Controlled Crumpling of Two-Dimensional Titanium Carbide (MXene

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Controlled Crumpling of Two-Dimensional Titanium Carbide (MXene) for Highly Stretchable, Bendable, Efficient Supercapacitors Ting-Hsiang Chang, Tianran Zhang, Haitao Yang, Kerui Li, Yuan Tian, Jim Yang Lee, and Po-Yen Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02908 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Controlled Crumpling of Two-Dimensional Titanium Carbide (MXene) for Highly Stretchable, Bendable, Efficient Supercapacitors Ting-Hsiang Chang,† Tianran Zhang,† Haitao Yang,† Kerui Li,† Yuan Tian,† Jim Yang Lee,† PoYen Chen*,† †Department

of Chemical and Biomolecular Engineering, National University of Singapore,

Singapore 117585 Email: [email protected] KEYWORDS. MXene nanocoatings; wrinkled and crumpled structures; controllable wetting surfaces; electrochemical energy storage; stretchable and bendable supercapacitors.

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ABSTRACT Two-dimensional MXene materials have demonstrated attractive electrical and electrochemical properties in energy storage applications. Adding stretchability to MXene remains challenging due to its high mechanical stiffness and weak intersheet interaction, so the assembling techniques for mechanically-stable MXene architectures require further development. We report a simple fabrication by harnessing the interfacial instability to generate higher dimensional MXene nanocoatings capable of programmed crumpling/unfolding. A sequential patterning approach enabled the design of sequence-dependent MXene textures across multiple length scales, which were utilized for controllable wetting surfaces and high-areal-capacitance electrodes. We next transferred the crumpled MXene nanocoating onto elastomer to fabricate a MXene/elastomer electrode with high stretchability. The accordion-like MXene can be reversibly folded/unfolded and still preserve efficient specific capacitances. We further fabricated asymmetric MXene supercapacitors, and the devices demonstrated efficient electrochemical performance and large deformability (180° bendability, 100% stretchability). Our texturing techniques can be applied to large MXene families for designing stretchable architectures in wearable electronics.


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Unconventional stretchable electronics have emerged as an important branch of modern electronics.1-4 The stretchable devices can sustain large deformations and conform to surfaces with complicated geometry while maintaining the electronic properties. Various stretchable electronic devices have been developed for different applications, including wearable electronics,5,

6

biomedical devices,7, 8 stretchable displays,9, 10 and artificial electronic skin.11-13 Since most of the unconventional electronics require electricity, the energy storage devices that can be integrated with stretchable electronics have become indispensable in achieving fully power-independent and mechanically-stable systems for future commercialization.11, 14, 15 Therefore, supercapacitors with high deformability become one mainstream in personalized electronics due to their high power density, fast rate of charge-discharge, and long cycling lifetime.16, 17 Transition metal carbides, carbonitrides, and nitrides (MXenes) are a new class of twodimensional nanomaterials with metal-like conductivity, hydrophilic surfaces, and excellent mechanical properties. MXenes have been rapidly recognized as promising electrode materials in electrochemical energy applications, including metal-ion batteries,18-20 electrochemical capacitors,21, 22 and micro-supercapacitors.23, 24 Previous reports have successfully demonstrated the crumpled MXene nanosheets with mesoporous microstructures by controlling the pH value of MXene colloidal suspension.25, 26 However, due to high mechanical stiffness (Young’s moduli ~375 GPa for dry Ti3C2Tx) and weak intersheet interaction,27-29 challenges remain in the incorporation of stiff MXene nanosheets into deformable energy storage devices, which require the electrode design to sustain large mechanical deformations (e.g., under large strain over 50% and/or under repeated stretching cycles over 1000 times) without sacrificing its electrochemical activities. Recently, Ti3C2Tx MXenes have been combined with carbon nanotubes (CNTs) or polymers for the purpose of strain sensors.30, 31 The stretchability of these devices was achieved


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by the incorporation of hairy CNTs or long polymeric chains that can knit the Ti3C2Tx sheets under large strains. The electrode design is very suitable for strain sensors, which can induce gaps and cracks to increase the electricity resistance of MXene nanocoatings to achieve high gauge factors. Yet, for stretchable energy storage devices (e.g., supercapacitors), different electrode design is required to maintain constant, efficient electrochemical activities while stretching and bending. To the best of our knowledge, stretchable MXene electrodes for supercapacitors have not yet been reported in the literature. Here we report a simple fabrication route by harnessing the interfacial instability to generate higher dimensional MXene nanocoatings capable of programmed crumpling and unfolding (Figure 1). The multifunctionality of mechanically-patterned MXene nanocoatings was demonstrated in (1) controllable wetting surfaces, (2) electrochemical electrodes with high areal capacitance (CA), and (3) highly stretchable, bendable supercapacitors. We first utilized a sequential patterning process to repeatedly deform the MXene hybrid nanocoatings via the actuation of thermally-responsive substrates, which can undergo controlled biaxial (2D) or uniaxial (1D) contractions at an elevated temperature. This sequential patterning approach enables the design of sequence-dependent surface topographies with structural memory across multiple length scales. These MXene hierarchies can be utilized as controllable wetting surfaces, showing both super-hydrophilicity (water static contact angles (θw) ~0°) and super-hydrophobicity (θw >150° after grafting with fluorocarbons). The high-generation MXene nanocoatings were next characterized as electrochemical electrodes, demonstrating a 27-fold improvement of CA relative to flat surfaces. The accordion-like MXene nanocoating can be further transferred on an elastic VHBTM substrate to achieve a MXene/elastomer electrode with high stretchability. The MXenebased electrode demonstrated a set of merits including high stretchability (up to 1D 80% and 2D


