- Email: [email protected]
Facile synthesis of porous nanostructures of NiCo2O4 grown on rGO sheet for high performance supercapacitors
Synthetic Metals 259 (2020) 116215
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Facile synthesis of porous nanostructures of NiCo2O4 grown on rGO sheet for high performance supercapacitors
T
Arvind Singh, Saurav K. Ojha, Animesh K. Ojha* Department of Physics, Motilal Nehru National Institute of Technology Allahabad, Prayagraj 211004, India
ARTICLE INFO
ABSTRACT
Keywords: rGO@NiCo2O4 Nanowires Nanoflower Supercapacitors
Porous nanowires (NWs) and nanoflowers (NFs) type structures of nickel cobaltite (NiCo2O4) were synthesized on rGO sheet (rGO@NiCo2O4) as supercapacitor electrode material. The synthesized materials are characterized using different experimental techniques. The growth of NiCo2O4 over rGO sheet is confirmed by the appearance of D and G bands in the Raman spectrum. The rGO@NiCo2O4 NFs electrode revealed relatively high value of specific capacity (436 C/g at current density of 1 A/g) with good capacity retention compared to other electrodes. The improved electrochemical activity of the rGO@NiCo2O4 NFs electrode may be due to the porous nanostructure of NiCo2O4 on rGO sheet. The rGO sheets provide easy access to ions, large surface area and large active sites with stable structural integrity. The Nyquist plot of the rGO@NiCo2O4 NFs electrode shows relatively low values of Rs and Rct compared to other electrodes, which essentially refer to its improved electrical conductivity. Thus, the rGO@NiCo2O4 NFs may be used as a supercapacitor electrode material for device fabrication.
1. Introduction The energy storage materials find numerous applications in designing hybrid electric vehicles, portable electronic and wearable devices [1–6]. A tradeoff between battery and traditional capacitor is known as supercapacitor [5,2–6]. Apart from simple charge storage application, the supercapacitors have advantage of long lifespan, cheap and almost maintenance free. In general, there are two types of supercapacitors (i) electrochemical double layer capacitors (EDLCs) which store energy by physical adsorption/desorption of the ions at the electrode/electrolyte interfaces (commonly carbon based materials) and (ii) pseudocapacitors (PCs) which work on the principle of fast and reversible faradic reactions at the surface or near the surface of the electrode [7,8]. Currently, PCs are preferred for applications because they offer high current density with acceptable power density and specific capacity/capacitance (Cs/SC) compared to EDLCs. It encourages us to develop new efficient electrode materials for energy storage applications [8,9]. The transition metal based hydroxides/oxides, sulphides, phosphides, mixed transition metal oxides/sulphides and conducting polymers are used as electrode materials which trigger the redox faradic reaction due to multiple oxidation states, electroactive sites, large capacity and promising electrochemical activity [10–14]. In this regard, NiCo2O4 acts as better electrode material for building supercapacitors ⁎
and lithium-sulfur based batteries [13,15,16]. It may be due to the facts that NiCo2O4 offers good electronic conductivity, environment friendly, and low cost deign with different morphological structures [17,18]. Recently, various types of NiCo2O4 nanostructures such as, 1D nanowires, 2D nanosheets and 3D-hierarchical structures were studied in detailed [17–19]. The electrochemical performance of the NiCo2O4 can be tuned by altering its structural morphology [17–19]. However, the agglomeration of nanosized NiCo2O4 nanostructures restricts to tune its properties effectively. The integrated electrode materials supported with conducting substrates as carbon cloth, titanium mesh, copper foil, stainless steel and nickel foam are being widely used as abundant electroactive sites for short diffusion path and to minimize the “dead surface” [20–22]. Thus, there is still a wide space to explore nanostructures of NiCo2O4 with different structural morphologies for its improved charge storage properties. Till date, a significant progress is made in the field of energy storage materials [13,17–19]. Supercapacitors counter some drawbacks e.g. high energy density, high capacity, safety, long cycle life, excellent rate capability and easy fabrication of devices. To overcome these limitations, the recent research in this field is focused on the integration of hybrid electrodes with binary/ternary transition metal hydroxides/ oxides and conductive carbon based materials [22,23]. Carbon based materials, especially graphene, is one of the fascinating electrode materials for high performance as it acts as both, current collectors and
Corresponding author. E-mail address: [email protected] (A.K. Ojha).
https://doi.org/10.1016/j.synthmet.2019.116215 Received 25 August 2019; Received in revised form 3 October 2019; Accepted 18 October 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
supporters due to its intrinsic properties such as, high electrical conductivity, high surface area, chemical stability, flexibility, network structure, low cost and large scale production [24,25]. The reduced graphene oxide (rGO) sheet is expected to enhance its electrical conductivity in terms of easy transfer of charge and uniform growth of the materials [25,26]. The synergistic effect of NiCo2O4 and rGO sheet provides fast electronic diffusion which enhanced rate capability with improved surface activity for charge storage [20,22]. The NiCo2O4-rGO composites have been synthesized by different methods [23,27] for charge storage applications. These synthesis methods are very complicated and expensive. Here, we explored a facile method to synthesize NiCo2O4 with different structural morphologies over the rGO sheets as efficient and stable electrode materials for supercapacitors application. In view of above discussed studies on various electrode materials and their applications in energy storage devices, we successfully synthesized nanoflowers (NFs) and nanowires (NWs) like architectures over the rGO sheets using a simple hydrothermal method followed by annealing at different temperatures. Due to the porous and conductive nature of Ni foam, the NFs and NWs type structures of NiCo2O4 are grown easily over the rGO sheets due to its interface affinity, which minimizes the contact resistance to improve the efficiency of the electron transport. The rGO@NiCo2O4 NFs like electrode material is expected to show improved specific capacity (436 C/g at 1 A/g) with excellent rate capability and long cycle life, as required for the fabrication of efficient energy storage devices.
were dissolved in 60 mL distilled water with stirring at room temperature for 30 min. The color of the solution turns out to pink. The dissolved pink solution was transferred into a Teflon-lined stainless steel autoclave of 100 ml capacity containing rGO treated Ni foam and kept at 120 °C for 12 h. Thereafter, as ready precursor was taken out and washed with distilled water and ethanol several times. The washed material is dried in electric oven for 3 h. Finally, the obtained materials were annealed at 300 and 400 °C for 3 h to grow NWs and NFs like structural morphologies of NiCo2O4. Moreover, pure NiCo2O4 NWs are also synthesized at same condition for comparison. The mass loading of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs on Ni foam was found to be ∼ 1.5, ∼ 1.7 and ∼ 1.7 mg cm−2, respectively. 2.2. Characterization The synthesized materials are characterized by different experimental techniques. The X-ray diffraction (XRD) measurement is done using Rigaku smart lab diffractometer with CuKα radiation (λ = 1.5406 Å) for the 2θ values lying in the range of 10°– 80° at a scan rate of 5°/min with the step size of 0.02. The structural morphologies and chemical elements present in the samples are characterized using a JEOL FESEM equipped with an EDX and elemental mapping equipment. The Fourier-transform infrared spectroscopy (FTIR) is employed to record the spectra using JASCO-6100 in the spectral range 400–3200 cm−1. The Raman spectra of the samples are measured on a Reinshaw Raman spectrometer (Renishaw, Wotton-under-Edge, UK) using a wavelength of 514 nm by Argon ion laser as excitation wavelength.
