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Polyhedral-Like NiMn-Layered Double Hydroxide/Porous Carbon as Electrode for Enhanced Electrochemical Performance Supercapacitors Mei Yu,* Ruili Liu, Jianhua Liu, Songmei Li, and Yuxiao Ma Therefore, large surface area and high porous structure are essential for highperformance supercapacitors. Layered double hydroxides (LDHs) are kinds of classic pseudocapacitive materials with lamellar structure and large surface area. The special layered structure is favorable for redox reaction, making LDHs one of the most promising electrode materials in supercapacitors.[13–15] Zhang et al. fabricated porous CoAl-LDH/ graphene composites with a Nickel foam grid a serving as working electrode, and the material shows a specific capacitance of 479.2 F g−1 at 1 A g−1.[16] Similarly, NiAl-based LDH growing on Ni foam could achieve a specific capacitance of 795 F g−1 at 1 A g−1 current density.[17] Despite high electrochemical performance has been achieved, serious agglomeration and intrinsic poorly electric conductivity are still the problems to be solved. The insufficient above will reduce the rate capability and cycling stability of LDH when served as electrode materials for supercapacitors. The way to improve the electric conductivity of active materials is in situ growing LDH on carbon materials, like carbon nanotubes (CNTs), graphene, carbon fibers, porous carbon (PC), and so on.[18] Compared with carbon materials above, PC possesses low density, high surface area, high thermal and chemical stability, and low cost.[19] PC has broad applications in adsorption, catalyst, and energy storage. PC with well-defined morphology and high porous attracts more attentions. Highly PC can be synthesized through a series of methods like activation of carbon, carbonization of polymer materials, and templating methods with zeolite and mesoporous silica as precursors.[20–24] Metal– organic frameworks (MOFs) structured with nanoscaled cavities and open channels offer a special template for synthesis of nanoporous carbons via carbonization. Compared with the traditional inorganic templating method, using inorganic–organic MOFs as template is relative simple, friendly, and convenient. Because it can partially avoid to use toxic chemicals to remove the templates under wicked conditions.[25] Besides, the PC has been applied as electrode materials for supercapacitors with favorable electrochemical performance.[26] In this study, zeolitic imidazolate frameworks-8 (ZIF-8) (a kind of MOFs) was chosen as the precursor of PC, successfully synthesizing LDH/PC-x composites by hydrothermal
Polyhedral-like NiMn-layered double hydroxide/porous carbon (NiMn-LDH/ PC-x) composites are successfully synthesized by hydrothermal method (x = 1, 2 means different mass percent of porous carbon (PC) in composites). The NiMn-LDH/PC-1 composites possess specific capacitance 1634 F g−1 at a current density of 1 A g−1, and it is much better than that of pure LDH (1095 F g−1 at 1 A g−1). Besides, the sample can retain 84.58% of original capacitance after 3000 cycles at 15 A g−1. An asymmetric supercapacitor with NiMn-LDH/PC-1 as anode and activated carbon as cathode is fabricated, and the supercapacitor can achieve an energy density of 18.60 Wh kg−1 at a power density of 225.03 W kg−1. The enhanced electrochemical performance attributes to the high faradaic pseudocapacitance of NiMn-LDH, the introduction of PC, and the 3D porous structure of LDH/PC-1 composites. The introduction of PC hinders serious agglomeration of LDH and further accelerates ions transport. The encouraging results indicate that these materials are one of the most potential candidates for energy storage devices.
1. Introduction In order to meet the rapid increasing demands of electronic devices and hybrid electric vehicles, it is significant to develop high energy storage system. Supercapacitors or electrochemical capacitors (ECs) are deemed to play more and more essential roles in energy storage by virtue of their outstanding power density, long cycle lifetime, and high reliability.[1–5] Supercapacitors can be separated into two kinds of parts (electric double-layer capacitors (EDLCs) and pseudocapacitors) in line with the charge-storage mechanism.[6,7] Compared with EDLCs, pseudocapacitors possess high specific capacitance and high energy density through fast redox reactions, so redox-active materials possessing pseudocapacitance are highly expected for ECs owing to their high specific capacitance.[8–12] Besides, the capacitance reactions mainly occur on the interfaces between electrode materials and electrolyte.
Dr. M.Yu, R. Liu, Prof. J. Liu, Prof. S. Li, Y. Ma School of Materials Science and Engineering Beihang University Beijing 100191, China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201702616.
DOI: 10.1002/smll.201702616
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method. Polyhedral-like NiMn-LDH/PC composites were successfully obtained, and the electrochemical performance was tested in three-electrode system and full-cell device. First, in the three-electrode system with 6 m KOH electrolyte, the achieved NiMn-LDH/PC-1, LDH/PC-2, and pure LDH showed maximum specific capacitance of 1634, 1518, and 1095 F g−1 at a current density of 1 A g−1, respectively. Furthermore, an asymmetric supercapacitor with optimized NiMn-LDH/PC-1 composites as positive electrode and AC as negative electrode could achieve an energy density of 18.60 Wh kg−1 at a power density of 225.03 W kg−1. It is obvious that the introduction of PC improves their electrochemical performance. Therefore, the optimized LDH/PC-1 composites can be used as promising electrode materials in energy storage and transformation application.
2. Results and Discussion 2.1. Morphological and Structural Characterization The synthesis process of NiMn-LDH/PC is shown in Scheme 1, and the detail information is shown in the Experimental Section. After carbonized, PC keeps the original morphology of ZIF-8 very well, and possesses high porosity. After growing NiMn-LDH nanosheets, polyhedral-like structured NiMn-LDH/ PC-x composites are obtained. X-ray powder diffraction (XRD) measurements were applied to confirm the crystallographic structure of products. In Figure 1a, the diffraction peaks of as-prepared ZIF-8 are identical to simulated ZIF-8, indicating a successful preparation of ZIF-8. Besides, in Figure 1b, the typical diffraction peaks of a hydrotalcite-like structure can be observed clearly at 11.34°, 22.74°, 34.41° in LDH and LDH/PC-1. These peaks are properly agreed to previous literatures on NiMn-LDH and can be ascribed to (003), (006), and (012) planes, respectively,[18,27–29] investigating a successful synthesis of NiMn-LDH. Nevertheless, the diffraction peaks of PC cannot be found clearly in the XRD pattern of LDH/PC, probably because much too weak to be detected. The microstructure of products was investigated by scanning electron microscopy (SEM) and transmission electron microscope (TEM) techniques, and the results are shown in Figure 2. In Figure 2a,d, the pure LDH nanosheets aggregate together to form a large-size cluster, and the LDH nanosheets are crosslinked in a disordered way. Besides, the pure LDHs present dilapidated nanosheets structure with few open pores.
