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Anchoring 2D NiMoO4 nano-plates on flexible carbon cloth as a binder-free electrode for efficient energy storage devices
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Anchoring 2D NiMoO4 nano-plates on flexible carbon cloth as a binder-free electrode for efficient energy storage devices Yasir Abbasa, Sining Yuna,∗, Muhammad Sufyan Javedb,c,∗∗, Jiageng Chena, Muhammad Faizan Tahird, Ziqi Wanga, Chao Yanga, Asim Arshada, Shahid Hussaine a
Functional Materials Laboratory (FML), School of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China Siyuan Laboratory, Department of Physics, Jinan University, Guangzhou, 510632, China c Department of Physics, COMSATS University Islamabad, Lahore Campus, Lahore, 54000, Pakistan d School of Electric Power, South China University of Technology, Guangzhou 510640, China e School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, China b
ARTICLE INFO
ABSTRACT
Keywords: Energy storage Mixed metal oxide Electrode Carbon cloth High-performance Supercapacitor
The energy security and mounting environmental issues compel the scientific community to allocate greatly efficient and economical energy renovation and storage systems. Among the energy storage devices, supercapacitors have become the forefront in energy storing systems in recent decades. The efficiency of supercapacitors mainly depend on the electrode's material and they usually suffer from a quick reduction in specific capacitance at higher current densities. Herein, we combined the nano-plates like bimetallic oxides (NiMiO4) with mixed valence states on the surface of a conductive substrate (carbon cloth) without any binder and additives (denoted NMO@CC). The as-prepared electrode NMO@CC showed marvelous electrochemical properties in the aqueous basic electrolyte by achieving a high capacity of 1500 C g−1 at current density of 5 A g−1 with high degree of rate capability. More interestingly, the NMO@CC electrode demonstrated excellent cycling stability of 94.63% after 5000 cycles during charge-discharge process. Further, the charge storage mechanism of NMO@CC electrode is investigated by analyzing the surface capacitive and diffusion controlled processes and it shows high surface capacitive storage (71%). These admirable results are based on the highly open channels for efficient diffusion of electrolyte ions and electronic transmission through the NMO and backbone carbon cloth, respectively. Therefore, accurate morphology and surface manufacturing engineering are highly appreciated to enhance the active surface area and inherent conductivity of electrode materials.
1. Introduction The rapidly depletion of fossil fuel reservoirs and environmental protection are the most significant issues of this century to be solved. To accurately meet these issues, sustained and clean energy sources are required on an urgent basis [1]. Meanwhile, the appropriate technology for the storage of energy is also a great challenge: in this regard, the new and latest advancements in energy storage devices, including batteries and supercapacitors (SCs) are gaining the more interest [2,3]. Among these storage devices, the SCs are superior due to its short charge-discharge time, extraordinary power density, longer working lifespan, great safety and low maintenance cost [4]. However, to fabricate small, flexible and lightweight SCs with high energy density is still a challenging task. To provide superior stability with high
∗
performance in foldable electronic gadgets, the flexible SCs are gaining great interest due to high flexibility. Generally, SCs are categorized into two classes depending on charge storage mechanism. (a) Electric double-layer capacitors (EDLCs) are charged using electrostatic adsorption and desorption of electrolyte ions on the surface of electrode. (b) The pseudocapacitors (PCs), which stores charge based on highly reversible redox reaction and also adsorption and desorption process simultaneously [5,6]. The specific capacitance of PCs is ~100 times higher than EDLCs [7]; however, still they need more improvement to fulfil the realistic applications demand. Thus, the latest research directions are focused on new interesting materials for PCs and supercapattery with high energy density and flexibility [8–10]. Several researchers suggested that bimetallic oxides based SCs have good charge storage performance because of their multiple oxidation
Corresponding author. School of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China. Corresponding author. Siyuan Laboratory, Department of Physics, Jinan University, Guangzhou, 510632, China. E-mail addresses: [email protected], [email protected] (S. Yun), [email protected] (M.S. Javed).
