Layer-Structured Copper Antimony Chalcogenides - American

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Layer-Structured Copper Antimony Chalcogenides (CuSbSeS ): Stable Electrode Materials for Supercapacitors x

2-x

Karthik Ramasamy, Ram K. Gupta, Soubantika Palchoudhury, Sergei Ivanov, and Arunava Gupta Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5041166 • Publication Date (Web): 12 Dec 2014 Downloaded from http://pubs.acs.org on December 15, 2014

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Chemistry of Materials

Layer-Structured Copper Antimony Chalcogenides (CuSbSexS2-x): Stable Electrode Materials for Supercapacitors Karthik Ramasamy,1,3* Ram. K. Gupta,2 Soubantika Palchoudhury,3 Sergei Ivanov,1 Arunava Gupta3* 1

Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Albuquerque, NM87185 2

3

Department of Chemistry, Pittsburg State University, Pittsburg, KS-66762

Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL35487 Email: [email protected]; [email protected]

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Abstract: The ever-growing need for energy generation and storage applications demands development of materials with high performance and long term stability. A sizeable number of chalcogenide-based materials have been investigated for supercapacitor applications. Layerstructured chalcogenides are advantageous in terms of providing large surface area with good ionic conductivity and ability to host a variety of atoms or ions between the layers. CuSbS2 is a ternary layered chalcogenide material that is composed of earth abundant and less-toxic elements. For the first time we have developed a simple colloidal method for the synthesis of CuSbSexS2-x mesocrystals over the whole composition range (0 ≤ x ≤ 2) by substitution of S with Se. Our approach yields mesocrystals with belt-like morphology for all the compositions. X-ray diffraction results show that substitution of sulfur with selenium in CuSbS2 enables tuning the width of the interlayer gap between the layers. In order to investigate the suitability of CuSbSexS2-x mesocrystals for supercapacitor applications, we have carried out electrochemical measurements by cyclic voltammetry and galvanostatic charge-discharge measurements in 3M KOH, NaOH and LiOH electrolytes. Our investigations reveal that the mesocrystals exhibit promising specific capacitance values with excellent cyclic stability. The unique properties of CuSbSexS2-x mesocrystals make them attractive both for solar energy conversion and energy storage applications. Keywords:

Copper

Antimony

Sulfide,

Copper

Antimony

Supercapacitors.


Selenide,

Mesocrystals,

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Introduction: Supercapacitors are emerging energy storage devices that are complementary to conventional batteries.1–3 As compared to batteries, supercapacitors offer high power density, rapid charge-discharge cycles, long-life durability and improved safety.4–6 Based on the storage mechanism, supercapacitors have been categorized either as electrochemical double layer capacitors that store energy in a non-Faradaic electrostatic process or pseudocapacitors in which Faradaic redox reactions are involved.7–9 Double layer capacitance behavior has largely been observed in carbon-based materials, including carbon nanotubes (CNTs), graphite and graphene.10–12 Energy storage in these materials is recognized as occurring through adsorption of electrolytes on the electrode surface.13 In contrast, a large group of transition metal oxides exhibit pseudocapacitance behavior.14 Besides oxides, a number of chalcogenide-containing materials have been investigated as electrodes for supercapacitors. These include transition metal chalcogenides (VS2, CuS, CoE2, NiE2, E = S or Se),15–19, rare-earth metal sulfides (La2S3, Sm2S3),20,21 and layer-structured chalcogenides (MoS2, SnSe).22,23

Amongst these, layer-

structured chalcogenides are especially attractive because of their large surface area and ability to host smaller atoms or ions between the layers.24,25 The use of ternary or higher-order layered chalcogenide-based materials with more than one metal ion is particularly appealing, since such materials provide opportunities for rich redox reactions in addition to the flexibility of tuning the van der Waals gaps between layers. The gap can host a variety of ions from the electrolytes, thereby increasing the prospect of enhanced specific capacitance and stability. However, such higher-order layered chalcogenide materials have thus far not been investigated for supercapacitor applications. Copper antimony sulfide (CuSbS2) is one of the layer-structured chalcogenide material containing more than one metal ion. CuSbS2 belongs to an orthorhombic crystal system with Pnma space group.26 It consists of SbS2 and CuS3 chains along the b-axis, which are formed by linkage of Sb square pyramids and CuS4 tetrahedral links. These two infinite chains are interconnected to produce sheets that are perpendicular to the c-axis.27 The interlayer distance between the layers in CuSbS2 is 2.051 Å, which is sufficiently wide for intercalation of a number of smaller atoms, ions or molecules. Interestingly, CuSbS2 has also been identified as a potential low-cost and less-toxic material for solar energy conversion because of its suitable band gap and



