CdS microspheres as promising electrode materials for high performance supercapacitors

CdS microspheres as promising electrode materials for high performance supercapacitors

Materials Science in Semiconductor Processing 105 (2020) 104677 Contents lists available at ScienceDirect Materials Science in Semiconductor Process...

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Materials Science in Semiconductor Processing 105 (2020) 104677

Contents lists available at ScienceDirect

Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

CdS microspheres as promising electrode materials for high performance supercapacitors

T

I. Rathinamalaa, I.Manohara Babub, J. Johnson Williamb, G. Muralidharanb, ⁎ N. Prithivikumaranc, a

Department of Physics, V. V. Vanniaperumal College for Women, Virudhunagar, Tamil Nadu, India Department of Physics, The Gandhigram Rural Institute (Deemed to be University), Gandhigram, Tamil Nadu, India c Department of Physics, V. H. N. Senthikumara Nadar College (Autonomous), Virudhunagar, Tamil Nadu, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: CdS microspheres Supercapacitor Asymmetric Cyclic voltammetry

Red carpet has always been prearranged for innovative structured electrode materials in supercapacitors gallery. Cadmium sulphide (CdS) microspheres have been successfully synthesized via affordable and energy budget approach. The structural, morphological and electrochemical properties of CdS structures were studied for supercapacitor applications. Electrochemical investigations (CV analysis) suggest that CdS microsphere electrodes exhibit pseudocapacitive behaviour and capable of delivering the specific capacitance of 592 F g−1 at a scan rate of 5 mV s−1. From galvanostatic charge/discharge method, a maximum specific capacitance of 854 F g−1 was estimated at a current density of 2 A g−1. Moreover, an asymmetric supercapacitor has been fabricated by utilizing CdS microsphere and activated carbon as electrodes and it could capable of deliver ultrahigh energy density of 8.4 W h kg−1 with a maximum power density of 7.56 kW kg−1 at a constant current density of 20 A g−1. These profitable features of CdS microsphere electrodes prepared in the present study are quite favourable electrode material for next generation supercapacitors.

1. Introduction Recently, huge energy demand has been evolved which intrinsically stimulates current research towards the development of advanced energy storage devices [1,2]. Supercapacitors have considered as prominent energy storage devices since they are capable of delivering superior capacitance, high energy/power density and excellent cyclic performance [3–5]. These excellent and superior properties make them as promising candidate in vast range of thrust areas that includes hybrid electric vehicles (HEVs) and portable electronic gadgets [6]. The supercapacitors, an energetic charge storage gadget which stores charge via Faradaic reversible phenomenon at electrode/electrolyte surfaces that results in instantaneous power delivery and extended cycle life. The electrochemical charge storage process in supercapacitors are primarily depends on the electrode materials used. The electrodes can be divided into two major groups: (i) Carbon based materials stores charges via ion adsorption at electrode surface. Carbonaceous materials (carbon nanotubes, activated carbon, and graphene) [7,8] employ as traditional electrode materials owing to its high mechanical strength, high conductivity and high specific area. But, it usually delivers poor capacitance even at high operating voltage. (ii) Pseudocapacitive ⁎

electrodes store charges through fast Faradaic process at electrode/ electrolyte interfaces. Materials that includes transition metal hydroxides/oxides and conducting polymers (NiOx, CoOx, MnOx, MoOx, FeOx, ZnOx, CdOx) [9,10] are serve as pseudocapacitive electrode materials. They possess high capacity in narrow potential range. However, it lacks in power density (103 W kg−1). To address the issues emerged in aforementioned electrodes, transition metal sulphides have explored as innovative electrodes for supercapacitors because of their superior Faradaic redox activity [11]. Solvothermal approach has been chosen to fabricate NiS Nanoflower electrodes by Yang and his co-workers [12] and it provides a maximum specific capacitance of 966 F g−1 at a specific current of 0.5 A g−1. Zhang et al. [13] exposed the supercapacitive performance of CoS2 ellipsoids, yields a specific capacitance of 750 F g−1 at 5 A g−1. Further, a metal sulphide with net-like nanostructure [14] was tested as electrode material for supercapacitors and it provides a specific capacitance of 516 F g−1 at a current density of 0.5 A g−1. Cadmium sulphide (CdSx), an excellent potential supercapacitor electrode has inspired by energy community in recent years due to their unique physicochemical features, electronic properties and electrochemical properties [15]. Cadmium nanostructures that include

