A Tubular Sandwich-Structured CNT@Ni@Ni - ACS Publications

Article pubs.acs.org/JPCC

A Tubular Sandwich-Structured CNT@Ni@Ni2(CO3)(OH)2 with High Stability and Superior Capacity as Hybrid Supercapacitor Shipei Chen,†,§ Qingnan Wu,†,§ Ming Wen,*,† Chenxiang Wang,† Qingsheng Wu,† Jiahao Wen,‡ Meng Zhu,† and Yansen Wang† †

School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092, China ‡ School of Electrical Engineering, Chongqing University, Chongqing 400044, China S Supporting Information *

ABSTRACT: Development of highly stabile battery-type electrode materials with superior capacity has been a critical challenge for hybrid supercapacitors. We report a high-performance electrode material, tubular sandwich-structured CNT@Ni@Ni2(CO3)(OH)2, synthesized via a scalable, dynamic, controlled in situ reduction− chemical deposition process. Applied as a battery-type electrode material, this novel nanostructure exhibits excellent electrochemical stability, majorly attributed to the Ni midshell serving a dual role as “capacity supplement” and “electron highway”, which, to our knowledge, was incorporated into the nanocomposite electrodes for the first time. Also benefiting from the high conductivity of carbon nanotubes (CNTs) and the high capacity of the amorphous NiOOH ultrathin film [converted from the Ni2(CO3)(OH)2 outer shell], the resulting CNT@Ni@Ni2(CO3)(OH)2 material as a battery-type electrode achieves a superior capacity of 221 mAh·g−1 at 5 A·g−1 with 76% capacity retention at 50 A·g−1 and maintains 81% capacity after 9000 cycles at 5 A·g−1. An advanced aqueous hybrid supercapacitor using activated carbon and CNT@Ni@Ni2(CO3)(OH)2 nanocomposite as the negative and positive electrodes, respectively, delivers a high energy density of 179 Wh·kg−1 at a power density of 2880 W·kg−1 with capacitance retention in excess of 85% over 5500 cycles. The outstanding performance demonstrates its practical potential in advanced hybrid supercapacitors.

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in EDLCs electrodes, most Ni compounds are flawed by poor electrical conductivity and drastic volume change over charge− discharge cycling, leading to low rate capability and poor cycling stability.22−24 Recently, the nanocomposites of carbon/ Ni compounds have been explored.25−30 They generally combine the advantages of high capacity from the Ni compounds, low electron transportation resistance and chemical inertness from conductive carbon backbones, and abundant electrochemical reaction sites from high-surface-area nanostructures. Nonetheless, the duration of these existing composite electrodes is still impeded by the capacity degradation due to the volume change of Ni compounds in the reversible charge−discharge process. To solve this problem, tubular sandwich-structured CNT@ Ni@Ni2 (CO3)(OH)2 was prepared through a scalable, dynamic, controlled in situ reduction−chemical deposition (ISRCD) process. In this nanocomposite, the outer shell of Ni2(CO3)(OH)2 converts into NiOOH with the same morphologic and amorphous structure as in the early stage of

INTRODUCTION In various electrochemical energy storage systems, a supercapacitor has been of considerable interest as an advanced energy storage device for portable electronic devices and hybrid electrical vehicles,1−3 owing to its high power density, fast charging−discharging rate, and operational safety.4−6 Yet, most commercial supercapacitors, usually categorized into electrochemical double-layer capacitors (EDLCs), suffer from a lower energy density (normally ≤10 Wh·kg−1) than rechargeable batteries.7−10 To address this issue, battery-type electrode materials have been extensively investigated to achieve a notable improvement of the energy density by fabrication into hybrid supercapacitors.11 The energy storage mechanism of battery-type electrode materials is mainly characterized by rapid and reversible Faradaic redox reactions at or near surfaces, rather than a sole accumulation of electrostatic charge at the interfaces of electrode materials in EDLCs.12−16 These redox reactions of battery-type materials are able to boost the capacity of working electrodes.2,15 In particular, nickel (Ni) compounds are able to accommodate high theoretical capacity and well-defined redox behavior, thus giving rise to a prospective species of batterytype electrode materials.17−21 However, unlike carbon materials © XXXX American Chemical Society