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225% stretching), high volumetric capacitance (395 F cm-3), and high mechanical stability (over 1,000 stretching/relaxation cycles). We further fabricated a stretchable asymmetric supercapacitor (ASC) based on the MXene and activated carbon (AC) electrodes. The electrochemical performance of stretchable MXene ASCs remained nearly unchanged under large mechanical deformations (~180° bendability and 100% 1D stretchability). The stretchable MXene ASCs also showed high cycling stability (over 3,000 cycles) under 0%, 50%, and 100% 1D strains. We believe our controlled crumpling strategies can be applied to large MXene families, allowing us to fabricate mechanically-stable architectures for stretchable and wearable energy-storage applications.

RESULTS AND DISCUSSION Preparation of MXene Nanosheet-Polymer Dispersion. Two-dimensional Ti3C2Tx MXene nanosheets were prepared by etching Ti3Al2C2 MAX crystals in an in situ hydrofluoric acid (in situ HF) solution (dissolving LiF salts in HCl solution) (SEM images shown in Figure S1a,b), followed by exfoliation with the assistance of ultrasonication. The yield of Ti3C2Tx MXene is about 60%. The XRD analysis in Figure S1e indicates that all MAX phase32 is fully transformed to MXene after the etching and exfoliation processes.33 The dimension of as-exfoliated MXene nanosheets was measured by using transmission electron microscope (TEM) and dynamic light scattering (DLS), which ranged from 100~600 nm (Figures S1c and Figures S1g). Detailed discussion of MXene synthesis and characterizations is shown in Supporting Note. The MXene nanosheets were further hybridized with the non-active polymer binder, sodium alginate (SA). The polymeric chains contain oxygen-containing functional groups (–OH, –COOH, and –C=O), which can form hydrogen bonding with the termination groups of MXene nanosheets.34 The hydrogen


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bonding enhances the mechanical stability of MXene nanocoatings and enables the retention of electrical conductivity.34, 35

Controlled Crumpling of MXene Nanocoatings by Harnessing the Surface Instability during Thermal Actuation. Figure S2 depicts a sequential deformation process to extremely compress planar MXene nanocoatings into multigenerational structures. The MXene-SA dispersion was first drop-cast and air-dried on a thermally-responsive polystyrene (PS) “shrink film”, forming a thin MXene nanocoating with a planar morphology. The thickness of MXene nanocoatings can be systematically controlled by depositing the MXene-SA dispersion with different dilution ratios (Figure S3). The deposition was facilitated by pretreatment of the PS substrate with oxygen plasma, which increased hydrophilic interactions and hydrogen bonding after the solvent evaporated.36 For simplicity, the planar MXene nanocoating is denoted by G0 (generation 0) and subsequently patterned structures are denoted Gn after undergoing n deformation(s). By heating the samples above the Tg of PS (~100 °C), the substrate contracted to release the pre-stretched strain, leading to G1 wrinkles (under 1D strain) or G1 crumples (under 2D strain). The pre-stretched 1D strain is about 150%, corresponding to the film shrinkage of 40% of original length (x = y = 40%) and 16% of original area (area = 16%). Without the incorporation of SA, the MXene-only coating exhibited many surface cracks after 2D deformation (Figure S4). The deformed patterns are highly dependent on the MXene-to-SA weight ratio (Figure S5), and the ratio was kept at 4to-1 in the following studies. The initial 1D or 2D deformation results in G1 architectures that display periodic wrinkles or disordered crumples, respectively (Figure S6). The characteristic wavelength of G1-1D wrinkled structures can be approximated by eq 1:37


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1/3

l = 2p h ( Ec / 3Es )

(1)

(

)

, where h is the G0 thickness and the plane-strain elastic modulus Ei = Ei / 1- n i2 is given in terms of Young’s modulus E and Poisson’s ratio ν of the coating (c) or shrink film (s), respectively. The coating thickness h can be controlled from 27 to 551 nm, resulting in wrinkling wavelengths, λ, that range from 6.4 to 33.2 µm (Figure S7a). Given νs = 0.34 and Es (at 140 °C) = 5 MPa,38-40 Young’s modulus of MXene-SA films can be further estimated at 24.5 GPa through eq 1 and Figure S7b. In contrast, 2D deformation results in isotropic crumples without orientational order, and the crumpled feature size shifts from 1×1 to 10×10 µm by controlling the coating thickness from 27 to 551 nm.

Multigenerational MXene Architectures Programmed by Deformation Sequences. Next, G1 structures were detached, transferred, and underwent a second round of mechanical deformation to achieve hierarchical G2 structures (Figure 2a-e). For simplicity, these experiments used a constant coating thickness of 551 nm, corresponding to a G1-1D wrinkle wavelength λ = 17.6 µm and G1-2D crumple size at 10×10 µm. There are five possible variations for G1G2 deformation sequences. The repeated 2D contraction case (2D2D) results in a denser crumpled topography with a larger feature size (~ 17×17 µm, Figure 2a). The mixed deformation cases (2D1D and 1D2D) qualitatively appear more wrinkled or more crumpled, respectively. The 2D1D structure consists of larger G2 wrinkles (λ = 32.2 µm, Figure 2b, Figure S7c,d) with the pre-existing G1 patterns; the 1D2D structures consist of smaller G1 wrinkles (λ = 2 µm) that decorate larger G2 crumples (Figure 2c). Finally, 1D contraction followed by a perpendicular 1D contraction (1D1D) results in smaller folding features that decorate larger wavelength wrinkles (λ = 9.3 µm) but are oriented


