Preparation of stable composite porous nanofibers carried SnOx-ZnO as a flexible supercapacitor material with excellent electrochemical and cycling performance

Journal Pre-proof Preparation of stable composite porous nanofibers carried SnOx-ZnO as a flexible supercapacitor material with excellent electrochemical and cycling performance Jing Liu, Tong Xu, Xingwei Sun, Jie Bai, Chunping Li PII:

S0925-8388(19)32879-8

DOI:

https://doi.org/10.1016/j.jallcom.2019.151652

Reference:

JALCOM 151652

To appear in:

Journal of Alloys and Compounds

Received Date: 29 April 2019 Revised Date:

29 July 2019

Accepted Date: 30 July 2019

Please cite this article as: J. Liu, T. Xu, X. Sun, J. Bai, C. Li, Preparation of stable composite porous nanofibers carried SnOx-ZnO as a flexible supercapacitor material with excellent electrochemical and cycling performance, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.151652. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Preparation of Stable Composite Porous Nanofibers Carried SnOx-ZnO as a Flexible Supercapacitor Material with Excellent Electrochemical and Cycling Performance

Jing Liu a, b, Tong Xu a, b, Xingwei Sun a, b, Jie Bai a, b, Chunping Li a, b, *

a

Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, 010051,

People’s Republic of China b

Inner Mongolia Key Laboratory of Industrial Catalysis, Hohhot, 010051, People’s Republic of

China Tel: +86471 6575722. Fax: +86471 6575722.

E-mail address:[email protected]

1

Abstract Flexible and portable supercapacitors as a variety of energy storage devices are used in wearable electronic devices. However, the practical application of flexible supercapacitors is still restricted by comparatively poor performance. Herein, we have successfully prepared porous carbon nanofibers supported Sn and Zn bimetallic composites as flexible and freestanding electrode material in supercapacitors. Sn element encapsulated in nano-fiber films were prepared by electrospinning technique, the SnOx-ZnO/MCNFs composites were formed by immersion of different proportions of Zn2+, and further high temperature carbonization process. In addition, on the basis of comparative experiments, a possible mechanism for effective synergy between the two metals were raised. The specific capacitance of SnOx-ZnO/MCNFs-10 mmol/L at 0.5 A g-1 is 783 F g-1 with the retention rate of 87% after 5000 cycles, which is higher than their respective counterparts SnOx/MCNFs (599 F g-1) and ZnO/MCNFs (429 F g-1). Moreover, a flexible all-solid-state asymmetric supercapacitor (ASC) was further assembled with the energy density of 37.7 Wh kg-1 at the power density of 374.9 W kg-1, and long-term cyclic stability (76.2% after 5000 cycles). The reason for the remarkable electrochemical performance could be the reasonable combination of the two electrode materials, which provides a new idea for the application of composite materials in supercapacitors.

Keywords: SnOx-ZnO; Porous carbon nanofibers; Flexible; Synergistic effect; All-solid-state asymmetric supercapacitors

2

1.

Introduction Supercapacitors (SCs) as a new description of energy storage device, has been widely used in

portable flexible electronic devices. The efficiency of SCs is attributed to their high power density, good stability and fast charging-discharging rate [1-4]. In recent years, SCs have been applied to flexible electronic products (such as notebook computers, photovoltaic cells, curtain displays and the like), and made lots of breakthroughs. At present, there are three kinds of materials used in SCs: carbonaceous materials, conducting polymers, and transition metal oxides/sulfides or other compounds [5-7]. Among them, carbon-based materials suffered from low energy density and limited by double layer superficial area. However, metal oxides have a high theoretical capacity, so they are considered as an ideal substitute for carbon materials. Electrode materials are considered to be the heart of supercapacitors. Therefore, the development of promising electrode materials were crucial to optimize the overall electrochemical performance of SCs. Among the pseudocapacitive materials studied before, RuO2 has been attracted much attention due to its high theoretical specific capacitance and high conductivity. However, the toxicity and high cost limit its large-scale utilized [8-9]. In consequence, the search for cost-effective and high-performance electrode materials have become the focus of research. In recent years, many materials, especially transition metals, such as cobalt (Co) [10], nickel (Ni) [11], copper (Cu) [12], vanadium (V) [13], manganese (Mn) [14] and their oxides, have been studied as possible candidate materials for SCs. However, due to the inherent poor electrical conductivity and low surface area of single metal oxides, their properties and stability were still not widely applicable constraints. Some recent studies have shown that by properly combining two metals to form a composite electrode material, the defects of single metal could be well overcome, and the performance will even surpass the precious metals such as RuO2. Therefore, the use of bimetal 3

