Controllable synthesis of Ni(OH)₂/Co(OH)₂ hollow nanohexagons wrapped in reduced graphene oxide for supercapacitors

Abstract

Reduced graphene oxide (rGO) wrapped hollow nanohexagons comprised of nickel hydroxide and cobalt hydroxide have been synthesized via in situ wet chemical approach. The 3D structure comprised of rGO and Co, Ni–OH can improve the electron transport ability and increase the contact of the active sites with electrolyte. The Ni(OH)₂/Co(OH)₂ hollow nanohexagons are uniform with outer diameter of ∼200 nm and inner diameter of ∼150 nm. The electrochemical performance of Ni(OH)₂/Co(OH)₂ hollow nanohexagon wrapped by rGO can be readily manipulated by adjusting the ratio between Ni, Co precursors and graphene oxide. High specific capacitance with enhanced electrochemical properties is attributed to the conductive network provided by graphene and synergetic effect between graphene and other components in electroactive material. Specially, the Ni(OH)₂/Co(OH)₂ hollow nanohexagon with 1.0 μg mL⁻¹ rGO exhibits a maximum specific capacitance of 1292.79 F g⁻¹. Meanwhile, such sample also exhibit smaller equivalent series resistance and charge transfer resistance compared with hollow hexagons wrapped by other rGO concentrations. Cycling performance performed at 5 A g⁻¹ shows that after 2500 cycles, the capacitance can still maintain 85.9% of the maximum.

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Introduction

Nowadays, renewable and clean energy supplies, such as solar, wind and electrochemical energies, have been employed to relieve the increasingly serious energy and environmental issues.^1–3 Highly efficient storage devices with superb energy and power densities, as well as excellent long-term stability are in urgent demand to solve the energy issues.^4–6 Supercapacitors, classified into electric double layer capacitors (EDLC) and pseudocapacitors, have attracted considerable attention due to their fast dynamics response, excellent power density and superb long-term stability.^7–10 By employing carbonaceous materials as electrodes, EDLCs store energy at the electrode material–electrolyte interface through physically opposite charge separation and possess high power density.¹¹ However, they also suffered from relatively low capacitance, which seriously hinder their practical applications.¹² Pseudocapacitors exhibit not only higher power density and better cyclic stability than secondary batteries but also higher specific capacitance than EDLCs. Hence, pseudocapacitors which carry out fast and reversible faradaic redox reactions occurring at the electrolyte/electrode interface become potential alternatives for practical use.^13–16

Cobalt base and nickel base compounds have been proven to possess great potential in the applications as electrochemical supercapacitor materials because of their excellent electrochemical properties.^13,17–19 In addition, their electrochemical performances can be well tuned by the precise control of their morphologies, including layered structure which allows fast guest ion insertion/desertion reaction in interlayer space. Among them, cobalt hydroxide has relatively high theoretical capacitance which attribute to the two-step reversible redox reactions from Co(II) ↔ Co(III) ↔ Co(IV). In addition, through construction of bimetallic hydroxide, excellent electrochemical activity is exhibited which results from synergistic effect of nickel and cobalt ions in faradaic redox reactions.^19,20 In Ni–Co binary hydroxide, the atomic substitution will also increase electrical conductivity and effectively enhance the active site density and roughness which result in enhanced electrochemical performance.^21,22 Hou et al. synthesized a nanoporous α-Co(OH)₂ mesocrystal nanosheets through biomolecule-assisted hydrothermal approach with a specific capacitance of 506 F g⁻¹.²³ Previously, we reported a unique 2D Ni(OH)₂@Co(OH)₂ nanohexagons synthesized via hydrothermal reaction, and a high specific capacitance of 369 F g⁻¹ could be exhibited at 1 A g⁻¹.²⁴ However, the limited specific capacitance of cobalt hydroxide or Co–Ni binary hydroxide still make it difficult in practical applications. Moreover, the electrode also suffered from great capacity loss at higher current densities which mostly attributed to the weak electron transport capability. The electrochemical performances of these materials need further optimization. In this case, carbonaceous materials are frequently blended with transition metal hydroxides, forming composite electrodes. On one hand, electron transport ability can be improved; on the other hand, the shortage of transition metal hydroxide materials can be overcome.^25–27

