- Email: [email protected]
Ag2S [email protected] core-shell architectures for electrochemical supercapacitors
Accepted Manuscript Alteration of Ag nanowires to Ag/Ag2S nanowires@CdS core-shell architectures for electrochemical supercapacitors Dipali S. Patil, Sachin A. Pawar, Jae Cheol Shin PII:
S0925-8388(18)32745-2
DOI:
10.1016/j.jallcom.2018.07.244
Reference:
JALCOM 46960
To appear in:
Journal of Alloys and Compounds
Received Date: 1 June 2018 Revised Date:
19 July 2018
Accepted Date: 21 July 2018
Please cite this article as: D.S. Patil, S.A. Pawar, J.C. Shin, Alteration of Ag nanowires to Ag/Ag2S nanowires@CdS core-shell architectures for electrochemical supercapacitors, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.07.244. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Alteration of Ag nanowires to Ag/Ag2S nanowires@CdS core-shell architectures for electrochemical supercapacitors Dipali S. Patil*, Sachin A. Pawar, Jae Cheol Shin*
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Department of Physics, Yeungnam University, Gyeongsan, Gyeongbuk 38541, South Korea. *Corresponding Author Emails: [email protected], [email protected] Abstract
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Silver nanowire@cadmium sulfide (AgNW@CdS) core-shell nanostructured electrodes were synthesized in two steps. Initially, a layer of AgNWs was coated onto Ni foam by drop
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casting. CdS was then deposited onto the pre-coated Ni foam by a successive ionic layer adsorption and reaction (SILAR) method. This process converted the Ag nanowires to Ag2S, which was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The effects of the CdS layer thickness on AgNWs on the electrochemical properties of the nanostructure were
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examined by varying the number of SILAR cycles. The resulting AgNWs@CdS electrode exhibited a high areal capacitance of approximately 2662 mFcm-2 at 10 mVs-1 and 810 mFcm-2 at 45 mA applied current.
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KEYWORDS
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Silver nanowires, CdS, supercapacitor.
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Introduction Supercapacitors or ultracapacitors, also known as electrochemical capacitors, are the most attractive energy storage device because of their intermediate energy density and power
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density. Supercapacitors can be classified into two types: electric double layer capacitors (EDLC) and pseudo-capacitors [1]. In pseudo-capacitors, the capacitance is due to faradaic reactions in the nanosize thick region at the electrode-electrolyte interface. Therefore, a large electrochemical
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interface is favorable for enhancing the electrochemical performance of the supercapacitor. In this direction, several efforts have been made to develop different electrode materials with a
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range of nanostructures to improve the energy and power density [2-7]. The electrochemical performance is improved significantly by having the least resistance between the electroactive material and current collector [8, 9].
One dimensional nanostructures, such as nanorods, nanowires, and nanotubes, are
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interesting candidates that possesses high surface area and also deliver short diffusion paths to ions, leading to high charge/ discharge rates. In this way, silver nanowires (AgNWs) with high metallic conductivity, high aspect ratio, mechanical stability, and excellent anti-oxidation
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property have potential applications as current collector materials [10, 11]. Recently, a conducting network of AgNWs has been used as a core material for metal oxides, metal sulfides,
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and polymers to enhance their conductivity and capacity, and has been used as an excellent electrode in energy storage. Yuksel et al. developed core@shell AgNWs@MoO2 (Specific capacitance, 500.7 Fg-1 at 0.25 Ag-1) [12] and AgNWs@Ni(OH)2 (Specific capacitance, 1165.2 Fg-1 at 3 Ag-1) [13] nanostructures for supercapacitor electrodes. They showed that the synergy of highly conductive AgNWs and high capacitive MoO2 or Ni(OH)2 assists in ion and electron transportation and boosts the electrochemical properties of the resulting electrode. In addition,
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polypyrrole was grown onto the surface of the AgNWs and its electrochemical stability (90 % after 10000 cycles) and electrochemical properties (Specific capacitance, 509 Fg-1; energy density, 4.27 Whkg-1; power density, 60.7 Wkg-1, at 0.31 Ag-1) were improved [14]. In a
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previous report, AgNWs@Cu2S core@shell nanostructures were synthesized by simple and inexpensive drop casting followed by successive ionic layer adsorption and reaction (SILAR) methods and its electrochemical performance (Specific capacitance, 707 Fg-1, at 10 mVS-1;
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energy density, 10.01 Whkg-1; power density, 25.33 Wkg-1, at 0.2 mA) was studied [15]. Chen et al. reported the direct growth of Fe2O3 on the surface conductive networks of AgNWs on a
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coffee filter as a current collector that showed good electrochemical performance (specific capacitance, 287.4 F g-1; energy density 64.6 W h kg-1; power density 18 kW kg-1 at 1 mAcm-2) [16]. Liu et al. fabricated AgNW/graphene composites on polyethylene terephthalate (PET) using an electrophoresis method to improve the conductivity of the graphene electrode. They
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achieved the highest areal capacitance of 7.6 mFcm-2 at 100 mVs-1 with 82.6% capacitance retention after 2500 cycles [17].
