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Flexible CuCo2O4@Ni-Co-S hybrids as electrode materials for high-performance energy storage devices
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Xiaowei Wanga, Yuchen Suna, Wei-Chao Zhangb, Xiang Wu*, a
Chinese Chemical Letters | 2023, 34(3) : 107593
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Chinese Chemical Letters | 2023, 34(3): 107593
• Communication •
Flexible CuCo2O4@Ni-Co-S hybrids as electrode materials for high-performance energy storage devices
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Xiaowei Wanga, Yuchen Suna, Wei-Chao Zhangb, Xiang Wu*, a
Affiliations
  • aSchool of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
  • bKey Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150080, China
Published: 2023-03-15 doi: 10.1016/j.cclet.2022.06.016
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Rational design of electrode meterials with unique core-shell nanostructures is of great significance for improving the electrochemical performance of supercapacitors. In this work, we prepare several CuCo2O4@Ni-Co-S composite electrodes by a controllable hydrothermal and electrodeposition route. One-dimensional nanowires can shorten the ions transport path, while two-dimensional nanosheets expose many active sites. This enables three-dimensional structured composite with high electrochemical activity. The as-prepared heterostructured materials show a specific of 1048 C/g at 1 A/g. It still maintains 75.6% of initial capacity after 20000 cycles at 10 A/g. The device delivers an energy density of 79.2 Wh/kg when the power density reaches to 2280 W/kg. Moreover, it possesses an excellent mechanical stability after repeated folding at different angles

Core-shell structure  /  CuCo2O4@Ni-Co-S  /  Supercapacitor  /  Energy density  /  Mechanical stability
Xiaowei Wang, Yuchen Sun, Wei-Chao Zhang, Xiang Wu. Flexible CuCo2O4@Ni-Co-S hybrids as electrode materials for high-performance energy storage devices[J]. Chinese Chemical Letters, 2023 , 34 (3) : 107593 - . DOI: 10.1016/j.cclet.2022.06.016
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Nowadays, people pay close attention to the flexible and portable energy storage devices with the quick development of smart wearable technology [1-4]. Among them, supercapacitors have been research focuses due to their high power density, fast charge-discharge process and long cycle life [5, 6]. However, the low energy density limits their wide applications compared with conventional batteries [7-12]. The main reasons are the low voltage window of electrode material and the low capacitance [13-15]. In addition, the composition and spatial structure of the materials have an important influence on the electrochemical performance of the devices [16-20]. Therefore, it is of great significance to solve the above problems through some effective strategies.
In general, ternary transition metal oxides possess more abundant oxidation states than their corresponding binary counterparts [21-23]. Thereinto, the spinel-structured CuCo2O4 material is regarded as an excellent electrode material for supercapacitor due to its high electrochemical activity and structural stability [24, 25]. Nevertheless, due to the poor conductivity of single CuCo2O4 materials, it is still difficult to improve their specific capacitance remarkably [26, 27]. Transition metal layered double hydroxides (LDHs), especially nickel-cobalt-based materials, are impressive among the various electrode materials [28-30]. LDH materials usually present high electrical conductivity and large specific surface area, which can be mainly attributed to their special hydrotalcite-like intercalation structure [31]. Meanwhile, the corresponding metal sulfides have been also widely studied due to their high intrinsic conductivity and abundant redox active sites [32]. For instance, Qu et al. fabricated hierarchical hollow NiCo2S4@NiS nanostructures on carbon fabrics. The results indicate that the unique structure improves their electrical conductivity and overall capacitive performance [33]. Our group reported a P-doped Ni-Co-S@C assembly composed of nanosheets. As an electrode material for supercapacitors, it delivers a specific capacitance of 1026 C/g at 1 A/g and maintains 89% of initial capacity after 10,000 cycles [34].
Herein, we synthesize core-shell CuCo2O4@Ni-Co-S composite by one-step hydrothermal method combined with electrodeposition. The construction of 3D porous structures can effectively increase the specific surface area of the prepared electrode materials. The CuCo2O4@Ni-Co-S-14 sample delivers a specific capacitance of 1048 C/g at 1 A/g. The composite electrode material still maintains a capacity of 75.6% after 20,000 cycles. In addition, an asymmetric device is assembled using CuCo2O4@Ni-Co-S-14 as cathode, which delivers 79.2 Wh/kg at 2280 W/kg. A series of bending experiments demonstrate its potential applications in future flexible electronic devices.
