Highly Oxidized Graphene Anchored Ni(OH) - American Chemical

Subscriber access provided by Columbia Univ Libraries

Article 2

Highly Oxidized Graphene Anchored Ni(OH) Nanoflakes as Pseudocapacitor Materials for Ultrahigh Loading Electrode With High Areal Specific Capacitance Yongfu Tang, Yanyan Liu, Wanchun Guo, Teng Chen, Hongchao Wang, Shengxue Yu, and Faming Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5075779 • Publication Date (Web): 08 Oct 2014 Downloaded from http://pubs.acs.org on October 15, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Figure 1 1203x662mm (96 x 96 DPI)

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2 500x399mm (300 x 300 DPI)


Page 2 of 36

Page 3 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 262x217mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4 389x155mm (299 x 299 DPI)


Page 4 of 36

Page 5 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1230x1030mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6 221x158mm (150 x 150 DPI)


Page 6 of 36

Page 7 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7 500x369mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8 950x600mm (96 x 96 DPI)


Page 8 of 36

Page 9 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9 559x410mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Graphical Abstract 221x274mm (150 x 150 DPI)


Page 10 of 36

Page 11 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Highly Oxidized Graphene Anchored Ni(OH)2 Nanoflakes as Pseudocapacitor Materials for Ultrahigh Loading Electrode with High Areal Specific Capacitance Yongfu Tang *, Yanyan Liu, Wanchun Guo, Teng Chen, Hongchao Wang, Shengxue Yu, Faming Gao Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, Hebei, 066004, China

Abstract The contact between Ni(OH)2 and graphene oxide (GO) determines the specific capacitance, high-rate performance and stability of Ni(OH)2-GO composites when they were used as capacitive materials with high/ultrahigh material loading. To improve this contact, the exfoliated GO from Hummers’ method is secondly oxidized for anchoring Ni(OH)2 nanoflakes. The X-ray photoelectron spectroscopy (XPS) results reveal that the further oxidation process increases the carbonyl (C-O) groups and oxygen content on the GO surface. The Ni(OH)2-GO composites were obtained through

a

simple

hydrothermal

process.

Morphology

and

microstructure

characterizations indicate that the further oxidation of GO improves the affinity of Ni(OH)2 and GO via the increased surface groups on the GO. Due to the high conductivity and suitable structure, the Ni(OH)2 anchored on the treated GO (Ni(OH)2/TGO) exhibits good capacitive performance and high areal specific capacitance. The Ni(OH)2/TGO exhibits high specific capacitance of 1236.4 F g-1 at 1.0 mV s-1 and 1374.8 F g-1 at 0.1 A g-1, respectively, which is higher than that of Ni(OH)2 on the untreated GO. The capacitance retention of Ni(OH)2/TGO is 52.2 % even at 10 A g-1, which is higher than that of Ni(OH)2/GO (48.8 %). For the high * Corresponding author: Tel.: +86 13780351724 E-mail address: [email protected] 1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

conductivity, the specific capacitance is still 996.2 F g-1 at 1.0 A g-1 even with ultra-high material loading of 12.48 mg cm-2, which can be transferred to 12.06 F cm-2 calculated by areal specific capacitance. Furthermore, low deterioration is observed in Ni(OH)2/TGO (8.8 % loss) after 1000-cycle charge-discharge test at 1.0 A g-1, which is lower than that of Ni(OH)2/GO (19.5 % loss). The asymmetric supercapacitor, using the Ni(OH)2/TGO and activated carbon as the positive material and negative material, respectively, exhibits high energy density of 22.5 Wh kg-1 at 86.3 W kg-1 and 17.8 Wh kg-1 even at 4.05 kW kg-1.

Keywords Supercapacitor, Nickel hydroxide, Graphene oxide, Further oxidation, Hydrothermal process, Nanocomposites

2


Page 12 of 36

Page 13 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Due to increasing demand for energy, using up of fossil fuels and growing concern on environmental protection, the researches on alternative energy sources and energy storage have been attracting more attentions.1 Supercapacitors, which can bridge the energy density and power density gaps between the traditional capacitors and the secondary batteries, have been used in many fields, such as portable electronics, military device and hybrid electric vehicles.2 As the only component contributing capacitance, electrode material directly dominates the performance of supercapacitor.3 Researches on active materials and hybrid electrodes have improved the performance of supercapacitors in a large extent. Carbon materials, transition metal oxides/hydroxides and conducting polymers are widely used as the active materials for supercapacitors. Nickel hydroxide, as a promising pseudocapacitive candidate of various kinds of electrode materials, has been investigated extensively due to its high theoretical capacitance (higher than 289 mAh g-1).4-8 In our previous work, flower-like nanostructured nickel hydroxide assembled by nanoflakes, with specific capacitance of 2653.2 F g-1, was obtained by a facile hydrothermal process.6 Jiang and co-workers obtained an uniform hierarchical nanostructure Ni(OH)2, assembled by 7.4 nm ultrathin nanoflakes, which exhibited high rate capability.7 However, the poor electrical conductivity of nickel hydroxide, caused by its intrinsic semiconducting nature, results in the poor cycling stability6 and the limited active materials loading on electrode in the supercapacitor application. As we known, only the active material contributes the capacitance of the supercapacitor, while all the other components including current collector, electrolyte and separator are also need to be considered to calculate the device-level performance metrics. Therefore, the active material loading