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perpendicularly (Figure 2d, Figure S7c,d). In contrast, repeated 1D contractions in the same direction (1D1D) result in a tightly squeezed wrinkled pattern with characteristic wavelengths at 4.4 µm (Figure 2e, Figure S7c,d). The third iteration of mechanical deformation results in additional variations for G3, shown in Figure 2f-h. In general, multiscale G3 structures display larger topographical features decorated with smaller features generated from the previous deformations. For instance, the triple 2D contractions (2D2D2D) result in a multiscale structure with large G3 crumples at 100×100 µm and smaller G1, G2 crumples at 5×5 µm (Figure 2f). Double 2D contractions followed by 1D contraction (2D2D1D) result in large anisotropic G3 wrinkles with a dominant wavelength about 57.1 µm, decorated with smaller G2 and G1 crumples (Figure 2g, Figure S7c,d). Instead, 2D then 1D then 2D contractions (2D1D2D) display large G3 crumpled features (20×20 µm) decorated with medium G2 wrinkles and small G1 crumples (Figure 2h). It should be noted that G3 structures exhibit reduced contact areas with responsive substrates relative to G1, G2, resulting in delamination if a G4 deformation is attempted. Figure S8 demonstrates the “family tree” of multigenerational MXene structures, starting from planar G0 coatings to multiscale G3 structures. The y-axis provides the ratio of the area of initial planar coating (A0) to the area of multigenerational MXene structure (A). This sequential patterning technique gives complex MXene topologies that emerge as the nanocoatings under extreme compression down to 0.6% of their initial area (A0/A ~ 160). Several design principles for engineering feature sizes and orientations of MXene topographies are similar to the graphene oxide (GO) hierarchies in the literature (see Figure S8 for the detailed discussion).41 The major difference between the MXene and GO families is their relaxation degree during transferring processes. As the G1, G2 structures were detached, the MXene structures relaxed back to 130 % of


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the compressed state, which is less than GO’s relaxation degree (~180% of the compressed state).41 This also explains why the structural features from previous generations preserve more significantly in MXene’s final structures than the GO case.

Multigenerational

MXene

Structures

for

Controllable

Wetting

Surfaces.

The

multigenerational MXene structures were further characterized by contact angle goniometry to probe the surface chemistry and roughness. For simplicity, only successively crumpled structures were characterized, and the θw was measured before and after the treatment of fluorinated trichlorosilane (Figure 3a,b). Before the treatment, the MXene films were hydrophilic, and the contact angles decreased with increasing generations, showing at θw ~61° for G0, θw ~45° for G12D, and the super-hydrophilicity (θw ~0°) was present in both G2-2D2D and G3-2D2D2D structures (Figure 3a). After the MXene samples were grafted with the fluorocarbons, they became hydrophobic and the θw progressively increased with increasing generations, demonstrating at θw ~120° for G1, θw ~142° for G2, and θw ~151° for G3 (Figure 3b). The super-hydrophilicity and super-hydrophobicity in higher-generation structures likely originate from increased structural complexity after sequential deformation steps. Take the hydrophobic surfaces for example, a water droplet on structured surfaces only remained in contact with the raised portion of the structures, and the air trapped in the cavities between micron- and nano-sized features. Therefore, the air embedded in the hierarchies and the interface beneath the water droplet could be considered a hydrophobic surface at a Cassie-Baxter state.42

Hierarchical MXene-CB Electrodes with High Areal Capacitance. Although the mechanical stability of MXene nanocoatings was enhanced by incorporating SA binder, the electrical


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conductivity was sacrificed. Thus, conductive carbon black nanoparticles (CBs), Super-P, were further incorporated into multiscale MXene nanocoatings to achieve the electrochemical electrodes with high CA. The CBs were first mixed with the MXene-SA dispersion, and the mixtures were ultrasonicated, filtered, and fabricated into a MXene-CB working electrode in a three-electrode electrochemical cell (with Ag/AgCl reference electrode and Pt counter electrode in 1.0 M aqueous H2SO4 electrolyte). The MXene-CB electrode is composed of MXene multilayers packed with CBs as shown in the cross-sectional SEM image in Figure 3c. Electrochemical impedance spectroscopy (EIS) was used to analyze the interfacial resistances and provided additional insight into the effect of CBs on the electrochemical performance of MXene-CB electrodes. Both MXene and MXene-CB electrodes were characterized, and their Nyquist plots are shown in Figure 3e. The semicircle at high frequency region is attributed to the resistance of charge transfer process, and the diameter indicates the magnitude of charge-transfer resistance (RCT). In addition, the 45°-sloped portion of the curves in the middle-frequency region (Warburg impedance) indicates the efficiency of ion diffusion. From the fitting results, RCT of the MXene-CB electrode (1 ) is smaller than that of the MXene electrode (>200 ), indicating the incorporation of CBs facilitates electron transport as a result of enhanced electrical conductivity. The MXene-CB electrodes also yield a near-vertical line in the middle region, which may result from the expanded interlayer spacing that maximizes the contact area at the electrolyte/electrode interface. The CA of MXene-CB electrochemical electrodes can be effectively increased by undergoing sequential mechanical deformations. The planar G0 electrodes were deformed sequentially to obtain G1-2D, G2-2D2D, and G3-2D2D2D electrodes, and the structural complexity increased with the generations (SEM images in Figure 3d). The cyclic voltammetry (CV) curves of G1, G2, and


