Highly Self-healable 3D Microsupercapacitor with MXene-Graphene

Apr 12, 2018 - The MSC based on 3D MXene-rGO aerogel delivers a large area specific capacitance of 34.6 mF cm-2 at the scan rate of 1 mV s−1 and an ...
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Highly Self-Healable 3D Microsupercapacitor with MXene−Graphene Composite Aerogel Yang Yue, Nishuang Liu,* Yanan Ma, Siliang Wang, Weijie Liu, Cheng Luo, Hang Zhang, Feng Cheng, Jiangyu Rao, Xiaokang Hu, Jun Su, and Yihua Gao* Center for Nanoscale Characterization & Devices (CNCD), School of Physics and Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology (HUST), Luoyu Road 1037, Wuhan 430074, P.R. China S Supporting Information *

ABSTRACT: High-performance microsupercapacitors (MSCs) with three-dimensional (3D) structure provide an effective approach to improve the ability of energy storage. Because the electrodes with 3D structure are generally easily destroyed under mechanical deformation in practical applications, we fabricated a self-healable 3D MSC consisting of MXene (Ti3C2Tx)−graphene (reduced graphene oxide, rGO) composite aerogel electrode by wrapping it with a selfhealing polyurethane as an outer shell. The MXene−rGO composite aerogel combining large specific surface area of rGO and high conductivity of the MXene can not only prevent the self-restacking of the lamella structure but also resist the poor oxidization of MXene to a degree. The MSC based on a 3D MXene−rGO aerogel delivers a large area specific capacitance of 34.6 mF cm−2 at a scan rate of 1 mV s−1 and an outstanding cycling performance with a capacitance retention up to 91% over 15 000 cycles. The 3D MSC presents an excellent self-healing ability with specific capacitance retention of 81.7% after the fifth healing. The preparation of this self-healable 3D MSC can provide a method for designing and manufacturing next-generation long-life multifunctional electronic devices further to meet the requirements of sustainable development. KEYWORDS: self-healable, MXene, aerogel, microsupercapacitor, three-dimensional was creatively synthesized in 2011 by Gogotsi and colleagues,12 it has been widely used in supercapacitors,13−16 electromagnetic interference shielding,17 lithium-ion batteries,18,19 and oxygen evolution reactions.20 Similar to graphene, self-restacking and aggregation of MXene nanosheets are usually inevitable during drying and electrode fabrication processes due to the strong van der Waals interaction between adjacent nanosheets, which limits the ion transport and reduces the active site in supercapacitors.15 In order to overcome this shortcoming, recently, it has been reported that graphene between MXene layers can act as an ideal “buffer” and spacer to prevent the stacking between the MXene nanosheets, which helps to improve the electrochemical property of MXene.13 However, the relatively small size of the MXene nanosheets used in that study (∼200 nm) increased the internal resistance due to the contact resistance among the sheets.15 In order to acquire high area energy density, a threedimensional (3D) structured electrode is a possible solution. Recently, the 3D structure electrode built of graphene

C

urrently, fast advances of portable and wearable electronic devices require lightweight, miniaturized, and highly efficient energy storage units and selfpowered systems.1−3 Recently, commercially available microbatteries and thin-film batteries showed rapid expansion. However, they still have the problems of low power density and short cycle life, which limits their application as reliable power sources. Microsupercapacitors (MSCs) as the new type of energy storage devices possess many advantages, such as high power density, ultralong cycle life, fast charge−discharge rate, and easy integration into micro-nano electronic systems as power supply sources, which make them a hot topic in related fields.2,4−9 Most importantly, the footprint area of the micronano device is a key factor. Therefore, for MSCs, the area energy density is the most important performance criteria. However, these typical thin-film MSCs have bottlenecks in area energy density because of the small volume size of active materials, which cannot provide sufficient energy to drive various electronic components.10 Two-dimensional (2D) transition metal nitrides and carbides (MXenes) are an emerging family with a general chemical composition of Mn+1XnTx, where M represents an early transition metal, X is C and/or N, Tx denotes surface functional groups,11 and n = 1, 2, or 3. Since the 2D Ti3C2Tx © 2018 American Chemical Society

