In situ co-deposition of nickel hexacyanoferrate nanocubes on the reduced graphene oxides for supercapacitors

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In situ co-deposition of nickel hexacyanoferrate nanocubes on the reduced graphene oxides for supercapacitors Yunyun Yang a, Yanfei Hao a, Junhua yuan

a,b,* ,

Li Niu c, Fang Xia

d

a Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Life Sciences and Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, China b School of Pharmacy, Hubei University of Science and Technology, Xianning, Hubei 437100, China c State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China d Guantang Middle School, Xianning, Hubei 437100, China

A R T I C L E I N F O

A B S T R A C T

Article history:

A facile co-precipitation strategy is developed to prepare nickel hexacyanoferrate nano-

Received 2 July 2014

cubes (NiHCF NBs) supported on the reduced graphene oxide (rGO) in the presence of

Accepted 1 December 2014

poly(diallyldimethylammonium chloride) (PDDA). The NiHCF NBs are uniformly deposited

Available online 5 December 2014

on the rGO by electrostatic interaction. Their size can be tuned from 10 nm to 85 nm by changing their content from 32.6% to 68.2%. Under the optimal condition, NiHCF/PDDA/ rGO hybrids are composed of 51.4% NiHCF NBs with an average size of 38 nm. The specific capacitance of NiHCF/PDDA/rGO hybrids reaches up to 1320 F g1 at a discharge density of 0.2 A g1, more than twice that of the pure NiHCF, as well as slight capacitance decay by 15% at 0.2 A g1 and excellent cycling stability with 87.2% of its initial capacitance after 10,000 discharge/charge cycles. More importantly, NiHCF/PDDA/rGO hybrids exhibit an ultrahigh energy density of 58.7 Wh kg1 at the power density of 80 W kg1. The superior storage energy performance of NiHCF/PDDA/rGO hybrids, such as high specific capacitance, good rate capacity and long cycling stability, positions them as a promising candidate for supercapacitor materials.  2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Ever increasing demand for sustainable and environmentally-friendly energy has speed up the pursuit of alternative energy conversion/storage systems, including fuel cells, solar cells, secondary lithium-ion batteries and supercapacitors [1]. Among them, supercapacitors are superior in term of ultralong cycling stability, high power density and fast

charge/discharge rate [2,3], demonstrating feasibility in potential application, such as portable electronics, back-up power storage and hybrid electric vehicles [4,5]. However, their typical energy density, <10 Wh kg1, is the lowest, e.g., in comparison with ca. 200 Wh kg1 of a lithium secondary battery, limiting their practical use as primary power sources for quick start of a hybrid vehicle in conjunction with a battery [1].

* Corresponding author at: Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Life Sciences and Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, China. Fax: +86 579 82282269. E-mail address: [email protected] (J. yuan). http://dx.doi.org/10.1016/j.carbon.2014.12.005 0008-6223/ 2014 Elsevier Ltd. All rights reserved.

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Supercapacitors can be divided into the electrical double layer capacitors (EDLCs) and pseudocapacitors on the basis of the energy storage mechanism. The EDLCs performance depends on the available surface area of the electrodes and the finite charge separation between the electrode materials and the electrolyte. But for pseudocapacitor, the energy storage capability was determined by the fast and reversible faradic reaction near the surface [6]. Pseudocapacitor can provide with a huge specific capacitance, which far exceeds that of an EDLC. Most of pseudocapacitors are based on redoxable transition metal oxides/hydroxides [7–9] and conducting polymers [10,11]. They are poor in conductivity, and usually suffer from cycling stability due to swelling and shrinkage of the electroactive species during charge/dedoping process [12]. On the other hand, EDLCs are made of carbonaceous materials [2,13], they are highly conductive and maintain particularly stable during the long running process. Therefore, supercapacitors can be commercialized in hope of a perfect combination of these pseudocapacitive materials with the high-surface-area carbonaceous materials [14]. Recently, transition metal hexacyanoferrates (MHCF, M represents as Fe, Co, Ni, et al.), a zeolite-like Prussian blue coordination compound, attract increasing interest in application as supercapacitors due to their unique structure, low cost, reversible redox ability and superior environmental benignancy [15–17] Lisowaka-Oleksiak et al. [15] prepared MHCF network inside poly(3,4-ethylenedioxythiophene) (pEDOT) films for electrochemical capacitors by the electrochemical deposition process. Specific capacitance of hybrid materials is equal to 70, 70 and 50 F cm3 for pEDOT/CoHCF, pEDOT/NiHCF and pEDOT/FeHCF, respectively. Chen et al. synthesized nanosized NiHCF by co-precipitation method. The specific capacitance of NiHCF was 574.7 F g1 obtained at the current density of 0.2 A g1 with 91% capacitance retention over 1000 charge/recharge cycles [16]. Graphene has been considered as an ideal electrode materials for supercapacitors because of its extremely high theoretical surface area (2630 m2 g1) and a superior theoretical capacitance (550 F g1) [18,19]. Herein, we demonstrate an in situ co-precipitation strategy to prepare NiHCF nanocubes (NBs) supported on the reduced graphene oxide (rGO) in the presence of poly(diallyldimethylammonium chloride) (PDDA). In this composite, PDDA serves as a stabilizer for rGO in the process of chemical reduction; it also acts as a linker to anchor NiHCF nanocubes onto the surface of rGO. The rGOs function as the substrate upon which to deposit the NiHCF NBs. Besides, the rGOs improve the electrical conductivity of the hybrids, and increase the effective utilization of NiHCF. As a result, the NiHCF/PDDA/rGO hybrids possess a huge specific capacitance, excellent rate capacity and superior cycling stability, providing with a promising candidate for supercapacitors in commercial utility.

