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Research Article pubs.acs.org/journal/ascecg

Ni(OH)2 Nanoflowers/Graphene Hydrogels: A New Assembly for Supercapacitors Ronghua Wang,†,‡ Anjali Jayakumar,† Chaohe Xu,*,§ and Jong-Min Lee*,† †

School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore College of Materials Science and Engineering and §College of Aerospace Engineering, Chongqing University, No. 174 Shazhengjie Road, Chongqing 400044, P.R. China

‡

S Supporting Information *

ABSTRACT: A novel structure of graphene-based hybrid hydrogels was constructed, in which α-Ni(OH)2 nanoflowers with nanopetals thicknesses of approximately 20 nm were uniformly anchored on a three-dimensional graphene framework. Benefiting from the unique morphological nickel hydroxide nanoflowers and hydrogels, the nickel hydroxide nanoflowers/ graphene hydrogels exhibited great specific capacitances (1 A·g−1; 1632 F·g−1), great rate capabilities, and longer cycle life (after 1000 cycles, 95.2% capacitance retention) when used as electrodes in supercapacitors.

KEYWORDS: Graphene, Hydrogels, Nickel hydroxide, Binder-free electrode, Supercapacitor

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INTRODUCTION Supercapacitors or ultracapacitors are emerging as the sustainable energy storage systems of the future for varied large-scale applications, mostly in portable electronics, biomedical devices, electric vehicles, and high power applications and so on.1,2 In comparison with the widely used and common double-layer capacitors, pseudocapacitors (for example, metal oxide, metal hydroxide, and polymers) have higher specific capacitance, as a result of which they are widely explored and considered for supercapacitor applications.3−5 But, the use of these materials has been disadvantaged by their low cyclic stability and rate capability, due to the low conductivity of these materials. Graphene (GS), has attractive characteristic properties, which makes it an outstanding material compared to many other carbonaceous materials; its excellent characteristic properties of high surface area and attractive electromechanical properties makes it the right material which can be used as a supporting surface for anchoring pseudocapacitive materials to improve both the overall capacitance and cycling stability. Though numerous graphene-based composites that were developed to date have succeeded in effectively improving the overall capacitive performance, many a time, the irreversible and unfavorable restacking of the graphene layers has led to a drastic decrease in the surface area, thereby limiting the free crusade of electron and ions.6−8 Porosity being one of the vital factors influencing the capacitive performance of electrode materials, graphene-based mesoporous materials, such as graphene-based foams, aerogels, and hydrogels, have been able to attract substantial interest.9−11 © XXXX American Chemical Society

Graphene-based hybrid hydrogels belong to a class of hydrated materials containing a large amount of water, which have received particular attention in the recent times.12,13 On one hand, the hydrogels have the advantage of being used as electrode materials without a binder, resulting in the reduction in the electrode preparation steps, and also any reduction in the specific capacitance caused by polymer binders used in the slurry preparation for the electrode material. On the other hand, these unique graphene-based hybrid hydrogels are highly hydrophilic in nature and have a high surface area (∼1000 m2· g−1 as per methylene blue molecular adsorption studies), allowing fast ion diffusions and electron transport through the hierarchical, porous and conductive graphene hydrogel network.14,15 These unique features favor hydrogels as capable electrode materials of high performance supercapacitors. For example, Yan et al. combined three-dimensional (3D) MnO2/ graphene hydrogels, which have a capacitance (1 A·g−1): 242 F· g−1.16 Xue et al. prepared 3D polyaniline/graphene hydrogels (334 F·g−1) by a method involving self-assembly of graphene layers.17 Duan et al. developed nickel hydroxide/GS hydrogels having 1247−785 F·g−1 from 5 to 40 mV·s−1 and great stability for cycling.18 To date, although several graphene-based hydrogels have been constructed and greatly improved the electrochemical performance, the research on graphene-based hybrid hydrogels for supercapacitors is still very limited despite their significant potential. Moreover, the study of 3D hybrid Received: February 23, 2016 Revised: June 3, 2016

