Partial Ion-Exchange of Nickel-Sulfide-Derived Electrodes for High

May 9, 2014 - ABSTRACT: A novel method to adjust the composition of a material while maintaining its morphology was described in this study. Nickel su...
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Partial Ion-Exchange of Nickel-Sulfide-Derived Electrodes for High Performance Supercapacitors Wutao Wei,† Liwei Mi,*,‡,§ Yang Gao,† Zhi Zheng,§ Weihua Chen,*,† and Xinxin Guan*,† †

College of Chemistry and Molecular Engineering, Zhengzhou University, Henan 450001, China Center for Advanced Materials Research, Zhongyuan University of Technology, Henan 450007, China § Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, Institute of Surface Micro and Nano Materials, Xuchang University, Henan 461000, China ‡

S Supporting Information *

ABSTRACT: A novel method to adjust the composition of a material while maintaining its morphology was described in this study. Nickel sulfide, the material investigated in this work, was found to be useful as a high surface area electrode material for supercapacitor applications. First, a nest-like Ni3S2@NiS composite electrode with 1D nanorod as structural unit was synthesized by simultaneously using Ni foam as template and Ni as a source through a one-step in situ growth method. Co and Se ions, which respectively acted as beneficial cation and anion, were successfully introduced into the nest-like Ni3S2@NiS material, resulting in the formation of Ni3S2@Co9S8 and NiS@NiSe2 composite electrodes with structures similar to those of the parent materials. The material structure was virtually retained and single-crystal-to-single-crystal transformation was achieved in the process. Introducing the cation and anion into the same type of material while maintaining topology could be important for the field of material synthesis and preparation of supercapacitor electrodes. Moreover, the electrochemical properties of these three materials were studied by cyclic voltammetry measurements and galvanostatic charge−discharge tests. The results indicated that the rate performance was improved significantly by ion exchange. In particular, the derived electrode with Se still showed superior oxidation and reduction ability at high scan rate of 10000 mV s−1. In addition, the second charge−discharge specific capacity also increased from 516 F g−1 to 925 F g−1 and 1412 F g−1 at the current density of 0.5 A g−1 and by Co and Se exchange, respectively. This work contributes to the knowledge on electrode materials for supercapacitors and can provide good reference for the fabrication of desired materials.

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INTRODUCTION Novel materials with excellent performance for storage and transformation of energy7−9 have been designed to cope with fossil fuel crisis1−3 and contribute to the study on renewable energy.4−6 Supercapacitors are promising energy storage devices10−14 with numerous irreplaceable and indispensable advantages over conventional capacitors, such as fast charge− discharge rates, long cycle life, high energy conversion efficiency, high operation stability, small size, and pollutionfree operation. However, current studies have mainly focused on the synthesis of special electrode materials with excellent electrochemical performance. Information on the design of a simple method that can synthesize a series of electrode materials is scarce. Moreover, improvement methods for the electrochemical performance of supercapacitor electrode materials should be studied. Transition metal sulfides, particularly Ni sulfide (NiS15,16 and Ni3S217−19) and Co sulfide (CoS1.09720 and Co9S821,22), have been extensively investigated in previous studies as electrode materials for Li-ion batteries23−25 and supercapacitors.15−22 These metal sulfides are suitable candidates for supercapacitors because of the satisfactory redox reaction of their metal ion. A © 2014 American Chemical Society

material composition mainly determines its properties. Ion exchange is an effective method to purposefully introduce beneficial ions into the parent material for adjustment of the composition and improvement of the material performance. This method has been widely used in superionic conductors,26 optoelectronic devices,27−29 catalysts,30 electrochemistry,23 and band gap tuning.31 However, ion exchange has rarely been applied to the preparation of novel electrode materials for supercapacitors. Co ion can be a representative of beneficial metal ions, and its introduction into Ni sulfides can produce a Co−Ni sulfide composite electrode that exhibits the complementary advantages of Co−Ni sulfide material, as well as improved electrochemical performance. The composite electrode with Se exhibits excellent cycling stability and satisfactory rate performance, and has been considered for use in Li-ion batteries.32−35 However, the potential application of these electrodes in supercapacitors has yet to be reported. Se, as a beneficial anion, is introduced into the Ni sulfide material to Received: February 23, 2014 Revised: April 16, 2014 Published: May 9, 2014 3418

