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DOI: 10.1039/C4RA06601D (Paper) RSC Adv., 2014, 4, 39875-39883

Surfactant free gram scale synthesis of mesoporous Ni(OH)2–r-GO nanocomposite for high rate pseudocapacitor application†

Upendra Singh‡ ab, Abhik Banerjee‡*ab, Dattakumar Mhamaneab, Anil Suryawanshiab, Kush Kumar Upadhyayab and Satishchandra Ogale*ab
aCentre of Excellence in Solar Energy, National Chemical Laboratory (CSIR-NCL), Dr Homi Bhabha Road, Pune, India
bNetwork Institute of Solar Energy (CSIR-NISE), New Delhi, India. E-mail: abhikchm@gmail.com; sb.ogale@ncl.res.in; Fax: +91 20 2590 2636; Tel: +91 20 2590 2260

Received 3rd July 2014 , Accepted 19th August 2014

First published on 19th August 2014

Abstract

We report a single-step surfactant-free gram scale hydrothermal synthesis of mesoporous Ni(OH)2 nanoparticles and the Ni(OH)2–reduced graphene oxide (Ni(OH)2–r-GO) nanocomposite. Interesting morphological features are noted. These nanomaterials are examined and compared as cathode materials for pseudo-capacitor application through detailed characterizations. A high specific capacitance (Cs) of 1538 F g−1 is observed for Ni(OH)2–r-GO even at a high current density of 40 A g−1, whereas at the same current rate, bare Ni(OH)2 shows Cs of only 936 F g−1.

Introduction

Depleting natural resources (fossil fuels) and growing energy demands have forced the development of high efficiency energy storage systems (Li-ion batteries (LIBs), supercapacitors (SCs), hybrid capacitors).1–3 Amongst these, the SCs are being widely explored because of their fast charge–discharge properties and long cyclability.4–6 SCs are used in conjugation with LIBs in laptops, portable media players, power backup, hybrid electrical vehicles etc.7

There are two types of SCs based on their charge storage mechanism. In electrical double layer capacitors (EDLCs) the charge storage is governed by diffusion and accumulation of charges at the electrode–electrolyte interface whereas in the pseudocapacitors it is dominated by Faradaic reactions at the electrode.8,9 Carbon based nanomaterials are being intensely investigated as electrode materials in EDLCs. However, these materials exhibit relatively low specific capacitance (Cs) values than desired.10 Metal oxides/hydroxides are being separately investigated as efficient electrode materials in pseudocapacitors. However, their performance is limited due to a low operating potential window.11

In comparison with the carbonaceous materials, metal oxides offer an advantage of easier and cost effective synthesis. Metal oxides such as MnO2, Co3O4, RuO2, NiO, Ni(OH)2 etc. have a very high theoretical capacitance values due to ultra fast and highly reversible redox reactions.2,12–17

It has been established that the specific capacitance and rate performance of such pseudocapacitive materials depends greatly upon the use of active material in the electrode during electrochemical performance, and the rates of electron and ion transmission.18 However, these pseudocapacitive materials usually suffer from low energy storage due to poor conductivity, low stability at high current rate, limited potential window, and less utilization of active material during electrochemical performance. Therefore, development of pseudocapacitive materials with high surface area, porosity and greater electronic conductivity is desired to achieve high energy storage.19,20 The problem of low operating potential window is being separately addressed by developing asymmetric SCs i.e. coupling of pseudocapacitive materials with carbon based materials. Such devices can provide higher energy density than individual electrode components.21–23

Amongst various pseudocapacitive materials Ni(OH)2 has emerged as one of the most promising candidates with high theoretical specific capacitance value (2082 F g−1 within 0.5 V), low cost and easy processing.24,25 To enhance the supercapacitor properties of bare Ni(OH)2, carbon based materials such as activated carbon, carbon nanotubes and graphene have been incorporated.26–29 The outstanding electronic conductivity and high surface area of graphene has helped in the enhancement of electrochemical performance of Ni(OH)2.28–30 Hybrid with graphene also forms a conducting network by connecting individual Ni(OH)2 nanostructures thereby facilitating the fast electron transfer between the active material and the current collector. Also, stacking of graphene sheets is prevented due to the anchoring of Ni(OH)2 nanostructures onto the graphene sheets resulting in an enhanced electrochemical performance of the composite. Tang et al.23 have reported a capacitance of 3300 F g−1 for Ni(OH)2–CNT based composite. Yang et al.31 have reported a capacitance of 3152 F g−1 for electrodeposited Ni(OH)2. Yan et al.32 have reported a capacitance of 2194 F g−1 for Ni(OH)2–r-GO composite. Although these results are quite interesting and impressive, they suffer from some disadvantages such as scalability in synthesis and fading capacity at high current densities that has prohibited their practical applicability.33 Also, the employed synthesis methods, e.g. chemical vapor deposition, electro-deposition are expensive to implement. This calls for the use of easy and cost effective materials synthesis methods which can render similar or better levels of performance.