synergies to enhance specific capacitance have attracted much attention. For instance, Li et al. reported tremelliform Co3O4@CeO2 hybrid electrodes grown on Ni foam as electrode materials showed a remarkable specific capacitance of 2260.8 F g-1 at 1 A g-1 [15]. As reported by Jiang et al., CuO embedded β-Ni(OH)2 nano-composite as advanced electrode materials with the specific capacitance of 855 F g-1 at 5 A g-1 [16]. Additionally, Dai et al. reported Co-Ni binary hydroxide nanotubes with three-dimensionally nanosheets as cathode materials for hybrid SCs with a specific capacity of 209.9 mA h g-1 at 1 A g-1 and outstanding cycling stability with 84.4% capacity retention after 10000 cycles at 20 A g-1 [17]. SnOx and ZnO are one of the SCs materials with cost-effective, the efficient combination in supercapacitors has rarely been reported as yet. SnOx, which has become a candidate material for efficient electrochemical energy storage systems due to its distinctive physical and chemical characters, such as electrical conductivity and cyclic stabilization. However, like other metal oxides, SnOx has poor conductivity [18-21]. ZnO is a transition metal oxide, is a semiconductor material with high conductivity and mechanical stability. Moreover, ZnO has made outstanding contributions to the performance and function of supercapacitors in terms of electron transport and ion diffusion. Therefore, it is a feasible scheme to overcome the shortcomings of single material by combining SnOx and ZnO reasonably. Nevertheless, both metals exist in the form of powder, which caused the large volume change of SnOx and ZnO powder in the process of circulation and insufficient cycle performance. In addition, the powder sample does not have the ability to support independently, which requires the introduction of conductive agents and binders in the application of SCs. The addition of these polymer materials inevitably hinders the weight and performance of supercapacitors [22-26]. Recent studies have shown that the combination of metal oxides and carbon nanomaterials, such as carbon nanofibers (CNF) and carbon nanotubes (CNT), not only could used as a cost-effective 4

flexible substrate, but also increase the number of active sites and promote the longitudinal transmission of electrons [27-29]. Combining the unique advantages of these different capacitor materials is a promising way to control, develop and optimize the structure and performance. Based on our previous studies [30], polyvinylpyrrolidone (PS) was introduced to increase the surface contact between active material and electrolyte, so as to realize the efficient chemical reaction. Herein, we have developed an economical method to synthesize SCs materials with high performance by simple electrospinning technology, impregnation and subsequent carbonization method, in which Sn nanoparticles were encapsulated in porous carbon nanofibers, and Zn2+ were distributed on the surface of carbon fibers (SnOx-ZnO/MCNFs). The optimum synergistic effect of Sn and Zn bimetallic oxides as electrode materials for SCs was discussed by changing the Zn2+ impregnation

amount

of

different

concentrations.

The

specific

capacitance

of

SnOx-ZnO/MCNFs-10 mmol/L at 0.5 A g-1 is 783 F g-1 with the retention rate of 87% after 5000 cycles. Moreover, SnOx-ZnO/MCNFs-10 mmol/L were assembled a flexible all-solid-state asymmetric supercapacitors (ASC) with the energy density of 37.7 Wh kg-1 at the power density of 374.9 W kg-1, and long-term cyclic stability (76.2% after 5000 cycles). These results would encourage researchers to provide new ideas for the preparation of high performance composite SCs. The detailed steps were shown in the Scheme. 1. 2.

Experimental

2.1 Materials Polyacrylonitrile (PAN, Mw = 80, 000, Kunshan Hongyu Plastics Co., Ltd.) and Polystyrene (PS, Mw = 110, 000, Tianjin Xindahui Chemical Company) was served as the solute in the electrospunning solution. N, N-Dimethyformamide (DMF, 99.5%, Aladdin) was served as the solvent without further depuration. Tin (Ⅱ) chloride (SnCl2·2H2O, 98%, Tianjin Ruejinte Chemical 5

Co., Ltd) and Zinc acetate dihydrate (Zn (CH3COO)2·2H2O, 99%, Tianjin Kemeng Chemical Industry) were used as metal sources. Preparation of multichannel SnOx-ZnO/MCNFs composite materials The fabrication process of multichannel carbon nanofibers based carried SnOx and ZnO (SnOx-ZnO/MCNFs) were described as follow. Transparent and homogeneous PAN-PS/DMF solutions involving 8 wt% PAN (polyacrylonitrile) were obtained by stirring continuously at room temperature for 24 h, and the molar ratio of PS to PAN monomer was 1:30. Then, SnCl2 was added to the above PAN-PS/DMF solution, in which the molar ratio of SnCl2 to AN was 1:40. The mixture was stirred for another 12 h under the same conditions. The precursor solution was then spun into composite nanofibers by electrospinning device, which consists of a high voltage power supply, an aluminum collector and a sharp nozzle dropper, with a voltage of 15 kV and the distance of 20 cm. After the completion of the electrospinning process, a whiteness film containing SnCl2/PAN-PS nanofibers were obtained. Adsorption of Zn2+ on the surface of the nanofibers was realized by immersing the SnCl2/PAN-PS nanofibers into the Zn precursor solution. The quality of Sn salt is fixed and changed the concentration of different Zn(CH3COOH)2. The concentrations of Zn(CH3COOH)2 were 1 mmol/L, 4 mmol/L, 7 mmol/L, 10 mmol/L, 13 mmol/L, 16 mmol/L and 19 mmol/L, respectively. SnCl2/PAN-PS nanofibers were then immersed in Zn(CH3COOH)2 at different concentrations for 12 h, and rinsed with deionized water to remove the unsorbed Zn2+. Afterwards, SnCl2-Zn2+/PAN-PS nanofibers, which was being freezedried, were heated 2 h at 250 Ⅱ in air atmosphere to underwent a pre-oxidation process, the heating rate begins at room temperature of 5 Ⅱ/min. And further annealed at 600 Ⅱ for 2 h in nitrogen atmosphere to achieve carbonization process. In addition, the PS polymer was removed during high temperature carbonization, and the 6

inner channel structure with uniform length direction of carbon nanofibers was formed [30]. This process obtained freestanding, flexible SnOx-ZnO/MCNFs. The obtained carbon nanofiber (CNF) mats

