C nanofibers as an efficient electrode material for high-performance supercapacitors

Journal of Power Sources 451 (2020) 227802

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Electrospun Fe2MoC/C nanofibers as an efficient electrode material for high-performance supercapacitors Xuxia Hao a, b, Jianqiang Bi a, b, *, Weili Wang a, b, **, Weikang Yan a, b, Xicheng Gao a, b, Xiaoning Sun a, b, Rui Liu c a b c

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan, 250061, PR China Suzhou Institute of Shandong University, Suzhou, 215123, PR China State Key Lab of Nonferrous Metals & Processes, General Research Institute for Nonferrous Metals, Beijing, 10088, PR China

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Fe2MoC nanofibers can be prepared via a facile electrospinning method. � Fe2MoC/CNFs-5 exhibits a superior rate capability and cycle stability. � ASC coin-cell device shows a promising energy and power density.

A R T I C L E I N F O

A B S T R A C T

Keywords: Fe2MoC/CNFs Electrospinning Heating rate Supercapacitor

Bimetallic carbides have aroused wide attention for energy-storage applications recently. In this work, onedimensional Fe2MoC/CNFs (Fe2MoC/C nanofibers) are successfully synthesized via a facile electrospinning method for the first time. To obtain the most integrated structure between the Fe2MoC nanoparticles and carbon nanofibers, we explore the optimal heating rate during the carbonization treatment. Fe2MoC/CNFs exhibits an integrated one-dimensional structure under 800 � C with a heating rate of 5 � C/min. As revealed in the experi­ mental results, Fe2MoC/CNFs possesses a high specific surface area of 196.9 m2/g, a high specific capacitance of 347.8 F/g at the current density of 1 A/g, an excellent rate capability of 91% capacitance retention from 1 A/g to 40 A/g, and shows superior cycling stability with the capacitance retention of about 85.6% and Coulombic ef­ ficiency of about 100% after 5000 cycles. An asymmetric supercapacitor coin-cell device using Fe2MoC/CNFs as the positive electrode displays an energy density of 14.5 Wh/kg at a power density of 300 W/kg and an outstanding cycling life of 93% retention after 5000 cycles. The impressive electrochemical performance in­ dicates that the Fe2MoC/CNFs composite is a promising material for efficient supercapacitors.

* Corresponding author. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan, 250061, PR China. ** Corresponding author. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan, 250061, PR China. E-mail addresses: [email protected] (J. Bi), [email protected] (W. Wang). https://doi.org/10.1016/j.jpowsour.2020.227802 Received 11 April 2019; Received in revised form 5 July 2019; Accepted 24 January 2020 Available online 31 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.

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1. Introduction

Table 1 Comparison of the morphology, Specific surface area (SSA), electrolyte, Ca­ pacitances (Cs) and cycle stability of some reported transition-metal carbides as electrode materials and the present work.