This disordered structure may inhibit ions transfer. Figure 2b shows the ZIF-8 with a regular rhombic dodecahedral structure, and the size is about 500 nm. After carbonized, the PC derived from ZIF-8 still retains its dodecahedral structure perfectly (Figure 2e), except a little diminution in size (around 300 nm). Furthermore, through an in situ growth method, equally distributed NiMn-LDH nanosheets are equally distributed covered on the framework of PC, and the nanosheets are crosslinking to each other to form a lot of mesoporous pores, accelerating the ions transport (Figure 2c,f). The polyhedral-like core–shell LDH/PC-1 is confirmed by TEM images (Figure 1g), in which the gauze-like NiMn-LDH nanosheets are uniformly grafted on dodecahedral PC. The high resolution transmission electron microscopy (HRTEM) image (Figure 2h) clearly displays an interplanar spacing of 3.314 Å, corresponding to the (015) plane of LDHs. The selected area electron diffraction (SAED) image (Figure 2i) further proves the crystalline structure of LDHs. The TEM image and relevant energy-dispersive X-ray spectrometry (EDS) elemental mapping of LDH/PC-1 (Figure 3a,b) unambiguously prove the core–shell structure of LDH/PC-1 with homogeneous distribution of Ni, Mn, O, and C elements. The X-ray photoelectron spectroscopy (XPS) was used to explore the valence state and composition information of LDH/PC-1. As presented in Figure 4, the survey XPS spectra (Figure 4a) show four elements (C, O, Ni, and Mn) in LDH/ PC and two elements (C and O) in PC. This result indicates that LDHs have been grown on PC. The C 1s XPS spectrum of PC possessing three distinguished peaks are well fitted to CC (284.8 eV), CO (286.5 eV), and OCO (288.86 eV) as depicted in Figure S1 in the Supporting Information.[30] The peaks located at 286.5 and 288.86 eV are ascribed to oxygencontaining functional groups, which will effectively provide anchoring sites for growth of inorganic nanoparticles during hydrothermal process.[31,32] In Ni 2p spectrum (Figure 4b), the Ni 2p3/2 (855.8 eV) and Ni 2p1/2 (873.3 eV) peaks coupled with two satellite bands (indicated as “Sat.”) suggest the existence of Ni2+ state.[33] In Mn 2p spectrum (Figure 4c), the major peaks for Mn 2p3/2 and Mn 2p1/2 stand at ≈643.5 and ≈653.0 eV, indicating the existence of Mn3+ in the materials.[33] The property of specific surface area and pore structure is essential for electrode materials in energy storage applications. Hence, N2 adsorption/desorption measurement was applied to characterize the surface area and porosity feature of LDH/PC-1 (Figure 4d), ZIF-8 and PC (Figure S2, Supporting Information). Figure 4d presents typical IV isotherms along with H3-type hysteresis loops (P/P0 > 0.4) for LDH/PC-1 and pure LDH,
Scheme 1. Growth and morphology schematic illustration of NiMn-LDH/PC-x.
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Figure 1. XRD spectra of a) ZIF-8. b) PC, LDH, and LDH/PC-1.
showing the existence of mesoporous structure. Moreover, the shape of the hysteresis loops proves that the particle aggregates to form slit-shape pores.[34] From the pore sizes distribution (Figure 4e), the LDH/PC-1 composites possess mesoporous at the range of 2–10 nm, and the pore size of LDH/PC-1 is a little larger than that of pure LDH. Large pore size is beneficial for ions transmission during electrochemical reaction process. Besides, the specific surface area and total pore volume of ZIF-8, PC, LDH/PC-1, and pure LDH are obtained through Brunauer–Emmett–Teller (BET) method, and the resulting data are listed in Table 1. On the one hand, the LDH/PC-1 composites display relative larger specific surface area (159.56 m2 g−1) and total pore volume (0.71 cm3 g−1) than that of pure LDH (124.20 m2 g−1) and (0.65 cm3 g−1), respectively, further demon strating that the introduction of PC inhibiting the agglomeration of LDH. On the other hand, the ZIF-8 possesses high specific surface area (1909.57 m2 g−1) and total pore volume (0.74 cm³ g−1), and after carbonization, the area still remains
1193.26 m2 g−1. Compared with the PC, the LDH/PC-1 composites exhibit relatively low specific surface area (159.56 m2 g−1), demonstrating that LDH nanosheets are grown on PC successfully once again. Moreover, the relatively high pore volume of LDH/PC composites is beneficial for kinetics of reaction and ions transport.
2.2. Electrochemical Performance Characterization 2.2.1. Three-Electrode System The electrochemical performance of LDH, LDH/PC-1, and LDH/PC-2 was investigated through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in three-electrode system including a Pt counter electrode, an Ag/AgCl reference electrode, and 6 m KOH electrolyte. Figure 5a presents a series
Figure 2. SEM images of a,d) pure LDH at different magnification. b) SEM image of ZIF-8. c,f) LDH/PC-1 at different magnification. e) SEM image of PC. TEM images of LDH/PC-1 at g) low magnification, h) high magnification. i) SAED images of LDH/PC-1.
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Figure 3. a) TEM image. b) EDS mapping of C, O, Mn, and Ni elements on local structure of LDH/PC-1.
of typical CV curves of LDH, LDH/PC-1, and LDH/PC-2 in a potential range −0.3 to 0.7 V at the scan rate of 10 mV s−1. All the curves show strong redox current peaks, coinciding to typical pseudocapacitive behavior of Ni2+/Ni3+ with the assistance of OH−, the reaction involved may be illustrated as:[35] Ni (OH)2 + OH− ↔ NiOOH + H2O + e −
(1)
It is quite clear that LDH/PC-1 composites present much higher redox activity than other samples, illustrating that LDH/ PC-1 composites possess the best electrochemical performance. The CV curves of NiMn-LDH and Ni(OH)2 were shown in Figure S3a in the Supporting Information. The oxidation peak
of NiMn-LDH shifts to the low potential, and this conclusion is agreed to other works.[36] Figure 5b manifests the charge– discharge curves of LDH, LDH/PC-x composites at 1 A g−1. It is obvious that the specific capacitance of LDH/PC-1, LDH/ PC-2, and LDH at 1 A g−1 is 1634, 1518, and 1095 F g−1. While the charge–discharge curves of LDH and Ni(OH)2 at 1 A g−1 were shown in Figure S3b in the Supporting Information. According to the charge–discharge curves of three materials, the specific capacitance of NiMn-LDH and Ni(OH)2 is 1095 and 857 F g−1, respectively. As the LDH/PC is a battery-type electrode,[37,38] its specific capacity (Cs) is calculated. The specific capacity of achieved NiMn-LDH/PC-1, LDH/PC-2, and pure LDH at 1 A g−1 is also calculated to investigate their electro-
Figure 4. a) XPS survey spectra of the as-obtained LDH/PC-1 and PC samples. b) Ni 2p, c) Mn 2p XPS spectrum of LDH/PC-1. d) N2 adsorption/ desorption isotherms and e) pore diameter distribution of pure LDH and LDH/PC-1 composites.
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Specific surface area [m2 g−1] 3
−1
Total pore volume [cm g ]
MICRO
ZIF-8
PC
LDH
LDH/PC-1
1909.57
1193.26
124.20
159.56
0.74
0.58
0.65
0.717
chemical performance, and the value of them is 686.28, 637.56, and 459.9 C g−1, respectively. While relative specific capacity of LDH and Ni(OH)2 is 459.9, 308.5, and 421.8 C g−1, respectively. It is clear that NiMn-LDH possesses relative high specific capacitance and wide potential window, demonstrating the contribution of Mn3+ to capacitance and other electrochemical performance. The results above demonstrate that an introduction of PC can improve the electrochemical performance, and an appropriate content of PC can provide optimized nucleation and growing sites, leading to relatively better electrochemical performance. The LDH/PC-1 composites are elected as further studying object. Figure 5c shows the CV curves of LDH/PC-1 at various scan rates, and the redox peaks are clearly presented in the plot, demonstrating a representative faradic redox capacitive behavior. In addition, with the scan rate increasing, the anodic peaks shift a little to high potential zone while the cathodic peaks shift a little to low potential zone. Whereas the shape of CV curves is still well kept, illustrating a well rate capability. The specific capacitance of LDH/PC-1 is 1634, 1451, 1296, 1115, 1047, and 988 F g−1 at various current densities of 1, 2, 3, 5, 7, and 10 A g−1, respectively (Figure 5d). At the same time, the specific capacity of LDH/PC-1 at different current densities
of 1, 2, 3, 5, 7, and 10 A g−1 is 686.28, 609.42, 544.32, 468.3, 439.74, and 414.96 C g−1, respectively. EIS of LDH and LDH/PC-x composites was carried in the frequency range 0.01–100 kHz to examine the conductivity (Figure 6a). The resulting Nyquist plots in the Figure 6a consist of a straight line and a semicircle. The equivalent circuit (Figure 6a inset image) contains bulk resistance Rs, charge transfer resistance Rct, Warburg impedance W, pseudocapacitance Cp, and double-layer capacitance Cdl. In low frequency, the straight line with real axis (Zʹ) with an angle at about 45° is related to the Warburg impedance (W), and it shows the results of ion transmission are frequency dependent in the electrolyte.[39] According to the Nyquist plots in Figure 6a, the W value of LDH/PC-1 (0.21 Ω) is dramatically smaller than that of pure LDH (2.18 Ω) and a little smaller than that of LDH/PC-2 (0.25 Ω). In high frequency, the Rs value of LDH/PC-1 (6.20 Ω) is lower than that of LDH (9.42 Ω), LDH/PC-2 (8.23 Ω). According to the diameter of semicircle arc of EIS, the Rct value of LDH/ PC-1 (6.91 Ω) is lower than that of LDH (9.04 Ω), but a little bit higher than that of LDH/PC-2 (5.80 Ω).[40–42] The results above indicate that the LDH/PC-1 composites possess lower resistance and better electric conductivity than pure LDH and LDH/ PC-2, owing to the introduction of proper mass of PC to provide moderate nucleation sites and hinder the agglomeration of LDH nanosheets. In addition, the results of EIS measurement are consistent with former CV and GCD measurement, that is, LDH/PC-1 composites possess the best electrochemical performance. As we all know, the cycling stability is an important influence factor for energy storage devices in application. The cycling measurement of LDH/PC-1 composites was applied in
Figure 5. a) Cyclic voltammetry curves and b) discharge curves of LDH, LDH/PC-1, and LDH/PC-2 at the scan rate of 10 mV s−1 and the current density of 1 A g−1, respectively. c) CV curves and d) charge–discharge curves of LDH/PC-1 at various scan rates and current densities.
