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DOI: 10.1039/C4RA02653E (Paper) RSC Adv., 2014, 4, 26094-26101

High performance supercapacitor electrode material based on vertically aligned PANI grown on reduced graphene oxide/Ni(OH)2 hybrid composite†

Debasis Ghosh, Soumen Giri, Manas Mandal and Chapal Kumar Das*
Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur-721302, India. E-mail: chapal12@yahoo.co.in

Received 26th March 2014 , Accepted 27th May 2014

First published on 2nd June 2014

Abstract

A simple and cost effective one pot hydrothermal process has been followed for the synthesis of flowery Ni(OH)2, reduced graphene oxide (rGO)/Ni(OH)2 hybrid composite. A ternary composite of rGO/Ni(OH)2/PANI has also been synthesised by in situ oxidative polymerisation of aniline with the binary composite of rGO/Ni(OH)2. A unique morphology of vertically aligned PANI on rGO surface and randomly connected PANI nanowires has been demonstrated following a heterogeneous nucleation on the rGO surface and homogeneous nucleation inside the bulk material. A comparative electrochemical analysis reveals a superior electrochemical behavior of the ternary composite over the rGO–Ni(OH)2, which again shows better electrochemical utility over the virgin Ni(OH)2. The proton insertion/deinsertion reversible pseudocapacitance of Ni(OH)2 combined with the pseudocapacitance of the vertically aligned conducting PANI nanowires and their synergistic effect with in situ reduced graphene oxide results in a high specific capacitance of 514 F g−1 at 2 A g−1 current density accompanied with 94.4% specific capacitance retention after 1000 charge discharge cycles at 5 A g−1 current density.

1. Introduction

Recent research on alternative energy resources is mainly centered on supercapacitors due to its high energy density at a high power delivery rate accompanied with high cycle life performance. The charge storage mechanism of the supercapacitor electrode material involves either electrical double layer (EDL) formation at the electrode/electrolyte interface or the faradaic reaction within the electrode material in the presence of suitable electrolyte.1 The total amount of charge stored in pseudocapacitor is many times higher than that of the EDLC materials, however, the later exhibits better cycle life than the pseudocapacitors.1 Carbonaceous materials such as activated carbon, carbon aerogel, carbon nanotube and graphene can act as active EDLC source due to their unique properties of high specific surface area and tunable porosity.2 Amongst the carbonaceous materials graphene attracts the modern research most due to the unique properties of high surface area, high flexibility, high thermal and electrical conductivity and high capacitance.3–6 Transition metal oxides/hydroxides such as, Co(OH)2, Ni(OH)2, MnO2, SnO2 are well known for their redox activity and effective pseudocapacitance in suitable electrolyte.7–10 Amongst these pseudocapacitive materials Ni(OH)2 is of most interest due to its easy processibility, various morphology, high redox activity, nontoxicity and overall lowcost.11 Recent researches have established that a hybrid type material containing both pseudocapacitive and EDLC materials to be more effective in terms of both high capacitance and long cycle life.12–14 Conducting polymers, such as polyaniline (PANI), polythiophene and polypyrrole are well known for their redox activity and can also act as pseudocapacitor. Amongst the various conducting polymers PANI is of most interest due to its unique properties of doping and dedoping in acid and base media, respectively, controllable electrical conductivity, easy and cost effective processability, etc.15,16 Several investigation on PANI has revealed that it can act as efficient supercapacitor electrode material stand alone or with in a composite with CNT or graphene.17–19 But there are a few instance of a ternary composite of conducting polymers with metal oxide/hydroxide and carbonaceous materials.20–23

Although the theoretical specific capacitance of Ni(OH)2 is very high (2358 F g−1 with potential region of 0.44 V), the very low conductivity decreases the full utilization of electrode material and a close to optimum value can be obtained only in a very thin layer of material.23 If the conductivity of the Ni(OH)2 can be increased, the low charge transfer resistance can result in high specific capacitance. The conductivity of Ni(OH)2 can be increased in a composite with conducting polymer (PANI) or highly conductive graphene. But with only PANI the problem of swelling and shrinking of the electrode material during charge discharge cannot be avoided. So if a ternary composite of Ni(OH)2/graphene/PANI is considered all the advantages of the three can come into act together for fabricating a superior electrode material, where, Ni(OH)2 can acts as pseudocapacitor; graphene acts as EDLC source and increase the conductivity, surface area, porosity and flexibility of the composite, and PANI, apart from its pseudocapacitance can form a conductive interconnected network between Ni(OH)2 and graphene.

