Highly Conductive Mo2C Nanofibers Encapsulated in Ultrathin MnO2

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Highly conductive Mo2C nanofibers encapsulated in ultrathin MnO2 nanosheets as self-supported electrode for high-performance capacitive energy storage Minjie Shi, Liping Zhao, Xuefeng Song, Jing Liu, Peng Zhang, and Lian Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10637 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016

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ACS Applied Materials & Interfaces

Highly conductive Mo2C nanofibers encapsulated in ultrathin

MnO2

nanosheets

as

self-supported

electrode for high-performance capacitive energy storage Minjie Shi, Liping Zhao*, Xuefeng Song, Jing Liu, Peng Zhang, Lian Gao* State Key Laboratory for Metallic Matrix Composite Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China KEYWORDS transition metal carbide, nanofiber, hybrid film, self-standing flexible electrode, supercapacitor

ABSTRACT Nano-structured transition metal carbides (TMCs) with superior electrochemical properties are promising materials for high-efficiency energy storage applications. Herein, onedimensional molybdenum carbide nanofibers (Mo2C NFs) have been fabricated by a facile and effective electrospinning strategy. Based on the cross-linked network architecture with ultrahigh electronic conductivity, each Mo2C NFs is uniformly encapsulated in lamellar manganese dioxide (MnO2) via electro-deposition method, forming a self-supported MnO2-Mo2C NFs film with excellent electrochemical activity. Remarkably, highly conductive inner layer of porous

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Mo2C NFs acts like “highway” to facilitate the charge transport and ionic diffusion, while the MnO2 nanosheets with abundant active area are favorable for the accumulation of effective electric charges. Benefiting from these features, the hybrid film is directly applied as the selfstanding electrode of supercapacitors (SCs) without any additives, which delivers considerably large specific capacitance with strong durability in both aqueous and organic (ionic liquid) electrolytes. This work elucidates a feasible way toward hetero-nanofiber engineering of TMCs on a promising additive-free electrode for flexible and high-performance SCs.

Flexible energy storage devices have received great interest for their potential application in wearable and portable electronics. 1-3 Supercapacitors (SCs) have been regarded as an attractive class of energy-storage devices associated with their quick charge-discharge capability, long cycle life, security and reliability.4-6 Unfortunately, the employment of inactive components including conductive additives and current collectors makes the SCs too heavy and rigid to meet the practical requirements for portable devices. Furthermore, the introduction of insulating binder blocks the diffusion channels of charge transport and compromise the electrochemical performance.5, 7 Therefore, additive-free electrode materials with flexibility and high capacitance are urgently required for the development of new-generation flexible SCs. Combining of pseudocapacitance materials and flexible conducting support without any additive agents is an effective approach to fabricate flexible electrodes.5, 8 Manganese dioxide (MnO2) is known to be a promising pseudocapacitance material due to its low-cost, environmental friendliness, wide electrochemical potential and substantial specific capacitance.6, 9

Some efforts have been dedicated to explore various MnO2 based electrodes for SCs. Ma et al.


