- Email: [email protected]
Surfactant determines the morphology, structure and energy storage features of CuO nanostructures
Results in Physics 13 (2019) 102185
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Surfactant determines the morphology, structure and energy storage features of CuO nanostructures
T
Balakrishnan Saravanakumara,b, Chandran Radhakrishnanb,c, Murugan Ramasamyd, ⁎ Rajendran Kaliaperumalb, Allen J. Brittenb, Martin Mkandawireb, a
Department of Physics, Dr. Mahalingam College of Engineering and Technology, Pollachi, Tamilnadu 642 003, India Chemistry Department, Cape Breton University, 1250 Grand Lake Rd. Sydney, Nova Scotia B1P 6L2, Canada c Department of Autombile Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi, Tamilnadu 642 003, India d National Centre for Earth Science Studies, Ministry of Earth Sciences, Govt. of India, Thiruvananthapuram, Kerala 695 011, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Copper oxide Neutral electrolyte Surfactant Supercapacitor
In quest for cost-affordable but high performing supercapacitor (SC), we explored fabrication of electrode material of copper oxide (CuO) nanostructures using simple solution-based synthesis procedure and different surfactants. Here, we report the influence of the surfactants on the morphological and structural evolutions as well as energy-storage capacity of the CuO. As an SC electrode material, CuO exhibits considerably high specific capacity (51 mAhg−1 @ 1Ag−1), good rate performance (31 mAhg−1 @ 10Ag−1) and better cyclic stability. In addition, it has low charge transfer resistance (1.6 Ω), which is very important when performing charge–discharge at high current rates. These results are highlighting the way for its use in design of advanced CuObased supercapacitor devices.
Introduction The mounting demand for highly improved Electrochemical Energy Storage (EES) devices is compelling researchers to innovate new material architectures. Among various energy storing mechanisms, Supercapacitors (SCs) have created significant impact on the overall development of EES technologies over last decade owing to their rapid charge-discharge ability, high power and excellent cycle life [1]. Presently, SCs are utilized as complementary device with batteries to facilitate additional peak power during higher power surge in many applications including hybrid electrical vehicles (HEV), wind turbine blade pitching systems, and optical zooming and flashing in camera. Generally, an SC device comprises of three prime components, namely: electrodes; separator; and, electrolyte. Among these, the electrochemical properties and morphology of an electrode are key to the overall efficiency of an SC device. SCs are commonly categorised as either electric double layer capacitors (EDLC) or pseudocapacitors, based on the charge storage process in the electrode active material. The EDLC store electric charges through adsorption of ion at electrolyte and electrode interface. In contrast, pseudocapacitors chemically store the charges through redox reaction in a few nanometres at the surface of the active material. The EDLC type utilizes carbon related materials (e.g., activated carbon, ⁎
graphene, CNT, and etc.) as electrode active material, while the pseudocapacitors have conducting polymers (e.g., PEDOT, PPY and etc.) and transition metal oxides (RuO2, V2O5, MnO2, NiO, Fe2O3, etc) as electrode materials [1–5]. To attain worthy enhancement in SC performance by means of innovative electrode material with electrochemically efficient sites, good number of pathways/channels for ion intercalation, eco-friendly production, cost effectiveness and good understanding of ion storage mechanism are necessary. Among various transitional metal oxides (TMOs) tested for SC electrode applications, copper oxide (CuO) is one among the best candidate due to its natural abundance, cost effectiveness and easy preparation procedures. Regardless of these attractive features, CuO has relatively high material volume expansion and poor electrical conductivity, which harms the capacity of the material as an electrode in SC. These issues may be overcome through the controlling the morphology of the CuO nanostructures [6]. Here, surfactants play an essential role in cost effective synthesis of nanostructured materials. It acts as a structure directing agents and leads different nanostructures including nanosheets, nanoflower, and nanofibers [7–9]. Electrolyte is also an important component, determining the overall performance of any SC device. Specifically, neutral electrolytes have several application advantages because of having high ionic conductivity and being non-flammable and inexpensive. Na+ (0.358 nm,
Corresponding author. E-mail address: [email protected] (M. Mkandawire).
https://doi.org/10.1016/j.rinp.2019.102185 Received 5 March 2019; Accepted 7 March 2019 Available online 09 March 2019 2211-3797/ © 2019 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
50.1 cm2/Ω mol) ions possess thin ionic radius and higher ionic conductivity compared to Li+ (0.38 nm, 38.6 cm2/Ω mol) [10]. Hence, it is reasonable to expect that an SC, utilizing eco-friendly Na2SO4 electrolyte, should show better supercapacitive features. Many researchers have investigated effects of different surfactants on synthesis and characterization of CuO nanostructures, but only few works have elaborated their complete electrochemical properties. Furthermore, most of the works do not report performance rate of the electrode material, while some of those reported show very poor performance rate (See Table S1, supporting data). Owing to this, we investigated the performance rates of different electrochemical morphologies of CuO nanostructures, synthesized using different surfactants (anionic, cationic and non-ionic) such as sodium dodecyl sulphate (SDS), cetyl trimethylammonium bromide (CTAB) and Poly Ethylene Glycol (PEG 4000) as structure directing agents. The influence of these agents on structure, morphology and electrochemical performance are investigated. When tested as a SC electrode active material in Na2SO4 electrolyte, CuO prepared using SDS showed superior electrochemical features, like higher capacitance, better rate performance and good cycle life than the others. Details regarding synthesis, physiochemical and electrochemical investigations are given in the supporting information.
Material characterization techniques The crystallographic information’s of the prepared samples were analyzed using using Bruker D8 Advance X-Ray diffractometer equipped with CuKα radiation (λ = 1.5406 Å) 10˚ to 80˚. The FT-IR spectrum of the samples were recorded using NICOLET 6700 spectrometer. The morphological features of the samples were analyzed using TESCAN VEGA 3 LMU scanning electron microscope and Hitachi transmission electron microscope. The Brunauer-Emmett-Teller (BET) method was used to determine the specific surface area of the samples. The nitrogen adsorption-desorption measurements were performed by Micrometeritics ASAP 2020 analyzer. The porosity distributions were derived from desorption part of the isotherm using the Barrett-JoynerHalenda (BJH) method.
