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Sb2S3 Nanoparticles Anchored or Encapsulated by SulfurDoped Carbon Sheet for High-Performance Supercapacitor Rakesh Kumar Sahoo, Saurabh Singh, Je Moon Yun, Se-Hun Kwon, and Kwang Ho Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11028 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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ACS Applied Materials & Interfaces

Sb2S3 Nanoparticles Anchored or Encapsulated by Sulfur-Doped Carbon Sheet for HighPerformance Supercapacitor Rakesh K. Sahoo,b Saurabh Singh,a Je Moon Yun b,* Se Hun Kwon a,* and Kwang Ho Kim a,b,*

a School

of Materials Science and Engineering, Pusan National University, 2, Busandaehak-ro,

63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea

b Global

Frontier R&D Center for Hybrid Interface Materials and National Core Research Center

for Hybrid Materials Solution, Pusan National University, 2, Busandaehak-ro, 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea

*Corresponding

author:

E-mail: [email protected] (Kwang Ho Kim) [email protected] (Je Moon Yun) [email protected] (Se Hun Kwon)

1 ACS Paragon Plus Environment

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ABSTRACT: The specific capacitance and energy density of antimony trisulfide (Sb2S3) @carbon supercapacitor (SC) have been limited and in need of significant improvement. In this work, Sb2S3 nanoparticles were selectively encapsulated or anchored in a sulfur-doped carbon (Scarbon) sheet depending on the use of microwave-assisted synthesis. The microwave-triggered Sb2S3 nanoparticle growth resulted in core-shell hierarchical spherical particles of uniform diameter assembled with Sb2S3 as the core and an encapsulated S-carbon layer as the shell (Sb2S3M@S-C). Without the microwave mediation, the other nanostructure was found to comprise fine Sb2S3 nanoparticles widely anchored in the sulfur-doped carbon sheet (Sb2S3-P@S-C). Structural and morphological analyses confirmed the presence of encapsulated and anchored Sb2S3 nanoparticles in the carbon. These two materials exhibited higher specific capacitance values of 1179 (0 to +1.0 V) and 1380 F∙g–1 (-0.8 to 0 V) at a current density of 1 A∙g–1, respectively, than those previously reported for Sb2S3 nanomaterials in considerable supercapacitors (SCs). Furthermore, both materials exhibited outstanding reversible capacitance and cycle stability when used as SC electrodes, while retaining over 98% of the capacitance after 10,000 cycles, which indicates their long-term stability. Furthermore, a hybrid Sb2S3-M@S-C/Sb2S3-P@S-C device was designed, which delivers a remarkable energy density of 49 Wh∙kg−1 at a power density of 2.5 kW∙kg−1 with long-term cycle stability (94% over 10,000 cycles) and is comparable to SCs in recent literature. Finally, a light-emitting diode (LED) panel comprising 32 LEDs was powered using three pencil-type hybrid SCs in series.

KEYWORDS: Sb2S3 nanoparticles; encapsulation; anchoring; Sulfur-doped carbon; supercapacitor


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1. INTRODUCTION The increasing energy demand of electric vehicles and portable electronic devices has necessitated the rethinking of supercapacitors (SCs) having a high specific capacitance and extraordinary cycle stability.

1-2

Over the past several years, considerable research efforts have been invested in

developing high-specific-capacity electrode materials for SCs.

3-4

In particular, carbon

nanomaterials have been actively proposed for use as electrode materials in SCs because of the availability of carbon, its high electrical conductivity, and its low cost, which compensate for its low specific capacitance value.5 However, to address the growing energy demand of high-powerconsumption devices, pristine carbon nanomaterials are not the optimal solution. Thus, it is necessary to design an electrode material having a high specific capacitance and long-term cycle stability for SC application. Recently, an antimony-based composite has exhibited excellent capacity, which could inspire a lot of research on energy storage devices.

6

Antimony chalcogenide is a well-known,

commonly used, and promising electrode material for batteries owing to its high theoretical capacity and suitable working voltage. Antimony (Sb) has immense alloying/dealloying potential and remarkable high capacity (660 mAh∙g–1 theoretically). 7 However, metallic Sb in a sodium ion battery (SIB) or lithium ion battery (LIB) demonstrates a large volume expansion during sodiation/desodiation or lithiation/delithiation in each battery system, which results in a deterioration in the long-term cycle performance of the device. Several methods have been adopted to reduce the volume expansion: (1) alloying with other metals, (2) forming a stable oxide heterostructure (e.g., Sb/S2bO3), and (3) tuning the morphology at the nanoscale and anchoring in a carbon network. By anchoring the Sb in or with carbon, its high performance and long and stable cyclic battery life can be developed. Recently, there have been several works on the synthesis of



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an Sb@S-C composite for use in SIBs. Yuan et al. reported ball-milled Sb/C, which is synthesized via the ball milling of CaC2 and Sb2O3, as a porous anode for SIBs. 7 Kong et al. 8 designed thin films of a freestanding metal carbon framework using a space-confined super-assembly strategy. Zhang et al. 9 presented a spherical Sb/C composite synthesized using spray pyrolysis. Li et al. 10 used a two-step microwave irradiation and chemical self-assembly route to accommodate Sb2O3/Sb nanoparticles in a graphene network. In addition, a metal organic framework template in the galvanic replacement reaction that forms the Sb/C nanocomposite

11

exhibited a superior

performance as the SIB anode. Liu et al. also adopted the galvanic replacement synthesis route to form unique hollow Sb@S-C yolk–shell sphere structures. Luo et al. encapsulated Sb in a peapodlike Sb@S-C structure via a simple oxidation–carbon-coating–reduction route.12 Wang et al. performed a unique two-step electrostatic spray deposition followed by heat treatment to capture Sb nanoparticles in a 3D reticular carbon network. 13 Song et al. 14 used a simple low-cost scalable approach via the annealing of polydopamine-coated Sb2O3 to form a yolk–shell-like Sb@S-C composite. The primary objectives of using carbon with Sb as the buffer layer are (i) to provide structural stability with superior conductivity and surface area, which enhances the capacitance, and (ii) to minimize the volume expansion and agglomeration of Sb particles during the cycle life with carbon as the buffer layer. Sb particles anchored in carbon and Sb-based chalcogenides have demonstrated superior potential in LIBs and in catalytic and supercapacitive applications. Simple freeze-drying and heat treatment were implemented to encapsulate Sb particles in the expanded graphite for application in LIBs. 15 Recently, a simple impregnation and in situ calcination method was used to simultaneously anchor 2–3-nm Sb and carbon particles in a porous palygorskite for catalytic applications. 16 Karade et al. 17 reported on chemical-bath-deposited Sb2S3 thin films on a stainless-steel substrate, and its supercapacitive behavior was estimated at 248 F∙g–1 at a current


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density of 0.5 A∙g–1. Ramasamy et al.