Fig. 1 depicts the synthetic schematic of the composite materials. Firstly, CuCo2O4 (CCO) nanowires are grown on 3D porous Ni foam (NF) substrate through one-step hydrothermal route. Then, a large amount of Ni-Co-S (NCS) nanosheets are covered vertically on the surface of CCO nanowires with controllable electrodeposition process to form a 3D core-shell heterostructures. The crystal structures of the as-prepared samples are first characterized by XRD (Fig. 2a). The CCO sample is scraped from the surface of NF before testing. It can be found that the diffraction peaks at (111), (220), (311), (400), (511) and (440) planes are coincide with CuCo2O4 spinel structure (JCPDS No. 76–1887). From the XRD patterns of composite product, three distinct diffraction peaks at 44.50°, 51.85° and 76.38° are indexed to Ni foam substrate (JCPDS No. 87–0712). In addition, the peaks located at 16.34°, 31.59°, 55.33° and 65.08° can be ascribed to (111), (311), (440), (533) planes of NiCo2S4 material (JCPDS No. 20–0782). The weak signal intensity of NCS nanosheets can be attributed to the low loading and small grain size.
XPS is then used to study the element composition and surface valence state of the samples. The survey spectra (Fig. 2b) depict the existence of Cu, Co, Ni, S, O and C elements in CCO@NCS-14 sample. Cu 2p spectra at binding energy of 933.3 and 953.1 eV can be indexed to Cu 2p3/2 and Cu 2p1/2, respectively, as shown in Fig. 2c. In addition, the presence of two shakeup satellite peaks (denoted as "Sat.") located at 941.0 and 961.5 eV further indicates the exsistence of Cu2+ [35]. As shown in Fig. 2d, the binding energies at 783.6/798.2 eV in Co 2p spectra can be assigned to Co2+, and those at 781.1/796.8 eV are ascribed to Co3+. The spin-orbit splitting gap in Co 2p1/2 and Co 2p3/2 is 16.0 eV, which further proves the existence of different valence states of Co element [36]. Similarly, for Ni 2p spectra in Fig. 2e, the binding energy between 855.6 eV (Ni 2p3/2) and 873.2 eV (Ni 2p1/2) is 17.6 eV, demonstrating the appearance of Ni2+ and Ni3+ [36]. Furthermore, two main peaks of S2p3/2 and S2p1/2 can be observed at 161.7 and 162.8 eV (Fig. 2f), indicating typical metal-sulfur bond (M-S) and low surface coordination sulfide [37]. The satellite peak at around 169.3 eV is mainly attributed to surface sulfur species at certain high oxidation states [38].
From SEM image in Fig. 3a, many CCO nanowires are uniformly covered on NF surface. High magnification SEM images (Fig. 3d) indicate that the average diameter of CCO product is 160 nm. Figs. 3b and e show that the nanowires are transformed into a layered core-shell structure after electrochemical deposition. Besides, the SEM images of NCS product and composites with different deposition time are shown in Fig. S1 (Supporting information). Compared with CCO@NCS-14 product, with the deposition time increasing, NCS nanosheets gradually agglomerate on the surface of the nanowires, which will reduce the active sites and hinder the ion transport. Then, TEM tests are conducted to further explore the CCO@NCS-14 product. As shown in Figs. 3c and f, ultrathin NCS nanosheets are uniformly coated on the surface of CCO@NCS-14 sample, which is consistent with SEM results. Furthermore, HRTEM images find that lattice fringe spacings of 0.468 nm, 0.286 nm and 0.203 nm can assigned to the (111), (220) and (400) planes of CCO phase, respectively (Fig. 3g). While the lattice spacing around 0.283 nm and 0.235 nm can be indexed to the NCS (311) and (400) planes (Fig. 3h). The polycrystalline structure of CCO@NCS-14 sample is confirmed through selected area electron diffraction (SAED) pattern in the insets of Figs. 3g and h. Among them, each diffraction ring can well be indexed to the crystal plane of CCO@NCS-14 nanostructure, which is consistent with XRD results. The EDS mapping further demonstrates the distribution of Cu, Co, Ni and S elements in CCO@NCS-14 heterostructure (Fig. 3i).
