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Green synthesis of NiO nanostructured materials using Hydrangea paniculata flower extracts and their efficient application as supercapacitor electrodes
Powder Technology 311 (2017) 132–136
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short communication
Green synthesis of NiO nanostructured materials using Hydrangea paniculata flower extracts and their efficient application as supercapacitor electrodes Manab Kundu a,b,⁎, Gopalu Karunakaran b,c,⁎⁎, Denis Kuznetsov b a b c
Department of Material Science and Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway Department of Functional Nanosystems and High-Temperature Materials, National University of Science and Technology “MISiS”, Leninskiy Pr. 4, Moscow, 119049, Russia Department of Biotechnology, K. S. Rangasamy College of Arts and Science (Autonomous), Tiruchengode 637215, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 2 November 2016 Received in revised form 18 December 2016 Accepted 29 January 2017 Available online 1 February 2017 Keywords: Green synthesis Hydrangea paniculata NiO-NPs Structural characterizations Electrochemical properties Supercapacitor
a b s t r a c t In this report, for the first time, we have applied green synthesis NiO nanoparticles (NiO-NPs) as an electrode material for supercapacitor (SC). Hydrangea paniculata flower extracts were used for the green synthesis of NiO-NPs. The green synthesized NiO-NPs is found to be 33 nm with surface area of 78.472 m2 g−1 along with pore volume and pore size of 0.149 cm3 g−1 and 4.061 nm. The green synthesized NiO-NPs based electrode exhibit high specific capacitance of 752, 709, 644, 603 and 581 F g−1 at current densities of 2.5, 5, 10, 20, and 40 A g−1, respectively, are revealing excellent capacitance retention even at a high current density. More notably, the NiO-NPs electrodes also show outstanding cycling stability up to 5000 cycles at 10 A g−1 without discernable capacitance fading. This superior electrochemical performance of NiO-NPs is mainly attributed to the nano-dimention of the particles, which shorten the diffusion path lengths for both ions and electrons, ease migration during the rapid charge-discharge process and consequently improve the effective electrochemical utilization of electroactive material. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors are attractive power sources because they can instantaneously deliver higher power density than batteries, and higher energy density than conventional dielectric capacitors [1,2]. The electrode materials are one of the important key components in order to determine the supercapacitor performance. In this pursuit, metal oxides, metal sulfides and conducting polymers are the most investigated materials [3–8]. Among the different metal oxides, RuO2 because of good reversibility of redox process and high proton conductivity, has been investigated widely. However, the high cost and toxicity still hamper its application prospect in commercial level [9]. As an alternative choice, nickel oxide (NiO) has been proposed as an encouraging electrode material due to its outstanding theoretical specific capacitance of 2584 F g−1, low toxicity, environmental friendly and thermal stability [10–13].
A variety of NiO nanostructures such as nanowires, nanorods, nanomembranes, nanotubes, nanostrips, and hollow nanospheres, have been synthesized in the recent years [14–17] by using various physical and chemical methods. However, the multiple complicated processes, involvement of toxic raw materials, and the difficulty in scaling up the product limits their practical application. In this circumstance, green synthesis concept might be a beneficial approach. This concept involves the synthesis and manipulation of nanoparticles by an eco-friendly approach for making an efficient, effective, and safest method that can be used for human welfare [18–19]. Moreover, it has also received special attention due to cost effectiveness, simplicity, and easy recoverability [20]. Motivated by these advantages, in this study, for the first time, we have applied green synthesis concept to synthesized nanostructured NiO as an electrode material for SC. Hydrangea paniculata flower extract has been used to synthesis NiO-NPs. 2. Experimental
⁎ Correspondence to: M. Kundu, Department of Material Science and Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway. ⁎⁎ Correspondence to: G. Karunakaran, Department of Functional Nanosystems and High-Temperature Materials, National University of Science and Technology “MISiS”, Leninskiy Pr. 4, Moscow 119049, Russia. E-mail addresses: [email protected] (M. Kundu), [email protected] (G. Karunakaran).
http://dx.doi.org/10.1016/j.powtec.2017.01.085 0032-5910/© 2017 Elsevier B.V. All rights reserved.
2.1. Materials used, biochemical and GC-MS analysis The Nickel nitrate Ni (NO3)2·3H2O (Reachem, Russia) used as such for the preparation of solution. The Hydrangea paniculata flower (Fig. 1a) was collected from Gorky Central Park of Culture and Leisure,
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Fig. 1. Characterization of NiO-NPs. a) Colour change during synthesis, b) XRD, c) FTIR spectrum, d) Elemental analysis (EDS), e) SEM (×10,000 magnification) and f) SEM (×25,000 magnification) image.