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rationally fabricated birnessite-type MnO2 nanoplates and nanosheets, and found their specific capacitance in aqueous electrolyte is as high as 308 F/g and 269 F/g, respectively.10, 11 Fan et al. successfully constructed a series of MnO2 hybrids with core-shell arrays, such as MnO2 nanosheets@Co3O4 nanowires and MnO2 nanosheets@NiO nanoflakes, which exhibit excellent electrochemical performances for SCs.12-14 Recent literatures have concentrated on directly growing or depositing MnO2 with various nanostructures on flexible conducting support to obtain flexible electrode.15, 16 In these works, carbon-based films have been used as flexible and conducting support, especially carbon nanotubes (CNTs) and graphene nanosheets (GNSs) films.15-18 However, CNTs or GNSs are inherent agglomerates. Surfactants or stabilizers are necessary to improve the dispersibility and film-forming ability. For those reasons, CNTs or GNSs film as conducting scaffold leads to a high fraction of dead volume in electrodes, resulting in drastic degradation of capacitance for SCs.19, 20 In addition, most methods require complicated fabricating process (GNSs) or high-cost raw materials (CNTs), which are difficult to upscale for their practical applications in SCs.21 It is highly desirable to develop a facile, cost-effective, and scalable way to fabricate a novel flexible and conducting support incorporating with electroactive materials, which can be directly applied as additive-free electrode for SCs. Recently, transition-metal carbides (TMCs), such as Fe5C2, Ti3C2, Mo2C, VC and WC, have attracted renewed research interest owing to their unique properties, combining the exceptional stability of ceramic and the excellent electronic conductivity of metal.22-24 Among them, molybdenum carbide (Mo2C) has shown great potential in applications of energy conversion and storage, especially as anode material for lithium ion batteries.25, 26 Benefiting from unique Mo-C chemical bonding and d-state density around Fermi level, Mo2C exhibits ultrahigh conductivity, excellent mechanical stiffness, great thermal and chemical stability.27, 28 However, techniques for


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organizing Mo2C on the nanoscale size are relatively scarce due to the harsh reaction conditions required (high temperature above 1200°C) and difficultly controlled carburization process,29-31 consequently resulting in mediocre performance. To achieve high performance of Mo2C in various fields, the development of a facile and mild synthesis strategy is highly desirable for organizing Mo2C with suitable morphology on the nanoscale. As a promising way to build one-dimensional (1D) nanostructures, electrospining has been widely used as a large-scale approach to prepare continuous fiber membranes.32-35 To the best of our knowledge, this is the first report on the research of Mo2C nanofibers and its hybrids as flexible SCs electrode. To this end, a novel self-supported hybrid film of lamellar MnO2 wrapping on Mo2C nanofibers (denoted as MnO2-Mo2C NFs hereafter) has been successfully synthesized, via the combination of facile electrospinning and electrochemical deposition approach. The electrospun Mo2C nanofibers (denoted as Mo2C NFs hereafter) with cross-linked network architecture are chosen as conducting support because of their desirable electronic conductivity, which offer abundant and efficient pathways for rapid ionic diffusion and charge transport. Furthermore, ultrathin MnO2 nanosheets uniformly grow around the Mo2C NFs and continuously interconnect with each other, providing high surface area to facilitate the accumulation of effective electric charges during the electrochemical process. As a result, the self-supported MnO2-Mo2C hybrid film is directly applied as binder-free electrode for SCs, which exhibits significant specific capacitance, conductive behaviours and cycling stability in both aqueous and organic electrolytes. The synthesis strategy presented in this work is simple and effective for the preparation of self-supported TMCs hybrid films with promising 1D nanostructure for energy storage applications.


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RESULTS AND DISCUSSION

Figure 1. Schematic illustrations of the preparation process of MnO2-Mo2C NFs hybrid film.

The synthetic route of MnO2-Mo2C NFs hybrid film is schematically illustrated in Fig. 1. Mo2C NFs are effectively synthesized by a electrospinning following heat-treatment strategy, which exhibit desirable electrical conductivity, large surface scaffold and inter-connectivity as well as flexibility. Subsequently, Mo2C NFs are chosen as the supporting substrate to allow the high mass-loading of MnO2 nanosheets via electro-deposition. As shown in Fig. 2a, a non-woven film of precursor nanofibers (denoted as PNFs) in white color is obtained after the electrospinning process. After thermal annealing, the white precursor film is transformed into a


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Figure 2. (a) A digital photograph of PNFs and Mo2C NFs films. (b) The corresponding XRD patterns of the PNFs and Mo2C NFs films.

carbonized film with black color. XRD pattern (Fig. 2b) of the carbonized black film shows diffraction peaks around 37º, 44º, 63º, 75º and 80º, which are well indexed to the reflections of monoclinic Mo2C phase (JCPDS card no. 15-0457).23,

