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Electrochemical energy storage properties studies of Cu0.2Ni0.8O-Reduced graphene oxide nano-hybrids
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Electrochemical energy storage properties studies of Cu0.2Ni0.8O-Reduced graphene oxide nano-hybrids Sheraz Yousafa,∗, Sonia Zulfiqarb, Muhammad Shahidc, Akmal Jamilc, Imran Shakird, Philips O. Agboolae, Muhammad Farooq Warsia,∗∗ a
Department of Chemistry, Baghdad-ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur, 63100, Pakistan Department of Chemistry, School of Sciences & Engineering, The American University in Cairo, New Cairo, 11835, Egypt c Department of Chemistry, College of Science, University of Hafr Al Batin, P.O.Box 1803, Hafr Al Batin, 31991, Saudi Arabia d Sustainable Energy Technologies (SET) Center, College of Engineering, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia e College of Engineering Al-Muzahmia Branch, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia b
A R T I C LE I N FO
A B S T R A C T
Keywords: NiO Doping XRD FTIR UV–Visible SEM Electrochemical properties
Cu0.2Ni0.8O nano-particles and their nano-hybrids were synthesized by wet chemical methods and examined by X-ray diffraction (XRD) technique, spectroscopic analysis, Field Emission scanning electron microscopic (FESEM) technique and energy dispersive x-rays (EDX) analysis. XRD results confirmed that the synthesized materials were crystallite in the cubic phase of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C. The crystallite size synthesized product was found to be around 5–20 nm. FTIR and EDX results were supporting XRD data. Morphological studies represented the spherical nature of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C with particle size < 70 nm. Electrochemical measurements of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nano-hybrids were also performed. Brilliant specific capacitance (303 F g-1) was observed in the case of rGO-based nano-hybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C. This additional noted specific capacitance was credited to more conducting nature and relatively high surface area of graphene.
Energy is all-important for the establishment of mankind. The global economy is critically affected by the rapid intake of fossil fuels, swift population growth, ever-increasing habitat destructions and imminent energy crises [1–3]. The crucial anxiety of the present-day world is to overcome this necessity by developing clean and effective alternatives for energy storage [4]. This energy catastrophe forces the researchers to pay exceptional attention to energy production from natural supplies i.e., wind, sun, and water, etc. [5,6]. In respect of various energy storage systems, electrochemical storage devices like batteries, fuel cells and supercapacitors are included [7]. Lithium-ion batteries being an imperative constituent of movable electronic devices exhibit high energy density and splendid cyclic power is of pronounced importance. Such as Guo et al. have adopted a novel route for the synthesis of highly stable silicon-based ternary composites as an electrode material for LIBs [8]. However, the paramount downside of LIBs is their short life and low power density which restricts the usage [9–11]. Electrochemical capacitors which are also known as ultracapacitors or supercapacitors are sought after contenders for storage of energy on grounds of their distinctive electrochemical achievements. However, Lithium-ion
∗
batteries and electrochemical capacitors are not competitors because both these technologies are compatible with their particular niches of applications [12]. High energy and power densities, large cyclic life, high specific capacitance, relatively high charge-discharge rates, and capability to release cache energy rapidly as compared to LIBs, fuel cells and dielectric capacitors make them best participant among different energy storage systems [12–14]. Supercapacitors may be electrical double-layer capacitors (EDLC) or pseudo capacitors (faradaic capacitors) depending on their charge storage mechanism. EDLC stores energy at the boundary of the electrolyte by virtue of ion adsorption while pseudo capacitor stores energy through speedy faradaic reversible reaction [15]. Hybrid supercapacitors are the amalgam of pseudocapacitors and electrical double-layer capacitors which can supply high currents accompanying high gravimetric and volumetric energy densities [2]. Capacitive performances of the supercapacitors depend upon electrolyte, assembly of the technology and most importantly the electrode material which is used in the supercapacitors [14]. Carbon active electrode materials (graphene, CNTs and activated carbon and metal-organic frameworks, etc) are used in EDLCs while pseudo
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Yousaf), [email protected] (M.F. Warsi).
∗∗
https://doi.org/10.1016/j.ceramint.2020.02.115 Received 15 December 2019; Received in revised form 10 February 2020; Accepted 12 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sheraz Yousaf, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.115
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of rGO, 5 cm3 of graphite oxide suspension was diluted with 5 cm3 of deionized water and mixed with 40 μL of ammonia solution and 5 μL of hydrazine. The mixture was kept at 95 °C with continuously stirring for 60 min which gave results to black colored reduce graphene oxide [15]. Cu doped NiO nanomaterials (90 mg) and rGO (10 mg) were suspended in deionized separately for 1 h. After the complete sonication process, both suspensions were mixed and further again sonicated for 1 h. After this, the drying of the resulted suspension was carried out in an oven and then it was used for more characterizations [35–37]. The cyclic voltammetry experiment was done by using Cu0.2Ni0.8O nano-particles electrodes. In these electrodes, 0.001 g Cu0.2Ni0.8O nano-particles were pasted on carbon paper. The electrodes were dipped in 6 M KOH aqueous solution. The voltammetry experiment was carried out in −0.44 to 0.55 V potential window in case of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their rGO based nano-hybrid [31]. XRD diffraction patterns of Cu0.2Ni0.8O nano-particles annealed at different temperatures are represented in Fig. 2. In this Figure, It can be seen that there were five main diffraction peaks corresponded to (111), (200), (220), (311) and (222) hkl values as compared with ICDD # 01078-0647. It can be seen from XRD results, synthesized material was fully pure without any peak observed due to secondary phases. Furthermore, it was also seen that while increasing annealing temperature, diffraction peaks become narrowed. This corresponded to an increase in the crystallite size of the material. The average crystallite size of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C was calculated by means of Scherrer equation [38] which is given in equation (1).
