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Co-based metal-organic frameworks as a flexible electrode for energy storage and conversion
Journal of Power Sources 438 (2019) 227057
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Design of 2D mesoporous Zn/Co-based metal-organic frameworks as a flexible electrode for energy storage and conversion Yunhe Zhao a, b, Hongxing Dong a, b, c, Xinyi He a, b, Jing Yu a, b, *, Rongrong Chen a, b, c, Qi Liu a, b, Jingyuan Liu a, b, Hongsen Zhang a, b, Rumin Li a, b, Jun Wang a, b, c, ** a b c
Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China College of Materials Science and Chemical Engineering, Harbin Engineering University, 150001, PR China Institute of Advanced Marine Materials, Harbin Engineering University, 150001, PR 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
� Zn/Co-based MOFs derived Zn0.76Co0.24S/NiCo2S4 nanosheets were prepared successfully. � The Zn0.76Co0.24S/NiCo2S4 electrode exhibited remarkable specific capacitance. � A flexible all-solid-state Zn0.76Co0.24S/ NiCo2S4//AC ASC device was fabricated. � The enhanced property is due to the nanostructure, porosity and conductivity.
A R T I C L E I N F O
A B S T R A C T
Keywords: Two-dimensional ZIF-L Transition metal sulfides Nanosheets Flexible asymmetric supercapacitors Binder-free flexible devices
Electrode materials with a large internal surface area, tunable pore size, efficient transport of electrons and high ion-accessibility are highly desired in the development of advanced flexible supercapacitors. Recently, transition metal sulfides with rationally designed nanostructures have attracted considerable attention as electrode ma terials for supercapacitors. In this study, we report the synthesis of Zn0.76Co0.24S/NiCo2S4 nanosheets grown on carbon cloth using a two-dimensional bimetallic Zn/Co zeolitic imidazolate framework (named as Zn/Co-ZIF-L) as a precursor through a simple and cost-effective chemical solution process. The ZIF-derived Zn0.76Co0.24S/ NiCo2S4 electrode nanosheets deliver an ultrahigh specific capacitance of 2674 F g 1 at 1 A g 1, a superior ratio performance and cycle stability. Furthermore, the as-fabricated Zn0.76Co0.24S/NiCo2S4//AC all-solid-state asymmetric supercapacitor (ASC) achieves a maximum energy density of 48.1 Wh kg 1 as well as a power density of 837 W kg 1, and a superior cycling performance of 91% retention after 5000 cycles. The detailed electrochemical kinetic analysis demonstrates that the total capacitance of Zn0.76Co0.24S/NiCo2S4 is derived from its capacitive-effective charge storage mechanism. This ZIF-derived strategy provides a reasonable and simple way to synthesize transition metal sulfides as potential active materials for next-generation flexible supercapacitors.
* Corresponding author. Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China. ** Corresponding author. Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China. E-mail addresses: [email protected] (J. Yu), [email protected] (J. Wang). https://doi.org/10.1016/j.jpowsour.2019.227057 Received 10 May 2019; Received in revised form 10 August 2019; Accepted 20 August 2019 Available online 23 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
preparation of various bimetallic metal sulfides. Shao et al. prepared mesoporous polyhedron-structured NiCo2S4 from Ni and Co bimetallic ZIFs, demonstrating an ultrahigh capacitance that reaches up to 1296 F g 1 at 1 A g 1 with a high rate performance [31]. The electro chemical contribution of bimetallic sulfides with both nickel and cobalt ions provides a beneficial redox process compared with mono-metallic sulfides (nickel sulfide, cobalt sulfide), leading to a larger specific capacitance. Nevertheless, MOF materials are usually made in the form of powders, and the agglomeration of powders will reduce the efficiency of active materials. Moreover, involving the polymer binder in the electrode preparation process will immensely reduce the electrical conductivity of the electrode materials and cause a “peeling off” reaction during the lengthy charge/discharge process, which severely limits the binder’s practical application in high-performance flexible SCs. One approach is to grow closely well-aligned arrays directly onto flexible substrates, which effectively decrease the “dead surface” in traditional slurry-derived electrodes, thereby facilitating more charge and efficient mass transportation [32]. Thus, directly growing MOF-derived transi tion metal sulfides onto flexible substrates is of great importance for binder-free electrodes of flexible SCs. Herein, we demonstrate an easy strategy to prepare porous Zn0.76Co0.24S/NiCo2S4 nanosheets from two-dimensional bimetallic Zn/ Co ZIF-L as a binder-free flexible electrode. The original structural and morphological features of the Zn0.76Co0.24S/NiCo2S4 electrode result in an increase of the surface area, providing rich active sites and sufficient contact with electrolytes. The synergistic effect among the components assures rich redox reactions. Moreover, this bind-free electrode grown on carbon cloth effectively averts the “dead surface”. As a result, the ZIFL derived Zn0.76Co0.24S/NiCo2S4 nanosheets exhibit an ultrahigh ca pacity, superior rate performances and an outstanding cycle ability because of the nanostructure and its full usage of the large porosity and favorable conductivity. This strategy may provide a new direction for constructing transition metal sulfide nanostructures for high energy storage application.
Recently, the ever-growing demand for miniaturized portable and wearable flexible electronic devices in applications of electronic textiles, electronic skins, and wearable health monitors have attracted world wide research interest in flexible electronic devices [1–4]. Flexible all-solid-state supercapacitors (SCs) have become promising energy storage devices that are used as various high-power supplies for portable and diverse functions of flexible electronics due to their high power density, fast charge-discharge capability, and super-long cycle life [5–7]. In addition, flexible all-solid-state SCs possess numerous advan tages, including a lightweight composition, strong durability, ease of handling and improved eco-safety compared with traditional liquid electrolyte SCs; therefore, flexible SCs are greatly desired for portable and wearable consumer electronics [8–10]. For flexible SCs, electrode materials significantly affect the overall performance of SCs. Therefore, the development of novel and reliable flexible electrode materials is urgent and highly desirable. Battery-type Faradaic materials, based on transition metal oxides or sulfides, have been widely studied as positive electrode materials for SCs with higher theoretical capacitances and energy densities than conventional carbon-based materials [11]. Nevertheless, battery-type materials can also enhance energy and power densities when combined with electric double layer capacitors in a system [12,13]. Transition metal sulfides (TMSs) offer tremendous potential as electrode materials for energy storage devices, owing to their superior conductivity and higher electrochemical activity compared to their ox ides and hydroxide counterparts [14]. However, large volume changes of TMSs during cycle processes causes a rapid drop in capacity, while a low charge usually leads to a poor rate capability. Various strategies have been proposed to solve these critical problems. A carefully crafted structure design of TMSs with porous nanostructures has been demon strated as an attractive way to greatly promote the TMSs’ electro chemical performance [15,16]. In these nanostructures, the void space can buffer the volume change and effectively facilitate ion/electron transport. On the other hand, recent reports have shown that multi component TMSs possess outstanding electrochemical performance when compared to single TMSs [17]. The multiple valences and syner gistic effects among the different compositions provide rich redox re actions, which can enhance the specific capacity. Accordingly, the simultaneous controlling nanostructures and the composition of TMSs may offer opportunities for improving their electrochemical perfor mance. Nevertheless, there are still several issues that limit the large-scale production of the electrodes, such as an additional post-heat-treatment step and a long synthesis time. Thus, high-performance TMS electrodes with simple and cost-effective syn thesis methods are highly desirable. Metal-organic frameworks (MOFs) have attracted increasing concern in various fields since it is a class of hybrid nanoporous materials with crystalline architectures linked by metal ions and organic ligands [18–20]. In recent years, MOFs have received additional attention and have been demonstrated to have enormous potential for advanced SCs, owing to their large interfacial area, tunable pore size and diversity of metal ions [21–23]. Interestingly, MOFs have been used as a new class of sacrificial template for constructing complex architectures [24]. The tunable structures enable MOFs to derive into a variety of functional materials, including metal-based composites, carbonaceous materials, hydroxides with porous or hollow nanostructures, simultaneously retaining its initial structural features [25,26]. These nanostructures display multiple structure- and composition-dependent advantages, providing considerable potential for superior SC performance. Until now, many studies reported porous TMS nanomaterials which derived from MOFs, such as cubic NiS nano frames, double-shelled CoS hollow nanoboxes, CoS1.097/nitrogen-doped carbon nanosheets, and Co9S8@N–C@MoS2 [27–30]. Moreover, MOFs are not just for the exploitation of mono-metallic sulfides but they also enable the