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Another diffraction peak located at

about 26° corresponds to the characteristic (002) reflection of residual carbon, resulting from the thermal decomposition of PVA molecules.36-38 The formation of Mo2C NFs include following processes: firstly, the AHM molecules are homogeneously distributed within the matrix of PVA to form PNFs by electrospinning; secondly, Mo6+ of AHM is reduced to Mo4+ and grow up to MoO2 nanoparticles during heat treatment in the Ar/H2 atmosphere. Meanwhile, carbon (either partially graphitized or amorphous) arises from thermal decomposition of PVA polymer; thirdly, MoO2 nanodomains are carbon-thermal reduced and form well crystallized Mo2C phase when the heating temperature is further increased to 800 oC. This leads to a homogeneously distribution of Mo2C within carbon matrix to construct nanofibers. (XRD patterns of different temperatures are shown in Fig. S1).


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Figure 3. FESEM images of PNFs (a and b), and Mo2C NFs (c and d) in different magnification. The inset of d is the corresponding surface and section image of Mo2C NFs.

FESEM images of the PNFs and Mo2C NFs are fully demonstrated in Fig. 3. As seen from Fig. 3a and b, PNFs exhibit the mesh architecture formed by long and smooth nanofibers continuously with average diameters around 450 nm (Fig. S2). As observed, the length of the nanofibers can reach the millimeter or even centimeter grade. After thermal treatment, the obtained Mo2C NFs preserve similar cross-linked network morphology as PNFs (Fig. 3c), while the diameters of Mo2C NFs (Fig. 3d) are reduced considerably to about 250 nm. It can be explained to the macroscopic size shrinkage of electrospun film before and after thermal carbonization process (Fig. 2a). The magnified FESEM image of surface and cross-section of Mo2C NFs is evinced in the inset of Fig. 3d. There are numerous nanoparticles with the average


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size of about 50 nm, which are dispersed not only on the surface of the nanofibers, but also within the interior of nanofibrous framework (see the region indicated by the circle). As discussed above, these nanoparticles are belong to crystalline Mo2C resulting from the carbon thermal reduced process of MoO2 nanodomains, which are uniformly distributed within the amorphous carbonaceous matrix to form nanofibers. Meanwhile, amorphous carbonaceous matrix effectively prevents the aggregation of Mo2C nanoparticles and offers high surface area for electrolyte ionic adsorption.39, 40

Figure 4. (a, b and c) TEM images of Mo2C NFs in different magnification. Inset b is the corresponding elemental mappings. (d) high-resolution TEM (HRTEM) image and (e) corresponding selected area electron diffraction (SAED) pattern.


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TEM characterization was performed to provide further insights into the morphology and structure of the Mo2C NFs. The representative TEM images of different magnified observations are shown in Fig. 4a, b and c. It can be seen that the nanofiber consists of densily packed crystalline nanoparticles surrounding with amorphous layer. The HRTEM image (Fig. 4d) reveals that the interplanar spacing of the nanoparticle is measured to be 0.24 and 0.15 nm, which can be assigned to the (111) and (220) reflections of the monoclinic Mo2C plane.23 These crystalline Mo2C nanoparticles in nanofibers represent a polycrystalline structure according to the selected-area electron diffraction (SAED) pattern in Fig. 4e. Corresponding elemental mapping images (Inset in Fig. 4b) show the amorphous layer is mainly composed of carbon elements, further suggesting that Mo2C nanoparticles are densely packed and closely surrounded by the carbonaceous layer in each nanofiber. These results are in good agreement with those observed in the FESEM images. The weight content of amorphous carbon in Mo2C NFs is about 38 wt% determined from TGA result in Fig. S3. The resulting Mo2C NFs film possesses an efficient conductive pathway about 18.67 S/cm for electrical conductivity as determined by the standard 4-point probe technique, which is much higher than that of carbon nanofibers (0.5~10 S/cm).9, 41 It is believed that Mo2C nanoparticles continuously distributed in the carbonaceous matrix ensure the excellent electronic conductivity of the nanofibers. Nitrogen adsorption–desorption isotherm of the Mo2C NFs film possesses a typical IV curve type with a H3 hysteretic loop (Fig. S4), implying the existence of mesoporous structure.41 Pore size distribution curve shows that the dominant pore size of Mo2C NFs ranges from 3 to 10 nm (Fig. S4). These mesopores mainly come from the interspace between the adjacent Mo2C nanoparticles, which can be easily accessible by electrolyte ions.4 Moreover, the Mo2C NFs film exhibits excellent mechanical flexibility that can be bent to any shape and is


