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C composite nanofibers with superior electrochemical performance for supercapacitor
Journal of Physics and Chemistry of Solids 140 (2020) 109385
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Electrospun ZnCo2O4/C composite nanofibers with superior electrochemical performance for supercapacitor Hongquan Yu a, *, Hengyan Zhao b, Yanbo Wu b, Baojiu Chen a, **, Jiashi Sun a a b
Department of Physics, Dalian Maritime University, Dalian, 116026, Liaoning, China College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian, 116028, Liaoning, China
A B S T R A C T
In recent years, supercapacitors have attracted much attention due to their excellent charge–discharge performance, long-term cycle lifetimes, and high specific power. Transition metal oxides are promising candidates for environmentally friendly, low-cost, and high-powered energy storage materials. In addition, the use of one-dimensional nanostructures can markedly enhance the characteristic properties of the transition metal oxides. In this work, ZnCo2O4/C composite nanofibers were successfully fabricated by using a simple electrospinning method. The nanofibers have an average diameter of 300–500 nm and consist of many ZnCo2O4 and carbon nanoparticles. A supercapacitor fabricated using the ZnCo2O4/C nanofibers was found to exhibit a high specific capacitance of 90.0 F/g at a current density of 8.0 A/g and good rate capability under current densities in the range 0.5–8.0 A/g at a voltage of 0.4 V. It also has superior working stability with a capacitance retention of 125% after 1000 cycles in air.
1. Introduction The storage of electrochemical energy continues to be a highly active field of research due to its wide range of applications, from portable devices and wearable electronics to electric vehicles and smart grids [1–4]. Among the various energy-storage technologies, supercapacitors have attracted a great deal of attention in recent years owing to their high-power density, outstanding cycle stability, and fast char ge–discharge characteristics. Generally speaking, there are two types of supercapacitors based on the charge-storage mechanism involved: electric double-layer capacitors and Faraday pseudocapacitors [5–8]. Traditional carbon-based mate rials rely on the double-layer capacitance mechanism, which means they can only store a limited amount of energy [9,10]. Recent cutting-edge research has focused on pseudocapacitive-type electrode materials, wherein the charge is stored not only via ion adsorption but also near surfaces where redox reactions occur. Therefore, the properties of such pseudocapacitors mainly depend on the electrochemical performance of the electrode materials [11–13]. Mixed transition metal oxides (MTMOs) can partake in a rich array of reversible oxygen anion redox reactions, which can significantly improve the electrochemical performance of the pseudocapacitor [14, 15]. Therefore, MTMOs with large surface areas, excellent energy and power densities, high charge–discharge efficiencies, and good cycle
stabilities, are promising electrode materials for use in supercapacitor devices. In particular, transition metal–cobalt based oxides (e.g. NiCo2O4, CuCo2O4, ZnCo2O4, and MnCo2O4) have interested a large number of researchers due to their high theoretical capacitances, low-cost, and relatively low pollution risks [16–21]. Recently, several applications of ZnCo2O4 for higher performance supercapacitors have been evaluated to meet these challenges. Venkatachalam et al. reported ZnCo2O4 nanostructures for supercapacitor applications with a specific capacitance (Cs) of 845.7 F/g at a current density of 1.0 A/g [22]. Rajesh et al. developed hierarchical coral-like ZnCo2O4 nanowires exhibiting a Cs of 264 F/g at 10.0 A/g [23]. Kim et al. reported tests on a Zn–Co-urea/ammonium fluoride/hexamethylenetetramine with a high Cs of 462.5 C/g at 1.0 A/g [24]. Since the ground-breaking discovery of carbon nanotubes in the 1990s, one dimensional (1D) nanostructures have also attracted a sig nificant amount of research interest owing to their remarkable physical and chemical properties [25–28]. 1D nanostructures have high length/diameter ratios and large specific surface areas (SSAs) and quantum confinement effects. Together with their unique electrical, optical, photovoltaic, and mechanical properties, this makes them highly suitable for use in various contexts, e.g. solar cells, field emission applications, and energy conversion and storage [29–31]. When it comes to electrochemical energy storage, and especially supercapacitor fabrication, 1D nanostructures enjoy several appealing
* Corresponding author. No. 1, Linghai Road, Dalian, 116026, Liaoning, China. ** Corresponding author. No. 1, Linghai Road, Dalian, 116026, Liaoning, China. E-mail addresses: [email protected] (H. Yu), [email protected] (B. Chen). https://doi.org/10.1016/j.jpcs.2020.109385 Received 17 December 2019; Received in revised form 21 January 2020; Accepted 28 January 2020 Available online 31 January 2020 0022-3697/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic diagram of the set-up used in the electrospinning process.
advantages: (i) using them can shorten electronic pathways and facili tate electrical transport in an axial direction (unlike their bulk coun terparts); (ii) 1D nanostructured electrodes have shorter ion diffusion lengths which are beneficial to the rate capability (the characteristic time taken for ions to diffuse through an electrode is t � l2/D, where l represents the diffusion length and D is the diffusion coefficient [32]); (iii) 1D nanostructures have large electroactive SSAs which increase the electrode/electrolyte contact surface/interface, which further enhances the electrode kinetics facilitating ultrafast charge–discharge capability; (iv) 1D nanoarchitectures can be readily utilized to form various higher-level multi-dimensional structures with improved SSAs, good mechanical stabilities, and enhanced cycling performance for use in supercapacitors [33,34]. Electrospinning is a simple, versatile, technique for fabricating nanofibers that are exceptionally long and uniform in diameter and is based on a variety of functional materials [35,36]. The diameters of the fibers prepared using this method can range from tens of nanometres to several microns. The morphology of the electrospun fibers depends on the solvent and solution properties, as well as on other process variables. Efforts have been made in the last ten years to develop 1-d materials for batteries and capacitors. Kim et al. fabricated ZnCo2O4 nanofibers, designed a porous nanostructure using a facile electrospinning process and used selective etching of ZnO to form the cathode material in lithium-oxygen batteries [37]. Chang et al. developed a combined electrospray/electrospinning technique to obtain NiC o2O4@Carbon/Carbon composite nanofiber to produce flexible elec trodes with enhanced performance [38]. In this work, 1D ZnCo2O4/C composite nanofibers were fabricated using a simple electrospinning method. The measured surface area of the nanofibers is 199.0 m2/g and the average pore size (calculated using the Barrett–Joyner–Halenda model) is ~3.83 nm. The ZnCo2O4/C com posite fibers have average diameters of 300–500 nm and consist of many ZnCo2O4 and carbon nanoparticles. The ZnCo2O4/C composite fibers have high specific capacitance 90.0 F/g (8.0 A/g) and good cycle sta bility which maintains 125% after 1000 cycles.
Fig. 2. TG and DSC curves of the ZnCo2O4/C precursor nanofibers.
between the two. The charged jet was attracted to the collector where upon it solidified as the solvent evaporates. In our experiments, the applied voltage was 12 kV and the collection distance was 140 mm. The as-prepared hybrid precursor samples were then dried in a vacuum oven for 24 h at room temperature before being subjected to pre-oxidation and carbonization. Pre-oxidation was carried out by heating the fibers to 200 � C at a rate of 5 � C/min and maintaining them at that temperature for 4 h in the presence of air. The stabilized fibers were then carbonized at 500, 600, 700, and 800 � C under an atmosphere of argon (a heating rate of 5 � C/min was again used). The final elec trospun ZnCo2O4/C composite nanofibers were thereby obtained. For comparison purposes, ZnCo2O4 powders were also prepared using a sol-gel technique appearing in the literature [39]. 2.2. Physical characterization The crystal structure of the samples was determined via X-ray diffraction (XRD). A Cu Kα radiation source was employed in this work (λ ¼ 0.15405 nm). The morphology of the samples was also observed using a field-emission scanning electron microscope (SEM; ZEISS SUPRA 55). Transmission electron microscope (TEM) images, high-resolution (HR) TEM images, and EDS) patterns were recorded by a TEM (JEM2010) at a working voltage of 200 kV. Thermogravimetric (TG) and differential scanning calorimetry (DSC) data were recorded using a simultaneous thermal analyzer (NETZSCH STA 449 F3). Specific surface areas and pore size distributions (PSDs) were determined by conducting Brunauer–Emmett–Teller (BET) measurements (Micromeritics ASAP 2020). 2.3. Electrochemical measurements The electrochemical performance of ZnCo2O4/C-based electrodes was investigated in aqueous KOH solution using an electrochemical workstation (CHI660B, CH Instruments Co, Shanghai, China). A threeelectrode cell configuration consisting of a platinum wire auxiliary electrode and a standard calomel electrode (SCE) as reference was employed. The working electrode was prepared by mixing ZnCo2O4/C composition nanofibers, carbon black, and solution of polymer binder (polytetrafluoroethylene) in proportions of 85 : 10 : 5 (by weight). A few drops of ethanol were also added. The mixture was then dispersed ul trasonically for 15 min to form a slurry. The slurry was then pasted onto nickel foam in the unit area and dried in an oven at 80 � C for 8 h.