In recent efforts on electrode material for supercapacitors, nanostructured metal sulfides
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(NiS, CoS, Cu2S, CdS, Bi2S3, and SnS2) have been used widely as a new class of energy storage material because of their excellent electrochemical properties [18, 19]. Among these, CdS
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exhibits attractive properties, such as good electronic conductivity, high theoretical capacitance (1675 Fg-1), lower cost, natural abundance, and environmental stability. These encouraging properties of CdS lead to potential applications in supercapacitors. Few reports on CdS-based electrode materials for supercapacitors are available and there is still scope to improve their capacitive performance. Xu et al. prepared porous CdS on nickel foam using a one-step hydrothermal method, which showed a specific capacitance of 909 Fg-1 at 2 mAcm-2 [20]. The
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three dimensional reduced graphene oxide/needle-like CdS hydrogel synthesized by a hydrothermal process displayed a specific capacitance of 300 Fg-1 at 5 mVs-1 [21]. Recently, CdS was used as a shell material for the nanostructured Ni3S2 [22] and Co3O4 [23] to improve their
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electrochemical properties.
In this study, AgNWs@CdS core shell nanostructured supercapacitor electrodes were synthesized by dip coating followed by SILAR methods. An interaction was observed between
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the AgNWs and sulfur during the SILAR process, leading to the formation of Ag2S, which again contributes to the pseudo-capacitance. To examine the effects of the CdS thickness on the
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electrochemical performance, the number of SILAR cycles was varied from one to four cycles. All the synthesized electrodes were characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance. The investigations showed that the CdS thickness had a significant impact on the electrochemical performance of AgNWs@CdS.
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Experimental Details
Commercially available AgNWs with a mean diameter ≈ 30 nm and length ≈ 15 µm were purchased from CNVISION CO., LTD. (Seoul, South Korea). For the synthesis of
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AgNWs@CdS core-shell nanostructure, initially, diluted AgNWs (AgNWs:ethanol = 1:3) were deposited uniformly on Ni foam using a dip coating technique. The formation of a shell (CdS)
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layer on the pre-coated AgNWs was developed using the SILAR technique. Briefly, the precoated AgNWs Ni foam was first dipped into 0.5 M (Cd(NO3)2 in methanol for 60 s and then rinsed with methanol for 60 s. Subsequently, it was dipped into a sodium sulfide nonahydrate (Na2S.9H2O) solution in methanol:water (7:3) for 60 s, and rinsed again with methanol for 60 s. This process is called a single SILAR cycle. The number of SILAR cycles is varied (1, 2, 3 and 4)
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and the corresponding electrodes are called AgNW-Cd-1, AgNW-Cd-2, AgNW-Cd-3, and AgNW-Cd-4, respectively. X-ray diffraction (XRD (PANalytical)) was carried out using Cu Kα radiation. X-ray
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photoelectron spectroscopy (XPS, K-alpha, Thermo Scientific, UK) was performed to determine the surface chemical composition. The surface morphology of the films was examined by field emission scanning electron microscopy (FE-SEM, S-4800 HITACHI, Ltd., Japan). Elemental
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mapping and the surface morphology were analyzed by high resolution transmission electron microscopy (HRTEM, JEOL JEM 2100F with Cs-corrected STEM). The electrochemical
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measurements, such as CV, electrical impedance spectroscopy (EIS), and galvanostatic charge/discharge were performed in a 3M KOH electrolyte in a conventional three-electrode arrangement consisting of a graphite counter electrode and a saturated calomel electrode (SCE), as the reference electrode, on a ZIVE SP5 electrochemical workstation (WonAtech).