2. Experimental details
2.3. Electrochemical measurements
2.1. Synthesis
The electrochemical performance of as synthesized electrode material is investigated using Metrohm Autolab PGSTAT302 N with a standard three electrodes configuration. The standard Ag/AgCl is used as a reference electrode, platinum wire as a counter electrode and individual synthesized material is used as working electrode without any binder. The electrochemical properties are investigated at room temperature in 6 M KOH aqueous electrolyte using cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements. The value of Cs is calculated with the help of GCD results using Eq. 1 [30,31]:
The chemicals procured herein were of analytical grade and used without any further purification. A schematic presentation of steps used to synthesize rGO@NiCo2O4 NFs and NWs like structures is presented in Fig. 1. To synthesize the material, some pieces of Ni foam (1 × 1 cm2) were cleaned to remove oxides using 3 M HCl, washed with ethanol and then pre-weighed before use. Graphene oxide (GO) was synthesized using the method reported in earlier studies [28,29]. The treated pieces of Ni foam were put vertically into a 50 mL Teflon-lined stainless steel autoclave containing brown solution of GO and heated them at 120 °C for 12 h. After being cooled naturally, the Ni foam was washed carefully with distilled water and ethanol and then dried at 60 °C in air for 3 h. Subsequently, 1.85 g CoCl2.6H2O, 0.95 g NiCl2.6H2O and 0.72 g urea
Cs =
I× t m
Fig. 1. Schematic presentation of steps used to synthesize rGO@NiCo2O4 NWs and NFs like structures. 2
(1)
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
where I (A), m and t are discharging current, mass on electrode and discharging time, respectively. Further, the values of SC of the electrode materials are calculated using Eq. 2, where V is applied potential window [31]:
SC =
I× t m× V
nanostructures [36]. 3.2. FESEM The FESEM micrographs are shown in Fig. 3. Fig. 3a shows the formation of rGO wrinkled type sheets grown on the surface of Ni foam and only carbon and oxygen related peaks are appeared in the EDX spectra (Fig. 3b). The rGO sheets are interconnected with Ni foam due to better mechanical strength of the rGO. FESEM micrographs of pure NiCo2O4 show NWs like structural morphology (Fig. 3c). The NWs are not grown uniformly over the surface of Ni foam. It may be due to the different rates of agglomeration of the seed particles [37]. However, NWs and NFs like structures of NiCo2O4 are grown uniformly over the rGO sheet (Fig. 3d–g). The uniform growth may be due to the presence of different functional groups on rGO sheet [38]. The low magnified FESEM micrographs of NWs and NFs (Fig. 3e–g) clearly show mesoporous features, which essentially needed for better performance of charge storage devices. The NFs like structures may be formed by temperature induced agglomeration of large number of NWs array interconnected with each other. These kind of hybrid nanostructures offer abundantly active surface area between the electrode and electrolyte which provide additional space for the collection of large electrolyte ions to minimize the ion diffusion length during charging and discharging [23]. The elemental mapping and EDX spectrum of the samples are shown in Fig. 3h and i. Fig. 3i shows presence of Ni, Co, O, and C in rGO@NiCo2O4.
(2)
3. Results and discussion The free metals (Ni2+ and Co2+) and hydroxide (OH−) ions generated during hydrolysis reaction assist to form mixed bimetallic hydroxide [32]. During hydrolysis, NixC2x(OH)6x is formed over the rGO sheets grown over the surface of Ni foam [32]. The product thus obtained is put for annealing to get the cubic spinel structure. Different annealing temperatures are applied for getting various structural morphologies of rGO@NiCo2O4. The formation of NiCo2O4 nanostructures may be governed by the following chemical reactions:
NO3 + 7H2 O + 8e
NH+4 + 10 OH x Ni2 + + 2x Co2 + + 6x OH Nix Co2x (OH)6x
NixCo2x(OH)6x + O2 → x NiCo2O4 + H2O ↑ 3.1. XRD
3.3. Raman and FTIR
The crystallographic information of the synthesized samples is obtained by analyzing the XRD patterns, as shown in Fig. 2. No peaks related to any impurity phases are observed. The peaks at 44.5˚ and 51.8˚ are assigned to (111) and (200) planes of Ni foam (JCPDF no. 040850). The peaks at 31.2 ˚, 36.6˚, 44.7˚ and 64.9˚ confirm the formation of NiCo2O4 [33]. No characteristic signature of rGO is observed in the XRD pattern. The XRD pattern of the samples annealed at 300 (rGO@ NiCo2O4 NWs) and 400 °C (rGO@NiCo2O4 NFs) are shown in Fig. 2, respectively. The peaks at 18.9 ˚, 31.2 ˚, 36.6 ˚, 44.7 ˚, 59.2˚ and 64.9 ˚ correspond to (111), (220), (311), (400), (511) and (440) planes of single cubic phase with the Fd3m space group of the ternary spinel NiCo2O4 structure (JCPDS card no. 00-020-0781) [34]. From the FESEM micrographs, as shown in Fig. 3, one can easily notice that the overall size of the composite is reduced. The smaller crystallite size has large active surface area [35]. Further, the lattice of rGO@NiCo2O4 crystals is strained compared to pure NiCo2O4. The improved lattice strain could be due to the incorporation of rGO with the NiCo2O4
Raman spectroscopy is a frequently used method for analysis of structural features and composition of low dimension materials, specially carbon and its composite materials [39]. Fig. 4 illustrates the Raman spectra of as synthesized GO, rGO and its rGO based composites. The presence of D (1350 cm−1) and G (∼1603 cm−1) bands (see Fig. 4a–c) confirms the growth of NiCo2O4 over the rGO sheet. The D band is assigned to disorder and structural defects present in the material, however, the G band is associated to the graphitic nature assigned to the E2g phonon of sp2 bonded carbon atoms in a 2D hexagonal lattice [39]. The presence of defect states and degree of graphitization is evaluated by taking the intensity ratio of D and G bands. The intensity ratio of D and G bands (ID/IG) for rGO, rGO@NiCo2O4 NWs and NFs is calculated to be 0.89, 0.93 and 0.94, respectively. A relatively larger value of ID/IG for rGO@NiCo2O4 NWs and rGO@NiCo2O4 NFs compare to rGO indicates that the NiCo2O4 interacts with rGO sheet [40]. The peaks at ∼151, ∼457, ∼455, ∼505, ∼656 and ∼1096 cm−1 correspond to the F2g, Eg, 1LO, A1g and 2LO modes of NiCo2O4, respectively [41]. Fig. 4d represents the FTIR spectra of pure NiCo2O4 NWs, rGO@ NiCo2O4 NWs and NFs like structures. In NiCo2O4 and rGO@NiCo2O4 NWs, two intense bands at ∼545 and ∼645 cm−1 are observed. These bands are assigned to Ni–O and Co–O stretching modes of NiCo2O4 [42,43]. The low intense peaks lying in the spectral range 10001400 cm−1 are assigned to epoxy and alkoxy groups [29,44]. The presence of peak corresponding to CeOH (1385 cm−1) and C]C (1630 cm−1) stretching vibrations confirm the presence of rGO sheets in rGO@NiCo2O4. The interaction between rGO and NiCo2O4 is confirmed by the changes in Raman peak parameters [44,45]. 3.4. Electrochemical analysis The electrochemical behavior of the electrode materials is evaluated by CV measurements using three electrodes configuration system with 6 M KOH aqueous electrolyte. Fig. 5a shows the CV curves of NiCo2O4 NWs, rGO@NiCo2O4 NWs, and rGO@NiCo2O4 NFs in the potential range –0.15 to + 0.55 V vs. Ag/AgCl, at scan rate of 5 mV/s. The CV curves show a pair of well-defined redox peaks. These peaks indicate that the capacitive nature is mainly governed by the Faradic reaction
Fig. 2. XRD patterns of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs grown on Ni-foam (* indicates the peak of Ni foam). 3
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
Fig. 3. (a) FESEM micrographs of rGO sheets grown on Ni foam, (b) EDX spectrum, (c) pure NiCo2O4 NWs, (d–e) rGO@NiCo2O4 NWs, (f–g) NFs like structures and (h–i) elemental mapping and EDX spectrum of rGO@NiCo2O4.
[34]. The integrated area under the CV curve of the rGO@NiCo2O4 NFs electrode is increased by the introduction of rGO. This signifies an increase in capacity due to the synergistic response of Ni-Co oxides and rGO [10,17]. The CV curve of all the electrodes revealed a pair of redox peak in the range of −0.15 V to +0.55 V due to the redox reaction of Ni2+/Ni3+ and Co3+/Co4+ [17,30]. The following redox reactions are expected to take place at surface of the electrode:
Fig. 5b shows typical CV curves of NFs like electrode at different scan rates. Apparently, as the scan rate is increased, the peak current is also increased while shape of the CV curves is retained with a pair of redox peak up to 70 mV/s scan rate. It is favorable for the redox reaction of the electrode. The increased value of peak current density with respect to scan rate suggests quick enrichment of electronic and ionic transport with low polarization value of the electrode [22,46]. The shifting of cathodic and anodic peaks towards positive and negative side with increasing scan rate corresponds to increased value of internal resistance of the electrode [47]. Fig. 5c represents the linear dependence of cathodic and anodic peak current densities with respect to scan rates. It also demonstrates the hybrid supercapacitive nature of the synthesized electrode materials. The surface coverage (Γ*) of redox species over the pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and rGO@ NiCo2O4 NFs electrodes is calculated using Eq. 3 [45,48]:
NiCo2O4 + H2O+OH− ↔ NiOOH + 2 CoOOH+e− (cathode) CoOOH+OH− ↔ CoO2 + H2O+e- (anode) Due to the conducting nature of rGO sheets, the separation between peak to peak values (ΔEP) in rGO@NiCo2O4 is comparatively less compared to NiCo2O4 electrode (see Table 1). The high anodic current density and less value of ΔEP for rGO@NiCo2O4 electrodes could lead to fast electron transfer between the electrode and electrolyte [46].
