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CoMoO4 nanosheets assembled 3D-frameworks for high-performance energy storage
Author’s Accepted Manuscript CoMoO4 nanosheets assembled 3D-frameworks for high-performance energy storage Huiwu Long, Tianmo Liu, Wen Zeng, Yifang Yang, Shuoqing Zhao www.elsevier.com/locate/ceri
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S0272-8842(17)32422-7 https://doi.org/10.1016/j.ceramint.2017.10.216 CERI16635
To appear in: Ceramics International Received date: 5 October 2017 Revised date: 27 October 2017 Accepted date: 28 October 2017 Cite this article as: Huiwu Long, Tianmo Liu, Wen Zeng, Yifang Yang and Shuoqing Zhao, CoMoO4 nanosheets assembled 3D-frameworks for highperformance energy storage, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.10.216 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CoMoO4 nanosheets assembled 3D-frameworks for high-performance energy storage Huiwu Long1, Tianmo Liu1,*1, Wen Zeng1, Yifang Yang1, Shuoqing Zhao1,2,* 1. College of Materials Science and Engineering, Chongqing University, Chongqing China 2. Centre for Clean Energy Technology, Faculty of Science, University of Technology, Sydney
Abstract The selection of electrode materials and the optimization of electrode configurations are two effective approaches to achieve high-performance energy storage. As a representative ternary metal oxide, CoMoO4 has been directly grown on the carbon cloth via a facile hydrothermal method and subsequent calcination, forming a binder-free electrode. Further measurements reveal that this electrode is composed of CoMoO4 nanosheets assembled 3D-framworks (NAFs) and thereby shows a superb electrochemical property in KOH solution. It is worth mentioning that the specific capacitance reaches as high as 1234 F/g at the current density of 1 A/g, which exceeds most of CoMoO4 and CoMoO4-based composites reported so far. Meanwhile, when the current density increases to 10 A/g, a considerable specific capacitance of 446 F/g is still obtained and can maintain nearly unchanged after 5000 cycles.
Keywords: Hydrothermal, Supercapacitor, Microstructure, Metal oxides 1. Introduction The ever-worsening energy crisis propels the innovation in energy production and consumption, which also reflects in a pressing demand towards high-performance energy storage
*
Corresponding author. Tel./fax: +86 23 65102465
E-mail: [email protected] (T.M. Liu)
[email protected] (S.Q. Zhao)
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devices (ESDs) [1, 2]. On one hand, the proportion of sustainable and renewable energy sources such as solar and wind has increased. Given their intermittence, ESDs with the excellent stability are required to bridge them with the smart grid [3]. On the other hand, the replace of petrol-powered vehicles by electric ones can diminish the depletion of fossil fuels in a large extent. But the convenience of filling up the petrol tank in several minutes and the rapid accelerating can only be replicated through ESDs with the high charging/discharging power density [4, 5]. In light of these features, supercapacitors which exhibit desirable performances of the fast charging/discharging rate as well as the long cycling life could serve as a promising candidate for efficient ESDs [1-3, 5]. Additionally, the utilization of the aqueous electrolyte also endows supercapacitors with the characters of low cost, high safety and environmental benignity in comparison with traditional batteries [6]. Supercapacitors can be subdivided into two categories according to the charge storage mechanism [2, 3]. Different from electrical double layer capacitors which physically absorb ions at the electrode/electrolyte interface, pseudo-capacitors, represented by transition metal oxides, store energy via a faradaic process of redox-active species and therefore display a higher capacitance [7, 8]. Ternary metal oxides (AxByOz) have more variable oxidation states and better electronic conductivity than simple ones (AxOy), and thus have been widely investigated as electrode materials for high-performance supercapacitors [9-16]. The traditional electrode fabrication of supercapacitors adopts the slurry-coating technique which normally employs polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) as the binder [17-19]. These polymeric binders are insulated and inevitably exert detrimental impacts on the electrochemical performance of electrodes [4, 21-23]. To solve this problem, electrochemical active materials are directly loaded on the conductive substrate, forming the 2
binder-free electrode [22-25]. Nickel (Ni) foam is universally used due to its high porosity and large specific surface area [24, 26], but the poor acid resistance of Ni foam confines its application only to the neutral or alkaline electrolyte [25, 26]. Meanwhile, its density reaches as high as 262 mg/cm3 which results in a low specific capacitance of the total electrode [23]. 3D-graphene usually exhibits the excellent chemical stability and a relative low density (approximately 20 mg/cm3), and thus attracts intensive attention as the substitution of the Ni foam [23, 27, 28]. Despite the improvement in electrochemical properties, the complicated and time-consuming synthesis strategy of 3D-graphene is much more likely to restrict its large-scale commercialization. In most cases, graphene is firstly deposited on Ni foam through the chemical vapor deposition (CVD) and then soaked in HCl solution to dissolve the initial Ni foam, eventually forming the 3D-structure [27, 28]. Based on aforementioned considerations, the commercialized carbon cloth, which integrates advantages of the Ni foam as well as the 3D-graphene, is expected to be an ideal scaffold to grow redox-active species on it. Except these merits, the unique fiber structure of the carbon cloth also offers it brilliant mechanical strength and flexibility, which would further extend the application range of the electrode, especially in portable and wearable electronics [29-31]. In this paper, we report the direct synthesis of CoMoO4 nanosheets assembled 3D-frameworks (NAFs) on the carbon cloth through a hydrothermal process and subsequent calcination, during which the addition of urea (CO(NH2)2) and polyvinyl alcohol (PVA) is regarded to play a significant role in the formation of such novel morphology. Electrochemical measurements confirm their structural superiority in energy storage with an excellent specific capacitance as high as 1234 F/g and a long life-span of 5000 cycles are discovered.