capacitors utilize conducting polymers, transition metal oxides (TMO) and transition metal sulfides (TMS) as electrode material [16–18]. As the electrode is the chief constituent of supercapacitor devices so it must have higher mechanical and electrical characteristics, superior cyclic stability and capacitance. It has enough space to provide accommodation for large strains without losing its efficiency [19]. Oxides of transition metals like NiO [20], Fe2O3 [21], MoO3 [22], Co3O4 [23], MnO2 [24], V2O5 [25], CuO [26] accomplish foreground interest of the scientist because these are electrochemically stable, have large energy density, low cost, and less toxic nature, large theoretical capacitance, rich in oxidation-reduction reactions and can possess various oxidation states. Ma and his collaborators have reported a comparative study of iron and iron oxide-based materials for electrochemical energy storage properties. It was found that α-Fe2O3 porous fibers exhibited higher power density and specific capacitance than nano-grains. This higher specific capacitance was observed due to the availability of more surface area in the case of porous fibrous α-Fe2O3 materials [27]. The only downside of these oxides is that these have fewer conductivity values which cause decrement in their rate electrochemical capabilities [28,29]. It becomes vital to overcoming the conductivity gap in order to encourage the demand for supercapacitors. Presently, metal-doped oxide of transition metal nanohybrids has pushed the rigorous attention of the researchers because of their best electrochemical performances, better conductivity values, and high structural stability. Oxides of nickel (NiO) is one of the charming candidates among different transition metal oxides and mesmerize attraction because of its low cost and easiest synthetic route, highest pseudocapacitance, expeditious kinetics and several electrochemical reversible reactions [30,31]. However, nickel oxide (NiO) generally experiences low approachable surface area and poor electrical conductivities. These shortcomings of NiO consequences low reversibility and capability rates during the process of electrical charge and discharge [32,33]. So it is crucial to defeat the drawbacks and enhance capability and conductivity values of nickel oxide in order to magnify its supercapacitor performances. In this work, we proposed the nanohybrids of Cu+2 doped NiO nanoparticles (Cu0.2Ni0.8O) with reduced graphene oxide (rGO) and employed to study their supercapacitor applications. Cu+2 substituted NiO nanoparticles were synthesized using their corresponding analytic grade salt solutions as such as received. 80 cm3 of nickel nitrate solution (0.1 M) and 20 cm3 of zinc nitrate solution (0.1 M) were taken in a beaker. The beaker was placed for magnetic stirring on the hotplate at 50 °C for 30 min. Then, a 5 mM standard solution of sodium hydroxide (50 cm3) was added to it slowly to yield dark green colored precipitates. The precipitates were further continuously stirred for 30 min at 100 °C temperature. After this, it was cool down naturally. The synthesized precipitates were filtered to remove the excess amount of solvent and washed with deionized water for neutralizing the pH and then dried at 100 °C. Finally, the obtained precipitates were ground and calcined for 2 h at 500 and 800 °C. The complete synthesis process is briefly described in Fig. 1. Graphite oxide was synthesized by modified Hummer's method. In this method, 3 g of Sodium Nitrate and 3 g of Graphite powder were taken and mixed with 150 cm3 of sulphuric acid. The obtained black colored suspension was stirred constantly for 30 min and then the suspension was taken in an ice bath. 18 g of KMnO4 was added to it slowly. The gained green colored suspension was stirred for about 2 h. After the desired time interval, the suspension was removed from the ice bath and was further stirred for 2 days. As a result of these, the brown-colored slurry was gotten. A mixture of warm water (840 cm3) and H2O2 (60 cm3) was added to it that resulted in yellow-colored suspension. The yellow color suspension was filtered and washed. Its washing was done using a mixture of concentrated sulphuric acid (6 wt%/500 cm3) and Hydrogen peroxide (1 wt%/500 cm3) to obtained brown color suspension. Thus, brown-colored suspension so obtained was neutralized by washing using deionized water to obtained graphite oxide [34]. For the synthesis
D=
Kλ βcosθ
(1)
In the above equation, “D” = crystallite size is denoted by “K” = Sherrer's constant and is 0.9 in the case spherical shaped particles, “β” = full width at half maxima, θ ” = Bragg's angle whereas “λ” = wavelength (Cu Kα 1.5406 Å). The calculated crystallite size of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C was 6.8 nm and 19.02 nm respectively. This increase in size was attributed as a result of increasing annealing temperature which can also be observed from the X-rays diffraction pattern. Apart from these, the crystallite size and micro-strain of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C were also calculated through the Williamson-hall method which is represented in equations (2) and (3) [39].
βhkl cosθ =
Kλ + 4εsinθ D
βhkl = βD +βs
(2)
(3a)
In this method, the crystallite size can be calculated from y-intercept and micro-strain from the slope of the graph plotted against “4sinθ” and “βhklcosθ” as shown in Fig. 3. From this method, it can be noticed that the crystallite size of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C was 7.30 nm and 28.30 nm respectively. Both of these methods gave almost similar results whereas the Williamson-Hall plot method was offering some larger size. It can be due to the presence of varied geometries of particles and micro-strain in the crystal lattice of nanoparticles. Moreover, Cu0.2Ni0.8O nano-particles annealed at 500 °C exhibited negative micro-strain (−0.00179), which was due to contraction of the crystal lattice, on the other hand, Cu0.2Ni0.8O nano-particles annealed at 800 °C exhibited positive micro-strain (0.000275) that which was attributed to the expansion of its crystal lattice. This positive strain is responsible for the increase in the crystallite size of nanoparticles. Information related to molecular structure as well as the nature of chemical bonding found in Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C were obtained from FTIR analysis. Fig. 4 shows the FTIR spectra of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C. The 2
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Fig. 1. Synthesis of Cu0.2Ni0.8O nano-particles.
Fig. 2. XRD diffraction pattern, (a) Cu0.2Ni0.8O nano-particles annealed at 500 °C and (b) Cu0.2Ni0.8O nano-particles annealed at 800 °C.