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Figure 6. a) Nyquist plots of the LDH, LDH/PC-1, and LDH/PC-2. b) Cycling performance of LDH/PC-1 at 15 A g−1.
three-electrode system, and the result was shown in Figure 6b. After 3000 cycles, the capacitance retention of LDH/PC-1 at 15 A g−1 can remain 84.58%. The good cycling stability is ascribed from 3D porous structure. As far as we know, the electrochemical performance of LDH/ PC-1 is comparable or superior to the results in relative literatures as listed in Table 2. From the Table 2, it is demonstrated that the material is a promising candidate for supercapacitors.
2.2.2. The Asymmetric Supercapacitor The electrochemical performance of the LDH/PC-1 composites was also investigated in a full-cell device, where LDH/PC-1 composites were served as the positive electrode, while AC was served as negative electrode (marked as LDH/PC-1//AC).
Figure 7a shows the schematic illustration of LDH/PC-1//AC asymmetric supercapacitor, and the mass of the positive electrode and negative electrode follows the Equation (2)[58,59] in order to coordinate the charge stores in both electrodes: m+ C− × ∆V− = m − C+ × ∆V+
(2)
In the equation, m+ represents the positive electrode mass, m− represents the negative electrode mass, C+ and C− are specific capacitance of anode and cathode, respectively, besides, ΔV+ and ΔV− are potential range of anode and cathode, respectively. Figure 7b shows CV plots of AC and LDH/PC-1 with potential windows of −1 to 0 V and −0.3 to 0.7 V, respectively. The charge–discharge curves of AC (Figure S4, Supporting Information) exhibit that the specific capacitance of
Table 2. Comparison of electrochemical performance of LDHs materials. Electrode materials
Electrolyte
Specific capacitance
NiMn-LDH/PC
6 M KOH
−1
1634 F g (1 A
MXene/NiAl-LDH
6 M KOH
1061 F g−1 (1 A g−1)
RGO@MgAl-LDH
1 M KOH
1334 F g−1 (1 A g−1)
Co–Fe-LDH@NiO
2 M KOH
903 F g−1 (1 A g−1) g−1
g−1)
60.5% at 10 A
52.4% at 10 A g−1
Reference This work [43] [44]
55.4% at 10 A g−1
[45]
69.4% at 10 A g−1
[46]
42.7% at 10 A g−1
[47]
g−1
[48]
2 M KOH
1268 F
CoAl-LDH nanosheets/rGO
2 M KOH
1296 F g−1 (1 A g−1) g−1
g−1
g−1)
NiMn-LDH/CNTs/rGO
(1 A
Rate capability
g−1)
Silver nanowire@hierarchical NiAl-LDH
6 M KOH
1247 F
Core–Shell NiAl-LDH
1 M KOH
735 F g−1 (2 A g−1)
75.0% at 25 A g−1
[49]
CoAl-LDH/PEDOT/core–shell nanoplatelet
6 M KOH
672 F g−1 (1 A g−1)
62.8% at 40 A g−1
[50]
g−1
(1 A
g−1)
60.9% at 10 A
g−1
[51]
NiMn-LDH/rGO
2 M KOH
Glucose-intercalated NiMn-LDH
6 M KOH
1464 F g−1 (0.5 A g−1)
59.4% at 10 A g−1
[29]
NiMn-layered double oxides arrays grown on GOS
1 M KOH
1648 F g−1 (0.5 A g−1)
75.9% at 10 A g−1
[52]
1635 F
g−1
(1 A
g−1)
71.0% at 10 A
g−1
[53]
NiMn-LDH/rGO
3 M KOH
NiCo2S4 nanotube@NiMn-LDH arrays/3D graphene sponge
6 M KOH
1740 mF cm−2 (1 mA cm−2)
72.9% at 10 mA cm−2
[54]
Ni–Co-LDH/graphene composites
1 M KOH
217.8 mF cm−2 (2 mA cm−2)
1250 F
g−1
(1 A
75.0% at 16 mA cm−2
[55]
g−1)
40.4% at 5 A g−1
[56]
52.4% at 10 A g−1
[27]
NiO/NiMn-LDH nanosheet array on Ni foam
3 M KOH
510 F
Colloidal NiMn-LDH nanosheets
1 M KOH
769 F g−1 (1 A g−1)
(1 A
g−1
g−1)
Sulfidation of NiMn-LDHs/graphene oxide
2 M KOH
NiMn-LDH/CNTs
1 M KOH
2960 F g−1 (1.5 A g−1)
NiMn-LDH/carbon cloth
PVA/LiCl
2239 F g−1 (5 mA cm−2)
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2247 F
36.0% at 5 A
(1 A
g−1
[18]
79.5% at 30 A g−1
[34]
74.4% at 10 A
[57]
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Figure 7. a) Schematic interpretation of the asymmetric supercapacitor device. b) CV curves of AC and LDH/PC-1 at scan rates of 10 mV s−1. c) CV curves and d) charge–discharge curves of LDH/PC-1//AC asymmetric supercapacitor at various scan rates and current density, respectively. e) Ragone plot of the asymmetric supercapacitor. f) Cycling performance of asymmetric supercapacitor.
AC is 106.34 F g−1. According to Equation (2), the mass of AC and LDH/PC-1 is about 9.2 mg and 1.3 mg, respectively. As presented in the Figure 7c, CV curves of the cell are carried in the potential window 0–1.5 V at various scan rates, and the extensive voltage of cell is ascribed to the combination of LDH/PC-1 electrode and AC electrode. Besides, the CV curves exhibit both double-layer capacitance and pseudocapacitance character at low scan rates. Besides, owing to both electrodes, there is no obvious transformation when the scan rates rise up to 80 mV s−1. The GCD curves of the cell are shown in Figure 7d, the specific capacitance of the cell is 59.51 F g−1 at 0.3 A g−1. Based on these data, the corresponding power density and energy density can be acquired, and the relation between power density and energy density is shown in the Ragone plot (Figure 7e). The asymmetric supercapacitor displays a high energy density 18.60 Wh kg−1 at a power density 225.03 W kg−1 and an energy density of 11.65 Wh kg−1 at a high power density of 2330.16 W kg−1. Besides, the asymmetric supercapacitor exhibits well cycling performance (Figure 7f). The specific capacitance can reserve 58.18% after cycling for 3000 cycles. The results further illustrate the potential of LDH/PC-1 composites applied for supercapacitors. The good electrochemical performance of the as-synthesized composites can be ascribed to the following reasons: (1) The NiMn-LDH exhibits good pseudocapacitive character; (2) PC is served as a framework to grow LDH nanosheets. An appropriate introduction of PC dramatically reduces the agglomeration and the dead surfaces of LDH, and increases its reactive area. Besides, the resulting LDH/PC-x composites possess open porous structure consists of interconnected nanosheets. While the porous structure makes it effective for ion transfer,
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the increase of electric conductivity, and further improving the electrochemical performance.