Graphene, a new allotrope of carbon with remarkable mechanical, electrical and thermal properties, has attracted tremendous attentions in recent decade.^28,29 Since its discovery in 2004, it has been widely investigated to improve the performance of various devices, such as transparent conductors, supercapacitors and sensors. Recent studies revealed that reduced graphene oxide (rGO) is potential in various applications such as separation, Li-ions batteries, and supercapacitors.³⁰ Reduced graphene oxide, which has good electrical conductivity and chemical stability, can suppress the volume change and particle agglomeration during the charge–discharge process by forming a uniform nanocomposite with metal compounds.^31–33 Moreover, a perfect contact can be formed between the metal compounds and the 2D graphene nanostructure, benefiting the ion diffusion and effective utilization of the surface area.^34–36 Considering the multiple characteristics of the graphene architectures, such as rich porosity and multidimensional electron-transport pathway,³⁷ the development of electrochemical performance of hybrid materials consist of graphene and 2D metal hydroxides have attract more and more attention for exploiting more potential applications of graphene.^18,38–40

Results and discussion

The EDS, XRD and XPS analysis

The energy dispersive X-ray spectroscopy (EDS) experiment is taken firstly. The EDS spectrum is shown in Fig. 1a. The C, Co, Ni elements can be observed in EDS spectrum clearly while Si peaks and Al peaks can be attributed to the substrate. The Co : Ni ratio of the material is about 2.5 : 1 according to the EDS spectrum. This result is in close agreement with our previous work.²⁴

Fig. 1 (a) EDS spectrum of the sample with 1.0 μg mL⁻¹ rGO; (b) XRD pattern of sample with 1.0 μg mL⁻¹ rGO; (c) Raman spectra of Ni(OH)₂/Co(OH)₂@rGO and graphene oxide.

The as-synthesized material was also identified as Ni–Co hydroxide by X-ray diffraction (XRD) and Fig. 1b shows the XRD pattern of the sample. The result demonstrated that the Ni, Co–OH exist as β-Co(OH)₂ (PDF# 30-0443) and β-Ni(OH)₂ (PDF# 14-0117) according to the XRD patterns.^20,41 No other peaks can be observed in XRD pattern, indicating high phase purity of the sample.

Raman spectrum analysis

Raman spectroscopy is an important tool to characterize carbon-based materials, especially C=C bonds which lead to high Raman intensities. Raman scattering is strongly dependent upon electronic structure which makes it an efficient tool to recognize the characteristic of the graphene oxide and reduced graphene oxide.⁴² In Raman spectrum of GO and rGO, the D vibration band arising from the breathing mode of j-point phonons of A_1g symmetry while the G vibration mode is owing to the first-order scattering of E_2g phonons by sp² carbon. Hence, the higher intensity of the D-band suggested the presence of a more isolated graphene domain in rGO in comparison to graphene oxide.⁴³

Raman spectrums of Ni(OH)₂/Co(OH)₂@rGO and the graphene oxide are shown in Fig. 1c. In Fig. 1c, the Raman spectrum of Ni(OH)₂/Co(OH)₂@rGO exhibits two obvious peaks centred at 1346 and 1592 cm⁻¹, which attributed to D and G-band, respectively. The D and G-band of graphene oxide can be observed at 1350 and 1599 cm⁻¹, respectively. According to Fig. 1b, the intensity ratio between the D-band and G-band (I_D/I_G) of bare graphene oxide is calculated to be 0.93 while Ni(OH)₂/Co(OH)₂@rGO shows a much higher I_D/I_G ratio of 2. Moreover, the D-band and the G-band of Ni(OH)₂/Co(OH)₂@rGO have been shifted toward lower frequency compared with bare GO, which indicating the reduction from graphene oxide to rGO.⁴³ However, the higher I_D/I_G ratio also indicated an intimate interaction between Ni(OH)₂/Co(OH)₂ and rGO which result in defects and destruction of sp² regions. Such interaction would improve the charge transfer property by defect-assisted propagation during electrochemical process.^44,45