FTIR bands that appeared under 1000 cm−1 are measured as fruitful. These bands provide information related to corresponding metal to oxygen bonds in the synthesized nanomaterials. FTIR spectra presented in Fig. 4 show several bands in the region from 501 to 420 cm−1, these were due to stretching vibrations of octahedral geometry of NiO6 groups in Face centered cubic structure of Cu0.2Ni0.8O nano-particles
annealed at 500 as well as 800 °C [40,41]. The formation of graphite oxide and its reduction to reduced graphene oxide was tested by matching the position of their absorption maximum peaks as given in Fig. 5. The maximum absorption peak in the case of Graphite oxide was at 226 nm which is considered to be due to π to π* transition of electrons offered by aromatic carbon frame.
Fig. 3. Williamson-Hall plot of (a) Cu0.2Ni0.8O nano-particles annealed at 500 °C and (b) Cu0.2Ni0.8O nano-particles annealed at 800 °C. 3
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Fig. 4. FTIR spectra of (a) Cu0.2Ni0.8O nano-particles annealed at 500 °C and (b) Cu0.2Ni0.8O nano-particles annealed at 800 °C.
material. The second weight loss was due to the combustion of carbon content added to the composite which was also traced in EDX result. From these results, it was concluded that a total of 10% organic content was found in the rGO-based nanocomposite. The amount founded from TGA measurement was closely matched with the quantity of rGO added in the formation of composite material [36]. The porosity, Brunauer–Emmett–Teller (BET) surface area and Langmuir surface area of typical hybrid sample was obtained using Micromeritics ASAP 2020 physisorption analyzer from N2 adsorption isotherms at 77 K. The samples were degassed at 100 °C for 4 h prior to analysis. Through this experiment, the calculated specific surface area of transition metal-based oxides nanomaterials synthesized via co-precipitation route is lower and is near to 4–5 m2/g [46]. This lower surface area is attributed due to aggregation, to compensate for their inherent high surface to volume ratio [47]. Moreover, rGO is famous due to its 3-dimensional structure and more conducting nature. This 3dimensional structure is responsible for its higher surface area. Fig. 8 shows the Nitrogen adsorption-desorption spectra of rGO-based nanohybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C. During this experiment, 0.05 g of material was used and the process was continued for approximately 6 h. It was found that the BET surface area of rGObased nano-hybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C was 34.78 m2/g whereas Langmuir surface area was 50.29 m2/g. This larger surface area might be attributed due to the higher surface area of rGO in composite material. The electrochemical properties of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nano-hybrids were inspected. Fig. 9 showed the cyclic voltammogram of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nano-hybrids recorded at different scan rates (5–100 mVs−1). The Cyclic voltammogram of Cu0.2Ni0.8O nano-particles exhibited noticeable oxidation and reduction peaks at scan rates considered. However, pair of well-defined oxidation-reduction reactions peaks were visible in the cyclic voltammogram of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nanohybrids. These indicate that the electrochemical capacitance of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nanohybrids mainly resulted from the pseudo-capacitance [48]. The chief advantage of pseudocapacitance is its extraordinary energy transfer through the faradaic reaction and given that 10 to 100 times extra capacitance than EDLCs [49].
Fig. 5. UV–Visible spectra of Graphite oxide and rGO.
Besides this, a weak shoulder peak at 299 nm was also observed. This weak shoulder peak is owing to n- π* electronic transition of C˭˭˭O bond of carbonyl group [42]. In the case of rGO, the absorption maximum peak was observed at 271 nm. The observed red shifting in the absorption peak in the case of rGO confirmed the reduction of GO [43]. Morphological studies Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and Cu0.2Ni0.8O/rGO nano-hybrid were carried out by field emission scanning electron microscopic technique. Before imaging the samples were gold sputtered at 15 mA for 120 s. The surface morphology of samples was investigated using ZEISS LEO SUPRA 55 field emission scanning electron microscope. FESEM image of Cu0.2Ni0.8O nano-particles annealed at 500 °C showed that they were in spherical morphology with average particle size ranges from 30 to 32 nm [44] as given in Fig. 6 (a). Similarly, Cu0.2Ni0.8O nano-particles annealed at 800 °C also showed spherical morphology with average particle size ranges from 60 to 70 nm as depicted in Fig. 6 (b). Metal oxide nanoparticles have a higher surface to volume ratio. In order to cope with high surface energy, they show aggregations [45]. FESEM image of Cu0.2Ni0.8O/rGO nano-hybrid shows that nano-particles are randomly dispersed on the surface of rGO sheets as presented in Fig. 6 (c). Thus confirmed the formation of rGO-based nano-hybrid material. Fig. 6 also showed the EDX results of Cu0.2Ni0.8O nano-particles annealed at 500 °C, 800 °C and Cu0.2Ni0.8O/rGO nano-hybrid. EDX analysis was carried out on JEOL JCM-6000Plus SEM. From Fig. 6 (a and b), it can be concluded that Cu+2 doped NiO nanoparticles annealed at two different temperatures contains significant amount of each element with different atomic percentage. Moreover Fig. 6 (c), clearly showed the presence of C-atom in the rGO based Cu+2 doped NiO nano-hybrid. Thermo-gravimetric analysis of Cu0.2Ni0.8O/rGO nano-hybrid was conducted in order to confirm carbon content found in it as represented in Fig. 7. The initial weight loss was observed due to the elimination of moisture content found as a waste of adsorption in the composite
Specific capacitance =
Q V × m
(3b)
By applying the above equation, the specific capacitance of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nanohybrids were calculated and given in Table 1. It was found that the Cu0.2Ni0.8O nano-particles annealed at 500 °C exhibited 203 F g-1 of specific capacitance at 100 mVs−1 scan rate. However, the specific capacitance of Cu0.2Ni0.8O nano-particles annealed at 800 °C drops to 60 F g-1 at a 100 mVs−1 scan rate. This drop-in specific capacitance was observed due to the increase in particle size and the decrease in surface area of nanoparticles with the increase in annealing temperature. The 4
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Fig. 6. FESEM and EDX images of (a) Cu0.2Ni0.8O nano-particles annealed at 500 °C, (b) Cu0.2Ni0.8O nano-particles annealed at 800 °C and (c) Cu0.2Ni0.8O/rGO nanohybrid.