Russia. The collected fresh flowers were subjected to washing with sterile de-ionized water followed by air dry for 4 to 6 h until it is free from water. Further, 5 g of the flower was crushed into smooth paste using mortar and pestle followed by collecting the clear extracts using filter paper and the separating funnel. 2.2. Green synthesis of NiO-NPs and characterization The nickel nitrate (1 mM concentration) solution was prepared using de-ionized distilled water. The synthesis of NiO-NPs is performed based on our previous research on biosynthesis of silver and magnesium nanoparticles [19]. The nickel nitrate solution and freshly prepared flower extracts were mixed in the equal ratio (1:1 v/v) and the mixture solution was incubated under the dark condition for a period of 12 to 24 h. After the incubation period the solution mixture was observed for the colour change. Further, the solution was undergone for centrifugation at a rotating speed of 6500 rpm for 30 min and the clear supernatant is removed to collect the pellet. The obtained dark pellet is re-suspended in de-ionised water and washed 2 to 3 times to eliminate any impurities
or unreacted solute present in it. After washing, the pellet was dried at 100 °C for 2 h. Then, the dried pellet is crushed into fine powder using mortar and pestle. The resultant powder was subjected to different analysis to evaluate its different characteristics. X-ray powder diffraction analysis (Difray, 401, Russia, Saint Petersburg) using the source as chromium (λ = 2.2909 Å) was used to analyse the crystalline phase of the nanoparticles. Fourier transform (FTIR) infrared spectrophotometer (Thermo Scientific, Nicolet 380, USA) was used to evaluate the elemental groups and chemical bonds present in the nanoparticles. Energy dispersive spectrum (EDS) (EDX SSD X-MAX, JAPAN) was used to confirm the elements present in the nanoparticles. Scanning electron (SEM- JEOL, JSM – 6610 LV, JAPAN) microscope and Transmission electron (TEM, JEM-2010, JEOL, Japan) microscopy techniques was used to evaluate the size and shapes of the green synthesized nanoparticles. Nitrogen adsorption desorption isotherms of the synthesized nanoparticles were analyzed by using Nova 1200e analyzer (Quantachrome Instruments, USA). The surface area of the powder was calculated using BrunauereEmmette-Teller (BET) method. The pore size and pore volume was calculated using Barrett-Joyner-Halenda (BJH) method.
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2.3. Electrochemical measurements To fabricate the working electrode, NiO-NPs were mixed with the Super P and PVDF (Polyvinylidene fluoride) binder in a weight ratio of 80:10:10 in NMP (N-Methyl-2-pyrrolidone) solution, the obtained slurry casted onto a Ni foam current collector and dried at 110 °C. The electrochemical tests were carried out in a three-electrode configuration using a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode and 3 M KOH aqueous electrolyte. In order to evaluate the electrochemical performance of as-prepared NiONPs, cyclic voltammetry (CV), galvanostatic charge/discharge technique and electrochemical impedance spectroscopy (EIS) tests were carried out using a Biologic VMP3 battery tester. The potential window was in the range of 0 to 0.45 V (vs. SCE), and the EIS measurements were performed in a frequency range of 0.1 Hz–1 M Hz with 10 mV sinusoidal perturbation. 3. Results and discussion The Hydrangea paniculata is known to contained different phytocompounds such as tannins, flavonoids, anthraquinones, terpenes and saponins, etc., [19]. The phytocompounds present in flower extract is highly potential to synthesize nanoparticles [19]. The phytochemicals convert Ni (NO3)2 to Ni (OH)2 (nickel hydroxide) NPs, which is confirmed by observing the colour change from light yellow to lime green which is shown in Fig. 1a. Further, the Ni (OH)2NPs upon heating at 100 °C, is converted into NiO-NPs. The XRD pattern of the green synthesized nanoparticles are shown in Fig. 1b which is found to be best matched with JCPDS card (JCPDS 00-002-1216) of NiO. The peaks observed are ~ 56.5° (111), ~ 66.6° (200), ~101.2° (220), and ~130.4° (311), confirms the cubic crystalline
phase with the lattice parameter of 4.17 Å and average crystal size of 9 nm for the obtained NiO-NPs. The involvements of phytocompounds in the synthesis of nanoparticles are confirmed by FTIR analysis (Fig. 1c). IR spectrum of NiO-NPs showed different bands at 3321, 2551, 2348, 2163, 2011, 1628, 1355, 1038 and 822 cm− 1, respectively. The observed bands at 3321 and 2551 cm−1 shows the presence of O-H stretching mode, it may be due to the small amount of hydroxyl group which is generally associated with phenols and carboxylic acids. Also, 2348, 2163, 2011 and 1628 cm−1 were due to the presence of \\CH, C_ O stretching and \\C\\H bending vibration which may be due to the presence of aldehyde groups, amides groups, and carboxylic acids. The bands at 1355, 1038 and 822 cm−1 confirmed the presence of nickel. Thus, it is clear that phytochemicals take part in the biotransformation of nitrates to nanoparticles. The phytochemicals mostly act as capping, reducing and stabilizing agent during the synthesis of nanoparticles. The EDS spectrum of NiO-NPs (Fig. 1d) reveals that the nanoparticles contain nickel, carbon, and oxygen. The presence of carbon is due to the presence of organic molecules with the nanoparticles. The morphology of the green synthesized nanoparticles is analyzed by SEM and TEM analysis (Figs. 1e, f and 2a). It is clear that the particles are of different shapes such as spherical, oval and rod with size ranges from 23 to 80 nm and the calculated average particle size is 33 nm. Nitrogen adsorption desorption isotherms of the synthesized nanoparticles was analyzed and the specific surface area of the powder was calculated using Brunauere-Emmette-Teller (BET) method and the pore volume and pore size of the nanocomposites were analyzed by Barrett-Joyner-Halenda (BJH) method. The obtained results are shown in Fig. 2b and c. The result reveals that the nanoparticles exhibits the surface area of 78.472 m2 g−1 with the pore volume and pore size of 0.149 cm3 g−1 and 4.061 nm.