were

hereafter

SnOx-ZnO/MCNFs-1,

denominated

SnOx-ZnO/MCNFs-4,

SnOx-ZnO/MCNFs-7, SnOx-ZnO/MCNFs-10, SnOx-ZnO/MCNFs-13, SnOx-ZnO/MCNFs-16 and SnOx-ZnO/MCNFs-19, respectively. For the completeness of the experiment, SnCl2/PAN-PS without immersion Zn(CH3COOH)2 and PAN-PS nanofibers impregnating 10 mmol/L Zn(CH3COOH)2 solution were carbonized under the same conditions mentioned above, and finally obtained the electrode materials. They were labeled as SnOx/MCNFs and ZnO/MCNFs, respectively. In addition, SnCl2/PS-PAN was first carbonized to form SnOx/MCNFs, followed by impregnating 10 mmol/L Zn(CH3COOH)2, which was labeled as SnOx/MCNFs-ZnO-10 2.3 Electrochemical measurement The electrochemical performance of the as-prepared samples, containing cyclic voltammetry (CV), constant current charge and discharge (GCD) and electrochemical impedance spectroscopy (EIS) in a three-electrode system, were studied at CHI660E electrochemical workstation (Chenhua, Shanghai). Pt mesh and Hg/HgO electrode were used for the counter electrode and the reference electrode, individually. 4 mol/L KOH was employed as electrolyte. The as-prepared samples with high flexibility could be straightly manufactured into work electrode without adding binder and conductive. In this process, SnOx-ZnO/MCNFs (0.7 mg/cm2) put in the middle of two nickel foams, the pressure was 10 MPa for about 60 s in the tablet press. The specific capacitance(C, F g-1) in the three-electrode collocation could be calculated on the basis of the formula below: I × ∆t

C= m × ∆V

(1)

In which C (F g-1) is specific capacitance, I (A) is the discharge current, t (s), m (g), ∆V (V) is the discharge time, the quality of the active materials and the potential charge during the discharge time 7

after IR drop, respectively. 2.4 Assembly of the all-solid-state asymmetric supercapacitor (ASC) devices In order to estimate the manufacturing material in practical application potential, ASC devices was fabricated. In 4 mol/L KOH, ASC took as-prepared SnOx-ZnO/MCNFs-10 as positive electrode and CNF (600 °C) as negative electrode. Positive and negative materials were put in the middle of the foam nickel, and a sandwich construction was formed under 10 MPa for 1 minutes to achieve positive and negative electrodes, respectively. Before assembling, electrodes were immersed in the KOH electrolyte and vacuum overnight. On the basis of the charge balance mechanism, Eq. (2) was employed by calculate the quality of active material on the negative and positive electrode, respectively. Specific capacitance (C), energy density (E) and power density (P) were calculated on the basis of the following Eqs. (3)-(5): m+ m-

C- × ∆V-

=

I × ∆t

C = E= P=

C+ × ∆V+

M × ∆V

1 2

C∆V 2

E ∆t

(2) (3) (4) (5)

In which m+/m- (mg) denotes the quality of positive and negative electrodes, C+/C- (F g-1) and △V (V) express the specific capacitance and potential windows of the two electrodes. M (mg) is the gross quality of the two electrodes. In addition, △ t (s), E (Wh kg-1) and P (W kg-1) represent discharge time, energy density and power density, respectively. 2.5 Characterization The microstructure of the samples were studied by scanning electron microscopy (SEM, Pro, Phenom, Netherlands) with energy-dispersive spectrum (EDS). X-ray photoelectron spectroscopy (XPS, Escalab 250xi, Thermo Fisher Scientific USA) was used to evaluate the chemical states of 8

elements in the films. X–ray diffraction (XRD, Rigaku Ultima IV, Japan) was used to discuss the crystal structures of the samples over a range of 2θ angles from 10° to 80° at a scanning rate of 10°/min. Raman spectra (InVia Microscope Raman, Renishaw, England) with an argon laser (λ=532 nm) excitation source was used to analyze the carbon content and carbon defects in the carbon nanofiber. 3 Results and discussion 3.1 Structure and morphology Firstly, the morphology, microstructure and element distribution of SnOx-ZnO/MCNF-10 were detected by SEM and surface scanning (Fig. 1). The SEM image shows a smooth, uniform and continuous fiber morphology, which have no orientation and the average size is about 428 nm. The surface morphology of the sample is free of agglomerated particles, indicating that SnOx and ZnO are uniformly distributed in the carbon fiber. As shown in Fig. 1 (b-f), it presents the result of element mapping, confirming the existence of these elements (ie C, N, O, Sn and Zn), which show that elements are evenly distributed along CNFs. The distribution diameters of C, N and Sn were 393 nm, and O, Zn were 428 nm. The results show that the distribution of ZnO in the outer layer of carbon fibers is about 17.5 nm. This can be interpreted as when the Zn2+ solution is impregnated, the lone pair electrons on the N in the PAN structure coordinate with the vacant orbital of Zn2+, so that the Zn2+ are tightly fixed on the surface of the fiber membrane. After the subsequent carbonization process, the ZnO formed by the combination of Zn2+ and oxygen were wrapped in the outer wall of the carbon fiber. The atomic percent of the SnOx-ZnO/MCNFs-10 composite (Fig. 1 (g)) assured by energy-dispersive X-ray spectroscopy using SEM. By focusing the electron beams on the CNF surface, only Sn, Zn, C, N, O signals appeared, which suggesting that the composite without impurity. The results showed that C 49.3%, O 18.41%, N 17.42%, Sn 5.11%, Zn 9.75%, 9