Received: October 24, 2017 Accepted: April 12, 2018 Published: April 12, 2018 4224

DOI: 10.1021/acsnano.7b07528 ACS Nano 2018, 12, 4224−4232

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Figure 1. (a) Schematic diagrams of the manufacturing process of MXene−rGO composite aerogels. (b) Schematic diagrams of the manufacturing process of MXene−rGO self-healing MSCs. (c) Photograph of the stable MXene−rGO (left), rGO (middle), and MXene (right) in water dispersion. (d) Flake size distributions of MXene. (e) Pressure−strain curves of the MXene−rGO aerogel and rGO composite aerogel.

aerogel has a very good mechanical elasticity, which facilitates its stability as an electrode when subjected to external mechanical strain. Self-healing materials have flourished in the past decade because their internal or external damage can be healed by itself.35 In addition to the ability to recover structural and mechanical properties after damage, functional recovery is also a feature of these materials. For example, relevant studies have shown that self-healing conductive materials prepared by casting inorganic silver nanowires on the self-healing polymer substrate can restore electrical properties after damage.36,37 Therefore, self-healing materials are expected to further develop powerful and long-lived functional devices. When operating in an actual application, the substrates and electrode materials of the supercapacitor are often subjected to various mechanical damage from the outside. These failures constrained the reliability and stability of the supercapacitor and led to whole scale breakdown of the device.38,39 In summary, we believe that self-healing materials have great potential for applications in MSCs to restore electrical properties and structural integrity of destroyed devices.35,40−42 In this work, we report a simple method to fabricate and assemble self-healable MSCs by using 3D MXene−rGO composite aerogels as an electrode material. The MXene− rGO composite aerogel electrode was fabricated through simple freeze-drying and laser cutting methods. By further wrapping the 3D structure electrode with a self-healing carboxylated polyurethane (PU) shell, a self-healable 3D MSC was fabricated. The MXene−rGO aerogel can recover to its original

derivatives or other 2D materials has attracted great attention in relevant industries and academia. This 3D structure electrode with high loading of active materials, rich porous structure, and large specific surface prevents the undesirable self-restacking of 2D materials.21−25 Therefore, the utilization of such a 3D structure material as the electrode of the supercapacitor is an effective strategy to improve the ability of energy storage. For example, Gogotsi and co-workers fabricated a 3D macroporous MXene electrode by poly(methyl methacrylate) spheres as the template.26,27 This structure effectively prevents the selfstacking of MXene and further provides more electrochemically active sites in supercapacitors. However, the 3D structure electrode generally tends to have poor mechanical properties and less resilience. Because the graphene aerogel usually has excellent mechanical properties,28,29 the composite aerogels of graphene−polypyrrole and graphene−carbon nanotubes have been validated as effective materials with enhanced mechanical properties.30,31 In addition, the graphene aerogel has high conductivity so that it can be combined with other active materials in the applications of catalysis and supercapacitors.32−34 We believe that the fabrication of the 3D MXene− rGO aerogel with a mechanically robust structure is an effective strategy to prevent self-stacking of MXene and further improve the electrochemical performance. This 3D structure has many advantages. First, the large nanosheet of rGO acts like an armor to protect MXene from the poor oxidization to some degree. Second, the rich porous structure of aerogel is conducive to the ion transport and effectively prevents self-restacking of the 2D layered material (MXene and rGO). Finally, this composite 4225

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Figure 2. Characterizations of the active materials in our device. (a) TEM image of two MXene nanoflakes. Inset: Corresponding selected area electron diffraction pattern of MXene in (c). (b,c) STEM and elemental mapping images of MXene nanoflakes. (d) SEM images of rGO aerogel. (e−g) SEM images of MXene−rGO composite aerogel. (h) EDX image of MXene−rGO composite aerogel. (i) XRD patterns of the rGO aerogel, MXene film, and MXene−rGO composite aerogel.