In our present study, we have synthesized a ternary composite of in situ reduced graphene oxide (rGO)/Ni(OH)2/PANI by a two-step method. Firstly, reduced graphene/Ni(OH)2 composite has been prepared by a one pot hydrothermal synthesis where graphene oxide is in situ reduced by urea and Ni2+. In the second step in situ oxidative polymerization of aniline in presence of rGO/Ni(OH)2 composite has been followed to prepare the ternary composite of rGO/Ni(OH)2/PANI.

2. Preparation of materials

2.1. Preparation of graphene oxide

Graphene oxide was prepared following by a modified Hummer's method.24 Briefly, in a mixture of 120 ml conc. H2SO4 and 13.3 ml H3PO4 1 g graphite powder was added and sonicated for 10 minute. In the mixture, 6 g KMnO4 was added slowly in the suspension while stirring and the stirring was continued for 12 h maintaining the suspension temperature around 50 °C. After that 1.5 ml 30% H2O2 in 135 ml ice water was slowly added to the mixture, cooled at room temperature and was stirred for another 4 h and centrifuged at a low rpm. The brown precipitation was collected and repeatedly sonicated and centrifuged in acetone for four times and the final product was collected and dried at 60 °C.

2.2. Preparation of reduced graphene oxide/Ni(OH)2 composite

50 mg of the as prepared GO was well dispersed in a mixture of 20 ml 0.1 M Ni(NO3)2 and 20 ml 0.5 M urea by ultra sonicating using a ultrasonic horn for 1 h. Then the whole suspension was immediately transferred into a teflon sealed stainless steel autoclave of 50 ml capacity and maintained at 120 °C for 8 h and then allowed to cool at room temperature. The precipitation was collected and washed with ethanol water for several times and dried at 60 °C. At high temperature in presence of Ni2+, GO is reduced to graphene (reduced graphene oxide). The resulting black material was leveled as rGO/Ni(OH)2.

2.3. Preparation of ternary composite

The ternary composite was prepared using oxidative polymerization of aniline monomer in presence of rGO/Ni(OH)2. Briefly, 0.3 ml aniline monomer was well dispersed in 100 ml ice water by ultrasonication for 5 minute. Then 190 mg of rGO/Ni(OH)2 composite containing 50 mg rGO was added to it and again sonicated for two minute at a low power. 2 g APS was dissolved in 100 ml ice water and the solution was poured drop by drop to the above suspension of rGO/Ni(OH)2 containing aniline monomer while continuous stirring at 250 rpm and the stirring was continued to 3 h. The temperature of the bulk material was kept at 0–5 °C during the whole stirring process. Then the whole material was kept in refrigerator overnight and then washed with water ethanol several times and the final precipitation was dried at 60 °C. The prepared material was leveled as rGO/Ni(OH)2/PANI.