Fig. 8. Nitrogen adsorption-desorption curve of rGO based nano-hybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C.
Fig. 7. Thermo-gravimetric analysis of Cu0.2Ni0.8O/rGO nano-hybrid.
specific capacitance of Cu0.2Ni0.8O nano-particles annealed at 500 °C is better than the pure NiO and nearly similar to previously reported Cu+2 doped NiO nanoparticles. Chen et al. [50] have reported specific capacitance of NiO nanoparticles as 161 F g-1 and Sathishkumar et al. [51]
have reported the specific capacitance of Cu+2 doped NiO nanoparticles as 242 F g-1. In the case of Cu0.2Ni0.8O nano-particles annealed at 500 °C, the 5
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Fig. 9. Cyclic-Voltammogram of Cu0.2Ni0.8O nano-particles annealed at 500 °C, Cu0.2Ni0.8O nano-particles annealed at 800 °C and Cu0.2Ni0.8O/rGO nano-hybrid at (a) 100 mVs−1, (b) 50 mVs−1, (c) 20 mVs−1, (d) 10 mVs−1 (e) 5 mVs−1scan rate.
tested. It was found that rGO based nano-hybrid material exhibited an excellent increment in specific capacitance of Cu0.2Ni0.8O nano-particles. The calculated specific capacitance value of Cu0.2Ni0.8O/rGO nano-hybrid was 353, 303, 287, 252 and 182 F g-1 at 100, 50, 20, 10 and 5 mVs−1 scan rates, respectively. This increment in specific capacitance was attributed due to more conductive nature and larger size of rGO sheets that were responsible for the greater surface area [46].
Table 1 Specific capacitance of Cu0.2Ni0.8O nanoparticles and rGO based nano-hybrids. Sample
Specific capacitance (Fg−1) Scan rate (mVs−1)
Cu0.2Ni0.8O annealed at 500 °C Cu0.2Ni0.8O annealed at 800 °C Cu0.2Ni0.8O/rGO nano-hybrid
100
50
20
10
5
204 60 353
147 52 303
115 49 287
99 48 252
87 43 182
Conclusion Wet chemical method was applied to synthesized Cu0.2Ni0.8O nanoparticles and their rGO based nano-hybrids. XRD results confirmed the single cubic phase of synthesized nanoparticles. FTIR results were supporting the XRD results. FESEM analysis confirmed the spherical morphology of the synthesized product. Thermal analysis of rGO-based nano-hybrid yielded the same amount of carbon content as used. Electrochemical properties of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their monohybrids were examined. Excellent specific capacitance (303 F g-1) was noticed in the case of rGO-based nanohybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C. This extra recorded capacitance was attributed due to more conducting nature and
calculated value of specific capacitance was 203, 147, 115, 99 and 86 F g-1 at 100, 50, 20, 10 and 5 mVs−1 scan rates, respectively. This increase in specific capacitance was due to the formation of positive defects in NiO crystal lattice, which induced due to the incorporation of Cu+ 2 into NiO crystal lattice. This was responsible for the increase in protons transfer and the escape of electrons from the Nickel surface more easily. Thus, the electrochemical properties of the material were improved. On the other hand, the electrochemical properties of rGO-based nano-hybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C were also 6
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relatively high surface area of rGO.
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Wang, Preparation of NiO nanoparticles as supercapacitor electrode by precipitation using carbon black powder, 2011 International Conference on Materials for Renewable Energy & Environment, 2011, pp. 640–643. [51] K. Sathishkumar, N. Shanmugam, N. Kannadasan, S. Cholan, G. Viruthagiri, Synthesis and characterization of Cu2+ doped NiO electrode for supercapacitor application, J. Sol. Gel Sci. Technol. 74 (2015) 621–630.