XPS survey spectrum of samples both with and without rGO wrapped are shown in Fig. S1.† All of the spectrum were calibrated with C 1s peak at 284.6 eV. Fig. S1a† displayed the Co 2p core-level XPS spectra. For Ni(OH)₂/Co(OH)₂ hollow nanohexagon without rGO wrapped, the peaks at the 780.50 eV and 796.28 eV are assigned to Co 2p_3/2 and Co 2p_1/2 energy levels, along with two satellite peaks, which are ascribed to the shakeup excitation of the high-spin Co²⁺ ions.⁴⁶ For Ni(OH)₂/Co(OH)₂ hollow nanohexagon wrapped by rGO, the Co 2p_3/2 and Co 2p_1/2 peaks shifted to 779.87 and 794.93 eV, respectively. These shifts were likely due to the influence of the reduced graphene oxide. In Fig. S1b,† the Ni 2p peaks of both samples positioned at 855.94 eV (Ni 2p_3/2) and 873.39 eV (Ni 2p_1/2), respectively.^18,47 Besides, the Ni 2p spectrum contains two prominent shake-up satellites (denoted as “Sat.”). The Ni 2p_3/2 and Ni 2p_1/2 peaks show no obvious shift after rGO wrapped, which indicated that rGO exert little influence on Ni 2p_3/2 and Ni 2p_1/2 peaks.

Morphology analysis

Fig. 2a and b demonstrate that the products have a typical hexagonal structure and are very uniform in large scale. The inner facets are found to be parallel to the outer facets in a hollow nanohexagon. The diameter of the Ni(OH)₂/Co(OH)₂ hollow nanohexagon is about 200 nm and the thickness is about 30 nm. As the hollow nanohexagons are lamellar and hexagonal nature, they tend to lie with (001) plane. The existence of rGO can be easily distinguished from the wrinkle around the hollow nanohexagon. Due to the soft and flexible structure of graphene, the hollow nanohexagons are successfully well wrapped by the rGO shown in Fig. 2b.

Fig. 2 SEM images of Ni(OH)₂/Co(OH)₂ hollow hexagons wrapped by 1.0 μg mL⁻¹ rGO in (a) large scale and (b) small scale. (c) TEM bright-field image of hollow hexagon and (d) its corresponding HRTEM. The inset in (d) shows the SAED pattern of the Ni(OH)₂/Co(OH)₂ hollow hexagons with 1.0 μg mL⁻¹ rGO.

Bright field transmission electron microscopy (BFTEM) image, high-resolution TEM image and the corresponding diffraction pattern are shown in Fig. 2c and d. It can be observed that the surface of the nanohexagon is scobinate with small grains attached on it. As shown in Fig. 2c, the outer diameter of the nanohexagon was calculated to be ∼200 nm while the inner diameter is ∼150 nm. The interior angle of the hollow nanohexagon is determined to be 120° which suits typical hexagonal structures well. The thickness is about 30 nm and close to the edge width of hollow nanohexagon. The lattice spacing of the as-synthesized sample was determined to be 0.271 nm depending on the HRTEM image in Fig. 2d, corresponding to the (100) plane of Co(OH)₂ or Ni(OH)₂. Investigation indicated that the small crystallites on the surface are estimated to be about 4 nm. The corresponding selected area electron diffraction (SAED) pattern discloses its polycrystalline nature shown in the inset of Fig. 2d, indicating that the small grains absorbed on the scobinate surface are in different orientations.

The growth mechanism

Based on the above characterization, a possible growth mechanism for the evolution from reduced graphene oxide wrapped Ni(OH)₂/Co(OH)₂ nanoplate to reduced graphene oxide wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagon. Both internal crystal factors and external factors, such as additives and pH values, have been considered.

The formation of the graphene wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagon can be divided into three steps: (I) the Ni²⁺/Co²⁺ ions were firstly absorbed on graphene oxide because of the electrostatic interaction, (II) the Ni(OH)₂/Co(OH)₂ nuclei formed before the formation of graphene wrapped Ni(OH)₂/Co(OH)₂ hexagonal nanoplates, (III) the formation of graphene wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagons.