3. Results and discussions

3.1. Materials characterizations

3.1.1. X-ray diffraction analysis. The XRD pattern of the as prepared rGO, Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI are shown in Fig. 1. For rGO film the strong peak at 2θ = 10.2° indicates the (001) plane. The XRD pattern of the as prepared Ni(OH)2 exhibiting a very sharp peak centered on 2θ = 12.3° indicates the (003) plane. The medium frequency peaks at 2θ = 24.8°, 33.4° and 59.5° are indicative of the (006), (101) and (110) crystalline plane, respectively. A broad peak can be observed within the range of 2θ = 34.5°–38°, which belongs to the (101), (012) and (015) planes.25 All these peaks confirms the α phase of the as prepared Ni(OH)2.25–27 In the XRD pattern of rGO–Ni(OH)2 the crystalline peak of GO disappears and all the peaks of α Ni(OH)2 re appears without any significant change confirming the reduction of the GO to graphene (rGO) by the Ni2+ and urea at 180 °C. In the XRD pattern of the ternary composite, only characteristic peaks of PANI can be observed indicating successful coating of the amorphous PANI over the binary composite of rGO/Ni(OH)2. The peak at 2θ = 20.4° and 25.2° indicates the periodicities parallel (100) and perpendicular (110) to the PANI chain respectively.28 The presence of reduced graphene oxide in the composites was also confirmed by the Raman spectra and is shown in Fig. S1 (see ESI†).
image file: c4ra02653e-f1.tif
Fig. 1 XRD analysis of the as synthesized GO, Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI.
3.1.2. Morphological analysis. The morphological analysis of the as prepared materials was performed in terms of FESEM and TEM analysis and the images are shown in Fig. 2 and 3, respectively. 3D porous flower like architecture can be seen for the Ni(OH)2. A close inspection of the flowery architecture reveals that the various interconnected sheet like petals with smooth surface and thickness of ca. 50 nm are self-assembled to form the flowery architecture. Formation of the hierarchical flowery structure with smooth surface petal by a hydrothermal method involves dissolution and recrystallization process and with time being in order to minimize surface strain hierarchical flowery structure is obtained following a Ostwald ripening mechanism.29,30 Surface morphology of the rGO–Ni(OH)2 composite exhibits that the flowery architecture of the Ni(OH)2 is well wrapped by the flexible rGO sheets (Fig. 2b). Here rGO not only act as the basal plane for the growth of the flowery architecture, but also interconnects the various Ni(OH)2 flowers. In case of the ternary composite (Fig. 2c–h) no individual Ni(OH)2 was found possibly due to the excellent coating by the amorphous PANI. For the PANI, two types of morphology were found. Vertically aligned PANI grown on rGO sheet (Fig. 2e–g) and random connected PANI nanowires in the bulk material. At the starting of the polymerization most active nucleation sites generate on the solid rGO surface by a process of heterogeneous nucleation. Once the active site is generated it reduces the inter-facial energy barrier between the solid surface and bulk solution and renders the polymerization process where further PANI grows on the solid surface forming vertically aligned PANI over rGO surface.31 Apart from the heterogeneous nucleation on solid surface, homogeneous nucleation occurs inside the bulk and randomly connected PANI nanowires are formed (Fig. 2h), where aniline micelles act as the “soft template”.32,33
image file: c4ra02653e-f2.tif
Fig. 2 FESEM micrograph of (a) Ni(OH)2 flower, (b) rGO wrapped Ni(OH)2 flower, rGO/Ni(OH)2/PANI composite (c) at low magnification, (d) at high magnification; (e–g) vertically aligned PANI over rGO surface in the rGO/Ni(OH)2/PANI composite, (h) interconnected PANI nanowire.

image file: c4ra02653e-f3.tif
Fig. 3 Tem image of (a) Ni(OH)2 flower, (b) rGO/Ni(OH)2 composite, (c) rGO/Ni(OH)2/PANI composite, (d) vertically aligned PANI on rGO surface in rGO/Ni(OH)2/PANI composite and (e) interconnected PANI nanowire network in rGO/Ni(OH)2/PANI composite.

The FESEM analysis was further supported by the TEM analysis as shown in Fig. 3. As shown in Fig. 3a, the Ni(OH)2 nano sheets are strongly interconnected to form Ni(OH)2 flower. For the rGO/Ni(OH)2 composites the Ni(OH)2 nanopetals are well anchored on the rGO sheet as observed from Fig. 3b. The little contrast between the rGO sheet and the carbon coated on copper grid indicates the ultrathin layer of the rGO in the binary composite. Vertically aligned PANI on rGO sheet in Fig. 3d and randomly connected PANI nanowire without any rGO substrate in Fig. 3e strongly supports the FESEM observation of the two types of PANI morphology in the ternary composite. However, for the rGO/Ni(OH)2/PANI composite no individual Ni(OH)2 flower was found from both the FESEM and TEM analysis indicating excellent coating of the PANI on the nanopetals forming the Ni(OH)2 flowers.

