Ultrathin Ni-Co Hydroxide Nanosheets and Reduced Graphene Oxide ...
Supporting information
Nickel Cobalt Hydroxide @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
2
Department of Materials Science and Engineering, EPIC, NUANCE Center, Northwestern University 2220 Campus Drive, Evanston, Illinois 60208, United States 3
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
S-1
Experimental Preparation of control samples The bulk Ni-Co hydroxide was synthesized as follow and denoted as Ni,Co-OH. 2mmol Ni(NO3)2·6H2O and 0.5mmol Co(NO3)2·6H2O dissolved in 50 mL deionized (DI) water was added and stirred for 1h. Subsequently, 1.25 mmol KOH was dissolved in another 10 mL DI water, and then slowly added to above solution under stirring. After 2h, the mixture was transferred to a Teflon lined autoclave of 100 mL capacity and heated at 180oC for 12h. The black mixture was collected by filtration, washed with DI water and absolute ethanol, and finally vacuum freeze dried overnight. Pure Ni-Co hydroxide nanolayer was prepared through a one pot hydrothermal route and denoted as Ni,Co-OH-LAA. Firstly, 60 mg L-ascorbic acid (LAA) was dissolved in 10 mL deionized (DI) water. After that, 2 mmol Ni(NO3)2·6H2O and 0.5 mmol Co(NO3)2·6H2O dissolved in 30 mL deionized (DI) water was added to the above solution and stirred for 1h. Subsequently, 1.25 mmol KOH dissolved in another 20 mL DI water was then slowly added into above solution and stirred for 2h. The resultant mixture was transferred to a 100 mL Teflon lined autoclave and heated at 180oC for 12h. The fluffy green solid was collected after washing with DI water and absolute ethanol, and then vacuum freeze dried overnight. The Ni-Co hydroxide/GO without adding L-ascorbic acid was synthesized as follow and denoted as Ni,Co-OH/GO. Firstly, 2mmol Ni(NO3)·6H2O and 0.5mmol Co(NO3)·6H2O dissolved in 20 mL deionized (DI) water were added to 30 mL 2 mg mL-1 GO colloid and stirred for 1h. Subsequently, 1.25 mmol KOH was dissolved in another 10 mL DI water, and then slowly added to above solution under stirring. After 2h, the mixture was transferred to a Teflon lined autoclave of 100 mL capacity and heated at 180oC for 12h. The black mixture was collected by filtration, washed with DI water and absolute ethanol, and finally vacuum freeze dried overnight. The pure rGO was prepared through a similar hydrothermal process. Firstly, 60mg LAA was added to 60 mL 1 mg mL-1 GO colloid and stirred for 3h. After that, the mixture was transferred to a Teflon lined autoclave of 100 mL capacity and heated at 180oC for 12h. The black mixture was collected by filtration, washed with DI water and absolute ethanol, and finally vacuum freeze dried overnight to form a 3D network. The Ni-Co hydroxide @ rGO composites with different Ni/Co ratios (1:0, 2:1, 1:1, and 0:1) were synthesized following similar procedure and denoted as Ni-OH/rGO, Ni2Co1-OH/rGO, Ni1Co1S-2
OH/rGO, and Co-OH/rGO, respectively. The Ni/Co ratio for Ni,Co-OH/rGO in the manuscript was 4:1, thus it is also named Ni4Co1-OH/rGO. In a typical preparation, 60 mg L-ascorbic acid (LAA) was added to 30 mL GO colloid of 2 mg mL-1. Then, desired amount of Ni(NO3)·6H2O and Co(NO3)·6H2O dissolved in 20 mL deionized (DI) water was added and stirred for 1h to form a uniform mixture. Note that, the total amount of nickel nitrate and cobalt nitrate is 2.5 mmol. 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. Preparation of hierarchical porous carbon (HPC) HPC was prepared as reported previously1. Artemia cyst shells were cleaned with deionized water, dried, and then ball-milled for 6 h at a speed of 300 rpm. The ball-milled Artemia cyst shells were heated to 300oC at Ar gas atmosphere with a heating rate of 5oC min-1 and maintained for 3 h for precarbonization, then heated up to 700oC at 5oC min-1 and kept for 4 h. The obtained products were sonicated in 67 wt% HNO3, then washed with deionized water and dried at 80oC for 12 h, the final product is named as HPC.
S-3
Supporting Figures
Figure S1 FTIR spectra of as prepared samples. The chemical structure of as-prepared samples was further characterized by FTIR (Figure S1). For all samples, common vibration bands are observed at 3641, 3440, 2917, 2857, 1624, 1384, 1100, 533, 446 cm-1. The sharp peak at 3641 cm-1 corresponds to the stretching vibration mode of nonhydrogen-bonded hydroxyl groups, which confirms the brucite structure of β-Ni,Co hydroxide phase2,3. The broad band at 3440 cm-1 can be assigned to hydrogen-bonded O-H groups stretching vibration. The band at 2917 and 2857 cm-1 are attributed to the –C-H vibration mode of –CH2. The band at 1624 cm-1 is attributed to the bending vibration of adsorbed water molecules and δO-H of hydroxyl group. The band of 1384 cm-1 is attributed to the vibration of NO3– which is the leftover from the nickel or cobalt nitrate. The band at 1100 cm-1 correspond to the C–O (alkoxy) stretching peak. The peaks of 535 and 446 cm-1 can be assigned to Ni/Co-O vibrations ascribed to Ni,Co-OH.
S-4
Figure S2 Raman spectra of Ni,Co-OH-LAA (a) and Ni,Co-OH/rGO (b) Raman spectra of Ni,Co-OH-LAA and Ni,Co-OH/rGO are tested and shown in Figure S2. The Raman peaks at 451 and 510 cm−1 can be assigned to the characteristic A1g, and A2u modes of the hexagonal brucite-like (Co,Ni)(OH)2 respectively, indicating the existence of Ni-Co hydroxide in Ni,Co-OH/rGO4-7. In the spectrum of Ni,Co-OH/rGO (Figure S2b), a D band at 1350 cm-1 and a G band at 1600 cm-1 for reduced graphite oxide are detected, corresponding to the A1g mode breathing vibrations of six member sp2 carbon rings, indicating the existence of rGO in Ni,Co-OH/rGO8,9. The Raman spectrum of Ni,Co-OH/rGO (Figure S2b) exhibits characteristics of both rGO and Ni-Co hydroxide, which further indicated the hybrid structure is composed of rGO and Ni-Co hydroxide10.