∗∗
https://doi.org/10.1016/j.ceramint.2019.10.173 Received 4 September 2019; Received in revised form 7 October 2019; Accepted 18 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yasir Abbas, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.173
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states and high electrical conductivities. Bimetallic oxides have two times higher electrical conductivity than their single counterpart oxide [11,12]. Such as Yanmei Li et al. reported that NiCo2O4 possessed a much smaller resistance as compared to their single counterparts NiO and Co3O4 [13]. It has been also investigated that the nanostructures reduce the diffusion distance for electrolyte ions and improve the electrochemical properties of SCs [9,14,15]. Moreover, bimetallic oxides are considered as environmentally friendly, low-cost and can enhance the energy density of SCs, which is produced through various oxidation-states and reversible fast faradaic reactions [12] However, number of nanostructured bimetallic oxides are explored and successfully used for SCs; such as ZnCo2O4 [16], NiCo2O4 [12], NiMoO4 [17], Zn2SnO4 [18] and CoMoO4 [17]. Recently, molybdate based bimetallic oxides such as NiMoO4, CoMoO4 have shown superior specific capacitance attributed to high electrochemical property of the Ni/Co atoms [19]. Several studies have exposed that NiMoO4 can improve ion penetration, increase specific capacitance and mass transfer; therefore improved cycling stability [20]. Ajay et al. revealed that amorphous NiMoO4 nanoflakes synthesized by speedy microwave method could improve ion permeation and showed great capacitance of 1739 F g−1 at a scan rates of 1 mV s−1 in alkali (NaOH) electrolyte [20]. Similarly, nanoporous NiMoO4 enhanced the mass transfer and pseudocapacitive reactions and exhibited the excellent specific capacitance of 2351 F g−1 at 2 A g−1, and it showed reasonable cycling stability after completing 3000 cycles at 5 A g−1 [21]. Further, NiMoO4 nanoplate fabricated by hydrothermal reaction exhibits a high capacitance of 2138 F g−1 at 2 mA cm−2 because of interconnected structure which encouraged the transportation of ions and electrons; however, it shows worst cycling stability 87% after 3000 cycles [22]. Furthermore, it was investigated that the as-prepared NiMoO4 nanosheets like morphology have shown better performance than nanorods because of its porous morphology and fine equally dispersed nature [23]. In one prospect, all these studies showed great specific capacitances but their cyclic stability should be need to be greatly improved, that might be due to the use of binders and conductive additives such as Nafion [24], polyvinylidene fluoride or polyvinylidene difluoride (PVDF) [25] and carbon/graphene. Binders and additives can help to improve the mechanical properties of the electrode and at the same time enhance the resistance for electrons and electrolyte ions, and reduce the power capability of the device during practical applications. In second prospect, binder-free electrodes can significantly enhance the conductivity and ionic diffusion paths and also improve the flexibility of the electrode. The active material can directly grow on various conductive substrates, for example nickel foam, stainless steel foil and carbon cloth using simple and low cost hydrothermal method with robust performance [26–30]. Among them, carbon cloth is a highly favorable substrate for direct growth of metal oxides nanostructures for SCs due to its good conductivity, flexibility, non-toxicity and good mechanical strength. Herein, we have fabricated a binder-free bimetallic NiMoO4 nanoplates electrode by an eco-friendly hydrothermal process and shortterm post-annealing treatment. The nano-plates were directly grown on carbon cloth to make it flexible, which increases its performance and durability and enhance the utilization of electroactive material. We combined the nano-plates like bimetallic oxides (NiMiO4) with mixed valence states on the surface of the conductive substrate (carbon cloth) without any binder and additives. The as-prepared electrode NMO@CC exhibited superb electrochemical properties in an aqueous basic electrolyte by achieving high specific capacity of 1500 C g−1 at a current density 5 A g−1 with high rate-capability. More interestingly, the NMO@CC electrode demonstrated excellent cycling stability by retaining 94.63% capacity after 5000 charge-discharge cycles.
2. Experimental 2.1. Synthesis process of NiMoO4 nano-plates arrays on flexible carbon cloth In this work, all chemicals are used as purchased from Aladdin Chemicals suppliers and manufacturer, China. The carbon cloth (thickness of 0.2 mm and density: 1.8 g/cm³) was purchased from Shanghai Lishuo Composite Material Technology Company. The typical hydrothermal method was used to directly grow the NiMoO4 nanoplates arrays on a flexible carbon cloth (CC). First of all, a solution mixture was prepared by dissolving 2 mmol of Ni(NO3)2.6H2O, 4 mmol of Na₂MoO₄·2H₂O, 2 mmol of NH4F and 9 mmol of urea in 40 mL of distilled water. Then as-prepared solution was put in ultra-sonicator for 1 h to get complete homogeneous mixture. Subsequently, this ultrasonicated homogeneous solution was poured into Teflon autoclave and filled it about 70%, after that a clean piece of CC (2 × 4 cm2) was inserted along the wall of Teflon autoclave. In order to establish a hydrothermal reaction, the autoclave was properly airtight closed and put it in electrical oven for 16 h at 120 °C. After the completion of hydrothermal process, let it to cool down naturally and the CC assembled with the NiMoO4 precursor was carefully cleaned by ethanol and deionized water. The MNO@CC precursor was sonicated for 10 min to washout the residual particles from the surface of CC. After that the MNO@CC was put in an air furnace at 90 °C for complete dehydration. Finally, dried MNO@CC was sintered at 350 °C in an air atmosphere for 2 h to improve its crystallinity. The trial methodology was performed multiple times and similar outcomes were obtained. 2.2. Fabrication of MNO@CC electrodes For testing the electrochemical performance as a supercapacitor electrode, without using any binder and conductive agents, the asprepared NMO@CC by cutting it into small pieces of 1 × 1 cm2, and then directly used as a freestanding electrode in 6 M KOH aqueous electrolyte. The active mass loading density of material was 1.26 mg cm−2 for electrode and carefully calculated by taking the mass difference of only CC and annealed NMO@CC. 2.3. Materials characterization In current study, field emission SEM (scanning electron microscopy) (FEI Nova 400) was used to analyze morphology as-prepared NMO. The powder X-ray diffraction (XRD) crystal composition of NMO samples were carefully characterized by PAN-analytical X–pert diffractometer with Cu–K radiation. X-ray source (Escalab (250Xi)) using Al Ka (1486.5 eV) was used to perform X-ray photoelectron spectroscopy (XPS). 2.4. Electrochemical measurements The electrochemical performance of NMO@CC electrode was measured through cyclic voltammetry (CV) analysis, galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS). The three-electrode configurations (Ag/AgCl as a reference electrode and platinum sheet as a counter electrode at room temperature) using electrochemical workstation (CHI660E, ChenHua, Shanghai) in 6 M (mol L−1) KOH electrolyte. The specific capacity (C) of NMO@CC in the three-electrode system was calculated on the basis of the following equation [31].
C=
I
t
(1)
m −1
where C, I, m, Δt are the specific capacity (C g ), discharge current (A), effective mass of the material (g) and discharge time (s). 2
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Fig. 1. Schematic diagram for the synthesis strategy followed to directly grow the nano plates like NMO on the surface of carbon cloth.