respectively. The ratio of theoretical Sn content to impregnated Zn content is approximately 1:2 (Table 2). Fig. 2 shows the XRD patterns of SnOx-ZnO/MCNFs-10 and SnOx/MCNFs. It is noteworthy that the XRD diagram of the samples show a wide peak near 25° and 43°, which corresponds to the graphite carbon obtained by PAN carbonization [29]. SnOx related peak was not detected in the original SnOx/MCNFs, which proved that SnOx existed in porous carbon fibers in an amorphous form. In addition, no peak related to ZnO was detected in SnOx-ZnO/MCNFs-10 composite mats, which perhaps owing to the fact that the stable pre-oxidation temperature at 250 Ⅱ (air atmosphere) is not enough to make ZnO crystallize. And N2 flow also inhibits the oxidation process during carbonization, thus SnOx and ZnO form nano or amorphous phases [31]. Therefore, both SnOx and ZnO in the mixed oxide sample exist in an amorphous form. The amorphous phase or nanoparticles contribute to the rapid proton embedding and de-embedding, and produce a fast and reversible oxidation/reduction reaction on the surface or in the bulk of the electrode to produce pseudocapacitance, without causing serious deformation of the material structure of the electrode, and without affecting the performance of the electrode [32]. To further study the surface state of the specimens, X-ray photoelectron spectroscopy (XPS) was applied to analyze the chemical composition and valence states of each element in SnOx-ZnO/MCNFs-10 (Fig. 3). The full-survey-scan spectrum in Fig. 3a shows that SnOx-ZnO/MCNFs are composed of Sn, Zn, C, N and O elements. By using the method of Gauss curve fitting, the elements in the sample were fitted by peaking. Two fitted peaks with binding energies at 487.2 and 495.6 eV are assigned to Sn4+ (the peak to peak separation is 8.4 eV) [21, 33], while the other fitted peaks at 486.5 and 494.9 eV are belong to Sn2+. The Zn 2p spectrum is shown in Fig. 3c, with the binding energies at 1044.4 and 1021.5 eV which consistent of Zn 2p1/2 and Zn 10

2p3/2, respectively. The binding energy and calculated splitting width (23.1 eV) are in good coincide with Zn2+ [34]. For C spectra (Fig. 3d), the C 1s could be split into four featured peaks at ~284.4, ~285.4, ~286.3 and ~288.9 eV, consistent of carbon sp2 (C-C), carbon-nitrogen (C-N), epoxy/hydroxyls(C-O) and carbonyl (C=O) bond. Furthermore, the peak for N 1s is presented in Fig. 3d. The peak can be further divided into two types, including pyridinic (398 eV) and pyrrolic (399.8 eV), respectively. Pyridinic N and pyrrolic N have different positions on the carbon skeleton and play a role in providing the pseudocapacitance in electrochemistry and improving the conductivity and wettability of the electrodes [35-37]. As for O 1s in Fig. 3e, the emission spectra fitted well and composed of three parts: lattice oxygen (Olatt), surface adsorption oxygen (Oads) and surface residual water [38]. The component at 530.3 eV is typical of lattice oxygen bonds (Sn-O), and the fitted peak at 533.6 eV is related to a variety of OH- that is physically and chemically adsorbed on or near the surface [39]. The fitted peaks at 531.3 and 533.7 eV belong to adsorbed oxygen, representing C=O and C-O bonds, respectively. Accordingly, the above-mentioned XPS results go a step further corroborate that SnOx-ZnO/MCNFs-10 samples possess SnO, SnO2 and ZnO components. Raman scattering spectra were used to keep a record of the graphitization of the composite carbon nanofibers in 532 nm laser excitation state. The Raman spectrum in the Fig. 4 shows the absorption bands at 1348 and 1586 cm-1, corresponding to the representative disorder-induced feature (D bands) and the E2g patterns of graphite (G bands) carbon of all samples, respectively, which confirms that PAN underwent successful decomposition and conversion into graphitized carbon after carbonization at 600 °C in N2. Wherein, the G band represents the in-plane vibration of the sp2 conjugated carbon, and the D band corresponds to the presence of the sp3 defect in the carbon chain. The ID/IG value of the peak is determined by calculating the area under the D and G 11

bands, where the ID/IG=1.08 of SnOx-ZnO/MCNFs-10 is calculated from the fitted peaks in the Fig. 5b. The calculation results of other samples are shown in the Fig. 5. The low ID/IG value indicates that the disordered degree of the composites increases and the graphitization is good, which is conducive to the rapid migration of electrons in the composites, reducing the internal resistance of SCs and improving their power density. 3.2. Electrochemical characterization of SnOx-ZnO/MCNFs In order to study the electrochemical properties of SnOx-ZnO/CNFs impregnated with Zn(CH3COO)2 at different concentrations, all the samples were measured in a three-electrode system in 4 mol/L KOH solution as the electrolyte. The films possess splendid mechanical strength and flexibility, therefore, carbon films as working electrodes can be directly used in SCs without introducing any adhesives and conductive agents. Fig. 7 (a) depicts the CV curves of distinct samples at a scanning rate of 70 mv s-1. A series of curves with obvious redox peaks have been obtained within 0-0.65 V, showing obvious pseudocapacitive features. At higher scan rates, the peak currents of all samples increased, revealing that these materials are advantageous to fast redox reactions. In addition, since the peaks of the anode and the cathode are symmetrical, the SnOx-ZnO/MCNFs have good redox reversibility. Owing to the difference of polarization behavior of electrodes, the redox peak position of CV curves in all samples shows a modicum deviation, which is attributed to the difference of physical morphology of electrode materials [40]. Among all samples, SnOx-ZnO/MCNFs-10 has brought out the largest current response along with the maximum integral area of the CV curves, revealing the maximum pseudocapacitance [41]. To further evaluate the potential applications of SnOx-ZnO/MCNFs as electrodes for electrochemical SCs, the constant current discharge curves (GCD) of these samples were analyzed (Fig. 7 (b)). The applied potential ranges from 0 to 0.52 V, and the current density was fixed at 0.5 A 12