2. Experimental 3
Before the material synthesis, the carbon cloth was ultrasonicated in ethanol, acetone and deionized water alternately, followed by drying at 60 ℃ overnight. In a typical synthetic process, 2 mmol CoCl2·4H2O, 2 mmol Na2MoO4·2H2O, 6 mmol CO(NH2)2 and 0.75 g PVA were dissolved in deionized water under the magnetic stirring. Subsequently, the as-prepared reaction solution and the dried carbon cloth were transferred into a Teflon-lined stainless-steel autoclave and heated at 180 ℃ for 24 h. The synthesized sample was washed with ethanol and deionized water for several times and dried at 60 ℃ overnight. Finally, the calcination process was carried out at 350 ℃ for 3 h with a relatively low heating rate. The crystal structure of the sample was identified by the X-ray powder diffraction (XRD, Rigaku D/Max-1200X diffractometry with Cu Kα radiation). The morphology was characterized by the scanning electron microscopy (SEM, Hitachi S-4300) and the transmission electron microscopy (TEM, ZEISS LIBRA200). Electrochemical measurements were operated on a CHI 660E electrochemical workstation in a standard three-electrode system using the aqueous KOH electrolyte. The obtained carbon cloth was directly utilized as the working electrode, while a Pt wire and a saturated Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively.
3. Results and Discussion XRD pattern (Fig. 1) reveals the obtained sample is CoMoO4 with a monoclinic structure (JCPDS No.21-0868, a=10.210 Å, b=9.268 Å, c=7.022 Å), where the strongest peak comes from the (002) plane and locates at 2θ=26.5°. Coincidentally, the strongest peak of the carbon cloth appears at nearly the same angle (JCPDS No.26-1077), which makes the overlapped peak obviously higher and broader than others. No noticeable impurity peaks can be detected, 4
indicating the high purity of the sample. SEM was then employed to investigate the morphology of CoMoO4 (Fig. 2a and 2b). During the hydrothermal process, each carbon wire functions as a substrate and is uniformly covered with CoMoO4 nanosheets. These nanosheets are almost vertical to the substrate and extend outwards along the radial direction. More subtle structural features were analyzed by TEM (Fig. 2c and 2d). The central part of the single nanosheet, which is somewhat thicker than the edge like a convex lens, reaches about 20-30 nm. Countless nanosheets interconnect with each other and create a large amount of space among them, forming an open and porous nanostructure. Based on these findings, growth details of CoMoO4 NAFs are put forward (Fig. 3). CoMoO4 can be formed through a simple non-redox reaction of Co2+ and MoO42-. NH3 which is generated by the decomposition of CO(NH2)2 has one lone pair of electrons on the nitrogen atom and shows the coordination ability to Co2+ [32]. Determined by the valance state and the steric limitation, six vacant orbitals of Co2+ take part in the coordination, resulting in [Co(NH3)6]2+ [33]. The formed [Co(NH3)6]2+ will further react with MoO42- and the precursor [Co(NH3)6]MoO4 is obtained subsequently, which will transform into CoMoO4 in the later calcination and re-generate NH3. Although the ultimate product is identical, the addition of CO(NH2)2 actually provides a new reaction path and changes the original reaction kinetics. PVA with the hydroxyl group (-OH) at each constitutional unit exhibits the hydrogen-bonding ability and therefore works as a nonionic surfactant [34]. In the reaction process, PVA adsorbs on the particle surface via hydrogen bonds and causes a macromolecular layer. This layer gives rise to the steric hindrance and avoids the dense stacking of particles, which is beneficial for the construction of the open and porous morphology [35]. It is the co-existence of NH3 and PVA in the hydrothermal system that 5
