Porous NiCo2O4-reduced graphene oxide (rGO) composite with superior capacitance retention for supercapacitors

Electrochimica Acta 132 (2014) 332–337

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Porous NiCo2 O4 -reduced graphene oxide (rGO) composite with superior capacitance retention for supercapacitors Yazi Luo, Haiming Zhang, Di Guo, Jianming Ma, Qiuhong Li, Libao Chen ∗ , Taihong Wang ∗ Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, and State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha, P. R. China

a r t i c l e

i n f o

Article history: Received 26 December 2013 Received in revised form 25 March 2014 Accepted 28 March 2014 Available online 6 April 2014 Keywords: Porous network nanocomposite supercapacitors binder-free

a b s t r a c t In present work, we developed a simple and green route to fabricate irregular porous network-like NiCo2 O4 -reduced graphene oxide (rGO) nanocomposite supported on the nickel foam substrates, which was directly used as a binder-free electrode for supercapacitors. The rGO served as a conductive network to facilitate the collection and transportation of electrons during the cycling, improved the conductivity of NiCo2 O4 , and allowed for a larger specific surface area. The irregular porous structure allowed for the easy diffusion of the electrolyte into the inner region of the electrode. Moreover, the nanocomposite directly fabricated on nickel foam with a better adhesion and avoided the use of polymer binder. This method efficiently reduced ohmic polarization and enhanced the rate capability. As a result, the NiCo2 O4 -rGO nanocomposite exhibited a specific capacitance of 777.1 F g−1 at 5 A g−1 and about 99.3% of the capacitance retained after 3000 cycles. The capacitance retention was about 87.6% when the current density increased from 1 A g−1 to 20 A g−1 . These results indicated that such the irregular porous network-like structure directly synthesized on the substrate could be a promising candidate for high performance energy storage application. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Supercapacitors (SCs) or electrochemical capacitors (ECs) have attracted considerable attention due to their high power density, long cycling life, and bridging function for the power/energy gap between traditional capacitors (which have high power output) and batteries/fuel cells (which have high energy storage).[1,2] SCs commonly store energy based on either ion adsorption (electrochemical double layer capacitors, EDLCs) or fast surface redox reactions (pseudocapacitors). However, pseudocapacitors exhibit far larger capacitance values and energy density than the EDLCs since the electrochemical processes occur both on the surface and in the bulk near the surface of the solid electrode, which meets the evergrowing need for peak-power assistance in electric vehicles.[3] To date, much effort has been concentrated on the demand of materials undergoing such redox reactions. Transition-metal oxides, such as MnO2 ,[4,5] NiO[6] and Co3 O4 ,[7] have long been intensively studied due to their excellent electrochemical performances. Especially, in recent years, binary metal oxides have been reported to deliver

∗ Corresponding author. Tel.: +86 731 88822332; fax: +86 731 88822332. E-mail addresses: [email protected], [email protected] (L. Chen), [email protected] (T. Wang). http://dx.doi.org/10.1016/j.electacta.2014.03.179 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

a higher performance than single component oxides due to their feasible oxidation state and high electrical conductivity such as NiCo2 O4 , NiMoO4 , ZnCo2 O4 .[8–10] Spinel nickel cobaltite (NiCo2 O4 ), as one of binary metal oxides, has been extensively studied as a promising pseudocapacitivetype electrode material owing to its outstanding electrochemical performances, low cost, abundant resources and environmental friendliness.[11–16] As we known, NiCo2 O4 exhibits two orders of magnitude higher electrical conductivity comparing to the pure NiO or Co3 O4 (10−3 to 10−2 S cm−1 ).[11] On the other hand, NiCo2 O4 is expected to offer richer redox reactions, including contributions from both nickel and cobalt ions, than those of pure NiO and Co3 O4 .[12–16] These characteristics lead to superior electrochemical performances. Until now, plenty of papers have been published using NiCo2 O4 . However, most of the reported electrodes using NiCo2 O4 are either rich of binders or unavoidably bearing corrosion of the substrates.[17–19] The shortcomings of the former are discussed later. The latter may lead to the inexact calculation of the synthesized eletro-active material, and a certain extent limit the quantity of the unit area of material. For the reasons, a new synthetic method is required. Graphene or reduced graphene oxide (rGO), endowed with desirable characteristics of excellent electron transport, large surface area, regular frameworks and strong thermal/chemical