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large absorption co-efficient in the visible region of solar spectrum.28,29 We have recently reported solution-based methods to synthesize nanoplates and mesobelts of CuSbS2, along with an intercalation-exfoliation process to obtain mono- to few layers of the material.30 In a separate study, we investigated the use of CuSbS2 as a counter electrode for dye-sensitized solar cells (DSSC) and found that it exhibits catalytic performance comparable to or better than Pt.31 It’s remarkable stability against the corrosive iodine/iodide redox couple encouraged us to investigate the material for supercapacitor applications. Moreover, advancing our synthesis and study of CuSbS2, herein we report the colloidal synthesis of mesocrystals of CuSbSexS2-x with Se substitution over the entire composition and their use as electrode materials for supercapacitors. Our studies indicate that CuSbSexS2-x are stable materials for a large number of charging and discharging cycles with promising specific capacitance values. In addition, with changing the ratio of sulfur to selenium in CuSbSexS2-x, the van der Waals gap can be continually varied leading to tunable capacitance values in different electrolytes. The demonstrated applicability of CuSbSexS2-x in supercapacitors makes them a unique class of materials that can be used both in energy conversion as well as charge storage processes. Experimental details Methods All chemicals were used as received and the solvents were dried over molecular sieves and purged with high purity argon for 30 minutes before use. 1-dodecanethiol (1-DDT, 98.0 %), t-dodecanethiol (t-DDT, 98.0 %), diphenyldiselenide (DPDS, 98.0%) and antimony chloride (SbCl3.6H2O, 99.5 %) were received from Alfa Aesar; copper acetylacetonate (Cu(acac)2, ≥99.0%), and oleylamine (OLA, ≥80-90.0%) were obtained from Acros Organics and Pfaltz and Bauer, respectively. Analytical grade hexane and ethanol were obtained from Aldrich Chemical Co. Synthesis of CuSbSexS2-x Mesocrystals All experiments were carried out in a fume hood under N2 atmosphere using a standard Schlenk technique. In a typical synthesis of CuSbSexS2-x, a mixture of 0.5 mmol of Cu(acac)2, 0.50 mmol of SbCl3. 6H2O and 10 mL of OLA was deaerated at room temperature for 15 min, placed under N2 and subsequently heated to reaction temperatures of 250-255 ˚C. In a separate vial, corresponding amount of DPDS was dissolved in a mixture of 0.25 mL of 1-DDT and 1.75


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mL of t-DDT. The solution was quickly injected into the metal source mixture held at the target synthesis temperature, with continued stirring of the resulting mix for 10-30 min. After cooling down to room temperature, a mixture of hexane (15 mL) and ethanol (15 mL) was added to precipitate the product. The black precipitate was then isolated via centrifugation (4000 rpm/5 min). The washing process was repeated three times to ensure the removal of the uncoordinated capping agent. For the synthesis of phase-pure CuSbSe2, the above mentioned procedure was also followed but using diphenyldiselenide in OLA instead of a mixture of 1-DDT and t-DDT Measurements Transmission electron microscopy (TEM) analysis was performed using a FEI-Tecnai, 200 kV transmission electron microscope equipped with a CCD camera for STEM, HAADF detector, and EDX. TEM image non-linear processing was carried out using Gatan digital micrograph version 3.4. Powder XRD patterns were recorded on a Bruker D8 instrument equipped with Cu Kα radiation source operated as a rotating anode at 40 kV and 20 mA. Scanning electron microscope (SEM) imaging and EDX mapping analysis were carried out using a JEOL 7000 FE SEM equipped with energy dispersive X-ray spectroscopy (EDX), wavelength dispersive X-ray spectroscopy (WDS), electron backscatter diffraction (EBSD), secondary electron (SE), backscattered electron (BE) and transmission electron (TE) detectors. Electrochemical measurements The electrochemical measurements of all the samples were performed in a standard threeelectrode cell system. Platinum wire and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The working electrode was prepared by mixing 80 wt.% of the prepared samples, 10 wt.% of acetylene black and 10 wt.% of polyvinylidenedifluoride in the presence of N-methyl pyrrolidinone. After thoroughly mixing the components, the slurry was pasted onto a nickel foam. The prepared electrode was dried overnight at 60 °C under vacuum. 3M aqueous solutions of KOH, NaOH and LiOH were used as electrolytes. The charge storage capability of the materials was tested using cyclic voltammetry (CV) and galvanostatic chargedischarge method.32 Electrochemical measurements were performed on a VersaSTAT 4-500 electrochemical workstation (Princeton Applied Research, USA).