18

presented a colloidal deposition method for a CuSbS2

electrode with a specific capacitance of 34 F∙g–1 at 0.4 mA. In addition, they also reported 19 the enhanced performance of a CuSbS2 ternary sulfide layer with a specific capacitance of 120 F∙g–1 and theoretically predicted that its capacitance can be enhanced to 1160 F∙g–1. Reddy et al.

20

demonstrated a conducting polymer comprising enclosed Sb2S3 nanorods with a remarkable capacitance of 1008 F∙g–1 at a current density of 1 A∙g–1 in a gel electrolyte. However, this conducting polymer coating on hydrothermally synthesized Sb2S3 nanorods is unreliable. In addition, its production-to-performance cost is high, and its device performance duration is limited. Thus, the challenges in designing facile, scalable, and conducting Sb2S3 nanostructures for highperformance supercapacitor applications are yet to be addressed. Despite the aforementioned progress in the study of nanoparticle/quantum dots21-22 anchored in carbon as nanocomposites for SC applications, developing Sb2S3 encapsulated in a carbon matrix is still a challenging task. Moreover, the scalability of the process is also a crucial issue. Herein, we report two types of composites with Sb2S3 nanoparticles encapsulated and anchored in a sulfur-doped carbon frame based on the use of a microwave-assisted synthesis. From the precursor comprising antimony chloride, thioacetamide, in 1,2-propanediol, and a polystyrene solution, a fine Sb2S3-anchored carbon composite is formed via a thermal annealing process; whereas, a spherical Sb2S3-encapsulated carbon composite is produced via a microwave treatment of the precursor solution prior to the thermal annealing process. The structure, morphology, crystalline phase, and surface area of both the samples are investigated using various techniques. A three-electrode electrochemical system is used to measure the SC performance of each material in a 1-M KOH electrolyte. Interestingly, Sb2S3-P@S-C electrode exhibits an electrostatic doublelayer capacitance behavior in a positive potential window (0 to 1 V), whereas a Sb2S3-M@S-C



electrode exhibits a pseudocapacitive behavior in a negative potential window (−0.8 to 0 V). When a hybrid supercapacitor (HSC) comprising Sb2S3-M@S-C/Sb2S3-P@S-C is assembled, its specific capacitance, rate capability, energy density, power density, and durability are then measured and presented. Three HSC devices connected in series are used to simultaneously power 32 light emitting diodes (LEDs). The carbon covering the Sb2S3 nanoparticles not only acted as a buffer layer that alleviated structural degradation, but also increased the effective contact area between the electrode and electrolyte. It is believed that this work can provide new ideas for the SC application of Sb2S3@S-C-based electrodes with high reversible capacities.

2. EXPERIMENTAL SECTION 2.1. Materials. Antimony chloride (SbCl3), thioacetamide (CH3C(S)NH2), 1,2-propanediol (CH3CH(OH)CH2OH), and powders were obtained from Sigma Aldrich, South Korea. 2.2. Synthesis of nanomaterials. Initially, 0.3-M SbCl3 was dissolved in 30 ml of 1,2propanediol using magnetic stirring. Then, in the 0.3-M SbCl3 solution, 0.4-M thioacetamide solution in 1,2-propanediol was slowly added dropwise, while continuously stirring for 10 min. The color of the solution began to change from transparent to faded yellow because SbCl2 reacted with thioacetamide to form Sb-complex ions ([Sb(CH3CSNH2)x]3+) in the solution. In a beaker, PS powder is swollen using 6-M NaOH (refer to S-1 in Supporting Information.). Then, the yellowish solution containing Sb-complex ions was mixed with the PS solution while stirring continuously for 30 min. The mixing ratio was 2:1 of the PS and Sb-complex solutions. This mixed solution was divided into two parts for the different processes to be used. One part was treated in a microwave oven for 10 min, and the other was stirred continuously for 1 h. In the next step, the


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precipitates in both the solutions were skimmed and annealed at 450 °C for 4 h in a nitrogen atmosphere. After the annealing, each sample was collected and used for further characterizations. 2.3. Physical Characterizations. XRD measurements were performed using a D8 Discovery Bruker (Cu-Kα, λ = 1.5405 Å) diffractometer. The SEM and EDAX analyses were performed using a Hitachi S-4800 at 15 kV equipped with an energy dispersive X-ray spectrometer (EDX). The TEM and HRTEM mages were recorded using a FEI TALOS F200X microscope operated at 200 kV. The EDX elemental mapping was obtained using a probe of 0.2-nm diameter and a camera length of 20-cm with a scanning transmission electron microscope. The adsorption surface area and total pore volumes were obtained from the nitrogen absorption–desorption isotherm for the pressure range of P/P0 = 0.05–0.1 using the BET equation. Furthermore, the pore size distribution was calculated using BJH theory. The XPS from VG Scientifics ESCALAB250 with an Al Kα X-ray source (1486.6 eV) and an X-ray beam of approximately 1 mm was used to analyze the composition and valence states of the ions present in each sample. The obtained XPS profiles were calibrated to a carbon peak located (C 1s) at 284.6 eV. The FTIR spectra were recorded using the MAGNA-IR 560 (Nicolet) system in the range of 400–4000 cm-1 at a resolution of 4 cm-1. The Raman spectra were obtained using a Vertex 80V spectrometer with a 514.5-nm wavelength. The TGA was conducted using a TGA 2050 (Thermal analysis instrument, USA) with a nitrogen flow rate of 100 ml/min and temperature ramp of 1 °C/min. 2.4. Electrode and device preparation and Electrochemical Measurements. The electrode is prepared using nickel foam as current collector. In a typical preparation, 85 wt. % of the as-synthesized powder, 10% wt. % acetylene black and 5 wt. % polytetrafluoroethylene (PTFE) are mixed properly and coated on the Ni foam substrate with a loading of 1-2 mg cm-2. This as-coated electrodes are dried overnight in a vacuum oven at 100 °C before measurement.