The formation of Ni(OH)2 flower and vertically aligned PANI on rGo/Ni(OH)2 is shown in Scheme 1.


image file: c4ra02653e-s1.tif
Scheme 1 Formation mechanism of Ni(OH)2 flower and rGO/Ni(OH)2/PANI composite.
3.1.3. BET analysis. In order to understand the effect of rGO and PANI on the surface area and pore size distribution of the binary and ternary composite with Ni(OH)2 the BET analysis was carried out and is shown in Fig. 4.
image file: c4ra02653e-f4.tif
Fig. 4 N2 adsorption and desorption isotherm of (a) Ni(OH)2, (c) rGO/Ni(OH)2 and (e) rGO/Ni(OH)2/PANI and their respective BJH pore size distribution (b), (d) and (f).

The inspection of the nitrogen adsorption and desorption isotherm of the Ni(OH)2, rGO–Ni(OH)2 and rGO/Ni(OH)2/PANI at 77 K is shown in Fig. 4a, c and e, respectively manifesting a typical type IV isotherm with respective specific surface area of 54.56 m2 g−1, 62.729 m2 g−1 and 66.371 m2 g−1. The type IV isotherm indicates the mesoporous nature of the as prepared materials. The increased surface area of the binary composite over virgin Ni(OH)2 is due to the high surface area of graphene, which increase the overall surface area. The nano dimension of the PANI nanowire and vertically aligned PANI is responsible for the increased surface area of the ternary composite over the binary composite. The BJH pore size distribution of the Ni(OH)2, rGO–Ni(OH)2 and rGO/Ni(OH)2/PANI is shown in Fig. 4b, d and f, respectively. The pore size distribution exhibited a minor microporosity and a predominant mesoporosity within the range of 5–90 Å for Ni(OH)2 and 15–90 Å for both the binary and ternary composites with respective maxima peak at 7.52 Å, 15 Å and 16.6 Å. An average pore diameter of 1.095e+02 Å, 2.39e+02 Å and 1.64 e+02 Å was calculated for the Ni(OH)2, rGO–Ni(OH)2 and rGO/Ni(OH)2/PANI, respectively. An increased of average pore size of the binary composite is attributed to the presence of highly porous graphene, alternatively in the ternary composite surface coating by PANI results an decreased average pore size of the rGO/Ni(OH)2/PANI composite.

3.2. Electrochemical characterizations

We first performed the GCD analysis of the as prepared electrode materials in order to evaluate the best working potential. The best working potential chosen for the Ni(OH)2 and rGO–Ni(OH)2 was from (−) 0.3 V to 0.4 V. For the Gr–Ni(OH)2–PANI composite an increased working potential from (−) 0.3 V to 0.55 V was chosen. The working potential was chosen by considering the maximum coulombic efficiency. The GCD plots of the as prepared Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI electrode are shown in Fig. 5 at various constant charge discharge current densities of 2, 3, 4 and 5 A g−1. The specific capacitances from the GCD plots was calculated using the equation
 
image file: c4ra02653e-t1.tif(1)
where, i/m, t, and Δv represents the current density, discharge time and potential limit, respectively. The highest specific capacitance obtained from the Ni(OH)2 electrode was 238 F g−1 at low current density of 2 A g−1. For the rGO–Ni(OH)2 electrode an increased specific capacitance of 359 F g−1 was obtained at 2 A g−1 current density. The improved specific capacitance generates from a combined EDLC effect of the rGO and pseudocapacitive effect of Ni(OH)2 and their synergistic interaction. Low conductivity of Ni(OH)2 causes a larger charge transfer resistance that restricts its specific capacitance to a lower value. Conductive graphene increases the overall electrical conductivity of the composite, which ensure high rate of charge transfer, hence high specific capacitance. For the ternary composite of rGO/Ni(OH)2/PANI still increased specific capacitance of 514 F g−1 was achieved at 2 A g−1 current density. PANI itself is pseudocapacitive in nature, besides the vertically aligned PANI grown on rGO surface of the ternary composite and the nanowire PANI grown on Ni(OH)2 results a better synergistic interaction amongst the three. PANI not only increases the pseudocapacitance of the composite but also being a conductive polymer increases the overall electrical conductivity of the composite. Besides, the nano dimension increases the surface area of the composite. While performing the GCD test at higher current density of 5 A g−1, high specific capacitance of 444 F g−1 was achieved for rGO/Ni(OH)2/PANI ternary composite, which was far higher than that of virgin Ni(OH)2 and rGO/Ni(OH)2 electrode exhibiting specific capacitance of 156 F g−1 and 281 F g−1, respectively. The ternary composite showed excellent rate capability in retaining specific capacitance of 86.3% at high current density of 5 A g−1 with respect to the low current density of 2 A g−1, which was better than the Ni(OH)2 and rGO/Ni(OH)2 composite with respective specific capacitance retention of 65.5% and 78.27%. The decreasing specific capacitance with increasing current density is due to the quick attainment of the voltage limit at high current in galvanostatic mode resulting in shortening of charging and discharging time. The variation of specific capacitance as a function of current density of all three electrodes is shown in Fig. 9a. The coulombic efficiency from the charge discharge plot can be calculated from the equation
 