3. Conclusion In summary, we successfully synthesized a kind of rhombic dodecahedral NiMn-LDH/PC-x (x = 1, 2) composites through hydrothermal method. The introduction of PC hinders the agglomeration of LDH nanosheets obviously and further improves the electric conductivity of the sample. Therefore, LDH/PC-1 possesses 3D porous structure and enhanced electrochemical performance for supercapacitor. Besides, it shows high specific capacitance of 1634 F g−1 at the current density of 1 A g−1, and it exhibits well cycling stability, remaining 84.58% of original specific capacitance after 3000 cycles at 15 A g−1. Furthermore, the asymmetric supercapacitor assembled with LDH/PC-1 as anode and AC as cathode manifests high energy density of 18.60 Wh kg−1 at a power density of 225.03 W kg−1. Finally, our work provides a new thinking of matrix for synthesizing LDH composites, and the enhanced electrochemical performance of the as-prepared products shows great prospect for practical application in energy storage devices in ahead.
4. Experimental Section Preparation of ZIF-8: All materials were bought for use without any further purification. Synthesis of ZIF-8 was described in other reports.[60] 0.35 g Zn(CH3COO)2·2H2O was added into 50 mL methanol and then stirred to form solution A, and 0.526 g 2-methylimidazole (2-MeIM) was
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www.advancedsciencenews.com added to 50 mL methanol with stirring to form solution B, which was slowly injected into solution A drop by drop under continuous stirring for 1 h. Then the obtained solution was kept still at room temperature for 24 h. ZIF-8 was gathered by centrifugation, washed by methanol for three times and then acetone for once, and finally dried at 60 °C overnight in vacuum. Preparation of PC: The PC was prepared by carbonization of ZIF-8. ZIF-8 powder was placed in a porcelain boat in tubular furnace to carbonize at 1000 °C with Ar atmosphere for 4 h, and the temperature rate is 5 °C min−1. Then the obtained PC was treated with the 12 m HCl solution for 24 h at room temperature to remove ZnO intermediate product from ZIF-8 and probable inorganic impurities. The solution was filtered with inorganic nanofiltration membranes under vacuum in deionized water for three times. After drying in vacuum at 60 °C overnight, the final product was acquired. Pretreatment of PC: For the chemical oxidation method of PC, 2 g PC was refluxed in 200 mL KMnO4 (0.5 mol L−1) and NaOH (0.5 mol L−1) mixing solution at 100 °C for 1 h with continuously stirring. One hundred milliliters H2SO4 (1.5 mol L−1) and 20.8 g NaHSO3 were added to mixing solution to acidize reaction liquid and remove MnO2 generated from the process, respectively. The pretreated PC was collected after washing with deionized water and subsequently dried at 60 °C. Synthesis of Ni(OH)2, NiMn-LDH, NiMn-LDH/PC-1, and NiMn-LDH/ PC-2: The synthesis method of LDH was proposed in other work.[48] In short, 0.47 g NiCl2·6H2O, 0.08 g MnCl2 6H2O, and 0.56 g hexamethylenetetramine (HMT) were dissolved in 30 mL deionized water with constant stirring for 30 min, then the uniform green mixed solution was moved into 50 mL Teflon stainless steel autoclave and kept at 90 °C for 6 h in oven. After cooling down to room temperature, LDHs were collected through centrifugation at 8000 r min−1 for 5 min. Deionized water was used to remove impurities, and then the obtained sample was dried at 60 °C in vacuum overnight. For the pure Ni(OH)2 sample, 0.55 g NiCl2·6H2O and 0.56 g HMT were dissolved in 30 mL deionized water, and the subsequent reaction condition was the same as LDH. LDH/PC-x composites were synthesized the same way as LDH, diluting as-obtained PC in 30 mL deionized water before adding Ni and Mn precursors and HMT, the subsequent reaction condition was the same as LDH. The resulting products were, respectively, marked LDH/PC-x (x = 1, 2) according to the various mass of PC (0.004 and 0.01 g) added into deionized water. Based on the thermogravimetric (TG) results (Figure S5, Supporting Information), the contents of LDH and PC in the composites were obtained. In LDH/PC-1, the mass ratio of LDH and PC was about 4:1, while in the LD/PC-2, the mass ratio of LDH and PC was about 1:4. Sample Characterization: The XRD measurements were conducted on Rigaku D/MAX-2500 to investigate the crystal structure of the products. The morphology of the materials was examined through SEM (JSM7500F) and TEM (Tecnai G2 F20 S-TWIN). The surface areas and pore volumes were obtained using BET method. The XPS (Al-Ka X-rays) were carried out with a Thermo escalab 250Xi spectrometer to investigate the surface properties. EDS elemental mapping was tested on Tecnai G2 F20 S-TWIN. The TG analysis (TG) was tested on TG-DTA 6300 to investigate the content of PC in the composites. Electrode Preparation and Electrochemical Measurements: The electrochemical characterizations (CV curves, GCD, and EIS) of all samples were applied in three-electrode system. It contained a platinum plate, an Ag/AgCl electrode, LDH/PC-x electrode, and 6 m KOH solution, separately working as counter electrode, reference electrode, working electrode, and electrolyte. All measurements were operated on CHI-660e electrochemical workstation. The LDH/PC-x electrode was fabricated by filtering the dimethyl formamide (DMF) with active materials, acetylene black, and polyvinylidene fluoride (PVDF) (85 wt%:10 wt%:5 wt%), stirring for 5 h. Then the slurry was pressed on carbon paper, drying at 60 °C in vacuum overnight. In particular, EIS was conducted on 5 mV amplitude of AC voltage at open-circuit voltage, and the frequency range was 0.01–100 kHz. The specific capacitance of the samples was calculated on the basis of the GCD curves by formula (3)[18]
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C=
I × ∆t m × ∆V
(3)
where I is the discharging current (A), Δt is the discharging time (s), and m is the mass of active material in working electrode (g). The covered area of activated material was about 4 cm2, and the mass loading of LDH, LDH/PC-1, LDH/PC-2, and Ni(OH)2 was 4, 3.2, 3.7, and 6.6 mg cm−2, respectively. Fabrication of LDH/PC-1//AC Asymmetrical Supercapacitor Devices: The fabrication of an asymmetric supercapacitor was using LDH/PC-1 as anode and AC as cathode. The fabrication of cathode was the same as positive electrode, by filtering the DMF with 85 wt% AC, 10 wt% acetylene black, and 5 wt% PVDF for 5 h. The mixture was coated on the carbon paper to synthesize the cathode electrode, drying at 60 °C in vacuum overnight. The area of single electrode was 1.76 cm2 with a 3.3–5.2 mg cm−2 mass loading. Subsequently, the LDH/PC-1 and the AC were wetted with 6 m KOH and divided by a piece of glass fiber and pressed together. The energy density and power density of the asymmetric supercapacitor were calculated according to the formulas (4) and (5):[28] 1 E = C m∆V 2 2 P=
(4)
E ∆t
(5)
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the Beijing Natural Science Foundation (2172032) and the Beijing Nova Program (Grant No. Z161100004916061). The authors also whole heartedly thank the efforts taken by our group members.
Conflict of Interest The authors declare no conflict of interest.
