Nanofoaming to Boost the Electrochemical Performance of Ni@Ni(OH

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Nanofoaming to boost the electrochemical performance of Ni@Ni(OH)2 nanowires for ultra-high-volumetric supercapacitors Shusheng Xu, Xiaolin Li, Zhi Yang, Tao Wang, Wenkai Jiang, Chao Yang, Shuai Wang, Nantao Hu, Hao Wei, and Yafei Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10700 • Publication Date (Web): 28 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016

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ACS Applied Materials & Interfaces

Nanofoaming to Boost the Electrochemical Performance of Ni@Ni(OH)2 Nanowires for Ultra-High-Volumetric Supercapacitors Shusheng Xu, Xiaolin Li, Zhi Yang*, Tao Wang, Wenkai Jiang, Chao Yang, Shuai Wang, Nantao Hu*, Hao Wei, Yafei Zhang

Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

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ABSTRACT：： Three dimensional free-standing film electrode has aroused great interest for energy storage devices. However, small volumetric capacity and low operating voltage limit its practical application for large energy storage applications. Herein, a facile and novel nanofoaming process was demonstrated to boost the volumetric electrochemical capacitance of the devices via activation of Ni nanowires to form ultrathin nanosheets and porous nanostructures. The as-designed free-standing Ni@Ni(OH)2 film electrodes display a significantly enhanced volumetric capacity (462 C/cm3 at 0.5 A/cm3) and excellent cycle stability. Moreover, the as-developed hybrid supercapacitor employed Ni@Ni(OH)2 film as positive electrode and graphene-carbon nanotube film as negative electrode exhibits a high volumetric capacitance of 95 F/cm3 (at 0.25 A/cm3) and excellent cycle performance (only 14% capacitance reduction for 4500 cycles). Furthermore, the volumetric energy density can attain to 33.9 mWh/cm3, which is much higher than most thin film lithium batteries (1–10 mWh/cm3). This work gives an insight for designing high-volumetric three dimensional electrodes and paves a new way to construct binder-free film electrode for high-performance HSC applications. KEYWRODS: Nanofoaming; Nickel nanowire; Supercapacitor; Volumetric capacity; Energy storage.

1. INTRODUCTION A lot of electrochemical energy storage devices have been prepared to moderate the energy and environment challenges. Obviously, the two most important for electrochemical energy storage devices are battery and supercapacitor. With the 2


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growing demand of energy, single battery or supercapacitor can’t satisfy the needs of high electricity consumption, so we urgently need an energy storage device have both high energy density and power density.1-4 Most recently, more and more attentions are attracted to combining the merits of these two devices, namely hybrid supercapacitors (HSC),5 which representatively compose of the battery-type as energy source electrode and the supercapacitor-type as power source electrode, such as TiO2@Ni(OH)2//mesoporous carbon,6 LiMn2O4//active carbon7 and Li4Ti5O12//CNTs.8 Compared with traditional electric double layer capacitor, HSC can ensure a wider working potential window and larger capacity, resulting in a higher energy density. While compared with battery, HSC can deliver an improved power density for fast charge-discharge and long time cycling stability. In consequence, HSC has opened up a new field for electrochemical energy storage and reduced the gap between battery and supercapacitor. Obviously, the electrochemical energy storage performance of HSC is directly affected by electrode materials, such as material structure, type and property. To enhance electrochemical performance of HSC, many efforts have been made to research electrode materials, especially transition metal oxides and metal hydroxides, such as NiO,9,10 Ni(OH)211,12 and Co3O4,13-15 due to its larger capacities and higher energy densities than traditional carbonaceous materials.16-20 Among them, Ni(OH)2 has aroused great interest owing to its high theoretical specific capacity, excellent chemical stability and easy preparation. However, its poor electric conductivity often results in low rate capability and short cycle life. The reason is that most faradic 3



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materials are diffusion-controlled, poor conductivity often lead to low coulombic efficiency and sluggish reaction kinetics.21 Up to now, one effective method to solve these problems is directly synthesis high performance electrode materials on desired substrates with high electrical conductivity. For example, Liu et al. have proposed a facile method to synthesis free-standing and additive-free three dimensional (3D) Ni@NiO film electrode with excellent rate capability.22 Peng et al. synthesized NiMoO4 nanosheet and nanorod arrays and studied electrochemical properties for hybrid asymmetric supercapacitor application.23 Go et al. directly synthesized ultra-thin NiCo2O4 nanosheets on conductive Ni nanofoam (NF) for energy storage electrode with excellent electrochemical performance.24 Compared with traditional methods add binder and conductive additives to fabricate electrode, this additive-free fabricate method can avoid using binder and conductive additives. Thus, all materials on the surface of the film can full access with the electrolyte solution and take part in redox reaction process, which significantly improves the overall electrochemical properties and exhibits promising possibility to improve the volumetric capacity of HSC. 3D nanostructures constructed from low-dimensional segments have attracted increasing interest for electrochemical energy storage. However, these 3D nanostructures are often subjected to low volumetric capacitance. Several methods, such as vacuum-assisted self-assembly,16 mechanical compression,17 and liquid electrolyte-mediated dense integration,20 have been reported to increase bulk density for the purpose of improving the volumetric capacity, while often at the expense of 4