Fig. 2. Physical characterizations of NMO@CC electrode: (a) XRD pattern, (b) Crystal structure of NMO, (c) XPS full scan survey, (d) Ni 2p spectrum, (e) Mo 3d spectrum, (f) C 1 s spectrum.
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Fig. 3. Morphological characterizations of NMO@CC electrode: (a) low resolution FE-SEM image, (b) selected part from Fig. (a). (c) High resolution FE-SEM image and (d) selected part from Fig. (c).
3. Results and discussion
respectively [34]. The multiple oxidations states in NMO@CC electrode combined with sp [2] and sp [3] hybridized carbon offers good charge storage characteristics through redox-reactions as well as high stability. The surface morphology of as-synthesized NiMoO4 was analyzed through FE-SEM with low and high magnifications as shown in Fig. 3. The low-resolution image exposed uniformly distribution of NiMoO4 nanostructure over the conductive carbon cloth as shown in Fig. 3(a–b), which is a very important surface for good electrochemical response. Furthermore, the FE-SEM image of NiMoO4 nano-plates exhibited a three-dimensional consistent roughed surface structure with ununiformed voids, which significantly enhanced the reaction transportation (Fig. 3(c)). The average thickness of nano-plates is in the range of ~33 nm with micro-meter in length (Fig. 3(d)), which considered as excellent morphology for the charge storage host. The three-dimensional nano-plates connected with each other makes an extremely porous nano-structure that surely increase the surface area, improve the electrochemical conductivity and allow quick transmission of electrons and electrolyte ion between the active material and current collector. Three electrode cell-system was employed to examine the electrochemical properties (GCD and CV) of as-prepared NiMoO4 nano-plates electrode in the aqueous 6 M KOH electrolyte. The characteristic CV curves of NMO@CC and pristine CC electrode in a potential window of −0.5 - 0.6 V vs Ag/AgCl at various scan rates are shown in Fig. 4(a). The area under the CV curve of pristine CC electrode is negligible as compared with NMO@CC electrode. Therefore, we neglected the contribution from pristine CC substrate in the capacity calculation of NMO@CC electrode. The enlarged potential window with prominent redox peaks of NMO@CC electrode signifies the excellent pseudocapacitive charge storage properties (Fig. 4(b)). The potential window is attributed to the diverse redox response transmission limitations in the aqueous electrolyte (KOH). The CV curve shape is not perfectly rectangular like carbon-based electrodes, representing the pseudocapacitive features of the NMO@CC electrode. Remarkably, on increasing the scan rates, the peaks of oxidation and reduction are stable in the same
Fig. 1 demonstrated the NMO@CC nano-plates like structure which was directly grown on the flexible conductive carbon cloth using hydrothermal process. The Ni atoms react with Mo atoms in the existence of urea and ammonium florid to start the growth of nano-plates at 120 °C. After the first stage, it was annealed in an air environment at 350 °C with ramp rate of 2 °C min−1 to improve its crystallinity. The XRD pattern of the annealed NiMoO4 electrode explained its structural nature and purity as represents in Fig. 2(a). The diffraction peaks of the NiMoO4 nano-plates at 15.1°, 25.1°, 37.5°, 44°, 53.2° and 57.1° could be freely indexed to the (110), (112), (220), (222), (003) and (330) crystal planes of NiMoO4, correspondingly. The sharp diffraction peaks reveal the crystallinity of the as-prepared NiMoO4 nano-plates. No indications of contaminations from MoO3 or NiO have been identified, indicating the nano-plates are phase pure NiMoO4. The three-dimensional crystal structures of NiMoO4 nano-plates shown in Fig. 2(b). The atoms of Ni and Mo interconnected mutually for establishing a tunnel structure, which might be helpful in fast ionic transmission between the electrolyte and the active material. In addition, the oxidation states and chemical compositions of NMO@CC electrode are also analyzed by X-ray photoelectron spectroscopy (XPS) to achieve more clarity. The spectrum investigation of NiMoO4 exposed the existence of Ni 2p, Mo 3d, C1s and O1s (Fig. 2(c)). The deconvoluted spectrum of the Ni 2p spectra, as shown in Fig. 2(d) exposed two noticeable spin-orbit peaks including such as Ni 2p1/2 and Ni 2p3/2 at 873 and 856 eV respectively; individually were fitted with spin-orbit doublet distinctive of the Ni3+ and Ni2+ binding states [32]. Fig. 2(e) demonstrated the deconvoluted spectrum of Mo 3d bands exhibited clearly two main peaks, which involved the binding energies of 232.5 and 236 eV with a spin energy difference of 3.5 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively [33]. Fig. 2(f) demonstrated the deconvoluted C 1s spectra, wherein the peaks at 285.2 and 286.2 were ascribed to the sp [2] and sp [3] hybridized carbon, 4
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Fig. 4. Electrochemical characterizations of NMO@CC electrode in 6 M KOH aqueous electrolyte: (a) Comparative CV curves of NMO@CC and pristine CC electrodes, (b) CV curves of NMO@CC electrode, (c) GCD curves of NMO@CC electrode, (d) IR-drop versus current density, (e) Nyquist plot of EIS spectrum (inset is the high resolution from low frequency region).