Received: February 17, 2017 Revised: April 1, 2017

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DOI: 10.1021/acs.jpcc.7b01551 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Scheme 1. (A) Fabrication Scheme of CNT@Ni@Ni2(CO3)(OH)2 Nanocomposites and (B) Working Mechanism of CNT@ Ni@Ni2(CO3)(OH)2//AC Hybrid Supercapacitor

electrochemical activation.31 The resulting amorphous ultrathin film of NiOOH abounds in defects and vacancies, thereby provides plentiful reaction sites on a large contact surfaces with the electrolyte, which endows the prepared electrode with a high capacity. Specially, supported by CNT with outstanding structural stability,26,27 the midshell Ni is designed to reinforce the capacity retention of the outer shell over charge/discharge cycles by converting to NiOOH subsequently in the activation process, serving as a capable capacity buffer.32 Moreover, the high electrical conductivity of Ni-coated CNT facilitates the electron diffusion and permits a high rate capability. In this work, the resultant CNT@Ni@Ni2(CO3)(OH)2 tubular sandwich structure, employed as a battery-type electrode nanomaterial, exhibits a high capacity of 221 mAh· g−1 at a current density of 5 A·g−1 with a capacity retention of 76% at 50 A·g−1. Meanwhile, it maintains 81% capacity after 9000 cycles at 5 A·g−1. Furthermore, the hybrid supercapacitor built on the basis of activated carbon (AC) and CNT@Ni@ Ni2(CO3)(OH)2 tubular sandwich structure as the negative and positive electrodes, respectively, shows a high energy density of 179 Wh·kg−1 at a power density of 2880 W·kg−1. On account of its superior electrochemical performance, the tubular sandwichstructured CNT@Ni@Ni2(CO3)(OH)2, as a battery-type electrode material, is an outstanding candidate for hybrid supercapacitors. The tubular sandwich structure also could provide robust construction of battery-type electrode materials for a desirable hybrid supercapacitor.

nanotubular CNT@Ni (5 mg) was uniformly dispersed into deionized water (50 mL) and immersed in an ultrasonic bath for 20 min. The NiCl2·6H2O (11.9 mg) was added into the pretreated solution under magnetic stirring for 1 h and sonicated (40 kHz) for 0.5 h. Then, the final mixture was sealed in a beaker by a presynthesized collodion membrane and placed upside down in a bigger beaker filled with Na2CO3 solution (26.5 mg, 50 mL). After 12 h of reaction, the products were collected, washed with deionized water and ethanol, and subsequently vacuum-dried at 60 °C overnight for further use. For comparison, the concentration gradient of solutions inside and outside the collodion membrane was adjusted by a similar method. Characterization. The morphology and size were measured by FE-SEM (JEOL, S-4800) and TEM (JEOL, JEM2100EX). Elemental analysis was conducted by EDXS (Oxford, TN-5400) at 15 kV. The nanostructure was characterized by XRD (Bruker, D8 Advance) with a Cu Kα X-ray radiation source (λ = 0.154 056 nm). The surface elemental valence analysis was performed by XPS (PerkinElmer, PHI-5 000C ESCA) with Al Kα radiation (hν = 1486.6 eV). The whole spectrum (0−1200 eV), the narrow spectrum, and the highresolution spectrum were all recorded by a RBD 147 interface (RBD Enterprises). Binding energy was calibrated using the containment carbon (C 1s = 284.6 eV), and the data analysis was carried out using XPSPeak4.1. The thermal behaviors and compositions were performed by TG (Netzsch, STA409PC) from 30 to 800 °C with the heating rate of 10 °C·min−1 in air. The specific surface area was determined by a Brunauer− Emmett−Teller (BET) instrument (Micromeritics, ASAP2020) at −196 °C based on the BET equation. The molecular structure was measured by FT-IR (Thermo, NEXUS).

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EXPERIMENTAL SECTION Synthesis of CNT@Ni@Ni2(CO3)(OH)2. Tubular sandwich structures of CNT@Ni@Ni2(CO3)(OH)2 were synthesized by dynamic controlled chemical deposition using a colloidal membrane as the structure director. Typically, the as-treated B


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Figure 1. TEM (a), high magnification TEM (b), HRTEM images (c), and corresponding SADE patterns (d) of CNT (A), CNT@Ni (B), and CNT@Ni@Ni2(CO3)(OH)2 (C).