g-1. As can be seen from the figure, each GCD curve has a voltage platform corresponding to the redox peak in the CV curve, which better prove the pseudopotential property of the electrode. At the same current density, the discharge time of SnOx-ZnO/MCNFs-10 is the longest of all samples, which revealing the maximum pseudocapacitance. According to the discharge curves, SnOx-ZnO/MCNFs-1,

SnOx-ZnO/MCNFs-4,

SnOx-ZnO/MCNFs-7,

SnOx-ZnO/MCNFs-10,

SnOx-ZnO/MCNFs-13, SnOx-ZnO/MCNFs-16 and SnOx-ZnO/MCNFs-19 show the specific capacitance as 642, 681, 746, 783, 779, 741 and 732 F g-1, respectively. In addition, it is noteworthy that the specific capacitance values in this study were higher than previously reported Zn-doped Sn hybrid electrodes using other methods, for instance, nano-urchin-enriched 3D carbonaceous framework Zn-doped SnO2 (635 F g-1 at 1 A g-1) [42]. The results showed that impregnating Zn(CH3COOH)2 with appropriate concentration can improve the specific capacitance, and make Sn and Zn metals play a better synergistic role. When the concentration of Zn(CH3COOH)2 reached 10 mmol/L, the specific capacitance value was the highest. This can be explained as follows: the number of adsorption sites on PAN fiber membranes is certain. When the concentration of Zn2+ is continuously increased, the adsorption sites will be saturated, and the specific capacitance can’t change greatly with the increase of the concentration of Zn2+. This was why the specific capacitance reaches its peak when the concentration of Zn2+ reaches 10 mmol/L, and remains stable when the concentration of Zn2+ continues to increase (13 mmol/L, 16 mmol/L, 19 mmol/L). It was proved that the saturated adsorption capacity of SnOx/PS-PAN was reached when the concentration of Zn(CH3COOH)2 was reached 10 mmol/L. Because only the interface changes or changes near the interface occur in the Faradaic process, we hope the electrode material have good coulomb efficiency [43]. Fig. 6c displays the specific capacitance of the hybrid electrode at distrinct discharge current densities. At a current density of 13

0.5 A g−1, the specific capacitances of the SnOx-ZnO/MCNFs (1, 4, 7, 10, 13, 16, 19 mmol/L) are 642, 681, 746, 783, 779, 741 and 732 F g-1, respectively. When the current density increases from 0.5 to 5 A g−1, the SnOx-ZnO/MCNFs (1, 4, 7, 10, 13, 16, 19 mmol/L) composite electrodes are remain emerged high specific capacitances of 484, 515, 570, 604, 568, 548 and 522 F g−1, consistent with capacitance retentions of 75.4%, 75.6%, 76.4% 77.1%, 72.9%, 73.9% and 71.4%, respectively. It can be seen that the samples not only has a high specific capacitance value, but also could better maintain these high values under the increasing current density. Obtained from the results, with the increase of current densities, the specific capacitance diminish gradually, which is because the limitation of ion migration/diffusion and the reduction of active material utilization. The EIS of the three electrode systems was measured in the frequency range of 100 KHz-0.1 Hz at an open circuit voltage of 5 mV. The corresponding Nyquist diagram was shown in Fig. 6d. The EIS spectrum confirmed the conductivity and electrochemical reactivity of Zn(CH3COOH)2 impregnated with different concentrations. The impedance spectra of the electrodes are basically resemble in morphology. In the low frequency region, the slope of the curve is concerned with the Warburg impedance (W), which stands for the diffusion of electrolytes in porous electrodes and protons in matrix materials. And in high frequency region, Z (equal to RS) at the intersection of the real part of the curve represents an ionic resistance of an electrolyte combined with a resistance, a resistance of the intrinsic electrode material, and a contact resistance of the active material/collector interface. It is worth noting that, when the high frequency region of the resistor is amplified, no semicircle is observed, which indicates that the contact resistance between charge transfer resistance and electrode-electrolyte is relatively small [44]. From the plots, the series resistance of SnOx-ZnO/MCNFs-10 (0.479 Ω) electrodes is smaller than that of SnOx-ZnO/MCNFs-1 (0.492 Ω), SnOx-ZnO/MCNFs-4 (0.495 Ω), SnOx-ZnO/MCNFs-7 (0.482 Ω), SnOx-ZnO/MCNFs-13 (0.481 Ω), 14