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reversible. It is ascribed to the formation of the carbonaceous outer layer and the interconnectivity network architecture, rendering a strong backbone for Mo2C NFs film to endure external force.9,

42, 43

Consequently, these characteristics, including cross-linked network

structure with high porosity, favorable electrical conductivity, desirable mechanical strength and flexibility, making such Mo2C NFs film suitable as an appropriate support for combining with electroactive materials. Applying Mo2C NFs film as supporting substrate, a self-supported hybrid film integrated with electro-active MnO2 was successfully obtained via the electrochemical deposition method. As seen from the side-view FESEM image of the MnO2-Mo2C NFs hybrid film (Fig. 5a), the thickness of hybrid film is about 12 µm. The top-view FESEM image of hybrid film is shown in Fig. 5b. It can be found that the cross-linked mesh structure is completely reserved after electrodeposition treatment. Correspondingly, the hybrid film maintains acceptable mechanical strength and flexibility (Inset of Fig. 5b). FESEM observation of an individual hybrid nanofiber is shown in Fig. 5c. Numerous MnO2 nanosheets are interweaved and assembled on the surface of nanofiber. Clearly, the Mo2C NFs are located in the inner layer of the hybrid structure (inset of Fig. 5c). TEM images and corresponding elemental mapping images (Fig. 5d and e) further evidence that the Mo2C NFs are uniformly encapsulated within MnO2 nanosheets, in which these MnO2 nanosheets (~70 nm long) are crumpled, wrinkled and lamellar (~4 nm thickness as obtained from the inset of Fig. 5e and Fig. S5). Benefiting from the novel nano-structure, MnO2Mo2C NFs exhibits large specific surface area of 153.7 m2/g (Fig. S4), which facilitates the accumulation of effective electron charges during the electrochemical reaction.9,44


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Figure 5. (a) Cross-section and (b) top-surface FESEM images of the MnO2-Mo2C NFs film. The inset in (b) shows the digital photo of flexible MnO2-Mo2C NFs film. (c) FESEM image of individual MnO2-Mo2C nanofiber. The inset in (c) is the corresponding enlarged image. (d and e) typical TEM images and corresponding elemental mapping images of the MnO2-Mo2C NFs. The inset in (e) is the magnified area of MnO2 nanosheets.


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Figure 6. (a) XPS spectrum of MnO2-Mo2C NFs film. The inset of a is the corresponding Mn 2p high-resolution spectrum. (b) XRD patterns of Mo2C NFs and MnO2-Mo2C NFs films.

XPS and XRD were carried out to determine the composition of hybrid MnO2-Mo2C NFs film. As shown in Fig. 6a, XPS full spectrum reveals the existence of Mo, C, Mn and O elements in the MnO2-Mo2C NFs hybrid film, which is consistent with the EDS results (Fig. S6). The highresolution spectrum of Mn 2p shows that the Mn 2p3/2 peak centered at 642.6 eV and the Mn 2p1/2 peak at 654.2 eV with a spin-energy separation of 11.6 ev (Inset of Fig. 6a), which are ascribed to the nature of a 4+ oxidation state for Mn element.45, 46 XRD pattern of the MnO2Mo2C NFs film is shown in Fig. 6b. Compared with the XRD result of Mo2C NFs film, new weak and broad peaks at around 10º, 55º and 66º are exhibited. These characteristic peaks are attributed to the α-MnO2·H2O (JCPDS No.44−0141),47, 48 indicating the existence of MnO2 in hybrid film. It is reported that α-MnO2·H2O presents drastically electrochemical activity, beneficial for improvement of the capacitance performance comparing to other phases of MnO2, particularly enhancing the fast proton/ion diffusion rate during the electrochemical process.14, 49 Mass content of MnO2 in the hybrid film was determined by TGA measurement, which is estimated to be 37.2 wt% as shown in Fig. S3.