2. Experimental section 2.1. Materials Co(NO3)2⋅6H2O, Ni(NO3)2⋅6H2O, Zn(NO3)2⋅6H2O, 2-Methylimida zole (C4H6N2, 2MI) were obtained from Tianjin Guangfu Technology Development Co., Ltd. All the chemicals used in this study were analytical grade and used without further purification. 2.2. Synthesis Synthesis of two-dimensional bimetallic Zn/Co ZIF-L samples: The Zn/Co ZIF-L grown on carbon cloth were fabricated by mixed Co (NO3)2⋅6H2O (8 mmol) and Zn(NO3)2⋅6H2O (4 mmol) in 20 mL deion ized water, and 2-methylimidazole (1 mmol in 20 mL deionized water) together rapidly with vigorous stirring. The clean carbon cloth (CC) was put inside the reaction solution for 2 h, then was taken out, washed and dried overnight. The loading for Zn/Co ZIF-L nanosheets on CC is ~2 mg cm 2. Synthesis of ZnNiCo-LDH samples: A piece of Zn/Co ZIF-L nano sheets on the CC was transferred into a mixed solution with Ni (NO3)2⋅6H2O (0.18 g in 50 mL ethanol solution), and magnetically stir red for 30 min. The as-fabricated ZnNiCo-LDH nanosheets on the CC were then washed again repeatedly, and dried at 60 � C in an oven overnight. Synthesis of Zn0.76Co0.24S/NiCo2S4 nanosheets on the carbon cloth: A solution containing 70 � 10 3 M Na2S (30 mL) with a piece of the ZnNiCo-LDH nanosheet on CC were put inside a 50-mL reaction vessel together, and were subsequently kept at 140 � C for 4 h. After the hydrothermal process, the as-prepared samples were washed with distilled water and ethanol, and dried at 60 � C. The mass loading of the 2
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Zn0.76Co0.24S/NiCo2S4 nanosheets was ~4 mg cm
Journal of Power Sources 438 (2019) 227057 2
. E¼
R I⋅ VðtÞdt m
(4)
P¼
E Δt
(5)
2.3. Flexible all-solid-state ASCs In the flexible all-solid-state ASC device, the Zn0.76Co0.24S/NiCo2S4 and activated carbon (AC) were used as positive and negative electrode materials with a PVA/KOH gel electrolyte as a separator. To achieve the ideal electrochemical performance of ASC, the charge quantity (q) on the two electrodes should be balanced, in which q can be estimated using the following equation: q ¼ m � Cs � ΔV
3. Results and discussion 3.1. Morphology and structure analysis The construction strategy of Zn0.76Co0.24S/NiCo2S4 is illustrated in Fig. 1. Two-dimensional bimetallic Zn/Co ZIF-L as the precursor was first prepared by a reaction of Co2þ and Zn2þ with 2-methylimidazole. Then, the ZnNiCo-LDH nanosheets were successfully synthesized by mixed Zn/Co ZIF-L with a solution of Ni(NO3)2⋅6H2O in ethanol. During this process, the Hþ protons are gradually generated by the hydrolysis of Ni2þ ions and etch the ZIF-L templates, and the released Co2þ ions are partially oxidized by the dissolved O2 and NO3 ions in the solution [26]. Then, the coprecipitation of Co2þ/Co3þ ions with Ni2þ and Zn2þ ions results in the formation of ZnNiCo LDH. Moreover, the hydrolysis of Ni(NO3)2 will be accelerated along with the Hþ protons consumption, further promoting the production of LDH. Subsequently, ZnNiCo-LDH transforms into Zn0.76Co0.24S/NiCo2S4 through a solvothermal process with Na2S as the sulfurization agent. The morphologies of Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/ NiCo2S4 were investigated by FESEM. The FESEM images in Fig. 2a show the uniform 2D Zn/Co ZIF-L cover on the carbon cloth surface. As shown in high-magnification FE-SEM images (Fig. 2b and c), these Zn/Co ZIF-L nanosheets present a 2D triangle feature with a glossy surface. The color of Zn/Co ZIF-L on the CC changes into a uniform purple. After the etching and ion exchange process in the ethanol solution of Ni (NO3)2⋅6H2O, the color of ZnNiCo-LDH sample changes from purple to light green (Fig. S1). A FESEM image (Fig. 2d) shows that the nano structures’ array was maintained effectively. The ZnNiCo-LDH samples appear to have a rougher surface constituted by small nanosheets in the magnified images (Fig. 2e and f). The etching process is affected by the concentration of nickel nitrate. When a low concentration of nickel ni trate is used, the Hþ protons generated from the hydrolysis of nickel nitrate become insufficient, and the Zn/Co ZIF-L templates cannot dissolve completely (Figs. S2a and b). When a high concentration of nickel nitrate is used, an over-rapid etching of Zn/Co ZIF-L can even tually lead to the destruction of the 2D nanosheet structure (Figs. S2c and d). After sulfurization in the Na2S solution, the color of the Zn0.76Co0.24S/NiCo2S4 sample changes to black (Fig. S1). Fig. 2g shows that the 2D vertically aligned structure with cross-linked features is mainly retained with no apparent collapse. The Zn0.76Co0.24S/NiCo2S4 nanosheets were interconnected and had a strong adhesion to the CC, thereby providing a high accessibility for the diffusion of electrolyte ions into the surface of the electrode. Thus, fast electrochemical reactions are expected to enhance the electro chemical performance. Close observations (Fig. 2h and i) further show the surface morphology of the nanosheets becoming rougher. In addi tion, the edges of the Zn0.76Co0.24S/NiCo2S4 nanosheets are denser than the other parts. Notably, a high S2 concentration will destroy the 2D arrays (Fig. S3). The presence of Zn, Co, Ni and S is proven by SEM-EDS (Fig. S4a). The X-ray elemental mappings also demonstrates successful synthesis of the Zn0.76Co0.24S/NiCo2S4 electrode (Fig. S4b), which clearly demonstrates the uniform distribution of the Zn, Co, Ni and S elements. Meanwhile, the TEM (Fig. 3a and b) further displays the as-prepared samples, the surface of Zn/Co ZIF-L is smooth and does not display any porous morphology. After the etching with Ni(NO3)2, the original morphology is largely reserved, and we distinctly observed small nanosheets (Fig. 3c). After sulfurization treatment, we clearly observe that numerous interconnected nanoparticles were dispersed throughout
(1)
where Cs (F g 1) is the specific capacitance, ΔV (V) is the operating voltage window, and m (g) is the mass of the active materials. The optimal mass ratio between the positive and negative electrodes materials was determined according to the following equation: mþ C � ΔV ¼ m Cþ � ΔVþ
(2)
where ΔVþ and ΔV are the voltage range for the positive electrode (þ) and negative electrode ( ), respectively. Cþ and C are the Cs of the Zn0.76Co0.24S/NiCo2S4 and active carbon, respectively. The optimal mass ratio of the Zn0.76Co0.24S/NiCo2S4 and active carbon was calcu lated to be 1:6.1. Typically, the PVA/KOH gel electrolyte was synthesized as follows: KOH (3 g) and PVA (6 g) were dissolved in 60 mL DI water with constant stirring at 80 � C for 1 h. After natural cooling, a homogeneous trans parent gel electrolyte was obtained. Prior to their assemblage, the two electrodes were soaked into the PVA/KOH gel electrolyte for approxi mately 30 min and were then spread on the PVA/KOH gel electrolyte for face-to-face construction. Finally, the as-assembled ASC device was maintained at 20 � C for 12 h to remove excess water. 2.4. Characterization The morphology and structures were conducted by a field emission scanning electron microscope (FE-SEM, Hitachi, S-4800) and trans mission electron microscopy (TEM, FEI Tecnai G2-TWIN). The crystal structures were analyzed by a Japanese Rigaku-MiniFlex 600 X-ray diffractometer with Cu Kα radiation (λ ¼ 0.15406 nm). The elemental information was explored by X-ray photoelectron spectra (XPS, Sigma probe ThermoVG, U.K). 2.5. Electrochemical characterization Electrochemical measurements were tested on an electrochemistry workstation (CHI 660D, Shanghai Chenhua Instrument, Inc.). The capacitive properties of the electrodes were performed in a threeelectrode system in a 2 M KOH aqueous electrolyte. The saturated calomel electrode (SCE) and platinum foil were used as reference and counter electrodes, with the Zn0.76Co0.24S/NiCo2S4 nanosheets as the working electrode. The specific capacitance Cs (F g 1) of the active material can be calculated as follows [33]: Z 2I CS ¼ VðtÞdt (3) Vf mV 2 j Vi where I (A) represents the current, t (s) is the discharging time, m (g) is the mass of active material (Zn/Co ZIF-L, ZnNiCo-LDH, Zn0.76Co0.24S/ R NiCo2S4 and active carbon), V(t)dt represents the integral current area under the experimental curve, and V is the potential with initial and final values of Vi and Vf, respectively. The energy density (E, Wh kg 1) and power density (P, W kg 1) can be estimated using the following equations: 3
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Journal of Power Sources 438 (2019) 227057
Fig. 1. Schematic illustration of the synthesis of the ZIF-derived Zn0.76Co0.24S/NiCo2S4 nanosheets.