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

is important for the energy and power density of the whole supercapacitor. However, if the electrical conductivity of active material is poor, the performance of electrode will be markedly deteriorated when the material loading increased.9 Therefore, developing the high conductive active material is an important strategy to increase the material loading with lower performance deterioration. Anchoring the Ni(OH)2 on high electrical conductive carbon materials and directly electrodepositing the Ni(OH)2 on the nickel substrate are the two main strategies to increase the conductivity of Ni(OH)2. Electrodeposition of nickel hydroxide is usually performed by electroreduction of nickel nitrate electrolyte.10, 11 The nitrate radical ion is reduced by cathodic electrochemical reaction to obtain hydroxyl ion on the surface of the electrode and to cause the in-situ precipitation of nickel hydroxide,10, 12 which constructs the excellent electrical contact between the nickel hydroxide and current collector without any binder. However, the loading of nickel hydroxide on the electrode by electrodeposition was limited. Preparing Ni(OH)2/carbon composites by anchoring the Ni(OH)2 on the high electrical conductive carbon materials (activated carbon, graphene, carbon nanotubes, and so on) is the effective strategy to obtain high-yield high electrical conductive Ni(OH)2-based active materials for high loading electrode.13-15 Graphene, possessing superior electrical conductivity, prominent electrochemical stability and good mechanical properties, as well as providing a large accessible surface area and fast passageway for the short transportation of ions,16-18 can be used as a good candidate for the support to anchoring the Ni(OH)2. The interaction/contact between nickel hydroxide and graphene plays important roles in the electronic conduct for Ni(OH)2/graphene materials due to the semiconductor characteristic of Ni(OH)2.13, 18-21 As reported by Wang et al,13, 19 the

4


Page 14 of 36

Page 15 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


particle pre-coated on graphene sheet with few oxygen-containing surface groups is prone to diffuse and recrystallize, while the particle pre-coated on the graphene oxide was anchored by the rich surface oxygen groups on the graphene oxide without recrystallization. Therefore, functionalizing the surface of graphene to enhance the interaction (containing Van der Waals force and bonding effect) between Ni(OH)2 and graphene is important for the electrical conductivity and the stability of Ni(OH)2-graphene oxide composites. In this work, the exfoliation of graphene oxide from Hummer’s method was further oxidized by HCl enhanced KMnO4 solution to increase the oxygen-containing functional groups. The further oxidized graphene oxide keeps the Ni(OH)2 nanoflakes from aggregation and anchors them uniformly dispersed on its surface. The enhanced contact between Ni(OH)2 and graphene oxide provides suitable electronic passage for charge transfer in the charge-discharge process of supercapacitor, which can possess the composite higher capacitive performance. Moreover, the enhanced hydrophilic surface functional groups are accessible to the hydrate ions, which will contribute the high rate capability of the Ni(OH)2-graphene

oxide

composites.

For

the

good

conductivity,

the

Ni(OH)2-graphene oxide composite exhibits high mass specific capacitance of 1374.8 F g-1 (calculated by total mass of composite, 2749.6 F g-1 calculated by Ni(OH)2). The specific capacitance is still 996.2 F g-1 even with high material loading of 12.48 mg cm-2, which can be transferred to 12.06 F cm-2 calculated by areal specific capacitance.

2. Experimental Section 2.1. Preparation of TGO. Graphene oxide (GO) was obtained by ultrasonic exfoliation of the Graphite oxide, which was prepared by Hummers method22 in an ultrasonic bath

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(FRQ-1006HT, 300 W, FRONT ULTRASONIC). Typically, 2.0 g graphite and 1.0 g NaNO3 were added to 46 mL concentrated H2SO4 in the ice-bath and stirred in a beaker. Then 6.0 g KMnO4 was mixed into the suspension. The beaker was placed in water-bath and was maintained the temperature 35 oC for 30 min. At the next 30 minutes, 92 ml of water was added drop by drop into the suspension, and the obtained suspension was heated to 98 oC. After 15 min, the as-prepared suspension was then further diluted to approximately 280 ml and treated by 3wt. % hydrogen peroxide. The graphite oxide was obtained after centrifugating, washing and drying. The GO was obtained by ultrasonic exfoliation of the as-prepared graphite oxide. To increase the surface functional groups, the GO was treated by further oxidization as follow. 10 mg of GO and 50 mg KMnO4 were mixed by ultrasonic bath in 10 ml of deioned water for 2 h to obtain a homogeneous dispersion. Then 1 ml HCl and 1 ml H2O2 were added into drop by drop to remove the residual KMnO4 and manganese oxides. The final product, denoted as TGO, was washed by deioned water and ethanol several times. 2.2. Preparation of Ni(OH)2/TGO and Ni(OH)2/GO Composites. Treated graphene oxide-Ni(OH)2 composite was obtained by a facile hydrothermal process. The as-prepared TGO and Ni(NO3)2 solution with concentration of 85 mM were mixed with the TGO/Ni mass ratio of 1:1 and stirred in a beaker. Then 2 M aqueous ammonia was dropped slowly into the above solution until pH value up to 10 and reacted for 2 h under stirring. Subsequently, the obtained approximate 50 ml mixture was put into a 100 mL autoclave and reacted at 120 oC for 4 h. The product, denoted as Ni(OH)2/TGO, was obtained after centrifuging, washing, drying in vacuum at 60 oC for 24 h. For comparison, the another sample with the

6


Page 16 of 36

Page 17 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


untreated GO instead of the TGO was prepared by the same process, which was denoted as Ni(OH)2/GO. 2.3. Fabrication of electrode. The preparation of electrode was similar to our previous work.6, 32 The active material, electric carbon black and polyvinylidene difluoride (PTFE) were mixed in the ethanol with the mass ratio of 80:15:5 by ultrasonic dispersion for 20 min. The slurry was spread on the pre-cleaned Ni foam and dried in vacuum at 80 oC for 12 h. Finally, the active material coated Ni foam was pressed at 10 M Pa to obtain the working electrode. 2.4. Materials characterization and electrochemical measurements. Typical X-ray diffraction (XRD) patterns were recorded using a Bruker AXS D8 diffractometer with Cu K Rradiation (λ) 0.15418 nm) generated at 40 kV and 30 mA. The Fourier Transform Infrared Spectroscopy (FTIR) measurements of the samples were characterized in KBr pellets by reflectance mode with Nicolet 380 FT-IR (Thermo Fisher Scientific Corp. USA). The surface species and their chemical states were analyzed by X-ray photoelectron spectroscopy (XPS) measurements. Raman spectra of the GO and TGO were measured by a Ramanstation 400 F. The morphology and microstructure of the composites were characterized by transmission electron microscope (TEM) with a HT 7700 model operating at 100 kV and 10 µA and

filed

emission

scanning

electron

microscope

(FESEM)

analysis

by

KYKY-2800B. Cyclic voltammetry (CV) measurements, galvanostatic charge-discharge (GC) measurements and electrochemical impedance spectroscopy (EIS) of the electrodes with active materials were performed on CHI 660A workstation and Land CT 2001A,

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

respectively, in 6.0 M KOH solution. Hg/HgO electrode and Pt foil (1 x 1 cm2) were used as reference electrode and counter electrode, respectively. To investigate the practical application potential, the Ni(OH)2/TGO was used as positive material to assemble the asymmetric supercapacitor with activated carbon (KetjenBlack EC 600J) as the negative material, which was denoted as TGN//AC supercapacitor. The positive and negative mass loadings were 4.6 and 8.8 mg cm-2, respectively. The electrochemical evaluation of the as-prepared TGN//AC supercapacitor was performed by CV and charge-discharge measurements.