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These electrodes are tested in a three-electrode configuration consisting of platinum foil and Hg/HgO as the counter and reference electrode, respectively, containing a 1-M KOH solution as an electrolyte. Electrochemical measurements of the CV, GCD, and EIS (100 kHz–0.01 Hz with an AC voltage amplitude of 5 mV rate) of the samples were conducted on an IVIUM potentiostat (Ivium, The Netherlands). The distance between the working electrode and counter electrode was fixed at approximately 1 cm. From the GCD plots, the specific capacitance (Cs) of the SCs was estimated using the following equation. 𝐼∆𝑡

(1)

𝐶𝑠 = 𝑚∆𝑉

where I(mA), ∆𝑉(𝑉), m (mg), and ∆t (s) represent the current, operating voltage window, loading mass, and discharge time, respectively. Furthermore, an HSC device was assembled using Sb2S3P@S-C and Sb2S3-M@S-C as the two counter electrodes. In order to prepare the HSC, assynthesized powder, acetylene black and PTFE (the weight percentage as mentioned above in individual electrode preparation) are mixed and coated on a 4x3 cm-2 area of the Ni-foam. The charge balance (𝑞 + = 𝑞 ― ) of each electrode for an appropriate mass loading was set as per the following equation. 𝑚+ 𝑚―

𝐶 ― × ∆𝑉 ―

= 𝐶 + × ∆𝑉 + (q = 𝐶 × ∆𝑉 × 𝑚)

(2)

where q, m, C, and ΔV are the corresponding stored charge, mass, specific capacitance, and voltage window during the CV operation, respectively. The optimal mass ratio for fabricating an HSC device was

𝑚+ 𝑚―

= 1.2. For the HSC, both the electrodes were coupled using a filter-paper 20-μm thick

separator to prevent direct contacting. The energy density and power density values of the HSC device were determined from the galvanostatic discharge profiles using the following equations: 𝐸𝑛𝑒𝑟𝑔𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 =

𝐼∫𝑉𝑑𝑡 3.6𝑀


(3)

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𝑃𝑜𝑤𝑒𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 =

𝐸 ∆𝑡

(4)

Furthermore, three HSC devices in series were used to light up the LED panel depicting the GFHIM logo that comprised 32 LEDs that require 3 V and 250 mA to light up simultaneously.

3. RESULTS AND DISCUSSION 3.1. Formation mechanism The additive-free synthesis procedure for the Sb2S3 particles encapsulated and anchored in a sulfur-doped carbon matrix is presented in Scheme 1. The hydrolysis of the Sb-based precursor has been a major drawback in preparing the Sb-based chalcogenide. Therefore, to minimize the hydrolysis of the precursor, 1,2-propanediol (CH3CH(OH)CH2OH) was used as a solvent instead of water to dissolve the antimony trichloride (SbCl3), and thioacetamide (CH3CSNH2) was used as the sulfur source. First, the antimony salt precursor reacted with 1,2-propanediol (CH3CH(OH)CH2OH) to form Sb3+ ions and a high coordination complex of antimony. In the subsequent step, these Sb3+ ions reacted with CH3CSNH2 to form a transparent solution containing Sb-complex as follows: 23 𝑆𝑏3 + + 𝑛 𝐶𝐻3𝐶𝑆𝑁𝐻2 + (𝑛 ― 𝑥)𝑂𝐻 ― →[Sb(C𝐻3𝐶𝑆𝑁𝐻2)𝑥]3 + + (𝑛 ― 𝑥)𝐻2𝑆 + (𝑛 ― 𝑥)𝐶𝐻3 𝐶𝑂𝑂𝑁𝐻4 (5) Subsequently, this solution containing the [𝑆𝑏(𝐶𝐻3𝐶𝑆𝑁𝐻2)𝑥]3 + complex was mixed with a dissolved polystyrene (PS) solution for 30 min. A part of the mixed solution was treated with microwave radiation for 10 min to obtain spherical Sb2S3 particles encapsulated with thioacetamide/hydrolyzed PS as follows: 2[𝑆𝑏(𝐶𝐻3𝐶𝑆𝑁𝐻2)𝑥]3 + + (𝑛 ― 𝑥)𝐻2𝑆 + ℎ𝑦𝑑𝑟𝑜𝑙𝑖𝑧𝑒𝑑 𝑃𝑆 + 6𝑂𝐻 ― + (𝑛 ― 𝑥 ― 3)𝐻2𝑆/𝑃𝑆 ∙ 𝑂𝐻


𝑀𝑖𝑐𝑟𝑜𝑤𝑎𝑣𝑒

𝑆𝑏2𝑆3 + 6𝐻2𝑂 (6)


The microwave radiation played an important role in the fast and equal growth of spherical Sb2S3 particles in the PS-thioacetamide matrix. In the process, the PS matrix acts as a soft template to aid in anisotropically growing the Sb2S3, thus resulting in the formation of spherical particles. The antimony–thioacetamide complex ions enclosed by the PS polymers are alchemized into uniform spherical Sb2S3 nanoparticles by a local thermal shock of microwaves. Furthermore, sulfur-ligand, thioacetamide groups are provided as the sulfur source for the PS-thioacetamide polymers to obtain a sulfur-doped carbon material subsequent to a thermal annealing process. [21] ___________________________________________________________________________

Scheme 1 (a) Schematic of evolution of Sb2S3 core–shell nanosphere encapsulated and (b) Sb2S3 nanoparticles anchored in sulfur-doped carbon matrix depending on the middle step. ___________________________________________________________________________ The other solution that was not exposed to a microwave process was stirred until the color of the solution turned yellow, which indicated the formation of Sb2S3 nanoparticles at around 25 ℃. The


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Sb2S3 particle size was limited by the confined immobilization of the nucleated sites on a PSthioacetamide matrix, which resulted in fine particles (ca. 7 nm) of Sb2S3 anchored in the PSthioacetamide matrix. ___________________________________________________________________________

Figure 1. SEM and TEM images of Sb2S3-M@S-C nanosphere. (a) Low-magnification area of the SEM images of the nanospheres with uniform diameter shown in the inset image of the particle size distribution, (b) low-magnification TEM image, (c) magnified image of nanospheres with the inset showing their core–shell structure, (d) HRTEM image of a single sphere, and (e–h) corresponding element mapping images of the core–shell spheres. ___________________________________________________________________________