Fig. 2. a) TEM image, b) nitrogen (N2) adsorption and desorption isotherms and c) BET- surface area plot.
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Fig. 3a shows the CVs of the NiO-NPs electrode at a various scanning rates ranging from 5 to 100 mV s−1. The well-defined redox reaction peaks within 0–0.45 V (vs. SCE) are visible at all scan rates, indicating that the electrochemical capacitance of the NiO-NPs electrode is distinct from that of electric double-layer capacitance with common rectangular shape. The anodic peak is due to the oxidation of NiO to NiOOH and the cathodic peak is for the reverse process, the corresponding surface faradaic reaction can be expressed as follows [21]: NiO þ OH− ↔NiOOH þ e−
ð1Þ
Fig. 3a is also revealed that with the increase in scan rates, the anodic peaks shift positively whereas the cathodic peaks shift negatively, which attributes the high surface area of the NiO-NPs and the fast ionic/electronic diffusion rate during the faradic redox reaction [22]. Moreover, the peak current increase linearly with the scan rate, (inset of Fig. 3a), indicating that a faradaic redox reaction is taking place at the NiO/electrolyte interface. To get more information about their potential application in supercapacitors, galvanostatic charge-discharge measurements were carried out at various current densities ranging from 2.5 to 40 A g− 1 (Fig. 3b). The pseudocapacitive nature of the NiO electrodes at all current densities is reflected by the nonlinear feature of the chargedischarge curves, which is consistent with the CV results. The sloped variation from ∼ 0.2 to 0.45 V indicates the pseudocapacitive nature of the electrodes arising from the redox reaction at the electrode/ electrolyte interface, whereas the linear variation in the range of 0–0.2 V represents the contribution of double-layer capacitance resulting from the electrostatic interaction of ions at the electrode
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surface. From the discharge curves, the specific capacitance can be calculated using the following formula: C¼
IΔt mΔV
ð2Þ
where I is the discharge current, m the loading mass of the active materials, ΔV the potential window, and Δt the discharge time in this potential window. The specific capacitance values obtained are 752, 709, 644, 603 and 581 F g−1 at current densities of 2.5, 5, 10, 20, and 40 A g−1, respectively (Fig. 3c). The cycling performance of electroactive material, specifically at high current density, is one of the most significant parameter. The cycle lifespan of NiO-NPs was tested at a current density of 10 A g−1 (Fig. 4a). The gradual increasing trend of specific capacitance is related to the improvement in the effective electrochemical utilization of electroactive material during charge-discharge reaction. Up to 3500 cycles, the specific capacitance gradually increased to 750 F g−1 and finally reached to 780 F g−1 after 5000 cycles, which is much higher than the value of NiO with different nano structures reported previously [13,23]. Such miraculous data with outstanding long cycling performance and rate capability further highlights the capability of this green synthesis concept which has the potential to meet the requirements of large scale application keeping our environment safe. In order to gain some insight into the good performance of the NiONPs electrodes, EIS measurement has been carried out and the corresponding Nyquist plot is shown in Fig. 4b. The equivalent circuit (inset of Fig. 4b) comprising of a resistor (Re) in series connection with a parallel combination of a capacitor (Cdl) and a resistor (Rct) plus a Warburg
Fig. 3. (a) Cyclic voltammograms recorded at scan rates of 5, 10, 30, 50, 80, 100 and 120 mV s−1. Inset is the linear relationship of the anodic current with the scan rates, the red curve is a linear fit, (b) galvanostatic charge-discharge profiles measured at current densities of 2.5, 5, 10 and 15 A g−1 and (c) the variation of specific capacitance as a function of the current density.
