Hierarchical ultrathin rolled-up Co(OH)(CO₃)_0.5 films assembled on Ni_0.25Co_0.75S_x nanosheets for enhanced supercapacitive performance

Abstract

A hierarchical nano-array of ultrathin rolled-up Co(OH)(CO₃)_0.5 films assembled on Ni_0.25Co_0.75S_x nanosheet arrays has been prepared for the construction of a supercapacitive electrode. This hierarchical arrayed structure has a high specific capacitance of 1710 F g⁻¹ at 5 mA cm⁻² and good rate capability.

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The serious contradiction between energy demand and environmental protection in recent years has led to urgent needs for the development of clean energy. In the past several years, scientists have made many efforts to develop new materials, such as supercapacitors,^1–3 solar cells,^4,5 and water-splitting photocatalysts, with which clean energy conversion or high efficiency storage could be sustained.⁶ Among them, supercapacitors with high power density, short charge–discharge time, long cycle life and low maintenance costs, have attracted a lot of attention.

Transitional metal oxides/hydroxides, such as Co(OH)₂,^1,7,8 Co₃O₄,^9,10 Ni(OH)₂,^11–13 NiO,^14–17 MnO₂,^18,19 and their composites,^2,20 have been considered one of the best candidates for pseudocapacitive electrode materials because of their high theoretical supercapacitive (SC) values, low toxicity, and great flexibility in structural and morphological design. In particular, hierarchical arrayed structures composed of transitional metal compounds have been widely researched in energy storage and conversion fields²¹ due to their high specific surface areas, improved permeability and more active surface sites, which could bring about faster electron transport ability and ionic permeability. Recently, many studies have reported hierarchical arrayed structures, showing high specific capacitance, good rate capability, and excellent cycling stability. Sun and coworkers have reported hierarchical Ni_0.25Co_0.75(OH)₂ nanowire@nanoplatelet nanowire arrays showing a 7 times larger areal capacitance (9.59 F cm⁻², specific capacitance of 928.4 F g⁻¹ at 5 mA cm⁻²) and better rate capability than the precursive Ni_0.5Co_1.5(OH)₂CO₃ nanowire arrays.²² Sun et al. have also illustrated that hierarchical Co₃O₄@Ni–Co–O nanosheets–nanorod arrays on Ni foam with high specific capacitance of 2098 F g⁻¹, give specific capacitances per area as high as ∼25 F cm⁻² at a current density of 5 mA cm⁻², much higher than those of pure Co₃O₄ nanosheet arrays on Ni foam.²³ Li et al. have demonstrated that hierarchical Mo-decorated Co₃O₄ nanowire arrays on nickel foam substrates provide an extraordinarily high area capacitance of 3.5 F cm⁻² at a current density of 17 mA cm⁻² (∼2000 F g⁻¹ at a current density of 10 A g⁻¹), compared with a bare Co₃O₄ nanowire array electrode with 2.2 F cm⁻² at 17 mA cm⁻² (∼1257 F g⁻¹ at 10 A g⁻¹).²¹ Liu et al. have reported the synthesis of a hierarchical structure of MnO₂–NiO nanoflake-assembled tubular arrays that exhibited better rate performance and cycle life than a pure MnO₂ array.²⁴ Huang et al. have indicated nickel–cobalt hydroxide nanosheets coated on NiCo₂O₄ nanowires grown on carbon fiber paper possesses high rate capability and excellent cycling stability.²⁵ However, although the active materials have high intrinsic supercapacitive values, the poor conductivity always hinders their actual performance. Therefore, in hierarchical arrayed structures, structural design of both supercapacitance active materials and substrates simultaneously is of great importance for a high supercapacitive performance.

Herein, a novel hierarchical nanostructure Co(OH)(CO₃)_0.5@Ni_0.25Co_0.75S_x array was designed, where Ni_0.25Co_0.75S_x nanosheet arrays were grown on a Ni foam as backbone materials, which were proved to be better than their oxides and hydroxides because of their improved conductivity. Moreover, ultrathin rolled-up Co(OH)(CO₃)_0.5 films assembled on the surface of the Ni_0.25Co_0.75S_x nanosheet arrays as high pseudocapacitive materials.^26–29 Because of the unique advantages of hierarchical arrayed structures, such as high surface areas, good conductivity and direct growth on conductive substrates, the nanomaterials indicated an extraordinarily high specific capacitance and good rate capability.