Fig. 4. (a) Raman spectra of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs {inset (b) Raman spectra of GO and rGO (c) magnified Raman spectra of rGO@NiCo2O4 NWs and NFs and (d) FTIR spectra of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs. 4
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
Fig. 5. (a) CV curves of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs like structures at 5 mV/s (b) CV curve of rGO@NiCo2O4 NFs electrode at various scan rates (5–100 mV/s) (c) linear dependence of anodic current and cathodic current densities Vs scan rate.
Ip =
n2F 2 *As 4RT
We performed cycling stability measurements of all the electrodes under the same conditions. Fig. 6d demonstrates cyclic stability of all the electrodes. Among all the electrodes, rGO@NiCo2O4 NFs showed better stability compared to other electrodes. It is noticed that when constant current density (30 A/g) is applied for the first cycle, the value of Cs turns out to be maximum. As the number of cycle is increased to 5000, the overall capacity is decreased to only 12% of its initial value. A relatively better stability of rGO@NiCo2O4 NFs electrode could be due to the interconnection of rGO sheets with NiCo2O4 NWs to form NFs like structures, leading to the barrier free contact which minimizes the internal resistance of the electrode remarkably [55]. The kinetics of the electrode materials are studied by performing electrochemical impedance spectroscopy (EIS) measurements in the frequency range, 0.01 Hz to 100 kHz at an open circuit voltage of 5 mV. The Nyquist plots of all the three electrodes are shown in Fig. 7 a. At high frequency side of the Nyquist plot, the intersection of the curve at the real axis represents the equivalent series resistance (Rs) of the electrochemical system [17,50,57]. The diameter of semicircle is associated with the charge transfer resistance (Rct). The slope of line at lower frequency range reflects the Warburg (w) [20,57]. It can be observed that the rGO@NiCo2O4 NFs electrode has a smaller semicircle compared to other electrodes. This could deliver a superior pathway for ion transfer and electron transport in the electrode material. The EIS data of all the electrodes have been simulated via NOVA software to get equivalent Randles circuit and the values of fitting parameters are presented in Table 2. The relatively less value of Rct and Rs for rGO@ NiCo2O4 NFs electrode may be due to hairy needle like porous structures of NiCo2O4 over the rGO sheets that provide easy access to ions [20]. Inset of Fig. 7b represents the simplified circuit diagram with different components, Rs, Rct, Cdl, and W. Evidently, the simulated graph nearly close to the experimental data, indicating that the simplified circuit provides the correct information of electrochemical reactions that take place inside the rGO@NiCo2O4 NFs electrode.
(3)
where, n is the number of electrons involved in the process, A is the area of the active electrode, F is the Faraday constant, s is the scan rate, R is gas constant and T is the temperature. A relatively large value of Γ* (8.9 × 10−5 mol cm−2) is calculated for the rGO@NiCo2O4 NFs electrode compared to other electrodes. The higher value of Γ* for rGO@ NiCo2O4 NFs electrode may be due to the interconnected and combined surface of numerous hairy needles like porous structures of NiCo2O4 and large surface area of rGO sheets [49]. Further from Figs. S1 and S2 (see the Supplementary information), it is clear that rGO@NiCo2O4 NFs like electrode shows highest value of Cs and SC compared to other electrodes. The Figs. S1 and S2 also suggest that as the scan rate is increased, the value of Cs and SC is decreased due to increase of internal diffusion resistance of the electrode materials [10]. The electrochemical performance of the synthesized materials is further investigated by performing GCD measurements. The results of GCD measurements within the potential window 0 to +0.42 V vs. Ag/ AgCl are shown in Fig. 6. In Fig. 6a, a relatively longer charge/discharge time is observed for rGO@NiCo2O4 NFs electrode compared to other electrodes at an applied current density of 5 A/g. The plateau type nature of GCD curve, indicates hybrid supercapacitive nature of the electrodes [50]. The GCD curves of rGO@NiCo2O4 NFs electrodes at various current densities are shown in Fig. 6b. All the GCD curves are found nearly symmetrical in nature, suggesting high coulombic efficiency of the electrode materials at different current densities [50]. The values of Cs of the rGO@NiCo2O4 NFs electrode turn out to be 436 (1040 F/g), 434 (1035 F/g), 430 (1029 F/g), 399 (950 F/g), 388 (924 F/g), 376 (895 F/g), 370 (882 F/g) and 353 (840 F/g) C/g at current densities of 1, 3, 5, 7, 10, 20, 30 and 50 A/g, respectively. The electrode retained its Cs up to 80 % (353 C/g) even if the current density is increased to 50 A/g. It indicates a better capacitive retention of the electrode materials. In the present study, an improved value of Cs is found compared to the values reported in the earlier studies [40,51–57]. The values of Cs and SC of all the electrode materials obtained at different densities are plotted and shown in Fig. 6c.
4. Conclusion NWs, and NFs type structures of NiCo2O4 are grown over the rGO
Table 1 The values of peak to peak separation (ΔEp), peak current density (ipa), specific capacity and capacitance. Electrode materials
Pure NiCo2O4 NWs rGO@ NiCo2O4 NWs rGO@ NiCo2O4 NFs
Mass loading (mg)
1.5 1.7 1.7
ΔEp (V)
0.27 0.16 0.17
Peak current density, ipa (A/ g)
Specific capacity (Cs) from GCD@ 1 A/g (C/g)
Specific capacitance (SC) (F/g) SC from CV@ 5 mv/s
SC from GCD@ 1 A/g
18.69 19.88 20.05
423 435 436
811 892 1003
1017 1036 1040
5
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
Fig. 6. (a) GCD profile of NiCo2O4 NWs, rGO@NiCo2O4 NWs and rGO@NiCo2O4 NFs electrodes (b) curves at different current densities (c) summary of specific capacities and specific capacitance as a function of current densities and (d) cycling stability performance of all the electrodes at 30 A/g.
sheet using hydrothermal method. The growth of NiCo2O4 nanostructures over the rGO sheet is confirmed by Raman and FTIR measurements. The FESEM micrographs confirm the formation of NWs and NFs like structures over the rGO sheet. The CV and GCD results showed that the rGO@NiCo2O4 NFs acts as an improved supercapacitive electrode material with excellent properties such as, enhanced surface
areas, short diffusion pathways for electrolyte ions, excellent rate capability, and cyclic stability at higher current density. The low value of Rct and Rs provides easy access to ions, resulting enhanced value of Cs for rGO@NiCo2O4 electrodes. Thus, the rGO@NiCo2O4 based electrodes may be used to design efficient energy storage devices.
Fig. 7. (a) Nyquist plots of the electrode materials formed by pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and rGO@NiCo2O4 NFs type structures (b) Nyquist plot of rGO@NiCo2O4 NFs type structures and corresponding fitted equivalent circuit is shown in the inset.