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eco-friendly nature would be the great benefit which will make this synthesis concept as one of the best and safest methods for human welfare.
Acknowledgement The work was carried out with financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (No. K4-2015-017), implemented by a governmental decree dated 16th of March 2013, N 211. References
Fig. 4. (a) Charge–discharge cycling test at a current density of 10 A g−1, inset is the potential profile of NiO-NPs before and after 2000 cycles at a current density of 10 A g−1, (b) electrochemical impedance spectra, inset is the equivalent circuit.
element (Zw). In Fig. 4b, the vertical nature of the plot explicitly exhibits an ideal capacitive behavior of the electrode. The intercept on the real axis in the high frequency range represents the equivalent series resistance (ESR, Re), which includes both electronic and ionic contributions. According to the fitting result, the calculated value of ESR is 1.87 Ω. This small ESR value implies that the high surface area offered by the NiONPs allows the ions to more easily access the NiO surface to accomplish the redox reaction. Moreover, the nano-dimention of NiO shortens the diffusion path lengths for both ions and electrons which ease migration during the rapid charge-discharge process and consequently improve the effective electrochemical utilization of electroactive material. This can reasonably explain the good rate performance at various current densities, as presented in Fig. 3b. 4. Conclusions In summary, for the first time, Hydrangea paniculata flower extracts mediated green synthesized NiO-NPs have been investigated as potential candidate in supercapacitors application. The average particle size of NiO-NPs is found to be 33 nm with surface area of 78.472 m2 g−1 along with pore volume and pore size of 0.149 cm3 g−1 and 4.061 nm. Remarkable rate capability at various current densities (ranging from 2.5 to 15 A g− 1) and outstanding cycling stability up to 5000 cycles at 10 A g−1 stimulate to employing green synthesized nano-metal oxides as electrode materials for different electrochemical energy storage application. The cost effectiveness, simplicity, easy recoverability and
[1] Y. Gogotsi, P. Simon, True performance metrics in electrochemical energy storage, Science 334 (2011) 917–918. [2] C.R. Zhu, P.H. Yang, D.L. Chao, X.L. Wang, X. Zhang, S. Chen, B.K. Tay, H. Huang, H. Zhang, W.J. Mai, H.J. Fan, All metal nitrides solid-state asymmetric supercapacitors, Adv. Mater. 27 (2015) 4566–4571. [3] J. Lang, L. Kong, W. Wu, Y. Luo, L. Kang, Facile approach to prepare loose-packed NiO nano-flakes materials for supercapacitors, Chem. Commun. 35 (2008) 4213–4215. [4] T. Yan, J.F. Cai, T.T. Yang, Z.J. Li, Template-free synthesis of alpha-Ni(OH)(2) hollow microspheres with flower-like morphology for high-performance supercapacitors, Mater. Res. Bull. 60 (2014) 612–620. [5] M. Kundu, L. Liu, Direct growth of mesoporous MnO2 nanosheet arrays on nickel foam current collectors for high-performance pseudocapacitors, J. Power Sources 243 (2013) 676–681. [6] L.S. Aravinda, K.K. Nagaraja, H.S. Nagaraja, K. Udaya Bhat, B. Ramachandra Bhat, Fabrication and performance evaluation of hybrid supercapacitor electrodes based on carbon nanotubes and sputtered TiO2, Nanotechnology 27 (2016) 314001. [7] L.S. Aravinda, K.K. Nagaraja, H.S. Nagaraja, K.U. Bhat, B.R. Bhat, ZnO/carbon nanotube nanocomposite for high energy density supercapacitors, Electrochim. Acta 95 (2013) 119–124. [8] X. Li, Q. Li, Y. Wu, M. Rui, H. Zeng, Two-dimensional, porous nickel–cobalt sulfide for high-performance asymmetric supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 19316–19323. [9] Z.-S. Wu, D.-W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li, H.-M. Cheng, Anchoring hydrous RuO2 on graphene sheets for high-performance electrochemical capacitors, Adv. Funct. Mater. 20 (2010) 3595–3602. [10] M. Kundu, L. Liu, Electrospun porous nickel oxide nanofibers for high-performance electrochemical energy storage, J. Nanosci. Lett. 5 (2015) 11. [11] M. Kundu, L. Liu, Binder-free electrodes consisting of porous NiO nanofibers directly electrospun on nickel foam for high-rate supercapacitors, Mater. Lett. 144 (2015) 114–118. [12] Z. Gonzalez, B. Ferrari, A.J. Sanchez-Herencia, A. Caballero, J. Morales, Use of polyelectrolytes for the fabrication of porous NiO films by electrophoretic deposition for supercapacitor electrodes, Electrochim. Acta 211 (2016) 110–118. [13] J. Liang, H. Tan, C. Xiao, G. Zhou, S. Guo, S. Ding, Hydroxyl-riched halloysite clay nanotubes serving as substrate of NiO nanosheets for high-performance supercapacitor, J. Power Sources 285 (2015) 210–216. [14] B. Ren, M. Fan, Q. Liu, J. Wang, D. Song, X. Bai, Hollow NiO nanofibers modified by citric