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C: Energy Conversion and Storage; Energy and Charge Transport

Microwave-Assisted Solvothermal Synthesis of Cuprous Oxide Nanostructures for High-Performance Supercapacitor Abhaya Kumar Mishra, Arpan Kumar Nayak, Ashok Kumar Das, and Debabrata Pradhan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02210 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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The Journal of Physical Chemistry

Microwave-Assisted Solvothermal Synthesis of Cuprous Oxide Nanostructures for High-Performance Supercapacitor Abhaya Kumar Mishra, Arpan Kumar Nayak, Ashok Kumar Das, and Debabrata Pradhan* Materials Science Centre, Indian Institute of Technology, Kharagpur, W. B. 721 302, India

Abstract Enhancing the performance and stability of the low-cost materials for electrochemical energy storage device is an important aspect. Herein, we report microwave-assisted solvothermal synthesis of three-dimensional (3D) spherical CuO structures composed of either onedimensional (rod-like) or two-dimensional (2D) flake-like building blocks by varying the reaction medium, i.e., water and ethylene glycol (EG). A higher EG in the reaction medium facilitates formation of the flake-like structures. A specific surface area of 168.47 m2 g−1 is achieved with the 3D flower-like CuO, synthesized using copper acetate precursor in 1:3 water:EG solvent ratio. The same sample delivers a specific capacitance of 612 F g−1 at an applied current density of 1 A g−1 and shown high stability with capacity retention of 98% after 4000 galvanostatic charge-discharge cycles. The high specific capacitance of flower-shaped CuO architecture is attributed to large surface area and availability of sufficient pores for ions diffusion. Furthermore, two-electrode asymmetric supercapacitor (ASC) device is fabricated using the 3D flower-shaped CuO as positive electrode and activated carbon as negative electrode, which shows an energy density of 27.27 Wh kg−1 at a power density of 800 W kg−1. This underlines the potential of inexpensive CuO architecture as an active material for energy storage devices.

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1. INTRODUCTION The ever-growing energy demands and depletion of the fossil fuels have stimulated intense research on the development of sustainable, efficient, clean energy conversion and storage devices.

1 , 2

In this regard, fuel cells, batteries, and supercapacitors are the important

electrochemical energy conversion or storage devices.3,4 Among these, supercapacitors are a new class of energy storage device with a property intermediated to that of capacitors and batteries, which have attracted tremendous attention in recent years.5 They deliver high power density (ten times more than that of batteries), fast charging (within a few seconds), and exhibit high-rate capability and cyclic stability. 6 These properties imply the importance of supercapacitors in cameras, electric vehicles, power back-up systems, pacemakers, and airbags.7,8 On the basis of general charge storage mechanism, supercapacitors are classified as (i) electrical double layer capacitor (EDLC) where the charge storage occurs due to non-faradaic capacitance arises from the charge separation in a Helmholtz double layer at the electrode/electrolyte interface9 and (ii) pseudocapacitor, which involves Faradaic electrostatic charge storage process with redox reaction to store the charges. The latter has gained great interest because of the high specific capacitance than that of EDLCs. 10 , 11 Moreover, all Faradaic process may not contribute to capacitive current and those noncapacitive energy-storage processes are called supercapattery as they show noncapacitive or battery-like charge-discharge characteristics. 12,13 Transition metal oxides are the most promising active electrode materials for the pseudocapacitors or supercapattery, as several of them have multivalent oxidation states for the active redox charge transfer.14,15 Ruthenium oxide (RuO2) and hydroxide have been extensively studied due to their high theoretical specific capacitance, wide potential window, and reversible redox reactions.16 However, high cost, toxic nature, and need of strong acidic electrolyte limit the large scale


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utilization of RuO2-based supercapacitor.17 Therefore, other transition metal oxides such as NiO, MnO2, Co3O4, VOx, and CuO have been focused as the low-cost alternatives for supercapacitor application.18,19,20,21,22,23 In addition to the abundance, nontoxicity, and ease synthesis process of CuO, it exhibits impressive electrochemical energy storage performance. Zhang et al. synthesized flower-like CuO nanostructure, which shows a specific capacitance of 133.6 F g−1 in KOH electrolyte, three-fold higher than that of commercial CuO powder. 24 Dubal et al. synthesized nanosheets, micro-roses, and micro-woolen-like nanostructures using chemical bath deposition with a specific capacitance of 303 F g−1, 279 F g−1, and 346 F g−1, respectively.25 Lu et al. demonstrated excellent electrochemical performance of graphene-like copper oxide nanofilms with a specific capacitance of 919 F g−1.26 Recently Moosavifard et al. found a specific capacitance of 431 F g−1 with nanoporous CuO electrode.27 These studies suggest the importance of CuO morphology to achieve high specific capacitance. In addition to the inherent properties of materials, the spatial arranagement of micro/nanostructrures of the electrode materials also play important role in the charge storage performance of the supercapacitors.28,29 Various approaches such as hydrothermal,30 thermal oxidation,31 and arc-discharge process32 have been employed to synthesize CuO nanostructures. However, these conventional methods generally require high temperature, high pressure, and relatively long duration. On the other hand, microwave-assisted hydrothermal/solvothermal method has been explored to synthesize nanostructures with diverse shapes and sizes. The advantages of the microwave synthesis technique include shorter reaction duration, i.e., in minutes instead of hours, fast heating rate which is eco-friendly and energy preserving, fast kinetics of crystallization and nucleation, better reproducibility, and higher yield of the products.33,34,35