Fig. 1 illustrates the synthesis procedures of the hierarchical arrayed structure. The Ni_0.25Co_0.75(OH)₂ nanosheet arrays were first synthesized on a nickel foam according to literatures (named H-NSAs).^23,30 Then, by immersing the as-prepared H-NSAs in a highly concentrated Na₂S solution, the nanosheets were converted to Ni_0.25Co_0.75S_x nanosheets at room temperature (named S-NSAs).³¹ At last, the ultrathin rolled-up Co(OH)(CO₃)_0.5 films were grown on the surface of S-NSAs in situ (named UTNFSAs). The details are presented in the ESI.†

Fig. 1 Schematic illustration of the fabrication procedures of the hierarchical arrayed structure of Co(OH)(CO₃)_0.5@Co_0.75Ni_0.25S_x.

The H-NSAs were first synthesized and the phase and morphology characterizations are presented in Fig. 1, S1 and S2,† from which it can be verified that H-NSAs present the Ni_0.25Co_0.75(OH)₂ nanosheet arrays grown on a Ni foam (details of proving procedures can be seen in the ESI†).

Then, the S-NSAs were fabricated by sulfurizing the H-NSAs. Because the solubility product constant (K_sp) of cobalt sulphide and nickel sulphide are near to equal, and compared with the K_sp of their respective hydroxides of cobalt and nickel, the K_sp of their respective sulphides is lower by six orders of magnitude, the cobalt sulphide or nickel sulphide can be achieved simultaneously by immersing the as-prepared Ni_0.25Co_0.75(OH)₂ nanosheets into a Na₂S solution at room temperature. After this process, the colour of the purple H-NSAs changed into black, which indicated that the Ni_0.25Co_0.75(OH)₂ nanosheets were sulfurized (Fig. 3A). Fig. 3B shows the SEM image of S-NSAs, which is composed by uniform hexagonal nanosheets with the thickness of ∼150 nm and the length of ∼6 μm on the Ni foam. In the XRD patterns (Fig. 2), when the curves were normalized according to the peak of Ni (111), the peaks of Ni_0.25Co_0.75(OH)₂ in S-NSAs were much lower than those in H-NSAs, suggesting that the quantity of unsulfurized Ni_0.25Co_0.75(OH)₂ included in S-NSAs was very low; therefore, they were neglected in the S-NSAs in order to simplify the structure. In addition, the XRD pattern of S-NSAs (the curve b in Fig. 2) does not have other extra diffraction peak appearances, indicating that the metal sulphide could be amorphous. Therefore, the nanosheets scraped from S-NSAs were also characterized by SEM element mapping, and the sulphur element was equally distributed on the nanosheet (Fig. S3†). Moreover, the XPS analysis can also prove that there is elemental sulphur in S-NSAs (Fig. S4†). It can be inferred that the S²⁻ anion has indeed reacted with the Ni_0.25Co_0.75(OH)₂ nanosheets to form cobalt and nickel sulphide. As previously mentioned, the sulphide should exist with the amorphous formation. According to literature,³² cobalt sulphide contains several phases, and the valence of cobalt in the compound may be 2, 3 or 4; thus, the sulphide in our experiment is named Ni_0.25Co_0.75S_x. Because cobalt and nickel sulphides show high conductivity and pseudocapacitive performance as previously reported,^{26,27,33–35} the Ni_0.25Co_0.75S_x nanosheet arrays on Ni foam are suitable backbone materials for docking electroactive materials.

Fig. 2 XRD patterns of H-NSAs (a), S-NSAs (b) and UTNFSAs (c).

