Carbon Nanotube Films

Jun 26, 2019 - Achieving Rich Mixed-Valence Polysulfide/Carbon Nanotube Films toward Ultrahigh Volume Energy Density and Largely Deformable ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25271−25282

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Achieving Rich Mixed-Valence Polysulfide/Carbon Nanotube Films toward Ultrahigh Volume Energy Density and Largely Deformable Pseudocapacitors Weijie Song,†,∥ Gengjie Wang,†,∥ Dongbo Zhao,‡ Yuewei Zhou,§ Yuying Ding,† Changbin Tan,† Shaochun Tang,*,† Hao Dong,*,‡ and Xiangkang Meng*,†

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†

National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, College of Engineering and Applied Sciences, Department of Materials Science & Engineering, Nanjing University, Nanjing, Jiangsu 210093, P. R. China ‡ Kuang Yaming Honors School, Nanjing University, Nanjing 210023, P. R. China § Nanjing Foreign Language School, Nanjing, Jiangsu 210008, P. R. China S Supporting Information *

ABSTRACT: In this work, new insights into dependence of electrochemical performance enhancement on transition metals’ rich mixed valence and their atomic ratio as well as redox active polysulfides are proposed. Especially, the influence of atomic ratio is further demonstrated by both experiments and density functional theoretical calculation where increasing Co/S leads to the enlargement of both interatom distance and hole diameter in a MnxCoySz cell. We rationally designed and prepared novel flexible electrodes of a rich mixed-valence polysulfide MnxCoySz/carbon nanotube film (CNTF) through acid activation of a dense CNTF into a hydrogel-like conductive matrix, growth of the MnxCoy(CO3)0.5OH precursor on each CNT, and controlled sulfidation. Nanostructure control allows us to obtain fast electron/ion transfer and increased availability of active sites/interfaces. The optimal MnCo9S10/CNTF shows a specific capacitance reaching 450 F cm−3 at 10 mA cm−2, much higher than reported values for CNT-based electrodes. Also, it exhibits remarkable cycling stability with only 1.6% capacity loss after 10 000 cycles at a high current density of 80 mA cm−2. An all-solid-state asymmetric supercapacitor (ASC) applying MnCo9S10/CNTF delivers an exceptionally high volumetric energy density of 67 mW h cm−3 (at 10 W cm−3). Particularly, integrated electric sources with adjustable output voltages can be obtained by connecting several ASCs in series, and there are no structural failure and capacity loss during repeated large-angle twisting and vigorous hammering. This work provides a general route to energy storage devices with ultrahigh volumetric energy density and outstanding reliability for wearable electronics. KEYWORDS: electrochemical energy-storage, carbon nanotubes, polysulfides, flexible electrodes, volume energy density

1. INTRODUCTION

Rapid development of carbon-based assemblies such as carbon fiber (CF) cloth,7,8 graphene paper,9 and carbon nanotube film (CNTF)10 has created substantial opportunities for the development of flexible SCs. Among them, the CNTF is the thinnest and lightest and thus advantageous for flexible film-shape electrodes.11−13 One-dimensional CNT with a large length-to-diameter ratio as an isolated unit possesses many inherent advantages of good electrical conductivity, large surface area, and high flexibility and strength, but in a CNTstacked dense film, high CNT−CNT contact resistance and limited accessible area greatly reduce capacitances of CNTFs.12

To meet the power requirements of wearable electronics, developing flexible, foldable, lightweight, and high-performance power sources is highly required. All-solid-state supercapacitors (SCs) have emerged as promising energy storage devices owing to their advantages including high power density, fast recharge capability, and long cycle life.1−3 For miniaturized SCs, specific volumetric energy density based on an entire device is a more important evaluation parameter than specific gravimetric and areal ones because the former reflects total amount of the stored energy.2,4−6 Therefore, realization of ultrahigh volumetric energy density without sacrificing the power density, cycle life, and other performance parameters while assuring reliability under repeated large deformations is of great significance. © 2019 American Chemical Society

Received: May 6, 2019 Accepted: June 26, 2019 Published: June 26, 2019 25271

DOI: 10.1021/acsami.9b06936 ACS Appl. Mater. Interfaces 2019, 11, 25271−25282

Research Article

ACS Applied Materials & Interfaces

optimization of sulfidation leads to a nanoparticle-built porous layer. The optimal MnCo9S10/CNTF electrode has a specific capacitance of 450 F cm−3 at 10 mA cm−2 and also exhibits remarkable cycling stability (only 1.6% capacity loss after 10 000 cycles), much superior over reported values for similar composite film electrodes. On the basis of density functional theory (DFT) calculations,34−37 dependence of performance enhancement on the crystal structure, atomic ratio, and polysulfides is comprehensively studied. An asymmetric flexible all-solid-state SC applying such an electrode delivers a volumetric energy density of 67 mW h cm−3 (at 10 W cm−3). Integrated sources with high output voltages are obtained by connecting several SCs in series. Particularly, no structural failure and performance loss are observed during the repeated large-angle deformations and vigorous hammering.