2. Experimental section 2.1. Preparation of materials The synthesis process typically involved dissolving Zn (CH3COO)2⋅4H2O (0.439 g), Co(CH3COO)2⋅4H2O (0.996 g), and polyvinylpyrrolidone (PVP; 3.610 g) in N,N-dimethlformamide (DMF; 20 mL). The solution was then stirred for 10 h to acquire a homogeneous hybrid sol for electrospinning. A schematic diagram of the electrospinning apparatus is shown in Fig. 1. Three key items of equipment could be attained: a high-voltage power supply, a spinneret, and a collector. Electrospinning was per formed by pumping the solution through a nozzle towards the collector. An electrically charged jet was formed because of the high voltage 2
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Journal of Physics and Chemistry of Solids 140 (2020) 109385
Fig. 3. SEM images of ZnCo2O4/C nanofibers prepared at different carbonization temperatures: (a) 500 � C, (b) 600 � C, (c) 700 � C, and (d) 800 � C. The scale bar corresponds to 1.0 μm.
Fig. 4. (a) A TEM image, (b) magnified TEM image, (c) HR-TEM image and (I) SAED pattern acquired using a single ZnCo2O4/C nanofiber obtained at a temperature of 600 � C. Elemental mapping images are shown in (d–h).
3. Results and discussion
the TG curve: (i) a loss of ~3% up to 100 � C which is attributed to the release of adsorbed water and water of crystallization (from the Zn (CH3COO)2⋅2H2O and Co(CH3COO)2⋅4H2O); (ii) a loss of ~7% up to 250 � C which is due to thermal decomposition of acetate in the precursor fibers; and (iii) a major weight loss of 70% at ~430 � C which is
Fig. 2 illustrates the thermal analysis (TG-DSC) curves obtained using ZnCo2O4/C precursor fibers in a nitrogen atmosphere (using a heating � rate of 5 C/min). Three major weight-loss stages could be identified in 3
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Journal of Physics and Chemistry of Solids 140 (2020) 109385
Fig. 5. (a) TEM image and SAED pattern (inset) of ZnCo2O4 nanopowders; (b) HR-TEM image of ZnCo2O4 nanopowders.
successfully prepared. Therefore, it ‘can be concluded that the single ZnCo2O4/C nanofiber consists of polycrystalline ZnCo2O4 nanoparticles and carbon nanoparticles. The TEM and HR-TEM images of ZnCo2O4 powders are shown in Fig. 5: the morphology of ZnCo2O4 powders is that of uniform, polycrystalline rhomboidal nanoparticles with diameters ranging from c. 10 nm to c. 100 nm. The lattice fringes of the ZnCo2O4 powders are shown in Fig. 5b: the fringe spacings were detected to be 0.461 nm and 0.285 nm, which correspond to the (111) and (220) lattice planes of a typical spinel-structure ZnCo2O4 [42]. The XRD patterns of ZnCo2O4/C nanofibers prepared using different carbonization temperatures are shown in Fig. 6. The diffraction peaks clearly become sharper as the carbonization temperature increases. The peaks located at 18.9� , 31.2� , 36.8� , 38.5� , 44.7� , 48.9� , 55.6� , and 59.3� can be attributed to the 111, 220, 311, 222, 400, 422, 511, and 440 planes of cubic ZnCo2O4 (JCPDS card: 23–1390) [43]. Moreover, the peaks detected at 34.4� and 36.3� are attributed to the 002 and 101 planes of hexagonal ZnO (JCPDS card: 80–0074), according to the literature [44]. Fig. 7 shows N2 adsorption–desorption isotherms and Bar rett–Joyner–Halenda pore size distribution curves obtained using the ZnCo2O4 nanofibers and ZnCo2O4 powder. The hysteresis apparent in the N2 adsorption–desorption isotherms can be classified, according to the International Union of Pure and Applied Chemistry, as belonging to category type IV (type H1 hysteresis loop). This behavior was typically observed in mesoporous materials formed by agglomerated particles [45]. The specific surface area calculated for the ZnCo2O4 nanofibers (199.0 m2/g) is much higher than that of the ZnCo2O4 powder (30.48 m2/g). The pore diameters of the ZnCo2O4/C composite nanofibers lie in the range 2–10 nm (Fig. 7b), giving an average pore diameter of 3.83 nm. The pore diameters determined for the ZnCo2O4 powders lie in the range 2–30 nm (Fig. 7d), with an average value of 16.83 nm. The ZnCo2O4/C composite nanofibers therefore have a fine meso porous structure that effectively facilitates the diffusion of electrolytes and rapid charge transfer, and hence endows the nanofibers with excellent electrochemical performance. The electrochemical characteristics of electrodes fabricated from ZnCo2O4/C composite nanofibers and ZnCo2O4 powder were investi gated in strongly alkaline solution (6 M KOH). Fig. 8 illustrates CV curves derived using these electrodes for a variety of scan rates. The shape of each CV curve deviates from a rectangular profile. This suggests that redox reactions are taking place and the MTMOs are contributing to the charge-storage process via reactions such as:
Fig. 6. XRD patterns obtained using ZnCo2O4/C nanofibers prepared using different carbonization temperatures: (a) 500 � C, (b) 600 � C, (c) 700 � C, and (d) 800 � C.
attributed to the thermal decomposition and volatilization of PVP and other organic compounds (and corresponds to the sharp peak observed in the DSC curve). No significant weight loss was observed above 500 � C, which confirms the formation of inorganic ZnCo2O4/C. Furthermore, to ensure complete decomposition of the precursor, calcination tempera tures in the range 500–800 � C were chosen to synthesize several ZnCo2O4/C nanofiber samples. Fig. 3 indicates SEM images of the ZnCo2O4/C composite nanofibers carbonized at different temperatures. It can be seen that the nanofibers undergo crispation as the carbonization temperature increases. The di ameters of the nanofibers also gradually decrease from ~500 to ~100 nm. Fig. 4a–i shows a TEM image, selected area electron diffraction (SAED) pattern, and HR-TEM image of the ZnCo2O4/C composite nanofibers, respectively. It can be seen from the images that the ZnCo2O4/C nanofibers are 1D. Some pin holes can be seen randomly distributed within the fibers (Fig. 4a). The SEAD pattern in Fig. 4i in dicates the presence of polycrystalline rings and the (311), (220), and (422) planes can be indexed according to the spinel-like structure of ZnCo2O4 [40]. The lattice fringes from the constituent nanoparticles that have a separation of 0.248 nm can be indexed to the (311) planes of ZnCo2O4 in a cubic phase [41]. This is in good agreement with the most intense diffraction peak in the XRD pattern. Accordingly, EDS mapping images of a single ZnCo2O4/C nanofiber are demonstrated in Fig. 4d–h. Elemental Zn, Co, and O atoms are seen to be evenly distributed over the length and width of the single ZnCo2O4/C nanofiber (except for the C supported carbon grid) which indicates that the required nanofiber was
ZnCo2O4 þ OH þ H2O ⇌ ZnOOH þ 2CoOOH þ e– –
CoOOH þ OH ⇌ CoO2 þ H2O þ e
(1) (2)
It is also clear that the redox peaks progressively shift and broaden as the sweep rate is increased. This behavior can be attributed to the lim itations placed on charge transfer and diffusion rate of the electrolyte when the charging and discharging processes were carried out at high speed [46,47]. The electrode fabricated using ZnCo2O4/C nanofibers 4
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Journal of Physics and Chemistry of Solids 140 (2020) 109385
Fig. 7. Nitrogen adsorption–desorption isotherms for ZnCo2O4/C nanofibers and ZnCo2O4 powders are shown in (a) and (c), respectively. The corresponding pore size distributions are shown in (b) and (d), respectively.