3. Results and Discussion The structure, morphology and surface functional groups of GO and TGO, which determines the properties of the Ni(OH)2/GO and Ni(OH)2/TGO composites, are investigated by physiochemical characterization firstly. Fig. 1 shows the typical TEM images and FESEM images of the raw nature graphite (Fig. 1a), exfoliation graphene oxide (GO) obtained by the Hummers’ method (Fig. 1b and 1d) and further oxidized graphene oxide (TGO, Fig. 1c and 1e). As shown in Fig. 1a, the pristine graphite is stacked by many graphite sheets with high stacking density. Thick and large graphene oxide sheets were observed in Fig. 1b, indicating that the graphene oxide is successfully obtained by ultrasonic exfoliation of graphite oxide from Hummers’ method. After further oxidation by KMnO4 and further ultrasonic exfoliation, the graphene oxide sheets become obviously smaller and less layers (Fig. 1c). The thinner and smaller graphene oxide sheets in TGO will be more suitable for the anchoring of the nanoparticles or nanoflakes. This result is confirmed by the FESEM images of GO and TGO. The larger graphene oxides with smooth surface are observed in GO sample (Fig. 1d). After further oxidation, the graphene oxide sheets become smaller

8


Page 18 of 36

Page 19 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Fig. 1e). The pleat-like surface is caused by the less layered structure of TGO sample. The surface atomic concentration and functional groups of the as-obtained GO and TGO were analyzed by XPS and shown in Fig. 2. As shown in the C1s spectrum of raw graphite (Fig. 2a), just one peak located at 282.2 eV, which is ascribed to the ring C-C, is observed. After oxidizing treatment, two major peaks, being attributed to the non-oxygenated C (C-C, lower binding energy) and carbonyl carbon (C-O, higher binding energy),20, 23-24 respectively, are observed in the XPS patterns of C 1s for both GO (Fig. 2b) and TGO (Fig. 2c). The peaks of GO are centered at 281.5 and 283.9 eV, while those of TGO are increased to 282.9 and 285.1 eV, respectively. The increased banding energy of the non-oxygenated C-C and the carbonyl carbon C-O after further oxidation treatment should be ascribed to the increased oxidation degree of the ambient functional groups. Furthermore, the further oxide process has increased the content of the carbonyl carbon (C-O) from 36.6 % to 59.4 %, also indicating the higher oxidation degree of TGO. The change of the surface oxygen contents resulted from the Hummers’ method process and the further oxidation process are shown in Table 1, which were calculated from the XPS full spectrum. After the first oxide process by Hummers’ method, the oxygen content is markedly increased from 3.2 % to 28.3 %, indicating the high oxidation degree of GO. Nevertheless, the GO from Hummers’ method can be further oxidized through the second oxidation process by KMnO4, corresponding to the slight increment of oxygen content from 28.3 % to 29.7 %. This results are in agreement with the enhancement of carbonyl oxygen (C-O) discussed in Figs. 2b and 2c. The XRD patterns of raw graphite, GO and TGO are given in Fig. 2d. As shown in the (a) pattern for raw graphite, peak at 26.4o is ascribed to the (002) facet of graphite. After oxidation and exfoliation by Hummer’s

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

method, diffraction peaks located at 25.4o, 19.2o and 9.8o, corresponding to the graphite carbon, the oxidized graphite and the stacked graphene oxide, respectively, are observed in the GO sample patterns, demonstrating the coexist of these three phases in the GO sample.24 The diffraction peak at 25.4o and 19.2o disappears in the TGO sample, which means the full disassembly of graphite structure and the enhanced dispersibility of GO sheets after being further oxidized by KMnO4 and exfoliated by ultrasonic process. The outstanding dispersion is advantageous to the composite of graphene oxide and nickel hydroxide. The positive shift of the peak around 10o for TGO should be attributed to the decrease of interlayer distance

24

due

to the increased surface functional groups from further oxidation. The peaks round 43o of the two samples may be ascribed to the residual manganese oxide from the reduction of KMnO4. Table 1 XPS data of raw graphite, GO and TGO Graphite

GO

TGO

Peak BE(ev)

At. %

Peak BE(ev)

At. %

Peak BE(ev)

At. %

C 1s

284.27

96.8

281.84

71.7

284.8

70.3

O 1s

531.99

3.2

529.52

28.3

530.37

29.7

To further investigate the structures of GO and TGO, the Raman spectra were carried out and shown in Fig. 3. For both GO and TGO, the broad D band and G band are observed at 1354 cm-1 and 1594 cm-1, respectively. As reported,25, 26 the G band indicates the C sp2 atoms vibration corresponding to E2g phonons, while the D band is assigned to the A1g phonons of breathing vibration. The higher intensities of D band (ID/IG > 1) in both GO and TGO should be attributed to their defects and partially disordered crystal structure. Furthermore, the ratio of D and G band intensities (ID/IG)