Currently, supercapacitors (SCs) is deemed as one of the most promising energy storage devices due to their rapid charge-discharge rate, high energy density, low cost, long cycle life, safety and reli­ ability [1–3]. As the key part of supercapacitors, active electrode ma­ terial is critical for high performance [4]. To date, conductive polymers [5–7], carbon-based materials [8–10], transition metal compounds [11–13] have received extensive attention as electrode materials. However, exploring novel electrode materials is still necessary to enhance the properties of SCs. In recent years, transition-metal carbides (TMCs) have attracted great interest as electrode materials for their intrinsic high electrical conductivity, chemical stability, and rich redox reactions for energy storage applications [14,15]. Among them, single-phase carbides, such as Mo2C [16], TiC [17], TaC [18] and NbC [19] have been proved to possess great electrochemical performance in SCs and electrocatalysis (Shown in Table .1). However, the electrochemical stability of single-phase carbides is not desirable under electrochemical conditions [14,20]. One effective method to solve the issue is to induce a second metal element to form bimetallic carbides, which can tune the electronic properties of the single-phase carbides [21]. Recently, many bimetallic carbides, including Co6Mo6C2 [22], Fe3Mo3C [23], Co3W3C [24] and Fe2MoC [25,26] have been reported in the electrochemical field. In particular, Fe2MoC is used in both elec­ trocatalysis and SCs [16,26,27]. Nevertheless, the electrochemical per­ formance of Fe2MoC as electrode material still requires significant improvement. The synthesis of bimetallic carbides remains very challenging due to the tendency to form two separated single-phase carbides under the high-temperature processing [28,29]. For the compound of carbides, the high-temperature treatment is inevitable, so the method to obtain appropriate precursor is critical. In previous reports, Yan et al. used ion-exchange method to synthesize precursor of Fe2MoC, and obtained Fe2MoC at 1300 � C [25,27]. Hao et al. synthesized precursor of Fe2MoC by hydrothermal method, followed by heat treatment at 900 � C [26]. Generally, the morphology of the electrode materials and their intrinsic characteristics are equally important for the performance of SCs [30–34]. Nowadays, one-dimensional (1D) nanomaterials (nanofibers, etc.) have drawn extensive interest for their special properties. During electrochemical processes, some 1D nanostructures can improve the ion collection, shorten the transport path of ions, and provide abundant active sites and high electrical conductivity along the axial orientation [35,36]. Electrospinning is a cost-efficient, facile and universal method to produce nanofibers [37–40]. Herein, electrospinning followed by stabilization and carbonization is used to synthesize the Fe2MoC/CNFs composites in an in-situ manner. The heating temperature and ramp rate of heat treatment is critical to the synthesis and morphology of Fe2MoC. Thus, we have explored the influence of both factors on the final products. More importantly, evaluated as electrode materials of supercapacitors, the novel 1D Fe2MoC/CNFs composites demonstrate excellent electrochemical per­ formance. In addition, this report provides a viable synthesis route of polymetallic carbides.

Material

Morphology

SSA

Electrolyte

Cs

Cycle life

Ref.

Mo2C/ CNF TiC/C

nanofibers

–

16

116.5 m2/g

Core/shell NSs

–

6 M KOH

NbC/C

nanofibers

1370 m2/g

1 M TEABF4

93.0% (25000 cycles) 94% (5000 cycles) 91% (10000 cycles)

17

TaC/C

Fe2MoC/ C

nanoparticles

133.7 m2/g

1 M KOH

nanofibers

196.9 m2/g

1 M KOH

83.9% (1000 cycles) 85.6% (5000 cycles)