6
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
Table 2 The values of parameters of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs electrodes obtained after the fitting. Parameters → Samples ↓
Rs (Ω)
Rct (Ω)
W (mMho)
Cdl
χ2
Pure NiCo2O4 NWs rGO@NiCo2O4 NWs rGO@NiCo2O4 NFs
2.70 2.06 1.33
11 3.19 1.79 × 10−6
12.6 13.9 12.7
963 μF 7.86 μF 119 mF
1.245 1.217 0.414
Adv. 5 (48) (2015) 38407–38416. [18] G. Gao, H.B. Wu, S. Ding, L.M. Liu, X.W. Lou, Small 11 (7) (2015) 804–808. [19] L. Wang, X. Jiao, P. Liu, Y. Ouyang, X. Xia, W. Lei, Q. Hao, Appl. Surf. Sci. 427 (2018) 174–181. [20] S. Chen, G. Yang, Y. Jia, H. Zheng, J. Mater. Chem. A 5 (3) (2017) 1028–1034. [21] S. Li, K. Yang, P. Ye, H. Jiang, Z. Zhang, Q. Huang, L. Wang, Appl. Surf. Sci. 473 (2019) 326–333. [22] L. Zhang, S. Diwen, L. Tao, Mietek Jaroniec, Yu Jiaguo, Mater. Today 25 (2019) (2018) 35–65, https://doi.org/10.1016/j.mattod.2018.11.002. [23] U. Patil, S.C. Lee, S. Kulkarni, J.S. Sohn, M.S. Nam, S. Han, S.C. Jun, Nanoscale 7 (16) (2015) 6999–7021. [24] Gao, W., The chemistry of graphene oxide, Springer, Cham. 2015, 61-95. [25] F. Li, X. Jiang, J. Zhao, S. Zhang, Nano Energy 16 (2015) 488–515. [26] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat. Mater. 14 (3) (2015) 271. [27] H. Chen, C.K. Hsieh, Y. Yang, X.Y. Liu, C.H. Lin, C.H. Tsai, Y.X. Zhang, Chem Electro Chem 4 (9) (2017) 2414–2422. [28] W.S. Hummers Jr, R.E. Offeman, J. Am. Chem. Soc. 80 (6) (1958) 1339-1339. [29] A. Singh, B. Ahmed, A. Singh, A.K. Ojha, Spectrochim. Acta A. Mol. Biomol. Spectrosc. 204 (2018) 603–610. [30] W. Jiang, F. Hu, Q. Yan, X. Wu, Inorg. Chem. Front. 4 (2017) 1642–1648. [31] Z. Xu, J. Ren, Q. Meng, X. Zhang, C. Du, J. Chen, ACS Sustain. Chem. Eng. 7 (14) (2019) 12447–12456. [32] C. Zheng, C. Cao, R. Chang, J. Hou, H. Zhai, J. Chem. Soc. Faraday Trans. 18 (8) (2016) 6268–6274. [33] C. Yuan, J. Li, L. Hou, X. Zhang, L. Shen, X.W. Lou, Adv. Funct. Mater. 22 (21) (2012) 4592–4597. [34] C. Zhang, C. Lei, C. Cen, S. Tang, M. Deng, Y. Li, Y. Du, Electrochim. Acta 260 (2018) 814–822. [35] U. Holzwarth, N. Gibson, Nat. Nanotechnol. 6 (9) (2011) 534. [36] X. Wang, Y. Orikasa, Y. Takesue, H. Inoue, M. Nakamura, T. Minato, N. Hoshi, Y. Uchimoto, J. Am. Chem. Soc. 135 (16) (2013) 5938–5941. [37] H. Cheng, Y.Z. Su, P.Y. Kuang, G.F. Chen, Z.Q. Liu, J. Mater. Chem. A 3 (38) (2015) 19314–19321. [38] Z. Ma, X. Huang, S. Dou, J. Wu, S. Wang, J. Phys. Chem. C 118 (31) (2014) 17231–17239. [39] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (18) (2006) 187401. [40] T.H. Ko, S. Radhakrishnan, M.K. Seo, M.S. Khil, H.Y. Kim, B.S. Kim, J. Alloys Compd. 696 (2017) 193–200. [41] D. Li, Y. Gong, Y. Zhang, C. Luo, W. Li, Q. Fu, C. Pan, Sci. Rep. 5 (2015) 12903. [42] G. He, L. Wang, H. Chen, X. Sun, X. Wang, Mater. Lett. 98 (2013) 164–167. [43] N. Padmanathan, S. Selladurai, Ionics 19 (11) (2013) 1535–1544. [44] X. Jiao, X. Xia, P. Liu, W. Lei, Y. Ouyang, Q. Hao, Nanotechnology 28 (31) (2017) 315403. [45] A.K. Das, R.K. Layek, N.H. Kim, D. Jung, J.H. Lee, Nanoscale 6 (18) (2014) 10657–10665. [46] B. Baruah, A. Kumar, Synth. Met. 245 (2018) 74–86. [47] Q. Zhang, Y. Deng, Z. Hu, Y. Liu, M. Yao, P. Liu, J. Chem. Soc. Faraday Trans. 16 (42) (2014) 23451–23460. [48] L. Shen, Q. Che, H. Li, X. Zhang, Adv. Funct. Mater. 24 (18) (2014) 2630–2637. [49] A.J. Bard, Faulkner, R. Larry, Electrochemical Methods: Fundamentals and Applications, 2nd ed., John Wiley & Sons, Inc., New York, 2001. [50] K.H. Oh, G.S. Gund, H.S. Park, J. Mater. Chem. A 6 (44) (2018) 22106–22114. [51] V. Veeramani, R. Madhu, S.M. Chen, M. Sivakumar, C.T. Hung, N. Miyamoto, S.B. Liu, Electrochim. Acta 247 (2017) 288–295. [52] W.W. Liu, C. Lu, K. Liang, B.K. Tay, J. Mater. Chem. A 2 (14) (2014) 5100–5107. [53] J.W. Mao, C.H. He, J.Q. Qi, A.B. Zhang, Y.W. Sui, Y.Z. He, F.X. Wei, J. Electron. Mater. 47 (1) (2018) 512–520. [54] Z. Xu, L. Yang, Q. Jin, Z. Hu, Electrochim. Acta 295 (2019) 376–383. [55] S. Ramesh, D. Vikraman, H.S. Kim, H.S. Kim, J.H. Kim, J. Alloys Compd. 765 (2018) 369–379. [56] Y. Li, F. Tang, R. Wang, C. Wang, J. Liu, ACS Appl. Mater. Interfaces 8 (2016) 30232–30238. [57] E. Umeshbabu, G. Rajeshkhanna, P. Justin, G.R. Rao, RSC Adv. 5 (82) (2015) 66657–66666.
Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgements AS is thankful to Motilal Nehru National Institute of Technology Allahabad, Prayagraj for providing the Senior Research Fellowship (SRF). AS and AKO are thankful to TEQIP-III for providing the financial support. The instrumentation facilities available in the Centre for Interdisciplinary Research (CIR) of the Institute are greatly acknowledged. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116215. References [1] B.E. Conway, Electrochemical Capacitors: Scientific Fundamentals and Technology Applications, Kluwer Academic/Plenum Publishers, New-York, 1999. [2] C. Li, M.M. Islam, J. Moore, J. Sleppy, C. Morrison, K. Konstantinov, S.X. Dou, C. Renduchintala, J. Thomas, Nat. Commun. 7 (2016) 13319. [3] C. Chen, N. Zhang, Y. He, B. Liang, R. Ma, X. Liu, ACS Appl. Mater. Interfaces 8 (35) (2016) 23114–23121. [4] I. Roger, M.A. Shipman, M.D. Symes, Nat. Rev. Chem. 1 (1) (2017) 0003. [5] M. Winter, R.J. Brodd, Chem. Rev. 2003 (104) (2005) 4245–4269 Published on the Web 09/28/2004.). Chemical reviews, 105(3), 1021-1021. [6] P. Simon, Y. Gogotsi, B. Dunn, Science 343 (6176) (2014) 1210–1211. [7] T. Brousse, D. Bélanger, J.W. Long, J. Electrochem. Soc. 162 (5) (2015) A5185–A5189. [8] R. Sahoo, A.K. Sasmal, C. Ray, S. Dutta, A. Pal, T. Pal, ACS Appl. Mater. Interfaces 8 (28) (2016) 17987–17998. [9] Y. Liu, N. Fu, G. Zhang, M. Xu, W. Lu, L. Zhou, H. Huang, Adv. Funct. Mater. 27 (8) (2017) 1605307. [10] Alberto Adán-Más, T.M. Silva, L.G. Demourgues, L. Bourgeois, C.L. Sarroste, M.F. Montemor, J. Power Sources 419 (2019) 12–26. [11] S. Gao, Y. Sui, F. Wei, J. Qi, Q. Meng, Y. Ren, Y. He, J. Colloid Interface Sci. 2018 (531) (2018) 83–90. [12] Z. Sun, F. Huang, Y. Sui, F. Wei, J. Qi, Q. Meng, H. Hu, Y. He, J. Mater. Sci. Mater. Electron. 2017 (28) (2017) 14019–14025. [13] L. Cao, G. Tang, J. Mei, H. Liu, J. Power Sources 359 (2017) 262–269. [14] Q. Meng, K. Cai, Y. Chen, L. Chen, Nano Energy 36 (2017) 268–285. [15] L. Fang, H. Qiu, P. Luo, W. Li, H. Zhang, Y. Wang, Appl. Surf. Sci. 403 (2017) 35–42. [16] Z. Fan, B. Wang, Y. Xi, X. Xu, M. Li, J. Li, P. Coxon, S. Cheng, G. Gao, C. Xiao, G. Yang, Carbon 99 (2016) 633–641. [17] P. Syedvali, G. Rajeshkhanna, E. Umeshbabu, G.U. Kiran, G.R. Rao, P. Justin, RSC
7
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Facile synthesis of porous nanostructures of NiCo2O4 grown on rGO sheet for high performance supercapacitors
T
Arvind Singh, Saurav K. Ojha, Animesh K. Ojha* Department of Physics, Motilal Nehru National Institute of Technology Allahabad, Prayagraj 211004, India
ARTICLE INFO
ABSTRACT
Keywords: rGO@NiCo2O4 Nanowires Nanoflower Supercapacitors
Porous nanowires (NWs) and nanoflowers (NFs) type structures of nickel cobaltite (NiCo2O4) were synthesized on rGO sheet (rGO@NiCo2O4) as supercapacitor electrode material. The synthesized materials are characterized using different experimental techniques. The growth of NiCo2O4 over rGO sheet is confirmed by the appearance of D and G bands in the Raman spectrum. The rGO@NiCo2O4 NFs electrode revealed relatively high value of specific capacity (436 C/g at current density of 1 A/g) with good capacity retention compared to other electrodes. The improved electrochemical activity of the rGO@NiCo2O4 NFs electrode may be due to the porous nanostructure of NiCo2O4 on rGO sheet. The rGO sheets provide easy access to ions, large surface area and large active sites with stable structural integrity. The Nyquist plot of the rGO@NiCo2O4 NFs electrode shows relatively low values of Rs and Rct compared to other electrodes, which essentially refer to its improved electrical conductivity. Thus, the rGO@NiCo2O4 NFs may be used as a supercapacitor electrode material for device fabrication.