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gravimetric capacity and rate capability. Thanks to enhanced physicochemical activity, large specific surface area, and excellent electrical conductivity, metal nanofoams (especially transition metals) have attracted considerable attentions as an important class of functional materials. In this work, we demonstrate an efficient Ni@Ni(OH)2 film electrode by a simple nanofoaming process using hydrothermal method. Unlike other reported methods, our nanofoaming process can in situ synthesize a layer of ultrathin Ni(OH)2 nanosheets with porous nanostructures on the surface of conductive Ni nanowires (NWs) substrate. This unique structure of electrode material has several advantages including large energy storage capacity of Ni(OH)2 and excellent conductivity of Ni, ease to prepare and low cost of Ni NWs. Furthermore, our film electrode with 3D conductive network is similar to the commercial nickel foam except that its pore is macroporous (pore size of 450–3200 µm), so we call this Ni@Ni(OH)2 film as nanofoam (NF). After nanofoaming process, the Ni@Ni(OH)2 NF electrodes display significantly enhanced volumetric capacity of 462 C/cm3 at 0.5 A/cm3, long time cycle stability and excellent rate capability. Graphene-carbon nanotube (G-CNT) film is chosen for negative electrode for HSC owing to its large specific surface area, excellent conductivity of 3D CNT network and large bulk density.25 Moreover, unlike traditional slurry-processed method, both positive and negative electrodes are binder-free, which not only ensures fast ion diffusion, but also makes sufficient use of active materials. As a consequence, the as-fabricated HSC device possesses large volumetric capacitance of 95 F/cm3 at 0.25 A/cm3. Furthermore, the device displays an ultra-high volumetric energy density of 33.9 mWh/cm3, which is even much higher 5



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than lithium thin film batteries. Our work may pave a new route to fabricate high performance nanofoam film electrodes for other metal nanomaterials with similar structure. 2. EXPERIMENTAL SECTION Synthesis of Ni@Ni(OH)2 NF positive electrode. All analytical grade (AR) chemicals were used as received in all experiments. Ultralong Ni NWs were synthesized using a facile one-step process under a magnetic field as reported in our work.26 The Ni film was acquired by dispersing Ni NWs in a 1 wt% polyvinylpyrrolidone (PVP) of ethanol solution, vacuum-filtered and then pressed at 10 MPa for 5 min. The obtained Ni film was dried in 60°C vacuum oven for 12 h. Ni@Ni(OH)2 NF film was prepared using a moderate hydrothermal reaction. Typically, 32 mg Ni film and 30 mL 15% H2O2 solution were put into a 50 mL Teflon-lined autoclave reactor, and heated to 180 °C for 12 h. After hydrothermal process, the obtained materials was cleaned using deionized water and dried at 60 °C for 6 h. For the following electrochemical characterizations, the film was cut into 0.5 × 0.5 cm2. To get optimal experiment conditions, the weight of Ni film (32 and 64 mg), concentration of H2O2 solution (0%, 15% and 30%), temperature (160, 180 and 200 °C) and reaction time (6, 12 and 24 h) were adjusted. NF-2mg-15% (as shown in table S1) was chosen as the experiment condition due to its optimum electrochemical properties. All characterizations in article refer to this sample (labeled as Ni@Ni(OH)2 NF) except for special statements. Characterizations. The microstructures and morphologies of the as-prepared 6


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materials were obtained by a scanning electron microscope (SEM, Ultra-55, Carl Zeiss, Germany) and transmission electron microscopy (TEM, JEM-2010, JEOL, Japan). The phase of the products was obtained using a D8 advanced X-ray diffractometer (XRD) with Cu-Kα source in a 2θ range of 10-90°. X-ray photoelectron spectroscopy (XPS) was characterized on a Kratos Axis UltraDLD with an Al Kα source (1486.6 eV). The chemical elements of the material was analyzed by using Energy dispersive X-ray spectroscopy (EDS, Oxford Instruments INCA PentaFET×3, Model: 7426). The specific surface area of the sample was obtained from the Brunauer-Emmett-Teller (BET) plot of the N2 desorption isotherm measured at 77 K with ASAP 2020 (Micromeritics, USA). The pore-size distribution curves were determined from desorption branch isotherms by a nonlocal density functional method. Electrochemical measurements. Electrochemical tests of individual electrodes (Ni@Ni(OH)2 and G-CNT), including cyclic voltammetry (CV), galvanostatic charge/discharge measurements (GCD), and the electrochemical impedance spectroscopy (EIS) were carried out in room temperature using CHI760E electrochemical working station (Chenhua, Shanghai, China). Film electrodes were directly used as the working electrode. A Pt foil electrode and an Hg/HgO electrode acted as counter electrode and reference electrode, respectively. An hybrid asymmetric supercapacitor was fabricated by using Ni@Ni(OH)2 NF positive electrode and G-CNT negative electrode, separate by porous nonwoven fabric into two face-to-face electrodes. For both three-electrode cells and two-electrode cells, 6 7