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DOI: 10.1021/acssuschemeng.6b00362 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

materials were immersed in a 2 M KOH aqueous solution overnight to exchange water in the materials. After that, the materials were prepared into small pieces and pressed between two nickel foams (1 × 1 cm) to form an electrode. The mass loading was about 1.5−2 mg. For the pure Ni(OH)2, a slurry-coating process was used to assemble the electrodes; a homogenized slurry was obtained on mixing the active materials, a PVDF binder and carbon black (ratio: 8:1:1 in mass), wetted by sufficient N-methyl-2-pyrrolidone. Then, it was casted on a foam (nickel) and placed under vacuum at 110 °C. Characterization. Their morphological data was obtained from transmission electron microscopy (TEM, JEOL-2010F) and field emission scanning microscopy (FE-SEM, JEOL-6700F). Their physical characteristics were analyzed from X-ray diffraction (XRD) Bruker D8, X-ray photoemission spectroscopy (AXIS, Kratos analytical LTD), and Raman spectroscopy (Thermal Scientific Corporation, USA). Thermogravimetric analysis of the samples was done in air. The hydrogels’s surface area was given via methylene blue adsorption studies. Methylene blue is a common dye probe used to determine the graphitic materials’ surface area, where 1 mg of methylene blue which gets adsorbed corresponds to a covered surface area of 2.54 m2.15,22 A standard methylene blue solution was used, into which, a known mass of hydrogel was laid and kept undisturbed for 24 h to attain maximum adsorption. A UV−vis spectrophotometer was used to analyze the supernatant which remained at the end of 24 h (at 665 nm). Afterward, its methylene blue content was tested by correlating the supernatant’s absorption spectra at a wavelength of 665 nm using UV−vis spectroscopy. The amount of methylene blue adsorbed can be obtained from the initial standard concentrations and the concentration of solution after the adsorption. This value will help in directly calculating the surface area as per the standard for graphitic materials’ adsorption mentioned earlier. Electrochemical Measurements. A single electrode was tested using an electrochemical three-electrode setup: Pt (counterelectrode), Hg/HgO (reference electrode), and 2 M KOH solution (electrolyte). A CHI660D electrochemical workstation was used to perform all the electrochemical measurements. For Ni(OH)2/GS hydrogels, the total active mass (including GS) on the electrode was used in calculating the current densities, while for the nickel hydroxide electrode, only its mass was used to calculate current densities. The electrochemical impedance study was done by obtaining the Nyquist plots (0.01−100 kHz, AC voltage, 10 mV amplitude). Calculation. Three-Electrode System. From Figure 4a, the capacitance (Csp, F g−1) was evaluated by the following:4

hydrogels is greatly limited to the attachment of nanoparticles, nanofibers, or nanosheets on to the graphene layers in the 3D network.15−18 As of today, there has been no systematic or detailed study based on the nanoflowers composited with graphene 3D structures, and thus, their capacitive performance remains unknown so far. Here in this study, we report a new assembly of graphenebased hybrid hydrogel, in which Ni(OH)2 nanoflowers with nanopetals thicknesses of approximately 20 nm were uniformly anchored on a 3D graphene framework. Ni(OH)2 was selected as the significant research entity because of its cost effectiveness, layered structure, and beneficially high theoretical specific capacitance.5,19 The flowerlike nanostructured Ni(OH)2 stands apart from other morphologies with respect to its 3D interconnected nanoplatelets that has plenty of macropores, which can enable fast electrolyte transport, consequently improving the effective electrochemical utilization.20 Benefiting from the unique structure of nickel hydroxide nanoflowers and hydrogels, the nanoflowers/GS hybrid hydrogels exhibited appreciable specific capacitances (1 A·g−1) 1632 F·g−1, great rate capabilities, and great stability (after 1000 cycles−95.2% capacitance retention).