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(911)

(820)

CuSbSexS (x = 2) 2-x

(130) (701) (002) (620) (521) (112) (800) (711)(131) (810) (621) (412)

(600) (501) (221)(601) (321)

(400)

(a)

Intensity (a.u)

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(410)(111) (020) (220)(120) (301) (311) (320) (510)


CuSbSexS (x = 1.5) 2-x CuSbSexS (x = 1) 2-x CuSbSexS (x = 0.5) 2-x CuSbSexS (x = 0) 2-x

30

(b)

40

50

60

70

2-Theta (deg) (d)

(e)

(c)

Figure 1. (a) Powder x-ray diffraction patterns of CuSbSexS2-x mesocrystals grown at 250 °C. Crystal structures of CuSbS2 ((b) & (c)) and CuSbSe2 ((d) & (e)) showing the van der Waals gap and distance between two quadruple layers. 3. Results and discussion Mesocrystals of CuSbSexS2-x have been synthesized following a conventional colloidal synthesis method. We recently reported a method for the synthesis of uniform large area mesobelts of CuSbS2 using Cu(acac)2, SbCl3 and mixture of 1-DDT and t-DDT.30 Here we have expanded our approach to the synthesis of CuSbSexS2-x using DPDS as the selenium source along 6 ACS Paragon Plus Environment

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with a mixture of 1-DDT and t-DDT. The ratio of sulfur to selenium in the final product can be adjusted by merely varying the amount of diphenyldiselenide used, and for the synthesis of pure CuSbSe2 mesocrystals, diphenyldiselenide dissolved in oleylamine is used. The phase purity, crystallinity and composition of the product have been investigated by powder X-ray diffraction analysis. The diffraction patterns in Figure 1(a) show that CuSbSexS2-x mesocrystals crystallize in the orthorhombic structure with no additional diffraction peaks arising from impurities or secondary phases of copper and antimony chalcogenides. The major diffraction peaks in the spectra can be indexed as (111), (410, (020), (301), (501), (321), (521), (131) and (212) planes of orthorhombic phase with Pmna space group (Indexed XRD pattern for CuSbS2 is given in Figure S1, ICDD: 044-1417; here a-axis is considered as the long axis of the unit cell). 15.0 14.8

Lattice Parameters (Å) (c) (a) (b)

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14.6 14.4 6.3 6.2 6.1 6.0 4.0 3.9 3.8

0.0

0.5

1.0

1.5

2.0

x in CuSbSexS2-x

Figure 2. Vegard’s plot indicating a linear increase in the lattice parameter values with increasing selenium content It can be noticed from the XRD patterns that with increase of selenium content in CuSbSexS2-x there is a systematic shift in the diffraction peaks towards lower angles, indicating the formation of a solid solution. Vegard’s graph (Figure 2) obtained by plotting the lattice parameter values ((a), (b) & (c)) versus composition show close to linear trend with increasing selenium content, with the values for the end members CuSbS2 and CuSbSe2 matching those 7 ACS Paragon Plus Environment


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reported in the literature.33,34 The crystal structures of orthorhombic CuSbS2 and CuSbSe2 are shown in Figure 1(b)-(e). They show the layered structure of CuSbS2 and CuSbSe2 consisting of covalently bonded quadruple layers bound together by van der Waals attractions between Sb and S or Se that are separated by a distance of ~3.115 Å in CuSbS2 and ~3.234 Å in CuSbSe2. The gap between two different quadruple layers is estimated to be 2.051 Å in CuSbS2 and 2.136 Å in CuSbSe2, which clearly indicate a significant widening of the gap upon replacing sulfur with selenium.33,34