Electrochemical Measurements. The CNT@Ni@ Ni2(CO3)(OH)2 working electrodes were prepared by mixing CNT@Ni@Ni2(CO3)(OH)2 (80 wt %), acetylene black (10 wt %), and poly(tetrafluoroethylene) (10 wt %). The mixture was ground with ethanol to form a homogeneous slurry that was then spread and pressed onto the Ni foam and subsequently vacuum-dried at 100 °C overnight. The final mass of activated materials was calculated on the current collector before testing. The AC working electrodes were prepared in a similar way. The electrochemical measurements for nanocomposite electrode were carried out in an aqueous KOH electrolyte (6.0 M) with a three-electrode cell where a platinum plate served as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. To fabricate the hybrid supercapacitor, the as-prepared CNT@Ni@Ni2(CO3)(OH)2 and AC were used as the positive and negative electrodes, respectively. The coin cell was constructed by using a commercial cellulose paper as the separator and 6 M KOH as the electrolyte. CV and GCD measurements were carried on a CHI660E electrochemical workstation (Chenhua, Shanghai, China). EIS was measured by a Versa STAT4 electrochemical workstation (Princeton Applied Research).

a tubular sandwich-structured CNT@Ni@Ni2(CO3)(OH)2 via a stepwise ISRCD process, as illustrated in Scheme 1A. The Ni2+ precursors initially immobilized on the functionalized CNT were reduced by N2H4·H2O, leading to epitaxial growth of a metallic shell of Ni on the CNT backbones, forming the CNT@Ni core−shell nanotubes (step I). Hydroxyl groups were brought onto the surface of the Ni shell because of the alkaline solution used during N2H4·H2O reduction. Subsequently, the semipermeable collodion membrane was used as the structure-director in a dynamic controlled chemical deposition driven by the ion concentration difference on the two sides of the collodion membrane.33−35 Ni2+, which was immobilized on the Ni shell via hydroxyl groups, CO32−, and OH−, which diffused from the Na2CO3 solution, coprecipitated as Ni2(CO3)(OH)2 amorphous ultrathin film grafted throughout the longitudinal axis of CNT@Ni, thereby giving rise to a tubular sandwich-structured CNT@Ni@Ni2(CO3)(OH)2 (step II). The morphology characterization of nanocomposites in different steps was sequentially studied by field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The TEM images of CNT@Ni reveal that a shell of Ni uniformly grew along the longitudinal axis of CNT (Figure 1Aa,Ab and Figure S1A in the Supporting Information), exhibiting an increased thickness of ∼15 nm (Figure 1Ba,Bb and Figure S1B in the Supporting Information). The representative high-resolution TEM (HRTEM) images

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RESULTS AND DISCUSSION Structural Characterization. The strategy, to achieve a high-performance battery-type electrode material with high stability based on superior capacity, was realized by developing C


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Figure 2. (A) XRD, (B) EDXS, and (C) TG patterns of CNT@Ni@Ni2(CO3)(OH)2 (a), CNT@Ni (b), and CNT (c).

Figure 3. XPS spectra of CNT@Ni@Ni2(CO3)(OH)2 (A) and CNT@Ni (B) with high-resolution spectra for Ni 2p (a), C 1s (b), and O 1s (c).

Ni2(CO3)(OH)2. This structure could benefit the long-term electrochemical stability when applied for a hybrid supercapacitor, owing to the isotropic strain and stress within the amorphous phase over charging/discharging cycles.36−38 It should be noted that all the reactants play crucial roles in the adjustment of the morphology (Figures S3 and S4 in the Supporting Information). Further structure characterization of as-prepared samples was traced by X-ray diffractometer analysis (XRD) in Figure 2A. All of the analyses show two explicit diffraction peaks observed at 25.9° and 44.5°, which correspond to the (002) and (101) planes of the hexagonal graphite in the CNT (JCPDS #656212), indicating the presence of CNT in all the nanostructures. As shown in Figure 2Ab, three diffraction peaks at 44.4° (111), 51.7° (200), and 76.1° (220) can be indexed to the cubic phase of Ni (JCPDS #65-0380), suggesting the assembly of the Ni shell onto CNT backbone. Two broad and weak peaks at 34.7° and 60.5° in Figure 2Aa are attributed to the outer-shelled Ni2(CO3)(OH)2, again certifying the amorphous nature of the ultrathin film of Ni2(CO3)(OH)2. Elemental analysis of CNT@Ni@Ni2(CO3)(OH)2 and CNT@Ni (Figure 2B) by energy dispersive X-ray spectroscopy (EDXS) unambiguously shows the consolidated existence of C, O, Ni elements throughout the heterostructures. The weights of Ni:C and O:Ni are found to increase after chemical deposition (Table S2 in Supporting Information), thereby confirming the