contributes to the final formation of CoMoO4 NAFs. However, considering the complexity of the hydrothermal system that reagents may affect each other and the reaction is influenced by multiple factors (e.g. the temperature and the time), a more comprehensive understanding of CO(NH2)2 and PVA still needs further research. The novel morphology endows CoMoO4 NAFs an excellent permeability in the aqueous electrolyte and abundant active sites for the electrochemical reaction [33, 36], so they are predicted to possess excellent energy storage properties. As a proof-of-concept, a serial of electrochemical measurements was carried out. The cyclic voltammetry (CV) was tested in the potential window from -0.1 to 0.4 V and well-defined redox peaks can be observed, testifying the charge storage ability of CoMoO4 NAFs (Fig. 4a). Moreover, the charge storage mechanism can be judged through the b value in the formula i=aνb, where i and ν stand for the peak current and the scan rate of CV curves, respectively [37-39]. If the peak current is linearly related to the square root of the scan rate (b=1/2), the reaction reflects an insertion process and is diffusion-controlled. In contrast, if the peak current is linearly related to the scan rate (b=1), the reaction represents a rapid capacitive process and is surface-controlled [40, 41]. In this work, the latter condition (b=1) is discovered (Fig. 4b), suggesting the fast charging/discharging ability of CoMoO4 NAFs. A complete CV curve involves an oxidation process with the voltage increasing from -0.1 to 0.4 V and a reduction process with the voltage decreasing from 0.4 to -0.1 V. Partial areas of the CV curve measured at the scan rate of 5 mV/s are magnified to reveal the redox detail of CoMoO4. Three oxidation peaks which locate at 0, 0.06, and 0.28 V can be observed, implying a multi-step oxidation process (Fig. 4c). With Co2+ in octahedral sites and Mo6+ in tetrahedral sites, only the cobalt atom in CoMoO4 can participate in the redox reaction and the voltage value of oxidation 6
peaks exactly corresponds to three valence changes of Co2+ [34, 36, 42]. Relevant equations are as follows: (1) 3[Co(OH)3]- = Co3O4 + 4H2O + OH- + 2e- (Co2+ to Co8/3+) (2) Co3O4 + H2O + OH- = 3CoOOH + e- (Co8/3+ to Co3+) (3) CoOOH + OH- = CoO2 + H2O+e- (Co3+ to Co4+) Distinguished from the oxidation process, only one reduction peak can be found in the reduction process (Fig. 4d) and the cobalt ion with a high valence (CoX+) is reduced to the primary Co2+ via a one-step reaction. This difference may be ascribed to the electronic configuration of the cobalt atom with two electrons exist in the outermost layer and the cobalt ion prefers to a two-valance state (Co2+) [36, 42]. The galvanostatic charging/discharging (GCD) test was carried out at the same potential window to the CV measurement (Fig. 5a). A full charging and discharging process can be accomplished in 1500 s at the current density of 1 A/g and this process will decrease to less than 50 s when the current density increases to 10 A/g. Meanwhile, the GCD curve keeps smooth and no fluctuation is observed even at this high current density, confirming the fast charging/discharging feature of CoMoO4 NAFs. The specific capacitance (C) is calculated by the formula C=It/V, where I is the current density, t is the discharging time and V is the potential window (Fig. 5b) [35, 43]. Moreover, the referential specific capacitance (CRef) of CoMoO4 can be calculated at diverse valances using the formula CRef=F(n-2)/VM, where F is the Faraday constant, (n-2) is the valence difference to initial Co2+, V is the potential window and M is the relative atomic mass of CoMoO4. If all cobalt atoms in CoMoO4 can transform from Co2+ to Co8/3+ (n=8/3), CoMoO4 will provide a referential specific capacitance of 588 F/g. Similarly, this value 7
will up to 882 and 1764 F/g when Co2+ is completely switches to Co3+ (n=3) and Co4+ (n=4), respectively (Fig. 5c). The referential specific capacitance means the full utilization of CoMoO4 in an absolutely ideal condition, and the real specific capacitance can be as close as possible to this value through the rational structural design. In this work, a high specific capacitance of 1234 F/g at the current density of 1 A/g is achieved (Fig. 5b), surpassing the threshold value of Co3+ (n=3, 882 F/g), which indicates the existence of a higher valance state (Co4+) and is in accordance with the previous analysis (Fig. 4c). More importantly, this high specific capacitance shows