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prepare nickel sulfide−selenide composite electrodes with the expected excellent electrochemical performance. This method of introducing the cation and anion into the same kind of material presents a challenge in the field of material synthesis. Moreover, this mechanism is significant in the synthesis of new materials, because novel materials with unique properties can be fabricated by the purposeful introduction of the corresponding ions. In addition, material performance is also restricted by its morphology and structure,36−38 which is also the case for electrode materials for supercapacitors. One-dimensional (1D) nanoarray electrode materials have shown excellent electrochemical performance because of their large specific surface area and short diffusion path lengths for ions.39,40 For example, Zhang et al. proved that NiCo2O4−Ni foam binder-free electrode has considerably high capacitance and excellent cycling stability.41 Xu and colleagues fabricated acicular Co9S8 nanorod arrays as positive materials that exhibit superior electrochemical performance.21 Although several electrode materials with 1D nanoarray structures have been reported for supercapacitors, designing a simple method for the preparation of more electrode materials with 1D nanoarray structure is necessary. Based on our previous studies,31 which showed how one can adjust the composition of material while retaining the structure and morphology, we propose that this approach is a promising method that can utilize the existing lattice as template for the targeted synthesis of the required electrode material by introducing beneficial ions. Even singlecrystal-to-single-crystal transformation may be achieved through this process.31 However, this method has not been applied for the preparation of supercapacitor electrode materials. In this work, a three-dimensional (3D) hierarchical nest-like Ni3S2@NiS electrode with nanorods as primary building blocks was successfully synthesized. The nest-like Ni3S2@NiS material was then used as template to prepare Ni3S2@Co9S8 and NiS@ NiSe2 composite electrodes with similar morphology to the parent material by injecting beneficial ions, namely, Co and Se. In addition, the electrochemical performance of the electrode materials has been significantly improved before and after ion exchange reactions. Cyclic voltammetry (CV) measurements have proven that the redox peaks of all as-prepared electrodes were still relatively sharp at high scan rates, particularly those of the NiS@NiSe2 composite electrode at scan rate of 10000 mV s−1. Galvanostatic charge−discharge tests showed that all asprepared electrodes have high specific capacity. The first charge−discharge specific capacities of Ni3S2@NiS, Ni3S2@ Co9S8, and NiS@NiSe2 composite electrodes were 2440, 6427, and 7717 F g−1, respectively, at a current density of 0.5 A g−1. To the best of our knowledge, these specific capacities are extremely high; that is, the rate performance and specific capacity of electrodes have been greatly increased by this method. The method used in this study can be a beneficial reference for the synthesis of other new materials. A series of characterization methods, such as XRD, EDX, SAED, and XPS, were performed to confirm the component and crystal form of electrodes before and after ion exchange reactions. Results revealed that all as-obtained electrodes had high purity and crystallinity, and that single-crystal-to-singlecrystal transformation was also achieved.

Article

EXPERIMENTAL SECTION

All chemical reagents in the present work were of analytical grade and used without further purification. The 3D hierarchical, nest-like Ni3S2@NiS electrode materials with a special core−shell (sample 1) were successfully synthesized using Ni foam as template and Ni source through a one-step in situ growth method. The Ni3S2@Co9S8 and NiS@NiSe2 electrode materials were obtained by introducing Co and Se ions into sample 1. A series of parallel experiments with different replacement times was conducted to investigate the displacement process. Samples 2, 3, 4, 5, 6, 7, 8, 9, and 10 were synthesized within a reaction time of 10 min, and 1, 2, 4, 6, 8, 10, 12, and 24 h, respectively, for the introduction of Co ion. Samples 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21 were synthesized within a reaction time of 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 24 h, respectively, to introduce the Se ion. The structure, component, phase, and element distribution of the asobtained materials were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), high-resolution TEM (HRTEM) analyses, and X-ray photoelectron spectroscopy (XPS). CV and galvanostatic charge−discharge tests were conducted to investigate the electrochemical performance of samples 1, 9, and 20 in a conventional three-electrode cell. The detailed experimental steps are shown in the Supporting Information (SI).

