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Novel Quaternary Chalcogenide/Reduced Graphene Oxide-Based Asymmetric Supercapacitor with High Energy Density Samrat Sarkar,† Promita Howli,‡ Biswajit Das,‡ Nirmalya Sankar Das,†,∥ Madhupriya Samanta,† G. C. Das,§ and K. K. Chattopadhyay*,†,‡ †

School of Materials Science and Nanotechnology, ‡Thinfilm and Nanoscience Laboratory, Department of Physics, and §Department of Metallurgical and Material Engineering, Jadavpur University, Kolkata 700032, India S Supporting Information *

ABSTRACT: In this work we have synthesized quaternary chalcogenide Cu2NiSnS4 (QC) nanoparticles grown in situ on 2D reduced graphene oxide (rGO) for application as anode material of solid-state asymmetric supercapacitors (ASCs). Thorough characterization of the synthesized composite validates the proper phase, stoichiometry, and morphology. Detailed electrochemical study of the electrode materials and ASCs has been performed. The as-fabricated device delivers an exceptionally high areal capacitance (655.1 mF cm−2), which is much superior to that of commercial micro-supercapacitors. Furthermore, a remarkable volumetric capacitance of 16.38 F cm−3 is obtained at a current density of 5 mA cm−2 combined with a very high energy density of 5.68 mW h cm−3, which is comparable to that of commercially available lithium thin film batteries. The device retains 89.2% of the initial capacitance after running for 2000 cycles, suggesting its long-term capability. Consequently, the enhanced areal and volumetric capacitances combined with decent cycle stability and impressive energy density endow the uniquely decorated QC/rGO composite material as a promising candidate in the arena of energy storage devices. Moreover, Cu2NiSnS4 being a narrow band gap photovoltaic material, this work offers a novel protocol for the development of self-charging supercapacitors in the days to come. KEYWORDS: hydrothermal, QC/rGO, specific capacitance, asymmetric supercapacitor, volumetric capacitance, volumetric energy density

1. INTRODUCTION

Supercapacitors form a bridge between batteries and conventional electrolytic capacitors. However, their high power densities come at the expense of relatively low energy densities. One of their incipient applications is in electric equipment, which entails delivery of very high electrical energy in a relatively short time interval. Examples include hybrid automotive vehicles, medical equipment such as defibrillators, photographic flashes in digital cameras, pulsed lasers, etc. Particularly in the automotive industry, supercapacitors offer regenerative braking and burst mode power delivery.9 According to the mechanism of charge storage, supercapacitors are categorized into electrical double layer capacitors (EDLCs)

In the era of energy crisis, energy production from renewable energy sources has been a major drive among the research community. Parallel research is also going on for the fabrication of devices to store this energy in electric form. Supercapacitors or ultracapacitors are in the forefront for efficient storage of electrical energy. Supercapacitors are constantly evolving energy storage devices that are designed to deliver very high power densities compared to conventional batteries. As a result, the past few years has witnessed a significant upsurge in scientific interest for the application of nanostructured materials in the design of supercapacitors.1−6 They have taken the upper hand compared to the conventional batteries due to their additional advantages, such as longer recyclability, fast charging, low cost, and easy maintenance.7,8 © 2017 American Chemical Society

Received: January 10, 2017 Accepted: June 15, 2017 Published: June 15, 2017 22652

DOI: 10.1021/acsami.7b00437 ACS Appl. Mater. Interfaces 2017, 9, 22652−22664

Research Article

ACS Applied Materials & Interfaces

GO to rGO is realized in this process.26,37 Cu2NiSnS4 with the incorporation of rGO effectively strengthens the properties required for a supercapacitor. The as-obtained composites with a layered structure provide an enhanced interaction of the active material in the electrode matrix with that of the electrolyte. This causes an effective decrease of the diffusion distance of charge carriers, leading to an improved electrochemical performance.38 Generally, oxide- or sulfur-based pseudocapacitors possess high specific capacitances but are limited to operate in a narrow voltage window (∼0.6 V). On the other hand, EDLCs in the form of carbon derivatives such as activated carbon, carbon fibers, carbon nanotubes, etc. can operate over a wide voltage window (∼1.0 V).39 A tangible application is to assemble these materials in asymmetric devices. Notable scientific improvement is now taking place for the enhancement of the performance of storage devices by the amalgamation of pseudocapacitive materials with suitable carbon derivatives, which augments the device’s conductivity, thereby reducing ohmic drop during the galvanostatic discharge process.40 In this work, electrochemical characterization including cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and galvanostatic electrochemical impedance spectroscopy (EIS) techniques has been performed. The supercapacitive performance of the QC/rGO electrode has been compared with that of the pristine QC electrode. An asymmetric supercapacitor (ASC) with QC/rGO and activated carbon (AC) as the positive and negative electrodes, respectively, has been fabricated, and poly(vinyl alcohol)−KOH gel is used as the electrolyte. Since Cu2NiSnS4 is also a potential photovoltaic material, by suitable design of a device, it can store energy as well as act as an active surface to harness solar power. Thus, in the future, it may give rise to a new generation of self-charging solar supercapacitors to integrate energy generation and storage for obtaining self-driven electronic devices.41−43