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Results and discussion
Fig.1 presents a schematic diagram of the synthesis of AgNW@CdS core-shell nanostructures. The AgNW@CdS core-shell nanostructure was formed by dip coating followed
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by the SILAR process. First, the dip coating technique was used for the uniform deposition of AgNWs onto the Ni foam. Subsequently, the SILAR process was used for the formation of CdS
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(shell), in which dipping of the pre-coated Ni foam in a cationic precursor (Cd(NO3)2 leads to the adsorption of Cd2+ ions and later dipping in an anionic precursor (Na2S.9H2O) leads to the adsorption of S2- ions [24]. The reaction on the pre-coated Ni foam leading to the formation of CdS is as follows [25]:
+ → ------------------- (1)
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XRD was performed to examine the crystal structure of the synthesized electrode. Fig. 2 presents the XRD pattern of AgNW-Cd-2 on stainless steel. In this figure, XRD peaks at 44.31°, 50.23°, 64.43°, 74.17° and 81.87° 2θ were observed for the electrode due to the stainless steel
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substrate, which are marked by ‘∆’. There are few XRD peaks observed at 44.30°, 64.58°, and 77.29° values of 2θ were assigned to the (200), (220), and (311) planes of Ag crystals,
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respectively [(JCPDS card no. 4-0783] [26]. The XRD peaks detected at 26.42°, 30.44°, 43.94°, 51.76°, 63.82°, 80.75°, and 86.53° of 2θ were assigned to the (111), (200), (220), (311), (400),
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(422), and (511), planes of the cubic phase of CdS, respectively. These XRD peaks are in good agreement with the JCPDS card no. 03-065-2887. In addition, several XRD peaks were observed from the AgNW-Cd-2 electrode. All these XRD peaks can be indexed to the planes of monoclinic Ag2S, (Fig.2) from JCPDS card no. 01-089-3840. This suggests that the AgNWs strongly affected during the deposition of CdS by SILAR. When AgNW-coated Ni foam was
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inserted into the sulfur precursor, there was an interaction between AgNWs and sulfur ions, resulting in the formation of Ag2S.
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XPS was used to confirm the formation of CdS on the surface of the AgNWs. Fig. 3 (a) presents a high resolution spectrum of the Ag 3d region, which displays two prominent peaks at
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368.08 and 374.08 eV analogous to Ag 3d5/2 and Ag 3d3/2, respectively, suggesting that Ag exists either in metallic form or as Ag2S [27, 28]. Fig. 3 (b) presents the high resolution spectra of the Cd 3d region, which clearly defines the two distinct peaks. The XPS peaks of Cd 3d located at 405.28 eV and 411.98 eV were assigned to Cd 3d5/2 and Cd 3d3/2, respectively, with an energy separation of 6.70 eV. These peak positions correspond to the Cd2+ cations of CdS crystal structure. For S 2p, two distinct peaks were observed at 161.38, 162.38 eV. The peaks at 161.38
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and 162.38 eV were attributed to S2p3/2 and S2p1/2, respectively, indicating S2- species in CdS. The results suggest that CdS had been deposited successfully on the AgNW-coated Ni foam. The synthesized samples were examined further for their morphological and
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microstructural features by SEM, TEM and energy dispersive X-ray spectroscopy (EDS) mapping. Fig. 4 shows SEM images of all the samples at different magnifications. For AgNWCd-1, the deposition of a thin non-uniform layer of CdS was clearly seen on the surface of
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AgNWs. Furthermore, for the deposition of a second SILAR cycle of CdS (AgNW-Cd-2), the well covered, rough appearance with interconnected CdS nanoparticles is clearly visible.