the structural superiority of CoMoO4 NAFs in the energy storage. When the current density increases to 10 A/g, a considerable specific capacitance of 446 F/g is still achieved, displaying an acceptable rate capability (Fig. 5b and 5d). Except the GCD curve, the specific capacitance can be calculated using the formula C=S/νmV, where S is the integral area of the CV curve, ν is the scan rate, m is the mass and V is the potential window [44, 45]. So, the rate capability can also be evaluated through the CV curve. With the rise of scan rates from 5 to 200 mV/s, the obtained capacitance retention is similar to that calculated by the GCD curve (Fig. 5d). This decent rate capability is associated with the unique morphology of CoMoO4 NAFs, which is further proved by the electrochemical impedance spectroscope (Fig. 5d, inset). The semicircle in the high-frequency region is negligible, implying the low interfacial resistance between the electrode and the electrolyte, which testifies the excellent permeability of CoMoO4 NAFs to KOH solution [46, 47]. The line in the low-frequency region shows a sharp slope, indicating the fast charge diffusion in the electrode, which can be attributed to the binder-free connection of CoMoO4 NAFs and the carbon cloth [30, 48, 49]. As the life span is another essential parameter to estimate the electrode material, the cycling 8
performance of CoMoO4 NAFs is performed using the GCD measurement with the current density of 10 A/g (Fig. 6). It is notable that CoMoO4 NAFs show a superb cycling life, which can keep nearly invariant after 5000 cycles (3.1% loss in the specific capacitance). This excellent cycling performance can be attributed to two aspects: (i) the stability of CoMoO4 and its oxidation products (Co3O4, CoOOH and CoO2) in the aqueous KOH solution and (ii) the novel nanostructure of CoMoO4 NAFs as well as the binder-free configuration of the electrode, which provide a low impedance and have been analyzed above. In fact, the utilization of CoMoO4 in supercapacitors has been widely studied in the research and a great deal of methods have been employed to improve its electrochemical property [30, 43, 44, 50-59]. On one hand, the morphology modulation is utilized and CoMoO4 is fabricated into varied nanostructures, such as nanorods and nanosheets. On the other hand, CoMoO4-based composites are synthesized via the recombination with electrochemically active materials, including carbon materials (carbon nanotube (CNT) and graphene), conductive polymers (polypyrrole (PPy) and polyaniline (PANI)) as well as other Co-containing inorganics (Co(OH)2 and Co3O4). Here, we report CoMoO4 NAFs on the carbon cloth with the superior specific capacitance, which outstrip most of CoMoO4 and CoMoO4-based composites ever utilized, as a promising electrode material of the high-performance supercapacitor (Fig. 7).
4. Conclusions In summary, CoMoO4 nanosheets assembled 3D-framworks (NAFs) were directly constructed on the carbon cloth through the hydrothermal process and subsequent calcination. The coordination effect of NH3 is considered to exert a critical influence on the reaction kinetics control by inducing the reaction along a new route, while the hydrogen-bonding effect makes PVA 9
a nonionic surfactant and can contribute to the non-dense stacking of particles. The final formation of CoMoO4 NAFs is attributed to the co-existence of NH3 and PVA in the reaction system. Owing to the open and porous morphology, CoMoO4 NAFs show an excellent permeability in the aqueous electrolyte and can provide a large number of active sites for the electrochemical reaction, resulting in their outstanding specific capacitance (1234 F/g at the current density of 1 A/g) and long cycling life (96.9% capacitance retention after 5000 cycles). Our work provides a novel avenue in preparing CoMoO4 utilized in high-performance supercapacitors, which may also enlighten the research of other ternary metal oxides.
Acknowledgment This research is funded by Chongqing Research Program of Basic Research and Frontier Tec hnology (No.cstc2016jcyjA0006) and Chongqing Graduate Student Research Innovation Project (No. CYS17001).