Especially, charge storage can only be achieved in nanometer thickness because of carbon materials’ electrical double-layer capacitance that stems from charge separation at the electrode/ electrolyte interface, which is unsatisfying for practical applications needing micrometer thick electrodes. In order to obtain higher volumetric capacitances, CNTFs as a conductive matrix are usually loaded with other pseudocapacitive materials that store charges via reversible redox reactions. Recently, some encouraging progress has been made in the synthesis of CNT scaffold-based composite electrodes for SC and pseudocapacitor/battery applications.14−16 A pristine CNTF itself has a dense network structure with low internal space and hydrophobic surface, which tends to block the electrolyte wetting and ionic diffusion from the surface to the interior, leading to a low loading of active materials, and thus, the large inner space of the CNTFs cannot be utilized. Although great progress has been made, for example, composite film electrodes using CNTFs coated with conducting polymers (e.g., polyaniline17 and polypyrrole18,19) or inorganic materials20 have been reported, their capacitances and energy densities of assembled SCs were far from the practical requirements because of the low loading mass of the pseudocapacitive materials. Pretreatment is an effective strategy to address this issue because adjustable surface groups on each CNT facilitate loading of other materials, which allows the CNTFs to act as both an active electrode and a current collector. Namely, it is freestanding, and a large enhancement in internal three-dimensional (3D) space and surface wettability are highly expected to avoid the thickness limitation. It is still extremely challenging, however, especially in a solution environment, to achieve high loading of inorganic materials with a fine nanostructure control. As for the grown pseudocapacitive materials, transitionalmetal oxides (e.g., MnO2, Co3O4, NiO, and so forth) suffer from unsatisfying cycling stability because of their low electrical conductivity. Experimental studies show that substitution of O with S is an effective technique for overcoming this drawback. In particular, ternary transitionmetal sulfides such as MCo2S4 (M = Ni,21−25 Cu,26,27 Mo,28 and Zn29−31) possess higher electrochemical activity and higher capacity than monometallic sulfides because of multiple oxidation states. However, research on MnxCoySz with nonstoichiometry and varying element molar atomic ratios has not been reported yet, and their specific effects on energy storage properties should be investigated in depth. Especially, the reported MCo2S4 nanostructures are mainly focused on solid ones with smooth outer and interior surfaces. Also, coexistence of rich mixed valences of multimetals and variable S benefits the formation of ionic defects (misplaced ions) and electronic defects (holes). Polysulfide binding and trapping has been demonstrated to be an effective strategy for improving the performance of transition-metal sulfides in Li−S batteries.32 Using polysulfides as the redox active electrolyte results in high specific capacitance and good cycling behavior of SCs.33 Nevertheless, the role of mixed metal polysulfides for SCs and the involved mechanisms are still unclear. In this work, a rich mixed-valence polysulfide MnxCoySz/ CNTF electrode is developed via an acid solution activation of dense CNTFs, growth of the MnxCoy(CO3)0.5OH precursor on each CNT, and controlled sulfidation. The activation leads to an open porous structure and transforms hydrophobic CNTF into highly hydrophilic one, which assures high loading because of separation of adjacent NTs in 3D space, and

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Materials. Cobalt acetate tetrahydrate Co(CH3COO)2· 4H2O, urea (CO(NH2)2), concentrated hydrochloric acid (HCl, 35 wt %), and concentrated nitric acid (HNO3, 65−67 wt %) were bought from Sinopharm Chemical Reagent Co. Ltd. Manganese acetate tetrahydrate Mn(CH3COO)2·4H2O, anhydrous ethanol, and thioacetamide (CH3CSNH2, TAA) were all provided by Aladdin. All chemicals were of analytical grade. Deionized water with a resistivity exceeding 18.0 MΩ cm was from a JL-RO 100 Millipore-Q Plus purifier and used throughout the experiments. 2.2. Synthesis of Mn/Co Precursor@CNTFs. High-quality pristine CNTFs were prepared at 1300 °C using anhydrous ethanol as the carbon source and ferrocene as the catalyst via a modified floating catalyst chemical vapor deposition (FCCVD) process. The asprepared pristine CNTF from the FCCVD had an area up to 2.0 m (length) × 1.5 m (width) and was then cut into rectangular pieces with a size of 1 × 2 cm2. Under continuous ultrasonic irradiation at a power of 100 W, the rectangular CNTF was soaked in 40 mL of 5 wt % diluted HCl aqueous solution for a period of 30 min and was then cleaned with deionized water. After that, CNTF was soaked in 20 mL of concentrated HNO3 (65−67 wt %) for another period of different times (tHNO3). Finally, CNTF was cleaned with deionized water and freeze-dried. As for the typical synthesis of Mn/Co precursor@CNTFs, under continuous stirring, 2 mmol of Co(CH3COO)2·4H2O and 0.22 mmol of Mn(CH3COO)2·4H2O were dissolved in a preprepared 40 mL water−ethanol (1:1 volume ratio) mixed solution, which resulted in 50 and 5.56 mM concentrations of Co2+ and Mn2, respectively. Then, 0.6 g of urea was added under continuous stirring for 30 min. A piece of dried, activated CNTF (defined as A-CNTF) was soaked into the clear mixed solution, and the mixed solution was transferred to a Teflon-lined autoclave with a volume of 50 mL. The autoclave was kept at a constant temperature of 140 °C in an oven for 3 h. After cooling down to room temperature (rt), the resulting Mn/Co precursor@CNT composite film was taken out and cleaned with deionized water and isopropyl alcohol several times. Finally, the composite film was dried at 80 °C in air for 6 h. 2.3. Synthesis of MnxCoySz@CNTF. Typically, the obtained Mn/ Co precursor@CNT composite film was immersed in 40 mL of anhydrous ethanol containing 0.01 g of TAA. This led to a TAA concentration (CTAA) of 3.3 mM. The mixture was then transferred to a 50 mL Teflon-lined stainless steel autoclave. After being kept for 12 h at 120 °C, the product was collected and washed with ethanol. In order to investigate the nanostructure control, CTAA was changed from 40 to 1.7 mM, while other reaction parameters were kept unchanged. Finally, the resulting product was annealed at 350 °C in the N2 atmosphere for 2 h. 2.4. Characterizations. The contacting angles of pristine CNTFs before and after the acid solution activation were measured by a DataPhysics OCA-30 contact angle measurement system. The morphology of the CNTFs and composite films was investigated 25272

DOI: 10.1021/acsami.9b06936 ACS Appl. Mater. Interfaces 2019, 11, 25271−25282

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the three main steps for achieving the MnxCoySz@CNTF composite. The synthesis includes the activation of pristine CNTF using 5 wt % HCl and concentrated HNO3 one after another (step I), precursor growth (step II), and controlled sulfidation (step III). Without activation, only the upper surface of whole pristine CNTF is covered by a dense precursor layer. (b) Schematic illustration of the sulfidation process of the precursor, leading to a porous layer. 2.6. Electrochemical Measurements. Electrochemical measurements were carried out using a computer-controlled Metrohm Autolab 302N electrochemical workstation. For three-electrode tests, a platinum plate was served as the counter electrode and a standard Ag/AgCl electrode as the reference, MnCo9S10@CNTF was used as the working electrode, and the effective area is 1 cm2, leaving 1 cm for connection clamps. The electrolyte was 3 M KOH aqueous solution. Cyclic voltammetry (CV) measurements were performed between −0.4 and 0.4 V (vs Ag/AgCl). Specific volumetric capacitance (Cv) can be calculated from the discharge curves based on Cv = IΔt/(VΔU), where i (A), Δt (s), V (cm3), and ΔU (V) represent the discharge current, total discharge time, the volume of the whole electrodes, and potential window during discharge, respectively. 2.7. Flexible All-Solid-State SCs. To assemble flexible all-solidstate SCs, a filter paper-based solid electrolyte film was prepared first. Typically, 1 g of KOH was poured into 18 mL of deionized water, and 1.5 g of poly(vinyl alcohol) (PVA) powder was added. The mixture was then heated to 90 °C under stirring until it became clear. The solution was mechanically stirred slowly for 2 h at rt in order to eliminate bubbles. Finally, the solution was dripped onto two sides of a filter paper and dried in air. The obtained KOH−PVA gel membrane-covered filter paper was used as a separator, which was sandwiched in between the MnCo9S10/CNT positive electrode and the A-CNTF negative electrodes. Finally, it was pressed into a threelayered solid device using two same polyimide films as the packaging material. To determine practical energy and power densities of a flexible allsolid-state device, the total area/volume of an entire device consisting of two electrodes, a solid-state electrolyte, a separator, and a packaging material should be taken into consideration. The volumetric energy density (Ev) and power density (Pv) of SCs can be calculated based on Ev = (1/2)Cv(ΔU)2 and Pv = Ev/Δt, where Cv is the specific volume capacitance based on the total volume of an allsolid-state device. In order to get high output voltages, integrated sources were made by connecting several SCs in series where a polyimide film was also used as the packaging material.