In step one, the Co²⁺ and Ni²⁺ ions were firstly absorbed with graphene oxide because of the electrostatic interaction. Next, the Co, Ni–OH nuclei tend to grow along the [001] axis. As the Co/Ni hydroxide has a hexagonal brucite structure, the absorbed nuclei will form hexagonal nanoplates.²⁴

In current synthesis system, N₂H₄·H₂O acts as a strong reducing agent. After the formation of graphene oxide wrapped Ni(OH)₂/Co(OH)₂ hexagonal nanoplates, the graphene oxide was reduced firstly to reduced graphene oxide. As a higher density of imperfections, such as planar defects, exists near the centre of the nanoplates, the dissolution energy of the nanocrystals in the centre is reduced and the centre of the nanoplates tend to have higher dissolution rate. Hence, several nanoscale holes will form in the centre with the existence of reducing agent. As the reaction processed, the nanoscale holes become larger and connect with each other and the nanoplates turn out to be hollow nanohexagons gradually. The Ni(OH)₂/Co(OH)₂ nanoplates was reduced to Ni(OH)₂/Co(OH)₂ hollow nanohexagons and metallic cobalt. The formation of reduced graphene oxide wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagons is schematically illustrated in Fig. 3, the corresponding time dependent SEM images have been given in Fig. S2.†

Fig. 3 A schematic illustration of the formation process of the rGO wrapped Ni(OH)₂/Co(OH)₂ hollow hexagons.

Electrochemical performance comparison

In order to verify the superiority of rGO wrapped hollow nanohexagons electrode, the electrochemical performance of hollow nanohexagons electrode with rGO concentrations ranging from 0 to 2.0 μg mL⁻¹ are measured. The concentrations of the GO in precursors are determined from 0.6 to 2.0 μg mL⁻¹ because of the reasons below:

If the concentration of GO is lower than 0.6 μg mL⁻¹, only few hollow nanohexagons have been wrapped in rGO because of the insufficiency of rGO. Meanwhile, as the concentration of GO is more than 2.0 μg mL⁻¹, the hollow nanohexagons are wrapped by the rGO tightly after the reaction. As the rGO layer is thicker and tighter compared with previous samples, it impossible to separate the reduced metallic cobalt from as-synthesized sample when they are wrapped in the rGO.

Fig. 4a shows the CV curves of the rGO wrapped hollow nanohexagons with different rGO concentration in a scan rate of 10 mV s⁻¹. Each sample exhibits two pairs of visible redox peaks: in the anodic process, Co²⁺ shifts to Co³⁺ and Co⁴⁺ step by step, and in the cathodic process, Co⁴⁺ shifts to Co²⁺ in the same way.⁴⁸ Meanwhile, the valence state changes between Ni²⁺ and Ni³⁺.⁴⁹ Corresponding reversible redox reaction is described below:

Co(OH)₂ + OH⁻ ↔ CoOOH + H₂O + e⁻ (1)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (2)

Ni(OH)₂ + OH⁻ ↔ NiOOH + H₂O + e⁻ (3)

Fig. 4 (a) CV curves of Ni(OH)₂/Co(OH)₂ hollow nanohexagons electrodes with different rGO concentrations at a scan rate of 10 mV s⁻¹; (b) galvanostatic discharge curves of hollow nanohexagons electrodes with different rGO concentrations at a current density of 1 A g⁻¹; (c) Nyquist plots of experimental impedance of hollow hexagons with different rGO concentrations.

The redox peaks attributed to Ni²⁺ → Ni³⁺ are close to those between Co²⁺ → Co³⁺ and Co³⁺ → Co⁴⁺, which result in the coupling redox peaks performed in Fig. 3a. Each curves show great symmetric shapes, which is result of good reversibility of the as-synthesized material.⁴¹

According to the equation for specific capacitance, a larger area surrounded by the CV curves indicate a higher specific capacitance.¹² The CV curve of rGO wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagons with a concentration of 1 μg mL⁻¹ shows the largest area than the other samples and the bare Ni(OH)₂/Co(OH)₂ hollow nanohexagons is the smallest. This result indicated that the rGO wrapped structure improved the specific capacitance of Ni(OH)₂/Co(OH)₂ hollow nanohexagon significantly.⁴⁴