Electrochemical measurements Electrochemical measurements were accomplished using three electrode configuration in electrochemical workstation (CHI 660 D, CH instruments Inc., USA). The Ag/AgCl and a platinum wire were used as reference and counter electrodes respectively. The working electrode was prepared using 85 wt% of electrode active material, 10 wt% carbon black, and 5 wt% polytetrafluoroethylene (Sigma-Aldrich) binder was blended with few drops of ethanol. This slurry was pasted onto a nickel foam current collector (1 cm2). The nickel foam was cleaned using ultrasonicator with HCl (37 wt%) for few minutes to eliminate the oxide layer present in the nickel foam. In addition, it is also cleaned using ethanol and DI water. Further the electrode was dried at 80˚C for 4 hr. The electrochemical parameters of the electrodes were determined by cyclic voltammetry (CV), galvanostatic charge discharge (GCD), electrochemical impedance spectroscopy (EIS) in 1 M sodium sulphate (Na2SO4) aqueous electrolyte solution. The specific capacitance, energy density, power density values were estimated based on the mass of the electrode active material (0.8 mg) present in the electrode and following equations, GCD curves are utilized for estimation of specific capacitance from following relation:
Experimental Fabrication of CuO nanostructures Copper nitrate (Cu(NO3)2·3H2O), Sodium dodecyl sulphate (SDS), Poly Ethylene Glycol (PEG 4000), Cetrimonium bromide (CTAB), (30%) HCl and NaOH were procured from Sigma Aldrich. All chemicals and reagents were used as supplied without further purification and all solutions were made with Millipore Milli- Qwater (18 M-cm). In the first step, 0.906 g of Cu (NO3)2·3H2O was dissolved in 100 mL of H2O and stirred for an hour to get homogeneous solution. Further, 0.090 g of SDS was added to 25 mL of H2O and stirred for 30 min. In the second step, the SDS solution was slowly added to the copper nitrate solution followed by adding 2 M NaOH to make the pH ∼ 10. Finally, this solution was magnetically stirred for 6 Hr to get the homogeneous solution and aged for 72 Hr. The samples were collected through centrifugation and dried over night at 100˚C to get the final product (CuOSDS). The same methodology was adopted to prepare other copper oxide nanostructures utilizing different surfactants such as CTAB and PEG 4000. These samples are named as CuOCTAB, CuOPEG.
Cs =
I Δt (mAhg − 1) m3.6
(1)
Where Cs, I (mA), Δt (s), and m (mg) are the specific capacitance, current, discharge time, and mass of the electro active material respectively.
Fig. 1. (a) XRD spectrum of CuObare, CuOsds, CuOctab and CuOpeg samples. (b) FT-IR spectrum of CuObare, CuOsds, CuOctab and CuOpeg samples. 2
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
Results and discussion Then structure of the CuO nanostructures were confirmed with Xray diffraction technique. Fig. 1a displays the X-ray diffraction patterns of CuO samples synthesized using different surfactants. The sharp dif− − fraction peaks observed at (1 1 0), (0 0 2), (111), (2 02), (0 2 0), (2 0 2), − − − (113), (3 11), (2 2 0), (3 1 1) and (2 22) indexed to monoclinic structure (JCPDS No: 80–0076) with lattice parameters of a = 4.688 A˚, b = 3.423 A˚, and c = 5.132 A˚. In addition, the absence of any other impurity peaks indicating the purity of the prepared samples. To further identify the surface functional groups present on the CuO, FT-IR analysis was performed and presented in Fig. 1b. The bands centred at 434, 535 and 621 cm−1 is characteristic peaks of CuO and attributed to Cu–O Stretching vibrations [11]. The band centred at 1657 and 3442 cm−1 corresponds to O–H stretching vibrations [12,13]. It is due to the presence of trace amount of OH in the samples. Moreover, there is no other prominent band identified in the spectrum represents to good interaction of surfactants with source materials used during synthesis. It shows the better quality of the sample, which is most favourable for SC electrode applications. The understand the influence of surfactants on morphology of CuO samples scanning electron microscopy (SEM) analysis was employed and respective SEM images are presented in Fig. 2(a-h). The CuO prepared without surfactant shows uneven coral like morphology (Fig. 2a, b). The variation of structures is due to the influence of the surfactants. The uneven coral like morphology of CuObare is due to the absence of surfactant, which leads to the very faster growth in different orientation. Further, the use of SDS leads the leaf like morphology (Fig. 2c, d) and PEG provides ball kind of morphology with size of ∼1–3 μm (Fig. 2e, f). The use of CTAB created particles with size ranges from ∼100 to 400 nm (Fig. 2g, h).The surfactants present in the solution forms micelles and may act as seeds for nucleation. These seeds further assist the aggregation in different orientations. It’s a natural tendency of surfactants (SDS, PEG and CTAB) having different degree of aggregation leads the different structures. It is reasonable to expect that the variation of CuO morphologies influence its electrochemical features. Fig. 3a presents cyclic voltammetric (CV) curves of CuObare, CuOsds, CuOpeg and CuOctab electrodes in 1 M Na2SO4 electrolyte solution at a scan rate of 5 mVs−1. The CV curves show the distorted rectangular shape with well-defined anodic and cathodic peaks, which indicates the influence of redox behaviour in the electrodes.The presence of redox peaks suggesting the material possesses battery like characteristics. CV curves of the samples at different scan rates (5-100mVs−1) are presented in Fig. S1. Here it is noted that the small shift in anodic and cathodic peaks of CuObare electrode compared to the other electrodes. This kind of trend may attribute due to the presence of voids and lesser active sites for electrolyte ion intercalation [14]. The anodic peak potential is changed towards to positive potential and the cathodic peak potential changed towards the negative potential suggesting the quasi reversible behaviour of redox reactions [15]. The GCD profiles of CuObare, CuOsds, CuOpeg and CuOctab electrodes at a current density of 1 Ag−1 are presented in Fig. 3b. The specific capacity values are derived from GCD curves are 22 mAhg−1 (193 Fg−1), 51 mAhg−1 (456 Fg−1), 39 mAhg−1 (350 Fg−1) and 27 mAhg−1 (306 Fg−1) for CuObare, CuOsds, CuOpeg and CuOctab electrodes respectively. It is valuable to note that, CuOsds electrode exhibits higher capacitance compared to other CuO electrodes. It’s meaningful to mention here that the charging time is greater than the discharging time. This drop (iR drop) in potential is due to the different contributions of the internal resistance of the electrode material including electrolyte ion migration resistance and bulk resistance of the electrolyte [6]. It is attributed to the leaf like morphology, which may provide more electro active sites for electrolyte ion intercalation than other morphologies presented in this work. This higher value of capacitance
Fig. 2. SEM micrographs of CuObare (a, b), CuOsds (c, d) CuOpeg (e, f) and CuOctab (g, h).