using scanning electron microscopy (SEM) (Hitachi S-4800). X-ray diffraction (XRD) was carried out on a D/Max-RA X-ray diffractometer (Cu Kα = 1.5418 Å radiation) at 2° min−1. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction analyses were performed on a JEOL JEM-2100 electron microscope at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was used to analyze the valence states of elements in a Thermo VG Scientific MultiLab ESCA2000 system with a CLAM4 hemispherical analyzer at a base pressure below 3 × 10−10 mbar. The quantity of the individual elements was analyzed by energy-dispersed X-ray spectroscopy (EDS). EDS analysis and elemental mappings were carried out on the same field emission scanning electron microscope. The EDS analysis was performed repeatedly via collecting information from a large area in a low-magnification SEM mode. Also, XPS measurements were performed at several different locations of one sample, and then the compositional ratios were averaged. The quantity of the individual elements was analyzed by inductively coupled plasma (ICP) on a J-A1100 ICP spectrometer (Jarrell-Ash Company, USA). The sample (5 mg) was dissolved in 10 mL of 4 M HNO3 aqueous solution and then diluted with deionized water to 100 mL. After that, 2 mL of solution was taken and diluted to parts per million level for subsequent ICP measurements. Brunauer−Emmett− Teller (BET) N2 adsorption/desorption isotherms were measured at 77 K using a surface area analyzer (Micromeritics ASAP 2020) to investigate the surface characteristics. The samples were kept in vacuum at 323 K for 2 h to remove adsorbed gases before measurements. Specific surface areas and the pore size distributions were determined using the BET and the Barrett−Joyner−Halenda (BJH) methods, respectively. 2.5. Theoretical Computational Details. The first-principles calculations based on the DFT were performed with spin-polarized generalized gradient approximation (GGA). Periodic DFT computations with the plane-wave pseudopotential method were implemented in the program Vienna ab initio simulation package (VASP 5.4).31 All the VASP calculations employed GGA with exchange and correlation, which were treated by Perdew−Burke−Ernzerhof.32 The Monkhorst−Pack type of k-point sampling of 5 × 5 × 5 was used, and the plane-wave cutoff was set to 450 eV to strike a balance between the accuracy and the computational cost. The projector augmented wave potentials were used to describe the electron−core interaction.33,34 Spin polarization was considered here to calculate both the energies and structures.

3. RESULTS AND DISCUSSION 3.1. Design of the MnxCoySz/CNTF Composite. The preparation process of the MnxCoySz/CNTF composite is schematically illustrated in Figure 1a. The synthesis includes 25273

DOI: 10.1021/acsami.9b06936 ACS Appl. Mater. Interfaces 2019, 11, 25271−25282

Research Article

ACS Applied Materials & Interfaces

schematically shown in Figure 1b. Experimental demonstration of specific nanostructure dependence on CTAA will be further discussed in detail later. 3.2. Morphology and Nanostructure Characterizations. Figure S1 shows a digital photograph of a pristine CNTF synthesized from a FCCVD method before (the upper half) and after (the lower half) activation pretreatment. Here, the rectangular shape (4 cm length × 2 cm width) was cut from a large film. Obviously, after activation of the half of a CNTF, its width of the rectangular film increases from 2 to 2.3 cm, with 15% size increment. The internal space of CNTF is highly enlarged, as confirmed by the increase in film thickness from 9.5 to 263.5 μm (with 27 times increment). The large increases in both the width and the thickness indicate that CNTF expands in 3D space. Interestingly, the size increases in the width and thickness (different directions) are largely different. This is because a film consists of ultralong CNTs that are intercrossed in 2D space. When it is expanded in volume, individual CNT is only getting straight from bending states to a certain extent because of an existence of large CNT−CNT complex drag force, while the adjacent CNTs along the direction perpendicular to the film are separated from each other. In spite of a large volume expansion, the mechanical stability and even the structural integrity of CNTF remained unchanged. The pristine CNTF is hydrophobic and its surface cannot be wetted by water (before the activation), but the activated films showed greatly improved surface wettability. The surface contacting angle changes from 120° before activation (the upper inset of Figure S1b) to 60° after activation (the lower inset), indicating that the activation transforms the hydrophobic CNTF to a highly hydrophilic one. This is mainly attributed to both the change of the network nanostructure and addition of active groups onto the CNT surface. The obvious volume expansion of CNTF is further confirmed by SEM observation. In the pristine CNTF, individual CNTs attach to each other and prefer to exist in thick bundles (Figure S1c). Its dense network structure and hydrophobic surface undoubtedly blocked ions from diffusion, thus it works in a way similar to a planar substrate. After activation, adjacent CNTs become well separated to form into a 3D network (Figure S1d), as is also confirmed by a high BET surface area of 276 m2 g−1. The mass density greatly reduces from being 2000 to ∼100 mg cm−3 because of ∼20 times enlargement in the volume of the film. The volume expansion provides a large internal space combined with active groups on each CNT, which facilitates subsequent anchoring of the MnxCoy(CO3)0.5OH precursor. With the A-CNTFs, by adjusting the molar ratio of Mn2+/Co2+ in the initial mixed reaction solutions (step II) and subsequent sulfidation (step III), high Co/S atomic ratios in the resulting MnxCoySz were obtained. Figure 2a shows a low-magnification SEM of a typical MnxCoySz/CNTF product. The film is composed of ultralong composite NTs with lengths reaching tens of micrometers, and the original 3D network nanostructure of the A-CNTF is inherited. As shown in the magnified SEM image (Figure 2b), the resulting MnxCoySz-coated CNT has a rough and uniform surface with an average diameter of ∼240 nm. A magnified SEM image showing a single tube (inset of Figure 2b) clearly indicates that MnxCoySz consists of a large number of loosely packed nanoparticles of ∼20 nm. Compared with the precursor-covered CNTs (see SEM and TEM images in Figure S2, Supporting Information), MnxCoySz/CNTs have rougher surfaces and a 50 nm increase