In this article, a microwave-assisted solvothermal process is demonstrated to synthesize high surface area 3D CuO structures assembled of nanosize building blocks. The novelty of the present work lies in controlling the morphology of CuO building blocks by varying the reaction parameters such as copper salt precursors (copper acetate and copper nitrate) and reaction medium (water or water+EG mixture). The precursor salt along with different solvent proportions is found to play a significant role in controlling the morphology of the products. With an increase in the EG content in the solvent, the building blocks of 3D spherical CuO are found to be flake-like structures. These nanoflakes are self-assembled in a manner yielding enormous pores, a highly desirable trait for the improved electrochemical performance due to large electrode/electrolyte interfacial contact. Another importance of the present work is thus on the synthesis of CuO nanostructures with a large surface area of 168.47 m2g−1, which is higher than that reported for CuO porous nanostructured assemblies.33, 36 , 37 , 38 The electrochemical energy-storage (supercapacitor) performance of the as-synthesized CuO architectures is measured using both the three- and two-electrode configurations. In a two-electrode asymmetry supercapacitor (ASC) device, flower-like CuO is used as a positive electrode and activated carbon as a negative electrode material. The fabricated CuO-based ASC device exhibits excellent energy storage performance with an energy density of 27.17 Wh kg−1 at a power density of 800 W kg−1 and a high cycle stability with capacity retention of 98.47% after 4000 cycles.

2. EXPERIMENTAL 2.1 Chemicals. Copper nitrate trihydrate [Cu(NO3)2·3H2O], copper acetate monohydrate [Cu(CH3COO)2⋅H2O], ethylene glycol (EG), polyvinylidene fluoride (PVDF), potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium sulfate (Na2SO4) from Merck, India, urea


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(NH2CONH2) from Hi-media and lithium hydroxide monohydrate (LiOH⋅H2O) from Loba Chemicals, India. All these reagents were analytical grade and used without further purification. 2.2 Synthesis of CuO nanostructures. Different shapes and sizes of CuO nanostructures were synthesized by varying the precursor salts [Cu(OAc)2 and Cu(NO3)2] and solvents (water and EG with different volume ratios) using a microwave-assisted solvothermal technique. Typically, in a 100 mL beaker, 40 mL of solvent was taken. The solvent was either water or mixed solvent of water and EG at a varied volume proportion (Water:EG), i.e., 4:0, 3:1, 1:1, and 1:3. Then 1 mmol of copper precursor salt [Cu(OAc)2 or Cu(NO3)2] and 10 mmol (600 mg) of urea were added and stirred for 10 min, which resulted a deep green solution. The solution was then transferred into a 100 mL Teflon-lined vessel and the microwave-assisted solvothermal synthesis was carried out with a microwave reactor (Multiwave PRO, Anton-Paar, Austria). The microwave reactor was programmed to supply a maximum power of 200 W with a ramping rate 20 W per min and the reaction temperature was reached to ~140 °C. The reaction was allowed for 10 min by holding the microwave power at 200 W prior to reducing the power. The timedependent experiments were also performed by varying the microwave-irradiation duration from 1 min to 20 min. The precipitated product was isolated via centrifuging using distilled water. The powder product was finally dried in an air-oven at 60 °C for 8 h. 2.3 Material Characterization. The surface morphology of the powder products was examined with a MERLIN field emission scanning electron microscope (FESEM). The energy dispersive x-ray (EDX) analysis and mapping were performed with a FESEM (TESCAN LYRA3) attached with Oxford Instrument’s EDX detector. The structural property of the as-synthesized samples was probed with a PANalytical high-resolution powder x-ray diffractometer (XRD) (PW 3040/60, operated at 40 kV and 30 mA) using Cu Kα x-rays in the two theta angular range