1. Introduction The energy storage materials find numerous applications in designing hybrid electric vehicles, portable electronic and wearable devices [1–6]. A tradeoff between battery and traditional capacitor is known as supercapacitor [5,2–6]. Apart from simple charge storage application, the supercapacitors have advantage of long lifespan, cheap and almost maintenance free. In general, there are two types of supercapacitors (i) electrochemical double layer capacitors (EDLCs) which store energy by physical adsorption/desorption of the ions at the electrode/electrolyte interfaces (commonly carbon based materials) and (ii) pseudocapacitors (PCs) which work on the principle of fast and reversible faradic reactions at the surface or near the surface of the electrode [7,8]. Currently, PCs are preferred for applications because they offer high current density with acceptable power density and specific capacity/capacitance (Cs/SC) compared to EDLCs. It encourages us to develop new efficient electrode materials for energy storage applications [8,9]. The transition metal based hydroxides/oxides, sulphides, phosphides, mixed transition metal oxides/sulphides and conducting polymers are used as electrode materials which trigger the redox faradic reaction due to multiple oxidation states, electroactive sites, large capacity and promising electrochemical activity [10–14]. In this regard, NiCo2O4 acts as better electrode material for building supercapacitors ⁎
and lithium-sulfur based batteries [13,15,16]. It may be due to the facts that NiCo2O4 offers good electronic conductivity, environment friendly, and low cost deign with different morphological structures [17,18]. Recently, various types of NiCo2O4 nanostructures such as, 1D nanowires, 2D nanosheets and 3D-hierarchical structures were studied in detailed [17–19]. The electrochemical performance of the NiCo2O4 can be tuned by altering its structural morphology [17–19]. However, the agglomeration of nanosized NiCo2O4 nanostructures restricts to tune its properties effectively. The integrated electrode materials supported with conducting substrates as carbon cloth, titanium mesh, copper foil, stainless steel and nickel foam are being widely used as abundant electroactive sites for short diffusion path and to minimize the “dead surface” [20–22]. Thus, there is still a wide space to explore nanostructures of NiCo2O4 with different structural morphologies for its improved charge storage properties. Till date, a significant progress is made in the field of energy storage materials [13,17–19]. Supercapacitors counter some drawbacks e.g. high energy density, high capacity, safety, long cycle life, excellent rate capability and easy fabrication of devices. To overcome these limitations, the recent research in this field is focused on the integration of hybrid electrodes with binary/ternary transition metal hydroxides/ oxides and conductive carbon based materials [22,23]. Carbon based materials, especially graphene, is one of the fascinating electrode materials for high performance as it acts as both, current collectors and
Corresponding author. E-mail address: [email protected] (A.K. Ojha).
https://doi.org/10.1016/j.synthmet.2019.116215 Received 25 August 2019; Received in revised form 3 October 2019; Accepted 18 October 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
supporters due to its intrinsic properties such as, high electrical conductivity, high surface area, chemical stability, flexibility, network structure, low cost and large scale production [24,25]. The reduced graphene oxide (rGO) sheet is expected to enhance its electrical conductivity in terms of easy transfer of charge and uniform growth of the materials [25,26]. The synergistic effect of NiCo2O4 and rGO sheet provides fast electronic diffusion which enhanced rate capability with improved surface activity for charge storage [20,22]. The NiCo2O4-rGO composites have been synthesized by different methods [23,27] for charge storage applications. These synthesis methods are very complicated and expensive. Here, we explored a facile method to synthesize NiCo2O4 with different structural morphologies over the rGO sheets as efficient and stable electrode materials for supercapacitors application. In view of above discussed studies on various electrode materials and their applications in energy storage devices, we successfully synthesized nanoflowers (NFs) and nanowires (NWs) like architectures over the rGO sheets using a simple hydrothermal method followed by annealing at different temperatures. Due to the porous and conductive nature of Ni foam, the NFs and NWs type structures of NiCo2O4 are grown easily over the rGO sheets due to its interface affinity, which minimizes the contact resistance to improve the efficiency of the electron transport. The rGO@NiCo2O4 NFs like electrode material is expected to show improved specific capacity (436 C/g at 1 A/g) with excellent rate capability and long cycle life, as required for the fabrication of efficient energy storage devices.
were dissolved in 60 mL distilled water with stirring at room temperature for 30 min. The color of the solution turns out to pink. The dissolved pink solution was transferred into a Teflon-lined stainless steel autoclave of 100 ml capacity containing rGO treated Ni foam and kept at 120 °C for 12 h. Thereafter, as ready precursor was taken out and washed with distilled water and ethanol several times. The washed material is dried in electric oven for 3 h. Finally, the obtained materials were annealed at 300 and 400 °C for 3 h to grow NWs and NFs like structural morphologies of NiCo2O4. Moreover, pure NiCo2O4 NWs are also synthesized at same condition for comparison. The mass loading of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs on Ni foam was found to be ∼ 1.5, ∼ 1.7 and ∼ 1.7 mg cm−2, respectively. 2.2. Characterization The synthesized materials are characterized by different experimental techniques. The X-ray diffraction (XRD) measurement is done using Rigaku smart lab diffractometer with CuKα radiation (λ = 1.5406 Å) for the 2θ values lying in the range of 10°– 80° at a scan rate of 5°/min with the step size of 0.02. The structural morphologies and chemical elements present in the samples are characterized using a JEOL FESEM equipped with an EDX and elemental mapping equipment. The Fourier-transform infrared spectroscopy (FTIR) is employed to record the spectra using JASCO-6100 in the spectral range 400–3200 cm−1. The Raman spectra of the samples are measured on a Reinshaw Raman spectrometer (Renishaw, Wotton-under-Edge, UK) using a wavelength of 514 nm by Argon ion laser as excitation wavelength.