S-5
Figure S3 XPS spectra of Ni,Co-OH/rGO: survey scan (a); C 1s (b); Ni 2p (c); and Co 2p (d) regions. In order to obtain more information and the surface electronic states, Ni,Co-OH/rGO was further examined and analyzed by XPS measurements (Figure S3). The survey spectrum (0-1200 eV) mainly shows carbon (C1s), oxygen (O1s), Ni2p and Co2p speicies (Figure S3a). Detail analysis of C1s region is shown in Figure S3b. The presence of three components of carbon bond, including non-oxygenated carbon (C-C: 284.3 eV), hydroxyl carbon (C-O: 285.3 eV) and carbonyl carbon (C=O: 288.0 eV), is in good agreement with the reported data. In the Ni 2p region (Figure S3c), two main peaks at 854.6 and 872.2 eV with a spin-energy separation of 17.6 eV are assigned to Ni 2p3/2 and Ni 2p1/2, which demonstrates the characteristic of Ni2+ in Ni(OH)2. In the Co 2p region (Figure S3d), two pairs of individual peaks at 781.4 and 797.3 eV of Co 2p3/2 and Co 2p1/2, respectively, which is identified as the major binding energies of Co2+ in Co(OH)2.11-13
S-6
Figure S4 Tapping mode AFM topography images of (a) rGO sheets; (b) Ni,Co-OH-LAA deposited on a mica substrate and the distribution of z height of (a1,a2) rGO sheets; (b1,b2) Ni,Co-OH-LAA . The AFM images of pure rGO sheets and Ni,Co-OH-LAA are shown in Figure S4, with the average height of 0.7 nm and 1.04 nm, respectively. Obviously, the thickness of Ni,Co-OH/rGO (1.37 nm) is less than the sum value (1.74 nm) of rGO (0.7 nm) and Ni-Co hydroxide (1.04 nm), which could be explained by two aspects as follows. First of all, nickel and cobalt ions can adsorb or chemically interact with layered rGO functionalized by oxygen functional groups such as hydroxyls, epoxides, carboxyls, and so on.14-16 These functional groups, namely active sites, are like rivets, fixing the Ni-Co hydroxide seeds onto GO sheets and further impelling the oriented growth of Ni-Co hydroxide (Figure 5), allowing a very intimate contact between Ni,Co-OH and rGO. On the other hand, as previously reported17,18, a synergistic enhanced dispersion could be observed when coupling rGO with metal compounds. The rGO with amphiphilic property can function as a surfactant, which could improve the dispersion of the composite, forming a uniform structure free of the agglomeration of Ni-Co hydroxide. Meanwhile the existence of Ni-Co hydroxide can also prevent the rGO sheets from agglomeration19-22. Therefore, the slightly smaller thickness of Ni,Co-OH/rGO than the sum value of their pure components could probably be benefited from the enhanced dispersion of the composite and the intimate contact between the metal hydroxide and rGO. S-7
Figure S5 The energy dispersive X-ray spectroscopic (EDS) spectrum of Ni,Co-OH/rGO Table S1 EDS analysis and the element content of Ni,Co-OH/rGO. Element
C
O
Ni
Co
Weight %
14.23
33.57
41.40
10.63
Atomic %
28.40
50.30
16.91
4.32
S-8
Figure S6 Capacitive performance of Ni,Co-OH/rGO with different Ni/Co ratios (1:0, 4:1, 2:1, 1:1, and 0:1). (a) Galvanic Charge-Discharge curves at the current density of 0.5 A g-1; (b) CV curves at the scan rate of 10 mV s-1; (c) SC values calculated from discharge curves; (d) EIS plots of Ni-Co hydroxide @ rGO composites with different Ni/Co ratios. The electrochemical performance NiCo-OH/rGO composites with different Ni/Co atomic ratios are also tested (Figure S6). It can be seen, the capacitive performance of binary composites is greatly enhanced as compared to those of the single component systems (Ni-OH/rGO and Co-OH/rGO). Notably, Ni4Co1-OH/rGO (Ni,Co-OH/rGO) presents the best capacitance behavior (Figure S6), mainly attributes to the synergistic effect between nickel and cobalt as well its unique nanolayer structure which can make most atoms exposed outside with high activity. The largest integral area of CV curve (Figure S6a), the longest discharge time (Figure S6b), and the highest capacitance values (Figure S6c) all imply that 4:1 is the optimal Ni/Co ratio of the composite, as confirmed by the relatively smaller equivalent series resistance (Figure S6d).
S-9
Figure S7 Capacitive performance of Ni,Co-OH/rGO with different LAA amount (0, 20, 60, and 80 mg LAA). (a) CV curves at the scan rate of 10 mV s-1; (b) Galvanic Charge-Discharge curves at the current density of 0.5 A g-1; (c) SC values calculated from discharge curves; (d) EIS plots of Ni-Co hydroxide @ rGO composites with different LAA amounts. Note that, the amount of LAA used has a significant effect on the electrochemical performance of metal hydroxide/rGO composites. Ni,Co-OH/rGO with different LAA amount (20 mg and 80 mg) were synthesized through the same process of Ni,Co-OH/rGO and were denoted as Ni,Co-OH/rGOLAA20 and Ni,Co-OH/rGO-LAA80, respectively. Ni,Co-OH/rGO with LAA amount of 60 mg was also named as Ni,Co-OH/rGO-LAA60. It is obvious that Ni,Co-OH/rGO-LAA60 (Ni,Co-OH/rGO) presents the best capacitance behavior (Figure S7), mainly attributed to its unique single layer or few layer structure which can make most atoms exposed outside with high activity. With the participation of LAA, the electrochemical performance of the Ni-Co hydroxide @ rGO composite increased markedly (Figure S7). However, the performance of Ni,Co-OH/rGO-LAA80 is worse than Ni,CoOH/rGO-LAA60 (Ni,Co-OH/rGO), it can be explained that excess content of reductant (LAA) may cause too much defects on rGO nanosheets and decrease the electrical conductivity and poor electrochemical reversibility, thus leading to the decrease in capacitive performance for the composite. S-10
Figure S8 Schematic illustration of fabricated asymmetric supercapacitor device, based on Ni,CoOH/rGO as the positive electrode and HPC as the negative electrode.
