Nickel Cobalt Hydroxide @Reduced Graphene Oxide Hybrid

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Nickel Cobalt Hydroxides @Reduced Graphene Oxide Hybrid Nanolayers for High Performance Asymmetric Supercapacitors with Remarkable Cycling Stability Hongnan Ma, Jing He, Ding-Bang Xiong, Jinsong Wu, Qianqian Li, Vinayak Dravid, and Yufeng Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10280 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 8, 2016

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ACS Applied Materials & Interfaces

Nickel Cobalt Hydroxides @Reduced Graphene Oxide Hybrid Nanolayers for High Performance Asymmetric Supercapacitors with Remarkable Cycling Stability Hongnan Ma1, Jing He 1, Ding-Bang Xiong3, Jinsong Wu2, Qianqian Li2, Vinayak Dravid2, Yufeng Zhao1* 1

Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, China. Corresponding Email:1*[email protected]; 2

Department of Materials Science and Engineering, EPIC, NUANCE Center, Northwestern University 2220 Campus Drive, Evanston, IL 60208. 3

State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Abstract Nanolayered structures present significantly enhanced electrochemical performance by facilitating the surface dependent electrochemical reaction processes for supercapacitors, which however, causes capacitance fade upon cycling due to their poor chemical stability. In this work, we report a simple and effective approach to develop a stable, high performance electrode material by integrating 2D transition metal hydroxide and reduced graphene oxide sheets at nanometer scale. Specifically, a hybrid nanolayer of Ni-Co hydroxide @reduced graphene oxide (Ni,Co-OH/rGO) with an average thickness of 1.37 nm is synthesized through an easy one-pot hydrothermal method. Benefiting from the face to face contact model between Ni-Co hydroxide and rGO sheets, such unique structure presents superior specific capacitance and cycling performance as compared to the

pure Ni-Co hydroxide nanolayers. An asymmetric supercapacitor based on Ni,Co-OH/rGO and three dimensional (3D) hierarchical porous carbon is developed, exhibiting a high energy density of 56.1 Wh kg-1 along with remarkable cycling stability (80% retention after 17,000 cycles), which holds great promise for practical applications in energy storage devices. Keywords: ultrathin, nanolayer, Ni-Co hydroxide, supercapacitor; cycling stability 1

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Introduction Supercapacitors with high power density, ultrafast charge-discharge speed, and long lifespan have recently received considerable attention due to increasing demand for advanced energy storage devices1-4, which however suffer from the low energy density in practical applications. Building an asymmetric supercapacitor (ASC) is considered as one of the most effective approaches to solve this problem. These ASCs usually combine a battery-like pseudocapacitive electrode (as energy source) and a capacitive electrode (as power source) in one system, which can make full use of the different operation voltages of the electrode materials, and then increase the energy density for the cell system5-9. Therefore, much effort has been devoted to recognizing good electrode materials for ASCs specially to improve the energy density without sacrificing power density and cycling stability. The specific capacitance (SC) of electrode materials depends heavily on their electrochemical active surface area3, 4. For this reason, layered structured materials with high electrochemical active surface area, including metal oxides/hydroxides10-15, metal chalcogenides16-19, transition metal carbides/nitrides (Mxenes)20-22, metal phosphates23 etc, have been extensively investigated for their potential as supercapacitor electrodes. In particular, the emergence of atomically-thick nanolayer materials, which can enhance the surface-dependent electrochemical performance significantly, offers a promising opportunity to optimize supercapacitor properties on an atomic level10-12,14,23-25. Among these, transition metal oxides or hydroxides have attracted most attention due to their superior electrochemical performance. For instance, Zhu et al.11 prepared a high-quality ultrathin (< 2 nm) α-Ni(OH)2 nanosheet via microwave-assisted liquid-phase growth under low-temperature atmospheric conditions, which suggests an ultrahigh surface atom ratio with unique electronic structure. Feng et al.12 demonstrated a large-scale preparation of sub-3 nm atomic layered Co3O4 nanofilms with a nonsurfactant and substrate-free hydrothermal method, which exhibited an SC of 1400 F g-1. More recently, Xie et al.10 reported a novel single atomic layered β-Co(OH)2 with 0.47 2