2. Experimental details
2.3. Electrochemical measurements
2.1. Synthesis
The electrochemical performance of as synthesized electrode material is investigated using Metrohm Autolab PGSTAT302 N with a standard three electrodes configuration. The standard Ag/AgCl is used as a reference electrode, platinum wire as a counter electrode and individual synthesized material is used as working electrode without any binder. The electrochemical properties are investigated at room temperature in 6 M KOH aqueous electrolyte using cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements. The value of Cs is calculated with the help of GCD results using Eq. 1 [30,31]:
The chemicals procured herein were of analytical grade and used without any further purification. A schematic presentation of steps used to synthesize rGO@NiCo2O4 NFs and NWs like structures is presented in Fig. 1. To synthesize the material, some pieces of Ni foam (1 × 1 cm2) were cleaned to remove oxides using 3 M HCl, washed with ethanol and then pre-weighed before use. Graphene oxide (GO) was synthesized using the method reported in earlier studies [28,29]. The treated pieces of Ni foam were put vertically into a 50 mL Teflon-lined stainless steel autoclave containing brown solution of GO and heated them at 120 °C for 12 h. After being cooled naturally, the Ni foam was washed carefully with distilled water and ethanol and then dried at 60 °C in air for 3 h. Subsequently, 1.85 g CoCl2.6H2O, 0.95 g NiCl2.6H2O and 0.72 g urea
Cs =
I× t m
Fig. 1. Schematic presentation of steps used to synthesize rGO@NiCo2O4 NWs and NFs like structures. 2
(1)
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
where I (A), m and t are discharging current, mass on electrode and discharging time, respectively. Further, the values of SC of the electrode materials are calculated using Eq. 2, where V is applied potential window [31]:
SC =
I× t m× V
nanostructures [36]. 3.2. FESEM The FESEM micrographs are shown in Fig. 3. Fig. 3a shows the formation of rGO wrinkled type sheets grown on the surface of Ni foam and only carbon and oxygen related peaks are appeared in the EDX spectra (Fig. 3b). The rGO sheets are interconnected with Ni foam due to better mechanical strength of the rGO. FESEM micrographs of pure NiCo2O4 show NWs like structural morphology (Fig. 3c). The NWs are not grown uniformly over the surface of Ni foam. It may be due to the different rates of agglomeration of the seed particles [37]. However, NWs and NFs like structures of NiCo2O4 are grown uniformly over the rGO sheet (Fig. 3d–g). The uniform growth may be due to the presence of different functional groups on rGO sheet [38]. The low magnified FESEM micrographs of NWs and NFs (Fig. 3e–g) clearly show mesoporous features, which essentially needed for better performance of charge storage devices. The NFs like structures may be formed by temperature induced agglomeration of large number of NWs array interconnected with each other. These kind of hybrid nanostructures offer abundantly active surface area between the electrode and electrolyte which provide additional space for the collection of large electrolyte ions to minimize the ion diffusion length during charging and discharging [23]. The elemental mapping and EDX spectrum of the samples are shown in Fig. 3h and i. Fig. 3i shows presence of Ni, Co, O, and C in rGO@NiCo2O4.
(2)
3. Results and discussion The free metals (Ni2+ and Co2+) and hydroxide (OH−) ions generated during hydrolysis reaction assist to form mixed bimetallic hydroxide [32]. During hydrolysis, NixC2x(OH)6x is formed over the rGO sheets grown over the surface of Ni foam [32]. The product thus obtained is put for annealing to get the cubic spinel structure. Different annealing temperatures are applied for getting various structural morphologies of rGO@NiCo2O4. The formation of NiCo2O4 nanostructures may be governed by the following chemical reactions:
NO3 + 7H2 O + 8e
NH+4 + 10 OH x Ni2 + + 2x Co2 + + 6x OH Nix Co2x (OH)6x
NixCo2x(OH)6x + O2 → x NiCo2O4 + H2O ↑ 3.1. XRD
3.3. Raman and FTIR
The crystallographic information of the synthesized samples is obtained by analyzing the XRD patterns, as shown in Fig. 2. No peaks related to any impurity phases are observed. The peaks at 44.5˚ and 51.8˚ are assigned to (111) and (200) planes of Ni foam (JCPDF no. 040850). The peaks at 31.2 ˚, 36.6˚, 44.7˚ and 64.9˚ confirm the formation of NiCo2O4 [33]. No characteristic signature of rGO is observed in the XRD pattern. The XRD pattern of the samples annealed at 300 (rGO@ NiCo2O4 NWs) and 400 °C (rGO@NiCo2O4 NFs) are shown in Fig. 2, respectively. The peaks at 18.9 ˚, 31.2 ˚, 36.6 ˚, 44.7 ˚, 59.2˚ and 64.9 ˚ correspond to (111), (220), (311), (400), (511) and (440) planes of single cubic phase with the Fd3m space group of the ternary spinel NiCo2O4 structure (JCPDS card no. 00-020-0781) [34]. From the FESEM micrographs, as shown in Fig. 3, one can easily notice that the overall size of the composite is reduced. The smaller crystallite size has large active surface area [35]. Further, the lattice of rGO@NiCo2O4 crystals is strained compared to pure NiCo2O4. The improved lattice strain could be due to the incorporation of rGO with the NiCo2O4
Raman spectroscopy is a frequently used method for analysis of structural features and composition of low dimension materials, specially carbon and its composite materials [39]. Fig. 4 illustrates the Raman spectra of as synthesized GO, rGO and its rGO based composites. The presence of D (1350 cm−1) and G (∼1603 cm−1) bands (see Fig. 4a–c) confirms the growth of NiCo2O4 over the rGO sheet. The D band is assigned to disorder and structural defects present in the material, however, the G band is associated to the graphitic nature assigned to the E2g phonon of sp2 bonded carbon atoms in a 2D hexagonal lattice [39]. The presence of defect states and degree of graphitization is evaluated by taking the intensity ratio of D and G bands. The intensity ratio of D and G bands (ID/IG) for rGO, rGO@NiCo2O4 NWs and NFs is calculated to be 0.89, 0.93 and 0.94, respectively. A relatively larger value of ID/IG for rGO@NiCo2O4 NWs and rGO@NiCo2O4 NFs compare to rGO indicates that the NiCo2O4 interacts with rGO sheet [40]. The peaks at ∼151, ∼457, ∼455, ∼505, ∼656 and ∼1096 cm−1 correspond to the F2g, Eg, 1LO, A1g and 2LO modes of NiCo2O4, respectively [41]. Fig. 4d represents the FTIR spectra of pure NiCo2O4 NWs, rGO@ NiCo2O4 NWs and NFs like structures. In NiCo2O4 and rGO@NiCo2O4 NWs, two intense bands at ∼545 and ∼645 cm−1 are observed. These bands are assigned to Ni–O and Co–O stretching modes of NiCo2O4 [42,43]. The low intense peaks lying in the spectral range 10001400 cm−1 are assigned to epoxy and alkoxy groups [29,44]. The presence of peak corresponding to CeOH (1385 cm−1) and C]C (1630 cm−1) stretching vibrations confirm the presence of rGO sheets in rGO@NiCo2O4. The interaction between rGO and NiCo2O4 is confirmed by the changes in Raman peak parameters [44,45]. 3.4. Electrochemical analysis The electrochemical behavior of the electrode materials is evaluated by CV measurements using three electrodes configuration system with 6 M KOH aqueous electrolyte. Fig. 5a shows the CV curves of NiCo2O4 NWs, rGO@NiCo2O4 NWs, and rGO@NiCo2O4 NFs in the potential range –0.15 to + 0.55 V vs. Ag/AgCl, at scan rate of 5 mV/s. The CV curves show a pair of well-defined redox peaks. These peaks indicate that the capacitive nature is mainly governed by the Faradic reaction
Fig. 2. XRD patterns of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs grown on Ni-foam (* indicates the peak of Ni foam). 3
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
Fig. 3. (a) FESEM micrographs of rGO sheets grown on Ni foam, (b) EDX spectrum, (c) pure NiCo2O4 NWs, (d–e) rGO@NiCo2O4 NWs, (f–g) NFs like structures and (h–i) elemental mapping and EDX spectrum of rGO@NiCo2O4.
[34]. The integrated area under the CV curve of the rGO@NiCo2O4 NFs electrode is increased by the introduction of rGO. This signifies an increase in capacity due to the synergistic response of Ni-Co oxides and rGO [10,17]. The CV curve of all the electrodes revealed a pair of redox peak in the range of −0.15 V to +0.55 V due to the redox reaction of Ni2+/Ni3+ and Co3+/Co4+ [17,30]. The following redox reactions are expected to take place at surface of the electrode:
Fig. 5b shows typical CV curves of NFs like electrode at different scan rates. Apparently, as the scan rate is increased, the peak current is also increased while shape of the CV curves is retained with a pair of redox peak up to 70 mV/s scan rate. It is favorable for the redox reaction of the electrode. The increased value of peak current density with respect to scan rate suggests quick enrichment of electronic and ionic transport with low polarization value of the electrode [22,46]. The shifting of cathodic and anodic peaks towards positive and negative side with increasing scan rate corresponds to increased value of internal resistance of the electrode [47]. Fig. 5c represents the linear dependence of cathodic and anodic peak current densities with respect to scan rates. It also demonstrates the hybrid supercapacitive nature of the synthesized electrode materials. The surface coverage (Γ*) of redox species over the pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and rGO@ NiCo2O4 NFs electrodes is calculated using Eq. 3 [45,48]:
NiCo2O4 + H2O+OH− ↔ NiOOH + 2 CoOOH+e− (cathode) CoOOH+OH− ↔ CoO2 + H2O+e- (anode) Due to the conducting nature of rGO sheets, the separation between peak to peak values (ΔEP) in rGO@NiCo2O4 is comparatively less compared to NiCo2O4 electrode (see Table 1). The high anodic current density and less value of ΔEP for rGO@NiCo2O4 electrodes could lead to fast electron transfer between the electrode and electrolyte [46].