(a)

(b)

(c)

(d)

Figure 3. Scanning electron microscope images of CuSbSexS2-x (a) x = 0, (b) x = 1, (c) x = 1.5, (d) x = 2 grown at 250 °C The shape and size of CuSbSexS2-x mesocrystals have been investigated using scanning electron microscopy (SEM). SEM images in Figure 3 (a)-(d) show belt-like morphology for all compositions. The dimensions of the mesobelts predominantly vary from ~10 µm (l), ~2.5 µm (w) and 45 nm (d) for CuSbS2 to ~47 µm (l), 4.5 µm (w) and 50 nm (d) for CuSbSe2, with clear indication of elongation of mesobelt length with increasing selenium content. However, few CuSbS2 mesocrystals with less than 4 µm (l) and 1 µm (w) were also observed occasionally. The average elemental composition of CuSbSexS2-x mesocrystals has been determined using EDX with the samples coated onto a carbon tape attached to SEM stubs. The results yield a composition ratio of 1:1:1.96 (Cu: Sb: S) for x = 0, 1:1:1.09:0.91 (Cu: Sb: S: Se) for x = 1 and 1:1:1.94 (Cu: Sb: Se) for x = 2. The representative EDX graphs are provided in Figure S2. These 8 ACS Paragon Plus Environment

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quantitative composition values are obtained from different locations, each consisting of a large number mesocrystals.

(a)

(b)

(c)

(d)

(e)

Figure 4. SEM-EDX mapping images of CuSbSexS2-x, x = 1 mesocrystals showing uniform distribution of Cu, Sb, S and Se. Further, to confirm the elemental homogeneity and distribution, we have carried out SEM-EDX mapping on CuSbSexS2-x (x =1) mesocrystals. The mapping images in Figure 4(a)-(e) show a uniform distribution of Cu, Sb, S and Se over the entire mesobelt. It can also be noted that the samples are free of any elemental islands or clusters of binary and ternary phases. TEM images of one of the mesobelt in Figure 5(a) and (b) show width of ~1.5 µm and thickness of ~45 nm. High resolution transmission electron microscope (HRTEM) imaging of mesobelts was not suitable to provide any information about crystal lattice because the mesobelts were too thick to be penetrable by the electron beam. However, the HRTEM image in Figure 5d from one of the edges of the CuSbSexS2-x (x =1) sample exhibits lattice fringes with a lattice spacing of 0.197 nm, corresponding to the (020) planes of orthorhombic phase, which is in good agreement with d-spacing value obtained from the XRD pattern. The measured lattice distance is noticeably larger than that of the corresponding (020) planes of CuSbS2 but lower than CuSbSe2, 9 ACS Paragon Plus Environment


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confirming CuSbSexS2-x solid solution. Further, to ensure uniform elemental distribution in the mesocrystal lattices, we have carried out high angle annular dark field (HAADF) imaging, which provides Z-contrast information. HAADF image of CuSbSexS2-x (x =1) mesocrystal in Figure 5(c) show no evidence of any elemental segregation, in agreement with the EDX elemental mapping data.

(a)

(b)

(c)

(d)

Figure 5. (a) TEM images showing (a) width and (b) thickness of CuSbSexS2-x, x = 1 mesocrystal (c) High angle annular dark field image (d) HRTEM image In order to evaluate the potential of layer-structured CuSbSexS2-x mesocrystals as electrode materials for supercapacitor applications, we have investigated their electrochemical properties by cyclic voltammetry and galvanostatic charge-discharge methods. Figure 6 shows the cyclic voltammetry (CV) curves of CuSbS2 and CuSbSe2 electrodes at various scan rates using KOH electrolyte. As seen in the voltammogram of CuSbS2 (Figure 6(a)), an obvious pair of reversible redox waves is present for each curve at different scan rates. This indicates the presence of redox pseudocapacitance feature in the material, similar to that reported for other chalcogenide containing materials.15 10 ACS Paragon Plus Environment