(Figure 1Ac,Bc) present dual lattice fringes with a spacing of 0.3350 and 0.2040 nm, corresponding to the (002) planes of CNT and the (111) planes of Ni, respectively (insets of Figure 1Ac,Bc, bottom-right corner). The selected-area electron diffraction (SAED) pattern displays a set of diffraction rings, assigned to the diffractions of the (002) and (100) planes of CNT, respectively (Figure 1Ad), and the SAED pattern of CNT@Ni is the superimposition of the polycrystalline diffraction rings with the lattice plane (002) of CNT and the lattice planes (111), (200), and (220) of Ni (Figure 1Bd). Both of them images verify the incorporation of a crystallized Ni shell on CNT. As the outer shell of the CNT@Ni@Ni2(CO3)(OH)2 tubular sandwich structure (Figure 1Ca,Cb and Figure S1C in the Supporting Information), the gauzelike Ni2(CO3)(OH)2 ultrathin film with a thickness of ∼1.8 nm (Figure 1Cc) exhibits an extremely large surface area of 293.53 m2·g−1 (Table S1 and Figure S5 in the Supporting Information), which is expected to provide ample activated sites for electrochemical reactions, further greatly facilitating ion transport and enhancing the capacity of fabricated electrodes. In addition, no clear lattice fringes in the HRTEM image and a broad diffused ring in the SAED pattern (Figure 1Cd) confirm the amorphous structure of Ni2(CO3)(OH)2. Furthermore, the line scan of EDXS reveals that the variation of the Ni element is consistent with the C element in Figure S2 of the Supporting Information, illustrating the tubular sandwich-structure of CNT@Ni@ D


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Figure 4. Electrochemical characterizations of CNT@Ni@Ni2(CO3)(OH)2: (A) CV cyclic curves at the scan rate of 5 mV·s−1, (B) CV curves at various scan rates, and (C) GCD curves at various current densities.

Figure 5. Contrastive electrochemical performances of CNT@Ni@Ni2(CO3)(OH)2 (a), CNT@Ni2(CO3)(OH)2 (b), and CNT@Ni (c): (A) CV curves at the same scan rate, (B) GCD curves at 5 A·g−1, and (C) The capacity at different current densities calculated by discharging curves. (D) The cycle performances at 5 A·g−1. (E) The Nyquist plots with an inset showing the magnified semicircle of Nyquist plots at high frequency. (F) The TEM images and SAED patterns of CNT@Ni@Ni2(CO3)(OH)2 before and after activation by CV: (a) before and (b) after.

close integration of Ni2(CO3)(OH)2 film on CNT@Ni. The TG pattern was used to measure the weight percentage of each component in the nanomaterials, as shown in Figure 2C. For CNT@Ni (marked in red), the contents of CNT and Ni in the architectures are estimated to be about 82 and 16 wt %, respectively, based on the changes in weight attributed to the combustion of CNT (500−600 °C) and the oxidation of Ni. As for CNT@Ni@Ni2(CO3)(OH)2 (marked in black), the CNT is calculated by the same method to be about 24 wt %, consistent with the result of EDXS quantitative analysis. Putting aside the impact from the solvent evaporation (below 100 °C) and the oxidation of Ni, the content of the Ni2(CO3)(OH)2 in the nanocomposites is calculated to be about 20% due to the decomposition of Ni2(CO3)(OH)2 (250−300 °C), which also agrees with the result of EDS quantitative analysis. X-ray photoelectron spectroscopy (XPS) was done to analyze the oxidation states and bonding information on each component. The survey spectra of CNT@Ni@Ni2(CO3)(OH) 2 and CNT@Ni (Figure S6 in the Supporting Information) confirm the elements characterized in the