Fig. 4. (a) Raman spectra of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs {inset (b) Raman spectra of GO and rGO (c) magnified Raman spectra of rGO@NiCo2O4 NWs and NFs and (d) FTIR spectra of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs. 4
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
Fig. 5. (a) CV curves of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs like structures at 5 mV/s (b) CV curve of rGO@NiCo2O4 NFs electrode at various scan rates (5–100 mV/s) (c) linear dependence of anodic current and cathodic current densities Vs scan rate.
Ip =
n2F 2 *As 4RT
We performed cycling stability measurements of all the electrodes under the same conditions. Fig. 6d demonstrates cyclic stability of all the electrodes. Among all the electrodes, rGO@NiCo2O4 NFs showed better stability compared to other electrodes. It is noticed that when constant current density (30 A/g) is applied for the first cycle, the value of Cs turns out to be maximum. As the number of cycle is increased to 5000, the overall capacity is decreased to only 12% of its initial value. A relatively better stability of rGO@NiCo2O4 NFs electrode could be due to the interconnection of rGO sheets with NiCo2O4 NWs to form NFs like structures, leading to the barrier free contact which minimizes the internal resistance of the electrode remarkably [55]. The kinetics of the electrode materials are studied by performing electrochemical impedance spectroscopy (EIS) measurements in the frequency range, 0.01 Hz to 100 kHz at an open circuit voltage of 5 mV. The Nyquist plots of all the three electrodes are shown in Fig. 7 a. At high frequency side of the Nyquist plot, the intersection of the curve at the real axis represents the equivalent series resistance (Rs) of the electrochemical system [17,50,57]. The diameter of semicircle is associated with the charge transfer resistance (Rct). The slope of line at lower frequency range reflects the Warburg (w) [20,57]. It can be observed that the rGO@NiCo2O4 NFs electrode has a smaller semicircle compared to other electrodes. This could deliver a superior pathway for ion transfer and electron transport in the electrode material. The EIS data of all the electrodes have been simulated via NOVA software to get equivalent Randles circuit and the values of fitting parameters are presented in Table 2. The relatively less value of Rct and Rs for rGO@ NiCo2O4 NFs electrode may be due to hairy needle like porous structures of NiCo2O4 over the rGO sheets that provide easy access to ions [20]. Inset of Fig. 7b represents the simplified circuit diagram with different components, Rs, Rct, Cdl, and W. Evidently, the simulated graph nearly close to the experimental data, indicating that the simplified circuit provides the correct information of electrochemical reactions that take place inside the rGO@NiCo2O4 NFs electrode.
(3)
where, n is the number of electrons involved in the process, A is the area of the active electrode, F is the Faraday constant, s is the scan rate, R is gas constant and T is the temperature. A relatively large value of Γ* (8.9 × 10−5 mol cm−2) is calculated for the rGO@NiCo2O4 NFs electrode compared to other electrodes. The higher value of Γ* for rGO@ NiCo2O4 NFs electrode may be due to the interconnected and combined surface of numerous hairy needles like porous structures of NiCo2O4 and large surface area of rGO sheets [49]. Further from Figs. S1 and S2 (see the Supplementary information), it is clear that rGO@NiCo2O4 NFs like electrode shows highest value of Cs and SC compared to other electrodes. The Figs. S1 and S2 also suggest that as the scan rate is increased, the value of Cs and SC is decreased due to increase of internal diffusion resistance of the electrode materials [10]. The electrochemical performance of the synthesized materials is further investigated by performing GCD measurements. The results of GCD measurements within the potential window 0 to +0.42 V vs. Ag/ AgCl are shown in Fig. 6. In Fig. 6a, a relatively longer charge/discharge time is observed for rGO@NiCo2O4 NFs electrode compared to other electrodes at an applied current density of 5 A/g. The plateau type nature of GCD curve, indicates hybrid supercapacitive nature of the electrodes [50]. The GCD curves of rGO@NiCo2O4 NFs electrodes at various current densities are shown in Fig. 6b. All the GCD curves are found nearly symmetrical in nature, suggesting high coulombic efficiency of the electrode materials at different current densities [50]. The values of Cs of the rGO@NiCo2O4 NFs electrode turn out to be 436 (1040 F/g), 434 (1035 F/g), 430 (1029 F/g), 399 (950 F/g), 388 (924 F/g), 376 (895 F/g), 370 (882 F/g) and 353 (840 F/g) C/g at current densities of 1, 3, 5, 7, 10, 20, 30 and 50 A/g, respectively. The electrode retained its Cs up to 80 % (353 C/g) even if the current density is increased to 50 A/g. It indicates a better capacitive retention of the electrode materials. In the present study, an improved value of Cs is found compared to the values reported in the earlier studies [40,51–57]. The values of Cs and SC of all the electrode materials obtained at different densities are plotted and shown in Fig. 6c.
4. Conclusion NWs, and NFs type structures of NiCo2O4 are grown over the rGO
Table 1 The values of peak to peak separation (ΔEp), peak current density (ipa), specific capacity and capacitance. Electrode materials
Pure NiCo2O4 NWs rGO@ NiCo2O4 NWs rGO@ NiCo2O4 NFs
Mass loading (mg)
1.5 1.7 1.7
ΔEp (V)
0.27 0.16 0.17
Peak current density, ipa (A/ g)
Specific capacity (Cs) from GCD@ 1 A/g (C/g)
Specific capacitance (SC) (F/g) SC from CV@ 5 mv/s
SC from GCD@ 1 A/g
18.69 19.88 20.05
423 435 436
811 892 1003
1017 1036 1040
5
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
Fig. 6. (a) GCD profile of NiCo2O4 NWs, rGO@NiCo2O4 NWs and rGO@NiCo2O4 NFs electrodes (b) curves at different current densities (c) summary of specific capacities and specific capacitance as a function of current densities and (d) cycling stability performance of all the electrodes at 30 A/g.
sheet using hydrothermal method. The growth of NiCo2O4 nanostructures over the rGO sheet is confirmed by Raman and FTIR measurements. The FESEM micrographs confirm the formation of NWs and NFs like structures over the rGO sheet. The CV and GCD results showed that the rGO@NiCo2O4 NFs acts as an improved supercapacitive electrode material with excellent properties such as, enhanced surface
areas, short diffusion pathways for electrolyte ions, excellent rate capability, and cyclic stability at higher current density. The low value of Rct and Rs provides easy access to ions, resulting enhanced value of Cs for rGO@NiCo2O4 electrodes. Thus, the rGO@NiCo2O4 based electrodes may be used to design efficient energy storage devices.
Fig. 7. (a) Nyquist plots of the electrode materials formed by pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and rGO@NiCo2O4 NFs type structures (b) Nyquist plot of rGO@NiCo2O4 NFs type structures and corresponding fitted equivalent circuit is shown in the inset.