The thermal annealing process of both the samples at 450 ℃ then provides sulfur-doped carbon composites, which are obtained from the thioacetamide-PS containing Sb2S3 nanoparticles. The resulting Sb2S3 morphology in the carbon matrix is dependent on the application of microwave radiation. 3.2. Structure–morphology corelation Fig. 1a shows the magnified image of the microwave-synthesized Sb2S3-M@S-C sample with a uniform diameter and spherical morphology forming a network structure. The average diameter of each particle is 0.6±0.2 μm (Fig 1a). The transmission electron microscopy (TEM) images confirm the spherical core–shell nature of the nanoparticles (Fig. 1b and 1c). The magnified image in Fig. 1c clearly shows the contrast of the core and shell of the spherical nanoparticles. The high-resolution TEM (HRTEM) image of this particle (highlighted portion of Fig. 1c inset image) confirms that the core consists of crystalline SB2S3 with a lattice spacing of 0.35 nm corresponding to the (130) of stibnite and shell, which contains amorphous carbon that forms the unique core–shell structure presented in Fig. 1d. The energy dispersive Xray analysis (EDAX) elemental mapping of one spherical particle (Fig. 1e–1h) clearly indicates the major elemental distribution of Sb and S at the particle core and the gradient layer of carbon and sulfur at the outer shell. These confirm that the core comprises Sb2S3 and the shell comprises hierarchical and porous sulfur dispersed in a carbon matrix. The outer hierarchical layer (Fig. S1) provides a large surface area for electrolyte accommodation and sulfur incorporation in the carbon matrix, which provides robust conductivity. In addition, it also acts as a mechanical buffer layer for protecting and accommodating the volume change of the chalcogenide during charge– discharge cycles. 12-13


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___________________________________________________________________________

Figure 2. SEM and TEM measurements of Sb2S3-P@S-C sample. (a−c) SEM images with various magnifications, (d−e) TEM images with various magnifications showing the dispersed Sb2S3 particles on a carbon material, (f) HRTEM image of a single particle showing the lattice planes of Sb2S3 with d = 0.35 nm, and (g–j) elemental mapping images of Sb2S3 nanoparticle on a carbon sheet. ___________________________________________________________________________ The scanning electron microscopy (SEM) images of the Sb2S3-P@S-C sample are presented in Fig. 2a–2c. The low-magnification image from the broad area (Fig. 2a) reveals the numerous fine 13 ACS Paragon Plus Environment


particles that are closely and uniformly wrapped in the carbon matrix in a 3D form. The fine nanoparticles appeared to be encapsulated inside with very few transparent well-interconnected layers of carbon forming a unique 3D honeycomb-like structure (Fig. 2b and 2c). The TEM images confirm the presence of a 3D honeycomb-like structure with abundant fine particles anchored in the 3D carbon framework (Fig. 2d and 2e). This observation is in accordance with the topographical observation made via SEM. These fine particles in the range of 5–10 nm are anchored and well dispersed in the carbon matrix. The HRTEM image of the individual nanoparticles confirms their superior crystallinity with a lattice spacing of 0.35 nm corresponding to (130) of Sb2S3 (Fig. 2f). Furthermore, an amorphous layer (oxysulfide phase of Sb2S3) surrounding the crystalline lattice (Fig. 2f and 2d; indicated by red arrows) is observed in the Xray diffraction (XRD) results (discussed in the next section). The elemental distribution of the sample was obtained via EDAX elemental mapping (Fig. 2g–2j). As expected, the particles are Sb, and the distribution of carbon and sulfur is uniform throughout the imaging area. The elemental mapping results (Fig. 2g–2i) further confirm the uniform distribution of S in the carbon matrix, which enhances the conductivity of the electrode and results in a higher capacitance value. Fig. 3a presents the XRD pattern of the Sb2S3-M@S-C sample with two distinct phases. The major phase matches with that of the joint committee on powder diffraction standards (JCPDS) 42-1393, which has an orthorhombic crystal system and Pnma space group symmetry with the first three intense peaks at 29.2, 32.3, and 25.04 corresponding to the (211), (212), and (103) planes, respectively, of the stibnite (Sb2S3) phase. The minor phase (JCPDS 50-0926) with intense peaks at 22.7°,14.8°, and 16.6° corresponds to the (120), (110), and (002) respective planes of hexagonal carbon. However, the XRD patterns for Sb2S3-P@S-C (Fig. 3b) show one major and two minor phases. The major phase of the stibnite (Sb2S3) (JCPDS 42-1393) has a similar crystal


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structure and space group as those of the Sb2S3-M@S-C sample. The first minor phase matches with JCPDS 79-1410 and has P63/mmc space-group symmetry with major peaks at 43.9, 41.9, and 47, respectively, corresponding to the (102), (101), and (103) planes of the hexagonal carbon. The second minor phase matches with JCPDS 71-1789 with the F1 space group with three major peaks at 28.4, 30.6, and 34.3, respectively, corresponding to the (−224), (224), and (−802) planes of kermesite (Sb2S2O). The presence of the sulphate phase could be owing to the partial oxidation of the product during annealing. To further confirm the XRD structural characteristics, the Raman and Fourier-transform infrared (FTIR) spectra are obtained and compared for both the samples. The Raman spectra, shown in Fig. 3c, provide the ultimate structural confirmation for both the carbon and stibnite-based system. According to group theory, 30 Raman active modes are expected for the Pnma phase of Sb2S3. 24-25 𝛤 = 10𝐴𝑔 + 5𝐵1𝑔 + 10𝐵2𝑔 + 5𝐵3𝑔

(7)

From Fig. 3c, the peaks observed at 118 and 156.8 cm–1 correspond to the 𝐵42𝑔 and 𝐵52𝑔 vibration of Sb2S3, respectively, for both the samples with the lesser peak intensity observed at 362.2 cm–1 assigned to the Sb-S vibration.

25-26

This comparative spectrum indicates several interesting

physical consequences of the electronic interaction of carbon and stibnite. Two strong characteristic peaks of carbon, i.e., the D and G bands at 1340 and 1580 cm–1, respectively, are observed for both the samples, which confirms the presence of carbon. In Raman spectrum of the microwave-synthesized sample (Fig. 3c-i), sharp peaks at 118 and 156 cm–1, corresponding to antimony compounds, were observed rather than those related to the carbon shell which well encapsulate a stibnite sphere as shown in Fig. 1d.27 Similar features are observed in the case of the extra loading of Sb2S3 with carbon composite reported by Zhong et al. 16 The ratio of the D-band and G-band intensities (ID/IG) for both the samples is calculated as 0.80 and 0.89 for Sb2S3-P@S-



C more than Sb2S3-M@S-C (Fig. 3c), respectively, which indicates the increased disorder in the graphitic matrix. ___________________________________________________________________________

Figure 3 Structural confirmation of samples. XRD patterns with JCPDS references of (a) Sb2S3M@S-C and (b) Sb2S3-P@S-C samples. Comparison of the (c) Raman spectra and (d) FTIR spectra. N absorption–desorption isotherm of (e1) Sb2S3-M@S-C, (e2) Sb2S3-P@S-C with inserts of the pore size distribution. XPS elemental confirmation (f) survey scan, (g) deconvoluted C1s spectra, (h) deconvoluted Sb spectra, and (i) deconvoluted S2p spectra, and (j) TGA spectra with different reaction zones (indicated by arrows). ___________________________________________________________________________ The presence of surface functional groups of carbon was analyzed using FTIR spectra. Fig. 3d shows the FTIR spectra for the both samples in the range of 400 to 4000 cm–1. The strong wide 16 ACS Paragon Plus Environment