previously mentioned EDXS patterns. The narrow spectrum of Ni 2p in CNT@Ni@Ni2(CO3)(OH)2 (Figure 3Aa) features two peaks at 856.0 and 873.7 eV appearing along with two satellite peaks at 861.7 and 880.0 eV, indicating the presence of Ni2+ in Ni2(CO3)(OH)2. In the high-resolution spectra of C 1s (Figure 3Ab) and O 1s (Figure 3Ac), the peaks split into five peaks in each. Compared with those of CNT@Ni (Figure 3Bb,Bc), additional peaks with a bonding energy of 290.2 and 532.0 eV correspond to the CO32− and OH−, suggesting the formation of Ni2(CO3)(OH)2. It should be noted that the Ni spectrum in CNT@Ni (Figure 3Ba) displays the peaks of Ni 2p3/2 and Ni 2p1/2 split into four peaks at 853.8 and 856.0 eV and 870.2 and 873.8 eV, which are assigned to Ni0 and Ni2+, respectively. The existence of Ni2+ on the surface of CNT@Ni arises from the oxidization of the Ni shell by atmospheric oxygen. Electrochemical Performances. The electrochemical performances of the resulting products are characterized with a three-electrode cell system in KOH (6 M) aqueous solution. Figure 4A shows the cyclic voltammetry (CV) curves of E


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The Journal of Physical Chemistry C CNT@Ni@Ni2(CO3)(OH)2 within the first 100 cycles at a scan rate of 5 mV·s−1 in the potential range from −0.2 to 0.6 V. The continuously increasing CV curve areas and redox peak intensities suggest an improving charge storage capability due to the electrochemical activation of CNT@Ni@Ni2(CO3)(OH)2. In the activation process, the outer-shell Ni2(CO3)(OH)2 (activated process I) and midshell Ni (activated process II) could transfer to NiOOH when exposed to alkaline electrolytes sequentially, as defined by eqs 1 and 2.31,32 The resulting NiOOH, known as an outstanding battery-type electrode material, provides a growing capacity, as shown in CV curves.39

Figure 5A compares the electrochemical performance of three samples, CNT@Ni, CNT@Ni2(CO3)(OH)2, and CNT@ Ni@Ni2(CO3)(OH)2, by collecting CV curve of each at the same scan rate. The area of the CV curve for CNT@Ni@ Ni2(CO3)(OH)2 is significantly larger than those of CNT@Ni and CNT@Ni2(CO3)(OH)2, indicating its excellent charge storage capability. Furthermore, the pairs of redox peaks reveal the typical battery-type behaviors. Likewise, the well-defined potential plateaus in GCD curves of all samples at the same current density in Figure 5B verify the presence of a Faradaic redox process in all samples. (Detailed CV and GCD curves of CNT@Ni2(CO3)(OH)2 and CNT@Ni are collected in Figure S9 of the Supporting Information.) The discharge time is negatively correlated with the increasing current density, due to the kinetics of the redox reaction being too sluggish to keep pace with the fast potential change. The capacities are calculated on the basis of GCD curves, as shown in Figure 5C, demonstrating a good consistency with the results displayed in CV curves. On the basis of GCD curves, the CNT@Ni@Ni2(CO3)(OH)2 exhibits a higher capacity of 221 mAh·g−1 than other samples [CNT@Ni2(CO3)(OH)2 with 117 mAh·g−1 and CNT@Ni with 92 mAh·g−1] at the same current density of 5 A·g−1. Also, the CNT@Ni@Ni2(CO3)(OH)2 maintains a high percentage (76%) of its capacity at 50 A·g−1, implying its high rate capability. The ultrahigh capacity of as-prepared electrode results from the battery-type behaviors of outer-shell amorphous Ni2(CO3)(OH)2 ultrathin film and midshell Ni and thereby will substantially contribute to the high energy density of a fabricated hybrid supercapacitor. Especially, the metallic Ni shell is capable of converting into NiOOH continuously and offers a highly activated redox couple, to further generate an increased capacity, even within numerous charge−discharge cycles after the initial electrochemical activation. Moreover, as another key parameter of supercapacitors with high performance, the superior rate capability of the CNT@Ni@Ni2(CO3)(OH)2 electrode is attributed to the reduced diffusion path of ions, the large contact surface, and the enhanced electrical conductivity. Metallic-Ni-coated CNT acts not only as the support for the deposition and conversion of Ni2(CO3)(OH)2 ultrathin film but also as the electronic conductive channels. The loose interfacial contact between outer-shell Ni2(CO3)(OH)2 and CNT@Ni is of great benefit to the fast transport of ions and electrons throughout the whole electrode matrix. Therefore, the CNT@Ni@Ni2(CO3)(OH)2 stands out with high capacity and good rate capability and is an ideal candidate for a battery-type electrode material. Additionally, the electrochemical performances of CNT and Ni@ Ni2(CO3)(OH)2 were listed in Figure S10 and S11 of the Supporting Information. Furthermore, the CNT@Ni@ Ni2(CO3)(OH)2 within different electrolytes were also measured in Figure S12 of the Supporting Information. Considering the fact that Faradaic redox reactions in the charge−discharge process often lead to sluggish reaction kinetics and structural damage, high cycling stability of battery-type electrode in hybrid supercapacitors is a crucial factor for practical applications. Figure 5D shows the capacity retentions of CNT@Ni@Ni 2 (CO 3 )(OH) 2 and CNT@ Ni2(CO3)(OH)2 measured on the basis of GCD curves at a current density of 5 A·g−1, respectively. Both curves exhibit a gradual decrease in capacity within the first few hundred cycles because of the initial activation process. Thereafter, the curve of CNT@Ni2(CO3)(OH)2 declines greatly due to the poor electrical conductivity of converted NiOOH. In comparison,

