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MnO2 nanocylinder array with high capacitance for supercapacitors
Results in Physics 12 (2019) 1411–1416
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Three-dimensional Ni/MnO2 nanocylinder array with high capacitance for supercapacitors
T
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W.L. Xiea, M.Y. Sunb, Y.Q. Lia, B. Zhanga, X.Y. Langa, Y.F. Zhua, , Q. Jianga a Key Laboratory of Automobile Materials, Ministry of Education (Jilin University), School of Materials Science and Engineering, Jilin University, Changchun 130022, China b School of Materials Science and Engineering, Fudan University, Shanghai 200433, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Supercapacitor Hybrid electrode Ni nanocylinder arrays δ-MnO2 3D nanostructure
Though high-capacitance transition metal oxides are considered as promising pseudocapacitive materials to achieve high-energy storage for supercapacitors, the practical capacitance is far from the theoretical values because of their intrinsic poor abilities about the electronic and ionic transports. Here, to tackle these problems, we develop a hybrid nanostructured electrode in a facile process, in which layer-structured δ-MnO2 nanosheets are anchored onto seamless three-dimensional Ni nanocylinder arrays. The resultant Ni/δ-MnO2 nanocylinder arrays exhibit high specific capacitance up to 883.2 Fg−1 at a scan rate of 10 mVs−1 and achieve high energy density about 108 Whkg−1 at a power of 2.4 kWkg−1. The improved electrochemical performance mainly benefits from the vertically aligned Ni/δ-MnO2 hybrid structure. Owing to such a hybrid structure the ion/ electron transports are facilitated along the nanocylinders due to the reduction in the transport distance, and large accessible surface area is also provided for anchoring more high-capacitance materials. Moreover, the additional contact resistance can be dramatically reduced owing to the large interplanar spacing of δ-MnO2 and ordered semicoherent interface of Ni/δ-MnO2.
Introduction With the global ever-growing energy demand, numerous kinds of clean and renewable energy, such as solar, wind and tide, are emerging and fast developing [1,2]. Corresponding energy storage devices, in which the massive energy can be stored and delivered at a fast rate costeffectively, have become a research hotpot in recent years [3,4]. Though conventional carbon-based supercapacitors show high power capability, intrinsic low theoretical capacitance due to the adsorption/ desorption mechanism hinders their applications on high-energy requiring occasions [5,6]. As promising alternatives, transition metal oxides (TMOs) as MxOy (M = Fe, Co, Ni, Mn and so on) possess high theoretical capacitance (∼1000 Fg−1) endowed with high energy storage [7–13]. However, intrinsic poor electronic/ionic conductivities of TMOs undermine their advantages, making their actual capacitance far below their theoretical values [14–16]. In order to address this issue, much efforts have been devoted to incorporate nanostructured TMOs with high conductive current collectors [17–22]. Among them, conventional planar electrodes typically combine the active materials with the current collectors by using insulative polymeric binders. However, the contact resistance is dramatically increased, hindering the electron
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transport in the electrodes [23,24]. Furthermore, poor ionic diffusion capability due to low porosity of planar electrodes also restricts the sufficient contact between the TMOs and the electrolyte. Recently, series of processes, such as alloying/dealloying [25], annealing [26], chemical vapor deposition [27] and hydrothermal reaction [28], have been utilized to successfully fabricate electrodes with 3D nanoporous structure, ensuring sufficient TMOs/electrolyte interfaces for fast redox reactions [29–31]. Several aspects of the benefits can be obtained through these methods [32,33]. Firstly, the contact area between the electrode and the electrolyte rises effectively, so do the active sites. Secondly, since the process of storing energy of active materials mainly occurs in the thin surface layer, reducing the size of active materials is conducive to improving the utilization rate of active materials. Thirdly, it can effectively inhibit the volume change of the active material during the charge/discharge process and maintain the integrity of the electrode structure. Fourthly, shortening the ions transport distance is beneficial for higher power performance. However, these methods usually involve complex procedures and high cost, limiting the extensive promotion in practical mass production. Hence, it is of necessity to develop manufacture methods of supercapacitors which are easy to fabricate and simple in steps. In order to
Corresponding author. E-mail address: [email protected] (Y.F. Zhu).
https://doi.org/10.1016/j.rinp.2019.01.041 Received 15 December 2018; Accepted 15 January 2019 Available online 18 January 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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W.L. Xie et al.
Fig. 1. Schematic illustration of the fabrication process of the Ni/MnO2 NCAs electrode. (a) Cleaned Ni foam as the substrate. (b) Ni nanocylinder arrays produced by electrochemical etching Ni foam in H3BO3 solution. (c) MnO2 nanosheets were anchored onto Ni NCAs by the pulse electrodeposition method.
evaluated in a three-electrode system in 1 M Na2SO4 aqueous electrolyte, using a Ni/MnO2 NCAs electrode as the work electrode, a carbon rod and an Ag/AgCl electrode as counter and reference electrode, respectively. Cyclic voltammetry (CV) tests were performed in a potential window ranging from 0 to 0.8 V at various scan rates, and the galvanostatic charge/discharge (GCD) tests were performed in a potential window ranging from 0 to 0.8 V at various current densities.