In this work, we report a single step bulk scale, surfactant free hydrothermal synthesis of mesoporous channeled Ni(OH)2 nanoparticles and their composite with reduced graphene oxide (r-GO). We term the composite as Ni(OH)2–r-GO. The synthesis procedure demonstrated here is easy, cost effective, highly scalable and can be used commercially. The specific capacitance values obtained for Ni(OH)2 and Ni(OH)2–r-GO at low current density are fairly comparable to the recent reports (please see ESI Table 1†). Importantly, Ni(OH)2–r-GO performs exceptionally well even at high current rates delivering a Cs of 1538 F g−1 at a high current density of 40 A g−1.

Experimental section

All the chemicals used for the synthesis were analytical grade and commercially available from Merck Pvt. Ltd. These were used as received without further purification.

Synthesis of mesoporous channelled Ni(OH)2 nanoparticles

In a typical procedure for the synthesis mesoporous channelled Ni(OH)2 nanostructures, solution of NiCl2·6H2O (0.33 M) was prepared by dissolving NiCl2·6H2O (11.9 g) in deionized water (150 mL) to form a uniform clear solution. Then aqueous solution NaOH (2 M, 75 mL) was added drop-wise to the above solution to form a Ni(OH)2 suspension, which was stirred for 30 min. The light green Ni(OH)2 suspension was filtered and washed with distilled water several times to remove Cl−, Na+ ions and other possible impurities. Subsequently, the filtered Ni(OH)2, without drying was directly added to the aqueous solution of NaOH (2.5 M, 200 mL) under fierce stirring for 30 min. Then, the suspension was transferred into a 250 mL Teflon-lined stainless steel autoclave. The sealed autoclave was heated to and maintained at 160 °C for 20 h in an electric oven and then air-cooled to room temperature. The resulting light green precipitate was collected by centrifugation, washed with de-ionized water and alcohol alternately several times until the filtrate pH became neutral. Finally, the product was dried in a vacuum oven at 60 °C for 12 h. The total yield obtained was 6.2 g.

Synthesis of graphene oxide (GO)

Initially GO was prepared by the modified hummer's method.34 Later, it was purified by repeated centrifugation process as discussed elsewhere.35

Synthesis of Ni(OH)2–r-GO

Ni(OH)2–reduced graphene oxide (Ni(OH)2–r-GO) composite was prepared using basically a similar approach as used for the synthesis of mesoporous channelled Ni(OH)2 nanostructures. Initially, GO (150 mg) was added in deionized water (150 mL). After that, the above mixture was ultrasonicated for 1 h to get homogeneous dispersion of GO in water (1 mg ml−1). Then, NiCl2·6H2O (11.9 g) was dissolved in the GO dispersion. Later, the same procedure was followed as for the synthesis of bare Ni(OH)2. In situ reduction of GO into reduced graphene oxide (r-GO) takes place when heated in the basic medium.36

Characterization

X-ray Diffraction (XRD, Philips X'Pert PRO) was used for structural determination. Field Emission Scanning Electron Microscopy (FESEM, Nova NanoSEM 450) and High Resolution-Transmission Electron Microscopy (HR-TEM, FEI Tecnai 300) were used for morphology analysis and diffraction. Cyclic voltammetry, galvanostatic charge–discharge and impedance measurements were performed using AutoLab potentiostat with Nova 1.7 [PGSTAT30 (Eco-Chemie)]. The porous structure of the sample was confirmed by N2 adsorption–desorption isotherm using Quantachrome (Nova 3200 e) surface area and pore size analyzer. UV-visible diffused reflectance spectra were recorded using a Jasco V-570 spectrophotometer. Raman spectroscopy was carried out by a confocal micro-Raman spectrometer LabRAM ARAMIS Horiba Jobin-Yvon apparatus with laser excitation wavelength of 532 nm with appropriate filters.

Preparation of pseudocapacitor electrodes

The electrodes for the electrochemical measurements were prepared by mixing the active material (Ni(OH)2/Ni(OH)2–r-GO), conducting carbon (Super P), and binder (Kynar) in the weight ratio of (75[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]5) in N-methyl pyrrolidone (NMP) solvent in an agate mortar homogeneously and coating on carbon fiber paper (Toray paper, Alfa Aesar). The slurry coated papers were directly used as electrodes for measuring electrochemical properties after drying at 80 °C for 10 h in an electric oven.
Electrochemical measurements. Cyclic voltammetry (CV) studies, galvanostatic charge–discharge measurements and electrochemical impedance analysis were carried out using a three-electrode system. Ni(OH)2 and Ni(OH)2–r-GO were used as working electrodes, Hg/HgO as reference electrode, and platinum strip as a counter electrode. Cyclic voltammetry was carried using 2 M aqueous KOH solution as the electrolyte, at different potential scan rates (2–100 mV s−1). The potential window used in the measurement was from 0 to 0.65 V. Charging and discharging were carried out galvanostatically by varying the current density from 1 to 40 A g−1 over a potential range of 0–0.55 V. Cyclic stability study was carried out by cyclic voltammetry at a constant scan rate of 10 mV s−1 up to 1000 cycles.