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EXPERIMENTAL PROCEDURES

Material Synthesis. Direct preparation of GO was seen from graphite powder obtained from Alfa-Aesar using the popularly used and established modified Hummers method.21 Ni(OH)2 Nanoflower Precursor. A 1.0 g portion of nickelous acetate (Ni(OAc)2·4H2O) was dissolved in 60 mL ethanol with continuous stirring. This was followed by the subsequent addition of 5 mL ammonium hydroxide. The resulting solution was then refluxed at 100 °C for 3 h forming a Ni(OH)2 precursor. A precipitate was acquired, which was washed with water and diluted to 1 mg mL−1 for further use. 3D Macroscopic Ni(OH)2/GS Hydrogels. The nickel hydroxide prepared previously was mixed into 10 mL GO dispersion (1 mg mL−1) dropwise. The solution was ultrasonicated for about 30 min until it was well mixed. This was later centrifuged and redispersed in 5 mL DMF to obtain a Ni(OH)2/GO dispersion. This dispersion was subjected to a solvothermal synthesis for 6 h to obtain reduced GO which would self-assemble into a 3D Ni(OH)2/GS monolith at 180 °C. The obtained monolith was taken and washed thoroughly with distilled water before using it further. Of note, it is important to maintain the right ratio of Ni(OH)2 and GS for being used as a good supercapacitor electrode since the capacitance is different for each of the individual constituents, the GS contributing much less toward the overall capacitance. In this study, two samples with graphene contents of 16.5% and 24.9% were prepared. After a detailed study on these two samples, the sample with 16.5% graphene has been used as the optimized sample hereafter (Figure S3 and S8). As a control, in addition, pure nickel hydroxide without GS was synthesized via the same procedure as Ni(OH)2/GS hydrogels except that no GO was added. The sovothermal reactions of the GO dispersion at 180 °C for 6 h were conducted to produce pure graphene hydrogels. Ni(OH)2/GS Aerogels. A freeze-drying technique was employed for making Ni(OH)2/GS aerogels (scanning electron microscopy samples), which does not usually cause shrinkage or toughening of the material being dried, therefore, is commonly used to dry graphenebased hydrogels so as to preserve the interconnected, porous 3D graphene framework. In our study, the hybrid hydrogel was first kept in the refrigerator for 12 h to convert water to ice. Afterward, the freezed hydrogel was kept into the freeze-dryer machine (SCIENTZ10N) for another 12 h, to allow the sublimation of frozen water to gas phase. Electrode Preparation. The hydrogels were directly used in the wet state as an electrode, without adding any conductor or binder. The

Csp =

∫ dt I ΔVm

(1)

where I is the applied current, ΔV indicates the potential window for one sweep segment, dt is the differential time, and m is active material mass (including GS for Ni(OH)2/GS hydrogel). Also, galvanostatic charge−discharge curves also provide us with the specific capacitance as per the following equation:4,9 Cs =

I Δt ΔVm

(2)

where I, ΔV, Δt, and m are the discharge current, voltage range, discharge time, and active material mass, respectively. Two-Electrode System. Calculation of specific capacitance in a twoelectrode system by eq 3 gives13,23 Cs =

2I Δt ΔVm

(3)

where I, ΔV, Δt, and m are the discharge current, potential window applied, discharge time, and active material mass in one electrode.

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RESULTS Figure 1a schematically illustrates the experimental procedure. The hydrogels were prepared via electrostatic attractive interactions, followed by a solvothermal−self-assembly induced B


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network of nanopetals which have a thicknesses of around 20 nm (see the Supporting Information, Figure S1a and b). As reported in the literature, with a hydroxyl-deficient phase, α-Ni(OH)2 maintains charge neutrality despite the large number of positively charged Ni(OH)2−x layered stacks by virtue of the water molecules and intercalated anions which exist between the interlayers.24 Here, the prepared Ni(OH)2 nanoflowers have an overall positive charge from the adsorption of cations (zeta potential of +36.3 mV), while GO, with many oxygen-functional groups, is negatively charged with a zeta potential of −30 mV. These negatively charged GO layers form the right ground for anchoring the positively charged Ni(OH)2 nanoflowers by means of strong electrostatic interactions (Figure S1c and d). During the solvothermal treatment, GO was reduced to GS. This process also saw the self-assembly of these electrostatically well-bound units of GS/Ni(OH)2 into 3D macroscopic hydrogels; the strong π−π interactions driving it (the digital photograph in Figures 1a and S2). From Figure 1b, the peaks of the nickel hydroxide correspond to hexagonal α-Ni(OH)2 (JCPDS 22-0752) after the reaction. This indicates the phase of the nickel hydroxide/GS did not change with respect Ni(OH)2 precursor, except that the peaks corresponding to (001) and (002) planes have undergone a small positive shift possibly because via the c-axis from the intercalated anions occupying the layers of the lattice, the d spacing reduced.24,25 Figure 2 shows the electronic structure and chemical composition of the hydrogels from XPS. The survey scan spectrum designates the sample to be containing Ni, O, and C elements (Figure 2a), without any other impurities. The C 1s core level peak consists mostly of the bonds (CC/C−C). The lower intensities of oxygen-rich functional groups compared to those of GO (Figure S4) indicate the successful reduction of GO to GS during the reaction.26,27 The synthesized α-Ni(OH)2 shows typical Ni 2p XPS spectrum as per the XPS studies. The major peaks occur at binding energies at 873.9 (2p1/2 Ni) and 856.2 (2p3/2 eV Ni), with 17.7 eV of the energy separation, which reiterates the formation for our