S-11
Figure S9 (a) CV curves of Ni,Co-OH/rGO and HPC electrodes performed in three-electrode cell in 6 M KOH aqueous electrolyte at a scan of 10 mV s-1; (b) CV curves of the asymmetric supercapacitor of Ni,Co-OH/rGO//HPC measured at different potential windows at a scan rate of 5 mV s-1; (c) Galvanostatic charge/discharge curves of Ni,Co-OH/rGO//HPC measured at different potential windows at a current density of 0.1 A g-1; (d) Specific capacitances of the asymmetric supercapacitor with the increase of potential window at 0.1 A g-1; The different operation voltages of Ni,Co-OH/rGO (0~0.5V) and HPC (-1~0V) can be seen in Figure S9a. Figure S9b and S9c show the CV plots and the galvanostatic charge/discharge curves of Ni,Co-OH/rGO//HPC with different potential windows. It indicates that, with the increasing of potential window from 1 to 1.6V, more severe Faradaic pseudocapacitive behavior occurs at the Ni,Co-OH/rGO//HPC ASC. Figure S9d demonstrates the variation of specific capacitance (SC) with the increase of potential window (1~1.6 V) for the ASC, and the corresponding SC value increase from 69 to 167 F g-1. It is well known that the energy density is proportional to the square of potential, and the enlarged potential window would give rise to a remarkably enhanced energy density.
S-12
Table S2 Comparison of electrochemical performance with similar work from literature based on three-electrode system Electrode materials
SC value (F g-1)
Rate performance (%)
Cycling performance (%)
RGO/Ni(OH)210
1828 (2 A g-1)
42.7% (1 to 10 A g-1)
76.9% (1000 cycles)
nickel cobalt oxide23
1479 (1 A g-1)
53.5% (1 to 30 A g-1)
-
ZnONF/NiCoLDH24
1624 (10 A g-1)
71.4 (10 to 100 A g-1)
-
Ni-Co oxide25
1846 (1 A g-1)
62.5 (1 to 10 A g-1)
-
Ni(OH)2/G26
1985.1 (5 mA cm-2) 41.9% (5 to 50 mA cm-2)
93.5% (500 cycles)
NiO-GNC-GC27
1495 (0.5 A g-1)
22.4% (0.5 to 10 A g-1)
89.3% (1000 cycles )
NiCo2O4@graphene28
778 (1 A g-1)
48.1% (1 to 80 A g-1)
90% (10000 cycles)
Ni-Co oxide29
1545 (5 A g-1)
75.5% (5 to 20 A g-1 )
93% (5000 cycles)
rGO/NiCoAl LDH30
1902 (1 A g-1)
75% (1 to 10 A g-1)
62% (1500 cycles)
Ni-Co hydroxide31
1734 (6 A g-1)
66.1% (6 to 30 A g-1)
86% (1000 cycles)
Ni(OH)2@Co(OH)232
369 (0.3 A g-1)
-
96.4% (2000 cycles)
β-Co(OH)24
2028(5 A g-1)
-
-
This work
1691(0.5 A g-1)
82.6% (0.5 to 10 A g-1) 78.5% (0.5 to 40 A g-1)
S-13
100% (1000 cycles)
References (1) Zhao, Y. F.; Ran, W.; He, J.; Song, Y. F.; Zhang, C. M.; Xiong, D. B.; Gao, F. M.; Wu, J. S.; Xia, Y. Y. Oxygen Rich Hierarchical Porous Carbon Derived from Artemia Cyst Shells with Superior Electrochemical Performance. ACS Appl. Mater. Interfaces. 2015, 7, 1132-1139. (2) Wu, Z.; Huang, X. L.; Wang, Z. L.; Xu, J. J.; Wang, H. G.; Zhang, X. B. Electrostatic Induced Stretch Growth of Homogeneous β-Ni(OH)2 on Graphene with Enhanced High-Rate Cycling for Supercapacitors. Sci. Rep. 2014, 4, 1-8. (3) Chen, X.; Long, C. L.; Lin, C. P.; Wei, T.; Yan, J.; Jiang, L. L.; Fan, Z. J. Al and Co Co-doped αNi(OH)2/Graphene Hybrid Materials with High Electrochemical Performances for Supercapacitors. Electrochim. Acta. 2014, 137, 352-358. (4) Gao, S.; Sun, Y. F.; Lei, F. C.; Liang, L.; Liu, J. W.; Bi, W. T.; Pan, B. C.; Xie, Y. Ultrahigh Energy Density Realized by a Single-Layer β-Co(OH)2 All-Solid-State Asymmetric Supercapacitor. Chem. Int. Ed. 2014, 53, 12789 -12793. (5) Shieh, S. R.; Duffy, T. S. Raman Spectroscopy of Co(OH)2 at High Pressures: Implications for Amorphization and Hydrogen Repulsion. Phys. Rev. B. 2002, 66, 134301-134308. (6) Lutz, H. D.; Moller, H.; Schmidt, M. Infrared and Raman Study of Polyaniline. Part I: Hydrogen Bonding and Electronic Mobility in Emeraldine Salts. J. Mol. Struct. 1994, 317, 261-271. (7) Murli, C.; Sharma, S.; Kulshreshtha, S. K.; Sikka, S. K. High-pressure Behavior of β-Ni(OH)2 — A Raman Scattering Study. Physica B. 2001, 307, 111-116. (8) Cui, P.; Lee, J.; Hwang, E.; Lee, H. One-pot Reduction of Graphene Oxide at Subzero Temperatures. Chem. Commun. 2011, 47, 12370-12372. (9) Lin, T. W.; Dai, C. S.; Hung, K. C. High Energy Density Asymmetric Supercapacitor Based on NiOOH/Ni3S2/3D Graphene and Fe3O4/Graphene Composite Electrodes. Sci. Rep. 2014, 4, 1-10. (10) Chang, J.; Xu, H.; Sun, J.; Gao, L. High Pseudocapacitance Material Prepared via in Situ Growth of Ni(OH)2 Nanoflakes on Reduced Graphene Oxide. J. Mater. Chem. 2012, 22, 11146-11150. (11) Zhu, J. X.; Huang, L.; Xiao, Y. X.; Shen, L.; Chen, Q.; Shi, W. Z. Hydrogenated CoOx Nanowire@Ni(OH)2 Nanosheet Core–shell Nanostructures for High-performance Asymmetric Supercapacitors. Nanoscale. 