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nickel-based manganese dioxide core-shell nanostructure for supercapacitor electrodes
Electrochimica Acta 212 (2016) 671–677
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hierarchical copper/nickel-based manganese dioxide core-shell nanostructure for supercapacitor electrodes Hao Chena , Xue Qiang Qib,** , Min Kuanga , Fan Dongc, Yu Xin Zhanga,d,* a
College of Material Science and Engineering, Chongqing University, Chongqing 400044, PR China College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, PR China d National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, PR China b c
A R T I C L E I N F O
Article history: Received 8 December 2015 Received in revised form 25 June 2016 Accepted 5 July 2016 Available online 6 July 2016 Keywords: Manganese dioxides Nanocomposites Core-shell nanostructure Supercapacitor
A B S T R A C T
A novel copper/nickel-based manganese dioxide core-shell nanostructure can be prepared for supercapacitor electrodes using facile hydrothermal methods. Attributed to Kirkendall-type diffusion process, the “core” can transfer to hollow structure during the reaction interestingly. The faradic reaction occurs effectively in the electrode and electrolyte due to the high conductivity of Cu/Ni and large hollow channel structure. As a result, the electrode displays a high specific capacitance (374 F g1 at current density of 0.25 A g1), fine cycling stability (86.9% retention after 3000 cycles) and outstanding rate capability. These interesting findings suggest that the hybrid material can be a promising candidate for supercapacitor electrodes. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the ever-increasing fossil fuels consumption, energy crisis has become more and more severe in the planet. Large amounts of energy storage technologies are emerging to solve this trouble [1–5]. Supercapacitors, with the features of high power density, fast charge-discharge characteristic and long cycling lifespan, play a potential role in energy storage devices [6–8]. These stunning properties are directly affected by the electrode materials. Based on the previous studies, electrode materials can be divided into three major types: (i) carbon materials, (ii) transition metal oxides, and (iii) conducting polymers [9–13]. Among these electrode materials, transition metal oxides (MnO2, Co3O4, NiO etc.) have drawn extensive research interests due to their advantages of low cost, simple operation, abundance and efficient Faradaic reaction [14–16]. MnO2 with different morphologies and structures has been investigated widely on account of many excellent characteristics, but most MnO2 materials constantly suffers from poor electrical
* Corresponding author at: College of Material Science and Engineering, Chongqing University, Chongqing 400044, PR China. ** Corresponding author. E-mail addresses: [email protected] (X.Q. Qi), [email protected] (Y.X. Zhang). http://dx.doi.org/10.1016/j.electacta.2016.07.024 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
conductivity (105–106 S cm1) and agglomeration [17]. Because of these deficiencies, MnO2 is arduous to be effectively utilized during Faradaic reactions. In other words, single-component nanomaterials can hardly satisfy the demand of pleasurable electrochemical performances for supercapacitors. To improve these issues, MnO2 is used to combine with other materials to increase electrical conductivity or design rational structures for favorable electrochemical properties. Based on these facts, numerous MnO2-based materials with different structures have been fabricated for supercapacitor electrodes, such as graphene@MnO2 [18], carbon nanotubes@MnO2 [19,20], CuO@MnO2 [21], Co3O4@MnO2 [22,23] nanocomposites. The electrochemical performances of these nanocomposites are effectively enhanced by the synergistic effect. Despite these improvements, it still remains a challenge to develop smart recombination and rational design for electrode materials with specific structure. Herein, we report a novel copper/nickel-based manganese dioxide core-shell nanostructure for supercapacitor electrodes via a facile hydrothermal method, in which the Cu/Ni-based nanotubes work as the “core” and porous MnO2 nanosheets serve as the “shell”. The unique core-shell nanostructure can benefit the improvements on the electrochemical properties of the electrode. Naturally, the core-shell nanostructure exhibits a high specific capacitance (374 F g1 at current density of 0.25 A g1), good cycling stability (86.9% retention after 3000 cycles), and
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remarkable rate capability. It is believed that the nanocomposites would be a promising electrode material for energy storage devices. 2. Experimental 2.1. Materials synthesis All the reagents in the experiments were of analytical purity and used without any further purification.