Results and discussion

X-ray diffraction patterns of the Ni(OH)2 and Ni(OH)2–r-GO samples are shown in Fig. 1. All the peaks of Ni(OH)2 match very well with the JCPDF data file no. 14-0117 (hexagonal primitive lattice). The presence of sharp peaks in the Ni(OH)2 and the Ni(OH)2–r-GO confirms the good crystalline nature of the hydroxide phase in both the cases. Moreover, no extra peaks are observed confirming the high purity of the phase. In the case of the composite involving hydroxide of high Z element Ni the r-GO contribution is not easily discernible in XRD due to its low content, and the low Z of carbon. In order to reveal the presence of r-GO more clearly we dissolved the Ni(OH)2 component of the composite with concentrated HCl. The resulting product gave an XRD is shown in the inset of Fig. 2a. The occurrence of a broad hump around 2θ value of 23–26° unambiguously represents r-GO, implying that we do have r-GO in our composite phase, as desired. Moreover, no peak is observed around 2θ value of 10° (corresponding to GO) which confirms the successful reduction of GO to r-GO during the synthesis of the composite.
image file: c4ra06601d-f1.tif
Fig. 1 XRD patterns for (a) Ni(OH)2–r-GO composite, (b) Ni(OH)2, and (c) Ni(OH)2 (PCPDF standard data).

image file: c4ra06601d-f2.tif
Fig. 2 (a) Raman spectrum of Ni(OH)2–r-GO. Inset shows XRD spectrum of r-GO obtained from Ni(OH)2–r-GO by dissolution of hydroxide, (b) comparison of the Raman spectra of GO used for the synthesis of composite and r-GO from the composite.

To further confirm the presence of r-GO in Ni(OH)2–r-GO, Raman spectroscopy was performed on this composite sample. Fig. 2a shows the Raman spectrum for the Ni(OH)2–r-GO nanocomposite.

The Raman spectra for the Ni(OH)2 and r-GO components present in the composite were recorded using 0.6D and 2D filters, respectively, to get higher spectral clarity. Three prominent peaks are observed in the spectrum. The peak at 512 cm−1 corresponds to the Ni–OH symmetric stretching mode.2 The peaks at 1326 cm−1 and 1608 cm−1 correspond to the D and G bands of r-GO present in the composite, respectively.

In Fig. 2b we compare the Raman spectra for GO which was used as the starting material during the synthesis of the composite with the r-GO in the Ni(OH)2–r-GO sample. The Raman spectra reflect the reduction of GO to r-GO via the changes in relative intensity of the two main peaks: D and G.37 Also, there is considerable shift of the D peak towards lower wave number as expected for the GO to r-GO transformation. The D band originates from the defect-induced breathing mode of sp2 rings38 and arises from the stretching of C–C bond. The G peak on the other hand is due to the first order scattering of the E2g phonon of sp2 C atoms.38 The intensity of the D band depends on the size of the in-plane sp2 domains39 and its increase indicates formation of more sp2 domains. The relative intensity ratio (ID/IG) reflects the degree of disorder and is inversely proportional to the average size of the sp2 clusters. In our case the ID/IG ratio for GO used as the starting material is 0.97 while that for the carbon obtained after hydroxide dissolution is 1.08. This increase in the intensity ratio implies that newer graphene domains are formed and the sp2 cluster number has increased38 after the composite formation process. In order to further confirm the presence of r-GO in the composite Diffuse Reflectance Spectroscopy (DRS) was performed on the samples. The DRS spectra of Ni(OH)2 and the Ni(OH)2–r-GO nanocomposite are shown in Fig. 3.


image file: c4ra06601d-f3.tif
Fig. 3 Diffused reflectance spectra (DRS) of Ni(OH)2 and Ni(OH)2–r-GO composite. The inset image shows the powders of bare Ni(OH)2 (green) and the Ni(OH)2–r-GO (black) composite.

The absorbance at 655 nm (marked as 1 in the figure) and 380 nm (marked as 2 in the figure) are the d–d transitions of Ni2+ in octahedral coordination arising due to 3A2g → 3T1g and 3A2g → 3T2g transitions, respectively. The DRS spectrum of bare Ni(OH)2 matches with the previous literature report.40 The spectrum of Ni(OH)2–r-GO shows a much higher absorbance as compared to bare Ni(OH)2 and the signature peaks for Ni2+ are not visible due to the presence of r-GO. The inset to Fig. 3 shows the images of Ni(OH)2 and Ni(OH)2–r-GO. The bare Ni(OH)2 is green in color whereas Ni(OH)2–r-GO appears blackish due to the presence of r-GO.