achieve these purposes, electrochemical corrosion and electrodeposition at room temperature are believed to be beneficial for simplifying the process. Besides this, MnO2 is a transition metal oxide with high theoretical capacity as the active material on electrodes. Some researchers reported that the capacitance of MnO2 in MnO2-based supercapacitors is mainly determined by its crystal structure [34]. Compared to α-MnO2, δ-MnO2 has a large specific capacitance due to its large interlayer spacing with a simple preparation process. Because of this, δ-MnO2 has become an ideal active material in supercapacitor applications. In this study, we developed a 3D nanoporous hybrid electrode via a simple electro-etching/depositing way, in which layer-structured δMnO2 nanosheets are anchored onto seamless Ni foam/Ni nanocylinder arrays current collector. The synthesis procedure of the Ni nanocylinder arrays by electro-etching is quite different from other reported conventional methods, such as hydrothermal synthesis and chemical vapor deposition. It is expected that the high electronic conductive Ni nanocylinder arrays can greatly increase the accessible surface area of the electrode and facilitate the ions/electrons transport through the hybrid electrodes. Meanwhile, the layer-structured δ-MnO2 nanosheets can offer wide-open channels for accommodating more ions and accelerating ion transports in the bulk of MnO2 [34]. As a result of large accessible surface area and the enhanced ionic/electronic transport, fast and reversible faradaic redox reactions of MnO2 are greatly boosted. Strikingly, the capacitance is achieved high as 883.2 Fg−1 at a scan rate of 10 mVs−1, and the energy density is high as of 108 Whkg−1 at a power of 2.4 kWkg−1.
Results and discussion Fig. 1 shows a facile fabrication process of the Ni/MnO2 NCAs hybrid electrode comprised of electrochemical etching of Ni foam and pulse electrodeposition of MnO2. During the etching process at a potential of 1.0 V, surface Ni atoms dissolve. However, the resultant Ni2+ ions react with oxygen in the environment, forming a passivation film to protect the underlying Ni against further dissolution. Since this passivation film will limit the electron transport from active materials to the current collector, to avoid this negative impact, the surface oxide was etched away in 1 M HCl, and 3D Ni nanocylinder arrays are thus obtained on the nickel foam substrate [29]. Fig. 2a shows a typical SEM image of the Ni foam with a smooth surface. After etching process at 1 V, Ni nanocylinder arrays are uniformly and vertically formed on the Ni foam substrate with the diameter of ∼100 nm and gap of ∼100 nm on the average (Fig. 2b). The vertical aligned structure of the Ni nanocylinder arrays can facilitate the transport of electrons along the nanocylinder wall, and the moderate interval between the Ni nanocylinders are also helpful for electrolyte ions diffusion in the whole hybrid electrode. Furthermore, the Ni nanocylinder arrays can offer large accessible surface area for anchoring more high-capacitance materials to achieve high energy density and power density. It is well known that the merit of supercapacitors is their ability to store large amount of energy at fast charge/discharge rate. The fast adsorption/desorption and/or reversible redox reaction just occurs on the surface (∼20 nm thick) of electro-active materials. Thick electro-active materials will lead to low utilization rate (high dead volume) and aggravate the overall electrochemical performance of the electrode. In view of this, a thin layer of MnO2 was electrodeposited onto the current collector in this work. During the deposition, MnO2 nanosheets with a thickness of ∼10 nm are uniformly anchored onto the Ni NCAs, forming a rough oxide surface morphology (Fig. 2c). This rough surface morphology of the Ni/MnO2 NCAs greatly increases the accessible surface area of the hybrid electrode, which is in stark contrast to the traditional planar electrodes with dense oxide layer resulting in inferior ion/electron transports. HRTEM image of the Ni/ MnO2 composite illustrates that the layered δ-MnO2 bonded with Ni via the semicoherent interfaces (Fig. 2d), where the interplanar distances are 0.239 and 0.204 nm corresponding to the lattice planes of the layerstructured δ-MnO2 (0 0 3) and Ni (1 1 1), respectively [31]. Undoubtedly, the wide interplanar spacing and the ordered semicoherent interface can provide freeways for the transportation of electrolyte ions and electrons, dramatically reducing the additional contact resistance. The crystallographic structure of the deposited MnO2 nanosheets is attested by the XRD characterization. As shown in Fig. 2e, three typical diffraction peaks at 2θ = 22.6°, 36.7° and 66° are attributed to the − (0 0 2), (1 1 1), and (0 2 1) plane of layer structured δ-MnO2, respectively [34]. The layered structure of δ-MnO2 can facilitate the accessibility and diffusion of electrolyte ions for fast faradic redox reaction. Raman spectrum in Fig. 2f further confirms the crystal structure of δ-
Experimental section Fabrication of Ni/MnO2 NCAs electrode At first, Ni foam was cleaned with acetone, alcohol, deionized water and 1 M HCl subsequently for 30 min to remove the surface organics, resident acetone and surface oxides, and then it was dried in vacuum at room temperature. The 3D nanostructured current collector of Ni nanocylinder arrays (Ni NCAs) was prepared via etching the Ni foam at 1.0 V for 12 h in 0.5 M H3BO3 at room temperature. After removing the surface oxides in 1 M HCl for another 30 min, the combination of MnO2 and Ni NCAs was carried out by pulse electrodeposition in 0.1 M KMnO4 with on-time of 1 s at −0.15 V and off-time of 10 s at 0.4 V for 8 cycles in a three-electrode system using a carbon rod and an Ag/AgCl electrode as counter and reference electrode, respectively. Characterization The microstructure of Ni/MnO2 NCAs was characterized using a field emission scanning electron microscope (JEOL JSM-6700F, 15 keV). The X-ray diffraction (XRD) pattern was investigated using a D/max2500pc diffractometer using Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS) was measured using an ESCALAB Mk II (Vacuum Generators) spectrometer with an Al anode. The Raman spectrum measurement was carried out on a Renishaw Raman spectrometer using 532 nm wavelength with intensity of 0.1 mW. Electrochemical measurement All electrochemical performances of the supercapacitors were 1412
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Fig. 2. Structure features of the Ni/MnO2 NCAs electrode. Typical SEM images for Ni foam (a) before and (b) after etching. (c) SEM image for Ni NCAs electrode after loading of MnO2. (d) HRTEM image of Ni/MnO2 structure. (e) XRD pattern of MnO2 nanosheets. (f) Raman spectrum of the Ni/MnO2 NCAs electrode. (g) Typical XPS survey spectrum for as-synthesized MnO2. High-resolution XPS spectrum for (h) Mn 2p and (i) O 1s.