26

Fe2MoC/ CNFs

74 F/g (1A/g) 130 F/ g (0.1 A/g) 223 F/ g (1 A/ g) 101 F/ g (0.02 A/g) 97.7 F/g (1 A/g) 347.8 F/g (1 A/g)

–

nanofibers

1M Na2SO4 6 M KOH

18 19

This work

2.2. Preparation of Fe2MoC/CNFs composite The Fe2MoC/CNFs was prepared through electrospinning method followed by the stabilization and carbonization. The preparation route was illustrated in Scheme 1 and detailed as follows. Typically, PVP (1.5 g) was dissolved in DMF (10 mL) and HAc (2 mL) to obtain a transparent solution. FeCl.24H2O (0.79 g) was added to the above solution with stirring for 3 h. Then, 1.86 g (NH4)6Mo7O24�4H2O aqueous solution (19.4 wt%) was dropped in it slowly, and stirred for 12 h to obtain the electrospinning precursor. The electrospinning was carried out under the voltage of 15 kV and the flow rate of 0.5 mL/h. The nanofibers were collected by a rotating drum coated aluminum foil with the rotation speed of 30 rpm. After electrospinning, the fibers were stabilized at 250 � C for 2 h under air with a rate of 1 � C/min. Then, the fibers were carbonized at 800 � C for 2 h under Ar. To explore the effects of the temperature and heating rate of high treatment, we synthesized different samples under the heat treatment of 600 � C, 700 � C and 800 � C, and the heating rate of 2 � C/min, 5 � C/min and 8 � C/min, respectively. What’s more, the heating rates at different carbonization temperature are 2 � C/min. The samples are named as Fe2MoC/CNFs-A (A represents the heating rate during the carbonation). For example, Fe2MoC/CNFs-5 represents the sample was synthesized with the heating rate of 5 � C/min. 2.3. Characterization X-ray diffraction (XRD) patterns of the products were studied on a Rigaku D/Max-Rb diffractometer with Ni-filtered Cu Kα radiation (λ ¼ 1.5406 Å). The Raman spectra were carried out on Invia Renishaw using a 633 nm laser. Thermogravimetric (TG) curve of the Fe2MoC/CNFs were determined on a TG analyzer (STA449F5, Netzsch) with a heating rate of 10 � C/min under air. The chemical states of Fe2MoC/CNFs were investigated by X-ray photoelectron spectroscopy (XPS, AXIS Supra, Shimadzu, Japan). The morphologies of Fe2MoC/CNFs were examined by a field-emission scanning electron microscopy (FESEM, JSM-7800F, JEOL, Japan), transmission electron microscope (TEM) and highresolution transmission electron microscope (HRTEM, JEM-2100F, JEOL, Japan). The specific surface areas were analyzed by nitrogen adsorption-desorption isotherms (BET, JW-BK300, JWGB Sci. & Tech. Co., China).

2. Experimental section 2.1. Materials and reagents Iron chloride tetrahydrate (FeCl2⋅4H2O) and Ammonium molybdate ((NH4)6Mo7O24⋅4H2O) are provided by Damao Chemical Reagent Fac­ tory. Polyvinylpyrrolidone (PVP, average Mw 130000) is purchased form Aladdin Industrial Corporation. N, N-dimethylformamidel (DMF) and Acetic acid (HAc) are bought from Sinopharm Chemical Reagent Co., Ltd.

2.4. Electrochemical measurements The working electrodes were prepared by mixing 80 wt% active 2

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Scheme 1. Schematic illustration for fabricating Fe2MoC/CNFs by electrospinning and asymmetric supercapacitor coin-cell device.

materials, 10 wt% polytetrafluoroethylene and 10 wt% acetylene black together in ethyl alcohol. The obtained slurry was loaded on the Ni foam (1 cm � 1 cm) and dried at 80 � C overnight in vacuum. The loading mass of electroactive materials in the working electrode was about 1–2 mg cm 2. In the three-electrode system, Pt and saturated calomel electrode were used as counter and reference electrode, respectively. An asym­ metric supercapacitor system (ASC) adopting a CR/LIR 2032 coin-type cell was fabricated using Fe2MoC/CNFs as positive electrode and acti­ vated carbon (AC) as negative electrode. The electrochemical mea­ surements including cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out on CHI660E electrochemical station (Shanghai CH Instruments Co., China). The long-term cycling measurements were carried out on a Land CT2001A battery test system. All the electro­ chemical measurements used 1 M KOH aqueous solution as electrolyte. The specific capacitance was calculated from galvanostatic charging and discharge tests as follows: Z t Idt ΔQ I⋅Δt Cm ¼ ¼ ¼ 0 (1) ΔV m⋅ΔV m⋅ΔV where Cm, ΔQ, I, Δt, m and ΔV represent the mass specific capacitance (F/g), quantity of electric charge (C), the discharge current (A), the discharge time (s), the mass of the active material (g) and voltage drop on discharge (V), respectively. In ASC coin-cell, the mass of the positive electrode and negative electrode were determined according to the Eq. (2). mþ = m ¼ ðC ΔU Þ=ðCþ ΔUþ Þ

(2)

The m, C and ΔU represent the mass of the active material (g), spe­ cific capacitance (F/g) and potential window (V), respectively, in addition, the “þ” and “-” refer to the positive and negative. And the loading contents of the Fe2MoC and active carbon are about 1.6 mg and 2.7 mg, respectively. The energy density and power density were calculated on the basis of Eqs. (3) and (4). 1 E ¼ CðΔUÞ2 2 P¼