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G3 electrodes were measured and compared with the G0 electrode in Figure 3f. The areal current densities (mA cm-2) and capacitance (F cm-2) are highly dependent on the areal density of active materials (g cm-2) on the electrodes. For instance, G2 electrodes achieved 70% higher CA than G1 electrodes (i.e., 6 times higher than G0 electrode). The improvement mainly originates from the extreme convolution of a planar MXene nanocoating from multiple processes, including G1 shrinkage (16% of original dimension, see Figure S9a), relaxation of G1 structures (back to 20%), and G2 shrinkage (re-compression to 3%). Furthermore, G3 electrodes demonstrated additional 120% improvement on CA compared to G2 (13-fold higher CA than G0 electrodes). This mechanical patterning approach can effectively increase the CA of electrochemical electrodes, which can be applied to fabricate high-performance MXene micro-supercapacitors in the future.23, 24 Next, the electrical resistance of multigenerational Ti3C2Tx MXene films were measured over a 1-cm distance by estimating the slope of their I-V curves and summarized in Figure S9b. Interestingly, the G1-2D crumpled MXene exhibited a ~35% decrease in the electrical resistance compared to the G0 planar MXene. The trend differed from what we observed in the reduced graphene oxide (rGO) case, which exhibit a 35% increase in resistance after G1 compression due to the tortuous electron pathway.41 The better electron transporting properties in G1 MXene may result from the facts that the small-sized MXene nanosheets can be packed tightly after biaxial compression, resulting in a decrease in the particle-to-particle contact resistance. The reduced contact resistance after G1 compression surpass the geometric effect of tortuous electron pathways, leading to an overall decrease in the film resistance. In crumpled G2-2D2D and G3-2D2D2D structures, electrons have to bypass more complex topographies, resulting in increased electrical resistance as the MXene undergo multiple compression steps.


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Stretchable MXene/Elastomer Bilayer Composites with High Mechanical Stability. Besides achieving high-CA MXene electrodes, the higher dimensional texturing concept also enables the fabrication of stretchable MXene/elastomer composites that can sustain large mechanical deformations. Instead of undergoing thermal actuation, we used relaxation of pre-stretched elastomer substrates to compress the MXene hybrid nanocoatings into complex out-of-plane micro-textures. We utilized VHBTM acrylic tapes as elastomer substrates and pre-stretched them biaxially before the deposition of active materials (Figure 4a). A thin layer of silver nanowires (AgNWs) as a current collector was first transferred onto the pre-stretched elastomer before the deposition of MXene nanocoatings. As the pre-stretched elastomer was relaxed, the lateral dimensions of adhered MXene nanocoatings reduced by the same ratio as those of elastomer (Figure 4a). Microscopically, under the 2D strain at 300%, the dimension of MXene nanocoating was compressed from 2.40 to 0.60 cm2, and the planar surface was deformed into a crumpled structure with the texture length scale at 70 µm due to localized mechanical instabilities (Figure 4b). A freestanding, stretchable, textured MXene/elastomer bilayer composite was finally produced. An important consequence of these accordion-like topographies of MXene nanocoatings is that they can be reversibly folded/unfolded by in-plane deformations, which diminish the effective strain on the bilayer devices while stretching or under stress. The stretchability of MXene/elastomer composites can be pre-programmed by the areal strain applied on the VHBTM substrates, which can be defined in eq 2: Areal Strain = ΔA/Af × 100%

(2)

, where ΔA is the change (A0 – Af) of superficial area of MXene nanocoatings before (A0 as the planar film is deposited) and after the substrate relaxation (Af as the film is fully compressed). The


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highest 2D strain at 300% corresponds to the 1D strain in each direction of εx = εy = 100%. An in situ tensile test was carried out within a SEM to observe the topographical changes of textured MXene nanocoatings at 125%, 225% 2D and 80% 1D stretching states (Figure 4b). While the MXene/elastomer bilayer was uniaxially stretched to 80%, we observed the wrinkling angle increased from 79° to 131°. Under 125% and 225% 2D strains, the texture length scale increased from 70 (0% strain) to 94 and 160 µm, respectively. By undergoing programmed crumpling and unfolding, the textured MXene nanocoatings can mimic the stretchability of elastomeric materials and maintain high material integrity under multiple cycles of large deformation. The conformal MXene nanocoatings were subjected to 200 cycles of stretching/relaxing (to 80% 1D strain) during which we did not observe changes in the relaxed state topographies (Figure 4c). The tensile stress test was further conducted on the bilayer device composed of a 5-μm-thick MXene coating on a 1-mm-thick elastomeric substrate, and the stress-stain curve (to 50% 1D strain) was recorded in Figure 4d and compared with the uncoated case. With conformal MXene coatings, the bilayer device exhibits a higher degree of elastic hysteresis, and about 44% more energy is required to obtain the same stretching percentage. A fatigue test in Figure 4e demonstrates that the hysteresis loops are stable after 100 cycles of tensile testing (to 50% 1D strain). It is worth to note that the MXene nanocoating will fracture and get detached from the elastomer if the stretching percentage exceeds the pre-strain. Electrochemical Performance of Stretchable MXene Electrodes. Owing to its high mechanical stability of crumple-patterned MXene nanocoatings, the MXene/elastomer bilayer composite is expected to be a promising electrode for stretchable energy-storage applications. To evaluate the electrochemical performance of conformal MXene nanocoatings at different stretching states, we conducted CV and galvanostatic charge-discharge (GCD) experiments in a three-electrode