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RESULTS AND DISCUSSION A 3D hierarchical nest-like Ni3S2@NiS material built using a special method of NiS nanorods firmly grown on the 3D network Ni3S2 substrate, was successfully synthesized by onepot in situ growth method. The low-magnification SEM image of sample 1 showed that the surface of the 3D framework was covered by a layer of nest-like product (Figure 1a). The caliber of the nest was approximately tens of micrometers, and the inner diameter of the nest decreased with further extension into the 3D skeleton, as shown in Figure 1b. The figure also proved that the nest-like product comprises numerous nanorods of approximately 40 μm in length (Figure 1d) and 50 nm in diameter (SI Figure S1a). Numerous nanorod arrays densely and firmly covered the surface of 3D framework in every direction. The space between nanorods increased in size with the outward growth of nanorods, which results in a nest-like morphology. Moreover, Figure 1d and SI Figure S1a also showed that the obtained product had a high draw ratio of approximately 800, which greatly increased the specific surface area and the number of active sites. Compared with the traditional Ni foam-based composite electrode prepared by deposition, the current method applies the advantage of the 3D skeleton of the Ni foam. The active material also covers the substrate more firmly. The ordered space between the nanorods provides a short diffusion path for electronic movement on the surface of the material, resulting in a high charge−discharge rate. Therefore, this morphology is expected to result in excellent electrochemical performance. XRD measurement (Figure 1c) was conducted to determine the phase of sample 1 and showed that sample 1 is composed of NiS (JCPDS no. 12-41, Space Group: R3m (no. 160), a = 90620 Å, c = 3.149 Å) and Ni3S2 (JCPDS no. 76-1870, Space Group: R32 (no. 155), a = 4.07300 Å) crystals. Therefore, sample 1 may be a mixture of NiS and Ni3S2 or may have a Ni3S2@NiS core−shell structure. No Ni peak was observed in the curve, which proved that the Ni with a 3D framework was completely converted into the products. The strong and sharp diffraction peaks indicated that the obtained products were thoroughly crystallized. The EDX spectra of the external nanorods and internal skeleton are shown in the insets of Figure 1d; these spectra can be used to investigate the 3419

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obtained by an average of 10 groups of parallel experiments. The mass and quantity of Ni added into the Teflon-lined autoclave were 0.0293 g and 0.5 mmol, respectively. Sample 1 contained only two elements, namely, Ni and S. Thus, the atomic ratio of Ni and S in sample 1 was approximately 1.15:1, which is an intermediate value between 1:1 and 1.5:1. This finding was in agreement with the characterization results. Thus, sample 1 comprised two components of NiS and Ni3S2. Depending on the known conditions, the components of sample 1 can also be quantified. Sample 1 contains approximately 0.0277 g (0.305 mmol) of NiS and 0.0156 g (0.065 mmol) of Ni3S2. The sample also had a special core− shell structure, which is a layer of nest-like NiS covering the surface of the 3D network Ni3S2 skeleton, and all the internal Ni3S2 or the external nest-like NiS have a 3D network structure. This composite electrode was successfully synthesized at 160 °C for 24 h and may exhibit excellent electrochemical performance for supercapacitors. To maintain the excellent morphology, the Co ion, as a helpful cation for supercapacitors, was introduced into sample 1 to prepare Ni−Co sulfide with excellent electrochemical performance. Therefore, sample 9 was obtained using sample 1 as the template at 160 °C for 12 h. The solution color in the Teflon-lined autoclave changed from dark red to orange red. The concentration of Co ion in the solution was possibly reduced, and a small amount of S was generated in the replacement step, indicating that ion exchange occurred. Figure 2b shows the SEM image of sample 9 and confirms that the

Figure 1. (a) Low-magnification SEM image of sample 1. (b) Highmagnification SEM image of sample 1. (c) XRD pattern of sample 1. (d) SEM image of the cross section of sample 1. Inset: EDX spectra of the external skeleton and internal nanorod of sample 1. (e) SAED pattern of NiS nanorod on the surface of sample 1. (f) The corresponding HRTEM image of NiS nanorod on the surface of sample 1.

distribution of NiS and Ni3S2 in sample 1. By contrast, only Ni and S were found in both the internal framework and external nanorods, but the ratio of Ni and S elements differed significantly between the internal framework and external nanorods. The elemental ratio of Ni and S in external nanorods was 51:49, close to 1:1, whereas in the internal framework, the ratio was 60:40. The external nanorods comprised NiS crystals, whereas the internal framework comprised Ni3S2 phase. Thus, sample 1 is not a normal mixture; the sample possesses a core− shell structure with a layer of NiS nanorods arraying on the surface of a 3D network Ni3S2 framework. HRTEM analysis results described the detailed geometrical structure of the nanorods on the surface of sample 1. In Figure 1f, the distinct set of visible lattice fringes with interplanar spacings of 1.605 and 2.71 Å can be clearly observed, corresponding to the (330) and (300) planes of NiS crystals (JCPDS no. 12-41), respectively. The orderly array of lattice points explains the extremely high crystallinity of the NiS nanorods. The corresponding selected area electron diffraction (SAED) pattern in Figure 1e projected along the [001] zone axis further confirmed the single crystallinity of the NiS nanorod. In the NiS nanorod, the lattice spacings of 1.60 and 2.74 Å correspond to the (330) and (300) planes of rhombohedral phase NiS, respectively. The SAED results were in good agreement with the HRTEM results and supported the above-mentioned conclusion that sample 1 is tightly covered by a layer of single-crystal NiS nanorods. The composition of sample 1 was further verified using its weight. Sample 1 weighed approximately 0.0433 g, which was