E Δt

(3) (4)

where E, C, ΔU, P and Δt are the energy density (Wh/kg), mass specific capacitance (F/g), the potential window (V), power density (W/kg) and the discharge time (s), respectively. Fig. 1. (a) XRD patterns of the Fe2MoC/CNFs composites with different heating rate, (b) TG curve of the Fe2MoC/CNFs-5 and XRD patterns of the derivedproduct, (c) Raman spectra of the Fe2MoC/CNFs with different heating rate.

3. Results and discussion The XRD patterns of the Fe2MoC/CNFs obtained with different heating rates are shown in Fig. 1(a). The peaks of all three samples agree 3

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Fig. 2. XPS spectra of Fe2MoC. (a) Wide-scan XPS spectra, (b) Fe 2p, (c) Mo 3d, (d) C 1s.

well with the characteristic peaks of Fe2MoC (JCPDS 17–0911), indi­ cating that the electrospinning under the heat treatment at 800 � C is an effective method to synthesize the bimetallic carbide Fe2MoC. It can be clearly proved by Fig. S1 that Fe2MoC couldn’t be obtained at 600 and 700 � C. In addition, with the increase of the heating rate, the crystal­ linity of Fe2MoC is enhanced. Moreover, the broad peaks located at 2θ ¼ 26� are assigned to the (002) peak of graphitic carbon (ICDD No. 65–6212) [41,42]. TG curve of the Fe2MoC/CNFs-5 is shown in Fig. 1(b). The mass decrease under 250 � C is ascribed to the evaporation of solvent on the surface [43]. With further increase of the temperature, the mass decrease is attributed to the oxidation of carbon fibers and Fe2MoC. After TG analysis, the derived products from Fe2MoC/CNFs are identi­ fied as Fe2(MoO4)3 and Fe2O3 by XRD (insertion in Fig. 1(b)). The car­ bon content was calculated based on Eq. (5), and the carbon content is about 47 wt%. The carbon content of other two samples (Fig. S1) are 23.6 wt% and nearly 48.2 wt%, respectively, demonstrating that the fast heating rate is beneficial to the synthesis of carbon fibers and inhibiting for that of Fe2MoC. 3Fe2MoC þ 12O2→Fe2ðMoO4Þ3 þ 2Fe2O3 þ 3CO2

spectrum confirm the presence of Fe, Mo, C, O and N elements. As shown in Fig. 2 (b), in the high-resolution spectrum of Fe 2p, the Fe 2p3/2 and 2p1/2 peaks associated with iron carbides [45] can be deconvoluted to two pairs of peaks including Fe2þ (710.7 and 724.1 eV), Fe3þ(712.2 and 726.3 eV) and a satellite peak at around 719.3 eV [46]. The Mo 3d XPS spectrum in Fig. 2(c) is deconvoluted to four peaks, corresponding to Mo2þ(228.7 eV), Mo4þ(232.3 and 233.1 eV) and Mo6þ(236.2 eV), respectively. The peak of Mo2þ at 228.2 eV is attributed to Mo 3d5/2 in molybdenum carbides [47,48]. In addition, the high-oxidation-state Mo4þ and Mo6þ are ascribed to the surface oxidation of Mo species [49]. Fig. 2(d) shows the deconvolution of the C 1s peak. The peak at 284.0 eV is ascribed to the sp2 C–C bonding from iron carbides and molybdenum carbides [50,51]. The peaks at around 285 eV and 286.2 – C, respectively. The eV are attributed to the sp3 C–C bonding and C– abovementioned analysis verifies the successful synthesis of Fe2MoC/CNFs composite via the electrospinning method. The morphologies of the Fe2MoC/C nanofibers with different heating rates are shown in Fig. 3. Typically, the Fe–Mo/PVP precursor fibers are highly uniform with a diameter of ~ 1 μm, and the surface is smooth, as shown in Fig. 3(a). The precursor fibers have obvious shrinkage after pre-oxidation, and the diameters are about 600 nm (Fig. 3(b)). After carbonization in Ar, the SEM images of three samples with different heating rates are shown in Fig. 3(c)–(f). Similarly, Fe2MoC/CNFs-2, Fe2MoC/CNFs-5 and Fe2MoC/CNFs-8 all display 1D nanostructure, and there are Fe2MoC nanoparticles formed within the fibers. Moreover, decrease of the heating rate causes remarkable reduction of the diameter of carbon fibers. In general, overly thin fibers are not ideal for sup­ porting the Fe2MoC nanoparticles or forming integrated fibers. Thus, the heating rate of 2 � C/min, which causes the short and fine fibers of Fe2MoC/CNFs-2 as shown in Fig. 3(c), is not suitable for our purpose. Under the high heating rate of 8 � C/min (Fig. 3(f)), the content of Fe2MoC nanoparticles decreases remarkably and their distributions are not uniform enough. Under the heating rate of 5 � C/min shown in Fig. 3 (d) and (e), abundant Fe2MoC nanoparticles and integrated carbon nanofibers are obtained. Thus, a suitable heating rate of 5 � C/min is