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nm thickness using an oriented-attachment strategy, the all-solid-state asymmetric supercapacitor based on this material exhibited extremely high energy density. However, most existing researches about ultrathin structure focus on single cation systems. Binary metal composites such as Ni-Co hydroxide are reported to present enhanced performance and distinct advantages over single element systems3-5. The introduction of cobalt for nickel can not only reduce the resistance of the electrode and raise the oxygen overpotential, but also offer rich redox reaction from both nickel and cobalt ions to increase the electrochemical activity5-7. Furthermore, reduced graphene oxide (rGO), which has good electrical conductivity and chemical stability, can suppress the volume change and particle agglomeration during the charge-discharge process by forming a uniform nanocomposite with metal compounds. This would provide an efficient solution to the above issues26-28. As reported, the morphology of the materials has a great impact on the combining way with other materials, a very thin 2D nanostructure will allow a perfect contact between the metal compounds and rGO29, benefiting the ion diffusion and effective utilization of the surface area30-32. Therefore, it would be worthwhile to develop a simple and effective approach for the integration of ultrathin nanolayered 2D transition metal compounds and rGO sheets at nanometer scale. In this work, we report a facile synthesis of novel ultrathin hybridized material composed of 2D Ni-Co hydroxide (Ni,Co-OH) and rGO via a one-pot hydrothermal process, in which the metal hydroxide nanolayers anchored tightly to the rGO surface. L-ascorbic acid (LAA), a normal reductant of GO32, was also used as the morphology controlling agent for metal hydroxide, and thus the reduction of GO and the deposition of hydroxide nanolayer on rGO can be realized simultaneously. The as prepared Ni,Co-OH/rGO hybrid exhibits significantly enhanced cycling

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stability with an average thickness of 1.37 nm. An asymmetric supercapacitor device operable in aqueous electrolyte using Ni,Co-OH/rGO as positive electrode, and an Artemia cyst shell derived hierarchical porous carbon (HPC)13,33 as negative electrode, is developed. The HPC with welldefined macropores and interconnected meso-/micropores combined in one system, was produced by a very simple carbonization of the natural Artemia cyst shells33. Electrochemical characterizations indicate that this asymmetric supercapacitor can be cycled reversibly between 0 and 1.6 V in 6M KOH solution, and a high energy density of 56.1 Wh kg-1 is achieved. Meanwhile the ASC exhibit remarkable long cycle life along with 80% capacitance retention after 17000 charge-discharge cycles. In comparison with those from literature, such rational designed supercapacitor shows advantages in energy density and cycling stability. Experimental section Synthesis of electrode materials GO was commercially available, and ultrasonically dispersed in deionized (DI) water to form a 2 mg mL-1 GO colloid. The Ni-Co hydroxide/rGO was prepared by a one pot hydrothermal route. In a typical experiment, 60 mg L-ascorbic acid (LAA) was added to 30 mL GO colloid. Then, 2 mmol Ni(NO3)·6H2O and 0.5 mmol Co(NO3)·6H2O dissolved in 20 mL deionized (DI) water was added and stirred for 1 h to form a uniform mixture. Subsequently, 1.25 mmol KOH was dissolved in another 10 mL DI water and added to the above mixture dropwise, and stirred for 2h. The resultant product was transferred to a 100 mL Teflon lined autoclave and heated at 180oC for 12h. The black solid was collected after washing with DI water and absolute ethanol, and then vacuum freeze dried overnight to form a 3D network, denoted as Ni,Co-OH/rGO. For contrast, pure Ni-Co hydroxides with bulk structure and nanolayered structure, as well as the composite of Ni-Co hydroxide with GO were also prepared and denoted as Ni,Co-OH-LAA, Ni,Co-OH, and Ni,Co-OH/GO respectively (see supplementary materials for detailed information). Characterization 4