2.

Experimental

2.1.

Chemicals and materials

K3[Fe(CN)6], Ni(NO3)2, H2SO4 (98%) and H2O2 (30%) were obtained from Shanghai reagent Co. Inc. Water was purified by a Milli-Q system.

2.2.

Preparation of graphene oxide (GO)

GO was synthesized by slight modification to Hummer’s method [20]. Graphite powder (0.4 g), NaNO3 (0.4 g) and H2SO4 (20 mL) were cooled and sonicated in an ice bath for 20 min. Then 2.4 g of KMnO4 was added slowly into the graphite powder solution at 20 C, and the mixture was vigorously stirred at 35 C for 4 h. The oxidation step was stopped by dropwise addition of 20 mL de-ionized water and 12 mL H2O2 (30%) solution, and the GO solution was washed and filtrated with 200 mL HCl (1 M). In order to remove ion and excess acid, this GO solution was dispersed in de-ionized water and dialyzed for 1 week until its pH value increased from 4 to 6.

2.3.

Preparation of PDDA/rGO hybrids

PDDA/rGO hybrids were prepared according to the reference in the presence of PDDA [21]. In brief, 20% PDDA (0.5 mL) solution was added to 0.05% GO solution (100 mL) and stirred for 30 min. The pH value of the mixture were adjusted to 10 by 30% NH3ÆH2O. Then 80% hydrazine hydrate (0.5 mL) was introduced. GO were reduced in the presence of PDDA under stirring for 24 h at 90 C. Finally, the PDDA/rGO hybrids were recovered by centrifugation, and the precipitates washed with distilled water, and then re-dispersed readily in water to form a black suspension. The preparation of pure rGO was similar to that of PDDA/ rGO, except that no PDDA was added.

2.4.

Preparation of NiHCF/PDDA/rGO hybrids

NiHCF/PDDA/rGO hybrids were prepared by co-precipitation of K3[Fe(CN)6] and Ni(NO3)2 on the surface of PDDA/rGO hybrids in acid aqueous solution. A certain amount of K3[Fe(CN)6] (20 mM) were added into 50 mL PDDA/rGO dispersion (0.2 mg mL1) in batches. The Fe(CN)3 6 will be adsorbed on to the surface of PDDA/rGO hybrids due to electrostatic interaction. This process will proceed under ultrasonic wave for 4 h. The mixture were adjusted to pH 1.0 by HCl, then a certain amount of NiNO3 (20 mM) equal to K3[Fe(CN)6] were added and stirred for 4 h. The resulting NiHCF/PDDA/rGO hybrids were collected by centrifugation and washed with de-ionized water and ethanol. For comparison, the hybrids, K3[Fe(CN)6] adsorbed on to the rGo is prepared according to the synthesis of the NiHCF/ PDDA/rGO hybrids without addition of Ni2+ ion (Fe(CN)3 6 / PDDA/rGO). The pure NiHCF was also prepared in the strategy similar to that of NiHCF/PDDA/rGO hybrids without PDDA/ rGO hybrids.