Fig. 2. FE-SEM images of (a–c) Zn/Co ZIF-L, (d–f) ZnNiCo-LDH and (g–i) Zn0.76Co0.24S/NiCo2S4 nanosheets.
the surface of the Zn0.76Co0.24S/NiCo2S4 nanosheets (Fig. 3d). The fully porous nanostructure is an extremely in-demand morphology in the field of supercapacitor applications. In addition, the sample demonstrates strong center-edge contrast from the TEM image (Fig. 3e); this is due to the higher density of the edge area for the nanosheets, which can also be proved by the SEM image (Fig. 2i). The HRTEM image (Fig. 3f) exhibits lattice plane distances of 0.31 nm and 0.28 nm, which are related to the (111) and (311) crystalline planes of Zn0.76Co0.24S and NiCo2S4, respectively. The SAED pattern indicates that the Zn0.76Co0.24S/NiCo2S4 is polycrystalline in nature. The elemental composition of the prepared sample was further analyzed by EDX spectroscopy. Apparently, the S, Co, Zn and Ni elements were uniformly distributed throughout the nanosheet (Fig. 3g). XRD analysis characterized the crystal structures of samples, in which Fig. 4 displays the corresponding patterns. The major peaks are located at approximately 26� and 43� , which is attributed to the CC substrate, while the other peaks are assigned to the diffraction pattern of ZIF-L [34]. After the reaction, the diffraction peaks of ZIF-L are completely absent in the XRD pattern, and the peaks corresponding to the (003), (012), (015), and (110) planes are evidently associated with the typical LDH structure. After sulfurization, a series of diffraction peaks appear at 16.3� , 31.6� , 38.3� , 50.5� and 55.3� , associated with the (111), (311), (400), (511) and (440) planes, respectively, and all the
characteristic peaks are assigned to NiCo2S4 (JCPDS No. 43–1477). In addition, the diffraction peaks at 28.6� , 47.6� and 56.6� are in agree ment with the (111), (220) and (311) planes of Zn0.76Co0.24S, respec tively (JCPDS No. 47–1656). These results show that the ZIF-L derived Zn0.76Co0.24S/NiCo2S4 was synthesized successfully. The N2 adsorption isotherm tests were investigated to explore the surface area and porous structure of the fabricated Zn0.76Co0.24S/ NiCo2S4. Zn0.76Co0.24S/NiCo2S4 exhibits type IV isotherms with a pro nounced hysteresis loop (Fig. S5), which indicates a mesoporous struc ture with a surface area of 122 m2 g 1. The Barrett-Joyner-Halenda (BJH) method can calculate the pore size distribution, which depicts that the majority of pores are of 12 nm in diameter (inset of Fig. S5). The moderate specific surface areas endow Zn0.76Co0.24S/NiCo2S4 with a large electrode/electrolyte contact area, while the hierarchical porous characteristics can promote the transport and the penetration of the electrolyte ions as well as accommodate volume variations of the active materials during the charging-discharging process. XPS measurements were further conducted to evaluate the changes in the chemical status of ZIF-L derived Zn0.76Co0.24S/NiCo2S4 nano sheets. As shown in Fig. 5a, the high-resolution Zn 2p spectrum is deconvoluted into two peaks at binding energies 1045.5 and 1021.5 eV that correspond to Zn 2p1/2 and Zn 2p3/2, respectively. By applying a Gaussian fitting method, the fine-resolution Co 2p and Ni 2p spectrum 4
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Journal of Power Sources 438 (2019) 227057
Fig. 3. TEM images of (a,b) Zn/Co ZIF-L, (c) ZnNiCo-LDH and (d,e) Zn0.76Co0.24S/NiCo2S4, (f) The HRTEM image of Zn0.76Co0.24S/NiCo2S4, the inset is the SAED pattern. (g) Corresponding elemental distribution mapping of Zn, Co, Ni and S.
fabricated samples, CV, GCD and EIS were measured. Fig. 6a com pares the CV curve characteristics of the Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes collected at 5 mV s 1 in the 0–0.6 V potential window. Importantly, the Zn0.76Co0.24S/NiCo2S4 electrode shows a larger current density than the other electrodes, revealing its superior capacitive performance. The CV curves display a set of intense redox peaks, which contributed to the Faradaic redox process of elec trodes, implying battery-type behaviors. The possible redox reactions of the Zn0.76Co0.24S/NiCo2S4 electrodes are expressed as follows [14,38]: ZnS þ OH ↔ ZnSOH þ e
(6)
CoS þ OH ↔ CoSOH þ e
(7)
CoSOH þ OH ↔ CoSO þ H2 O þ e
(8)
NiS þ OH ↔ NiSOH þ e
(9)
Fig. 6b depicts a comparison of GCD curves for the Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 at the 0–0.5 V potential win dow (1 A g 1). As expected, the obviously prolonged discharge time of the Zn0.76Co0.24S/NiCo2S4 nanosheets electrode displays a higher spe cific capacity contrast to the Zn/Co ZIF-L and ZnNiCo-LDH electrodes, which is in accordance with the CV results (Fig. 6a). The symmetric GCD profiles with a voltage platform suggest a typical pseudocapacitive behavior, which is consistent with the CV tests. Fig. 6c and Fig. S6 represent the GCD curves of Zn0.76Co0.24S/NiCo2S4, ZIF-L and ZnNiCoLDH at various current densities (1–20 A g 1). The nearly symmetrical triangles for each curve prove that the Faradaic electrode redox process is highly reversible and has a low degree of polarization. Integrating the charge/discharge profiles, the corresponding specific capacitances of the Zn0.76Co0.24S/NiCo2S4 electrode are displayed in Fig. 6d. Encour agingly, the Zn0.76Co0.24S/NiCo2S4 electrode possesses specific capaci tances of 2674, 2514, 2380, 2277, 2202 and 2112 F g 1 at 1, 2, 4, 8, 12 and 20 A g 1, respectively, which are much larger than their ZIF-L or ZnNiCo-LDH counterparts. Importantly, the Zn0.76Co0.24S/NiCo2S4 electrode reveals an outstanding rate capability (79% retention of its capacitance). Fig. 6e presents the cycling performance of ZIF-L, ZnNiCoLDH and Zn0.76Co0.24S/NiCo2S4 electrodes at 8 A g 1. Impressively, the Zn0.76Co0.24S/NiCo2S4 electrode shows a cycle stability of 93% retained
Fig. 4. XRD patterns of Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4.
corresponds to the two spin-orbit doublets and their satellites (marked as “Sat.“) (Fig. 5b and c). For the Co 2p spectrum, the binding energies appear at 779.5 and 794.4 eV, which are particularly characteristic of Co2þ ions, and the other two peaks at 782.4 and 796.1 eV are associated with Co3þ ions [35]. The spectrum of Ni 2p can also be equipped with Ni2þ at 853.2 and 872.6 eV, and two satellites (sat.) 856.2 and 874.4 eV are correspond to Ni3þ ions [36]. The S 2p spectrum is shown in Fig. 5d, with two major peaks of S 2p1/2 and S 2p3/2 located at 161.8 and 162.9 eV, respectively. In detail, the S 2p1/2 peak is confirmed by metal-sulfur bonds, and the S 2p3/2 peak is attributed to the surface low-coordinated sulfur ions [37]. The results show that the chemical composition of Zn0.76Co0.24S/NiCo2S4 contains Zn2þ, Ni2þ, Ni3þ, Co2þ, Co3þ and S2 . 3.2. Electrochemical performance analysis To more effectively assess the electrochemical characteristics of as5
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Fig. 5. XPS spectra of Zn0.76Co0.24S/NiCo2S4: (a) Zn 2p, (b) Co 2p, (c) Ni 2p, (d) S 2p.
after 2000 charge/discharge cycles, whereas the ZIF-L and ZnNiCo-LDH show a capacitance retention rates of 81% and 86%, respectively. The Zn0.76Co0.24S/NiCo2S4 electrode’s outstanding electrochemical perfor mance is probably due to: (i) the interconnection of Zn0.76Co0.24S/ NiCo2S4 nanosheets with unique porous features and a large interfacial area, leading to exposures of additional active edge sites for redox re actions; (ii) the presence of plentiful pores and void spaces between nanosheets, facilitating rapid ion diffusion and electrons transport dur ing the electrochemical process; (iii) its high electrical conductivity as well as strong electrical contact and mechanical adhesion with the conductive substrate. The Nyquist plots from the EIS analysis of the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes are shown in Fig. 6f. Thereafter, the inset in Fig. 6f shows the equivalent circuit. The Nyquist plots can be segmented into a semicircle part at the high frequency region and a linear part at the low frequency region. Meanwhile, the semicircle rep resents an electron-transfer-limited process, which mainly relies on the wettability between the electrode and electrolyte, the morphology, and the conductivity of the electrode. Furthermore, the serial resistance consists of the contact resistance between the active materials and the current collector and the intergranular electronic resistance between active particles. Based on the first intersection with the real axis, the Zn0.76Co0.24S/NiCo2S4 electrode exhibits the smallest serial resistance (0.85 Ω) and charge transfer resistance (1.1 Ω), signifying an improved electronic conductivity in both the serial and charge transfer compo nents. Otherwise, the straight slope in the low frequency corresponds to a limited ion diffusion process. The Zn0.76Co0.24S/NiCo2S4 electrode presents a more vertical line, demonstrating faster ion diffusion and
electron transport than the ZIF-L or ZnNiCo-LDH electrodes. To further distinguish the charge storage mechanism of the Zn0.76Co0.24S/NiCo2S4 electrode, its electrochemical reaction kinetics were evaluated. Fig. 7a displays the anodic and cathodic peaks of CV curves shift toward the higher and lower potential with the scan rates increase, which is ascribed to the polarization effect of the electrode [39]. All the CV curves maintain similar shapes at different scan rates, indicating a remarkable electrochemical reversibility and rate capa bility. The linear relationship between the scan rate and peak current was also analyzed based on the following equation [40]: i ¼ avb
(10)
where a is a constant, and b is an adjustable parameter reflecting the possible electrochemical reaction kinetics. Notably, b ¼ 0.5 indicated the diffusion-controlled process, and b ¼ 1 represented the capacitivecontrolled process. As shown in Fig. 7b, the adjustable values of b ¼ 0.86, and b ¼ 0.85 for the anodic and cathodic peak currents, respectively, indicate that a typical diffusion-controlled characteristic and surface capacitive effect both exist between the electrolyte and electrode. The capacitive contribution (k1ν) and diffusion-controlled contribu tions (k2ν1/2) of the charge storage are analyzed in detail by using the equation shown below [41–44]: iðVÞ ¼ k1 v þ k2 v1=2
(11)
The above equation can also be converted to the following: iðVÞv1=2 ¼ k1 v1=2 þ k2 6
(12)
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Fig. 6. (a) CV curves of the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes at 5 mV s 1; (b) GCD curves of the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/ NiCo2S4 electrodes at 1 A g 1; (c) GCD curves of the Zn0.76Co0.24S/NiCo2S4 electrode at different current densities; (d) Specific capacitance as a function of current density for the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes; (e) Cycling performance of the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes at a constant current density of 12 A g 1 for 2000 cycles; (f) Nyquist plots of the Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes.