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band observed at 3385 cm−1 is ascribed to the stretching vibration of −OH groups (indicated by the watermark). The other prominent bands are observed in the range of 400 to 1800 cm–1. The vibrations at 1,632 and 1,385 cm–1 are the signature absorption peaks of the stretching vibration of the six-member aromatic carbon containing double bonds, whereas the peaks at 1116 cm–1 are the characteristic peaks of the carbonyl group. 28 An intense peak at 615 cm–1 and the shoulder peak at 997 cm–1 are deformed skeleton peaks of the C–H bond in styrene or mono-substituted benzene rings, indicating the presence of S atoms in the sp2 carbon matrix (peak at 1373 cm–1). 26 Thus, the absence of carboxylic stretching (1700 cm-1) and bending (1235 cm-1) vibrational peaks and 29 the presence of prominent peaks at 1632, 1116, and 615 cm–1 confirm the restoration of the aromatic sp2 hybrid carbon skeleton owing to the reduction and monoatomic substitution in the styrene. Furthermore, the reduction and atomic substitution is more pronounced in the Sb2S3-M@S-C sample than in Sb2S3-P@S-C (based on the peak intensity change). The surface areas of both the samples are estimated from the N absorption–desorption isotherm. A type-IV isotherm with a distinct hysteresis loop at the relative pressure P/P0 range of 0.5 to 1 (Fig. 3e1) was observed for the Sb2S3-M@S-C sample, which suggests the existence of micropores and mesopores. However, the Barrett–Joyner–Halenda (BJH) pore-size distribution depicts a sharp peak centered at 5 nm, which confirms a higher mesopore density than the micropore density. Moreover, the Sb2S3-P@SC sample also shows a hysteresis loop in the relative pressure P/P0 range of 0.5 to 1 (Fig. 3e2) with the BJH distribution (Fig. 3e2 inserts), which suggests the existence of micropores and mesopores in the sample. The Brunauer–Emmett–Teller (BET) result shows that the Sb2S3-M@S-C and Sb2S3-P@S-C samples possess a surface area of 5.99 and 7.58 m2g−1, respectively. In addition, Xray photoelectron spectroscopy (XPS) was used to obtain the surface elemental composition and electronic structure of both the materials. Fig. 3f shows the survey scan of both the samples, which



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indicates the presence of Sb, S, and C without other foreign impurities. As expected, regular signals from S 2p, Sb 3d, and C 1s are observed with the S:Sb peak ratio of approximately 1.5 for both samples (Fig. 3f), thus confirming the presence of stibnite in the form of Sb2S3.

29

The high-

resolution spectra of C 1s (Fig. 3g) is deconvoluted to three peaks in both cases that are positioned at 284.4, 285.9, and 287.6 eV corresponding to sp2, sp3, and C−OH bonds, respectively.

30

The

peak position and peak intensity are of close correspondence for both the samples. In the case of Sb 3d, a spin-orbit doublet separated by a peak separation of 9.4 eV (Fig. 3h) corresponding to Sb 3d3/2 (538.2 eV) and Sb 3d5/2 (528.8 eV), respectively, was observed, which confirms the presence of Sb3+ in Sb2S3. 31 This Sb3d peak was deconvoluted to five peaks (Fig. 3h), i.e., two peaks of 3d3/2, two for 3d5/2, and one for O1s at the binding energy of 539.4, 538.2, 528.8, 530, and 532.4 eV, respectively. This O1s peak in both the samples corresponds to surface hydroxyl groups.

32

This is in accordance with the XRD and FTIR results. The S 2p peak of both the samples are deconvoluted and presented in Fig. 3i. The S 2p peaks at 160 and 161.3 eV correspond to the S2states of Sb2S3. Moreover, the additional peaks (observed for the Sb2S3-P@S-C sample) at 163.5 and 162.7 correspond to the –C–S–C– covalent bond of the thiophene-type or –C–SOx–C sulfur. This result is in agreement with the extant literature.

24-25

A thermogravimetric analysis (TGA)

under a nitrogen atmosphere was performed to determine the thermal stability and mass content of both the samples. Fig. 3j shows the 3D TGA graph with the typical reaction zones indicated by arrows. Three reaction zones are observed for both the samples (indicated by the cyan-colored arrow). In the first reaction zone (below 100 °C), weight is lost owing to the evaporation of physisorbed atmospheric water because the S atom has a strong affinity for oxygen. 33 The second reaction zone is in the range of 248 to 345 °C where the weight loss is ascribed to the decomposition of Sb2S3 in both the samples. The weight loss in this zone is 15% and 12% for the


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Sb2S3-M@S-C and Sb2S3-P@S-C samples, respectively. In the temperature range of 400 to 657 °C, the large weight loss of approximately 62% and 70% for the Sb2S3-M@S-C and Sb2S3-P@S-C samples, respectively, is attributed to the carbon combustion. The rate of weight loss in this zone is lower for the Sb2S3-P@S-C sample than that of the Sb2S3-M@S-C owing to the presence of surface functional groups of carbon in Sb2S3-P@S-C as observed in the FTIR and XPS analyses. In the range of 630 °C to 750 °C, a weight loss of 1% is observed for the Sb2S3-P@S-C sample, whereas that for the Sb2S3-M@S-C sample is minimal. The weight loss in this region for the Sb2S3P@S-C sample is attributed to the decomposition of the antimony oxide present in the sample, 34 which is in accordance with the XRD and TEM results. Thus, from the above observations, an Sb content of 17% and 22% was estimated for the Sb2S3-P@S-C and Sb2S3-M@S-C samples, respectively. 3.3. Electrochemical half-cell performance Owing to their compelling structure and morphology, the electrochemical performance of the Sb2S3M@S-C and Sb2S3-P@S-C samples were obtained in a three-electrode configuration using a 1-M KOH electrolyte. Fig. 4a shows the cyclic voltammograms of Sb2S3-M@S-C at various scan rates (20, 30, 50, 70, and 100 mV∙s−1. The cyclic voltammetry (CV) curves form an oval shape with the specific capacitance varying linearly with the scan rate, which indicates the electrical double layer behavior of the material. At a relatively low scan rate of 5 mV∙s-1 shown in Fig. S2, Faradic humps were observed at 0.35 and 0.38 V, which reflects the reversible transitions of Sb3+/Sb4+ with the aid of K+ and OH− ions. The disappearance of the redox peak at a higher scan rate indicates the dominance of the surface absorption/desorption behavior over the diffusion-mediated intercalation/deintercalation. 35-36 Thus, the reinforced redox peaks can be attributed to the redox reaction of the active Sb atoms from Sb2S3 with the electrolyte ions following the reaction:



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𝑆𝑏2𝑆3@𝐶 +𝐾 + + 𝑒 ― →(𝑆𝑏2𝑆3@𝐶𝐾)𝑆𝑢𝑟𝑓𝑎𝑐𝑒

(8)

𝑆𝑏2𝑆3@𝐶 + 𝑥𝐾 + + 𝑥𝑒 ― ↔𝐾𝑥+ 𝑆𝑏2𝑆3@𝐶

(9)

which is an important contribution to the overall capacitance. 17 Furthermore, at various scan rates, the CV curves of Sb2S3-P@S-C completely follow the electric double-layer capacitor (EDLC) nature of reversible absorption/desorption of the ions (Fig. 4b) without a redox peak at a low scan rate in a potential window of 0 to 1 V with respect to the Hg/HgO electrode (Fig S2-c). This typical change in the CV characteristics of the Sb2S3-P@S-C electrode at a lower scan rate indicates its superior rate capability.

37-38

To observe the high-power performance of the hybrid

nanostructures, the galvanostatic charge–discharge (GCD) curves of the electrodes are measured at various current densities ranging from 1 to 12 A∙g−1. Fig. 4c shows the GCD curve of Sb2S3-M@S-C at various current densities in a potential window range of 0 to − 0.8 V not – 1.0 V to maximize its Coulombic efficiency. ___________________________________________________________________________

Figure 4. (a, b) CV and (c, d) GCD curves at various scan rates (5–100 mV∙s−1) and current densities (1–12 A∙g−1); (e) specific capacitance at various current densities; (f) comparative Nyquist plots with the insets of Randle’s equivalent circuit and magnifed plot in a higher frequency region; (g) comparative analysis of specific capacitance of both electrodes with literature and (h)


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showing the existence of individual potential windows for the Sb2S3-P@S-C positive and Sb2S3M@S-C negative electrodes at a 5-mV∙s−1 scan rate. ___________________________________________________________________________ Nearly symmetric curves without curvature were observed for the Sb2S3-P@S-C electrode (Fig. 4d). The EDLC charge–discharge behavior of the Sb2S3-P@S-C electrode is a collaborative contribution from the carbon and fine Sb2S3 particles instead of the limited charge–discharge duration of the pristine carbon (derived from the same precursor Fig. S3). From the GCD curves, the specific capacitance of the electrodes was estimated as 1179 and 1380 F∙g-1 at 1 A∙g-1 for the Sb2S3P@S-C and Sb2S3-M@S-C electrodes, respectively (Fig. 4c and 4d). The comparison of the specific capacitance for both the electrodes at various current densities is shown in Fig. 4e. Approximately 24% and 15 % of the specific capacitance is lost when the current density varies from 1 to 12 A∙g–1 for the Sb2S3-M@S-C and Sb2S3-P@S-C electrodes, respectively, which indicates the excellent rate performance of the Sb2S3-P@S-C electrodes. The 3D network carbon structure encapsulates the fine nanoparticles of Sb2S3 in Sb2S3-P@S-C, which provides large electroactive sites and transport channels in the electrolyte. The ion diffusion and charge transfer behavior of the electrode reflects its electrochemical performance. The charge transfer behavior of the electrodes was studied based on the electrochemical impedance spectroscopy (EIS) measurements. Fig. 4f shows the comparative Nyquist plots of the EIS data obtained for both the electrodes and the insets with the magnified high-frequency semicircle spectra and the Randle’s equivalent circuit. In the inset, the Re (X-axis intercept) value defines the ohmic resistance of the electrodes. Rct+Re defines the sum of bulk electrolyte resistance and the charge transfer resistance at the electrolyte/electrode interface. 39 The series resistances (Re,) of the Sb2S3-M@S-C and Sb2S3-P@S-C electrodes are 1.01 and 1.09 Ω, respectively, which confirms the good contact resistance between the material and nickel foam. Both the electrodes demonstrated a minimal charge transfer resistance (Rct, calculated from the semicircle diameter), i.e., 1.33 and 2.18 Ω,



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respectively, which matches with the Rct caused by the faradaic reactions and EDLC at the electrode/electrolyte interface. The variations in the values are related to the structure morphology changes of the materials. Moreover, the low-frequency linear graphs confirm the superior capacitive nature of both the materials. However, the more vertical line in the low-frequency EIS profile confirms the superior SC performance of the Sb2S3-P@S-C electrode over that of the Sb2S3-M@S-C electrode owing to the presence of high active surface pores. ___________________________________________________________________________

Figure 5. Post-mortem report of both electrodes. (a) Cycle stability after 10,000 GCD cycles at 10 A∙g−1. Comparison of the spectra of the pristine and after-2,000 GCD cycle (b) for Sb2S3-M@S-C and (c) Sb2S3-P@S-C. SEM images and EDAX spectrum of electrodes after 10,000 cycles (d, e and f) for Sb2S3-M@S-C and (g, h and i) Sb2S3-P@S-C, and (j) XRD patterns of Sb2S3-P@S-C and Sb2S3M@S-C electrodes after the 10000-cycle test. ___________________________________________________________________________ Fig. 4g shows the comparative analysis of the specific capacitances of both the Sb2S3-M@S-C and Sb2S3-P@S-C electrodes with those in the extant literature.

17-20

The presence of oppositely

charged storage kinetics was involved in Sb2S3-M@S-C and Sb2S3-P@S-C owing to the change in the chemical states of the electrodes as evidenced on screening their CV curves in the -1.0–0-V 22 ACS Paragon Plus Environment

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and 0– +1.0-V potential windows in 1-M KOH electrolyte at a scan rate of 5 mV∙s−1 (Fig. 4h). As reported in the literature, Sb2S3@carbon composite demonstrates the performance as a dual electrode for rechargeable battery.