self-assembly. However, in the hydrothermal process, high temperature can destroy the MXene structure because the antioxidant activity of MXene is relatively poor (Figure S1). An ice-template method has been proven to be an efficient way to prepare graphene aerogels.43,44 Therefore, we selected the icetemplate method for the preparation of the GO−MXene composite aerogel. Followed by a relatively mild chemical reduction method, the freeze-dried method can further prepare MXene−rGO composite aerogels. The preparation procedures of self-healable 3D MSC is shown in Figure 1b. First, the 3D MXene−rGO composite aerogel was patterned by using laser cutting to form the predesigned interdigitated patterns. Then, the MXene−rGO composite aerogels were wrapped with PVA−H2SO4 gel as the solid electrolyte. Finally, we chose a highly industrially compatible self-healing material, carboxylated polyurethane, as the outermost shell of the 3D MSC to provide the self-healing properties of the entire device. The carboxylated PU has a rich interfacial hydrogen bond in the supramolecular network, providing the source of self-healing properties of this self-healing material.45−47 In addition, PVA gel is also a self-adhesive material and shows a certain degree of self-healing.48,49 More importantly, the 3D structure of the electrodes facilitates the accurate reconnection of disconnected electrodes. The PU shell can effectively guide the reconnection process and restore the device performance when the separated parts of the destroyed 3D electrode are put in suitable places. The colloidal states of GO, MXene, and MXene−GO in water

volume from the state of around 70% strain after the load is released. The MSCs based on the 3D MXene−rGO aerogel display a large specific area capacitance of 34.6 mF cm−2 at a scan rate of 1 mV s−1 and an outstanding cycling performance with a capacitance retention up to 91% over 15 000 cycles. The 3D MSC also presents an excellent self-healing ability, keeping 81.7% specific capacitance after the fifth healing. The preparation of these self-healable 3D MSCs can provide a method for designing and manufacturing next-generation longlife multifunctional electronic devices further to meet the requirements of sustainable development.

RESULTS AND DISCUSSION The preparation procedures of the MXene−rGO composite aerogel is shown in Figure 1a. First, a mixture of solution (GO and MXene) was injected into a vessel, followed by freezedrying and chemical reduction by a mixed HI and HAc solution. After complete reaction, the MXene−rGO composite aerogels were removed to carry out the second freeze-drying process. When the mixture solution of GO and MXene was frozen, the GO and MXene nanosheets were forced to gradually align along the ice crystal boundary and finally cross-linked by the π−π interaction to form a porous network. In this process, the small-sized MXene nanosheets were wrapped in GO nanosheets, and the mechanical strength of the composite aerogel can be improved. In general, the hydrothermal method of graphene is an effective strategy of 3D 4226

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Figure 3. Electrochemical characterizations of MXene−rGO composite aerogel electrodes. (a) CV curves of the MXene−rGO composite aerogel with various initial GO/MXene weight ratios at the scan rate of 0.05 V s−1. (b) GCD curves of the MXene−rGO composite aerogel with various initial GO/MXene weight ratios at the current densities of 0.8 mA cm−2. (c) Area capacitances vs various initial GO/MXene weight ratios. (d) CV curves of the MXene−rGO composite aerogel with an initial GO/MXene weight ratio of 16:6 at various scan rates. (e) Galvanostatic discharge curves of the MXene−rGO composite aerogel with an initial GO/MXene weight ratio of 16:6 at different current densities. (f) Nyquist curves of pure rGO aerogel and MXene−rGO composite aerogel with an initial GO/MXene weight ratio of 16:6.