is comparable with recent works including bare CuO nanostructures and many CuO based composites. For instance, CuO nanorod/carbon nanofiber (398 Fg−1 at 1Ag−1) [16] CuO/rGO nanostructures (340 Fg−1 at 0.5 Ag−1) [16], CuO nanoflowers (130 Fg−1 at 1Ag−1) [15] CuO/rGO nanostructures (326 Fg−1 at 0.5 Ag−1) [18], CuO/rGO (80 Fg−1 at 1 mAcm−2) [19], and CuOmicrosphers/ rGO (244 Fg−1 at 0.125 Ag−1) [20] showing the lower capacitance compared to CuOsds sample. Fig. 3c presents the variation of specific capacitance with current density. It is noted that CuOsds electrode holds 31.25 mAhg−1 (282 Fg−1, 62% of initial capacitance) at a higher current density of 10 Ag−1.This rate performance of this as prepared CuOsdsexhibits better rate performance than other CuO based systems including CuO-rGO (170 Fg−1 at 5Ag−1) [17], CuO-Microspheres -rGO (200 at 0.5Ag−1) [20], CuO/Co3O4 (209 Fg−1 at 10 Ag−1) [21], CuO nanosheets (232 Fg−1 at 2 mA/cm2) [22], polypyrrole/CuO (92 Fg−1 at 10 Ag−1) [23], Diatom@CuO@MnO2 hybrid (134 at 5Ag−1) [24]. Further, the specific capacitances of the all electrodes are decreasing with increasing of current density. At higher rates the electrolyte ions do not have ample
3
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
Fig. 3. (a) CV curves (vs Ag/AgCl) of CuObare, CuOsds, CuOpeg and CuOctab samples at a scan rate of 5 mVs−1. (b) GCD profiles of (vs Ag/AgCl) of CuObare, CuOsds, CuOpeg and CuOctab samples at a current density of 1 Ag−1. (c) Variation of specific capacity with current density. (d) Nyquist plot of CuObare, CuOsds, CuOpeg and CuOctab samples.
CuObare, CuOsds, CuOpegandCuOctabelectrodes. The solution resistance (Rs) is originated from the internal ohmic resistance of the electrolyte [28]. The measured Rs values are 2, 2, 2.7 and 4.1 for CuObare, CuOsds, CuOpeg and CuOctab electrodes. At lower frequencies the Warburg (W) slope remains near 45˚ indicating the better capacitive nature of the electrode [29]. It is valuable to note that the lower Rs and RCT value of the CuOsds electrode is one among the reason for its high specific capacitance. It is interesting to note that CuOsds electrode show better electrochemical performance compared to all other electrodes. We have further explored the reason behind this better performance of CuOsds electrode by using TEM, BET surface area and pore size distribution (PSD) measurements. TEM images of CuOsds (Fig. 4a, b) indicating the presence of well-defined leaf morphology. These two-dimensional leaves may offer additional electro active sites for electrolyte ion intercalation. Further, the N2 adsorption- desorption analysis was performed to understand the porosity of the CuOsds sample. The BJH model was used to calculate pore size distribution. The isotherm of CuOsds (Fig. 4c) have capillary condensation with a mild slope, indicating the presence of small pores in the CuOsds and the CuOsdsbelonging to type IV isotherm based on IUPAC classification [30,31]. The pore sizes range between 2 and 50 nm are classified as mesopores. The presence of mesopores in the sample is highly beneficial to improve the electrochemical features of the electrode active material owing to the better intercalation–deintercalation into the interior part of the electrode materials [32]. Inset of the Fig. 4c presents the PSD curve of CuOsds
time to approach the interior part of the electro active material. But, the electrolyte ions may get enough time to access the electrode completely at lower current density leads the higher capacitance [25]. The cyclic stability test was carried out for all electrodes at a current density of 10Ag-1up to 2000 repeated charge – discharge cycles (Fig. S3). It is noted that the CuOsds electrode retains 81% of initial capacitance after 1000 cycles and 52% after 2000 cycles, which is higher than the other electrodes. Charge transport kinetics of the electro active materials was investigated using electrochemical impedance spectroscopy (EIS) analysisin the frequency range of 0.01 Hz to 100 KHz. The respective Nyquist plot and equivalent circuit used to fit the experimental data for all CuO electrodes is presented in Fig. 3d and Fig. S4. In this circuit model (Fig S4), electrolyte ohmic resistance is mentioned as RS. The charge transfer resistance (RCT) attached parallelto the double layer capacitance CDL to represent the lower left segment of the curve (higher frequency region). Then the semi-circle in the high frequency areaowes to charge transfer resistance (RCT) between electrolyte and electrode [26]. Further the inclination of the spectra with real axis is mentioned as leakage resistance (RL) combined with mass capacitance CL. The vertical lines nearly parallel to the imaginary axis of the spectra suggest the ideal capacitive nature of the electrodes [27]. The semicircle present in the high frequency region facilitates the information reading charge transfer resistance (RCT) at electrode-electrolyte interface. The observed RCT values are 7.8, 1.6, 1.8 and 2.4 for 4
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
agreement between Cape Breton University, Nova Scotia Canada and Dr. Mahalingam College of Engineering and Technology (MCET), Tamil Nadu, India. The authorsacknowledge the support from NachimuthuIndustrial Association and its Secretary Dr. C. Ramaswamy. Authors are also indebted to Ms Judy MacInnes for technical support in the lab, especially on Cryo-TEM. This work was partly financed through NSERC, CFI, Public Works and Government Service Canada (formally Devco arm of ECBC) to the Industrial Research Chair of Mine Water Management at CBU, ACOA and IRAP grants. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2019.102185. References [1] Wang K, Wu H, Meng Y, Wei Z. Conducting polymer nanowire arrays for high performance supercapacitors. Small 2011;10(1):14–31. [2] Kong S, Cheng K, Ouyang T, Gao Y, Ye K, Wang G, et al. Facile electrodepositing processed of RuO2-graphene nanosheets-CNT composites as a binder-free electrode for electrochemical supercapacitors. Electrochim Acta 2017;246:433–42. [3] Purushothaman KK, Saravanakumar B, Muralidharan G, Dhanashankar M. Design of additive free 3D floral shaped V2O5@ Ni foam for high performance supercapacitors. Mater Technol 2017;32(9). https://doi.org/10.1080/10667857.2017. 