Fig. 8. CV curves for (a) ZnCo2O4/C nanofibers, and (b) ZnCo2O4 powders using different scan rates.
exhibits a substantially higher current density compared to the one fabricated using powder. Also, the larger areas under the CV curves indicate that they have greatly enhanced capacitive performance compared to the ZnCo2O4 powder electrode. Fig. 9a and c presents galvanostatic charge–discharge (GCD) curves obtained using the electrodes fabricated from ZnCo2O4/C nanofibers
and ZnCo2O4 powders for a variety of current densities. Distinct plateaus are found in each GCD curve indicating the pseudocapacitive behavior of ZnCo2O4. The specific capacitances of the electrodes (Cs) can be calculated using these GCD curves and other electrode data via the following expression:
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Journal of Physics and Chemistry of Solids 140 (2020) 109385
Fig. 9. GCD curves obtained using a ZnCo2O4/C nanofiber electrode and ZnCo2O4 powder electrode are illustrated in (a) and (c), respectively, for different current densities. (b) and (d) show the corresponding specific capacitances of the electrodes, respectively. Nyquist plots for the ZnCo2O4/C nanofiber electrode and ZnCo2O4 powder electrode at open circuit potential are shown in (e) and (f), respectively (the insets are expanded views of the high frequency regions of the impedance plots). The equivalent electrical circuit used to fit the impedance spectra is shown in (g) and (h) presents plots of the cyclic stabilities of the ZnCo2O4 electrodes over 1000 GCD cycles.
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Cs ¼ IΔt/AΔV
Declaration of competing interest
where I is the discharge current, A represents the area of the film elec trode, Δt represents the discharge time, and ΔV is the voltage change during the discharge time after the voltage (IR) drop. Fig. 9b and d show plots of the calculated specific capacitances as a function of current density. The specific capacitances of the ZnCo2O4/C nanofibers were found to be 327.5, 266.8, 213.0, 155.0, and 90.0 F/g at current densities of 0.5, 1.0, 2.0, 4.0, and 8.0 A/g, respectively. On the other hand, the specific capacitances of the ZnCo2O4 powders were calculated to be 93.8, 90.6, 86.7, 81.5, and 71.0 F/g at current densities of 0.25, 0.5, 1.0, 2.0, and 4.0 A/g, respectively. Thus, the specific capacitance of the ZnCo2O4/C nanofiber electrode is 1–2 times that of the ZnCo2O4 powder electrode for current densities in the range 0.5–4.0 A/g. Furthermore, it still retains a specific capacity of 90.0 F/g when the current density is 8.0 A/g. To identify the relationship between electrochemical performance and electrode kinetics, the impedances of the ZnCo2O4/C nanofiber and ZnCo2O4 powder electrodes were also measured at frequencies from 100 kHz to 0.01 Hz (Fig. 9e and f). These electrochemical impedance spectra consist of ‘depressed semicircles’ in the high frequency region and essentially straight lines in the low frequency region. The equivalent circuit commonly used to demonstrate the capacitive behavior of the ZnCo2O4/C nanofiber and ZnCo2O4 powder electrodes is shown in Fig. 9g. The intercept on the Re (Z) axis at the high frequency end cor responds to the ohmic resistance (Rs), which includes the resistance of the intrinsic substrate, contact resistance at the active material/current collector interface, and ionic resistance of the electrolyte. The main contribution here comes from the ionic resistance of the electrolyte. The resistance in the low frequency region corresponds to the chargetransfer resistance (Rct). This term is governed by fast diffusion pro cesses and accumulation of charge/discharge on the accessible surface of the electrodes. The Warburg impedance (ZW) relates to the diffusion/ transport of electrolyte through the electrode. Our results indicate that the ZnCo2O4/C nanofiber electrode has smaller Rs and Rct values (0.370 and 0.622 Ω, respectively) compared to the ZnCo2O4 powder electrode (0.375 and 0.757 Ω, respectively). Long cycle stability of the active materials is critical to the perfor mance of supercapacitors [48,49]. Curves illustrating the cycling per formance of the two electrodes are shown in Fig. 9h. The figure shows that the nanofiber electrode has ultrahigh electrochemical durability (125% real capacitance retention after 1000 cycles) and performs markedly better than the ZnCo2O4 powder electrode (91% capacitance retention) when the current density is 1.0 A/g. We speculate that the fibrous morphology of the ZnCo2O4/C nanofiber electrode effectively can inhibit the destruction of the microstructure of the ZnCo2O4 nano particle units in the electrode during the charge–discharge cycle process. Thus, the ZnCo2O4/C nanofiber electrode exhibits a much better per formance as a Faraday pseudocapacitor than the ZnCo2O4 powder electrode.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant nos. 11774042 and 11704056), the Programme for the Education Department Project of Liaoning Province (grant no. JDL2017001), Joint Research Fund Liaoning-Shenyang National Labo ratory for Materials Science (grant no. 20180510045), the Fundamental Research Funds for the Central Universities (grant no. 3132019338), the China Postdoctoral Science Foundation (grant no. 2016M591420), and the Open Fund of the State Key Laboratory of Integrated Optoelectronics (Grant nos. IOSKL2019KF06 and IOSKL2018KF02). References [1] M.A. Green, P.B. Stephen, Energy conversion approaches and materials for highefficiency photovoltaics, Nat. Mater. 16 (2017) 23–34. [2] J.H. Montoya, L.C. Seitz, P. Chakthranont, A. Vojvodic, T.F. Jaramillo, J. K. Nørskov, Materials for solar fuels and chemicals, Nat. Mater. 16 (2017) 70–81. [3] H. Sun, J. Zhu, D. Baumann, L. Peng, Y. Xu, I. Shakir, Hierarchical 3D electrodes for electrochemical energy storage, Nat. Rev. Mater. 4 (2019) 45–60. [4] J. Nai, W. Xiong, L. David, Hollow structures based on prussian blue and its analogs for electrochemical energy storage and conversion, Adv. Mater. 38 (2019) 1706825. [5] Y. Wang, Y. Song, Y. Xia, Electrochemical capacitors: mechanism, materials, systems, characterization and applications, Chem. Soc. Rev. 45 (2016) 5925–5950. [6] M.F. El-Kady, Y. Shao, R.B. Kaner, Graphene for batteries, supercapacitors and beyond, Nat. Rev. Mater. 1 (2016) 16033. [7] C. Costentin, R.P. Thomas, S. Jean-Michel, How do pseudocapacitors store energy? Theoretical analysis and experimental illustration, ACS Appl. Mater. Interfaces 9 (2017) 8649–8658. [8] Z. Pan, Y. Jiang, P. Yang, Z. Wu, W. Tian, L. Liu, In situ growth of layered bimetallic ZnCo hydroxide nanosheets for high-performance all-solid-state pseudocapacitor, ACS Nano 12 (2018) 2968–2979. [9] A. Belhboub, S. Patrice, M. C�eline, On the development of an original mesoscopic model to predict the capacitive properties of carbon-carbon supercapacitors, Electrochim. Acta 327 (2019) 135022. [10] J. Wang, C. Cui, W. Qian, Highly electroconductive mesoporous activated carbon fibers and their performance of the ionic liquid-based electrical double-layer capacitors, Carbon 154 (2019) 1–6. [11] B. Muhammad, Y. Gogotsi, MXene-conducting polymer asymmetric pseudocapacitors, Adv. Energy Mater. 9 (2019) 1802917. [12] X. Zang, C. Shen, E. Kao, R. Warren, R. Zhang, K.S. Teh, M. Sanghadasa, Titanium disulfide coated carbon nanotube hybrid electrodes enable high energy density symmetric pseudocapacitors, Adv. Mater. 30 (2018) 1704754. [13] L. Quan, T. Liu, M. Yi, Q. Chen, D. Cai, H. Zhan, Construction of hierarchical nickel cobalt selenide complex hollow spheres for pseudocapacitors with enhanced performance, Electrochim. Acta 281 (2018) 109–116. [14] C. Yuan, H.B. Wu, Y. Xie, X.W. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014) 1488–1504. [15] T. Ling, P. Da, X. Zheng, B. Ge, Z. Hu, M. Wu, S.Z. Qiao, Atomic-level structure engineering of metal oxides for high-rate oxygen intercalation pseudocapacitance, Sci. Adv. 4 (2018) eaau6261. [16] C. Hao, S. Zhou, J. Wang, X. Wang, H. Gao, C. Ge, Preparation of hierarchical spinel NiCo2O4 nanowires for high-performance supercapacitors, Ind. Eng. Chem. Res. 57 (2018) 2517–2525. [17] A.T. Aqueel Ahmed, B. Hou, H.S. Chavan, Y. Jo, S. Cho, J. Kim, H. Im, Selfassembled nanostructured CuCo2O4 for electrochemical energy storage and the oxygen evolution reaction via morphology engineering, Small 14 (2018) 1800742. [18] X. Han, Y. Yang, J.J. Zhou, Q. Ma, K. Tao, L. Han, Metal-organic framework templated 3D hierarchical ZnCo2O4@Ni(OH)2 core-shell nanosheet arrays for highperformance supercapacitors, Chem. Eur J. 24 (2018) 18106–18114. [19] H. Chen, J. Wang, X. Han, F. Liao, Y. Zhang, L. Gao, C. Xu, Facile synthesis of mesoporous ZnCo2O4 hierarchical microspheres and their excellent supercapacitor performance, Ceram. Int. 45 (2019) 8577–8584. [20] J. Xu, Y. Sun, M. Lu, L. Wang, J. Zhang, E. Tao, X. Liu, Fabrication of the porous MnCo2O4 nanorod arrays on Ni foam as an advanced electrode for asymmetric supercapacitors, Acta Mater. 152 (2018) 162–174. [21] F. Liao, X. Han, Y. Zhang, X. Han, C. Xu, H. Chen, Hydrothermal synthesis of mesoporous MnCo2O4/CoCo2O4 ellipsoid-like microstructures for highperformance electrochemical supercapacitors, Ceram. Int. 45 (2019) 7244–7252. [22] ] V. Venkatachalam, A. Alsalme, A. Alswieleh, R. Jayavel, Double hydroxide mediated synthesis of nanostructured ZnCo2O4 as high performance electrode material for supercapacitor applications, Chem. Eng. J. 321 (2017) 474–483.