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Powder X-ray diffraction (XRD) patterns of as prepared samples were recorded on a D-max2500/PC X-ray diffractometer (Japan, Rigaku Corporation）using Cu Kα radiation (λ=1.5406 Å) as the source. Hitachi-SU8030 field emission scanning electron microscope (FESEM, Japan) was under the acceleration voltage of 2 kV and transmission electron microscopy (TEM, JEM 2010) at 200 kV. The scanning transmission electron microscopy (STEM) images and X-ray energy dispersive spectroscopy (EDS) maps were collected by a Hitachi HD-2300 STEM at 200 kV. The selected area electron diffraction (SAED) patterns were recorded by a Hitachi H-8100 TEM at 200 kV. Atomic force microscope (AFM) images were recorded using a multimode 8 scanning probe microscope (Veeco Instrument, Plainview, NY, USA). The Fourier transform infrared spectroscopy (FTIR) measurements of the samples were characterized in KBr pellets by EQUINOX55 FTIR Spectrometer ranged from 4000 to 400cm-1. Raman spectra from 200 cm-1 to 2000 cm-1 were recorded using a RenishawinVia Raman microscope with an Ar+ laser (λ=514.5 nm). X-ray photoelectron spectrum (XPS) was measured by a VG ESCALAB MKIIX-ray photoelectron spectrometer using Mg-Kα as the exciting source (1253.6 eV). Preparation of electrodes and electrochemical measurements The electrochemical properties of as-prepared materials were evaluated in a three-electrode system. The working electrodes were prepared as follows: 80 wt% of as-prepared samples, 15 wt% of acetylene carbon black and 5 wt% PTFE (polytetrafluoroethylene) were mixed to obtain a homogeneous slurry. The slurry was then coated on a 1 x 1 cm2 area foam nickel substrate and dried in vacuum at 120 °C for 12 h. Platinum foil and Hg/HgO electrodes were used as counter and reference electrodes, respectively. The mass loading of active materials is controlled as about 3 mg cm-2. The asymmetric supercapacitor was assembled with Ni,Co-OH/rGO as a positive electrode, and hierarchical porous carbon (HPC) (see supplementary for preparation method) as a negative electrode (denoted as Ni,Co-OH /rGO //HPC). Cyclic voltammetry (CV) and electrochemical spectroscopy (EIS) of electrodes were performed on CHI 660E, and galvanostatic charge-discharge (GCD) measurement was implemented on a LAND battery program-control test system (Land, CT2001A, China). All tests were carried out in 6M KOH aqueous electrolyte at room temperature. The SC (C (F g-1)), energy density (E (W h kg-1)) and power density (P (W kg-1)) values were calculated from discharge curves according to 5



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Equations (1, 2, 3): (1) (2) (3) where, I (A) is the discharge current, m (g) is the mass of active materials, dV/dt (V s-1) is the gradient of discharge curve, ∆V (V) is the potential window of discharge curve, ∆t is the discharge time consumed in the potential range of ∆V. Results and discussion Figure 1 shows the XRD patterns of Ni,Co-OH, Ni,Co-OH /GO, Ni,Co-OH-LAA and Ni,CoOH/rGO, and the corresponding results demonstrate that Ni,Co-hydroxide exist as β-Ni(OH)2 (PDF#14-0117) and β-Co(OH)2 (PDF#30-0443) in all the samples. Narrow and sharp peaks of sample Ni,Co-OH and Ni,Co-OH/GO demonstrate the high crystallinity of Ni-Co hydroxide. The significantly broadening and weakening characteristic peaks for both Ni,Co-OH-LAA and Ni,CoOH/rGO suggest the fine particle size and inferior crystallinity under the effect of LAA. The missing of GO and rGO peaks in the XRD patterns should be attributed to the exfoliation of regular stacks of GO/rGO, which correlates well with the previous studies34, 35. The FTIR spectra (Figure S1) also confirm the brucite structure of β (Ni-Co hydroxide) phase. Figure 2 shows the typical FESEM and TEM images of the as prepared samples. Thick nanoflakes were formed for pure Ni,Co-OH (Figure 2a,b), while sample Ni,Co-OH/GO presents more transparent and dispersive feature, indicating that GO can effectively decrease the agglomeration and slightly reduce the thickness of Ni-Co hydroxide nanoflakes (Figure 2c,d). The presence of LAA can effectively reduce the thickness by forming ultrathin nanolayers for Ni,Co-OH-LAA (Figure 2e,f). For sample Ni,Co-OH/rGO, a uniform metal hydroxide/rGO nanocomposite was formed, of which the hydroxide nanolayers are hardly distinguishable from rGO in both FESEM and TEM images, implying the ultrathin character of the metal hydroxide layer (Figure 2g,h). To investigate the detailed information about the morphology and composition, Ni,Co-OH/rGO was further examined under STEM (Figure 3). From the bright-field image (Figure 3a) and SE 6