Then we study the electrochemical performance of the electrode materials in a three-electrode system in 3 mol/L KOH. Firstly, the CV tests are performed in 0–0.6 voltage interval at 20 mV/s (Fig. 4a). It is found that the composite samples possess larger CV area than the CCO samples, indicating their high capacitance. Additionally, four samples show distinct redox peaks. It shows that the faradaic reaction proceeds on/near the surface in an alkaline environment. The existence of redox peaks also proves their typical battery-type capacitive behavior. The GCD curves of the electrode materials are shown in Fig. 4b. The CCO@NCS-14 electrode possesses excellent charge-discharge performance compared with the others at 1 A/g. Fig. 4c shows the CV curves of CCO@NCS-14 products at different scan rates. As the scan rate increases, the redox peaks of composite electrode shift to positive and negative directions. It can be observed that there is an obvious polarization phenomenon at high scan rate, which is related to the redox reaction.
Electrochemical mechanism of the electrode materials can be described by the following equations [39, 40]:
The GCD curves of CCO@NCS-14 product show symmetry and obvious potential plateaus at different current densities (Fig. 4d). It suggests the excellent reversibility of the charge-discharge behavior. The electrode delivers specific capacitances of 2232, 1048, 980, 839, 714 and 608 C/g at 0.5, 1, 2, 4, 6 and 8 A/g, respectively. The excellent electrochemical performance is mainly attributed to the encapsulation of the highly conductive NCS nanosheet networks. The 3D heterostructures can greatly increase the electrochemically active sites and facilitate the ion transport. The CV and GCD curves of CCO, CCO@NCS-7 and CCO@NCS-21 samples are shown in Fig. S2 (Supporting information). Meanwhile, Table S1 (Supporting information) presents electrochemical performance comparison of CCO@NCS nanostructures and CCO products. The results demonstrate that the specific capacitances are 236, 529 and 678 C/g at 1 A/g, respectively. It further demonstrates that the deposition cycles have a great influence on the electrochemical performance of the electrode materials. This may be attributed to the decrease in the effective active area of electrode materials with increasing deposition time, which affects electron transport and redox reaction process.
In addition, the conductivity of electrode materials is further investigated by the EIS measurement in the range of 100 kHz-0.01 Hz, as shown in Fig. 4e. The intercept in high frequency region and the diameter of semicircle represent equivalent series resistance (Rs) and charge transfer resistance (Rct), respectively. As a result, the Rs value of CCO@NCS-14 composite is 0.65 Ω, which is much smaller than 0.83 Ω of CCO product. The slope of straight line in low frequency region indicates the interface diffusion resistance (Zw). It can be clearly seen that the Zw of CCO@NCS-14 electrode is smaller than those of others, revealing its excellent kinetic behavior and ion diffusion rate. Meanwhile, the equivalent circuit is shown in the inset. Fig. 4f presents the capacitive contribution of CCO and CCO@NCS-14 materials. The diffusion controlled capacity gradually reduces with scan rate increasing. The dominance of surface controlled capacitance can be attributed to the limited ion diffusion time at high scan rates.
Fig. 4g shows the specific capacitance of four materials at various current densities. CCO@NCS-14 sample still maintains 58% of its original capacitance even at 8 A/g, indicating its excellent rate performance. From Fig. 4h, CCO@NCS-14 electrode delivers the largest specific capacitance compared with previous work, such as CuCo2O4@MnO2 products (416 F/g at 1 A/g) [41], NiCo2S4 nanoneedles (936 F/g at 1 A/g) [42], NiCo2O4@NiWS nanosheets (580 C/g at 1 A/g) [43], CuCo2O4/MnCo2O4 composite (1434 F/g at 0.5 A/g) [44] and CuCo2O4@Carbon fiber nanostructure (1737 F/g at 1 A/g) [45]. Furthermore, Fig. 4i shows the cycling curves of the electrode materials during long-term cycling. After 20,000 cycles, CCO@NCS-14 product delivers 75.6% of initial capacitance, while the capacitance retention of CCO, CCO@NCS-7 and CCO@NCS-21 samples are 88.2%, 68.8% and 55.8%, respectively. In addition, the high capacity retention of the CCO nanostructure demonstrates that it can serve as a backbone to support the NCS nanosheets. The capacitances of the other two composites drop significantly after many charge-discharge process, which can be attributed to surface shedding off or agglomeration during long-term cycling.