(5)

The Raman spectra of the Fe2MoC/CNFs with different heating rates are shown in Fig. 1(c). The characteristic D and G bands of carbon materials centered at around 1330 cm 1 and 1590 cm 1 demonstrate the presence of carbon species. To determine the graphitization degree of the carbon species, the ratios of ID/IG are compared in Fig. 1(c). The value of ID/IG is the lowest when the rate is 5 � C/min, suggesting high graphitization degree and a low level of carbon defects [44]. According to the electrochemical results which will be mentioned later, the high electrochemical activity of Fe2MoC/CNFs comes from graphitic carbon rather than the defective sites of the carbon. Thus, optimal heating rate of 5 � C/min is critical to the electrochemical properties of Fe2MoC/CNFs. X-ray photon spectroscopy (XPS) is used to investigate the chemical states of the elements in Fe2MoC/CNFs-5. The peaks appeared in the XPS 4

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Journal of Power Sources 451 (2020) 227802

Fig. 4. TEM images of Fe2MoC/CNFs-5 (a) (b), HRTEM image of Fe2MoC/ CNFs-5 (c).

significantly important for the formation of integrated Fe2MoC/CNFs. To look further into the morphologies of Fe2MoC/CNFs-5, the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images are shown in Fig. 4. The low magnification TEM (Fig. 4 (a)) reveals a single nanofiber with a diameter around 500 nm, in which Fe2MoC nanoparticles are evenly dispersed within the carbon fiber (Fig. 4(b)). Such structural features have several advantages for the Fe2MoC/CNFs composite: firstly, the well-dispersion of Fe2MoC nano­ particles in the carbon fibers can reduce aggregation and increase the specific area; secondly, the carbon fibers can provide the efficient electron transportation along the axis direction; last but not least, the carbon fibers can accommodate the volume change of Fe2MoC during electrochemical processes and thus improve the stability of Fe2MoC/

Fig. 3. SEM images of the Fe–Mo/PVP precursor (a) and the product of stabi­ lization (b), SEM images of Fe2MoC/CNFs-2 (c), Fe2MoC/CNFs-5 (d) (e), Fe2MoC/CNFs-8 (f).