composite aerogel exhibits an obvious 3D porous structure similar to that of pure rGO. From the magnified SEM image of the composite aerogel (Figure 2f,g and Figure S7), it is observed that the composite nanosheets still maintain a 2D flat structure, and the MXene is wrapped in the large nanosheet of rGO. Figure 2h displays the EDS spectrum of the MXene−rGO aerogel, where the Ti signal is from MXene and C and the O signal is from MXene and rGO. The corresponding EDX elemental mapping proved that elements of Ti, C, and O are evenly distributed throughout the composite aerogel, as presented in Figure S8. In order to further prove the composite structure of the 3D MXene−rGO aerogel, the MXene−rGO composite aerogel was sonicated and dispersed into alcohol and water to form a dispersion. Its STEM image and its corresponding EDS mapping (as shown in Figure S9) demonstrate the larger rGO nanosheets wrapping the smaller MXene nanosheets. In addition, the integrity and stability of the composite aerogel after freeze-drying and chemical reduction were also confirmed by the X-ray photoelectron spectroscopy (XPS) and the corresponding element analysis (Figure S10 and Table S3). Figure 2i shows the X-ray diffraction (XRD) pattern of the MXene thin film, rGO aerogel, and MXene−rGO aerogel. The peak located at about 7° belongs to MXene (Ti3C2Tx), and the peak of 25° is ascribed to rGO, which further proved the stability of the composite aerogel after chemical reduction.50 In order to obtain better performance of the fabricated device, different initial weight ratios of MXene− rGO composite aerogels were used to find the optimization parameter. We performed a series of related electrochemical tests on rGO−MXene composite aerogels through a universal three-electrode configuration. Figure 3a and Figure S11 show the cyclic voltammogram (CV) curves of pure rGO aerogel, MXene−rGO aerogel with different initial weight ratios of 16:1, 2, 3, 4, 5, and 6 of GO/ MXene. At a scan rate of 50 mV s−1, all 3D MXene−rGO

were identified by the Tyndall scattering effect with passing of the green laser through the dispersion even after 1 week (Figure 1c and Figure S2). The stable existence of MXene−GO uniform dispersion provides the possibility of preparing composite aerogels by the ice-template method. Figure 1d shows that the lateral size distribution of MXene is concentrated in several hundred nanometers. This size is significantly smaller than the size of the GO nanosheets (Figure S3), which favors the MXene nanosheets attached to the surface of GO. The pressure−strain curve of the rGO aerogel and MXene−rGO aerogel is shown in Figure 1e, which shows that the introduction of MXene did not destroy the original structure of the rGO aerogels but increased the mechanical properties. Other physical properties of MXene−graphene composite aerogels are shown in Figures S4 and S5 and Table S2. The MXene−rGO aerogel can recover to its original volume from the state of around 70% strain after the load is released. This strong mechanical property benefits the stability of the electrode under external force damage. The structure and composition of synthetic MXene were examined by transmission electron microscopy (TEM) and energy-dispersive spectrometry (EDS) mapping. Figure 2a presents the TEM image of MXene and revealed that MXene is obviously a two-dimensional lamellar structure, and its lateral diameter is several hundred nanometers. This lateral diameter matches the previous flake size distributions of MXene (Figure 1d). As shown in the AFM image (Figure S6), the thickness of MXene is ∼1.52 nm. The selected area electron diffraction (SAED, inset in Figure 2a) exhibits a typical hexagonal symmetry diffraction pattern, indicating the high crystallinity of a typical MXene nanosheet.13 The high-angle annular darkfield scanning TEM and elemental mapping images obviously show the Ti, C, and O elements in the MXene (Figure 2b,c). Figure 2d,e shows the SEM images of the rGO and MXene− rGO composite aerogel, respectively. The MXene−rGO 4227

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Figure 4. Electrochemical behaviors of MXene−rGO composite aerogel MSCs. (a) CV curves of MXene−rGO composite aerogel MSCs at different scan rates. (b) GCD curves of MXene−rGO composite aerogel MSCs at different current densities. (c) Area capacitances vs scan rate of MXene−rGO composite aerogel MSCs. (d) Cycling stability of the MXene−rGO composite aerogel MSC at the 2 mA cm−2 current density (the inset presents the GCD curve from the 14990th to the 15000th cycle).

