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electrical vehicles, etc.[1–4] Nevertheless, the low energy density has hindered the applications of supercapacitors in ever-developing electronic world. According to the key parameters that determine the energy density (E = 1/2CV2) of super-capacitor devices, enormous efforts have been devoted to promoting the energy density by enhancing the specific capaci-tance (C) and cell voltage (V). Configu-ration of asymmetric supercapacitors (ASCs) has arisen to be an attractive strategy due to the enlarged cell voltage resulted from separate potential windows of positive electrode and negative elec-trode, respectively. Therefore, increasing research interest has been focused on the development of highly capacitive anodes and cathodes. However, comparing with the remarkable progress achieved by cathodes, the lack of high-performance anodes impeded the promotion of ASCs (1/CASC = 1/Cp + 1/Cn) with high energy

density. Previously, carbon materials were usually used as the anode material based on the electric-double layer capacitance (EDLC) mechanism, resulting in relatively low specific capaci-tance.[5–8] In this regard, pseudocapacitive anode materials were proposed to be promising alternatives, whereas the limited reports and inferior performance by far suggest that further exploration of high-performance negative electrodes for ASCs is urgently demanded.[9–11]

Active efforts have been attracted to explore novel pseudo-capacitive anode materials to achieve large specific capacitance and good rate performance, including VOx, MoOx, VN, RuO2, FeOx, and FeOOH.[12–16] Among them, iron oxides/hydrox-ides (α-Fe2O3, β-Fe2O3, γ-Fe2O3, Fe3O4, α-FeOOH, β-FeOOH, γ-FeOOH, and δ-FeOOH) have received tremendous research interest as promising anode materials for ASCs because of the multiple valences of iron element, rich redox chemistry in the negative potential window, low cost, and abundant resource.[17–21] Particularly, α-Fe2O3 is regarded as one of the most promising pseudocapacitive materials owing to its stable crystal structure, environment compatibility, and high theoret-ical capacitance.[15,20,22–25] Currently, the reported specific capac-itance of α-Fe2O3 is, however, still far below the theoretical value, which can be mainly attributed to the inferior conduc-tivity and insufficient ionic diffusion rate of electrodes.[22,26–28]

Fe2O3 Nanoneedles on Ultrafine Nickel Nanotube Arrays as Efficient Anode for High-Performance Asymmetric Supercapacitors

Yang Li, Jing Xu,* Tao Feng, Qiaofeng Yao, Jianping Xie, and Hui Xia*

High performance of electrochemical energy storage devices depends on the smart structure engineering of electrodes, including the tailored nanoarchi-tectures of current collectors and subtle hybridization of active materials. To improve the anode supercapacitive performance of Fe2O3 for high-voltage asymmetric supercapacitors, here, a hybrid core-branch nanoarchitecture is proposed by integrating Fe2O3 nanoneedles on ultrafine Ni nanotube arrays (NiNTAs@Fe2O3 nanoneedles). The fabrication process employs a bottom-up strategy via a modified template-assisted method starting from ultrafine ZnO nanorod arrays, ensuring the formation of ultrafine Ni nanotube arrays with ultrathin tube walls. The novel developed NiNTAs@Fe2O3 nanoneedle electrode is demonstrated to be a highly capacitive anode (418.7 F g−1 at 10 mV s−1), matching well with the similarly built NiNTAs@MnO2 nanosheet cathode. Contributed by the efficient electron collection paths and short ion diffusion paths in the uniquely designed anode and cathode, the asymmetric supercapacitors exhibit an excellent maximum energy density of 34.1 Wh kg−1 at the power density of 3197.7 W kg−1 in aqueous electrolyte and 32.2 Wh kg−1 at the power density of 3199.5 W kg−1 in quasi-solid-state gel electrolyte.

Y. Li, Dr. J. Xu, Prof. H. XiaSchool of Materials Science and EngineeringNanjing University of Science and TechnologyNanjing 210094, ChinaE-mail: [email protected]; [email protected]. Li, Dr. J. Xu, Prof. T. Feng, Prof. H. XiaHerbert Gleiter Institute of NanoscienceNanjing University of Science and TechnologyNanjing 210094, ChinaDr. Q. Yao, Prof. J. XieDepartment of Chemical and Biomolecular EngineeringFaculty of EngineeringNational University of SingaporeSingapore 117576, Singapore

DOI: 10.1002/adfm.201606728

1. Introduction

Alternative and sustainable energy conversion and storage systems have been urgently demanded for the solution to the exhaustible fossil fuels in this century, in which the super-capacitors with high power density, fast charge–discharge rate, long cycling life, nonmemory effect, low cost, and environ-ment friendliness have been demonstrated as efficient power sources in high-power laser devices, backup power devices, and

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To address these issues, researchers have focused on the fab-rication of α-Fe2O3 nanostructures, such as nanoparticles, nanorods, nanotubes, nanosheets, and nanoflowers, with high surface area and short ion diffusion paths for improved elec-trochemical performance.[21,23,29–33] Nevertheless, the poor electrical conductivities of bulk α-Fe2O3 (10−4 Ω−1 m−1) and nanostructures (e.g., 2.5 × 10−3 Ω−1 m−1 of the α-Fe2O3 nano-wires) still restrain the complete utilization of active material, resulting in limited improvement in specific capacitance and rate performance.[34,35] Therefore, it is still a great challenge to develop α-Fe2O3-based electrodes with both high electron and ion transport capabilities for high-performance ASCs.