Ni 2(OH)2 CO3 + 4OH− → 2NiOOH + CO32 − + 2H 2O + 2e−

(1)

2Ni + O2 + 2OH− → 2NiOOH

(2)

The TEM images and SADE patterns of CNT@Ni@ Ni2(CO3)(OH)2 before and after activation are displayed in Figure 5F. The ultrathin film grafted outside the CNT@Ni nanotubes is NiOOH in Figure 5Fb, which has almost the same morphological and amorphous characteristics as the former Ni2(CO3)(OH)2 film in Figure 5Fa. To further prove the conversion of Ni2(CO3)(OH)2 and Ni to NiOOH, Fouriertransform infrared spectroscopy (FT-IR) was used to analyze the molecular structures of all samples. In Figure S8 of the Supporting Information, ignoring the peak of atmospheric CO2 at 2349 cm−1, the curve of CNT@Ni@Ni2(CO3)(OH)2 after activation shows a broad peak of OH− at 3424 cm−1, which is absent in the curve of CNT@Ni, indicating the conversion of Ni. Meanwhile, the peak of CO32− existing in the curve of CNT@Ni@Ni2(CO3)(OH)2 before activation vanishes in the curve after activation, illustrating the conversion of Ni2(CO3)(OH)2. Unlike the rectangular CV curves of EDLCs, the CV curves of CNT@Ni@Ni2(CO3)(OH)2 exhibit a pair of broad redox couple peaks in Figure 4B, illustrating that the capacity characteristic is dominantly generated by Faradaic redox reactions. These two peaks arise from fast and reversible electrochemical redox reactions of the Ni3+/Ni2+ redox couple, according to the following eq 3: NiOOH + H 2O + e− ↔ Ni(OH)2 + OH−

(3) −1

As the scan rate increases from 5 to 100 mV·s , the redox peaks undergo a slight shift because the rate of electron diffusion is much faster than the rate of redox reaction on the material’s surface (polarization phenomenon).40 Meanwhile, the shape and symmetricity of the CV curves hardly change, indicating the excellent electron conductivity of the resulting CNT@Ni@Ni2(CO3)(OH)2. The potential voltage plateaus observed in the galvanostatic charging−discharging (GCD) curves of CNT@Ni@Ni2(CO3)(OH)2 in Figure 4C demonstrate the typical battery-type behavior, confirmed by the CV curves. On the basis of the GCD curves (eq S1 in the Supporting Information), the capacity of the CNT@Ni@ Ni2(CO3)(OH)2 is calculated as 221 mAh·g−1 at a current density of 5 A·g−1, and 168 mAh·g−1 at a current density of 50 A·g−1. The capacity retained 76% of its initial value, implying an excellent rate capability for the sample. These superior electrochemical properties of the resulting materials are majorly attributed to the multifunctioning midshell Ni with a high electrical conductivity and potential capacity. F


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Table 1. Capacity and Cycling Life in Aqueous Electrolytes of Various Carbon/Ni-Compound Nanomaterials Reported in the Literature material

capacity (mAh·g−1) (calcd from GCD curves)

current density (A·g−1)