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Figure 7. (a) CV curves scanning at 10 mV/s and 50 mV/s, (b) Nyquist plots (Inset is the high frequency range) of Mo2C NFs and MnO2-Mo2C NFs film electrodes in 1 M Na2SO4 aqueous electrolyte.

In order to evaluate the electrochemical and capacitance characteristics of SCs, both Mo2C NFs film and MnO2-Mo2C NFs hybrid film were directly applied as additive-free electrodes in 1 M Na2SO4 aqueous electrolyte. As Fig. 7a shows, the cycle voltammetry (CV) curves of Mo2C NFs and MnO2-Mo2C NFs film electrodes represent fairly regular rectangle in sharp at 10 and 50 mV/s with the operation voltage region of 0~1 V. The ideal capacitive behaviors are ascribed to the highly conductive Mo2C NFs, which act like “highway” to facilitate the charge transport and ionic diffusion during electrochemical process. The specific capacitances of Mo2C NFs film electrode at various current densities are listed in Fig S7. Owing to the pseudocapacitance effect and the enlarged surface area of ultrathin MnO2 nanosheets, MnO2-Mo2C NFs electrode exhibits much larger area of CV curve than Mo2C NFs electrode, indicating the enhanced specific capacitance of hybrid film. As shown in Fig. 7b, the Nyquist plots of both Mo2C NFs and MnO2Mo2C NFs film electrodes display small semicircles at high frequency followed by transition to


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linearity at low frequency, which demonstrate low diffusion resistance of electrolyte ions and electronic charges in electrodes.50 However, a slight larger intercept of the semicircle is observed for MnO2-Mo2C NFs film electrode (inset in Fig. 7b). It is well-known that the intercept of the semicircle with the real axis reflects the total inner resistance of the electrode.50,

51

This

discrepancy is attributed to the intrinsic insufficient conductivity of MnO2 wrapped on Mo2C NFs, leading to the relatively large bulk resistance of MnO2-Mo2C NFs film electrode compared with that of highly conductive Mo2C NFs film electrode. It is believed that excessive MnO2 deposited on Mo2C NFs film can affect the overall electrochemical properties of the hybrid film electrode. In our work, the electrochemical behaviors of samples deposited with different time periods were compared (Fig. S8). Since the electro-deposition time of 5 min yielded MnO2Mo2C NFs films with the best performance, it has been determined as the optimal synthetic condition. The electrochemical performance of MnO2-Mo2C NFs film electrode in Na2SO4 electrolyte was further investigated in detail. As seen in Fig. 8a, all the CV curves at different scan rates ranging from 5 to 100 mV/s exhibit fairly regular rectangle in sharp, indicating the great electrochemical reversibility during the charge/discharge processes even at the high scan rate. It is also supported by the linear and symmetrical shape of galvanostatic charge-discharge (GCD) curves (Fig. 8b and Fig. S9). Meanwhile, negligible “IR drop” of discharge curve strongly evinces the favorable conductive behaviors of the MnO2-Mo2C NFs film electrode.52 As shown in Fig. 8c, the hybrid film electrode delivers the large specific capacitance of 430 F/g at a current density of 0.1 A/g and 302 F/g at a current density of 1 A/g, respectively. This value of specific capacitance is considerably higher than that of previously reported state-of-the-art MnO2-based film electrode materials used for SCs (Table S1). Furthermore, the cycling stability with a high


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retention of 92.6% after 5000 cycles is shown in Fig. 8d. More importantly, the CV curve of MnO2-Mo2C NFs film electrode after cycling shows almost no change compared with the initiative curve (Inset in Fig. 8d). It is noted that the Mo2C NFs play an important role in achieving the excellent electrochemical stability and reversibility of hybrid film electrode. High conductive Mo2C NFs as the substrate not only provide efficient pathways for rapid transfer of electron charge, but also maintain structural integrity of the electrode by buffering the volume change of MnO2 nanosheets. Accordingly, the comparable capacitance and strong durability are highly desirable for the practical application of high-performance SCs.