40

Moreover, Sb2S3 nanomaterials coated with poly(3,4-

ethylenedioxythiophene) have demonstrated superior supercapacitive performance in a positive electrode window, 20 whereas pure Sb2S3 thin-film as a negative electrode (in a potential window of −0.9 to 0.15 V).17 To demonstrate the structural stability of both the electrode materials, their long-term cycling stability is tested and presented. As shown in Fig. 5a, both the electrodes exhibit excellent cycle stability at 10 A∙g–1. Nearly 95% and 98% of the specific capacitance retention was estimated after 10,000 continuous GCD cycles for Sb2S3-M@S-C and Sb2S3-P@S-C, respectively. Both the electrodes demonstrated a superior Coulombic efficiency of nearly 100% retention after 10,000 cycles of GCD. To understand the change in capacitance with increase in cycle life of the electrodes, impedance measurements are obtained after every 2,000 charge–discharge cycles. Fig. 5b and 5c show the comparative EIS profiles of the pristine and after 2,000 cycles of GCD for Sb2S3-M@S-C and Sb2S3-P@S-C, respectively. In the Randle’s equivalent circuit, the Rct+Re values increase to 4,000 cycles, which indicates the degradation of the surface functional groups present in active material and reduced infiltration of electrolyte into the electrode. After 4,000 cycles, very minor changes are observed owing to the increased infiltration of the electrolyte into the electrodes 41 To further validate this, the SEM images of the samples after 10,000 cycles are obtained and presented in Fig. 5d and 5e. In the case of the Sb2S3-M@S-C sample, spherical particles with a slightly increased volume were observed owing to the electrolyte ion loading (Fig. 5d and 5e). This confirms the role of carbon in preventing unwanted volume expansion during the charge–discharge cycles. The low-magnification image of the Sb2S3-P@S-C depicts the distinct area of metal ion



phases and distortion of the 3D integrity (Fig. 5g and 5h). The EDAX patterns of both the samples also indicated the presence of Sb, C, O, S, and K in both the samples (Fig. 5f and 5i). To understand the exact phase formation, the XRD patterns of the samples are obtained. The XRD pattern of Sb2S3-P@S-C in Fig. 5j is shown for two different phases of K0.51Sb2.67O6.26 (JCPDS 29-0132) and an additional phase of K(HCO3) (JCPDS 33-0111). In the case of the Sb2S3-M@S-C electrode, Sb6O7(SO4)2 and Sb6O13 appeared owing to the bulky Sb2S3 content in the sample and their participation in the charge-discharge processes. The relatively poor cycling performance of approximately 95% less in the case of the Sb2S3-M@S-C electrode as compared to that of the Sb2S3P@S-C electrode could be owing to the larger Sb2S3 sphere and less open space for electrolyte interaction resulting in more stresses and defects in comparison to those in Sb2S3-P@S-C. 3.4. Electrochemical Sb2S3-M@S-C/Sb2S3-P@S-C hybrid device performance To evaluate the practical application of the electrodes in SC as an energy storage device, HSC devices comprising Sb2S3-M@S-C/Sb2S3-P@S-C were fabricated with a 20-μm filter paper as the separator. The operating voltage of the HSC assembled with Sb2S3-M@S-C/Sb2S3-P@S-C could extend to 2.0 V; however, considering the water hydrolysis voltage of commercial electrolyzers (1.6–2.0 V), we restricted its operation to under 1.5 V (Fig. 4h). The schematic of these devices, consisting of the electrodes and electrolyte, is presented in Fig. 6a. Fig. 6b shows the nearly rectangular CV curves of the HSC at scan rates of 20 to 100 mV∙s–1 in a potential window of 0–1.5 V. With the increased scan rate, the device retains its capacity values and nearly rectangular shape even at a high scan rate of 100 mV∙s−1, which indicates an SC performance with good reversibility, a fast charge–discharge feature, and good rate performance. 42 The redox peaks in the CV curves correspond to the pseudocapacitive behavior of Sb2S3. 2, 35 (Fig. S4) Slight peak position shifts in the cathodic and anodic peaks are observed with the increase in the sweep rate from 5 to 100 mV∙s−1 (Fig. S4). This slight shift indicates a very low resistance


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in both the electrodes owing to the superior electric conductivity of Sb2S3 and sulfur-doped carbon channels for ionic and electronic transport between the active Sb2S3@S-C and nickel-foam current collector.

42

From the CV curves at different scan rates (Fig. 6b), the cathodic and anodic peaks are

observed over the entire scan range; however, they are more prominent at a lower scan rate. To further confirm this, the GCD curves of the device were obtained for various current densities from 1 to 12 A∙g−1 (Fig. 6c). The nearly triangular GCD curves with a slight curvature in the charging–discharging curves clearly confirm the combined pseudocapacitive and EDLC contribution to the overall capacitance. 43 Simultaneously, the EIS profile was acquired to represent the superior electrochemical performance of the device. Fig. 6d represents the EIS profile of the device with a charge transfer resistance (Rct) that is slightly greater than the Sb2S3-M@S-C electrodes (1.5 versus 1 Ω). This value is very low, which confirms the faster electrochemical charge transfer between the electrode and electrolyte. Furthermore, Re at a high frequency and inclination angle at a low frequency were estimated as 1 Ω and 78° (shown in Fig. 6d inserts), respectively, which confirms the SC nature of the charge transfer and superior rate capability of the device. The capacitance of the device was calculated from the GCD curves as a function of the current density based on Eq. (1) with determining both loading mass from Eq. (2) (Fig. 6f). The HSC device demonstrates a maximum capacitance of 628 F∙g–1 at 1 A∙g–1. It should also be noted that the HSC retains 60 % of the capacitance when the current density is increased by six times. This drop-in capacitance with the increased current density results in a high current, which deficits the active materials in the redox reaction. 44 The cycle stability and rate capability are important device parameters for real time application. The cycle life of the device was calculated from the GCD curves measured at 10 A∙g–1 (Fig. 6e) over 10,000 cycles. After 10,000 cycles, the device could preserve 94% of its initial capacitance. The coulombic efficiency of the HSC device remains 95 % after 10, 000 GCD cycles at 10 A∙g–1. The energy density and power density are the main parameters in



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confirming the device potential for actual application. Using Eq. (3) and (4), the energy density and power density, respectively, of the HSC device was calculated and presented as the Ragone plot in Fig. 6f with a comparative correlation to the conventional capacitor and batteries. The highest energy density of 49 Wh∙kg−1 at a power density of 2.5 kW∙kg−1 was estimated for the HSC device, far beyond the Sb2S3-based asymmetric devices and carbon@ MoS2 nanospheres anode for SCs (Table S1). 18, 20 45-51 This superior energy density and power density of the device was the contribution of the large surface area of unique morphologies, conduction of sulfur-doped carbon networks, and squeezing of electrolyte-carrying active surface area between Sb2S3 and carbon. Three HSC devices are used in series to light an LED panel comprising 32 LEDs depicting the global frontier hybrid and interface materials (GFHIM) logo (requires 3 V and 250 mA) for 2 min (Fig. 6g), which demonstrates their potential in actual applications. In terms of electrochemical characteristics, the device demonstrated a superior rate performance, cycle stability, and energy density. To further explore its superior performance, a quantitative analysis was performed based on the charge-storage mechanism. As reported in the extant literature, 47, 52-53 in

a linear sweep voltammetry, the peak current follows the power law with a scan rate as 𝑖 = 𝑎𝜗𝑏