4. Conclusions In summary, ZnCo2O4/C composite nanofiber electrodes were suc cessfully prepared in this work using an electrospinning process fol lowed by pre-oxidation and carbonization. Their electrochemical performance was then studied and compared with that of ZnCo2O4 powder electrodes generated using a sol-gel method. Our results show that the specific capacitances of the ZnCo2O4/C nanofibers and ZnCo2O4 powders are 327.5 and 90.6 F/g at a current density of 0.5 A/g, respectively. Moreover, after 1000 charge–discharge cycles at a current density of 1.0 A/g, the ZnCo2O4/C nanofibers and ZnCo2O4 powders retain 125% and 91% of their initial specific capaci tances, respectively. The ZnCo2O4/C nanofibers thus exhibit excellent faradic pseudocapacitance behavior and good cycling stability due to their large surface area and fibrous structure.
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[23] J. Rajesh, B. Min, J. Kim, S. Kang, H. Kim, K. Ahn, Facile hydrothermal synthesis and electrochemical supercapacitor performance of hierarchical coral-like ZnCo2O4 nanowires, J. Electroanal. Chem. 785 (2017) 48–57. [24] Y.A. Kumar, K.D. Kumar, H.J. Kim, Reagents assisted ZnCo2O4 nanomaterial for supercapacitor application, Electrochim. Acta 330 (2020) 135261. [25] Z. Lou, G. Shen, Flexible photodetectors based on 1D inorganic nanostructures, Adv. Sci. 3 (2016) 1500287. [26] H. Long, W. Zeng, H. Wang, M. Qian, Y. Liang, Z. Wang, Self-assembled biomolecular 1D nanostructures for aqueous sodium-ion battery, Adv. Sci. 5 (2018) 1700634. [27] P. Wang, Y. Wang, L. Tong, Functionalized polymer nanofibers: a versatile platform for manipulating light at the nanoscale, Light Sci. Appl. 2 (2013) e102. [28] P. Lutsyk, R. Arif, J. Hruby, A. Bukivskyi, et al., A sensing mechanism for the detection of carbon nanotubes using selective photoluminescent probes based on ionic complexes with organic dyes, Light Sci. Appl. 5 (2016) e16028, Crossref. [29] F.X. Xiao, J. Miao, H.B. Tao, S.F. Hung, H.Y. Wang, H.B. Yang, B. Liu, Onedimensional hybrid nanostructures for heterogeneous photocatalysis and photoelectrocatalysis, Small 11 (2015) 2115–2131. [30] L. Mai, J. Sheng, L. Xu, S. Tan, J. Meng, One-dimensional hetero-nanostructures for rechargeable batteries, Accounts Chem. Res. 51 (2018) 950–959. [31] Z. He, Y. Yang, H.W. Liang, J.W. Liu, S.H. Yu, Nanowire genome: a magic toolbox for 1D nanostructures, Adv. Mater. 31 (2019), 1902807. [32] Y. Wang, J. Zeng, J. Li, X. Cui, A.M. Al-Enizi, L. Zhang, G. Zheng, One-dimensional nanostructures for flexible supercapacitors, J. Mater. Chem. 3 (2015) 16382–16392. [33] Z. Yu, L. Tetard, L. Zhai, J. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions, Energy Environ. Sci. 8 (2015) 702–730. [34] G. Zhang, X. Xiao, B. Li, P. Gu, H. Xue, H. Pang, Transition metal oxides with onedimensional/one-dimensional-analogue nanostructures for advanced supercapacitors, J. Mater. Chem. 5 (2017) 8155–8186. [35] H. Yu, Y. Song, Y. Tang, Y. Li, S. Liu, Synthesis of aligned titanium-based oxide fibre arrays, Ceram. Int. 44 (2018) 12149–12156. [36] K. Liu, Z. Zhang, C. Shan, Z. Feng, J. Li, A flexible and super hydrophobic upconversion luminescence membrane as an ultrasensitive fluorescence sensor for single droplet detection, Light Sci. Appl. 5 (2016) e16136.
[37] J. Kim, G. Lee, S. Lee, S. Oh, Y. Kang, D. Kim, Tailored porous ZnCo2O4 nanofibrous electrocatalysts for lithium-oxygen batteries, Adv. Mater. Interfaces 5 (2018) 1701234. [38] L. Chang, C. Li, H. Ouyang, J. Huang, Q. Huang, Z. Xu, Flexible NiCo2O4@carbon/ carbon nanofiber electrodes fabricated by a combined electrospray/ electrospinning technique for supercapacitors, Mater. Lett. 240 (2019) 21–24. [39] X. Wei, D. Chen, W. Tang, Preparation and characterization of the spinel oxide ZnCo2O4 obtained by sol-gel method, Mater. Chem. Phys. 103 (2007) 54–58. [40] L.S. Lobo, A.R. Kumar, Structural and electrical properties of ZnCo2O4 spinel synthesized by sol-gel combustion method, J. Non-Cryst. Solids 505 (2019) 301–309. [41] H. Liu, J. Wang, One-pot synthesis of ZnCo2O4 nanorod anodes for high power Lithium ions batteries, Electrochim. Acta 92 (2013) 371–375. [42] B.H. Qu, L.L. Hu, Q.H. Li, Y.G. Wang, L.B. Chen, T.H. Wang, High-performance lithium-ion battery anode by direct growth of hierarchical ZnCo2O4 nanostructures on current collectors, ACS Appl. Mater. Interfaces 6 (2014) 731–736. [43] S.H. Choi, Y.C. Kang, Yolk-shell, hollow, and single-crystalline ZnCo2O4 powders: preparation using a simple one-pot process and application in lithium-ion batteries, ChemSusChem 6 (2013) 2111–2116. [44] Q. Xie, D. Zeng, Y. Ma, L. Lin, L. Wang, D.L. Peng, Synthesis of ZnO-ZnCo2O4 hybrid hollow microspheres with excellent lithium storage properties, Electrochim. Acta 169 (2015) 283–290. [45] N. Joshi, L.F. da Silva, H.S. Jadhav, F.M. Shimizu, P.H. Suman, J.C. M’Peko, O. N. Oliveira Jr., Yolk-shelled ZnCo2O4 microspheres: surface properties and gas sensing application, Sensor. Actuator. B Chem. 257 (2018) 906–915. [46] Y.A. Kumar, K.D. Kumar, H.J. Kim, Reagents assisted ZnCo2O4 nanomaterial for supercapacitor application, Electrochim. Acta 11 (2019) 135261. [47] Y. Shang, T. Xie, Y. Gai, L. Su, L. Gong, H. Lv, F. Dong, Self-assembled hierar-chical peony-like ZnCo2O4 for high-performance asymmetric supercapacitors, Electrochim. Acta 253 (2017) 281–290. [48] A.J.C. Mary, A.C. Bose, Surfactant assisted ZnCo2O4 nanomaterial for supercapacitor application, Appl. Surf. Sci. 449 (2018) 105–112. [49] G. Zhou, J. Zhu, Y. Chen, L. Mei, X. Duan, G. Zhang, L. Chen, T. Wang, B. Lu, Simple method for the preparation of highly porous ZnCo2O4 nanotubes with enhanced electrochemical property for supercapacitor, Electrochim. Acta 123 (2014) 450–455.