20°−80°. The microstructure of the samples was analyzed by using a TECNAI G2 (FEI) transmission electron microscope (TEM) operated at 200 kV. The surface composition of a representative sample was studied by x-ray photoelectron spectroscopy (XPS) with a PHI5000 Versa Probe II XPS Microprobe with a monochromatic Al Kα source (1486.6 eV). The Brunauer-Emmett-Teller (BET) surface area of the as-synthesized powder products was measured using an Autosorb iQ2 BET surface analyzer (Quantachrome, USA). 2.4 Electrochemical Study. All the electrochemical analysis was performed using a CHI 760D electrochemical workstation (CH Instruments, Inc., USA) either in a three-electrode configuration or a two-electrode configuration. In three-electrode mode, active material (CuO) coated Ni-foam was used as working electrode, while saturated calomel electrode (SCE) and the platinum wire were used as the reference and counter electrodes, respectively. The working electrode material was prepared by mixing active material (different morphologies of CuO), acetylene black, and polyvinylidene fluoride (PVDF) in an agate mortar with a weight ratio of 70%, 15%, and 15%, respectively. To make homogeneous slurry of the mixture, 2 mL of 1methyl-2-pyrrolidone (NMP) was added to the preceding mixture, and the solution was grinded followed by sonication for 30 min. The prepared slurry was drop casted onto a Ni-foam of area of 2 cm × 1 cm and then dried at 80 °C for 6 h. Prior to drop-casting, the Ni-foam was cleaned in 3 M HCl to remove the oxide layer present on its surface. More specifically, for the electrode preparation, 7 mg of active electrode material (CuO) along with 1.5 mg of both acetylene black and PVDF were taken. The weight of Ni-foam was measured before and after the mass loading. The mass difference of approximately 10 mg confirmed the loading of the electrode material. To study the role of substrate, glassy carbon and graphite sheet (Nickunj Eximp Entp P. Ltd., India) substrates were also used instead of Ni-foam.


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3. RESULTS AND DISCUSSION 3.1 Crystal structure. The phase purity of the as-synthesized powder products was characterized by powder XRD. Figure 1 shows the XRD patterns of the powder products obtained with 1 mmol of Cu(OAc)2 precursor in different solvents, i.e., (a) water, (b) water:EG = 3:1, (c) water:EG =1:1, and (d) water:EG = 1:3. All the samples show similar XRD patterns and the diffraction peaks were readily indexed to the monoclinic symmetry of CuO that match JCPDS File No. 005-0661. 39 No possible impurity peak from Cu(OH)2 and/or Cu2O was observed. 40 Moreover, with increasing EG content in the solvent, the diffraction peaks were found to be broader, which is ascribed to decrease in the size of the nanostructured units in the spherical 3D morphology as observed in the morphology analysis (discussed later). The crystal structure of the sample was further probed by performing Raman analysis of a representative CuO sample synthesized with Cu(OAc)2 as precursor in 1:3 water:EG solvent. The Raman spectrum (Figure S1, Supporting Information) matches the monoclinic crystal structure of CuO with

6 C2h

space group.41,42 Out of twelve phonon branches at the zone center of the crystalline

CuO, there are three acoustic modes (Au+2Bu), six infrared active mode (3Au+3Bu), and three Raman active modes (Ag+2Bg).42 All the three Raman active bands are observed for the assynthesized CuO with the Raman vibration peak at 285.5 cm−1 assigned to Ag mode, while the peaks at 333.7 cm−1 and 590 cm−1 correspond to the Bg modes, correlating the literature.41


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(c)

Water:EG, 1:1

(b)

Water:EG, 3:1

20

40

(202) (-113) (022) (310) (311) (004)

(112)

Water:EG, 4:0 (-202)

(a)

(111)

Water:EG, 1:3

(002)

(d)

(110)

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Intensity (a.u.)


60

80

2θ (degree) Figure 1. Powder XRD patterns of CuO microstructures obtained with 1 mmol of Cu(OAc)2 by varying solvent volume ratio of water:EG, i.e., (a) water:EG=4:0, (b) water:EG= 3:1, (c) water:EG=1:1, and (d) water:EG =1:3.

3.2 Morphology and microstructure. The surface morphology of the synthesized powder products was examined by FESEM. Figure 2 shows the FESEM images of CuO architectures obtained with 1 mmol of Cu(OAc)2 in different solvent medium, i.e., (a,a1) water, (b,b1) water:EG = 3:1, (c,c1) water:EG = 1:1, and (d,d1) water:EG = 1:3. Broadly, 3D spherical CuO architectures were found to be composed of one-dimensional (1D) or two-dimensional (2D) units. Figures 2a and 2a1 show highly compact spherical CuO microstructures (diameter 1−1.5 µm) obtained in only aqueous medium. With water:EG volume ratio of 3:1, CuO

microarchitectures (Figure 2b) were obtained. These structures are not completely spherical and are found to be assembled of elongated structures (Figure 2b1). In addition, they appear less compact than that of the structures obtained with only water as the solvent. Anandan et al. 8 ACS Paragon Plus Environment

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reported similar quasi-spherical CuO microarchitecture using Cu(CH3COO)2 and urea by highintensity ultrasound vibration.43 With equal volume (20 mL) of each solvent, i.e., water and EG, 3D spherical morphology (Figures 2c and 2c1) composed of thin flakes-like structures was obtained. The thickness of these flakes is measured to be

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