2014, 6, 6772-6781. (12) Zhang, J. L.; Yang, H. J.; Shen, G. X.; Cheng, P.; Zhang, J. Y.; Guo, S. W. Reduction of Graphene Oxide via L-ascorbic Acid. Chem. Commun. 2010, 46, 1112-1114. (13) Sun, C. C.; Ma, M. Z.; Yang, J.; Zhang, Y. F.; Chen, P.; Huang, W.; Dong, X. C. Phase-controlled Synthesis of α-NiS Nanoparticles Confined in Carbon Nanorods for High Performance Supercapacitors. Sci. Rep. 2014, 4, 1-6. (14) Lerf, A.; He, H.; Forster, M.; Klinowski, J. Structure of Graphite Oxide RevisitedⅡ. J. Phys. Chem. B. 1998, 102, 4477-4482. (15) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228-240. (16) Zhang, H. T.; Zhang, X.; Zhang, D. C.; Sun, X. Z.; Lin, H.; Wang, C. H.; Ma, Y. W. One-Step Electrophoretic Deposition of Reduced Graphene Oxide and Ni(OH)2 Composite Films for Controlled Syntheses Supercapacitor Electrodes. J. Phys. Chem. B. 2013, 117, 1616-1627. (17) 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, 72-78. (18) Xiang, C. C.; Li, M.; Zhi, M. J.; Manivannan, A.; Wu, N. Q. Reduced Graphene Oxide/titanium Dioxide Composites for Supercapacitor Electrodes: Shape and Coupling Effects. J. Mater. Chem. 2012, 22, 19161-19167. (19) Xi, P. X .;Chen, F. J.; Xie, G. Q.; Ma, C.; Liu, H. Y.; Shao, C. W.; Wang, J.; Xu, Z. H.; Xu, X. M.; Zeng, Z. Z. S-14
Surfactant Free RGO/Pd Nanocomposites as Highly Active Heterogeneous Catalysts for the Hydrolytic Dehydrogenation of Ammonia Borane for Chemical Hydrogen Storage. Nanoscale. 2012, 4, 5597-5601. (20) Phong, D. T.; Sudip, K.; Batabyal.; Stevin, S.; Pramana.; James, B.; Lydia, H.; Wong.; Say, C. J. L. A Cuprous Oxide-reduced Graphene Oxide (Cu2O-rGO) Composite Photocatalyst for Hydrogen Generation: Employing rGO as an Electron Acceptor to Enhance the Photocatalytic Activity and Stability of Cu2O. Nanoscale. 2012, 4, 38753878. (21) Yu, X.; Kuaia, L.; Geng, B. Y. CeO2/rGO/Pt Sandwich Nanostructure: rGO-enhanced Electron Transmission Between Metal Oxide and Metal Nanoparticles for Anodic Methanol Oxidation of Direct Methanol Fuel Cells. Nanoscale. 2012, 4, 5738-5743. (22) Chen, F. J.; Xi, P. X.; Ma, C.; Shao, C. W.; Wang, J.; Wang, S.; Liu. G. Z.; Zeng, Z. Z. In Situ Preparation, Characterization, Magnetic and Catalytic Studies of Surfactant Free RGO/FexCo100-x Nanocomposites. Dalton Trans. 2013, 42, 7936-7942. (23) Wang, Xu.;Yan, C. Y.; Sumboja, A.;Lee, P. S. High Performance Porous Nickel Cobalt Oxide Nanowires for Asymmetric Supercapacitor. Nano Energy, 2014, 3, 119-126. (24) Trang, N. T. H.; Ngoc, H. V.; Lingappan, N.; Kang, D. J. A Comparative Study of Supercapacitive Performances of Nickel Cobalt Layered Double Hydroxides Coated on ZnO Nanostructured Arrays on Textile Fiber as electrodes for wearable energy storage devices. Nanoscale. 2014, 6, 2434-2439. (25) Wang, R. T.; Yan, X. B. Superior Asymmetric Supercapacitor Based on Ni-Co Oxide Nanosheets and Carbon Nanorods. Sci. Rep., 2014, 4, 1-9. (26) Yan, H. J.; Bai, J. W.; Wang, J.; Zhang, X. Y.; Wang, B.; Liu, Q.; Liu, L. H. Graphene Homogeneously Anchored with Ni(OH)2 Nanoparticles as Advanced Supercapacitor Electrodes. CrystEngComm. 2013, 15, 1000710015. (27) Zhou, M. L.; Chai, H.; Jia, D. Z.; Zhou, W. Y. The Glucose-assisted Synthesis of a Graphene Nanosheet-NiO Composite for High-performance Supercapacitors. New J. Chem. 2014, 38, 2320-2326. (28) Wei, Y. Y.; Chen, S. Q.; Su, D. W.; Sun, B.; Zhu, J. G.; Wang, G. X. 3D Mesoporous Hybrid NiCo2O4@Graphene Nanoarchitectures as Electrode Materials for Supercapacitors with Enhanced Performances. J. Mater. Chem. A. 2014, 2, 8103-8109. (29) Cheng, J. B.; Lu, Y.; Qiu, K. W.; Zhang, D. Y.; Wang, C. L.; Yan, H. L.; Xu, J. Y.; Zhang, Y. H.; Liu, X. M.; Luo, Y. S. Hierarchical Multi-villous Nickel–cobalt Oxide Nanocyclobenzene Arrays: Morphology Control and Electrochemical Supercapacitive Behaviors. CrystEngComm. 2014, 16, 9735-9742. (30) Xu, J.; Gai, S. L.; He, F.; Niu, N.; Gao, P.; Chen, Y. J.; Yang, P. P. Reduced Graphene Oxide/Ni 1-xCoxAllayered Double Hydroxide Composites: Preparation and High Supercapacitor Performance. Dalton Trans. 2014, 43, 11667-11675. (31) Pu, J.; Tong, Y.; Wang, S. B.; Sheng, E. H.; Wang, Z. H. Nickelecobalt Hydroxide Nanosheets Arrays on Ni Foam for Pseudocapacitor Applications. J. Power Sources. 2014, 250, 250-256. (32) Zhou, D.; Su, X. R.; Boese, M.; Wang, R. M.; Zhang, H. Z. Ni(OH)2@Co(OH)2 Hollow Nano-hexagons: Controllable Synthesis, Facet-selected Competitive Growth and Capacitance Property. Nano Energy. 2014, 5, 52-59.