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Figures
Fig. 1 XRD pattern of the sample
Fig. 2. (a, b) SEM images of the sample; (c, d) TEM images of the sample
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Fig. 3 Schematic description of growth details of CoMoO4 NAFs
Fig. 4. (a) CV curves of CoMoO4 NAFs at different scan rates; (b) The cathodic/anodic peak current as a function of the scan rate; (c, d) The oxidation and reduction process of CoMoO4 NAFs
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Fig. 5. (a, b) GCD curves of CoMoO4 NAFs at different current densities; (c) The referential specific capacitance of CoMoO4 calculated at different valences; (d) The capacitance retention at different current densities/scan rates (The inset is the Nyquist plot of CoMoO4 NAFs)
Fig. 6 The cycling performance of CoMoO4 NAFs tested at the current density of 10 A/g
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Fig. 7 Comparison of CoMoO4 NAFs with other CoMoO4 and CoMoO4-based composites utilized in supercapacitors
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PII: DOI: Reference:
S0272-8842(17)32422-7 https://doi.org/10.1016/j.ceramint.2017.10.216 CERI16635
To appear in: Ceramics International Received date: 5 October 2017 Revised date: 27 October 2017 Accepted date: 28 October 2017 Cite this article as: Huiwu Long, Tianmo Liu, Wen Zeng, Yifang Yang and Shuoqing Zhao, CoMoO4 nanosheets assembled 3D-frameworks for highperformance energy storage, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.10.216 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CoMoO4 nanosheets assembled 3D-frameworks for high-performance energy storage Huiwu Long1, Tianmo Liu1,*1, Wen Zeng1, Yifang Yang1, Shuoqing Zhao1,2,* 1. College of Materials Science and Engineering, Chongqing University, Chongqing China 2. Centre for Clean Energy Technology, Faculty of Science, University of Technology, Sydney
Abstract The selection of electrode materials and the optimization of electrode configurations are two effective approaches to achieve high-performance energy storage. As a representative ternary metal oxide, CoMoO4 has been directly grown on the carbon cloth via a facile hydrothermal method and subsequent calcination, forming a binder-free electrode. Further measurements reveal that this electrode is composed of CoMoO4 nanosheets assembled 3D-framworks (NAFs) and thereby shows a superb electrochemical property in KOH solution. It is worth mentioning that the specific capacitance reaches as high as 1234 F/g at the current density of 1 A/g, which exceeds most of CoMoO4 and CoMoO4-based composites reported so far. Meanwhile, when the current density increases to 10 A/g, a considerable specific capacitance of 446 F/g is still obtained and can maintain nearly unchanged after 5000 cycles.
Keywords: Hydrothermal, Supercapacitor, Microstructure, Metal oxides 1. Introduction The ever-worsening energy crisis propels the innovation in energy production and consumption, which also reflects in a pressing demand towards high-performance energy storage
*
Corresponding author. Tel./fax: +86 23 65102465
E-mail: [email protected] (T.M. Liu)
[email protected] (S.Q. Zhao)
1
devices (ESDs) [1, 2]. On one hand, the proportion of sustainable and renewable energy sources such as solar and wind has increased. Given their intermittence, ESDs with the excellent stability are required to bridge them with the smart grid [3]. On the other hand, the replace of petrol-powered vehicles by electric ones can diminish the depletion of fossil fuels in a large extent. But the convenience of filling up the petrol tank in several minutes and the rapid accelerating can only be replicated through ESDs with the high charging/discharging power density [4, 5]. In light of these features, supercapacitors which exhibit desirable performances of the fast charging/discharging rate as well as the long cycling life could serve as a promising candidate for efficient ESDs [1-3, 5]. Additionally, the utilization of the aqueous electrolyte also endows supercapacitors with the characters of low cost, high safety and environmental benignity in comparison with traditional batteries [6]. Supercapacitors can be subdivided into two categories according to the charge storage mechanism [2, 3]. Different from electrical double layer capacitors which physically absorb ions at the electrode/electrolyte interface, pseudo-capacitors, represented by transition metal oxides, store energy via a faradaic process of redox-active species and therefore display a higher capacitance [7, 8]. Ternary metal oxides (AxByOz) have more variable oxidation states and better electronic conductivity than simple ones (AxOy), and thus have been widely investigated as electrode materials for high-performance supercapacitors [9-16]. The traditional electrode fabrication of supercapacitors adopts the slurry-coating technique which normally employs polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) as the binder [17-19]. These polymeric binders are insulated and inevitably exert detrimental impacts on the electrochemical performance of electrodes [4, 21-23]. To solve this problem, electrochemical active materials are directly loaded on the conductive substrate, forming the 2
binder-free electrode [22-25]. Nickel (Ni) foam is universally used due to its high porosity and large specific surface area [24, 26], but the poor acid resistance of Ni foam confines its application only to the neutral or alkaline electrolyte [25, 26]. Meanwhile, its density reaches as high as 262 mg/cm3 which results in a low specific capacitance of the total electrode [23]. 3D-graphene usually exhibits the excellent chemical stability and a relative low density (approximately 20 mg/cm3), and thus attracts intensive attention as the substitution of the Ni foam [23, 27, 28]. Despite the improvement in electrochemical properties, the complicated and time-consuming synthesis strategy of 3D-graphene is much more likely to restrict its large-scale commercialization. In most cases, graphene is firstly deposited on Ni foam through the chemical vapor deposition (CVD) and then soaked in HCl solution to dissolve the initial Ni foam, eventually forming the 3D-structure [27, 28]. Based on aforementioned considerations, the commercialized carbon cloth, which integrates advantages of the Ni foam as well as the 3D-graphene, is expected to be an ideal scaffold to grow redox-active species on it. Except these merits, the unique fiber structure of the carbon cloth also offers it brilliant mechanical strength and flexibility, which would further extend the application range of the electrode, especially in portable and wearable electronics [29-31]. In this paper, we report the direct synthesis of CoMoO4 nanosheets assembled 3D-frameworks (NAFs) on the carbon cloth through a hydrothermal process and subsequent calcination, during which the addition of urea (CO(NH2)2) and polyvinyl alcohol (PVA) is regarded to play a significant role in the formation of such novel morphology. Electrochemical measurements confirm their structural superiority in energy storage with an excellent specific capacitance as high as 1234 F/g and a long life-span of 5000 cycles are discovered.