Figure 8. (a) CV curves at various scan rates (5~100 mV/s) and (b) GCD curves at different current densities (1~8 A/g) of MnO2-Mo2C NFs film electrode in 1 M Na2SO4 aqueous


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electrolyte. (c) The value of specific capacitance as a function of corresponding current density. Inset of c is the specific capacitance at relatively low current densities. (d) Cycle life at current density of 1 A/g during 5000 cycles. Inset of d is the CV curves at scan rate of 50 mV/s before and after cycles.

Figure 9. (a) Structure of the cation and anion of ionic liquid used in this study. (b) CV curves at various scan rates (2~50 mV/s). (c) GCD curves at different current densities (1~8 A/g). (d) Nyquist plot (Inset is the high frequency range) of MnO2-Mo2C NFs film electrode in 2 M EMIMBF4/AN organic electrolyte.


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To extend the working voltage window, the electrochemical performance of self-supported MnO2-Mo2C NFs film electrode in 2 M EMIMBF4/AN organic electrolyte was systematically investigated. The MnO2-Mo2C NFs film electrode represents good electrochemical behaviors especially in allowing to operate at a wide working voltage region of 0~2.5 V, which delivers a specific capacitance of 106 F/g at a current density of 1 A/g (Fig. 9). Since the energy density of SCs is proportional to the square of their operating voltage, the wide voltage range of MnO2Mo2C NFs film in organic electrolyte can achieve high energy density and broad the applications of SCs.4,

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Furthermore, acceptable rate and cycle performances of self-supported MnO2-

Mo2C NFs film electrode in organic (ionic liquid) electrolyte are shown in Fig. S10. More information about the electrochemical behaviors of MnO2-Mo2C NFs film for two electrode SCs are provided in Fig. S11. The excellent electrochemical performance of SCs based on MnO2-Mo2C NFs film electrode is benefited to the nano-architecture composed by the 1D Mo2C NFs with high conductivity and uniformly assembled ultrathin MnO2 nanosheets, which can be summarized to the following aspects: (1) Ultrathin MnO2 nanosheets (~4 nm thickness) wrapped on each of porous Mo2C NFs possess large surface area, which offer numerous accommodations for the electron charge steadily stored. Meanwhile, lamellar structure of MnO2 can greatly shorten the migrated distance of electrolyte ions and electron charges between electrode and electrolyte. Both of them assure the large reversible specific capacitance of SCs. (2) Mo2C NFs with ultrainhigh electronic conductivity (18.67 S/cm) located in the inner layer of MnO2-Mo2C NFs, which act like “highway” to provide efficient pathways for rapid transfer of electron charge especially at high


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Figure 10 Schematic diagram of ionic diffusion and charge transport in MnO2-Mo2C NFs film during the process of charging and discharging.

charge/discharge current densities. It results in the favorable power density and rate performance of SCs. (3) Mo2C NFs serve as numerous active units to couple with MnO2 nanosheets, which effectively buffer the strain from volume change and inhibit the fusion of MnO2 nanosheets, rendering the stability and integrity of film electrode during the cycling process. (4) Cross-linked network architecture constructed by MnO2-Mo2C NFs not only ensure the hybrid film with good mechanical stability to meet the need of flexible or wearable electrode, but also provide abundant and continuous ion/charge carrier transport. A proposed electrochemical process of MnO2-Mo2C NFs film electrode is schematically illustrated in Fig. 10. Large number of electrolyte ions (blue transparent balls) not only rapidly diffuse into the redox active sites to involve in pseudocapacitance reaction with ultrathin MnO2 nanosheets (purple parts), but also enable to migrate into the interspace between the adjacent Mo2C nanoparticles (black circles) to generate