Thermal behavior of the samples was also investigated using Thermogravimetric Analysis (TGA). Fig. 4 shows the TGA plots for both Ni(OH)2 and Ni(OH)2–r-GO.


image file: c4ra06601d-f4.tif
Fig. 4 Thermo-gravimetric analysis (TGA) of Ni(OH)2–r-GO and Ni(OH)2 in oxygen.

It can be seen from the TG curves that both of them begin to decompose at 230 °C and the process is complete at 295 °C. This decomposition is associated with the following reaction:

Ni(OH)2 → NiO + H2O

Further weight loss in the case of the composite can be attributed to the oxidation of r-GO to gaseous forms such as CO or CO2. From the comparative study of weight loss it can be concluded that around 2 wt% of r-GO is present in the composite. The TGA data thus further confirm the presence of r-GO in the composite.

The specific area and pore size distribution of Ni(OH)2–r-GO and bare Ni(OH)2 were studied using the N2 adsorption and desorption isotherms represented in Fig. 5. The specific surface area of bare Ni(OH)2 is only 24.842 m2 g−1 whereas that of the Ni(OH)2–r-GO composite is 43.752 m2 g−1. This increase in the specific surface area of the composite can be attributed to the addition of r-GO that acts as an anchor for the Ni(OH)2 nanoparticles thereby avoiding the stacking of these nanoparticles. It is seen from the adsorption–desorption isotherm that at low relative pressure the adsorbed volume does not increase rapidly, indicating the presence of fewer number of micropores in the sample. However, as the relative pressure increases, the adsorbed volume increases and at high relative pressure small hysteresis loop is observed, which a characteristic of Type-V isotherm. The presence of hysteresis loop indicates the presence of mesoporosity in the samples.41,42 Due to the capillary condensation in the mesopores, there is an increase in the adsorption isotherm in the relative pressure region of 0.4 to 0.8. The hysteresis loop observed here is the Type H3 loop, which does not represent any limiting adsorption at high relative pressures.42 This kind of loop is observed for slit-shaped pores in aggregated plate-like particles. The hysteresis loop in the case of our composite is bigger when compared to bare Ni(OH)2. The inset of the Fig. 5 represents the pore size distribution present in the samples. It is evident from the figure that distribution of pores in both the samples is similar and predominantly mesoporous in nature, supporting the inference from the isotherm. However, it is observed that the pore volume in case of Ni(OH)2–r-GO composite is relatively higher which relates to the fatter hysteresis loop and higher surface area (double).


image file: c4ra06601d-f5.tif
Fig. 5 Adsorption–desorption isotherm and the pore size distribution (inset) for (a) Ni(OH)2–r-GO and (b) Ni(OH)2.

Fig. 6 shows the TEM images for the Ni(OH)2 and Ni(OH)2–r-GO samples. Fig. 6a clearly shows the presence of r-GO with hexagonal and elongated rod-like structures of Ni(OH)2 (SEM image, elemental map and Energy Dispersive X-ray Analysis (EDAX) are discussed in ESI-I†). The image also shows the interconnected nickel hydroxide nanoparticles on r-GO sheets along with the mesoporous channels. These kinds of mesoporous channels are very useful for charge storage applications as they can decrease the ionic diffusion length and also the electrolyte resistance. Fig. 6b shows the HR-TEM image of Ni(OH)2–r-GO with interplanar distance of 0.23 nm, corresponding to the (101) plane of Ni(OH)2. Fig. 6c shows the image for bare Ni(OH)2 which reveals porous hexagonal as well as elongated structures. The observed interplanar distance for bare Ni(OH)2 is 0.24 nm (Fig. 6d).


image file: c4ra06601d-f6.tif
Fig. 6 HRTEM images for (a and b) Ni(OH)2–r-GO and (c and d) Ni(OH)2. Mesoporous channels present in the sample are marked in red.

It is important to mention here that these mesopores nanoplates are obtained without the use of any surfactants and structural template. This reduces the extra step of removal of these structure-directing reagents; thereby minimizing the chance of structural collapse. The formation of these mesoporous structures is achieved through an aggregation based growth as demonstrated by Banfield et al.43 The mechanism for the growth of these nanostructures is illustrated in Fig. 7.


image file: c4ra06601d-f7.tif
Fig. 7 Schematic illustration of the formation of mesoporous nanoplates formed by hydrothermal treatment of Ni(OH)2 precipitate.

In current protocol, the bulk Ni(OH)2 obtained by precipitation is subjected to hydrothermal treatment, which leads to the formation of spherical Ni(OH)2 nanoparticles.44 In the absence of any inorganic and organic templates these particles flocculate forming colloidal aggregates. Under the hydrothermal conditions of high temperature and pressure, these aggregates undergo rapid Brownian motion driven collisions which result in the elimination of water molecules and formation of nickel–oxygen bond at the interface of these aggregates resulting in the formation of porous plate like morphology.

