- Email: [email protected]
Decoration of ultrathin porous zeolitic imidazolate frameworks on zinc–cobalt layered double hydroxide nanosheet arrays for ultrahigh-performance supercapacitors
Journal of Power Sources 450 (2020) 227689
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Decoration of ultrathin porous zeolitic imidazolate frameworks on zinc–cobalt layered double hydroxide nanosheet arrays for ultrahigh-performance supercapacitors Xiaopeng Li, Caixia Wu, Ziran Zhu, Zhongwei Lu, Yirong Zhao, Xudong Zhang, Jin-Yuan Zhou, Zhenxing Zhang, Xiaojun Pan *, Erqing Xie School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� The Zn–Co-ZIFs was prepared by chem ical vapor deposition. � The composite material contributes to storing and adsorbing electrolyte ions. � The Zn–Co-ZIFs nanosheets exhibits high capacitance and good cycle stability.
A R T I C L E I N F O
A B S T R A C T
Keywords: Zn–Co LDH nanosheets Supercapacitors MOFs ZIFs
In this work, ultra-thin zeolitic imidazolate frameworks (ZIFs) layers are coated on Zn–Co layered double hy droxide (LDH) nanosheet electrodes by chemical vapor deposition. The ZIF-coated Zn–Co LDH nanosheets are revealed to present excellent electrochemical performances (specific capacitance of 2068 F g 1 at scan rate of 1 A g 1, and retention ratio of 92.6% after 10,000 cycles), which are much higher than those of the untreated ones (specific capacitance of 1444 F g 1 at 1 A g 1, and retention ratio of 63.6% after 10,000 cycles). The enhanced electrochemical performance may be attributed to the cavity confinement effect and more active sites exposed by the rough Zn–Co LDH nanosheets coated with ZIFs. Meanwhile, the ZIFs with porous structure and ultra-high specific surface area could promote the redox reaction on the surface of the Zn–Co LDH nanosheets by elec trolyte ions storing and adsorbing during the electrochemical process. The surface-coated ZIFs can effectively avoid unnecessary electrochemical reaction between the electrolyte and the electrode material during electro chemical cycling, avoiding irreversible dissolution during cycling. Furthermore, the Zn–Co-130-ZIFs-10h//NC950 device delivers a higher energy density of 42.5 Wh kg 1 at a power density of 424 W kg 1.
* Corresponding author. E-mail address: [email protected] (X. Pan). https://doi.org/10.1016/j.jpowsour.2019.227689 Received 11 July 2019; Received in revised form 16 December 2019; Accepted 31 December 2019 0378-7753/© 2020 Elsevier B.V. All rights reserved.
X. Li et al.
Journal of Power Sources 450 (2020) 227689
1. Introduction
130 LDH nanosheets with 2-methylimidazole vapor for 10 h showed the best electrochemical performance and the sample was named Zn-Co130-ZIFs-10h. Moreover, electrochemical results demonstrated that the Zn–Co-130-ZIFs-10h nanosheets composite electrode exhibited a high capacitance of 2068 F g 1 at current density of 1 A g 1, which shows a significant improvement compared to that of the Zn–Co-130 LDH nanosheets (1444 F g 1 at 1 A g 1). And the Zn–Co-130-ZIFs-10h nanosheets composite electrode could retain 92.6% of the initial value after 10,000 cycles, which is highly improved compared to that of the Zn–Co-130 LDH nanosheets (63.9%). Experimental results show that surface-coated ZIFs have no capacity performance, and the enhanced electrochemical performance is mainly attributed to the effect of cavity confinement and rough Zn–Co LDH nanosheets exposed more active sites after ZIFs coated. There are two main reasons for the higher elec trochemical capacity of Zn-Co-ZIFs. Firstly, the CVD process exposes more active sites of Zn-Co LDH. Secondly, the ZIFs grown on the surface of Zn-Co LDH, which hardly contribute to the capacity, but could transfer and store electrolytes, during the electrochemical process, the applied electric field increases the electrolyte concentration of OH at the interface between the ZIFs and Zn-Co LDH nanosheets, and then make the Zn-Co LDH nanosheets more sufficiently react with the elec trolyte [41,42]. At the same time, the surface-coated ZIF can effectively avoid unnecessary electrochemical reactions between the electrolyte and the electrode material during the electrochemical cycle and avoid irreversible dissolution during the cycle. Furthermore, the Zn–Co-130-ZIFs-10h//NC-950 device gets a higher energy density of 42.5 Wh kg 1 at a power density of 424 W kg 1. This research would provide a new insight for the preparation of high-performance MOFs composite electrode materials for SCs.
Supercapacitors (SCs), as a significant accessory power supply, have gained a lot of attentions due to their excellent features such as high power density (fast charge-discharge rate), and low manufacturing cost [1]. According to the energy storage mechanism, SCs can be mainly divided into two categories, i.e., electrochemical double-layer capaci tors (EDLCs) usually using carbon-based electrodes [2–8], and the faradaic pseudocapacitors employing metal oxides or hydroxides as electrodes [9–11]. Usually, the electrode materials for pseudocapacitors show much higher specific capacitance than those for the EDLCs. Taken Co-based materials for instance, they possess a high theoretical specific capacitance (over 3000 F g 1), which makes them one of the most promising materials for application in high-performance SCs [12,13]. However, the poor conductivity and limited number of active sites of these Co-based compound materials led to poor capacitive and kinetic behaviors in their electrode applications in SCs. In order to obtain high electrochemical performances of Co-based materials, much effort has been devoted on the following three aspects: i) introducing heteroatoms to increase the active sites on the surfaces of Co-based materials, such as ZnCo2S4 [14–20]; ii) growing hierarchical structures to obtain large specific surface area, such as ZnCo2O4@Ni(OH)2 and ZnCo2O4@ZnWO4 [21–27]; and iii) combining the carbon nanostructures with Co-based materials, such as ZnCo2O4/NG [28]. Recently, carbonized metal-organic frameworks (MOFs) have also been incorporated into SC electrodes, due to their unique porous structure and large specific surface area [29,30]. Huang et al. reported that ZIF-67 derived porous hollow Co3O4@C frameworks had a large specific capacitance of 1100 F g 1 at a current density of 1.25 A g 1, which was much superior to the performance of pure Co3O4 (Co3O4 nanowire electrode exhibited 336 F g 1 at a current density of 1 A g 1) [31,32]. Zhao et al. found that the CoNi@S,N-doped carbon composites derived from Co/Ni MOFs had a specific capacitance of 1970 F g 1 at a current density of 1 A g 1 with a high rate capability (remaining over 87% at a current density of 5 A g 1), and this MOF-derived nanocarbon material had also a considerably improved performance as compared with the conventional carbon composites (Ni–Co–S@Graphene Frame works electrode exhibited 1492 F g 1 at the current density of 1 A g 1) [33,34]. These enhancements have been mainly attributed to that the unique porous structure of MOFs which are more conducive to the full reaction of electrolyte and porous hollow pseudocapacitance nano materials [31,33,35]. More recently, it was also reported that MOFs could be directly used as an SC electrode material without any carbon ization treatment. For examples, Wang et al. reported that the cobalt-based layered MOFs had a specific capacity of 2474 F g 1 at a current density of 1 A g 1 [36], and Yao et al. found the Co-MOFs exhibited an ultrahigh areal specific capacitance of 13.6 F cm 2 at 2 mA cm 2 [37]. These results implied that the uncarbonized MOFs might also be incorporated with pseudocapacitance nanomaterials to enhance the electrochemical performance of electrode materials. And many re searchers found that the pores in the MOFs can be acted as precise channels for charge transporting, then the charges confining in them could effectively react with the metal element inside of the MOFs through these channels and resulted into a greatly improved electro chemical performances [30,36–40]. Thus, it is highly important and instructive to study the enhancement effect of the uncarbonized ZIFs on the electrochemical performances of Co-based materials. Herein, an ultrathin layer of ZIFs were designed and coated on a type of Zn–Co layered double hydroxide (LDH) nanosheets via chemical vapor deposition. It is noted here that the ZIFs usually consists of Zn2þ and/or Co2þ ions, and imidazole ligands attached to metal atoms, and these metal atoms were obtained from in the Zn–Co LDH nanosheets matrix. The Brunauer-Emmett-Teller (BET) tests indicated that the deposited ZIFs possessed a specific pore size distribution less than 2 nm, which was more conducive to storing and transporting electrolyte ions during energy storage process. The sample obtained by reacting Zn–Co-
2. Experimental 2.1. Materials synthesis The Zn–Co-130-ZIFs electrodes were synthesized according to the following experimental details. Firstly, the reaction solutions were ob tained via mixing Co(NO3)2⋅6H2O (2 mM), Zn(NO3)2⋅6H2O (1 mM) and Urea (5 mM) in 35 mL deionized water (DI) under agitated stirring. After stirring for 30 min, the mixed solution was transferred to a 50 mL stainless steel autoclaves with a Teflon liner. And then a piece of Ni foam (1 cm � 2 cm) was placed into the reaction solution. Secondly, the stainless steel autoclaves was transferred to an oven at 130 � C for 6 h to synthesize the Zn–Co-130 LDH nanosheets arrays on Ni foam. After drying, the pink active materials were grown successfully on the surface of the Ni foam. After the oven cooled to room temperature, the surface of the Ni Foam was rinsed with DI water. Thirdly, the dried Zn–Co-130 LDH nanosheets on Ni foam was fixed above the Teflon liner and 7 g 2methylimidazole was placed in the Teflon liner. Then the stainless steel autoclave was transferred to an oven at 100 � C for 10 h to synthesize the Zn–Co-130-ZIFs. After the reaction, the color of surface was turned to purple and the mass of the active materials is about 3.5 mg. The Zn-Co130 and Zn-Co-130-ZIFs powders were ultrasonically collected for XRD and XPS tests. The NC-950 powder (950 represents the carbonization temperature of ZIF-8) derived from the ZIF-8 [29,43,44]. The ZIF-8 was prepared according to a standard preparation method. In short, the 25 mL solution containing 3.5 g 2-methylimidazole was magnetically stirred and then added to the 25 mL methanol solution containing 1 g zinc nitrate. Stir ring at room temperature for 24 h, centrifuge collection, then drying in vacuum oven 80 � C for 6 h. The collected powder was moved to an oven. Then the oven was raised from room temperature to 300 � C under Ar gas protection for 1 h at the rate of 5 � C⋅min 1 and to 950 � C for 3 h at the rate of 5 � C⋅min 1. The black powder was cleaned in 2 M HCl solution for 24 h, then dried at 80 � C in the vacuum. The NC-950 electrode was consisted of the NC-950 powder (80 wt%), acetylene black (10 wt%), polyvinylidene fluoride (PVDF, 10 wt%) and mixed 2
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 1. Schematic illustration of the preparation of Zn–Co-130-ZIFs on Ni Foam.