2. Experimental 3
Before the material synthesis, the carbon cloth was ultrasonicated in ethanol, acetone and deionized water alternately, followed by drying at 60 ℃ overnight. In a typical synthetic process, 2 mmol CoCl2·4H2O, 2 mmol Na2MoO4·2H2O, 6 mmol CO(NH2)2 and 0.75 g PVA were dissolved in deionized water under the magnetic stirring. Subsequently, the as-prepared reaction solution and the dried carbon cloth were transferred into a Teflon-lined stainless-steel autoclave and heated at 180 ℃ for 24 h. The synthesized sample was washed with ethanol and deionized water for several times and dried at 60 ℃ overnight. Finally, the calcination process was carried out at 350 ℃ for 3 h with a relatively low heating rate. The crystal structure of the sample was identified by the X-ray powder diffraction (XRD, Rigaku D/Max-1200X diffractometry with Cu Kα radiation). The morphology was characterized by the scanning electron microscopy (SEM, Hitachi S-4300) and the transmission electron microscopy (TEM, ZEISS LIBRA200). Electrochemical measurements were operated on a CHI 660E electrochemical workstation in a standard three-electrode system using the aqueous KOH electrolyte. The obtained carbon cloth was directly utilized as the working electrode, while a Pt wire and a saturated Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively.
3. Results and Discussion XRD pattern (Fig. 1) reveals the obtained sample is CoMoO4 with a monoclinic structure (JCPDS No.21-0868, a=10.210 Å, b=9.268 Å, c=7.022 Å), where the strongest peak comes from the (002) plane and locates at 2θ=26.5°. Coincidentally, the strongest peak of the carbon cloth appears at nearly the same angle (JCPDS No.26-1077), which makes the overlapped peak obviously higher and broader than others. No noticeable impurity peaks can be detected, 4
indicating the high purity of the sample. SEM was then employed to investigate the morphology of CoMoO4 (Fig. 2a and 2b). During the hydrothermal process, each carbon wire functions as a substrate and is uniformly covered with CoMoO4 nanosheets. These nanosheets are almost vertical to the substrate and extend outwards along the radial direction. More subtle structural features were analyzed by TEM (Fig. 2c and 2d). The central part of the single nanosheet, which is somewhat thicker than the edge like a convex lens, reaches about 20-30 nm. Countless nanosheets interconnect with each other and create a large amount of space among them, forming an open and porous nanostructure. Based on these findings, growth details of CoMoO4 NAFs are put forward (Fig. 3). CoMoO4 can be formed through a simple non-redox reaction of Co2+ and MoO42-. NH3 which is generated by the decomposition of CO(NH2)2 has one lone pair of electrons on the nitrogen atom and shows the coordination ability to Co2+ [32]. Determined by the valance state and the steric limitation, six vacant orbitals of Co2+ take part in the coordination, resulting in [Co(NH3)6]2+ [33]. The formed [Co(NH3)6]2+ will further react with MoO42- and the precursor [Co(NH3)6]MoO4 is obtained subsequently, which will transform into CoMoO4 in the later calcination and re-generate NH3. Although the ultimate product is identical, the addition of CO(NH2)2 actually provides a new reaction path and changes the original reaction kinetics. PVA with the hydroxyl group (-OH) at each constitutional unit exhibits the hydrogen-bonding ability and therefore works as a nonionic surfactant [34]. In the reaction process, PVA adsorbs on the particle surface via hydrogen bonds and causes a macromolecular layer. This layer gives rise to the steric hindrance and avoids the dense stacking of particles, which is beneficial for the construction of the open and porous morphology [35]. It is the co-existence of NH3 and PVA in the hydrothermal system that 5
contributes to the final formation of CoMoO4 NAFs. However, considering the complexity of the hydrothermal system that reagents may affect each other and the reaction is influenced by multiple factors (e.g. the temperature and the time), a more comprehensive understanding of CO(NH2)2 and PVA still needs further research. The novel morphology endows CoMoO4 NAFs an excellent permeability in the aqueous electrolyte and abundant active sites for the electrochemical reaction [33, 36], so they are predicted to possess excellent energy storage properties. As a proof-of-concept, a serial of electrochemical measurements was carried out. The cyclic voltammetry (CV) was tested in the potential window from -0.1 to 0.4 V