and pseudocapacitors. EDLCs rely on the nonfaradic surface charge at the interface of the electrode and electrolyte, while pseudocapacitors store charge on the basis of the redox reactions at the interfaces.10,11 Transition-metal oxides are the foremost choice as pseudocapacitive electrode materials,12 but low electrical conductivity hinders their applications in certain fields.13,14 Hence, researchers are turning toward some metal chalcogenides which provide higher electrical conductivity owing to the lower electronegativity of sulfur as compared to oxygen. Most of these metal chalcogenides, mainly sulfides, are derived from earth-abundant and low-cost transition metals. Sulfides such as Ni3S2,15 NiCo2S4,16 Bi2S3,17 CuS,18 etc. are widely used for this purpose. Moreover, they are generally semiconducting and undergo redox reactions among valence states of the metallic ions.19 Lately, apart from transition-metal oxides and chalcogenides, researchers have reported the use of some metal nitrides as electrodes in lithium ion batteries and supercapacitors with remarkable performance. These metal nitrides, for example, Ni3N, TiN, etc., possess excellent electrochemical properties with high chemical stability.20,21 To further enhance performance, suitable composites of various compounds with conducting materials such as graphene, carbon fiber, and carbon nanotubes can be employed.22−25 Chemically converted graphene (CCG) can be used to concoct an effective composite material for application in electrochemistry with improved performance. Graphene, a 2D nanosheet of graphite, has proven itself as a noble material and has attracted tremendous research interest due to its excellent carrier mobility, large surface area, high thermal stability, and chemical inertness.26,27 On the basis of these properties, CCG is a desirable candidate to be an excellent additive to wrap/decorate nanoparticles for enhancement of the composite’s electrochemical performance. Building suitable architects of these nanoparticles with graphene not only gives good electrical conductivity but provides robustness, stability, and a high surface area.24,25,28 Recently, some researchers have reported supercapacitors based on multiwalled carbon nanotube (CNT)/Ag nanoparticle ink,29 metal oxide/graphene,30 carbon fibers coated with a metal oxide and polymer,31 and selfhealable reduced graphene oxide (rGO) fibers.32 These carboncomposite-based supercapacitors show excellent performance with high energy densities and hence are promising materials for next-generation supercapacitors. Here, quaternary chalcogenide Cu2NiSnS4 (QC) nanoparticles have been grown in situ on rGO sheets to form a novel composite electrode material (QC/rGO). Cu2NiSnS4 is a relatively new material belonging to the I2−II−IV−VI4 group of quaternary chalcogenides. It is a narrow band gap semiconductor which is scientifically important in the field of photovoltaics.33,34 Furthermore, in contrast to single-phase metal sulfides, ternary or quaternary compounds show better performance because of the synergistic effect of two or more metallic cations.35 As far as the literature is concerned, there are few reports available on the synthesis and applications of Cu2NiSnS4. Very recently, Yuan et al. reported the application of 3D flowerlike Cu2NiSnS4 as an anode material in Na ion batteries.36 However, to the best of our knowledge, no work exists on the application of Cu2NiSnS4 in supercapacitors. Here, we are reporting a facile and in situ hydrothermal synthesis of Cu2NiSnS4/rGO for application in supercapacitors. Hydrothermal conditions provide the dual platform for the growth of quaternary Cu2NiSnS4 nanoparticles, and also the reduction of

2. EXPERIMENTAL SECTION 2.1. Materials. Copper(II) chloride dihydrate (CuCl2·2H2O; 99%), nickel(II) chloride hexahydrate (NiCl2·6H2O; 99%), and thiourea (NH2CSNH2; 99%) were purchased from Merck, and tin(IV) chloride pentahydrate (SnCl4·5H2O; 99%) was obtained from Lobachemie. All deionized water used in the synthesis procedure was obtained from the Millipore water purification plant. 2.2. Synthesis of QC/rGO Nanocomposites. All the chemicals are analytical grade and are used without further purification. At first, graphene oxide (GO) is synthesized by the modified Hummers method, which is described in the Supporting Information. A 10 mg sample of as-synthesized GO is then dispersed ultrasonically in 40 mL of ultrapure deionized water for about 1 h. This results in full exfoliation of the GO flakes to obtain a uniform aqueous dispersion. Then 1 mmol of copper(II) chloride dihydrate, 0.5 mmol of nickel(II) chloride hexahydrate, and 0.5 mmol of tin(IV) chloride pentahydrate are dissolved in the dispersion, and the resulting solution is stirred in a magnetic stirrer for 30 min. A 4 mmol sample of thiourea is then added and kept under stirring for another 1 h. The gray mixture is then poured into a stainless steel autoclave with a Teflon liner of 50 mL capacity. The sealed autoclave is placed in a hot air oven at 180 °C for 24 h. The hydrothermal reaction takes place under elevated conditions of temperature and autogenerated pressure. The oven is equipped with a blower to make the inside temperature uniform. The autoclave is then cooled; the contents are taken out, separated by centrifugation, and washed with deionized water and ethanol. The washed black product is then dried in a vacuum oven at 60 °C for 8 h. The final dry sample is used for characterization. A similar experiment is carried out without using GO to prepare a pristine Cu2NiSnS4 sample. The synthesis procedure is schematically presented in Figure 1. 22653