three main steps. An activation pretreatment using two kinds of acid solutions (step I) plays a vital role for subsequent growth of the precursor on each CNT. Without the activation, only upper surface of whole pristine CNTF is covered by a dense precursor layer, while individual NTs in the interior cannot be coated because of an inaccessibility of ions. With the activation instead, the dense-packed films are converted to hydrogel-like ones that have greatly enlarged internal space and largely enhanced surface wettability. The activation process includes immersing the pristine CNTFs in diluted HCl aqueous solution (5 wt %) and concentrated HNO3 (65 wt %) one after another. Interestingly, no obvious size change of a film is observed when only diluted HCl solution is used for activation; such result cannot be obtained when using only HNO3. This is because HCl solution removes a small quantity of the ferrocene catalyst (confirmed by the increased conductivity of CNTF after immersion) and concentrated HNO3 oxidizes the surface of individual CNTs sufficiently and endows their surface with many active groups such as hydroxyl, carbonyl, and carboxyl. The activated surface of the groups helps to generate repulsive force to expand the dense CNT network, leading to an open porous structure, and also transforms hydrophobic CNTF into highly hydrophilic one. After that, the precursor grows on each NT in A-CNTFs (step II). The cations of Co2+, Ni2+, and NH4+ react with the CO32− and OH− anions released from the slow hydrolysis of urea, which leads to the precursor of MnxCoy(CO3)0.5OH.38 The active groups serve as nucleation centers for the subsequent uniform surface growth. The open porous structure and greatly improved wettability facilitate ion accessibility, which assures an ultrahigh mass loading. In step III, a MnxCoySz coating was obtained after sulfidation of the precursor by TAA (CH3CSNH2). The involved possible chemical reaction equations are as follows CO(NH 2)2 + 2H 2O → CO32 − + 2NH4 +

(1)

Mn(CH3COO)2 + Co(CH3COO)2 → Mn 2 + + Co2 + + 4CH3COO− CH3COO− + H 2O → CH3COOH + OH−

(2) (3)

x Mn 2 + + yCo2 + + 0.5CO32 − + (2z − 1)OH− → MnxCoy(CO3)0.5 (OH)2z − 1 CH3CSNH 2 + H 2O → CH3CONH 2 + H 2S

(4) (5)

MnxCoy(CO3)0.5 (OH)2z − 1 + z H 2S → MnxCoySz + (2z − 0.5)H 2O + CO2

(6)

In step III, the anion-exchange rate is determined by the concentration of TAA (defined as CTAA), and thus, controlled sulfidation is also important for the nanostructure modulation. Optimization of CTAA leads to a nanoparticle-built nanoporous MnCoxSy coating. Specifically, such a nanostructure is achieved at relatively slow reaction rates (CTAA ≤ 20 mM), while dense and smooth coatings are formed at CTAA > 20 mM where the anion-exchange rate is relatively high. The former is desirable for SC electrode materials because of its larger porosity as well as for providing much more active sites. The microscopic nanostructure change during the sulfidation process of the precursor leading to a nanoparticle-built nanoporous layer is 25274

DOI: 10.1021/acsami.9b06936 ACS Appl. Mater. Interfaces 2019, 11, 25271−25282

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ACS Applied Materials & Interfaces

combustion of CNT. When the temperature is higher than 670 °C, no weight loss happens any longer, which is consistent with the differential scanning calorimetry (DSC) heat flow plot (blue curve in Figure 2f), where a strong exothermic peak at around 600 °C corresponds to CNT combustion. On the basis of the weight loss, the loading percentage of MnxCoySz in the typical composite film is calculated to be 67.7 wt %. This value is close to that calculated from the weight change before and after deposition (69.5 wt %), higher than reported values regarding the CNTF-based composite in the literature.12,38 Therefore, our method assures a high mass loading of the grown active material MnxCoySz in the resulting composite. Figure 3a shows XRD patterns of pristine CNTF (black curve) and the resulting MnxCoySz/CNTF (blue curve). A

Figure 2. Detailed characterizations of the typical MnxCoySz-coated CNT composite film. (a,b) SEM at different magnifications and (c) BET results with the inset showing BJH average pore diameter distribution. (d) TEM image of several MnCo9S10-coated CNTs from the composite film. The inset of (b) is a magnified SEM, and the insets of (d) are the TEM image of a single bare CNT (on the left) and a single MnxCoySz-covered CNT (on the right). (e) HRTEM image recorded from an edge of the MnxCoySz layer and (f) TG and DSC curves recorded from the typical composite.

Figure 3. (a) XRD patterns of pristine CNTF and the resulting MnxCoySz/CNTF. (b) SEM image (on the left) with X-ray elemental mappings recorded from a single CNT coated with loosely packed MnxCoySz (the upper right images) and SEM−EDS with the Mn, Co, and S elemental atomic percentages (the lower right spectrum).