MnO2. The characteristic band at 638 cm−1 belongs to the transverse vibration of MnO2, while the one at 578 cm−1 is attributed to the longitudinal vibration of MnO2. The latter is usually related to the intrinsic vibration frequency of the Mn4+ ions. The high characteristic band strength at 578 cm−1 can also certify the layered structure of the as-synthesized δ-MnO2. The nanostructure characteristics of the hybrid Ni/MnO2 NCAs electrode exhibits excellent electronic and ionic conductivity, while the favorable crystal structure of δ-MnO2 endows fast transport of electrolyte ions inside the active materials [34]. The chemical states of Mn and O components in MnO2 are further investigated by XPS (Fig. 2g). High-resolution Mn 2p spectrum demonstrates the existence of Mn species. The peaks at the binding energy of 640.5 and 651.9 eV are attributed to 2p3/2 and 2p1/2 for Mn4+, while those at 640.2 and 644.1 eV and at 644.0 and 655.2 eV are ascribed to small amounts of Mn2+ and Mn3+, respectively (Fig. 2h) [31]. The O 1s spectrum consists of three typical peaks centered at 529.7, 531.4 and 532.8 eV, which correspond to the metaleoxygen bond, O in OH−, and O contained in absorbed H2O, respectively (Fig. 2i) [35]. The FTIR and EDAX results are shown in Supporting Information. The FTIR absorption peaks of 708 cm−1 are attributed to the vibrations of the Mn-O bonds in the [MnO6] eight plane of the MnO2 crystal, and 3368 cm−1 corresponds to the stretching vibration peak of surface hydroxyl groups of manganese dioxide (Fig. S1), which are consistent with the data in the literature results [36]. The presence of Mn, O and Ni is clearly seen in the EDAX profiles. The Mn and O signals come from MnO2, and the Ni signals is originated from the Ni NCAs. Pseudocapactive performance of the Ni/MnO2 NCAs electrode was evaluated in 1 M Na2SO4. Fig. 3a shows the representative CV curves of
the Ni/MnO2 NCAs electrode at scan rates ranging from 10 to 200 mVs−1. The CV curves show ideally symmetric rectangle shape even at 200 mVs−1, which indicates fast reversible faradaic redox reactions and low contact resistance in the Ni/MnO2 NCAs electrodes [37,38]. By calculation, the Ni/MnO2 NCAs electrode achieves high specific capacitance of 883.2 Fg−1 at 10 mVs−1, which decreases to 407.6 Fg−1 as the scan rate increases to 200 mVs−1. The approximate rectangular and symmetrical shape of CV curves reveal the fast reversible faradaic redox reactions on the electrode. Fig. 3b presents the variation of the areal capacitance of the Ni/MnO2 NCAs electrodes as a function of scan rate, compared with the bare Ni foam and the Ni NCAs. The areal capacitance of the Ni/MnO2 NCAs electrode exhibits 22.7 mFcm−2 at 10 mVs−1, and it is maintained at 8.3 mFcm−2 as the scan rate increases by 50-folds (up to 500 mVs−1). The areal capacitances of the bare Ni NCAs and the Ni foam electrodes only achieve 3.8 and 2.9 mFcm−2 at 10 mVs−1 and remain as 1 and 0.6 mFcm−2 at 500 mVs−1 under the same condition. Compared the Ni foam and the Ni NCAs electrodes, the corrosion process effectively increases the accessible surface area of the substrates, and the capacitance significantly increases after the incorporation of the MnO2 nanosheets. The galvanostatic charge/discharge test of the Ni/MnO2 NCAs electrode also confirms the rate characteristic under different current densities in Fig. 3c. The symmetric charge/discharge profiles demonstrate the fast and reversible redox process of the Ni/MnO2 NCAs electrode. At current density of 1 mAcm−2, the specific capacitance reaches 138.7 mFcm−2, which can be maintained as 112.5 mFcm−2 at 2 mAcm−2 and 75 mFcm−2 at 5 mAcm−2. Fig. 3d shows the galvanostatic charge/ discharge curves of the Ni/MnO2 NCAs electrode at 1 mAcm−2 1413
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Fig. 3. Electrochemical characterization of the Ni/MnO2 NCAs electrode. (a) CV curves at scan rates ranging from 10 to 200 mVs−1. (b) Specific capacitance of the Ni/MnO2 NCAs as a function of scan rates ranging from 10 to 200 mVs−1, compared with the values of Ni/MnO2 electrode, Ni NCAs and bare Ni foam. (c) Galvanostatic charge/discharge curves at current ranging from 1 to 5 mAcm−2. (d) Galvanostatic charge/discharge curves of the Ni/MnO2 NCAs electrode at 1 mAcm−2, compared with the values of the Ni NCAs and the bare Ni foam.