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stability, is widely utilized to synthesize various hybrid materials.[20,21] So far, many metal oxide-rGO nanocomposites have been extensively investigated, such as MnO2 -rGO,[22,23] NiO-rGO,[24] and Co3 O4 -rGO,[25] and exhibit enhanced electrochemical performances in comparison with their bare counterparts due to the participation of rGO. Thus, it is meaningful to develop new methods for preparing binary oxide/rGO nanocomposites with novel structures. The traditional preparation of electrodes for SCs is mainly based on a slurry-casting method that requires mixing active materials with binders and active carbon additives such as carbon black and CNTs, and then cast the mixture onto a metal foil or mesh current collector.[3] The binder works as the mechanical connections between active materials, conductive additives and current collectors. However, it can decrease the electric conductivity, prevent the access of ions to the surface of active materials and increase the polarization of electrodes due to their insulating and electrochemically inactive properties. This “dead surface” can block from the contact with the electrolyte to participate in the Faradaic reactions for energy storage.[26–28] Moreover, binder-rich electrodes are unstable at high temperature because most of binders are not stable over 200 ◦ C.[22] The drawbacks of binders limits the extensive application of the SCs. Therefore, it is highly desirable to prepare binder-free electrodes with stable structures. In this work, we have successfully developed an electrostatic spray deposition (ESD) route to fabricate the NiCo2 O4 -rGO nanocomposite supported on the nickel foam substrates with excellent electrochemical performance. The NiCo2 O4 -rGO composite exhibits a specific capacitance of 777.1 F g−1 at a current density of 5 A g−1 and about 99.3% of the capacitance retained after 3000 cycles. The excellent performance can be contributed to the following points: i) The rGO serves as a conductive network to facilitate the collection and transportation of electrons during the cycling, improving the conductivity of NiCo2 O4 , and allowing for a larger specific surface area owing to its network-like structure, effectively enhancing the electrochemical performance of the composite; ii) The irregular porous structure also allows for easy diffusion of the electrolyte into the inner region of the electrode, reducing diffusion resistance of the electrolyte; iii) Binder-free electrodes with a better adhesion reduces Ohmic polarization, enhances the rate capability, and the direct contact between the active material and the substrate was favorable for electron collection and also ensured them participate as much as possible in reaction. The as-fabricate NiCo2 O4 -rGO composites are promising electrodes for SCs in future.

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the flow rate to feed the precursor solution was controlled at 2 ml h−1 by a syringe pump. During the deposition, the nickel foam substrate was heated to 623 K. After deposition, the black nickel foam was continuely treated at 623 K for 3 h in air to obtain the final composite. The thickness of the layer was controlled by the deposition time and the uniformity of the layer was undoubted as long as the dispersion of the solution was uniform. Considering that the quantity of the sprayed materials is proportional to the deposition time when all the other factors such as feeding rate, solution concentration are constant, it’s reasonable believed that the ESD technique exhibits excellent reproducibility.[30] As a contrast, pure NiCo2 O4 and pure rGO deposited on nickel foam were also prepared under the same procedure. 2.2. Materials characterization The morphology of the composite was characterized by field emission scanning electron microscopy (SEM) [Hitachi S-4800], transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) [JEOL2010]. The crystal structure of the samples was characterized by X-ray diffraction ˚ (XRD) [Rigaku Dmax-2500] with Cu-K␣ radiation (␭ = 0.15406 A) operating at 40 kV, 60 mA. Raman spectra were measured and collected using a 1064 nm laser with RFS 100/S under ambient conditions, with a laser spot size of about 1 micron. 2.3. Electrochemical measurement The electrochemical tests of samples were conducted using a three electrode system in 2 M KOH using a CHI 660B electrochemical working station. The nickel foam-supported composite was directly used as the work electrode with a platinum plate counter electrode and a standard calomel electrode (SCE) as the reference electrode. The specific capacitance (C) was calculated from the chronopotentiometry curves based on the following equation: C=