in diameter. The porosity can be further demonstrated by a high BET surface area of 416.34 m2 g−1 (Figure 2c) and the BJH average pore size distribution (see the inset) in a narrow range of 3−7 nm. The TEM image (Figure 2d) reveals that each CNT is coated by loose MnxCoySz where hollow interior of the CNT and rough coating with many nanopores are both visible. MnxCoySz is tightly bound with each CNT, presenting a robust and stable hybrid structure. The bare CNT (before coating) is multiwalled with an outer diameter of around 40 nm, as shown in the TEM image (inset of Figure 2d). Thus, the thickness of the MnxCoySz layer is about 100 nm. In the HRTEM image (Figure 2e), the lattice fringes exhibit interplanar spacings of 0.212 and 0.479 nm, which are slightly larger than those for the corresponding (200) and (111) planes for MnCo2S4, implying a difference in the lattice parameters between our MnxCoySz and MnCo2S4, respectively. In order to determine the mass loading of MnxCoySz in the composite, thermogravimetric analysis (TGA) was performed. The TGA curve (black curve in Figure 2f) shows the first weight fluctuation at about 280−500 °C and even a slight increase of mass is observed, which is because the lightweight sample floats as the temperature rises. Furthermore, it shows that the most significant changes take place at 500 °C, whereas at residual, a little Fe catalyst-facilitated oxidation of defective or small-diameter CNTs takes place.39 After that point, a continuous weight loss from 520 to 670 °C originates from the

broad peak at around 26.6° is associated with the carbon. The 2θ scan peaks at 31.7°, 35.62°, 38.5°, 54.2°, and 58.8° should be assigned to the (200), (210), (211), (311), and (230) crystal planes of MnxCoySz, respectively. The observed diffraction peaks do not match well to those for MnCo2S4 (black lines for standard peak positions), although there is a slight shift toward the smaller degrees. The diffraction peak shift should be attributed to the difference in the crystal structure originated from varying Mn/Co/S atomic ratio. On the basis of the peaks, the lattice cell parameter of MnxCoySz compounds is calculated to be 9.723 Å, larger than that for MnCo2S4 (a = 9.407 Å from XRD peaks in ref 40). This combined with HRTEM indicates that the lattice cell parameter and unit cell volume slightly increase with the Co/S ratio. In particular, the value of 9.723 Å is also close to that from our theoretical calculations (discussed latter). X-ray elemental mappings recorded from a single MnxCoySz/CNT (Figure 3b) reveal that different elements of Co (green), S (red), and Mn (cyan) are homogeneously distributed along the whole tube, which implies a uniform MnxCoySz coverage on 25275

DOI: 10.1021/acsami.9b06936 ACS Appl. Mater. Interfaces 2019, 11, 25271−25282

Research Article

ACS Applied Materials & Interfaces CNT. The SEM−EDS shows the characteristic peaks of the elements, and the atomic percentages of Mn, Co, and S are 5.1, 44.8, and 50.1 at. %, respectively (see the inset). Note that the atomic percentages from repeated EDS measurements at three different positions are averaged in order to obtain accurate values. Their atomic molar ratio is thus 1:8.79:9.82 (about 1:9:10). This was further corroborated by ICP analysis (see Table 3 below) where the weight percentages of Mn, Co, and S elements are 6.15, 56.68, and 37.18 wt %, respectively. The Mn/Co/S atomic ratio is thus 0.05:0.43:0.52 (1:8.60:10.40). In addition, according to XPS analysis (Figure S2, Supporting Information), the atomic ratio of Mn, Co, and S is 1:8.52:9.68. As summarized in Table 1, the Mn/Co/S atomic molar ratios from the XPS and ICP measurements are both

different HNO3 immersion times (defined as tHNO3). After activation with a short time, the resistances are still high because of a small change of the CNTF. With an increase of tHNO3, Rct (the faradaic charge-transfer resistance) and Re (the bulk resistance of the system including the electrolyte and internal resistance of the electrode), both increase initially and then decline. Among the composite films, MnCo9S10/CNTF obtained with tHNO3 = 6 h has the smallest resistances (Rct = 1.51 Ω, Re = 0.15 Ω). This further indicates that the optimal tHNO3 = 6 h leads to the highest electrical conductivity. The loading percentages of MnCo9S10 with varying tHNO3 are shown in the red curve of Figure S4b. With a gradual increase of tHNO3, the loading percentage of MnCo9S10 increases first and then declines, which reaches the highest value of 66.7 wt % at tHNO3 = 6 h. It means that longer tHNO3 does not facilitate an increase in active materials’ loading percentage. This is because a newly formed Mn/Co precursor cannot grow on the surfaces of individual CNTs but in the solution instead. Also, specific volume capacitance of the resulting composite films is maximized at this point (tHNO3 = 6 h), as shown in the black curve of Figure S4b. Obviously, the change trend in the red and black curves is the same, but the increase rate (slope of curve) is different in two separate stages. When tHNO3 ≤ 6 h, the load percentages of MnCo9S10 and specific capacitance both increase rapidly (the slope is steeper), which indicates that the volume expansion of CNTF is large at this stage. On the basis of the results, tHNO3 = 6 h is the optimal time considering structure integrity and uniformity of the films, high loading of MnCo9S10, electrical conductivity, and capacitance. As is known, the electrochemical performance depends on the nanostructure of electrode materials. It is found that the porosity of the MnCo9S10 layer can be controllable by CTAA that determines the anion-exchange reaction rate. With a gradual increase of CTAA, specific BET surface area increases initially, declines, and then reaches the largest at CTAA = 3.3 mM (Figure S5a, Supporting Information). This indicates that a relatively low CTAA is necessary for obtaining porous nanostructures. As seen from the BJH adsorption average pore size distribution (insets of Figure S5), when CTAA is below 20 mM, all the coverings have nanopores, and the pore diameters obtained at lower CTAA are relatively large. At the optimal CTAA of 3.3 mM, nanoparticle-built porous covering on individual CNTs is achieved, while dense and smooth ones are formed at higher CTAA than 20 mM. More importantly, the largest area circled in the CV curves at 100 mV s−1 (Figure S5d), the longest discharge time in galvanostatic charge/ discharge (GCD) curves at 10 mA cm−2 (Figure S5e), and the corresponding highest specific capacitances (Figure S5f) are all achieved at that point, which further demonstrates the nanostructure optimization. 3.3. Electrochemical Performance of MnCo9S10/CNTF Electrodes. Figure 4a shows CV curves of the optimal MnCo9S10/CNTF electrode at various scan rates from 10 to 100 mV s−1. The presence of redox peaks corresponding to reversible faradaic reactions indicates its battery-type behavior, and near-rectangular shape of the CV curves indicates a pseudocapacitive behavior. Figure 4b shows GCD curves of the electrode at various current densities. The good symmetry reveals charge/discharge reversibility. On the basis of the discharge curves, the corresponding specific volumetric