electrode demonstrates the enhanced electrolyte ion access/diffusion ability due to the wide-open structure of the nanocylinder arrays and the δ-MnO2 component [39]. Fig. 4c investigates the cycling stability of the Ni/MnO2 NCAs supercapacitor at a high current density of 1 mAcm−2. After 2000 cycles, the specific capacitance is maintained at 73.8% of the initial value, suggesting a great stability of the 3D hybrid electrode. Fig. 4d shows the Ragone plot to compare the power and energy densities of the Ni/MnO2 NCAs supercapacitor with recently reported results [40–47]. By anchoring layer structured δ-MnO2 nanosheets onto the seamless Ni foam/Ni nanocylinder arrays current collector without using polymer binders as well as tuning the crystallographic structure of the electrode materials, we minimize the internal resistances and significantly improve the pseudocapacitive energy storage. The Ni/ MnO2 NCAs supercapacitor exhibits high energy density of 108 Whkg−1 at 2.4 kWkg−1. When the power density increases by about 20 times (up to 44.4 kWkg−1), the energy density can still be maintained at ∼49.4 Whkg−1, much higher than other different electrodes reported recently, such as the MnO2/RGO [40], the MnO2 nanorods [41], the LDH/PEDOT-Ni foam [42], the MnO2/Graphene/CNF [43], the MnO2/NPG hybrid electrode [44], the CNPs/MnO2 nanorods hybrid electrode [45], the MnO2/NiCo2O4 composite electrode [46], and the Ni-Mn LDH/MnO2 electrode [47]. In summary, we have developed a 3D nanoporous hybrid electrode via a simple electro-etching/depositing way. The resultant Ni/MnO2 NCAs deliver a high specific capacitance and high energy density. This achievement is benefited from not only the large interplanar spacing of δ-MnO2 but also the fast ion and electron transports provided by the high conductive Ni NCAs and the ordered semicoherent Ni/MnO2 interfaces.
compared with the values of the Ni NCAs and the Ni foam. It can be deduced that the capacitance of the electrode is significantly improved after the MnO2 deposition, as shown in Fig. 3b. The areal capacitance of the Ni/MnO2 NCAs electrode achieves 138.7 mFcm−2 at 1 mAcm−2, while those of the Ni NCAs and the Ni foam electrodes are only 10 and 6.2 mFcm−2, respectively. Fig. 4a shows a thorough inquiry about the influence of the surface morphology of the Ni/MnO2 NCAs electrodes under the ion transport condition. Due to the proton penetration of electrolyte ions in the Na2SO4 solution, the profile of voltammetric charge (q*) and scan rate (υ) approximates a straight line dependence. Herein, q0* is used to express the q* value of the voltammetric charge as v → 0, and the value is calculated as 202.7 Cg−1, which is thought to be associated with the whole active material oxide surface. Correspondingly, q∞* is used as v → ∞ and calculated as 769.2 Cg−1. This denotes that only the outermost oxide surface participates fast reversible faradaic redox reactions [15]. Therefore, the q∞* only accounts for 25.5% of the whole oxide surface, indicating that the 3D architectures of Ni nanocylinder arrays can greatly enhance the accessibility/diffusion of ions. EIS analysis is tested to investigate the superb electrochemical performance of the Ni/MnO2 NCAs electrodes in a frequency range from 100 kHz to 10 mHz. Fig. 4b is conducted to show the impedance result of the Ni/MnO2 NCAs electrode. The intrinsic resistance between electrolyte and electrode (Ri) is calculated to be 4.6 Ω, and the charge transfer resistance (Rct) is 0.12 Ω for the Ni/MnO2 NCAs electrode. The low value of Rct represents that the fast electron transfer can be achieved at the interface of electrode and electrolyte, and the power delivery of the pseudocapacitance can thus be enhanced. This enhancement is benefited from the fast electron and ion transports provided by the high conductive Ni NCAs, as well as the absence of insulative polymer binder between the oxides and the current collectors. Moreover, the low Warburg resistance (Zw) of the Ni/MnO2 NCAs 1414
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Fig. 4. (a) The relationship between voltammetric charge (q*) and scan rate (υ). (b) Nyquist plots of the Ni/MnO2 NCAs electrodes over frequency ranges from 100 kHz to 10 mHz. Inset: A magnification of EIS at the high-frequency region and the electrical equivalent circuit used for fitting EIS. (c) Cycling performance of the Ni/MnO2 NCAs electrode at a current density of 1 mAcm−2 for 2000 cycles. (d) Ragone plot for comparing the energy and power densities of the Ni/MnO2 NCAs electrodes with the literature results.