Keywords asymmetric supercapacitors, NiMn-layered double hydroxides, porous carbon Received: July 29, 2017 Revised: August 30, 2017 Published online:
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Polyhedral-Like NiMn-Layered Double Hydroxide/Porous Carbon as Electrode for Enhanced Electrochemical Performance Supercapacitors Mei Yu,* Ruili Liu, Jianhua Liu, Songmei Li, and Yuxiao Ma Therefore, large surface area and high porous structure are essential for highperformance supercapacitors. Layered double hydroxides (LDHs) are kinds of classic pseudocapacitive materials with lamellar structure and large surface area. The special layered structure is favorable for redox reaction, making LDHs one of the most promising electrode materials in supercapacitors.[13–15] Zhang et al. fabricated porous CoAl-LDH/ graphene composites with a Nickel foam grid a serving as working electrode, and the material shows a specific capacitance of 479.2 F g−1 at 1 A g−1.[16] Similarly, NiAl-based LDH growing on Ni foam could achieve a specific capacitance of 795 F g−1 at 1 A g−1 current density.[17] Despite high electrochemical performance has been achieved, serious agglomeration and intrinsic poorly electric conductivity are still the problems to be solved. The insufficient above will reduce the rate capability and cycling stability of LDH when served as electrode materials for supercapacitors. The way to improve the electric conductivity of active materials is in situ growing LDH on carbon materials, like carbon nanotubes (CNTs), graphene, carbon fibers, porous carbon (PC), and so on.[18] Compared with carbon materials above, PC possesses low density, high surface area, high thermal and chemical stability, and low cost.[19] PC has broad applications in adsorption, catalyst, and energy storage. PC with well-defined morphology and high porous attracts more attentions. Highly PC can be synthesized through a series of methods like activation of carbon, carbonization of polymer materials, and templating methods with zeolite and mesoporous silica as precursors.[20–24] Metal– organic frameworks (MOFs) structured with nanoscaled cavities and open channels offer a special template for synthesis of nanoporous carbons via carbonization. Compared with the traditional inorganic templating method, using inorganic–organic MOFs as template is relative simple, friendly, and convenient. Because it can partially avoid to use toxic chemicals to remove the templates under wicked conditions.[25] Besides, the PC has been applied as electrode materials for supercapacitors with favorable electrochemical performance.[26] In this study, zeolitic imidazolate frameworks-8 (ZIF-8) (a kind of MOFs) was chosen as the precursor of PC, successfully synthesizing LDH/PC-x composites by hydrothermal
Polyhedral-like NiMn-layered double hydroxide/porous carbon (NiMn-LDH/ PC-x) composites are successfully synthesized by hydrothermal method (x = 1, 2 means different mass percent of porous carbon (PC) in composites). The NiMn-LDH/PC-1 composites possess specific capacitance 1634 F g−1 at a current density of 1 A g−1, and it is much better than that of pure LDH (1095 F g−1 at 1 A g−1). Besides, the sample can retain 84.58% of original capacitance after 3000 cycles at 15 A g−1. An asymmetric supercapacitor with NiMn-LDH/PC-1 as anode and activated carbon as cathode is fabricated, and the supercapacitor can achieve an energy density of 18.60 Wh kg−1 at a power density of 225.03 W kg−1. The enhanced electrochemical performance attributes to the high faradaic pseudocapacitance of NiMn-LDH, the introduction of PC, and the 3D porous structure of LDH/PC-1 composites. The introduction of PC hinders serious agglomeration of LDH and further accelerates ions transport. The encouraging results indicate that these materials are one of the most potential candidates for energy storage devices.
1. Introduction In order to meet the rapid increasing demands of electronic devices and hybrid electric vehicles, it is significant to develop high energy storage system. Supercapacitors or electrochemical capacitors (ECs) are deemed to play more and more essential roles in energy storage by virtue of their outstanding power density, long cycle lifetime, and high reliability.[1–5] Supercapacitors can be separated into two kinds of parts (electric double-layer capacitors (EDLCs) and pseudocapacitors) in line with the charge-storage mechanism.[6,7] Compared with EDLCs, pseudocapacitors possess high specific capacitance and high energy density through fast redox reactions, so redox-active materials possessing pseudocapacitance are highly expected for ECs owing to their high specific capacitance.[8–12] Besides, the capacitance reactions mainly occur on the interfaces between electrode materials and electrolyte.
Dr. M.Yu, R. Liu, Prof. J. Liu, Prof. S. Li, Y. Ma School of Materials Science and Engineering Beihang University Beijing 100191, China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201702616.
DOI: 10.1002/smll.201702616
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method. Polyhedral-like NiMn-LDH/PC composites were successfully obtained, and the electrochemical performance was tested in three-electrode system and full-cell device. First, in the three-electrode system with 6 m KOH electrolyte, the achieved NiMn-LDH/PC-1, LDH/PC-2, and pure LDH showed maximum specific capacitance of 1634, 1518, and 1095 F g−1 at a current density of 1 A g−1, respectively. Furthermore, an asymmetric supercapacitor with optimized NiMn-LDH/PC-1 composites as positive electrode and AC as negative electrode could achieve an energy density of 18.60 Wh kg−1 at a power density of 225.03 W kg−1. It is obvious that the introduction of PC improves their electrochemical performance. Therefore, the optimized LDH/PC-1 composites can be used as promising electrode materials in energy storage and transformation application.
2. Results and Discussion 2.1. Morphological and Structural Characterization The synthesis process of NiMn-LDH/PC is shown in Scheme 1, and the detail information is shown in the Experimental Section. After carbonized, PC keeps the original morphology of ZIF-8 very well, and possesses high porosity. After growing NiMn-LDH nanosheets, polyhedral-like structured NiMn-LDH/ PC-x composites are obtained. X-ray powder diffraction (XRD) measurements were applied to confirm the crystallographic structure of products. In Figure 1a, the diffraction peaks of as-prepared ZIF-8 are identical to simulated ZIF-8, indicating a successful preparation of ZIF-8. Besides, in Figure 1b, the typical diffraction peaks of a hydrotalcite-like structure can be observed clearly at 11.34°, 22.74°, 34.41° in LDH and LDH/PC-1. These peaks are properly agreed to previous literatures on NiMn-LDH and can be ascribed to (003), (006), and (012) planes, respectively,[18,27–29] investigating a successful synthesis of NiMn-LDH. Nevertheless, the diffraction peaks of PC cannot be found clearly in the XRD pattern of LDH/PC, probably because much too weak to be detected. The microstructure of products was investigated by scanning electron microscopy (SEM) and transmission electron microscope (TEM) techniques, and the results are shown in Figure 2. In Figure 2a,d, the pure LDH nanosheets aggregate together to form a large-size cluster, and the LDH nanosheets are crosslinked in a disordered way. Besides, the pure LDHs present dilapidated nanosheets structure with few open pores.