Fig. 3 (A) Digital images of (a) H-NSAs, (b) S-NSAs and (c) UTNFSAs. (B) SEM image of S-NSAs. (C) Low, (D) middle and (E) high magnification SEM images of UTNFSAs. Typical rolled-up Co(CO₃)_0.5(OH) films in (E) FE-SEM image and (F) low magnification of TEM image. (G) TEM image of UTNFSAs, the yellow arrows in (G) point toward the rolled-up edges. The inset is the SAED of the area surrounded by the yellow circle in (H). (I) and (J) HRTEM image of the yellow boxed area in (G).

The ultrathin rolled-up Co(OH)(CO₃)_0.5 films were grown on S-NSAs by the hydrothermal method, and the SEM image of the structure is shown in Fig. 3C. Fig. 3D shows the SEM image of a typical hierarchical arrayed structure, and the branch on the nanosheet is composed of ultrathin rolled-up films, as evidenced by the high magnification SEM image shown in Fig. 3E. The thickness of these nanofilms is ∼2.7 nm and the length is ∼1 μm, which can be measured from Fig. 3E–G. The selected-area electron diffraction (SAED) pattern, detected from the area surrounded by the yellow circle covering the ultrathin films in Fig. 3H, shows well-defined diffraction rings, which coincidently match the (300), (221), (340) and (142) diffraction peaks of Co(OH)(CO₃)_0.5 (JCPDF: 48-0083). High-resolution TEM (Fig. 3I and J) revealed that the ultrathin films had reasonable crystallinity with lattice spacings of 0.205 nm, 0.194 nm and 0.445 nm, which corresponded to the interplanar spacings of Co(OH)(CO₃)_0.5 (050), (340) and (001), respectively. The XRD pattern of UTNFSAs is no different from that of S-NSAs (the curve c and b in Fig. 2), which could be ascribed to the small quantity of ultrathin rolled-up films added in UTNFSAs; thus, the UTNFSAs diffraction peaks are too weak. To further confirm the result, the deposition, which was obtained from the solution after the hydrothermal process for growing the ultrathin rolled-up films, was characterized by XRD (Fig. S5†), and its phase could also be indexed to the Co(OH)(CO₃)_0.5 (JCPDF: 48-0083). The above results suggest that the ultrathin films were indeed Co(OH)(CO₃)_0.5 crystals. The formation of ultrathin rolled-up Co(OH)(CO₃)_0.5 films may be related to the low reacting temperature and long time in the hydrothermal process.³⁶ Since the thickness of the as-produced Co(OH)(CO₃)_0.5 films is really reduced, these films rolled-up in order to decrease the high surface energy.

This hierarchical arrayed structure, UTNFSAs, can be divided into two parts for the backbone materials, S-NSAs, and the ultrathin rolled-up Co(OH)(CO₃)_0.5 films. In backbone materials, the Ni_0.25Co_0.75S_x nanosheet arrays not only serve as electroactive materials for a pseudocapacitor, but also supply larger surface area for scaffold electroactive materials; moreover, the ultrathin rolled-up Co(OH)(CO₃)_0.5 films for high pseudocapacitive materials were grown on the surface of S-NSAs. The integrated electrode brings about high supercapacitive performance, which was proved by the subsequent experiment shown in Fig. 4. The specific capacitance of the electrode is as high as 1710 F g⁻¹ at 5 mA cm⁻² with a high areal capacitance of 5.9 F cm⁻². The good electrochemical performance of the UTNFSAs could be related to both the backbone and the ultrathin films. The novel backbone provides large area for loading electroactive materials and high specific capacitance (372 F g⁻¹ or 1.4 F cm⁻² at 5 mA cm⁻²), and the ultrathin films offer larger reaction areas leading to higher utility. In a word, the enhanced supercapacitive performance of a hierarchical arrayed structure can be attributed to the unique hierarchical arrayed structures. It is also expected that the designation of the hierarchical arrayed structures can be easily extended to other fields related to energy storage or conversion.