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image (Figure 3b), the thin Ni-Co hydroxide nanosheets are almost invisible, while they have faint contrast in the ZC (Figure 3c) image due to Ni and Co. The EDS maps of elements Ni, Co, O and C (Figure 3d) further clearly demonstrate that the ultrathin Ni-Co hydroxide sheets are uniformly distributed on rGO surfaces. Figure 3e depicts a selected area electron diffraction pattern (SAED) of Ni,Co-OH/rGO obtained from the marked region (Figure 3c), which consists of graphene and Ni-Co hydroxide. The second ring with d spacing of 0.20 nm can be well indexed as the {110} lattice planes of rGO. And six diffraction spots coexisted in the diffraction ring shows that the rGO retain the hexagonal symmetry of the [0001] diffraction pattern36. Another two clear diffraction rings can be referred to the crystallographic directions of (100) and (111) plane of β-(Ni,Co-OH). The coexistence of rGO and Ni-Co hydroxide could be further confirmed by Raman Spectra (Figure S2) and XPS results (Figure S3). The AFM image and corresponding height profiles (Figure 4) depict the thickness of as prepared samples. The average height of Ni,Co-OH/rGO is about 1.37 nm, and the hump in the middle part could be due to the overlapped nanosheets (Figure 4a). The ultrathin structure of Ni,Co-OH/rGO possesses much increased density-of-states (DOS) near the Fermi level as compared to their bulk counter-parts, which could facilitate electron transportation along the two-dimensional conducting channels to react with OH- ions, thus achieving fast diffusion kinetics10. Besides, this unique structure allows better contact between the electrode materials and the electrolyte, hence can provide a larger zone for ion diffusion and electron transport during charge/discharge processes, enhancing the overall electrochemical performance. As noticed, the rGO and metal hydroxide are indistinguishable from each other, for comparison, the thickness of pure rGO and Ni,Co-OH-LAA prepared in a similar procedure (see supporting information for detailed information) are also tested (Figure S4). The average thickness of pure rGO and pure Ni,Co-OH is measured as 0.7 nm and 1.04 nm respectively, indicating the ultrathin character of both components. The smaller thickness of 1.37 nm as compared to the sum value of the pure components (1.74 nm) could be attributed to the synergistic interaction and the extremely intimate contact between rGO and Ni,Co-OH. A more detailed discussion is given in the supporting information. A possible mechanism for the structure evolution is proposed in Figure 5. For Ni,Co-OH-LAA, after the injection of KOH solution into the mixture of (Ni,Co)(NO3)2 and LAA, the OH- ions firstly 7



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react with LAA leading to the formation of L-dehydroascorbic acid (DAA), followed enable the Ni2+ and Co2+ to form abundant small 2D Ni,Co-OH seeds without obvious orientation. DAA tends to adsorb on the (001) facet of the small 2D seeds to further reduce the surface energy, owing to the fact that the low surface energy renders the (001) facet much more stable than the (100) and (010) facets with high surface energy37. Driven by DAA, the small seeds gradually grow into 2D nano structure through an epitaxial lateral overgrowth process, and simultaneously evolve into ultrathin nanolayers. For the LAA assisted growth of Ni,Co-OH/rGO, the mechanism is quite similar, except for that, the reduction of GO can happen through a back-side SN2 nucleophilic attack, where LAA serves as a reduction reagent38. The rGO with amphiphilic property39, will enable better dispersive behavior and then further reduce the thickness of the Ni,Co-OH nanolayers, and eventually form a uniform metal hydroxide @rGO layer on layer nanostructure. Meanwhile, the abundant functional groups on LAA will also prevent the rGO sheets from restacking and enable a much better dispersion as compared to the Ni,Co-OH/GO sample (Figure 2). The electrochemical performance of the as-prepared samples was all evaluated in a three-electrode cell in 6 M KOH aqueous electrolyte. For the Ni,Co-OH/rGO, good symmetry of anodic and cathodic peaks from the CV curves reveals the excellent reversibility (Figure 6a). Each curve manifests a pair of strong redox peak, which results from superior pseudocapacitor feature achieved by following redox reactions:

