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One-step synthesis of free-standing -Ni(OH)2 nanosheets on reduced graphene oxide for high-performance supercapacitors

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Nanotechnology Nanotechnology 25 (2014) 435403 (7pp)

doi:10.1088/0957-4484/25/43/435403

One-step synthesis of free-standing α-Ni (OH)2 nanosheets on reduced graphene oxide for high-performance supercapacitors Bitao Dong, Han Zhou, Jin Liang, Lusi Zhang, Guoxin Gao and Shujiang Ding State Key Laboratory for Mechanical Behavior of Materials and MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter and Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China E-mail: [email protected] and [email protected] Received 2 July 2014, revised 3 September 2014 Accepted for publication 5 September 2014 Published 9 October 2014 Abstract

In this work, a hierarchical hybrid structure of reduced graphene oxide (rGO) supported ultrathin α-Ni(OH)2 nanosheets (denoted as α-Ni(OH)2@rGO NSs) has been developed successfully via an environmentally friendly one-step solution method. The resulting product of α-Ni (OH)2@rGO NSs was further characterized by scanning electron microscope, transmission electron microscope, x-ray diffraction, Raman spectroscopy, x-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller. The ultrathin α-Ni(OH)2 nanosheets of around 6 nm in thickness are uprightly coated on the double sides of rGO substrate. When evaluated as electrodes for supercapacitors, the hybrid α-Ni(OH)2@rGO NSs demonstrate excellent supercapacitor performance and cycling stability, compared with the self-aggregated α-Ni(OH)2 powder. Even after 2000 cycles, the hybrid electrodes still can deliver a specific capacitance of 1300 F g−1 at the current density of 5 A g−1, corresponding to no capacity loss of the initial cycle. Such excellent electrochemical performance should be attributed to the ultrathin, free-standing, and hierarchical nanosheets of α-Ni(OH)2, which not only promote efficient charge transport and facilitate the electrolyte diffusion, but also prevent aggregation of electro-active materials effectively during the charge–discharge process. Keywords: supercapacitor, reduced graphene oxide, electrochemical performance (Some figures may appear in colour only in the online journal) 1. Introduction

capacitance of carbon-based materials is generally low, while the price of RuO2-based materials is very high. Recently, the nanostructured metal hydroxides or oxides (Ni(OH)2 [3–6], NiO [7, 8], Co3O4 [9], NiCo2O4 [10, 11], MnO2 [12] etc) have drawn growing attention as electrode materials for electrochemical energy storage due to their low cost, abundance, environmental benignity, and high theoretical capacities. Among them, Ni(OH)2 has been widely investigated as a high-performance electrode material for supercapacitors, due to its low cost, environmentally benign nature, natural abundance, and high theoretical capacitance (ca. 3750 F g−1) [13, 14]. Generally, there are two polymorphs of Ni(OH)2, which are denoted as α-Ni(OH)2 and β-Ni(OH)2, respectively.

The fast-growing demand for high-power applications such as electric vehicles and hybrid electric vehicles has triggered significant research efforts in the design and development of novel electrode materials for an advanced energy storage device [1, 2]. Supercapacitors, as an important next-generation energy storage device, have received extensive attention, mainly due to their higher power density than batteries and higher energy density than conventional dielectric capacitors [3–5]. Unfortunately, their practical applications are largely hindered due to the lack of high performance electrode materials at a reasonable cost. For example, the specific 0957-4484/14/435403+07$33.00