Fig. 8 shows the FE-SEM images for Ni(OH)2–r-GO (a) and Ni(OH)2 (b) electrodes used for electrochemical measurements discussed below.


image file: c4ra06601d-f8.tif
Fig. 8 FESEM images of the electrodes of (a) Ni(OH)2–r-GO, (b) Ni(OH)2 and (c) enhanced contrast image for Ni(OH)2–r-GO revealing the presence of r-GO.

It is seen that the average particle size in the case of Ni(OH)2–r-GO is much smaller as compared to the case of bare Ni(OH)2. The smaller particle size in the case of composite clearly results in higher density of slit-type pores and increased surface area, which is expected to enhance the electrochemical performance. This observation is consistent with the BET data and better performance of the composite when compared to the bare Ni(OH)2. In Fig. 8c we show a FESEM image with enhanced contrast to reveal the presence of r-GO in the composite case. Sheet-like features (marked) are clearly noted near the edges of the nanoparticulate portions of the image which correspond to r-GO, as also seen in the TEM images presented above.

From the data derived from the above characterizations, it is clear that these mesoporous channel based Ni(OH)2 and Ni(OH)2–r-GO can be useful in pseudocapacitive charge storage. To find out the detailed electrochemical charge storage properties, we measured the cyclic voltammogram (CV) in three electrode assembly in 2 M KOH with platinum and mercury–mercury oxide (Hg/HgO, 30% KOH) as counter and reference electrodes, respectively.

Fig. 9a and b represent the CV data for Ni(OH)2–r-GO and Ni(OH)2, respectively. The CV was performed at different scan rates from 2 to 100 mV s−1. Since Ni(OH)2 is known to be a good cathode material in supercapacitor performance, the cyclic voltammetry was carried out by applying positive potential with respect to the reference electrode.


image file: c4ra06601d-f9.tif
Fig. 9 Cyclic voltammetry (CV) at different scan rates from 2 to 100 of mV s−1 for (a) Ni(OH)2–r-GO, (b) Ni(OH)2, (c) the CV plot comparison between Ni(OH)2–r-GO and Ni(OH)2 at the scan rate of 10 mV s−1.

During sweep, the potential window range was selected in such way as to avoid strong polarization due to water splitting issue. The CV curves in both the cases show two distinct, ideal, and symmetric oxidation–reduction peaks.

From the CV signatures it is very clear that the charge storage contribution is only through Faradaic reactions. This confirms the pure pseudocapacitive nature of Ni(OH)2. The oxidation and reduction peaks originate from the following reversible reaction.

Ni(OH)2 ⇄ NiOOH + e−

In both the samples under study, on increasing the scan rate from 2 to 100 mV s−1, the oxidation peaks shift towards the right and the reduction peaks shift towards the left. This is very common for metal oxide based materials having low surface area and it is mainly due to the diffusion resistance of the ions at the high scan rates. Symmetric oxidation and reduction peaks observed at low scan rate (2 mV s−1) are even maintained at a very high scan rate (100 mV s−1) for both the electrodes (Fig. 9a and b). This proves high rate performance of these materials.

Fig. 9c compares the CV plots for the Ni(OH)2 and Ni(OH)2–r-GO with same mass loading and scan rate of 10 mV s−1. The peak current is significantly less for the bare Ni(OH)2 when compared with composite. There are three major differences which are very important to notice. First, the oxidation and reduction peak current value in the Ni(OH)2–r-GO case is much higher than that for bare Ni(OH)2, which yields higher Cs value for the composite. Second, the oxidation and reduction peaks of Ni(OH)2–r-GO are sharp whereas the peaks for only Ni(OH)2 case are relatively broad in nature. This sharpness implies higher porosity and surface area which give faster ion transport into the electrode (decreasing the diffusion path length of the electrolyte). The role of r-GO cannot be neglected here as it gives electrical conductive channel for electrons consumed or generated by the Faradaic reactions on the Ni(OH)2 surfaces, and it also provides the high surface area and better accessibility for the electrolyte. Third is the observation of more reversible nature of Ni(OH)2–r-GO as compared to the Ni(OH)2 electrode. The potential difference between the oxidation and reduction peaks for Ni(OH)2–r-GO (0.25 V) is less than that for Ni(OH)2 (0.27 V). Thus, the Ni(OH)2–r-GO electrodes offer kinetically smaller barrier for redox reactions over bare Ni(OH)2.

From the points discussed above, the superiority of the Ni(OH)2–r-GO electrodes over Ni(OH)2 in the context of electrochemical charge storage is clearly revealed. To calculate the gravimetric capacitance, galvanostatic charge–discharge measurements were performed for both the electrode materials in 2 M KOH solution, using three electrode configuration, by varying the current density from 1 to 40 A g−1. The discharge plots for both the cases are shown in Fig. 10a and b. It is worth mentioning that the initial 200 cycles were required for both the electrodes to get stabilized (current density increases with cycling). Hence the charge–discharge measurements were carried out after stabilizing the electrodes for 200 cycles. The observed long discharge plateau features in both the cases confirm the Faradaic nature of these electrodes.


image file: c4ra06601d-f10.tif
Fig. 10 Discharge plots at different current densities for (a) Ni(OH)2–r-GO (from 1 to 40 A g−1), (b) Ni(OH)2 (from 1 to 40 A g−1), (c) discharge plot for the Ni(OH)2–r-GO and Ni(OH)2 at a current density of 40 A g−1, (d) IR drop against current density for the Ni(OH)2–r-GO and Ni(OH)2 samples.