With scan rates increasing from 5 to 50 mV s-1, the current density increases and the positions of anodic and cathodic peaks shift to the more positive and negative direction, respectively, attributed to the internal resistance of the electrode material. Figure 6b displays the GCD curves of Ni,CoOH/rGO at various current densities. Nearly symmetric potential-time curves imply the high chargedischarge columbic efficiency and low polarization of Ni,Co-OH/rGO. For comparison, the CV curves at 10 mV s-1 and the GCD curves at 0.5 A g-1 of all the as prepared samples were carried out 8


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(Figure 6c and 6d). The SC values were calculated from the discharge curves according to Equation (1) and compared in Figure 6e. The nanolayered Ni,Co-OH-LAA exhibits a decent SC value of 1316 F g-1 at the current density of 0.5 A g-1, which is much higher than the bulk sample of Ni,Co-OH (525 F g-1) and Ni,Co-OH/GO (385 F g-1). This should be attributed to its nanolayered structure, which can make most atoms exposed outside with high activity, and thus the surface-dependent electrochemical reaction would be significantly enhanced, and finally improve the electrochemical performance. However this structure suffers from poor cycling stability (Figure 6f), only 75.1% capacitance retained after 1000 cycles. The Ni,Co-OH/rGO electrode, on the other hand, exhibits an excellent SC value of 1691 F g-1 with superior rate performance and excellent long cycle life. The capacitance retention of this hybrid structure can reach 82.6% from 0.5 to 10 A g-1, and remains 78.5% from 0.5 to 40 A g-1, which should be attributed to the improved electrical conductivity. No obvious capacitance decay was detected from Ni,Co-OH/rGO after 1000 cycles (Figure 6f), which is significantly enhanced as compared to Ni,Co-OH-LAA. Figure 6g demonstrates the EIS plots of the as prepared samples. The intersection of the semi-circle on the real axis at high frequency represents the equivalent series resistance (Rs) of the electrode, while the diameter of the semicircle corresponds to the charge-transfer resistance (Rct) of the electrodes and electrolyte interface40. From the inset of Figure 6g, the Rs values are almost identical for all the samples, while the Ni,CoOH/rGO owns the lowest Rct value, indicating the lower intrinsic resistance and improved capacitive behavior. The straight line in the low frequency range is related to the diffusive resistance of the electrolyte into the interior of the electrode and ion diffusion into the electrode. The almost vertical line of Ni,Co-OH/rGO, represents the fast ion diffusion in electrolyte and the fast adsorption onto the electrode surface, suggesting the ideal capacitive behavior of the electrodes41. The good conductivity of Ni,Co-OH/rGO composite not only thanks to the good conductivity of rGO, but also benefits from its unique hybrid nanolayer structure. The face to face contact between Ni,Co-OH and rGO will facilitate the continuous electron transportation, while the large interface area between the two phases of Ni,Co-OH and rGO, as well as the channels formed by the 3D structure would provide a high speed pathway for ionic transportation. Therefore the composite material presents improved conductivity for both electronic and ionic conduction. Furthermore, the rGO composition, which is regarded as an ideal matrix to prevent hydroxide nanolayers from agglomeration, can also provide enhanced mechanical strength and chemical stability, which endow 9



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Ni,Co-OH/rGO with excellent cycling stability42. Meanwhile, this hybrid structure with good electronic and ionic conduction can improve the charge-discharge efficiency and relax the tension caused by the volume change induced by phase transformation of Ni,Co-OH, thus making sure the good reversibility upon cycling43,44. The intimate contact of Ni,Co-OH and rGO is also critical to the enhanced stability. The planar contact between the metal hydroxide and rGO results in a much higher contact area at the interface, which can benefit the well dispersion without any agglomeration or size variation and maintaining the utrathin structure during the charge and discharge process, thus achieving enhanced the cycling stability45,46. Note that, Ni,Co-OH/rGO presents superior rate and cycling performance as compared to similar work from literatures (Table S2). In order to certify the practical application of the Ni,Co-OH/rGO for energy storage, an ASC cell was assembled with Ni,Co-OH/rGO as the positive electrode, and HPC (see supporting information for detailed information) as the negative electrode material, which is denoted as Ni,CoOH/rGO//HPC (Figure S8). The potential window of the Ni,Co-OH/rGO//HPC ASC is confirmed in Figure S9 as 1.6V, and the optimal mass ratio between Ni,Co-OH/rGO and HPC electrodes is calculated as m+ /m- = 0.48 according to Equation 4: (4) Specifically, the loading mass of positive and negative active materials is 2 mg cm-2 and 4.16 mg cm2