potential range. This redox peaks change indicates the interior resistance of the NMO@ CC electrode, moreover, it supports the pseudocapacitive characteristics with improved charge storage performance [35]. Fig. 4(c) demonstrates the distinctive GCD curves of NMO@CC electrode at different current densities from 5 to 20 A g−1 in the potential window of −0.5 - 0.6 V. It was observed that GCD curves seem very closely symmetric (columbic efficiency ~ 94%) with a strong redox performance and supported to the results of CV analysis, indicating the pseudocapacitive charge storage characteristics are closely symmetric pair of redox peaks, which could be attributed to a Faradaic reaction. The potential drop attributed to the inner resistance of electrode and electrolyte. It means that if the IR-drop is large, then the material of electrode has poor conductivity and vice versa. The NMO electrode represents the very small IR-drop (0.06 V) at a current density of 20 A g−1, which specifies the great electrical conductivity, as shown in Fig. 4(d). In addition, it can be observed that IR-drop goes linearly with an increase of current density and is very consistent with the published literature [36]. The specific capacities of the NMO@CC electrode were
measured instead of capacitance from the discharge curves as shown in Fig. 4(e). The specific capacity of NMO@CC electrode reach at the highest value of 1500, 1350, 1300, 1230 and 1200 C g−1 at the current densities of 5, 7, 10, 15 and 20 A g−1, respectively as shown in Fig. 4(e). The NMO@CC electrode provides a great specific capacity of 1500 C g−1 at a current density of 5 A g−1 and maintains up to 80% at 20 A g−1, indicating the outstanding rate capability. These results are ascribed to the binder- and additive-free nature of the electrode with high exposed surface area, which offers improved availability to the electrolyte, empowered the transport of ions and decreased the resistance for charge transfer. In addition, electrochemical impedance spectroscopy (EIS) test was carried out for a better understanding of the improved electrochemical performance of the NMO@CC electrode. In Fig. 4(f), the Nyquist plot of NMO@CC electrode shows two distinct parts; one is a semicircle at high-frequency range, which exhibits the charge transmission kinetics and the second is a quasi-vertical line at low-frequency range, which is related to the partial diffusion process [37]. The calculated solution impedance of NMO@CC (Rs ~ 3.86 Ω) is quite low, as it consist of the internal resistance of the electrode 5
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Fig. 5. Charge storage kinetics and cycling stability tests of NMO@CC electrode in aqueous electrolyte: (a) Logarithm plots of anodic and cathodic peaks versus scan rate, (b) Capacitive and diffusion based stored charge, (c) Cycling stability versus number of cycles over 5000 cycles.
material, electrolytic resistance and the resistance of electrolyte/electrode interface [2,38,39]. The low resistance of charge transmission (Rct ~ 4.6 Ω) can be ascribed to the good electrical connection between CC and active material. The total charge originated by the capacitive and diffusion-controlled process was calculated by areas under the CV curves at different scanning rates, by using equations (2) and (3).
i (v ) = av b
(2)
log (i ) = blog (v ) + log(a)
(3)
NiMoO4@CoMoO4 hierarchical nanospheres (79.5% retaining after 2000 cycles) [42], and core-shell NiCo2O4@NiMoO4 nanowires (90.6% retention after 2000 cycles) [43]. These results clearly exposed that the NMO@CC electrode has excellent cycling stability over 5000 cycles. Therefore, the binder-free NMO@CC electrode is suitable for highperformance energy storage devices. 4. Conclusion
where, i(v) is current density, v represents the scan rate, and a, b are adjustable coefficient. Fig. 5(a) represents both anodic and cathodic reactions peaks, which exhibit a linear correlation between them, where b-values of cathodic and anodic reaction peaks are 0.81 and 0.70, respectively. The calculated b-values for anodic and cathodic peaks are in the range of 0.7–0.85, which demonstrated the hybrid nature of the charge storage in NMO@CC electrode [31]. Fig. 5(b) represents the total stored charge from both diffusions controlled and capacitive process, among them 71% charge stored by capacitive process and represented by the area under the blue shaded curve, and 29% charge is stored by the diffusion-controlled process. This kind of hybrid charge storage process is very promising for high energy application in practical fields. Besides the good electrochemical performance, cycling stability is one of the most important parameters to characterize the SC's electrode. To test the cycling stability, we conduct a continuous 5000 GCD cycles at a high current density of 20 A g−1, and the obtained results are demonstrated in Fig. 5(c). After the successful completion of 5000 cycles, the NMO@CC electrode retains almost 94.63% of its initial capacity, which is much superior to those of previously published results on the same material as well as for other bimetallic oxides, such as hierarchical Co3O4@NiMoO4 nanosheets (70% retaining after 1000 cycles) [40], Co3O4@NiMoO4 nanowires (70% retaining after 3000 cycles) [41],