worked as the reference electrode and platinum plate was used as the counter electrode. A nickel foam (1 1 cm2) coated by the slurry was used as working electrode, which was composed of active materials, carbon black, and polyvinylidene fluoride (PVDF) with proportion of 7: 2: 1. The active materials were about 2.3 mg. The electrochemical impedance spectroscopy (EIS) was performed with a perturbation amplitude of 5 mV in the frequency between 100 kHz and 0.01 Hz. The specific capacitances are calculated according to the following equation [25]: Cm ¼
2.1.1. Synthesis of Cu/Ni nanowires In a typical synthesis [24], 15 ml NaOH (7 M) and 0.15 ml ethylenediamine (EDA) were mixed sufficiently in a reactor. Then, 0.26 ml Ni(NO3)26H2O (0.5 M) and 0.14 ml Cu(NO3)26H2O (0.5 M) were added and shaken until it turned blue, following by the sequential addition of 15 ml NaOH (7 M) and 0.15 ml hydrazine hydrate. Finally, the mixture was put into a Teflon-lined stainless steel autoclave maintained at 80 C for 2 h. The precipitate was washed and dried at 60 C in a vacuum for 12 h. 2.1.2. Synthesis of Cu/Ni-based manganese dioxide Briefly, Cu/Ni NWs (20 mg) were dissolved into 35 ml of 0.5 M KMnO4 solution to form homogeneous solution, and the mixture was transferred to a 50 ml Teflon-lined stainless steel autoclave which was sealed and maintained 140 C for 24 h. In the end, the samples were washed several times and dried at 60 C in a vacuum for 12 h. 2.2. Materials characterization The compositions of the samples were charicaterized by powder X-ray diffraction (XRD, D/max 2500, Cu Ka) and X-ray photoelectron spectroscope (XPS, Kratos XSAM800). The morphology and structure were observed by focused ion beam scanning electron microscopy (ZEISS AURIGA FIB/SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscopy (TEM, ZEISS LIBRA 200). The nitrogen adsorption-desorption isotherms were measured at 77 K by using micrometritics ASAP 2020 sorptometer. 2.3. Electrochemical measurements All the electrochemical measurements were carried out in 1 M Na2SO4 solution by using an electrochemical workstation (CHI 660E) in a three-electrode system. The saturated calomel (SCE) was
I Dt mDV
where I is the discharge current (A), Dt is the discharge time (s), m is the weight (g) of active materials, and DV is the discharging potential window (V). 3. Results and Discussion In order to confirm the composition and phase purity of the sample, XRD patterns of Cu/Ni-based manganese dioxide are shown in Fig. 1. It is clear that the diffraction peaks of Cu and Ni phases are in accord with the values in the standard card (Cu: JCPDS card no. 89-2838; Ni: JCPDS card no. 87-0712). [24] Meanwhile, the diffraction peaks of the CuO and NiO are well in line with the standard XRD patterns (CuO: JCPDS card no. 48-1548; NiO: JCPDS card no. 47-1049). The three strong diffraction peaks of the MnO2 at about 12.5 , 25.2 and 65.6 are indexed to (001), (002) and (020) planes of birnessite-type MnO2 (JCPDS card no. 80-1098) [21]. Obviously, no other diffraction peaks are detectable, indicating the high purity of the sample. Furthermore, the SEM image and corresponding EDS mapping of Cu/Ni-based manganese dioxide are shown in Fig. 2. As can be seen from the pictures, Cu, Ni, Mn and O elements are well distributed in the structure. Apparently, Cu and Ni elements are internally distributed, demonstrating the core-shell nanostructure. The strong C signal is observed because of the carbon-conducing tap for sample uploading. These results further confirm the composition of the sample. To better understand the chemical composition and oxidation states, XPS spectra of the composite is presented in Fig. 3. The Mn 2p XPS spectrum displays two major peaks with a spin-energy separation of 11.8 eV (Fig. 3a) [21]. Fig. 3b shows XPS of Ni 2p core level at binding energy of 871.9 and 860.5 eV, confirming the oxide in the sample as NiO. The peak at 854.7 eV is connected with Ni. In the Cu 2p spectrum, two peaks at binding energy of 954.2 and 934.4 eV are corresponding to Cu 2p1/2 and Cu 2p3/2. On the other
Fig. 1. XRD patterns of Cu/Ni-based manganese dioxide.
H. Chen et al. / Electrochimica Acta 212 (2016) 671–677
Fig. 2. Energy dispersive spectroscopy (EDS) analysis (a), typical SEM image (b) and the corresponding EDS mapping (c) of Cu/Ni-based manganese dioxide.
hand, two satellite peaks at 962.7 and 943.6 eV demonstrate the oxide in the sample as CuO. [21] Moreover, another peak at 932.4 eV proves the presence of Cu (0). The fitted O 1 s spectrum is characterized by three bands in Fig. 3d, and the one at 530.4 eV corresponds to the O2 band with Cu, Ni and Mn. Two other peaks are observed, which is ascribed to the water molecules and absorption of oxygen on the surface. These results are consistent with the XRD analysis. A possible mechanism of growth is proposed for the core-shell nanotubes structure (Fig. 4a), and the process is mainly divided into two steps. MnO4 ions are firstly adsorbed on the surface of Cu/Ni nanowires. In this way, MnO4 ions and H2O molecules are in the progress to form MnO2 nuclei on the surface, and also partial
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oxidize Cu/Ni nanowires into CuO/NiO nanowires. Then, with the growth mechanism of Ostwald ripening process, the MnO2 nuclei aggregate and transform into MnO2 nanosheets as the time increases, which is driven by the minimization of surface energy [26–29]. On the other hand, hollow nanotubes take shape because of the Kirkendall-type diffusion process: metal ions diffuse outward faster than oxygen inward caused by the self-decomposition of KMnO4, and the vacancies can aggregate to form voids. [30–32] Impressively, the unique highly porous morphology with large hollow channel structure can provide a high surface area and more active sites for effective faradic reaction. Fig. 4 presents SEM images of Cu/Ni nanowires (b and c) and Cu/Ni-based manganese dioxide (d and e). As Fig. 4b and c show, plenty dispersed Cu/Ni nanowires are clearly observed [25]. The diameters of Cu/Ni nanowires rang from 100 to 200 nm, and the length of the nanowires are up to a few micrometres. After hydrothermal reaction, Cu/Ni-based nanowires are uniformly coated by MnO2 nanosheets with rough appearance (Fig. 4d). From the red dotted circle, the fracture structure can be clearly observed. After hydrothermal reaction, the length of core-shell nanostructures gets shorter than the original nanowires, because the long nanowires are broken under the strong oxidation. Interestingly, the core gradually transfers to hollow nanostructure with the progress of reaction, which is attributed to Kirkendall effect (Fig. 4e). Naturally, the core-shell nanotubes are beneficial to ions permeation and electrons transport, giving rise to good electrochemical properties of the electrode. In order to investigate the hierarchical core-shell structure, the TEM images of the Cu/Ni-based manganese dioxide are also displayed. As Fig. 5a presented, the core-shell nanowires are
Fig. 3. XPS spectra of Cu/Ni-based manganese dioxide: a) Mn 2p spectrum; b) Ni 2p spectrum; c) Cu 2p spectrum; d) O 1s spectrum.