S-15
Nickel Cobalt Hydroxide @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
2
Department of Materials Science and Engineering, EPIC, NUANCE Center, Northwestern University 2220 Campus Drive, Evanston, Illinois 60208, United States 3
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
S-1
Experimental Preparation of control samples The bulk Ni-Co hydroxide was synthesized as follow and denoted as Ni,Co-OH. 2mmol Ni(NO3)2·6H2O and 0.5mmol Co(NO3)2·6H2O dissolved in 50 mL deionized (DI) water was added and stirred for 1h. Subsequently, 1.25 mmol KOH was dissolved in another 10 mL DI water, and then slowly added to above solution under stirring. After 2h, the mixture was transferred to a Teflon lined autoclave of 100 mL capacity and heated at 180oC for 12h. The black mixture was collected by filtration, washed with DI water and absolute ethanol, and finally vacuum freeze dried overnight. Pure Ni-Co hydroxide nanolayer was prepared through a one pot hydrothermal route and denoted as Ni,Co-OH-LAA. Firstly, 60 mg L-ascorbic acid (LAA) was dissolved in 10 mL deionized (DI) water. After that, 2 mmol Ni(NO3)2·6H2O and 0.5 mmol Co(NO3)2·6H2O dissolved in 30 mL deionized (DI) water was added to the above solution and stirred for 1h. Subsequently, 1.25 mmol KOH dissolved in another 20 mL DI water was then slowly added into above solution and stirred for 2h. The resultant mixture was transferred to a 100 mL Teflon lined autoclave and heated at 180oC for 12h. The fluffy green solid was collected after washing with DI water and absolute ethanol, and then vacuum freeze dried overnight. The Ni-Co hydroxide/GO without adding L-ascorbic acid was synthesized as follow and denoted as Ni,Co-OH/GO. Firstly, 2mmol Ni(NO3)·6H2O and 0.5mmol Co(NO3)·6H2O dissolved in 20 mL deionized (DI) water were added to 30 mL 2 mg mL-1 GO colloid and stirred for 1h. Subsequently, 1.25 mmol KOH was dissolved in another 10 mL DI water, and then slowly added to above solution under stirring. After 2h, the mixture was transferred to a Teflon lined autoclave of 100 mL capacity and heated at 180oC for 12h. The black mixture was collected by filtration, washed with DI water and absolute ethanol, and finally vacuum freeze dried overnight. The pure rGO was prepared through a similar hydrothermal process. Firstly, 60mg LAA was added to 60 mL 1 mg mL-1 GO colloid and stirred for 3h. After that, the mixture was transferred to a Teflon lined autoclave of 100 mL capacity and heated at 180oC for 12h. The black mixture was collected by filtration, washed with DI water and absolute ethanol, and finally vacuum freeze dried overnight to form a 3D network. The Ni-Co hydroxide @ rGO composites with different Ni/Co ratios (1:0, 2:1, 1:1, and 0:1) were synthesized following similar procedure and denoted as Ni-OH/rGO, Ni2Co1-OH/rGO, Ni1Co1S-2
OH/rGO, and Co-OH/rGO, respectively. The Ni/Co ratio for Ni,Co-OH/rGO in the manuscript was 4:1, thus it is also named Ni4Co1-OH/rGO. In a typical preparation, 60 mg L-ascorbic acid (LAA) was added to 30 mL GO colloid of 2 mg mL-1. Then, desired amount of Ni(NO3)·6H2O and Co(NO3)·6H2O dissolved in 20 mL deionized (DI) water was added and stirred for 1h to form a uniform mixture. Note that, the total amount of nickel nitrate and cobalt nitrate is 2.5 mmol. 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. Preparation of hierarchical porous carbon (HPC) HPC was prepared as reported previously1. Artemia cyst shells were cleaned with deionized water, dried, and then ball-milled for 6 h at a speed of 300 rpm. The ball-milled Artemia cyst shells were heated to 300oC at Ar gas atmosphere with a heating rate of 5oC min-1 and maintained for 3 h for precarbonization, then heated up to 700oC at 5oC min-1 and kept for 4 h. The obtained products were sonicated in 67 wt% HNO3, then washed with deionized water and dried at 80oC for 12 h, the final product is named as HPC.
S-3
Supporting Figures
Figure S1 FTIR spectra of as prepared samples. The chemical structure of as-prepared samples was further characterized by FTIR (Figure S1). For all samples, common vibration bands are observed at 3641, 3440, 2917, 2857, 1624, 1384, 1100, 533, 446 cm-1. The sharp peak at 3641 cm-1 corresponds to the stretching vibration mode of nonhydrogen-bonded hydroxyl groups, which confirms the brucite structure of β-Ni,Co hydroxide phase2,3. The broad band at 3440 cm-1 can be assigned to hydrogen-bonded O-H groups stretching vibration. The band at 2917 and 2857 cm-1 are attributed to the –C-H vibration mode of –CH2. The band at 1624 cm-1 is attributed to the bending vibration of adsorbed water molecules and δO-H of hydroxyl group. The band of 1384 cm-1 is attributed to the vibration of NO3– which is the leftover from the nickel or cobalt nitrate. The band at 1100 cm-1 correspond to the C–O (alkoxy) stretching peak. The peaks of 535 and 446 cm-1 can be assigned to Ni/Co-O vibrations ascribed to Ni,Co-OH.
S-4
Figure S2 Raman spectra of Ni,Co-OH-LAA (a) and Ni,Co-OH/rGO (b) Raman spectra of Ni,Co-OH-LAA and Ni,Co-OH/rGO are tested and shown in Figure S2. The Raman peaks at 451 and 510 cm−1 can be assigned to the characteristic A1g, and A2u modes of the hexagonal brucite-like (Co,Ni)(OH)2 respectively, indicating the existence of Ni-Co hydroxide in Ni,Co-OH/rGO4-7. In the spectrum of Ni,Co-OH/rGO (Figure S2b), a D band at 1350 cm-1 and a G band at 1600 cm-1 for reduced graphite oxide are detected, corresponding to the A1g mode breathing vibrations of six member sp2 carbon rings, indicating the existence of rGO in Ni,Co-OH/rGO8,9. The Raman spectrum of Ni,Co-OH/rGO (Figure S2b) exhibits characteristics of both rGO and Ni-Co hydroxide, which further indicated the hybrid structure is composed of rGO and Ni-Co hydroxide10.