N-methylpyrrolidone (NMP), then mixed a homogeneous slurry. Finally, the suspension was adhered to a 1 cm � 2 cm commercial Ni foam and dried in a vacuum oven at 80 � C for 12 h. To eliminate the effects of the capacity of the ZIFs on the Zn-Co-130 LDH, a neutral 2 M Na2SO4 electrolyte to test the electrochemical performance of Zn–Co-130-ZIFs. The voltage window is 0–0.55 V consistent with the Zn–Co-130-ZIFs in the KOH electrolyte. The Na2SO4 electrolyte as a neutral electrolyte does not react with the Zn–Co-130 LDH electrode and the ZIFs can only possible provide EDLC capacitance. The ZIFs were prepared by means of the literatures reported and the amount of Zn Co added is consistent with the Zn-Co-130 LDH [29,45]. The electrodes prepared by the coating method and tested the electrochemical properties of the ZIFs to eliminate the electrochemical effects of the ZIFs on the Zn-Co-130-ZIFs.
electrolyte (2.0 M) with a three-electrode configuration, using Pt elec trode as the counter electrode and saturated Ag/AgCl electrode as the reference electrode. Cycling voltammetry (CV) and galvanostatic charge-discharge (GCD) were used to analyze the electrochemical properties of the electrodes. Electrochemical impedance spectroscopy (EIS) is performed in a frequency range of 100 kHz to 0.01 Hz. It is noted that the metal atoms for ZIFs growth were derived from the Zn–Co-130 LDH nanosheets. Also, because ZIFs act only as electrolyte transport layer and ZIFs have almost no electrochemical properties, the quality of active substances does not include the quality of ZIFs [29,46,47]. The following computational formula was used to analyze specific capacity of the electrode according to GCD curves (C, F g 1): C¼
2.2. Material characterization
I⋅t m⋅ΔU R U2
The morphologies and micro structures of the nanosheets were characterized by scanning electron microscopy (SEM, Apreo S), and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F30) equipped with an energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD, X’pert pro, Philips) with Cu Kα radiation (0.154056 nm) was used to investigate the crystal structures of the samples. The Brunauer-Emmett-Teller (BET) surface areas of the sam ples were measured by nitrogen (N2) adsorption method at 77K (ASAP 2020, micromeritics).
C¼
U1
(1) IðUÞdU
2 � m � ΔU � S
(2)
where I is the current density (A) of charge-discharge processes, t (s) represents discharge time, m (g) is the mass of active material, ΔU represents the potential window during GCD process. The GCD curves of asymmetric supercapacitor (ASC) are used to calculate energy density (E, Wh kg 1) and power density (P, W kg 1) according to following formula: � E ¼ 1 2C⋅ΔU 2 (3)
2.3. Electrochemical characterization
P¼
All the electrochemical tests in the experiment were carried out on an electrochemical workstation (CS 310, Wuhan CorrTest) in KOH aqueous
E Δt
(4)
Where C (F g 1) represent the specific capacitance at different scan rates
Fig. 2. The SEM images of the Zn–Co-110 LDH (a), Zn–Co-130 LDH (c), Zn–Co-150 LDH (e), Zn–Co-110-ZIFs (b), Zn–Co-130-ZIFs (d), Zn–Co-150-ZIFs (f). The insert images are high magnification of the SEM images. 3
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 3. (a) XRD patterns and (b) XPS spectra of the Zn–Co-130 LDH and Zn–Co-130-ZIFs, (c) Zn 2p of Zn–Co-130 LDH, (d) Zn 2p of the Zn–Co-130-ZIFs, (e) Co 2p of the Zn–Co-130 LDH, (f) Co 2p of the Zn–Co-130-ZIFs.
of the ASC supercapacitor, ΔU (V) is the potential windows, Δt (s) is the discharge time of the ASC.
Zn–Co-130-ZIFs nanosheets It can clearly see that the surface roughness of the Zn–Co-130-ZIF is more than that of the Zn–Co-130 LDH. This phenomenon is mainly due to the metal atom on the surface of Zn–Co-130 LDH react with 2-Methylimidazole vapor. BET could be used to investigate the pore structures and specific surface area of materials. Fig. S3(a) shows the nitrogen adsorption–desorption isotherms of the Ni foam, Zn–Co-130 LDH and Zn–Co-130-ZIFs, and the corresponding specific BET surface areas of the samples are 2.3, 4.3 and 25.0 m2 g 1, respectively. The large difference of the specific surface area between Zn–Co-130 LDH and Zn–Co-130-ZIFs is mainly due to the fact that the ZIFs on the surface of Zn–Co-130 LDH is porous [50]. Fig. S3(b) presents the pore size distributions of the Ni foam, Zn–Co-130 LDH and Zn–Co-130-ZIFs. It found that the large surface area of the Zn–Co-130-ZIFs is mainly due to micropores size less than 2 nm, which also consistent with the pore characteristics of the ZIFs [44]. Fig. 3(a) presents the XRD patterns of the Zn–Co-130 LDH and Zn–Co-130-ZIFs. The peaks at 12.8� , 32.6� and 59.5� could match well with the standard diffraction date of the Zn5(CO3)2(OH)6 (JCPDS No. 19–1458). In Fig. S4, the EDS verify that the Zn and Co elements co-exist in the Zn–Co-130 LDH nanosheets. Thus, the Zn–Co-130 LDH nano sheets can be regarded as Co-doped Zn5(CO3)2(OH)6 with a chemical formula of ZnxCo5-x (CO3)2(OH)6 [51,52]. In XRD pattern, the diffrac tion peaks of the ZIFs were clear, and suggest that the ZIFs were grown successfully on the surface of the Zn–Co-130 LDH nanosheets [46,50,53,
3. Results and discussion The Zn–Co-T-ZIFs (T represents the hydrothermal temperature) nanosheets on the Ni Foam were prepared by a simple one-step hydro thermal process and then coated by followed chemical vapor deposition, as depicted in Fig. 1. The color change of the samples represents that different matter grown on the surface of the Ni foam, as shown in Fig. S1. This phenomenon indicates that ZIFs are formed on the surface of the Zn–Co LDH nanosheets successfully [29]. Fig. 2 represents the SEM images of the samples. Fig. 2(a, c and e) represents the SEM images of the Zn–Co LDH nanosheets synthesized at 110 � C, 130 � C and 150 � C, respectively. From Fig. 2(a, c, and e), it can be observed that the Zn–Co-130 LDH presents large scale homogeneity than that of others [48,49]. After that, a simple chemical vapor method was adopted to synthesize the ZIFs on the surface of the Zn–Co LDH nanosheets. Under a high temperature of 100 � C, 2-methylimidazole vapor was introduced to react directly with the Zn2þ and Co2þ ions from the Zn–Co LDH nanosheets, and then form a layer of ZIFs. Fig. 2(b, d and f) represent the SEM images of the ZIFs grown on the Zn–Co LDH nanosheets prepared under different synthesized temperature. Fig. S2 shows the higher magnification SEM images of the Zn–Co-130 LDH and 4
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 4. (a, b) TEM images of the Zn–Co-130-ZIFs, (c, d) High-resolution TEM images of the Zn–Co-130-ZIFs, (e) EDX elemental mapping of C, N, O, Zn and Co in the Zn–Co-130-ZIFs nanosheets.
54], as shown in Fig. 3(a). In XPS of Fig. 3(b), the peak of element N could be observed clearly observed in the Zn–Co-130-ZIFs, which may be due to N atoms introduced in the synthesis process of the ZIFs and the atomic ratios of different elements for the Zn–Co-130 LDH and Zn–Co-130-ZIFs are shown in Table S1. It can be seen from Table S1 that as the growth time of the ZIFs continues to increase, the amount of the Zn Co elements that can be detected becomes less and less, for the X-ray is shielded by the ZIFs layer. The 2p peaks of the Zn and Co are weak, due to the surrounding of the ZIFs on the surface of the Zn–Co-130 LDH nanosheets. In the Zn 2p spectra (Fig. 3(c and d)), the binding energies of 1021.5 eV of 2p3/2 and 1044.5 eV of 2p1/2 suggest that the existence mode of the Zn element is bivalence [55,56]. As shown in Fig. 3(e and f) of high-resolution XPS spectrum of Co 2p, the binding energy of Co at 780.8 eV and 783.1 eV is mainly derived from Co3þ and Co2þ in Co 2p3/2, respectively. Another two peaks at 797.1 eV and 800.4 eV, cor responding to the spin-orbit feature of Co 2p1/2, also prove that the existence format of the Co element is positive tri- and bivalence [48,49, 57,58]. The peaks at 785.4 eV and 804.5 eV are the shake-up satellites (labelled as Sat. in Fig. 3(e and f)). TEM was further used to illustrate the structural information of the Zn–Co-130 LDH and Zn–Co-130-ZIFs nanosheets. Figs. S5(a–d) shows the TEM images of the Zn–Co-130 LDH nanosheets. The lattice spacing of 0.300 and 0.258 nm represents the ( 401) and ( 202) planes of Zn5(CO3)2(OH)6 (JCPDS No. 19–1458), as shown in Fig. S5. Fig. 4 shows the TEM images of the Zn–Co-130-ZIFs nanosheets. Fig. 4 (a) and Fig. S5 (f) shows that the thickness of the uniform ZIFs coated on the surface of the Zn-Co-130 is about 25 nm. Fig. S5(e) shows the thickness of the ZnCo-130 LDH nanosheets in the vertical state, and the results show that the thickness is only about 34 nm. In high resolution image, it also can
be observed that the lattice spacing of 0.204 and 0.258 nm corre sponding to the (022) and ( 202) planes of Zn5(CO3)2(OH)6 (JCPDS No. 19–1458), respectively. Fig. 4(e) shows the corresponding EDX element mapping of the Zn–Co-130-ZIFs. The existence of N element further proves that the ZIFs was successfully grown on surface of the Zn–Co-130 LDH nanosheets. GCD, CV and EIS are used here to evaluate the electrochemical properties of the electrode materials. Fig. 5(a and b) shows the CV curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs nanosheets at different scan rates. From the CV curves, it found that the curves present similar redox peak, indicating that the same redox reaction occurs in electrochemical process of the Zn–Co-130 LDH and Zn–Co-130-ZIFs nanosheets. Fig. 5(c and d) shows the GCD curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs nanosheets. The Zn–Co-130-ZIFs nanosheets exhibits the capacitance of 2068 F g 1 to 1444 F g 1 of the Zn–Co-130 LDH nanosheets at 1 A g 1. Fig. 5(e) reflects the performance of the contrast samples before and after ZIFs growth at 110 � C, 130 � C and 150 � C. The experimental results show that the electrochemical performance make a great improvement after ZIFs growing, as shown in Fig. 5(e and f). The ZIFs grown on the surface of the Zn-Co LDH nanosheets, which hardly contribute to the capacity, but could transfer and store electrolytes. During the electro chemical process, OH will be transferred to the surface of the Zn CoZIFs along the pores in the ZIFs under the action of an applied electric field. This result will make the concentration of the electrolyte stored in the pores of the ZIFs greater than that in the aqueous solution and this effect will increase the concentration of OH at the interface between the ZIFs and Zn Co LDH nanosheets, and then make the Zn Co LDH nanosheets more sufficiently react with the electrolyte [41,42]. Also, the CVD process needs to consume the Zn Co element in Zn-Co-LDH, this 5
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 5. (a, b) CV curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs at different scan rates, (c, d) GCD curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs at different scan rates, (e) GCD curves of the samples at 1 A g 1, (f) the capacitance as a function of current density.