acid and the performances as supercapacitor electrode, Electrochim. Acta 92 (2013) 197–204. [15] L. Fan, L. Tang, H. Gong, Z. Yao, R. Guo, Carbon-nanoparticles encapsulated in hollow nickel oxides for supercapacitor application, J. Mater. Chem. 22 (2012) 16376–16381. [16] K.B. Xu, R.J. Zou, W.Y. Li, Q. Liu, T.N. Wang, J.M. Yang, Z.G. Chen, J.Q. Hu, Carboncoated mesoporous NiO nanoparticles as an electrode material for high performance electrochemical capacitors, New J. Chem. 37 (2013) 4031–4036. [17] S. Vijayakumar, S. Nagamuthu, G. Muralidharan, Supercapacitor Studies on NiO nanoflakes synthesized through a microwave route, ACS Appl. Mater. Interfaces 5 (2013) 2188–2196. [18] T. Wang, S. Liu, Green synthesis and photoluminescence property of AlOOH nanoflakes, Powder Technol. 294 (2016) 280–283. [19] G. Karunakaran, M. Jagathambal, M. Venkatesh, G.S. Kumar, E. Kolesnikov, A. Dmitry, A. Gusev, D. Kuznetsov, Hydrangea paniculata flower extract-mediated green synthesis of MgNPs and AgNPs for health care applications, Powder Technol. 305 (2017) 488–494. [20] G. Xu, X. Zhang, H. Cui, Z. Zhang, J. Ding, J. Wu, Facile synthesis of mesoporous SnO2 microspheres using bioactive yeast cell, Powder Technol. 301 (2016) 96–101. [21] X.L. Sun, W.P. Si, X.H. Liu, J.W. Deng, L.X. Xi, L.F. Liu, C. Yan, O.G. Schmidt, Multifunctional Ni/NiO hybrid nanomembranes as anode materials for high-rate Li-ion batteries, Nano Energy 9 (2014) 168–175. [22] K. Liang, X. Tang, W. Hu, High-performance three-dimensional nanoporousNiO film as a supercapacitor electrode, J. Mater. Chem. 22 (2012) 11062–11067. [23] Y. Zhang, J. Wang, H. Wei, J. Hao, J. Mu, P. Cao, J. Wang, S. Zhao, Hydrothermal synthesis of hierarchical mesoporous NiO nanourchins and their supercapacitor application, Mater. Lett. 162 (2016) 67–70.
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short communication
Green synthesis of NiO nanostructured materials using Hydrangea paniculata flower extracts and their efficient application as supercapacitor electrodes Manab Kundu a,b,⁎, Gopalu Karunakaran b,c,⁎⁎, Denis Kuznetsov b a b c
Department of Material Science and Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway Department of Functional Nanosystems and High-Temperature Materials, National University of Science and Technology “MISiS”, Leninskiy Pr. 4, Moscow, 119049, Russia Department of Biotechnology, K. S. Rangasamy College of Arts and Science (Autonomous), Tiruchengode 637215, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 2 November 2016 Received in revised form 18 December 2016 Accepted 29 January 2017 Available online 1 February 2017 Keywords: Green synthesis Hydrangea paniculata NiO-NPs Structural characterizations Electrochemical properties Supercapacitor
a b s t r a c t In this report, for the first time, we have applied green synthesis NiO nanoparticles (NiO-NPs) as an electrode material for supercapacitor (SC). Hydrangea paniculata flower extracts were used for the green synthesis of NiO-NPs. The green synthesized NiO-NPs is found to be 33 nm with surface area of 78.472 m2 g−1 along with pore volume and pore size of 0.149 cm3 g−1 and 4.061 nm. The green synthesized NiO-NPs based electrode exhibit high specific capacitance of 752, 709, 644, 603 and 581 F g−1 at current densities of 2.5, 5, 10, 20, and 40 A g−1, respectively, are revealing excellent capacitance retention even at a high current density. More notably, the NiO-NPs electrodes also show outstanding cycling stability up to 5000 cycles at 10 A g−1 without discernable capacitance fading. This superior electrochemical performance of NiO-NPs is mainly attributed to the nano-dimention of the particles, which shorten the diffusion path lengths for both ions and electrons, ease migration during the rapid charge-discharge process and consequently improve the effective electrochemical utilization of electroactive material. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors are attractive power sources because they can instantaneously deliver higher power density than batteries, and higher energy density than conventional dielectric capacitors [1,2]. The electrode materials are one of the important key components in order to determine the supercapacitor performance. In this pursuit, metal oxides, metal sulfides and conducting polymers are the most investigated materials [3–8]. Among the different metal oxides, RuO2 because of good reversibility of redox process and high proton conductivity, has been investigated widely. However, the high cost and toxicity still hamper its application prospect in commercial level [9]. As an alternative choice, nickel oxide (NiO) has been proposed as an encouraging electrode material due to its outstanding theoretical specific capacitance of 2584 F g−1, low toxicity, environmental friendly and thermal stability [10–13].