Declaration of competing interest None. Acknowledgement Authors are thankful to the Islamia University of Bahawalpur and Higher Education Commission of Pakistan. Authors from King Saud University (KSU) sincerely appreciate the KSU for their contribution through Researchers Supporting Project (RSP-2019/49). Dr. Sonia Zulfiqar is highly grateful to American University in Cairo (AUC) for financial support through STRC mini-grant and research project No. SSE-CHEMS.Z.-FY19-FY20-FY21-RG (1–19)-2018-Oct-01-17-53-22. References [1] Poonam, K. Sharma, A. Arora, S.K. Tripathi, Review of supercapacitors: materials and devices, J. Energy Storage 21 (2019) 801–825. [2] J. Libich, J. Máca, J. Vondrák, O. Čech, M. Sedlaříková, Supercapacitors: properties and applications, J. Energy Storage 17 (2018) 224–227. [3] W. Shaheen, M.F. Warsi, M. Shahid, M.A. Khan, M. Asghar, Z. Ali, M. Sarfraz, H. Anwar, M. Nadeem, I. 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Zhang, Q. Li, D.H. Anjum, H. Liang, Y. Liu, C.-s. Liu, Husam N. Alshareef, H. Pang, A novel strategy for the synthesis of highly stable ternary SiOx composites for Li-ion-battery anodes, J. Mater. Chem. 7 (2019) 15969–15974. [9] J.-G. Lee, B.N. Joshi, J.-H. Lee, T.-G. Kim, D.-Y. Kim, S.S. Al-Deyab, I.W. Seong, M.T. Swihart, W.Y. Yoon, S.S. Yoon, Stable high-capacity lithium ion battery anodes produced by supersonic spray deposition of hematite nanoparticles and self-healing reduced graphene oxide, Electrochim. Acta 228 (2017) 604–610. [10] G. Zhu, K. Wen, W. Lv, X. Zhou, Y. Liang, F. Yang, Z. Chen, M. Zou, J. Li, Y. Zhang, W. He, Materials insights into low-temperature performances of lithium-ion batteries, J. Power Sources 300 (2015) 29–40. [11] B. Ahmed, M. Shahid, D.H. Nagaraju, D.H. Anjum, M.N. Hedhili, H.N. Alshareef, Surface passivation of MoO3 nanorods by atomic layer deposition toward high rate durable Li ion battery anodes, ACS Appl. Mater. Interfaces 7 (2015) 13154–13163. [12] R. Vicentini, W.G. Nunes, L.H. Costa, L.M. Da Silva, A. Pascon, P. Jackson, G. Doubek, H. Zanin, Highly stable nickel-aluminum alloy current collectors and highly defective multi-walled carbon nanotubes active material for neutral aqueous-based electrochemical capacitors, J. Energy Storage 23 (2019) 116–127. [13] W.H. Low, P.S. Khiew, S.S. Lim, C.W. Siong, E.R. Ezeigwe, Recent development of mixed transition metal oxide and graphene/mixed transition metal oxide based hybrid nanostructures for advanced supercapacitors, J. Alloys Compd. 775 (2019) 1324–1356. [14] Y. Bai, X. Yang, Y. He, J. Zhang, L. Kang, H. Xu, F. Shi, Z. Lei, Z.-H. Liu, Formation process of holey graphene and its assembled binder-free film electrode with high volumetric capacitance, Electrochim. Acta 187 (2016) 543–551. [15] M. Aadil, W. Shaheen, M.F. Warsi, M. Shahid, M.A. Khan, Z. Ali, S. Haider, I. Shakir, Superior electrochemical activity of α-Fe2O3/rGO nanocomposite for advance energy storage devices, J. 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7
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Short communication
Electrochemical energy storage properties studies of Cu0.2Ni0.8O-Reduced graphene oxide nano-hybrids Sheraz Yousafa,∗, Sonia Zulfiqarb, Muhammad Shahidc, Akmal Jamilc, Imran Shakird, Philips O. Agboolae, Muhammad Farooq Warsia,∗∗ a
Department of Chemistry, Baghdad-ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur, 63100, Pakistan Department of Chemistry, School of Sciences & Engineering, The American University in Cairo, New Cairo, 11835, Egypt c Department of Chemistry, College of Science, University of Hafr Al Batin, P.O.Box 1803, Hafr Al Batin, 31991, Saudi Arabia d Sustainable Energy Technologies (SET) Center, College of Engineering, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia e College of Engineering Al-Muzahmia Branch, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia b
A R T I C LE I N FO
A B S T R A C T
Keywords: NiO Doping XRD FTIR UV–Visible SEM Electrochemical properties
Cu0.2Ni0.8O nano-particles and their nano-hybrids were synthesized by wet chemical methods and examined by X-ray diffraction (XRD) technique, spectroscopic analysis, Field Emission scanning electron microscopic (FESEM) technique and energy dispersive x-rays (EDX) analysis. XRD results confirmed that the synthesized materials were crystallite in the cubic phase of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C. The crystallite size synthesized product was found to be around 5–20 nm. FTIR and EDX results were supporting XRD data. Morphological studies represented the spherical nature of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C with particle size < 70 nm. Electrochemical measurements of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nano-hybrids were also performed. Brilliant specific capacitance (303 F g-1) was observed in the case of rGO-based nano-hybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C. This additional noted specific capacitance was credited to more conducting nature and relatively high surface area of graphene.
Energy is all-important for the establishment of mankind. The global economy is critically affected by the rapid intake of fossil fuels, swift population growth, ever-increasing habitat destructions and imminent energy crises [1–3]. The crucial anxiety of the present-day world is to overcome this necessity by developing clean and effective alternatives for energy storage [4]. This energy catastrophe forces the researchers to pay exceptional attention to energy production from natural supplies i.e., wind, sun, and water, etc. [5,6]. In respect of various energy storage systems, electrochemical storage devices like batteries, fuel cells and supercapacitors are included [7]. Lithium-ion batteries being an imperative constituent of movable electronic devices exhibit high energy density and splendid cyclic power is of pronounced importance. Such as Guo et al. have adopted a novel route for the synthesis of highly stable silicon-based ternary composites as an electrode material for LIBs [8]. However, the paramount downside of LIBs is their short life and low power density which restricts the usage [9–11]. Electrochemical capacitors which are also known as ultracapacitors or supercapacitors are sought after contenders for storage of energy on grounds of their distinctive electrochemical achievements. However, Lithium-ion
∗
batteries and electrochemical capacitors are not competitors because both these technologies are compatible with their particular niches of applications [12]. High energy and power densities, large cyclic life, high specific capacitance, relatively high charge-discharge rates, and capability to release cache energy rapidly as compared to LIBs, fuel cells and dielectric capacitors make them best participant among different energy storage systems [12–14]. Supercapacitors may be electrical double-layer capacitors (EDLC) or pseudo capacitors (faradaic capacitors) depending on their charge storage mechanism. EDLC stores energy at the boundary of the electrolyte by virtue of ion adsorption while pseudo capacitor stores energy through speedy faradaic reversible reaction [15]. Hybrid supercapacitors are the amalgam of pseudocapacitors and electrical double-layer capacitors which can supply high currents accompanying high gravimetric and volumetric energy densities [2]. Capacitive performances of the supercapacitors depend upon electrolyte, assembly of the technology and most importantly the electrode material which is used in the supercapacitors [14]. Carbon active electrode materials (graphene, CNTs and activated carbon and metal-organic frameworks, etc) are used in EDLCs while pseudo
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Yousaf), [email protected] (M.F. Warsi).