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specific capacitances of the three samples are 278.0 F/g, 347.8 F/g and 290.2 F/g for Fe2MoC/CNFs-2, Fe2MoC/CNFs-5 and Fe2MoC/CNFs-10, respectively, at the current density of 1 A/g calculated by the chargedischarge curves as shown in Fig. 6 (b). The results indicate that en­ ergy storage capability of the Fe2MoC/CNFs is further improved from good structural design. 1D nanofibers possess large specific surface area, which not only create large contact area between electrode material and electrolyte, but also provide more electron transport channels and shorten the transport path of ions. Fig. 6(c) shows the CV curves of the Fe2MoC/CNFs-5 electrode at different scan rates from 5 to 100 mV/s. With the increase of scan rate, the shapes of the curves show no remarkable change, suggesting the excellent reversible Faradaic reaction and good rate capability of the Fe2MoC/CNFs for energy storage [56,57]. At the same time, the redox peaks of the CV curves shift slightly, which may be caused by the po­ larization due to the increase of the scan rate [58]. The charge-discharge curves of the Fe2MoC/CNFs-2 at different current densities from 1 A/g to 40 A/g are shown in Fig. 6(d). An obvious voltage plateau of the curves also proves the typical pseudo­ capacitive characteristics of the electrode material. The specific capac­ itance of Fe2MoC/CNFs-5 decreases from 347.8 F/g to 315.08 F/g, with the current density increases from 1 A/g to 40 A/g. The values of the specific capacitance at the current density from 1 A/g to 40 A/g for Fe2MoC/CNFs-2, Fe2MoC/CNFs-5, Fe2MoC/CNFs-8 are presented in Fig. 6(e). All the samples possess the proper specific capacitances and capacitance retention even up to 40 A/g. In particular, Fe2MoC/CNFs-5 always maintains the maximum specific capacitance at different current densities and possesses the largest capacitance retention of 91% up to 40 A/g. The results verify the excellent rate capability of Fe2MoC/CNFs composites and the optimized heating rate of 5 � C/min. The CV and GCD curves of Fe2MoC/CNFs-2 and Fe2MoC/CNFs-8 are shown in Fig. S3. Nyquist plots of the Fe2MoC/CNFs composites with different heating rates are depicted in Fig. 6(f). The slopes of the linear sections of the three samples in the low frequency refer to the diffusion resistance (Warburg impedance Rw). The largest slope of the Fe2MoC/CNFs-5 il­ lustrates the more efficient diffusion of electrons and electrolyte during the electrochemical process compared to that in the other two samples. The unobvious semicircles in the high frequency region as shown in Fig. 6(f) inset indicate the low charge-transfer resistance (Rct) [59], implying the good charge transfer kinetics of the Fe2MoC/CNFs elec­ trodes [60]. Moreover, the intersections on X-axis are related to the intrinsic resistance (Rs) of the active materials, including the resistance of electrolyte and the contact resistance between the electrode material and the current collector. For the same testing environment, the small intercepts of the all curves reveal low intrinsic resistances of the Fe2MoC/CNFs. The low Rct, Rw and Rs verify the high ion conductivity of the Fe2MoC/CNFs composites, especially the Fe2MoC/CNFs-5 elec­ trode [61]. To have an insight into the reaction kinetics, we investigated the redox peaks of CV curves. The anodic current (i) and scan rate (v) obeys a power-law relationship [62,63]:

Fig. 5. N2 sorption isotherms (a) and pore size distributions (b) of the Fe2MoC/ CNFs with different heating rate.

CNFs. From the HRTEM image (Fig. 4(c)), the lattice fringes of 0.228 nm are associated with the (340) plane of Fe2MoC, and the lattice fringes of 0.338 nm are ascribed to the (002) facets of graphite. The N2 sorption isotherms curves and pore distributions of the Fe2MoC/CNFs are shown in Fig. 5. All the isotherms with typical hys­ teresis loops can be classified as type IV, which indicate the mesoporous structure of the Fe2MoC/CNFs [52,53]. The Fe2MoC/CNFs-5 exhibits the largest surface area of 196.9 m2/g, higher than those of Fe2MoC/CNFs-2 (87.1 m2/g) and Fe2MoC/CNFs-8 (126.1 m2/g). The pore size distribu­ tions of all samples reveal the abundance of mesopores around 4–5 nm, which favor the permeation of electrolyte. The SSA of Fe2MoC/CNFs-5 is competitive to some well-known metallic carbides as shown in Table 1. The electrochemical performance is evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectra (EIS) and long-term cycling measurement in a three-electrode system. Fig. 6 (a) shows the CV curves of the Fe2MoC/CNFs compos­ ites with different heating rates ranging from 0 to 0.6 V. It can be found that there are a couple of redox peaks, indicating the electrochemical reaction is responsible for the pesudocapacitance behavior [54]. And the faradaic redox reaction mechanism of the Fe2MoC/CNFs with the 1 M KOH electrolyte can be described by the following reaction [26,55]: þ