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Subsequently, as the number of SILAR cycles was increased (AgNW-Cd-3, AgNW-Cd-4), the aggregation of CdS was observed within the network of AgNWs.
Fig. 5 presents TEM and HRTEM images of the AgNWs and AgNWs-CdS sample. A straight and smooth surface was observed for AgNWs, whereas the Ag NWs-CdS sample
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showed rough CdS nanoparticles. The HRTEM images from Fig. 5 clearly show that the spacing between the lattice fringes (d) is 0.238 nm for both the samples, which belongs to the (111) lattice plane of the AgNWs [29]. Furthermore, for AgNW-Cd2, the lattice spacings were
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approximately 0.33 and 0.67 nm, which corresponds to the (200) lattice plane of CdS [30] and the (010) lattice plane of Ag2S, respectively [31]. These outcomes again specify the formation of
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Ag2S during the deposition of CdS. The silver, cadmium, and sulfur distribution on AgNW-Cd2 was evaluated by quantitative EDS mapping. As shown in Fig.5, the green, red and yellow regions in the image are the silver, cadmium, and sulfur-containing regions, respectively, revealing the distribution of CdS nanoparticles on the surface of AgNWs. The Brunauer-Emmett-Teller (BET) surface area of sample AgNW-Cd is performed. Fig.6 shows the nitrogen adsorption and desorption isotherms and the corresponding pore size
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distribution curve (inset in Fig.6) of sample AgNW-Cd. The isotherm of the sample is characteristic of type IV with a hysteresis loop observed in the range of 0.7 - 1.0 P/P0, indicating
and the pore size distribution range from 20 to 90 Å.
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the existence of mesoporous structure. The BET surface area of the AgNW-Cd is 49.74 m2g-1,
To examine the potential applications of the as-synthesized electrodes as an electrode for supercapacitors, electrochemical characterization, such as cyclic voltammetry, charge discharge,
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and EIS were carried out. Fig. 7 (a) shows the CV curves of all the synthesized samples and pure CdS recorded within a potential window of -0.2 to 0.6 volts vs. SCE at 10 mVs-1 in 3M KOH.
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After the deposition of CdS on the Ag nanowires, remarkable change was observed in the shape and an area of the CV curve. The CV curves indicate the number of redox peaks for the CdScoated AgNWs electrodes, suggesting that the capacitance is due mainly to the pseudo-capacitive behavior of the active materials. One pair of redox peaks was attributed to the
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oxidation/reduction of CdS, whereas other corresponds to the oxidation/reduction of Ag nanowires and Ag2S. The oxidation of CdS takes place at 0.32 V, whereas CdS reduction occurs at 0.24 V. The oxidation of Ag nanowires takes place at 0.15 V, whereas Ag reduction occurs at -
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0.09 V. The electrochemical reversible oxidation/reduction reactions of Ag2S occurs at 0.32 V (overlapping of oxidation potential of CdS) and 0.13 V (reduction potential), respectively. The
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electrochemical behavior of AgNW@CdS in the KOH electrolyte can be described by the following reversible electrochemical reactions: + ↔ + --------- (1)
↔ +
--------- (2)
S + ↔ + --------- (3)
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As the number of SILAR cycles of CdS was varied from one to two, there was drastic enhancement in the CV current. A further increase in the number of SILAR cycles resulted in a decrease in the current of CV. This was attributed to the morphological features of the
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synthesized electrodes, where AgNWs coated with two CdS SILAR cycles showed a wellcovered uniform coating of CdS nanoparticles over the AgNWs. The areal capacitances and densities of each film were calculated using the equations as mentioned in the supporting