Ni/Ni(OH)2@MCNT CNT@Ni(OH)2 Ni(OH)2@Ni-coated CNTs Ni(OH)2/graphene TiO2@Ni(OH)2 CNT@Ni@Ni2(CO3)(OH)2

∼215 ∼167 ∼194 ∼238 264 221

5 2 2 0.8 1 5

capacity retention (%)

cycling numbers

81 92 98 91.6

1000 1000 2000 2000

81

9000

ref (year) 42 (2014) 43 (2015) 44 (2015) 45 (2016) 46 (2016) this work

Figure 6. Electrochemical performances of the CNT@Ni@Ni2(CO3)(OH)2//AC hybrid supercapacitor. (A) CV curves in 6 M KOH electrolyte at different scan rates. (B) GCD curves at various current densities. (C) Cycle performance within 1.6 V at 10 A·g−1. (D) The Nyquist plots. (E) The Ragone plot. (F) Photograph of the hybrid supercapacitor powering one light-emitting diode (1.5 V).

corresponds to the charge-transfer resistance (Rct) caused by the Faradaic reactions and the double-layer capacitance on the grain surface. A smaller Rct value of CNT@Ni@Ni2(CO3)(OH)2, compared with that of CNT@Ni2(CO3)(OH)2, indicates the lower charge-transfer resistance in the interface between the electrode and electrolyte. Meanwhile, the curve of CNT@Ni@Ni2(CO3)(OH)2 in the low-frequency region is more vertical than that of CNT@Ni2(CO3)(OH)2, demonstrating a higher diffusion rate within the electrode. In Figure 5F and Figure S7 of the Supporting Information, the CNT@ Ni@Ni2(CO3)(OH)2, which was activated and test after cycling, has almost the same morphological and amorphous characteristics as the former CNT@Ni@Ni2(CO3)(OH)2. This structure could benefit the long-term electrochemical stability

the capacity of CNT@Ni@Ni2(CO3)(OH)2 still retains 81% after 9000 cycles. It is believed that the midshell Ni is capable of converting into electrochemically activated NiOOH continuously, further delaying the decay of capacity, enabling the ultrahigh cycling stability of CNT@Ni@Ni2(CO3)(OH)2 battery-type electrode. To estimate the enhanced electrical conductivity of as-prepared products, the electrochemical impedance spectroscopy (EIS, under open circuit potential) of CNT@Ni@Ni2(CO3)(OH)2 and CNT@Ni2(CO3)(OH)2 was measured, as shown in Figure 5E. In the high-frequency region of the Nyquist plots, equivalent values of Rs (the real axis intercept) are observed in both curves, representing the same solution resistance in two systems.41 A major difference occurs in the magnified semicircles in Figure 5E, which G


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feasibility. These results also prove that the CNT@Ni@ Ni2(CO3)(OH)2//AC hybrid supercapacitor holds great hope to realize battery-level energy density and capacitor-level power density, as well as long cycling life.