Fe2 MoC þ xK þ xe →Kx Fe2 MoC

i ¼ avb

(6)

(7)

where a is a constant, b is an adjustable value. b ¼ 0.5 indicates a semiinfinite diffusion process, and b ¼ 1 indicates a capacitive process. Where b can be determined by the slope of the diagram of log i and log v, shown in Fig. S3(e). The b values of Fe2MoC/CNFs-2, Fe2MoC/CNFs-5, Fe2MoC/CNFs-8 are all between 0.5 and 1, manifesting both mecha­ nisms existing during the redox process. To determine the contribution of the capacitive effect, we calculated the values of k1 (capacitive pro­ cess) and k2 (diffusion process) in the Eq. (8) by plotting v1/2 vs i/v1/2 shown in Fig. S3(f) [64]:

which is due to the conversation reaction between Kþ and KxFe2MoC, as shown in Scheme 1. During the charge process, the Kþ inserts into Fe2MoC nanoparticles and reacts with e to form KxFe2MoC, and the KxFe2MoC releases the Kþ and e to the electrolyte during the discharge. The redox peaks in CV curves are attributed to the conversation reaction above. The small dimensions of Fe2MoC nanoparticles are beneficial for Kþ diffusion, while the continuous carbon fibers facilitate the charge transfer and avoid agglomeration of nanoparticles. Generally, the capacitance of the electrode materials can be esti­ mated by the encircled area of the CV curves. It is obvious that the capacitance of Fe2MoC/CNFs-5 is largest among all samples. The

i ¼ k1 v þ k2 v1=2

(8)

As the results showing in Fig. 6(g), the contributions of the capacitive 6

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Fig. 6. Electrochemical performance in three-system. (a) Comparison of CV curves of Fe2MoC/CNFs with different heating rate, (b) Comparison of GCD curves of Fe2MoC/CNFs with different heating rate, (c) CV curves and (d) GCD curves of Fe2MoC/CNFs-5, (e) Specific capacitance retention at various current densities, (f) EIS spectra, (g) Capacitive contribution ratio of Fe2MoC/CNFs with different heating rate, (h) Cycling stability and the coulombic efficiency up to 5000 cycles.