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Fig. 4. Schematic illustration of the formation of Cu/Ni-based manganese dioxide (a); SEM images of Cu/Ni nanowires (b and c) and Cu/Ni-based manganese dioxide (c and d); The inset is corresponding detailed image.
clearly visible with a rough surface, and the average thickness of the MnO2 is about 150 nm. The core-shell structure is composed of hollow nanotube supported porous MnO2 nanosheets (Fig. 5b). Moreover, the high-magnification TEM image (the inset) of the hollow structure suggests that the large hollow channel and high surface area can promote the effective interaction on the surface of the core and inside of the nanotube, which is in favor of pleasurable electrochemical performances. Besides, the contour of core-shell nanostructure can be distinctly observed. Moreover, the interplanar distances of 0.14 and 0.35 nm are indexed as the (020) and (002) plane of birnessite-type MnO2, in line with the previous XRD data (Fig. 5c). The polycrystalline structure of the MnO2 nanosheets is investigated by the SAED pattern (the inset of Fig. 5d), which is pointed to (020) and (002) diffractions of birnessite-type MnO2. These results further demonstrated the formation of the core-shell nanostructure. The porous structure and surface area of the sample are further investigated by nitrogen adsorption-desorption measurements (Fig. 6). The adsorption-desorption isotherm is characteristic of type IV, manifesting the open mesoporous structure. The Brunauer-Emmett-Teller (BET) surface area of the sample is calculated to be 51 m2 g1, and the pore volume of the sample is about 0.19 cm3 g1. What’s more, the pore size distribution by the Barrett-Joyner-Halenda (BJH) method makes it clear that the average pore size is 14.9 nm. Apparently, the large BET surface
area and porous structure can promote efficient transport of electrons and permeation of ions between the electrode and electrolyte, resulting in high specific capacitance. The electrochemical performances of the electrode are systematically measured using 1 M Na2SO4 solution as electrolyte in a three-electrode system. The CV curves of the electrode measured at different scan rates are shown in Fig. 7a. The nearly rectangular profiles of the CV curves without distinct redox peaks are clearly detected, indicating faradic pseudocapacitive nature of the electrode [33–35]. The appearance suggests the fast and reversible consecutive faradic reaction between alkali cations (Na+) and electrode [36]. With the scan rates increase, the CV curve doesn’t change visibly, which shows good electrochemical properties. The one-dimensional core mainly serves as an efficient media for charge transport and improves the loading efficiency of the MnO2 in neutral electrolyte [37]. As for the charge storage mechanisms in MnO2, two kinds of mechanisms are proposed. First one involves the intercalation/extraction of Na+ with concomitant reduction/ oxidation of the Mn ion (Eq. (1)). The other one is a surface process, including the adsorption/desorption of Na+ (Eq. (2)) [38,39]. The “M” represents Na+ or H+: MnO2 + M+ + e $ MnOOM
(MnO2)
+ surface + M + e $ (MnOOM) surface
(1)
(2)
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Fig. 5. Low-magnification TEM image of the Cu/Ni-based manganese dioxide (a); High-magnification TEM image of the sample (b); HRTEM image of MnO2 sheets (c) and Cu/ Ni-based composites (d); The inset is the SAED pattern of corresponding MnO2.
In addition, the nearly symmetrical triangular outlines manifest the capacitive and reversible character of the electrode. The specific capacitances of the electrode are calculated to be 374, 354, 328, 307, 281 F g1 at 0.25, 0.5, 1.0, 4.0, 8.0 A g1, displaying good rate performance with 75.1% retention of the original. This value exceeds many other electrode materials compared with previous published results (Table S1). The pleasurable electrochemical properties of the electrode are ascribed to the hierarchical architecture and the synergistic effect of composites. Specifically, a highly porous structure has been developed by interwoven MnO2 nanosheets on the core, which not only aggrandize efficient utilization of active MnO2, but also provide sufficient contact area between the active materials and the electrolytes. Other than that, the porous MnO2 and hollow channel can shorten the electron and ion diffusion paths, and make possible fast and reversible redox reactions on the surface and inside of the nanotubes. Still, although
the incorporation of CuO and NiO in the composites can improve the loading efficiency of the MnO2, they could suffer from low conductivity. Fortunately, the Cu and Ni can improve the electrical conductivity of the electrode, which further promote the transport and collection of electrons, leading to a fast charge-discharge rate [16,21,40]. Cycling performance of the electrode is a significant evaluation index of supercapacitors for practical applications, which is presented in Fig. 7d with the current density of 6 A g1. Obviously, the specific capacitance still maintains 86.9% after 3000 cycles. Nyquist plots for the electrode before and after cycles are further demonstrated in Fig. 7e. The internal resistances (Rs) of the electrode are 1.3 and 1.5 V before and after 3000 cycles, and the charge transfer resistance (Rct) varies from 11.3 to 12.5 V with a slight change. Furthermore, as shown in Fig. 7f, the morphology and structure of the electrode changes little compared with the
Fig. 6. Nitrogen adsorption/desorption isotherms (a) and corresponding pore size distribution (b) of the Cu/Ni-based manganese dioxide.