Conclusions
Appendix A. Supplementary data
Supercapacitors with high energy storage and fast charge/discharge performance have been fabricated by combining MnO2 nanosheets with 3D seamless Ni foam/Ni NCAs arrays. Here, the Ni NCAs can greatly increase the accessible surface area of the electrode for anchoring more high-capacitance materials to achieve high energy density and power density. It can also facilitate the transport of electrolyte ions due to the reduction in the transport distance inside the hybrid electrodes. Besides this, the layer structured δ-MnO2 nanosheets can offer large channels for further accelerating ion transports in the bulk of MnO2, and the ordered semicoherent interfaces lead to reduced additional contact resistances between MnO2 and Ni foam. These features of crystallographic structure and nanostructure enable superb electronic and ionic conductivities in the Ni/MnO2 NCAs electrodes. As a result, the fast reversible faradaic redox reaction of MnO2 is greatly boosted. The capacitance is achieved high as 883.2 Fg−1 at a scan rate of 10 mVs−1, and the energy density is high as 108 Whkg−1 at 2.4 kWkg−1. The outstanding performance makes the Ni/MnO2 NCAs electrodes possess great potential as a stable energy storage device that can be widely used.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 61674069 and No. 51631004), the Key Scientific and Technological Research and Development Project of Jilin Province (20180201080GX), the Fundamental Research Funds for the Central Universities, the Program for Innovative Research Team (in Science and Technology) in University of Jilin Province and the Program for JLU Science and Technology Innovative Research Team (2017TD-09). 1415
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Results in Physics journal homepage: www.elsevier.com/locate/rinp
Three-dimensional Ni/MnO2 nanocylinder array with high capacitance for supercapacitors
T
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W.L. Xiea, M.Y. Sunb, Y.Q. Lia, B. Zhanga, X.Y. Langa, Y.F. Zhua, , Q. Jianga a Key Laboratory of Automobile Materials, Ministry of Education (Jilin University), School of Materials Science and Engineering, Jilin University, Changchun 130022, China b School of Materials Science and Engineering, Fudan University, Shanghai 200433, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Supercapacitor Hybrid electrode Ni nanocylinder arrays δ-MnO2 3D nanostructure
Though high-capacitance transition metal oxides are considered as promising pseudocapacitive materials to achieve high-energy storage for supercapacitors, the practical capacitance is far from the theoretical values because of their intrinsic poor abilities about the electronic and ionic transports. Here, to tackle these problems, we develop a hybrid nanostructured electrode in a facile process, in which layer-structured δ-MnO2 nanosheets are anchored onto seamless three-dimensional Ni nanocylinder arrays. The resultant Ni/δ-MnO2 nanocylinder arrays exhibit high specific capacitance up to 883.2 Fg−1 at a scan rate of 10 mVs−1 and achieve high energy density about 108 Whkg−1 at a power of 2.4 kWkg−1. The improved electrochemical performance mainly benefits from the vertically aligned Ni/δ-MnO2 hybrid structure. Owing to such a hybrid structure the ion/ electron transports are facilitated along the nanocylinders due to the reduction in the transport distance, and large accessible surface area is also provided for anchoring more high-capacitance materials. Moreover, the additional contact resistance can be dramatically reduced owing to the large interplanar spacing of δ-MnO2 and ordered semicoherent interface of Ni/δ-MnO2.
Introduction With the global ever-growing energy demand, numerous kinds of clean and renewable energy, such as solar, wind and tide, are emerging and fast developing [1,2]. Corresponding energy storage devices, in which the massive energy can be stored and delivered at a fast rate costeffectively, have become a research hotpot in recent years [3,4]. Though conventional carbon-based supercapacitors show high power capability, intrinsic low theoretical capacitance due to the adsorption/ desorption mechanism hinders their applications on high-energy requiring occasions [5,6]. As promising alternatives, transition metal oxides (TMOs) as MxOy (M = Fe, Co, Ni, Mn and so on) possess high theoretical capacitance (∼1000 Fg−1) endowed with high energy storage [7–13]. However, intrinsic poor electronic/ionic conductivities of TMOs undermine their advantages, making their actual capacitance far below their theoretical values [14–16]. In order to address this issue, much efforts have been devoted to incorporate nanostructured TMOs with high conductive current collectors [17–22]. Among them, conventional planar electrodes typically combine the active materials with the current collectors by using insulative polymeric binders. However, the contact resistance is dramatically increased, hindering the electron
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transport in the electrodes [23,24]. Furthermore, poor ionic diffusion capability due to low porosity of planar electrodes also restricts the sufficient contact between the TMOs and the electrolyte. Recently, series of processes, such as alloying/dealloying [25], annealing [26], chemical vapor deposition [27] and hydrothermal reaction [28], have been utilized to successfully fabricate electrodes with 3D nanoporous structure, ensuring sufficient TMOs/electrolyte interfaces for fast redox reactions [29–31]. Several aspects of the benefits can be obtained through these methods [32,33]. Firstly, the contact area between the electrode and the electrolyte rises effectively, so do the active sites. Secondly, since the process of storing energy of active materials mainly occurs in the thin surface layer, reducing the size of active materials is conducive to improving the utilization rate of active materials. Thirdly, it can effectively inhibit the volume change of the active material during the charge/discharge process and maintain the integrity of the electrode structure. Fourthly, shortening the ions transport distance is beneficial for higher power performance. However, these methods usually involve complex procedures and high cost, limiting the extensive promotion in practical mass production. Hence, it is of necessity to develop manufacture methods of supercapacitors which are easy to fabricate and simple in steps. In order to
Corresponding author. E-mail address: [email protected] (Y.F. Zhu).
https://doi.org/10.1016/j.rinp.2019.01.041 Received 15 December 2018; Accepted 15 January 2019 Available online 18 January 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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Fig. 1. Schematic illustration of the fabrication process of the Ni/MnO2 NCAs electrode. (a) Cleaned Ni foam as the substrate. (b) Ni nanocylinder arrays produced by electrochemical etching Ni foam in H3BO3 solution. (c) MnO2 nanosheets were anchored onto Ni NCAs by the pulse electrodeposition method.
evaluated in a three-electrode system in 1 M Na2SO4 aqueous electrolyte, using a Ni/MnO2 NCAs electrode as the work electrode, a carbon rod and an Ag/AgCl electrode as counter and reference electrode, respectively. Cyclic voltammetry (CV) tests were performed in a potential window ranging from 0 to 0.8 V at various scan rates, and the galvanostatic charge/discharge (GCD) tests were performed in a potential window ranging from 0 to 0.8 V at various current densities.