, respectively, with the total loading of 6.16 mg cm-2. The CV curves Ni,Co-OH/rGO//HPC

asymmetric supercapacitors at different scan rates were tested (Figure 7a), and just slight shape distortion of CV curves is observed from 5 to 100 mV s-1, indicating high rate capacity of the as constructed ASC, this result is in good agreement with GCD analysis (Figure 7b). SC values were calculated by the total active materials in both positive Ni,Co-OH/rGO electrodes and negative HPC electrodes. A high SC of 162 F g-1 is achieved at current density of 0.1 A g-1 (Figure 7c). The asymmetric supercapacitor demonstrates excellent cycling stability with 80% of capacitance retention and 100 % of coulomb efficiency after 17000 charge/discharge cycles (Figure 7d). Figure 7e illustrates the Ragone plots of the Ni,Co-OH/rGO//HPC asymmetric supercapacitors, the energy density can reach up to 56.1 Wh kg-1 at 76 W kg-1, and still remain 37.5 Wh kg-1 even at 7.12 kW kg1

, which is superior to the previous works (Figure 7f). The detailed information of the relative ASCs is given in Table 1, suggesting both competitive 10


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energy density and long cycle performance of the as fabricated ASC. The cycling stability of ASCs depends not only on advanced positive materials but also negative materials47. The as constructed ASC adopted HPC33 as cathode, which exhibits an excellent long life cycle (100% after 10,000 charge/discharge cycles) in alkaline electrolyte, laying a foundation for the remarkable cycling stability of the asymmetric supercapacitor. As for the anode (Ni,Co-OH/rGO), which has demonstrated good chemical stability due to its unique structure, further ensure the superior cycle of the Ni,Co-OH/rGO//HPC supercapacitor. Combined with the advantages (low costs, friendly environment and safety) of electrode materials, this asymmetric supercapacitor of Ni,Co-OH/rGO //HPC is a very promising energy storage system for future application. Conclusions In this work, ultrathin nanolayered Ni-Co binary hydorxides/rGO hybrid material with high electric conductivity through a simple one-pot hydrothermal method. The as prepared material exhibits a high specific capacitance of 1691 F g-1 at 0.5 A g-1, and remains 1327 F g-1 at 40 A g-1, suggesting a 78.5% capacitance retention from 0.5 to 40 A g-1. The as fabricated Ni,Co-OH/rGO //HPC asymmetric supercapacitor shows high energy densities of 56.1 Wh kg-1 at power densities of 76 W kg-1, and 37.5 Wh kg-1 at 7120 W kg-1, respectively. Significantly remarkable long cycle life (80% specific capacitance retains after 17000 cycles) is achieved. This suggests that the as prepared Ni-Co hydroxides/rGO composite is a promising electrode material to fabricate high performance supercapacitor. Moreover, this method can be extended to synthesize other ultrathin layered metal hydroxides/rGO with high electrochemical activity for application in supercapacitors, sensors, catalysis, and so on. ASSOCIATED CONTENT Supporting Information FTIR spectra of as prepared samples Ni,Co-OH, Ni,Co-OH/GO, Ni,Co-OH-LAA and Ni,CoOH/rGO; Raman spectra of Ni,Co-OH-LAA and Ni,Co-OH/rGO; XPS spectra of Ni,Co-OH/rGO; Tapping mode AFM topography images of rGO sheets and Ni,Co-OH-LAA; Capacitive performance of Ni,Co/rGO with different Ni/Co ratios; Capacitive performance of Ni,Co/rGO with different LAA amount;