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electric double layer capacitance. At the same time, electric charge can rapidly transport (red arrows) not only through the efficient pathways of highly conductive Mo2C NFs within the MnO2-Mo2C NFs, but also across the continuous channels constructed by inter-connected MnO2Mo2C NFs. Therefore, we present this hybrid film with hierarchical nanostructures obtained by direct electro-deposition of electro-active MnO2 nanosheets on highly conductive Mo2C NFs, which exhibits great promise as superior binder-free electrode for new generation SCs. Further investigation on flexible energy-storage device capable of governing high energy density is continuing in our following works.

CONCLUSIONS In this work, flexible Mo2C film with promising 1D nanofibers structure was successfully prepared by a straightforward and large-scale electrospinnig strategy for the first time. The synthesis strategy is facile and effective, which is expected to be applied to the preparation of other transition metal carbides (TMCs) with intriguing nanofiber architectures for various applications. Subsequently, a self-standing MnO2-Mo2C hybrid film was obtained by the growth of MnO2 nanosheets on Mo2C NFs via the electro-deposition method, in which the unltrathin MnO2 nanosheets are uniformly wrapped on highly conductive Mo2C nanofibers to form crosslinked network morphology. This unique hybrid architecture not only ensures the desirable mechanical stability and conductivity of the hybrid film, but also offers numerous channels to rapid ionic diffusion and charge transport. Benefiting from these intriguing features, the selfsupported MnO2-Mo2C NFs film is directly applied as additive-free electrode for SCs, which exhibits high specific capacitance, favorable rate capability and cycling stability. This work provides a new insight to fabricating superior additive-free electrode material for next generation


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of wearable energy-storage devices. It is believe that the present synthesis strategy for the MnO2Mo2C hybrid would contribute to develop a series of TMCs hybrid materials with promising architectures and desirable energy storage performances.

METHODS Material preparation. Mo2C NFs film was prepared by a simple and mature electrospinning method following heat-treatment. In a typical synthesis, 1g (NH4)2Mo6O24 powder (AHM) was added into 10 g of the 10 wt% polyvinyl alcohol (PVA) solution to obtain the electrospinning precursor. A constant flow rate of 0.5 mL/h, a 15 kV high voltage with a collecting distance 15 cm were adopted for electrospinning. The as-prepared electrospun film was peeled off from the collector directly to the following heat-treatment. It was first stabilized at 350 oC for 1 h and then carbonized and reduced at 800 oC for 2 h in Ar/H2 (95:5 v/v) atmosphere with a ramp rate of 5 o

C/min. A self-supported film of carbon-coated Mo2C NFs in black color was obtained as a result.

MnO2-Mo2C hybrid film was fabricated via a facile electro-deposition technique. MnO2 nanosheets were electrochemically deposited on self-supported Mo2C films from an mixed solution of 1 M MnSO4, 1 M Na2SO4, and 1 M NaC2H3O2 at a constant potential of 0.8 V for 5 min. Mo2C NFs film (1 cm × 2 cm) was directly used as the working electrode benefiting from its excellent conductivity. A platinum foil and an Ag/AgCl electrode were set as the counter and the reference electrodes, respectively. After electro-deposition, the MnO2-Mo2C NFs film was finally washed with Milli-Q water and ethanol to remove residual solvents, and dried at 60 °C in a vacuum oven. The total mass of resultant MnO2-Mo2C NFs film was about 0.84 mg.