To further evaluate its potential application of the prepared samples. An asymmetric flexible supercapacitor is assembled with CCO@NCS-14 electrode as cathode and the AC as anode. Fig. 5a shows the CV curves of positive and negative materials at 30 mV/s, and the total voltage window can reach 1.6 V. As described in Fig. 5b, the CV curves of the device present a rectangular-like shape at different voltage windows, indicating a capacitive-dominant behavior. Additionally, the GCD curves show that the device has no obvious polarization phenomenon at the voltage window up to 1.8 V (Fig. 5c), further confirming that it can operate stably at the voltage window of 1.6 V. From Fig. 5d, the shapes of CV curves remains almost unchanged with the scan rate increasing, which demonstrates its efficient electron transport capability and excellent capacitive characteristic. The GCD curves of the device shows a triangular shape, which is consistent with CV results (Fig. 5e). It delivers capacitances of 106.3, 99.2, 88.6, 72.4 and 60.1 C/g at 0.5, 1, 2, 4, 6 and 8 A/g, respectively. As seen from EIS plot in Fig. 5f, the Rs value of the device is 1.32 Ω, and the corresponding equivalent circuit is shown in the inset. Fig. 5g presents the Ragone plots of CCO@NCS-14//AC device. It delivers an energy density of 79.2 Wh/kg at the power density of 2280 W/kg, and still maintains 38.4 Wh/kg at 23,040 W/kg, which is superior to previously reported work [46-50]. Furthermore, cycling performance of the device is conducted at 1 A/g. It shows 83.3% of initial specific capacitance after 10,000 cycles. It can be clearly seen from the inset that the device still maintains excellent charge-discharge capability after long cycles (Fig. 5h).
Finally, a series of experiments are conducted to study the practical application of CCO@NCS-14//AC device. Firstly, multiple folding experiments are conducted. As shown in Figs. 6a and b, when the folding angles are 30° and 60°, the areas of CV curves increases significantly. It may be attributed to the fact that the electrode material is in full contact with electrolyte during the folding process, and it returns to its original state in the subsequent folding process (Fig. 6c). The CV curves almost completely overlap during reverse folding at different angles (Fig. 6d). It demonstrates the excellent stability and mechanical flexibility of the assembled device. Additionally, the practical application of the device can be explored by connecting three devices in series to power a blue LED. It can light up for up to 7 min, as shown in Fig. 6e.
In conclusion, Ni-Co-S nanosheets decorated CuCo2O4 nanowires are successfully prepared through a combination of hydrothermal and electrodeposition methods. 3D CuCo2O4@Ni-Co-S hybrids integrate the internal characteristics and external spatial effects of each component. It provides much convenience for the transport of electrons, which makes faradaic redox reaction more likely occur, thereby induce large specific capacitance. Furthermore, the assembled device presents excellent cycling stability and remarkable mechanical durability. The synthesized product is expected to be candidate for next-generation flexible devices.
Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
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The work is supported by National Natural Science Foundation of China (No. 52172218) and Key Laboratory of Engineering Dielectrics and Its Application (Harbin University of Science and Technology), Ministry of Education (No. KFZ202002).
Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.06.016.
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Year 2023 volume 34 Issue 3
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Article Info
doi: 10.1016/j.cclet.2022.06.016
  • Receive Date：2022-05-15
  • Online Date：2025-11-21
  • Published：2023-03-15
Article Data
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  • Received：2022-05-15
  • Revised：2022-06-04
  • Accepted：2022-06-07
Affiliations
    aSchool of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
    bKey Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150080, China
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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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