achieve these purposes, electrochemical corrosion and electrodeposition at room temperature are believed to be beneficial for simplifying the process. Besides this, MnO2 is a transition metal oxide with high theoretical capacity as the active material on electrodes. Some researchers reported that the capacitance of MnO2 in MnO2-based supercapacitors is mainly determined by its crystal structure [34]. Compared to α-MnO2, δ-MnO2 has a large specific capacitance due to its large interlayer spacing with a simple preparation process. Because of this, δ-MnO2 has become an ideal active material in supercapacitor applications. In this study, we developed a 3D nanoporous hybrid electrode via a simple electro-etching/depositing way, in which layer-structured δMnO2 nanosheets are anchored onto seamless Ni foam/Ni nanocylinder arrays current collector. The synthesis procedure of the Ni nanocylinder arrays by electro-etching is quite different from other reported conventional methods, such as hydrothermal synthesis and chemical vapor deposition. It is expected that the high electronic conductive Ni nanocylinder arrays can greatly increase the accessible surface area of the electrode and facilitate the ions/electrons transport through the hybrid electrodes. Meanwhile, the layer-structured δ-MnO2 nanosheets can offer wide-open channels for accommodating more ions and accelerating ion transports in the bulk of MnO2 [34]. As a result of large accessible surface area and the enhanced ionic/electronic transport, fast and reversible faradaic redox reactions of MnO2 are greatly boosted. Strikingly, the capacitance is achieved high as 883.2 Fg−1 at a scan rate of 10 mVs−1, and the energy density is high as of 108 Whkg−1 at a power of 2.4 kWkg−1.
Results and discussion Fig. 1 shows a facile fabrication process of the Ni/MnO2 NCAs hybrid electrode comprised of electrochemical etching of Ni foam and pulse electrodeposition of MnO2. During the etching process at a potential of 1.0 V, surface Ni atoms dissolve. However, the resultant Ni2+ ions react with oxygen in the environment, forming a passivation film to protect the underlying Ni against further dissolution. Since this passivation film will limit the electron transport from active materials to the current collector, to avoid this negative impact, the surface oxide was etched away in 1 M HCl, and 3D Ni nanocylinder arrays are thus obtained on the nickel foam substrate [29]. Fig. 2a shows a typical SEM image of the Ni foam with a smooth surface. After etching process at 1 V, Ni nanocylinder arrays are uniformly and vertically formed on the Ni foam substrate with the diameter of ∼100 nm and gap of ∼100 nm on the average (Fig. 2b). The vertical aligned structure of the Ni nanocylinder arrays can facilitate the transport of electrons along the nanocylinder wall, and the moderate interval between the Ni nanocylinders are also helpful for electrolyte ions diffusion in the whole hybrid electrode. Furthermore, the Ni nanocylinder arrays can offer large accessible surface area for anchoring more high-capacitance materials to achieve high energy density and power density. It is well known that the merit of supercapacitors is their ability to store large amount of energy at fast charge/discharge rate. The fast adsorption/desorption and/or reversible redox reaction just occurs on the surface (∼20 nm thick) of electro-active materials. Thick electro-active materials will lead to low utilization rate (high dead volume) and aggravate the overall electrochemical performance of the electrode. In view of this, a thin layer of MnO2 was electrodeposited onto the current collector in this work. During the deposition, MnO2 nanosheets with a thickness of ∼10 nm are uniformly anchored onto the Ni NCAs, forming a rough oxide surface morphology (Fig. 2c). This rough surface morphology of the Ni/MnO2 NCAs greatly increases the accessible surface area of the hybrid electrode, which is in stark contrast to the traditional planar electrodes with dense oxide layer resulting in inferior ion/electron transports. HRTEM image of the Ni/ MnO2 composite illustrates that the layered δ-MnO2 bonded with Ni via the semicoherent interfaces (Fig. 2d), where the interplanar distances are 0.239 and 0.204 nm corresponding to the lattice planes of the layerstructured δ-MnO2 (0 0 3) and Ni (1 1 1), respectively [31]. Undoubtedly, the wide interplanar spacing and the ordered semicoherent interface can provide freeways for the transportation of electrolyte ions and electrons, dramatically reducing the additional contact resistance. The crystallographic structure of the deposited MnO2 nanosheets is attested by the XRD characterization. As shown in Fig. 2e, three typical diffraction peaks at 2θ = 22.6°, 36.7° and 66° are attributed to the − (0 0 2), (1 1 1), and (0 2 1) plane of layer structured δ-MnO2, respectively [34]. The layered structure of δ-MnO2 can facilitate the accessibility and diffusion of electrolyte ions for fast faradic redox reaction. Raman spectrum in Fig. 2f further confirms the crystal structure of δ-
Experimental section Fabrication of Ni/MnO2 NCAs electrode At first, Ni foam was cleaned with acetone, alcohol, deionized water and 1 M HCl subsequently for 30 min to remove the surface organics, resident acetone and surface oxides, and then it was dried in vacuum at room temperature. The 3D nanostructured current collector of Ni nanocylinder arrays (Ni NCAs) was prepared via etching the Ni foam at 1.0 V for 12 h in 0.5 M H3BO3 at room temperature. After removing the surface oxides in 1 M HCl for another 30 min, the combination of MnO2 and Ni NCAs was carried out by pulse electrodeposition in 0.1 M KMnO4 with on-time of 1 s at −0.15 V and off-time of 10 s at 0.4 V for 8 cycles in a three-electrode system using a carbon rod and an Ag/AgCl electrode as counter and reference electrode, respectively. Characterization The microstructure of Ni/MnO2 NCAs was characterized using a field emission scanning electron microscope (JEOL JSM-6700F, 15 keV). The X-ray diffraction (XRD) pattern was investigated using a D/max2500pc diffractometer using Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS) was measured using an ESCALAB Mk II (Vacuum Generators) spectrometer with an Al anode. The Raman spectrum measurement was carried out on a Renishaw Raman spectrometer using 532 nm wavelength with intensity of 0.1 mW. Electrochemical measurement All electrochemical performances of the supercapacitors were 1412
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Fig. 2. Structure features of the Ni/MnO2 NCAs electrode. Typical SEM images for Ni foam (a) before and (b) after etching. (c) SEM image for Ni NCAs electrode after loading of MnO2. (d) HRTEM image of Ni/MnO2 structure. (e) XRD pattern of MnO2 nanosheets. (f) Raman spectrum of the Ni/MnO2 NCAs electrode. (g) Typical XPS survey spectrum for as-synthesized MnO2. High-resolution XPS spectrum for (h) Mn 2p and (i) O 1s.