etching process also exposes a relatively large number of active sites in the Zn-Co-LDH. Fig. 5(f) shows the relationship between scan rates and capacitance performance of the different samples, the electrochemical performance of Zn–Co-130-ZIFs nanosheets is the best one of the six samples. Fig. S6 (a and b) shows the electrochemical performances of the Zn-Co-130-ZIFs in different electrolytes at a scan rate of 20 mV s 1 and Fig. S6 (c and d) shows the electrochemical performances of ZIFs at a scan rate of 20 mV s 1. The results indicate that the ZIFs on the surface of the Zn-Co-130-LDH provide scarcely any capacitance. Hence, the capacity was calculated by CV curves (Fig. 5(a and b)) using total mass loading (3.5 mg) of the Zn-Co-130-ZIFs, and the capacity is 1228 F g 1 at 5 mV s 1 (Fig. S7). If don’t consider the mass loss of the Zn-Co-130 nanosheets in CVD process, the capacity is 1691 F g 1 at 5 mV s 1 (2.5 mg, the original mass of the Zn-Co-130 nanosheets) (Fig. S7). Also, the cycling performance of the Zn-Co-130 LDH is improved after ZIFs grown. In above analysis, the ZIFs is benefit for the electrochemical performance of the Zn Co-130 nanosheets. Fig. 6 shows the electrochemical performances of the Zn–Co LDH nanosheets with different ZIFs coated time of 0 h, 5 h, 10 h and 15 h. Fig. 6(a) and (b) represent the GCD curves at scan rates at 1 A g 1 and 20 A g 1, respectively. The results show that the grown time of the ZIFs is an important influence factor to the electrochemical performance of the samples. Fig. 6(c) shows the relationships between scan rates and the capacitance performance of the samples. It was found that the Zn–Co130 LDH nanosheets with ZIFs grown of 10 h can achieve the best electrochemical performance. There are two mainly reasons for such
results. First, the growth time of the ZIFs is critical to obtaining a uni form electrolyte ion transfer channel. Secondly, the more ZIFs grow, the more Zn–Co-130 LDH nanosheets that participate in the redox reaction are consumed. As increasing of the deposition time, the total mass of the Zn-Co-130-ZIFs-H also shows an increasing trend. The total masses of the deposition time of 5 h, 10 h and 15 h are 3.1 mg, 3.5 mg and 4 mg, respectively. Fig. S8 reflects the XRD and XPS data for the Zn-Co-130ZIFs-5h and Zn-Co-130-ZIFs-15h. The data show that the longer the ZIFs grown, the weaker the signal of the Zn-Co-130-LDH can be detected by XPS and XRD, the reason for this phenomenon is that the X-ray is shielded by the ZIFs layer. Fig. 6(d) reflects the EIS of the samples. From the slopes at the low frequency region of the EIS curve, it can be found that the longer ZIFs grown time, the worse conductivity of the material [3,30,59]. This phenomenon is also consistent with the poor conduc tivity of the ZIFs. The cycle stability is critical issue for practical application of the electrode materials of supercapacitor. Fig. 7(a) shows cycle stability of the Zn–Co-130 LDH and Zn–Co-130-ZIFs-10h. The results present that the Zn–Co-130-ZIFs-10h nanosheets electrode remained 96.9%, beyond that of the Zn–Co-130 LDH with 63.9% after 10,000 cycles. Fig. 7(c) shows SEM image of the Zn–Co-130 electrode after 10,000 GCD cycles. The Zn–Co-130 LDH on the Ni foam has been cracked after the cycle and the nanosheets are etched more seriously in the electrochemical process. These changes are the main cause of the degradation of electrochemical performance. Fig. 7(d) is the SEM image of the Zn-Co-130-ZIFs-10h electrode after 10,000 GCD cycles. The Zn-Co-130-ZIFs-10h 6
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 6. (a) The GCD curves of the Zn–Co-130-ZIF-H nanosheets at current density of 1 A g 1, (b) The GCD curves of Zn–Co-130-ZIF-H nanosheets at current density of 20 A g 1, (c) The capacitance as a function of current density, (d) EIS of the Zn–Co-130-ZIF-H (H represents the growth time of ZIFs).
Fig. 7. (a) Cycling performance of the Zn–Co-130 LDH and Zn–Co-130-ZIFs-10h nanosheets electrodes beyond 10,000 cycles at current density of 20 A g 1, (b) EIS curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs-10h nanosheets electrodes, (c) SEM image of Zn–Co-130 LDH electrode after 10000 GCD cycles, (d) SEM image of Zn–Co-130-ZIFs-10h electrode after 10000 GCD cycles.
nanosheets still retain their original morphology and do not undergo significant structural changes like that of the Zn-Co-130 LDH, which further demonstrates that the ZIFs with excellent alkali resistance play an important role in the electrochemical cycle [60]. After cycling, the powder of the Zn-Co-130-ZIFs-10h was collected using Ultrasonic method, and then drip onto FTO to test the XRD. Fig. S9 shows the XRD pattern of the Zn–Co-130-ZIFs-10h electrode after 10,000 GCD cycles, the experimental results show that the original signal can still be
detected after the cycle, which further indicates that the Zn-Co-130-ZIFs have better cycle performance. The main reason for the excellent cycle performance of the Zn–Co-130-ZIFs-10h is that the surface-coated ZIFs with excellent alkali resistance can effectively avoid unnecessary elec trochemical reaction between the electrolyte and the electrode material during electrochemical cycling, thus reducing the amount of the Zn–Co-130 LDH nanosheets lost during electrochemical cycling. Fig. 7 (b) shows the EIS curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs-10h 7
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 8. (a) CV curves of the NC-950 and Zn–Co-130-ZIFs-10h electrodes at scan rate of 20 mV s 1 in a three-electrode configuration in 2 M KOH, (b) CV curves of the Zn–Co-130-ZIFs-10h//NC-950 device at different voltage window, (c) The specific capacity of the Zn–Co-130-ZIFs-10h//NC-950 device, (d) Ragone plots.
nanosheets electrodes. In both the low frequency region and the high frequency region of the EIS curve, the Zn–Co-130-ZIFs-10h nanosheets electrode presents lower electrochemical conductivity to the Zn–Co-130 LDH nanosheets electrode. This phenomenon could be explained with the larger impedance of the ZIFs to the Zn–Co LDH nanosheets, and be consistent with previous experimental results. The asymmetric supercapacitors were assembled using the NC-950 as negative electrode and the Zn–Co-130-ZIFs-10h as positive elec trode. Fig. S10 shows the CV and GCD curves of the NC-950 electrode, and the NC-950 electrode exhibits the capacitance of 132 F g 1 at 1 A g 1 and 94 F g 1 at 20 A g 1. Fig. S11 shows the SEM images of the ZIF-8 and NC-950 powders. The structure of the ZIF-8 has collapsed signifi cantly before and after carbonization, which is mainly related to un stable porous structures of the ZIF-8 at high temperatures [61]. Fig. 8(a) shows the CV curves of the positive and negative electrodes at scan rate of 20 mV s 1. The voltage window of the NC-950 electrode is from 1.0 to 0 V, while the voltage range of the Zn–Co-130-ZIFs-10h electrode is from 0 to 0.55 V. Thus, the potential of the Zn–Co-130- ZIFs-10h//NC-950 supercapacitor may extend to about 1.6 V. In order to explore the actual operating voltage range of this device, the voltage window of the CV curve was attempted to increase, as shown in Fig. 8 (b). When the voltage higher than 1.7 V, the CV curve has a less pro nounced polarization phenomenon, thus the actual operating voltage range is 1.7 V. The CV curve does not show significant redox peaks, indicating the good performance of the device combined the perfor mance of double-layer capacitors and pseudocapacitor. The GCD test (Fig. S12) is used to analyze the performances of the ASC. The specific capacitances of the device were 106, 97.2, 85.2, 67.9 and 52 F g 1 at current density of 0.5, 1, 2, 5 and 10 A g 1, respectively, as shown in the inset of Fig. 8(c). Ragone plots between energy density and power density is key factor for practical application of the supercapacitors, as shown in Fig. 8(d). The Zn–Co-130-ZIFs-10h//NC-950 device gets a higher energy density of 42.5 Wh kg 1 at a power density of 424 W kg 1. The ASC device presents a higher energy density than that have been reported, such as NiCo2S4/RGO//AC (24.4 Wh kg 1 @ 750 W kg 1) [62], Ni–Co–S/NF//AC/NF (24.8 Wh kg 1 @ 849.5 W kg 1) [57], ZnCo2O4/NG//AC (28.3 Wh kg 1 @ 500 W kg 1) [28], NiC o2O4//WSs-2 (21 Wh kg 1 @ 424.5 W kg 1) [63].
4. Conclusion In summary, the porous zeolitic imidazolate frameworks (ZIFs) ultrathin layer was successfully grown on the surface of the Zn–Co-130 LDH (layered double hydroxide) nanosheets by chemical vapor deposition process. The Zn–Co-130-ZIFs electrode exhibits an excellent capacity of 2068 F g 1 to 1444 F g 1 of the Zn–Co-130 LDH nanosheets at current density of 1 A g 1. After 10,000 cycles, the Zn–Co-130-ZIFs can maintain a specific capacity of 92.6%, compared to 63.9% of the Zn–Co-130 LDH nanosheets. The main reasons are that the porous ZIFs can promote the contract between the electrolyte ions and the surface of the Zn–Co-130 LDH nanosheets. Moreover, the assembled Zn–Co-130-ZIFs//NC-950 device exhibits a higher energy density of 42.5 Wh kg 1 at a power density of 424 W kg 1, as well as distinguished rate capability. Declaration of competing interest 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. Acknowledge This work is supported by the National Natural Science Foundation of China (Nos. 51572118, 11474135 and 11674140). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227689. References [1] J. Li, S. Chen, X. Zhu, X. She, T. Liu, H. Zhang, S. Komarneni, D. Yang, X. Yao, Adv. Sci. 4 (2017) 1700345. [2] B. Gangaja, S.V. Nair, D. Santhanagopalan, Nanotechnology 29 (2018), 095402. [3] X. Li, C. Hao, B. Tang, Y. Wang, M. Liu, Y. Wang, Y. Zhu, C. Lu, Z. Tang, Nanoscale 9 (2017) 2178–2187.