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Current (A/g)

10.00

5.00

5 mV/s 10 mV/s 20 mV/s 30 mV/s 40 mV/s

(a)


0.00

-5.00

-10.00

CuSbS2- 3M KOH

0.2

0.3

5.00

0.4

0.5

0.6

Potential (V vs SCE)

10.00

Current (A/g)

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(b)


0.00

-5.00 CuSbSe2- 3M KOH

0.2

0.3

0.4

0.5

0.6

Potential (V vs SCE) Figure 6. Cyclic voltammogram curves of (a) CuSbS2 and (b) CuSbSe2 at various scan rates. With increasing scan rate, CV curves of both mesocrystals maintain their shapes but exhibit increased wave currents. The ∆Ewave (difference in the peak potential of cathodic and anodic curves) increases concomitantly with scan rate. A linear relationship is observed for the wave current as a function of the square root of scan rate (Figure S3). Such linear behavior indicates that the reaction kinetics during the redox process is likely controlled by diffusion processes.14 The involvement of non-Faradaic reaction due to the formation of a double layer at the electrode/electrolyte interface has been reported in a similar layer-structured sulfide, MoS2.35 However, in our case the absence of any non-Faradaic process is clearly evident from the CV curves. The redox reactions in copper-based ternary chalcogenide materials studied for lithium 11 ACS Paragon Plus Environment


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ion batteries indicate the occurrence of a combination displacement/intercalation (CDI) mechanism rather than traditional intercalation pathway.36 If we consider CDI mechanism in our system, it is possible to have the following reaction. MOH (M = Li, Na or K)

CuSbS2

MOH (M = Li, Na or K)

MxCu1-xSbS2

MSbS2 + Cu0

The CDI mechanism in CuSbS2 would lead to eventual replacement of copper with alkali metal ion (in this case K+, Na+ or Li+) while leaving the structure intact or partially distorted. XRD analysis of CuSbSe2 electrode on Ni foam sample after a few CV cycles showed diffraction peaks corresponding to CuSbSe2, along with peaks for Ni, indicating the retention of initial orthorhombic structure (Figure S4). Further, we did not observe any appreciable shift in XRD peaks positions suggesting the absence of alkali metal substitution. SEM images of electrodes after a few cycles and 1000 cycles of CV are shown in Figure S5. The image of CuSbSe2 electrode after a few CV cycles exhibits mesobelt morphology, same as the parent CuSbSe2 sample, whereas the SEM image of CuSbSe2 electrode after 1000 cycles shows broken mesobelts. A similar broken morphology was observed for CuSbS2 mesobelts when we intentionally exfoliated CuSbS2 after intercalating lithium ion between the layers.30 These observations suggest the intercalation of alkali metal ions between the layers during the electrochemical process. In another scenario, CuSbS2 might produce Cu2O and Sb2O3 in alkaline solution giving rise to the formation of polysulfide. This polysulfide formation pathway will degrade the electrode and lead to the loss of capacitance after a few cycles.37 Nevertheless, longterm cyclic stability study using CuSbSexS2-x mesocrystals electrode have shown no indication of loss of capacitance even after 1000 cycles, ruling out the polysulfide formation pathway. The excellent cyclic stability in these materials is intriguing, and definitely warrants additional investigation that is presently being carried out. The charge storage capacity of these materials was further investigated using constant current charge/discharge method. Figure 7(a) and (b) show the charge-discharge characteristics of CuSbS2 and CuSbSe2 as a function of discharge current. As seen, the discharge time decreases with increase in discharge current. The specific capacitance (Csp) of these materials was calculated using the following expression.38


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=

×∆

-- (1)

∆ ×

where I is the discharge current (A), ∆t is the discharge time (s), ∆V is the potential window, and m is the mass (g) of the active material. The variation of the specific capacitance with discharge current for all the samples is shown in Figure 7(c). As seen in the figure, a capacitance of 34 F/g (Energy density = 0.97 Wh/kg, Power density = 59.58 W/kg) is observed for CuSbSe2 at a discharge current of 0.4 mA using KOH as an electrolyte. The observed specific capacitance decreases with increase in the discharge current likely due to higher potential drop and insufficient Faradic redox reaction of the active materials under higher discharge currents.39