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Capacitive performance of fabricated asymmetric supercapacitor device with increased potential window; EDS analysis and the element content of Ni,Co-OH/rGO; Comparison of electrochemical performance with similar work from literature based on three-electrode system. This material is available free of charge via the Internet at http://pubs.acs.org/ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through equal contributions of Hongnan Ma and Jing He. All authors have given approval to the final version of the manuscript. Funding The authors gratefully acknowledge NSFC (Grant 51202213), CPSF (Grant 2013M530889, 2014T70230), EYSFHP (Grant Y2012005), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (CG2014003002). Notes The authors declare no competing financial interest. Acknowledgments Financial support from the NSFC (Grant 51202213), CPSF (Grant 2013M530889, 2014T70230), EYSFHP (Grant Y2012005), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (CG2014003002) is acknowledged. Part of this work made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); and the State of Illinois, through the IIN. References (1) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845-854. (2) Zhi, M. J.; Xiang, C. C.; Li, J. T.; Li, M.; Wu, N. Q. Nanostructured Carbon–metal Oxide Composite Electrodes for Supercapacitors: a Review. Nanoscale. 2013, 5, 72-88. (3) Wang, G. P.; Zhang, L.; Zhang, J. J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. (4) Feng, J.; Sun, X.; Wu, C. Z.; Peng, L. L.; Lin, C. W.; Hu, S. L.; Yang, J. L.; Xie, Y. Supercapacitor Electrode 12


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Figure Captions Figure 1 XRD patterns of as prepared samples. Figure 2 FSEM images of Ni,Co-OH (a), Ni,Co-OH/GO (c), Ni,Co-OH-LAA, (e) Ni,Co-OH/rGO 15



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(g); TEM images of Ni,Co-OH (b) Ni,Co-OH/GO (d), Ni,Co-OH-LAA, (f) Ni,Co-OH/rGO (h). Figure 3 STEM images of Ni,Co-OH/rGO collected by: (a) bright-field detector (BF-STEM image), (b) secondary electron detector (SE image), (c) high-angle annular dark-field detector showing Zcontrast (ZC image) and (d) corresponding EDS mapping of Ni, Co, O, C elements, respectively, (e) SEAD pattern of Ni,Co-OH/rGO. Figure 4 Tapping mode AFM topography images of (a) Ni,Co-OH/rGO deposited on a mica substrate and their distribution of z height, (b) crystal structures of Ni,Co-hydroxide and Ni,CoOH/rGO. Figure 5. Schematic illustration of the formation mechanism for nanolayered Ni,Co-OH-LAA and Ni,Co-OH/rGO. Figure 6 CV curves at different scan rates (a), Galvanostatic charge/discharge curves measured at different current densities (b) of Ni,Co-OH/rGO; CV curves at the scan rate of 10 mV s-1 (c), Galvanic Charge-Discharge curves at the current density of 0.5 A g-1 (d), SC values calculated from discharge curves (e), long cycle performance (f), EIS plots (g) of Ni,Co-OH, Ni,Co-OH/GO, Ni,CoOH-LAA and Ni,Co-OH/rGO. Figure 7 (a) CV curves of Ni,Co-OH/rGO //HPC asymmetric supercapacitors measured with a voltage of 1.6V at different scan rates; (b) Galvanostatic charge/discharge curves Ni,Co-OH/rGO //HPC asymmetric supercapacitor measured at different current densities; (c) Specific capacitances and capacitance retention ratio as a function of discharge current densities; (d) Cycling performance and coulomb efficiency with a voltage of 1.6 V at a current density of 2 A g-1; (e) Ragone plots related to energy and power densities of Ni,Co-OH/rGO //HPC; (f) A comparison of energy densities among as-constructed Ni,Co-OH/rGO //HPC with ASCs from literature. Table 1 Comparision of the performance with work from literatures based on similar asymmetric supercapacitors systems.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Table 1 Energy density (Wh⋅kg−1)

Power Density (W⋅kg−1)

Number

Retention

Ni,Co-OH/rGO //HPC

56.1

76

17000

80 %

This work

Co0.45Ni0.55O-rGO//rGO

35.3

330

1000

96 %

48

Ni(OH)2//AC

42.3

110

1000

82 %

49

NiCo2O4–rGO//AC

23.32

324.9

2500

83 %

50

[email protected](OH)2//CMK-3

31.2

396

3000

82 %

51

NixCo1-x LDHs // AC

23.7

284.2

5000

92.7 %

52

rGO//rGO

6.8

49.8

--

--

53

Ni-Zn-Co oxide/hydroxide//PC

41.65

85

1000

84 %

54

Ni(OH)2/GNs/NF//AC

11.11

93

--

--

55

Ni(OH)2/UGF//a-MEGO

13.4

65

10000

63.2 %

56

Positive materials// negative materials

Cycle

Capacitance Ref.

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TOC graphic

25


Nickel Cobalt Hydroxide @Reduced Graphene Oxide Hybrid

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