In the current investigation, we have invented the fabrication of a binder-free NiMoO4 nan-plates based electrode for high-performance SCs. The NiMoO4 nano-plates are directly assembled on the surface of carbon cloth substrate through a simple hydrothermal route. The asprepared electrode NMO@CC exhibites exceptional electrochemical properties in an aqueous basic electrolyte by achieving impressive capacity of 1500 C g−1 at 5 A g−1 with a good retention at high current density of 20 A g−1. The charge storage mechanism of NMO@CC electrode is investigated by analyzing the surface capacitive and diffusion controlled processes and it shows high surface capacitive storage (71%). More interestingly, the NMO@CC electrode demonstrates an excellent stability of 94.63% after 5000 charge-discharge cycles. The admirable results are based on the highly open channels for electrolyte ions diffusion and electronic transmission through the NMO and backbone carbon cloth, respectively. Therefore, accurate morphology and surface manufacturing engineering is highly appreciated to increase the active surface area and enhance the inherent conductivity of the electrode materials. Declaration of competing interest There are no conflicts to declare. 6
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Acknowledgement
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Anchoring 2D NiMoO4 nano-plates on flexible carbon cloth as a binder-free electrode for efficient energy storage devices Yasir Abbasa, Sining Yuna,∗, Muhammad Sufyan Javedb,c,∗∗, Jiageng Chena, Muhammad Faizan Tahird, Ziqi Wanga, Chao Yanga, Asim Arshada, Shahid Hussaine a
Functional Materials Laboratory (FML), School of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China Siyuan Laboratory, Department of Physics, Jinan University, Guangzhou, 510632, China c Department of Physics, COMSATS University Islamabad, Lahore Campus, Lahore, 54000, Pakistan d School of Electric Power, South China University of Technology, Guangzhou 510640, China e School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, China b
ARTICLE INFO
ABSTRACT
Keywords: Energy storage Mixed metal oxide Electrode Carbon cloth High-performance Supercapacitor
The energy security and mounting environmental issues compel the scientific community to allocate greatly efficient and economical energy renovation and storage systems. Among the energy storage devices, supercapacitors have become the forefront in energy storing systems in recent decades. The efficiency of supercapacitors mainly depend on the electrode's material and they usually suffer from a quick reduction in specific capacitance at higher current densities. Herein, we combined the nano-plates like bimetallic oxides (NiMiO4) with mixed valence states on the surface of a conductive substrate (carbon cloth) without any binder and additives (denoted NMO@CC). The as-prepared electrode NMO@CC showed marvelous electrochemical properties in the aqueous basic electrolyte by achieving a high capacity of 1500 C g−1 at current density of 5 A g−1 with high degree of rate capability. More interestingly, the NMO@CC electrode demonstrated excellent cycling stability of 94.63% after 5000 cycles during charge-discharge process. Further, the charge storage mechanism of NMO@CC electrode is investigated by analyzing the surface capacitive and diffusion controlled processes and it shows high surface capacitive storage (71%). These admirable results are based on the highly open channels for efficient diffusion of electrolyte ions and electronic transmission through the NMO and backbone carbon cloth, respectively. Therefore, accurate morphology and surface manufacturing engineering are highly appreciated to enhance the active surface area and inherent conductivity of electrode materials.
1. Introduction The rapidly depletion of fossil fuel reservoirs and environmental protection are the most significant issues of this century to be solved. To accurately meet these issues, sustained and clean energy sources are required on an urgent basis [1]. Meanwhile, the appropriate technology for the storage of energy is also a great challenge: in this regard, the new and latest advancements in energy storage devices, including batteries and supercapacitors (SCs) are gaining the more interest [2,3]. Among these storage devices, the SCs are superior due to its short charge-discharge time, extraordinary power density, longer working lifespan, great safety and low maintenance cost [4]. However, to fabricate small, flexible and lightweight SCs with high energy density is still a challenging task. To provide superior stability with high
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performance in foldable electronic gadgets, the flexible SCs are gaining great interest due to high flexibility. Generally, SCs are categorized into two classes depending on charge storage mechanism. (a) Electric double-layer capacitors (EDLCs) are charged using electrostatic adsorption and desorption of electrolyte ions on the surface of electrode. (b) The pseudocapacitors (PCs), which stores charge based on highly reversible redox reaction and also adsorption and desorption process simultaneously [5,6]. The specific capacitance of PCs is ~100 times higher than EDLCs [7]; however, still they need more improvement to fulfil the realistic applications demand. Thus, the latest research directions are focused on new interesting materials for PCs and supercapattery with high energy density and flexibility [8–10]. Several researchers suggested that bimetallic oxides based SCs have good charge storage performance because of their multiple oxidation
Corresponding author. School of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China. Corresponding author. Siyuan Laboratory, Department of Physics, Jinan University, Guangzhou, 510632, China. E-mail addresses: [email protected], [email protected] (S. Yun), [email protected] (M.S. Javed).