Corresponding author. E-mail addresses: [email protected] (I. Rathinamala), [email protected] (N. Prithivikumaran).

https://doi.org/10.1016/j.mssp.2019.104677 Received 15 March 2019; Received in revised form 24 July 2019; Accepted 11 August 2019 1369-8001/ © 2019 Published by Elsevier Ltd.

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impedance properties. Nair et al. [19] reported the pseudocapacitance performance (181 F g−1 at a scan rate of 5 mV s−1) of CdS nanoforest 3D architecture via solution chemistry approach. For the circumstance, Xu and his research [20] studied the supercapacitive performance of porous cadmium sulphide on nickel foam, delivers a specific capacitance of 909 F g−1 at a current density of 2 mA cm−2. In this context, a facile and affordable solution chemistry route has been adopted to fabricate CdS microsphere without any structure directing agents. Different analytical and electrochemical investigations were made on CdS microspheres and its potential towards supercapacitors were studied and reported. 2. Experimental section 2.1. Chemicals Analytical grade of cadmium nitrate (Sigma Aldrich, India), thiourea (Merck, India) and ammonia solution (CDH, India) were purchased and used without further processing. Fig. 1. XRD pattern of CdS microsphere electrode.

2.2. Synthesis of CdS microspheres

nanosheets, nonorods, nanoflakes and nanowires have drawn their significance in variety of applications like solar cells, light sensors, electrochemical biosensors, photocatalysis and supercapacitors [16]. For instance, Moorthy and his group [17] reported surfactant assisted cadmium nanostructures for electrochemical charge storage applications. CdS nanoparticles decorated polyaniline nanorods were synthesized by Ameen and his lab mates [18] for electrochemical

In the typical laboratory synthesis, 0.5 g of thiourea was dissolved in 100 mL of double distilled water and allowed to stir using magnetic stirrer for 30 min at room temperature. 0.5 g of cadmium nitrate was dissolved in 50 mL of distilled water and the same has been added to the thiourea solution in dropwise manner (20 drops/min). After 60 min, the pH of the solution was adjusted to 10 by the addition of ammonia

Fig. 2. (a–c) SEM images of CdS electrodes at different magnifications (d) EDS profile of CdS microspheres. 2

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Fig. 3. (a–c) HR-TEM images of CdS at different magnifications; (d) SAED pattern of CdS.

Fig. 4. (a) Nitrogen adsorption-desorption isotherms of CdS microsphere samples; (b) Pore size distribution profile of the prepared material.

yield cadmium sulphide microspheres.

solution. The resultant solution was allowed to stir for 150 min to get a homogenous solution. Later, the solution containing precipitate was kept undisturbed for next 2880 min. Then, the solution was washed thrice by distilled water and twice by ethanol. Finally, the precipitate was collected using centrifuge and dried in air atmosphere (80 ̊C) to

2.3. Characterization techniques The crystal structure and phase purity of the prepared materials 3

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Scheme 1. Schematic representation for the formation of CdS microspheres.

were characterized by XRD analysis using XPERT-PRO diffractometer with Cu Kα radiation and scanned from 20° to 80°. The morphological features associated with the prepared products were analyzed using scanning electron microscopy and high resolution transmission electron microscopy (SEM, Model-TESCAN VEGA-3 LMU & HRTEM, ModelJEOL/JEM 2100 having LaB6-source) at different magnifications. Nitrogen adsorption/desorption experiments were performed Quantachrome® ASiQwin analyzer at −77 K. Electrochemical measurements like cyclicvoltammetry, galvanostatic charge-discharge method and a.c impedance spectroscopy were executed using CHI 660 D electrochemical workstation (CH Instruments, India).