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Fig. 7. CV curves of the sample measured at different scan rates (a); Charge-discharge curves of the sample at different current densities (b); Specific capacitances of the sample under different current densities (c); Cycling performance of the sample at the current density of 6 A g1 (d), the inset shows the charge-discharge curves after different cycles; Nyquist plots for the sample before and after cycle tests (e), the inset shows the equivalent circuit; SEM images of the sample after 3000 cycles (f).
initial, which elaborated the good cycling performance. These results imply that this nanocomposite is appropriate for supercapacitor electrodes. 4. Conclusions In this study, the hierarchical Cu/Ni-based manganese dioxide core-shell nanostructure has been synthesized successfully by hydrothermal methods. Impressively, the “core” can transfer to hollow structure, which is initiated by the Kirkendall effect during the reaction process. The theoretical mechanism is certainly irradiative to the development of materials with unique nanostructure. With this novel structure and a specific redox, the
electrochemical properties of the electrode are certainly improved. 1) Cu and Ni can improve the electrical conductivity of the electrode. 2) Cu/Ni-based MnO2 nanostructure with porous nansheets with large hollow channel can own high surface area and boost up the effective interaction on the surface and inside of the nanotubes between the electrode and electrolyte. 3) The composite material brings about an effective faradic reaction of the electrode. The electrode exhibits a high specific capacitance (374 F g1 at current density of 0.25 A g1), excellent rate capability and good cycling performance (86.9% retention after 3000 cycles). These consequences suggest that the new copper/nickel-based materials with remarkably enhanced capacitive behaviors could be a potential electrode material for energy storage devices.
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Acknowledgements The authors sincerely acknowledge the financial supports provided by National Natural Science Foundation of China (Grant no. 51104194 and 21576034), International S&T Cooperation Projects of Chongqing (CSTC2013gjhz90001), National Key laboratory of Fundamental Science of Micro/Nano-device and System Technology (2013MS06, Chongqing University), State Education Ministry and Fundamental Research Funds for the Central Universities (Project no. CDJZR14135501, Chongqing University, PR China). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.07.024. References [1] J.R. Miller, P. Simon, Electrochemical Capacitors for Energy Management, Science 321 (2008) 651–652. [2] C.-C. Hu, K.-H. Chang, M.-C. Lin, Y.-T. Wu, Design and Tailoring of the Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors, Nano. Lett. 6 (2006) 2690–2695. [3] I. Hadjipaschalis, A. Poullikkas, V. Efthimiou, Overview of current and future energy storage technologies for electric power applications, Renew. Sust. Energ. Rev. 13 (2009) 1513–1522. [4] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical Energy Storage for the Grid: A Battery of Choices, Science 334 (2011) 928–935. [5] H. Ibrahim, A. Ilinca, J. Perron, Energy storage systems-Characteristics and comparisons, Renew. Sust. Energ. Rev. 12 (2008) 1221–1250. [6] M. Winter, R.J. Brodd, What Are Batteries Fuel Cells, and Supercapacitors? Chem. Rev. 104 (2004) 4245–4269. [7] J.J. Yoo, K. Balakrishnan, J. Huang, V. Meunier, B.G. Sumpter, A. Srivastava, M. Conway, A.L. Reddy, J. Yu, R. Vajtai, P.M. Ajayan, Ultrathin planar graphene supercapacitors, Nano Lett. 11 (2011) 1423–1427. [8] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797–828. [9] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520–2531. [10] H. Xia, C. Hong, B. Li, B. Zhao, Z. Lin, M. Zheng, S.V. Savilov, S.M. Aldoshin, Facile Synthesis of Hematite Quantum-Dot/Functionalized Graphene-Sheet Composites as Advanced Anode Materials for Asymmetric Supercapacitors, Adv. Funct. Mater. 25 (2015) 627–635. [11] R.V. Kumar, Y. Diamant, A. Gedanken, Sonochemical Synthesis and Characterization of Nanometer-Size Transition Metal Oxides from Metal Acetates, Chem. Mater. 12 (2000) 2301–2305. [12] H. Guan, L.-Z. Fan, H. Zhang, X. Qu, Polyaniline nanofibers obtained by interfacial polymerization for high-rate supercapacitors, Electrochim. Acta 56 (2010) 964–968. [13] H.-F. Ju, W.-L. Song, L.-Z. Fan, Rational design of graphene/porous carbon aerogels for high-performance flexible all-solid-state supercapacitors, J. Mater. Chem. A 2 (2014) 10895–10903. [14] S. Devaraj, N. Munichandraiah, Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties, J. Phys. Chem. C 112 (2008) 4406–4417. [15] R. Li, J. Liu, Mechanistic investigation of the charge storage process of pseudocapacitive Fe3O4 nanorod film, Electrochim. Acta 120 (2014) 52–56. [16] H. Xia, D. Zhu, Z. Luo, Y. Yu, X. Shi, G. Yuan, J. Xie, Hierarchically structured Co3O4@Pt@MnO2 nanowire arrays for high-performance supercapacitors, Sci. Rep. 3 (2013) 2978. [17] D. Kong, J. Luo, Y. Wang, W. Ren, T. Yu, Y. Luo, Y. Yang, C. Cheng, ThreeDimensional Co3O4@MnO2 Hierarchical Nanoneedle Arrays: Morphology Control and Electrochemical Energy Storage, Adv. Funct. Mater. 24 (2014) 3815–3826.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hierarchical copper/nickel-based manganese dioxide core-shell nanostructure for supercapacitor electrodes Hao Chena , Xue Qiang Qib,** , Min Kuanga , Fan Dongc, Yu Xin Zhanga,d,* a