when applied for hybrid supercapacitors, owing to the isotropic strain and stress within the amorphous phase over charging/ discharging cycles. The electrochemical performance of CNT@ Ni@Ni2(CO3)(OH)2, which excelled at high capacity and had good cycling capability, is superior to most recently reported carbon/Ni compound materials, as summarized in Table 1.42−46 To analyze the electrochemical performances at a device level, a CNT@Ni@Ni2(CO3)(OH)2//AC hybrid supercapacitor in a 6 M KOH aqueous electrolyte was fabricated for detailed discussion. A schematic illustration for the working mechanism of the hybrid supercapacitor built of CNT@Ni@ Ni2(CO3)(OH)2 and AC nanocomposites is shown in Scheme 1B. To obtain the best possible comprehensive electrochemical performances in the hybrid supercapacitor, the charge balance between anode [CNT@Ni@Ni2(CO3)(OH)2] (Figure 4) and cathode (AC; see Figure S13 in the Supporting Information) is used to calculate the mass loading (eq S3 in the Supporting Information). In Figure 6A, the CV curves for the as-fabricated hybrid supercapacitor were obtained at different scan rates within a voltage range from 0.0 to 1.6 V. The curves display a rectangle-like shape combined with a pair of weak redox peaks, indicating the hybrid capacitive behavior of AC (non-Faradaic) and CNT@Ni@Ni2(CO3)(OH)2 (Faradaic). Furthermore, the GCD curves in Figure 6B show a nearly linear behavior in the discharge curves, indicating the capacitor-like behavior of CNT@Ni@Ni2(CO3)(OH)2//AC. The specific capacitance values were calculated on the basis of the GCD curves (eq S2 in the Supporting Information), and the maximum specific capacitance could reach 140 F·g−1 at the current density of 2 A· g−1. Specifically, 77% of the capacitance retention takes place for CNT@Ni@Ni2(CO3)(OH)2 when the current density increases to 50 A·g−1. Figure 6C reveals the cycling performance of hybrid supercapacitor at a current density of 10 A·g−1. After 5500 cycles, the capacitance retention can reach 85% compared with the first cycle, indicating a good cycling stability of the fabricated hybrid supercapacitor. Furthermore, the EIS analysis of the hybrid supercapacitor before and after 5500 cycles was measured and is shown in Figure 6D. The slightly smaller Rct value before cycling compared with the value of the last cycle in the Nyquist plots suggests the enhanced charge transfer resistance on the electrode interface in the charging/ discharging process. Meanwhile, the corresponding lines in the low-frequency region prove the faster ion transport of the hybrid supercapacitor before cycling. To further evaluate the energy storage performances of the CNT@Ni@Ni2(CO3)(OH)2//AC hybrid supercapacitor, the relationship of power density versus energy density was given in a Ragone plot (Figure 6E). Impressively, the hybrid supercapacitor can reach a maximum energy density of 179 Wh·kg−1 at a power density of 2880 W·kg−1 (eqs S4 and S5 in the Supporting Information). It surpasses many previously reported hybrid supercapacitors, such as CNT@Ni(OH)2//3D GN (a maximum energy density of 44.0 Wh·kg−1 at a power density of 800 W·kg−1),42 TiO2@ Ni(OH)2//mesoporous carbons (the energy and power density are 53.54 Wh·kg−1 and 77.71 W·kg−1),46 NiCo2O4@NiO//AC (an energy density of 31.5 Wh·kg−1 at a power density of 215.2 W·kg−1),47 Ni−Co2(CO3)1.5(OH)3@NiCo2S4//AC (an energy density of 32.3 Wh·kg−1 at a power density of 1835 W·kg−1),48 and Ni(OH)2−CFG//activated carbon (an energy density of 10.4 Wh·kg−1 at a power density of 6200 W·kg−1).49 Furthermore, in Figure 6F, the hybrid supercapacitor was used to power one light-emitting diode (1.5 V) to evaluate its

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CONCLUSIONS In summary, we have designed and prepared a tubular sandwich-structured CNT@Ni@Ni2(CO3)(OH)2 as a batterytype electrode material through a stepwise ISRCD process. In addition to the outstanding electrical conductivity of the CNT and the robust electrochemical activation of Ni2(CO3)(OH)2 amorphous ultrathin film, the newly designed Ni midshell is able not only to reinforce the capacity retention by continuously converting to NiOOH but also to promote the diffusion and migration of electrons during the rapid charge/ discharge process due to its high electrical conductivity. In a comprehensive comparison, the electrochemical performance of the CNT@Ni@Ni2(CO3)(OH)2 battery-type electrode surpasses that of most of the previously reported carbon/Nicompound electrodes. It exhibits a high capacity of 221 mAh· g−1 at a current density of 5 A·g−1 and maintains a high capacity retention of 81% after 9000 cycles at 5 A·g−1. The as-fabricated hybrid supercapacitor, employing AC and CNT@Ni@ Ni2(CO3)(OH)2 nanocomposites as the negative and positive electrodes, respectively, shows a higher energy density of 179 Wh·kg−1 at a power density of 2880 W·kg−1. This unique tubular sandwich-structured CNT@Ni@Ni2(CO3)(OH)2 is proved to be a promising battery-type electrode material for construction of an advanced hybrid supercapacitor and potentially other electrochemical devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01551. The details of experimental methods and more characterization and electrochemical properties of all samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ming Wen: 0000-0002-2327-5459 Author Contributions §

S.C. and Q.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51271132, 21471114, and 91122103).

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A Tubular Sandwich-Structured CNT@Ni@Ni - ACS Publications

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