Figure 2. (a) SEM image of sample 1. (b) SEM image of sample 9. (c) HRTEM image of Co9S8 nanorod. The red dots, which were marked, represent the lattice points. (d) The corresponding SAED pattern of Co9S8 nanorod.

nest-like morphology was faultlessly inherited from sample 1. In addition, the diameter of the nanorods forming the nest-like structure was maintained at approximately 50 nm (SI Figure S1b). The XRD spectra of sample 9 showed that sample 9 contains two components, that is, Co9S8 and Ni3S2 (Figure 3). The NiS peaks have completely disappeared, whereas Ni3S2 peaks are retained. We speculated that the NiS nanorods on the surface of sample 1 were transformed into the Co9S8 nanorods in this exchange process. Figure 2c shows the typical HRTEM image of external nanorods with lattice spacings of 1.79 and 2.84 Å, which corresponded to the (440) and (222) interplanar 3420

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crystal NiS nanorods of sample 1 transformed completely into single-crystal Co9S8 nanorods, but the structure and composition of the internal skeleton remained unchanged, possibly because of the compact structure and the markedly small surface area. Furthermore, the EDX surface scanning image (Figure 4c) of sample 9 surface showed that elements Co, Ni, and S were uniformly distributed in the replacement product. Figure 4d shows the EDX surface scanning image of the skeleton section, revealing that elements Co and Ni existed in the external nanorods and internal skeleton, respectively. This result is in agreement with the above-mentioned conclusion. Therefore, sample 9 with the core−shell structure comprised the nest-like single crystal Co9S8 nanorods on the surface of sample 9, as well as Ni3S2 as the internal skeleton. The XPS measurements of sample 9 (Figure 5a) indicated the presence of Ni, Co, S, and C from the reference, and the

Figure 3. XRD patterns of samples 1, 3, 5, and 9.

spacings of Co9S8 (JCPDS no. 73-1442), respectively. The array of lattice points was highly orderly, which explains the extremely high crystallinity of the external Co9S8 nanorods. This conclusion is supported by SAED observations (Figure 2d). The corresponding SAED spots projected along the [110] zone axis can be indexed as Co9S8 (222) and (440) planes (JCPDS no. 73-1442). This result further verifies that the external nanorod of sample 9 is the single crystal Co9S8 with high crystallinity. Therefore, single-crystal-to-single-crystal transformation from NiS to Co9S8 in micro/nano inorganic materials is also perfectly achieved in this step. EDX spectroscopy was performed to further characterize the element distribution of Ni, Co, and S. Figure 4b shows the

Figure 5. XPS spectra of (a) survey spectrum, (b) Ni 2p, (c) Co 2p, (d) S 2p for sample 9.

absence of other impurities. Peaks assignable to the core levels of Ni 2p3/2, Ni 2p1/2, Co 2p3/2, Co 2p1/2, S 2p3/2, and S 2p1/2 were identified in the survey spectrum. The Ni 2p spectrum (Figure 5b) was deconvoluted into two spin−orbit doublets and two shakeup satellites (identified as “Sat.”). The first doublet at 851.9 and 869 eV,42 and the second at 854.9 and 872.5 eV,42,43 were assigned to Ni2+ and Ni3+, respectively. Figure 3c shows the division of Co 2p into Co 2p3/2 and Co 2p1/2. The Co 2p spectrum (Figure 5c) showed the best fit, considering two spin−orbit doublets at 777.3 and 792.4 eV,43,44 and 780.1 and 795.4 eV;43 and the doublets were assigned to Co2+ and Co3+, respectively. The strong peak at around 160.5 eV42 and 161.4 eV45 (Figure 5d) belonged to S 2p3/2, and the peak at 162.5 eV45 can be assigned to S 2p1/2. These values approached the reported values and confirmed that Co was successfully introduced into sample 1. Based on the above analysis, NiS nanorods transformed into Co9S8 nanorods that covered the 3D network Ni3S2 skeleton. In addition, a small amount of S may have been generated in this step. Thus, the following reaction may occur in this system:

Figure 4. (a) Curves of element ratio of S, Co, and Ni in samples 3−8, which was obtained by the EDX measure. (b) SEM image of the cross section of sample 9. Inset: EDX spectra of the internal skeleton and external nanorods of sample 9. (c) EDX mapping of the surface of sample 9. (d) EDX mapping of the cross section of sample 9.