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The α-Ni(OH)2 phase displays a more disorderly and larger interlayer spacing (>7.5 Å) as the interlamellar space to contain anions such as nitrate, carbonate, sulfate, and water molecules [15, 16]. It has been found that α-Ni(OH)2 possesses better electrochemical properties than β-Ni(OH)2 because α-Ni(OH)2 can be oxidized to γ-NiOOH in a lower potential than the corresponding oxidation state compared with β-Ni(OH)2. Also α-Ni(OH)2 can deliver larger electrochemical capacity than β-Ni(OH)2/β-NiOOH [17]. Recently, Gao and colleagues synthesized α-Ni(OH)2 nanosheets on graphite nanosheets as the active material in supercapacitor electrodes, and the result indicated that the materials showed improved supercapacitor performance compared to pure α-Ni (OH)2 nanosheets [18]. However, like most metal hydroxides/ oxides, α-Ni(OH)2-based electrodes still have to face fast capacity fading and poor rate capability due to their poor electronic conductivity and ion diffusion, in addition to severe aggregation during the charging–discharging cycling. Nowadays, one effective strategy is to fabricate the electrode materials directly on the conductive substrate (such as graphene nanosheets, carbon nanofibers and nanotubes, Ni foam, etc) to improve the capacity performance and cycling stability of supercapacitors by enhancing the electrical conductivity of the active materials [19, 20]. Graphene, as a novel two-dimensional (2D) carbon material, has been widely investigated for electrochemical energy storage due to its many unusual features, including superior electrical conductivity, excellent mechanical flexibility, large specific surface area, and high thermal and chemical stability [21–23]. Recent progress in the large-scale synthesis of graphene nanosheets from graphene oxide (GO) has promoted extensive exploration. Graphene-based materials have recently shown promising application in nano-electronics and energy storage [24, 25]. Because of their high specific surface area and excellent conductivity, graphene sheets are expected to be excellent electrode materials for supercapacitors. However, graphene materials exhibit unsatisfactory capacitance performance (generally tens-264 F g−1) due to unavoidable aggregation or re-stacking of graphene nanosheets. Therefore, fabricating redox-active materials (such as α-Ni(OH)2) on graphene sheets will improve the electrochemical performance of graphene-based supercapacitors. In this work, we prepared a hierarchical hybrid structure of reduced graphene oxide (rGO) supported ultrathin α-Ni (OH)2 nanosheets (denoted as α-Ni(OH)2@rGO NSs) via a facile one-step solution method. The as-obtained α-Ni (OH)2@rGO NSs hybrid structure is believed to be very suitable as the electrode material for supercapacitors for at least three reasons: (i) The higher specific surface area of these ultrathin α-Ni(OH)2 nanosheets uprightly standing on an rGO surface can provide more electrochemical sites for redox reaction between the active materials and electrolyte ions, leading to outstanding rate performance and higher specific capacity; (ii) The free-standing nanosheets feature may help the separation of neighboring ultrathin nansoheets, thus restraining the active material from aggregating and making the active materials have sufficient contact with the electrolyte; (iii) The adequate hydroxyl groups existing in

rGO could further quicken the diffusion rate of Ni2+ and OHto enhance their electrochemical kinetics. Furthermore, its remarkable cation/anion exchange capacity can keep the high reaction rate and maintain supercapacitors with better cycling stability. In terms of the above-mentioned advantages, the integrated hybrid electrodes of α-Ni(OH)2@rGO NSs have exhibited an excellent specific capacitance and cycling stability, even at a high charge/discharge current density.

2. Experimental section 2.1. Synthesis of α-Ni(OH)2@rGO NSs and self-aggregated αNi(OH)2

All reagents were analytical grade and used without further purification. GO was synthesized from natural graphite powder according to the modified Hummer’s method [26]. In a typical synthesis of α-Ni(OH)2@rGO NSs, 5 mg of GO nanosheets were well dispersed in 40 mL of deionized water by sonication for 30 min. Then, 0.25 mmol of Ni (NO3)2 • 6H2O, 0.25 mmol of hexamethylenetetramine (HMT) and 0.25 mmol of trisodium citric (TSC) were dissolved in the above suspension, and magnetically stirred for 30 min. Next, the resulting solution was heated to 90 °C in an oil bath for 6 h with violent stirring. After reaction, the precipitate was collected by centrifugation, washed thoroughly with ethanol, and dried at 60 °C for 12 h. For comparison, self-aggregated α-Ni(OH)2 was synthesized as well in the same procedure without adding the GO template. 2.2. Characterization

The product morphology was examined using field-emission scanning electron microscopy (FESEM; JEOL, JSM-7000F) and transmission electron microscopy (TEM; JEOL, JEM2100). The Raman spectra was performed on a Raman spectrometer under a backscattering geometry (λ = 633 nm; HORIBA JOBIN YVON, HR 800). The specific surface area and pore-size distribution of the products were measured using a BET analyzer (Autosorb-iQ, Quantachrome Instruments U.S.) at 77 K. The crystallographic information of the samples was collected using powder x-ray diffraction (XRD; SHIMADZU, Lab X XRD-6000). The chemical states of the products were studied using the x-ray photoelectron spectroscopy (XPS) measurement performed on an Axis Ultra, Kratos (UK) at monochromatic Al K a radiation (150 W, 15 kV and 1486.6 eV). The thermogravimetric analysis (METTLER-TOLEDO TGA 1) was carried out under a flow of nitrogen with a temperature ramp of 10 °C min−1 from room temperature to 700 °C. 2.3. Electrochemical measurements

For electrochemical measurements, the working electrode consisted of active material (α-Ni(OH)2@rGO NSs), carbon black (super-P-Li), and polymer binder (polyvinylidene difluoride, PVDF, Aldrich) in a weight ratio of 70:20:10. For the supercapacitor test, the slurry was pasted to Ni foam and 2

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Figure 1. (A), (B) SEM and TEM images of GO; (C), (D) SEM and TEM images of self-aggregated α-Ni(OH)2.

then dried at 120 °C overnight under vacuum. The electrochemical tests were conducted with a CHI 660D workstation in an aqueous KOH (4 M) with a three-electrode cell, where Pt foil serves as the counter electrode and a standard calomel electrode (SCE) serves as the reference electrode. In this paper, the mass loadings of the active materials are about 1.03 mg cm−2 on the nickel foam.