College of Material Science and Engineering, Chongqing University, Chongqing 400044, PR China College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, PR China d National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, PR China b c
A R T I C L E I N F O
Article history: Received 8 December 2015 Received in revised form 25 June 2016 Accepted 5 July 2016 Available online 6 July 2016 Keywords: Manganese dioxides Nanocomposites Core-shell nanostructure Supercapacitor
A B S T R A C T
A novel copper/nickel-based manganese dioxide core-shell nanostructure can be prepared for supercapacitor electrodes using facile hydrothermal methods. Attributed to Kirkendall-type diffusion process, the “core” can transfer to hollow structure during the reaction interestingly. The faradic reaction occurs effectively in the electrode and electrolyte due to the high conductivity of Cu/Ni and large hollow channel structure. As a result, the electrode displays a high specific capacitance (374 F g1 at current density of 0.25 A g1), fine cycling stability (86.9% retention after 3000 cycles) and outstanding rate capability. These interesting findings suggest that the hybrid material can be a promising candidate for supercapacitor electrodes. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the ever-increasing fossil fuels consumption, energy crisis has become more and more severe in the planet. Large amounts of energy storage technologies are emerging to solve this trouble [1–5]. Supercapacitors, with the features of high power density, fast charge-discharge characteristic and long cycling lifespan, play a potential role in energy storage devices [6–8]. These stunning properties are directly affected by the electrode materials. Based on the previous studies, electrode materials can be divided into three major types: (i) carbon materials, (ii) transition metal oxides, and (iii) conducting polymers [9–13]. Among these electrode materials, transition metal oxides (MnO2, Co3O4, NiO etc.) have drawn extensive research interests due to their advantages of low cost, simple operation, abundance and efficient Faradaic reaction [14–16]. MnO2 with different morphologies and structures has been investigated widely on account of many excellent characteristics, but most MnO2 materials constantly suffers from poor electrical
* Corresponding author at: College of Material Science and Engineering, Chongqing University, Chongqing 400044, PR China. ** Corresponding author. E-mail addresses: [email protected] (X.Q. Qi), [email protected] (Y.X. Zhang). http://dx.doi.org/10.1016/j.electacta.2016.07.024 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
conductivity (105–106 S cm1) and agglomeration [17]. Because of these deficiencies, MnO2 is arduous to be effectively utilized during Faradaic reactions. In other words, single-component nanomaterials can hardly satisfy the demand of pleasurable electrochemical performances for supercapacitors. To improve these issues, MnO2 is used to combine with other materials to increase electrical conductivity or design rational structures for favorable electrochemical properties. Based on these facts, numerous MnO2-based materials with different structures have been fabricated for supercapacitor electrodes, such as graphene@MnO2 [18], carbon nanotubes@MnO2 [19,20], CuO@MnO2 [21], Co3O4@MnO2 [22,23] nanocomposites. The electrochemical performances of these nanocomposites are effectively enhanced by the synergistic effect. Despite these improvements, it still remains a challenge to develop smart recombination and rational design for electrode materials with specific structure. Herein, we report a novel copper/nickel-based manganese dioxide core-shell nanostructure for supercapacitor electrodes via a facile hydrothermal method, in which the Cu/Ni-based nanotubes work as the “core” and porous MnO2 nanosheets serve as the “shell”. The unique core-shell nanostructure can benefit the improvements on the electrochemical properties of the electrode. Naturally, the core-shell nanostructure exhibits a high specific capacitance (374 F g1 at current density of 0.25 A g1), good cycling stability (86.9% retention after 3000 cycles), and
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remarkable rate capability. It is believed that the nanocomposites would be a promising electrode material for energy storage devices. 2. Experimental 2.1. Materials synthesis All the reagents in the experiments were of analytical purity and used without any further purification.