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Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs
Electrospun ZnCo2O4/C composite nanofibers with superior electrochemical performance for supercapacitor Hongquan Yu a, *, Hengyan Zhao b, Yanbo Wu b, Baojiu Chen a, **, Jiashi Sun a a b
Department of Physics, Dalian Maritime University, Dalian, 116026, Liaoning, China College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian, 116028, Liaoning, China
A B S T R A C T
In recent years, supercapacitors have attracted much attention due to their excellent charge–discharge performance, long-term cycle lifetimes, and high specific power. Transition metal oxides are promising candidates for environmentally friendly, low-cost, and high-powered energy storage materials. In addition, the use of one-dimensional nanostructures can markedly enhance the characteristic properties of the transition metal oxides. In this work, ZnCo2O4/C composite nanofibers were successfully fabricated by using a simple electrospinning method. The nanofibers have an average diameter of 300–500 nm and consist of many ZnCo2O4 and carbon nanoparticles. A supercapacitor fabricated using the ZnCo2O4/C nanofibers was found to exhibit a high specific capacitance of 90.0 F/g at a current density of 8.0 A/g and good rate capability under current densities in the range 0.5–8.0 A/g at a voltage of 0.4 V. It also has superior working stability with a capacitance retention of 125% after 1000 cycles in air.
1. Introduction The storage of electrochemical energy continues to be a highly active field of research due to its wide range of applications, from portable devices and wearable electronics to electric vehicles and smart grids [1–4]. Among the various energy-storage technologies, supercapacitors have attracted a great deal of attention in recent years owing to their high-power density, outstanding cycle stability, and fast char ge–discharge characteristics. Generally speaking, there are two types of supercapacitors based on the charge-storage mechanism involved: electric double-layer capacitors and Faraday pseudocapacitors [5–8]. Traditional carbon-based mate rials rely on the double-layer capacitance mechanism, which means they can only store a limited amount of energy [9,10]. Recent cutting-edge research has focused on pseudocapacitive-type electrode materials, wherein the charge is stored not only via ion adsorption but also near surfaces where redox reactions occur. Therefore, the properties of such pseudocapacitors mainly depend on the electrochemical performance of the electrode materials [11–13]. Mixed transition metal oxides (MTMOs) can partake in a rich array of reversible oxygen anion redox reactions, which can significantly improve the electrochemical performance of the pseudocapacitor [14, 15]. Therefore, MTMOs with large surface areas, excellent energy and power densities, high charge–discharge efficiencies, and good cycle
stabilities, are promising electrode materials for use in supercapacitor devices. In particular, transition metal–cobalt based oxides (e.g. NiCo2O4, CuCo2O4, ZnCo2O4, and MnCo2O4) have interested a large number of researchers due to their high theoretical capacitances, low-cost, and relatively low pollution risks [16–21]. Recently, several applications of ZnCo2O4 for higher performance supercapacitors have been evaluated to meet these challenges. Venkatachalam et al. reported ZnCo2O4 nanostructures for supercapacitor applications with a specific capacitance (Cs) of 845.7 F/g at a current density of 1.0 A/g [22]. Rajesh et al. developed hierarchical coral-like ZnCo2O4 nanowires exhibiting a Cs of 264 F/g at 10.0 A/g [23]. Kim et al. reported tests on a Zn–Co-urea/ammonium fluoride/hexamethylenetetramine with a high Cs of 462.5 C/g at 1.0 A/g [24]. Since the ground-breaking discovery of carbon nanotubes in the 1990s, one dimensional (1D) nanostructures have also attracted a sig nificant amount of research interest owing to their remarkable physical and chemical properties [25–28]. 1D nanostructures have high length/diameter ratios and large specific surface areas (SSAs) and quantum confinement effects. Together with their unique electrical, optical, photovoltaic, and mechanical properties, this makes them highly suitable for use in various contexts, e.g. solar cells, field emission applications, and energy conversion and storage [29–31]. When it comes to electrochemical energy storage, and especially supercapacitor fabrication, 1D nanostructures enjoy several appealing
* Corresponding author. No. 1, Linghai Road, Dalian, 116026, Liaoning, China. ** Corresponding author. No. 1, Linghai Road, Dalian, 116026, Liaoning, China. E-mail addresses: [email protected] (H. Yu), [email protected] (B. Chen). https://doi.org/10.1016/j.jpcs.2020.109385 Received 17 December 2019; Received in revised form 21 January 2020; Accepted 28 January 2020 Available online 31 January 2020 0022-3697/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic diagram of the set-up used in the electrospinning process.
advantages: (i) using them can shorten electronic pathways and facili tate electrical transport in an axial direction (unlike their bulk coun terparts); (ii) 1D nanostructured electrodes have shorter ion diffusion lengths which are beneficial to the rate capability (the characteristic time taken for ions to diffuse through an electrode is t � l2/D, where l represents the diffusion length and D is the diffusion coefficient [32]); (iii) 1D nanostructures have large electroactive SSAs which increase the electrode/electrolyte contact surface/interface, which further enhances the electrode kinetics facilitating ultrafast charge–discharge capability; (iv) 1D nanoarchitectures can be readily utilized to form various higher-level multi-dimensional structures with improved SSAs, good mechanical stabilities, and enhanced cycling performance for use in supercapacitors [33,34]. Electrospinning is a simple, versatile, technique for fabricating nanofibers that are exceptionally long and uniform in diameter and is based on a variety of functional materials [35,36]. The diameters of the fibers prepared using this method can range from tens of nanometres to several microns. The morphology of the electrospun fibers depends on the solvent and solution properties, as well as on other process variables. Efforts have been made in the last ten years to develop 1-d materials for batteries and capacitors. Kim et al. fabricated ZnCo2O4 nanofibers, designed a porous nanostructure using a facile electrospinning process and used selective etching of ZnO to form the cathode material in lithium-oxygen batteries [37]. Chang et al. developed a combined electrospray/electrospinning technique to obtain NiC o2O4@Carbon/Carbon composite nanofiber to produce flexible elec trodes with enhanced performance [38]. In this work, 1D ZnCo2O4/C composite nanofibers were fabricated using a simple electrospinning method. The measured surface area of the nanofibers is 199.0 m2/g and the average pore size (calculated using the Barrett–Joyner–Halenda model) is ~3.83 nm. The ZnCo2O4/C com posite fibers have average diameters of 300–500 nm and consist of many ZnCo2O4 and carbon nanoparticles. The ZnCo2O4/C composite fibers have high specific capacitance 90.0 F/g (8.0 A/g) and good cycle sta bility which maintains 125% after 1000 cycles.
Fig. 2. TG and DSC curves of the ZnCo2O4/C precursor nanofibers.
between the two. The charged jet was attracted to the collector where upon it solidified as the solvent evaporates. In our experiments, the applied voltage was 12 kV and the collection distance was 140 mm. The as-prepared hybrid precursor samples were then dried in a vacuum oven for 24 h at room temperature before being subjected to pre-oxidation and carbonization. Pre-oxidation was carried out by heating the fibers to 200 � C at a rate of 5 � C/min and maintaining them at that temperature for 4 h in the presence of air. The stabilized fibers were then carbonized at 500, 600, 700, and 800 � C under an atmosphere of argon (a heating rate of 5 � C/min was again used). The final elec trospun ZnCo2O4/C composite nanofibers were thereby obtained. For comparison purposes, ZnCo2O4 powders were also prepared using a sol-gel technique appearing in the literature [39]. 2.2. Physical characterization The crystal structure of the samples was determined via X-ray diffraction (XRD). A Cu Kα radiation source was employed in this work (λ ¼ 0.15405 nm). The morphology of the samples was also observed using a field-emission scanning electron microscope (SEM; ZEISS SUPRA 55). Transmission electron microscope (TEM) images, high-resolution (HR) TEM images, and EDS) patterns were recorded by a TEM (JEM2010) at a working voltage of 200 kV. Thermogravimetric (TG) and differential scanning calorimetry (DSC) data were recorded using a simultaneous thermal analyzer (NETZSCH STA 449 F3). Specific surface areas and pore size distributions (PSDs) were determined by conducting Brunauer–Emmett–Teller (BET) measurements (Micromeritics ASAP 2020). 2.3. Electrochemical measurements The electrochemical performance of ZnCo2O4/C-based electrodes was investigated in aqueous KOH solution using an electrochemical workstation (CHI660B, CH Instruments Co, Shanghai, China). A threeelectrode cell configuration consisting of a platinum wire auxiliary electrode and a standard calomel electrode (SCE) as reference was employed. The working electrode was prepared by mixing ZnCo2O4/C composition nanofibers, carbon black, and solution of polymer binder (polytetrafluoroethylene) in proportions of 85 : 10 : 5 (by weight). A few drops of ethanol were also added. The mixture was then dispersed ul trasonically for 15 min to form a slurry. The slurry was then pasted onto nickel foam in the unit area and dried in an oven at 80 � C for 8 h.