10


Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in TGO sample (ID/IG = 1.08) is slightly higher than that in GO (ID/IG =1.01), which indicates the more defects in TGO sample than GO sample. The XRD patterns of Ni(OH)2/GO and Ni(OH)2/TGO composites are given in Fig. 4a. The obvious diffraction peaks round 19.8°, 33.1°, 38.56°, 52.0°, 59.1°, 62.9° and 70.1°, being assigned to the (002), (100), (101), (102), (110), (111) and (201) facets of β-phase nickel hydroxide (JCPDS 14-0117), respectively, are observed in both Ni(OH)2/GO and Ni(OH)2/TGO samples. The peaks located at high angles (72.9° and 82.7°) can be assigned to the high index facets of β-phase nickel hydroxide. The absence of peaks around 10o, corresponding to the diffraction pattern of graphene oxides, demonstrates the formation of Ni(OH)2/GO and Ni(OH)2/TGO composites. The similar XRD patterns illustrate that the similar structured Ni(OH)2 flakes were obtained in the as-synthesized Ni(OH)2/GO and Ni(OH)2/TGO. However, the broader peaks of Ni(OH)2/TGO demonstrate its smaller grain size than that of Ni(OH)2/GO according to Scherrer formula.27 The calculated grain sizes of Ni(OH)2/GO and Ni(OH)2/TGO for different facets are shown in Table 2. Obviously, the remarkable difference in the grain sizes calculated via different facets indicates the flake-like structure of Ni(OH)2 in Ni(OH)2/TGO and Ni(OH)2/GO composites, which can be confirmed in TEM images. The breadth of Ni(OH)2 flakes in the Ni(OH)2/GO and Ni(OH)2/TGO samples, corresponding to the largest grain size’s facets, are 30.1 and 21.2 nm, respectively. Their thicknesses are 9.6 and 9.5 nm, respectively. The further oxidation on graphene oxide reduce the sizes of Ni(OH)2 in breadth rather than in thickness. The decrease of Ni(OH)2 crystallite size should be ascribed to the increased carbonyl (C-O) groups in TGO, which prevents the aggregation of nickel hydroxide and improves the dispersibility of Ni(OH)2 on the surface of graphene oxide. The similar FTIR spectrum (Fig. 4b) further confirms the similar structure of Ni(OH)2 in

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

the Ni(OH)2/GO and Ni(OH)2/TGO composites. The broad peak from 3400 to 3640 cm-1 in both Ni(OH)2/GO and Ni(OH)2/TGO should be assigned to the -OH vibration and the intercalated water molecules in composites.28 The band at 1630 cm-1 is corresponding to vibrations of graphene oxide (C-O) or Ni-O bond of Ni(OH)2. Moreover, the narrow and sharp band at 1380 cm-1 corresponds to the interlayer nitrate anion. The band at the range of 490-510 cm-1 is ascribed to the νNi-OH vibrations.20 Table 2 Crystalline sizes of Ni(OH)2/GO and Ni(OH)2/TGO composites calculated by different facets Crystal Facets

(002)

(100)

(101)

(102)

(110)

(111)

9.6

30.1

13.9

11.4

24.9

17.2

9.5

21.2

8.6

10.4

15.7

10.7

Crystalline Size of Ni(OH)2/GO (nm) Crystalline Size of Ni(OH)2/TGO (nm)

The morphology and microstructures of the as-prepared Ni(OH)2/GO (Figs. 5a and 5b) and Ni(OH)2/TGO (Figs. 5c and 5d) composites are characterized by TEM and FE-SEM images. As shown, the Ni(OH)2 nanoflakes are anchored on the graphene oxide sheets, forming a hierarchical network-like structure. In the Ni(OH)2/GO, the nickel hydroxide flakes are inclined to agglomerate together and “stand” on the graphene oxide sheets. The enrichment of surface functional groups in the TGO makes Ni(OH)2 nanoflakes uniformly dispersed and “recumbed” on the graphene oxides. Clearly, the more surface functional groups C-O in TGO can anchor Ni(OH)2 flakes more closer and prevent them from agglomeration, which made the growth of nickel hydroxide nanoflakes along with TGO surface direction. This microstructure increases the contact area between the Ni(OH)2 nanoflakes and

12


Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


graphene oxide sheets, as well as provides a convenient passageway for electrical conduction, which is suitable for the high performance supercapacitor. A possible mechanism, which demonstrates the effects of graphene oxide surface functional groups on the composite processes for Ni(OH)2 and graphene oxide, is given in Fig. 6. The synthetic processes of Ni(OH)2/GO and Ni(OH)2/TGO composites are proposed as follows: Firstly, the Ni2+ ions adsorb on the graphene oxide sheets via electrostatic attraction when the nickel nitrate solution was added in graphene oxide suspension. When the aqueous solution of ammonia was added drop by drop slowly, Ni2+ ions combined with NH3 to form the [Ni(NH3)n]2+ ion complexs (Eq.1). With the increase of ammonia, the hydrolyzation of ammonia augmented to form Ni(OH)2 precipitate (Eqs. 2 and 3). When the reaction time was extended at hydrothermal condition, the particle size of Ni(OH)2 grew larger. The growth of Ni(OH)2 flakes stacked facilely and could not be restricted in the Ni(OH)2/GO composites due to the poor dispersibility of GO and the aggregation tendency of Ni(OH)2 flakes. On the contrary, after further oxidizing, the carbonyl carbon (C-O) groups markedly increase on the surface and the edges of the TGO. These surface functinonal groups directionally guide the Ni(OH)2 nanoflakes to grow along with the graphene oxide owing to the forces of Ni(OH)2 and carbonyl of TGO.

Ni 2+ + nNH 3 ↔ [ Ni( NH 3 ) n ]2+ +

(1)

H 2 O + NH 3 ↔ NH 4 + OH −

(2)

Ni 2+ + 2OH − ↔ Ni(OH) 2

(3)

To understand the electrochemical performances of Ni(OH)2/GO and Ni(OH)2/TGO electrodes for supercapacitor, CV and GC measurements are curried out. The CV curves at 1 mV s-1 are shown in Fig. 7a. Obvious redox peaks, which demonstrates the capacitive characteristics of typical pseudocapacitance, are present 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 36

in the CV curves of both Ni(OH)2/GO and Ni(OH)2/TGO samples. The anodic peaks are located around 0.5 V and the cathodic peaks are located around 0.25 V, corresponding to the electrochemical oxidation of Ni(OH)2 to NiOOH and the reverse reduction process from NiOOH to Ni(OH)2, respectively. This process is expressed by Eq. 4.20-21