To construct fast electron and ion pathways, in this work, we developed a novel strategy to engineer ultrafine Ni nanotube arrays as robust conductive backbones for depositing ultrafine Fe2O3 active material, namely, a hybrid hollow core-branch nanoarchitecture. Herein, we first fabricated the ultrafine Ni nanotube arrays with ultrathin tube walls by a modified tem-plate-assisted method, resulting in a much larger surface area compared to the previously reported Ni micro/nanotube arrays built up with big Ni particles.[36–39] α-Fe2O3 nanoneedles are then controllably electrodeposited on ultrafine Ni nanotube arrays (NiNTAs@Fe2O3 nanoneedles) through a bottom-up strategy, which can effectively enhance the specific capacitance (418.7 F g−1 at 10 mV s−1) and rate capability (215.3 F g−1 at a current density of 64 A g−1). Moreover, this effective strategy was also adopted for the cathode design, and the NiNTAs@MnO2 nanosheets were successfully fabricated, exhibiting excellent electrochemical performance as cathode for supercapacitors. Finally, high-performance 1.6 V ASCs integrated based on the NiNTAs@Fe2O3 nanoneedle anode and the NiNTAs@MnO2 nanosheet cathode were successfully demonstrated, manifesting the potential application of the proposed novel engineering strategy in developing highly efficient energy storage devices.

2. Results and Discussion

2.1. Morphology and Structure Characterization

The schematic fabrication process is shown in Figure 1. First, ZnO nanorod arrays were grown on the surface of Ti foil as a template by a hydrothermal method.[36] Subsequently, a thin Au layer was sputtered on the surface of ZnO nanorods fol-lowed by the electrodeposition of Ni thin films as the outer conductive shell. It should be noted that the thin layer of Au (with a thickness of ≈5 nm) on ZnO nanorod is significantly essential for the deposition of homogeneous and continuous Ni thin film by generating a finely tuned electrical field (inset in Figure 1). In contrast, inhomogeneous and noncontinuous Ni films composed of large Ni particles are formed around the ZnO nanorods without the assistance of Au film, making it difficult to maintain the nanorod morphology and form con-tinuous conductive backbones (Figure S1, Supporting Informa-tion). After that, ZnO/Ni nanorod arrays (ZnO/NiNRAs) are immersed in the FeCl2 solution for electrochemical deposition by a static potential method at 1.5 V (vs Ag/AgCl). During the deposition process, FeOOH nanoneedles are grown on the Ni film while the hydrolysis of Fe3+ generates weak acid environ-ment that dissolves the ZnO interior, resulting in the hollow structure. The final NiNTAs@Fe2O3 nanoneedles can be obtained by annealing the sample in air at 450 °C for 3 h. As the NiNTAs are also beneficial for cathode design, NiNTAs@MnO2 nanosheets with delicate structure were also fabricated using a similar electrodeposition method in the Mn(CH3COO)2-based electrolyte. It should be noticed that the electrodeposition of MnO2 cannot generate acid environment in the solution and the ZnO core should be removed by HCl solution. In this way, both anode and cathode with similar hybrid core-branch nano-architectures can be fabricated.

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Figure 1. Schematic illustration of the synthesis procedure for the NiNTAs@Fe2O3 nanoneedles and NiNTAs@MnO2 nanosheets. The inset shows the different Ni films formed on the ZnO nanorod with and without Au layer.

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The scanning electron microscopy (SEM) images of ZnO nanorod arrays, ZnO/NiNRAs, and NiNTAs@Fe2O3 nanonee-dles display the evolution of surface morphology of the samples at different fabrication steps. Vertically aligned ZnO nanorods with an average diameter of 150 nm (Figure 2a) were coated by a conformal Ni film, resulting in thicker nanorods with the diameter around 180 nm (Figure 2b). A transmission electron microscopy (TEM) image of pure Ni nanotubes without ZnO template is shown in Figure S1 in the Supporting Information, revealing the hollow structure of Ni nanotubes with a wall thick-ness of 10–20 nm and a diameter of around 180 nm. After the electrodeposition of Fe2O3, the surface of nanorods was deco-rated by interconnected tiny Fe2O3 nanoneedles with the length of about 10 nm (Figure 2c). The TEM image in Figure 2d clearly verifies the Ni nanotube core and interconnected Fe2O3 nano -needle branches, revealing an overall thickness of 70 nm for the nanotube walls. The high-resolution TEM (HRTEM) images of Fe2O3 nanoneedles (e, red circle) and Ni nanoparticles (f, red circle) are shown in Figure 2e,f, in which the measured inter-planar spacings of 0.27 and 0.23 nm for the well-defined lat-tice fringes consist well with the α-Fe2O3 (104) planes and Ni (010) planes, respectively. Finally, this unique nanoarchitec-ture of NiNTAs@Fe2O3 nanoneedles was also demonstrated by the energy-dispersive X-ray spectroscopy (EDS) elemental mappings of Au, Ni, Fe, and O elements from the designated

area in the scanning transmission electron microscopy (STEM) image (Figure 2g). Under the same electrodeposition condi-tions, only dense Fe2O3 film was obtained (Figure S2a, Sup-porting Information) on the Ti foil without Ni nanotubes, indi-cating that a finely tuned electrical field is important for the growth of ultrathin Fe2O3 nanoneedles. For comparison, we also deposited α-Fe2O3 nanorod arrays directly on the Ti foil by a hydrothermal method (Figure S2b, Supporting Informa-tion). As for the NiNTAs@MnO2 nanosheets, tiny and ultrathin MnO2 nanosheets with the thickness of about 5 nm are elec-trodeposited on Ni nanotubes, exhibiting a similar hybrid hollow core-branch nanoarchitecture (Figure 3a,b; Figure S3, Supporting Information). According to the HRTEM image in the bottom-right inset of Figure 3b, the MnO2 nanosheets are not highly crystallized with some vague lattice fringes being observed. Further analysis of the selected area electron diffrac-tion (SAED, the top-right inset) indicates that the as-obtained MnO2 is polycrystalline α-phase. The STEM image and corre-sponding EDS elemental mapping images of Au, Ni, Mn, and O elements in Figure 3c further reveal the hierarchically archi-tectured building blocks of this novel electrode structure for MnO2-based cathode materials.