2. Experimental section 2.1. Preparation of materials The synthesis process typically involved dissolving Zn (CH3COO)2⋅4H2O (0.439 g), Co(CH3COO)2⋅4H2O (0.996 g), and polyvinylpyrrolidone (PVP; 3.610 g) in N,N-dimethlformamide (DMF; 20 mL). The solution was then stirred for 10 h to acquire a homogeneous hybrid sol for electrospinning. A schematic diagram of the electrospinning apparatus is shown in Fig. 1. Three key items of equipment could be attained: a high-voltage power supply, a spinneret, and a collector. Electrospinning was per formed by pumping the solution through a nozzle towards the collector. An electrically charged jet was formed because of the high voltage 2
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Fig. 3. SEM images of ZnCo2O4/C nanofibers prepared at different carbonization temperatures: (a) 500 � C, (b) 600 � C, (c) 700 � C, and (d) 800 � C. The scale bar corresponds to 1.0 μm.
Fig. 4. (a) A TEM image, (b) magnified TEM image, (c) HR-TEM image and (I) SAED pattern acquired using a single ZnCo2O4/C nanofiber obtained at a temperature of 600 � C. Elemental mapping images are shown in (d–h).
3. Results and discussion
the TG curve: (i) a loss of ~3% up to 100 � C which is attributed to the release of adsorbed water and water of crystallization (from the Zn (CH3COO)2⋅2H2O and Co(CH3COO)2⋅4H2O); (ii) a loss of ~7% up to 250 � C which is due to thermal decomposition of acetate in the precursor fibers; and (iii) a major weight loss of 70% at ~430 � C which is
Fig. 2 illustrates the thermal analysis (TG-DSC) curves obtained using ZnCo2O4/C precursor fibers in a nitrogen atmosphere (using a heating � rate of 5 C/min). Three major weight-loss stages could be identified in 3
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Fig. 5. (a) TEM image and SAED pattern (inset) of ZnCo2O4 nanopowders; (b) HR-TEM image of ZnCo2O4 nanopowders.
successfully prepared. Therefore, it ‘can be concluded that the single ZnCo2O4/C nanofiber consists of polycrystalline ZnCo2O4 nanoparticles and carbon nanoparticles. The TEM and HR-TEM images of ZnCo2O4 powders are shown in Fig. 5: the morphology of ZnCo2O4 powders is that of uniform, polycrystalline rhomboidal nanoparticles with diameters ranging from c. 10 nm to c. 100 nm. The lattice fringes of the ZnCo2O4 powders are shown in Fig. 5b: the fringe spacings were detected to be 0.461 nm and 0.285 nm, which correspond to the (111) and (220) lattice planes of a typical spinel-structure ZnCo2O4 [42]. The XRD patterns of ZnCo2O4/C nanofibers prepared using different carbonization temperatures are shown in Fig. 6. The diffraction peaks clearly become sharper as the carbonization temperature increases. The peaks located at 18.9� , 31.2� , 36.8� , 38.5� , 44.7� , 48.9� , 55.6� , and 59.3� can be attributed to the 111, 220, 311, 222, 400, 422, 511, and 440 planes of cubic ZnCo2O4 (JCPDS card: 23–1390) [43]. Moreover, the peaks detected at 34.4� and 36.3� are attributed to the 002 and 101 planes of hexagonal ZnO (JCPDS card: 80–0074), according to the literature [44]. Fig. 7 shows N2 adsorption–desorption isotherms and Bar rett–Joyner–Halenda pore size distribution curves obtained using the ZnCo2O4 nanofibers and ZnCo2O4 powder. The hysteresis apparent in the N2 adsorption–desorption isotherms can be classified, according to the International Union of Pure and Applied Chemistry, as belonging to category type IV (type H1 hysteresis loop). This behavior was typically observed in mesoporous materials formed by agglomerated particles [45]. The specific surface area calculated for the ZnCo2O4 nanofibers (199.0 m2/g) is much higher than that of the ZnCo2O4 powder (30.48 m2/g). The pore diameters of the ZnCo2O4/C composite nanofibers lie in the range 2–10 nm (Fig. 7b), giving an average pore diameter of 3.83 nm. The pore diameters determined for the ZnCo2O4 powders lie in the range 2–30 nm (Fig. 7d), with an average value of 16.83 nm. The ZnCo2O4/C composite nanofibers therefore have a fine meso porous structure that effectively facilitates the diffusion of electrolytes and rapid charge transfer, and hence endows the nanofibers with excellent electrochemical performance. The electrochemical characteristics of electrodes fabricated from ZnCo2O4/C composite nanofibers and ZnCo2O4 powder were investi gated in strongly alkaline solution (6 M KOH). Fig. 8 illustrates CV curves derived using these electrodes for a variety of scan rates. The shape of each CV curve deviates from a rectangular profile. This suggests that redox reactions are taking place and the MTMOs are contributing to the charge-storage process via reactions such as:
Fig. 6. XRD patterns obtained using ZnCo2O4/C nanofibers prepared using different carbonization temperatures: (a) 500 � C, (b) 600 � C, (c) 700 � C, and (d) 800 � C.
attributed to the thermal decomposition and volatilization of PVP and other organic compounds (and corresponds to the sharp peak observed in the DSC curve). No significant weight loss was observed above 500 � C, which confirms the formation of inorganic ZnCo2O4/C. Furthermore, to ensure complete decomposition of the precursor, calcination tempera tures in the range 500–800 � C were chosen to synthesize several ZnCo2O4/C nanofiber samples. Fig. 3 indicates SEM images of the ZnCo2O4/C composite nanofibers carbonized at different temperatures. It can be seen that the nanofibers undergo crispation as the carbonization temperature increases. The di ameters of the nanofibers also gradually decrease from ~500 to ~100 nm. Fig. 4a–i shows a TEM image, selected area electron diffraction (SAED) pattern, and HR-TEM image of the ZnCo2O4/C composite nanofibers, respectively. It can be seen from the images that the ZnCo2O4/C nanofibers are 1D. Some pin holes can be seen randomly distributed within the fibers (Fig. 4a). The SEAD pattern in Fig. 4i in dicates the presence of polycrystalline rings and the (311), (220), and (422) planes can be indexed according to the spinel-like structure of ZnCo2O4 [40]. The lattice fringes from the constituent nanoparticles that have a separation of 0.248 nm can be indexed to the (311) planes of ZnCo2O4 in a cubic phase [41]. This is in good agreement with the most intense diffraction peak in the XRD pattern. Accordingly, EDS mapping images of a single ZnCo2O4/C nanofiber are demonstrated in Fig. 4d–h. Elemental Zn, Co, and O atoms are seen to be evenly distributed over the length and width of the single ZnCo2O4/C nanofiber (except for the C supported carbon grid) which indicates that the required nanofiber was
ZnCo2O4 þ OH þ H2O ⇌ ZnOOH þ 2CoOOH þ e– –
CoOOH þ OH ⇌ CoO2 þ H2O þ e
(1) (2)
It is also clear that the redox peaks progressively shift and broaden as the sweep rate is increased. This behavior can be attributed to the lim itations placed on charge transfer and diffusion rate of the electrolyte when the charging and discharging processes were carried out at high speed [46,47]. The electrode fabricated using ZnCo2O4/C nanofibers 4
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Fig. 7. Nitrogen adsorption–desorption isotherms for ZnCo2O4/C nanofibers and ZnCo2O4 powders are shown in (a) and (c), respectively. The corresponding pore size distributions are shown in (b) and (d), respectively.