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Figure 1. Schematic representation of the synthesis procedure of the QC/rGO composite.

Figure 2. (a) X-ray diffractograms of QC and the QC/rGO composite. (b) Room temperature Raman spectra of GO and QC/rGO. Raman imaging of the composite sample showing the distribution of (c) QC and (d) rGO. (e) Combined bitmap Raman image of the composite. 2.3. Materials Characterization. The powder X-ray diffraction (XRD) patterns of the synthesized samples are obtained by an X-ray diffractometer (Bruker D8 Advance) employing Cu Kα radiation (λ = 1.5406 Å) operating at 40 kV and 40 mA in a 2θ range of 20−80° with a scan rate of 6 deg min−1. The morphology and microstructure are analyzed by field emission scanning electron microscopy (FESEM; Hitachi S-4800 operated at an accelerating voltage of 5 kV). The elemental composition with proper stoichiometry and mapping of the spatial distribution of elements is examined by energy-dispersive spectroscopy (EDS; Thermo Scientific attached to a Hitachi S-4800 operated at 15 kV). The morphology and nanostructure with lattice spacing and selected area electron diffraction (SAED) pattern are further examined using high-resolution transmission electron microscopy (HRTEM; JEOL, JEM 2100 operating at an accelerating voltage of 200 kV). Room temperature Raman spectra are recorded using a confocal Raman spectrometer (alpha 300, Witec, Germany, laser source of λ = 532 nm). The spectrograph is equipped with a Peltiercooled back-illuminated charge-coupled device (CCD) camera. X-ray

photoelectron spectroscopy (XPS) is performed using monochromatic Al Kα X-ray radiation to investigate the oxidation states and phase of the composite sample (SPECS HSA-3500 hemispherical analyzer). The specific surface area and pore size distribution of the as-prepared QC and QC/rGO composite are characterized by Brunauer− Emmett−Teller (BET) measurements (Nova 1000e, Quantachrome, United States). 2.4. Electrochemical Measurements. CV, GCD, and EIS have been carried out to explore the electrochemical properties of pure and composite samples. The experiment has been performed in a 6 M KOH aqueous solution in a three-electrode cell of Gamry Interface 1000 (potentiostat/galvanostat/ZRA). The working electrode potential ranges from 0.0 to 0.6 V with various scan rates in the range of 5− 100 mV s−1. All tests are carried out at room temperature. The working electrode is prepared by using a mixture of 80 wt % active material, 10 wt % acetylene black, and 10 wt % poly(tetrafluoroethylene) (PTFE) binder followed by the dropwise addition of N-methylpyrrolidone (NMP) with continuous stirring to 22654


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Figure 3. FESEM images of (a) QC, (b) as-synthesized GO, and (c) the QC/rGO composite. (d) EDS spectra of QC/rGO. form a slurry paste. Ni foam substrates are carefully washed in very dilute HCl, deionized water, and ethanol followed by drying in an oven. After this process, the prepared paste is coated on the Ni foam substrates (active area of 1.0 cm2). After drying (at 80 °C for 12 h), the electrode material is pressed for better compactness and adhesion to the substrate. The active masses of the working electrodes are 6.3 mg (QC) and 3.2 mg (QC/rGO). A platinum wire and Ag/AgCl electrode are used as the counter and reference electrodes, respectively. 2.5. Fabrication of the Asymmetric Supercapacitor. Initially, the gel electrolyte is procured by taking 3.0 g of poly(vinyl alcohol) (PVA) and 1.5 g of KOH in 30 mL of deionized water. The contents are then stirred vigorously, and the temperature is raised to 85 °C. After about 1 h, gel formation takes place. Ni foam is cut into rectangular strips with dimensions of 1.4 cm × 3.0 cm and washed carefully. To make the positive electrode, QC/rGO is used as the active material, and activated carbon on Ni foam is used as the negative electrode. Electrode fabrication is similar to the procedure for preparing the working electrodes in the three-electrode setup. The foams are pressed for better compactness. The thickness of the pressed foam is ∼0.04 cm. The active area of the device electrode is ∼2.0 cm2. Next the as-prepared positive and negative electrodes are immersed into the PVA/KOH gel for 5 min, keeping a portion of the foam outside. A Whatman filter paper is cut into rectangular strips and immersed into the gel. Both the electrodes are exposed to air at room temperature for a few minutes to evaporate the excess water. Finally, they are assembled carefully with the soaked filter paper in between as the separator. The assembly is then pressed securely by binder clips and covered by poly(ethylene terephthalate) (PET) films on both sides, leaving a bare portion of the electrodes uncovered for making electrical contacts. The volume (v) of the device is determined by multiplying the active area by the total thickness of the ASC (∼2.0 cm2 × 0.085 cm).