Fig. 7. (a) CV curves of the Zn0.76Co0.24S/NiCo2S4 electrode at various scan rates; (b) Log (peak current) vs. log(scan rate) plots used to obtain b-values; (c, d) Capacitive charge storage contributions at the scan rates of 5 mV s 1 and 50 mV s 1; (e) Capacitive and diffusion-controlled charge storage contributions at different scan rates.
respectively. As expected, the data in Fig. 7e indicate that the capacitive contribution ratio value rose while the diffusion contribution ratio value declined with the sweep rate increases. The capacitive-controlled pro cess contributes 76%, 82%, 86%, 88%, 91% and 94% of the total charge storage at 5, 10, 20, 30, 40 and 50 mV s 1, respectively, indicating a
which enables us to evaluate the k1 (slope) and k2 (intercept) of the straight lines under the different potentials (V) with ease. Thus, based on the calculated k1 and k2, we determined the k1ν and k2ν1/2 at the specific fixed potentials. Fig. 7c and d displays the voltage profiles of capacitive current responses (total shaded areas) at 5 mV s 1 and 50 mV s 1, 7
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dominant capacitive charge-storage mechanism in the Zn0.76Co0.24S/ NiCo2S4 electrode. As a result, this electrode displays an excellent overall electrochemical performance, which is essential for the practical application of SCs.
examined at different potential windows. Fig. S9a shows the CV curves of the constructed Zn0.76Co0.24S/NiCo2S4//AC ASC at different poten tials, confirming the stabilized voltage window at 0–1.6 V. However, when the potential window extends to 1.7 V, the CV curve becomes distorted due to irreversible oxygen evolution reactions. Thus, the optimized voltage window is determined as 1.6 V. The GCD curves tested at various potentials in Fig. S9b further demonstrate that the voltage window of the ASC device is 0–1.6 V. Fig. 8b shows the CV curves of the ASC device tested at various scan rates (5–50 mV s 1). Unlike the sharp redox peaks in the CV curves in the three-electrode configuration, the ASC device exhibited quasirectangular CV curves with small peaks, which are due to the com bined features of the Faradaic and non-Faradaic behaviors of Zn0.76Co0.24S/NiCo2S4 and AC. Furthermore, the changes of CV curves shapes are insensitive during the oxidation and reduction processes, when the sweep rate increases. Notably, Fig. 8c shows that the corre sponding GCD curves at various current densities are nonlinear, which are in accordance with the CV curves. Fig. 8d depicts the rate perfor mance of the Zn0.76Co0.24S/NiCo2S4//AC ASC device, which can be reversibly cycled at 1, 2, 4, 8, 12 and 20 A g 1 with corresponding ca pacitances of 135.2, 126.8, 118.1, 108.8, 105.1 and 97.4 F g 1, respec tively. When the current density reverts to 1 A g 1 after 60 cycles, the
3.3. Electrochemical characterization of the Zn0.76Co0.24S/NiCo2S4//AC ASC device The practical application of the Zn0.76Co0.24S/NiCo2S4 electrode in SCs was evaluated by fabricating an asymmetric ASC, with Zn0.76Co0.24S/NiCo2S4 as the positive electrode and AC as the negative electrode, as schematically displayed in Fig. 8a. The characteristics of active carbon (AC) are demonstrated in Fig. S7. The electrochemical characteristics of AC are shown in Fig. S8a, the CV curves of AC show a quasi-rectangular shape within a stable potential window of 1~0 V, illustrating the typical EDLC features. The GCD curves of AC show almost linear characteristics, thus demonstrating the ideal EDLC nature (Fig. S8b). The specific capacitances of AC is 220 F g 1 at 1 A g 1. To achieve ideal electrochemical performance, prior to the construction of the ASC device, the active mass ratio of Zn0.76Co0.24S/NiCo2S4 to AC was determined as 1:6.1 according to charge balance relationship. To further confirm the stable voltage, the CV feature of the ASC device was
Fig. 8. (a) A schematic illustration of the assembled Zn0.76Co0.24S/NiCo2S4//AC flexible all-solid-state ASC device; (b) CV curves of the ASC device at various scan rates; (c) GCD curves of the ASC device at various current densities; (d) Specific capacitance variations with the current density for the ASC device; (e) Ragone plot of the Zn0.76Co0.24S/NiCo2S4//AC ASC device compared with several reported ASCs; (f) Cycling performance of the ASC device at a constant current density of 12 A g 1 for 5000 cycles; (g) CV curves of the ASC device in bent states; (h) GCD curves of ASC devices connected in series; (i) a demonstration of the ASC devices powering one LED. 8
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reversible capacitance recovers to 132 F g 1, revealing a strong rate ability and the stability of the ASC device. Fig. 8e delivered the Ragone plot of the energy and power density of the Zn0.76Co0.24S/NiCo2S4//AC ASC device, which are two additional and critical parameters for assessing the practicability of SCs. A high energy density of 48.1 Wh kg 1 was attained at the power density of 837 W kg 1, and maintained at 34.6 Wh kg 1 at an extremely high power density of 16.6 kW kg 1. The device performance recorded in this study compares closely to or even exceeds the most recently reported ASCs when using TMSs as the positive electrode [35,45–53]. Moreover, the cycling performance was measured at 12 A g 1 with up to 5000 cycles (Fig. 8f) and maintained 91% of the initial capacitances, signi fying the superior long-term electrochemical stability of the ASC device. Furthermore, the as-fabricated Zn0.76Co0.24S/NiCo2S4//AC ASC de vice can be applied to energy storage textiles and wearable electronics, and its mechanical stability was investigated. Fig. 8g depicts the GCD curves of ASC device with various bending angles. The shape of the GCD curves did not show a substantial change, confirming the superior flexibility and extraordinary mechanical stability of the device. Furthermore, we connected the advanced ASC in a series to reach a high output voltage (Fig. 8h). For demonstration purposes, the photographic image in Fig. 8i shows the actual application of our device to power a 5 mm diameter commercial LED (2.2 V, 20 mA) by connecting three ASC devices in series. In order to apply to wearable electronic devices, the ASC device should work stably at human body temperature or higher temperature. As shown in Fig. S10, the CV curves of ASC device reveal little changes, indicating that the effect of temperature on the ASC de vice is small.
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4. Conclusions In summary, we demonstrated a simple method for fabricating Zn0.76Co0.24S/NiCo2S4 nanosheets onto a conductive carbon cloth using the ZIF-L precursor and employed for flexible all-solid-state Zn0.76Co0.24S/NiCo2S4//AC ASC. A facile and efficient synthesis involved the growth of the Zn/Co ZIF-L precursor, which was subse quently transformed into ZnNiCo-LDH and finally converted to Zn0.76Co0.24S/NiCo2S4 nanosheets via sulfurization. The integrated design of active materials anchored on a conductive substrate not only avoids the use of polymer binder and conductive additives, but also provides fast charge transport. Beneficial from the unique nanostructure design and the synergistic effects of the electrode materials, the pre pared Zn0.76Co0.24S/NiCo2S4 nanosheets show a high specific capaci tance of 2674 F g 1 at 1 A g 1, and a strong rate capability and cycling performance. We assembled the ASC device with a positive Zn0.76Co0.24S/NiCo2S4 electrode and negative AC electrodes. Further more, the device possesses a maximum energy density at 48.1 Wh kg 1 and power density at 837 W kg 1. Moreover, the capacitance retention of ASC maintained at 91% with up to 5000 cycles. This study opens new avenues for the development of transition metal sulfides as potential electrode materials for next-generation flexible devices. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 51603053, 51872057, 51901055, 21576060), Natural Sci ence Foundation of Heilongjiang Province (LH2019E025), Fundamental Research Funds of the Central University (3072019CF1003), China Postdoctoral Science Foundation (2019M651260), and Defense Indus trial Technology Development Program (JCKY2016604C006) and the National Key Research and Development Program of China (2016YFE0202700). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 9
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Design of 2D mesoporous Zn/Co-based metal-organic frameworks as a flexible electrode for energy storage and conversion Yunhe Zhao a, b, Hongxing Dong a, b, c, Xinyi He a, b, Jing Yu a, b, *, Rongrong Chen a, b, c, Qi Liu a, b, Jingyuan Liu a, b, Hongsen Zhang a, b, Rumin Li a, b, Jun Wang a, b, c, ** a b c
Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China College of Materials Science and Chemical Engineering, Harbin Engineering University, 150001, PR China Institute of Advanced Marine Materials, Harbin Engineering University, 150001, PR 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
� Zn/Co-based MOFs derived Zn0.76Co0.24S/NiCo2S4 nanosheets were prepared successfully. � The Zn0.76Co0.24S/NiCo2S4 electrode exhibited remarkable specific capacitance. � A flexible all-solid-state Zn0.76Co0.24S/ NiCo2S4//AC ASC device was fabricated. � The enhanced property is due to the nanostructure, porosity and conductivity.