C_P = IΔt/MΔv (4)

In Fig. 4b, a series of galvanostatic discharge measurements are conducted on electrodes with different rGO concentrations at 1 A g⁻¹ as well. Based on the eqn (4), longer discharge time will lead to higher specific capacitance. As illustrated in Fig. 4b, all electrodes displayed typical pseudocapacitive behaviours with non-linear discharge curves and exhibit discharge voltage plateaus, which correspond to the reduction peaks in CV curves.^50–52 The Ni(OH)₂/Co(OH)₂ hollow nanohexagon with 1 μg mL⁻¹ rGO shows the longest discharge time. Calculations determined that specific capacitance of the Ni(OH)₂/Co(OH)₂ hollow nanohexagons with 1.0 μg mL⁻¹ rGO exhibits the highest specific capacitance of 1292.8 F g⁻¹. The specific capacitance of Ni(OH)₂/Co(OH)₂ hollow nanohexagons with 0 μg mL⁻¹, 0.6 μg mL⁻¹, 1.4 μg mL⁻¹ and 2.0 μg mL⁻¹ are 358.7 F g⁻¹, 504.9 F g⁻¹, 526.7 F g⁻¹ and 434.2 F g⁻¹, respectively.

This result indicated that the introduction of rGO enhanced the specific capacitance of Ni(OH)₂/Co(OH)₂ hollow nanohexagons significantly. The specific capacitance has increased to the 3.6 times of the Ni(OH)₂/Co(OH)₂ hollow nanohexagons one after the proper addition of rGO.

Electrochemical impedance spectroscopy (EIS) is an effective method to evaluate the bulk resistance and the electron transport resistance.⁵³

As shown in Fig. 4c, electrochemical impedance spectroscopy was carried out and the Nyquist plot is given. The inset is the EIS spectrum in high frequency region. In low frequency region, the slopes of rGO wrapped samples are close and nearly vertical compared with sample without rGO which indicated that with the participation of rGO, the material shows rapid electrolytic diffusion and more ideal capacitive behaviour during the electrochemical performance.^54,55 In high frequency region, the first intercept with the real axis reveals the equivalent series resistance (ESR) of the electrodes and the diameter of the semicircles corresponds to the charge transfer resistance of the reaction.^27,56 The Ni(OH)₂/Co(OH)₂ hollow nanohexagons with 1.0 μg mL⁻¹ rGO shows the smallest ESR of 0.338 Ω. Moreover, it also shows the smallest diameter in semicircle, which demonstrated the lowest charge transfer resistance according to the spectrum. In EIS spectrums, the bare Ni(OH)₂/Co(OH)₂ hollow nanohexagon shows a relatively high ESR in the spectrum. When the rGO concentration increased from 0 to 1.0 μg mL⁻¹, the ESR decreased rapidly. After that, further increase of the rGO concentration will enlarge the ESR as shown in Table S1.† Charge transfer resistances of these samples show similar trend with that of ESR. The charge transfer resistance firstly decreased and then increased with the increasing of the rGO concentration. Among them, the Ni(OH)₂/Co(OH)₂ hollow nanohexagon with 1.0 μg mL⁻¹ rGO exhibits the best in electrochemical impedance properties. This result is in good accordance with the result displayed in Fig. 4b. These results can be attributed to the redundant rGO in samples, which improve the electric conductivity firstly and become thicker in synthesis and block the charge transfer during the electrochemical processes afterwards.³³

Based on the above analyses, the reasons for the optimal graphene wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagons can be well understood considering the following factors:

Firstly, the high quality rGO provided efficient conductive 3D networks for fast ion and electron transport and the layered structure rGO also provide double-layer capacitance.⁵⁷ Secondly, in graphene wrapped hollow nanohexagon structure, the hollow nanohexagons are closely protected by the adjacent graphene sheets and prohibit it from aggregation.⁵⁸ Moreover, the rGO wrapped structure can also enlarge the specific area of the material (56.1 m² g⁻¹) compared with the bare Ni(OH)₂/Co(OH)₂ hollow nanohexagon (23.3 m² g⁻¹).⁵⁹