2.5. Asymmetric supercapacitor assembly

2.4. Electrodes fabrication & electrochemical assessment

3.1. Structural, morphological and textural studies

The electrochemical behaviour of CdS nanostructures were evaluated by three electrode configuration. It comprises of working electrode (CdS microspheres), counter electrode (Pt wire), reference electrode (Ag/AgCl) and an aqueous alkaline electrolyte (2 M KOH). The working electrode was prepared as follows: Initially, the current collector (Ni foam) was washed with 5% HCl and acetone. 85 wt % of synthesized CdS microspheres, 10 wt % of activated carbon and 5 wt of conducting polymer (polytetrafluro ethylene) were properly mixed along with few drops of ethanol. The syrup like material was coated onto the Ni foam (1*1 cm2) and dried at 80 ̊C. The mass loading of the electroactive material is 1 mg. The specific capacitance (Cs, F g−1) of the active material has been estimated using the relation [21],

The X-ray diffraction analysis of cadmium sulfide microspheres is presented in Fig. 1. The diffraction peaks observed in XRD pattern could be indexed to hexagonal structured CdS with the space group of P63mc. Besides, the XRD pattern agrees well with the standard JCPDS file card no: 80-0006. Addition to that, some extra peaks (indicated by *) has been observed and it could be indexed to orthorhombic structured sulphur, well coincides with the JCPDS file card no: 72–0410. In addition, the average crystallite size was calculated using Debye Scherrer relation and it was found to be 26.4 nm. Fig. 2a-c shows the surface morphology of cadmium sulphide electrodes. The low magnification SEM image clearly reveals that the dried product is comprised of microspheres with an average diameter of 0.5 μm (~). Particularly, this spherical configuration provides multichannels for the absorption of OH− ion that may endow excellent electrochemical activity. Purty et al. [23] synthesized mesoporous ball like CdS/PPY decorated rGO nanocomposite for enhanced supercapacitive performance. Spherical morphology takes an advantage of ion insertion/de-insertion in all possible direction. EDS analysis was helpful to identify the presence of elements in the prepared material. Fig. 2d depicts the EDS profile of CdS microspheres. The atomic weight percentage of Cd & S is displayed in inset of Fig. 2d. Further, to examine the morphological features in nanoscale, HR-TEM images of the sample has been recorded in different magnifications and presented in the boards of Fig. 3. Nanosheets/flakes are the building blocks of microspheres. From HR-TEM images (Fig. 3 a-c), we can notice that numerous sheets/flakes are assembled in periodic manner and conglutinates with each other due to low van der Waals force that results in the formation of microspheres. Moreover, the SAED pattern (Fig. 3d) with bright diffusion spots reveals the good crystallographic nature of CdS. Added to that, the inter spacing between the crystal lattice has been measured and it was found to be 0.34 nm and it was consistent with the crystal plane (111). The textural features of the prepared CdS electrodes were evaluated by BET measurements using N2 adsorption/desorption isotherms. Fig. 4a. Shows the nitrogen adsorption/desorption isotherms of CdS microsphere electrodes, exhibits a strong capillary condensation at a high relative pressure (~1) which further confirms the typical type IV isotherms with H2 hysteresis loop. The estimated BET surface area of

Cs =

To examine the potential of the fabricated electrodes in two way platform, an asymmetric supercapacitor was designed. We have devised to fabricate the asymmetric supercapacitor in sandwich model, utilizing CdS microspheres and activated carbon as electrodes. A thin polypropylene sheet soaked in 6 M KOH, served as separator cum charge reservoir. All the electrochemical measurements of the fabricated device have been done in room temperature and discussed in section 3.4. 3. Results and discussion