The phase and composition of products are further con-firmed by X-ray diffraction (XRD) analysis and X-ray photoelec-tron spectroscopy (XPS). In Figure 4a, the characteristic peaks

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Figure 2. SEM images of a) ZnO nanorod arrays, b) ZnO/NiNRAs, and c) NiNTAs@Fe2O3 nanoneedles. d) TEM image of NiNTAs@Fe2O3 nano-needles. e,f) High-resolution TEM images of Fe2O3 nanoneedle and Ni nanoparticles of the circled areas are provided in part (d). g) STEM image of NiNTAs@Fe2O3 nanoneedles and corresponding EDS mappings of Au, Ni, Fe, and O elements.

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of hexagonal ZnO phase (JCPDS 36–1451) are clearly seen in the XRD spectra for the samples after the hydrothermal growth of ZnO nanorods and the electrodeposition of Ni, but vanish after the growth of Fe2O3 nanoneedles, agreeing well with the TEM observation. The diffraction peaks of the as-synthesized Fe2O3 for the final product can be well assigned to the hexagonal α-Fe2O3 phase (JCPDS 33-0664), so do the hydrothermally fab-ricated Fe2O3 nanorods (Figure S2c, Supporting Information). Furthermore, XPS analysis was carried out to understand the electronic structures of the as-prepared samples. In the high-resolution Fe 2p spectrum (Figure 4b), two distinct peaks at the binding energies of 710.9 eV for Fe 2p3/2 and 724.1 eV for Fe 2p1/2 with two shake-up satellites at 719.5 and 733.2 eV can be observed, which agree well with literature reports for Fe2O3.[40] The high-resolution O 1s spectrum could be deconvoluted into three different components at the binding energies of 529.9, 531.1, and 532.5 eV, representing the existence of the lattice oxygen (Olatt), hydroxyl oxygen (Ohyd), and physically adsorbed oxygen (Oads), respectively (Figure 4c).[41] It is generally accepted that Oads oxygen species are mainly induced by surface oxygen vacancies, and the indicated oxygen vacancies on the surface of Fe2O3 nanoneedles could further facilitate the electron transport in the NiNTAs@Fe2O3 nanoneedle electrode.[42] The XRD pat-tern of the NiNTAs@MnO2 nanosheets is recorded in Figure 4d as the bottom line. It is observed that there is no typical

diffraction peaks for MnO2 except for peaks of Ti and Ni, indicating the electrodepos-ited MnO2 nanosheets have a low degree of crystallinity. To further verify the oxidation state of Mn, XPS analysis was carried out on the NiNTAs@MnO2 nanosheets. Two dis-tinct peaks at binding energies of 641.8 and 653.6 eV with the spin-orbital splitting of 11.8 eV were observed in the Mn 2p core-level spectrum (Figure 4e), corresponding well to the Mn 2p3/2 and Mn 2p1/2 of Mn4+, respec-tively.[43–45] The O 1s core-level spectrum (Figure 4f) has also three distinct components located at 530.0, 531.1, and 532.4 eV, respec-tively, which can be assigned to the three kinds of oxygen species mentioned above. The survey-scan XPS spectra in Figure S4a,b (Supporting Information) confirm the exist-ence of Ni, O, and corresponding metal ele-ments, while no Zn signal is detected, indi-cating the ZnO template has been completely dissolved. For the core-level Ni 2p spectrum of NiNTAs@Fe2O3 nanoneedles (Figure S4c, Supporting Information), two peaks located at 855.8 and 873.2 eV correspond to Ni 2p1/3 and Ni 2p2/3, matching well with electronic state of Ni2+ species. In the core-level Ni 2p spectrum of NiNTAs@MnO2 nanosheets (Figure S4d, Supporting Information), peaks at 855.5 and 873.3 eV are also ascribed to Ni2+, revealing the surface oxidation of Ni after electrodeposi-tion and storage in air.[46]

2.2. Electrochemical Properties

The electrochemical performance of the hybrid hollow core-branch electrode was measured in a three-electrode cell with 1 m Na2SO4 as the electrolyte in a potential window from −0.8 to 0 V (vs Ag/AgCl). To demonstrate the superior capaci-tive performance of the NiNTAs@Fe2O3 nanoneedle electrode, two counterparts including the Fe2O3 film electrodeposited on Ti foil (Figure S4a, Supporting Information) and the α-Fe2O3 nanorod arrays on Ti foil fabricated by the hydrothermal method (Figure S4b, Supporting Information) were also tested for comparison. In Figure 5a, the cyclic voltammetry (CV) curve of the NiNTAs@Fe2O3 nanoneedle electrode exhibits much larger capacitive current density than the other two counterparts. According to the CV shapes, it is indicated that the capacitance of NiNTAs@Fe2O3 nanoneedle electrode can be attributed to the EDLC by surface adsorption of electrolyte ions and the pseudocapacitance of Fe2O3 by the redox couple of Fe2+/Fe3+.[22] The expanded CV curve of the NiNTAs@Fe2O3 nanoneedle electrode suggests that the hybrid core-branch structure can provide abundant active sites for both EDLC and redox reactions to substantially enhance the specific capaci-tance of Fe2O3. Figure 5b shows the CV curves of the NiNTAs@Fe2O3 nanoneedle electrode at various scan rates from 10 to 2000 mV s−1. Even at the high scan rate of 2000 mV s−1, the

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Figure 3. a) TEM image of NiNTAs@MnO2 nanosheets. b) Magnified TEM image of NiNTAs@MnO2 nanosheets. The insets show the high-resolution TEM (bottom right) and the SAED pattern (top right) of MnO2 nanosheets. c) STEM image of NiNTAs@MnO2 nanosheets and corresponding EDS mappings of Au, Ni, Mn and O elements.