S-5
Figure S3 XPS spectra of Ni,Co-OH/rGO: survey scan (a); C 1s (b); Ni 2p (c); and Co 2p (d) regions. In order to obtain more information and the surface electronic states, Ni,Co-OH/rGO was further examined and analyzed by XPS measurements (Figure S3). The survey spectrum (0-1200 eV) mainly shows carbon (C1s), oxygen (O1s), Ni2p and Co2p speicies (Figure S3a). Detail analysis of C1s region is shown in Figure S3b. The presence of three components of carbon bond, including non-oxygenated carbon (C-C: 284.3 eV), hydroxyl carbon (C-O: 285.3 eV) and carbonyl carbon (C=O: 288.0 eV), is in good agreement with the reported data. In the Ni 2p region (Figure S3c), two main peaks at 854.6 and 872.2 eV with a spin-energy separation of 17.6 eV are assigned to Ni 2p3/2 and Ni 2p1/2, which demonstrates the characteristic of Ni2+ in Ni(OH)2. In the Co 2p region (Figure S3d), two pairs of individual peaks at 781.4 and 797.3 eV of Co 2p3/2 and Co 2p1/2, respectively, which is identified as the major binding energies of Co2+ in Co(OH)2.11-13
S-6
Figure S4 Tapping mode AFM topography images of (a) rGO sheets; (b) Ni,Co-OH-LAA deposited on a mica substrate and the distribution of z height of (a1,a2) rGO sheets; (b1,b2) Ni,Co-OH-LAA . The AFM images of pure rGO sheets and Ni,Co-OH-LAA are shown in Figure S4, with the average height of 0.7 nm and 1.04 nm, respectively. Obviously, the thickness of Ni,Co-OH/rGO (1.37 nm) is less than the sum value (1.74 nm) of rGO (0.7 nm) and Ni-Co hydroxide (1.04 nm), which could be explained by two aspects as follows. First of all, nickel and cobalt ions can adsorb or chemically interact with layered rGO functionalized by oxygen functional groups such as hydroxyls, epoxides, carboxyls, and so on.14-16 These functional groups, namely active sites, are like rivets, fixing the Ni-Co hydroxide seeds onto GO sheets and further impelling the oriented growth of Ni-Co hydroxide (Figure 5), allowing a very intimate contact between Ni,Co-OH and rGO. On the other hand, as previously reported17,18, a synergistic enhanced dispersion could be observed when coupling rGO with metal compounds. The rGO with amphiphilic property can function as a surfactant, which could improve the dispersion of the composite, forming a uniform structure free of the agglomeration of Ni-Co hydroxide. Meanwhile the existence of Ni-Co hydroxide can also prevent the rGO sheets from agglomeration19-22. Therefore, the slightly smaller thickness of Ni,Co-OH/rGO than the sum value of their pure components could probably be benefited from the enhanced dispersion of the composite and the intimate contact between the metal hydroxide and rGO. S-7
Figure S5 The energy dispersive X-ray spectroscopic (EDS) spectrum of Ni,Co-OH/rGO Table S1 EDS analysis and the element content of Ni,Co-OH/rGO. Element
C
O
Ni
Co
Weight %
14.23
33.57
41.40
10.63
Atomic %
28.40
50.30
16.91
4.32
S-8
Figure S6 Capacitive performance of Ni,Co-OH/rGO with different Ni/Co ratios (1:0, 4:1, 2:1, 1:1, and 0:1). (a) Galvanic Charge-Discharge curves at the current density of 0.5 A g-1; (b) CV curves at the scan rate of 10 mV s-1; (c) SC values calculated from discharge curves; (d) EIS plots of Ni-Co hydroxide @ rGO composites with different Ni/Co ratios. The electrochemical performance NiCo-OH/rGO composites with different Ni/Co atomic ratios are also tested (Figure S6). It can be seen, the capacitive performance of binary composites is greatly enhanced as compared to those of the single component systems (Ni-OH/rGO and Co-OH/rGO). Notably, Ni4Co1-OH/rGO (Ni,Co-OH/rGO) presents the best capacitance behavior (Figure S6), mainly attributes to the synergistic effect between nickel and cobalt as well its unique nanolayer structure which can make most atoms exposed outside with high activity. The largest integral area of CV curve (Figure S6a), the longest discharge time (Figure S6b), and the highest capacitance values (Figure S6c) all imply that 4:1 is the optimal Ni/Co ratio of the composite, as confirmed by the relatively smaller equivalent series resistance (Figure S6d).
S-9
Figure S7 Capacitive performance of Ni,Co-OH/rGO with different LAA amount (0, 20, 60, and 80 mg LAA). (a) CV curves at the scan rate of 10 mV s-1; (b) Galvanic Charge-Discharge curves at the current density of 0.5 A g-1; (c) SC values calculated from discharge curves; (d) EIS plots of Ni-Co hydroxide @ rGO composites with different LAA amounts. Note that, the amount of LAA used has a significant effect on the electrochemical performance of metal hydroxide/rGO composites. Ni,Co-OH/rGO with different LAA amount (20 mg and 80 mg) were synthesized through the same process of Ni,Co-OH/rGO and were denoted as Ni,Co-OH/rGOLAA20 and Ni,Co-OH/rGO-LAA80, respectively. Ni,Co-OH/rGO with LAA amount of 60 mg was also named as Ni,Co-OH/rGO-LAA60. It is obvious that Ni,Co-OH/rGO-LAA60 (Ni,Co-OH/rGO) presents the best capacitance behavior (Figure S7), mainly attributed to its unique single layer or few layer structure which can make most atoms exposed outside with high activity. With the participation of LAA, the electrochemical performance of the Ni-Co hydroxide @ rGO composite increased markedly (Figure S7). However, the performance of Ni,Co-OH/rGO-LAA80 is worse than Ni,CoOH/rGO-LAA60 (Ni,Co-OH/rGO), it can be explained that excess content of reductant (LAA) may cause too much defects on rGO nanosheets and decrease the electrical conductivity and poor electrochemical reversibility, thus leading to the decrease in capacitive performance for the composite. S-10
Figure S8 Schematic illustration of fabricated asymmetric supercapacitor device, based on Ni,CoOH/rGO as the positive electrode and HPC as the negative electrode.