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M KOH was used as the electrolyte. The electrochemical impedance spectroscopy (EIS) measurements were performed at open-circuit potential over a frequency ranging of 100 KHz to 0.01 Hz. 3. RESULTS AND DISCUSSION Nanofoaming techniques were exploited to fabricate Ni@Ni(OH)2 NF film with ultrathin nanosheets and porous nanostructure by a facile one-step hydrothermal process in our work, as shown in Scheme 1. The formation of ultrathin porous nanosheets on the surface of Ni NWs may be due to strong hydroxidizability of nickel by H2O2 in the autoclave at high pressure and temperature conditions. The Ni NWs act as Ni sources for growing of Ni(OH)2 ultra-thin nanosheets, as well as the skeleton and current collector to support active materials. The reaction equation can be summarized as: Ni + H2O2 ⟶ Ni(OH)2.

(1)

We proposed a formation mechanism of ultrathin and porous Ni(OH)2 nanosheets into two processes as follows. (i) The transformation Ni to β-Ni(OH)2 on the surface of NWs. This can be demonstrated by HRTEM and XRD characterizations in Figure S1. (ii) β-Ni(OH)2 transform to amorphous Ni(OH)2 with mesopores, which can be proved by following TEM, XRD and XPS characterizations. Different from traditional method, our nanofoaming techniques can generate a layer of porous ultrathin nanosheets on the surface of conductive Ni NWs. This unique porous core-sheath structure can ensure excellent contact between conductive Ni core and active Ni(OH)2 sheath, provide a quick electronic transfer channel, leading to the 8


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boosted energy storage ability for Ni@Ni(OH)2 NF film electrode.

Scheme 1. Schematic illustration of nanofoaming process for Ni@Ni(OH)2 nanofoams fabrication. (a) The formation of β-Ni(OH)2 after 6 h hydrothermal reaction, (b) Transformation of β-Ni(OH)2 to amorphous Ni(OH)2 with mesopores after 12 h hydrothermal reaction.

The morphologies and nanostructures of as-synthesized raw materials Ni NWs and Ni@Ni(OH)2 NF were obtained by SEM and TEM, as exhibited in Figure 1. It can be easily observed from Figure 1a that ultralong Ni NWs have been obtained by a simple liquid-phase method under a magnet field. Moreover, the magnification SEM inset image exhibits that Ni NWs have an acicular structure with an uneven surface, which can efficiently improve the specific surface area of Ni NWs. Figure 1b displays the TEM image of a Ni NW, many thorns can be observed and evenly distributed on the surface of Ni NW. To study crystal structure of Ni NWs, HRTEM image obtained from the edge of a thorn was displayed in Figure 1c. Clear and ordered lattice fringes can be detected with a crystal spacing of about 0.21 nm, in well agreement with the {111} spacing planes of the Ni face-centered cubic (fcc) structure.27 Compared with Ni NWs in Figure 1a, for Ni@Ni(OH)2 NF, the embossments on the surface of Ni 9



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NWs disappeared, and the diameter of the NWs become much thinner after nanofoaming treatments, as exhibited in Figure 1d. Furthermore, the surface of the nanowire become uneven, which facilitates full contact between electrode material and electrolyte. Figure 1e exhibits the TEM image of Ni@Ni(OH)2 NF. It is clear that embossments on the surface disappeared after nanofoaming treatments, indicating that Ni on the surface of NWs has reacted with H2O2. From the HRTEM image in Figure 1f, numerous nanosheets and considerable mesopores (marked with red circles) can be observed, which may be ascribed to the release of gas during hydrothermal reaction. The nanosheets show a transparent feature, further demonstrating its ultrathin nature. Moreover, these nanosheets are aligned and uniform wrapped on the Ni NWs. This resultant porous nanostructure is believed to facilitate the ion diffusion and charge transport without any additives and binders. The porous nanostructures can be further demonstrated by pore size distribution curves shown in Figure S2. For Ni NWs without any treatments, the pore size is mainly distributed around 4 nm. After a facile one-step nanofoaming treatments, the distribution of Ni@Ni(OH)2 NF pore size become much wider and can be increased to more than 10 nm. For battery-type material Ni(OH)2, a substantial fraction of charge storage arise from redox reaction. Hence, a larger pore size is more conducive to electrolyte diffusion and ion transport, which can further enhance the energy storage ability for Ni@Ni(OH)2 NF electrode. It is worth mentioning that BET surface area decreased after nanofoaming treatment, this is due to the smaller surface area change compared with macroscopic foaming process. HRTEM image of Ni@Ni(OH)2 NF in Figure 1f displays a portion of 10