∗∗
https://doi.org/10.1016/j.ceramint.2020.02.115 Received 15 December 2019; Received in revised form 10 February 2020; Accepted 12 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sheraz Yousaf, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.115
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of rGO, 5 cm3 of graphite oxide suspension was diluted with 5 cm3 of deionized water and mixed with 40 μL of ammonia solution and 5 μL of hydrazine. The mixture was kept at 95 °C with continuously stirring for 60 min which gave results to black colored reduce graphene oxide [15]. Cu doped NiO nanomaterials (90 mg) and rGO (10 mg) were suspended in deionized separately for 1 h. After the complete sonication process, both suspensions were mixed and further again sonicated for 1 h. After this, the drying of the resulted suspension was carried out in an oven and then it was used for more characterizations [35–37]. The cyclic voltammetry experiment was done by using Cu0.2Ni0.8O nano-particles electrodes. In these electrodes, 0.001 g Cu0.2Ni0.8O nano-particles were pasted on carbon paper. The electrodes were dipped in 6 M KOH aqueous solution. The voltammetry experiment was carried out in −0.44 to 0.55 V potential window in case of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their rGO based nano-hybrid [31]. XRD diffraction patterns of Cu0.2Ni0.8O nano-particles annealed at different temperatures are represented in Fig. 2. In this Figure, It can be seen that there were five main diffraction peaks corresponded to (111), (200), (220), (311) and (222) hkl values as compared with ICDD # 01078-0647. It can be seen from XRD results, synthesized material was fully pure without any peak observed due to secondary phases. Furthermore, it was also seen that while increasing annealing temperature, diffraction peaks become narrowed. This corresponded to an increase in the crystallite size of the material. The average crystallite size of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C was calculated by means of Scherrer equation [38] which is given in equation (1).
capacitors utilize conducting polymers, transition metal oxides (TMO) and transition metal sulfides (TMS) as electrode material [16–18]. As the electrode is the chief constituent of supercapacitor devices so it must have higher mechanical and electrical characteristics, superior cyclic stability and capacitance. It has enough space to provide accommodation for large strains without losing its efficiency [19]. Oxides of transition metals like NiO [20], Fe2O3 [21], MoO3 [22], Co3O4 [23], MnO2 [24], V2O5 [25], CuO [26] accomplish foreground interest of the scientist because these are electrochemically stable, have large energy density, low cost, and less toxic nature, large theoretical capacitance, rich in oxidation-reduction reactions and can possess various oxidation states. Ma and his collaborators have reported a comparative study of iron and iron oxide-based materials for electrochemical energy storage properties. It was found that α-Fe2O3 porous fibers exhibited higher power density and specific capacitance than nano-grains. This higher specific capacitance was observed due to the availability of more surface area in the case of porous fibrous α-Fe2O3 materials [27]. The only downside of these oxides is that these have fewer conductivity values which cause decrement in their rate electrochemical capabilities [28,29]. It becomes vital to overcoming the conductivity gap in order to encourage the demand for supercapacitors. Presently, metal-doped oxide of transition metal nanohybrids has pushed the rigorous attention of the researchers because of their best electrochemical performances, better conductivity values, and high structural stability. Oxides of nickel (NiO) is one of the charming candidates among different transition metal oxides and mesmerize attraction because of its low cost and easiest synthetic route, highest pseudocapacitance, expeditious kinetics and several electrochemical reversible reactions [30,31]. However, nickel oxide (NiO) generally experiences low approachable surface area and poor electrical conductivities. These shortcomings of NiO consequences low reversibility and capability rates during the process of electrical charge and discharge [32,33]. So it is crucial to defeat the drawbacks and enhance capability and conductivity values of nickel oxide in order to magnify its supercapacitor performances. In this work, we proposed the nanohybrids of Cu+2 doped NiO nanoparticles (Cu0.2Ni0.8O) with reduced graphene oxide (rGO) and employed to study their supercapacitor applications. Cu+2 substituted NiO nanoparticles were synthesized using their corresponding analytic grade salt solutions as such as received. 80 cm3 of nickel nitrate solution (0.1 M) and 20 cm3 of zinc nitrate solution (0.1 M) were taken in a beaker. The beaker was placed for magnetic stirring on the hotplate at 50 °C for 30 min. Then, a 5 mM standard solution of sodium hydroxide (50 cm3) was added to it slowly to yield dark green colored precipitates. The precipitates were further continuously stirred for 30 min at 100 °C temperature. After this, it was cool down naturally. The synthesized precipitates were filtered to remove the excess amount of solvent and washed with deionized water for neutralizing the pH and then dried at 100 °C. Finally, the obtained precipitates were ground and calcined for 2 h at 500 and 800 °C. The complete synthesis process is briefly described in Fig. 1. Graphite oxide was synthesized by modified Hummer's method. In this method, 3 g of Sodium Nitrate and 3 g of Graphite powder were taken and mixed with 150 cm3 of sulphuric acid. The obtained black colored suspension was stirred constantly for 30 min and then the suspension was taken in an ice bath. 18 g of KMnO4 was added to it slowly. The gained green colored suspension was stirred for about 2 h. After the desired time interval, the suspension was removed from the ice bath and was further stirred for 2 days. As a result of these, the brown-colored slurry was gotten. A mixture of warm water (840 cm3) and H2O2 (60 cm3) was added to it that resulted in yellow-colored suspension. The yellow color suspension was filtered and washed. Its washing was done using a mixture of concentrated sulphuric acid (6 wt%/500 cm3) and Hydrogen peroxide (1 wt%/500 cm3) to obtained brown color suspension. Thus, brown-colored suspension so obtained was neutralized by washing using deionized water to obtained graphite oxide [34]. For the synthesis
D=
Kλ βcosθ
(1)
In the above equation, “D” = crystallite size is denoted by “K” = Sherrer's constant and is 0.9 in the case spherical shaped particles, “β” = full width at half maxima, θ ” = Bragg's angle whereas “λ” = wavelength (Cu Kα 1.5406 Å). The calculated crystallite size of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C was 6.8 nm and 19.02 nm respectively. This increase in size was attributed as a result of increasing annealing temperature which can also be observed from the X-rays diffraction pattern. Apart from these, the crystallite size and micro-strain of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C were also calculated through the Williamson-hall method which is represented in equations (2) and (3) [39].