∫ IdV 2mΔVv

(1)

where, ∫ IdV represents the area enclosed by the CV curve (A V), m is the mass of the electroactive material (mg), ΔV denotes the potential window (V), ν is the potential sweep rate (mV s−1). The specific capacitance of the electrode was also estimated by charge-discharge analysis using the relation,

Cs =

2I∫ Vdt mΔV 2

(2)

∫ Vdt indicates the area under the discharge curve (V s). Further, the energy density (W h kg−1) and power density (W kg−1) of the CdS based electrodes were calculated from the discharge curves using the relations [22],

E=

1 CV 2 2

(3)

p=

E t

(4)

where, Cs is the specific capacitance (F g−1) obtained from GCD measurements and t is the discharge time (s). 4

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Fig. 5. (a) CV profile of CdS microspheres at different scan rates (vs Ag/AgCl) in 2 M KOH; (b) Peak current vs square root of scan rate; (c) Charge/discharge plateaus of CdS electrodes for various current densities; (d) Effect of current density on specific capacitance; (e) Stability profile of CdS electrodes; (f) Impedance spectrum of CdS.

CdS microsphere electrodes was found to be 89 m2 g−1. The pore size distribution profile of CdS sample (Fig. 4b) reveals the presence of mesopores (pores present in the range of 2.5 nm–20 nm) and obtained pore volume is 0.171 cm3 g−1. This high specific BET surface area and mesoporous nature of CdS electrodes could provide effective channels for ion mobilization and results in enhanced charge storage process.

transport at electrode/electrolyte interface. Formation of microspheres includes can be explained by three major stages [24,25]: (i) Complex formation, (ii) Nucleation and (iii) Oriented growth. Initially, S2− ions are released from thiourea in aqueous medium and they are mobilized towards Cd2+ (released from precursor). Due to the weak Vander Wall force between the molecules, they tend to attract each other and form the unstable complex. After addition of ammonia, the molecules in the aqueous medium gets stabilized (Cd–S). Later, the growth of the molecules has initiated along one particular direction and results in the development of microspheres.

3.2. Formation mechanism of CdS microspheres Scheme 1 represents the possible formation mechanism of CdS microspheres. Spherical morphology facilitates most favourable electron 5

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854 F g−1 at a current density of 2 A g−1. It is worth to mention that, this superior performance has been achieved by unique spherical morphology that opens up fresh pathways for ion insertion in multi-direction. Purty et al. [23] prepared CdS-PPY decorated rGO nanocomposite electrodes for supercapacitor applications (844 F g−1 at a current density of 1 A g−1). Morphology controlled RGO/CdS [27] hydrogels provided a specific capacitance of 204 F g−1 at a current density of 0.5 A g−1. As revealed in Fig. 5d, specific capacitance has started to decay while increase in current density. There are two possible facts behind the phenomenon [28]: (i) Internal resistance developed within the electrode surface; (ii) Negligible IR drop at low current densities that results in better charge storage. The electrochemical stability of CdS electrodes was tested by executing 5000 continuous charge/discharge cycles at a constant current density of 10 A g−1. As shown in Fig. 5e and 4.9% degradation been monitored after 5000 cycles. Initially, there was a gradual increase in capacitance for first 2500 cycles (activation process of the electrode material) and reaches a maximum specific capacitance (100%). After that, the specific capacitance decreases slowly which is due to the restriction of pathways for ion intercalation/deintercalation. Finally, 4.9% degradation in capacitance has been observed after 5000 continuous galvanostatic cycles. For an instance, 92% of capacitance retention has been reported for [email protected] core-shell nanostructures [29]. The impedance and capacitive elements associated with CdS microsphere electrodes (Fig. 5f) were measured by a.c. impedance analysis within the frequency range 0.01 Hz–100 kHz at a bias potential of 0.5 V. The observed impedance data is fitted to equivalent Randle's circuit that (inset of Fig. 5f) contains solution resistance (RS), charge transfer resistance (RCT), pseudocapacitance (CP) and Warburg resistance (W). The impedance plot consists of high frequency semi-circle region and low frequency straight line. The semi-circle in the plot (high frequency region) corresponds to the Faradaic redox reactions held in the surface of the electrode material and the diameter denotes the charge transfer resistance RCT. The straight line makes an angle 45° (approximately) with X-axis and it corresponds to ion diffusion nature of electrode materials. The low RCT (0.15 Ω) of CdS microspheres shows high conductivity that highly favours for electrochemical phenomenon. Also, the capacitive and resistive elements associated with CdS electrodes (while it was coated in graphite sheet) were diagnosed and it was found to be 2.48 Ω (Fig. S2.)