1311526. [4] Vijayakumar S, Nagamuthu S, Muralidharan G. Supercapacitor studies on NiO nanoflakes synthesized through a microwave route. ACS Appl Mater Interfaces 2013;5(6):2188–96. [5] Sethuraman B, Purushothaman KK, Muralidharan G. Synthesis of mesh-like Fe2O3/C nanocomposite via greener route for high performance supercapacitors. RSC Adv 2014;4:4631–7. [6] Pendashteh A, Mousavia MF, Rahmanifar MS. Fabrication of anchored copper oxide nanoparticles on graphene oxide nanosheets via an electrostatic coprecipitation and its application as supercapacitor. Electrochim Acta 2013;88:347–57. [7] Wu Y, Xu M, Chen X, Yang S, Wu H, Pan J, et al. CTAB-assisted synthesis of novel ultrathin MoSe2nanosheets perpendicular to graphene for the adsorption and photo degradation of organic dyes under visible light. Nanoscale 2016;8:440. [8] Miao R, Zeng W, Gao Q. SDS-assisted hydrothermal synthesis of NiO flake-flower architectures with enhanced gas-sensing properties. App Sur Sci 2016;384:304–10. [9] Marichi RB, Sahu S, Lalwani S, Mishra M, Gupta G, Sharma RK, et al. Nickel-shell assisted growth of nickel-cobalt hydroxide nanofibres and their symmetric/asymmetric supercapacitive characteristics. J Power Sources 2016;325:762–71. [10] Zhong C, Deng Y, Hu W, Qiao J, Zhang L, Zhang J. A review of electrolyte materials and compositions for electrochemical supercapacitors. Chem Soc Rev 2015;44:7484–539. [11] Husnain SM, Uma W, Chang YY, Chang YS. Recyclable superparamagnetic adsorbent based on mesoporous carbon for sequestration of radioactive Cesium. Chem Engg J 2013;308:798–808. [12] Yu Q, Huang H, Chen R, Wang P, Yang H, Gao M, et al. Synthesis of CuOnanowalnuts and nanoribbons from aqueous solution and their catalytic and electrochemical properties. Nanoscale 2012;4:2613–20. [13] Ethiraj AS, JoonKang D. Synthesis and characterization of CuO nanowires by a simple wet chemical method. Nanoscale Res Lett 2012;7:70–5. [14] Lu Y, Yan H, Qiu K, Cheng J, Wang W, Liu X, et al. Hierarchical porous CuO nanostructures with tunable properties for high performance supercapacitors. RSC Adv 2015;5:10773. [15] Heng B, Qing C, Sun D, Wang B, Wang H, Tang Y. Rapid synthesis of CuO nanoribbons and nanoflowers from the same reaction system, and a comparison of their supercapacitor performance. RSC Adv 2013;3:15719–26. [16] Moosavifard SE, Shamsi J, Fanic S, Kadkhodazade S. Facile synthesis of hierarchical CuO nanorod arrays on carbon nanofibers for high-performance supercapacitors. Ceram Int 2014;40:15973–9. [17] Li Y, Ye K, Cheng K, Cao D, Pan Y, Kong S, et al. Anchoring CuO nanoparticles on nitrogen-doped reduced graphene oxide nanosheets as electrode material for supercapacitors. J Electro Anal Chem 2014;727:154–62. [18] Purushothaman KK, Saravanakumar B, Babu IM, Sethuraman B, Muralidharan G. Nanostructured CuO/reduced graphene oxide composite for hybrid supercapacitors. RSC Adv 2014;4:23485–91. [19] Bu IYY, Huang R. Fabrication of CuO-decorated reduced graphene oxide nanosheets for supercapacitor applications. Ceram Int 2017;43:45–50. [20] Saraf M, Dar RA, Natarajan K, Srivastava AK, Mobin SM. A Binder-Free hybrid of CuO-microspheres and rGO nanosheets as an alternative material for next generation energy storage application. Chem Select 2016;1:2826–33. [21] Kim HJ, Kim SY, Lim LJ, Reddy AE, Muralee Gopi CVV. Facile one-step synthesis of a composite CuO/Co3O4 electrode material on Ni foam for flexible supercapacitor applications. New J Chem 2017;41:5493–7. [22] Gund GS, Dubal DP, Dhawale DS, Shinde SS, Lokhande CD. Porous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitors. RSC Adv 2013;3:24099–107.
Fig. 4. (a, b) TEM images of CuOsds; (c) N2 adsorption and desorption measurements of CuOsds and inset showing respective the pore size distribution curve.
possesses pore size distribution maxima at 2.1 and 2.4 nm. Presence of mesopores in the CuOsds sample is facilitates the better electrochemical performance. In addition, this material exhibits BET surface area of 3.29 m2/g. Even though this material possesses lesser surface area, showing better electrochemical performance due to the presence of mesopores and well-defined morphology present in the sample. Conclusions This study has shown that CuO nanostructures can be fabricated using simple and cost-effective methods, but still maintain superior characteristics desired in SC electrode materials. The trick lies in the surfactants used during synthesis because it influences the morphology, physiochemical and electrochemical properties CuO nanostructures. In the current study, the CuO prepared using SDS exhibits best supercapacitive features compared to those prepared with other surfactants. For instance, the SDS-prepared CuO nanostructures has especially better performance rate and lower value of charge transfer resistance than all others, and further exceeding well known TMOs electrode material reported in previously. Thus, this study has demonstrated that affordable material of CuO nanostructures can be used to produce high performing SC electrodes through simple and cheap preparation method and choice of appropriate surfactant. Conflicts of interest There are no conflicts to declare. Acknowledgements This work has been done within the scope of collaboration 5
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
[23] Majumder M, Choudhary RB, Thakur AK, Karbhal I. Impact of rare-earth metal oxide (Eu2O3) on the electrochemical properties of a polypyrrole/CuO polymeric composite for supercapacitor applications. RSC Adv 2017;7:20037–48. [24] Zhang Y, Guo WW, Zheng TX, Zhang YX, Fan X. Engineering hierarchical Diatom@CuO@MnO2 hybrid for high performance supercapacitor. Appl Surf Sci 2018;427:1158–65. [25] Saravanakumar B, Purushothaman KK, Muralidharan G. MnO2 grafted V2O5 nanostructures: formation mechanism, morphology and supercapacitive features. CrystEngComm 2014;16:10711–20. [26] Masarapu C, Zeng HF, Hung KH, Wei B. Effect of temperature on the capacitance of carbon nanotube supercapacitors. ACS Nano 2009;3:2199–206. [27] Saravanakumar B, Purushothaman KK, Muralidharan G. Interconnected V2O5 nanoporous network for high-performance supercapacitors. ACS Appl Mater Interfaces 2012;4:484–4490. [28] Raut SS, Sankapal BR, Hossain MdSA, Pradhan S, Salunkhe RR, Yamauchi Y. Zinc
[29]
[30]
[31]
[32]
6
ferrite anchored multiwalled carbon nanotubes for high performance supercapacitor Applications. Eur J Inorg Chem 2018:137–42. Sharma V, Singh I, Chandra A. Hollow nanostructures of metal oxides as next generation electrode materials for supercapacitors. Sci Rep 2018;8:1307. https:// doi.org/10.1038/s41598-018-19815-y. Cheng J, Wang B, Xin HL, Yang G, Cai H, Nie F, et al. Self-assembled V2O5 nanosheets/reduced graphene oxide hierarchical nanocomposite as a highperformance cathode material for lithium ion batteries. J Mater Chem A 2013;1:10814–20. Xiong W, Liu M, Gan L, Lv Y, Li Y, Yang L, et al. novel synthesis of mesoporous carbon microspheres for supercapacitor electrodes. J Power Sources 2011;196:10461–4. Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nat Mater 2008;20:845–54.