2.5.

Graphite powders (320 meshes) and PDDA (20%, Mw ca. 65,000) were purchased from Acros. NaNO3, KMnO4,

175

Characterization

Thermogravimetric analysis (TGA) was recorded on Netzsch STA 449 C analyzer at a heating rate of 10 C/min under nitrogen.

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X-ray powder diffraction (XRPD) data were collected on a ˚) Philips PW3040/60 diffractometer with Cu KR (1.5406 A radiation. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB-MKII spectrometer with an unmonochromated Al KR X-ray source (1486.6 eV) for excitation. High-resolution spectra were obtained at a perpendicular takeoff angle, using the pass energy of 20 and 0.05 eV steps. Inductively coupled plasma-atom emission spectroscopy (ICP-AES) was performed on Agilent 7500ce quadrupole ICPAES. The pure NiHCF and NiHCF/PDDA/rGO, Fe(CN)3 6 /PDDA/ rGO hybrids were sampled by treatment with HNO3. The elements, Ni and Fe were dissolved, and left in solution for ICPAES analysis. Transmission electron microscopy (TEM) observation was performed on a JEOL 2010 microscope equipped with an electron energy dispersive X-ray spectroscopy (EDS), operating at 200 kV using a high-brightness LaB6 electron gun. The composition of different catalysts was determined by its EDS at 150 kV.

2.6.

Electrochemical investigation

All electrochemical measurements were conducted using a three-electrode system equipped with a standard calomel reference electrode (SCE) and a platinum rod counter electrode. A platinum foil electrode were used as the working electrode, and this electrode was modified with the electroactive materials as described elsewhere [22]. Briefly, a small amount of water (100 lL) was added to samples (5 mg), and ground to form homogeneous slurry. This sample slurry and poly(tetrafluoroethylene) (PTFE) were mixed in a mass ratio of 10:1 and dispersed in ethanol. Then the resulting mixture was uniformly coated onto the glassy carbon slide (0.5 cm · 0.5 cm) with a spatula, following by drying at 60 C for 12 h in a vacuum oven. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests were performed by using a CHI 660C electrochemical workstation in N2 saturated 0.5 M Na2SO4 solution. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 100 kHz to 0.1 Hz at open circuit potential with an ac perturbation of 5 mV.

3.

Results and discussion

3.1.

Materials characterization

The PDDA/rGO hybrids were prepared by reduction in the presence of PDDA. This PDDA/rGO dispersion (0.2 mg mL1) remains stable without rGO coagulation after more than 1 month due to electrostatic repulsion, indicating that the PDDA on the surface rGO can protect rGO from self-aggregation. Noted that the adsorption of PDDA on the rGO may derive from by Van de Waals’ force between PDDA and rGO, because the negative oxygenated species on GO, such as carboxyl, epoxy, will be converted into hydroxyl on rGO after reduction. The presence of PDDA on rGO hybrids is confirmed by XPS. As shown in Fig. 1A, N1s signal can be observed at ca.