It V

where I, t and V are discharging current density, discharging time and discharging potential range, respectively. Electrochemical impedance spectroscopies (EIS) tests were carried out with a frequency loop from 105 Hz to 0.1 Hz with perturbation amplitude of 5 mV under the open-circuit potential. 3. Results and discussion

2. Experimental 2.1. Materials synthesis All the chemicals were of analytical grade and were used without further purification. Graphene oxide (GO) was synthesized from graphite powder by modified Hummers method.[29] The as-synthesized GO was exfoliated in distilled water by the ultrasonication for 30 min to form homogeneous GO dispersions with a concentration of 0.7 mg ml−1 for further use. Ni(NO3 )2 ·6H2 O and Co(NO3 )2 ·6H2 O in the ratio required for the formation of NiCo2 O4 were dissolved at a concentration of 0.03 M in a mixture of 20 vol.% ethyl alcohol and 80 vol.% ethylene glycol. Subsequently, a certain amount of GO dispersions were added into the above solution to obtain the spray precursor. The as-prepared solution was transferred to a syringe. The nickel foam (2 cm × 1 cm, carefully cleaned with acetone, ethanol and DI-water in an ultrasound bath) was used as a substrate and the distance between the needle and the substrate was 3 cm. While applying a direct current (DC) voltage of about 16 kV between the needle and the substrate,

The electro-spray setup and the preparation process of the composite are illustrated schematically in Fig. 1. The process can be divided into four steps. Firstly, a mixed liquid solution in which the Co-Ni hydroxide is dispersed among the graphene oxide sheets

Fig. 1. Schematics of electrostatic spray setup and the preparation process of the composite: (I) the solution became charged as it was fed through a stainless steel needle connected to a high voltage power supply; (II) the solvent evaporated partly and fission happened as the droplets traveled to the substrate; (III) solidification; (IV) deposition.

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Fig. 2. (a) XRD pattern of the NiCo2 O4 -rGO composite on the Ni foam. (b) Raman spectra of the composite.

becomes charged as it’s fed through a stainless steel needle connected to a high voltage power supply. Secondly, on exiting the needle, the fluid is forced into fine droplets and elongated into a jet-like fog under the influence of the electric force and the electric field-induced shear stress on the liquid surfaces. Thirdly, solvent partly evaporated as they travels towards the substrate, leading to a increase of the concentration within the droplets. Simultaneously, changes happen between Co-Ni hydroxide and GO. Last, once the fog arrives at the substrate, the rest solvent can be removed due to the temperature control of the underlying substrate by a heater, which leads to an irregular porous structure, and NiCo2 O4 nanoparticles are homogeneously anchored on the surface of rGO nanosheets (rGNS). 3.1. Structural characterization The structure and phase of samples were investigated by X-ray diffraction (XRD) measurement. Fig. 2a shows the XRD pattern of the NiCo2 O4 -rGO nanocomposites supported on nickel foams. As observed in Fig. 2a, except for the three typical peaks originating from the Ni substrate, other three well-defined diffraction peaks are observed at 2␪ value of 18.9◦ , 36.6◦ and 64.9◦ . All of these peaks can be successfully indexed to (111), (311) and (440) plane reflection of the spinel NiCo2 O4 crystalline structure (JCPDF file no. 20-0781), with the standard peaks indicated by the red lines in Fig. 2a. However, the GO or rGO peaks are not present in the sample, because its content in the composite is not high so that the XRD signals are suppressed by NiCo2 O4 . The samples were also investigated by Raman spectroscopy and the results were displayed in Fig. 2b. The peak at about 1587 cm−1 (G band), corresponding to an E2g mode of graphite, is related to the vibration of sp2 -bonded carbon atoms in a 2-dimensional hexagonal lattice, while the peak at about 1327 cm−1 (D band) is related to the defects and disorders in the hexagonal graphitic layers.[31] The intensity ratio of the D to G band (ID /IG ) is calculated as 1.25, higher than that of GO (1.10) reported by our previous research.[32] This enhancement could be ascribed to the exfoliation of GO and the presence of NiCo2 O4 nanoparticles between the graphene sheets. The morphology and microstructure of the samples were examined by SEM and TEM. Fig. 3a-d display the SEM images of the composite at different magnifications, and the inset of Fig. 3c shows the SEM image of pure NiCo2 O4 . As it’s shown in the SEM images, the composite is uniformly deposited on the nickel foam surface and an irregular porous network-like structure can be observed. As materials synthesized by ESD exhibited the morphology of homogeneous nanoparticles reported by many groups, and the results shown in the TEM tests, it’s reasonable to believe that NiCo2 O4 nanoparticles were homogeneously