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Characterization. Field emission scanning electron microscope (FESEM, FEI Sirion 200) and transmission electron microscopy (TEM, JEM-2010F) were carried out to characterize the morphology of the samples. X-ray diffraction (XRD) patterns were characterized on a powder XRD system with Cu Ka radiation and X-ray photoelectron spectroscopic (XPS) measurements were performed on a Kratos AXIS Ultra DLD spectrometer with Al Ka X-ray source. Thermogravimetric analyses (TGA) were run on a SDT Q600 analyzer at a heating rate of 10 o

C/min from 50 to 800 °C in air. Nitrogen absorption and desorption measurements were

performed with an Autosorb IQ instrument at 77 K. The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distributions were determined from the adsorption branch of the isotherms based on the density functional theory (DFT). Electrical conductivity of as-prepared films was measured by the standard 4-point probe technique (Loresta-GP, Mitsubishi Chemical). The weight of films was obtained using electronic autobalance XS105 DualRange with high precision to 0.01 mg. Electrochemical Measurements. All the electrochemical measurements were carried out using a VMP3 multi-functional electrochemical analysis instrument (Bio-Logic, France). The selfsupported MnO2-Mo2C NFs film (about 1 cm × 2 cm) was directly used as the working electrode, a platinum foil and an Ag/AgCl electrode were employed as the counter and reference electrodes. The electrochemical characterization was performed in 1 M Na2SO4 aqueous electrolyte and 2 M 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid and acetonitrile (EMIMBF4/AN) organic electrolyte by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemistry impedance (EIS) methods. The CV and GCD tests were measured at various scan rates and current densities. The EIS measurements were carried out in the frequency range from 0.05 Hz to 100 kHz with 5 mV ac amplitude. The specific capacitance was


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calculated from the galvanostatic discharge curve according to the equation of C=(I·∆t)/(m·∆V), where I, ∆t, m, and ∆V are discharge current, discharge time, film electrode weight, and voltage variation during the discharge process after IR drop, respectively. For comparison, a similar process was applied in the measurement of pure Mo2C NFs film. ASSOCIATED CONTENT Supporting Information: Figures giving (1) XRD patterns of the sameples at different temperatures; (2) Magnified FESEM image and TEM image of PNFs.; (3) TGA curves (under air flow) of Mo2C NFs and MnO2-Mo2C NFs films; (4) Nitrogen adsorption/desorption isotherms and corresponding pore size distribution curves of Mo2C NFs and MnO2-Mo2C NFs films; (5) Magnified TEM image of MnO2 nanosheets on MnO2-Mo2C NFs film; (6) EDS spectra of Mo2C NFs and MnO2-Mo2C NFs films; (7) the specific capacitances of Mo2C NFs film electrode at various current densities in 1 M Na2SO4 electrolyte; (8) CV curves and GCD curves of MnO2-Mo2C NFs film electrode prepared at different deposition times measured in 1 M Na2SO4 aqueous electrolyte; (9) GCD curves of MnO2-Mo2C NFs film electrode in 1 M Na2SO4 aqueous electrolyte at relatively low current densities; (10) Cycle performance of MnO2Mo2C NFs film electrode in 2 M EMIMBF4/AN organic electrolyte at the current density of 1 A/g during 5000 cycles; (11) two electrode systematic characterization of MnO2-Mo2C NFs film and graphene film. Table giving (1) Representative specific capacitance and cycle stability ever reported for state-of-the-art MnO2-based film electrode in SCs for comparison with our results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*Email (L. Gao): [email protected]. *Email (L. P. Zhao): [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors greatly thank Cheng Yang, Xunbao Guan, Zhuang Sun and Dr. Shuhua Yang for the valuable discussions. This work was supported by National Natural Science Foundation of China (51302169, 51172142), and the Third Phase of 211 Project for Advanced Materials Science (WS3116205007), Shanghai Jiao Tong University Cooperation Grant of Medicine, Science, Engineering (YG2015MS04).

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Table of Contents Graphic and Synopsis

A new class of flexible self-supported film has been developed through wrapping lamellar MnO2 nanosheets on Mo2C nanofibers with high conductivity. Remarkably, the hybrid film as additivefree electrode delivers large specific capacitance, comparable rate capacity and strong durability in aqueous/organic electrolytes, which are highly desirable for the high-performance flexible supercapacitors.


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Highly Conductive Mo2C Nanofibers Encapsulated in Ultrathin MnO2

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