This disordered structure may inhibit ions transfer. Figure 2b shows the ZIF-8 with a regular rhombic dodecahedral structure, and the size is about 500 nm. After carbonized, the PC derived from ZIF-8 still retains its dodecahedral structure perfectly (Figure 2e), except a little diminution in size (around 300 nm). Furthermore, through an in situ growth method, equally distributed NiMn-LDH nanosheets are equally distributed covered on the framework of PC, and the nanosheets are crosslinking to each other to form a lot of mesoporous pores, accelerating the ions transport (Figure 2c,f). The polyhedral-like core–shell LDH/PC-1 is confirmed by TEM images (Figure 1g), in which the gauze-like NiMn-LDH nanosheets are uniformly grafted on dodecahedral PC. The high resolution transmission electron microscopy (HRTEM) image (Figure 2h) clearly displays an interplanar spacing of 3.314 Å, corresponding to the (015) plane of LDHs. The selected area electron diffraction (SAED) image (Figure 2i) further proves the crystalline structure of LDHs. The TEM image and relevant energy-dispersive X-ray spectrometry (EDS) elemental mapping of LDH/PC-1 (Figure 3a,b) unambiguously prove the core–shell structure of LDH/PC-1 with homogeneous distribution of Ni, Mn, O, and C elements. The X-ray photoelectron spectroscopy (XPS) was used to explore the valence state and composition information of LDH/PC-1. As presented in Figure 4, the survey XPS spectra (Figure 4a) show four elements (C, O, Ni, and Mn) in LDH/ PC and two elements (C and O) in PC. This result indicates that LDHs have been grown on PC. The C 1s XPS spectrum of PC possessing three distinguished peaks are well fitted to CC (284.8 eV), CO (286.5 eV), and OCO (288.86 eV) as depicted in Figure S1 in the Supporting Information.[30] The peaks located at 286.5 and 288.86 eV are ascribed to oxygencontaining functional groups, which will effectively provide anchoring sites for growth of inorganic nanoparticles during hydrothermal process.[31,32] In Ni 2p spectrum (Figure 4b), the Ni 2p3/2 (855.8 eV) and Ni 2p1/2 (873.3 eV) peaks coupled with two satellite bands (indicated as “Sat.”) suggest the existence of Ni2+ state.[33] In Mn 2p spectrum (Figure 4c), the major peaks for Mn 2p3/2 and Mn 2p1/2 stand at ≈643.5 and ≈653.0 eV, indicating the existence of Mn3+ in the materials.[33] The property of specific surface area and pore structure is essential for electrode materials in energy storage applications. Hence, N2 adsorption/desorption measurement was applied to characterize the surface area and porosity feature of LDH/PC-1 (Figure 4d), ZIF-8 and PC (Figure S2, Supporting Information). Figure 4d presents typical IV isotherms along with H3-type hysteresis loops (P/P0 > 0.4) for LDH/PC-1 and pure LDH,
Scheme 1. Growth and morphology schematic illustration of NiMn-LDH/PC-x.
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Figure 1. XRD spectra of a) ZIF-8. b) PC, LDH, and LDH/PC-1.
showing the existence of mesoporous structure. Moreover, the shape of the hysteresis loops proves that the particle aggregates to form slit-shape pores.[34] From the pore sizes distribution (Figure 4e), the LDH/PC-1 composites possess mesoporous at the range of 2–10 nm, and the pore size of LDH/PC-1 is a little larger than that of pure LDH. Large pore size is beneficial for ions transmission during electrochemical reaction process. Besides, the specific surface area and total pore volume of ZIF-8, PC, LDH/PC-1, and pure LDH are obtained through Brunauer–Emmett–Teller (BET) method, and the resulting data are listed in Table 1. On the one hand, the LDH/PC-1 composites display relative larger specific surface area (159.56 m2 g−1) and total pore volume (0.71 cm3 g−1) than that of pure LDH (124.20 m2 g−1) and (0.65 cm3 g−1), respectively, further demon strating that the introduction of PC inhibiting the agglomeration of LDH. On the other hand, the ZIF-8 possesses high specific surface area (1909.57 m2 g−1) and total pore volume (0.74 cm³ g−1), and after carbonization, the area still remains
1193.26 m2 g−1. Compared with the PC, the LDH/PC-1 composites exhibit relatively low specific surface area (159.56 m2 g−1), demonstrating that LDH nanosheets are grown on PC successfully once again. Moreover, the relatively high pore volume of LDH/PC composites is beneficial for kinetics of reaction and ions transport.
2.2. Electrochemical Performance Characterization 2.2.1. Three-Electrode System The electrochemical performance of LDH, LDH/PC-1, and LDH/PC-2 was investigated through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in three-electrode system including a Pt counter electrode, an Ag/AgCl reference electrode, and 6 m KOH electrolyte. Figure 5a presents a series
Figure 2. SEM images of a,d) pure LDH at different magnification. b) SEM image of ZIF-8. c,f) LDH/PC-1 at different magnification. e) SEM image of PC. TEM images of LDH/PC-1 at g) low magnification, h) high magnification. i) SAED images of LDH/PC-1.
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Figure 3. a) TEM image. b) EDS mapping of C, O, Mn, and Ni elements on local structure of LDH/PC-1.
of typical CV curves of LDH, LDH/PC-1, and LDH/PC-2 in a potential range −0.3 to 0.7 V at the scan rate of 10 mV s−1. All the curves show strong redox current peaks, coinciding to typical pseudocapacitive behavior of Ni2+/Ni3+ with the assistance of OH−, the reaction involved may be illustrated as:[35] Ni (OH)2 + OH− ↔ NiOOH + H2O + e −
(1)
It is quite clear that LDH/PC-1 composites present much higher redox activity than other samples, illustrating that LDH/ PC-1 composites possess the best electrochemical performance. The CV curves of NiMn-LDH and Ni(OH)2 were shown in Figure S3a in the Supporting Information. The oxidation peak
of NiMn-LDH shifts to the low potential, and this conclusion is agreed to other works.[36] Figure 5b manifests the charge– discharge curves of LDH, LDH/PC-x composites at 1 A g−1. It is obvious that the specific capacitance of LDH/PC-1, LDH/ PC-2, and LDH at 1 A g−1 is 1634, 1518, and 1095 F g−1. While the charge–discharge curves of LDH and Ni(OH)2 at 1 A g−1 were shown in Figure S3b in the Supporting Information. According to the charge–discharge curves of three materials, the specific capacitance of NiMn-LDH and Ni(OH)2 is 1095 and 857 F g−1, respectively. As the LDH/PC is a battery-type electrode,[37,38] its specific capacity (Cs) is calculated. The specific capacity of achieved NiMn-LDH/PC-1, LDH/PC-2, and pure LDH at 1 A g−1 is also calculated to investigate their electro-
Figure 4. a) XPS survey spectra of the as-obtained LDH/PC-1 and PC samples. b) Ni 2p, c) Mn 2p XPS spectrum of LDH/PC-1. d) N2 adsorption/ desorption isotherms and e) pore diameter distribution of pure LDH and LDH/PC-1 composites.
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www.advancedsciencenews.com Table 1. Specific surface areas and total pore volumes of ZIF-8, PC, and LDH/PC-1.
Specific surface area [m2 g−1] 3
−1
Total pore volume [cm g ]
MICRO
ZIF-8
PC
LDH
LDH/PC-1
1909.57
1193.26
124.20
159.56
0.74
0.58
0.65
0.717
chemical performance, and the value of them is 686.28, 637.56, and 459.9 C g−1, respectively. While relative specific capacity of LDH and Ni(OH)2 is 459.9, 308.5, and 421.8 C g−1, respectively. It is clear that NiMn-LDH possesses relative high specific capacitance and wide potential window, demonstrating the contribution of Mn3+ to capacitance and other electrochemical performance. The results above demonstrate that an introduction of PC can improve the electrochemical performance, and an appropriate content of PC can provide optimized nucleation and growing sites, leading to relatively better electrochemical performance. The LDH/PC-1 composites are elected as further studying object. Figure 5c shows the CV curves of LDH/PC-1 at various scan rates, and the redox peaks are clearly presented in the plot, demonstrating a representative faradic redox capacitive behavior. In addition, with the scan rate increasing, the anodic peaks shift a little to high potential zone while the cathodic peaks shift a little to low potential zone. Whereas the shape of CV curves is still well kept, illustrating a well rate capability. The specific capacitance of LDH/PC-1 is 1634, 1451, 1296, 1115, 1047, and 988 F g−1 at various current densities of 1, 2, 3, 5, 7, and 10 A g−1, respectively (Figure 5d). At the same time, the specific capacity of LDH/PC-1 at different current densities
of 1, 2, 3, 5, 7, and 10 A g−1 is 686.28, 609.42, 544.32, 468.3, 439.74, and 414.96 C g−1, respectively. EIS of LDH and LDH/PC-x composites was carried in the frequency range 0.01–100 kHz to examine the conductivity (Figure 6a). The resulting Nyquist plots in the Figure 6a consist of a straight line and a semicircle. The equivalent circuit (Figure 6a inset image) contains bulk resistance Rs, charge transfer resistance Rct, Warburg impedance W, pseudocapacitance Cp, and double-layer capacitance Cdl. In low frequency, the straight line with real axis (Zʹ) with an angle at about 45° is related to the Warburg impedance (W), and it shows the results of ion transmission are frequency dependent in the electrolyte.[39] According to the Nyquist plots in Figure 6a, the W value of LDH/PC-1 (0.21 Ω) is dramatically smaller than that of pure LDH (2.18 Ω) and a little smaller than that of LDH/PC-2 (0.25 Ω). In high frequency, the Rs value of LDH/PC-1 (6.20 Ω) is lower than that of LDH (9.42 Ω), LDH/PC-2 (8.23 Ω). According to the diameter of semicircle arc of EIS, the Rct value of LDH/ PC-1 (6.91 Ω) is lower than that of LDH (9.04 Ω), but a little bit higher than that of LDH/PC-2 (5.80 Ω).[40–42] The results above indicate that the LDH/PC-1 composites possess lower resistance and better electric conductivity than pure LDH and LDH/ PC-2, owing to the introduction of proper mass of PC to provide moderate nucleation sites and hinder the agglomeration of LDH nanosheets. In addition, the results of EIS measurement are consistent with former CV and GCD measurement, that is, LDH/PC-1 composites possess the best electrochemical performance. As we all know, the cycling stability is an important influence factor for energy storage devices in application. The cycling measurement of LDH/PC-1 composites was applied in
Figure 5. a) Cyclic voltammetry curves and b) discharge curves of LDH, LDH/PC-1, and LDH/PC-2 at the scan rate of 10 mV s−1 and the current density of 1 A g−1, respectively. c) CV curves and d) charge–discharge curves of LDH/PC-1 at various scan rates and current densities.