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effect about Fe2MoC/CNFs-2, Fe2MoC/CNFs-5 and Fe2MoC/CNFs-8 are 81.0%, 93.6% and 74.5%, respectively, suggesting a capacitive behavior of the Fe2MoC/CNFs composites. The specific capacitances of Fe2MoC/ CNFs composites are larger than the typical double-layer electrode materials, indicating the capacitive effect is dominated by the pseudo­ capacitive effect [65]. Furthermore, the capacitive contribution of Fe2MoC/CNFs-5 is the largest than the other two samples, manifesting the favorable ratios between Fe2MoC and carbon fibers can realize the efficient reaction kinetics, which benefits from the 1D structure, nano­ sized particles of Fe2MoC and large specific surface area of Fe2MoC/CNFs composites. The long-term cyclic performance and the coulombic efficiency of the three electrodes at 4 A/g are illustrated in Fig. 6(h). After 5000 cycles, the capacitance retentions of the three samples are 82.0%, 85.6% and 76.2%, respectively, indicating the excellent cycling stabilities. 1D nanofiber structure accommodates the volume change of the Fe2MoC/ CNFs composite during the electrochemical procedure, which, in com­ bination with the chemical stability of the Fe2MoC, results in the supe­ rior cycling stability of the Fe2MoC/CNFs composite. What’s more, the Coulombic efficiency of Fe2MoC/CNFs-5 maintains about 100% after 5000 cycles, revealing the superior reversibility of the electrode material [66,67]. As shown in Table 1, the performance of Fe2MoC/CNFs-5 is comparable or superior to those of well-known metallic carbides. To furthur explore the practicality of the Fe2MoC/CNFs, an asym­ metric supercapacitor (ASC) coin-cell device was assembled, in which the Fe2MoC/CNFs-5 was used as positive electrode and active carbon (AC) as negative electrode (Scheme 1). The electrochemical tests including CV, GCD and long-term cycling measurement are all con­ ducted with the ASC coin-cell device. Fig. 7(a) depicts the CV curves of the ASC cell at various scan rates with the potential window from 0 V to 1.5 V. It is obvious that there are a couple of oxidation and reduction peaks, whose shapes remained unchanged at different scan rates, sug­ gesting a high-rate charge/discharge performance [68]. Fig. 7(b) shows the GCD curves of the ASC cell at different current densities, and the specific capacitance is 46.3 F/g at the current density of 0.4 A/g. With the current density increased from 0.4 A/g to 10 A/g, the ASC cell possesses a specific capacitance of 33.5 F/g with 72% capacitance retention (Fig. S4), revealing an excellent rate property [69]. The energy density (E) and power density (P) are the most vital factors to evaluate the electrochemical performance of the cell devices. As shown in Fig. 7 (c), the ASC cell delivers a maximum energy density of 14.5 Wh/kg at a power density of 300 W/kg, and a maximum power density of 6718.5 W/kg at a energy density of 8.4 Wh/kg. Besides, the red LED can light driven by the coin-cell device, indicating the practicability of Fe2MoC/CNFs composite. In addition, the cycle stability of the ASC cell is great as shown in Fig. 7(d), which maintains 93% of the initial capacitance after 5000 cycles, showing a greater cycling stability than the three-electrode system of 85.6%. The reason are as follows: 1. In the ASC coin-cell device, activated carbon is used as the negative electrode. As a double-layer electrode material, active carbon pos­ sesses an excellent cycle stability for nonexistence of redox reaction during electrochemical process [70]. Thus, the synergistic effects of active carbon and Fe2MoC/CNFs-5 achieve a higher cycle stability of ASC coin-cell device than three system; 2. ASC coin-cell device is a stable system, which is not easily affected by the environment factors in three-electrode system, such as fluctua­ tion of electrolyte [71]. 4. Conclusion In conclusion, the Fe2MoC/CNFs with 1D structure are successfully prepared via a facile electrospinning method. 5 � C/min has been verified as the optimal heating rate for the carbonization. Fe2MoC/CNFs-5 with the heating rate of 5 � C/min not only displays the integrated structure between the Fe2MoC nanoparticles and carbon nanofibers, but also

Fig. 7. Electrochemical performance of ASC coin-cell device. (a) CV curves, (b) GCD curves, (c) Ragone plot, (d) Cycling stability after 5000 cycles. 8

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Journal of Power Sources 451 (2020) 227802

exhibits the excellent electrochemical performances. Fe2MoC/CNFs-5 possesses a high specific surface area of 196.9 m2/g, a high specific capacitance of 347.8 F/g at the current density of 1 A/g, an excellent rate capability of 91% capacitance retention from 1 A/g to 40 A/g, and shows the superior cycling stability with the capacitance retention of about 85.6% and Coulombic efficiency of about 100% after 5000 cycles. In addition, ASC coin-cell device exhibites an energy density of 14.5 Wh/kg at a power density of 300 W/kg and an outstanding cycling life of 93% retention after 5000 cycles. The excellent electrochemical perfor­ mance proves that the Fe2MoC/CNFs composite is a promising material for supercapacitors. In addition, this work confirms that it is feasible to obtain the bimetallic carbides via electrospinning method, and provides an idea for the preparation of the polymetallic carbides.

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