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Surfactant determines the morphology, structure and energy storage features of CuO nanostructures
T
Balakrishnan Saravanakumara,b, Chandran Radhakrishnanb,c, Murugan Ramasamyd, ⁎ Rajendran Kaliaperumalb, Allen J. Brittenb, Martin Mkandawireb, a
Department of Physics, Dr. Mahalingam College of Engineering and Technology, Pollachi, Tamilnadu 642 003, India Chemistry Department, Cape Breton University, 1250 Grand Lake Rd. Sydney, Nova Scotia B1P 6L2, Canada c Department of Autombile Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi, Tamilnadu 642 003, India d National Centre for Earth Science Studies, Ministry of Earth Sciences, Govt. of India, Thiruvananthapuram, Kerala 695 011, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Copper oxide Neutral electrolyte Surfactant Supercapacitor
In quest for cost-affordable but high performing supercapacitor (SC), we explored fabrication of electrode material of copper oxide (CuO) nanostructures using simple solution-based synthesis procedure and different surfactants. Here, we report the influence of the surfactants on the morphological and structural evolutions as well as energy-storage capacity of the CuO. As an SC electrode material, CuO exhibits considerably high specific capacity (51 mAhg−1 @ 1Ag−1), good rate performance (31 mAhg−1 @ 10Ag−1) and better cyclic stability. In addition, it has low charge transfer resistance (1.6 Ω), which is very important when performing charge–discharge at high current rates. These results are highlighting the way for its use in design of advanced CuObased supercapacitor devices.
Introduction The mounting demand for highly improved Electrochemical Energy Storage (EES) devices is compelling researchers to innovate new material architectures. Among various energy storing mechanisms, Supercapacitors (SCs) have created significant impact on the overall development of EES technologies over last decade owing to their rapid charge-discharge ability, high power and excellent cycle life [1]. Presently, SCs are utilized as complementary device with batteries to facilitate additional peak power during higher power surge in many applications including hybrid electrical vehicles (HEV), wind turbine blade pitching systems, and optical zooming and flashing in camera. Generally, an SC device comprises of three prime components, namely: electrodes; separator; and, electrolyte. Among these, the electrochemical properties and morphology of an electrode are key to the overall efficiency of an SC device. SCs are commonly categorised as either electric double layer capacitors (EDLC) or pseudocapacitors, based on the charge storage process in the electrode active material. The EDLC store electric charges through adsorption of ion at electrolyte and electrode interface. In contrast, pseudocapacitors chemically store the charges through redox reaction in a few nanometres at the surface of the active material. The EDLC type utilizes carbon related materials (e.g., activated carbon, ⁎
graphene, CNT, and etc.) as electrode active material, while the pseudocapacitors have conducting polymers (e.g., PEDOT, PPY and etc.) and transition metal oxides (RuO2, V2O5, MnO2, NiO, Fe2O3, etc) as electrode materials [1–5]. To attain worthy enhancement in SC performance by means of innovative electrode material with electrochemically efficient sites, good number of pathways/channels for ion intercalation, eco-friendly production, cost effectiveness and good understanding of ion storage mechanism are necessary. Among various transitional metal oxides (TMOs) tested for SC electrode applications, copper oxide (CuO) is one among the best candidate due to its natural abundance, cost effectiveness and easy preparation procedures. Regardless of these attractive features, CuO has relatively high material volume expansion and poor electrical conductivity, which harms the capacity of the material as an electrode in SC. These issues may be overcome through the controlling the morphology of the CuO nanostructures [6]. Here, surfactants play an essential role in cost effective synthesis of nanostructured materials. It acts as a structure directing agents and leads different nanostructures including nanosheets, nanoflower, and nanofibers [7–9]. Electrolyte is also an important component, determining the overall performance of any SC device. Specifically, neutral electrolytes have several application advantages because of having high ionic conductivity and being non-flammable and inexpensive. Na+ (0.358 nm,
Corresponding author. E-mail address: [email protected] (M. Mkandawire).
https://doi.org/10.1016/j.rinp.2019.102185 Received 5 March 2019; Accepted 7 March 2019 Available online 09 March 2019 2211-3797/ © 2019 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
50.1 cm2/Ω mol) ions possess thin ionic radius and higher ionic conductivity compared to Li+ (0.38 nm, 38.6 cm2/Ω mol) [10]. Hence, it is reasonable to expect that an SC, utilizing eco-friendly Na2SO4 electrolyte, should show better supercapacitive features. Many researchers have investigated effects of different surfactants on synthesis and characterization of CuO nanostructures, but only few works have elaborated their complete electrochemical properties. Furthermore, most of the works do not report performance rate of the electrode material, while some of those reported show very poor performance rate (See Table S1, supporting data). Owing to this, we investigated the performance rates of different electrochemical morphologies of CuO nanostructures, synthesized using different surfactants (anionic, cationic and non-ionic) such as sodium dodecyl sulphate (SDS), cetyl trimethylammonium bromide (CTAB) and Poly Ethylene Glycol (PEG 4000) as structure directing agents. The influence of these agents on structure, morphology and electrochemical performance are investigated. When tested as a SC electrode active material in Na2SO4 electrolyte, CuO prepared using SDS showed superior electrochemical features, like higher capacitance, better rate performance and good cycle life than the others. Details regarding synthesis, physiochemical and electrochemical investigations are given in the supporting information.
Material characterization techniques The crystallographic information’s of the prepared samples were analyzed using using Bruker D8 Advance X-Ray diffractometer equipped with CuKα radiation (λ = 1.5406 Å) 10˚ to 80˚. The FT-IR spectrum of the samples were recorded using NICOLET 6700 spectrometer. The morphological features of the samples were analyzed using TESCAN VEGA 3 LMU scanning electron microscope and Hitachi transmission electron microscope. The Brunauer-Emmett-Teller (BET) method was used to determine the specific surface area of the samples. The nitrogen adsorption-desorption measurements were performed by Micrometeritics ASAP 2020 analyzer. The porosity distributions were derived from desorption part of the isotherm using the Barrett-JoynerHalenda (BJH) method.