A R T I C L E I N F O
A B S T R A C T
Keywords: Two-dimensional ZIF-L Transition metal sulfides Nanosheets Flexible asymmetric supercapacitors Binder-free flexible devices
Electrode materials with a large internal surface area, tunable pore size, efficient transport of electrons and high ion-accessibility are highly desired in the development of advanced flexible supercapacitors. Recently, transition metal sulfides with rationally designed nanostructures have attracted considerable attention as electrode ma terials for supercapacitors. In this study, we report the synthesis of Zn0.76Co0.24S/NiCo2S4 nanosheets grown on carbon cloth using a two-dimensional bimetallic Zn/Co zeolitic imidazolate framework (named as Zn/Co-ZIF-L) as a precursor through a simple and cost-effective chemical solution process. The ZIF-derived Zn0.76Co0.24S/ NiCo2S4 electrode nanosheets deliver an ultrahigh specific capacitance of 2674 F g 1 at 1 A g 1, a superior ratio performance and cycle stability. Furthermore, the as-fabricated Zn0.76Co0.24S/NiCo2S4//AC all-solid-state asymmetric supercapacitor (ASC) achieves a maximum energy density of 48.1 Wh kg 1 as well as a power density of 837 W kg 1, and a superior cycling performance of 91% retention after 5000 cycles. The detailed electrochemical kinetic analysis demonstrates that the total capacitance of Zn0.76Co0.24S/NiCo2S4 is derived from its capacitive-effective charge storage mechanism. This ZIF-derived strategy provides a reasonable and simple way to synthesize transition metal sulfides as potential active materials for next-generation flexible supercapacitors.
* Corresponding author. Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China. ** Corresponding author. Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China. E-mail addresses: [email protected] (J. Yu), [email protected] (J. Wang). https://doi.org/10.1016/j.jpowsour.2019.227057 Received 10 May 2019; Received in revised form 10 August 2019; Accepted 20 August 2019 Available online 23 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Y. Zhao et al.
Journal of Power Sources 438 (2019) 227057
1. Introduction
preparation of various bimetallic metal sulfides. Shao et al. prepared mesoporous polyhedron-structured NiCo2S4 from Ni and Co bimetallic ZIFs, demonstrating an ultrahigh capacitance that reaches up to 1296 F g 1 at 1 A g 1 with a high rate performance [31]. The electro chemical contribution of bimetallic sulfides with both nickel and cobalt ions provides a beneficial redox process compared with mono-metallic sulfides (nickel sulfide, cobalt sulfide), leading to a larger specific capacitance. Nevertheless, MOF materials are usually made in the form of powders, and the agglomeration of powders will reduce the efficiency of active materials. Moreover, involving the polymer binder in the electrode preparation process will immensely reduce the electrical conductivity of the electrode materials and cause a “peeling off” reaction during the lengthy charge/discharge process, which severely limits the binder’s practical application in high-performance flexible SCs. One approach is to grow closely well-aligned arrays directly onto flexible substrates, which effectively decrease the “dead surface” in traditional slurry-derived electrodes, thereby facilitating more charge and efficient mass transportation [32]. Thus, directly growing MOF-derived transi tion metal sulfides onto flexible substrates is of great importance for binder-free electrodes of flexible SCs. Herein, we demonstrate an easy strategy to prepare porous Zn0.76Co0.24S/NiCo2S4 nanosheets from two-dimensional bimetallic Zn/ Co ZIF-L as a binder-free flexible electrode. The original structural and morphological features of the Zn0.76Co0.24S/NiCo2S4 electrode result in an increase of the surface area, providing rich active sites and sufficient contact with electrolytes. The synergistic effect among the components assures rich redox reactions. Moreover, this bind-free electrode grown on carbon cloth effectively averts the “dead surface”. As a result, the ZIFL derived Zn0.76Co0.24S/NiCo2S4 nanosheets exhibit an ultrahigh ca pacity, superior rate performances and an outstanding cycle ability because of the nanostructure and its full usage of the large porosity and favorable conductivity. This strategy may provide a new direction for constructing transition metal sulfide nanostructures for high energy storage application.
Recently, the ever-growing demand for miniaturized portable and wearable flexible electronic devices in applications of electronic textiles, electronic skins, and wearable health monitors have attracted world wide research interest in flexible electronic devices [1–4]. Flexible all-solid-state supercapacitors (SCs) have become promising energy storage devices that are used as various high-power supplies for portable and diverse functions of flexible electronics due to their high power density, fast charge-discharge capability, and super-long cycle life [5–7]. In addition, flexible all-solid-state SCs possess numerous advan tages, including a lightweight composition, strong durability, ease of handling and improved eco-safety compared with traditional liquid electrolyte SCs; therefore, flexible SCs are greatly desired for portable and wearable consumer electronics [8–10]. For flexible SCs, electrode materials significantly affect the overall performance of SCs. Therefore, the development of novel and reliable flexible electrode materials is urgent and highly desirable. Battery-type Faradaic materials, based on transition metal oxides or sulfides, have been widely studied as positive electrode materials for SCs with higher theoretical capacitances and energy densities than conventional carbon-based materials [11]. Nevertheless, battery-type materials can also enhance energy and power densities when combined with electric double layer capacitors in a system [12,13]. Transition metal sulfides (TMSs) offer tremendous potential as electrode materials for energy storage devices, owing to their superior conductivity and higher electrochemical activity compared to their ox ides and hydroxide counterparts [14]. However, large volume changes of TMSs during cycle processes causes a rapid drop in capacity, while a low charge usually leads to a poor rate capability. Various strategies have been proposed to solve these critical problems. A carefully crafted structure design of TMSs with porous nanostructures has been demon strated as an attractive way to greatly promote the TMSs’ electro chemical performance [15,16]. In these nanostructures, the void space can buffer the volume change and effectively facilitate ion/electron transport. On the other hand, recent reports have shown that multi component TMSs possess outstanding electrochemical performance when compared to single TMSs [17]. The multiple valences and syner gistic effects among the different compositions provide rich redox re actions, which can enhance the specific capacity. Accordingly, the simultaneous controlling nanostructures and the composition of TMSs may offer opportunities for improving their electrochemical perfor mance. Nevertheless, there are still several issues that limit the large-scale production of the electrodes, such as an additional post-heat-treatment step and a long synthesis time. Thus, high-performance TMS electrodes with simple and cost-effective syn thesis methods are highly desirable. Metal-organic frameworks (MOFs) have attracted increasing concern in various fields since it is a class of hybrid nanoporous materials with crystalline architectures linked by metal ions and organic ligands [18–20]. In recent years, MOFs have received additional attention and have been demonstrated to have enormous potential for advanced SCs, owing to their large interfacial area, tunable pore size and diversity of metal ions [21–23]. Interestingly, MOFs have been used as a new class of sacrificial template for constructing complex architectures [24]. The tunable structures enable MOFs to derive into a variety of functional materials, including metal-based composites, carbonaceous materials, hydroxides with porous or hollow nanostructures, simultaneously retaining its initial structural features [25,26]. These nanostructures display multiple structure- and composition-dependent advantages, providing considerable potential for superior SC performance. Until now, many studies reported porous TMS nanomaterials which derived from MOFs, such as cubic NiS nano frames, double-shelled CoS hollow nanoboxes, CoS1.097/nitrogen-doped carbon nanosheets, and Co9S8@N–C@MoS2 [27–30]. Moreover, MOFs are not just for the exploitation of mono-metallic sulfides but they also enable the