Figure 5. (a) Schematic diagrams of the self-healable mechanism. The self-healing feature comes from the interface hydrogen bond. (b) Photographs of self-healing PU: original (left), after the damage (middle) and after healing (right). (c) Demonstration of after self-healing PU: under bending state (top left), under flat state (bottom left), supporting a 500 g mass (right). Rectangles indicate the wound/healing positions.

electrodes present current densities significantly greater than those of the rGO aerogel electrodes. The reason is that MXene has better electrochemical performance than rGO. Figure 3b and Figure S12 show the galvanostatic charge−discharge (GCD) curves of rGO and MXene−rGO with various initial weight ratios of GO/MXene, which further proved that the performance of 3D MXene−rGO is better than that of rGO aerogel. Figure 3c shows the relationship of the specific capacitances and the various initial weight ratios of GO/ MXene. We can find that the optimal initial weight ratios of GO/MXene was about 16:6, which indicates that the performance of composite aerogel can be further promoted with the increase of MXene. However, the limited concentration of fabricated MXene nanosheet solution hindered the performance optimization by further increasing the proportion of MXene. Therefore, we could fabricate an all-solid-state 3D MSC with high performance based on the optimization process.

Figure 3d presents the CV curves of the 3D MXene−rGO aerogel with scanning rates from 5 to 100 mV s−1. As expected, the 3D MXene−rGO aerogel electrode exhibited good capacitive performance with similar rectangular CV curves. In addition, GCD measurements were performed to further test the performance of the 3D MXene−rGO aerogel (Figure 3e). The equivalent series resistance (ESR) of the 3D MXene−rGO electrode was 3.66 Ω, as shown in Figure 3f. This value of ESR is obviously better than 7.9 Ω of the rGO aerogel. In addition, the curve is nearly parallel to the imaginary axis, further demonstrating the ideal capacitive performance of the 3D MXene−rGO aerogel. In conclusion, these performance test results of the MXene−rGO aerogel demonstrate that it is an ideal candidate for electrode materials for supercapacitors. Generally, the electrochemical performance of a supercapacitor is evaluated using a two-electrode setup that is closer to the actual application.51−56 Therefore, a series of electro4228

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Figure 6. Performance and application of 3D self-healing MSCs. (a) Cyclic voltammogram curves (50 mV s−1), (b) galvonostatic charge− discharge (0.4 mA cm−2) curves, and (c) Nyquist curves of the original and after several self-healings of MSC. (d) Photographs of the 3D selfhealing MSC (the left is the original MSC, the middle is the MSC after cutting, the right is MSC after self-healing). (e) SEM image of perovskite nanowires. (f) XRD patterns of PbI2 and perovskite nanowires. (g,h) Illustration and circuit diagram of MSCs driving a perovskite nanowire based photodetector. (i) Performances of a photodetector driven by the original and after a healing cycle on MSC (red curve corresponds to an original MSC, and the black curve corresponds to the MSC after a healing cycle).

Figure 5a,b shows the cutting−healing process of a PU belt to demonstrate its excellent self-healing property. First, the PU belt is bisected using a scalpel; then the fractures are rejoined under a gentle pressure. The self-repairing process proceeds for 3 min. After the wound healed, only a scar on the surface was left behind. The carboxylated PU has a rich interfacial hydrogen bond in the supramolecular network, which is the source of self-healing properties of this self-healing material. This superior self-healing performance can meet stringent mechanical requirements under bending and tensile states (Figure 5c), which indicates that the self-healing process of the device should not be a simple self-adhering process. In order to further study the self-healing mechanism of PU, we observed its selfhealing process by SEM, as shown in Figure S14. Figure S14a shows the SEM image in a completely cut state. After a few minutes of self-healing (Figure S14b), the wound was healed. Figure S14c shows the SEM diagram of the repaired PU in a tensile state. Figure S14d is a partial enlargement image of Figure S14c. It is obvious that the electrode has a certain degree of rupture due to the different degree of healing, as shown in Figure S14e,g,h. However, the internal remains are still connected. Due to the high degree of repair in some areas, as shown in Figure S14g, the whole structure is still connected to each other in the stretching state.