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Figure 6. a) Nyquist plots of the LDH, LDH/PC-1, and LDH/PC-2. b) Cycling performance of LDH/PC-1 at 15 A g−1.
three-electrode system, and the result was shown in Figure 6b. After 3000 cycles, the capacitance retention of LDH/PC-1 at 15 A g−1 can remain 84.58%. The good cycling stability is ascribed from 3D porous structure. As far as we know, the electrochemical performance of LDH/ PC-1 is comparable or superior to the results in relative literatures as listed in Table 2. From the Table 2, it is demonstrated that the material is a promising candidate for supercapacitors.
2.2.2. The Asymmetric Supercapacitor The electrochemical performance of the LDH/PC-1 composites was also investigated in a full-cell device, where LDH/PC-1 composites were served as the positive electrode, while AC was served as negative electrode (marked as LDH/PC-1//AC).
Figure 7a shows the schematic illustration of LDH/PC-1//AC asymmetric supercapacitor, and the mass of the positive electrode and negative electrode follows the Equation (2)[58,59] in order to coordinate the charge stores in both electrodes: m+ C− × ∆V− = m − C+ × ∆V+
(2)
In the equation, m+ represents the positive electrode mass, m− represents the negative electrode mass, C+ and C− are specific capacitance of anode and cathode, respectively, besides, ΔV+ and ΔV− are potential range of anode and cathode, respectively. Figure 7b shows CV plots of AC and LDH/PC-1 with potential windows of −1 to 0 V and −0.3 to 0.7 V, respectively. The charge–discharge curves of AC (Figure S4, Supporting Information) exhibit that the specific capacitance of
Table 2. Comparison of electrochemical performance of LDHs materials. Electrode materials
Electrolyte
Specific capacitance
NiMn-LDH/PC
6 M KOH
−1
1634 F g (1 A
MXene/NiAl-LDH
6 M KOH
1061 F g−1 (1 A g−1)
RGO@MgAl-LDH
1 M KOH
1334 F g−1 (1 A g−1)
Co–Fe-LDH@NiO
2 M KOH
903 F g−1 (1 A g−1) g−1
g−1)
60.5% at 10 A
52.4% at 10 A g−1
Reference This work [43] [44]
55.4% at 10 A g−1
[45]
69.4% at 10 A g−1
[46]
42.7% at 10 A g−1
[47]
g−1
[48]
2 M KOH
1268 F
CoAl-LDH nanosheets/rGO
2 M KOH
1296 F g−1 (1 A g−1) g−1
g−1
g−1)
NiMn-LDH/CNTs/rGO
(1 A
Rate capability
g−1)
Silver nanowire@hierarchical NiAl-LDH
6 M KOH
1247 F
Core–Shell NiAl-LDH
1 M KOH
735 F g−1 (2 A g−1)
75.0% at 25 A g−1
[49]
CoAl-LDH/PEDOT/core–shell nanoplatelet
6 M KOH
672 F g−1 (1 A g−1)
62.8% at 40 A g−1
[50]
g−1
(1 A
g−1)
60.9% at 10 A
g−1
[51]
NiMn-LDH/rGO
2 M KOH
Glucose-intercalated NiMn-LDH
6 M KOH
1464 F g−1 (0.5 A g−1)
59.4% at 10 A g−1
[29]
NiMn-layered double oxides arrays grown on GOS
1 M KOH
1648 F g−1 (0.5 A g−1)
75.9% at 10 A g−1
[52]
1635 F
g−1
(1 A
g−1)
71.0% at 10 A
g−1
[53]
NiMn-LDH/rGO
3 M KOH
NiCo2S4 nanotube@NiMn-LDH arrays/3D graphene sponge
6 M KOH
1740 mF cm−2 (1 mA cm−2)
72.9% at 10 mA cm−2
[54]
Ni–Co-LDH/graphene composites
1 M KOH
217.8 mF cm−2 (2 mA cm−2)
1250 F
g−1
(1 A
75.0% at 16 mA cm−2
[55]
g−1)
40.4% at 5 A g−1
[56]
52.4% at 10 A g−1
[27]
NiO/NiMn-LDH nanosheet array on Ni foam
3 M KOH
510 F
Colloidal NiMn-LDH nanosheets
1 M KOH
769 F g−1 (1 A g−1)
(1 A
g−1
g−1)
Sulfidation of NiMn-LDHs/graphene oxide
2 M KOH
NiMn-LDH/CNTs
1 M KOH
2960 F g−1 (1.5 A g−1)
NiMn-LDH/carbon cloth
PVA/LiCl
2239 F g−1 (5 mA cm−2)
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2247 F
36.0% at 5 A
(1 A
g−1
[18]
79.5% at 30 A g−1
[34]
74.4% at 10 A
[57]
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Figure 7. a) Schematic interpretation of the asymmetric supercapacitor device. b) CV curves of AC and LDH/PC-1 at scan rates of 10 mV s−1. c) CV curves and d) charge–discharge curves of LDH/PC-1//AC asymmetric supercapacitor at various scan rates and current density, respectively. e) Ragone plot of the asymmetric supercapacitor. f) Cycling performance of asymmetric supercapacitor.
AC is 106.34 F g−1. According to Equation (2), the mass of AC and LDH/PC-1 is about 9.2 mg and 1.3 mg, respectively. As presented in the Figure 7c, CV curves of the cell are carried in the potential window 0–1.5 V at various scan rates, and the extensive voltage of cell is ascribed to the combination of LDH/PC-1 electrode and AC electrode. Besides, the CV curves exhibit both double-layer capacitance and pseudocapacitance character at low scan rates. Besides, owing to both electrodes, there is no obvious transformation when the scan rates rise up to 80 mV s−1. The GCD curves of the cell are shown in Figure 7d, the specific capacitance of the cell is 59.51 F g−1 at 0.3 A g−1. Based on these data, the corresponding power density and energy density can be acquired, and the relation between power density and energy density is shown in the Ragone plot (Figure 7e). The asymmetric supercapacitor displays a high energy density 18.60 Wh kg−1 at a power density 225.03 W kg−1 and an energy density of 11.65 Wh kg−1 at a high power density of 2330.16 W kg−1. Besides, the asymmetric supercapacitor exhibits well cycling performance (Figure 7f). The specific capacitance can reserve 58.18% after cycling for 3000 cycles. The results further illustrate the potential of LDH/PC-1 composites applied for supercapacitors. The good electrochemical performance of the as-synthesized composites can be ascribed to the following reasons: (1) The NiMn-LDH exhibits good pseudocapacitive character; (2) PC is served as a framework to grow LDH nanosheets. An appropriate introduction of PC dramatically reduces the agglomeration and the dead surfaces of LDH, and increases its reactive area. Besides, the resulting LDH/PC-x composites possess open porous structure consists of interconnected nanosheets. While the porous structure makes it effective for ion transfer,
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the increase of electric conductivity, and further improving the electrochemical performance.