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amorphous phase without any lattice structure on the surface of NWs corresponding to amorphous Ni(OH)2. A crystal plane spacing of 0.21 nm corresponding to {111} spacing of Ni fcc structure is also obtained. Expressively, when applied to energy storage electrode, ultrathin porous Ni(OH)2 nanosheets directly growing on the surface of Ni NWs was expected to obtain large capacity and excellent rate performance due to its mesoporous nanostructure and short paths way for ion diffusion and electron transfer. The detail experimental conditions of Ni@Ni(OH)2 samples to determine the optimum ratio of Ni NWs with H2O2 are shown in Table S1. SEM images of samples from NF-32mg-0% to NF-32mg-10% (32 mg Ni nanowires hydrothermal react with 30 mL 10 wt% H2O2) are displayed in Figure S3. Interestingly, when the relative content of H2O2 is low, as with NF-32mg-0% and NF-64mg-5%, some β-Ni(OH)2 hexagonal platelets can be found around the NWs, which can be demonstrated by XRD and XPS characterizations shown in Figure S4. With the H2O2 content increasing, no β-Ni(OH)2 hexagonal platelets can be found from NF-64mg-10% to NF-32mg-10% (Figure S3e to S3l). Figure S5 shows the EDS of NF-32mg-0% to NF-32mg-15%. Only Ni and O elements can be found in each samples. Moreover, except for NF-32mg-0% and NF-64mg-5%, the contents of O element increases with the increase of H2O2 content, indicating that more Ni has been transformed into amorphous Ni(OH)2 phase rather than β-Ni(OH)2. So we can deduce that when the content of H2O2 is low, Ni would prefer to be oxidized into β-Ni(OH)2. While Ni would prefer to transform into amorphous Ni(OH)2 with the increasing of the H2O2 11



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content, which is summarized in Figure S5h. For NF-32mg-0% and NF-64mg-5%, the formation of hexagonal β-Ni(OH)2 destroy the Ni NWs structures and 3D conductive network, resulting in poor electrochemical performance. While for NF-64mg-10% to NF-32mg-15%, amorphous Ni(OH)2 nanosheets in situ grow on the surface of Ni NWs, thus porous nanostructures and 3D conductive network is well retained, leading to enhanced electrochemical performance. Additionally, amorphous phase Ni(OH)2 can possess enhanced electrochemical efficiency compared with hexagonal β-Ni(OH)2 due to disorder. Thus, the results mentioned above suggested that as-synthesized Ni@Ni(OH)2 NF can provide a new insight for high capacity electrochemical energy storage electrode.

Figure 1. (a) SEM images of Ni NWs. Inset is the magnified SEM image. (b) TEM image and (c) HRTEM image of Ni NWs. Inset is the magnified HRTEM image. (d) SEM images of Ni@Ni(OH)2 NF. Inset is the magnified SEM image. (e) TEM and (f) HRTEM images of 12


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Ni@Ni(OH)2 NF. Inset is the magnified HRTEM image.

The crystallographic structure and chemical composition of Ni@Ni(OH)2 NF were characterized by XRD and XPS, the results were exhibited in Figure 2. The diffraction angles (2θ) in Figure 2a located at 44.62°, 51.94° and 76.58°, which can be assigned to (111), (200) and (220) reflections of Ni, respectively.28,29 All diffraction angles of Ni NWs and Ni@Ni(OH)2 NF are consistent well with fcc Ni phase (JCPDS No. 04-0850). No impurities or any Ni(OH)2 characteristic peaks can be found from XRD due to the fact that Ni(OH)2 is amorphous phase rather than crystalline. This can be further demonstrated by XPS spectrums as exhibited in Figure 2b-d. From survey spectrum in Figure 2b, C, O and Ni element can be detected from Ni@Ni(OH)2 NF, among which C should come from residues of ethylene glycol (EG). Two major peaks around 855.9 and 873.5 eV can be found from the high resolution Ni 2p spectra, along with two satellite peaks at 861.4 and 879.6 eV, which consistent well with amorphous Ni(OH)2 from previous reports.30 The peak at 531.5 eV in the O 1s high-resolution spectra (Figure 2d), which is assigned to Ni-OH. The peak around 533.1 eV is corresponded to C-OH from residues of EG.31 Hence, the formation of amorphous Ni(OH)2 on the surface of Ni NWs after nanofoaming process has been further confirmed.

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Figure 2. (a) XRD patterns of Ni NWs and Ni@Ni(OH)2 NF. XPS spectrums of Ni@Ni(OH)2 NF. (b) Survey spectrum, (c) Ni 2p and (d) O 1s.