SnOx-ZnO/MCNFs-16 (0.497 Ω) and SnOx/MCNFS-19 (0.489 Ω). Compared with other electrodes, the line of SnOx-ZnO/MCNFs-10 is closer to the Y axis than other electrodes, which indicates that its diffusion resistance is small and has faster charge transfer rate and better electronic conductivity. This is because SnOx-ZnO/MCNFs-10 internal porous structure enhances the utilization of electrode materials, thus promoting the diffusion of OH- to the surface of the electrode. Besides comparing specific capacitance, cyclic stability is considered to be another major factor in assessing the performance of SCs. Because the cyclic stability is limited by the current density, the cyclic stability test of SnOx-ZnO/MCNFs was carried out 5000 cycles at the current density of 5 A g-1 (Fig. 4e), in which SnOx-ZnO/MCNFs-10 electrode retained about 87% of the initial capacitance (Fig. 4f). This indicates that the materials in this experiment have good cyclic stability and may become candidate materials for long-life SCs. For the experimental integrity, a series of comparative experiments have been made (Fig. 7(a-d)). As shown in the figures, the SnOx-ZnO/MCNFs-10 prepared by impregnating Zn2+ on the fiber membrane exhibits higher electrochemical activity than SnOx/MCNFs-ZnO-10 prepared by impregnating Zn2+ on carbon nanofibers. The specific capacitance of SnOx/MCNFs-ZnO-10, SnOx/MCNFs and ZnO/MCNFs were computed as 631 F g-1, 599 F g-1 and 429 F g-1, respectively. The corresponding resistance values are 0.43 Ω, 0.373 Ω and 0.656 Ω. Due to the addition of Zn2+, SnOx-ZnO/CNFs-10 composite exhibits a specific capacitance of 819 F g-1 at 0.5 A g-1, which is exceed their respective counterparts. In addition, on the basis of comparative experiments, a possible mechanism for effective synergy between the two metals is raised (Scheme. 2). When the PAN nano-fiber was immersed in the Zn2+ solution, the N atom in the PAN structure contains a lone pair of electrons and coordinates with the vacancy orbital of the Zn2+, which caused the Zn2+ was fixed on the surface of the nano-fiber. 15

During the pre-oxidation process, the PAN molecules underwent physic and chemical changes, which were transformed from a linear chain of PAN molecules to an aromatic step structure. In the subsequent carbonization process, the unreacted cyano group was further cyclize and crosslink, and the formed small trapezoidal structure starts thermal crosslinking and thermal polymerization to finally form a larger conjugated structure. At the same time, Zn2+ combine with oxygen in the air to form ZnO. According to XRD, tetrahedral ZnO is not aggregated into nanoparticles and existed in an amorphous form. It’s further proved that ZnO is uniformly dispersed on the surface of carbon fiber. In this process, the tetrahedral structure of ZnO was bound in the carbon fiber network, which limits the free movement of ZnO. Under these circumstances, the hybrid electrode would provide conditions for better coordination of Zn atoms and Sn atoms. Because of the movement of ZnO was limited, it reduces the loss rate during electrochemical testing and life testing, thereby ensuring its excellent electrochemical performance. In addition, the synergistic effect of SnOx and ZnO in electrochemical reaction and cycle performance is mainly due to the carbon layer between the two metal oxides. The role of the carbon layer comes mainly from the following points: (1) The carbon layer has good electrical conductivity, providing a main high-speed channel for electron and ion transport, and a good internal passage facilitates electrolyte penetration into the active material [45]. (2) The carbon layer can also act as an electrical bridge connecting the SnOx and ZnO, which plays an important role in the connection between the two metal oxides [46].(3) ZnO, as the framework of layered structure, is a semiconductor material with high conductivity and mechanical stability. Moreover, ZnO has made outstanding contributions to the performance and function of supercapacitors in terms of electron transport and ion diffusion. Due to the good electrical conductivity of ZnO and the carbon species between the internal SnOx and the external ZnO, the transport capacity of ions from the outside to the inside is enhanced, thus improving the synergistic 16

effect of the two metal oxides [46-47].(4) The existence of carbon layer not only protects SnOx from corrosion in alkaline electrolyte solutions, but also reduces the volume expansion of metal oxides during charge-discharge process, and improves the stability of materials, making a prominent contribution to the cycle performance [48]. The excellent properties of SnOx-ZnO/CNFs-10 electrodes could be attributed to the following factors. First, the high flexibility and independent support properties of the sample itself could be directly used for the electrode material of SCs, averting the use of polymer binders or conductive Additives ensure fast electron transfer and good structural integrity. Secondly, the internal porous structure of the carbon fiber increased the contact area between the active material and the electrolyte, and temporarily stores the electrolyte to supply ions during rapid charging and reduced the diffusion resistance of electrolyte. In addition, nitrogen doping could generate additional pseudo-capacitance to improve the conductivity and wettability of the electrode. Finally, since the carbon layer between the internal SnOx and the external ZnO has excellent conductivity, the synergistic ability between the two metal oxides is enhanced. 3.3 Electrochemical performance of ASC (SnOx-ZnO/MCNFs//CNFs) In order to further assess the application potential of as-prepared material, a flexible all-solid-state asymmetric supercapacitor (ASC) was manufactured. The preparation method of the positive and negative electrode materials of the assembled ASC has been introduced above. Fig. 8a gives that the carbon nanofibers electrode and SnOx-ZnO/MCNFs electrode possess stable potential windows of -0.9-0 V and 0-0.65 V, respectively. For SnOx-ZnO/MCNFs//CNFs ASC, the CV curves at diversity scan rates are shown in Fig. 8b. The voltage window of SnOx-ZnO/MCNFs//CNFs ASC device can reach 1.5 V, widening the voltage window of single electrode. The asymmetric supercapacitor has good capacitance characteristics and no obvious polarization curve. It has been 17