chemical measurements were performed to evaluate our device by two-electrode configuration. Figure 4a presents the CV curves of the 3D MSC. The curves have a large closed area and very symmetrical rectangular shape at a scan rate of 1 to 100 mV s−1 with a 0.6 V potential window. We also performed GCD measurements to further test the electrochemical performance of the device, as presented in Figure 4b. The results show that the device has good linear potential−time curves at different current densities varying from 0.2 to 0.6 mA cm−2. Figure 4c shows area capacitances versus scan rate of the device. At a scan rate of 1 mV s−1, 3D MSCs exhibit an area capacitance of up to 34.6 mF cm−2. Even though the scan rate is further increased to 100 mV s−1, the device still has an area capacitance of 9.2 mF cm−2. This performance is significantly better than that of similar structural supercapacitors (Table S1). Due to effective encapsulation and hierarchical porous structure of MXene−rGO aerogel, our device presents outstanding cycling performance at a current density of 2 mA cm−2. The capacitance retention is as high as 91% over 15 000 cycles, as shown in Figure 4d. As shown in Figure S13, the Ragone plot was used to display the performance of our 3D MSC. At a power density of 60 μW cm−2, the as-fabricated 3D MSC exhibits a maximum energy density of 2.18 μWh cm−2. At an energy density of 1.33 μWh cm−2, the MSC exhibits a maximum power density of 180 μW cm−2. 4229

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prepared hydrochloric acid solution. After being agitated, 1 g of the MAX phase precursor was slowly added (in order to prevent overheating, the process should exceed 25 min), immediately followed by magnetic stirring for 24 h at 35 °C. The final product was washed by multiple centrifugations until pH ≥6. The precipitate after centrifugation was sonicated in an Ar atmosphere and a lowtemperature environment for 1 h, followed by centrifugation for 1 h at 3500 rpm. The final collected suspension is the Ti3C2Tx nanosheet dispersion. Preparation of the GO Dispersion. The GO is fabricated by a modified Hummers method according to our previous work.52 Preparation of the MXene−rGO Aerogel Based MSC. First, the preprepared GO solution was directly mixed with the MXene solution and then further sonicated for about 10 min to further obtain a homogeneous solution. In order to optimize the performance of the MXene−rGO aerogel, various ratios of GO/MXene (16:0, 1, 2, 3, 4, 5, and 6) mixed solutions were prepared. Second, we transfered the mixed solution into a container of a specific shape and then freezedried it to form an aerogel. Third, the prepared MXene−GO and GO aerogels were immersed in a mixture of HI and HAc (volume ratio is 1:2) for 10 min at room temperature and then heated to 60 °C for 3 h. Fourth, the obtained MXene−rGO aerogel was washed several times with ethanol and water alternately to completely remove the residue and then freeze-dried again. Fifth, the designed MSC device was patterned directly by laser processing of 3D MXene−rGO using a laser machine. Finally, it was wrapped with the gel electrolyte of PVA− H2SO4 and further vacuum-dried at room temperature. Self-repairing PU (Wanhua Chemical Group Co., Ltd.) was wrapped on the MSC and further dried in air. Fabrication of Photodetectors of Perovskite Nanowires. Photodetectors of perovskite nanowires were fabricated according to our previous reported work. First, preconfigured 1 M PbI2 solution (solvent is N,N-dimethylformamide, Aldrich) was spin-coated onto the preprepared interdigitated electrode and then heated at 70 °C for 30 min. The spin-coating parameters were 500 rpm for 30 s and then 3000 rpm for 30 s. Then, 10 mg/mL of CH3NH3I solution (solvent: 200 μL of N,N-dimethylformamide and isopropyl alcohol) was spincoated on the prepared PbI2 layer and then heated at 70 °C for 30 min. The spin-coating parameters were 500 rpm for 30 s and then 5000 rpm for 30 s. Thus, perovskite nanowire based photodetectors were fabricated. Characterization. All electrochemical measurements were performed by an electrochemical workstation (Chen Hua CHI660E). The configuration of the three-electrode test was specifically as follows: The MXene−rGO aerogel was used as a working electrode; a platinum plate and a Ag/AgCl electrode were used as a counter electrode and reference electrode, respectively. The electrolyte is 1 M H2SO4 solution. The structure and morphology of the MXene−rGO aerogel and the MXene nanosheet were examined by scanning electron microscopy (FEI Nova NanoSEM 450, 10 kV) with an EDS system, transmission electron microscopy (Tecnai G220 U-TWIN) with an EDS system, and X-ray diffraction (Empyrean) and X-ray photoelectron spectroscopy (Empyrean). The size distributions of GO and MXene nanosheets were examined by a Nano-ZEN 3600 (Malvern Instruments, UK).