To get the optimal experiment condition, the samples with different proportions of Ni NWs to H2O2, different reaction temperature and reaction time were tested by electrochemical measurements, the results are shown from Figure S6 to Figure S8. Comprehensive comparison, the optimum condition was chosen as 32 mg Ni NWs and 30 mL 15% H2O2 heating at 180 °C for 12 h (NF-32mg-15% in Table S1). Then we studied the influence of thickness on electrochemical property of the electrode, the results are shown in Figure S9. It is clear that the film with thickness of 45 µm possesses the largest capacity. When the film is too thin, the active material is not enough for redox reaction. While along with the increasing thickness of film electrode, the electrolyte can’t sufficiently diffuse into film to contact with active material for redox reaction. Therefore, the electrochemical property of the electrode would be 14


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attenuated when the film is too thick or too thin. To demonstrate the potential application of Ni@Ni(OH)2 NF film electrode for energy storage applications, electrochemical performances electrode were measured using CV and GCD techniques in a three-electrode cell. Figure 3a presents the CV curves of Ni@Ni(OH)2 NF at scan rate ranging of 2 to 100 mV/s at the potential of 0−0.6 V vs Hg/HgO. A couple of strong reversible redox peaks can be obtained in each curve, which unlike electric double layer capacitor materials with nearly rectangular CV curve. This is because Ni(OH)2 belong to battery-type electrode, whose capacity is mainly originated from Faradaic redox reaction as below29,32: Ni(OH)2 + OH– ↔ NiO(OH) + H2O + e–

(2)

The oxidation peak and reduction peak are nearly symmetric in each curve, demonstrating the excellent reversibility of as-prepared Ni@Ni(OH)2 NF. With the scan rate increasing, the oxidation peak move positively and the reduction peak move negatively, mainly owing to the existence of internal resistance for electrode material. However, the shape of CV curves show no significant change even at a high scan rate of 100 mV/s, suggesting the small equivalent series resistance of the as-prepared electrode based on the close contact between active Ni(OH)2 nanosheets and Ni substrates. The GCD tests were studied to further assess the electrochemical property of Ni@Ni(OH)2 NF film electrode, as shown in Figure 3b. The potential of two voltage plateaus on GCD curves match well with peak positions of CV curves (Figure 3a), corresponding to the reversible redox reaction. Moreover, GCD curves are nearly 15



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symmetric at current density ranging of 0.5 to 5 A/cm3, implying the high columbic efficiency and low polarization of Ni@Ni(OH)2 NF electrode. The volumetric capacity at different current densities calculated from GCD curves is exhibited in Figure S10a. The Ni@Ni(OH)2 NF film electrode shows a volumetric capacity as high as 462 C/cm3 at 0.5 A/cm3, suggesting that electrolytic ions can efficiency diffuse and migrate into the Ni(OH)2 nanosheets when the current density is small. It is clear that the volumetric capacity deduces with current density increasing. This is because at high current density, the migration of the electrolytic ions is limited, which resulting in a portion of Ni(OH)2 nanosheets on the surface become inaccessible for electrochemical charge storage. Remarkably, the volumetric capacity decreases to 208 C/cm3 while current density increases to 5 A/cm3, indicating capacity retention of 45% was obtained compared with the volumetric capacity at 0.5 A/cm3. These results indicate the excellent capacity and rate capability behavior of Ni@Ni(OH)2 NF electrode. Cycle performance is another crucial criterion to evaluate electrochemical energy storage ability of electrode material. Figure S10b reveals the cycle stability obtained at a large current density of 4 A/cm3 for 4500 cycles. The volumetric capacity increases up to 122% of original capacity at first 500 cycles. This can be attributed to the activation process of active material Ni(OH)2 inside the 3D structure film electrode. After 4500 cycles, the capacity could remain 83% of the initial capacity, suggesting the remarkable long-time cycle performance of the as-prepared Ni@Ni(OH)2 NF film electrode. Furthermore, the Coulombic efficiency calculated from GCD curves increases from 86% to 98% after cycling. Hence, the 16


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above-mentioned results reveal the ultrahigh volumetric capacity, remarkable rate capability and excellent cycle performance of Ni@Ni(OH)2 NF electrode material. We also compared the electrochemical properties of Ni@Ni(OH)2 NF with other previously reported materials, as listed in Table S2. It’s obvious that our electrode possess a competitive energy storage ability among listed electrode materials. To explore the remarkable electrochemical property of as-prepared Ni@Ni(OH)2 NF, nickel foam treated by hydrothermal reaction (marked as Ni-F) and Ni@Ni(OH)2 NW film treated by electrochemical reaction were chosen as comparison. Figure 3c displays the CV curves of different samples at scan rate of 10 mV/s. All curves have two strong symmetric peaks, suggesting that the capacity of these three samples are mainly derived from active material Ni(OH)2 owing to Faradaic redox reaction. Furthermore, Ni@Ni(OH)2 NF possesses the maximum CV area, indicating larger capacity than that of Ni-F and Ni@Ni(OH)2 NW. The same results can be found from GCD curves at 1 A/cm3 exhibited in Figure 3d. Ni@Ni(OH)2 NF electrode shows the longest discharge time, indicating the higher electrochemical capacity than other samples. Volumetric capacity of Ni-F, Ni@Ni(OH)2 NW and Ni@Ni(OH)2 NF with different current density is shown in Figure 3e, which is calculated to be 8, 88 and 462 C/cm3 at 0.5 A/cm3 for Ni-F, Ni@Ni(OH)2 NW and Ni@Ni(OH)2 NF, respectively. It is clear that the volumetric capacity of Ni@Ni(OH)2 NF is much higher than Ni-F and Ni@Ni(OH)2 NW, indicating nanofoaming process can boost the electrochemical capacity of Ni@Ni(OH)2 NF film electrode. When current density increases to 5A/cm3, Ni@Ni(OH)2 NF possesses a volumetric capacity of 208 C/cm3, which is 17