Ni(OH) 2 + OH − ↔ NiOOH + H 2 O + e −

(4)

The good symmetry of current peaks indicates a rapid current-potential response and good reversibility of the composite, which can contributes high rate performance of nickel hydroxides. The Ni(OH)2/TGO composite exhibits a bigger CV curve area the than the Ni(OH)2/GO composite, which signifies that the further oxidation for graphene oxide is beneficial for the improvement of electrochemical properties and the enhancement of specific capacitance. The specific capacitance (C), calculated via CV curve, is given by Eq. 5, where I(V) is the current density, m is the mass of active material, v representes the scan rate and (V-V0) is the potential window.29 The mass loading of Ni(OH)2/GO and Ni(OH)2/TGO in the working electrode are approximately 5.68 and 6.16 mg cm-2, respectively, which are considered as the high mass loading for supercapacitor electrodes.30 V

C=

∫

V0

I(V)dV

mν(V − V0 )

(5)

The specific capacitances of Ni(OH)2/GO and Ni(OH)2/TGO, calculated according to the CV curves by Eq. 5, are 1236.4 F g-1 and 981.8 F g-1 at 1.0 mV s-1, respectively. Obvious inclined platforms rather than sloped line are shown in the discharge curves of both Ni(OH)2/GO and Ni(OH)2/TGO composites at the current density of 1.0 A g-1 (inlet of Fig.6a), indicating their pseudocapacitive characteristics. The calculation of mean specific capacitances (C) is shown in Eq. 6, where I, m, dt 14


Page 25 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and ΔV are the current density, active material mass, differential of time and potential range, respectively.

C = ∫ Idt (m ⋅ ∆V )

(6)

The performance curves of Ni(OH)2/GO and Ni(OH)2/TGO samples at various current densities are given in Fig. 7b. At any current density, the specific capacitance of Ni(OH)2/TGO is higher than that of Ni(OH)2/GO. It indicates that the further oxidation process on the graphene oxide, which enhanced the adsorption of graphene oxide and nickel hydroxides in the composite by increasing the content of the carbonyl carbon (C-O), can markedly increase the specific capacitance. The specific capacitances of the Ni(OH)2/TGO composite are 1374.8, 1324.5, 1229.0, 1151.3, 1082.7, 935.4, and 718.2 F g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g-1, respectively. The high capacitance retention (52.2 %) at even 10.0 A g-1 demonstrates the good high-rate performance of Ni(OH)2/TGO. Since the ratio of TGO/Ni(OH)2 is 1:1, the specific capacitance calculated only by Ni(OH)2 will be 2749.6 F g-1. For comparison, the specific capacitance of Ni(OH)2/GO at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g-1 are 1164.2, 1079.3, 1011.6, 940.2, 839.3, 685.9 and 568.8 F g-1, respectively. The capacitance retention at 10.0 A g-1 is 48.8 %, which is lower than that of Ni(OH)2/TGO. The Ni(OH)2/TGO electrode reveals higher specific capacitance and higher rate capability than Ni(OH)2/GO electrode, which should be attributed to the better dispersion of Ni(OH)2 on the TGO than that on the GO resulting in the excellent electronic contact. The cycle life test of Ni(OH)2/GO and Ni(OH)2/TGO electrodes are performed during 1000 cycles by repeating the galvanostatic charge-discharge test at 1.0 A g-1, which is shown in Fig. 7c. The capacitance retention of Ni(OH)2/GO after 1000 cycles is 80.5 %, while that of Ni(OH)2/TGO is 91.2 %. Both the two composites exhibit higher cycling stability than the pure 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni(OH)2 nanoflakes reported in our previous work (80.6 % retention after 600 cycles).6 It is noted that the specific capacitance of Ni(OH)2/TGO electrode increases slightly in the first 600 cycles. This may be assigned to the surface wetting of the active material in the initial cycles, which can increase the active material utilization. Furthermore, the EIS spectra of the two electrodes were executed in the frequency from 1.0 Hz to 1×105 Hz to investigate the electrochemical properties of the two electrodes. As shown in Fig. 7d, both the two EIS spectra contain two parts: a semicircle and a straight line. The contact resistance, which mainly is from the resistance of the active material, current collector and their contacts, is demonstrated in the intersection of the EIS plots and the real axis of high frequencies. The semicircle in EIS spectrum is attributed to the charge-transfer resistance from the interface of the electrode to electrolyte.31-32 Obviously, both the ohmic resistance and the charge transfer resistance of the Ni(OH)2/TGO composite electrode are lower than Ni(OH)2/GO composite electrode, which demonstrates that the Ni(OH)2/TGO composite possesses higher electronic conductivity and more suitable structure for the charge transfer. This is consistent with the higher specific capacitance and the good high-rate performance of the Ni(OH)2/TGO composite. In summary, the higher performance of Ni(OH)2/TGO than Ni(OH)2/GO can be mainly ascribed to the reasons as follow, which are demonstrated in the Fig. 8. Firstly, the more hydrophilic surface functional groups, which are demonstrated by the XPS data on the TGO, provide easy access for the hydroxyl ions. The more adsorbed hydroxide ions offer abundant reactant for the charge process as Eq. 4. Secondly, the “recumbent” Ni(OH)2 nanoflakes in the Ni(OH)2/TGO (Fig. 8A) possess more contact with the graphene oxide substrate than the standing Ni(OH)2 nanoflakes in the Ni(OH)2/GO (Fig. 8B), which makes easy for the electron transfer from Ni(OH)2 to

16


Page 26 of 36

Page 27 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


graphene oxide. This result has been clarified by the TEM and FESEM images. These two reasons contribute the high specific capacitance and rate-capacity. Furthermore, the good dispersion of Ni(OH)2 on the graphene oxide for Ni(OH)2/TGO electrode can effectively prevent the nickel hydroxide from being dissolved into electrolyte by graphene wrapping, which prohibits the capacitance loss during the charge-discharge cycling. Table 3 Mass specific capacitance and areal specific capacitance of working electrodes with different mass loading of active material Mass loading / mg cm-2