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electrochemical cell. The as-fabricated MXene (5-µm-thick)/elastomer composites were first stretched uniaxially (to 0%, 50%, and 80% strains) and characterized as working electrodes. The 1D strain applied on stretchable MXene electrodes can be defined as follows (eq 3): Uniaxial Strain = ΔL/L0 × 100%

(3)

, where ΔL is the change (Ls – L0) of length of MXene electrodes before and after stretching, L0 is the length of MXene electrode at the relaxed state, and Ls is the length of MXene electrode at the stretched state. Suitable electrolyte (1.0 M Li2SO4) and a potential window (-0.8 – 0.0 V) were selected to avoid the oxidation of AgNWs and MXene.21 The CV curves of unstretched MXene electrodes are nearly rectangular at various scan rate (Figure 5a), indicating the textured MXene nanocoatings can serve as ideal electrochemical double layer capacitor (EDLC) electrodes. Figure 5b further compares the CV curves of MXene/elastomer electrode at 0%, 50%, and 80% 1D stretching states. The CV curves are similar under three stretching states and still exhibit nearly rectangular shape, indicating the ideal EDLC behaviors are not affected by the strains applied. Figure S10 reveals that AgNW current collectors show no capacitance in Li2SO4 aqueous solution. The GCD curves of MXene electrodes at 0%, 50%, and 80% stretching states are shown in Figure S11a,b, and Figure 5c, respectively. The MXene-based electrodes show similar chargedischarge behaviors at various stretching states, and the specific capacitances are calculated from the GCD curves and plotted in Figure 5d. The volumetric (Cv) and gravimetric capacitances (Cg) of the MXene nanocoatings are 395, 390, 362 F cm-3 and 118, 117, 108 F g-1 at 0%, 50%, and 80% stretching states, respectively. The detailed comparison with other MXene-based electrochemical electrodes is listed in Table S1. It can be observed that there is a decline in both specific capacitances at higher discharge current densities, since the Li+ intercalation becomes relatively slow and limits the charge transfer. The 80% stretched electrodes show slightly inferior


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capacitance, which may result from the mild cracks on the MXene nanocoatings that increase the resistance of electron collection. We further used EIS to investigate the interfacial resistance of MXene/elastomer electrodes (the Nyquist plots in Figure 5e), showing that 80% stretched MXene electrode exhibited a slightly higher RCT (~0.61 ) than the unstretched case (~0.49 ). The stretchable MXene electrodes demonstrated a durable cycling performance and mechanical stability. The MXene electrodes were stretched to 0 %, 50%, and 100% states and retained 98%, 96%, and 93% of initial capacitance after 3,000 charge-discharge cycles, respectively (Figure 5f). A fatigue test was further conducted on stretchable MXene electrodes. Figure 5g shows the specific capacitance can be preserved (96% retention) after 1,000 stretching/relaxation cycles (to 50% 1D strain). On the other hand, a decrease in capacitance (76%) was observed after 1,000 stretching/relaxation cycles (to 80% strain), which may result from mild fractures and delamination of MXene nanocoatings under large strains.

Device Performance of Highly Bendable, Stretchable MXene ASCs. A highly deformable ASC was constructed with the stretchable MXene and AC electrodes (denoted as AC//MXene ASC). The stretchable AC electrodes were fabricated by following similar approaches as the MXene electrodes (Figure S12). AC was selected as the anode material to expand the narrow voltage window of symmetric MXene devices (~0.6 V in symmetric Ti3C2Tx configuration, since MXene inclines to get oxidized at high anodic potential).23,

43

The detailed fabrication protocol of

stretchable AC//MXene ASCs is shown in the Figure S13. A gel electrolyte containing Li2SO4 and poly(vinyl alcohol) (PVA) was used. The AC//MXene ASC was operated at a voltage window of 1.0 V, and the CV curves at different scan rates (from 25 to 200 mV s-1) are shown in Figure S14.


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The AC//MXene ASCs exhibited efficient electrochemical performance while largely bending and stretching. The ASC can be reversibly bent (to 180°) and stretched (to 1D 100% strain), and all the CV curves maintain a rectangular shape without distortion (Figure 6a,b), indicating ideal capacitive behaviors of the devices are not disturbed by the applied strain. The photos of the ASC under large mechanical deformations are shown in Figure 6c. The GCD profiles of stretchable ASCs are presented in Figure 6d. When 0%, 50%, 100% strains were applied, Cv was calculated to be 95, 89, 85 F cm-3 (based on the total volume of active materials), and the corresponding Cg was 46, 43, 41 F g-1 at 25 mV s-1 (based on the total active materials mass of two electrodes), respectively. An inferior capacitance in 100% stretched ASCs may result from the strain-induced cracks that increase the electron transporting resistance. The MXene-based ASC also demonstrated excellent electrochemical stability, and 98%, 96%, and 93% of the specific capacitance can be preserved under 0%, 50%, and 100% strains after 3,000 charge-discharge cycles (Figure 6e). The stable cycling indicates MXene can be exploited as cathode materials in ASCs and does not exhibit possible electrochemical oxidation that occurred in the symmetric configuration.24