worked as the reference electrode and platinum plate was used as the counter electrode. A nickel foam (1 1 cm2) coated by the slurry was used as working electrode, which was composed of active materials, carbon black, and polyvinylidene fluoride (PVDF) with proportion of 7: 2: 1. The active materials were about 2.3 mg. The electrochemical impedance spectroscopy (EIS) was performed with a perturbation amplitude of 5 mV in the frequency between 100 kHz and 0.01 Hz. The specific capacitances are calculated according to the following equation [25]: Cm ¼
2.1.1. Synthesis of Cu/Ni nanowires In a typical synthesis [24], 15 ml NaOH (7 M) and 0.15 ml ethylenediamine (EDA) were mixed sufficiently in a reactor. Then, 0.26 ml Ni(NO3)26H2O (0.5 M) and 0.14 ml Cu(NO3)26H2O (0.5 M) were added and shaken until it turned blue, following by the sequential addition of 15 ml NaOH (7 M) and 0.15 ml hydrazine hydrate. Finally, the mixture was put into a Teflon-lined stainless steel autoclave maintained at 80 C for 2 h. The precipitate was washed and dried at 60 C in a vacuum for 12 h. 2.1.2. Synthesis of Cu/Ni-based manganese dioxide Briefly, Cu/Ni NWs (20 mg) were dissolved into 35 ml of 0.5 M KMnO4 solution to form homogeneous solution, and the mixture was transferred to a 50 ml Teflon-lined stainless steel autoclave which was sealed and maintained 140 C for 24 h. In the end, the samples were washed several times and dried at 60 C in a vacuum for 12 h. 2.2. Materials characterization The compositions of the samples were charicaterized by powder X-ray diffraction (XRD, D/max 2500, Cu Ka) and X-ray photoelectron spectroscope (XPS, Kratos XSAM800). The morphology and structure were observed by focused ion beam scanning electron microscopy (ZEISS AURIGA FIB/SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscopy (TEM, ZEISS LIBRA 200). The nitrogen adsorption-desorption isotherms were measured at 77 K by using micrometritics ASAP 2020 sorptometer. 2.3. Electrochemical measurements All the electrochemical measurements were carried out in 1 M Na2SO4 solution by using an electrochemical workstation (CHI 660E) in a three-electrode system. The saturated calomel (SCE) was
I Dt mDV
where I is the discharge current (A), Dt is the discharge time (s), m is the weight (g) of active materials, and DV is the discharging potential window (V). 3. Results and Discussion In order to confirm the composition and phase purity of the sample, XRD patterns of Cu/Ni-based manganese dioxide are shown in Fig. 1. It is clear that the diffraction peaks of Cu and Ni phases are in accord with the values in the standard card (Cu: JCPDS card no. 89-2838; Ni: JCPDS card no. 87-0712). [24] Meanwhile, the diffraction peaks of the CuO and NiO are well in line with the standard XRD patterns (CuO: JCPDS card no. 48-1548; NiO: JCPDS card no. 47-1049). The three strong diffraction peaks of the MnO2 at about 12.5 , 25.2 and 65.6 are indexed to (001), (002) and (020) planes of birnessite-type MnO2 (JCPDS card no. 80-1098) [21]. Obviously, no other diffraction peaks are detectable, indicating the high purity of the sample. Furthermore, the SEM image and corresponding EDS mapping of Cu/Ni-based manganese dioxide are shown in Fig. 2. As can be seen from the pictures, Cu, Ni, Mn and O elements are well distributed in the structure. Apparently, Cu and Ni elements are internally distributed, demonstrating the core-shell nanostructure. The strong C signal is observed because of the carbon-conducing tap for sample uploading. These results further confirm the composition of the sample. To better understand the chemical composition and oxidation states, XPS spectra of the composite is presented in Fig. 3. The Mn 2p XPS spectrum displays two major peaks with a spin-energy separation of 11.8 eV (Fig. 3a) [21]. Fig. 3b shows XPS of Ni 2p core level at binding energy of 871.9 and 860.5 eV, confirming the oxide in the sample as NiO. The peak at 854.7 eV is connected with Ni. In the Cu 2p spectrum, two peaks at binding energy of 954.2 and 934.4 eV are corresponding to Cu 2p1/2 and Cu 2p3/2. On the other
Fig. 1. XRD patterns of Cu/Ni-based manganese dioxide.
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Fig. 2. Energy dispersive spectroscopy (EDS) analysis (a), typical SEM image (b) and the corresponding EDS mapping (c) of Cu/Ni-based manganese dioxide.
hand, two satellite peaks at 962.7 and 943.6 eV demonstrate the oxide in the sample as CuO. [21] Moreover, another peak at 932.4 eV proves the presence of Cu (0). The fitted O 1 s spectrum is characterized by three bands in Fig. 3d, and the one at 530.4 eV corresponds to the O2 band with Cu, Ni and Mn. Two other peaks are observed, which is ascribed to the water molecules and absorption of oxygen on the surface. These results are consistent with the XRD analysis. A possible mechanism of growth is proposed for the core-shell nanotubes structure (Fig. 4a), and the process is mainly divided into two steps. MnO4 ions are firstly adsorbed on the surface of Cu/Ni nanowires. In this way, MnO4 ions and H2O molecules are in the progress to form MnO2 nuclei on the surface, and also partial
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oxidize Cu/Ni nanowires into CuO/NiO nanowires. Then, with the growth mechanism of Ostwald ripening process, the MnO2 nuclei aggregate and transform into MnO2 nanosheets as the time increases, which is driven by the minimization of surface energy [26–29]. On the other hand, hollow nanotubes take shape because of the Kirkendall-type diffusion process: metal ions diffuse outward faster than oxygen inward caused by the self-decomposition of KMnO4, and the vacancies can aggregate to form voids. [30–32] Impressively, the unique highly porous morphology with large hollow channel structure can provide a high surface area and more active sites for effective faradic reaction. Fig. 4 presents SEM images of Cu/Ni nanowires (b and c) and Cu/Ni-based manganese dioxide (d and e). As Fig. 4b and c show, plenty dispersed Cu/Ni nanowires are clearly observed [25]. The diameters of Cu/Ni nanowires rang from 100 to 200 nm, and the length of the nanowires are up to a few micrometres. After hydrothermal reaction, Cu/Ni-based nanowires are uniformly coated by MnO2 nanosheets with rough appearance (Fig. 4d). From the red dotted circle, the fracture structure can be clearly observed. After hydrothermal reaction, the length of core-shell nanostructures gets shorter than the original nanowires, because the long nanowires are broken under the strong oxidation. Interestingly, the core gradually transfers to hollow nanostructure with the progress of reaction, which is attributed to Kirkendall effect (Fig. 4e). Naturally, the core-shell nanotubes are beneficial to ions permeation and electrons transport, giving rise to good electrochemical properties of the electrode. In order to investigate the hierarchical core-shell structure, the TEM images of the Cu/Ni-based manganese dioxide are also displayed. As Fig. 5a presented, the core-shell nanowires are
Fig. 3. XPS spectra of Cu/Ni-based manganese dioxide: a) Mn 2p spectrum; b) Ni 2p spectrum; c) Cu 2p spectrum; d) O 1s spectrum.