Electrochemical measurements Electrochemical measurements were accomplished using three electrode configuration in electrochemical workstation (CHI 660 D, CH instruments Inc., USA). The Ag/AgCl and a platinum wire were used as reference and counter electrodes respectively. The working electrode was prepared using 85 wt% of electrode active material, 10 wt% carbon black, and 5 wt% polytetrafluoroethylene (Sigma-Aldrich) binder was blended with few drops of ethanol. This slurry was pasted onto a nickel foam current collector (1 cm2). The nickel foam was cleaned using ultrasonicator with HCl (37 wt%) for few minutes to eliminate the oxide layer present in the nickel foam. In addition, it is also cleaned using ethanol and DI water. Further the electrode was dried at 80˚C for 4 hr. The electrochemical parameters of the electrodes were determined by cyclic voltammetry (CV), galvanostatic charge discharge (GCD), electrochemical impedance spectroscopy (EIS) in 1 M sodium sulphate (Na2SO4) aqueous electrolyte solution. The specific capacitance, energy density, power density values were estimated based on the mass of the electrode active material (0.8 mg) present in the electrode and following equations, GCD curves are utilized for estimation of specific capacitance from following relation:
Experimental Fabrication of CuO nanostructures Copper nitrate (Cu(NO3)2·3H2O), Sodium dodecyl sulphate (SDS), Poly Ethylene Glycol (PEG 4000), Cetrimonium bromide (CTAB), (30%) HCl and NaOH were procured from Sigma Aldrich. All chemicals and reagents were used as supplied without further purification and all solutions were made with Millipore Milli- Qwater (18 M-cm). In the first step, 0.906 g of Cu (NO3)2·3H2O was dissolved in 100 mL of H2O and stirred for an hour to get homogeneous solution. Further, 0.090 g of SDS was added to 25 mL of H2O and stirred for 30 min. In the second step, the SDS solution was slowly added to the copper nitrate solution followed by adding 2 M NaOH to make the pH ∼ 10. Finally, this solution was magnetically stirred for 6 Hr to get the homogeneous solution and aged for 72 Hr. The samples were collected through centrifugation and dried over night at 100˚C to get the final product (CuOSDS). The same methodology was adopted to prepare other copper oxide nanostructures utilizing different surfactants such as CTAB and PEG 4000. These samples are named as CuOCTAB, CuOPEG.
Cs =
I Δt (mAhg − 1) m3.6
(1)
Where Cs, I (mA), Δt (s), and m (mg) are the specific capacitance, current, discharge time, and mass of the electro active material respectively.
Fig. 1. (a) XRD spectrum of CuObare, CuOsds, CuOctab and CuOpeg samples. (b) FT-IR spectrum of CuObare, CuOsds, CuOctab and CuOpeg samples. 2
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
Results and discussion Then structure of the CuO nanostructures were confirmed with Xray diffraction technique. Fig. 1a displays the X-ray diffraction patterns of CuO samples synthesized using different surfactants. The sharp dif− − fraction peaks observed at (1 1 0), (0 0 2), (111), (2 02), (0 2 0), (2 0 2), − − − (113), (3 11), (2 2 0), (3 1 1) and (2 22) indexed to monoclinic structure (JCPDS No: 80–0076) with lattice parameters of a = 4.688 A˚, b = 3.423 A˚, and c = 5.132 A˚. In addition, the absence of any other impurity peaks indicating the purity of the prepared samples. To further identify the surface functional groups present on the CuO, FT-IR analysis was performed and presented in Fig. 1b. The bands centred at 434, 535 and 621 cm−1 is characteristic peaks of CuO and attributed to Cu–O Stretching vibrations [11]. The band centred at 1657 and 3442 cm−1 corresponds to O–H stretching vibrations [12,13]. It is due to the presence of trace amount of OH in the samples. Moreover, there is no other prominent band identified in the spectrum represents to good interaction of surfactants with source materials used during synthesis. It shows the better quality of the sample, which is most favourable for SC electrode applications. The understand the influence of surfactants on morphology of CuO samples scanning electron microscopy (SEM) analysis was employed and respective SEM images are presented in Fig. 2(a-h). The CuO prepared without surfactant shows uneven coral like morphology (Fig. 2a, b). The variation of structures is due to the influence of the surfactants. The uneven coral like morphology of CuObare is due to the absence of surfactant, which leads to the very faster growth in different orientation. Further, the use of SDS leads the leaf like morphology (Fig. 2c, d) and PEG provides ball kind of morphology with size of ∼1–3 μm (Fig. 2e, f). The use of CTAB created particles with size ranges from ∼100 to 400 nm (Fig. 2g, h).The surfactants present in the solution forms micelles and may act as seeds for nucleation. These seeds further assist the aggregation in different orientations. It’s a natural tendency of surfactants (SDS, PEG and CTAB) having different degree of aggregation leads the different structures. It is reasonable to expect that the variation of CuO morphologies influence its electrochemical features. Fig. 3a presents cyclic voltammetric (CV) curves of CuObare, CuOsds, CuOpeg and CuOctab electrodes in 1 M Na2SO4 electrolyte solution at a scan rate of 5 mVs−1. The CV curves show the distorted rectangular shape with well-defined anodic and cathodic peaks, which indicates the influence of redox behaviour in the electrodes.The presence of redox peaks suggesting the material possesses battery like characteristics. CV curves of the samples at different scan rates (5-100mVs−1) are presented in Fig. S1. Here it is noted that the small shift in anodic and cathodic peaks of CuObare electrode compared to the other electrodes. This kind of trend may attribute due to the presence of voids and lesser active sites for electrolyte ion intercalation [14]. The anodic peak potential is changed towards to positive potential and the cathodic peak potential changed towards the negative potential suggesting the quasi reversible behaviour of redox reactions [15]. The GCD profiles of CuObare, CuOsds, CuOpeg and CuOctab electrodes at a current density of 1 Ag−1 are presented in Fig. 3b. The specific capacity values are derived from GCD curves are 22 mAhg−1 (193 Fg−1), 51 mAhg−1 (456 Fg−1), 39 mAhg−1 (350 Fg−1) and 27 mAhg−1 (306 Fg−1) for CuObare, CuOsds, CuOpeg and CuOctab electrodes respectively. It is valuable to note that, CuOsds electrode exhibits higher capacitance compared to other CuO electrodes. It’s meaningful to mention here that the charging time is greater than the discharging time. This drop (iR drop) in potential is due to the different contributions of the internal resistance of the electrode material including electrolyte ion migration resistance and bulk resistance of the electrolyte [6]. It is attributed to the leaf like morphology, which may provide more electro active sites for electrolyte ion intercalation than other morphologies presented in this work. This higher value of capacitance
Fig. 2. SEM micrographs of CuObare (a, b), CuOsds (c, d) CuOpeg (e, f) and CuOctab (g, h).