Scheme 2. Schematic of CdS and AC based asymmetric supercapacitor.

3.3. Electrochemical performance of CdS microsphere electrodes The supercapacitive performance of the prepared electrodes was examined by cyclicvoltammetry, galvanostatic charge-discharge method and a. c. impedance measurements in an aqueous alkaline solution (2 M KOH). Fig. 5a depicts the CV traces of CdS microspheres electrode at different sweep rates (5 mV s−1 to 100 mV s−1) within the potential range −0.2 Vto0.8 V. CV plot clearly exhibits a pair of strong Faradaic peaks (anionic peak-0.5 V & cathodic peak-0.2 V) that confirms the pseudocapacitive nature of CdS electrodes. Addition to that, shift in redox peaks has been observed in the CV plot and this is due to the low internal resistance and less polarization developed within the electrode material. The possible (reversible) reaction kinetics involved in CdS electrodes in the medium of alkaline can be expressed as

CdS + OH− ↔ CdS (OH ) + e−

(5)

This Faradaic nature of charge storage mechanism in CdS microsphere electrodes can be further confirmed by the plot between the peak current density and the square root of scanning rates (Fig. 5b). The linear plot reveals that charge storage process is accomplished by pseudocapacitive phenomenon. A slight deviation in the higher scan rate is attributed to the charge diffusion polarization. The specific capacitance was calculated to be 592 F g−1 at a scan rate of 5 mV s−1. This could be attributed to the facile morphology which reduces the pathways for ion insertion and makes easy diffusion of electrolyte ions. Ali et al. showed hydrothermal synthesis of CdS/rGO/CeO2 nanocomposite by estimating the specific capacitance of 407 F g−1 (at 5 mV s−1) [15]. Nair et al. [19] fabricated CdS Nanoforest like structure with superior electrochemical performance (181 F g−1 at 5 mV s−1). Added to that, an interesting phenomenon could be seen from CV studies (Fig. S1), i.e. while increasing the scan rates, the specific capacitance of the electrodes decreases. At higher scan rate, the possibility of surface utilization is not fulfill whereas, the electrolyte ions utilize both interior and exterior surface of the active material in lower scan rates and results in high capacitance [26]. Beyond that, the CdS electrode retains 57.6% capacity even at higher scan rate of 100 mV s−1. This withstanding capacity of CdS electrodes at higher scan rates is quite attractive, a necessary characteristic for an ideal supercapacitor. To further explore the electrochemical behaviour of CdS microsphere electrode, galvanostatic charge/discharge measurements were carried out. The charge/discharge curves of CdS electrode, recorded at various current densities between the potential range −0.2 V–0.8 V is presented in Fig. 5c. Evidently, the charge/discharge curves are nonlinear over the potential range showing pseudocapacitive behaviour. Consequently, the specific capacitance can be calculated to be