1.52

3.04

6.16

6.48

12.48

1746.7

1392.0

1151.3

1141.6

966.2

2.65

4.32

7.09

7.40

12.06

Mass specific capacitance / F g-1 Areal specific capacitance / F cm-2

To investigate the effect of active material loading on the performance of the electrode, the specific capacitance and areal specific capacitance of working electrodes with various mass loading are demonstrated in Table 3. The mass specific capacitance reaches 1746.7 F g-1 when the mass loading is 1.52 mg cm-1. Although the mass specific capacitance decreases with the increase of the loading per area, the areal specific capacitance is increased continuously. The mass specific capacitance is still 996.2 F g-1 even with ultrahigh material loading of 12.48 mg cm-2. The areal specific capacitance in this laoding can reach up to 12.06 F cm-2, which is much higher than the previously reported CoxNi1−x(OH)2/NiCo2O4 (2.3 F cm-2),33 CoxNi1−x(OH)2/NiCo2S4 (2.86 F cm-2),34 MnO2/NiCo2O4 (2.01 F cm-2),35 NiO/Co3O4 (1.35 F cm-2)36 electrodes, and so on. Due to active materials are the only component contributing to the capacitance, the high active material loading with high areal

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

specific capacitance will be meaning for the improvement of the energy density of the whole supercapacitor. To develop the application of Ni(OH)2/TGO in the practical supercapacitor, an asymmetric cell is assembled with activated carbon (KetjenBlack EC 600J) as the negative material, which is denoted as TGN//AC supercapacitor. CV and GC measurements are carried out to investigate the TGN//AC supercapacitor. Fig. 9a presents the CV curves. Redox peaks, representing pseudo-capacitive characteristic, are present in the CV curves. The CV curve in high scan rate (100 mV s-1) retains the shape of those in low scan rates (< 10 mV s-1), indicating the high rate-capacity of TGN//AC supercapacitor. The GC curves of TGN//AC supercapacitor are shown in Fig. 9b. The sloped voltage plateaus, rather than lines, confirm the pseudo-capacitive behavior of the TGN//AC supercapacitor. Good symmetry of these curves indicates the good reversibility of the redox reactions in the supercapacitor. The as-prepared asymmetric supercapacitor shows higher cell voltage (1.6 V) than the AC-based aqueous symmetric supercapacitors (~ 1 V). Fig. 9c presents the specific capacitances calculated by the GC curves in Fig. 9b. The specific capacitances at 1.3, 2.7, 6.7, 13.4, 26.9 and 67.2 mA, corresponding to the current densities of 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 A g-1 calculated by total mass containing the positive material and the negative material, are 56.5, 52.2, 47.3, 46.8, 46.5 and 45.0 F g-1, respectively. This result is higher than those of carbon based (AC//AC, CNT//CNT, etc) symmetric supercapacitors,37-38 MnO2//carbon

material

asymmetric supercapacitor39 and

NiO//AC asymmetric supercapacitor.40 The specific capacitance at 67.2 mA retains 79.6 % of that at 1.3 mA, which indicates the high-rate capacity of the TGN//AC supercapacitor. The Ragone plots of the as-prepared TGN//AC supercapacitor are shown in Fig. 9d. The energy density is 22.5 Wh kg-1 at 86.3 W kg-1 and 17.8 Wh kg-1

18


Page 28 of 36

Page 29 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


even at 4.05 kW kg-1, which is higher than those of carbon materials based symmetric supercapacitor (< 10 Wh kg-1)37-38 and comparable to those of MnO2//AC (~ 20 Wh kg-1)39 and NiO//AC (~ 20 Wh kg-1)40 asymmetric supercapacitor. As known, the energy density is calculated by E = 1/2 CV2, where E, C and V are the energy density, specific capacitance and voltage of supercapacitor, respectively. The high energy density of the as-prepared TGN//AC supercapacitor should be assigned to the high specific capacitance of Ni(OH)2/TGO (1374.8 F g-1) and the high cell voltage (1.6 V). The high power density (4.05 kW kg-1) confirms the high-rate capacity of the TGN//AC supercapacitor. The superior properties of the TGN//AC supercapacitor boosts its application potential in energy storage devices and electric vehicles.

Conclusions In this paper, nickel hydroxide nanoflakes anchored on the surface of highly oxidized graphene sheets are synthesized successfully via a facile hydrothermal method. The physicochemical characterizations demonstrate that the graphene oxide with rich surface functional groups can be obtained by the further oxidation on graphene oxide from Hummers’ method. Furthermore, the second oxidation process enhances the carbonyl carbon (C-O) content on the surface of highly oxidized graphene oxide, which is significant for the electronic contact between Ni(OH)2 and graphene oxide as well as the microstructure of Ni(OH)2-graphene oxide composite. Due to that the further oxidation on graphene oxide can improve the Ni(OH)2 dispersion on graphene oxide and their electronic contact with each other, the highly oxidized graphene oxide supported Ni(OH)2 (denoted as Ni(OH)2/TGO) exhibits markedly higher specific capacitance (1374.8 F g-1 at 0.1 A g-1) than that of graphene oxide supported Ni(OH)2 (denoted as Ni(OH)2/GO, 1164.2 F g-1 at 0.1 A g-1). Moreover, the Ni(OH)2/TGO exhibits better rate performance and cycle life than

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni(OH)2/GO. Due to the high conductivity, Ni(OH)2/TGO composite exhibits the ultrahigh areal specific capacitance of 12.06 F cm-2 with the high active material loading of 12.48 mg cm-2. The asymmetric supercapacitor, assembled with the as-prepared Ni(OH)2/TGO as the positive material and the activated carbon as the negative material, exhibits energy density of 22.5 Wh kg-1 at 86.3 W kg-1 and 17.8 Wh kg-1 even at 4.05 kW kg-1. The good properties of the as-synthesized Ni(OH)2/TGO composite and TGN//AC supercapacitor will promote their application potential in advanced energy storage.

Author Information Corresponding information *Phone: +86 13780351724.

E-mail address: [email protected].

Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by China Postdoctoral Science Foundation (No. 2012M520597), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20131333120011), Key Technology Research and Development Program of Qinhuangdao (No. 2012021A072) and Young Independent Project of Yanshan University (No. 13LGA015). Furthermore, this project was supported by State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology) (No. 2013-KF-11).