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https://doi.org/10.1016/j.ceramint.2019.10.173 Received 4 September 2019; Received in revised form 7 October 2019; Accepted 18 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yasir Abbas, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.173
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states and high electrical conductivities. Bimetallic oxides have two times higher electrical conductivity than their single counterpart oxide [11,12]. Such as Yanmei Li et al. reported that NiCo2O4 possessed a much smaller resistance as compared to their single counterparts NiO and Co3O4 [13]. It has been also investigated that the nanostructures reduce the diffusion distance for electrolyte ions and improve the electrochemical properties of SCs [9,14,15]. Moreover, bimetallic oxides are considered as environmentally friendly, low-cost and can enhance the energy density of SCs, which is produced through various oxidation-states and reversible fast faradaic reactions [12] However, number of nanostructured bimetallic oxides are explored and successfully used for SCs; such as ZnCo2O4 [16], NiCo2O4 [12], NiMoO4 [17], Zn2SnO4 [18] and CoMoO4 [17]. Recently, molybdate based bimetallic oxides such as NiMoO4, CoMoO4 have shown superior specific capacitance attributed to high electrochemical property of the Ni/Co atoms [19]. Several studies have exposed that NiMoO4 can improve ion penetration, increase specific capacitance and mass transfer; therefore improved cycling stability [20]. Ajay et al. revealed that amorphous NiMoO4 nanoflakes synthesized by speedy microwave method could improve ion permeation and showed great capacitance of 1739 F g−1 at a scan rates of 1 mV s−1 in alkali (NaOH) electrolyte [20]. Similarly, nanoporous NiMoO4 enhanced the mass transfer and pseudocapacitive reactions and exhibited the excellent specific capacitance of 2351 F g−1 at 2 A g−1, and it showed reasonable cycling stability after completing 3000 cycles at 5 A g−1 [21]. Further, NiMoO4 nanoplate fabricated by hydrothermal reaction exhibits a high capacitance of 2138 F g−1 at 2 mA cm−2 because of interconnected structure which encouraged the transportation of ions and electrons; however, it shows worst cycling stability 87% after 3000 cycles [22]. Furthermore, it was investigated that the as-prepared NiMoO4 nanosheets like morphology have shown better performance than nanorods because of its porous morphology and fine equally dispersed nature [23]. In one prospect, all these studies showed great specific capacitances but their cyclic stability should be need to be greatly improved, that might be due to the use of binders and conductive additives such as Nafion [24], polyvinylidene fluoride or polyvinylidene difluoride (PVDF) [25] and carbon/graphene. Binders and additives can help to improve the mechanical properties of the electrode and at the same time enhance the resistance for electrons and electrolyte ions, and reduce the power capability of the device during practical applications. In second prospect, binder-free electrodes can significantly enhance the conductivity and ionic diffusion paths and also improve the flexibility of the electrode. The active material can directly grow on various conductive substrates, for example nickel foam, stainless steel foil and carbon cloth using simple and low cost hydrothermal method with robust performance [26–30]. Among them, carbon cloth is a highly favorable substrate for direct growth of metal oxides nanostructures for SCs due to its good conductivity, flexibility, non-toxicity and good mechanical strength. Herein, we have fabricated a binder-free bimetallic NiMoO4 nanoplates electrode by an eco-friendly hydrothermal process and shortterm post-annealing treatment. The nano-plates were directly grown on carbon cloth to make it flexible, which increases its performance and durability and enhance the utilization of electroactive material. We combined the nano-plates like bimetallic oxides (NiMiO4) with mixed valence states on the surface of the conductive substrate (carbon cloth) without any binder and additives. The as-prepared electrode NMO@CC exhibited superb electrochemical properties in an aqueous basic electrolyte by achieving high specific capacity of 1500 C g−1 at a current density 5 A g−1 with high rate-capability. More interestingly, the NMO@CC electrode demonstrated excellent cycling stability by retaining 94.63% capacity after 5000 charge-discharge cycles.
2. Experimental 2.1. Synthesis process of NiMoO4 nano-plates arrays on flexible carbon cloth In this work, all chemicals are used as purchased from Aladdin Chemicals suppliers and manufacturer, China. The carbon cloth (thickness of 0.2 mm and density: 1.8 g/cm³) was purchased from Shanghai Lishuo Composite Material Technology Company. The typical hydrothermal method was used to directly grow the NiMoO4 nanoplates arrays on a flexible carbon cloth (CC). First of all, a solution mixture was prepared by dissolving 2 mmol of Ni(NO3)2.6H2O, 4 mmol of Na₂MoO₄·2H₂O, 2 mmol of NH4F and 9 mmol of urea in 40 mL of distilled water. Then as-prepared solution was put in ultra-sonicator for 1 h to get complete homogeneous mixture. Subsequently, this ultrasonicated homogeneous solution was poured into Teflon autoclave and filled it about 70%, after that a clean piece of CC (2 × 4 cm2) was inserted along the wall of Teflon autoclave. In order to establish a hydrothermal reaction, the autoclave was properly airtight closed and put it in electrical oven for 16 h at 120 °C. After the completion of hydrothermal process, let it to cool down naturally and the CC assembled with the NiMoO4 precursor was carefully cleaned by ethanol and deionized water. The MNO@CC precursor was sonicated for 10 min to washout the residual particles from the surface of CC. After that the MNO@CC was put in an air furnace at 90 °C for complete dehydration. Finally, dried MNO@CC was sintered at 350 °C in an air atmosphere for 2 h to improve its crystallinity. The trial methodology was performed multiple times and similar outcomes were obtained. 2.2. Fabrication of MNO@CC electrodes For testing the electrochemical performance as a supercapacitor electrode, without using any binder and conductive agents, the asprepared NMO@CC by cutting it into small pieces of 1 × 1 cm2, and then directly used as a freestanding electrode in 6 M KOH aqueous electrolyte. The active mass loading density of material was 1.26 mg cm−2 for electrode and carefully calculated by taking the mass difference of only CC and annealed NMO@CC. 2.3. Materials characterization In current study, field emission SEM (scanning electron microscopy) (FEI Nova 400) was used to analyze morphology as-prepared NMO. The powder X-ray diffraction (XRD) crystal composition of NMO samples were carefully characterized by PAN-analytical X–pert diffractometer with Cu–K radiation. X-ray source (Escalab (250Xi)) using Al Ka (1486.5 eV) was used to perform X-ray photoelectron spectroscopy (XPS). 2.4. Electrochemical measurements The electrochemical performance of NMO@CC electrode was measured through cyclic voltammetry (CV) analysis, galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS). The three-electrode configurations (Ag/AgCl as a reference electrode and platinum sheet as a counter electrode at room temperature) using electrochemical workstation (CHI660E, ChenHua, Shanghai) in 6 M (mol L−1) KOH electrolyte. The specific capacity (C) of NMO@CC in the three-electrode system was calculated on the basis of the following equation [31].
C=
I
t
(1)
m −1
where C, I, m, Δt are the specific capacity (C g ), discharge current (A), effective mass of the material (g) and discharge time (s). 2
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Fig. 1. Schematic diagram for the synthesis strategy followed to directly grow the nano plates like NMO on the surface of carbon cloth.