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CONCLUSIONS The Ragone plot is shown in Figure 6f. The AC//MXene ASC exhibited a maximal energy density of 5.5 W h kg-1 at a power density of 0.5 kW kg-1. The detailed comparison with other MXene-based supercapacitors is listed in Table S2.44-46 Most of the MXene supercapacitors showed the specific capacitance ranging 32-70 F g-1, while the electrodes and devices were not designed for stretchable purposes.44-49 Our MXene supercapacitors achieved high deformability and efficient electrochemical performance at the same time. In summary, we have demonstrated a mechanically-driven assembly strategy to fabricate highly stretchable, bendable, efficient MXene supercapacitors with high mechanical stability. We first harnessed the surface instability during substrate contraction to sequentially deform the MXene nanocoatings into multiscale structures. The complex MXene topographies can be preprogrammed through applying various deformation sequences, and the textured MXene nanocoatings were characterized as controllable wetting surfaces and electrochemical electrodes (G3 electrodes exhibit 13-time higher CA than G0). The MXene coating can be transferred onto an elastomeric substrate to achieve a MXene/elastomer electrode with high stretchability (225% areal strain). The electrodes demonstrated efficient electrochemical performance of 395, 390, and 362 F cm-3 under 0%, 50% and 80% strains, respectively. Afterwards, an ASC composed of stretchable MXene and AC electrodes was successfully fabricated with mechanical deformability (180° bendability and 100% 1D stretchability). The stretchable ASC exhibited an efficient energy density of 5.5 Wh kg-1 in Li2SO4-PVA gel electrolyte, and the device also exhibited high mechanical stretchability (100% 1D strain) and still showed efficient energy density at 4.9 Wh kg-1. The MXene-based ASCs also demonstrated high cycling performance at various stretching/bending states with no loss of performance. With the rapid development of the MXene family, we envision


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our approaches are applicable to other stiff MXene materials and make them into promising stretchable electrodes in wearable energy-storage applications. The production of stretchable MXene electrodes can be facilated by the use of printable MXene ink. The MXene ink has been recently reported by Nicolosi’s group,50 and we believe the MXene ink can be directly imprinted or stamped onto the pre-stretched elastomers with the assistance of inkjet printing technology. This approach inherits the advantages of the printing technique and renders the electrode stretchability at the same time, which may open an avenue for manufacturing stretchable electronic devices with high performance and desired deformability.


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METHODS Materials. Anhydrous ethanol, acetone, dichloromethane (DCM), (tridecafluoro-1,1,2,2tetrahydrooctyl)trichlorosilane, lithium fluoride (LiF), sodium alginate (SA), hydrochloric acid (HCl), sulfuric acid (H2SO4), poly(vinyl alcohol) (PVA), and 1-butyl-3-methylimidazolium chloride (BMIMCl) were purchased from Sigma-Aldrich. Conductive carbon black nanoparticles (CBs, Super-P), were purchased from Alfa Aesar. Silver nanowires (AgNWs, diameter ~30 nm, length ~12 µm) were purchased from Cheap Tubes Incorporation. Ti3AlC2 MAX phase powders were purchased from Xincailiao (200 mesh, >98%). Clear polystyrene (PS) heat shrink films were purchased from Grafix. Clear VHBTM Tape 4910 was purchased from 3M. All water was deionized (DI) (18.2 MΩ, mill-Q pore). All reagents were used as received without further purification.

Preparation of Exfoliated Ti3C2Tx MXene Nanosheets. Ti3C2Tx MXene nanosheets were prepared by in situ HF etching according to the previous literature with some modification.22, 51 In a typical experiment, 1.0 g Ti3AlC2 and 7.5 g of LiF were slowly added to 9.0 M HCl solution (40 mL) under vigorous stirring. The mixture was heated to 45 °C and kept at this temperature about 5 days. The solid residue was obtained by centrifugation and washed with DI water for several times until the pH value reached around 6.0. Subsequently, 100 mL of DI water was added to the residue, and the mixture was ultrasonicated for 4-6 hours under N2 and centrifuged at 2,500 rpm for 30 minutes. The supernatant was collected as the final suspension of Ti3C2Tx MXene nanosheets. The concentration of obtained MXene suspension was about 1.2 mg mL-1.

Preparation of MXene-SA and MXene-CB Dispersion. The MXene-SA dispersion was prepared by mixing the MXene suspension with SA, and the MXene-to-SA weight ratio was kept 19 ACS Paragon Plus Environment

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at 4:1. The MXene-SA dispersion was stirred overnight before deposition. The MXene-SA dispersion was further mixed with CBs (MXene:CB = 4:1) and ultrasonicated for 2 hours to obtain MXene-CB mixture.

Fabrication of Multigenerational MXene Structures. PS shrink film was cut into 16-cm2 squares and washed with ethanol. The substrates were treated with air plasma in a Harrick plasma cleaner for 2 minutes. After plasma treatment, the MXene-SA solution was deposited on the shrink films (75 µL cm-2). Once dry, the planar MXene-SA film (we called it G0) was obtained, and the samples were placed in an oven at 140 °C and allowed to shrink for 10 minutes. For 1D deformation, two sides of the samples were constrained by clamps; for 2D deformation, the samples were shrunk without any clamps or constrains. Afterwards, the samples were removed from the oven and allowed to cool on the bench, and the G1 wrinkles/crumples were obtained. To achieve subsequent deformation(s), the PS substrates were dissolved in DCM, and the G1 MXene structures were left in the solvent. The freestanding MXene films were sequentially rinsed with DCM for 2 hours, acetone for 15 minutes, and then transferred to ethanol. The freestanding MXene-SA films were transferred to new plasma-treated shrink films, and the G2 hierarchy can be obtained after shrinkage. After the processes (dissolving the substrates, washing, transferring, G3 shrinkage) were repeated, the multiscale G3 films were obtained.