Coulombic efficiency (η) = (td/tc) × 100% (2)
where, td and tc are the discharging and charging time, respectively. It is notable to mention that as the current density increases the coulombic efficiency increases for all the electrode material and for a particular current density of 2 A g−1 the coulombic efficiency was calculated as 96.2%, 98.1% and 97.6%, respectively for the Ni(OH)2 and its binary and ternary composite, respectively. Higher the coulombic efficiency better is the reversibility of the electrode material. Presence of rGO increase the reversibility of the rGO/Ni(OH)2 composite electrode, however another pseudocapacitive PANI in the ternary composite lowers the reversibility. The above observation demonstrates that it is only the rGO, which is responsible for the increased reversibility of the electrode material.
image file: c4ra02653e-f5.tif
Fig. 5 Galvanostatic charge discharge plot of (a) Ni(OH)2, (b) rGO/Ni(OH)2 and (c) rGO/Ni(OH)2/PANI at various constant current densities of 2, 3, 4 and 5 A g−1.

Energy storage material is known by its energy density and power density. Energy density from the GCD test can be calculated from the following equation

 
Energy density (E) = 1/2Cs × (ΔV)2 (3)
 
Power density (P) = E/T (4)
where, Cs is the specific capacitance, ΔV is the potential limit and T is the discharge time of the charge discharge cycle from where the specific capacitance was calculated. At a power delivery rate of 700 W kg−1 the energy density of the Ni(OH)2, rGO/Ni(OH)2 was calculated to be 16.17, 24.45 W h kg−1, respectively. The larger working potential of the ternary composite resulted in a higher power density and the maximum energy density of 51.62 W h kg−1 was obtained at a power delivery rate of 850 W kg−1. A significant energy density of 44.58 W h kg−1 was obtained even at high power density of 2125 W kg−1 indicating the excellent utility of the ternary composite as electrode material at high current. The various energy density and power density obtained for all the three electrodes are shown in a Ragone plot in Fig. 6a.34 In order to understand the cycle life of the electrode materials the charge discharge test was continued to 1000 consecutive cycles at 5 A g−1 current density and a specific capacitance retention of 86.5%, 94.7% was achieved respectively for NI(OH)2 and rGO/Ni(OH)2 at the end. The low cycle stability of Ni(OH)2 can be attributed to its pseudocapacitive nature and the faradaic reaction results in swelling of electrode material. Flexible rGO sheets releases the strain involved with the consecutive charging and discharging of Ni(OH)2, resulting in an increased cycle life of the binary composite. In case of the ternary composite it may be think that the cycle life would be low due to the possible deterioration of PANI after several dramatic shrinking and swelling charge discharge cycles. However stable electrochemical behavior of the ternary composite with 94.4% specific capacitance retention after 1000 cycles at 5 A g−1 current density can be attributed to the vertically aligned PANI nanowire arrays on flexible rGO surface and their better synergistic effect. rGO sheets minimizes the mechanical deformation in the redox process of Ni(OH)2 and PANI nanowire arrays and thus avoids electrode swelling and is beneficiary to high electrochemical stability of the ternary composite. Moreover, the vertical aligned PANI nanowire arrays were facile to strain reduction, which endorsed them to reduce the breaking during the process of counter ions doping/dedoping.32


image file: c4ra02653e-f6.tif
Fig. 6 (a) Ragone plot of the Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI electrode with a comparison of other energy storage systems, (b) variation of specific capacitance as a function of cycle number of the Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI electrode.