smaller peaks at higher angles of 69.23° and 76.44° with (400) and (331) lattice planes are also observed (JCPDS card no. 260552).33,36 However, no observable peak related to the rGO counterpart is found in the composite X-ray pattern (near about 26°), which may be attributed to the less ordered stacking of the rGO sheets.44,45 A low-intensity peak, if present, may be masked by the presence of a nearby (111) intense peak of Cu2NiSnS4. Nevertheless, the presence of rGO in the composite is later revealed and confirmed by Raman spectroscopy and other imaging studies. The XRD pattern of the assynthesized GO is shown in Figure S1 of the Supporting Information. 3.2. Raman Spectroscopy and Imaging. Raman spectroscopy is an indispensable tool for studying and characterizing samples involving graphene-related compounds. Raman spectroscopy of as-prepared graphene oxide (GO) and the composite samples is carried out in the range of 200−2000 cm−1 at room temperature as shown in Figure 2b. For the GO sample, the very familiar D and G peaks at 1350 and 1595 cm−1 are observed.46,47 For the composite sample, along with the D and G bands, additional peaks at 285 and 334 cm−1 are recorded. These peaks are related to Cu2NiSnS4 and similar quaternary chalcogenide materials as reported in our previous studies.33,48 However, the D and G bands for the GO and composite differ in their relative intensity ratios. Graphenerelated materials usually possess disorder and structure-related defects, which are a result of the sp3 C atoms incorporated into the lattice. The D band is related to the presence of this disorder, whereas the G band correlates to the sp2-hybridized C atoms in the regular honeycomb lattice. The intensity ratio (ID/ IG) is an important parameter deciding the relative abundance of disordered domains in the regular graphene lattice. A higher value of ID/IG implies more disorder or defects and a lesser abundance of sp2 domains.26,37 In this case, the ID/IG ratio (1.27) in the composite is higher than that of GO (0.91). For the composite, a substantial increase in the value of ID is due to an increase of disorder and incorporation of structural defects due to the hydrothermal treatment. Moreover, hydrothermal

3. RESULTS AND DISCUSSION 3.1. XRD Pattern. The phase identification of the asprepared pure and composite samples is ascertained from the powder diffraction data as shown in Figure 2a. The prominent peaks at 28.48°, 33.02°, 47.36°, and 56.21° for both the samples indicate the formation of Cu2NiSnS4 with crystallographic planes of (111), (200), (220), and (311), respectively. Two 22655


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Figure 4. (a, b) Low-magnification TEM images of the composite revealing good incorporation of QC nanoparticles in wrinkled rGO sheets. (c) HRTEM image of the composite. Well-resolved lattice images with measured d values are shown in the insets. (d) SAED pattern of the composite.

reduction results in the incorporation of hydrogen atoms into the composite system, which results in the diminution in size of the sp2 domains and hence a decrease in the value of IG, can be explained. Thus, it can be conferred that reduction of GO to rGO has been realized under elevated conditions of hydrothermal treatment.26,47 Raman image mapping has been carried out over a selected area (20 μm × 20 μm) of the composite sample. As shown in Figure 2c,d, signals corresponding to the Raman peak located around 334 cm−1 and the characteristic G band of rGO are detected, and images corresponding to the spatial distribution of peak intensities have been mapped. The scale bars in the insets correspond to intensities (CCD counts), yellow being the maximum (∼4.0 × 105 counts) and black being the minimum of zero counts. Figure 2e shows the combined bitmap image of the two components in the composite. 3.3. Morphology and Elemental Analysis. Figure 3a shows the FESEM image of the as-synthesized pristine QC nanoparticles. The nanoparticles with some larger agglomerates are observed in the image. Figure 3b is the image of assynthesized graphene oxide. The image of the composite sample is shown in Figure 3c. In this image, the good incorporation of the QC nanoparticles over the wrinkled rGO sheets is observed. Not only does the composite form a conjugated structure, but the incorporation of rGO effectively increases the surface area by virtue of its 2D layered structure, which augments the effective ion transfer between the embedded nanoparticles in the 2D sheets with that of the electrolyte. Figure 3d shows the EDS spectra of the constituent elements in the composite. There is a peak for Si as Si is used as