Table 1. Composition Results of Our Typical MnxCoySz/ CNTF Product by Three Different Quantitative Analysis Methods Including ICP, EDS, and XPS analytical method

measured Mn/Co/S atomic ratio

EDS ICP XPS

5.1:44.8:50.1 (1:8.79:9.82) 0.05:0.43:0.52 (1:8.60:10.40) 2.16:18.4:20.9 (1:8.52:9.68)

consistent with the EDS result (about 1:9:10). Therefore, the deposited layer is suggested to be MnCo9S10. The atomic molar ratio is determined by the used reactant stoichiometry (nMn2+/nCo2+) and the concentration-dependent sulfidation (CTAA). Because metal−metal ratios are adjustable and the sulfidation degree is controllable, this indicates that the method offers the possibility of engineering other MnxCoySz materials with a tunable atomic ratio and specific nanostructure. The oxidation states of Mn, Co, and S elements are further evaluated by XPS. By using Gaussian fitting, two types of manganese species were observed in the Mn 2p spectrum (Figure S3b), which are ascribed to the coexistence of mixed Mn4+ and Mn3+. The peaks located at 641.6 and 653.9 eV are ascribed to Mn4+ and those at 642.8 and 654.9 eV are related to Mn3+.41 From the Co 2p spectrum (Figure S3c), two distinct peaks with binding energies of 781.4 and 797.4 eV correspond to Co 2p3/2 and Co 2p1/2 for the characteristic peaks of Co2+. The peak at 781.4 eV could be ascribed to the oxidation state of Co, and two peaks at 778.9 and 794.0 eV are assigned to Co3+. The binding energy of 778.9 eV is slightly higher than that of metallic cobalt (778.0 eV), indicating that Co carries a partially positive charge (Coδ+).42 In the S 2p spectrum (Figure S3d), the main peaks at 162.0 and 163.0 eV belong to S 2p3/2 and S 2p1/2, respectively. The peaks are the typical characteristic binding between Sn2− and metal ions (Co/Sn and Mn/Sn bonds).43 The presence of polysulfides might be from the nonstoichiometry. In addition, an extra peak at 168.6 eV should be attributed to surface sulfur with high oxide state such as metal sulfates.44 Therefore, the coexistence of rich mixed-valence states (Co3+ and Co2+), (Mn3+ and Mn4+), and Sn2− for the Mn, Co, and S elements has been demonstrated. As discussed above, the activation with concentrated HNO3 (after immersion in HCl solution) plays a decisive role for the volume expansion of the CNT network in 3D space. Figure S4a in Supporting Information compares electrochemical impedance spectroscopy (EIS) curves of resulting MnCo9S10/CNTF composites obtained using A-CNTF with 25276

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inapparent redox peaks in the CV curves corresponding to the reversible faradaic reactions can be observed, which indicates its battery-type electrochemical behavior.56 Also, the nearrectangular shape of the CV curves demonstrates a pseudocapacitive behavior, which is due to the fact that the formed MnxS−OH and CoySz−1−OH undergo reversible redox processes (see eqs 8 and 9).57 This is similar to that of transitional-metal oxides with electrochemical characteristics that are neither purely capacitive nor purely faradaic.58 Such a 3D network with the largely opened geometry facilitating the electron transfer and OH− ion diffusion is schematically illustrated in Figure 4d. The possible reaction equations for the reversible redox reactions could be expressed as follows MnxCoySz + 2OH− V MnxSOH + CoySz − 1OH + 2e− (7)

MnxSOH + OH− V MnxSO + H 2O + e−

(8)

CoySz − 1OH + OH− V CoySz − 1O + H 2O + e−

(9)

Long-term cycling is another critical criterion for SCs. Remarkably, 98.4% of the initial capacitance remained unchanged after 10 000 charge/discharge cycles at a high current density of 80 mA cm−2 (Figure 4e), and the charge/ discharge behavior becomes stable after the 2000th cycle. The inset shows the initial and the last several periods, revealing a regular charging/discharging behavior. Coulombic efficiency is calculated from an equation η = td/tc × 100%, where tc and td are the charge and discharge times. The Coulombic efficiency is about 91.3% after 10 000 cycles. This indicates remarkable cycling stability of the electrode. After the cycling measurements, MnCo9S10 was still well attached on the CNT networks (Figure S6, Supporting Information), confirming the strong bonding between MnCo9S10 and each CNT. The excellent cycling stability is mainly attributed to three points: (1) the presence of networked channels helped to stabilize the expansion/contraction of the film electrode during the redox reactions. (2) After covering, the CNT−CNT contact is replaced by the contacting resistance of highly conductive MnCo9S10. The ion transfer is accelerated, which effectively reduces the internal resistance of the whole film and thus is of benefit for repeated charging/discharging. (3) The presence of nanopores in the MnCo9S10 layer leads to faster kinetics and preserves the structure, which enables extended cycling with OH− ions to occur with minimal decrease in charge storage. 3.4. Theoretical Calculations. In order to clarify the underlying mechanisms for the capacitance enhancement, the first-principles calculations based on the DFT method are performed, and theoretical calculation crystal structures are shown in Figure S7 (Supporting Information). By replacing O atoms with S atoms, the O−O bond length changes from 2.713 Å for MnCo2O4 to 3.121 Å for the S−S bond length in MnCo2 S 4 . The lattice cell parameter of MnCo 2 S 4 is theoretically calculated using DFT to be a = 9.347, close to the value from XRD analysis (a = 9.407 Å).37 Also, the calculated lattice cell parameter increases from 8.146 Å for MnCo2O4 to 9.347 Å for MnCo2S4, 15% increase in the lattice cell parameter (see Table 2). Such change is also observed in the ZnCoO4 system with 16% increase in bond length. In particular, the lattice cell parameter of MnCo9S10 is calculated to be 9.776 Å. This indicates that increasing Co/S is helpful to enlarge the interatom bond length and hole size. The

Figure 4. (a) CV curves at various scan rates and (b) GCD curves at different current densities of the MnCo9S10/CNTF electrode. (c) Specific capacitance vs current density, a comparison between our Cv = 449 F cm−3 with other similar CNT or CF-based composite film electrodes. (d) Schematic illustration of the 3D network with largely opened geometry, facilitating the electron transfer and OH− ion diffusion. (e) Cycle performance of the optimal MnCo9S10/CNTF electrode measured at a current density of 80 mA cm−2, and the inset shows GCD curves of the initial and last five cycles.