400 eV on the XPS survey profile of PDDA/rGO hybrids, indicating the successful modification of PDDA on the surface of rGO [23]. The content of PDDA on rGO can is determined to be 15% according to TDA curves (Supplementary materials, Fig. S1), which can be tuned by controlling the PDDA mass added into the reaction system. It should be noted the as-prepared NiHCF is deposited onto the rGO which has been loaded with 15% PDDA. These NiHCF/PDDA/rGO hybrids were synthesized by in situ co-precipitation of Ni(NO3)2 and K3[Fe(CN)6] in the stable dispersion system of PDDA/rGO containing l M HCl. The composition of NiHCF/PDDA/rGO hybrids (in this case, NiHCF2/ PDDA/rGO) was investigated by XPS analysis. For the XPS spectrum of NiHCF2/PDDA/rGO hybrids, Ni 2p and Fe 2p signals appear at the binding energy of ca. 717 eV and ca. 868 eV, respectively. But K 2p signal is difficultly distinguished from C1s spectra, since it overlaps at the binding energy of ca. 293 eV. The high-resolution spectra of K, Fe and Ni elements are also presented in Fig. 1B–D. demonstrating the successful deposition of NiHCF on the rGO [24]. Fig. 1E shows the highresolution C1s spectra of PDDA/rGO and NiHCF2/PDDA/rGO hybrids. The C1s fitted spectra curves display three peaks assigned to oxygenous species, such as hydroxyl (286.6 eV), carbonyl (288.0 eV) and carboxylate (289.0 eV), which totally account for 8.5 at.% of carbon for PDDA/rGO hybrids and 7.2 at.% for NiHCF2/PDDA/rGO hybrids [25]. This small ratio of oxygen to carbon is consistent with the reduction of GO in these hybrids. For comparison, the oxygenous species on rGO is also determined on its C1s fitted XPS spectrum, which is estimated to be 8.6 at.% of carbon as shown in Fig. S1 (Supplementary materials). In addition, another shoulder peak, which is ascribed to quaternary ammonium (QA) group, is located at 285.6 eV on the C1s spectrum of PDDA/rGO hybrids [25]. For NiHCF2/PDDA/rGO hybrids, the signal of nitrogenous species increases significantly due to the deposit of NiHCF on the rGO. The QA group can be observed in the high-resolution N1s spectra of PDDA/rGO and NiHCF2/PDDA/rGO hybrids at 400 eV [26], another N1s signal for cyano group appears on the XPS spectrum of NiHCF2/PDDA/rGO hybrids at 397.7 eV [27]. The XPS variation of C1s and N1s confirms the presence of NiHCF on the rGO. The composition of NiHCF2/PDDA/rGO hybrids was further determined by EDS and ICP-AES. EDS data are shown in Fig. 2, K Ka peak appears at 3.3 keV. Fe Ka and Kb peaks take place at 6.4 keV and 7.1 keV [30], respectively. Ni Ka peak occurs at 7.4 keV, and its relative Ni Kb peak cannot be discerned due to its overlap with Cu Ka signal [28]. Based on the intensity of the EDS curves, the relative stoichiometric ratio of K, Fe and Ni elements is estimated to be 1:3:2.5 for NiHCF2/PDDA/ rGO hybrids, and 1:2:2 for the pure NiHCF, respectively. The lower K peak intensity can be interpreted as a few K ion intercalated into the NiHCF unit cell in the process of its formation. In this case, H+ ion from acidic solution will enter into the matrix of NiHCF to balance the charge. Besides, QA groups of PDDA will take place of K+ ion on the surface of NiHCF during its deposition on the rGO. As based on the analysis of ICPAES, the content of NiHCF in NiHCF2/PDDA/rGO hybrids was calculated to be 51.4% with 0.85 Ni/Fe atom ratios, which is in good agreement with EDS data (Supplementary materials, Table S1).

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Fig. 1 – XPS survey profiles of PDDA/rGO, and NiHCF2/PDDA/rGO hybrids (A). The high-resolution spectra of (B) K 2p, (C) Fe 2p and (D) Ni 2p in NiHCF2/PDDA/rGO hybrids and (E) C1s and (F) N1s in PDDA/rGO and NiHCF2/PDDA/rGO hybrids. (A color version of this figure can be viewed online.)

The crystal structure of NiHCF2/PDDA/rGO hybrids was measured by XRD technique. Fig. 3 shows a typical XRD pattern of the pure NiHCF, all its peaks can be indexed as cubic ˚ , which nickel iron cyanide hydrate with a lattice of 10.23 A conform to the standard card (JPCDS No. 82-2283) [16]. NiHCF2/PDDA/rGO hybrids share the same XRD pattern with the pure NiHCF. Besides, they also present another insignificant broad peak associated with C (0 0 2) from the rGO on their XRD profile at the diffraction angle of 15–30 [29]. The morphology of NiHCF2/PDDA/rGO hybrids is observed by TEM. Fig. 4 shows the typical TEM images of NiHCF2/PDDA/ rGO hybrids under different magnification. NiHCF is presented as nanocube shape with an average diameter of 38 nm. These NiHCF NBs are uniformly dispersed on the surface of rGO. For comparison, most of the pure NiHCF consists of spherical-like particles with average size of 102 nm (Supplementary materials, Fig. S3).