proved that the working voltage is the key factor determining the energy density. According to the equation of E = 1/2 C∆V2, which was mentioned above. When the scanning rate increases from 2 mv s−1 to 50 mv s−1, the CV curve shape of SnOx-ZnO/MCNFs//CNFs ASC device still has no large deformation relative to the original shape. With the increase of scanning potential, the power supply has excellent reversibility and ideal quick charging and discharging performance. These results show that SnOx-ZnO/MCNFs-10 plays an important role in the asymmetric high reversibility equipment. The electrochemical performance of ASC devices were further assessed by GCD, measured at different current density, as shown in Fig. 8c. The specific capacitance of SnOx-ZnO/MCNFs//CNFs ASC devices is calculated on the basis of the total mass of positive and negative electrode materials. When the current density is 0.5, 0.8, 1, 2 and 3 A g-1, the specific capacitance of the device is 120.74, 67.68, 53.87, 18.60 and 17.64 F g-1, respectively. From the calculated results, it can be seen that the specific capacitance diminish with the increase of current density, which is due to the reduction of the utilization ratio of active materials and the limitation of ion diffusion/migration. Long cyclic stability is another vital demand for practical application of asymmetric supercapacitors. Due to the dependence of cycle performance on current density, the current density is fixed at 2 A g-1 and 5000 cycles are tested (Fig. 8f). The specific capacitance is 64.3 F g-1 in the first cycle, and gradually reduces to 43.0 F g-1 in 5000 cycles, resulting in a loss of 23.8% of the total capacitance. Power density (P) and energy density (E) are usually two vital parameters to reflect the practical application of electrochemistry. Ragon diagram (Fig. 8e) compares the power and energy densities of ASC devices fabricated in this work with those reported previously. Our asymmetric supercapacitors have a maximum energy density of 37.7 Wh kg-1 with a power density of 374.9 W 18

kg-1 at current density of 0.5 A g-1, which is higher than that of many reported asymmetric devices: NiO/Graphene nanocomposites (21.8 Wh kg-1 at 250 W kg-1) [49], MnO2//FeSe2 (27.14 Wh kg-1 at 571.3 W kg-1) [50], ZnxNi1-xS//AC (27.72 Wh kg-1 at 301 W kg-1) [51], Co-Ni-Zn//AC (84.2 Wh kg-1 at 374.8 W kg-1) [52], PVA/KOH/Cu5Sn2S7-ZnS (9.67 Wh kg-1 at 461 W kg-1) [53], NixCo1-x LDH-ZTO (ZnO:SnO2:C)//AC (23.7 Wh kg-1 at 284.2 W kg-1) [54]. Conclusion In summary, SnOx-ZnO/MCNFs as flexible and freestanding electrode material in SCs has been successfully fabricated via electrospinning technology, impregnation and subsequent carbonization method, in which Sn nanoparticles were encapsulated in porous carbon nanofibers, and Zn2+ were distributed on the surface of carbon fibers. The distribution diameters of Sn was 393 nm, and Zn was 428 nm, which indicates the distribution of ZnO in the outer layer of carbon fibers is about 17.5 nm. By comparing the Zn2+ impregnated with different concentration, it was found that the best electrochemical performance was obtained when the Zn2+ concentration was 10 mmol/L. The specific capacitance of SnOx-ZnO/MCNFs-10 at 0.5 A g-1 is 819 F g-1, and the retention rate is 87% after 5000 cycles. The electrochemical performance of the composites is higher than their respective counterparts SnOx/MCNFs (599 F g-1) and ZnO/MCNFs (429 F g-1). In addition, SnOx-ZnO/MCNFs are assembled into all-solid-state asymmetric supercapacitors (ASCs), which have the maximum energy density of 37.7 Wh kg-1 at a power density of 374.9 W kg-1 and a capacity retention rate of 76.2% after 5000 cycles. Overall, these freestanding, flexible composite provide higher specific capacitance and long cycle life, making them prospective candidates as electrode materials for SCs. Acknowledgements

19

The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 21766022).

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Figure captions Scheme. 1. Schematic illustration of the systhesis processes. Table. 1. Operating conditions for freestanding nanofibers. Fig. 1. Morphology and elemental composition of SnOx-ZnO/MCNFs-10: (a) SEM image, (b-f) elemental mapping of C, N, Sn, O and Zn, respectively, (g) EDX analysis. Fig. 2. XRD patterns of (a) SnOx-ZnO/MCNFs-10, (b) SnOx/MCNFs. Fig. 3. XPS spectra of SnOx-ZnO/MCNFs-10: (a) survey spectra; (b) Sn 3d; (c) Zn 2p; (d) C 1s; (e) N 1s; and (f) O 1s. Fig. 4. Raman spectra of the different samples. Fig. 5. Raman spectra of (a) SnOx-ZnO/MCNFs-7; (b) SnOx-ZnO/MCNFs-10; (c) SnOx-ZnO/MCNFs-13;

(d)

MCNFs/ZnO-10

(e)

SnOx/MCNFs-10

and

(f)

