3C2/Ni-Co-Al-LDH //active carbon (AC). The CV curves (Fig. S22a-22b) are firstly measured in a three electrode system, displaying a stable voltage window of Ti3C2/Ni-Co-Al-LDH and AC electrodes from -1 to 0 V and 0 to 0.8 V, respectively. Fig. 4a shows photograph and the schematic components of an all-solid-state flexible device. AC is ultimately selected as the negative electrode and Ti3C2/Ni-Co-Al-LDH as positive electrode. Furthermore, PVA-KOH gel serves as both the electrolyte and separator, while carbon cloth acts as current collector and PET film serves as a protective layer of electrodes. Based on charge balance theory and the specific capacitance of AC (Fig. S22c-22d) and Ti3C2/Ni-Co-Al-LDH, the mass ratio of cathode to anode is matched to approximately 1:3.9. Fig. 4b shows typical CV curves of the Ti3C2/Ni-Co-Al-LDH//AC all-solid-state flexible asymmetric supercapacitor from 0 to 1.6 V at various scan rates of 5, 10, 30, 50, 70, and 100 mV s−1, respectively. The obvious anodic and cathodic peaks correspond to the typical pseudocapacitve behavior of reversible redox reactions between Co2+/Co3+, Ni2+/Ni3+ and Ti, which also shows that the operating potential window is suitable to supercapacitor and usually higher than symmetric supercapacitors. The CV curve changes a little with the scan 6


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ACS Energy Letters

rate increasing, suggesting an excellent electrochemical performance of the device. The CV curves in Fig. 4c measured at a scan rate of 70 mV·s-1 at various bending angles almost overlapping with each other, showing the high stability and excellent flexibility of the Ti3C2/Ni-Co-Al-LDH//AC all-solid-state flexible asymmetric supercapacitor. According to GCD curves (Fig. S23a), the specific capacitance of the Ti3C2/Ni-Co-Al-LDH//AC all-solid-state flexible asymmetric supercapacitor at different current density can be calculated (Fig. 4d). At a current density of 0.5 A·g-1, the device exhibits a specific capacitance of 128.89 F·g-1, which is comparable or superior to that of other all-solid-state supercapacitors (Table S6). Furthermore, the Ragone plot (Fig. 4e) shows that the energy density of this all-solid-state flexible asymmetric supercapacitor is 45.8 Wh· kg-1 at a power density of 346 W·kg-1. Even at a high power density of 6.93 kW· kg-1, the energy density still remains 23.6 W h kg-1, which is comparable or superior to those of other all-solid-state supercapacitors in Fig. 4e and Table S7. 43-54 The cycling stability of the Ti3C2/Ni-Co-Al-LDH //AC all-solid-state flexible asymmetric supercapacitor (Fig. 4f) exhibits 97.8% retention of its initial capacitance after 10000 cycles at a current density of 5 A·g-1, and the GCD curves of the device (Fig. S23b) changes a little, indicating good cycling stability. The slight decrease of the capacitance should be derived from the decomposition of the PVA-KOH gel electrolyte during the charge/discharge process and nonreversible reaction of the active materials.49 Moreover, temperature may also inevitably induce a slight fluctuation of specific capacity.55 EIS spectrum of the Ti3C2/Ni-Co-Al-LDH//AC all-solid-state flexible asymmetric supercapacitor (Fig. S23c) shows an ideal capacitive behavior of the device and low charge transport resistance, which demonstrates high electron conductivity and ion diffusion of the device for redox reaction. A yellow LED is lighted by one Ti3C2/Ni-Co-Al-LDH//AC all-solid-state flexible asymmetric intercalation pseudocapacitor shown in the inset of Fig. 4f, indicating the feasibility and potential application of the device. In summary, the genuine layered Ti3C2/Ni-Co-Al-LDH heterostructures alternatively stacked with molecularlevel nanosheets are successfully prepared for the first time by a liquid phase co-feeding and electrostatic attraction hetero-assemble strategy between negatively charged Ti3C2 and positively charged Ni-Co-Al-LDH nanosheets. When the Ti3C2/Ni-Co-Al-LDH heterostructure is acted as active electrode, an excellent specific capacity up to 748.2 F·g-1 at a current density of 1 A·g-1 can be achieved. Even at a high current density of 20 A·g-1, a high capacity of 569.8 F·g-1 is still maintained, showing an excellent rate performance. Furthermore, a maximum energy density of 45.8 Wh· kg-1 is obtained at a power density of 346 W·kg-1 when acted as positive electrode and active carbon (AC) as the negative electrode of an all-solid-state flexible asymmetric supercapacitor. In this unique heterostructure, electrons penetrate in Ti3C2 layers and ions diffuse rapidly along two-dimensional galleries, which provide the shortest diffusion pathway and highest efficiency for ion diffusion and electron transfer. More importantly, the unique Ti3C2/Ni-Co-Al-LDH heterostructure can make a sufficiently contact between both Ti3C2 and Ni-Co-Al-LDH nanosheets, which can provide more efficient active sites for making full use of pseudocapacitive proficiency during fast reversible redox reactions, thus greatly enhances the electrochemical storage capacitance. The unique Ti3C2/Ni-Co-Al-LDH heterostructure is novel and encouraging, indicating that molecular-level heterotructure is a promising candidate for future high-energy supercapacitors. Supporting Information Detailed experimental procedure, schematic illustration for the preparation process of Ti3C2/Ni-Co-Al-LDH nanosheet heterostructure, XRD patterns, FTIR spectra, XPS spectra, SEM images, EDS spectra, AFM images, 7