3. Results and discussion

Figure 2. (A) and (B) FESEM image of α-Ni(OH)2@rGO NSs; (C) and (D) TEM image of α-Ni(OH)2@rGO NSs; (E) EDX image of αNi(OH)2@rGO; (F) XRD patterns of self-aggregated α-Ni(OH)2 (I) and α-Ni(OH)2@rGO NSs (II).

The morphology and structure of a pristine GO nanosheet, self-aggregated α-Ni(OH)2 and as-prepared α-Ni(OH)2@rGO NSs hybrids were characterized by FESEM and TEM. Figures 1(A) and (B) show the typical FESEM and TEM images of a pristine GO nanosheet, respectively. As can be seen, the surface of the pristine GO nanosheet is very clean and flat except for some wrinkles. Figure 1(C) and D display the FESEM and TEM images of the self-aggregated α-Ni (OH)2. Clearly, the α-Ni(OH)2 nanosheets aggregate each other severely to form flower-like structures due to the continuous increase of the pH value of the aqueous solution, which is mainly ascribed to the gradual decomposition of HMT. However, after some amount of GO is used as the template in this solution reaction, a large quantity of freestanding Ni(OH)2 nanosheets are uniformly deposited onto the double sides of the GO nanosheets to form a large-scale conformal coating (figures 2(A) and (B)), indicating that the GO nanosheets can effectively prevent the self-aggregation of the α-Ni(OH)2 nanosheets due to the functional groups on the surface of GO providing preferred nucleation sites. Meanwhile, GO substrates are expected to be partially reduced by the reducing species generated from HMT and TSC [27]. As a result, a hierarchical Ni(OH)2 layer with a uniform nanosheet structure is generated on both sides of resultant rGO sheets. As shown in figure 2(C), TEM images further reveal the details of the 2D hybrid nanostructure with a lateral dimension in micrometer size. From a representative TEM image

(figure 2(D)), it can be clearly observed that most of the α-Ni (OH)2 nanosheets are less than 6 nm in thickness, suggesting the ultrathin feature. Those ultrathin α-Ni(OH)2 nanosheets are uniformly standing uprightly on the double surfaces of rGO sheets to form a hybrid 2D nanostructure. As a result, the specific surface area and the mechanical strength of this hybrid structure are improved greatly to show the enhanced capacitance and outstanding cycle life of the supercapacitor. Furthermore, an energy dispersive x-ray (EDX) profile (figure 2(E)) reveals that the hybrid α-Ni(OH)2@rGO NSs are only composed of Ni, C and O elements with the atomic percent of 3.0%, 3.7% and 32.9%, respectively, indicating the high purity of the hybrid product. It must be mentioned that the strong peak of Si originates from the silicon slice when carrying out FESEM. In addition, the purity of self-aggregated α-Ni(OH)2 and hybrid α-Ni(OH)2@rGO NSs were once again analyzed by XRD as shown in figure 2(F). Apparently, the main identified diffraction peaks of self-aggregated α-Ni (OH)2 (Cure I) and hybrid α-Ni(OH)2@rGO NSs (Cure II) at 11.3°, 23.6°, 33.4°, 59.9° can be well assigned to (003), (006), (101) and (110) planes of α-Ni(OH)2, which is in good conformity with standard power diffraction patterns of α-Ni (OH)2 (JCPDS No: 38- 0715). More importantly, curve (II) 3

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Figure 3. (A) Raman spectra of GO (I) and α-Ni(OH)2@rGO NSs (II);(B) Nitrogen adsorption/desorption isotherms for the sample of α-Ni (OH)2@rGO NSs (inset pore-size distribution); (C) XPS survey spectrum of α-Ni(OH)2@rGO NSs and (D) Ni2p core level.