EDX spectroscopy results of the external nanorod and internal skeleton. Only Ni and S were found in the internal framework, and only Co and S were found in the external nanorods. The elemental ratio of Ni and S in the internal framework was 59:41, which is a close ratio to 3:2. The elemental ratio of Co and S in the external nanorods was 54:46, which approximates 9:8. This phenomenon further illustrates that the external single

9NiS + 9Co2 + → Co9S8 + 9Ni 2 + + S

(1)

For the quantization of Co9S8 and Ni3S2, the weight of sample 9 was maintained at approximately 0.0422 g, which is the average weight of 10 groups of parallel products. The 3421

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Figure 6. Diagram of morphology evolution during the replacement process with selenium. (a−e) Low-magnification SEM images of samples 11, 13, 16, 18, and 20, respectively. (f) High-magnification SEM image of sample 20. (g) Proposed mechanism for the transformation from sample 1 to sample 20.

by cation exchange method using sample 1 as template. In this process, morphology from samples 1 to 9 and single-crystal-tosingle-crystal transformation from NiS crystal to Co9S8 phase were achieved simultaneously. To introduce the cation and anion into the same kind of material, Se powder was selected as the anion representative to exchange with the S element of sample 1, resulting in the completion of the anion exchange. Se was selected as anion representative because Se and S were located in the same main group of the periodic table, and the addition of Se improved the cycling stability and rate performance of the supercapacitor electrode. Therefore, a series of products was obtained by anion exchange using sample 1 as template at 160 °C for different replacement times. SEM, XRD, and EDX measurements were carried out to confirm the morphology, phase, and elemental composition of the replacement products, respectively. Figure 6a−e shows the SEM images of samples 11, 13, 16, 18, and 20. At a replacement time of 1 h, the morphology of sample 11 was similar to that of sample 1. However, when the replacement time reached 4 h, fine particles were generated on the surface of the external nanorods. With continued replacement reaction, the particles on the surface of nanorods gradually developed and increased in number. When the replacement time was increased to 14 h, the number and size of particles stabilized, as shown in Figure 6d and e. Samples 18 and 20 have similar morphologies, which comprised a layer of nanorods covered by numerous particles growing on the surface of the products. The high-magnification SEM image of a single particle is shown in Figure 6f. These particles have a regular octahedron structure. The surface of each particle was very smooth, and the side length of the regular octahedron was approximately 4 μm. To clarify the growth process, the proposed mechanism for the transformation from sample 1 to 20 is shown in Figure 6g, suggesting that the replacement products inherited the 1D nanoarray structure of sample 1. However, the difference of these samples from Sample 1 is that the surface of the replacement products is covered with the newly generated product. The XRD patterns of samples 11, 13, 16, 18, and 20 are shown in Figure 7. As expected, the XRD curves of the replacement products contain the peaks of the NiSe2 phase, which gradually strengthened. Unexpectedly, the peaks of Ni3S2 crystal gradually weakened and subsequently disappeared when

weight of sample 1 was approximately 0.0433 g. The mass and amount of Co9S8 in sample 9 (0.0266 g and 0.0338 mmol, respectively) were subsequently calculated. Sample 9 contained approximately 0.0266 g (0.0338 mmol) of Co9S8 and 0.0156 g (0.065 mmol) Ni3S2. The amount of NiS in sample 1 was approximately 9 times that of Co9S8 in sample 9, in agreement with the characterization results. The cation exchange phenomenon only occurred on the surface of sample 1. The 3D hierarchical nest-like Ni3S2@Co9S8 material with a core− shell structure was successfully prepared, retaining topology. Therefore, single-crystal-to-single-crystal transformation at both micro and nanoscales for these inorganic materials was achieved. Time-dependent experiments were performed to investigate the exchange process. The morphology, phase, and elemental composition of the replacement products were recorded by SEM, XRD, and EDX, respectively, and several measurement results were selected as representatives. SI Figure S2 shows the SEM images of samples 2, 4, 6, and 10, with respective replacement times of 1, 4, 8, and 24 h. The morphology of the replacement products did not change with the replacement time. The XRD patterns of Samples 3, 5, and 9 are shown in Figure 3, whereas that of sample 10 is shown in SI Figure S3. The figures show that the peaks of NiS crystal gradually weakened, the peaks of Co9S8 crystal strengthened, and the peaks of Ni3S2 phase almost remained unchanged. The XRD pattern of sample 10 was similar to that of sample 9, and this similarity illustrates that the components of the replacement product remained stable up to 12 h of replacement time. The EDX spectra of samples 3 to 8 are shown in Figure 4a. When the replacement time was extended, the amount of Ni rapidly decreased for 8 h, after which, the amount remained constant. The content of Co rapidly increased and then remained constant beyond 8 h. The replacement rate was initially rapid and then gradually slowed to zero. The S content gradually decreased, and the total decrease in S content was considerably low during the replacement process. This finding may be caused by the higher anion ratio in the NiS crystal phase than in the Co9S8 phase. The difference between the anion ratios of NiS and Co9S8 crystals is low, and this finding supported the above-mentioned conclusion. Therefore, the 3D hierarchical nest-like Ni3S2@Co9S 8 material with a core−shell structure was successfully prepared 3422