and well-defined redox peaks can be observed, testifying the charge storage ability of CoMoO4 NAFs (Fig. 4a). Moreover, the charge storage mechanism can be judged through the b value in the formula i=aνb, where i and ν stand for the peak current and the scan rate of CV curves, respectively [37-39]. If the peak current is linearly related to the square root of the scan rate (b=1/2), the reaction reflects an insertion process and is diffusion-controlled. In contrast, if the peak current is linearly related to the scan rate (b=1), the reaction represents a rapid capacitive process and is surface-controlled [40, 41]. In this work, the latter condition (b=1) is discovered (Fig. 4b), suggesting the fast charging/discharging ability of CoMoO4 NAFs. A complete CV curve involves an oxidation process with the voltage increasing from -0.1 to 0.4 V and a reduction process with the voltage decreasing from 0.4 to -0.1 V. Partial areas of the CV curve measured at the scan rate of 5 mV/s are magnified to reveal the redox detail of CoMoO4. Three oxidation peaks which locate at 0, 0.06, and 0.28 V can be observed, implying a multi-step oxidation process (Fig. 4c). With Co2+ in octahedral sites and Mo6+ in tetrahedral sites, only the cobalt atom in CoMoO4 can participate in the redox reaction and the voltage value of oxidation 6
peaks exactly corresponds to three valence changes of Co2+ [34, 36, 42]. Relevant equations are as follows: (1) 3[Co(OH)3]- = Co3O4 + 4H2O + OH- + 2e- (Co2+ to Co8/3+) (2) Co3O4 + H2O + OH- = 3CoOOH + e- (Co8/3+ to Co3+) (3) CoOOH + OH- = CoO2 + H2O+e- (Co3+ to Co4+) Distinguished from the oxidation process, only one reduction peak can be found in the reduction process (Fig. 4d) and the cobalt ion with a high valence (CoX+) is reduced to the primary Co2+ via a one-step reaction. This difference may be ascribed to the electronic configuration of the cobalt atom with two electrons exist in the outermost layer and the cobalt ion prefers to a two-valance state (Co2+) [36, 42]. The galvanostatic charging/discharging (GCD) test was carried out at the same potential window to the CV measurement (Fig. 5a). A full charging and discharging process can be accomplished in 1500 s at the current density of 1 A/g and this process will decrease to less than 50 s when the current density increases to 10 A/g. Meanwhile, the GCD curve keeps smooth and no fluctuation is observed even at this high current density, confirming the fast charging/discharging feature of CoMoO4 NAFs. The specific capacitance (C) is calculated by the formula C=It/V, where I is the current density, t is the discharging time and V is the potential window (Fig. 5b) [35, 43]. Moreover, the referential specific capacitance (CRef) of CoMoO4 can be calculated at diverse valances using the formula CRef=F(n-2)/VM, where F is the Faraday constant, (n-2) is the valence difference to initial Co2+, V is the potential window and M is the relative atomic mass of CoMoO4. If all cobalt atoms in CoMoO4 can transform from Co2+ to Co8/3+ (n=8/3), CoMoO4 will provide a referential specific capacitance of 588 F/g. Similarly, this value 7
will up to 882 and 1764 F/g when Co2+ is completely switches to Co3+ (n=3) and Co4+ (n=4), respectively (Fig. 5c). The referential specific capacitance means the full utilization of CoMoO4 in an absolutely ideal condition, and the real specific capacitance can be as close as possible to this value through the rational structural design. In this work, a high specific capacitance of 1234 F/g at the current density of 1 A/g is achieved (Fig. 5b), surpassing the threshold value of Co3+ (n=3, 882 F/g), which indicates the existence of a higher valance state (Co4+) and is in accordance with the previous analysis (Fig. 4c). More importantly, this high specific capacitance shows the structural superiority of CoMoO4 NAFs in the energy storage. When the current density increases to 10 A/g, a considerable specific capacitance of 446 F/g is still achieved, displaying an acceptable rate capability (Fig. 5b and 5d). Except the GCD curve, the specific capacitance can be calculated using the formula C=S/νmV, where S is the integral area of the CV curve, ν is the scan rate, m is the mass and V is the potential window [44, 45]. So, the rate capability can also be evaluated through the CV curve. With the rise of scan rates from 5 to 200 mV/s, the obtained capacitance retention is similar to that calculated by the GCD curve (Fig. 5d). This decent rate capability is associated with the unique morphology of CoMoO4 NAFs, which is further proved by the electrochemical impedance spectroscope (Fig. 5d, inset). The semicircle in the high-frequency region is negligible, implying the low interfacial resistance between the electrode and the electrolyte, which testifies the excellent permeability of CoMoO4 NAFs to KOH solution [46, 47]. The line in the low-frequency region shows a sharp slope, indicating the fast charge diffusion in the electrode, which can be attributed to the binder-free connection of CoMoO4 NAFs and the carbon cloth [30, 48, 49]. As the life span is another essential parameter to estimate the electrode material, the cycling 8