Fig. 8. CV curves for (a) ZnCo2O4/C nanofibers, and (b) ZnCo2O4 powders using different scan rates.
exhibits a substantially higher current density compared to the one fabricated using powder. Also, the larger areas under the CV curves indicate that they have greatly enhanced capacitive performance compared to the ZnCo2O4 powder electrode. Fig. 9a and c presents galvanostatic charge–discharge (GCD) curves obtained using the electrodes fabricated from ZnCo2O4/C nanofibers
and ZnCo2O4 powders for a variety of current densities. Distinct plateaus are found in each GCD curve indicating the pseudocapacitive behavior of ZnCo2O4. The specific capacitances of the electrodes (Cs) can be calculated using these GCD curves and other electrode data via the following expression:
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Fig. 9. GCD curves obtained using a ZnCo2O4/C nanofiber electrode and ZnCo2O4 powder electrode are illustrated in (a) and (c), respectively, for different current densities. (b) and (d) show the corresponding specific capacitances of the electrodes, respectively. Nyquist plots for the ZnCo2O4/C nanofiber electrode and ZnCo2O4 powder electrode at open circuit potential are shown in (e) and (f), respectively (the insets are expanded views of the high frequency regions of the impedance plots). The equivalent electrical circuit used to fit the impedance spectra is shown in (g) and (h) presents plots of the cyclic stabilities of the ZnCo2O4 electrodes over 1000 GCD cycles.
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Cs ¼ IΔt/AΔV
Declaration of competing interest
where I is the discharge current, A represents the area of the film elec trode, Δt represents the discharge time, and ΔV is the voltage change during the discharge time after the voltage (IR) drop. Fig. 9b and d show plots of the calculated specific capacitances as a function of current density. The specific capacitances of the ZnCo2O4/C nanofibers were found to be 327.5, 266.8, 213.0, 155.0, and 90.0 F/g at current densities of 0.5, 1.0, 2.0, 4.0, and 8.0 A/g, respectively. On the other hand, the specific capacitances of the ZnCo2O4 powders were calculated to be 93.8, 90.6, 86.7, 81.5, and 71.0 F/g at current densities of 0.25, 0.5, 1.0, 2.0, and 4.0 A/g, respectively. Thus, the specific capacitance of the ZnCo2O4/C nanofiber electrode is 1–2 times that of the ZnCo2O4 powder electrode for current densities in the range 0.5–4.0 A/g. Furthermore, it still retains a specific capacity of 90.0 F/g when the current density is 8.0 A/g. To identify the relationship between electrochemical performance and electrode kinetics, the impedances of the ZnCo2O4/C nanofiber and ZnCo2O4 powder electrodes were also measured at frequencies from 100 kHz to 0.01 Hz (Fig. 9e and f). These electrochemical impedance spectra consist of ‘depressed semicircles’ in the high frequency region and essentially straight lines in the low frequency region. The equivalent circuit commonly used to demonstrate the capacitive behavior of the ZnCo2O4/C nanofiber and ZnCo2O4 powder electrodes is shown in Fig. 9g. The intercept on the Re (Z) axis at the high frequency end cor responds to the ohmic resistance (Rs), which includes the resistance of the intrinsic substrate, contact resistance at the active material/current collector interface, and ionic resistance of the electrolyte. The main contribution here comes from the ionic resistance of the electrolyte. The resistance in the low frequency region corresponds to the chargetransfer resistance (Rct). This term is governed by fast diffusion pro cesses and accumulation of charge/discharge on the accessible surface of the electrodes. The Warburg impedance (ZW) relates to the diffusion/ transport of electrolyte through the electrode. Our results indicate that the ZnCo2O4/C nanofiber electrode has smaller Rs and Rct values (0.370 and 0.622 Ω, respectively) compared to the ZnCo2O4 powder electrode (0.375 and 0.757 Ω, respectively). Long cycle stability of the active materials is critical to the perfor mance of supercapacitors [48,49]. Curves illustrating the cycling per formance of the two electrodes are shown in Fig. 9h. The figure shows that the nanofiber electrode has ultrahigh electrochemical durability (125% real capacitance retention after 1000 cycles) and performs markedly better than the ZnCo2O4 powder electrode (91% capacitance retention) when the current density is 1.0 A/g. We speculate that the fibrous morphology of the ZnCo2O4/C nanofiber electrode effectively can inhibit the destruction of the microstructure of the ZnCo2O4 nano particle units in the electrode during the charge–discharge cycle process. Thus, the ZnCo2O4/C nanofiber electrode exhibits a much better per formance as a Faraday pseudocapacitor than the ZnCo2O4 powder electrode.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant nos. 11774042 and 11704056), the Programme for the Education Department Project of Liaoning Province (grant no. JDL2017001), Joint Research Fund Liaoning-Shenyang National Labo ratory for Materials Science (grant no. 20180510045), the Fundamental Research Funds for the Central Universities (grant no. 3132019338), the China Postdoctoral Science Foundation (grant no. 2016M591420), and the Open Fund of the State Key Laboratory of Integrated Optoelectronics (Grant nos. IOSKL2019KF06 and IOSKL2018KF02). References [1] M.A. Green, P.B. Stephen, Energy conversion approaches and materials for highefficiency photovoltaics, Nat. Mater. 16 (2017) 23–34. [2] J.H. Montoya, L.C. Seitz, P. Chakthranont, A. Vojvodic, T.F. Jaramillo, J. K. Nørskov, Materials for solar fuels and chemicals, Nat. Mater. 16 (2017) 70–81. [3] H. Sun, J. Zhu, D. Baumann, L. Peng, Y. Xu, I. Shakir, Hierarchical 3D electrodes for electrochemical energy storage, Nat. Rev. Mater. 4 (2019) 45–60. [4] J. Nai, W. Xiong, L. David, Hollow structures based on prussian blue and its analogs for electrochemical energy storage and conversion, Adv. Mater. 38 (2019) 1706825. [5] Y. Wang, Y. Song, Y. Xia, Electrochemical capacitors: mechanism, materials, systems, characterization and applications, Chem. Soc. Rev. 45 (2016) 5925–5950. [6] M.F. El-Kady, Y. Shao, R.B. Kaner, Graphene for batteries, supercapacitors and beyond, Nat. Rev. Mater. 1 (2016) 16033. [7] C. Costentin, R.P. Thomas, S. Jean-Michel, How do pseudocapacitors store energy? Theoretical analysis and experimental illustration, ACS Appl. Mater. Interfaces 9 (2017) 8649–8658. [8] Z. Pan, Y. Jiang, P. Yang, Z. Wu, W. Tian, L. Liu, In situ growth of layered bimetallic ZnCo hydroxide nanosheets for high-performance all-solid-state pseudocapacitor, ACS Nano 12 (2018) 2968–2979. [9] A. Belhboub, S. Patrice, M. C�eline, On the development of an original mesoscopic model to predict the capacitive properties of carbon-carbon supercapacitors, Electrochim. Acta 327 (2019) 135022. [10] J. Wang, C. Cui, W. Qian, Highly electroconductive mesoporous activated carbon fibers and their performance of the ionic liquid-based electrical double-layer capacitors, Carbon 154 (2019) 1–6. [11] B. Muhammad, Y. Gogotsi, MXene-conducting polymer asymmetric pseudocapacitors, Adv. Energy Mater. 9 (2019) 1802917. [12] X. Zang, C. Shen, E. Kao, R. Warren, R. Zhang, K.S. Teh, M. Sanghadasa, Titanium disulfide coated carbon nanotube hybrid electrodes enable high energy density symmetric pseudocapacitors, Adv. Mater. 30 (2018) 1704754. [13] L. Quan, T. Liu, M. Yi, Q. Chen, D. Cai, H. Zhan, Construction of hierarchical nickel cobalt selenide complex hollow spheres for pseudocapacitors with enhanced performance, Electrochim. Acta 281 (2018) 109–116. [14] C. Yuan, H.B. Wu, Y. Xie, X.W. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014) 1488–1504. [15] T. Ling, P. Da, X. Zheng, B. Ge, Z. Hu, M. Wu, S.Z. Qiao, Atomic-level structure engineering of metal oxides for high-rate oxygen intercalation pseudocapacitance, Sci. Adv. 4 (2018) eaau6261. [16] C. Hao, S. Zhou, J. Wang, X. Wang, H. Gao, C. Ge, Preparation of hierarchical spinel NiCo2O4 nanowires for high-performance supercapacitors, Ind. Eng. Chem. Res. 57 (2018) 2517–2525. [17] A.T. Aqueel Ahmed, B. Hou, H.S. Chavan, Y. Jo, S. Cho, J. Kim, H. Im, Selfassembled nanostructured CuCo2O4 for electrochemical energy storage and the oxygen evolution reaction via morphology engineering, Small 14 (2018) 1800742. [18] X. Han, Y. Yang, J.J. Zhou, Q. Ma, K. Tao, L. Han, Metal-organic framework templated 3D hierarchical ZnCo2O4@Ni(OH)2 core-shell nanosheet arrays for highperformance supercapacitors, Chem. Eur J. 24 (2018) 18106–18114. [19] H. Chen, J. Wang, X. Han, F. Liao, Y. Zhang, L. Gao, C. Xu, Facile synthesis of mesoporous ZnCo2O4 hierarchical microspheres and their excellent supercapacitor performance, Ceram. Int. 45 (2019) 8577–8584. [20] J. Xu, Y. Sun, M. Lu, L. Wang, J. Zhang, E. Tao, X. Liu, Fabrication of the porous MnCo2O4 nanorod arrays on Ni foam as an advanced electrode for asymmetric supercapacitors, Acta Mater. 152 (2018) 162–174. [21] F. Liao, X. Han, Y. Zhang, X. Han, C. Xu, H. Chen, Hydrothermal synthesis of mesoporous MnCo2O4/CoCo2O4 ellipsoid-like microstructures for highperformance electrochemical supercapacitors, Ceram. Int. 45 (2019) 7244–7252. [22] ] V. Venkatachalam, A. Alsalme, A. Alswieleh, R. Jayavel, Double hydroxide mediated synthesis of nanostructured ZnCo2O4 as high performance electrode material for supercapacitor applications, Chem. Eng. J. 321 (2017) 474–483.