Fig. 7 demonstrates the CV plot of the porous Ni(OH)2 electrode, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI electrode at various scan rates of 2, 10, 30 and 50 mV s−1. All the plots deviate from perfect rectangular nature, which demonstrate the non-ideality of the electrode material and strongly support the predominant pseudocapacitor behavior. Unlike Ni(OH)2 and rGO/Ni(OH)2/PANI, the CV plots of rGO/Ni(OH)2 shows some symmetrical behavior, possibly due to the rGO which act as EDLC source itself and its synergistic effect with pseucapacitive Ni(OH)2 that results a close proximity towards ideal behavior of the rGO/Ni(OH)2 electrode. The current response of all the electrodes increased with increasing the scan rate indicating that the electrode phenomenon is associates with a diffusion control process.


image file: c4ra02653e-f7.tif
Fig. 7 Cyclic voltammetry plots of (a) Ni(OH)2, (b) rGO/Ni(OH)2 and (c) rGO/Ni(OH)2/PANI at different scan rates of 2, 10, 30 and 50 mV s−1.

The specific capacitance from the CV plots was calculated from the equation below

 
image file: c4ra02653e-t2.tif(5)
where i is the instantaneous current response at any potential (V), V2 and V1 are upper and lower potential limit, ν and m are the scan rate and mass of the electroactive material, respectively. The various specific capacitance obtained at different scan rates are given in Table 1 and also presented in graphical form in Fig. 9b. The highest specific capacitance obtained from the Ni(OH)2 electrode was 269 F g−1 at 2 mV s−1 scan rate. The pseudocapacitance in Ni(OH)2 in KOH electrolyte arises from the reversible redox reaction of proton insertion and de-insertion
Ni(OH)2 ↔ NiOOH + H+ + e−

Table 1 Various specific capacitance obtained from the CV plot of the Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI at different scan rates of 2, 10, 30 and 50 mV s−1
Scan rate 2 mV s−1 10 mV s−1 30 mV s−1 50 mV s−1
Ni(OH)2 269 (F g−1) 210 (F g−1) 177 (F g−1) 160 (F g−1)
rGO/Ni(OH)2 392 (F g−1) 331 (F g−1) 287 (F g−1) 261 (F g−1)
rGO/Ni(OH)2/PANI 539 (F g−1) 471 (F g−1) 412 (F g−1) 390 (F g−1)

The synergistic interaction of the porous Ni(OH)2 with flexible rGO sheet results in improved specific capacitance of 392 F g−1 for the rGO–Ni(OH)2 composite. The redox peaks of the Ni(OH)2 is not prominent, this is a clear indication of the faradaic reaction to occur at a pseudo-constant rate over the entire volumetric cycle.35 In case of the ternary composite the total specific capacitance increases as high as 539 F g−1 at 2 mV s−1 scan rate. The added PANI increases the total pseudocapacitance of the ternary composite that can be considered as the individual pseudocapacitance contribution from Ni(OH)2, PANI and their synergistic interaction. PANI grown on the graphene surface are of nano dimension with excellent porosity and high surface area, which thereby increase the overall surface area of the rGO/Ni(OH)2 composite. This ensures that plenty of ions can come in contact with the electrode material for a time and result high rate of charge transfer, hence high specific capacitance.