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Figure 4. a) XRD patterns of the ZnO nanorods, ZnO/NiNAs, and NiNTAs@Fe2O3 nanoneedles. b) XPS Fe 2p and c) O 1s core-level spectra of NiNTAs@Fe2O3 nanoneedles. d) XRD patterns of the ZnO nanorods, ZnO/NiNRAs, and NiNTAs@MnO2 nanosheets. e) XPS Mn 2p and f) O 1s core-level spectra of NiNTAs@MnO2 nanosheets.

Figure 5. a) CV curves of the NiNTAs@Fe2O3 nanoneedles, Fe2O3 film, and α-Fe2O3 nanorod electrodes at a scan rate of 100 mV s−1. b) CV curves of the NiNTAs@Fe2O3 nanoneedle electrodes at varied scan rates. c) GCD curves of the NiNTAs@Fe2O3 nanoneedle electrode at different current densi-ties. d) The specific capacitances of different electrodes calculated from CV and GCD curves. e) Nyquist plots for the NiNTAs@Fe2O3 nanoneedles, Fe2O3 film, and α-Fe2O3 nanorod electrodes. f) Cycle performance of the NiNTAs@Fe2O3 nanoneedle electrode at 100 mV s−1 for 5000 cycles. The inset shows the first and 5000th CV curves of the NiNTAs@Fe2O3 nanoneedle electrode.

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CV curve can still retain a definite rectangular shape without obvious distortion. Figure 5c shows the galvanostatic charge/discharge (GCD) curves of the NiNTAs@Fe2O3 nanoneedle electrode at different current densities from 4 to 64 A g−1. The specific capacitances of the electrode are calculated from both CV and GCD curves (Figure 5d). It can be seen that the elec-trode delivers the highest specific capacitance of 418.7 F g−1 at 10 mV s−1, whereas only 76.4 and 177.1 F g−1 are obtained for Fe2O3 film and Fe2O3 nanorods, respectively. Even at a high cur-rent density of 64 A g−1, the NiNTAs@Fe2O3 nanoneedle elec-trode can still retain a large specific capacitance of 215.3 F g−1, which is much larger than those of previously reported Fe2O3-based electrodes in the same potential window at similar current densities (Table S1, Supporting Information). Electro-chemical impedance spectroscopy (EIS) measurements were carried out on the three Fe2O3 electrodes. All EIS spectra exhibit two distinct parts including a semicircle in the high-frequency region (charge transfer process) and a sloped straight line in the low-frequency region (diffusion-limited process). Among the three electrodes, the NiNTAs@Fe2O3 nanoneedle electrode presents the smallest equivalent series resistance (Rs) of 2.3 Ω and the charge transfer resistance (Rct) of 2.3 Ω. It is known that 1D nanostructures exhibit better conductivity than the bulk counterpart.[9] The Ni nanotubes act as efficient electron trans-portation bridges between Fe2O3 nanoneedles and current col-lectors. The ultrafine nanostructure of Fe2O3 also endows faster charge transfer rate on the surface of electrode. Therefore, the hybrid core-branch nanoarchitecture is essential for the elec-tron collection and fast Faradaic reactions. The corresponding cycling stability of the NiNTAs@Fe2O3 nanoneedle electrode was also evaluated by CV at a scan rate of 100 mV s−1, demon-strating higher capacitance retention of 93.3% after 5000 cycles compared with other Fe2O3-based electrodes (Table S1, Sup-porting Information). These aforementioned results demon-strate the remarkably high specific capacitance, rate capability, and cycling stability of the uniquely engineered Fe2O3 anode for supercapacitors.

Similarly, the electrochemical performance of the NiNTAs@MnO2 nanosheet electrode was investigated in a three-electrode cell with 1 m Na2SO4 electrolyte in a potential window of 0–0.8 V (vs Ag/AgCl). A CV comparison between the NiNTAs@MnO2 nanosheet electrode and the MnO2 film electrode without Ni nanotube arrays is displayed in Figure S5a (Supporting Infor-mation), showing significantly improved specific capacitance of the hybrid electrode. When the scan rate was increased from 5 to 1000 mV s−1, the quasirectangular shape of the CV curves was well preserved, demonstrating excellent rate capability and highly reversible redox reactions (Figure S5b, Supporting Infor-mation). A large specific capacitance of 440.7 F g−1 at the scan rate of 5 mV s−1 can be achieved by the hybrid MnO2 electrode (Figure S5c, Supporting Information), outperforming most of previously reported MnO2-based electrodes. Moreover, the good durability of the NiNTAs@MnO2 nanosheet electrode was verified by 92.2% capacitance retention after 5000 cycles at 100 mV s−1 (Figure S5d, Supporting Information).