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element Ni was distributed in the whole product, and the Ni content in the internal skeleton was greater than that in the external material. The elemental ratios of Ni, S, and Se in samples 11, 14, 17, 20, and 21 were measured by EDX and are shown in Figure 8c. During the replacement process, the content of S rapidly decreased until 18 h of replacement time, after which the content was maintained at approximately 12%. The Ni content also decreased to approximately 36% for up to 18 h of replacement time. The Se content rapidly increased, and then remained constant at approximately 52%. The sum of the content of S and half the content of Se was nearly equal to the content of Ni. This finding also supported the abovementioned conclusion. The goal of anion exchange was achieved from samples 1 to 20. However, anion exchange with Se resulted in significant changes with regards to the composition of sample 1.

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Figure 7. XRD patterns of samples 1, 11, 13, 16, 18, and 20.

ELECTROCHEMICAL PROPERTIES The electrochemical properties of samples 1, 9, and 20 were evaluated by CV and galvanostatic charge−discharge techniques. Figure 9a−c shows the typical initial two or three CV

the replacement time was extended to 18 h. The peaks of NiS remained constant. Sample 20 comprised NiS and NiSe2. The XRD pattern of sample 21 is shown in Figure S4, which showed that sample 21 contained NiS and NiSe2. NiS and NiSe2 were also present in sample 20. Therefore, the compositions of the replacement products remained unchanged until 18 h. Based on the above data, the replacement process may be divided into two steps. First, Se displaced S in the NiS nanorods on the surface of sample 1. Therefore, the morphology of the product was unchanged at a replacement time of 1 h. In this step, the NiS nanorods changed into NiSe2 nanorods. Second, Ni may spill from the skeleton of sample 1 and react with Se to form NiSe2, which developed on the surface of the NiSe2 nanorods formed in the first step. Thus, the skeleton was transformed into pure NiS phase. EDX was employed to investigate the elemental distribution in the skeleton section of sample 20 in detail, as shown in Figure 8a. Only Ni and S were found in the internal framework, and Ni and Se were found in the external material. The elemental ratio of Ni and S elements in the internal skeleton was 51:49, close to 1:1. The ratio of Ni and Se in the external material was 33:67, which was close to 1:2. sample 20 has a core−shell structure composed of a layer of NiSe2 regular octahedron and nanorods, which covered the surface of the NiS skeleton with a 3D network structure. The EDX map (Figure 8b) of the cross section of sample 20 also confirmed this structure. The map displayed elements Se and S in the external and internal parts of the product, respectively. However,

Figure 9. (a) Typical initial two CV curves of sample 1 at the scan rate of 1 mV s−1. (b) Typical initial three CV curves of sample 9 at the scan rate of 1 mV s−1. (c) Typical initial three CV curves of sample 20 at the scan rate of 1 mV s−1. (d) Normalized CV curves of sample 1, 9, and 20 at the scan rate of 5 mV s−1.

Figure 8. (a) EDX spectra of the external skeleton and internal material of sample 20. Inset: SEM image of the cross section of sample 20. (b) EDX mapping of the cross section of sample 9. (c) Curves of element ratio of Se, Ni and S in samples 11, 14, 17, 20, and 21, obtained by EDX. 3423