A variety of NiO nanostructures such as nanowires, nanorods, nanomembranes, nanotubes, nanostrips, and hollow nanospheres, have been synthesized in the recent years [14–17] by using various physical and chemical methods. However, the multiple complicated processes, involvement of toxic raw materials, and the difficulty in scaling up the product limits their practical application. In this circumstance, green synthesis concept might be a beneficial approach. This concept involves the synthesis and manipulation of nanoparticles by an eco-friendly approach for making an efficient, effective, and safest method that can be used for human welfare [18–19]. Moreover, it has also received special attention due to cost effectiveness, simplicity, and easy recoverability [20]. Motivated by these advantages, in this study, for the first time, we have applied green synthesis concept to synthesized nanostructured NiO as an electrode material for SC. Hydrangea paniculata flower extract has been used to synthesis NiO-NPs. 2. Experimental
⁎ Correspondence to: M. Kundu, Department of Material Science and Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway. ⁎⁎ Correspondence to: G. Karunakaran, Department of Functional Nanosystems and High-Temperature Materials, National University of Science and Technology “MISiS”, Leninskiy Pr. 4, Moscow 119049, Russia. E-mail addresses: [email protected] (M. Kundu), [email protected] (G. Karunakaran).
http://dx.doi.org/10.1016/j.powtec.2017.01.085 0032-5910/© 2017 Elsevier B.V. All rights reserved.
2.1. Materials used, biochemical and GC-MS analysis The Nickel nitrate Ni (NO3)2·3H2O (Reachem, Russia) used as such for the preparation of solution. The Hydrangea paniculata flower (Fig. 1a) was collected from Gorky Central Park of Culture and Leisure,
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Fig. 1. Characterization of NiO-NPs. a) Colour change during synthesis, b) XRD, c) FTIR spectrum, d) Elemental analysis (EDS), e) SEM (×10,000 magnification) and f) SEM (×25,000 magnification) image.
Russia. The collected fresh flowers were subjected to washing with sterile de-ionized water followed by air dry for 4 to 6 h until it is free from water. Further, 5 g of the flower was crushed into smooth paste using mortar and pestle followed by collecting the clear extracts using filter paper and the separating funnel. 2.2. Green synthesis of NiO-NPs and characterization The nickel nitrate (1 mM concentration) solution was prepared using de-ionized distilled water. The synthesis of NiO-NPs is performed based on our previous research on biosynthesis of silver and magnesium nanoparticles [19]. The nickel nitrate solution and freshly prepared flower extracts were mixed in the equal ratio (1:1 v/v) and the mixture solution was incubated under the dark condition for a period of 12 to 24 h. After the incubation period the solution mixture was observed for the colour change. Further, the solution was undergone for centrifugation at a rotating speed of 6500 rpm for 30 min and the clear supernatant is removed to collect the pellet. The obtained dark pellet is re-suspended in de-ionised water and washed 2 to 3 times to eliminate any impurities
or unreacted solute present in it. After washing, the pellet was dried at 100 °C for 2 h. Then, the dried pellet is crushed into fine powder using mortar and pestle. The resultant powder was subjected to different analysis to evaluate its different characteristics. X-ray powder diffraction analysis (Difray, 401, Russia, Saint Petersburg) using the source as chromium (λ = 2.2909 Å) was used to analyse the crystalline phase of the nanoparticles. Fourier transform (FTIR) infrared spectrophotometer (Thermo Scientific, Nicolet 380, USA) was used to evaluate the elemental groups and chemical bonds present in the nanoparticles. Energy dispersive spectrum (EDS) (EDX SSD X-MAX, JAPAN) was used to confirm the elements present in the nanoparticles. Scanning electron (SEM- JEOL, JSM – 6610 LV, JAPAN) microscope and Transmission electron (TEM, JEM-2010, JEOL, Japan) microscopy techniques was used to evaluate the size and shapes of the green synthesized nanoparticles. Nitrogen adsorption desorption isotherms of the synthesized nanoparticles were analyzed by using Nova 1200e analyzer (Quantachrome Instruments, USA). The surface area of the powder was calculated using BrunauereEmmette-Teller (BET) method. The pore size and pore volume was calculated using Barrett-Joyner-Halenda (BJH) method.