Given that the NiNTAs@Fe2O3 nanoneedle electrode and NiNTAs@MnO2 nanosheet electrode possess stable potential windows of −0.8 to 0 V and 0–0.8 V, respectively (Figure 6a), it is expected that a 1.6 V ASC can be constructed in 1 m Na2SO4

electrolyte by integrating these two electrodes. To balance the charges in cathode and anode, the mass ratio of Fe2O3 to MnO2 is set as 1:1 because the specific capacitances of both elec-trodes are close to each other. By using an Na2SO4/poly(vinyl alcohol) (PVA) polymer gel electrolyte, a quasisolid-state ASC was also fabricated by adopting these two electrodes. The elec-trochemical performance of both liquid-state ASC (denoted as LASC) and solid-state ASC (denoted as SASC) was investigated to demonstrate the advantage of the unique hybrid electrode design. In Figure 6b, CV curves of the LASC were collected at different scan rates from 10 to 800 mV s−1, and the nearly rectangular shape was well retained even at a high scan rate of 800 mV s−1, indicating superior rate capability for the full cell device. Similarly, the SASC can also achieve a cell voltage of 1.6 V, and the CV curves at different scan rates from 10 to 800 mV s−1 are shown in Figure 6c. Due to the relatively lower ionic conductivity of gel electrolyte, the CV curves of the SASC exhibit slight distortion at high scan rates caused by the increased polarization. Nevertheless, the distortion and polari-zation of CV curves at high scan rates of the present SASC are much less compared to the previously reported SASCs, indicating greatly reduced cell resistance. The electrochemical properties of LASC and SASC were further investigate by GCD measurements (Figure S6, Supporting Information) at different current densities of 4, 8, 16, 32, and 64 A g−1. All charge/dis-charge curves for both ASCs show good symmetry and fairly linear slopes, exhibiting good electrochemical reversibility and ideal supercapacitive behavior in the voltage window of 0–1.6 V. The specific capacitances (based on the mass of active mate-rials) and areal capacitances (based on the area of electrodes) of the ASCs were calculated according to the charge–discharge curves (Figure 6d). In comparison, the LASC shows relatively higher specific capacitance and better rate performance (66.9% capacitance retention at the current density of 64 A g−1) than the SASC (51.1% capacitance retention at the current density of 64 A g−1). The discrepancy between LASC and SASC can be attributed to the relatively poor ionic conductivity of gel elec-trolyte compared to that of aqueous electrolyte, which would increase the cell resistance and result in larger polarization and limited rate performance. In the present work, the specific capacitances of the as-fabricated LASC and SASC devices can achieve 95.9 and 90.6 F g−1 (based on the total mass of active materials from cathode and anode), respectively, at a current density of 4 A g−1, which are 1.5 times higher than the values of the latest reported LASC and SASC in literature.[47] The internal resistance (IR) voltage drops of both ASCs versus current den-sity are plotted in Figure 6e, and corresponding liner functions are fitted. Obviously, the ultrasmall slope values demonstrate the low resistance of both devices.[48]

EIS spectra of both ASCs were measured in the frequency range from 100 kHz to 0.01 Hz with an AC perturbation of 5 mV (Figure 6f). According to the fitting using equivalent cir-cuit, the intercepts of the Nyquist curves on the real axis are about 2.0 and 2.61 Ω for LASC and SASC, respectively, mani-festing the very low internal resistance of two ASC devices. The small semi-circles (1.16 Ω for LASC and 3.96 Ω for SASC) in the high-frequency region illustrate the fast charge transfer pro-cess of both devices at the electrolyte–electrode interface. More-over, in the Bode plots (Figure S7, Supporting Information),

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the characteristic frequencies fo for a phase angle of −45° are 2.7 Hz for LASC and 8.6 Hz for SASC, at which the resistive and capacitive impedances are equal. The corresponding time constants τ0 ( = 1/f0) are calculated to be 0.37 s for LASC and 0.12 s for SASC, showing much faster frequency response than some conventional active-carbon-based SCs (about 10 s).[1] In this respect, the abundant electron collecting paths and short-ened ion diffusion paths benefited from this hybrid core-branch nanoarchitecture ensure low resistance and fast charge trans-port for the ASC devices.

The electrochemical stabilities of both ASC devices were evaluated through a CV cycling test at a scan rate of 100 mV s−1 for 5000 cycles (Figure 6g). The LASC exhibits excellent cycling stability with 92.3% capacitance retention while the SASC can maintain 79.3% capacitance after 5000 cycles. The CV curves of both ASC devices after 5000 cycles nearly retain their original shapes, demonstrating good reversibility and cycling stability. These capacitance retention rates at such a

high charge–discharge rate are comparable and even better than those reported for other aqueous and solid-state super-capacitors (SSCs).[47,49–51] Importantly, the energy densities and power densities of the present two ASCs are compared with those of previously reported supercapacitor devices, as shown in the Ragone plots in Figure 6h. The maximum energy density of 34.1 Wh kg−1 was obtained at the power density of 3197.7 W kg−1 for LASC, which is much higher than those in some latest reported supercapacitor devices, such as MnO2 @graphen oxide-based ASCs (18.9 Wh kg−1 at 2000 W kg−1, 1 m Na2SO4),[52] NiCo2O4/NiO//Fe2O3-based ASCs (19 Wh kg−1 at 157 W kg−1),[53] MnO2@3D graphene-based SSC (6.8 Wh kg−1 at 62 W kg−1, 0.5 m Na2SO4),[54] graphite foam-CNT@Fe2O3//grapheite foam-CoMoO4-based ASC (1.4 Wh kg−1 at 74.7 W kg−1, 2 m KOH).[28] Moreover, the calculated maximum energy and power density of SASC (32.2 Wh kg−1 at the power den-sity of 3199.5 W kg−1) are comparable with the LASC, and exceed most of the reported values in literature for SSC devices,

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Figure 6. a) CV curves of the NiNTAs@Fe2O3 nanoneedle electrode and NiNTAs@MnO2 nanosheet electrode at a scan rate of 100 mV s−1. CV curves of the b) LASC and c) SASC at varied scan rates in a voltage range between 0 and 1.6 V. d) The specific capacitances (based on the mass of active materials) and areal capacitances (based on the area of electrodes) of the LASC and SASC. e) The IR voltage drops of both LASC and SASC as a function of current density. f) Nyquist plots for the LASC and SASC. The insets show enlarged EIS spectra (bottom) and the corresponding equivalent circuit (top). g) Cycle performance of the LASC and SASCs at 100 mV s−1 for 5000 cycles. The insets show the first and 5000th CV curves of LASC and SASC, respectively. h) Ragone plots of LASC and SASC and previously reported ASCs in literature.