βhkl cosθ =
Kλ + 4εsinθ D
βhkl = βD +βs
(2)
(3a)
In this method, the crystallite size can be calculated from y-intercept and micro-strain from the slope of the graph plotted against “4sinθ” and “βhklcosθ” as shown in Fig. 3. From this method, it can be noticed that the crystallite size of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C was 7.30 nm and 28.30 nm respectively. Both of these methods gave almost similar results whereas the Williamson-Hall plot method was offering some larger size. It can be due to the presence of varied geometries of particles and micro-strain in the crystal lattice of nanoparticles. Moreover, Cu0.2Ni0.8O nano-particles annealed at 500 °C exhibited negative micro-strain (−0.00179), which was due to contraction of the crystal lattice, on the other hand, Cu0.2Ni0.8O nano-particles annealed at 800 °C exhibited positive micro-strain (0.000275) that which was attributed to the expansion of its crystal lattice. This positive strain is responsible for the increase in the crystallite size of nanoparticles. Information related to molecular structure as well as the nature of chemical bonding found in Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C were obtained from FTIR analysis. Fig. 4 shows the FTIR spectra of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C. The 2
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Fig. 1. Synthesis of Cu0.2Ni0.8O nano-particles.
Fig. 2. XRD diffraction pattern, (a) Cu0.2Ni0.8O nano-particles annealed at 500 °C and (b) Cu0.2Ni0.8O nano-particles annealed at 800 °C.
FTIR bands that appeared under 1000 cm−1 are measured as fruitful. These bands provide information related to corresponding metal to oxygen bonds in the synthesized nanomaterials. FTIR spectra presented in Fig. 4 show several bands in the region from 501 to 420 cm−1, these were due to stretching vibrations of octahedral geometry of NiO6 groups in Face centered cubic structure of Cu0.2Ni0.8O nano-particles
annealed at 500 as well as 800 °C [40,41]. The formation of graphite oxide and its reduction to reduced graphene oxide was tested by matching the position of their absorption maximum peaks as given in Fig. 5. The maximum absorption peak in the case of Graphite oxide was at 226 nm which is considered to be due to π to π* transition of electrons offered by aromatic carbon frame.
Fig. 3. Williamson-Hall plot of (a) Cu0.2Ni0.8O nano-particles annealed at 500 °C and (b) Cu0.2Ni0.8O nano-particles annealed at 800 °C. 3
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Fig. 4. FTIR spectra of (a) Cu0.2Ni0.8O nano-particles annealed at 500 °C and (b) Cu0.2Ni0.8O nano-particles annealed at 800 °C.
material. The second weight loss was due to the combustion of carbon content added to the composite which was also traced in EDX result. From these results, it was concluded that a total of 10% organic content was found in the rGO-based nanocomposite. The amount founded from TGA measurement was closely matched with the quantity of rGO added in the formation of composite material [36]. The porosity, Brunauer–Emmett–Teller (BET) surface area and Langmuir surface area of typical hybrid sample was obtained using Micromeritics ASAP 2020 physisorption analyzer from N2 adsorption isotherms at 77 K. The samples were degassed at 100 °C for 4 h prior to analysis. Through this experiment, the calculated specific surface area of transition metal-based oxides nanomaterials synthesized via co-precipitation route is lower and is near to 4–5 m2/g [46]. This lower surface area is attributed due to aggregation, to compensate for their inherent high surface to volume ratio [47]. Moreover, rGO is famous due to its 3-dimensional structure and more conducting nature. This 3dimensional structure is responsible for its higher surface area. Fig. 8 shows the Nitrogen adsorption-desorption spectra of rGO-based nanohybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C. During this experiment, 0.05 g of material was used and the process was continued for approximately 6 h. It was found that the BET surface area of rGObased nano-hybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C was 34.78 m2/g whereas Langmuir surface area was 50.29 m2/g. This larger surface area might be attributed due to the higher surface area of rGO in composite material. The electrochemical properties of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nano-hybrids were inspected. Fig. 9 showed the cyclic voltammogram of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nano-hybrids recorded at different scan rates (5–100 mVs−1). The Cyclic voltammogram of Cu0.2Ni0.8O nano-particles exhibited noticeable oxidation and reduction peaks at scan rates considered. However, pair of well-defined oxidation-reduction reactions peaks were visible in the cyclic voltammogram of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nanohybrids. These indicate that the electrochemical capacitance of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nanohybrids mainly resulted from the pseudo-capacitance [48]. The chief advantage of pseudocapacitance is its extraordinary energy transfer through the faradaic reaction and given that 10 to 100 times extra capacitance than EDLCs [49].
Fig. 5. UV–Visible spectra of Graphite oxide and rGO.