indicates that the α-Ni(OH)2@rGO NSs synchronously possess the characteristic diffraction peaks of self-aggregated αNi(OH)2, confirming the composition of the hybrid material. No other additional diffraction peaks from possible impurities are observed, indicating the high phase purity of α-Ni(OH)2 in the hybrids [28, 29]. To confirm the presence of rGO in the hybrid structure, Raman spectroscopy was performed on pristine rGO and α-Ni (OH)2@rGO NSs as shown in figure 3(A). Apparently, the Raman spectra of rGO and α-Ni(OH)2@rGO NSs displays two prominent peaks at round 1338 cm−1 (D band) and 1609 cm−1 (G band), respectively, suggesting the existence of rGO in the hybrid composites. However, the peak intensity ratio (ID/IG) of α-Ni(OH)2@rGO NSs is higher than that of rGO, indicating the presence of localized sp3 defects within the sp2 carbon network upon reduction of the exfoliated rGO nanosheets [30]. The porous feature and pore sizes of asprepared α-Ni(OH)2@rGO NSs are further characterized by Brunauer–Emmett–Teller (BET) analysis. From the N2 adsorption–desorption isotherm (figure 3(B)), a type H4 hysteresis loop can be identified at relative pressures of 0.5–0.95, indicating the presence of a mesoporous structure with a high specific surface area of 40 m2 g−1. In addition, the main pore size inside the mesoporous hybrid structure (inset of figure 3(B)) ranges from 2–10 nm [31, 32]. Moreover, to

determine the chemical composition of the α-Ni(OH)2@rGO NSs, x-ray photoelectron spectroscopy (XPS) measurements were carried out in the region of 0–1200 eV. Figure 3(C) displays the full scan spectra of the hybrid composites, which mainly contain C1s, O1s, and Ni1s core-level peaks. In the Ni2p region (figure 3(D)), the spectrum shows two major peaks with binding energies at 852.8 and 870.5 eV, corresponding to Ni 2p3/2 and Ni 2p1/2, respectively, with a spin–energy separation of 17.7 eV, which is characteristic of α-Ni(OH)2 phase and in good agreement with the literature [33]. The solid redox couple of Ni2+/Ni3+ can afford enough active sites, which may be one of the important factors contributing to the high electro-catalytic performance of α-Ni(OH)2. In order to confirm the amount of electro-active materials (Ni(OH)2) in the final hybrid structure, a TGA analysis of αNi(OH)2@rGO NSs has been carried out in air at a heating rate of 10 °C min−1 as shown in figure 4. It can be observed that the residual weight of the TGA sample is round 66.70 wt% at 400 °C in air, indicating that the final substance of NiO is about 66.70 wt% after such a thermal oxidation process. Therefore, we can calculate that the amount of active materials (Ni(OH)2) is about 82.77 wt% in the as-prepared αNi(OH)2@rGO NSs hybrids. We next investigated the electrochemical properties of the α-Ni(OH)2@rGO NSs as electrodes for the supercapacitor 4

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reaching 300 cycles. Then it gradually decreases to 1300 F g−1 after 2000 cycles, the result being that there is almost no capacity loss compared with their initial cycle, suggesting an outstanding cycling stability of this novel metal hydroxide hybrid electrode. To make a further comparison, the cycling performance of self-aggregated α-Ni(OH)2 is also evaluated at the same current density and shown in figure 4(D). As can be seen, the specific capacitance of the aggregated α-Ni(OH)2 NSs is only 690 F g−1 in the first cycle, but it decreases sharply to 400 F g−1 after 1000 cycles. In addition, the Nyquist impedance spectroscopy was conducted to further understand the electrochemical performance of α-Ni(OH)2@rGO NSs as shown in figure 6. The GO electrode shows the least diameter of a semicircle and the self-aggregated Ni(OH)2 demonstrates the largest diameter of a semicircle, indicating that the electrical conductivity of Ni (OH)2@rGO NS has been improved greatly due to the hybrid effect between rGO and Ni(OH)2. Therefore, the hybrids can display very high specific capacitance and excellent rate performance The enhanced electrochemical performance of α-Ni (OH)2@rGO NSs can be attributed to reasonable structural design. First, the ultrathin, free-standing, and hierarchical nanosheets of α-Ni(OH)2 on a GO substrate make the active materials sufficiently contact with the electrolyte and greatly shorten the diffusion pathways of ions, ensuring good electrochemical performance of the supercapacitor. Secondly, the higher specific surface area of these uprightly standing α-Ni (OH)2 nanosheets can provide more electrochemical reaction sites for the redox reaction between the active materials and electrolyte ions, leading to higher specific capacity and outstanding rate performance. Finally, the GO substrate with relatively good electrical conductivity and flexible nature serves as the electron ‘highway’ and buffering matrix to counteract the pulverization problem. Just benefiting from these advantageous features, the hybrid α-Ni(OH)2@rGO NSs nanostructure is able to manifest excellent electrochemical performance for supercapacitors.