8
X. Li et al.
Journal of Power Sources 450 (2020) 227689 [34] J. Yang, C. Yu, X. Fan, S. Liang, S. Li, H. Huang, Z. Ling, C. Hao, J. Qiu, Energy Environ. Sci. 9 (2016) 1299–1307. [35] J.L. Niu, C.H. Zeng, H.J. Peng, X.M. Lin, P. Sathishkumar, Y.P. Cai, Small 13 (2017) 1702150. [36] X. Liu, C. Shi, C. Zhai, M. Cheng, Q. Liu, G. Wang, ACS Appl. Mater. Interfaces 8 (2016) 4585–4591. [37] G. Zhu, H. Wen, M. Ma, W. Wang, L. Yang, L. Wang, X. Shi, X. Cheng, X. Sun, Y. Yao, Chem. Commun. 54 (2018) 10499–10502. [38] M. Wei, J. Yang, C. Zheng, P. Xiong, Y. Li, J. Mater. Chem. 2 (2014) 19005–19010. [39] Y. Tang, H.J. Xu, J. Cao, C.F. Shan, B.K. Wang, P.X. Xi, W.S. Liu, Angew Chem Int 57 (2018) 8654–8658. [40] C. Kyung Min, J. Hyung Mo, P. Jung Hyo, Z. Yue-Biao, K. Jeung Ku, O.M. Yaghi, ACS Nano 8 (2014) 7451–7457. [41] J. Liu, J. Li, G. Wang, W. Yang, J. Yang, Y. Liu, J. Colloid Interface Sci. 555 (2019) 234–244. [42] C. Yang, W. Zhang, J. Wang, S. Li, X. Liu, L. Dou, T. Yue, J. Sun, J. Wang, Inorganic Chemistry Frontiers 6 (2019) 2667–2674. [43] R.R. Salunkhe, C. Young, J. Tang, T. Takei, Y. Ide, N. Kobayashi, Y. Yamauchi, Chem. Commun. 52 (2016) 4764–4767. [44] W. Chaikittisilp, M. Hu, H. Wang, H.S. Huang, T. Fujita, K.C. Wu, L.C. Chen, Y. Yamauchi, K. Ariga, Chem. Commun. 48 (2012) 7259–7261. [45] R.R. Salunkhe, J. Tang, Y. Kamachi, T. Nakato, J.H. Kim, Y. Yamauchi, ACS Nano 9 (2015) 6288–6296. [46] J. Tang, R.R. Salunkhe, J. Liu, N.L. Torad, M. Imura, S. Furukawa, Y. Yamauchi, J. Am. Chem. Soc. 137 (2015) 1572–1580. [47] S.D. Worrall, H. Mann, A. Rogers, M.A. Bissett, M.P. Attfield, R.A.W. Dryfe, Electrochim. Acta 197 (2016) 228–240. [48] C. Zhang, C. Lei, C. Cen, S. Tang, M. Deng, Y. Li, Y. Du, Electrochim. Acta 260 (2018) 814–822. [49] C. Zhang, X. Geng, S. Tang, M. Deng, Y. Du, J. Mater. Chem. 5 (2017) 5912–5919. [50] M. Jiang, X. Cao, D. Zhu, Y. Duan, J. Zhang, Electrochim. Acta 196 (2016) 699–707. [51] X. Wu, X. Han, X. Ma, W. Zhang, Y. Deng, C. Zhong, W. Hu, ACS Appl. Mater. Interfaces 9 (2017) 12574–12583. [52] H. Chen, J. Jiang, L.I. Zhang, D. Xia, Y. Zhao, D. Guo, Q.I. Tong, H. Wan, J. Power Sources 254 (2014) 249–257. [53] Y. Pan, Y. Liu, G. Zeng, L. Zhao, Z. Lai, Chem. Commun. 47 (2011) 2071–2073. [54] P. Zhang, F. Sun, Z. Shen, D. Cao, J. Mater. Chem. 2 (2014). [55] Z. Pan, Y. Jiang, P. Yang, Z. Wu, W. Tian, L. Liu, Y. Song, Q. Gu, D. Sun, L. Hu, ACS Nano 12 (2018) 2968–2979. [56] X. Zhou, B. Wang, H. Sun, C. Wang, P. Sun, X. Li, X. Hu, G. Lu, Nanoscale 8 (2015) 5446–5453. [57] K. Tao, X. Han, Q. Ma, L. Han, Dalton Trans. 47 (2018) 3496–3502. [58] S.G. Mohamed, I. Hussain, J.-J. Shim, Nanoscale 10 (2018) 6620–6628. [59] X. Xu, M. Wang, Y. Liu, T. Lu, L. Pan, J. Mater. Chem. 4 (2016) 5467–5473. [60] A.J. Howarth, Y. Liu, P. Li, Z. Li, T.C. Wang, J.T. Hupp, O.K. Farha, Nature Reviews Materials 1 (2016). [61] Y. Zhong, B. Li, S. Li, S. Xu, Z. Pan, Q. Huang, L. Xing, C. Wang, W. Li, Nano-Micro Lett. 10 (2018) 56. [62] Z. Li, X. Ji, J. Han, Y. Hu, R. Guo, J. Colloid Interface Sci. 477 (2016) 46–53. [63] W. Wang, J. Qi, Y. Sui, Y. He, Q. Meng, F. Wei, Y. Jin, J. Nanosci. Nanotechnol. 18 (2018) 5600–5608.
[4] L. Wan, E. Shamsaei, C.D. Easton, D. Yu, Y. Liang, X. Chen, Z. Abbasi, A. Akbari, X. Zhang, H. Wang, Carbon 121 (2017) 330–336. [5] W. Cao, E. Zhang, J. Wang, Z. Liu, J. Ge, X. Yu, H. Yang, B. Lu, Electrochim. Acta 293 (2019) 364–370. [6] Y. Feng, S. Chen, J. Wang, B. Lu, Journal of Energy Chemistry 43 (2020) 129–138. [7] P. Wang, G. Zhang, M.-Y. Li, Y.-X. Yin, J.-Y. Li, G. Li, W.-P. Wang, W. Peng, F.F. Cao, Y.-G. Guo, Chem. Eng. J. 375 (2019). [8] S. Kumagai, Y. Abe, T. Saito, T. Eguchi, M. Tomioka, M. Kabir, D. Tashima, J. Power Sources 437 (2019). [9] M. Kim, J. Kim, Nanotechnology 28 (2017) 195401. [10] J. Zhao, C. Li, R. Liu, Nanoscale 10 (2018) 5882–5887. [11] L. Wang, Q. Zhang, J. Zhu, X. Duan, Z. Xu, Y. Liu, H. Yang, B. Lu, Energy Storage Mater 16 (2019) 37–45. [12] A.A. Lobinsky, V.P. Tolstoy, RSC Adv. 8 (2018) 29607–29612. [13] J. Zhu, D. Song, T. Pu, J. Li, B. Huang, W. Wang, C. Zhao, L. Xie, L. Chen, Chem. Eng. J. 336 (2018) 679–689. [14] N. Zhang, W. Wang, C. Teng, Z. Wu, Z. Ye, M. Zhi, Z. Hong, RSC Adv. 8 (2018) 27574–27579. [15] X. Wang, A. Sumboja, M. Lin, J. Yan, P.S. Lee, Nanoscale 4 (2012) 7266–7272. [16] S. Balasubramaniam, S. Sivabalan, M.-K. Seo, H.Y. Kim, B.-S. Kim, Nanoscale 9 (2017) 18819–18834. [17] A. Mohammadi, N. Arsalani, A.G. Tabrizi, S.E. Moosavifard, Z. Naqshbandi, L. S. Ghadimi, Chem. Eng. J. 334 (2018) 66–80. [18] M. Li, X. Li, Z. Li, Y. Wu, ACS Appl. Mater. Interfaces 10 (2018) 34254–34264. [19] C. Cheng, X. Zhang, C. Wei, Y. Liu, C. Cui, Q. Zhang, D. Zhang, Ceram. Int. 44 (2018) 17464–17472. [20] X. Lu, Q. Zhang, J. Wang, S. Chen, J. Ge, Z. Liu, L. Wang, H. Ding, D. Gong, H. Yang, X. Yu, J. Zhu, B. Lu, Chem. Eng. J. 358 (2019) 955–961. [21] H. Pang, X. Li, Q. Zhao, H. Xue, W.-Y. Lai, Z. Hu, W. Huang, Nano Energy 35 (2017) 138–145. [22] R. Li, S. Wang, Z. Huang, F. Lu, T. He, J. Power Sources 312 (2016) 156–164. [23] Y. Yuan, R. Chen, H. Zhang, Q. Liu, J. Liu, J. Yu, C. Wang, Z. Sun, J. Wang, Electrochim. Acta 294 (2019) 325–336. [24] Y. Wang, Z. Yin, Z. Wang, X. Li, H. Guo, J. Wang, D. Zhang, Electrochim. Acta 293 (2019) 40–46. [25] X. Bai, Q. Liu, J. Liu, H. Zhang, Z. Li, X. Jing, P. Liu, J. Wang, R. Li, Chem. Eng. J. 315 (2017) 35–45. [26] X. Han, Y. Yang, J.J. Zhou, Q. Ma, K. Tao, L. Han, Chemistry 24 (2018) 18106–18114. [27] L. Xie, Y. Liu, H. Bai, C. Li, B. Mao, L. Sun, W. Shi, J. Colloid Interface Sci. 531 (2018) 64–73. [28] X. Ma, P. Zhang, Y. Zhao, Y. Liu, J. Li, J.Y. Zhou, X. Pan, E. Xie, Chem. Eng. J. 327 (2017) 1000–1010. [29] D. Saliba, M. Ammar, M. Rammal, M. Al-Ghoul, M. Hmadeh, J. Am. Chem. Soc. 140 (2018) 1812–1823. [30] T. Wang, S. Zhang, X. Yan, M. Lyu, L. Wang, J. Bell, H. Wang, ACS Appl. Mater. Interfaces 9 (2017) 15510–15524. [31] Y.Z. Zhang, Y. Wang, Y.L. Xie, T. Cheng, W.Y. Lai, H. Pang, W. Huang, Nanoscale 6 (2014) 14354–14359. [32] S.K. Meher, G.R. Rao, J. Phys. Chem. C 115 (2011) 25543–25556. [33] M. Tong, S. Liu, X. Zhang, T. Wu, H. Zhang, G. Wang, Y. Zhang, X. Zhu, H. Zhao, J. Mater. Chem. 5 (2017) 9873–9881.