Fig. 10c indicates the comparison of the discharge curves at 40 A g−1 for the Ni(OH)2 and Ni(OH)2–r-GO. The composite electrode shows substantially longer (almost double) discharge time over that for the bare Ni(OH)2 sample, suggesting that Ni(OH)2–r-GO offers higher capacitance. Also, it is important to highlight here that the resistive part arising from the voltage drop (IR drop) due to the equivalent series resistance (ESR) of the electrode configuration is more pronounced in the case of bare Ni(OH)2 over its composite (Ni(OH)2–r-GO). Fig. 10d shows the plots for IR drop vs. current density. It can be seen that Ni(OH)2–r-GO displays much smaller equivalent series resistance (ESR) as compared to the bare Ni(OH)2 electrode. This observation was further supported by electrochemical impedance spectroscopy (EIS).

Fig. 11a shows the Nyquist plots for the Ni(OH)2 and Ni(OH)2–r-GO samples in the frequency range of 100 mHz to 100 kHz. The real axis intercept which determines equivalent series resistance (ESR) is observed to be higher (1.74 Ω) in the case of Ni(OH)2 as compared to Ni(OH)2–r-GO (0.8 Ω). This can be attributed to the enhanced conductivity in the case of the composite due to r-GO. This further supports the result (please see Fig. 10d) of smaller potential drop in the case of Ni(OH)2–r-GO as compared to bare Ni(OH)2. Further, a close look at the impedance plot shows a longer Warburg line for Ni(OH)2–r-GO as compared to Ni(OH)2, implying better diffusion of electrolyte ions towards the electrode material in the former case. This is consistent with the BET data.


image file: c4ra06601d-f11.tif
Fig. 11 (a) Electrochemical impedance spectra for the Ni(OH)2–r-GO composite and Ni(OH)2. The inset shows the impedance spectra over the higher frequency region. (b) Cs vs. current density plot for the Ni(OH)2–r-GO and Ni(OH)2 from 1 to 40 A g−1.

The specific capacitance (Cs) for both electrodes was calculated after 200 cycles. The Cs values at various current densities were calculated from the charge–discharge curves by the equation,

Cs = IΔt/ΔV
where, Cs is the specific capacitance (F g−1), I is the current density (A g−1), ΔV is the potential window (V) and Δt is the discharge time (s). Cs vs. various current densities are plotted in Fig. 11b.

The Ni(OH)2–r-GO sample shows a maximum capacitance of 1795 F g−1 at a current density of 1 A g−1 whereas bare Ni(OH)2 exhibits a capacitance of 1707 F g−1 at the same current density.

The difference in Cs value is thus not that significant at very low current density. At high current density of 40 A g−1, however, the Ni(OH)2–r-GO shows a capacitance of 1538 F g−1 (85.68% retention) whereas bare Ni(OH)2 shows a capacitance of only 936 F g−1 (only 60.85% retention).

The high capacitance retention even at very high current density is mainly because of the high electrical conductivity of the composite due to r-GO (as confirmed by EIS and galvanostatic charge–discharge profile). Thus a faster ionic transport facilitated by porosity as well as the electronic conductivity enhanced by the presence of r-GO are responsible for the high rate performance of the composite.

Fig. 12a and b shows the CV profiles of Ni(OH)2–r-GO and Ni(OH)2 samples in the 1st, 200th and 1000th cycles, at the scan rate of 10 mV s−1. It is observed that after 200 cycles, there is nominal decrease in current for both of these electrodes which imply good cyclic stability. Cyclic stability data was also recorded after every 100 cycles up to 1000 cycles (Fig. 12c). After stabilization of 200 cycles, Ni(OH)2–r-GO has shown capacitance retention of about 96% whereas bare Ni(OH)2 is 80% which represent good cyclability of the composite.


image file: c4ra06601d-f12.tif
Fig. 12 Cyclic voltammetry of the Ni(OH)2–r-GO (a) and Ni(OH)2 (b) for 1st, 200th and 1000th cycle at the scan rate of 10 mV s−1. (c) Capacity retention plot upto 1000 cycles.

ESI-II Table 1† provides the detailed comparison of the present work with the recently published reports on Cs values of Ni(OH)2 based electrodes in KOH electrolyte.45–52 It can be concluded from the data that the achieved value in our case for Ni(OH)2–r-GO (1538 F g−1 at 40 A g−1) is one of the best values especially at the high current density.