the substrate on which the powder sample is spread for performing the EDS analysis. The elemental composition determined from EDS is presented in Table S1 in the Supporting Information. The EDS data indicate the near stoichiometry of Cu, Ni, Sn, and S in Cu2NiSnS4. Figure S2a in the Supporting Information shows the mapping of the constituent elements, which illustrates their even distribution in a selected area. EDS analysis of as-synthesized GO is also performed (cf. Figure S2b of the Supporting Information). Here, the C/O ratio obtained for GO is 1.67, whereas C/O for rGO in the composite (cf. Table S1) is ∼7.0. Thus, the relative oxygen content in the composite for rGO is much less than that for GO, indicating successful reduction of GO to rGO and validating the results of Raman analysis. 3.4. TEM Analysis. To further elucidate the morphology and to confirm the phase, transmission electron microscopy is carried out for the composite. Parts a and b of Figure 4 show the low-magnification images of the QC nanoparticles embedded in the crumpled rGO sheets, which can be clearly observed. High-resolution images in Figure 4c reveal the lattice of the individual nanoparticles. The lattice images with measured d spacings of 0.31 and 0.27 nm are shown in the two insets. These correspond to (111) and (200) crystallographic planes as seen in XRD. Figure 4d depicts the SAED image pattern in reciprocal lattice space. The pattern indicates the polycrystalline nature of the nanoparticles. All the measured d values and the designated lattice planes are in exact agreement with the XRD data.33 3.5. XPS and BET Analysis. To analyze the electronic structure and oxidation states of the constituent elements 22656


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Figure 5. (a) Survey XPS scan of QC/rGO. (b−f) High-resolution XPS scans of Cu 2p, Ni 2p, Sn 3d, S 2p, and C 1s, respectively. (g, h) N2 adsorption−desorption isotherm curves obtained by BET analysis (the insets show the corresponding pore size distribution based on the BJH method) for the pristine QC and QC/rGO composite, respectively.

present in the QC/rGO composite, high-resolution X-ray photoelectron spectra are recorded, and the results are depicted in Figure 5. For the C 1s and S 2p spectra, deconvolution is done employing the CASA XPS software program. The survey scan of the sample is shown in Figure 5a. Figure 5b show the Cu 2p spectrum with two distinct symmetric peaks at 932.2 and 952.2 eV for Cu 2p3/2 and Cu 2p1/2, respectively. Peak splitting of 20.0 eV indicates that Cu is in the +1 state. Similarly, the Ni 2p spectrum in Figure 5c has two peaks at 854.6 and 872.2 eV for Ni 2p3/2 and Ni 2p1/2, respectively. The peak separation of 17.6 eV suggests the presence of Ni2+. In Figure 5d, the Sn 3d5/2 peak at 486.8 eV and Sn 3d3/2 peak at 495.2 eV with a peak splitting of 8.4 eV strongly indicate the presence of Sn4+. By deconvolution, two peaks are present for S 2p3/2 and 2p1/2 at 161.9 and 163.5 eV with a splitting of 1.6 eV as seen in Figure 5e. These findings are consistent with the existing literature and our previous work.34,48 Deconvoluted C 1s spectra yield peaks at 284.7, 286.5, and 288.9 eV, which can be ascribed to C−C, C−O, and CO for the rGO counterpart in the composite (cf. Figure 5f).49 It is evident that the two oxygen-containing peaks have relatively low intensities, which indicate the successful hydrothermal reduction of GO to rGO. Nevertheless, this has already been established from the Raman spectral analysis. Thus, all the elements in the composite material have the desired chemical states, validating the formation of a pure phase. The specific surface area and pore size distribution of the asprepared QC and QC/rGO composite are characterized by Brunauer−Emmett−Teller (BET) measurements. To inves-

tigate the surface areas and porosity of our samples, N2 adsorption−desorption isotherms are measured using liquid N2 at a temperature of 77 K. Parts g and h of Figure 5 represent the N2 adsorption−desorption isotherms of pure QC and QC/ rGO composite and their corresponding Barrett−Joyner− Halenda (BJH) pore size distributions (shown in the insets). The BET surface areas are 27.2 and 33.9 m2 g−1 for pure QC and the QC/rGO composite, respectively, which indicate that the addition of 2D rGO sheets helps to enhance the surface area.50 Conferring to the IUPAC classification, at high P/P0 between 0.4 and 1.0, the composite exhibits a type IV nature with an H3 hysteresis loop, which validates the presence of mesopores having a pore diameter of 4.03 nm and a total pore volume of 0.062 cm3 g−1. At the same time, the pure sample exhibits a similar nature, and its pore diameter is found to be 3.59 nm and its total pore volume 0.046 cm3 g−1. For the composite, the total pore volume has increased by approximately 1.5-fold compared to that of the pure sample. The high specific surface area and total pore volume are key factors to enhance the electrochemical performance of an electrode material.51 3.6. Supercapacitive Performance of QC and QC/rGO Composite Electrodes. CV studies of QC and QC/rGO electrodes are performed in 6 M aqueous KOH electrolyte at various scan rates (5, 10, 20, 50, and 100 mV s−1) in a potential window of 0.0−0.6 V (cf. Figure 6a,b). The integral area under the CV curve for the composite is larger than that of the pure sample counterpart. This specifies superior capacitive performance and hence a larger specific capacitance (CS) of the 22657