4. Conclusions In summary, ZnCo2O4/C composite nanofiber electrodes were suc cessfully prepared in this work using an electrospinning process fol lowed by pre-oxidation and carbonization. Their electrochemical performance was then studied and compared with that of ZnCo2O4 powder electrodes generated using a sol-gel method. Our results show that the specific capacitances of the ZnCo2O4/C nanofibers and ZnCo2O4 powders are 327.5 and 90.6 F/g at a current density of 0.5 A/g, respectively. Moreover, after 1000 charge–discharge cycles at a current density of 1.0 A/g, the ZnCo2O4/C nanofibers and ZnCo2O4 powders retain 125% and 91% of their initial specific capaci tances, respectively. The ZnCo2O4/C nanofibers thus exhibit excellent faradic pseudocapacitance behavior and good cycling stability due to their large surface area and fibrous structure.
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[23] J. Rajesh, B. Min, J. Kim, S. Kang, H. Kim, K. Ahn, Facile hydrothermal synthesis and electrochemical supercapacitor performance of hierarchical coral-like ZnCo2O4 nanowires, J. Electroanal. Chem. 785 (2017) 48–57. [24] Y.A. Kumar, K.D. Kumar, H.J. Kim, Reagents assisted ZnCo2O4 nanomaterial for supercapacitor application, Electrochim. Acta 330 (2020) 135261. [25] Z. Lou, G. Shen, Flexible photodetectors based on 1D inorganic nanostructures, Adv. Sci. 3 (2016) 1500287. [26] H. Long, W. Zeng, H. Wang, M. Qian, Y. Liang, Z. Wang, Self-assembled biomolecular 1D nanostructures for aqueous sodium-ion battery, Adv. Sci. 5 (2018) 1700634. [27] P. Wang, Y. Wang, L. Tong, Functionalized polymer nanofibers: a versatile platform for manipulating light at the nanoscale, Light Sci. Appl. 2 (2013) e102. [28] P. Lutsyk, R. Arif, J. Hruby, A. Bukivskyi, et al., A sensing mechanism for the detection of carbon nanotubes using selective photoluminescent probes based on ionic complexes with organic dyes, Light Sci. Appl. 5 (2016) e16028, Crossref. [29] F.X. Xiao, J. Miao, H.B. Tao, S.F. Hung, H.Y. Wang, H.B. Yang, B. Liu, Onedimensional hybrid nanostructures for heterogeneous photocatalysis and photoelectrocatalysis, Small 11 (2015) 2115–2131. [30] L. Mai, J. Sheng, L. Xu, S. Tan, J. Meng, One-dimensional hetero-nanostructures for rechargeable batteries, Accounts Chem. Res. 51 (2018) 950–959. [31] Z. He, Y. Yang, H.W. Liang, J.W. Liu, S.H. Yu, Nanowire genome: a magic toolbox for 1D nanostructures, Adv. Mater. 31 (2019), 1902807. [32] Y. Wang, J. Zeng, J. Li, X. Cui, A.M. Al-Enizi, L. Zhang, G. Zheng, One-dimensional nanostructures for flexible supercapacitors, J. Mater. Chem. 3 (2015) 16382–16392. [33] Z. Yu, L. Tetard, L. Zhai, J. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions, Energy Environ. Sci. 8 (2015) 702–730. [34] G. Zhang, X. Xiao, B. Li, P. Gu, H. Xue, H. Pang, Transition metal oxides with onedimensional/one-dimensional-analogue nanostructures for advanced supercapacitors, J. Mater. Chem. 5 (2017) 8155–8186. [35] H. Yu, Y. Song, Y. Tang, Y. Li, S. Liu, Synthesis of aligned titanium-based oxide fibre arrays, Ceram. Int. 44 (2018) 12149–12156. [36] K. Liu, Z. Zhang, C. Shan, Z. Feng, J. Li, A flexible and super hydrophobic upconversion luminescence membrane as an ultrasensitive fluorescence sensor for single droplet detection, Light Sci. Appl. 5 (2016) e16136.
[37] J. Kim, G. Lee, S. Lee, S. Oh, Y. Kang, D. Kim, Tailored porous ZnCo2O4 nanofibrous electrocatalysts for lithium-oxygen batteries, Adv. Mater. Interfaces 5 (2018) 1701234. [38] L. Chang, C. Li, H. Ouyang, J. Huang, Q. Huang, Z. Xu, Flexible NiCo2O4@carbon/ carbon nanofiber electrodes fabricated by a combined electrospray/ electrospinning technique for supercapacitors, Mater. Lett. 240 (2019) 21–24. [39] X. Wei, D. Chen, W. Tang, Preparation and characterization of the spinel oxide ZnCo2O4 obtained by sol-gel method, Mater. Chem. Phys. 103 (2007) 54–58. [40] L.S. Lobo, A.R. Kumar, Structural and electrical properties of ZnCo2O4 spinel synthesized by sol-gel combustion method, J. Non-Cryst. Solids 505 (2019) 301–309. [41] H. Liu, J. Wang, One-pot synthesis of ZnCo2O4 nanorod anodes for high power Lithium ions batteries, Electrochim. Acta 92 (2013) 371–375. [42] B.H. Qu, L.L. Hu, Q.H. Li, Y.G. Wang, L.B. Chen, T.H. Wang, High-performance lithium-ion battery anode by direct growth of hierarchical ZnCo2O4 nanostructures on current collectors, ACS Appl. Mater. Interfaces 6 (2014) 731–736. [43] S.H. Choi, Y.C. Kang, Yolk-shell, hollow, and single-crystalline ZnCo2O4 powders: preparation using a simple one-pot process and application in lithium-ion batteries, ChemSusChem 6 (2013) 2111–2116. [44] Q. Xie, D. Zeng, Y. Ma, L. Lin, L. Wang, D.L. Peng, Synthesis of ZnO-ZnCo2O4 hybrid hollow microspheres with excellent lithium storage properties, Electrochim. Acta 169 (2015) 283–290. [45] N. Joshi, L.F. da Silva, H.S. Jadhav, F.M. Shimizu, P.H. Suman, J.C. M’Peko, O. N. Oliveira Jr., Yolk-shelled ZnCo2O4 microspheres: surface properties and gas sensing application, Sensor. Actuator. B Chem. 257 (2018) 906–915. [46] Y.A. Kumar, K.D. Kumar, H.J. Kim, Reagents assisted ZnCo2O4 nanomaterial for supercapacitor application, Electrochim. Acta 11 (2019) 135261. [47] Y. Shang, T. Xie, Y. Gai, L. Su, L. Gong, H. Lv, F. Dong, Self-assembled hierar-chical peony-like ZnCo2O4 for high-performance asymmetric supercapacitors, Electrochim. Acta 253 (2017) 281–290. [48] A.J.C. Mary, A.C. Bose, Surfactant assisted ZnCo2O4 nanomaterial for supercapacitor application, Appl. Surf. Sci. 449 (2018) 105–112. [49] G. Zhou, J. Zhu, Y. Chen, L. Mei, X. Duan, G. Zhang, L. Chen, T. Wang, B. Lu, Simple method for the preparation of highly porous ZnCo2O4 nanotubes with enhanced electrochemical property for supercapacitor, Electrochim. Acta 123 (2014) 450–455.
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