Fig. 4 Electrochemical properties of the UTNFSAs. (A) CV curves at the scanning rates of 1 mV s⁻¹, 5 mV s⁻¹ and 10 mV s⁻¹. (B) Galvanostatic charge–discharge curves at current densities of 5 mA cm⁻², 10 mA cm⁻², 20 mA cm⁻² and 30 mA cm⁻². (C) Cycling stability of UTNFSAs at 30 mA cm⁻². (D) Comparison of the charge and discharge curves of UTNFSAs at current density 5 mA cm⁻² after and before the cycling test. (E) Specific capacitance versus charge and discharge current densities. (F) Impedance plots of the H-NSAs (a), S-NSAs (b) and UTNFSAs (c) measured at an open circuit potential.

To investigate the charge storage mechanism and electrochemical performance of the as-designed hierarchical arrayed structure, cyclic voltammetry (CV) and chronopotentiometry were measured in an alkaline electrolyte with a three-electrode system. Fig. 4A shows the CV curve of the UTNFSAs at scan rates of 1, 5 and 10 mV s⁻¹ in the potential range of −0.1–0.4 V for each curve, and a couple of redox peaks that correspond to the reversible reactions between Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺/Co⁴⁺ associated with OH⁻ anions indicate a typical pseudocapacitive behavior arising from the reversible faradaic process.^20,23,37 Increasing of the scan rate, the cathodic peak potential shifts to a more negative position, which is attributed to the polarization effect of electrode.³⁸ Fig. 4B shows the galvanostatic charge–discharge curve for UTNFSAs at different current densities (5–30 mA cm⁻²), during which the symmetric triangular shape with more well-defined plateaus suggests the good pseudocapacitive behaviours. Notably, the specific capacitance can be calculated to be as extremely high as 1710 F g⁻¹ (5.9 F cm⁻²) at a low current density of 5 mA cm⁻². Moreover, the hierarchical UTNFSAs also exhibit a good rate capacitance (64.4%, at 30 mA cm⁻²). Furthermore, the UTNFSAs retain 80% of the initial specific capacitance value at a charge–discharge current density of 30 mA cm⁻² after 1000 repetitive cycles (Fig. 4C), whereas 85% is preserved at a lower current density of 5 mA cm⁻² (Fig. 4D).

In order to pursue the origin of this fantastic hierarchical arrayed structure (UTNFSAs) bringing such high specific capacitance, similar performance tests were also carried out for H-NSAs and S-NSAs (the results are shown in Fig. 4E and all of the calculated values are also summarized in Table S1†). The specific capacitance of all three samples at different current densities is calculated according to the charge–discharge curves, respectively, as shown in Fig. 4E. It can be clearly seen that the UTNFSAs exhibits the largest specific capacitance (1710 F g⁻¹ at 5 mA cm⁻²), while the other two samples (H-NSAs and S-NSAs) showed just ∼1/13 and ∼1/4 of the SC UTNFSAs value, respectively, indicating that the existence of ultrathin rolled-up Co(OH)(CO₃)_0.5 films is significant and beneficial for the enhanced specific capacitance of the UTNFSAs nanostructure. Moreover, it is noteworthy that the integrated electrode of UTNFSAs maintains 64.4% (1101 F g⁻¹ or 3.8 F cm⁻²) of the specific capacitance value at a high current density (30 mA cm⁻²), while the electrode of H-NSAs maintains 25.4%, and that of S-NSAs 58.8%, demonstrating that the hierarchical structure of UTNFSAs owns higher rate capability than pure Ni_0.25Co_0.75(OH)₂ nanosheet arrays and Ni_0.25Co_0.75S_x nanosheet arrays.