The electrodes were also analyzed by electrochemical impedance spectroscopy with in the frequency range of 1 MHz to 10 mHz, and have been represented in terms of nyquist plot in Fig. 8a after fitting with an equivalent electrical circuit (Fig. 8b). All the nyquist plots exhibited similar nature of a depressed semicircle at initial high frequency followed by a straight line at an angle close to 45°. The high frequency semicircle indicates the presence of charge transfer resistance (Rct), the diameter of which determines the magnitude of Rct and the intercept with real impedance axis indicates the solution resistance (Rs). Although the electrolyte resistance are comparable for all three electrode materials, they notably differ in the Rct with maximum of 93.8 ohm for Ni(OH)2 and a gradual decreased Rct of 26.13 ohm and 13.89 ohm for the rGO/Ni(OH)2 and rGO/NI(OH)2/PANI, respectively. The decreased Rct can be attributed to the presence of highly conducting rGO which increases the overall conductivity of the composite material. In case of the ternary composite conducting PANI forms several conducting tunnel within the composite material through which the charge can effectively pass ensuring the lowest Rct. The deviation of the post semicircle straight line from becoming parallel to the imaginary impedance axis indicates the non-ideality of the electrode materials and can be represented in terms of constant phase element (CPE). CPE (Q) exclude the real world capacitor and generates due to different defect in the electrode materials i.e. the edge defect, different coating thickness of electrode material on current collector, distribution of reaction sites, etc.14 CPE constant (n) determines the ideality of the electrode material and a value closer to 1 indicates nearer ideal behavior. For the Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI the CPE constant ‘n’ was found to be 0.76, 0.81 and 0.89, respectively. The post semicircle straight line inclined at an angle close to 45° indicates diffusion behavior of the electrode. A straight line with higher slope indicates lower diffusion resistance, hence high surface availability and higher ion accessibility. The presence of rGO is the main source of double layer capacitance of 1.1 F in the rGO/Ni(OH)2 and 1.3 F in the rGO/Ni(OH)2/PANI composite. The specific capacitance from the EIS was also calculated using the equation:

 
Specific capacitance = (−)1/(m × 2π × f × Zimg) (6)
here, f, Zimg, and m, indicates the frequency, imaginary impedance and the electrode mass. The maximum specific capacitance obtained at low frequency of 0.01 Hz was 176 F g−1, 281 F g−1 and 402 F g−1, respectively for the Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI composite electrode. The frequency dependent specific capacitance of the three electrodes is shown in Fig. 9c. It is interesting to observe that the specific capacitance sharply decreases with increasing frequency, and the frequency response can be divided into three regions, a low frequency capacitor behavior, high frequency resistor behavior and a capacitor resistor combined behavior at lower middle frequency region.


image file: c4ra02653e-f8.tif
Fig. 8 (a) Nyquist plot of Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI and their magnified portion at high frequency (inset); (b) the equivalent electrical circuit to which the nyquist plots were fitted.

image file: c4ra02653e-f9.tif
Fig. 9 Variation of Specific capacitance as a function of (a) current density, (b) scan rate and (c) frequency of Ni(OH)2, rGO/Ni(OH)2 and rGO/Ni(OH)2/PANI.

4 Conclusions

In this manuscript we have represented a cost effective hydrothermal procedure for the synthesis of Ni(OH)2 and in situ reduced graphene oxide/Ni(OH)2 composite, followed by an in situ oxidative polymerization of aniline with rGO/Ni(OH)2 for the synthesis of rGO/Ni(OH)2/PANI ternary composite. The porous flowery architecture and the effective pseudocapacitance of Ni(OH)2 resulted in a specific capacitance of 238 F g−1 at 2 A g−1. The excellent effectiveness of the pseudocapacitive Ni(OH)2 was achieved in its composite with reduced graphene oxide with an increased specific capacitance of 359 F g−1 at 2 A g−1 current density accompanied with high cycle life. The unique morphology of the vertically aligned PANI on rGO surface and its high electrochemical activity resulted in a superior electrochemical behavior of the rGO/Ni(OH)2/PANI ternary composite with 514 F g−1 specific capacitance at 2 A g−1 current density, which can be considered as the effective combined contribution of the pseudocapacitance of Ni(OH)2 and PANI and their improved synergistic effect with the reduced graphene oxide. The high energy density of 51.62 W h kg−1 at a power density of 850 W kg−1 accompanied with long term cycle stability signify the rGO/Ni(OH)2/PANI composite to be an superior supercapacitor electrode material.

Acknowledgements

The authors are thankful to UGC INDIA for funding and IIT Kharagpur for the instrumental help.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials and instruments, electrode preparation, Raman spectra, GCD plots of rGO/Ni(OH)2/PANI. See DOI: 10.1039/c4ra02653e
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