2. Experimental section 2.1. Materials Co(NO3)2⋅6H2O, Ni(NO3)2⋅6H2O, Zn(NO3)2⋅6H2O, 2-Methylimida zole (C4H6N2, 2MI) were obtained from Tianjin Guangfu Technology Development Co., Ltd. All the chemicals used in this study were analytical grade and used without further purification. 2.2. Synthesis Synthesis of two-dimensional bimetallic Zn/Co ZIF-L samples: The Zn/Co ZIF-L grown on carbon cloth were fabricated by mixed Co (NO3)2⋅6H2O (8 mmol) and Zn(NO3)2⋅6H2O (4 mmol) in 20 mL deion ized water, and 2-methylimidazole (1 mmol in 20 mL deionized water) together rapidly with vigorous stirring. The clean carbon cloth (CC) was put inside the reaction solution for 2 h, then was taken out, washed and dried overnight. The loading for Zn/Co ZIF-L nanosheets on CC is ~2 mg cm 2. Synthesis of ZnNiCo-LDH samples: A piece of Zn/Co ZIF-L nano sheets on the CC was transferred into a mixed solution with Ni (NO3)2⋅6H2O (0.18 g in 50 mL ethanol solution), and magnetically stir red for 30 min. The as-fabricated ZnNiCo-LDH nanosheets on the CC were then washed again repeatedly, and dried at 60 � C in an oven overnight. Synthesis of Zn0.76Co0.24S/NiCo2S4 nanosheets on the carbon cloth: A solution containing 70 � 10 3 M Na2S (30 mL) with a piece of the ZnNiCo-LDH nanosheet on CC were put inside a 50-mL reaction vessel together, and were subsequently kept at 140 � C for 4 h. After the hydrothermal process, the as-prepared samples were washed with distilled water and ethanol, and dried at 60 � C. The mass loading of the 2
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Zn0.76Co0.24S/NiCo2S4 nanosheets was ~4 mg cm
Journal of Power Sources 438 (2019) 227057 2
. E¼
R I⋅ VðtÞdt m
(4)
P¼
E Δt
(5)
2.3. Flexible all-solid-state ASCs In the flexible all-solid-state ASC device, the Zn0.76Co0.24S/NiCo2S4 and activated carbon (AC) were used as positive and negative electrode materials with a PVA/KOH gel electrolyte as a separator. To achieve the ideal electrochemical performance of ASC, the charge quantity (q) on the two electrodes should be balanced, in which q can be estimated using the following equation: q ¼ m � Cs � ΔV
3. Results and discussion 3.1. Morphology and structure analysis The construction strategy of Zn0.76Co0.24S/NiCo2S4 is illustrated in Fig. 1. Two-dimensional bimetallic Zn/Co ZIF-L as the precursor was first prepared by a reaction of Co2þ and Zn2þ with 2-methylimidazole. Then, the ZnNiCo-LDH nanosheets were successfully synthesized by mixed Zn/Co ZIF-L with a solution of Ni(NO3)2⋅6H2O in ethanol. During this process, the Hþ protons are gradually generated by the hydrolysis of Ni2þ ions and etch the ZIF-L templates, and the released Co2þ ions are partially oxidized by the dissolved O2 and NO3 ions in the solution [26]. Then, the coprecipitation of Co2þ/Co3þ ions with Ni2þ and Zn2þ ions results in the formation of ZnNiCo LDH. Moreover, the hydrolysis of Ni(NO3)2 will be accelerated along with the Hþ protons consumption, further promoting the production of LDH. Subsequently, ZnNiCo-LDH transforms into Zn0.76Co0.24S/NiCo2S4 through a solvothermal process with Na2S as the sulfurization agent. The morphologies of Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/ NiCo2S4 were investigated by FESEM. The FESEM images in Fig. 2a show the uniform 2D Zn/Co ZIF-L cover on the carbon cloth surface. As shown in high-magnification FE-SEM images (Fig. 2b and c), these Zn/Co ZIF-L nanosheets present a 2D triangle feature with a glossy surface. The color of Zn/Co ZIF-L on the CC changes into a uniform purple. After the etching and ion exchange process in the ethanol solution of Ni (NO3)2⋅6H2O, the color of ZnNiCo-LDH sample changes from purple to light green (Fig. S1). A FESEM image (Fig. 2d) shows that the nano structures’ array was maintained effectively. The ZnNiCo-LDH samples appear to have a rougher surface constituted by small nanosheets in the magnified images (Fig. 2e and f). The etching process is affected by the concentration of nickel nitrate. When a low concentration of nickel ni trate is used, the Hþ protons generated from the hydrolysis of nickel nitrate become insufficient, and the Zn/Co ZIF-L templates cannot dissolve completely (Figs. S2a and b). When a high concentration of nickel nitrate is used, an over-rapid etching of Zn/Co ZIF-L can even tually lead to the destruction of the 2D nanosheet structure (Figs. S2c and d). After sulfurization in the Na2S solution, the color of the Zn0.76Co0.24S/NiCo2S4 sample changes to black (Fig. S1). Fig. 2g shows that the 2D vertically aligned structure with cross-linked features is mainly retained with no apparent collapse. The Zn0.76Co0.24S/NiCo2S4 nanosheets were interconnected and had a strong adhesion to the CC, thereby providing a high accessibility for the diffusion of electrolyte ions into the surface of the electrode. Thus, fast electrochemical reactions are expected to enhance the electro chemical performance. Close observations (Fig. 2h and i) further show the surface morphology of the nanosheets becoming rougher. In addi tion, the edges of the Zn0.76Co0.24S/NiCo2S4 nanosheets are denser than the other parts. Notably, a high S2 concentration will destroy the 2D arrays (Fig. S3). The presence of Zn, Co, Ni and S is proven by SEM-EDS (Fig. S4a). The X-ray elemental mappings also demonstrates successful synthesis of the Zn0.76Co0.24S/NiCo2S4 electrode (Fig. S4b), which clearly demonstrates the uniform distribution of the Zn, Co, Ni and S elements. Meanwhile, the TEM (Fig. 3a and b) further displays the as-prepared samples, the surface of Zn/Co ZIF-L is smooth and does not display any porous morphology. After the etching with Ni(NO3)2, the original morphology is largely reserved, and we distinctly observed small nanosheets (Fig. 3c). After sulfurization treatment, we clearly observe that numerous interconnected nanoparticles were dispersed throughout
(1)
where Cs (F g 1) is the specific capacitance, ΔV (V) is the operating voltage window, and m (g) is the mass of the active materials. The optimal mass ratio between the positive and negative electrodes materials was determined according to the following equation: mþ C � ΔV ¼ m Cþ � ΔVþ
(2)
where ΔVþ and ΔV are the voltage range for the positive electrode (þ) and negative electrode ( ), respectively. Cþ and C are the Cs of the Zn0.76Co0.24S/NiCo2S4 and active carbon, respectively. The optimal mass ratio of the Zn0.76Co0.24S/NiCo2S4 and active carbon was calcu lated to be 1:6.1. Typically, the PVA/KOH gel electrolyte was synthesized as follows: KOH (3 g) and PVA (6 g) were dissolved in 60 mL DI water with constant stirring at 80 � C for 1 h. After natural cooling, a homogeneous trans parent gel electrolyte was obtained. Prior to their assemblage, the two electrodes were soaked into the PVA/KOH gel electrolyte for approxi mately 30 min and were then spread on the PVA/KOH gel electrolyte for face-to-face construction. Finally, the as-assembled ASC device was maintained at 20 � C for 12 h to remove excess water. 2.4. Characterization The morphology and structures were conducted by a field emission scanning electron microscope (FE-SEM, Hitachi, S-4800) and trans mission electron microscopy (TEM, FEI Tecnai G2-TWIN). The crystal structures were analyzed by a Japanese Rigaku-MiniFlex 600 X-ray diffractometer with Cu Kα radiation (λ ¼ 0.15406 nm). The elemental information was explored by X-ray photoelectron spectra (XPS, Sigma probe ThermoVG, U.K). 2.5. Electrochemical characterization Electrochemical measurements were tested on an electrochemistry workstation (CHI 660D, Shanghai Chenhua Instrument, Inc.). The capacitive properties of the electrodes were performed in a threeelectrode system in a 2 M KOH aqueous electrolyte. The saturated calomel electrode (SCE) and platinum foil were used as reference and counter electrodes, with the Zn0.76Co0.24S/NiCo2S4 nanosheets as the working electrode. The specific capacitance Cs (F g 1) of the active material can be calculated as follows [33]: Z 2I CS ¼ VðtÞdt (3) Vf mV 2 j Vi where I (A) represents the current, t (s) is the discharging time, m (g) is the mass of active material (Zn/Co ZIF-L, ZnNiCo-LDH, Zn0.76Co0.24S/ R NiCo2S4 and active carbon), V(t)dt represents the integral current area under the experimental curve, and V is the potential with initial and final values of Vi and Vf, respectively. The energy density (E, Wh kg 1) and power density (P, W kg 1) can be estimated using the following equations: 3
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Journal of Power Sources 438 (2019) 227057
Fig. 1. Schematic illustration of the synthesis of the ZIF-derived Zn0.76Co0.24S/NiCo2S4 nanosheets.