S-11
Figure S9 (a) CV curves of Ni,Co-OH/rGO and HPC electrodes performed in three-electrode cell in 6 M KOH aqueous electrolyte at a scan of 10 mV s-1; (b) CV curves of the asymmetric supercapacitor of Ni,Co-OH/rGO//HPC measured at different potential windows at a scan rate of 5 mV s-1; (c) Galvanostatic charge/discharge curves of Ni,Co-OH/rGO//HPC measured at different potential windows at a current density of 0.1 A g-1; (d) Specific capacitances of the asymmetric supercapacitor with the increase of potential window at 0.1 A g-1; The different operation voltages of Ni,Co-OH/rGO (0~0.5V) and HPC (-1~0V) can be seen in Figure S9a. Figure S9b and S9c show the CV plots and the galvanostatic charge/discharge curves of Ni,Co-OH/rGO//HPC with different potential windows. It indicates that, with the increasing of potential window from 1 to 1.6V, more severe Faradaic pseudocapacitive behavior occurs at the Ni,Co-OH/rGO//HPC ASC. Figure S9d demonstrates the variation of specific capacitance (SC) with the increase of potential window (1~1.6 V) for the ASC, and the corresponding SC value increase from 69 to 167 F g-1. It is well known that the energy density is proportional to the square of potential, and the enlarged potential window would give rise to a remarkably enhanced energy density.
S-12
Table S2 Comparison of electrochemical performance with similar work from literature based on three-electrode system Electrode materials
SC value (F g-1)
Rate performance (%)
Cycling performance (%)
RGO/Ni(OH)210
1828 (2 A g-1)
42.7% (1 to 10 A g-1)
76.9% (1000 cycles)
nickel cobalt oxide23
1479 (1 A g-1)
53.5% (1 to 30 A g-1)
-
ZnONF/NiCoLDH24
1624 (10 A g-1)
71.4 (10 to 100 A g-1)
-
Ni-Co oxide25
1846 (1 A g-1)
62.5 (1 to 10 A g-1)
-
Ni(OH)2/G26
1985.1 (5 mA cm-2) 41.9% (5 to 50 mA cm-2)
93.5% (500 cycles)
NiO-GNC-GC27
1495 (0.5 A g-1)
22.4% (0.5 to 10 A g-1)
89.3% (1000 cycles )
NiCo2O4@graphene28
778 (1 A g-1)
48.1% (1 to 80 A g-1)
90% (10000 cycles)
Ni-Co oxide29
1545 (5 A g-1)
75.5% (5 to 20 A g-1 )
93% (5000 cycles)
rGO/NiCoAl LDH30
1902 (1 A g-1)
75% (1 to 10 A g-1)
62% (1500 cycles)
Ni-Co hydroxide31
1734 (6 A g-1)
66.1% (6 to 30 A g-1)
86% (1000 cycles)
Ni(OH)2@Co(OH)232
369 (0.3 A g-1)
-
96.4% (2000 cycles)
β-Co(OH)24
2028(5 A g-1)
-
-
This work
1691(0.5 A g-1)
82.6% (0.5 to 10 A g-1) 78.5% (0.5 to 40 A g-1)
S-13
100% (1000 cycles)
References (1) Zhao, Y. F.; Ran, W.; He, J.; Song, Y. F.; Zhang, C. M.; Xiong, D. B.; Gao, F. M.; Wu, J. S.; Xia, Y. Y. Oxygen Rich Hierarchical Porous Carbon Derived from Artemia Cyst Shells with Superior Electrochemical Performance. ACS Appl. Mater. Interfaces. 2015, 7, 1132-1139. (2) Wu, Z.; Huang, X. L.; Wang, Z. L.; Xu, J. J.; Wang, H. G.; Zhang, X. B. Electrostatic Induced Stretch Growth of Homogeneous β-Ni(OH)2 on Graphene with Enhanced High-Rate Cycling for Supercapacitors. Sci. Rep. 2014, 4, 1-8. (3) Chen, X.; Long, C. L.; Lin, C. P.; Wei, T.; Yan, J.; Jiang, L. L.; Fan, Z. J. Al and Co Co-doped αNi(OH)2/Graphene Hybrid Materials with High Electrochemical Performances for Supercapacitors. Electrochim. Acta. 2014, 137, 352-358. (4) Gao, S.; Sun, Y. F.; Lei, F. C.; Liang, L.; Liu, J. W.; Bi, W. T.; Pan, B. C.; Xie, Y. Ultrahigh Energy Density Realized by a Single-Layer β-Co(OH)2 All-Solid-State Asymmetric Supercapacitor. Chem. Int. Ed. 2014, 53, 12789 -12793. (5) Shieh, S. R.; Duffy, T. S. Raman Spectroscopy of Co(OH)2 at High Pressures: Implications for Amorphization and Hydrogen Repulsion. Phys. Rev. B. 2002, 66, 134301-134308. (6) Lutz, H. D.; Moller, H.; Schmidt, M. Infrared and Raman Study of Polyaniline. Part I: Hydrogen Bonding and Electronic Mobility in Emeraldine Salts. J. Mol. Struct. 1994, 317, 261-271. (7) Murli, C.; Sharma, S.; Kulshreshtha, S. K.; Sikka, S. K. High-pressure Behavior of β-Ni(OH)2 — A Raman Scattering Study. Physica B. 2001, 307, 111-116. (8) Cui, P.; Lee, J.; Hwang, E.; Lee, H. One-pot Reduction of Graphene Oxide at Subzero Temperatures. Chem. Commun. 2011, 47, 12370-12372. (9) Lin, T. W.; Dai, C. S.; Hung, K. C. High Energy Density Asymmetric Supercapacitor Based on NiOOH/Ni3S2/3D Graphene and Fe3O4/Graphene Composite Electrodes. Sci. Rep. 2014, 4, 1-10. (10) Chang, J.; Xu, H.; Sun, J.; Gao, L. High Pseudocapacitance Material Prepared via in Situ Growth of Ni(OH)2 Nanoflakes on Reduced Graphene Oxide. J. Mater. Chem. 2012, 22, 11146-11150. (11) Zhu, J. X.; Huang, L.; Xiao, Y. X.; Shen, L.; Chen, Q.; Shi, W. Z. Hydrogenated CoOx Nanowire@Ni(OH)2 Nanosheet Core–shell Nanostructures for High-performance Asymmetric Supercapacitors. Nanoscale. 