Fig. 2. Physical characterizations of NMO@CC electrode: (a) XRD pattern, (b) Crystal structure of NMO, (c) XPS full scan survey, (d) Ni 2p spectrum, (e) Mo 3d spectrum, (f) C 1 s spectrum.
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Fig. 3. Morphological characterizations of NMO@CC electrode: (a) low resolution FE-SEM image, (b) selected part from Fig. (a). (c) High resolution FE-SEM image and (d) selected part from Fig. (c).
3. Results and discussion
respectively [34]. The multiple oxidations states in NMO@CC electrode combined with sp [2] and sp [3] hybridized carbon offers good charge storage characteristics through redox-reactions as well as high stability. The surface morphology of as-synthesized NiMoO4 was analyzed through FE-SEM with low and high magnifications as shown in Fig. 3. The low-resolution image exposed uniformly distribution of NiMoO4 nanostructure over the conductive carbon cloth as shown in Fig. 3(a–b), which is a very important surface for good electrochemical response. Furthermore, the FE-SEM image of NiMoO4 nano-plates exhibited a three-dimensional consistent roughed surface structure with ununiformed voids, which significantly enhanced the reaction transportation (Fig. 3(c)). The average thickness of nano-plates is in the range of ~33 nm with micro-meter in length (Fig. 3(d)), which considered as excellent morphology for the charge storage host. The three-dimensional nano-plates connected with each other makes an extremely porous nano-structure that surely increase the surface area, improve the electrochemical conductivity and allow quick transmission of electrons and electrolyte ion between the active material and current collector. Three electrode cell-system was employed to examine the electrochemical properties (GCD and CV) of as-prepared NiMoO4 nano-plates electrode in the aqueous 6 M KOH electrolyte. The characteristic CV curves of NMO@CC and pristine CC electrode in a potential window of −0.5 - 0.6 V vs Ag/AgCl at various scan rates are shown in Fig. 4(a). The area under the CV curve of pristine CC electrode is negligible as compared with NMO@CC electrode. Therefore, we neglected the contribution from pristine CC substrate in the capacity calculation of NMO@CC electrode. The enlarged potential window with prominent redox peaks of NMO@CC electrode signifies the excellent pseudocapacitive charge storage properties (Fig. 4(b)). The potential window is attributed to the diverse redox response transmission limitations in the aqueous electrolyte (KOH). The CV curve shape is not perfectly rectangular like carbon-based electrodes, representing the pseudocapacitive features of the NMO@CC electrode. Remarkably, on increasing the scan rates, the peaks of oxidation and reduction are stable in the same
Fig. 1 demonstrated the NMO@CC nano-plates like structure which was directly grown on the flexible conductive carbon cloth using hydrothermal process. The Ni atoms react with Mo atoms in the existence of urea and ammonium florid to start the growth of nano-plates at 120 °C. After the first stage, it was annealed in an air environment at 350 °C with ramp rate of 2 °C min−1 to improve its crystallinity. The XRD pattern of the annealed NiMoO4 electrode explained its structural nature and purity as represents in Fig. 2(a). The diffraction peaks of the NiMoO4 nano-plates at 15.1°, 25.1°, 37.5°, 44°, 53.2° and 57.1° could be freely indexed to the (110), (112), (220), (222), (003) and (330) crystal planes of NiMoO4, correspondingly. The sharp diffraction peaks reveal the crystallinity of the as-prepared NiMoO4 nano-plates. No indications of contaminations from MoO3 or NiO have been identified, indicating the nano-plates are phase pure NiMoO4. The three-dimensional crystal structures of NiMoO4 nano-plates shown in Fig. 2(b). The atoms of Ni and Mo interconnected mutually for establishing a tunnel structure, which might be helpful in fast ionic transmission between the electrolyte and the active material. In addition, the oxidation states and chemical compositions of NMO@CC electrode are also analyzed by X-ray photoelectron spectroscopy (XPS) to achieve more clarity. The spectrum investigation of NiMoO4 exposed the existence of Ni 2p, Mo 3d, C1s and O1s (Fig. 2(c)). The deconvoluted spectrum of the Ni 2p spectra, as shown in Fig. 2(d) exposed two noticeable spin-orbit peaks including such as Ni 2p1/2 and Ni 2p3/2 at 873 and 856 eV respectively; individually were fitted with spin-orbit doublet distinctive of the Ni3+ and Ni2+ binding states [32]. Fig. 2(e) demonstrated the deconvoluted spectrum of Mo 3d bands exhibited clearly two main peaks, which involved the binding energies of 232.5 and 236 eV with a spin energy difference of 3.5 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively [33]. Fig. 2(f) demonstrated the deconvoluted C 1s spectra, wherein the peaks at 285.2 and 286.2 were ascribed to the sp [2] and sp [3] hybridized carbon, 4
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Fig. 4. Electrochemical characterizations of NMO@CC electrode in 6 M KOH aqueous electrolyte: (a) Comparative CV curves of NMO@CC and pristine CC electrodes, (b) CV curves of NMO@CC electrode, (c) GCD curves of NMO@CC electrode, (d) IR-drop versus current density, (e) Nyquist plot of EIS spectrum (inset is the high resolution from low frequency region).