We would like to point out that most of the other interesting results tabulated for Ni(OH)2 based nanostructured electrodes are synthesized either by using electrodeposition or chemical bath deposition. Although the results for Ni(OH)2 based electrodes prepared by these techniques are very good, the difficulty associated with them is large scale production and high mass loading. For commercial applications, one needs to manipulate the mass loading and lower the cost of manufacturing with satisfactory performance. This important fact motivated us to develop a simple methodology for bulk scale, surfactant-free synthesis of Ni(OH)2 and its r-GO based composite.

Conclusion

In summary, we have demonstrated a gram scale, single step, surfactant-free and cost effective synthesis of mesoporous channelled Ni(OH)2 and Ni(OH)2–r-GO composite via a simple hydrothermal route. We have employed these materials as electrode materials for supercapacitors (SCs). The charge storage behaviour for both Ni(OH)2 and Ni(OH)2–r-GO was studied in 2 M KOH and compared. Interestingly, the Ni(OH)2–r-GO exhibits Cs of 1538 F g−1 at a high current density of 40 A g−1, whereas Ni(OH)2 alone shows the Cs of only 936 F g−1 at the same current density. This excellent performance of composite can be attributed to the increase in the conductivity and surface area due to addition of r-GO.

Acknowledgements

U.S. acknowledges Director IISER Pune and A. B. acknowledges the fellowship support from UGC-CSIR Govt of India. The authors acknowledge funding support from CSIR's TAPSUN solar energy program.