is comparable with recent works including bare CuO nanostructures and many CuO based composites. For instance, CuO nanorod/carbon nanofiber (398 Fg−1 at 1Ag−1) [16] CuO/rGO nanostructures (340 Fg−1 at 0.5 Ag−1) [16], CuO nanoflowers (130 Fg−1 at 1Ag−1) [15] CuO/rGO nanostructures (326 Fg−1 at 0.5 Ag−1) [18], CuO/rGO (80 Fg−1 at 1 mAcm−2) [19], and CuOmicrosphers/ rGO (244 Fg−1 at 0.125 Ag−1) [20] showing the lower capacitance compared to CuOsds sample. Fig. 3c presents the variation of specific capacitance with current density. It is noted that CuOsds electrode holds 31.25 mAhg−1 (282 Fg−1, 62% of initial capacitance) at a higher current density of 10 Ag−1.This rate performance of this as prepared CuOsdsexhibits better rate performance than other CuO based systems including CuO-rGO (170 Fg−1 at 5Ag−1) [17], CuO-Microspheres -rGO (200 at 0.5Ag−1) [20], CuO/Co3O4 (209 Fg−1 at 10 Ag−1) [21], CuO nanosheets (232 Fg−1 at 2 mA/cm2) [22], polypyrrole/CuO (92 Fg−1 at 10 Ag−1) [23], Diatom@CuO@MnO2 hybrid (134 at 5Ag−1) [24]. Further, the specific capacitances of the all electrodes are decreasing with increasing of current density. At higher rates the electrolyte ions do not have ample
3
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
Fig. 3. (a) CV curves (vs Ag/AgCl) of CuObare, CuOsds, CuOpeg and CuOctab samples at a scan rate of 5 mVs−1. (b) GCD profiles of (vs Ag/AgCl) of CuObare, CuOsds, CuOpeg and CuOctab samples at a current density of 1 Ag−1. (c) Variation of specific capacity with current density. (d) Nyquist plot of CuObare, CuOsds, CuOpeg and CuOctab samples.
CuObare, CuOsds, CuOpegandCuOctabelectrodes. The solution resistance (Rs) is originated from the internal ohmic resistance of the electrolyte [28]. The measured Rs values are 2, 2, 2.7 and 4.1 for CuObare, CuOsds, CuOpeg and CuOctab electrodes. At lower frequencies the Warburg (W) slope remains near 45˚ indicating the better capacitive nature of the electrode [29]. It is valuable to note that the lower Rs and RCT value of the CuOsds electrode is one among the reason for its high specific capacitance. It is interesting to note that CuOsds electrode show better electrochemical performance compared to all other electrodes. We have further explored the reason behind this better performance of CuOsds electrode by using TEM, BET surface area and pore size distribution (PSD) measurements. TEM images of CuOsds (Fig. 4a, b) indicating the presence of well-defined leaf morphology. These two-dimensional leaves may offer additional electro active sites for electrolyte ion intercalation. Further, the N2 adsorption- desorption analysis was performed to understand the porosity of the CuOsds sample. The BJH model was used to calculate pore size distribution. The isotherm of CuOsds (Fig. 4c) have capillary condensation with a mild slope, indicating the presence of small pores in the CuOsds and the CuOsdsbelonging to type IV isotherm based on IUPAC classification [30,31]. The pore sizes range between 2 and 50 nm are classified as mesopores. The presence of mesopores in the sample is highly beneficial to improve the electrochemical features of the electrode active material owing to the better intercalation–deintercalation into the interior part of the electrode materials [32]. Inset of the Fig. 4c presents the PSD curve of CuOsds
time to approach the interior part of the electro active material. But, the electrolyte ions may get enough time to access the electrode completely at lower current density leads the higher capacitance [25]. The cyclic stability test was carried out for all electrodes at a current density of 10Ag-1up to 2000 repeated charge – discharge cycles (Fig. S3). It is noted that the CuOsds electrode retains 81% of initial capacitance after 1000 cycles and 52% after 2000 cycles, which is higher than the other electrodes. Charge transport kinetics of the electro active materials was investigated using electrochemical impedance spectroscopy (EIS) analysisin the frequency range of 0.01 Hz to 100 KHz. The respective Nyquist plot and equivalent circuit used to fit the experimental data for all CuO electrodes is presented in Fig. 3d and Fig. S4. In this circuit model (Fig S4), electrolyte ohmic resistance is mentioned as RS. The charge transfer resistance (RCT) attached parallelto the double layer capacitance CDL to represent the lower left segment of the curve (higher frequency region). Then the semi-circle in the high frequency areaowes to charge transfer resistance (RCT) between electrolyte and electrode [26]. Further the inclination of the spectra with real axis is mentioned as leakage resistance (RL) combined with mass capacitance CL. The vertical lines nearly parallel to the imaginary axis of the spectra suggest the ideal capacitive nature of the electrodes [27]. The semicircle present in the high frequency region facilitates the information reading charge transfer resistance (RCT) at electrode-electrolyte interface. The observed RCT values are 7.8, 1.6, 1.8 and 2.4 for 4
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
agreement between Cape Breton University, Nova Scotia Canada and Dr. Mahalingam College of Engineering and Technology (MCET), Tamil Nadu, India. The authorsacknowledge the support from NachimuthuIndustrial Association and its Secretary Dr. C. Ramaswamy. Authors are also indebted to Ms Judy MacInnes for technical support in the lab, especially on Cryo-TEM. This work was partly financed through NSERC, CFI, Public Works and Government Service Canada (formally Devco arm of ECBC) to the Industrial Research Chair of Mine Water Management at CBU, ACOA and IRAP grants. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2019.102185. References [1] Wang K, Wu H, Meng Y, Wei Z. Conducting polymer nanowire arrays for high performance supercapacitors. Small 2011;10(1):14–31. [2] Kong S, Cheng K, Ouyang T, Gao Y, Ye K, Wang G, et al. Facile electrodepositing processed of RuO2-graphene nanosheets-CNT composites as a binder-free electrode for electrochemical supercapacitors. Electrochim Acta 2017;246:433–42. [3] Purushothaman KK, Saravanakumar B, Muralidharan G, Dhanashankar M. Design of additive free 3D floral shaped V2O5@ Ni foam for high performance supercapacitors. Mater Technol 2017;32(9). https://doi.org/10.1080/10667857.2017. 