capacitances (Cv) are calculated to be 449, 437, 403, 398, 391, and 370 F cm−3 at 10, 15, 20, 30, 50, and 80 mA cm−2, respectively. The Cv value of 449 F cm−3 (at 10 mA cm−2) is much higher than those of recently reported similar flexible film electrodes (Figure 4c) with CNT or CF-based composites, particularly at a high current density of 10 mA cm−2. For example, this value is roughly two times that of MnO2 nanosheets grown on NT fibers (253.2 F cm−3 at 10 mA cm−2),45 MoS2@CNTs (223.7 F cm−3 at 10 mA cm−2),46 bamboo-like O-doped porous CNTs (213 F cm−3 at 10 mA cm−2),47 and even several orders of magnitude higher than those for CNT forest on unidirectional CF (only 3.4 F cm−3 at 11 mA cm−2),48 mesoporous vanadium nitride (VN)/CNT (5.9 F cm−3 at 11 mA cm−2),49 and NiS-anchored carbon nanofibers (29.5 F cm−3 at 2.3 mA cm−2).50 Also, our MnCo9S10/CNTF electrode shows a superior rate capability (82.3% capacitance value remained at 10−80 mA cm−2) over other similar flexible film electrodes (data are listed in Table S1).45−55 It is reported that the reversible redox reactions of NiCo2S4 give rise to the formation of CoSOH and NiSOH.25 MnxCoySz is similar to NiCo2S4, thus it is believed that its energy storage mechanism in an alkaline electrolyte should be via reversible redox reaction of MnxCoySz to form MnxSOH, CoySz−1OH, MnxSO, and CoySz−1O (see eq 7). The presence of two pairs of 25277

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ACS Applied Materials & Interfaces Table 2. Comparison of Lattice Parameters and the Shole Values

Table 3. Comparison of Lattice Parameters and the Area of Holes

crystal

cell types

a (Å)

Shole (Å2)

bond length (Å)

crystal

cell types

a (Å)

Shole (Å2)

MnCo2O4 MnCo2S4 MnCo9S10a

unit cell unit cell supercell

8.146 9.347 9.776

3.8 4.9 5.3

2.713 3.121 3.125

MnCo2S4 MnCo2S4a MnCo2S4b MnCo2S4c

unit cell supercell supercell supercell

9.347 9.431 9.441 9.430

4.9 5.0 5.1 5.3

The supercell is constructed via a 1 × 1 × 1 unit cell of the optimized crystal structure of MnCo2S4, followed by replacement of the corresponding atoms in the supercell. a

One S atom is changed to Co in the supercell constructed via a 1 × 1 × 1 unit cell of the optimized crystal structure of MnCo2S4. bTwo S atoms are changed to Co. cFour S atoms are changed to Co. a

experimental and theoretical results for our MnxCoySz are both larger than the standard values for MnCo2S4, which further confirms the nonstoichiometry-induced increase in lattice cell parameters. The definition of a hole between three S atoms is presented in Figure 5a. The hole area (in red dashed lines, Shole) can be obtained through Heron’s formula: a+b+c S = p(p − a)(p − b)(p − c) , p = 2 , where a, b, and c are the interatomic distances. Lower electronegativity of S than O leads to a more flexible structure through elongation between the layers, which has been confirmed by the DFT calculation results where the interatomic channel becomes larger, and thus, the volume of the unit cell is enlarged when the O atoms in MnCo2O4 are replaced by S atoms. The influence of varying Co/S atomic ratio on the crystal structure was further studied. Figure 5b−d shows DFT theoretical calculation results of crystal structures and the Shole values after replacing Co with S atoms. As listed in Table 3, when only one S atom is changed to Co in the supercell, the lattice cell parameter increases to 9.431 Å. Interestingly, with a gradual increase of Co/S atomic ratio (more S atoms are changed to Co), the lattice cell parameter further increases to 9.441 Å but then reduces to 9.430 Å. The Shole size increases gradually with the Co/S atomic ratio from 4.9 to 5.0, 5.1, and 5.3 Å. Taking Mn2Co19S21, for example, the interatomic channel is further enlarged with a S−S bond length

of 3.33 Å (9.436 × 9.436 × 9.436). Thus, increasing the concentration of Co in the unit cell can enlarge the size of the holes. This means that adjusting the Mn/Co atomic ratio can further enlarge the interatomic channels, which is beneficial for ultrafast ion diffusion. The obvious change in the interatomic channel is due to the advantage of the improved charge transfer because of energy difference between O 2p and S 2p levels. The above calculated results indicate that obtaining MnxCoySz with a relatively higher Co/S ratio is more favorable for the electron- and ion-transfer kinetics compared with MnCo2S4, which is in good agreement with those from the experimental results. On the basis of the above experimental and theoretical results, the high Cv is suggested to be attributed to the following: (1) taking advantage of the CNT network as a conductive scaffold, the loose and porous network structure allows a small resistance and facilitates ion and electron transportation. (2) The generation of redox active polysulfide M/Sn (M = Co, Mn) can increase the number of electrons exchanged, facilitate electrons transport, reduce the energy barrier, and contribute to the remarkably improved capacitor performance. (3) Our MnCo9S10 with an optimal Co/S ratio allows larger Shole size and S−S bond lengths than those for typical MnCo2S4, which is more favorable for enhanced reversibility of faradaic reactions. (4) Mn4+/Mn3+ can attract

Figure 5. (a) Definition of a hole between adjacent three S atoms in the supercell constructed via a 1 × 1 × 1 unit cell of the optimized crystal structure of MnCo2S4. The hole area (in red dashed lines, Shole) can be obtained through Heron’s formula. Schemes showing a gradual change of Shole after replacing Co atoms with S atoms. (b) One S atom, (c) two S atoms, and (d) four S atoms are changed to Co, respectively. 25278

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mW h cm−3 at 0.4 W cm−3)49 and CNT forest grown on unidirectional CF (0.67 mW h cm−3 at 1.5 W cm−3).48 Even at a higher power density of 30.2 W cm−3, our MnCo9S10/ CNTF//A-CNTF ASC still delivers an energy density of 46.6 mW h cm−3. Long-term cycling performance was measured by repeated charging and discharging of the device at a high current density 12 mA cm−2 (Figure 6e). Remarkably, 94.2% of the initial capacitance of the device still remained even after 10 000 charge/discharge cycles. The applicability can be proved visibly by powering light-emitting diodes (LEDs). We connected several such advanced ASCs in series to reach a high output voltage, as illustrated in Figure 6f. An integrated source with three connecting ASCs in series can obtain a high voltage of 4.8 V and high capacity, which allows them to continuously power 17 connecting LEDs (2.0 V, 20 mA) in parallel, as shown in the inset of Figure 6f. Therefore, integrated sources with adjustable output voltages can be obtained in such a way. It is known that the intrinsic characteristic voltage of water splitting (1.23 V) makes an aqueous electrolyte limited to a potential of around 1 V, constraining the operating voltage to a maximum of 1.8−2.0 V. Obviously, this scalable technique can provide high output voltages for wearable electronics. In order to further test the stability of CNTF/MnCo9S10// A-CNTF devices, the electrochemical behaviors of an isolated ASC under large deformations were also evaluated. Figure 7a

more electrons than Mn3+/Mn2+ in an oxidation−reduction reaction. 3.5. Flexible All-Solid-State SCs. Figure 6a illustrates a scheme of the sandwich structure of two-electrode all-solid-