The size of NiHCF NBs can be tuned by changing the load of NiHCF on the rGO. As shown in Fig. 5, all of the NiHCF NBs are uniformly deposited on the rGO. Fig. 5A exhibits the NiHCF/PDDA/rGO composed of 32.6% mass of NiHCF NBs with an average diameter of 10 nm (defined as NiHCF1/PDDA/rGO, Fig. S3 and Table S1). When the mass content of NiHCF NBs is up to 68.2%, their average diameter is estimated to be 83 nm (named as NiHCF3/PDDA/rGO, Supplementary materials, Fig. S3 and Table S1). As discussed above, NiHCF NBs in the NiHCF/PDDA/rGO hybrids are smaller than the pure NiHCF, which make the NiHCF/PDDA/rGO hybrids possess a higher specific surface area by comparison with the pure NiHCF. Moreover, the introduction of rGO is beneficial to accelerate the electron transfer of NiHCF at the electrode interface due to its high conductivity. On the other hand, the content of PDDA on rGO may control the size of NiHCF NBs, as shown in Fig. S4 (Supplementary materials), the

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Fig. 2 – EDS curves of (A) NiHCF2/PDDA/rGO hybrids and (B) pure NiHCF. (A color version of this figure can be viewed online.)

Therefore, PDDA may act as linker and stabilizer for NiHCF NBs to control their deposition and growth on the surface of RGO.

3.2.

Fig. 3 – XRPD patterns of NiHCF2/PDDA/rGO hybrids and pure NiHCF. (A color version of this figure can be viewed online.)

average diameter of NiHCF NBs is increased to be 79 nm with 50.2% NiHCF loading when the content of PDDA on rGO is reduced to be 5% (Supplementary materials, Fig. S1).

Electrochemical behavior and capacitive performance

The electrochemical behavior of NiHCF2/PDDA/rGO hybrids was conducted by cyclic voltammetry in 0.5 M Na2SO4 solution. Fig. 6A shows the CV curves of the pure NiHCF, rGO and its hybrids modified electrode at scan rate of 50 mV s1. For the rGO, the rectangular shape of the CV curves without obvious redox peaks is the typical of conductive carbon surface [30]. But for the pure NiHCF, obviously, a couple of redox peaks (Peaks I) occurs at the midpoint potential (EF) of 0.41 V in the potential region from 0.2 to 0.8 V. Since the Ni2+ ion cannot be reduced in this potential range, this faradic current can be attributed to electron shift between Fe2+ and Fe3+ ions accompanied by Na+ uptake/release in the unit cell of NiHCF as follows [28]:   uptake II  II  KNi FeIII ðCNÞ6 þ Naþ þ e ƒƒƒƒ! NaKNi FeII ðCNÞ6

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Fig. 4 – TEM images of NiHCF2/PDDA/rGO hybrids under different magnificent (A–C) and pure NiHCF (D). Inset: histogram of size distributions for NiHCF NBs on NiHCF2/PDDA/rGO hybrids.

Fig. 5 – TEM images of NiHCF1/PDDA/rGO (A) and NiHCF3/PDDA/rGO hybrids (B).

 release  II  II  NaKNi FeII ðCNÞ6 ƒƒƒƒ! KNi FeIII ðCNÞ6 þ Naþ þ e Likewise, these redox reactions are also presented on the CV curves of NiHCF2/PDDA/rGO hybrids at 0.362 V. The peak-topeak separation (EppI), and peaks current ratio (IpaI/IpcI) of the pure NiHCF and NiHCF/PDDA/rGO hybrids are collected in Table S2 (Supplementary materials). By comparison with the pure NiHCF, NiHCF2/PDDA/rGO hybrids have a smaller EppI value, and their IpaI/IpcI ratio is nearer 1.0, indicating that this electrochemical process is more reversible and faster at the electrode modified with NiHCF2/PDDA/rGO hybrids than that of the pure NiHCF. Moreover, the peak current density of NiHCF2/PDDA/rGO hybrids (192 A g1) is 2.2 times that of the pure NiHCF (88 A g1). The superior electrochemical performance may be attributed to a combination of the small size of NiHCF NBs and the synergic effect from the rGO as discussed above. In addition, another couple of redox peaks (Peaks II) appears on the CV curves of NiHCF2/PDDA/rGO hybrids at the midpoint potential of 0.04 V, which is associspecies adsorbed on PDDA/rGO hybrids ated with Fe(CN)3 6 via electrostatic interaction. Its current decays continuously