anchored on the surface of rGO in the composite. The regions marked by red circles in Fig. 3e are the regions of rGNS. Moreover, the HRTEM image in Fig. 3f shows that the spacing between adjacent fringes is ca. 0.468 nm, close to the theoretical interplane spacing of spinel NiCo2 O4 (111) planes. 3.2. Electrochemical characteristics The electrochemical performance of the NiCo2 O4 -rGO composites on nickel foams for SCs were investigated by cycling voltammetry (CV) and galvanostatic charge-discharge techniques in a three-electrode system with Pt foil as the counter electrode and a saturated calomel electrode (SCE) as reference electrode in 2 M KOH. Fig. 4a shows the CV curves with various scanning rates ranging from 5 to 50 mV s−1 . Two pairs of redox current peaks can be found in each voltammogram. The redox couples C1 /C2 and N1 /N2 respectively correspond to the reversible reactions of Co2+ /Co3+ and Ni2+ /Ni3+ transitions. Such a feature that differs from CV curves where only Co2+ /Co3+ transitions are observed was reported by Hu and co-workers.[15] This may be due to strong chemical interactions between the nanoscale NiCo2 O4 nanoparticles and the residual oxygen-containing functional groups on the rGO, or van der Waals interactions between NiCo2 O4 and rGO. This intimate binding affords facile electron transport between individual NiCo2 O4 and the rGO, which may be a key to the presence of redox peaks Ni2+ /Ni3+ transitions. The galvanostatic discharge curves between -0.1 and 0.45 V at different specific currents are shown in Fig. 4b. The nanocomposite exhibited the specific capacitances of 783, 777, 744, 720 and 686 F g−1 at 1, 5, 10, 15 and 20 A g−1 , respectively, which were much higher than those of pure NiCo2 O4 (726, 663, 581 and 508 F g−1 at 5, 10, 15 and 20 A g−1 , respectively) (Fig. 4b and c). In addition, the cycling performance was also evaluated by the repeated chargingdischarging measurement at a constant current density of 5 A g−1 , as shown in Fig. 4d. The specific capacitance of the nanocomposite was 777.1 F g−1 in the first cycle and it gradually decreased to 772 F g−1 after 3000 times, with a retention of 99.3%, whereas the specific capacitances of pure NiCo2 O4 and pure rGO in the first cycle were only 719.6 F g−1 (67% retention, 2600 cycles) and 163.6 F g−1 (77.7% retention, 3000 cycles). Such good performances could be explained as follows. On the one hand, rGO helped facilitate the collection and transportation of electrons during the cycling, improve the conductivity of NiCo2 O4 , and allow for a larger specific surface area. On the other hand, the presence of NiCo2 O4 nanoparticles between the rGNS can efficiently avoid the agglomeration of the rGNS during cycling. Accordingly, rGO helped enhance the electrochemical performance of the nanocomposite. The microstructure of the electrode after cycling is revealed in Fig. 5a. As the figure

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Fig. 3. Characterization of the NiCo2 O4 -rGO composite: (a-d) SEM images (the inset of c shows the SEM image of pure NiCo2 O4 ); (e) TEM images; (f) High-resolution TEM images.