2014, 6, 6772-6781. (12) Zhang, J. L.; Yang, H. J.; Shen, G. X.; Cheng, P.; Zhang, J. Y.; Guo, S. W. Reduction of Graphene Oxide via L-ascorbic Acid. Chem. Commun. 2010, 46, 1112-1114. (13) Sun, C. C.; Ma, M. Z.; Yang, J.; Zhang, Y. F.; Chen, P.; Huang, W.; Dong, X. C. Phase-controlled Synthesis of α-NiS Nanoparticles Confined in Carbon Nanorods for High Performance Supercapacitors. Sci. Rep. 2014, 4, 1-6. (14) Lerf, A.; He, H.; Forster, M.; Klinowski, J. Structure of Graphite Oxide RevisitedⅡ. J. Phys. Chem. B. 1998, 102, 4477-4482. (15) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228-240. (16) Zhang, H. T.; Zhang, X.; Zhang, D. C.; Sun, X. Z.; Lin, H.; Wang, C. H.; Ma, Y. W. One-Step Electrophoretic Deposition of Reduced Graphene Oxide and Ni(OH)2 Composite Films for Controlled Syntheses Supercapacitor Electrodes. J. Phys. Chem. B. 2013, 117, 1616-1627. (17) 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, 72-78. (18) Xiang, C. C.; Li, M.; Zhi, M. J.; Manivannan, A.; Wu, N. Q. Reduced Graphene Oxide/titanium Dioxide Composites for Supercapacitor Electrodes: Shape and Coupling Effects. J. Mater. Chem. 2012, 22, 19161-19167. (19) Xi, P. X .;Chen, F. J.; Xie, G. Q.; Ma, C.; Liu, H. Y.; Shao, C. W.; Wang, J.; Xu, Z. H.; Xu, X. M.; Zeng, Z. Z. S-14
Surfactant Free RGO/Pd Nanocomposites as Highly Active Heterogeneous Catalysts for the Hydrolytic Dehydrogenation of Ammonia Borane for Chemical Hydrogen Storage. Nanoscale. 2012, 4, 5597-5601. (20) Phong, D. T.; Sudip, K.; Batabyal.; Stevin, S.; Pramana.; James, B.; Lydia, H.; Wong.; Say, C. J. L. A Cuprous Oxide-reduced Graphene Oxide (Cu2O-rGO) Composite Photocatalyst for Hydrogen Generation: Employing rGO as an Electron Acceptor to Enhance the Photocatalytic Activity and Stability of Cu2O. Nanoscale. 2012, 4, 38753878. (21) Yu, X.; Kuaia, L.; Geng, B. Y. CeO2/rGO/Pt Sandwich Nanostructure: rGO-enhanced Electron Transmission Between Metal Oxide and Metal Nanoparticles for Anodic Methanol Oxidation of Direct Methanol Fuel Cells. Nanoscale. 2012, 4, 5738-5743. (22) Chen, F. J.; Xi, P. X.; Ma, C.; Shao, C. W.; Wang, J.; Wang, S.; Liu. G. Z.; Zeng, Z. Z. In Situ Preparation, Characterization, Magnetic and Catalytic Studies of Surfactant Free RGO/FexCo100-x Nanocomposites. Dalton Trans. 2013, 42, 7936-7942. (23) Wang, Xu.;Yan, C. Y.; Sumboja, A.;Lee, P. S. High Performance Porous Nickel Cobalt Oxide Nanowires for Asymmetric Supercapacitor. Nano Energy, 2014, 3, 119-126. (24) Trang, N. T. H.; Ngoc, H. V.; Lingappan, N.; Kang, D. J. A Comparative Study of Supercapacitive Performances of Nickel Cobalt Layered Double Hydroxides Coated on ZnO Nanostructured Arrays on Textile Fiber as electrodes for wearable energy storage devices. Nanoscale. 2014, 6, 2434-2439. (25) Wang, R. T.; Yan, X. B. Superior Asymmetric Supercapacitor Based on Ni-Co Oxide Nanosheets and Carbon Nanorods. Sci. Rep., 2014, 4, 1-9. (26) Yan, H. J.; Bai, J. W.; Wang, J.; Zhang, X. Y.; Wang, B.; Liu, Q.; Liu, L. H. Graphene Homogeneously Anchored with Ni(OH)2 Nanoparticles as Advanced Supercapacitor Electrodes. CrystEngComm. 2013, 15, 1000710015. (27) Zhou, M. L.; Chai, H.; Jia, D. Z.; Zhou, W. Y. The Glucose-assisted Synthesis of a Graphene Nanosheet-NiO Composite for High-performance Supercapacitors. New J. Chem. 2014, 38, 2320-2326. (28) Wei, Y. Y.; Chen, S. Q.; Su, D. W.; Sun, B.; Zhu, J. G.; Wang, G. X. 3D Mesoporous Hybrid NiCo2O4@Graphene Nanoarchitectures as Electrode Materials for Supercapacitors with Enhanced Performances. J. Mater. Chem. A. 2014, 2, 8103-8109. (29) Cheng, J. B.; Lu, Y.; Qiu, K. W.; Zhang, D. Y.; Wang, C. L.; Yan, H. L.; Xu, J. Y.; Zhang, Y. H.; Liu, X. M.; Luo, Y. S. Hierarchical Multi-villous Nickel–cobalt Oxide Nanocyclobenzene Arrays: Morphology Control and Electrochemical Supercapacitive Behaviors. CrystEngComm. 2014, 16, 9735-9742. (30) Xu, J.; Gai, S. L.; He, F.; Niu, N.; Gao, P.; Chen, Y. J.; Yang, P. P. Reduced Graphene Oxide/Ni 1-xCoxAllayered Double Hydroxide Composites: Preparation and High Supercapacitor Performance. Dalton Trans. 2014, 43, 11667-11675. (31) Pu, J.; Tong, Y.; Wang, S. B.; Sheng, E. H.; Wang, Z. H. Nickelecobalt Hydroxide Nanosheets Arrays on Ni Foam for Pseudocapacitor Applications. J. Power Sources. 2014, 250, 250-256. (32) Zhou, D.; Su, X. R.; Boese, M.; Wang, R. M.; Zhang, H. Z. Ni(OH)2@Co(OH)2 Hollow Nano-hexagons: Controllable Synthesis, Facet-selected Competitive Growth and Capacitance Property. Nano Energy. 2014, 5, 52-59.
S-15