in the process of potential cyclic sweeping, suggesting the absorbed Fe(CN)3 6 species will be desorbed from the matrix (Supplementary materials, Fig. S5). All of the NiHCF/PDDA/rGO hybrids display two couples of redox peaks as discussed above (Fig. 6B). Obviously, the shape of CV curves is associated with the content of NiHCF on the rGO, the less content of NiHCF in the NiHCF/PDDA/rGO hybrids, the higher ratio value of oxidative peak IaI to peak IaII on their CV curves (Table S2). Note that PDDA can act as a stabilizer for NiHCF NBs. It is well-known that NiHCF NBs with a smaller size will possess a higher specific surface, and thus a higher coverage of PDDA on the surface of NiHCF NBs. In the synthetic process of NiHCF NBs, PDDA can adsorb Fe(CN)3 6 species by electrostatic interaction, and then Ni2+ ion will chelate with Fe(CN)3 6 species to form NiHCF NBs. Therefore, NiHCF NBs with a smaller size will have a higher content of species on their surface. For NiHCF1/PDDA/rGO Fe(CN)3 6 hybrids, the content of NiHCF is least, and the size of NiHCF NBs is smallest among the NiHCF/PDDA/rGO hybrids. Correspondingly, the faradic current of peaks I is higher than that of peak II on the curves of NiHCF1/PDDA/rGO hybrids. For

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Fig. 6 – CV curves of NiHCF, rGO and its hybrids (A) and different NiHCF/PDDA/rGO hybrids (B) in 0.5 M Na2SO4 solution at the scan rate of 50 mV s1. CV curves of NiHCF2/PDDA/rGO hybrids in 0.5 M Na2SO4 solution under different scan rate (C) and the corresponding dependence (D) of the peak (I) current on the square root of the scan rate. (A color version of this figure can be viewed online.) NiHCF3/PDDA/rGO hybrids, the highest load of NiHCF gives a poor conductivity, and slower the electron transfer at the electrode interface. Consequently, their redox reaction shows low reversibility as illustrated with a deformation of CV curves in Fig. 6B. Fig. 6C shows the CV curves of the NiHCF2/PDDA/rGO hybrids at different scan rate in the potential region from 0.2 to 0.8 V. Obviously, each curve exhibits two pairs of redox peaks. Fig. 6D shows the linear dependence of the current density upon the square root of scan rate, which can be indexed as a diffusion-controlled process [31], further confirming the good reversible stability and fast kinetic of electron transfer of the NiHCF [32]. The storage energy performance of the NiHCF/PDDA/rGO hybrids was determined by GCD test in 0.5 M Na2SO4 solution. Fig. 7A shows the GCD curves of rGO, pure NiHCF and NiHCF/ PDDA/rGO hybrids modified electrode at a current density of 0.2 A g1 within voltage between 0 and 0.8 V. The total gravimetric specific capacitances were calculated from the GCD curves using the following equation [33]: C¼

IDt mDV

where I (A) is the discharge current, Dt (s) is the discharge time, m (g) is the mass of the electroactive materials, and DV (V) is the voltage range of the discharge portion. The specific discharge capacitances sequence of these electrode materials follows the order of decrease: NiHCF2/PDDA/rGO (1320 F g1), NiHCF3/PDDA/rGO (1030 F g1) NiHCF1/PDDA/ rGO hybrids (753 F g1), NiHCF (606 F g1), rGO (189 F g1). Clearly, NiHCF2/PDDA/rGO hybrids present a largest specific

capacitance mainly contributed from NiHCF component. This result accords with the discussion on the CV curves. The specific capacitance of NiHCF2/PDDA/rGO hybrids is comparable or superior to those of the NiHCF and rGO hybrids reported up to date [2,16,34–39] (Table 1). Fig. 7B shows GCD curves of NiHCF2/PDDA/rGO hybrids in the potential range from with different current densities. All these GCD curves are almost symmetrical, demonstrating a good capacitive behavior for the NiHCF2/PDDA/rGO hybrids. Rate capability is identified as an important parameter in the evaluation of supercapacitors. Fig. 7C shows the specific capacitances of rGO, pure NiHCF and NiHCF/PDDA/rGO hybrids modified electrode as a function of discharge density. The capacitance of pure NiHCF and NiHCF/PDDA/rGO hybrids decreases gradually with increasing current density. However, the capacitance of NiHCF2/PDDA/rGO hybrids is still as high as 1122 F g1 at 16 A g1, amounting up to 85% of the capacitance at 0.2 A g1. The pure NiHCF maintain 81% of the capacitance under the same conditions. Fig. 7C also reveals a capacitance decrease of 10% for the rGO, 21.4% for NiHCF3/ PDDA/rGO and 25.6% for NiHCF1/PDDA/rGO hybrids, respectively. As discussed above, the high reversibility of NiHCF faradic reactions make it possible for NiHCF/PDDA/rGO hybrids to charge/discharge under high current density [16]. Besides, the good rate capability of NiHCF/PDDA/rGO hybrids is closely related to the excellent conductivity of the rGO and the unique structure of NiHCF. To begin with, the rGO in the hybrids can act as the current collector to decrease the electrode resistance [34]. In addition, the uniform dispersion of NiHCF NBs on the rGO can increase the basal spacing between the rGOs, which is more accessible for the electrolyte ions to