Fig. 2. FE-SEM images of (a–c) Zn/Co ZIF-L, (d–f) ZnNiCo-LDH and (g–i) Zn0.76Co0.24S/NiCo2S4 nanosheets.
the surface of the Zn0.76Co0.24S/NiCo2S4 nanosheets (Fig. 3d). The fully porous nanostructure is an extremely in-demand morphology in the field of supercapacitor applications. In addition, the sample demonstrates strong center-edge contrast from the TEM image (Fig. 3e); this is due to the higher density of the edge area for the nanosheets, which can also be proved by the SEM image (Fig. 2i). The HRTEM image (Fig. 3f) exhibits lattice plane distances of 0.31 nm and 0.28 nm, which are related to the (111) and (311) crystalline planes of Zn0.76Co0.24S and NiCo2S4, respectively. The SAED pattern indicates that the Zn0.76Co0.24S/NiCo2S4 is polycrystalline in nature. The elemental composition of the prepared sample was further analyzed by EDX spectroscopy. Apparently, the S, Co, Zn and Ni elements were uniformly distributed throughout the nanosheet (Fig. 3g). XRD analysis characterized the crystal structures of samples, in which Fig. 4 displays the corresponding patterns. The major peaks are located at approximately 26� and 43� , which is attributed to the CC substrate, while the other peaks are assigned to the diffraction pattern of ZIF-L [34]. After the reaction, the diffraction peaks of ZIF-L are completely absent in the XRD pattern, and the peaks corresponding to the (003), (012), (015), and (110) planes are evidently associated with the typical LDH structure. After sulfurization, a series of diffraction peaks appear at 16.3� , 31.6� , 38.3� , 50.5� and 55.3� , associated with the (111), (311), (400), (511) and (440) planes, respectively, and all the
characteristic peaks are assigned to NiCo2S4 (JCPDS No. 43–1477). In addition, the diffraction peaks at 28.6� , 47.6� and 56.6� are in agree ment with the (111), (220) and (311) planes of Zn0.76Co0.24S, respec tively (JCPDS No. 47–1656). These results show that the ZIF-L derived Zn0.76Co0.24S/NiCo2S4 was synthesized successfully. The N2 adsorption isotherm tests were investigated to explore the surface area and porous structure of the fabricated Zn0.76Co0.24S/ NiCo2S4. Zn0.76Co0.24S/NiCo2S4 exhibits type IV isotherms with a pro nounced hysteresis loop (Fig. S5), which indicates a mesoporous struc ture with a surface area of 122 m2 g 1. The Barrett-Joyner-Halenda (BJH) method can calculate the pore size distribution, which depicts that the majority of pores are of 12 nm in diameter (inset of Fig. S5). The moderate specific surface areas endow Zn0.76Co0.24S/NiCo2S4 with a large electrode/electrolyte contact area, while the hierarchical porous characteristics can promote the transport and the penetration of the electrolyte ions as well as accommodate volume variations of the active materials during the charging-discharging process. XPS measurements were further conducted to evaluate the changes in the chemical status of ZIF-L derived Zn0.76Co0.24S/NiCo2S4 nano sheets. As shown in Fig. 5a, the high-resolution Zn 2p spectrum is deconvoluted into two peaks at binding energies 1045.5 and 1021.5 eV that correspond to Zn 2p1/2 and Zn 2p3/2, respectively. By applying a Gaussian fitting method, the fine-resolution Co 2p and Ni 2p spectrum 4
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Fig. 3. TEM images of (a,b) Zn/Co ZIF-L, (c) ZnNiCo-LDH and (d,e) Zn0.76Co0.24S/NiCo2S4, (f) The HRTEM image of Zn0.76Co0.24S/NiCo2S4, the inset is the SAED pattern. (g) Corresponding elemental distribution mapping of Zn, Co, Ni and S.
fabricated samples, CV, GCD and EIS were measured. Fig. 6a com pares the CV curve characteristics of the Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes collected at 5 mV s 1 in the 0–0.6 V potential window. Importantly, the Zn0.76Co0.24S/NiCo2S4 electrode shows a larger current density than the other electrodes, revealing its superior capacitive performance. The CV curves display a set of intense redox peaks, which contributed to the Faradaic redox process of elec trodes, implying battery-type behaviors. The possible redox reactions of the Zn0.76Co0.24S/NiCo2S4 electrodes are expressed as follows [14,38]: ZnS þ OH ↔ ZnSOH þ e
(6)
CoS þ OH ↔ CoSOH þ e
(7)
CoSOH þ OH ↔ CoSO þ H2 O þ e
(8)
NiS þ OH ↔ NiSOH þ e
(9)
Fig. 6b depicts a comparison of GCD curves for the Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 at the 0–0.5 V potential win dow (1 A g 1). As expected, the obviously prolonged discharge time of the Zn0.76Co0.24S/NiCo2S4 nanosheets electrode displays a higher spe cific capacity contrast to the Zn/Co ZIF-L and ZnNiCo-LDH electrodes, which is in accordance with the CV results (Fig. 6a). The symmetric GCD profiles with a voltage platform suggest a typical pseudocapacitive behavior, which is consistent with the CV tests. Fig. 6c and Fig. S6 represent the GCD curves of Zn0.76Co0.24S/NiCo2S4, ZIF-L and ZnNiCoLDH at various current densities (1–20 A g 1). The nearly symmetrical triangles for each curve prove that the Faradaic electrode redox process is highly reversible and has a low degree of polarization. Integrating the charge/discharge profiles, the corresponding specific capacitances of the Zn0.76Co0.24S/NiCo2S4 electrode are displayed in Fig. 6d. Encour agingly, the Zn0.76Co0.24S/NiCo2S4 electrode possesses specific capaci tances of 2674, 2514, 2380, 2277, 2202 and 2112 F g 1 at 1, 2, 4, 8, 12 and 20 A g 1, respectively, which are much larger than their ZIF-L or ZnNiCo-LDH counterparts. Importantly, the Zn0.76Co0.24S/NiCo2S4 electrode reveals an outstanding rate capability (79% retention of its capacitance). Fig. 6e presents the cycling performance of ZIF-L, ZnNiCoLDH and Zn0.76Co0.24S/NiCo2S4 electrodes at 8 A g 1. Impressively, the Zn0.76Co0.24S/NiCo2S4 electrode shows a cycle stability of 93% retained
Fig. 4. XRD patterns of Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4.
corresponds to the two spin-orbit doublets and their satellites (marked as “Sat.“) (Fig. 5b and c). For the Co 2p spectrum, the binding energies appear at 779.5 and 794.4 eV, which are particularly characteristic of Co2þ ions, and the other two peaks at 782.4 and 796.1 eV are associated with Co3þ ions [35]. The spectrum of Ni 2p can also be equipped with Ni2þ at 853.2 and 872.6 eV, and two satellites (sat.) 856.2 and 874.4 eV are correspond to Ni3þ ions [36]. The S 2p spectrum is shown in Fig. 5d, with two major peaks of S 2p1/2 and S 2p3/2 located at 161.8 and 162.9 eV, respectively. In detail, the S 2p1/2 peak is confirmed by metal-sulfur bonds, and the S 2p3/2 peak is attributed to the surface low-coordinated sulfur ions [37]. The results show that the chemical composition of Zn0.76Co0.24S/NiCo2S4 contains Zn2þ, Ni2þ, Ni3þ, Co2þ, Co3þ and S2 . 3.2. Electrochemical performance analysis To more effectively assess the electrochemical characteristics of as5
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Fig. 5. XPS spectra of Zn0.76Co0.24S/NiCo2S4: (a) Zn 2p, (b) Co 2p, (c) Ni 2p, (d) S 2p.
after 2000 charge/discharge cycles, whereas the ZIF-L and ZnNiCo-LDH show a capacitance retention rates of 81% and 86%, respectively. The Zn0.76Co0.24S/NiCo2S4 electrode’s outstanding electrochemical perfor mance is probably due to: (i) the interconnection of Zn0.76Co0.24S/ NiCo2S4 nanosheets with unique porous features and a large interfacial area, leading to exposures of additional active edge sites for redox re actions; (ii) the presence of plentiful pores and void spaces between nanosheets, facilitating rapid ion diffusion and electrons transport dur ing the electrochemical process; (iii) its high electrical conductivity as well as strong electrical contact and mechanical adhesion with the conductive substrate. The Nyquist plots from the EIS analysis of the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes are shown in Fig. 6f. Thereafter, the inset in Fig. 6f shows the equivalent circuit. The Nyquist plots can be segmented into a semicircle part at the high frequency region and a linear part at the low frequency region. Meanwhile, the semicircle rep resents an electron-transfer-limited process, which mainly relies on the wettability between the electrode and electrolyte, the morphology, and the conductivity of the electrode. Furthermore, the serial resistance consists of the contact resistance between the active materials and the current collector and the intergranular electronic resistance between active particles. Based on the first intersection with the real axis, the Zn0.76Co0.24S/NiCo2S4 electrode exhibits the smallest serial resistance (0.85 Ω) and charge transfer resistance (1.1 Ω), signifying an improved electronic conductivity in both the serial and charge transfer compo nents. Otherwise, the straight slope in the low frequency corresponds to a limited ion diffusion process. The Zn0.76Co0.24S/NiCo2S4 electrode presents a more vertical line, demonstrating faster ion diffusion and
electron transport than the ZIF-L or ZnNiCo-LDH electrodes. To further distinguish the charge storage mechanism of the Zn0.76Co0.24S/NiCo2S4 electrode, its electrochemical reaction kinetics were evaluated. Fig. 7a displays the anodic and cathodic peaks of CV curves shift toward the higher and lower potential with the scan rates increase, which is ascribed to the polarization effect of the electrode [39]. All the CV curves maintain similar shapes at different scan rates, indicating a remarkable electrochemical reversibility and rate capa bility. The linear relationship between the scan rate and peak current was also analyzed based on the following equation [40]: i ¼ avb
(10)
where a is a constant, and b is an adjustable parameter reflecting the possible electrochemical reaction kinetics. Notably, b ¼ 0.5 indicated the diffusion-controlled process, and b ¼ 1 represented the capacitivecontrolled process. As shown in Fig. 7b, the adjustable values of b ¼ 0.86, and b ¼ 0.85 for the anodic and cathodic peak currents, respectively, indicate that a typical diffusion-controlled characteristic and surface capacitive effect both exist between the electrolyte and electrode. The capacitive contribution (k1ν) and diffusion-controlled contribu tions (k2ν1/2) of the charge storage are analyzed in detail by using the equation shown below [41–44]: iðVÞ ¼ k1 v þ k2 v1=2
(11)
The above equation can also be converted to the following: iðVÞv1=2 ¼ k1 v1=2 þ k2 6
(12)
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Fig. 6. (a) CV curves of the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes at 5 mV s 1; (b) GCD curves of the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/ NiCo2S4 electrodes at 1 A g 1; (c) GCD curves of the Zn0.76Co0.24S/NiCo2S4 electrode at different current densities; (d) Specific capacitance as a function of current density for the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes; (e) Cycling performance of the ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes at a constant current density of 12 A g 1 for 2000 cycles; (f) Nyquist plots of the Zn/Co ZIF-L, ZnNiCo-LDH and Zn0.76Co0.24S/NiCo2S4 electrodes.