potential range. This redox peaks change indicates the interior resistance of the NMO@ CC electrode, moreover, it supports the pseudocapacitive characteristics with improved charge storage performance [35]. Fig. 4(c) demonstrates the distinctive GCD curves of NMO@CC electrode at different current densities from 5 to 20 A g−1 in the potential window of −0.5 - 0.6 V. It was observed that GCD curves seem very closely symmetric (columbic efficiency ~ 94%) with a strong redox performance and supported to the results of CV analysis, indicating the pseudocapacitive charge storage characteristics are closely symmetric pair of redox peaks, which could be attributed to a Faradaic reaction. The potential drop attributed to the inner resistance of electrode and electrolyte. It means that if the IR-drop is large, then the material of electrode has poor conductivity and vice versa. The NMO electrode represents the very small IR-drop (0.06 V) at a current density of 20 A g−1, which specifies the great electrical conductivity, as shown in Fig. 4(d). In addition, it can be observed that IR-drop goes linearly with an increase of current density and is very consistent with the published literature [36]. The specific capacities of the NMO@CC electrode were
measured instead of capacitance from the discharge curves as shown in Fig. 4(e). The specific capacity of NMO@CC electrode reach at the highest value of 1500, 1350, 1300, 1230 and 1200 C g−1 at the current densities of 5, 7, 10, 15 and 20 A g−1, respectively as shown in Fig. 4(e). The NMO@CC electrode provides a great specific capacity of 1500 C g−1 at a current density of 5 A g−1 and maintains up to 80% at 20 A g−1, indicating the outstanding rate capability. These results are ascribed to the binder- and additive-free nature of the electrode with high exposed surface area, which offers improved availability to the electrolyte, empowered the transport of ions and decreased the resistance for charge transfer. In addition, electrochemical impedance spectroscopy (EIS) test was carried out for a better understanding of the improved electrochemical performance of the NMO@CC electrode. In Fig. 4(f), the Nyquist plot of NMO@CC electrode shows two distinct parts; one is a semicircle at high-frequency range, which exhibits the charge transmission kinetics and the second is a quasi-vertical line at low-frequency range, which is related to the partial diffusion process [37]. The calculated solution impedance of NMO@CC (Rs ~ 3.86 Ω) is quite low, as it consist of the internal resistance of the electrode 5
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Fig. 5. Charge storage kinetics and cycling stability tests of NMO@CC electrode in aqueous electrolyte: (a) Logarithm plots of anodic and cathodic peaks versus scan rate, (b) Capacitive and diffusion based stored charge, (c) Cycling stability versus number of cycles over 5000 cycles.
material, electrolytic resistance and the resistance of electrolyte/electrode interface [2,38,39]. The low resistance of charge transmission (Rct ~ 4.6 Ω) can be ascribed to the good electrical connection between CC and active material. The total charge originated by the capacitive and diffusion-controlled process was calculated by areas under the CV curves at different scanning rates, by using equations (2) and (3).
i (v ) = av b
(2)
log (i ) = blog (v ) + log(a)
(3)
NiMoO4@CoMoO4 hierarchical nanospheres (79.5% retaining after 2000 cycles) [42], and core-shell NiCo2O4@NiMoO4 nanowires (90.6% retention after 2000 cycles) [43]. These results clearly exposed that the NMO@CC electrode has excellent cycling stability over 5000 cycles. Therefore, the binder-free NMO@CC electrode is suitable for highperformance energy storage devices. 4. Conclusion
where, i(v) is current density, v represents the scan rate, and a, b are adjustable coefficient. Fig. 5(a) represents both anodic and cathodic reactions peaks, which exhibit a linear correlation between them, where b-values of cathodic and anodic reaction peaks are 0.81 and 0.70, respectively. The calculated b-values for anodic and cathodic peaks are in the range of 0.7–0.85, which demonstrated the hybrid nature of the charge storage in NMO@CC electrode [31]. Fig. 5(b) represents the total stored charge from both diffusions controlled and capacitive process, among them 71% charge stored by capacitive process and represented by the area under the blue shaded curve, and 29% charge is stored by the diffusion-controlled process. This kind of hybrid charge storage process is very promising for high energy application in practical fields. Besides the good electrochemical performance, cycling stability is one of the most important parameters to characterize the SC's electrode. To test the cycling stability, we conduct a continuous 5000 GCD cycles at a high current density of 20 A g−1, and the obtained results are demonstrated in Fig. 5(c). After the successful completion of 5000 cycles, the NMO@CC electrode retains almost 94.63% of its initial capacity, which is much superior to those of previously published results on the same material as well as for other bimetallic oxides, such as hierarchical Co3O4@NiMoO4 nanosheets (70% retaining after 1000 cycles) [40], Co3O4@NiMoO4 nanowires (70% retaining after 3000 cycles) [41],
In the current investigation, we have invented the fabrication of a binder-free NiMoO4 nan-plates based electrode for high-performance SCs. The NiMoO4 nano-plates are directly assembled on the surface of carbon cloth substrate through a simple hydrothermal route. The asprepared electrode NMO@CC exhibites exceptional electrochemical properties in an aqueous basic electrolyte by achieving impressive capacity of 1500 C g−1 at 5 A g−1 with a good retention at high current density of 20 A g−1. The charge storage mechanism of NMO@CC electrode is investigated by analyzing the surface capacitive and diffusion controlled processes and it shows high surface capacitive storage (71%). More interestingly, the NMO@CC electrode demonstrates an excellent stability of 94.63% after 5000 charge-discharge cycles. The admirable results are based on the highly open channels for electrolyte ions diffusion and electronic transmission through the NMO and backbone carbon cloth, respectively. Therefore, accurate morphology and surface manufacturing engineering is highly appreciated to increase the active surface area and enhance the inherent conductivity of the electrode materials. Declaration of competing interest There are no conflicts to declare. 6
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Acknowledgement
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