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2.3. Electrochemical measurements To fabricate the working electrode, NiO-NPs were mixed with the Super P and PVDF (Polyvinylidene fluoride) binder in a weight ratio of 80:10:10 in NMP (N-Methyl-2-pyrrolidone) solution, the obtained slurry casted onto a Ni foam current collector and dried at 110 °C. The electrochemical tests were carried out in a three-electrode configuration using a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode and 3 M KOH aqueous electrolyte. In order to evaluate the electrochemical performance of as-prepared NiONPs, cyclic voltammetry (CV), galvanostatic charge/discharge technique and electrochemical impedance spectroscopy (EIS) tests were carried out using a Biologic VMP3 battery tester. The potential window was in the range of 0 to 0.45 V (vs. SCE), and the EIS measurements were performed in a frequency range of 0.1 Hz–1 M Hz with 10 mV sinusoidal perturbation. 3. Results and discussion The Hydrangea paniculata is known to contained different phytocompounds such as tannins, flavonoids, anthraquinones, terpenes and saponins, etc., [19]. The phytocompounds present in flower extract is highly potential to synthesize nanoparticles [19]. The phytochemicals convert Ni (NO3)2 to Ni (OH)2 (nickel hydroxide) NPs, which is confirmed by observing the colour change from light yellow to lime green which is shown in Fig. 1a. Further, the Ni (OH)2NPs upon heating at 100 °C, is converted into NiO-NPs. The XRD pattern of the green synthesized nanoparticles are shown in Fig. 1b which is found to be best matched with JCPDS card (JCPDS 00-002-1216) of NiO. The peaks observed are ~ 56.5° (111), ~ 66.6° (200), ~101.2° (220), and ~130.4° (311), confirms the cubic crystalline
phase with the lattice parameter of 4.17 Å and average crystal size of 9 nm for the obtained NiO-NPs. The involvements of phytocompounds in the synthesis of nanoparticles are confirmed by FTIR analysis (Fig. 1c). IR spectrum of NiO-NPs showed different bands at 3321, 2551, 2348, 2163, 2011, 1628, 1355, 1038 and 822 cm− 1, respectively. The observed bands at 3321 and 2551 cm−1 shows the presence of O-H stretching mode, it may be due to the small amount of hydroxyl group which is generally associated with phenols and carboxylic acids. Also, 2348, 2163, 2011 and 1628 cm−1 were due to the presence of \\CH, C_ O stretching and \\C\\H bending vibration which may be due to the presence of aldehyde groups, amides groups, and carboxylic acids. The bands at 1355, 1038 and 822 cm−1 confirmed the presence of nickel. Thus, it is clear that phytochemicals take part in the biotransformation of nitrates to nanoparticles. The phytochemicals mostly act as capping, reducing and stabilizing agent during the synthesis of nanoparticles. The EDS spectrum of NiO-NPs (Fig. 1d) reveals that the nanoparticles contain nickel, carbon, and oxygen. The presence of carbon is due to the presence of organic molecules with the nanoparticles. The morphology of the green synthesized nanoparticles is analyzed by SEM and TEM analysis (Figs. 1e, f and 2a). It is clear that the particles are of different shapes such as spherical, oval and rod with size ranges from 23 to 80 nm and the calculated average particle size is 33 nm. Nitrogen adsorption desorption isotherms of the synthesized nanoparticles was analyzed and the specific surface area of the powder was calculated using Brunauere-Emmette-Teller (BET) method and the pore volume and pore size of the nanocomposites were analyzed by Barrett-Joyner-Halenda (BJH) method. The obtained results are shown in Fig. 2b and c. The result reveals that the nanoparticles exhibits the surface area of 78.472 m2 g−1 with the pore volume and pore size of 0.149 cm3 g−1 and 4.061 nm.
Fig. 2. a) TEM image, b) nitrogen (N2) adsorption and desorption isotherms and c) BET- surface area plot.