In order to satisfy the specific power and energy requirements in various practical applications, the relevant electrochemical properties of self-healing 3D MSCs encapsulated with a PU shell were investigated. As a proof of concept, we tested the capacitive performance of our device before and after several cutting and healing cycles. As shown in the CV curve of Figure 6a, our self-healing 3D MSC still approximates a rectangular shape after the fifth healing. The capacitance retention rate of the device after the fifth healing was approximately 81.7%. In the process of cutting−healing, the MSC inevitably undergoes vigorous operation, resulting in a deviation in the reconnection between the disconnected electrodes. The GCD results (Figure 6b) further demonstrate the good recovery of 3D MSCs. To further understand the selfhealing properties of the device, we studied the electrochemical impedance spectroscopy of the 3D self-healing MSC. The ESR of the devices before cutting and after the first through fifth cycles are 99.5, 110, 115, 126, 129, and 139 Ω, respectively (Figure 6c). Figure 6d shows the photographs of the 3D selfhealing MSC at different states. To further prove the selfhealable performance of our devices, the self-healable 3D MSCs were used to drive a photodetector. We selected perovskite nanowires as photoresponsive materials for the device due to their good light response performance. A simple two-step process (details in the Methods section) was used to prepare perovskite nanowires. The SEM image of Figure 6e demonstrates the nanowire structure of perovskite. As shown in the XRD patterns of Figure 6f, the main diffraction peaks are indexed, matching well with CH3NH3PbI.57,58 Figure 6g,h presents the schematic circuit diagram of the real product, in which a fully charged device drives a photodetector of perovskite nanowires. We use an original and an after-healing MSC to drive a photodetector. The performance comparison results are shown in Figure 6i. The on/off ratio of the photodetector driven by the healed MSCs only had a slight decay, proving the good restoration of the 3D MSCs. In summary, self-healing 3D MSCs have practical applications in wearable electronic devices.

CONCLUSION We have successfully fabricated 3D MXene−rGO composite aerogels by a simple ice-template method. This composite structure of composite aerogels exhibits excellent mechanical properties compared to those of pure rGO aerogels, and a selfhealable 3D MSC was further prepared with the 3D MXene− rGO composite aerogel wrapped with gel electrolyte and PU. The carboxylated PU shell gives the device excellent mechanically self-healing performance. The MSC still maintains 81.7% specific capacitance after the fifth healing. The preparation of this self-healable 3D MSC can provide a method for designing and manufacturing next-generation long-life multifunctional electronic devices further to meet the requirements of sustainable development.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b07528. SEM images of MXene−rGO composites; photograph of the stable MXene−rGO, rGO, and MXene water dispersion after 1 week and after 2 months, respectively; flake size distributions of GO; nitrogen adsorption− deposition isotherms and pore size distributions for MXene−graphene composite aerogel; mechanical performance test of MXene−graphene composite aerogel; AFM image of MXene nanoflake; SEM images of