MnO2. The characteristic band at 638 cm−1 belongs to the transverse vibration of MnO2, while the one at 578 cm−1 is attributed to the longitudinal vibration of MnO2. The latter is usually related to the intrinsic vibration frequency of the Mn4+ ions. The high characteristic band strength at 578 cm−1 can also certify the layered structure of the as-synthesized δ-MnO2. The nanostructure characteristics of the hybrid Ni/MnO2 NCAs electrode exhibits excellent electronic and ionic conductivity, while the favorable crystal structure of δ-MnO2 endows fast transport of electrolyte ions inside the active materials [34]. The chemical states of Mn and O components in MnO2 are further investigated by XPS (Fig. 2g). High-resolution Mn 2p spectrum demonstrates the existence of Mn species. The peaks at the binding energy of 640.5 and 651.9 eV are attributed to 2p3/2 and 2p1/2 for Mn4+, while those at 640.2 and 644.1 eV and at 644.0 and 655.2 eV are ascribed to small amounts of Mn2+ and Mn3+, respectively (Fig. 2h) [31]. The O 1s spectrum consists of three typical peaks centered at 529.7, 531.4 and 532.8 eV, which correspond to the metaleoxygen bond, O in OH−, and O contained in absorbed H2O, respectively (Fig. 2i) [35]. The FTIR and EDAX results are shown in Supporting Information. The FTIR absorption peaks of 708 cm−1 are attributed to the vibrations of the Mn-O bonds in the [MnO6] eight plane of the MnO2 crystal, and 3368 cm−1 corresponds to the stretching vibration peak of surface hydroxyl groups of manganese dioxide (Fig. S1), which are consistent with the data in the literature results [36]. The presence of Mn, O and Ni is clearly seen in the EDAX profiles. The Mn and O signals come from MnO2, and the Ni signals is originated from the Ni NCAs. Pseudocapactive performance of the Ni/MnO2 NCAs electrode was evaluated in 1 M Na2SO4. Fig. 3a shows the representative CV curves of
the Ni/MnO2 NCAs electrode at scan rates ranging from 10 to 200 mVs−1. The CV curves show ideally symmetric rectangle shape even at 200 mVs−1, which indicates fast reversible faradaic redox reactions and low contact resistance in the Ni/MnO2 NCAs electrodes [37,38]. By calculation, the Ni/MnO2 NCAs electrode achieves high specific capacitance of 883.2 Fg−1 at 10 mVs−1, which decreases to 407.6 Fg−1 as the scan rate increases to 200 mVs−1. The approximate rectangular and symmetrical shape of CV curves reveal the fast reversible faradaic redox reactions on the electrode. Fig. 3b presents the variation of the areal capacitance of the Ni/MnO2 NCAs electrodes as a function of scan rate, compared with the bare Ni foam and the Ni NCAs. The areal capacitance of the Ni/MnO2 NCAs electrode exhibits 22.7 mFcm−2 at 10 mVs−1, and it is maintained at 8.3 mFcm−2 as the scan rate increases by 50-folds (up to 500 mVs−1). The areal capacitances of the bare Ni NCAs and the Ni foam electrodes only achieve 3.8 and 2.9 mFcm−2 at 10 mVs−1 and remain as 1 and 0.6 mFcm−2 at 500 mVs−1 under the same condition. Compared the Ni foam and the Ni NCAs electrodes, the corrosion process effectively increases the accessible surface area of the substrates, and the capacitance significantly increases after the incorporation of the MnO2 nanosheets. The galvanostatic charge/discharge test of the Ni/MnO2 NCAs electrode also confirms the rate characteristic under different current densities in Fig. 3c. The symmetric charge/discharge profiles demonstrate the fast and reversible redox process of the Ni/MnO2 NCAs electrode. At current density of 1 mAcm−2, the specific capacitance reaches 138.7 mFcm−2, which can be maintained as 112.5 mFcm−2 at 2 mAcm−2 and 75 mFcm−2 at 5 mAcm−2. Fig. 3d shows the galvanostatic charge/ discharge curves of the Ni/MnO2 NCAs electrode at 1 mAcm−2 1413
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Fig. 3. Electrochemical characterization of the Ni/MnO2 NCAs electrode. (a) CV curves at scan rates ranging from 10 to 200 mVs−1. (b) Specific capacitance of the Ni/MnO2 NCAs as a function of scan rates ranging from 10 to 200 mVs−1, compared with the values of Ni/MnO2 electrode, Ni NCAs and bare Ni foam. (c) Galvanostatic charge/discharge curves at current ranging from 1 to 5 mAcm−2. (d) Galvanostatic charge/discharge curves of the Ni/MnO2 NCAs electrode at 1 mAcm−2, compared with the values of the Ni NCAs and the bare Ni foam.