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Fig. 4. Schematic illustration of the formation of Cu/Ni-based manganese dioxide (a); SEM images of Cu/Ni nanowires (b and c) and Cu/Ni-based manganese dioxide (c and d); The inset is corresponding detailed image.
clearly visible with a rough surface, and the average thickness of the MnO2 is about 150 nm. The core-shell structure is composed of hollow nanotube supported porous MnO2 nanosheets (Fig. 5b). Moreover, the high-magnification TEM image (the inset) of the hollow structure suggests that the large hollow channel and high surface area can promote the effective interaction on the surface of the core and inside of the nanotube, which is in favor of pleasurable electrochemical performances. Besides, the contour of core-shell nanostructure can be distinctly observed. Moreover, the interplanar distances of 0.14 and 0.35 nm are indexed as the (020) and (002) plane of birnessite-type MnO2, in line with the previous XRD data (Fig. 5c). The polycrystalline structure of the MnO2 nanosheets is investigated by the SAED pattern (the inset of Fig. 5d), which is pointed to (020) and (002) diffractions of birnessite-type MnO2. These results further demonstrated the formation of the core-shell nanostructure. The porous structure and surface area of the sample are further investigated by nitrogen adsorption-desorption measurements (Fig. 6). The adsorption-desorption isotherm is characteristic of type IV, manifesting the open mesoporous structure. The Brunauer-Emmett-Teller (BET) surface area of the sample is calculated to be 51 m2 g1, and the pore volume of the sample is about 0.19 cm3 g1. What’s more, the pore size distribution by the Barrett-Joyner-Halenda (BJH) method makes it clear that the average pore size is 14.9 nm. Apparently, the large BET surface
area and porous structure can promote efficient transport of electrons and permeation of ions between the electrode and electrolyte, resulting in high specific capacitance. The electrochemical performances of the electrode are systematically measured using 1 M Na2SO4 solution as electrolyte in a three-electrode system. The CV curves of the electrode measured at different scan rates are shown in Fig. 7a. The nearly rectangular profiles of the CV curves without distinct redox peaks are clearly detected, indicating faradic pseudocapacitive nature of the electrode [33–35]. The appearance suggests the fast and reversible consecutive faradic reaction between alkali cations (Na+) and electrode [36]. With the scan rates increase, the CV curve doesn’t change visibly, which shows good electrochemical properties. The one-dimensional core mainly serves as an efficient media for charge transport and improves the loading efficiency of the MnO2 in neutral electrolyte [37]. As for the charge storage mechanisms in MnO2, two kinds of mechanisms are proposed. First one involves the intercalation/extraction of Na+ with concomitant reduction/ oxidation of the Mn ion (Eq. (1)). The other one is a surface process, including the adsorption/desorption of Na+ (Eq. (2)) [38,39]. The “M” represents Na+ or H+: MnO2 + M+ + e $ MnOOM
(MnO2)
+ surface + M + e $ (MnOOM) surface
(1)
(2)
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Fig. 5. Low-magnification TEM image of the Cu/Ni-based manganese dioxide (a); High-magnification TEM image of the sample (b); HRTEM image of MnO2 sheets (c) and Cu/ Ni-based composites (d); The inset is the SAED pattern of corresponding MnO2.
In addition, the nearly symmetrical triangular outlines manifest the capacitive and reversible character of the electrode. The specific capacitances of the electrode are calculated to be 374, 354, 328, 307, 281 F g1 at 0.25, 0.5, 1.0, 4.0, 8.0 A g1, displaying good rate performance with 75.1% retention of the original. This value exceeds many other electrode materials compared with previous published results (Table S1). The pleasurable electrochemical properties of the electrode are ascribed to the hierarchical architecture and the synergistic effect of composites. Specifically, a highly porous structure has been developed by interwoven MnO2 nanosheets on the core, which not only aggrandize efficient utilization of active MnO2, but also provide sufficient contact area between the active materials and the electrolytes. Other than that, the porous MnO2 and hollow channel can shorten the electron and ion diffusion paths, and make possible fast and reversible redox reactions on the surface and inside of the nanotubes. Still, although
the incorporation of CuO and NiO in the composites can improve the loading efficiency of the MnO2, they could suffer from low conductivity. Fortunately, the Cu and Ni can improve the electrical conductivity of the electrode, which further promote the transport and collection of electrons, leading to a fast charge-discharge rate [16,21,40]. Cycling performance of the electrode is a significant evaluation index of supercapacitors for practical applications, which is presented in Fig. 7d with the current density of 6 A g1. Obviously, the specific capacitance still maintains 86.9% after 3000 cycles. Nyquist plots for the electrode before and after cycles are further demonstrated in Fig. 7e. The internal resistances (Rs) of the electrode are 1.3 and 1.5 V before and after 3000 cycles, and the charge transfer resistance (Rct) varies from 11.3 to 12.5 V with a slight change. Furthermore, as shown in Fig. 7f, the morphology and structure of the electrode changes little compared with the
Fig. 6. Nitrogen adsorption/desorption isotherms (a) and corresponding pore size distribution (b) of the Cu/Ni-based manganese dioxide.