References (1) Jin, Y.; Chen, H.; Chen, M.; Liu, N.; Li, Q. Graphene-Patched CNT/MnO2 Nanocomposite Papers for The Electrode of High-Performance Flexible Asymmetric Supercapacitors. ACS Appl. Mater. Interfaces 2013, 5, 3408-3416. 20


Page 30 of 36

Page 31 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2) Fang, D. L.; Chen, Z. D; Liu, X.; Wu, Z. F.; Zheng, C. H. Homogeneous Growth of Nano-sized β-Ni(OH)2 on Reduced Graphene Oxide for High-Performance Supercapacitors. Electrochim. Acta 2012, 81, 321-329. (3) Pognon, G.; Cougnon, C.; Mayilukila, D.; Bélanger, D. Catechol-Modified Activated Carbon Prepared by The Diazonium Chemistry for Application as Active Electrode Material in Electrochemical Capacitor. ACS Appl. Mater. Interfaces 2012, 4, 3788-3796. (4) Yang, D.; Wang, R.; He, M.; Zhang, J.; Liu, Z. Ribbon- and Boardlike Nanostructures of Nickel Hydroxide: Synthesis, Characterization and Electrochemical Properties. J. Phys. Chem. B 2005, 109, 7654-7658. (5) Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H. Ni(OH)2 Nanoplates Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials. J. Am. Chem. Soc. 2010, 132, 7472-7477. (6) Tang, Y.; Liu, Y.; Yu, S.; Zhao, Y.; Mu, S.; Gao, F. Hydrothermal Synthesis of A Flower-Like Nano-Nickel Hydroxide for High Performance Supercapacitors. Electrochim. Acta 2014, 123, 158-166. (7) Jiang, H.; Zhao, T.; Li, C.; Ma, J. Hierarchical Self-Assembly of Ultrathin Nickel Hydroxide Nanoflakes for High-Performance Supercapacitors. J. Mater. Chem. 2011, 21, 3818-3823. (8) Xing, S.; Wang, Q.; Ma, Z.; Wu, Y.; Gao, Y. Synthesis of Mesoporous α-Ni(OH)2 for High-Performance Supercapacitors. Mater. Lett. 2012, 78, 99-101. (9) Cheng, Y.; Zhang, H.; Varanasiac, C. V.; Liu, J. Improving The Performance of Cobalt-Nickel Hydroxidebased Self-Supporting Electrodes for Supercapacitors Using Accumulative Approaches. Energy Environ. Sci. 2013, 6, 3314-3321.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(10) Jayashree, R.S.; Kamath, P. V. Nickel Hydroxide Electrodeposition from Nickel Nitrate Solutions: Mechanistic Studies. J. Power Sources 2001, 93, 273-278. (11) Hu, C. C.; Chen, J. C.; Chang, K. H. Cathodic Deposition of Ni(OH)2 and Co(OH)2 for Asymmetric Supercapacitors: Importance of The Electrochemical Reversibility of Redox Couples. J. Power Sources 2013, 221, 128-133. (12) Kulkarni, S. B.; Jagadale, A. D.; Kumbhar, V. S.; Bulakhe, R. N.; Joshi, S. S.; Lokhande, C. D. Potentiodynamic Deposition of Composition Influenced Co1-xNix LDHs Thin Film Electrode for Redox Supercapacitors. Int. J. Hydrogen Energy 2013, 38, 4046-4053. (13) Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H. Ni(OH)2 Nanoplates Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials. J. Am. Chem. Soc. 2010, 132, 7472-7477. (14) Dubal, D. P.; Gund, G. S.; Lokhande, C. D.; Holze, R. Decoration of Spongelike Ni(OH)2 Nanoparticles onto MWCNTs Using an Easily Manipulated Chemical Protocol for Supercapacitors. ACS Appl. Mater. Interfaces 2013, 5, 2446-2454. (15) Wang, H.; Liang, Y.; Mirfakhrai, T.; Chen, Z.; Casalongue, H. S.; Dai, H. Advanced Asymmetrical Supercapacitors Based on Graphene Hybrid Materials. Nano Res. 2011, 4, 729-736. (16) Zhu, J.; Chen, S.; Zhou, H.; Wang, X. Fabrication of A Low Defect Density Graphene-Nickel Hydroxide Nanosheet Hybrid with Enhanced Electrochemical Performance. Nano Res. 2012, 5, 11-19. (17) Lee, J. W.; Hall, A. S.; Kim, J. D.; Mallouk, T. E. A Facile and Template-Free Hydrothermal Synthesis of Mn3O4 Nanorods on Graphene Sheets for Supercapacitor Electrodes with Long Cycle Stability. Chem. Mater. 2012, 24, 1158-1164.

22


Page 32 of 36

Page 33 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Perera, S. D.; Mariano, R. G.; Nijem, N.; Chabal, Y.; Ferraris, J. P.; Balkus, J. K. J. Alkaline Deoxygenated Graphene Oxide for Supercapacitor Applications: An Effective Green Alternative for Chemically Reduced Graphene. J. Power Sources 2012; 215, 1-10. (19) Wang, H.; Robinson, J. T.; Diankov, G.; Dai, H. Nanocrystal Growth on Graphene with Various Degrees of Oxidation. J. Am. Chem. Soc. 2010, 132, 3270-3271. (20) Yang, S.; Wu, X.; Chen, C.; Dong, H.; Hu, W.; Wang, X. Spherical α-Ni(OH)2 Nanoarchitecture Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials. Chem. Comm. 2012, 48, 2773-2775. (21) Sun, Z.; Lu, X. A Solid-State Reaction Route to Anchoring Ni(OH)2 Nanoparticles on Reduced Graphene Oxide Sheets for Supercapacitors. Ind. Eng. Chem. Res. 2012, 51, 9973-9979. (22) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339-1339. (23) Yan, J.; Wei, T.; Shao, B.; Ma, F.; Fan, Z.; Zhang, M.; Zheng, C.; Shang, Y.; Qian, W.; Wei, F. Electrochemical Properties of Graphene Nanosheet/Carbon Black Composites as Electrodes for Supercapacitors. Carbon 2010, 48, 1731-1737. (24) Hu, J.; Kang, Z.; Li, F.; Huang, X. Graphene with Three-Dimensional Architecture for High Performance Supercapacitor. Carbon 2014, 67, 221-229. (25) Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys Rev B 2000, 61, 14095-14107. (26) Cancado, L. G.; Pimenta, M. A.; Neves B. R. A.; Dantas M. S. S.; Jorio, A. Influence of The Atomic Structure on The Raman Spectra of Graphite Edges. Phys Rev Lett 2004;93(24):247401.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