TEM images, TG-DSC curves, raman spectra, surface areas and pore size distribution curves, theoretical calculation of mass ratio bewteen MXene and LDH nanosheets, CV curves, galvanostatic charge/discharge profiles, EIS curves, comparison of specific capacitance, energy density and power density (PDF) Acknowledgements We acknowledge support from the project supported by the Sate Key Program of National Natural Science of China (No.: 51532005), National Nature Science Foundation of China (No.: 51472148, 51602181，51272137), the Tai Shan Scholar Foundation of Shandong Province, General Financial Grant from the China Postdoctoral Science Foundation (No: 2015M582088), and the Fundamental Research Fund of Shandong University. References (1) Nystrom, G.; Marais, A.; Karabulut, E.; Wagberg, L.; Cui, Y.; Hamedi, M. M. Self-Assembled Three-Dimensional and Compressible Interdigitated Thin-Film Supercapacitors and Batteries. Nat. Commun. 2015, 6, 7259. 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Figure files

Figure 1. Characterization of Ti3C2/Ni-Co-Al-LDH heterostructure material: (a) TEM image. Inset shows photograph of the Ti3C2/Ni-Co-Al-LDH composites. (b) HRTEM image, the lattice fringe of 0.4 and 0.7 nm corresponds to the thickness of ex-Ni-Co-Al-LDH and ex-Ti3C2 nanosheets, respectively. (c) ED pattern, the diffraction rings are indexed to be those of ex-Ni-Co-Al-LDH (L100, L110) and ex-Ti3C2 nanosheets (T100, T110), respectively. (d) XRD pattern. Inset shows structure schematic illustration of the Ti3C2/Ni-Co-Al-LDH heterostructure. (e) EDS spectrum. (f) SEM image and corresponding element mapping images of Ti, C, O, F, Ni, Co, Al elements, respectively.

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Figure 2. (a) CVs of Ti3C2/Ni-Co-Al-LDH electrode at different scan rates. (b) CVs of different electrodes at a scan rate of 30 mV s-1. (c) Galvanostatic charge/discharge profiles of Ti3C2/Ni-Co-Al-LDH electrode at different current density. (d) Comparison of galvanostatic charge/discharge profiles of different electrodes at a current density of 3 A g-1. (e) Specific capacitance of different electrodes at different current density with a potential window. (f) Cycle performance of different electrodes within 10000 cycles at a current density of 2 A g-1. The electrochemical measurements are carried out with a three-electrode system in 1.0 M KOH electrolytes at room temperature.

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Figure 3. (a) Ragone plots of average power density vs. energy density for different electrodes. (b) Nyquist plots for different electrodes before cycling in the frequency range from 100 KHz to 10.0 m Hz. (c) Bode polts of different electrodes, respectively. (d) Charge transfer mechanism of Ti3C2/Ni-Co-Al-LDH nanosheet heterostructure electrode. Inset shows the equivalent circuit.

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Figure 4. The fabricated Ti3C2/Ni-Co-Al-LDH//AC all-solid-state flexible asymmetric supercapacitor device. (a) Photograph and Schematic illustration. (b) CV curves measured at different scan rates between 0 and 1.6 V. (c) CV curves collected at a scan rate of 70 mV·s-1 under different bending angle. (d) Specific capacitance at different current density. (e) Ragone plot of average power density vs energy density. (f) Variation in the specific capacitance at a current density of 5 A·g-1. Inset shows a yellow LED powered by one Ti3C2/Ni-Co-Al-LDH//AC all-solid-state flexible asymmetric supercapacitor.

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80x46mm (300 x 300 DPI)


Molecular-Level Heterostructures Assembled from Titanium Carbide

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