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Fig. 7. CV curves of the sample measured at different scan rates (a); Charge-discharge curves of the sample at different current densities (b); Specific capacitances of the sample under different current densities (c); Cycling performance of the sample at the current density of 6 A g1 (d), the inset shows the charge-discharge curves after different cycles; Nyquist plots for the sample before and after cycle tests (e), the inset shows the equivalent circuit; SEM images of the sample after 3000 cycles (f).
initial, which elaborated the good cycling performance. These results imply that this nanocomposite is appropriate for supercapacitor electrodes. 4. Conclusions In this study, the hierarchical Cu/Ni-based manganese dioxide core-shell nanostructure has been synthesized successfully by hydrothermal methods. Impressively, the “core” can transfer to hollow structure, which is initiated by the Kirkendall effect during the reaction process. The theoretical mechanism is certainly irradiative to the development of materials with unique nanostructure. With this novel structure and a specific redox, the
electrochemical properties of the electrode are certainly improved. 1) Cu and Ni can improve the electrical conductivity of the electrode. 2) Cu/Ni-based MnO2 nanostructure with porous nansheets with large hollow channel can own high surface area and boost up the effective interaction on the surface and inside of the nanotubes between the electrode and electrolyte. 3) The composite material brings about an effective faradic reaction of the electrode. The electrode exhibits a high specific capacitance (374 F g1 at current density of 0.25 A g1), excellent rate capability and good cycling performance (86.9% retention after 3000 cycles). These consequences suggest that the new copper/nickel-based materials with remarkably enhanced capacitive behaviors could be a potential electrode material for energy storage devices.
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Acknowledgements The authors sincerely acknowledge the financial supports provided by National Natural Science Foundation of China (Grant no. 51104194 and 21576034), International S&T Cooperation Projects of Chongqing (CSTC2013gjhz90001), National Key laboratory of Fundamental Science of Micro/Nano-device and System Technology (2013MS06, Chongqing University), State Education Ministry and Fundamental Research Funds for the Central Universities (Project no. CDJZR14135501, Chongqing University, PR China). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.07.024. References [1] J.R. Miller, P. Simon, Electrochemical Capacitors for Energy Management, Science 321 (2008) 651–652. [2] C.-C. Hu, K.-H. Chang, M.-C. Lin, Y.-T. Wu, Design and Tailoring of the Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors, Nano. Lett. 6 (2006) 2690–2695. [3] I. Hadjipaschalis, A. Poullikkas, V. Efthimiou, Overview of current and future energy storage technologies for electric power applications, Renew. Sust. Energ. Rev. 13 (2009) 1513–1522. [4] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical Energy Storage for the Grid: A Battery of Choices, Science 334 (2011) 928–935. [5] H. Ibrahim, A. Ilinca, J. Perron, Energy storage systems-Characteristics and comparisons, Renew. Sust. Energ. Rev. 12 (2008) 1221–1250. [6] M. Winter, R.J. Brodd, What Are Batteries Fuel Cells, and Supercapacitors? Chem. Rev. 104 (2004) 4245–4269. [7] J.J. Yoo, K. Balakrishnan, J. Huang, V. Meunier, B.G. Sumpter, A. Srivastava, M. Conway, A.L. Reddy, J. Yu, R. Vajtai, P.M. Ajayan, Ultrathin planar graphene supercapacitors, Nano Lett. 11 (2011) 1423–1427. [8] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797–828. [9] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520–2531. [10] H. Xia, C. Hong, B. Li, B. Zhao, Z. Lin, M. Zheng, S.V. Savilov, S.M. Aldoshin, Facile Synthesis of Hematite Quantum-Dot/Functionalized Graphene-Sheet Composites as Advanced Anode Materials for Asymmetric Supercapacitors, Adv. Funct. Mater. 25 (2015) 627–635. [11] R.V. Kumar, Y. Diamant, A. Gedanken, Sonochemical Synthesis and Characterization of Nanometer-Size Transition Metal Oxides from Metal Acetates, Chem. Mater. 12 (2000) 2301–2305. [12] H. Guan, L.-Z. Fan, H. Zhang, X. Qu, Polyaniline nanofibers obtained by interfacial polymerization for high-rate supercapacitors, Electrochim. Acta 56 (2010) 964–968. [13] H.-F. Ju, W.-L. Song, L.-Z. Fan, Rational design of graphene/porous carbon aerogels for high-performance flexible all-solid-state supercapacitors, J. Mater. Chem. A 2 (2014) 10895–10903. [14] S. Devaraj, N. Munichandraiah, Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties, J. Phys. Chem. C 112 (2008) 4406–4417. [15] R. Li, J. Liu, Mechanistic investigation of the charge storage process of pseudocapacitive Fe3O4 nanorod film, Electrochim. Acta 120 (2014) 52–56. [16] H. Xia, D. Zhu, Z. Luo, Y. Yu, X. Shi, G. Yuan, J. Xie, Hierarchically structured Co3O4@Pt@MnO2 nanowire arrays for high-performance supercapacitors, Sci. Rep. 3 (2013) 2978. [17] D. Kong, J. Luo, Y. Wang, W. Ren, T. Yu, Y. Luo, Y. Yang, C. Cheng, ThreeDimensional Co3O4@MnO2 Hierarchical Nanoneedle Arrays: Morphology Control and Electrochemical Energy Storage, Adv. Funct. Mater. 24 (2014) 3815–3826.
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