9
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Decoration of ultrathin porous zeolitic imidazolate frameworks on zinc–cobalt layered double hydroxide nanosheet arrays for ultrahigh-performance supercapacitors Xiaopeng Li, Caixia Wu, Ziran Zhu, Zhongwei Lu, Yirong Zhao, Xudong Zhang, Jin-Yuan Zhou, Zhenxing Zhang, Xiaojun Pan *, Erqing Xie School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� The Zn–Co-ZIFs was prepared by chem ical vapor deposition. � The composite material contributes to storing and adsorbing electrolyte ions. � The Zn–Co-ZIFs nanosheets exhibits high capacitance and good cycle stability.
A R T I C L E I N F O
A B S T R A C T
Keywords: Zn–Co LDH nanosheets Supercapacitors MOFs ZIFs
In this work, ultra-thin zeolitic imidazolate frameworks (ZIFs) layers are coated on Zn–Co layered double hy droxide (LDH) nanosheet electrodes by chemical vapor deposition. The ZIF-coated Zn–Co LDH nanosheets are revealed to present excellent electrochemical performances (specific capacitance of 2068 F g 1 at scan rate of 1 A g 1, and retention ratio of 92.6% after 10,000 cycles), which are much higher than those of the untreated ones (specific capacitance of 1444 F g 1 at 1 A g 1, and retention ratio of 63.6% after 10,000 cycles). The enhanced electrochemical performance may be attributed to the cavity confinement effect and more active sites exposed by the rough Zn–Co LDH nanosheets coated with ZIFs. Meanwhile, the ZIFs with porous structure and ultra-high specific surface area could promote the redox reaction on the surface of the Zn–Co LDH nanosheets by elec trolyte ions storing and adsorbing during the electrochemical process. The surface-coated ZIFs can effectively avoid unnecessary electrochemical reaction between the electrolyte and the electrode material during electro chemical cycling, avoiding irreversible dissolution during cycling. Furthermore, the Zn–Co-130-ZIFs-10h//NC950 device delivers a higher energy density of 42.5 Wh kg 1 at a power density of 424 W kg 1.
* Corresponding author. E-mail address: [email protected] (X. Pan). https://doi.org/10.1016/j.jpowsour.2019.227689 Received 11 July 2019; Received in revised form 16 December 2019; Accepted 31 December 2019 0378-7753/© 2020 Elsevier B.V. All rights reserved.
X. Li et al.
Journal of Power Sources 450 (2020) 227689
1. Introduction
130 LDH nanosheets with 2-methylimidazole vapor for 10 h showed the best electrochemical performance and the sample was named Zn-Co130-ZIFs-10h. Moreover, electrochemical results demonstrated that the Zn–Co-130-ZIFs-10h nanosheets composite electrode exhibited a high capacitance of 2068 F g 1 at current density of 1 A g 1, which shows a significant improvement compared to that of the Zn–Co-130 LDH nanosheets (1444 F g 1 at 1 A g 1). And the Zn–Co-130-ZIFs-10h nanosheets composite electrode could retain 92.6% of the initial value after 10,000 cycles, which is highly improved compared to that of the Zn–Co-130 LDH nanosheets (63.9%). Experimental results show that surface-coated ZIFs have no capacity performance, and the enhanced electrochemical performance is mainly attributed to the effect of cavity confinement and rough Zn–Co LDH nanosheets exposed more active sites after ZIFs coated. There are two main reasons for the higher elec trochemical capacity of Zn-Co-ZIFs. Firstly, the CVD process exposes more active sites of Zn-Co LDH. Secondly, the ZIFs grown on the surface of Zn-Co LDH, which hardly contribute to the capacity, but could transfer and store electrolytes, during the electrochemical process, the applied electric field increases the electrolyte concentration of OH at the interface between the ZIFs and Zn-Co LDH nanosheets, and then make the Zn-Co LDH nanosheets more sufficiently react with the elec trolyte [41,42]. At the same time, the surface-coated ZIF can effectively avoid unnecessary electrochemical reactions between the electrolyte and the electrode material during the electrochemical cycle and avoid irreversible dissolution during the cycle. Furthermore, the Zn–Co-130-ZIFs-10h//NC-950 device gets a higher energy density of 42.5 Wh kg 1 at a power density of 424 W kg 1. This research would provide a new insight for the preparation of high-performance MOFs composite electrode materials for SCs.
Supercapacitors (SCs), as a significant accessory power supply, have gained a lot of attentions due to their excellent features such as high power density (fast charge-discharge rate), and low manufacturing cost [1]. According to the energy storage mechanism, SCs can be mainly divided into two categories, i.e., electrochemical double-layer capaci tors (EDLCs) usually using carbon-based electrodes [2–8], and the faradaic pseudocapacitors employing metal oxides or hydroxides as electrodes [9–11]. Usually, the electrode materials for pseudocapacitors show much higher specific capacitance than those for the EDLCs. Taken Co-based materials for instance, they possess a high theoretical specific capacitance (over 3000 F g 1), which makes them one of the most promising materials for application in high-performance SCs [12,13]. However, the poor conductivity and limited number of active sites of these Co-based compound materials led to poor capacitive and kinetic behaviors in their electrode applications in SCs. In order to obtain high electrochemical performances of Co-based materials, much effort has been devoted on the following three aspects: i) introducing heteroatoms to increase the active sites on the surfaces of Co-based materials, such as ZnCo2S4 [14–20]; ii) growing hierarchical structures to obtain large specific surface area, such as ZnCo2O4@Ni(OH)2 and ZnCo2O4@ZnWO4 [21–27]; and iii) combining the carbon nanostructures with Co-based materials, such as ZnCo2O4/NG [28]. Recently, carbonized metal-organic frameworks (MOFs) have also been incorporated into SC electrodes, due to their unique porous structure and large specific surface area [29,30]. Huang et al. reported that ZIF-67 derived porous hollow Co3O4@C frameworks had a large specific capacitance of 1100 F g 1 at a current density of 1.25 A g 1, which was much superior to the performance of pure Co3O4 (Co3O4 nanowire electrode exhibited 336 F g 1 at a current density of 1 A g 1) [31,32]. Zhao et al. found that the CoNi@S,N-doped carbon composites derived from Co/Ni MOFs had a specific capacitance of 1970 F g 1 at a current density of 1 A g 1 with a high rate capability (remaining over 87% at a current density of 5 A g 1), and this MOF-derived nanocarbon material had also a considerably improved performance as compared with the conventional carbon composites (Ni–Co–S@Graphene Frame works electrode exhibited 1492 F g 1 at the current density of 1 A g 1) [33,34]. These enhancements have been mainly attributed to that the unique porous structure of MOFs which are more conducive to the full reaction of electrolyte and porous hollow pseudocapacitance nano materials [31,33,35]. More recently, it was also reported that MOFs could be directly used as an SC electrode material without any carbon ization treatment. For examples, Wang et al. reported that the cobalt-based layered MOFs had a specific capacity of 2474 F g 1 at a current density of 1 A g 1 [36], and Yao et al. found the Co-MOFs exhibited an ultrahigh areal specific capacitance of 13.6 F cm 2 at 2 mA cm 2 [37]. These results implied that the uncarbonized MOFs might also be incorporated with pseudocapacitance nanomaterials to enhance the electrochemical performance of electrode materials. And many re searchers found that the pores in the MOFs can be acted as precise channels for charge transporting, then the charges confining in them could effectively react with the metal element inside of the MOFs through these channels and resulted into a greatly improved electro chemical performances [30,36–40]. Thus, it is highly important and instructive to study the enhancement effect of the uncarbonized ZIFs on the electrochemical performances of Co-based materials. Herein, an ultrathin layer of ZIFs were designed and coated on a type of Zn–Co layered double hydroxide (LDH) nanosheets via chemical vapor deposition. It is noted here that the ZIFs usually consists of Zn2þ and/or Co2þ ions, and imidazole ligands attached to metal atoms, and these metal atoms were obtained from in the Zn–Co LDH nanosheets matrix. The Brunauer-Emmett-Teller (BET) tests indicated that the deposited ZIFs possessed a specific pore size distribution less than 2 nm, which was more conducive to storing and transporting electrolyte ions during energy storage process. The sample obtained by reacting Zn–Co-
2. Experimental 2.1. Materials synthesis The Zn–Co-130-ZIFs electrodes were synthesized according to the following experimental details. Firstly, the reaction solutions were ob tained via mixing Co(NO3)2⋅6H2O (2 mM), Zn(NO3)2⋅6H2O (1 mM) and Urea (5 mM) in 35 mL deionized water (DI) under agitated stirring. After stirring for 30 min, the mixed solution was transferred to a 50 mL stainless steel autoclaves with a Teflon liner. And then a piece of Ni foam (1 cm � 2 cm) was placed into the reaction solution. Secondly, the stainless steel autoclaves was transferred to an oven at 130 � C for 6 h to synthesize the Zn–Co-130 LDH nanosheets arrays on Ni foam. After drying, the pink active materials were grown successfully on the surface of the Ni foam. After the oven cooled to room temperature, the surface of the Ni Foam was rinsed with DI water. Thirdly, the dried Zn–Co-130 LDH nanosheets on Ni foam was fixed above the Teflon liner and 7 g 2methylimidazole was placed in the Teflon liner. Then the stainless steel autoclave was transferred to an oven at 100 � C for 10 h to synthesize the Zn–Co-130-ZIFs. After the reaction, the color of surface was turned to purple and the mass of the active materials is about 3.5 mg. The Zn-Co130 and Zn-Co-130-ZIFs powders were ultrasonically collected for XRD and XPS tests. The NC-950 powder (950 represents the carbonization temperature of ZIF-8) derived from the ZIF-8 [29,43,44]. The ZIF-8 was prepared according to a standard preparation method. In short, the 25 mL solution containing 3.5 g 2-methylimidazole was magnetically stirred and then added to the 25 mL methanol solution containing 1 g zinc nitrate. Stir ring at room temperature for 24 h, centrifuge collection, then drying in vacuum oven 80 � C for 6 h. The collected powder was moved to an oven. Then the oven was raised from room temperature to 300 � C under Ar gas protection for 1 h at the rate of 5 � C⋅min 1 and to 950 � C for 3 h at the rate of 5 � C⋅min 1. The black powder was cleaned in 2 M HCl solution for 24 h, then dried at 80 � C in the vacuum. The NC-950 electrode was consisted of the NC-950 powder (80 wt%), acetylene black (10 wt%), polyvinylidene fluoride (PVDF, 10 wt%) and mixed 2
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 1. Schematic illustration of the preparation of Zn–Co-130-ZIFs on Ni Foam.