3. Conclusion In summary, we successfully synthesized a kind of rhombic dodecahedral NiMn-LDH/PC-x (x = 1, 2) composites through hydrothermal method. The introduction of PC hinders the agglomeration of LDH nanosheets obviously and further improves the electric conductivity of the sample. Therefore, LDH/PC-1 possesses 3D porous structure and enhanced electrochemical performance for supercapacitor. Besides, it shows high specific capacitance of 1634 F g−1 at the current density of 1 A g−1, and it exhibits well cycling stability, remaining 84.58% of original specific capacitance after 3000 cycles at 15 A g−1. Furthermore, the asymmetric supercapacitor assembled with LDH/PC-1 as anode and AC as cathode manifests high energy density of 18.60 Wh kg−1 at a power density of 225.03 W kg−1. Finally, our work provides a new thinking of matrix for synthesizing LDH composites, and the enhanced electrochemical performance of the as-prepared products shows great prospect for practical application in energy storage devices in ahead.
4. Experimental Section Preparation of ZIF-8: All materials were bought for use without any further purification. Synthesis of ZIF-8 was described in other reports.[60] 0.35 g Zn(CH3COO)2·2H2O was added into 50 mL methanol and then stirred to form solution A, and 0.526 g 2-methylimidazole (2-MeIM) was
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www.advancedsciencenews.com added to 50 mL methanol with stirring to form solution B, which was slowly injected into solution A drop by drop under continuous stirring for 1 h. Then the obtained solution was kept still at room temperature for 24 h. ZIF-8 was gathered by centrifugation, washed by methanol for three times and then acetone for once, and finally dried at 60 °C overnight in vacuum. Preparation of PC: The PC was prepared by carbonization of ZIF-8. ZIF-8 powder was placed in a porcelain boat in tubular furnace to carbonize at 1000 °C with Ar atmosphere for 4 h, and the temperature rate is 5 °C min−1. Then the obtained PC was treated with the 12 m HCl solution for 24 h at room temperature to remove ZnO intermediate product from ZIF-8 and probable inorganic impurities. The solution was filtered with inorganic nanofiltration membranes under vacuum in deionized water for three times. After drying in vacuum at 60 °C overnight, the final product was acquired. Pretreatment of PC: For the chemical oxidation method of PC, 2 g PC was refluxed in 200 mL KMnO4 (0.5 mol L−1) and NaOH (0.5 mol L−1) mixing solution at 100 °C for 1 h with continuously stirring. One hundred milliliters H2SO4 (1.5 mol L−1) and 20.8 g NaHSO3 were added to mixing solution to acidize reaction liquid and remove MnO2 generated from the process, respectively. The pretreated PC was collected after washing with deionized water and subsequently dried at 60 °C. Synthesis of Ni(OH)2, NiMn-LDH, NiMn-LDH/PC-1, and NiMn-LDH/ PC-2: The synthesis method of LDH was proposed in other work.[48] In short, 0.47 g NiCl2·6H2O, 0.08 g MnCl2 6H2O, and 0.56 g hexamethylenetetramine (HMT) were dissolved in 30 mL deionized water with constant stirring for 30 min, then the uniform green mixed solution was moved into 50 mL Teflon stainless steel autoclave and kept at 90 °C for 6 h in oven. After cooling down to room temperature, LDHs were collected through centrifugation at 8000 r min−1 for 5 min. Deionized water was used to remove impurities, and then the obtained sample was dried at 60 °C in vacuum overnight. For the pure Ni(OH)2 sample, 0.55 g NiCl2·6H2O and 0.56 g HMT were dissolved in 30 mL deionized water, and the subsequent reaction condition was the same as LDH. LDH/PC-x composites were synthesized the same way as LDH, diluting as-obtained PC in 30 mL deionized water before adding Ni and Mn precursors and HMT, the subsequent reaction condition was the same as LDH. The resulting products were, respectively, marked LDH/PC-x (x = 1, 2) according to the various mass of PC (0.004 and 0.01 g) added into deionized water. Based on the thermogravimetric (TG) results (Figure S5, Supporting Information), the contents of LDH and PC in the composites were obtained. In LDH/PC-1, the mass ratio of LDH and PC was about 4:1, while in the LD/PC-2, the mass ratio of LDH and PC was about 1:4. Sample Characterization: The XRD measurements were conducted on Rigaku D/MAX-2500 to investigate the crystal structure of the products. The morphology of the materials was examined through SEM (JSM7500F) and TEM (Tecnai G2 F20 S-TWIN). The surface areas and pore volumes were obtained using BET method. The XPS (Al-Ka X-rays) were carried out with a Thermo escalab 250Xi spectrometer to investigate the surface properties. EDS elemental mapping was tested on Tecnai G2 F20 S-TWIN. The TG analysis (TG) was tested on TG-DTA 6300 to investigate the content of PC in the composites. Electrode Preparation and Electrochemical Measurements: The electrochemical characterizations (CV curves, GCD, and EIS) of all samples were applied in three-electrode system. It contained a platinum plate, an Ag/AgCl electrode, LDH/PC-x electrode, and 6 m KOH solution, separately working as counter electrode, reference electrode, working electrode, and electrolyte. All measurements were operated on CHI-660e electrochemical workstation. The LDH/PC-x electrode was fabricated by filtering the dimethyl formamide (DMF) with active materials, acetylene black, and polyvinylidene fluoride (PVDF) (85 wt%:10 wt%:5 wt%), stirring for 5 h. Then the slurry was pressed on carbon paper, drying at 60 °C in vacuum overnight. In particular, EIS was conducted on 5 mV amplitude of AC voltage at open-circuit voltage, and the frequency range was 0.01–100 kHz. The specific capacitance of the samples was calculated on the basis of the GCD curves by formula (3)[18]
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C=
I × ∆t m × ∆V
(3)
where I is the discharging current (A), Δt is the discharging time (s), and m is the mass of active material in working electrode (g). The covered area of activated material was about 4 cm2, and the mass loading of LDH, LDH/PC-1, LDH/PC-2, and Ni(OH)2 was 4, 3.2, 3.7, and 6.6 mg cm−2, respectively. Fabrication of LDH/PC-1//AC Asymmetrical Supercapacitor Devices: The fabrication of an asymmetric supercapacitor was using LDH/PC-1 as anode and AC as cathode. The fabrication of cathode was the same as positive electrode, by filtering the DMF with 85 wt% AC, 10 wt% acetylene black, and 5 wt% PVDF for 5 h. The mixture was coated on the carbon paper to synthesize the cathode electrode, drying at 60 °C in vacuum overnight. The area of single electrode was 1.76 cm2 with a 3.3–5.2 mg cm−2 mass loading. Subsequently, the LDH/PC-1 and the AC were wetted with 6 m KOH and divided by a piece of glass fiber and pressed together. The energy density and power density of the asymmetric supercapacitor were calculated according to the formulas (4) and (5):[28] 1 E = C m∆V 2 2 P=
(4)
E ∆t
(5)
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the Beijing Natural Science Foundation (2172032) and the Beijing Nova Program (Grant No. Z161100004916061). The authors also whole heartedly thank the efforts taken by our group members.
Conflict of Interest The authors declare no conflict of interest.
Keywords asymmetric supercapacitors, NiMn-layered double hydroxides, porous carbon Received: July 29, 2017 Revised: August 30, 2017 Published online:
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