(27) Patterson, A. L. The Scherrer Formula for X-Ray Particle Size Determination. Phys. Rev. 1939, 56, 978-982. (28) Hu, Z. A.; Xie, Y. L.; Wang, Y. X.; Wu, H. Y.; Yang, Y. Y.; Zhang, Z. Y. Synthesis and Electrochemical Characterization of Mesoporous CoxNi1-x Layered Double Hydroxides as Electrode Materials for Supercapacitors. Electrochim. Acta 2009, 54, 2737-2741. (29) Bai, Y.; Rakhi, R. B.; Chen, W.; Alshareef, H. N., Effect of pH-induced Chemical

Modification

of

Hydrothermally

Reduced

Graphene

Oxide

on

Supercapacitor Performance. J. Power Sources 2013, 233, 313-319. (30) Chen, H.; Jiang, J.; Zhang, L.; Xia, D.; Zhao, Y.; Guo, D.; Qi, T.; Wan H.; In Situ Growth of NiCo2S4 Nanotube Arrays on Ni Foam for Supercapacitors: Maximizing Utilization Efficiency at High Mass Loading to Achieve Ultrahigh Areal Pseudocapacitance. J. Power Sources 2014, 254, 249-257. (31) Xia, X.; Tu, J.; Mai, Y.; Chen, R.; Wang, X.; Gu, C.; Zhao, X. Graphene Sheet/Porous NiO Hybrid Film for Supercapacitor Applications. Chem. Eur. J. 2011, 17, 10898-10905. (32) Tang, Y.; Liu, Y.; Yu, S.; Mu, S.; Xiao, S.; Zhao, Y.; Gao, F. Morphology Controlled Synthesis of Monodisperse Cobalt Hydroxide for Supercapacitor with High Performance and Long Cycle Life. J. Power Sources 2014, 256, 160-169. (33) Huang, L.; Chen, D.; Ding, Y.; Feng, S.; Wang, Z. L.; Liu, M. Nickel–Cobalt Hydroxide Nanosheets Coated on NiCo2O4 Nanowires Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors. Nano Lett. 2013, 13, 3135-3138. (34) Xiao, J.; Wan, L.; Yang, S.; Xiao, F.; Wang S. Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for High- Performance Pseudocapacitors. Nano Lett. 2014, 14, 831-838.

24


Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35) Yu, L.; Zhang, G.; Yuan, C.; Lou, X. W. Hierarchical NiCo2O4@MnO2 Core-Shell Heterostructured Nanowire Arrays on Ni Foam as High-Performance Supercapacitor Electrodes. Chem. Commun. 2013, 49, 137-139. (36) Xia, X.; Tu, J.; Zhang, Y.; Wang, X.; Gu, C.; Zhao, X.; Fan, H.J. High-Quality Metal

Oxide Core/Shell

Nanowire

Arrays

on

Conductive Substrates

for

Electrochemical Energy Storage. ACS Nano 2012, 6, 5531-5538. (37) Guo Y. G. ; Hu J. S.; Wan L. J. Nanostructured Materials For Electrochemical Energy Conversion and Storage Devices. Adv. Mater. 2008, 20, 2878-2887. (38) Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G. Printable Thin Film Supercapacitors Using Single-Walled Carbon Nanotubes. Nano Lett. 2009, 9, 1872-1876. (39) Xu, C.; Du, H.; Li, B.; Kang, F.; Zeng, Y. Asymmetric Activated Carbon-Manganese Dioxide Capacitors in Mild Aqueous Electrolytes Containing Alkaline-Earth Cations. J. Electrochem. Soc. 2009, 156, A435-A411. (40) Inoue, H.; Namba, Y.; Higuchi, E. Preparation and Characterization of Ni-based Positive Electrodes for Use in Aqueous Electrochemical Capacitors. J. Power Sources 2010, 195, 6239-6244.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Capations Fig. 1 TEM images of (a) graphite, (b) GO, (c) TGO and FESEM images of (d) GO, (e) TGO Fig. 2 XPS spectrum of (a) raw graphite, (b) GO and (c) TGO; (d) XRD patterns of GO, TGO and Graphite Fig. 3 Raman spectra of the GO and the TGO. Fig. 4 XRD patterns (a) and FTIR patterns (b) of Ni(OH)2/GO and Ni(OH)2/TGO. Fig. 5 TEM images of a) Ni(OH)2/GO and c) Ni(OH)2/TGO; FESEM images of b) Ni(OH)2/GO and d) Ni(OH)2/TGO. Fig. 6 Schematic illustration for the fabrication procedures of Ni(OH)2/GO and Ni(OH)2/TGO composites Fig. 7 a) CV curves at 1 mV/s and galvanostatic discharge curves at 1 A g-1 of Ni(OH)2/GO and Ni(OH)2/TGO electrodes; b) the specific capacitance of Ni(OH)2/GO and Ni(OH)2/TGO electrodes at different current densities; c) Long cyclic performance of Ni(OH)2/GO and Ni(OH)2/TGO electrodes at the current density of 1 A g-1; d) Electrochemical impedance spectroscopy plots of Ni(OH)2/GO and Ni(OH)2/TGO electrodes. Fig. 8 Schematic illustration for the capacitive performance improvement of Ni(OH)2/TGO. Fig. 9 CV curves (a), GC curves (b), specific capacitances at various currents (c) and Ragone plots (d) of TGN//AC supercapacitor.

26


Page 36 of 36

Highly Oxidized Graphene Anchored Ni(OH) - American Chemical

Recommend Documents