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Figure 6. (a, b) CV curves of QC and QC/rGO, respectively. (c) Graphical comparison of the specific capacitances of the samples at various scan rates. (d) Nyquist plots of the samples as obtained from the EIS study.

and composite samples. As seen from the figure, the QC/rGO composite sample exhibits a smaller arc compared to the pure sample in the high-frequency region. This accounts for the lower value of interfacial charge transfer resistance at the electrode−electrolyte boundary. Also, in the low-frequency region, the composite electrode shows a more vertical line (closer to 90° with the x-axis) than the pure sample, which can be explained by the improved conductivity of the QC/rGO by the incorporation of conducting rGO sheets into the composite framework.53 Enhanced conductivity and a lower interfacial charge transfer resistance are the imminent causes of superior capacitive behavior and transport of charge carriers in solution. Therefore, rGO is used together with QC for procuring a composite with optimum electrochemical properties, and hence, the composite is a better choice as an electrode material for fabricating devices. To investigate the charge−discharge property, GCD for both the samples is performed in the same electrolytic setup at various current densities. The discharge curves of QC and QC/ rGO at various current densities are shown in parts a and b, respectively, of Figure 7. The charge−discharge curves are also consistent since the discharge time for the composite is greater than that of either rGO or QC (cf. Figure S3 in the Supporting Information). Figure 7c shows the values of the specific capacitances of the samples at various current densities. The composite electrode also exhibits excellent recyclability with a stable capacitance retention of 94.1% after running for 2000 charge−discharge cycles. Generally, the long cycle results in fading in the capacitance value, possibly due to the mechanical stress generated from insertion and deinsertion of electrolyte ions into the active material. This can also result in physical strain, especially for 2D layered structures. The slight degradation of the morphological structure of the material

composite material. From the CV curves of the composite, a shift of the redox peaks with a higher scan rate is observed. This is due to the electrical polarization effect and irreversible electrode reactions taking place at higher scan rates.52 A detailed comparative study of the CV and charge−discharge behavior of QC, rGO, and the QC/rGO composite has been carried out, the results of which are given in the Supporting Information (Figure S3). We clearly observe that the integral CV area for the composite is more than that for both pure rGO and QC. Also addition of rGO increases the conductivity and specific surface area of the material prepared under similar experimental conditions. Enhancement of the specific capacitance of the composite over that of the pure sample can be attributed to the increase in surface area of the composite, the increase in conductivity, and the synergistic effect of providing more active sites by the rGO nanosheets for effective transport of the ions in solution. Figure 6c gives a graphical representation of the comparative CS values of the two electrodes at various scan rates. It is observed that the capacitance of both the samples decreases with an increase in the scan rate, signifying the measured capacitance is related to the redox reaction. At lower scan rates, the outer and inner regions of the material could be reached, whereas with an increase of the scan rate, the diffusion of the ions might occur on the external superficial region and not the inner surface of the layered nanostructure of the composite material. CS at various scan rates is calculated from the CV curves. The composite material has a CS value of 1425 F g−1 at a scan rate of 5 mV s−1, which is higher than that of the pure sample (916 F g−1 at 5 mV s−1). EIS is essential to study the electrochemical properties of the electrode materials in a wide spectrum of frequencies (0.1 Hz to 1 MHz). Figure 6d displays the Nyquist plots of the pure 22658


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Figure 7. (a, b) Discharge curves of QC and QC/rGO, respectively. (c) Comparison of specific capacitance values of the samples at various current densities. (d) Capacitance retention and Coulombic efficiency versus cycle number of the QC/rGO electrode. The inset shows the last 10 of the 2000 cycles.

3.7. Performance of the Asymmetric Supercapacitor. The as-fabricated ASC is subjected to CV and GCD tests to assess its performance. The CV curves of activated carbon and the composite are shown in Figure 8a at a scan rate of 50 mV s−1. The working potential windows of the activated carbon and QC/rGO counterparts are from −1.0 to 0.0 V and from 0.0 to 0.6 V. Figure 8b shows the CV curves of the as-fabricated device at various potential windows of 0.0−1.0, 0.0−1.2, 0.0− 1.4, and 0.0−1.6 V at a scan rate of 50 mV s−1. The CV plots exhibit a quasi rectangular nature even at a high potential window, which establishes the contribution due to both EDLC and pseudocapacitive materials.57 Figure 8c reveals the charge− discharge curves of the device. The charging−discharging is performed at a potential of 1.6 V. From the discharge curve, the calculated maximum areal capacitance of the device is 655.1 mF cm−2, which is remarkably higher than or comparable to those reported for carbon-composite-based supercapacitors53,58−60 (cf. Figure S5a in the Supporting Information). The device also shows high values of the volumetric and gravimetric capacitances of 16.38 F cm−3 and 164 F g−1, respectively, at a current density of 5 mA cm−2. The values of the capacitances are calculated on the basis of the equations which are provided in the Supporting Information. Figure 8d shows the Nyquist plots as obtained from the EIS study of the ASC before and after 2000 cycles. At the high-frequency region, the series resistance (Rs) for the sample increases from 0.7 to 1.1 Ω, whereas the interfacial charge transfer resistance (Rct) increases slightly from 4.9 to 5.5 Ω. The linear portions of the Nyquist curves at the low-frequency region however show almost the