electrode demonstrates the enhanced electrolyte ion access/diffusion ability due to the wide-open structure of the nanocylinder arrays and the δ-MnO2 component [39]. Fig. 4c investigates the cycling stability of the Ni/MnO2 NCAs supercapacitor at a high current density of 1 mAcm−2. After 2000 cycles, the specific capacitance is maintained at 73.8% of the initial value, suggesting a great stability of the 3D hybrid electrode. Fig. 4d shows the Ragone plot to compare the power and energy densities of the Ni/MnO2 NCAs supercapacitor with recently reported results [40–47]. By anchoring layer structured δ-MnO2 nanosheets onto the seamless Ni foam/Ni nanocylinder arrays current collector without using polymer binders as well as tuning the crystallographic structure of the electrode materials, we minimize the internal resistances and significantly improve the pseudocapacitive energy storage. The Ni/ MnO2 NCAs supercapacitor exhibits high energy density of 108 Whkg−1 at 2.4 kWkg−1. When the power density increases by about 20 times (up to 44.4 kWkg−1), the energy density can still be maintained at ∼49.4 Whkg−1, much higher than other different electrodes reported recently, such as the MnO2/RGO [40], the MnO2 nanorods [41], the LDH/PEDOT-Ni foam [42], the MnO2/Graphene/CNF [43], the MnO2/NPG hybrid electrode [44], the CNPs/MnO2 nanorods hybrid electrode [45], the MnO2/NiCo2O4 composite electrode [46], and the Ni-Mn LDH/MnO2 electrode [47]. In summary, we have developed a 3D nanoporous hybrid electrode via a simple electro-etching/depositing way. The resultant Ni/MnO2 NCAs deliver a high specific capacitance and high energy density. This achievement is benefited from not only the large interplanar spacing of δ-MnO2 but also the fast ion and electron transports provided by the high conductive Ni NCAs and the ordered semicoherent Ni/MnO2 interfaces.
compared with the values of the Ni NCAs and the Ni foam. It can be deduced that the capacitance of the electrode is significantly improved after the MnO2 deposition, as shown in Fig. 3b. The areal capacitance of the Ni/MnO2 NCAs electrode achieves 138.7 mFcm−2 at 1 mAcm−2, while those of the Ni NCAs and the Ni foam electrodes are only 10 and 6.2 mFcm−2, respectively. Fig. 4a shows a thorough inquiry about the influence of the surface morphology of the Ni/MnO2 NCAs electrodes under the ion transport condition. Due to the proton penetration of electrolyte ions in the Na2SO4 solution, the profile of voltammetric charge (q*) and scan rate (υ) approximates a straight line dependence. Herein, q0* is used to express the q* value of the voltammetric charge as v → 0, and the value is calculated as 202.7 Cg−1, which is thought to be associated with the whole active material oxide surface. Correspondingly, q∞* is used as v → ∞ and calculated as 769.2 Cg−1. This denotes that only the outermost oxide surface participates fast reversible faradaic redox reactions [15]. Therefore, the q∞* only accounts for 25.5% of the whole oxide surface, indicating that the 3D architectures of Ni nanocylinder arrays can greatly enhance the accessibility/diffusion of ions. EIS analysis is tested to investigate the superb electrochemical performance of the Ni/MnO2 NCAs electrodes in a frequency range from 100 kHz to 10 mHz. Fig. 4b is conducted to show the impedance result of the Ni/MnO2 NCAs electrode. The intrinsic resistance between electrolyte and electrode (Ri) is calculated to be 4.6 Ω, and the charge transfer resistance (Rct) is 0.12 Ω for the Ni/MnO2 NCAs electrode. The low value of Rct represents that the fast electron transfer can be achieved at the interface of electrode and electrolyte, and the power delivery of the pseudocapacitance can thus be enhanced. This enhancement is benefited from the fast electron and ion transports provided by the high conductive Ni NCAs, as well as the absence of insulative polymer binder between the oxides and the current collectors. Moreover, the low Warburg resistance (Zw) of the Ni/MnO2 NCAs 1414
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Fig. 4. (a) The relationship between voltammetric charge (q*) and scan rate (υ). (b) Nyquist plots of the Ni/MnO2 NCAs electrodes over frequency ranges from 100 kHz to 10 mHz. Inset: A magnification of EIS at the high-frequency region and the electrical equivalent circuit used for fitting EIS. (c) Cycling performance of the Ni/MnO2 NCAs electrode at a current density of 1 mAcm−2 for 2000 cycles. (d) Ragone plot for comparing the energy and power densities of the Ni/MnO2 NCAs electrodes with the literature results.
Conclusions
Appendix A. Supplementary data
Supercapacitors with high energy storage and fast charge/discharge performance have been fabricated by combining MnO2 nanosheets with 3D seamless Ni foam/Ni NCAs arrays. Here, the Ni NCAs can greatly increase the accessible surface area of the electrode for anchoring more high-capacitance materials to achieve high energy density and power density. It can also facilitate the transport of electrolyte ions due to the reduction in the transport distance inside the hybrid electrodes. Besides this, the layer structured δ-MnO2 nanosheets can offer large channels for further accelerating ion transports in the bulk of MnO2, and the ordered semicoherent interfaces lead to reduced additional contact resistances between MnO2 and Ni foam. These features of crystallographic structure and nanostructure enable superb electronic and ionic conductivities in the Ni/MnO2 NCAs electrodes. As a result, the fast reversible faradaic redox reaction of MnO2 is greatly boosted. The capacitance is achieved high as 883.2 Fg−1 at a scan rate of 10 mVs−1, and the energy density is high as 108 Whkg−1 at 2.4 kWkg−1. The outstanding performance makes the Ni/MnO2 NCAs electrodes possess great potential as a stable energy storage device that can be widely used.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 61674069 and No. 51631004), the Key Scientific and Technological Research and Development Project of Jilin Province (20180201080GX), the Fundamental Research Funds for the Central Universities, the Program for Innovative Research Team (in Science and Technology) in University of Jilin Province and the Program for JLU Science and Technology Innovative Research Team (2017TD-09). 1415
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