Figure 4. TGA analysis of α-Ni(OH)2@rGO NSs in air at a heating rate of 10 °C min−1.

according to a three-electrode system in a 4 M KOH aqueous solution. Figure 5(A) shows the representative cyclic voltammograms (CV) curves of α-Ni(OH)2@rGO NSs electrodes with various sweep rates ranging from 1–50 mV s−1. The shapes of the CV curves clearly reveal their pseudocapacitive characteristics. Especially, a pair of redox peaks can be observed within the potential range from 0–0.55 V (vs. SCE) for all sweep rates, which are mainly associated with the Faradaic redox reactions related to M-O/M-O-OH, where M refers to Ni ions [34]. Obviously, with the increased scan rate, the shapes of the CV curves remain nearly unchanged except for the small shift of the peak position, indicating excellent electrochemical reversibility and outstanding high-rate performance. To get more information about their potential application in supercapacitors, the galvanostatic discharge profiles at different current densities ranging from 5–15 A g−1 are subsequently investigated and shown in figure 5(B). The potential plateau at around 0.2–0.4 V appears in the discharge curve at a current density of 5 A g−1, implying that the reduction process of nickel is dominant for the electrode materials. With the increased current density, the discharge potential plateaus are weakened gradually and almost disappear at the current density of over 15 A g−1. The specific capacitance is calculated by the formula C = IΔt/mΔV, where I is the discharge current, Δt is the discharge time, ΔV is the discharge voltage range and m is the mass of the active material. The calculated specific capacitance as a function of the discharge current density is plotted in figure 5(C). Notably, the hybrid α-Ni(OH)2@rGO NSs has delivered a specific capacitance of 1233 F g−1 at a low current density of 5 A g−1. In addition, even at a higher current density of 15 A g−1, the specific capacitance as high as 1044 F g−1 still remains, indicating the excellent rate performance of these hybrid electrodes. The cycling stability of α-Ni(OH)2@rGO NS electrodes is also evaluated by the repeated charging–discharging measurement at a current of 5 A g−1, as shown in figure 5(D). The specific capacitance is around 1233 F g−1 in the first cycle, and it slightly increases to 1376 F g−1 until

Conclusions Herein, we developed a novel hybrid structure of reduced graphene oxide (rGO) supported ultrathin α-Ni(OH)2 nanosheets (denoted as α-Ni(OH)2@rGO NSs) via a facile one-step solution method. The resulting product was further characterized by FESEM, TEM, XRD, Raman spectroscopy, and BET. FESEM and TEM images indicate that the ultrathin α-Ni(OH)2 nanosheets of around 6 nm in thickness are uprightly coated on the double sides of GO substrate. When evaluated as electrodes for supercapacitors, the hybrid α-Ni (OH)2@rGO NSs demonstrate excellent supercapacitor performance and cycling stability, compared with the selfaggregated α-Ni(OH)2 powder. Even after 2000 cycles, the hybrid electrodes still can deliver a specific capacitance of 1300 F g−1 at the current density of 5 A g−1, corresponding to no capacity loss of the initial cycle. This simple, synthetic approach could be extended to other transition metal 5

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−1

Figure 5. Electrochemical characterizations of the α-Ni(OH)2@rGO NSs. (A) CV curves at various scan rates ranging from 1–50 mV s ; (B) −1

Charge/discharge voltage profiles at various current densities ranging from 5–15 A g ; (C) the calculated capacitance as a function of current density according to the data in (B); (D) the capacitance cycling performance at current density of 5 A g−1: α-Ni(OH)2@rGO NSs (I), α-Ni (OH)2 (II).

Acknowledgements This research was supported partially by the National Natural Science Foundation of China (No. 51273158, 21303131); Natural Science Basis Research Plan in Shaanxi Province of China (No. 2012JQ6003, 2013KJXX-49); PhD Programs Foundation of Ministry of Education of China (No. 20120201120048); Program for New Century Excellent Talents in University (NCET-13-0449). The authors are grateful to the Fundamental Research Funds for the Central Universities for financial support.

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One-step synthesis of free-standing α-Ni(OH)₂ nanosheets on reduced graphene oxide for high-performance supercapacitors.

In this work, a hierarchical hybrid structure of reduced graphene oxide (rGO) supported ultrathin α-Ni(OH)2 nanosheets (denoted as α-Ni(OH)2@rGO NSs) ...
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