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also much higher than Ni-F (1.5 C/cm3) and Ni@Ni(OH)2 NW (76 C/cm3). Detailed electrochemical tests of Ni-F and Ni@Ni(OH)2 NW are displayed in Figure S11. Figure 3f exhibits the EIS curves of three samples which reveal that three samples have the similar and very low solution resistance (Rs), indicating the excellent conductivity of samples. The remarkable electrochemical property of Ni@Ni(OH)2 NF electrode can be owing to the reasons as follows: Firstly, unlike Ni-F, nickel foam which presents micron scale (Figure S12 a and b), Ni@Ni(OH)2 NF electrode material exists in nanoscale with ultrathin and porous nanostructures, greatly improving the surface area of electrode, promoting the electrolyte access and exposing more active sites to electrolyte. Secondly, compared with Ni@Ni(OH)2 NW (Figure S12 c and d), conductive Ni NWs are wrapped with microporous and amorphous Ni(OH)2 nanosheets on the surface, which further improves surface area and ensures efficient charge transport and ion diffusion. Thirdly, the unique mesoporous core-sheath structure ensures full contact between Ni substrate and active Ni(OH)2 nanosheets, which ensures an excellent electrical contact and short paths way for ion diffusion and electron transfer.

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Figure 3. (a) CV curves of Ni@Ni(OH)2 NF at scan rate from 2 to 100 mV/s. (b) GCD curves of the sample ranging from 0.5 to 5 A/cm3. (c) CV curves at scan rate of 10 mV/s, (d) GCD curves at a current density of 1 A/cm3, (e) Volumetric capacity as a function of current density, (f) Nyquist plots of Ni-F, Ni@Ni(OH)2 NW and Ni@Ni(OH)2 NF.

To evaluate Ni@Ni(OH)2 NF film for supercapacitor application, G-CNT was chosen as negative material for its large volumetric capacitance and large bulk density. Cross-sectional SEM image of G-CNT in Figure 4a displays a dense layered and wrinkled structure of as-prepared film. Furthermore, CNTs are uniformly incorporated with graphene to avoid the restacking of the sheets, and also efficiently improve the conductivity and mechanical strength of the film. The electrochemical property of the G-CNT film electrode was tested at room temperature in 6 M KOH solution. CV curves (Figure 4b) exhibit a near rectangular shapes without significant deformation with the scan rate increase, indicating an excellent capacitance behavior. The GCD 19



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curves of G-CNT are shown in Figure 4c. It is clear that each curves are nearly linear and symmetrical as current density increase from 0.5 to 5 A/cm3, demonstrating the typical characteristic of capacitor behavior. The volumetric capacitance at different current density is displayed in Figure 4d. The G-CNT shows a volumetric capacitance of 164, 146, 125, 95, 84 F/cm3 at 0.5, 1, 2, 4, 5 A/cm3, respectively. EIS is also performed and the homologous Nyquist plots are exhibited in Figure S13a. The Rs value is about 1.8 Ω which reveals the excellent conductivity of G-CNT. The total equivalent series resistance (ESR) is estimated as 1.1 Ω, indicating the excellent charge transfer between electrode and electrolyte. Additionally, the G-CNT electrode possesses an outstanding long time cycle stability with only 6% capacitance decrease after 2250 cycles (Figure S13b). All these results mentioned above demonstrate the excellent electrochemical properties of as-prepared G-CNT electrode for HSC applications.

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Figure 4. (a) Cross-sectional SEM image of G-CNT film. Inset is the magnified SEM image. (b) CV curves of G-CNT at different scan rates in 6 M KOH. (c) GCD curves of G-CNT at different constant current densities. (d) Volumetric capacitance of G-CNT as a function of current densities calculated from GCD curves.