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including the MnO2 @carbon nanotube-based textile SSC with 31.1 Wh kg−1 at 22.1 W kg−1 (polyethylene oxide/Na2SO4),[55] MnO2 nanosheet//Fe2O3 nanoparticle ASC with 19.4 Wh kg−1 at 5102 W kg−1 (carboxymethyl cellulose gel/Na2SO4),[47] Fe2O3/CNT//MnO2/CNT ASC with 19.2 Wh kg−1 at 32 700 W kg−1 (PVA/Na2SO4).[56]

In general, the excellent capacitive performance of the as-fabricated ASCs can be schematically elaborated by Figure 7. In neutral aqueous electrolyte, the cations are suggested to be incorporated into the redox reactions of Fe2O3 and MnO2 (see the equations in Figure 7).[4,57] When the supercapacitor is charged, NaxFe2O3 is formed on the surface of Fe2O3 due to the Faradaic reaction; while in the discharge process, the reac-tion occurs backward and NaxMnO2 is formed on the surface of MnO2. However, different from the deeply investigated pseu-docapacitive mechanism of MnO2, the specific mechanism of Fe2O3 is still not clear and need further investigation.[4,58] In terms of the structure engineering of electrodes and devices, the excellent performance can be attributed to the following reasons: (i) finely tuned electrical field induced by ultrafine Ni nanoframework during electrodeposition of Fe2O3 results in the unique nanoneedle morphology, providing enhanced active sites that further increase the specific capacitance of Fe2O3 anode. In contrast, only dense film with low capacitance was obtained without the assistance of ultrafine Ni nanotubes; (ii) smart ultrafine and hollow Ni nanoframework provides fast electron collection paths and acts as ion reservoir for fast ion diffusion, which not only increase the utilization of active materials for both anode and cathode but also contribute to the low resistance and fast response of devices; (iii) the plenty of nanospaces and the hierarchical core-branch nanoarchitecture in the electrodes are essential to alleviate volume change and preserve the structure stability of electrodes under repeated

ion insertion/extraction;[59] (iv) The sepa-rate potential windows and matchable spe-cific capacitances of the NiNTAS@Fe2O3 nano needle anode and NiNTAS@MnO2 nanosheet cathode maximize the energy den-sity of the ASC devices using either liquid or solid electrolytes.

3. Conclusions

In summary, an efficient strategy has been developed to fabricate a hybrid core-branch nanoarchitecture with ultrafine Fe2O3 nanoneedles grafted on vertically aligned Ni nanotubes. The intermediate Au layers presputtered on the ZnO nanorods play a key role in the later conformal deposition of Ni films. By adopting this core-branch hierarchical architecture, the NiNTAS@ Fe2O3 nanoneedle electrode exhibits signifi-cantly improved specific capacitance up to 417.8 F g−1 as well as good rate performance, which can be attributed to the high utiliza-tion of active material and fast charge trans-port in the electrode. The rational electrode

design has also been successfully applied to MnO2 and the developed core-branch NiNTAS@MnO2 nanosheet electrode accordingly display remarkably improved cathode performance. With well-separated potential windows and matchable specific capacitances, both aqueous and solid-state ASCs with 1.6 V cell voltage have been constructed by using NiNTAS@Fe2O3 anode and NiNTAS@MnO2 cathode. Consequently, the NiNTAS@Fe2O3//NiNTAS@MnO2 ASCs exhibit superior supercapacitive performance with greatly enhanced energy density and power density compared to previously reported supercapacitor devices, demonstrating the benefits of the smart electrode design for both anode and cathode. Moreover, this facile methodology ena-bles developing new functional hybrid systems with 3D metal current collectors and nanostructured active materials, which could find various applications in electrochemical devices in the future.

4. Experimental Section

Fabrication of ZnO/NiNRAs on Ti Foil: ZnO/NiNRA on Ti foil was fabricated by a ZnO-template-assisted electrodeposition method. Typically, in order to obtain well-aligned ZnO/NiNRAs, three steps were carried out. First, a layer of ZnO crystal seeds was deposited on Ti foil by dipping the Ti foil in 5 × 10−3 m Zn(CH3COO)2 solution for 10 s with three times of repeat, following by the postannealing treatment in air at 350 °C for 20 min. Second, ZnO nanorod arrays were synthesized by a hydrothermal method with treated substrate standing against the wall of autoclave liner. The reaction was carried out in 40 mL of 0.025 m Zn(NO3)2 and C6H12N4 solution at 95 °C for 6 h. After that, the as-obtained samples were washed by ethanol and deionized water for several times, and then dried at 60 °C overnight. Third, Ni films were electrodeposited on the ZnO nanorod arrays at a current density of −1.5 mA cm−2 in the solution of 0.02 m NiSO4 and 0.01 m NH4Cl for 10 min. To deposit the conformal Ni films on ZnO nanorods, a thin

Figure 7. Charge-storage mechanism of the NiNTAs@Fe2O3 nanoneedle anode and the NiNTAs@MnO2 nanosheet cathode in the two-electrode asymmetric supercapacitor system.

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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (9 of 10) 1606728Adv. Funct. Mater. 2017, 27, 1606728


Au layer with a thickness of ≈5 nm was precoated on the ZnO nanorod arrays by DC sputtering method before the electrodeposition.

Fabrication of NiNTAs@Fe2O3 Nanoneedle Electrode: The ZnO/NiNRAs on Ti foil was used as the scaffold for the growth of Fe2O3 nanoneedles. Initially, the electrochemical deposition was performed in 0.02 m FeCl2 solution under water-bath condition (70 °C) at a constant voltage of 1.5 V for 10 min in a three-electrode cell, using an Ag/AgCl reference electrode and a platinum foil counter electrode. During the electrodeposition, ZnO was simultaneously removed because of the acidic environment associated with Fe3+ hydrolysis (pH = 4 at 70 °C). After being washed with distilled water, the final products were annealed at 450 °C for 3 h in Ar atmosphere.