Hydrophobic Surface Functionalization. A solution of 95% anhydrous ethanol and 5% (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane was prepared and sonicated for 15 minutes. Multigenerational MXene structures were first treated by air plasma for 2 minutes and then dipped into the solution. The reaction was allowed to proceed for 120 minutes. After the reaction, the


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solution was removed from each well and replaced with an equivalent volume of anhydrous ethanol to rinse the MXene structures. The ethanol was then removed, and an equivalent volume of distilled, deionized water was added to quench the reaction. The samples were further rinsed with ethanol, dried with N2 and heated at 70 °C overnight.

Fabrication of MXene/Elastomer Bilayer Architectures with High Stretchability. The wellmixed MXene-CB solution (1.5 mL) was collected by a PVDF membrane (0.1 µm, Merck Millipore) through vacuum-assisted filtration. After dried, the membrane with filtered MXene film was rinsed with DI water, and the MXene film was detached from the membrane. The AgNW thin films (200 µL) were also prepared by undergoing similar processes. A clear VHBTM tape was first stretched biaxially to reach the areal strain about 225%, which corresponds to x = y = 80%. The stretched VHBTM tape was stabilized by a circular holder. The freestanding AgNW and MXeneCB films were sequentially transferred to the stretched VHBTM substrates. The strain was released by cutting the tape from the edge of the holder, and the MXene and AgNW films were compressed gradually. The MXene/elastomer layered samples were then used as stretchable electrochemical electrodes for characterization and device fabrication.

Uniaxial Tensile Testing. The fabricated MXene/elastomer bilayer devices were cut into 1 × 1 cm2 squares and secured onto the tensile tester (Instron 5543, Instron, USA) with clamps in a 500 N load cell. The sample was pulled with an extension rate at 5 mm min-1 until the strain of the sample reached 50%.


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Electrochemical Testing of Stretchable MXene Electrodes. A three-electrode cell was employed for the electrochemical measurements in the aqueous solution, where an Ag/AgCl electrode (Metrohm) (3 M KCl) and a Pt foil were used as the reference and counter electrodes, respectively. The MXene nanocoatings were used as the working electrodes. We used 1.0 M H2SO4 solution for characterizing the multigenerational MXene films, and 1.0 M Li2SO4 solution for characterizing the MXene/elastomer electrodes. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were performed at room temperature using an electrochemical workstation (Autolab PGSTAT302N). The area of MXene films was scanned and measured by ImageJ. The electrochemical impedance spectroscopy (EIS) was conducted at open circuit potential (OCP), with an amplitude of 5 mV and frequencies ranging from 0.1 Hz to 100 kHz. The areal capacitance of multigenerational MXene, CA (F cm-2), is calculated according to the following equation (eq 4): =

∫

(4)

, where j is the discharging current density (A cm-2); ∫

is the integrated area of CV curves;

ΔV is the voltage window (V); s is the scan rate (V s-1).

Fabrication of AC//MXene ASCs. The asymmetrical cell consisted of two stretchable electrodes (MXene and AC) and a piece of gel polymer electrolyte. The AC/elastomer electrode was fabricated by the same process with MXene/elastomer mentioned in Figure S12. The PVABMIMCl-Li2SO4 electrolyte was prepared according to the literature.52 Briefly, PVA-BMIMClLi2SO4 (the weight ratio of PVA:BMIMCl:Li2SO4 = 1:3:2) gel was poured onto the top of the MXene/elastomer electrode and then placed in a vacuum desiccator to ensure complete infiltration


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of the gel into electrodes. The asymmetric MXene supercapacitors were characterized as a standard

two-electrode system via an electrochemical work station.

Characterizations. Surface morphology of the multigenerational MXene structures was investigated using scanning electron microscopes (SEM) (FEI Quanta 600 and FESEM; JEOLJSM-6610LV) operating at 15.0 kV for low- and high-resolution imaging. Before the SEM imaging, the MXene structures were coated with a layer of Pt (~5 nm). Particle size distributions were measured using dynamic light scattering (DLS, Malvern Nano ZetaSizer Analyzer). The electrical conductivities of multigenerational MXene films were measured by a four-point probe (ResTest, Jandel Engineering). The characteristic wavelength of the wrinkled/crumpled features was quantified by sampling gray-scale line profiles from the micrographs using ImageJ, followed by Fast Fourier Transformation (FFT) in MATLAB (Mathworks).