Furthermore, comparing the pure Co₃O₄ nanosheet arrays (390 F g⁻¹), Co₃O₄ nanowire arrays (751 F g⁻¹),³⁹ Co₃O₄ nanosheet arrays (1081 F g⁻¹),³⁵ and even hierarchical Ni_0.25Co_0.75(OH)₂ nanoarrays (928.4 F g⁻¹ at 5 mA cm⁻²)²² from previous reports, the specific capacitance of UTNFSAs is even higher. Moreover, the rate capacity is also better than that of the foregoing pure Co₃O₄ nanosheet arrays (58%) and Co₃O₄ nanowire arrays (42%) at a current density of 30 mA cm⁻² in the report.³⁹ Therefore, we can easily conclude that the reason for the high specific capacitance and rate capability of the UTNFSAs electrode is that the Co(OH)(CO₃)_0.5 films of ultrathin nanostructure and regular channels with hierarchical arrayed structure lead to achieving a sufficient faradaic reaction. Moreover, it could be inferred that a larger specific capacitance value can be obtain, if the electrode with only ultrathin rolled-up Co(OH)(CO₃)_0.5 films as active material was fabricated as electrode for supercapacitors. To the best of our knowledge, this is first time that the ultrathin Co(OH)(CO₃)_0.5 films structure was reported, which might enlighten the researchers in this field of study.

To rationally prove the hierarchical arrayed structure designed, electrochemical impedance spectroscopy (Fig. 4F) was also performed for the three samples. From the intersection value of the real axis in high frequency, S-NSAs show smaller ohmic resistance (1.3 Ω) than H-NSAs (2.2 Ω), indicating that the sulfurizing process is efficient to increase the electrode conductivity, which suggests that S-NSAs are better than H-NSAs as backbone materials. However, unfortunately, UTNFSAs exhibit the largest resistance (2.5 Ω), which might be a response to the intrinsic resistance of ultrathin rolled-up films and the contact resistance between ultrathin Co(OH)(CO₃)_0.5 films and S-NSAs. Significantly, the UTNFSAs show the smallest semicircle and the most vertical line in low frequency, demonstrating the fast charge transfer process and the high charge store capability could be achieved by the construction of the hierarchical arrayed structure.

The above results demonstrate that the enhanced capacitive performance of UTNFSAs origins from the existence of ultrathin rolled-up Co(OH)(CO₃)_0.5 films and the reasonable engineering of hierarchical arrayed structure. The ultrathin rolled-up Co(OH)(CO₃)_0.5 films offer large contact areas with the electrolyte and a short distance for diffusion for the OH⁻ ion in the internal active materials, which make the more electroactive materials available for the redox reaction. On the other hand, the hierarchical arrayed structure enhanced porosity and channels facilitate the transfer and diffusion of both ions and the electrolyte, leading to a fast redox reaction and good rate capacity. Therefore, combining the two factors, it is easily understood that the UTNFSAs electrode achieved high capacitive performance.

Conclusions

In summary, we have successfully designed and prepared a novel hierarchical arrayed structure, in which Ni_0.25Co_0.75S_x nanosheet arrays stand on the surfaces of Ni foam and are covered with ultrathin rolled-up Co(OH)(CO₃)_0.5 nanofilms. In this unique nanoarchitecture, the Ni_0.25Co_0.75S_x nanosheet arrays not only supply large surface areas for loading a large amount of electroactive materials, but also serve as a high pseudocapacitive material by themselves. This fantastic structure as electrode for supercapacitor applications brings about an excellent supercapacitive performance, which is compared with that of the prepared Ni_0.25Co_0.75(OH)₂ nanosheet arrays and Ni_0.25Co_0.75S_x nanosheet arrays, respectively. The ultrahigh capacitive properties are attributed to the existence of ultrathin rolled-up Co(OH)(CO₃)_0.5 nanofilms and the reasonable hierarchical nanostructure. The designed hierarchical architecture may also be applicable in other energy storage and conversion fields like lithium ion batteries, water splitting cells, and photodetectors.

Acknowledgements

This work was financially supported by the National 973 Project of China (2015CB654902), the Chinese National Natural Science Foundation (11374174, 51390471 and 51102145), the Foundation for the Author of National Excellent Doctoral Dissertation (201141), and the National Program for Thousand Young Talents of China. This work made use of the resources of Beijing National Center for Electron Microscopy and Tsinghua National Laboratory for Information Science and Technology.

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Footnotes

† Electronic supplementary information (ESI) available: The EDS mapping and XPS of S-NSAs, EDS and SEM image of H-NSAs, and details about experiments and calculations can be obtained. See DOI: 10.1039/c4ra09248a

‡ These authors contributed equally to this work.

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