Figure 6. (a) Schematic illustration of the assembled structure of an all-solid-state CNTF/MnCo9S10//A-CNTF ASC; (b) CV curves at various scan rates and (c) GCD curves at different current densities of the ASC; (d) Ragone plots of the device in comparison with previous reports of similar solid-state SCs applying CNT or CF-based composite electrode materials. (e) Cycle performance of the ASC measured at a current density of 12 mA cm−2 and (f) scheme with digital photo (the inset) showing 17 connecting LEDs (2.0 V, 20 mA) in parallel that were powered by a high-voltage integrated source made by combining three ASCs.

Figure 7. (a) CV curves at a high scan rate of 100 mV s−1 for the CNTF/MnCo9S10//A-CNTFs ASC under normal, twisted, and bent states. (b,d) Flexibility test of a high output voltage integrated source consisting of several connected ASCs in series. (c,e) The source is still working without structural failure and performance loss during repeated large-angle folding and vigorous hammering.

state SC using MnCo9S10/CNT as the positive electrode and A-CNTFs as the negative electrode. Figure 6b shows CV curves of the MnCo9S10/CNTF//A-CNTF asymmetric SC (ASC) device at various scan rates. With a gradual increase of scan rate from 5 to 100 mV s−1, no obvious distortion of CV curve shape is observed, indicating an excellent fast charge/ discharge behavior of the device. Figure 6c shows GCD curves of the device at various current densities, which reveals a reversible charge/discharge behavior. Specific capacitances calculated from the discharge curves are 214, 183, 170, and 149 F cm−3 at 4, 6, 8, and 12 mA cm−2, respectively. The device achieves a high energy density of 67 mW h cm−3 at a power density of 10.0 W cm−3 (Figure 6d). This value is much superior over that of many asymmetric all-solid-state SCs with similar positive electrodes. The comparison data are listed in Table S2 in the Supporting Information.45−55 For example, this value is higher than those for the asymmetric devices of Ni@ Ni3S2 nanowire films//graphene-CNTs (44.5 mW h cm−3 at 0.375 W cm−3)51 and NiS-anchored carbon nanofibers// carbon nanofibers (13.3 mW h cm−3 at 0.18 W cm−3).50 Particularly, the energy density is more than 100 times of those for the asymmetric devices with mesoporous VN/CNT (0.54

shows CV curves of an asymmetric device under normal, twisted, and bent states. There is no obvious change in the CV curve shape under bent and twisted states, and there is a small difference in the curves (see the inset). Even a high output voltage integrated source consisting of several connected ASCs in series also has an outstanding flexibility and strong stability. In particular, in the case of face-to-face folding (see the scheme shown in Figure 7b, with a bending angle of 180 °C), the source can keep the current output smoothly (Figure 7c). Particularly, there are no structural failure and capacity loss during repeated large-angle twisting and vigorous hammering during discharging (Figure 7d,e). An integrated source is successfully operated under repeated large folding, twisting, and hammering (see an attached Video). The excellent stability promises many opportunities for wearable electronics.

4. CONCLUSIONS We demonstrated the synthesis of high-performing flexible electrodes based on MnxCoySz/CNTF composite films via first activation of CNTF, Mn/Co precursor growth, and controlled 25279

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sulfidation. The activation time was optimized to be 6 h to assure high loadings of the active material, and control over concentration-dependent sulfidation kinetics leads to loosely packed nanoparticle-built porous coating. The optimal MnCo9S10/CNTF electrode has an ultrahigh volumetric capacitance reaching 450 F cm−3 at 10 mA cm−2, much superior over previously reported values for CNT or CF-based composites. Also, only 1.6% capacitance loss was observed after 10 000 cycles at a high current density of 80 mA cm−2. Theoretical calculation allows us to identify the mechanism for how lattice cell parameter and polysulfide trapping dominate charge storage. All-solid-state CNTF/MnCo9S10//A-CNTF ASC delivers an exceptionally high energy density of 67 mW h cm−3 (at 10 W cm−3) with an excellent cycling stability. In particular, integrated sources with desirable output voltages can be obtained by connecting several ASCs in series. The integrated sources have outstanding flexibility and stability without structural failure and performance loss even after repeated hammering and folding during discharging. This work opens a new route to high-performance CNTF-based power sources for wearable electronics.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06936. Detailed comparisons of a CNT film before and after activation; SEM and TEM images of a single Mn−Co precursor-covered CNT; XPS spectra of the typical MnxCoySz /CNTF; EIS curves of resulting MnCo9S10/ CNTF composites; BET results of the resulting MnxCoySz/CNTF obtained with different CTAA; electrochemical performance comparison; SEM image of the MnCo9S10/CNTF electrode after 10 000 cycles; theoretical calculation crystal structures; and comparison between energy and power densities (PDF) Integrated source successfully operated under repeated large folding, twisting, and hammering (MP4)

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.T.). *E-mail: [email protected] (H.D.). *E-mail: [email protected] (X.M.). ORCID

Shaochun Tang: 0000-0003-4400-708X Hao Dong: 0000-0001-7280-7506 Author Contributions ∥

W.S. and G.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors kindly acknowledge the joint support by the National Natural Science Foundation of China (grant nos. 11374136, 21833002, 51771090), the Natural Science Foundation of Jiangsu Province (grant no. BK20161396), the Fundamental Research Funds for the Central Universities (grant no. 021314380073), the Technology Innovation Foundation of Nanjing University (02131480608203), and the “Jiangsu Specially Appointed Professor” program. 25280

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DOI: 10.1021/acsami.9b06936 ACS Appl. Mater. Interfaces 2019, 11, 25271−25282

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DOI: 10.1021/acsami.9b06936 ACS Appl. Mater. Interfaces 2019, 11, 25271−25282