N-methylpyrrolidone (NMP), then mixed a homogeneous slurry. Finally, the suspension was adhered to a 1 cm � 2 cm commercial Ni foam and dried in a vacuum oven at 80 � C for 12 h. To eliminate the effects of the capacity of the ZIFs on the Zn-Co-130 LDH, a neutral 2 M Na2SO4 electrolyte to test the electrochemical performance of Zn–Co-130-ZIFs. The voltage window is 0–0.55 V consistent with the Zn–Co-130-ZIFs in the KOH electrolyte. The Na2SO4 electrolyte as a neutral electrolyte does not react with the Zn–Co-130 LDH electrode and the ZIFs can only possible provide EDLC capacitance. The ZIFs were prepared by means of the literatures reported and the amount of Zn Co added is consistent with the Zn-Co-130 LDH [29,45]. The electrodes prepared by the coating method and tested the electrochemical properties of the ZIFs to eliminate the electrochemical effects of the ZIFs on the Zn-Co-130-ZIFs.
electrolyte (2.0 M) with a three-electrode configuration, using Pt elec trode as the counter electrode and saturated Ag/AgCl electrode as the reference electrode. Cycling voltammetry (CV) and galvanostatic charge-discharge (GCD) were used to analyze the electrochemical properties of the electrodes. Electrochemical impedance spectroscopy (EIS) is performed in a frequency range of 100 kHz to 0.01 Hz. It is noted that the metal atoms for ZIFs growth were derived from the Zn–Co-130 LDH nanosheets. Also, because ZIFs act only as electrolyte transport layer and ZIFs have almost no electrochemical properties, the quality of active substances does not include the quality of ZIFs [29,46,47]. The following computational formula was used to analyze specific capacity of the electrode according to GCD curves (C, F g 1): C¼
2.2. Material characterization
I⋅t m⋅ΔU R U2
The morphologies and micro structures of the nanosheets were characterized by scanning electron microscopy (SEM, Apreo S), and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F30) equipped with an energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD, X’pert pro, Philips) with Cu Kα radiation (0.154056 nm) was used to investigate the crystal structures of the samples. The Brunauer-Emmett-Teller (BET) surface areas of the sam ples were measured by nitrogen (N2) adsorption method at 77K (ASAP 2020, micromeritics).
C¼
U1
(1) IðUÞdU
2 � m � ΔU � S
(2)
where I is the current density (A) of charge-discharge processes, t (s) represents discharge time, m (g) is the mass of active material, ΔU represents the potential window during GCD process. The GCD curves of asymmetric supercapacitor (ASC) are used to calculate energy density (E, Wh kg 1) and power density (P, W kg 1) according to following formula: � E ¼ 1 2C⋅ΔU 2 (3)
2.3. Electrochemical characterization
P¼
All the electrochemical tests in the experiment were carried out on an electrochemical workstation (CS 310, Wuhan CorrTest) in KOH aqueous
E Δt
(4)
Where C (F g 1) represent the specific capacitance at different scan rates
Fig. 2. The SEM images of the Zn–Co-110 LDH (a), Zn–Co-130 LDH (c), Zn–Co-150 LDH (e), Zn–Co-110-ZIFs (b), Zn–Co-130-ZIFs (d), Zn–Co-150-ZIFs (f). The insert images are high magnification of the SEM images. 3
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 3. (a) XRD patterns and (b) XPS spectra of the Zn–Co-130 LDH and Zn–Co-130-ZIFs, (c) Zn 2p of Zn–Co-130 LDH, (d) Zn 2p of the Zn–Co-130-ZIFs, (e) Co 2p of the Zn–Co-130 LDH, (f) Co 2p of the Zn–Co-130-ZIFs.
of the ASC supercapacitor, ΔU (V) is the potential windows, Δt (s) is the discharge time of the ASC.
Zn–Co-130-ZIFs nanosheets It can clearly see that the surface roughness of the Zn–Co-130-ZIF is more than that of the Zn–Co-130 LDH. This phenomenon is mainly due to the metal atom on the surface of Zn–Co-130 LDH react with 2-Methylimidazole vapor. BET could be used to investigate the pore structures and specific surface area of materials. Fig. S3(a) shows the nitrogen adsorption–desorption isotherms of the Ni foam, Zn–Co-130 LDH and Zn–Co-130-ZIFs, and the corresponding specific BET surface areas of the samples are 2.3, 4.3 and 25.0 m2 g 1, respectively. The large difference of the specific surface area between Zn–Co-130 LDH and Zn–Co-130-ZIFs is mainly due to the fact that the ZIFs on the surface of Zn–Co-130 LDH is porous [50]. Fig. S3(b) presents the pore size distributions of the Ni foam, Zn–Co-130 LDH and Zn–Co-130-ZIFs. It found that the large surface area of the Zn–Co-130-ZIFs is mainly due to micropores size less than 2 nm, which also consistent with the pore characteristics of the ZIFs [44]. Fig. 3(a) presents the XRD patterns of the Zn–Co-130 LDH and Zn–Co-130-ZIFs. The peaks at 12.8� , 32.6� and 59.5� could match well with the standard diffraction date of the Zn5(CO3)2(OH)6 (JCPDS No. 19–1458). In Fig. S4, the EDS verify that the Zn and Co elements co-exist in the Zn–Co-130 LDH nanosheets. Thus, the Zn–Co-130 LDH nano sheets can be regarded as Co-doped Zn5(CO3)2(OH)6 with a chemical formula of ZnxCo5-x (CO3)2(OH)6 [51,52]. In XRD pattern, the diffrac tion peaks of the ZIFs were clear, and suggest that the ZIFs were grown successfully on the surface of the Zn–Co-130 LDH nanosheets [46,50,53,
3. Results and discussion The Zn–Co-T-ZIFs (T represents the hydrothermal temperature) nanosheets on the Ni Foam were prepared by a simple one-step hydro thermal process and then coated by followed chemical vapor deposition, as depicted in Fig. 1. The color change of the samples represents that different matter grown on the surface of the Ni foam, as shown in Fig. S1. This phenomenon indicates that ZIFs are formed on the surface of the Zn–Co LDH nanosheets successfully [29]. Fig. 2 represents the SEM images of the samples. Fig. 2(a, c and e) represents the SEM images of the Zn–Co LDH nanosheets synthesized at 110 � C, 130 � C and 150 � C, respectively. From Fig. 2(a, c, and e), it can be observed that the Zn–Co-130 LDH presents large scale homogeneity than that of others [48,49]. After that, a simple chemical vapor method was adopted to synthesize the ZIFs on the surface of the Zn–Co LDH nanosheets. Under a high temperature of 100 � C, 2-methylimidazole vapor was introduced to react directly with the Zn2þ and Co2þ ions from the Zn–Co LDH nanosheets, and then form a layer of ZIFs. Fig. 2(b, d and f) represent the SEM images of the ZIFs grown on the Zn–Co LDH nanosheets prepared under different synthesized temperature. Fig. S2 shows the higher magnification SEM images of the Zn–Co-130 LDH and 4
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 4. (a, b) TEM images of the Zn–Co-130-ZIFs, (c, d) High-resolution TEM images of the Zn–Co-130-ZIFs, (e) EDX elemental mapping of C, N, O, Zn and Co in the Zn–Co-130-ZIFs nanosheets.
54], as shown in Fig. 3(a). In XPS of Fig. 3(b), the peak of element N could be observed clearly observed in the Zn–Co-130-ZIFs, which may be due to N atoms introduced in the synthesis process of the ZIFs and the atomic ratios of different elements for the Zn–Co-130 LDH and Zn–Co-130-ZIFs are shown in Table S1. It can be seen from Table S1 that as the growth time of the ZIFs continues to increase, the amount of the Zn Co elements that can be detected becomes less and less, for the X-ray is shielded by the ZIFs layer. The 2p peaks of the Zn and Co are weak, due to the surrounding of the ZIFs on the surface of the Zn–Co-130 LDH nanosheets. In the Zn 2p spectra (Fig. 3(c and d)), the binding energies of 1021.5 eV of 2p3/2 and 1044.5 eV of 2p1/2 suggest that the existence mode of the Zn element is bivalence [55,56]. As shown in Fig. 3(e and f) of high-resolution XPS spectrum of Co 2p, the binding energy of Co at 780.8 eV and 783.1 eV is mainly derived from Co3þ and Co2þ in Co 2p3/2, respectively. Another two peaks at 797.1 eV and 800.4 eV, cor responding to the spin-orbit feature of Co 2p1/2, also prove that the existence format of the Co element is positive tri- and bivalence [48,49, 57,58]. The peaks at 785.4 eV and 804.5 eV are the shake-up satellites (labelled as Sat. in Fig. 3(e and f)). TEM was further used to illustrate the structural information of the Zn–Co-130 LDH and Zn–Co-130-ZIFs nanosheets. Figs. S5(a–d) shows the TEM images of the Zn–Co-130 LDH nanosheets. The lattice spacing of 0.300 and 0.258 nm represents the ( 401) and ( 202) planes of Zn5(CO3)2(OH)6 (JCPDS No. 19–1458), as shown in Fig. S5. Fig. 4 shows the TEM images of the Zn–Co-130-ZIFs nanosheets. Fig. 4 (a) and Fig. S5 (f) shows that the thickness of the uniform ZIFs coated on the surface of the Zn-Co-130 is about 25 nm. Fig. S5(e) shows the thickness of the ZnCo-130 LDH nanosheets in the vertical state, and the results show that the thickness is only about 34 nm. In high resolution image, it also can
be observed that the lattice spacing of 0.204 and 0.258 nm corre sponding to the (022) and ( 202) planes of Zn5(CO3)2(OH)6 (JCPDS No. 19–1458), respectively. Fig. 4(e) shows the corresponding EDX element mapping of the Zn–Co-130-ZIFs. The existence of N element further proves that the ZIFs was successfully grown on surface of the Zn–Co-130 LDH nanosheets. GCD, CV and EIS are used here to evaluate the electrochemical properties of the electrode materials. Fig. 5(a and b) shows the CV curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs nanosheets at different scan rates. From the CV curves, it found that the curves present similar redox peak, indicating that the same redox reaction occurs in electrochemical process of the Zn–Co-130 LDH and Zn–Co-130-ZIFs nanosheets. Fig. 5(c and d) shows the GCD curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs nanosheets. The Zn–Co-130-ZIFs nanosheets exhibits the capacitance of 2068 F g 1 to 1444 F g 1 of the Zn–Co-130 LDH nanosheets at 1 A g 1. Fig. 5(e) reflects the performance of the contrast samples before and after ZIFs growth at 110 � C, 130 � C and 150 � C. The experimental results show that the electrochemical performance make a great improvement after ZIFs growing, as shown in Fig. 5(e and f). The ZIFs grown on the surface of the Zn-Co LDH nanosheets, which hardly contribute to the capacity, but could transfer and store electrolytes. During the electro chemical process, OH will be transferred to the surface of the Zn CoZIFs along the pores in the ZIFs under the action of an applied electric field. This result will make the concentration of the electrolyte stored in the pores of the ZIFs greater than that in the aqueous solution and this effect will increase the concentration of OH at the interface between the ZIFs and Zn Co LDH nanosheets, and then make the Zn Co LDH nanosheets more sufficiently react with the electrolyte [41,42]. Also, the CVD process needs to consume the Zn Co element in Zn-Co-LDH, this 5
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 5. (a, b) CV curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs at different scan rates, (c, d) GCD curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs at different scan rates, (e) GCD curves of the samples at 1 A g 1, (f) the capacitance as a function of current density.