1311526. [4] Vijayakumar S, Nagamuthu S, Muralidharan G. Supercapacitor studies on NiO nanoflakes synthesized through a microwave route. ACS Appl Mater Interfaces 2013;5(6):2188–96. [5] Sethuraman B, Purushothaman KK, Muralidharan G. Synthesis of mesh-like Fe2O3/C nanocomposite via greener route for high performance supercapacitors. RSC Adv 2014;4:4631–7. [6] Pendashteh A, Mousavia MF, Rahmanifar MS. Fabrication of anchored copper oxide nanoparticles on graphene oxide nanosheets via an electrostatic coprecipitation and its application as supercapacitor. Electrochim Acta 2013;88:347–57. [7] Wu Y, Xu M, Chen X, Yang S, Wu H, Pan J, et al. CTAB-assisted synthesis of novel ultrathin MoSe2nanosheets perpendicular to graphene for the adsorption and photo degradation of organic dyes under visible light. Nanoscale 2016;8:440. [8] Miao R, Zeng W, Gao Q. SDS-assisted hydrothermal synthesis of NiO flake-flower architectures with enhanced gas-sensing properties. App Sur Sci 2016;384:304–10. [9] Marichi RB, Sahu S, Lalwani S, Mishra M, Gupta G, Sharma RK, et al. Nickel-shell assisted growth of nickel-cobalt hydroxide nanofibres and their symmetric/asymmetric supercapacitive characteristics. J Power Sources 2016;325:762–71. [10] Zhong C, Deng Y, Hu W, Qiao J, Zhang L, Zhang J. A review of electrolyte materials and compositions for electrochemical supercapacitors. Chem Soc Rev 2015;44:7484–539. [11] Husnain SM, Uma W, Chang YY, Chang YS. Recyclable superparamagnetic adsorbent based on mesoporous carbon for sequestration of radioactive Cesium. Chem Engg J 2013;308:798–808. [12] Yu Q, Huang H, Chen R, Wang P, Yang H, Gao M, et al. Synthesis of CuOnanowalnuts and nanoribbons from aqueous solution and their catalytic and electrochemical properties. Nanoscale 2012;4:2613–20. [13] Ethiraj AS, JoonKang D. Synthesis and characterization of CuO nanowires by a simple wet chemical method. Nanoscale Res Lett 2012;7:70–5. [14] Lu Y, Yan H, Qiu K, Cheng J, Wang W, Liu X, et al. Hierarchical porous CuO nanostructures with tunable properties for high performance supercapacitors. RSC Adv 2015;5:10773. [15] Heng B, Qing C, Sun D, Wang B, Wang H, Tang Y. Rapid synthesis of CuO nanoribbons and nanoflowers from the same reaction system, and a comparison of their supercapacitor performance. RSC Adv 2013;3:15719–26. [16] Moosavifard SE, Shamsi J, Fanic S, Kadkhodazade S. Facile synthesis of hierarchical CuO nanorod arrays on carbon nanofibers for high-performance supercapacitors. Ceram Int 2014;40:15973–9. [17] Li Y, Ye K, Cheng K, Cao D, Pan Y, Kong S, et al. Anchoring CuO nanoparticles on nitrogen-doped reduced graphene oxide nanosheets as electrode material for supercapacitors. J Electro Anal Chem 2014;727:154–62. [18] Purushothaman KK, Saravanakumar B, Babu IM, Sethuraman B, Muralidharan G. Nanostructured CuO/reduced graphene oxide composite for hybrid supercapacitors. RSC Adv 2014;4:23485–91. [19] Bu IYY, Huang R. Fabrication of CuO-decorated reduced graphene oxide nanosheets for supercapacitor applications. Ceram Int 2017;43:45–50. [20] Saraf M, Dar RA, Natarajan K, Srivastava AK, Mobin SM. A Binder-Free hybrid of CuO-microspheres and rGO nanosheets as an alternative material for next generation energy storage application. Chem Select 2016;1:2826–33. [21] Kim HJ, Kim SY, Lim LJ, Reddy AE, Muralee Gopi CVV. Facile one-step synthesis of a composite CuO/Co3O4 electrode material on Ni foam for flexible supercapacitor applications. New J Chem 2017;41:5493–7. [22] Gund GS, Dubal DP, Dhawale DS, Shinde SS, Lokhande CD. Porous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitors. RSC Adv 2013;3:24099–107.
Fig. 4. (a, b) TEM images of CuOsds; (c) N2 adsorption and desorption measurements of CuOsds and inset showing respective the pore size distribution curve.
possesses pore size distribution maxima at 2.1 and 2.4 nm. Presence of mesopores in the CuOsds sample is facilitates the better electrochemical performance. In addition, this material exhibits BET surface area of 3.29 m2/g. Even though this material possesses lesser surface area, showing better electrochemical performance due to the presence of mesopores and well-defined morphology present in the sample. Conclusions This study has shown that CuO nanostructures can be fabricated using simple and cost-effective methods, but still maintain superior characteristics desired in SC electrode materials. The trick lies in the surfactants used during synthesis because it influences the morphology, physiochemical and electrochemical properties CuO nanostructures. In the current study, the CuO prepared using SDS exhibits best supercapacitive features compared to those prepared with other surfactants. For instance, the SDS-prepared CuO nanostructures has especially better performance rate and lower value of charge transfer resistance than all others, and further exceeding well known TMOs electrode material reported in previously. Thus, this study has demonstrated that affordable material of CuO nanostructures can be used to produce high performing SC electrodes through simple and cheap preparation method and choice of appropriate surfactant. Conflicts of interest There are no conflicts to declare. Acknowledgements This work has been done within the scope of collaboration 5
Results in Physics 13 (2019) 102185
B. Saravanakumar, et al.
[23] Majumder M, Choudhary RB, Thakur AK, Karbhal I. Impact of rare-earth metal oxide (Eu2O3) on the electrochemical properties of a polypyrrole/CuO polymeric composite for supercapacitor applications. RSC Adv 2017;7:20037–48. [24] Zhang Y, Guo WW, Zheng TX, Zhang YX, Fan X. Engineering hierarchical Diatom@CuO@MnO2 hybrid for high performance supercapacitor. Appl Surf Sci 2018;427:1158–65. [25] Saravanakumar B, Purushothaman KK, Muralidharan G. MnO2 grafted V2O5 nanostructures: formation mechanism, morphology and supercapacitive features. CrystEngComm 2014;16:10711–20. [26] Masarapu C, Zeng HF, Hung KH, Wei B. Effect of temperature on the capacitance of carbon nanotube supercapacitors. ACS Nano 2009;3:2199–206. [27] Saravanakumar B, Purushothaman KK, Muralidharan G. Interconnected V2O5 nanoporous network for high-performance supercapacitors. ACS Appl Mater Interfaces 2012;4:484–4490. [28] Raut SS, Sankapal BR, Hossain MdSA, Pradhan S, Salunkhe RR, Yamauchi Y. Zinc
[29]
[30]
[31]
[32]
6
ferrite anchored multiwalled carbon nanotubes for high performance supercapacitor Applications. Eur J Inorg Chem 2018:137–42. Sharma V, Singh I, Chandra A. Hollow nanostructures of metal oxides as next generation electrode materials for supercapacitors. Sci Rep 2018;8:1307. https:// doi.org/10.1038/s41598-018-19815-y. Cheng J, Wang B, Xin HL, Yang G, Cai H, Nie F, et al. Self-assembled V2O5 nanosheets/reduced graphene oxide hierarchical nanocomposite as a highperformance cathode material for lithium ion batteries. J Mater Chem A 2013;1:10814–20. Xiong W, Liu M, Gan L, Lv Y, Li Y, Yang L, et al. novel synthesis of mesoporous carbon microspheres for supercapacitor electrodes. J Power Sources 2011;196:10461–4. Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nat Mater 2008;20:845–54.