METHODS Preparation of the MAX Phase (Ti3C2Tx) Precursor. First, Al, C, and TiH2 powders were mixed in a 3:1.1:2 molar ratio. Then, the mixed powders were transferred to a ball mill for milling (18 h). Finally, the above mixture was heated to 1400 °C under Ar atmosphere for 2 h. Preparation of the MXene (Ti3C2Tx) Nanosheet Dispersion. First, water was added to a hydrochloric acid solution to prepare 9 M hydrochloric acid. Then, 1 g of LiF powder was slowly added to the 4230

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ACS Nano

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MXene−rGO composites; SEM images and elemental mapping images of MXene−rGO composite aerogel; STEM and elemental mapping images of MXene nanoflakes coated on rGO nanoflakes; XPS survey spectrum, high-resolution Ti 2p and C 1s spectra of MXene−graphene composite aerogel; CV curves of pure rGO and MXene−rGO composite aerogel with initial weight ratios of 16:1, 2, 3, 4, 5 of GO/MXene; GCD curves of pure rGO aerogel and MXene−rGO composite aerogel with initial weight ratios of 16:1, 2, 3, 4, 5 of GO/ MXene; Ragone plots of the 3D MSCs with MXene− rGO composite aerogel compared with other energy storage devices; self-healing process of PU was observed through SEM; tables summarizing the areal capacitance of supercapacitors, physical properties of MXene− graphene composite aerogel, and elements ratio of MXene−graphene composite aerogel from XPS (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Nishuang Liu: 0000-0002-2507-7229 Yihua Gao: 0000-0003-1905-9531 Author Contributions

N.L. and Y.G. devised the original concept, designed the experiments, discussed the interpretation of results and revised the paper. Y.Y. and Y.M. performed almost all the experiments. Y.Y. wrote the draft of the manuscript. W.L., S.W., and C.L. participated in the analysis of the experimental results. H.Z., X.H., and J.S. contributed some SEM observations. F.C. completed the TEM observations. J.R. contributed to the discussion of the manuscript. All authors participated in manuscript revision. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Hubei Province (2016CFB432) and the National Natural Science Foundation of China (11674113). We thank Prof. Zhong Lin Wang for the facility support. The authors also thank Prof. Dawen Zeng (School of Materials Science and Engineering, HUST) and Dr. Qing Wu Huang and the Analysis and Testing Center of HUST for the characterizations. REFERENCES (1) Wang, Z. L. Self-Powered Nanosensors and Nanosystems. Adv. Mater. 2012, 24, 280−285. (2) Niu, Z.; Zhang, L.; Liu, L.; Zhu, B.; Dong, H.; Chen, X. All-SolidState Flexible Ultrathin Micro-Supercapacitors Based on Graphene. Adv. Mater. 2013, 25, 4035−4042. (3) Yu, D.; Goh, K.; Wang, H.; Wei, L.; Jiang, W.; Zhang, Q.; Dai, L.; Chen, Y. Scalable Synthesis of Hierarchically Structured Carbon Nanotube-graphene Fibres for Capacitive Energy Storage. Nat. Nanotechnol. 2014, 9, 555−562. (4) Chmiola, J.; Largeot, C.; Taberna, P. L.; Simon, P.; Gogotsi, Y. Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors. Science 2010, 328, 480−483. (5) Peng, L.; Peng, X.; Liu, B.; Wu, C.; Xie, Y.; Yu, G. Ultrathin TwoDimensional MnO2/Graphene Hybrid Nanostructures for High4231

DOI: 10.1021/acsnano.7b07528 ACS Nano 2018, 12, 4224−4232

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DOI: 10.1021/acsnano.7b07528 ACS Nano 2018, 12, 4224−4232