ZnO/MCNFs-10. Fig. 6. (a) CV curves of the sample impregnation at 1, 4, 7, 10, 13, 16 and 19 mmol/L at a scan rate of 70 mV s-1; (b) GD curves of the different samples at a current density of 0.5 A g-1; (c) Specific capacitance of the different samples at different current density; (d) Nyquist plots measured at an open-circuit voltage (inset: the magnified version at high frequency diagram); (e) Cycling performance of the different samples after 5000 cycles at a current density of 5 A g-1 and (f) Cycling stability of the SnOx-ZnO/MCNFs-10 tested after 5000 cycles at 5.0 A g-1. Fig. 7. (a) CV curves of SnOx-ZnO/MCNFs-10, SnOx/MCNFs-ZnO -10, SnOx/MCNFs and ZnO/MCNFs at a scan rate of 70 mV s-1; (b) GD curves of the different samples at a current density of 0.5 A g-1; (c) Specific capacitance of the 28

different samples at different current density and (d) Nyquist plots of the different samples measured at the frequency range of 0.01-105 Hz (the inserted graph in Nyquist plots is partially enlarged details at high frequency). Scheme. 2. Schematic of the formation mechanism of composite materials. Fig. 8. (a) CV curves of the SnOx-ZnO/MCNFs-10 and CNFs at a scan rate of 70 mv s-1 in a three electrode system; (b) CV curves of the asymmetric supercapacitor at different scan rates; (c) GCD curves of the device at different current densities; (d) Specific capacitance of the device at a current densities in the range of 0.5-3 A g-1; (e) Ragone plots of the device and some other hybrid materials reported in the literatures and (f) Cycling stability of the device tested after 5000 cycles at 2.0 A g-1.

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Scheme. 1. Schematic illustration of the systhesis processes.

30

parameter

condition

Voltage (kV)

15

Tip to colltctor distance (cm)

20

Needle (mm)

1

Stabilization temperature in air (Ⅱ) Stabilization time (h)

250 2

Carbonization temperature (Ⅱ) Carbonization time (h)

600 2

Table. 1. Operating conditions for freestanding nanofibers of SnCl2/PAN-PS.

31

Fig. 1. Morphology and elemental composition of SnOx-ZnO/MCNFs-10: (a) SEM image, (b-f) elemental mapping of C, N, Sn, O and Zn, respectively, (g) EDX analysis.

32

Sample SnOx-ZnO/MCNFs-10

Element (wt %) Sn Zn 5.11 9.75

C 49.30

O 18.41

N 17.42

Table. 2. EDX spectra of sample SnOx-ZnO/MCNFs-10 with the atomic composition.

Fig. 2. XRD patterns of (a) SnOx-ZnO/MCNFs-10, (b) SnOx/MCNFs.

33

Fig. 3. XPS spectra of SnOx-ZnO/MCNFs-10: (a) survey spectra; (b) Sn 3d; (c) Zn 2p; (d) C 1s; (e) N 1s; and (f) O 1s.

34

Fig. 4. Raman spectra of the different samples.

35

Fig. 5. Raman spectra of (a) SnOx-ZnO/MCNFs-7; (b) SnOx-ZnO/MCNFs-10; (c) SnOx-ZnO/MCNFs-13; (d) MCNFs/ZnO-10 (e) SnOx/MCNFs and (f) ZnO/MCNFs.

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Fig. 6. (a) CV curves of the sample impregnation at 1, 4, 7, 10, 13, 16 and 19 mmol/L at a scan rate of 70 mV s-1; (b) GD curves of the different samples at a current density of 0.5 A g-1; (c) Specific capacitance of the different samples at different current density; (d) Nyquist plots measured at an open-circuit voltage (inset: the magnified version at high frequency diagram); (e) Cycling performance of the different samples after 5000 cycles at a current density of 5 A g-1 and (f) Cycling stability of the SnOx-ZnO/MCNFs-10 tested after 5000 cycles at 5.0 A g-1.

37

Fig.

7.

(a)

CV

curves

of

SnOx-ZnO/MCNFs-10,

SnOx/MCNFs-ZnO-10,

SnOx/MCNFs and ZnO/MCNFs at a scan rate of 70 mV s-1; (b) GD curves of the different samples at a current density of 0.5 A g-1; (c) Specific capacitance of the different samples at different current density and (d) Nyquist plots of the different samples measured at the frequency range of 0.01-105 Hz (the inserted graph in Nyquist plots is partially enlarged details at high frequency).

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Scheme. 2. Schematic of the formation mechanism of composite materials.

39

Fig. 8. (a) CV curves of the SnOx-ZnO/MCNFs-10 and CNFs at a scan rate of 70 mv s-1 in a three electrode system; (b) CV curves of the asymmetric supercapacitor at different scan rates; (c) GCD curves of the device at different current densities; (d) Specific capacitance of the device at a current densities in the range of 0.5-3 A g-1; (e) Ragone plots of the device and some other hybrid materials reported in the literatures and (f) Cycling stability of the device tested after 5000 cycles at 2.0 A g-1.

40

Highlights

1. Porous carbon nanofibers supported Sn and Zn bimetallic composites as flexible and freestanding electrode material in supercapacitors have been successfully prepared. 2. The specific capacitance of SnOx-ZnO/MCNFs-10 mmol/L at 0.5 A g-1 is 783 F g-1 with the retention rate of 87% after 5000 cycles 3. The sample showed good electrochemical performance due to ZnO with tetrahedral structure which was embedded in carbon fiber structure.