immersed for a long time in the electrolytic solution during the long cycle study can also contribute to capacitance fading. Figure 7d shows the plot for cycle stability of the composite electrode sample. The long cycle is performed at 20 A g−1 in 6 M aqueous KOH. The inset of Figure 7d shows the final 10 cycles. During the initial 200 cycles, it is observed that the specific capacitance value gradually increases. This is a result of electrode activation due to the proliferation of available active sites in solution during the charge−discharge cyclic process.54,55 The Coulombic efficiency after 2000 cycles shows an impressive value of 96.4%. The improved pseudocapacitive performance of the composite electrode is ascribed to the unique decoration/ arrangement of the chalcogenide nanoparticles on the rGO sheets. Also rGO incorporation facilitates efficient transfer of the ions in the electrolyte at the electrode−electrolyte interface as well as within the bulk. Parts a and c of Figure S4 in the Supporting Information show the high-magnification FESEM images of the composite electrode before and after 2000 cycles. We can see that there is no noticeable change in the morphology of the nanoparticles even after the long cycle run. The corresponding low-magnification images in Figure S4b,d show the distribution of the composite material within the Ni foam framework. Here, it is clearly observed that the foam network itself undergoes slight degradation without appreciably affecting the particle morphology. Thus, it is can be established that the rGO impedes the self-aggregation of the nanoparticles and acts as a framework providing mechanical strength to the active material.56 22659


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Figure 8. (a) CV curve of activated carbon and QC/rGO at 50 mV s−1. (b) CV curves of ASC at various potential windows. (c) GCD profiles of ASC at various current densities. (d) Nyquist plot obtained from EIS spectra of the ASC before and after 2000 cycles. (e) Gravimetric and volumetric capacitances of ASC at various current densities as calculated from the GCD curves. (f) Digital photograph of the as-fabricated ASC.

loadings is a decisive factor in supercapacitors. Not only the gravimetric capacitance, but also the capacitance as a whole that can be derived from a single supercapacitor is important. It is often found that electrodes possess very high values of gravimetric capacitances but at the cost of ultralow mass loadings of active material. In that case, the imminent drawback is that the gravimetric capacitance is high but results in a poor areal or volumetric capacitance, which cannot be overlooked in designing an asymmetric device.62 Therefore, to obtain a high volumetric capacitance, we use relatively high mass loadings (∼16−20 mg of active materials for both electrodes taken together). In the characterization of the performance of supercapacitors, two important parameters to be taken into consideration are the energy density and power density. Our fabricated device possesses a remarkably high energy density (5.68 mW h cm−3) with a power density of 98.8 mW cm−3 at 5 mA cm−2. At a higher current density (12.5 mA cm−2), the energy density is still 2.95 mW h cm−3 and the power density reaches a value of 246.9 mW cm−3. The energy density values are strikingly high and better than many reported for similar carbon-based

same Warburg impedance (W) due to electrolyte diffusion in the electrode. A similar nature of the Nyquist plots indicates the good stability of the composite material even after 2000 charge−discharge cycles. The volumetric and gravimetric capacitance values are presented in a graph (cf. Figure 8e). A digital snapshot of the as-fabricated ASC is also shown (cf. Figure 8f). Digital snapshots of the individual electrodes and whole device assembly are shown in the Supporting Information (Figure S6). The excellent electrochemical behavior of our device can be gauged from the fact that the porous nature and high surface area of the electrodes provide an effective interconnection of the active material with the electrolyte, which in turn provides easy diffusion of the electrons and ions. Irrespective of the importance of the gravimetric capacitance of devices in the majority of the literature, for practical applications, volumetric capacitance and volumetric energy and power densities are vital considerations, where areas and volumes of electrodes are important parameters for designing devices in a compact form with many rolled and layered internal structures.61 Procuring electrodes with high mass 22660


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Figure 9. (a) Ragone plot (volumetric power density vs volumetric energy density) of the ASC. (b) Capacitance retention vs cycle number of the ASC at a current density of 50 mA cm−2. The inset shows the last 10 of the 2000 cycles. (c) GCD profiles of two devices in series and four devices in series/parallel combination at 7.5 mA cm−2. (d) Photographs of the glowing red LED at different time intervals. (e) Photograph of a running motor fan connected with the ASC.