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Fig. 3a shows the CVs of the NiO-NPs electrode at a various scanning rates ranging from 5 to 100 mV s−1. The well-defined redox reaction peaks within 0–0.45 V (vs. SCE) are visible at all scan rates, indicating that the electrochemical capacitance of the NiO-NPs electrode is distinct from that of electric double-layer capacitance with common rectangular shape. The anodic peak is due to the oxidation of NiO to NiOOH and the cathodic peak is for the reverse process, the corresponding surface faradaic reaction can be expressed as follows [21]: NiO þ OH− ↔NiOOH þ e−
ð1Þ
Fig. 3a is also revealed that with the increase in scan rates, the anodic peaks shift positively whereas the cathodic peaks shift negatively, which attributes the high surface area of the NiO-NPs and the fast ionic/electronic diffusion rate during the faradic redox reaction [22]. Moreover, the peak current increase linearly with the scan rate, (inset of Fig. 3a), indicating that a faradaic redox reaction is taking place at the NiO/electrolyte interface. To get more information about their potential application in supercapacitors, galvanostatic charge-discharge measurements were carried out at various current densities ranging from 2.5 to 40 A g− 1 (Fig. 3b). The pseudocapacitive nature of the NiO electrodes at all current densities is reflected by the nonlinear feature of the chargedischarge curves, which is consistent with the CV results. The sloped variation from ∼ 0.2 to 0.45 V indicates the pseudocapacitive nature of the electrodes arising from the redox reaction at the electrode/ electrolyte interface, whereas the linear variation in the range of 0–0.2 V represents the contribution of double-layer capacitance resulting from the electrostatic interaction of ions at the electrode
135
surface. From the discharge curves, the specific capacitance can be calculated using the following formula: C¼
IΔt mΔV
ð2Þ
where I is the discharge current, m the loading mass of the active materials, ΔV the potential window, and Δt the discharge time in this potential window. The specific capacitance values obtained are 752, 709, 644, 603 and 581 F g−1 at current densities of 2.5, 5, 10, 20, and 40 A g−1, respectively (Fig. 3c). The cycling performance of electroactive material, specifically at high current density, is one of the most significant parameter. The cycle lifespan of NiO-NPs was tested at a current density of 10 A g−1 (Fig. 4a). The gradual increasing trend of specific capacitance is related to the improvement in the effective electrochemical utilization of electroactive material during charge-discharge reaction. Up to 3500 cycles, the specific capacitance gradually increased to 750 F g−1 and finally reached to 780 F g−1 after 5000 cycles, which is much higher than the value of NiO with different nano structures reported previously [13,23]. Such miraculous data with outstanding long cycling performance and rate capability further highlights the capability of this green synthesis concept which has the potential to meet the requirements of large scale application keeping our environment safe. In order to gain some insight into the good performance of the NiONPs electrodes, EIS measurement has been carried out and the corresponding Nyquist plot is shown in Fig. 4b. The equivalent circuit (inset of Fig. 4b) comprising of a resistor (Re) in series connection with a parallel combination of a capacitor (Cdl) and a resistor (Rct) plus a Warburg
Fig. 3. (a) Cyclic voltammograms recorded at scan rates of 5, 10, 30, 50, 80, 100 and 120 mV s−1. Inset is the linear relationship of the anodic current with the scan rates, the red curve is a linear fit, (b) galvanostatic charge-discharge profiles measured at current densities of 2.5, 5, 10 and 15 A g−1 and (c) the variation of specific capacitance as a function of the current density.
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eco-friendly nature would be the great benefit which will make this synthesis concept as one of the best and safest methods for human welfare.
Acknowledgement The work was carried out with financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (No. K4-2015-017), implemented by a governmental decree dated 16th of March 2013, N 211. References
Fig. 4. (a) Charge–discharge cycling test at a current density of 10 A g−1, inset is the potential profile of NiO-NPs before and after 2000 cycles at a current density of 10 A g−1, (b) electrochemical impedance spectra, inset is the equivalent circuit.
element (Zw). In Fig. 4b, the vertical nature of the plot explicitly exhibits an ideal capacitive behavior of the electrode. The intercept on the real axis in the high frequency range represents the equivalent series resistance (ESR, Re), which includes both electronic and ionic contributions. According to the fitting result, the calculated value of ESR is 1.87 Ω. This small ESR value implies that the high surface area offered by the NiONPs allows the ions to more easily access the NiO surface to accomplish the redox reaction. Moreover, the nano-dimention of NiO shortens the diffusion path lengths for both ions and electrons which ease migration during the rapid charge-discharge process and consequently improve the effective electrochemical utilization of electroactive material. This can reasonably explain the good rate performance at various current densities, as presented in Fig. 3b. 4. Conclusions In summary, for the first time, Hydrangea paniculata flower extracts mediated green synthesized NiO-NPs have been investigated as potential candidate in supercapacitors application. The average particle size of NiO-NPs is found to be 33 nm with surface area of 78.472 m2 g−1 along with pore volume and pore size of 0.149 cm3 g−1 and 4.061 nm. Remarkable rate capability at various current densities (ranging from 2.5 to 15 A g− 1) and outstanding cycling stability up to 5000 cycles at 10 A g−1 stimulate to employing green synthesized nano-metal oxides as electrode materials for different electrochemical energy storage application. The cost effectiveness, simplicity, easy recoverability and
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