Fig. 7. (a) CV curves of the Zn0.76Co0.24S/NiCo2S4 electrode at various scan rates; (b) Log (peak current) vs. log(scan rate) plots used to obtain b-values; (c, d) Capacitive charge storage contributions at the scan rates of 5 mV s 1 and 50 mV s 1; (e) Capacitive and diffusion-controlled charge storage contributions at different scan rates.
respectively. As expected, the data in Fig. 7e indicate that the capacitive contribution ratio value rose while the diffusion contribution ratio value declined with the sweep rate increases. The capacitive-controlled pro cess contributes 76%, 82%, 86%, 88%, 91% and 94% of the total charge storage at 5, 10, 20, 30, 40 and 50 mV s 1, respectively, indicating a
which enables us to evaluate the k1 (slope) and k2 (intercept) of the straight lines under the different potentials (V) with ease. Thus, based on the calculated k1 and k2, we determined the k1ν and k2ν1/2 at the specific fixed potentials. Fig. 7c and d displays the voltage profiles of capacitive current responses (total shaded areas) at 5 mV s 1 and 50 mV s 1, 7
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dominant capacitive charge-storage mechanism in the Zn0.76Co0.24S/ NiCo2S4 electrode. As a result, this electrode displays an excellent overall electrochemical performance, which is essential for the practical application of SCs.
examined at different potential windows. Fig. S9a shows the CV curves of the constructed Zn0.76Co0.24S/NiCo2S4//AC ASC at different poten tials, confirming the stabilized voltage window at 0–1.6 V. However, when the potential window extends to 1.7 V, the CV curve becomes distorted due to irreversible oxygen evolution reactions. Thus, the optimized voltage window is determined as 1.6 V. The GCD curves tested at various potentials in Fig. S9b further demonstrate that the voltage window of the ASC device is 0–1.6 V. Fig. 8b shows the CV curves of the ASC device tested at various scan rates (5–50 mV s 1). Unlike the sharp redox peaks in the CV curves in the three-electrode configuration, the ASC device exhibited quasirectangular CV curves with small peaks, which are due to the com bined features of the Faradaic and non-Faradaic behaviors of Zn0.76Co0.24S/NiCo2S4 and AC. Furthermore, the changes of CV curves shapes are insensitive during the oxidation and reduction processes, when the sweep rate increases. Notably, Fig. 8c shows that the corre sponding GCD curves at various current densities are nonlinear, which are in accordance with the CV curves. Fig. 8d depicts the rate perfor mance of the Zn0.76Co0.24S/NiCo2S4//AC ASC device, which can be reversibly cycled at 1, 2, 4, 8, 12 and 20 A g 1 with corresponding ca pacitances of 135.2, 126.8, 118.1, 108.8, 105.1 and 97.4 F g 1, respec tively. When the current density reverts to 1 A g 1 after 60 cycles, the
3.3. Electrochemical characterization of the Zn0.76Co0.24S/NiCo2S4//AC ASC device The practical application of the Zn0.76Co0.24S/NiCo2S4 electrode in SCs was evaluated by fabricating an asymmetric ASC, with Zn0.76Co0.24S/NiCo2S4 as the positive electrode and AC as the negative electrode, as schematically displayed in Fig. 8a. The characteristics of active carbon (AC) are demonstrated in Fig. S7. The electrochemical characteristics of AC are shown in Fig. S8a, the CV curves of AC show a quasi-rectangular shape within a stable potential window of 1~0 V, illustrating the typical EDLC features. The GCD curves of AC show almost linear characteristics, thus demonstrating the ideal EDLC nature (Fig. S8b). The specific capacitances of AC is 220 F g 1 at 1 A g 1. To achieve ideal electrochemical performance, prior to the construction of the ASC device, the active mass ratio of Zn0.76Co0.24S/NiCo2S4 to AC was determined as 1:6.1 according to charge balance relationship. To further confirm the stable voltage, the CV feature of the ASC device was
Fig. 8. (a) A schematic illustration of the assembled Zn0.76Co0.24S/NiCo2S4//AC flexible all-solid-state ASC device; (b) CV curves of the ASC device at various scan rates; (c) GCD curves of the ASC device at various current densities; (d) Specific capacitance variations with the current density for the ASC device; (e) Ragone plot of the Zn0.76Co0.24S/NiCo2S4//AC ASC device compared with several reported ASCs; (f) Cycling performance of the ASC device at a constant current density of 12 A g 1 for 5000 cycles; (g) CV curves of the ASC device in bent states; (h) GCD curves of ASC devices connected in series; (i) a demonstration of the ASC devices powering one LED. 8
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reversible capacitance recovers to 132 F g 1, revealing a strong rate ability and the stability of the ASC device. Fig. 8e delivered the Ragone plot of the energy and power density of the Zn0.76Co0.24S/NiCo2S4//AC ASC device, which are two additional and critical parameters for assessing the practicability of SCs. A high energy density of 48.1 Wh kg 1 was attained at the power density of 837 W kg 1, and maintained at 34.6 Wh kg 1 at an extremely high power density of 16.6 kW kg 1. The device performance recorded in this study compares closely to or even exceeds the most recently reported ASCs when using TMSs as the positive electrode [35,45–53]. Moreover, the cycling performance was measured at 12 A g 1 with up to 5000 cycles (Fig. 8f) and maintained 91% of the initial capacitances, signi fying the superior long-term electrochemical stability of the ASC device. Furthermore, the as-fabricated Zn0.76Co0.24S/NiCo2S4//AC ASC de vice can be applied to energy storage textiles and wearable electronics, and its mechanical stability was investigated. Fig. 8g depicts the GCD curves of ASC device with various bending angles. The shape of the GCD curves did not show a substantial change, confirming the superior flexibility and extraordinary mechanical stability of the device. Furthermore, we connected the advanced ASC in a series to reach a high output voltage (Fig. 8h). For demonstration purposes, the photographic image in Fig. 8i shows the actual application of our device to power a 5 mm diameter commercial LED (2.2 V, 20 mA) by connecting three ASC devices in series. In order to apply to wearable electronic devices, the ASC device should work stably at human body temperature or higher temperature. As shown in Fig. S10, the CV curves of ASC device reveal little changes, indicating that the effect of temperature on the ASC de vice is small.
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4. Conclusions In summary, we demonstrated a simple method for fabricating Zn0.76Co0.24S/NiCo2S4 nanosheets onto a conductive carbon cloth using the ZIF-L precursor and employed for flexible all-solid-state Zn0.76Co0.24S/NiCo2S4//AC ASC. A facile and efficient synthesis involved the growth of the Zn/Co ZIF-L precursor, which was subse quently transformed into ZnNiCo-LDH and finally converted to Zn0.76Co0.24S/NiCo2S4 nanosheets via sulfurization. The integrated design of active materials anchored on a conductive substrate not only avoids the use of polymer binder and conductive additives, but also provides fast charge transport. Beneficial from the unique nanostructure design and the synergistic effects of the electrode materials, the pre pared Zn0.76Co0.24S/NiCo2S4 nanosheets show a high specific capaci tance of 2674 F g 1 at 1 A g 1, and a strong rate capability and cycling performance. We assembled the ASC device with a positive Zn0.76Co0.24S/NiCo2S4 electrode and negative AC electrodes. Further more, the device possesses a maximum energy density at 48.1 Wh kg 1 and power density at 837 W kg 1. Moreover, the capacitance retention of ASC maintained at 91% with up to 5000 cycles. This study opens new avenues for the development of transition metal sulfides as potential electrode materials for next-generation flexible devices. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 51603053, 51872057, 51901055, 21576060), Natural Sci ence Foundation of Heilongjiang Province (LH2019E025), Fundamental Research Funds of the Central University (3072019CF1003), China Postdoctoral Science Foundation (2019M651260), and Defense Indus trial Technology Development Program (JCKY2016604C006) and the National Key Research and Development Program of China (2016YFE0202700). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 9
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