performance of CoMoO4 NAFs is performed using the GCD measurement with the current density of 10 A/g (Fig. 6). It is notable that CoMoO4 NAFs show a superb cycling life, which can keep nearly invariant after 5000 cycles (3.1% loss in the specific capacitance). This excellent cycling performance can be attributed to two aspects: (i) the stability of CoMoO4 and its oxidation products (Co3O4, CoOOH and CoO2) in the aqueous KOH solution and (ii) the novel nanostructure of CoMoO4 NAFs as well as the binder-free configuration of the electrode, which provide a low impedance and have been analyzed above. In fact, the utilization of CoMoO4 in supercapacitors has been widely studied in the research and a great deal of methods have been employed to improve its electrochemical property [30, 43, 44, 50-59]. On one hand, the morphology modulation is utilized and CoMoO4 is fabricated into varied nanostructures, such as nanorods and nanosheets. On the other hand, CoMoO4-based composites are synthesized via the recombination with electrochemically active materials, including carbon materials (carbon nanotube (CNT) and graphene), conductive polymers (polypyrrole (PPy) and polyaniline (PANI)) as well as other Co-containing inorganics (Co(OH)2 and Co3O4). Here, we report CoMoO4 NAFs on the carbon cloth with the superior specific capacitance, which outstrip most of CoMoO4 and CoMoO4-based composites ever utilized, as a promising electrode material of the high-performance supercapacitor (Fig. 7).
4. Conclusions In summary, CoMoO4 nanosheets assembled 3D-framworks (NAFs) were directly constructed on the carbon cloth through the hydrothermal process and subsequent calcination. The coordination effect of NH3 is considered to exert a critical influence on the reaction kinetics control by inducing the reaction along a new route, while the hydrogen-bonding effect makes PVA 9
a nonionic surfactant and can contribute to the non-dense stacking of particles. The final formation of CoMoO4 NAFs is attributed to the co-existence of NH3 and PVA in the reaction system. Owing to the open and porous morphology, CoMoO4 NAFs show an excellent permeability in the aqueous electrolyte and can provide a large number of active sites for the electrochemical reaction, resulting in their outstanding specific capacitance (1234 F/g at the current density of 1 A/g) and long cycling life (96.9% capacitance retention after 5000 cycles). Our work provides a novel avenue in preparing CoMoO4 utilized in high-performance supercapacitors, which may also enlighten the research of other ternary metal oxides.
Acknowledgment This research is funded by Chongqing Research Program of Basic Research and Frontier Tec hnology (No.cstc2016jcyjA0006) and Chongqing Graduate Student Research Innovation Project (No. CYS17001).
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Figures
Fig. 1 XRD pattern of the sample
Fig. 2. (a, b) SEM images of the sample; (c, d) TEM images of the sample
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Fig. 3 Schematic description of growth details of CoMoO4 NAFs
Fig. 4. (a) CV curves of CoMoO4 NAFs at different scan rates; (b) The cathodic/anodic peak current as a function of the scan rate; (c, d) The oxidation and reduction process of CoMoO4 NAFs
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Fig. 5. (a, b) GCD curves of CoMoO4 NAFs at different current densities; (c) The referential specific capacitance of CoMoO4 calculated at different valences; (d) The capacitance retention at different current densities/scan rates (The inset is the Nyquist plot of CoMoO4 NAFs)
Fig. 6 The cycling performance of CoMoO4 NAFs tested at the current density of 10 A/g
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Fig. 7 Comparison of CoMoO4 NAFs with other CoMoO4 and CoMoO4-based composites utilized in supercapacitors
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