To estimate the energy storage ability of as-prepared film electrode in practical application, we have fabricated a hybrid asymmetric supercapacitor with Ni@Ni(OH)2 NF film and G-CNT film as positive electrode and negative electrode, respectively. The active electrode materials were placed on Ni foams which served as current collector, and then sandwiched with a separator using 6 M KOH as electrolyte, as illustrated in Figure 5a. The charge balance between two electrodes abide by the 21



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relation: q+=q-. Electrode charge storage often bases on capacitance (C), the voltage window (∆‫ ) ܧ‬and the volume of the electrode material (V) according to the equation below33: q=C × ∆‫ܸ × ܧ‬

(3)

We maintain the same surface area of two electrode and adjust the thickness ratio between two electrodes as V+(Ni@Ni(OH)2)/V-(G-CNT)=0.53, depending on the charge balance between two electrodes. To assess the electrochemical performance and obtain the suitable potential windows of our HSC, we performed CV tests of the device under various voltage window at 20 mV/s before characterizing the electrochemical performance of the full-cell, as shown in Figure S14a. It shows a capacitive properties with nearly rectangular-like shape CV curves even at a voltage window up to 1.6 V. With the increase of the operation voltage window, the existence of redox peaks indicates that more Faradaic reactions occurred on Ni@Ni(OH)2 NF electrode. When the operation potential of the HSC is increased from 1.1 to 1.6 V, the volumetric capacitance obtained from CV curves can be increased from 27.2 to 53.6 F/cm3 (Figure S14a inset), with an enhancement of 197%. Importantly, the energy density of the HSC is increased to 417% calculate by the equation E=0.5 CV2. However, when the voltage increases to much higher, there is a serious limitation related with H2 evolution at the negative electrode. Thus, the operation window was set as 1.6 V for further evaluate the capacitive behavior of as-fabricated HSC in subsequent study. Figure 5b exhibits CV curves of the assembled HSC from 2 to 100 mV/s tested 22


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at 1.6 V voltage window in 6 M KOH. It is clear that all CV curves have nearly rectangular-like shapes, demonstrating an excellent capacitive property and fast charge/discharge behaviors of as-fabricated HSC. With the scan rate increase, the form of the CV curves still maintain symmetry and display no obvious change, suggesting the excellent electrochemical reversibility and rate capability of the device. Additionally, the GCD curves exhibit in Figure 5c are approximate triangular shape with linear behavior, further confirming high electrochemical properties and capacitor-like behaviors of Ni@Ni(OH)2//G-CNT HSC. The volumetric capacitance of a supercapacitor can be obtained from GCD curve depend on the follow equation34:

‫ܷ∆ ∙ ܸ ∕ ݐ∆ ∙ ܫ = ܥ‬

(4)

Where C refer to volumetric capacitance (F/cm3), I refer to the charge/discharge current (A), ∆‫ ݐ‬refer to the discharge time (s), V is the total volume of active material on both electrodes (cm3) and ∆ܷ refer to the voltage window (V). The volumetric capacitance of the HSC at different current densities is exhibited in Figure 5d. The volumetric capacitance of assembled HSC is calculated to be 95, 85, 79, 70 and 50 F/cm3 at current density of 0.25, 0.5, 0.75, 1 and 2 A/cm3, respectively. Moreover, an areal capacitance from 1.6 F/cm2 at 4.2 mA/cm2 to 0.4 F/cm2 at 67.2 mA/cm2 were achieved. EIS spectrum in Figure S14b shows that as-fabricated HSC has a low ionic and electronic resistances at high frequency area. A nearly vertical line at low frequency area is the representative characteristics of capacitive behavior, suggesting the fast charge/discharge behavior of assembled HSC. Long time cycle stability of supercapacitor is very essential for practical 23



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application, so a cycle stability was performed for Ni@Ni(OH)2//G-CNT HSC by repeating GCD tests between 0 and 1.6 V with a current density of 1 A/cm3, as displayed in Figure 5e. The increase of capacitance at first 300 cycles can be related to the surface wetting enhancement of electrode material by electrolyte in cycling process.35 The volumetric capacitance can reach up to 110% of original capacitance. After a total number of 4500 galvanostatic charge/discharge cycling, a capacitance loss of 14% was observed for our HSC device. It suggests our Ni@Ni(OH)2//G-CNT HSC possess excellent long time electrochemical stability, which is superior to asymmetric supercapacitors reported by previous works, such as Ni(OH)2//AC (82% retention after 1000 cycles),36 Co7Ni3//AC (75.4% retention after 3000 cycles)37 and Co3O4@Ni(OH)2//RGO (84% retention after 1000 cycles).38 The volumetric energy density (E, mWh/cm3) and power density (P, Wh/cm3) of a supercapacitor are obtained according to following relationship:

E=0.5 CV2

(5)

P=E/t

(6)

Where C refer to the volumetric capacitance (F/cm3), V refer to the cell potential (V) and t is the discharge time (s).39 Figure 5f displays the Ragone plot of Ni@Ni(OH)2//G-CNT HSC obtained from GCD curves (Figure 5c) at different current densities. At a low current density of 0.25 A/cm3, the energy density and power density are calculated to be 33.9 mWh/cm3 and 200 mW/cm3, respectively. The improved energy density can be attributed to the synergistic effects between Ni@Ni(OH)2 positive electrode and G-CNT negative electrodes. On one hand, the 24