ACKNOWLEDGEMENTS The authors acknowledge the financial support provided by the Faculty Research Committee (FRC) Start-Up Grant of University of Singapore R-279-000-515-133, and the Ministry of Education (MOE) Academic Research Fund (AcRF) R-279-000-532-114.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI:


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(49) Rakhi, R. B.; Ahmed, B.; Hedhili, M. N.; Anjum, D. H.; Alshareef, H. N. Effect of Postetch Annealing Gas Composition on the Structural and Electrochemical Properties of Ti2CTx MXene Electrodes for Supercapacitor Applications. Chem. Mater. 2015, 27, 5314-5323. (50) Zhang, C.; Kremer, M. P.; Seral‐Ascaso, A.; Park, S. H.; McEvoy, N.; Anasori, B.; Gogotsi, Y.; Nicolosi, V. Stamping of Flexible, Coplanar Micro‐Supercapacitors Using MXene Inks. Adv. Funct. Mater. 2018, 28, 1705506. (51) Alhabeb, M.; Maleski, K.; Anasori, B.; Lelyukh, P.; Clark, L.; Sin, S.; Gogotsi, Y. Guidelines for Synthesis and Processing of Two-Dimensional Titanium Carbide (Ti3C2Tx MXene). Chem. Mater. 2017, 29, 7633-7644. (52) Zhang, X.; Wang, L.; Peng, J.; Cao, P.; Cai, X.; Li, J.; Zhai, M. A Flexible Ionic Liquid Gelled PVA-Li2SO4 Polymer Electrolyte for Semi-Solid-State Supercapacitors. Adv. Mater. Interfaces 2015, 2, 1500267.


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Figure 1. Controlled crumpling of MXene hybrid nanocoatings for the fabrication of high-arealcapacitance electrodes and highly bendable, stretchable supercapacitors. The planar MXene hybrid nanocoatings undergo multigenerational shrinkage processes to fabricate multiscale MXene structures, which can serve as electrochemical electrodes with high CA. The crumpled MXene nanocoating can be further transferred on soft elastomer to achieve the MXene/elastomer electrodes for bendable, stretchable supercapacitors.


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Figure 2. Multiscale G2 and G3 structures by controlling G1-G2-G3 shrinkage sequences. The G2 and G3 structures were deformed sequentially from planar MXene nanocoatings. The hierarchical G2 structures were achieved by undergoing (a) 2D2D, (b) 2D1D, (c) 1D2D, (d) 1D1D, and (e) 1D1D shrinkage sequences. The multiscale G3 structures were formed by experiencing (f) 2D2D2D, (g) 2D2D1D, and (h) 2D1D2D shrinkage sequences.


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Figure 3. Controllable surface wettability and electrochemical characterization of multiscale MXene hybrid nanocoatings. The θw of multiscale MXene structures (a) before and (b) after fluorocarbon functionalization are shown. The photos show the corresponding water droplets (volume = 10 µL) on the crumpled MXene structures at different generations. (c) Cross-sectional SEM image of MXene-CB hybrid nanocoatings. (d) Top-down SEM images of G1-2D, G2-2D2D,


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and G3-2D2D2D MXene-CB structures. (e) EIS curves of MXene and MXene-CB electrochemical electrodes. (f) Area normalized CV curves of G0, G1-2D, G2-2D2D, and G3-2D2D2D MXene-CB electrodes.

Figure 4. Mechanical stability of stretchable MXene/elastomer bilayer composites. (a) Schematic


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illustration of fabrication processes for stretchable MXene/elastomer architectures; the photographs of MXene hybrid nanocoating before and after 2D compression (the diameter shrinks from 1.75 to 0.85 cm) are shown. (b) Top-down SEM images of crumpled MXene nanocoatings (5-μm-thick) at 0% unstretched state, 125% and 200% 2D stretching, and 80% 1D stretching. (c) Top-down SEM images of crumpled MXene nanocoating after 1, 10, 50, 100, and 200 cycles of 1D stretching (to 80% 1D stretching). (d) Comparison of stress-strain curves between bare VHB elastomer and MXene/elastomer bilayer composites (5-μm-thick MXene nanocoatings). (e) Fatigue test for MXene/elastomer devices. The bilayer devices with 5-μm-thick MXene nanocoatings have undergone 100 cycles of tensile testing, and we do not observe the degradation of stress-strain properties.


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Figure 5. Electrochemical performance of stretchable MXene electrodes in a three-electrode system (aqueous Li2SO4 electrolyte). (a) CV curves of MXene electrode in 1.0 M aqueous Li2SO4 solution at different scan rates. (b) CV curves of stretchable MXene electrodes at 0%, 50%, and 80% 1D stretching states (scan rate at 25 mV s-1). (c) GCD curves of MXene electrode at 80% 1D 33 ACS Paragon Plus Environment

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stretching state at the different current densities. (d) Volumetric and gravimetric capacitances of stretchable MXene electrodes under 1D strains from 0% to 80%. (e) EIS curves of stretchable MXene electrodes under 0% and 80% strains. (f) Cycling stability of MXene electrodes at different stretching states (from 0% to 80%) and (g) mechanical stability of stretchable MXene electrodes after multiple times of stretching/relaxation (1,000 cycles of 50% and 80% 1D stretching).


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Figure 6. Electrochemical performance of the bendable, stretchable AC//MXene ASC (Li2SO4PVA quasi-solid electrolyte). The CV curves of the ASC at different (a) bending (0° to ~180°) and (b) stretching states (0% to 1D 100%). (c) Images of ASCs under ~180° bending and 100% 1D stretching. (d) GCD profiles of the stretchable ASCs under various 1D stains (0%, 50%, 100%) and at various current densities (from 1 to 10 A g-1). (e) Cycling stability of the ASCs at different


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1D stretching states (from 0% to 100%) up to 3,000 cycles at the charging/discharging rate of 10 A g-1. (f) Gravimetric energy versus power densities plot of stretchable ASCs at various 1D stretching states and the comparison with other non-stretchable MXene-based supercapacitors.


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Table of content 309x167mm (150 x 150 DPI)

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Controlled Crumpling of Two-Dimensional Titanium Carbide (MXene

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