devices, i.e., for powering equipment, cyclic stability with good capacitive retention is a prerequisite. A supercapacitor with a high cycle life is desirable for its active running over a longer period without any degradation of performance and at the same time with maintenance of its resilience. After 2000 charge− discharge cycles (at 50 mA cm−2), it is found that the device still retains 89.2% of the initial value of capacitance (cf. Figure 9b), signifying good capacitive retention. The final 10 cycles are shown in the inset. It is interesting to note that the device delivers a maximum energy density which is comparable to that of a standard 4 V/500 μA h Li thin-film battery but with a power density 1 order of magnitude higher. It also possesses superior E and P values compared to commercial 2.75 V/44 mF

supercapacitors such as the laser scribe graphene planar SC (0.6 mW h cm−3),63 the carbonaceous-fiber-based micro-ASC (5.0 mW h cm−3),64 the graphene-based in-plane micro-SC (2.5 mW h cm−3),65 carbon-nanoparticle-coated carbon fibers (2.1 mW h cm−3),66 and metal ternary selenides/porous graphene (2.85 mW h cm−3).67 Figure 9a shows the Ragone plot of E vs P. The Ragone plot is a convenient symbolic representation of the energy and power density regions where energy storage devices such as batteries, capacitors, fuel cells, and supercapacitors operate. The formerly mentioned volumetric capacitances and energy densities of our device are comparable to or better than those of related asymmetric devices reported in the literature.68−75 For practical application of asymmetric 22661



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supercapacitors. The operating voltage of the asymmetric system is 1.6 V, which is greater than that of alkaline cells (1.5 V). Nevertheless, the voltage and current requirements of some small electronic appliances cannot be provided by a single device. Hence, it is essential to have few similar devices connected in series or parallel or both to produce enough voltage and current. Figure 9c show the GCD curves of “two in series” (arrangement 1) and “four in series/parallel” (arrangement 2) assemblies charged at a current density of 7.5 mA cm−2. As expected, the series assembly can be charged to 2 times the voltage (3.2 V) compared to a single device. However, to obtain a greater current, two such series assemblies in parallel should be used. It is observed that the discharge time for arrangement 2 is almost double that of arrangement 1. As a test, this ASC series/parallel assembly is charged to 3.2 V with a 20 mA current for 12 s and then connected to the terminals of a commercial red light-emitting diode (LED) (2 V, 20 mA) which stayed alight for 6 min. The same assembly is again charged to 3.2 V but now with a higher current of 100 mA for just 20 s, which can store enough charge to operate a motor fan (3 V, 80 mA) at high speed for about 40 s (cf. Figure 9d,e). A movie of the fan in operation is given in the Supporting Information (Video S1). This demonstration verifies the highly competent energy storage capability of our device.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91 94 3338 9445. Fax: +91 33 2414 6007. ORCID

K. K. Chattopadhyay: 0000-0002-4576-2434 Present Address ∥

N.S.D.: Department of Basic Science and Humanities, Techno IndiaBatanagar, Kolkata 700141, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.S. thanks the University Grants Commission (UGC) (Grant No. 19-06/2011 (i) EU-IV), Government of India, for providing a fellowship. We also acknowledge the UGC for providing financial support of this work under the University with Potential for Excellence (UPE II) scheme.

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REFERENCES

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4. CONCLUSIONS In summary, a novel quaternary chalcogenide/rGO composite has been successfully synthesized by an in situ hydrothermal route. The composite material exhibits a specific capacitance of 1425 F g−1 (at 5 mV s−1). The composite also shows remarkably high cyclic stability of 94.1% capacitance retention with 96.4% Coulombic efficiency after 2000 cycles. The asfabricated asymmetric supercapacitor possesses outstanding areal and volumetric capacitances of 655.1 mF cm−2 and 16.38 F cm−3 (at 5 mA cm−2), respectively, combined with superior energy and power densities of 5.68 mW h cm−3 and 246.9 mW cm−3. Furthermore, the device shows impressive cyclic stability with 89.2% capacitance retention after 2000 cycles. As a demonstration, a series/parallel assembly of the device can light up a red LED and rotate a mini motor fan. It is noteworthy that the quaternary chalcogenide Cu2NiSnS4 is a potential photovoltaic material; hence, in the future, by suitable design, it open doors for the development of high-performance supercapacitors which can charge themselves using solar power.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00437. Synthesis of GO by the modified Hummers method, XRD of as-synthesized GO (Figure S1), complete elemental analysis of the composite by EDS (Table S1), elemental mapping (Figure S2), comparative CV curve, comparative electrochemical studies of rGO in Ni foam, pure QC and QC/rGO composite electrodes (Figure S3), FESEM images of electrodes before and after 2000 long cycles (Figure S4), areal capacitance plot and areal Ragone plot (Figure S5), photographs of the electrodes and device assembly (Figure S6), and additional formulas and calculations (PDF) Video of the fan in operation driven by the charge stored in the ASC assembly (Video S1) (AVI) 22662


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