Figure 1. (a) Graphic representation of preparation for the nickel hydroxide/GS hydrogels. (b) XRD spectra of the nickel hydroxide precursor (i), the freeze-dried nickel hydroxide/GS hydrogels (ii), and the nickel hydroxide precursor after the reaction (iii).

process. Briefly, the preparation of nickel hydroxide was done via a simple procedure which employed ammonia. XRD reveals that the reflection peaks of the precursor match to pure hexagonal α-Ni(OH)2 (JCPDS 22-0752, Figure 1b, curve i), though small shifts can be seen in the peak positions of the (001) and (002), and these shifts can be explained by the various intercalated anions that exist in their interlayer spaces.24,25 From SEM, the Ni(OH)2 precursor has flowerlike architectures, formed from an assembly of a well-connected

Figure 2. (a) Survey XPS spectra of the freeze-dried nickel hydroxide/GS hydrogels. (b) Spectra C 1s core level. (c) XPS spectrum Ni 2p. (d) TG curves of the pure nickel hydroxide and the freeze-dried nickel hydroxide/GS hydrogels. C


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Figure 3. (a and b) SEM images of nickel hydroxide/GS hydrogel after vacuum freeze-drying. (c and d) TEM of nickel hydroxide/GS hydrogel. (e) EDX for C, Ni, and O.

research entity.5,20 Raman spectra further confirm their successful formation (Tables S1 and S2, Figure S5). The graphene content was estimated from the TGA study. In Figure 2d, under 100 °C, the loss can be explained by the loss of any interfacial -adsorbed water molecules in both pristine nickel hydroxide and freeze-dried nickel hydroxide/GS. In addition, pure nickel hydroxide shows a considerable weight loss between 240−300 °C which is mainly from the change of nickel hydroxide to the formation of nickel oxide and partly by some combustion reactions.1 The graphene contents were calculated to be 16.5% from the residual weights from the two samples. The morphological studies and understanding of microstructure of the hybrid hydrogels were done by SEM and TEM (Figure 3). SEM exhibits an ordered and continuous interlinked 3D porous graphene network, containing macropores which are continuous and in micrometer sizes (Figure 3a and b). Close observation reveals Ni(OH)2 still keeps the flowerlike shape, and Ni(OH)2 nanoflowers are unvaryingly anchored on 3D GS frameworks. A high-magnified SEM image discloses that the thickness of the nanopetals remain about 20 nm. The thin thickness of the nanopetals can shorten the electron diffusion distance and facilitate the surface-dependent Faradaic reactions, even at high scan/current rates.20,28 Moreover, as demonstrated in literatures, the 3D interconnected nanopetals contain macropores, improving the electrolyte transport and the effective electrochemical utilization.4,29 Figure 3c and d shows the TEM images of nickel hydroxide/ GS hydrogels. Clearly, nickel hydroxide nanoflowers are highly disseminated and firmly attached on GS, well consistent with

SEM images. EDX mapping discloses the components of C, O, and Ni are strewn in the system, reiterating the completion of nickel hydroxide nanoflowers/GS mixtures (Figure 3e). Of note, although there is some research on graphene-based hybrid hydrogels for supercapacitors, most of it is limited to nanoparticles or nanosheets anchored on a graphene framework.15−18 This is the first time that a new structure, nanoflowers hybrid with graphene hydrogels, was constructed and reported. As for pure Ni(OH)2, it also keeps the flowerlike shape after the solvothermal reaction, similar to that of Ni(OH)2 in hydrogels (Figure S6). Considering the wet hydrogels were directly used as electrode materials, we adopted the dye adsorption method using methylene blue to measure their surface areas. With this approach, the specific surface area of our composite reached ∼887 m2 g−1. Such a 3D porous structure is highly beneficial for easy transport of electrolytes through their porous conductive layers promoting more charge-storage reactions. This hierarchical assembly also prevents the common restacking of graphene sheets.13,15 Based on the previous discussions, Ni(OH)2/GS hydrogel has shown to have very high specific surface area and a wellconnected and continuous porous structure. The major advantage here was that no polymer binders or conductive additives were used for electrode preparation and the hydrogel was directly sandwiched between two nickel foams as mentioned earlier. Figure 4a gives the CV plots of nickel hydroxide/GS electrodes collected (5−100 mV·s−1). The peaks are clearly exhibiting uniformity within a voltage of 0−0.7 V, which highlights the reversibility of the reaction taking place D