0.4 mA 0.5 mA 0.6 mA 0.8 mA 1.0 mA

(a)

CuSbSe2- 3M KOH

0.6


CuSbS2- 3M KOH

0.6

0.4

0.2

0.4 mA 0.5 mA 0.6 mA 0.8 mA 1.0 mA

(b)

0.4

0.2

0.0

0.0

40

50

100

150

Time (s)

3M KOH

0

200

CuSbS1.5Se0.5 CuSbSSe

30

50

7.50

CuSbSe2 CuSbS2

(c)

20

Current (A/g)

0

Specific Capacitance (F/g)

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100

Time (s)

Cycle 1 Cycle 50 Cycle 100 Cycle 150 Cycle 200

5.00

150

200

(d)

2.50

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10

CuSbSe2-3 M KOH 0.4

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0.6


Applied Current (mA)

Figure 7. Galvanostatic charge-discharge characteristics of (a) CuSbS2, (b) CuSbSe2 for various applied currents. (c) Variation of specific capacitance of the CuSbSexS2-x mesocrystals with applied current. (d) CV curves of CuSbSe2 for different CV cycles The long term cyclic stability of these compounds was investigated in detail. The shape and the area of the voltammograms were observed to be nearly identical (Figure 7(d)), indicating high cyclic voltammetric stability of these compounds. In fact, the area under the CV curve was 13 ACS Paragon Plus Environment


observed to increase by 0.5% after 200 cycles of CV measurements, confirming a slight improvement in charge-storage capacity. The observed long term cyclic stability of these materials could be due to the sheet-like structure helping in the insertion and extraction of electrolyte ions during cyclic measurements without much volume change. 10.0

Current (A/g)

CuSbS2 @50 mV/s

(a)

LiOH NaOH KOH 5.0

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-5.0

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Potential (V, vs SCE) 7.50

5.00

Current (A/g)

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(b)

LiOH NaOH KOH

CuSbSe2 @ 50mV/s

2.50

0.00

-2.50

-5.00 0.2

0.3

0.4

0.5

0.6

Potential (V, vs SCE) Figure 8. Cyclic voltammogram curves of (a) CuSbS2 (b) CuSbSe2 using different electrolytes. The interlayer distance between the layers in these compounds depends on the amount of the anionic substituents and we have demonstrated that the gap can be systematically varied by changing the sulfur to selenium ratio. By tuning the gap between the layers, it is possible to host a variety of intercalated atoms or ions. In order to test this hypothesis, we have carried out cyclic 14 ACS Paragon Plus Environment

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voltammetry measurements using three different electrolytes, LiOH, NaOH and KOH. It has been shown that the diffusion barrier is the major factor that determines the rate of charge and discharge processes and thereby the specific capacitance.40 CV measurements on CuSbS2 electrode using LiOH, NaOH and KOH in Figure 8(a) show the highest current density value for NaOH electrolyte. 40

Sp. Capacitance (F/g)

(a)

CuSbS2

30

20 LiOH NaOH KOH

10

0 0 50


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1

2

3

Current (mA)

(b)

4 CuSbSe2

40

30

20

10

LiOH NaOH KOH

0 0

1

2

3

Current (mA)

4

Figure 9. Variation of specific capacitance of (a) CuSbS2 and (b) CuSbSe2 with applied current using different electrolytes The capacitance value for CuSbS2 electrode, deduced from the area of CV curves obtained using NaOH electrolyte, was estimated to be the highest, which is quite interesting as the ionic radius of Na+ ions is intermediate between that of Li+ and K+ ions and its diffusion rate is expected to 15 ACS Paragon Plus Environment


be slower than Li+. It can be noted from the CV curves (Figure 8(a)) that the redox potentials are somewhat lower for the case of NaOH, indicating the easier charge transfer process between NaOH electrolyte and CuSbS2 electrode. Similar experiments on CuSbSe2 electrode (Figure 8(b) & 9(b)) using the different electrolytes have shown highest specific capacitance value for LiOH electrolyte and lowest for KOH. The specific capacitance follows the expected trend since Li+ ion has the smallest ionic radius and it can diffuse faster than Na+ and K+ ions. In addition, the expansion created in the interlayer distances by substitution of sulfur with selenium may also help in decreasing the diffusion barrier for Li+ ions to access the inner surfaces of the electrode. The effect of different electrolyte is quite evident in the charge-discharge characteristics of the samples (Figure 9). The results correlate very well with the cyclic voltammertic response of these materials with different electrolytes.