Fig. 4. Electrochemical performance of NiCo2 O4 -rGO composite: (a) CV curves at various scan rates. (b) Galvanostatic discharge curves at various specific currents. Comparison of the electrochemical performances of NiCo2 O4 -rGO composite and pure NiCo2 O4 . (c) Specific capacitance at different specific currents. (d) Evolution of the specific capacitance versus the number at 5 A g−1 . Table 1 The comparison of rate and cycle performances of some other NiCo2 O4 /rGO electrodes reported in previous literature. Electrode structure

Capacitance degradation after cycling

Capacitance retention

Refs.

Layer-by-layer NiCo2 O4 -rGO SDS- induced ultrasmall NiCo2 O4 nanocrystals on rGO Nanowires NiCo2 O4 on rGO Nanowires and ultrafine nanoparticles NiCo2 O4 on rGO

13% after 3550 cycles at 2 A g−1 8.4% after 3000 cycles at 20 A g−1 3% after 3000 cycles at 4 A g−1 2% after 500 cycles at 7 A g−1 and then 6% after 700 cycles at 16 A g−1

73.6% from 1 to 20 A g−1 62.8% from 0.5 to 40 A g−1 50% from 1 to 20 A g−1 83% from 1.5 to 33 A g−1

15 34 35 36

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Fig. 5. (a) SEM image of NiCo2 O4 -rGO composite after 3000 cycles. (b) Nyquist plots of NiCo2 O4 -rGO composite and pure NiCo2 O4 .

shows, after 3000 cycles, the irregular porous network-like structure of the nanocomposite can still be observed. Comparing with some other NiCo2 O4 -rGO electrodes reported in previous literature (Table 1), the as-fabricate nanocomposite exhibited remarkable cycle performance[15,34–36] . It should be pointed out that the nanocomposite exhibited a specific capacitance of 686 F g−1 at 20 A g−1 , which remained 87.6% of the specific capacitance at 1 A g−1 , and the specific capacitance at 10 A g−1 remained even 95% of the specific capacitance at 1 A g−1 . The capacity retention at high current densities of the nanocomposite in this study was much higher than those of the previous reported NiCo2 O4 -rGO electrodes (Table 1), due to its excellent rate performance. The EIS analysis is generally used to predict the behavior of electrochemical capacitor, and to determine the parameters affecting the performance of an electrode.[33] In the low frequency area, the slope of the curve shows the Warburg impedance which represents the electrolyte diffusion in the porous electrode and proton diffusion in host materials. The nanocomposite showed lower diffusion resistance (Fig. 5), which could be attributed to its irregular porous structure. In the high frequency area, the intersection of the curve at real part indicates the bulk resistance of the electrochemical system (electrolyte resistance, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface). The nanocomposite showed lower bulk resistance, which further prove that rGO can improve the conductivity of NiCo2 O4 . 4. Conclusions In summary, the irregular porous network-like NiCo2 O4 -rGO composite on the nickel foam was synthesized by electrostatic spray deposition (ESD). The rGO served as a conductive network to facilitate the collection and transportation of electrons during the cycling, improving the conductivity of NiCo2 O4 , and allowing for a larger specific surface area. The irregular porous structure also allowed for easy diffusion of the electrolyte into the inner region of the electrode, reducing diffusion resistance of the electrolyte. Moreover, the avoiding of binders reduced Ohmic polarization, and brought a better adhesion. As a result, the NiCo2 O4 -rGO composite exhibited a specific capacitance of 777.1 F g−1 at a current density of 5 A g−1 and about 99.3% of the capacitance retained after 3000 cycles. The capacitance retention was about 87.6% from 1 A g−1 to 20 A g−1 . It’s worth noting that the fabrication process was cheap (no cost for binders and conductive additives) and can easily and quickly harvest mass goods, which met the market demands. In addition, the ESD method can protect the substrate from corrosion, making the quantity of the synthesized material more accurate and controllable. The results indicate that this irregular porous

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