Besides this, a weak shoulder peak at 299 nm was also observed. This weak shoulder peak is owing to n- π* electronic transition of C˭˭˭O bond of carbonyl group [42]. In the case of rGO, the absorption maximum peak was observed at 271 nm. The observed red shifting in the absorption peak in the case of rGO confirmed the reduction of GO [43]. Morphological studies Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and Cu0.2Ni0.8O/rGO nano-hybrid were carried out by field emission scanning electron microscopic technique. Before imaging the samples were gold sputtered at 15 mA for 120 s. The surface morphology of samples was investigated using ZEISS LEO SUPRA 55 field emission scanning electron microscope. FESEM image of Cu0.2Ni0.8O nano-particles annealed at 500 °C showed that they were in spherical morphology with average particle size ranges from 30 to 32 nm [44] as given in Fig. 6 (a). Similarly, Cu0.2Ni0.8O nano-particles annealed at 800 °C also showed spherical morphology with average particle size ranges from 60 to 70 nm as depicted in Fig. 6 (b). Metal oxide nanoparticles have a higher surface to volume ratio. In order to cope with high surface energy, they show aggregations [45]. FESEM image of Cu0.2Ni0.8O/rGO nano-hybrid shows that nano-particles are randomly dispersed on the surface of rGO sheets as presented in Fig. 6 (c). Thus confirmed the formation of rGO-based nano-hybrid material. Fig. 6 also showed the EDX results of Cu0.2Ni0.8O nano-particles annealed at 500 °C, 800 °C and Cu0.2Ni0.8O/rGO nano-hybrid. EDX analysis was carried out on JEOL JCM-6000Plus SEM. From Fig. 6 (a and b), it can be concluded that Cu+2 doped NiO nanoparticles annealed at two different temperatures contains significant amount of each element with different atomic percentage. Moreover Fig. 6 (c), clearly showed the presence of C-atom in the rGO based Cu+2 doped NiO nano-hybrid. Thermo-gravimetric analysis of Cu0.2Ni0.8O/rGO nano-hybrid was conducted in order to confirm carbon content found in it as represented in Fig. 7. The initial weight loss was observed due to the elimination of moisture content found as a waste of adsorption in the composite
Specific capacitance =
Q V × m
(3b)
By applying the above equation, the specific capacitance of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their nanohybrids were calculated and given in Table 1. It was found that the Cu0.2Ni0.8O nano-particles annealed at 500 °C exhibited 203 F g-1 of specific capacitance at 100 mVs−1 scan rate. However, the specific capacitance of Cu0.2Ni0.8O nano-particles annealed at 800 °C drops to 60 F g-1 at a 100 mVs−1 scan rate. This drop-in specific capacitance was observed due to the increase in particle size and the decrease in surface area of nanoparticles with the increase in annealing temperature. The 4
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Fig. 6. FESEM and EDX images of (a) Cu0.2Ni0.8O nano-particles annealed at 500 °C, (b) Cu0.2Ni0.8O nano-particles annealed at 800 °C and (c) Cu0.2Ni0.8O/rGO nanohybrid.
Fig. 8. Nitrogen adsorption-desorption curve of rGO based nano-hybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C.
Fig. 7. Thermo-gravimetric analysis of Cu0.2Ni0.8O/rGO nano-hybrid.
specific capacitance of Cu0.2Ni0.8O nano-particles annealed at 500 °C is better than the pure NiO and nearly similar to previously reported Cu+2 doped NiO nanoparticles. Chen et al. [50] have reported specific capacitance of NiO nanoparticles as 161 F g-1 and Sathishkumar et al. [51]
have reported the specific capacitance of Cu+2 doped NiO nanoparticles as 242 F g-1. In the case of Cu0.2Ni0.8O nano-particles annealed at 500 °C, the 5
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Fig. 9. Cyclic-Voltammogram of Cu0.2Ni0.8O nano-particles annealed at 500 °C, Cu0.2Ni0.8O nano-particles annealed at 800 °C and Cu0.2Ni0.8O/rGO nano-hybrid at (a) 100 mVs−1, (b) 50 mVs−1, (c) 20 mVs−1, (d) 10 mVs−1 (e) 5 mVs−1scan rate.
tested. It was found that rGO based nano-hybrid material exhibited an excellent increment in specific capacitance of Cu0.2Ni0.8O nano-particles. The calculated specific capacitance value of Cu0.2Ni0.8O/rGO nano-hybrid was 353, 303, 287, 252 and 182 F g-1 at 100, 50, 20, 10 and 5 mVs−1 scan rates, respectively. This increment in specific capacitance was attributed due to more conductive nature and larger size of rGO sheets that were responsible for the greater surface area [46].
Table 1 Specific capacitance of Cu0.2Ni0.8O nanoparticles and rGO based nano-hybrids. Sample
Specific capacitance (Fg−1) Scan rate (mVs−1)
Cu0.2Ni0.8O annealed at 500 °C Cu0.2Ni0.8O annealed at 800 °C Cu0.2Ni0.8O/rGO nano-hybrid
100
50
20
10
5
204 60 353
147 52 303
115 49 287
99 48 252
87 43 182
Conclusion Wet chemical method was applied to synthesized Cu0.2Ni0.8O nanoparticles and their rGO based nano-hybrids. XRD results confirmed the single cubic phase of synthesized nanoparticles. FTIR results were supporting the XRD results. FESEM analysis confirmed the spherical morphology of the synthesized product. Thermal analysis of rGO-based nano-hybrid yielded the same amount of carbon content as used. Electrochemical properties of Cu0.2Ni0.8O nano-particles annealed at 500 and 800 °C and their monohybrids were examined. Excellent specific capacitance (303 F g-1) was noticed in the case of rGO-based nanohybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C. This extra recorded capacitance was attributed due to more conducting nature and
calculated value of specific capacitance was 203, 147, 115, 99 and 86 F g-1 at 100, 50, 20, 10 and 5 mVs−1 scan rates, respectively. This increase in specific capacitance was due to the formation of positive defects in NiO crystal lattice, which induced due to the incorporation of Cu+ 2 into NiO crystal lattice. This was responsible for the increase in protons transfer and the escape of electrons from the Nickel surface more easily. Thus, the electrochemical properties of the material were improved. On the other hand, the electrochemical properties of rGO-based nano-hybrid of Cu0.2Ni0.8O nano-particles annealed at 500 °C were also 6
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relatively high surface area of rGO.
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