etching process also exposes a relatively large number of active sites in the Zn-Co-LDH. Fig. 5(f) shows the relationship between scan rates and capacitance performance of the different samples, the electrochemical performance of Zn–Co-130-ZIFs nanosheets is the best one of the six samples. Fig. S6 (a and b) shows the electrochemical performances of the Zn-Co-130-ZIFs in different electrolytes at a scan rate of 20 mV s 1 and Fig. S6 (c and d) shows the electrochemical performances of ZIFs at a scan rate of 20 mV s 1. The results indicate that the ZIFs on the surface of the Zn-Co-130-LDH provide scarcely any capacitance. Hence, the capacity was calculated by CV curves (Fig. 5(a and b)) using total mass loading (3.5 mg) of the Zn-Co-130-ZIFs, and the capacity is 1228 F g 1 at 5 mV s 1 (Fig. S7). If don’t consider the mass loss of the Zn-Co-130 nanosheets in CVD process, the capacity is 1691 F g 1 at 5 mV s 1 (2.5 mg, the original mass of the Zn-Co-130 nanosheets) (Fig. S7). Also, the cycling performance of the Zn-Co-130 LDH is improved after ZIFs grown. In above analysis, the ZIFs is benefit for the electrochemical performance of the Zn Co-130 nanosheets. Fig. 6 shows the electrochemical performances of the Zn–Co LDH nanosheets with different ZIFs coated time of 0 h, 5 h, 10 h and 15 h. Fig. 6(a) and (b) represent the GCD curves at scan rates at 1 A g 1 and 20 A g 1, respectively. The results show that the grown time of the ZIFs is an important influence factor to the electrochemical performance of the samples. Fig. 6(c) shows the relationships between scan rates and the capacitance performance of the samples. It was found that the Zn–Co130 LDH nanosheets with ZIFs grown of 10 h can achieve the best electrochemical performance. There are two mainly reasons for such
results. First, the growth time of the ZIFs is critical to obtaining a uni form electrolyte ion transfer channel. Secondly, the more ZIFs grow, the more Zn–Co-130 LDH nanosheets that participate in the redox reaction are consumed. As increasing of the deposition time, the total mass of the Zn-Co-130-ZIFs-H also shows an increasing trend. The total masses of the deposition time of 5 h, 10 h and 15 h are 3.1 mg, 3.5 mg and 4 mg, respectively. Fig. S8 reflects the XRD and XPS data for the Zn-Co-130ZIFs-5h and Zn-Co-130-ZIFs-15h. The data show that the longer the ZIFs grown, the weaker the signal of the Zn-Co-130-LDH can be detected by XPS and XRD, the reason for this phenomenon is that the X-ray is shielded by the ZIFs layer. Fig. 6(d) reflects the EIS of the samples. From the slopes at the low frequency region of the EIS curve, it can be found that the longer ZIFs grown time, the worse conductivity of the material [3,30,59]. This phenomenon is also consistent with the poor conduc tivity of the ZIFs. The cycle stability is critical issue for practical application of the electrode materials of supercapacitor. Fig. 7(a) shows cycle stability of the Zn–Co-130 LDH and Zn–Co-130-ZIFs-10h. The results present that the Zn–Co-130-ZIFs-10h nanosheets electrode remained 96.9%, beyond that of the Zn–Co-130 LDH with 63.9% after 10,000 cycles. Fig. 7(c) shows SEM image of the Zn–Co-130 electrode after 10,000 GCD cycles. The Zn–Co-130 LDH on the Ni foam has been cracked after the cycle and the nanosheets are etched more seriously in the electrochemical process. These changes are the main cause of the degradation of electrochemical performance. Fig. 7(d) is the SEM image of the Zn-Co-130-ZIFs-10h electrode after 10,000 GCD cycles. The Zn-Co-130-ZIFs-10h 6
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 6. (a) The GCD curves of the Zn–Co-130-ZIF-H nanosheets at current density of 1 A g 1, (b) The GCD curves of Zn–Co-130-ZIF-H nanosheets at current density of 20 A g 1, (c) The capacitance as a function of current density, (d) EIS of the Zn–Co-130-ZIF-H (H represents the growth time of ZIFs).
Fig. 7. (a) Cycling performance of the Zn–Co-130 LDH and Zn–Co-130-ZIFs-10h nanosheets electrodes beyond 10,000 cycles at current density of 20 A g 1, (b) EIS curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs-10h nanosheets electrodes, (c) SEM image of Zn–Co-130 LDH electrode after 10000 GCD cycles, (d) SEM image of Zn–Co-130-ZIFs-10h electrode after 10000 GCD cycles.
nanosheets still retain their original morphology and do not undergo significant structural changes like that of the Zn-Co-130 LDH, which further demonstrates that the ZIFs with excellent alkali resistance play an important role in the electrochemical cycle [60]. After cycling, the powder of the Zn-Co-130-ZIFs-10h was collected using Ultrasonic method, and then drip onto FTO to test the XRD. Fig. S9 shows the XRD pattern of the Zn–Co-130-ZIFs-10h electrode after 10,000 GCD cycles, the experimental results show that the original signal can still be
detected after the cycle, which further indicates that the Zn-Co-130-ZIFs have better cycle performance. The main reason for the excellent cycle performance of the Zn–Co-130-ZIFs-10h is that the surface-coated ZIFs with excellent alkali resistance can effectively avoid unnecessary elec trochemical reaction between the electrolyte and the electrode material during electrochemical cycling, thus reducing the amount of the Zn–Co-130 LDH nanosheets lost during electrochemical cycling. Fig. 7 (b) shows the EIS curves of the Zn–Co-130 LDH and Zn–Co-130-ZIFs-10h 7
X. Li et al.
Journal of Power Sources 450 (2020) 227689
Fig. 8. (a) CV curves of the NC-950 and Zn–Co-130-ZIFs-10h electrodes at scan rate of 20 mV s 1 in a three-electrode configuration in 2 M KOH, (b) CV curves of the Zn–Co-130-ZIFs-10h//NC-950 device at different voltage window, (c) The specific capacity of the Zn–Co-130-ZIFs-10h//NC-950 device, (d) Ragone plots.
nanosheets electrodes. In both the low frequency region and the high frequency region of the EIS curve, the Zn–Co-130-ZIFs-10h nanosheets electrode presents lower electrochemical conductivity to the Zn–Co-130 LDH nanosheets electrode. This phenomenon could be explained with the larger impedance of the ZIFs to the Zn–Co LDH nanosheets, and be consistent with previous experimental results. The asymmetric supercapacitors were assembled using the NC-950 as negative electrode and the Zn–Co-130-ZIFs-10h as positive elec trode. Fig. S10 shows the CV and GCD curves of the NC-950 electrode, and the NC-950 electrode exhibits the capacitance of 132 F g 1 at 1 A g 1 and 94 F g 1 at 20 A g 1. Fig. S11 shows the SEM images of the ZIF-8 and NC-950 powders. The structure of the ZIF-8 has collapsed signifi cantly before and after carbonization, which is mainly related to un stable porous structures of the ZIF-8 at high temperatures [61]. Fig. 8(a) shows the CV curves of the positive and negative electrodes at scan rate of 20 mV s 1. The voltage window of the NC-950 electrode is from 1.0 to 0 V, while the voltage range of the Zn–Co-130-ZIFs-10h electrode is from 0 to 0.55 V. Thus, the potential of the Zn–Co-130- ZIFs-10h//NC-950 supercapacitor may extend to about 1.6 V. In order to explore the actual operating voltage range of this device, the voltage window of the CV curve was attempted to increase, as shown in Fig. 8 (b). When the voltage higher than 1.7 V, the CV curve has a less pro nounced polarization phenomenon, thus the actual operating voltage range is 1.7 V. The CV curve does not show significant redox peaks, indicating the good performance of the device combined the perfor mance of double-layer capacitors and pseudocapacitor. The GCD test (Fig. S12) is used to analyze the performances of the ASC. The specific capacitances of the device were 106, 97.2, 85.2, 67.9 and 52 F g 1 at current density of 0.5, 1, 2, 5 and 10 A g 1, respectively, as shown in the inset of Fig. 8(c). Ragone plots between energy density and power density is key factor for practical application of the supercapacitors, as shown in Fig. 8(d). The Zn–Co-130-ZIFs-10h//NC-950 device gets a higher energy density of 42.5 Wh kg 1 at a power density of 424 W kg 1. The ASC device presents a higher energy density than that have been reported, such as NiCo2S4/RGO//AC (24.4 Wh kg 1 @ 750 W kg 1) [62], Ni–Co–S/NF//AC/NF (24.8 Wh kg 1 @ 849.5 W kg 1) [57], ZnCo2O4/NG//AC (28.3 Wh kg 1 @ 500 W kg 1) [28], NiC o2O4//WSs-2 (21 Wh kg 1 @ 424.5 W kg 1) [63].
4. Conclusion In summary, the porous zeolitic imidazolate frameworks (ZIFs) ultrathin layer was successfully grown on the surface of the Zn–Co-130 LDH (layered double hydroxide) nanosheets by chemical vapor deposition process. The Zn–Co-130-ZIFs electrode exhibits an excellent capacity of 2068 F g 1 to 1444 F g 1 of the Zn–Co-130 LDH nanosheets at current density of 1 A g 1. After 10,000 cycles, the Zn–Co-130-ZIFs can maintain a specific capacity of 92.6%, compared to 63.9% of the Zn–Co-130 LDH nanosheets. The main reasons are that the porous ZIFs can promote the contract between the electrolyte ions and the surface of the Zn–Co-130 LDH nanosheets. Moreover, the assembled Zn–Co-130-ZIFs//NC-950 device exhibits a higher energy density of 42.5 Wh kg 1 at a power density of 424 W kg 1, as well as distinguished rate capability. Declaration of competing interest 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. Acknowledge This work is supported by the National Natural Science Foundation of China (Nos. 51572118, 11474135 and 11674140). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227689. References [1] J. Li, S. Chen, X. Zhu, X. She, T. Liu, H. Zhang, S. Komarneni, D. Yang, X. Yao, Adv. Sci. 4 (2017) 1700345. [2] B. Gangaja, S.V. Nair, D. Santhanagopalan, Nanotechnology 29 (2018), 095402. [3] X. Li, C. Hao, B. Tang, Y. Wang, M. Liu, Y. Wang, Y. Zhu, C. Lu, Z. Tang, Nanoscale 9 (2017) 2178–2187.