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Fig. 7 – GCD curves for rGO, NiHCF and its hybrids at a current density of 0.2 A g1 (A) and NiHCF2/PDDA/rGO hybrids at different current density (B), the specific discharge capacitance plots of rGO, NiHCF and its hybrids at different current densities (C), and comparison of energy densities of supercapacitors using NiHCF or graphene hybrids in aqueous electrolytes (D). (A color version of this figure can be viewed online.)

Table 1 – Summary of the recent literature data on NiHCF and graphene-based hybrids for supercapacitors. Graphene hybrids

Ni3(Fe(CN)6)2(H2O) rGO rGO/PANI/CNT MEGO/MnO2 RuO2/RGM GOBIN G-PANI-Co3O4 Ni(OH)2/GS NiHCF2/PDDA/rGO

Performance Electrolyte solution

Current density (A g1)

Specific capacitance (F g1)

Refs.

1 M KNO3 6 M NaOH 1 M HCl 1 M H2SO4 2 M Li2SO4 1 M H2SO4 1 M KOH 1 M KOH 0.5 M Na2SO4

0.2 1.0 0.1 0.25 0.1 0.1 1 2.8 0.2

574.7 456 569 850 502.8 370 1063 1335 1320

[16] [2] [34] [35] [36] [37] [38] [39] This work

transport into the rGOs, leading to a significant enhancement of the double-layer capacitance and faradic capacitance of NiHCF/PDDA/rGO hybrids. Moreover, the wide basal spacing of the rGOs can form ‘‘ion-buffering reservoir’’ to sustain the supply of electrolyte ions at high current densities, resulting in sufficient faradic reactions generated for energy storage [34]. The energy density (E) and power density (P) are two vital factors to characterize the electrochemical performance of supercapacitors, which can be calculated from CDG curves at various discharging current [3]: 1 P ¼ Cs ðDVÞ2 2 E¼

P t

where Cs is the specific capacitance from the GCD curves, t is the time of discharge, and DV is the potential range. Fig. 7D represents the Ragone plots of NiHCF2/PDDA/rGO hybrids in the aforementioned electrolytes. The energy density can approach 58.7 Wh kg1 at a power of 80 W kg1 in 0.5 M Na2SO4 solution, higher than previous reported graphenebased supercapacitors in aqueous electrolyte [2,30,35,40,41]. Even at a high power density of 6400 W kg1, the energy density still can be kept at approximately 35.8 Wh kg1. Therefore, NiHCF2/PDDA/rGO hybrids exhibit a large power range that can be obtained while maintaining a relatively high energy density. To further understand the superior power performance of these NiHCF/PDDA/rGO hybrids, the kinetic feature of the modified electrodes was investigated using electrochemical