(a)

30 0.6

20

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15

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5

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LiOH NaOH KOH

CuSbSe1.5S0.5 0

200

400

600

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Number of Cycles Figure 10. (a) Cycling performance of the CuSbS2 and CuSbSe2 electrodes at a constant current of 0.5 mA using NaOH electrolyte. The inset shows the first few cycles of the charge-discharge curves. (b) Cyclic performance of CuSbSe1.5S0.5 using different electrolytes 16

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The cyclic stability of the compounds for their potential application as rechargeable storage devices was further studied using galvanostatic charge-discharge characteristics. Figure. 10 shows the change in the specific capacitance of CuSbS2 and CuSbSe2 for 1000 chargedischarge cycles. It can be observed that most of the samples have essentially 100 % charge retention even after 1000 cycles. In some cases, such as for CuSbSe2, the specific capacitance even increases slightly (~ 16 %) with increase in cyclic charge-discharge measurement. The inset to Figure 10(a) shows the last few charge-discharge cycles for CuSbS2. As evident from the figure, these materials deliver very stable specific capacitance over 1000 cycles, indicating their potential application as stable power storage and delivery system. It is worth comparing the specific capacitance behavior of our material systems with a similar layer-structured material MoS2. The specific capacitance value of MoS2 is reported to decrease gradually with increasing number of charge-discharge cycles,41 but in our case the values increase or are retained even after 5000 charge-discharge cycles (Figure S7). One of the reasons for the observed excellent cyclic stability in these materials could be the attractive interaction between Sb in one layer and S or Se in the adjacent layer, creating cavities to host ions without altering the structure. The high stability of our electrodes at high current densities suggests that these energy storage devices are suitable for fast charging applications. The effect of different electrolytes on the cyclic stability of one of the intermediate composition material CuSbSexS2-x (x = 1.5) was also investigated. As evident from representative plot in Figure 10 (b), the specific capacitance is very stable in all the electrolytes although the value varies depending on the electrolyte used for the measurements.

Conclusion:

We have developed a simple solution-based approach for the synthesis of mesocrystals of CuSbSexS2-x over the entire composition range. Our method of synthesis yields mesocrystals of CuSbSexS2-x with belt-like morphology for all compositions. A systematic increase in the lattice constant values with substitution of selenium for sulfur in CuSbS2 confirms the formation of CuSbSexS2-x solid solution. Electrochemical measurements to evaluate their applicability as electrode material for supercapacacitor application have yielded promising specific capacitance values with excellent cyclic stability. Further, measurements on CuSbSexS2-x electrodes using different electrolytes, such as LiOH, NaOH and KOH, provide specific capacitance values as 17 ACS Paragon Plus Environment


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high as 48 F/g (Energy density = 1.31 Wh/kg, Power density = 60.88 W/kg) for CuSbSe2 in LiOH. The capacitance behavior in these materials occurs through Faradaic redox reaction, unlike the double layer capacitance nature observed for other layer-structured sulfides. The excellent cyclic stability of these materials is a significant motivation to investigate further possibilities of improving the specific capacitance value by synthesizing them in the form of nano-dimensional crystals and/or creating composites with carbon-based materials. These are currently under investigation and the results will be reported elsewhere. Thus far, our investigations have revealed the potential of these materials for energy storage applications in addition to their attractive solar energy conversion properties. Supporting Information. XRD patterns, EDX data, SEM images. This material is available free

of charge via the Internet at http://pubs.acs.org. Notes

The authors declare no competing financial interest Acknowledgement

Synthesis, X-ray, TEM and SEM characterization work was done at the University of Alabama, supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-08ER46537. Partial analysis were performed at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. Electrochemical measurements were carried out at Pittsburg State University. Dr. Ram Gupta expresses his sincere acknowledgment to Polymer Chemistry Initiative, Pittsburg State University for providing financial and research support. References:

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