Fabrication of NiNTAs@MnO2 Nanosheet Electrode: Similarly, the MnO2 nanosheets were also deposited via the electrodeposition method in an electrolyte containing 0.01 m Na2SO4 and Mn(CH3COO)2. Before the deposition, the ZnO template was first removed by immersing the ZnO/NiNRAs in a 0.01 m HCl solution for 5 min. Then, the electrodeposition of MnO2 was carried out using CV scan between 0.1 and 0.8 V (vs Ag/AgCl) at a scan rate of 50 mV s−1 for 100 cycles. Finally, the samples were washed with distilled water and dried at 60 °C for further characterization.

Materials Characterizations: The crystallographic information and phase purity of the samples were characterized by XRD (Shimadzu Model XRD-6000 X-ray diffractometer with Cu Kα radiation), EDS (Tecnai G2 F30 S-TWIN), and XPS (ThermoFisher Scientific, ESCALAB250Xi). The morphology and microstructure of the samples were investigated by SEM (Hitachi, Model S4300), TEM (FEI-Philips, Model CM300 UT/FEG), and HRTEM.

Electrochemical Measurements: The electrochemical performances of as-prepared electrodes were investigated with CV, GCD, and EIS measurements using a CHI 660D electrochemical workstation (Chenhua, Shanghai) in a three-electrode configuration for single electrodes and in a two-electrode configuration for asymmetric supercapacitor devices, respectively. In the aqueous supercapacitor system, 1 m Na2SO4 was used as the electrolyte. While in the quasi-solid-state supercapacitor system, PVA/Na2SO4 gel electrolyte was employed. In a typical gel electrolyte preparation, 2.13 g of Na2SO4 and 3 g of PVA were mixed with 50 mL of distilled water, and then the resulted mixture was heated at 90 °C for 2 h under constant stirring to obtain the final jelly-like electrolyte.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by the National Natural Science Foundation of China (Grant Nos. 51572129 and U1407106), International S&T Cooperation Program of China (Grant No. 2016YFE0111500), QingLan Project of Jiangsu Province, a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Fundamental Research Funds for the Central Universities (Grant No. 30915011204).

Received: December 20, 2016Revised: January 08, 2017

Published online: March 1, 2017

[1] M. F. El-Kady, V. Strong, S. Dubin, R. B. Kaner, Science 2012, 335, 1326.

[2] Z. Yu, L. Tetard, L. Zhai, J. Thomas, Energy Environ. Sci. 2015, 8, 702.[3] D.-H. Kim, Y.-S. Kim, J. Wu, Z. Liu, J. Song, H.-S. Kim, Y. Y. Huang,

K.-C. Hwang, J. A. Rogers, Adv. Mater. 2009, 21, 3703.

[4] Q. Xia, M. Xu, H. Xia, J. Xie, ChemNanoMat 2016, 2, 588.[5] J. Liu, L. Zhang, H. B. Wu, J. Lin, Z. Shen, X. W. Lou, Energy Environ.

Sci. 2014, 7, 3709.[6] X. Peng, L. Peng, C. Wu, Y. Xie, Chem. Soc. Rev. 2014, 43, 3303.[7] G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 2012, 41, 797.[8] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li,

L. Zhang, Int. J. Hydrogen Energy 2009, 34, 4889.[9] X. Tang, R. Jia, T. Zhai, H. Xia, ACS Appl. Mater. Interfaces 2015, 7,

27518.[10] G.-R. Li, H. Xu, X.-F. Lu, J.-X. Feng, Y.-X. Tong, C.-Y. Su, Nanoscale

2013, 5, 4056.[11] J. Xu, Q. Wang, X. Wang, Q. Xiang, B. Hang, D. Chen, G. Shen,

ACS Nano 2013, 7, 5453.[12] H. Wang, H. Yi, X. Chen, X. Wang, J. Mater. Chem. A 2014, 2, 1165.[13] W. Tang, L. Liu, S. Tian, L. Li, Y. Yue, Y. Wu, K. Zhu, Chem. Commun.

2011, 47, 10058.[14] Y.-G. Wang, Z.-D. Wang, Y.-Y. Xia, Electrochim. Acta 2005, 50, 5641.[15] Y. Zeng, Y. Han, Y. Zhao, Y. Zeng, M. Yu, Y. Liu, H. Tang, Y. Tong,

X. Lu, Adv. Energy Mater. 2015, 5, 1402176.[16] J. Liu, M. Zheng, X. Shi, H. Zeng, H. Xia, Adv. Funct. Mater. 2016,

26, 919.[17] P. H. Yang, Y. Ding, Z. Y. Lin, Z. W. Chen, Y. Z. Li, P. F. Qiang,

M. Ebrahimi, W. J. Mai, C. P. Wong, Z. L. Wang, Nano Lett. 2014, 14, 731.

[18] C. Fu, A. Mahadevegowda, P. S. Grant, J. Mater. Chem. A 2016, 4, 2597.

[19] Y. Jiao, Y. Liu, B. Yin, S. Zhang, F. Qu, X. Wu, Nano Energy 2014, 10, 90.

[20] L. Liu, J. Lang, P. Zhang, B. Hu, X. Yan, ACS Appl. Mater. Interfaces 2016, 8, 9335.

[21] S. Shivakumara, T. R. Penki, N. Munichandraiah, ECS Electrochem. Lett. 2013, 2, A60.