(10)

where a and b are rate constants. b is calculated for both the samples, i.e., slope of log ip versus log V (Fig. S5). The value of b of the HSC device for the oxidation and reduction peaks was estimated as 0.73 and 0.86, respectively, thus confirming their surface Faradic and capacitive mechanism-controlled capacitance at the electrode and electrolyte interface. 53 Further calculations were performed to obtain the individual contributions to the overall capacitance of the device. According to the Randel–Sevcik interpretation of CV, the total capacitive contribution to the current response can be presented as follows. 𝑖(𝑉) = 𝑘1𝜗 + 𝑘2𝜗1/2


(11)

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where k1 and k2 are constants for a particular scan rate. The term 𝑘1𝜗 in Eq. (11) defines the capacitive contribution, and the term 𝑘2𝜗1/2 represents the diffusion-controlled processes with respect to the peak current at a particular voltage. Eq. (11) can also be written as follows. 𝑖 𝜗1/2

= 𝑘1𝜗1/2 + 𝑘2

(12)

The linear fitting of the voltammetry current at each potential, k1 and k2, can be calculated. From the estimated values of k1 and k2, the capacitive current and diffusion-controlled current can be separately obtained. 54As shown in Fig. 6i, the green shaded area represents the surface capacitive contribution (55% at a 5-mV∙s–1 scan rate). Fig. 6j presents the bar diagram of the capacitive and diffusion-controlled contributions at different scan rates and shows the decrease in the diffusive contribution with the increase in the scan rate from 55% at 5 mV∙s–1 to 12% at 100 mV∙s–1. As reported in the extant literature, a more diffusion-controlled process could result in a system with a poorer rate capability; however, in this case, the majority of the charging in the Sb2S3@S-C electrodes is caused by the immediate supercapacitive contribution, which provides the system with a high rate capability. Thus, the unique structural design of the electrodes plays a vital role in its electrochemical performance. Based on the above observations, both the electrodes showed minimal changes in the EIS profiles (Re and Rct values) measured after the 2,000-charge–discharge-cycles interval and minimal alteration in their morphologies before and after the charge–discharge cycles, thus confirming a minimal change in their conductivity and morphology. ___________________________________________________________________________



Figure 6. (a) Schematic of the HSC device; (b) CV, (c) GCD curves of the HSC at various scan rates (5 to 100 mV∙s–1) and current densities (1 to 12 A∙g–1); (d) Nyquist plots of the device (the inset presents the magnified high-frequency region of the impedance spectra); (e) cycling stability of the device over 10,000 cycles at a current density of 10 A∙g−1 with inserts of the (f) specific capacitance as a function of the current density and (g) digital image of the 32 red LEDs lit by three HSC devices in series; (h) Ragone plots of the HSC, (i) separation of capacitive-controlled and diffusion-controlled charges at 50 mV∙s-1, and (j) contribution ratios of the capacitivecontrolled and diffusion-controlled processes at different scan rates for the HSC device. ___________________________________________________________________________ The carbon matrix not only provides a large surface area, but also supports the Sb2S3 nanostructure for agglomeration, prevents volume expansion, and provides conductivity in the composite structure. Moreover, the Sb2S3 nanostructures provide additional redox sites, which shorten the electron transfer paths, thus enhancing the electron transport kinetics in the composite structure. Therefore, the above analysis in correlation to semi-quantitative measurements helps in 28 ACS Paragon Plus Environment

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understanding the electrodes synergistical change in structural morphology and in transfer kinetics and their contribution to the electrochemical SC performance and rate capability of the electrodes. This study also provided a platform to scale-up the facile technology in an industrial scale for SC application.

4. CONCLUSIONS A simple microwave-mediated process was used to fabricate spherically uniform Sb2S3 nanoparticles encapsulated by sulfur-doped carbon. The perfect shielding of Sb2S3 nanoparticles in the carbon matrix doped with sulfur is a key in achieving a cycle-stable electrode material that endures long-term charge–discharge cycles. In addition, in comparison to the conductivity of Sb2S3, that of the composite was enhanced by the carbon sheet, which has a high inherent conductivity and serves as a buffer matrix for accommodating volume changes, thus maintaining the integrity of the electrode. In the case wherein microwave radiation was not used, the Sb2S3 anchored in the sulfur-doped carbon matrix exhibited a predominantly superior electrochemical performance with the property of electrostatic double-layered capacitance. The two composite electrodes demonstrated high specific capacitance values of 1179 and 1380 F∙g–1, respectively, with an excellent life span (98% over 10,000 cycles). The HSC assembled using Sb2S3-M@S-C as a negative electrode and Sb2S3-P@S-C as a positive electrode exhibited impressive energy and power densities of 49 Wh∙kg−1 and 2.5 kW∙kg−1 at 2 A∙g-1 and an excellent cycle stability (94%, 10,000 cycles). Using three HSC devices in series, 32 LEDs in an LED panel depicting the GFHIM logo (Fig. 6g) were powered for demonstrating their application. It should be noted that Sb2S3 is a good solar absorbing material, which can be effectively used in future rational designs of solarpowered SCs in eco-friendly applications. We believe that this work could potentially motivate



more researchers to conduct studies on scaling up of Sb2S3@carbon-based supercapacitor devices and, thus, pave the way for the development of high performance SCs.

AUTHOR INFORMATION *Corresponding

author:

E-mail: [email protected] (Kwang Ho Kim) E-mail: [email protected] (Je Moon Yun) E-mail: [email protected] (Se Hun Kwon) *Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2013M3A6B1078874). We would like to thank Jeong Seonghee, the FE-SEM and BET operations technician, managed by the National Core Research Center for Hybrid Materials Solution (NCRC), for their excellent work on acquiring morphology images.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxx. Preparation of hydrolyzed polystyrene, SEM of Sb2S3-M@S-C, CV curves of Sb2S3-M@S-C and Sb2S3-P@S-C electrodes, Comparative GCD curves at a current 30 ACS Paragon Plus Environment

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density of 1 A g-1, CV plots of HSC device at different scan rate, and Variation of log ip versus log (scan rate) of HSC (PDF)



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Sb2S3 Nanoparticles Anchored or Encapsulated ... - ACS Publications

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