8
X. Li et al.
Journal of Power Sources 450 (2020) 227689 [34] J. Yang, C. Yu, X. Fan, S. Liang, S. Li, H. Huang, Z. Ling, C. Hao, J. Qiu, Energy Environ. Sci. 9 (2016) 1299–1307. [35] J.L. Niu, C.H. Zeng, H.J. Peng, X.M. Lin, P. Sathishkumar, Y.P. Cai, Small 13 (2017) 1702150. [36] X. Liu, C. Shi, C. Zhai, M. Cheng, Q. Liu, G. Wang, ACS Appl. Mater. Interfaces 8 (2016) 4585–4591. [37] G. Zhu, H. Wen, M. Ma, W. Wang, L. Yang, L. Wang, X. Shi, X. Cheng, X. Sun, Y. Yao, Chem. Commun. 54 (2018) 10499–10502. [38] M. Wei, J. Yang, C. Zheng, P. Xiong, Y. Li, J. Mater. Chem. 2 (2014) 19005–19010. [39] Y. Tang, H.J. Xu, J. Cao, C.F. Shan, B.K. Wang, P.X. Xi, W.S. Liu, Angew Chem Int 57 (2018) 8654–8658. [40] C. Kyung Min, J. Hyung Mo, P. Jung Hyo, Z. Yue-Biao, K. Jeung Ku, O.M. Yaghi, ACS Nano 8 (2014) 7451–7457. [41] J. Liu, J. Li, G. Wang, W. Yang, J. Yang, Y. Liu, J. Colloid Interface Sci. 555 (2019) 234–244. [42] C. Yang, W. Zhang, J. Wang, S. Li, X. Liu, L. Dou, T. Yue, J. Sun, J. Wang, Inorganic Chemistry Frontiers 6 (2019) 2667–2674. [43] R.R. Salunkhe, C. Young, J. Tang, T. Takei, Y. Ide, N. Kobayashi, Y. Yamauchi, Chem. Commun. 52 (2016) 4764–4767. [44] W. Chaikittisilp, M. Hu, H. Wang, H.S. Huang, T. Fujita, K.C. Wu, L.C. Chen, Y. Yamauchi, K. Ariga, Chem. Commun. 48 (2012) 7259–7261. [45] R.R. Salunkhe, J. Tang, Y. Kamachi, T. Nakato, J.H. Kim, Y. Yamauchi, ACS Nano 9 (2015) 6288–6296. [46] J. Tang, R.R. Salunkhe, J. Liu, N.L. Torad, M. Imura, S. Furukawa, Y. Yamauchi, J. Am. Chem. Soc. 137 (2015) 1572–1580. [47] S.D. Worrall, H. Mann, A. Rogers, M.A. Bissett, M.P. Attfield, R.A.W. Dryfe, Electrochim. Acta 197 (2016) 228–240. [48] C. Zhang, C. Lei, C. Cen, S. Tang, M. Deng, Y. Li, Y. Du, Electrochim. Acta 260 (2018) 814–822. [49] C. Zhang, X. Geng, S. Tang, M. Deng, Y. Du, J. Mater. Chem. 5 (2017) 5912–5919. [50] M. Jiang, X. Cao, D. Zhu, Y. Duan, J. Zhang, Electrochim. Acta 196 (2016) 699–707. [51] X. Wu, X. Han, X. Ma, W. Zhang, Y. Deng, C. Zhong, W. Hu, ACS Appl. Mater. Interfaces 9 (2017) 12574–12583. [52] H. Chen, J. Jiang, L.I. Zhang, D. Xia, Y. Zhao, D. Guo, Q.I. Tong, H. Wan, J. Power Sources 254 (2014) 249–257. [53] Y. Pan, Y. Liu, G. Zeng, L. Zhao, Z. Lai, Chem. Commun. 47 (2011) 2071–2073. [54] P. Zhang, F. Sun, Z. Shen, D. Cao, J. Mater. Chem. 2 (2014). [55] Z. Pan, Y. Jiang, P. Yang, Z. Wu, W. Tian, L. Liu, Y. Song, Q. Gu, D. Sun, L. Hu, ACS Nano 12 (2018) 2968–2979. [56] X. Zhou, B. Wang, H. Sun, C. Wang, P. Sun, X. Li, X. Hu, G. Lu, Nanoscale 8 (2015) 5446–5453. [57] K. Tao, X. Han, Q. Ma, L. Han, Dalton Trans. 47 (2018) 3496–3502. [58] S.G. Mohamed, I. Hussain, J.-J. Shim, Nanoscale 10 (2018) 6620–6628. [59] X. Xu, M. Wang, Y. Liu, T. Lu, L. Pan, J. Mater. Chem. 4 (2016) 5467–5473. [60] A.J. Howarth, Y. Liu, P. Li, Z. Li, T.C. Wang, J.T. Hupp, O.K. Farha, Nature Reviews Materials 1 (2016). [61] Y. Zhong, B. Li, S. Li, S. Xu, Z. Pan, Q. Huang, L. Xing, C. Wang, W. Li, Nano-Micro Lett. 10 (2018) 56. [62] Z. Li, X. Ji, J. Han, Y. Hu, R. Guo, J. Colloid Interface Sci. 477 (2016) 46–53. [63] W. Wang, J. Qi, Y. Sui, Y. He, Q. Meng, F. Wei, Y. Jin, J. Nanosci. Nanotechnol. 18 (2018) 5600–5608.
[4] L. Wan, E. Shamsaei, C.D. Easton, D. Yu, Y. Liang, X. Chen, Z. Abbasi, A. Akbari, X. Zhang, H. Wang, Carbon 121 (2017) 330–336. [5] W. Cao, E. Zhang, J. Wang, Z. Liu, J. Ge, X. Yu, H. Yang, B. Lu, Electrochim. Acta 293 (2019) 364–370. [6] Y. Feng, S. Chen, J. Wang, B. Lu, Journal of Energy Chemistry 43 (2020) 129–138. [7] P. Wang, G. Zhang, M.-Y. Li, Y.-X. Yin, J.-Y. Li, G. Li, W.-P. Wang, W. Peng, F.F. Cao, Y.-G. Guo, Chem. Eng. J. 375 (2019). [8] S. Kumagai, Y. Abe, T. Saito, T. Eguchi, M. Tomioka, M. Kabir, D. Tashima, J. Power Sources 437 (2019). [9] M. Kim, J. Kim, Nanotechnology 28 (2017) 195401. [10] J. Zhao, C. Li, R. Liu, Nanoscale 10 (2018) 5882–5887. [11] L. Wang, Q. Zhang, J. Zhu, X. Duan, Z. Xu, Y. Liu, H. Yang, B. Lu, Energy Storage Mater 16 (2019) 37–45. [12] A.A. Lobinsky, V.P. Tolstoy, RSC Adv. 8 (2018) 29607–29612. [13] J. Zhu, D. Song, T. Pu, J. Li, B. Huang, W. Wang, C. Zhao, L. Xie, L. Chen, Chem. Eng. J. 336 (2018) 679–689. [14] N. Zhang, W. Wang, C. Teng, Z. Wu, Z. Ye, M. Zhi, Z. Hong, RSC Adv. 8 (2018) 27574–27579. [15] X. Wang, A. Sumboja, M. Lin, J. Yan, P.S. Lee, Nanoscale 4 (2012) 7266–7272. [16] S. Balasubramaniam, S. Sivabalan, M.-K. Seo, H.Y. Kim, B.-S. Kim, Nanoscale 9 (2017) 18819–18834. [17] A. Mohammadi, N. Arsalani, A.G. Tabrizi, S.E. Moosavifard, Z. Naqshbandi, L. S. Ghadimi, Chem. Eng. J. 334 (2018) 66–80. [18] M. Li, X. Li, Z. Li, Y. Wu, ACS Appl. Mater. Interfaces 10 (2018) 34254–34264. [19] C. Cheng, X. Zhang, C. Wei, Y. Liu, C. Cui, Q. Zhang, D. Zhang, Ceram. Int. 44 (2018) 17464–17472. [20] X. Lu, Q. Zhang, J. Wang, S. Chen, J. Ge, Z. Liu, L. Wang, H. Ding, D. Gong, H. Yang, X. Yu, J. Zhu, B. Lu, Chem. Eng. J. 358 (2019) 955–961. [21] H. Pang, X. Li, Q. Zhao, H. Xue, W.-Y. Lai, Z. Hu, W. Huang, Nano Energy 35 (2017) 138–145. [22] R. Li, S. Wang, Z. Huang, F. Lu, T. He, J. Power Sources 312 (2016) 156–164. [23] Y. Yuan, R. Chen, H. Zhang, Q. Liu, J. Liu, J. Yu, C. Wang, Z. Sun, J. Wang, Electrochim. Acta 294 (2019) 325–336. [24] Y. Wang, Z. Yin, Z. Wang, X. Li, H. Guo, J. Wang, D. Zhang, Electrochim. Acta 293 (2019) 40–46. [25] X. Bai, Q. Liu, J. Liu, H. Zhang, Z. Li, X. Jing, P. Liu, J. Wang, R. Li, Chem. Eng. J. 315 (2017) 35–45. [26] X. Han, Y. Yang, J.J. Zhou, Q. Ma, K. Tao, L. Han, Chemistry 24 (2018) 18106–18114. [27] L. Xie, Y. Liu, H. Bai, C. Li, B. Mao, L. Sun, W. Shi, J. Colloid Interface Sci. 531 (2018) 64–73. [28] X. Ma, P. Zhang, Y. Zhao, Y. Liu, J. Li, J.Y. Zhou, X. Pan, E. Xie, Chem. Eng. J. 327 (2017) 1000–1010. [29] D. Saliba, M. Ammar, M. Rammal, M. Al-Ghoul, M. Hmadeh, J. Am. Chem. Soc. 140 (2018) 1812–1823. [30] T. Wang, S. Zhang, X. Yan, M. Lyu, L. Wang, J. Bell, H. Wang, ACS Appl. Mater. Interfaces 9 (2017) 15510–15524. [31] Y.Z. Zhang, Y. Wang, Y.L. Xie, T. Cheng, W.Y. Lai, H. Pang, W. Huang, Nanoscale 6 (2014) 14354–14359. [32] S.K. Meher, G.R. Rao, J. Phys. Chem. C 115 (2011) 25543–25556. [33] M. Tong, S. Liu, X. Zhang, T. Wu, H. Zhang, G. Wang, Y. Zhang, X. Zhu, H. Zhao, J. Mater. Chem. 5 (2017) 9873–9881.
9