To further investigate the electrochemical performance of Ni(OH)₂/Co(OH)₂ hollow nanohexagons with 1.0 μg mL⁻¹ rGO, detailed electrochemical experiments have been taken. As shown in Fig. 5a, the CV curves with different scan rates (ranging from 1–50 mV s⁻¹) are performed from −0.1 to 0.4 V. The Ni(OH)₂/Co(OH)₂ hollow nanohexagons wrapped by 1.0 μg mL⁻¹ rGO shows two pairs of redox peaks with relatively small rectangle CV geometry, which indicated a mainly pseudocapacitive type.⁶⁰ The rectangle geometries are ascribed to the existence of rGO which is electric double-layer capacitance. The shapes of the CV curves show similar geometry as the scan rate increased from 1 mV s⁻¹ to 50 mV s⁻¹. As the scan rate increased, the oxidation peaks tends to move towards positive potential direction while the reduction peaks towards the negative direction.¹⁷ At lower scan rates, the individual peaks can be distinguished which corresponding with the Co²⁺ → Co³⁺ process and Co³⁺ → Co⁴⁺ process.⁶¹ As the scan rate increased, the oxidation peaks are broadened gradually with the vanish of individual peak. This is the result of the movement of redox pairs as the scan rate increased.⁶² The peak pair which ascribed to Ni²⁺ → Ni³⁺ are missing in the curves. This is mainly attributed to the coincident peak position between the redox processes of cobalt hydroxide and nickel hydroxide.⁶⁰

Fig. 5 (a) CV curves of Ni(OH)₂/Co(OH)₂ hollow nanohexagons with 1.0 μg mL⁻¹ rGO at various scan rates; (b) galvanostatic discharge curves of hollow nanohexagons electrodes wrapped by 1.0 μg mL⁻¹ rGO concentrations at different current densities; (c) dependence of specific capacitances of samples on current densities; (d) specific capacitance of Ni(OH)₂/Co(OH)₂ hollow hexagons as functions of cycle number at a current density of 5 A g⁻¹.

The galvanostatic discharge curves of the electrode prepared by Ni(OH)₂/Co(OH)₂ hollow nanohexagons with 1.0 μg mL⁻¹ rGO in various current densities has also been further illustrated in Fig. 5b. According to the CV curves, a high specific capacitance of 1292.8 F g⁻¹ has been achieved at current densities of 1 A g⁻¹. The specific capacitances of the electrode in other current densities are calculated to be 1141.3, 1035.6 and 902.2 F g⁻¹ at current densities of 3, 5 and 10 A g⁻¹, respectively.

The rate capacitive performance has also been investigated, and has been shown in Fig. 5c. As the current density increased, the specific capacitance of the electrode tends to decreased exponentially. The specific capacitance was calculated to be about 69.8% when the discharge rate increased from 1 to 10 A g⁻¹.

Compared with bare Ni(OH)₂/Co(OH)₂ hollow nanohexagons, the rGO wrapped structure improved the rate capacity performance significantly while bare Ni(OH)₂/Co(OH)₂ hollow nanohexagons show only 48.3% of the highest from 1 to 10 A g⁻¹, as shown in Fig. S3.†

The durability performance is one of the most important parameters for the application of electrode material. The cycling test of rGO wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagons has been taken at a current densities of 5 A g⁻¹. As displayed in Fig. 5d, the specific capacitance increased in the first 200 cycles, which attributed to the activation of the electroactive materials. According to the figure, the rGO wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagons exhibit extraordinary stability in alkali solution. The capacitance retention rate is 85.9% after 2500 cycles.

Experimental methods

Reagents and materials

The graphene oxide was synthesized from natural graphite powder according to the modified Hummers' method. Nickel chloride hexahydrate (NiCl₂·6H₂O, ≥99%), cobalt chloride hexahydrate (CoCl₂·6H₂O, ≥98%), sodium hydroxide (≥96%), hydrazine hydrate (N₂H₄·H₂O, 80 wt%) and ethanol were all purchased from Beijing Reagent Company. All reagents were of analytical purity grade and directly used for experiments without any further purification.