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curves of samples 1, 9, and 20 at a scan rate of 1 mV s−1 and with a potential window ranging from 0 to 0.65 V vs Hg/HgO in 2 M KOH, respectively. The samples were similar in terms of the initial curve, which differed from that in the subsequent cycles. These results indicate the phase transformation from NiS or Ni3S2 to Ni(OH)2 during the first positive sweep, as described in reactions 2 and 3.46 However, the initial curves of samples 1, 9, and 20 varied. This variation may be due to the introduction of Co or Se element into sample 1. Thus, the phase transformations from Co9S8 to Co(OH)2 [reaction 4] and NiSe2 to Ni(OH)2 [reaction 5] occurred during the first positive sweep of samples 9 and 20, respectively. Therefore, the phase transformation reactions occurred first, followed by the oxidation reaction from Ni(OH)2 (or Co(OH)2) to NiOOH (or CoOOH) during the first positive sweep. In addition, only the reversible reduction reaction from NiOOH (or CoOOH) to Ni(OH)2 (or Co(OH)2) occurred during the first negative sweep. Only the reversible redox reaction [reactions 6 and 7] occurred in subsequent CV cycle tests. Meanwhile, the XPS spectra of sample 9 before and after the first positive sweep (SI Figure S5) exhibited that sample 9 contains four kind of cations (Ni2+, Ni3+, Co2+, and Co3+), and the sample contains only two kind of cations (Ni3+ and Co3+) after the first positive sweep. All divalent cations were transformed into trivalent cations during the oxidation process, supporting the above conclusion. The reaction mechanism occurred as shown below. The phase transformation reactions were as follows: NiS + H 2O +

1 O2 → Ni(OH)2 + S 2

3 Ni3S2 + 3H 2O + O2 → 3Ni(OH)2 + 2S 2 Co9S8 + 9H 2O + NiSe2 + H 2O +

9 O2 → 9Co(OH)2 + 8S 2

1 O2 → Ni(OH)2 + 2Se 2

Figure 10. (a, b) CV curves of sample 1 at various scan rates. (c, d) CV curves of sample 9 at various scan rates. (e, f) CV curves of sample 20 at various scan rates.

(2)

observed at 0.43 and 0.22 V (overlapping with the peak at 0.29 V) in the CV curves of sample 9,47 and the peaks may have been caused by the reversible redox reaction 7. The potential difference between the redox peaks decreased with the introduction of the Se element. Parts b, d, and f of Figure 10 respectively show the CV curves of samples 1, 9, and 20 at high scan rates, which further proved the increased rate performance of the replacement product. The redox peaks still exist at high scan rates, thereby proving that all three ss have excellent rate performance. These rate performance can be attributed to the firm combination of the active material and the substrate. However, the redox peaks almost disappeared at a scan rate of 1000 mV s−1 for sample 1 and 2000 mV s−1 for sample 9. This phenomenon shows that the introduction of Co improved the rate performance of sample 1. The redox peaks of sample 20 were remained considerably strong at a scan rate 10000 mV s−1, indicating that Se is a beneficial ion that can improve the rate performance for supercapacitors. Similar results can also be obtained from charge−discharge tests. Parts a−c of Figure 11 show the galvanostatic charge− discharge behavior of samples 1, 9, and 20 at various current densities, respectively. The maximum current densities of all obtained products reached 10 or 20 A g−1, and the obtained samples showed excellent rate performance. The calculated specific capacitance as a function of cycle number from 1 to 18 at various current densities is plotted in Figure 11d. The specific capacitance calculations showed that the replacement products, especially with Se, had a higher specific capacity than sample 1, and that the first charge−discharge specific capacities of samples 1, 9, and 20 were 2440, 6427, and 7717 F g−1 at a current density of 0.5 A g−1, respectively (SI Figure S6). Meanwhile, Figure 11d shows the discharge specific capacitance increased with the introduction of Co and Se elements.

(3) (4) (5)

The reversible redox reactions were as follows: Ni(OH)2 + OH− ↔ NiOOH + H 2O + e−

(6)

Co(OH)2 + OH− ↔ CoOOH + H 2O + e−

(7)

Parts a, c, and e of Figure 10 show the typical CV curves of samples 1, 9, and 20, respectively, at scan rates of 1, 5, 10, 20, 30, 40, 50, and 100 mV s−1. A pair of peaks was visible in each voltammogram, indicating that the measured capacitance was mainly based on the redox mechanism. When the scan rate was increased, the shape of the curves was maintained, and the peak current increased, suggesting that all electrodes have excellent rate performance and can result in rapid redox reactions. The redox peaks became sharper after ion exchange, particularly when Se was involved, demonstrating that the rate performance improvement. Furthermore, the redox peaks shifted to lower and higher voltages at higher scan rates. The charge transfer kinetics may have been the limiting step of the reaction. In addition, the normalized CV curves of samples 1, 9, and 20 at a scan rate of 5 mV s−1 are shown in Figure 9d, and the curves show details of the electrochemical performance of the samples before and after replacement. A pair of redox peaks at 0.45 and 0.29 V was observed in CV curves of sample 1, which corresponds to the reversible conversion between Ni(OH)2 and NiOOH by eq 6.46 However, a new pair of redox peaks was 3424

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increased the number of valid ions from NiS to Co9S8 and confirmed the complementary advantages between Co and Ni ions. The Ni ions from the internal Ni3S2 overflowed into the NiSe2 crystal that grew on the surface of sample 20. The number of effective Ni ions sharply increased, which resulted in the increase in specific capacities from samples 1−20. In addition, the charge−discharge specific capacities of samples 1, 9, and 20 were relatively high at 436, 600, and 754 F g−1 at a current density of 0.5 A g−1. As a consequence, Ni3S2@NiS, Ni3S2@Co9S8, and NiS@ NiSe2, which showed excellent rate performance, were successfully synthesized by a simple method. The introduction of beneficial ions contributed to the improvement in rate performance and specific capacity.