References

  1. M. A. Roscher, J. Vetter and D. U. Sauer, J. Power Sources, 2010, 195, 3922–3927 CrossRef CAS PubMed.
  2. H. B. Li, M. H. Yu, F. X. Wang, P. Liu, Y. Liang, J. Xiao, C. X. Wang, Y. X. Tong and G. W. Yang, Nat. Commun., 2013, 4, 1894 CrossRef CAS PubMed.
  3. M. Ghaffari, S. Kosolwattana, Y. Zhou, N. Lachman, M. Lin, D. Bhattacharya, K. K. Gleason, B. L. Wardle and Q. M. Zhang, Electrochim. Acta, 2013, 112, 522–528 CrossRef CAS PubMed.
  4. C. Liu, F. Li, L. P. Ma and H.-M. Cheng, Adv. Mater., 2010, 22, E28–E62 CrossRef CAS PubMed.
  5. E. Frackowiak and F. Béguin, Carbon, 2001, 39, 937–950 CrossRef CAS.
  6. Y. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach and R. S. Ruoff, Science, 2011, 332, 1537–1541 CrossRef CAS PubMed.
  7. M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245–4270 CrossRef CAS.
  8. D. Liu, Q. Zhang, P. Xiao, B. B. Garcia, Q. Guo, R. Champion and G. Cao, Chem. Mater., 2008, 20, 1376–1380 CrossRef CAS.
  9. J. R. Miller and P. Simon, Science, 2008, 321, 651–652 CrossRef CAS PubMed.
  10. L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38, 2520–2531 RSC.
  11. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854 CrossRef CAS PubMed.
  12. J. Yan, E. Khoo, A. Sumboja and P. S. Lee, ACS Nano, 2010, 4, 4247–4255 CrossRef CAS PubMed.
  13. W. Yang, Z. Gao, J. Ma, J. Wang, B. Wang and L. Liu, Electrochim. Acta, 2013, 112, 378–385 CrossRef CAS PubMed.
  14. H. S. Huang, K. H. Chang, N. Suzuki, Y. Yamauchi, C. C. Hu and K. C. W. Wu, Small, 2013, 9, 2520–2526 CrossRef CAS PubMed.
  15. D. T. Dam and J. M. Lee, Electrochim. Acta, 2013, 108, 617–623 CrossRef CAS PubMed.
  16. X. Zhang, P. Yu, H. Zhang, D. Zhang, X. Sun and Y. Ma, Electrochim. Acta, 2013, 89, 523–529 CrossRef CAS PubMed.
  17. H. Zhang, X. Zhang, D. Zhang, X. Sun, H. Lin, C. Wang and Y. Ma, J. Phys. Chem. B, 2012, 117, 1616–1627 CrossRef PubMed.
  18. J. Chang, J. Sun, C. Xu, H. Xu and L. Gao, Nanoscale, 2012, 4, 6786–6791 RSC.
  19. Q. Zhang, E. Uchaker, S. L. Candelaria and G. Cao, Chem. Soc. Rev., 2013, 42, 3127–3171 RSC.
  20. M.-R. Gao, Y. F. Xu, J. Jiang and S.-H. Yu, Chem. Soc. Rev., 2013, 42, 2986–3017 RSC.
  21. D. W. Wang, F. Li and H. M. Cheng, J. Power Sources, 2008, 185, 1563–1568 CrossRef CAS PubMed.
  22. N. W. Duffy, W. Baldsing and A. G. Pandolfo, Electrochim. Acta, 2008, 54, 535–539 CrossRef CAS PubMed.
  23. Z. Tang, C. Tang and H. Gong, Adv. Funct. Mater., 2012, 22, 1272–1278 CrossRef CAS PubMed.
  24. H. Wang, Y. Liang, T. Mirfakhrai, Z. Chen, H. Casalongue and H. Dai, Nano Res., 2011, 4, 729–736 CrossRef CAS PubMed.
  25. D. Zhao, W. Zhou and H. Li, Chem. Mater., 2007, 19, 3882–3891 CrossRef CAS.
  26. J. Liu, J. Sun and L. Gao, J. Phys. Chem. C, 2010, 114, 19614–19620 CAS.
  27. Z. Gao, J. Wang, Z. Li, W. Yang, B. Wang, M. Hou, Y. He, Q. Liu, T. Mann, P. Yang, M. Zhang and L. Liu, Chem. Mater., 2011, 23, 3509–3516 CrossRef CAS.
  28. S. Min, C. Zhao, G. Chen and X. Qian, Electrochim. Acta, 2014, 115, 155–164 CrossRef CAS PubMed.
  29. L. L. Zhang, Z. Xiong and X. S. Zhao, J. Power Sources, 2013, 222, 326–332 CrossRef CAS PubMed.
  30. L. Yan, R. Li, Z. Li, J. Liu, Y. Fang, G. Wang and Z. Gu, Electrochim. Acta, 2013, 95, 146–154 CrossRef CAS PubMed.
  31. G.-W. Yang, C.-L. Xu and H. L. Li, Chem. Commun., 2008, 6537–6539 RSC.
  32. J. Yan, W. Sun, T. Wei, Q. Zhang, Z. Fan and F. Wei, J. Mater. Chem., 2012, 22, 11494–11502 RSC.
  33. G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, 797–828 RSC.
  34. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339 CrossRef CAS.
  35. D. Mhamane, S. M. Unni, A. Suryawanshi, O. Game, C. Rode, B. Hannoyer, S. Kurungot and S. Ogale, J. Mater. Chem., 2012, 22, 11140–11145 RSC.
  36. X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang and F. Zhang, Adv. Mater., 2008, 20, 4490–4493 CrossRef CAS PubMed.
  37. A. Jorio, M. Dresselhaus, R. Saito and G. F. Dresselhaus, Raman Spectroscopy in Graphene Related Systems, Wiley-VCH, 2011 Search PubMed.
  38. J. Robertson and A. C. Ferrari, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 61, 14095–14107 CrossRef.
  39. Y. Guo, X. Sun, Y. Liu, W. Wang, H. Qiu and J. Gao, Carbon, 2012, 50, 2513–2523 CrossRef CAS PubMed.
  40. P. Levecque, H. Poelman, P. Jacobs, D. De Vos and B. Sels, Phys. Chem. Chem. Phys., 2009, 11, 2964–2975 RSC.
  41. J. G. Yu, Y. R. Su and B. Cheng, Adv. Funct. Mater., 2007, 17, 1984–1990 CrossRef CAS PubMed.
  42. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouqerol and T. Sienieniewska, Pure Appl. Chem., 1984, 57, 603–609 Search PubMed.
  43. J. F. Banfield, S. A. Welch, H. Zhang, T. T. Ebert and R. L. Penn, Science, 2000, 289, 751–754 CrossRef CAS.
  44. J. C. Yu, A. Xu, L. Zhang, R. Song and L. Wu, J. Phys. Chem. B, 2004, 108, 64–70 CrossRef CAS.
  45. J. Ji, L. L. Zhang, H. Ji, Y. Li, X. Zhao, X. Bai, X. Fan, F. Zhang and R. S. Ruoff, ACS Nano, 2013, 7, 6237–6243 CrossRef CAS PubMed.
  46. J. Chang, H. Xu, J. Sun and L. Gao, J. Mater. Chem., 2012, 22, 11146–11150 RSC.
  47. H. Yan, J. Bai, J. Wang, X. Zhang, B. Wang, Q. Liu and L. Liu, CrystEngComm, 2013, 15, 10007–10015 RSC.
  48. J. T. Zhang, S. Liu, G. L. Pan, G. R. Li and X. P. Gao, J. Mater. Chem. A, 2014, 2, 1524–1529 CAS.
  49. D. L. Fang, Z. D. Chen, X. Liu, Z. F. Wu and C.-H. Zheng, Electrochim. Acta, 2012, 81, 321–329 CrossRef CAS PubMed.
  50. H. Yin, D. Wang, H. Zhu, W. Xiao and F. Gan, Electrochim. Acta, 2013, 99, 198–203 CrossRef CAS PubMed.
  51. S. Min, C. Zhao, G. Chen and X. Qian, Electrochim. Acta, 2014, 115, 155–164 CrossRef CAS PubMed.
  52. X. a. Chen, X. Chen, F. Zhang, Z. Yang and S. Huang, J. Power Sources, 2013, 243, 555–561 CrossRef CAS PubMed.

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06601d
‡ Contributed equally.
This journal is © The Royal Society of Chemistry 2014
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