Synthesis procedure

The graphene wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagon was prepared by the following optimal procedure. In a typical synthesis, CoCl₂·6H₂O and NiCl₂·6H₂O with a molar ratio of 9 : 1 were dissolved in de-ionized water. The concentrations of CoCl₂ and NiCl₂ are 0.009 M and 0.001 M, respectively. GO suspension was then dissolved in precursor with concentration of 0.6 μg mL⁻¹, 1.0 μg mL⁻¹, 1.4 μg mL⁻¹ and 2.0 μg mL⁻¹, respectively. The mixtures were stirred for more than 24 hours before reaction. N₂H₄·H₂O (0.3 mL) was added to the solution (40 mL) as reducing agent. During the reaction, the pH value of the solution was kept at about 13 by adding NaOH. The solution was then heated at 180 °C for 100 minutes in a sealed Teflon-lined autoclave. The autoclave was then cooled to room temperature in air. The product was collected by centrifugation and washed several times with de-ionized water and ethanol.

Apparatus and crystal structure measurements

The overall crystallinity and phase purity of the as-synthesized samples were analysed by X-ray powder diffraction on a Rigaku D-max 2200 X-ray diffractometer using Cu K_α incident radiation. The morphologies and chemical compositions of the products were characterized by a Hitachi S-4800 scanning electron microscope (SEM) equipped with an Oxford X-max energy-dispersive spectrometer (EDS) and a Carl Zeiss Supra 55 Ultra SEM. The crystal structures are analysed on an JEM-2200FS transmission electron microscope (TEM, 200 kV) equipped with a Gatan electron energy filter. The specific surface areas of the as-synthesized samples were determined by the Brunauer–Emmett–Teller (BET) measurement by Quantachrome Autosorb-IQ-MP automated gas sorption analyser.

Preparation of electrodes and electrochemical measurements

The working electrodes were prepared by mixing the as-synthesized materials, carbon black, and polytetrafluoroethylene (PTFE) in a weight ratio of 7 : 2 : 1 to obtain homogeneous slurry. Then we coated the homogeneous slurry into nickel foam substrates. The loading masses of the electroactive material were kept about 4 mg. The electrodes were dried in vacuum for 10 hours before tableting. The electrochemical properties of as-prepared material were evaluated in a three-electrode system. Platinum wire and saturated calomel electrodes were used as counter and reference electrodes, respectively. Electrochemical measurement was conducted on a CHI760E electrochemical workstation. All the measurements were carried out in a three-electrode system containing 6 M KOH aqueous solution as the electrolyte at room temperature.

Conclusions

Reduced graphene oxide has been introduced in the synthesis of Ni(OH)₂/Co(OH)₂ hollow nanohexagons via facile hydrothermal synthesis. The optimal wrapped Ni(OH)₂/Co(OH)₂ hollow nanohexagon by rGO can be obtained by the introducing GO with 1.0 μg mL⁻¹ in precursor solution. The Ni(OH)₂/Co(OH)₂ hollow nanohexagons which are homogeneously wrapped by rGO show 200 nm in outer diameter and 150 nm of inner diameter. The as-synthesized sample shows significant improvement in electrochemical performance compared with the bare Ni(OH)₂/Co(OH)₂ hollow nanohexagons. The optimal Ni(OH)₂/Co(OH)₂ hollow nanohexagon wrapped by rGO exhibits a specific capacitance of 1292.8 F g⁻¹ which is 3.6 times of that of bare Ni(OH)₂/Co(OH)₂ hollow nanohexagons and about 2.5 times of that of Ni(OH)₂/Co(OH)₂ hollow nanohexagons with rGO of other concentration. The enhanced performance can be attributed to efficient conductive networks for fast ion and electron transport provided by rGO networks. Moreover, with further increase of rGO in hollow hexagon samples, the rGO become thicker and block the charge transfer during the electrochemical processes which result in the decrease of specific capacitance. The optimal Ni(OH)₂/Co(OH)₂ hollow nanohexagon wrapped by rGO shows extraordinary long-term stability in cycling test and maintained 85.9% of the maximum of the specific capacitance after 2500 cycles.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51371015, 51571006 and 51331002), the Beijing Natural Science Foundation (No. 2142018) and the Fundamental Research Funds for the Central Universities (FRF-BR-15-009B).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20361b

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