6
Synthetic Metals 259 (2020) 116215
A. Singh, et al.
Table 2 The values of parameters of pure NiCo2O4 NWs, rGO@NiCo2O4 NWs and NFs electrodes obtained after the fitting. Parameters → Samples ↓
Rs (Ω)
Rct (Ω)
W (mMho)
Cdl
χ2
Pure NiCo2O4 NWs rGO@NiCo2O4 NWs rGO@NiCo2O4 NFs
2.70 2.06 1.33
11 3.19 1.79 × 10−6
12.6 13.9 12.7
963 μF 7.86 μF 119 mF
1.245 1.217 0.414
Adv. 5 (48) (2015) 38407–38416. [18] G. Gao, H.B. Wu, S. Ding, L.M. Liu, X.W. Lou, Small 11 (7) (2015) 804–808. [19] L. Wang, X. Jiao, P. Liu, Y. Ouyang, X. Xia, W. Lei, Q. Hao, Appl. Surf. Sci. 427 (2018) 174–181. [20] S. Chen, G. Yang, Y. Jia, H. Zheng, J. Mater. Chem. A 5 (3) (2017) 1028–1034. [21] S. Li, K. Yang, P. Ye, H. Jiang, Z. Zhang, Q. Huang, L. Wang, Appl. Surf. Sci. 473 (2019) 326–333. [22] L. Zhang, S. Diwen, L. Tao, Mietek Jaroniec, Yu Jiaguo, Mater. Today 25 (2019) (2018) 35–65, https://doi.org/10.1016/j.mattod.2018.11.002. [23] U. Patil, S.C. Lee, S. Kulkarni, J.S. Sohn, M.S. Nam, S. Han, S.C. Jun, Nanoscale 7 (16) (2015) 6999–7021. [24] Gao, W., The chemistry of graphene oxide, Springer, Cham. 2015, 61-95. [25] F. Li, X. Jiang, J. Zhao, S. Zhang, Nano Energy 16 (2015) 488–515. [26] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat. Mater. 14 (3) (2015) 271. [27] H. Chen, C.K. Hsieh, Y. Yang, X.Y. Liu, C.H. Lin, C.H. Tsai, Y.X. Zhang, Chem Electro Chem 4 (9) (2017) 2414–2422. [28] W.S. Hummers Jr, R.E. Offeman, J. Am. Chem. Soc. 80 (6) (1958) 1339-1339. [29] A. Singh, B. Ahmed, A. Singh, A.K. Ojha, Spectrochim. Acta A. Mol. Biomol. Spectrosc. 204 (2018) 603–610. [30] W. Jiang, F. Hu, Q. Yan, X. Wu, Inorg. Chem. Front. 4 (2017) 1642–1648. [31] Z. Xu, J. Ren, Q. Meng, X. Zhang, C. Du, J. Chen, ACS Sustain. Chem. Eng. 7 (14) (2019) 12447–12456. [32] C. Zheng, C. Cao, R. Chang, J. Hou, H. Zhai, J. Chem. Soc. Faraday Trans. 18 (8) (2016) 6268–6274. [33] C. Yuan, J. Li, L. Hou, X. Zhang, L. Shen, X.W. Lou, Adv. Funct. Mater. 22 (21) (2012) 4592–4597. [34] C. Zhang, C. Lei, C. Cen, S. Tang, M. Deng, Y. Li, Y. Du, Electrochim. Acta 260 (2018) 814–822. [35] U. Holzwarth, N. Gibson, Nat. Nanotechnol. 6 (9) (2011) 534. [36] X. Wang, Y. Orikasa, Y. Takesue, H. Inoue, M. Nakamura, T. Minato, N. Hoshi, Y. Uchimoto, J. Am. Chem. Soc. 135 (16) (2013) 5938–5941. [37] H. Cheng, Y.Z. Su, P.Y. Kuang, G.F. Chen, Z.Q. Liu, J. Mater. Chem. A 3 (38) (2015) 19314–19321. [38] Z. Ma, X. Huang, S. Dou, J. Wu, S. Wang, J. Phys. Chem. C 118 (31) (2014) 17231–17239. [39] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (18) (2006) 187401. [40] T.H. Ko, S. Radhakrishnan, M.K. Seo, M.S. Khil, H.Y. Kim, B.S. Kim, J. Alloys Compd. 696 (2017) 193–200. [41] D. Li, Y. Gong, Y. Zhang, C. Luo, W. Li, Q. Fu, C. Pan, Sci. Rep. 5 (2015) 12903. [42] G. He, L. Wang, H. Chen, X. Sun, X. Wang, Mater. Lett. 98 (2013) 164–167. [43] N. Padmanathan, S. Selladurai, Ionics 19 (11) (2013) 1535–1544. [44] X. Jiao, X. Xia, P. Liu, W. Lei, Y. Ouyang, Q. Hao, Nanotechnology 28 (31) (2017) 315403. [45] A.K. Das, R.K. Layek, N.H. Kim, D. Jung, J.H. Lee, Nanoscale 6 (18) (2014) 10657–10665. [46] B. Baruah, A. Kumar, Synth. Met. 245 (2018) 74–86. [47] Q. Zhang, Y. Deng, Z. Hu, Y. Liu, M. Yao, P. Liu, J. Chem. Soc. Faraday Trans. 16 (42) (2014) 23451–23460. [48] L. Shen, Q. Che, H. Li, X. Zhang, Adv. Funct. Mater. 24 (18) (2014) 2630–2637. [49] A.J. Bard, Faulkner, R. Larry, Electrochemical Methods: Fundamentals and Applications, 2nd ed., John Wiley & Sons, Inc., New York, 2001. [50] K.H. Oh, G.S. Gund, H.S. Park, J. Mater. Chem. A 6 (44) (2018) 22106–22114. [51] V. Veeramani, R. Madhu, S.M. Chen, M. Sivakumar, C.T. Hung, N. Miyamoto, S.B. Liu, Electrochim. Acta 247 (2017) 288–295. [52] W.W. Liu, C. Lu, K. Liang, B.K. Tay, J. Mater. Chem. A 2 (14) (2014) 5100–5107. [53] J.W. Mao, C.H. He, J.Q. Qi, A.B. Zhang, Y.W. Sui, Y.Z. He, F.X. Wei, J. Electron. Mater. 47 (1) (2018) 512–520. [54] Z. Xu, L. Yang, Q. Jin, Z. Hu, Electrochim. Acta 295 (2019) 376–383. [55] S. Ramesh, D. Vikraman, H.S. Kim, H.S. Kim, J.H. Kim, J. Alloys Compd. 765 (2018) 369–379. [56] Y. Li, F. Tang, R. Wang, C. Wang, J. Liu, ACS Appl. Mater. Interfaces 8 (2016) 30232–30238. [57] E. Umeshbabu, G. Rajeshkhanna, P. Justin, G.R. Rao, RSC Adv. 5 (82) (2015) 66657–66666.
Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgements AS is thankful to Motilal Nehru National Institute of Technology Allahabad, Prayagraj for providing the Senior Research Fellowship (SRF). AS and AKO are thankful to TEQIP-III for providing the financial support. The instrumentation facilities available in the Centre for Interdisciplinary Research (CIR) of the Institute are greatly acknowledged. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116215. References [1] B.E. Conway, Electrochemical Capacitors: Scientific Fundamentals and Technology Applications, Kluwer Academic/Plenum Publishers, New-York, 1999. [2] C. Li, M.M. Islam, J. Moore, J. Sleppy, C. Morrison, K. Konstantinov, S.X. Dou, C. Renduchintala, J. Thomas, Nat. Commun. 7 (2016) 13319. [3] C. Chen, N. Zhang, Y. He, B. Liang, R. Ma, X. Liu, ACS Appl. Mater. Interfaces 8 (35) (2016) 23114–23121. [4] I. Roger, M.A. Shipman, M.D. Symes, Nat. Rev. Chem. 1 (1) (2017) 0003. [5] M. Winter, R.J. Brodd, Chem. Rev. 2003 (104) (2005) 4245–4269 Published on the Web 09/28/2004.). Chemical reviews, 105(3), 1021-1021. [6] P. Simon, Y. Gogotsi, B. Dunn, Science 343 (6176) (2014) 1210–1211. [7] T. Brousse, D. Bélanger, J.W. Long, J. Electrochem. Soc. 162 (5) (2015) A5185–A5189. [8] R. Sahoo, A.K. Sasmal, C. Ray, S. Dutta, A. Pal, T. Pal, ACS Appl. Mater. Interfaces 8 (28) (2016) 17987–17998. [9] Y. Liu, N. Fu, G. Zhang, M. Xu, W. Lu, L. Zhou, H. Huang, Adv. Funct. Mater. 27 (8) (2017) 1605307. [10] Alberto Adán-Más, T.M. Silva, L.G. Demourgues, L. Bourgeois, C.L. Sarroste, M.F. Montemor, J. Power Sources 419 (2019) 12–26. [11] S. Gao, Y. Sui, F. Wei, J. Qi, Q. Meng, Y. Ren, Y. He, J. Colloid Interface Sci. 2018 (531) (2018) 83–90. [12] Z. Sun, F. Huang, Y. Sui, F. Wei, J. Qi, Q. Meng, H. Hu, Y. He, J. Mater. Sci. Mater. Electron. 2017 (28) (2017) 14019–14025. [13] L. Cao, G. Tang, J. Mei, H. Liu, J. Power Sources 359 (2017) 262–269. [14] Q. Meng, K. Cai, Y. Chen, L. Chen, Nano Energy 36 (2017) 268–285. [15] L. Fang, H. Qiu, P. Luo, W. Li, H. Zhang, Y. Wang, Appl. Surf. Sci. 403 (2017) 35–42. [16] Z. Fan, B. Wang, Y. Xi, X. Xu, M. Li, J. Li, P. Coxon, S. Cheng, G. Gao, C. Xiao, G. Yang, Carbon 99 (2016) 633–641. [17] P. Syedvali, G. Rajeshkhanna, E. Umeshbabu, G.U. Kiran, G.R. Rao, P. Justin, RSC
7