[22] X. Lu, Y. Zeng, M. Yu, T. Zhai, C. Liang, S. Xie, M. S. Balogun, Y. Tong, Adv. Mater. 2014, 26, 3148.

[23] M.-S. Wu, R.-H. Lee, J. Electrochem. Soc. 2009, 156, A737.[24] T. Liu, Y. Ling, Y. Yang, L. Finn, E. Collazo, T. Zhai, Y. Tong, Y. Li,

Nano Energy 2015, 12, 169.[25] L. Wang, H. Yang, X. Liu, R. Zeng, M. Li, Y. Huang, X. Hu, Angew.

Chem. 2016, 128, 1.[26] N. K. Chaudhari, S. Chaudhari, J. S. Yu, ChemSusChem 2014, 7,

3102.[27] Y. Lin, X. Wang, G. Qian, J. J. Watkins, Chem. Mater. 2014, 26,

2128.[28] C. Guan, J. Liu, Y. Wang, L. Mao, Z. Fan, Z. Shen, H. Zhang,

J. Wang, ACS Nano 2015, 9, 5198.[29] M. Zhu, Y. Wang, D. Meng, X. Qin, G. Diao, J. Phys. Chem. C 2012,

116, 16276.[30] S. Chaudhari, D. Bhattacharjya, J.-S. Yu, RSC Adv. 2013, 3, 25120.[31] D. Wang, Q. Wang, T. Wang, Nanotechnology 2011, 22, 135604.[32] K. Xie, J. Li, Y. Lai, W. Lu, Z. A. Zhang, Y. Liu, L. Zhou, H. Huang,

Electrochem. Commun. 2011, 13, 657.[33] B. Sarma, A. L. Jurovitzki, Y. R. Smith, R. S. Ray, M. Misra, J. Power

Sources 2014, 272, 766.[34] J. A. Glasscock, P. R. F. Barnes, I. C. Plumb, N. Savvides, J. Phys.

Chem. C 2007, 111, 16477.[35] M. A. Lukowski, S. Jin, J. Phys. Chem. C 2011, 115, 12388.[36] X. Xia, Y. Zhang, Z. Fan, D. Chao, Q. Xiong, J. Tu, H. Zhang,

H. J. Fan, Adv. Energy Mater. 2015, 5, 1401709.[37] G.-F. Chen, X.-X. Li, L.-Y. Zhang, N. Li, T. Y. Ma, Z.-Q. Liu, Adv.

Mater. 2016, 28, 7680.[38] Z. Sun, S. Firdoz, E. Y.-X. Yap, L. Li, X. Lu, Nanoscale 2013, 5, 4379.[39] D. Chao, X. Xia, C. Zhu, J. Wang, J. Liu, J. Lin, Z. Shen, H. J. Fan,

Nanoscale 2014, 6, 5691.[40] Q. Tang, W. Wang, G. Wang, J. Mater. Chem. A 2015, 3, 6662.

full

paper

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.com1606728 (10 of 10) Adv. Funct. Mater. 2017, 27, 1606728


[41] X. Yang, H. Sun, L. Zhang, L. Zhao, J. Lian, Q. Jiang, Sci. Rep. 2016, 6, 31591.

[42] X. Lu, Y. Zeng, M. Yu, T. Zhai, C. Liang, S. Xie, M.-S. Balogun, Y. Tong, Adv. Mater. 2014, 26, 3148.

[43] N. Sui, Y. Duan, X. Jiao, D. Chen, J. Phys. Chem. C 2009, 113, 8560.[44] H. Xia, D. Zhu, Z. Luo, Y. Yu, X. Shi, G. Yuan, J. Xie, Sci. Rep. 2013,

3, 2978.[45] H. Xia, J. Feng, H. Wang, M. O. Lai, L. Lu, J. Power Sources 2010,

195, 4410.[46] Z. Zhao, H. Wu, H. He, X. Xu, Y. Jin, Adv. Funct. Mater. 2014, 24,

4698.[47] G. S. Gund, D. P. Dubal, N. R. Chodankar, J. Y. Cho,

P. Gomez-Romero, C. Park, C. D. Lokhande, Sci. Rep. 2015, 5, 12454.[48] A. Izadi-Najafabadi, S. Yasuda, K. Kobashi, T. Yamada, D. N. Futaba,

H. Hatori, M. Yumura, S. Iijima, K. Hata, Adv. Mater. 2010, 22, E235.

[49] Q. Tang, W. Wang, G. Wang, J. Mater. Chem. A 2015, 3, 6662.

[50] S. Sun, J. Lang, R. Wang, L. Kong, X. Li, X. Yan, J. Mater. Chem. A 2014, 2, 14550.

[51] B. Zhu, S. Tang, S. Vongehr, H. Xie, X. Meng, ACS Appl. Mater. Interfaces 2016, 8, 4762.

[52] Y. Zhao, W. Ran, J. He, Y. Huang, Z. Liu, W. Liu, Y. Tang, L. Zhang, D. Gao, F. Gao, Small 2015, 11, 1310.

[53] A. Shanmugavani, R. K. Selvan, Electrochim. Acta 2016, 189, 283.[54] Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao, E. Xie, ACS Nano

2013, 7, 174.[55] T. G. Yun, B. Hwang, D. Kim, S. Hyun, S. M. Han, ACS Appl. Mater.

Interfaces 2015, 7, 9228.[56] T. Gu, B. Wei, J. Mater. Chem. A 2016, 4, 12289.[57] M. Toupin, T. Brousse, D. Bélanger, Chem. Mater. 2004, 16, 3184.[58] T. Brousse, D. Bélanger, J. W. Long, J. Electrochem. Soc. 2015, 162,

A5185.[59] C. Zhu, P. Yang, D. Chao, W. Mai, H. J. Fan, ChemNanoMat 2015,

1, 458.

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