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high volumetric energy density is obtained because of the ultrahigh volumetric capacity of positive electrode owing to the Faradaic reaction of active Ni(OH)2 nanosheets. On the other hand, the operation potential window is extended to 1.6 V due to the negative electrode G-CNT (1.0 V). Furthermore, energy density of as-prepared HSC is far more larger than VOx//VN ASC (0.61 mWh/cm3 and 850 mWh/cm3),40 CNT//Ni(OH)2 ASC (0.3 mWh/cm3and 14 Wh/cm3),41 Mn3O4//Ni(OH)2 APSC (0.35 mWh/cm3 and 7 Wh/cm3),42 VN//CNT ASC (0.54 mWh/cm3 and 400 Wh/cm3),43 MnO2//CoSe2/carbon cloth ASC,44 Fe2O3//MnO2 SCs (0.4 mWh/cm3 and 60 Wh/cm3),45 TiO2@MnO2//TiO2@C-based SCs(0.3 mWh/cm3 and 190 Wh/cm3)46 and Co9S8//Co3O4@Ru2O ASC (1.21 mWh/cm3 and 13290 Wh/cm3).47 It is even higher than thin film lithium batteries (1–10 mWh/cm3).48,49 Even the current density improve to 4 A/cm3, the energy density still remains 8.4 mWh/cm3 with a power density

of

3217

Wh/cm3.

The

excellent

electrochemical

property

of

Ni@Ni(OH)2//G-CNT HSC should be ascribed to the novel and unique electrode architecture. Particularly, our facile nanofoaming process could in situ grow Ni(OH)2 nanosheets on the surface of high conductivity Ni NWs, which could substantially alleviate the conductivity problem of Ni(OH)2. Moreover, the porous Ni(OH)2 nanosheets are beneficial to shorten the paths way for ions diffusion and electrons transfer

during

charge

storage

processes,

thus

significantly

improve

the

electrochemical performance and rate capability. Hence, our Ni@Ni(OH)2//G-CNT HSC device demonstrate an excellent potential for energy storage applications.

25



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Figure 5. (a) Schematic of the assembled structure of HSC based on Ni@Ni(OH)2 NF positive electrode and G-CNT negative electrode. (b) CV curves of HSC device at different scan rates. (c) GCD curves of HSC device at different current densities. (d) Volumetric capacitance and area of the HSC device as a function of current densities calculated from GCD curves. (e) Cycle performance of HSC device at 1 A/cm3. Inset is the GCD curves of 1st and 4500th cycle. (f) Ragone plots of HSC device.

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4. CONCLUSIONS In conclusion, we have proposed an efficient nanofoaming technique to boost the electrochemical energy storage property of Ni@Ni(OH)2 NF film electrode for ultra-high volumetric capacity supercapacitors. Ultrathin Ni(OH)2 nanosheets with porous nanostructures can be obtained on the surface of Ni NWs by a facile vacuum-assisted filtration method and subsequent nanofoaming treatments. The as-obtained Ni@Ni(OH)2 NF film electrodes display a significantly enhanced volumetric capacity of 462 C/cm3 at 0.5 A/cm3 and excellent cycle stability (83% capacity retention after 4500 cycles). A hybrid supercapacitor with Ni@Ni(OH)2 NF as positive electrode and G-CNT as negative electrode was developed. The HSC possesses an ultra-high volumetric capacitance of 95 F/cm3 at 0.25 A/cm3, a large energy density of 33.9 mWh/cm3, and a long time cycle stability of 86% retention after 4500 cycles at 1.6 V operation voltage window. These remarkable properties demonstrate that our facile nanofoaming method can open up new possibilities for metal oxide or hydroxide materials for high energy supercapacitor applications.

ASSOCIATED CONTENT Supporting Information. Ni-F, Ni@Ni(OH)2 NWs and G-CNT fabrication process; SEM, TEM, XRD, XPS, BET and EDS characterizations of Ni@Ni(OH)2 NF; detailed CV, GCD and EIS data for Ni-F, Ni@Ni(OH)2 NWs and Ni@Ni(OH)2 NF. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 27



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Corresponding Author * E-mail: [email protected] and [email protected]. Author Contributions All authors contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The support of National Key Research and Development Program of China (2016YFC0102700), the National Natural Science Foundation of China (61671299), Shanghai Science and Technology Grant (16JC1402000), Program of Shanghai Academic/Technology Research Leader (15XD1525200), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning are gratefully acknowledged. The Instrumental Analysis Center of Shanghai Jiao Tong University and the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University are also acknowledged.

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for


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Table of Contents

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Nanofoaming to Boost the Electrochemical Performance of Ni@Ni(OH

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