3.4. Two electrode system: Device performance (CdS//AC) An asymmetric supercapacitor has been fabricated to showcase the usefulness of the CdS microspheres electrodes in real life applications. Cell voltage and mass loaded in the device are the key parameters that elevate the electrochemical performance of the fabricated asymmetric supercapacitor. An effective operating voltage of 1.5 V has been fixed for the fabricated design (without any oxygen evolution). In order to extend the potential window of the asymmetric supercapacitor, we have chosen activated carbon as the negative electrode. The activated carbon serves as counter electrode in the device fabrication. The electrochemical features of the activated carbon have been studied in the potential window −0.7 V to −0.1 V and the results have been provided in the supporting information (Fig. S3). Then, the optimum mass has been loaded in the device, estimated using mass balance theory. The calculated mass balance ratio was found to be 0.3. Scheme 2. Depicts the schematic of the asymmetric supercapacitor based on CdS microsphere electrodes. To assess the energy storage capabilities of the fabricated asymmetric supercapacitor, CV, GCD, a.c impedance measurements were performed in room temperature and their results were presented in the panels of Fig. 6. Cyclic voltammetric responses of the fabricated device were depicted in Fig. 6a, indicating that (redox peaks) the charge storage process accomplished by Faradaic phenomenon. While increasing 6

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Fig. 6. (a) CV profile of asymmetric supercapacitor based on CdS electrodes; (b) GCD plot of CdS//AC at different current densities; (c) Ragone plot; (d) Impedance spectrum of the device; (e) Cyclic life of the asymmetric supercapacitor for 1500 cycles.

the sweep rates (5, 10, 15 and 20 mV s−1), the area under the CV plot is also increases (3.8, 5.2, 5.8 and 6.4 mA V). But, the poor rate performance and irreversibility of OH− ions results in lower area (in cathodic part). The rate performance, specific capacitance of the cell was evaluated from GCD measurements. GCD analysis reveals the non-linearity in charge/discharge curves (Fig. 6b) that suggests the pseudocapacitive charge storage. The estimated specific capacitance of the cell was found to be 38 F g−1 at a current density of 5 A g−1. Energy/power density is the significant factor that decides the

performance of the device. The energy and power densities were estimated by the discharge area and time obtained from the GCD analysis. CdS//AC asymmetric supercapacitor provides an energy density of 12 W h kg−1 with the power density of 1853 W kg−1 at a constant current density of 5 A g−1 (Fig. 6c). Moreover, the resistive/capacitive elements associated with the prepared asymmetric supercapacitor were diagnosed by a.c. impedance measurements (Fig. 6d). Thus, the obtained data was fitted to the Randle's equivalent circuit. The key components in the Randles circuit 7

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were briefly discussed in three electrode cell. The device shows the charge transfer resistance of 3.9 Ω, This low RCT probably enhances the OH‾ insertion into the material that results in excellent electrochemical performance. Furthermore, cycle life of the asymmetric supercapacitor was checked to examine the practical utility of the electrodes over 1500 continuous charge/discharge cycles at much larger current density of 10 A g−1. For the first 300 cycles, a gradual increase in capacitance has been observed (Fig. 6e) and it could be ascribed to the activation of the electrode materials. After the end of 1500 cycles, 93% of capacitance retention has been retained.

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4. Conclusions In conclusion, cadmium sulphide microspheres were synthesized by simple and affordable approach. CdS microspheres exhibit outstanding electrochemical performances that include high specific capacitance (854 F g−1 at a current density of 2 A g−1) and excellent cycle life (95.1% capacitance retention). This superior performance is chiefly due to the presence of spherical morphology with efficient channels for passage of ions during electrochemical phenomenon. Besides, an asymmetric supercapacitor has been fabricated and it provides an admirable power density of 7.56 kW kg−1. CdS microspheres electrodes with these remarkable features are expected to be a prominent nominee for future energy storage applications. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mssp.2019.104677. References [1] C.H. Tang, X. Yin, H. Gong, ACS Appl. Mater. Interfaces 5 (2013) 10574. [2] J. Zhang, H. Feng, J. Yang, Q. Qin, H. Fan, C. Wei, W. Zheng, ACS Appl. Mater.

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