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impedance spectroscopy (EIS) in 0.5 M Na2SO4 solution. Fig. 8 shows the Nyquist plots extracted from the EIS measurements on the rGO, pure NiHCF and NiHCF2/PDDA/rGO hybrids. The Nyquist plots of the rGO display a typical characteristic of EDLCs with nearly vertical shape at the lower frequencies (15–0.01 Hz) [41]. As shown in inset, the inconspicuous arc in the high frequency region (10,000– 55 Hz) is modeled by a parallel combination of an interfacial charge transfer resistance and the double-layer capacitance [42]. The slope of the 45 portion of the curve in the transition zone (55–15 Hz) refers to the Warburg resistance, which is a result of the frequency dependence of ion diffusion in the electrolyte to the electrode interface [43]. The very small length of the Warburg curve reveals a rapid ion transport within the rGO. For NiHCF2/PDDA/rGO hybrids, they exhibit an excellent capacitive behavior as an ideal supercapacitor characterized with the slope of Nyquist plots equal to 4.5 at the low frequencies (90–0.01 Hz). In addition, their Nyquist plot in the transition zone (254–90 Hz) also displays a small Warburg resistance, indicative of a short ion diffusion path in NiHCF2/PDDA/rGO hybrids. Differently, their high frequency loop (10,000–254 Hz) of NiHCF2/PDDA/rGO hybrids is larger than that of the rGO, suggesting another resistance contribution generated from the electron transfer of NiHCF. For the pure NiHCF, it exhibits an interfacial redox process as illustrated by its Nyquist plot. It is composed of a semicircle in the high frequencies (10,000–1205 Hz) and a straight line with 45 inclination angle in the low frequencies (1205– 0.01 Hz). The semicircle is corresponding to faradic resistances caused by the interfacial electron transport in NiHCF [16], and the 45 line is associated with Warburg resistance resulted from ion diffusion. Both the faradic resistances and Warburg resistance of NiHCF2/PDDA/rGO hybrids are much less than those of the pure NiHCF. Therefore, the rGO can dramatically improve the energy storage performance of NiHCF, because it can favor the electron transfer of NiHCF and increase the efficient access of electrolyte ions. On the other hand, The X-intercept of the Nyquist plots represents the equivalent series resistance (ESR) [42], which is estimated to be 2.09, 4.26 and 13.84 X cm1 for the rGO, NiHCF2/PDDA/

Fig. 9 – Capacitance retention of NiHCF2/PDDA/rGO hybrids and NiHCF at a current density of 1 A g1. Inset: the successive CV curves of NiHCF2/PDDA/rGO hybrids for 500 cycles. (A color version of this figure can be viewed online.)

rGO hybrids and pure NiHCF, respectively. NiHCF2/PDDA/rGO hybrids possess a lower ESR data relative to the pure NiHCF, demonstrating that rGO with its ultrahigh specific surface area can indeed act as a highly conductive substrate for NiHCF deposition. The cycle charge/discharge test has been employed to examine the service life of the NiHCF/PDDA/rGO hybrids. The variation of specific capacitance as a function of cycle numbers at the current density of 1 A g1 was shown in Fig. 9. The specific recharge capacitance of NiHCF2/PDDA/ rGO hybrids maintains 87.2% in contrast with 74.8% capacitance retention of the NiHCF for 10,000 cycles under same experimental condition, indicating that the rGO can significantly improve the cyclic stability of NiHCF. The inset shows the successive CV curves of NiHCF2/PDDA/rGO hybrids for 500 cycles of potential scanning. All of their CV curves exhibit an identical profile, demonstrating the good reversibility of the faradic reaction occurs at the NiHCF2/PDDA/rGO hybrids.

4.

Fig. 8 – Nyquist plots of rGO, NiHCF2/PDDA/rGO hybrids and NiHCF at open circuit potential. Inset: the high frequency region of the plot. (A color version of this figure can be viewed online.)

Conclusion

We demonstrate a co-precipitation strategy for the first time to prepare NiHCF/rGO hybrids. The NiHCF NBs can be uniformly deposited onto the surface of rGO with different diameters in high density using PDDA as a linker. Due to their special nanostructure and unique composition, these as-prepared NiHCF/PDDA/rGO hybrids show a great enhancement in capacity, rate capability and cyclic stability of supercapacitors by comparison with the pure NiHCF. Their maximum specific capacitance is 1320 F g1 at a discharge density of 0.2 A g1 in 0.5 M Na2SO4 solution with an 85% capacitance retention at 16 A g1 and 12.8% capacitance loss after 10,000 discharge/charge cycles at 1.0 A g1. They exhibit an energy density up to 58.7 Wh kg1 at the power density of 80 W kg1, which is comparable or superior to those so far reported for the rGO hybrids in aqueous electrolytes. Therefore, these superior properties of NiHCF/PDDA/rGO hybrids will prompt their extensive use as the promising electrode materials for supercapacitors on increasingly large scale.

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Acknowledgments This work was financially supported by Open Research Fund of Top Key Discipline of Chemistry in Zhejiang Provincial Colleges and Key Laboratory of the Ministry of Education for Advanced Catalysis Materials (Zhejiang Normal University, No. ZJHX201410).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2014.12.005.

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