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CONCLUSION This paper designed a simple method for fabricating novel electrode materials with high electrochemical performance for supercapacitors. Although the material composition was altered in these electrodes, the structure was preserved. When the onestep in situ growth method was performed, Ni foam was used as the template and Ni source for the synthesis of a Ni3S2@NiS composite electrode with a high rate performance for supercapacitors. The cation and anion were introduced into the same kind of material to improve the material performance. The composition of 1D nanorod was successfully altered in the process, but the excellent structure was unchanged. The singlecrystal-to-single-crystal transformation in micro/nano inorganic materials, particularly with Co ion, was achieved. Ni selenide, which has an excellent rate performance (i.e., superior oxidation−reduction ability at a scan rate of 10 V s−1) and specific capacity (1412 F g−1 at a current density of 0.5 A g−1), was used as a supercapacitor electrode. The electrode materials for supercapacitor applications were successfully improved. A method for introducing beneficial ions into a parent material is presented. The results can aid in improving product properties as a response to the energy crisis.

Figure 11. (a) Galvanostatic charge−discharge curve of sample 1 at various current densities of 0.5, 1, 2, 5, 10 A g−1. (b) Galvanostatic charge−discharge curve of sample 9 at various current densities of 0.5, 1, 2, 5, 10 A g−1. (c) Galvanostatic charge−discharge curve of sample 20 at various current densities of 0.5, 1, 2, 5, 10, 20 A g−1. (d) Discharge specific capacitance curve at different current density and the Coulombic efficiency curve at 0.5 A/g of sample 1, 9, and 20.

Moreover, the Coulombic efficiency of all samples, except the first cycle, was relatively high at about 97%, which may be due to the phase transformation reactions. In addition, the discharge specific capacitance of Sample 3 (Ni0.62Co0.38S) and 5 (Ni0.26Co0.74S) at various current densities is displayed in SI Figure S8, in which sample 3 had the highest specific capacitance among the samples. This finding may be due to the different elemental ratio of Co and Ni in the samples. The Co element content approached that of Ni, resulting in the maximized complementary advantages of Co and Ni. Therefore, sample 3 has the highest specific capacitance, supported by the previous report.48 Moreover, SI Figure S8 shows the variation curves of the specific capacity of sample 1 and 9 with the number of cycles among 500 cycles at a current density of 5 A/g. This result exhibited good cycling stability of sample 1, and that sample 9 had better cycling stability than that of sample 1. SI Figure S9 shows the first galvanostatic charge−discharge curve of samples 1, 9, and 20 at a current density of 0.5 A g−1, indicating that the charge time was higher than the discharge time. However, the second charge−discharge specific capacity of samples 1, 9, and 20 sharply declined to 516, 925, and 1412 F g−1 at the same current density. This phenomenon corresponds to the CV tests, thereby proving the existence of an irreversible phase transformation from the sulfide or selenide to hydroxide, followed by the oxidation reaction from Ni(OH)2 (or Co(OH)2) to NiOOH (or CoOOH) during the charge process. Only the reversible conversion between Ni(OH)2 (or Co(OH)2) and NiOOH (or CoOOH) occurred in the subsequent cycle because the specific capacities of samples 1, 9, and 20 were not significantly altered, or were only slightly decreased compared with those in the subsequent cycles. To clarify the change in specific capacity, the enlarged figure of the specific capacity of cycle numbers from 2 to 18 is shown in the inset of Figure 11d. The figure demonstrates that the specific capacity of sample 1 improved during the exchange with Co and Se. For sample 9, the improvement in specific capacity may have been caused by the introduction of the Co ion, which

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimentation section and Figures S1−S9 (SEM images, TEM image, XRD patterns) and the galvanostatic charge−discharge curve of the part of obtained samples. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. *Email: [email protected]. *Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant Nos. 21001090 and 21103153) and Program for Innovative Research Team (in Science and Technology) in University of Henan Province (Grant No. 2012IRTSTHN021), Education Department of Henan Province (Grant No. 13A150648), and the Outstanding Talented Persons Foundation of Henan Province. 3425

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