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Figure 4. (a) CV plots for the nickel hydroxide/GS electrode. (b) Galvanostatic plots for the nickel hydroxide/GS. (c) Galvanostatic curves for nickel hydroxide. (d) Corresponding capacitance for nickel hydroxide/GS, pure nickel hydroxide, and pure GS versus current density. (e) EIS of the nickel hydroxide/GS hydrogel electrodes in high frequency parts as insets. (f) Cycle life test (at 4 A·g−1) for the nickel hydroxide/GS and the pure nickel hydroxide.

here: Ni(OH)2 + OH− ↔ NiOOH + H2O + e−.18,20 Their plots do not get distorted as we raise scan rate, demonstrating a good electrochemical stability of our material. As calculated, it has a capacitance: 1440−1358 F g−1 from 5 to 10 mV·s−1. With the rate increase (40, 80, and 100 mV·s−1), its values remain 1098, 924, and 864 F·g−1, respectively, showing good rate capability. To quantify its capacitance, charge−discharge measurements were further made. In Figure 4b, the discharge curves for the nickel hydroxide/GS display a well-defined potential plateau at the given current density range, illustrating a typical pseudocapacitive behavior. The capacitance obtained from here is plotted against the corresponding current density in Figure 4d. Notably, the electrode gives a capacitance, 1632− 1088 F·g−1 from 1 to 6 A·g−1, more than pure nickel hydroxide and pure graphene hydrogels (Figures 4c and S7). Even when the current density is a high values (8 and 10 A·g−1), the values of capacitance still reach 1029 and 940 F·g−1, maintaining capacitance retention of 63% and 58%, respectively, showing that its specific capacitance and rate capability are greatly improved after compositing with graphene into hydrogels, which may be benefiting from the following: (i) The 3D porous structure of the hydrogel has a porous network to enable effective electrolyte diffusion leading to good ionic transport. (ii) As shown by the EIS of Ni(OH)2/GS hydrogel electrodes

(Figure 4e), the estimates of equivalent series resistance (ESR) is ∼1.24 and charge transfer resistance (Rct) is 0.16 Ω, indicating good conductivity and low internal resistance of the electrode.9,30 Thus, this conductive path provided by the graphene framework acts as a major element in improving the overall electrochemical performance.1 Apart from these, the Ni(OH)2/GS electrodes also display superior cycling stability. When cycled at 4 A g−1, the nickel hydroxide/GS retention capacitance is 95.2% after 1000 continuous cycles, which is substantially superior to pristine Ni(OH)2 with 85.6% (Figure 4f). These results easily validate the scope of our material, Ni(OH)2/GS hydrogel, as one which holds promise as an extremely good material for high performance supercapacitors.

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CONCLUSION A new structure of hybrid hydrogels, with nanoflowers uniformly anchored on a 3D graphene framework, were constructed via electrostatic attractive interactions combined with a solvothermal-induced self-assembly process. These novel hybrid hydrogels inherit the advantages of both graphene hydrogels and Ni(OH)2 nanoflowers thus exhibiting a grander electrochemical performance. These results we hope will go a E


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long way in the development of real-life applications pertaining to supercapacitors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00362. SEM images, TEM images, digital photographs, TG curves, C 1s XPS spectrum, Raman spectra, charge− discharge curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C. Xu). *E-mail: [email protected]. Tel.: +65 6513-8129 (J.-M. Lee). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Academic Research Fund (RGT27/ 13) of Ministry of Education in Singapore and Fundamental Research Funds for the Central Universities (Project No. 0903005203377).

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REFERENCES

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ACS Sustainable Chemistry & Engineering (30) Wang, R.; Xu, C.; Lee, J.-M. High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels. Nano Energy 2016, 19, 210−221.

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