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information. The calculated areal capacitances for AgNW-Cd-1, AgNW-Cd-2, AgNW-Cd-3, and AgNW-Cd-4 were 1012, 2662, 1388, and 616 mFcm-2, respectively, at 10 mVs-1. As the number
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of CdS SILAR cycles was varied from one to two, the areal capacitance increased from 1012 to 2662 mFcm-2; with further increases in the SILAR cycles there was a decrease in areal capacitance. Initially, with increasing number of CdS SILAR cycles, there is an effective interaction between the AgNWs@CdS and electrolyte due to the uniform distribution of CdS
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nanoparticles onto the surface of AgNWs. On the other hand, a further increase in the number of CdS SILAR cycles led to the aggregation of CdS onto the AgNWs surface, which interrupted the interaction between the electrolyte and active materials, restricting the pseudo-capacitive
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contribution from Ag2S. The highest specific capacitance 1901 Fg-1 (with deposited mass ~ 1.4 mg) is obtained for AgNW-Cd-2 at 10 mVs-1. This value was greater than those of previously
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reported electrodes, such as porous CdS (909 Fg-1) [20], reduced graphene oxide/CdS (300 Fg-1) [21], Ag2S nanowires (268.4 Fg-1) [32] and graphene/Ag2S (1063 Fg-1) [33]. Fig. 7 (c) presents the Nyquist plots of all the electrodes synthesized over the frequency
range of 106–10−1 Hz. All Nyquist plots consisted of a small distorted semicircle in the high frequency region and a straight line in the low frequency region. Very small arc diameter was associated with the rapid ion diffusion in the synthesized electrode. The intercept of the curve
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with the real impedance axis at very high frequency region demonstrates the equivalent series resistance (ESR). From the magnified Nyquist plots, an ESR of 0.61, 0.51, 0.40, 0.49 and 0.64 Ω was achieved for CdS, AgNW-Cd-1, AgNW-Cd-2, AgNW-Cd-3, and AgNW-Cd-4, respectively.
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The AgNW-Cd-2 electrode denotes the lower ESR (0.40 Ω) compared to the other electrodes (CdS, AgNW-Cd-1, AgNW-Cd-3, and AgNW-Cd-4), indicating the more conducting behavior of the electrode, which is beneficial for the supercapacitor.
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Fig. 7 (d) presents the galvanostatic charge/discharge curves of all the synthesized electrodes recorded at an applied current 45 mA. The deviation of the charge/discharge curves
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from the linear voltage-time relation suggested the purely pseudocapacitive charge storage mechanism of the AgNWs@CdS electrodes. The AgNWs coated with two SILAR cycles of CdS showed a considerably longer discharge time than the other electrodes, which was well coordinated with the results obtained from CV. AgNW-Cd-1, AgNW-Cd-2, AgNW-Cd-3, and
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AgNW-Cd-4 showed areal capacitances (areal energy densities) of 436 mFcm-2 (213 mWhcm-2), 810 mFcm-2 (397 mWhcm-2), 538 mFcm-2 (263 mWhcm-2), and 373 mFcm-2 (183 mWhcm-2), respectively, at 45 mA. Fig. 7(b) and inset of 7(e) presents the CVs and galvanostatic
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charge/discharge curves of the AgNW-Cd-2 electrode recorded at different scan rates and current densities. The areal capacitance of AgNW-Cd-2 was 764 mFcm-2 at a higher scan rate of 100
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mVs-1, indicating the good rate capability of the optimized electrode. The Ragone plot for AgNW-Cd-2 at different applied currents is as shown in Fig. 7(e). Higher energy density of 397 mWhcm-2 with power density of 31.5 Wcm-2 is achieved for AgNW-Cd-2 at an applied current of 45 mA. Energy density decreases with increasing applied current, since at higher current the only outer layers of active materials can able to contribute to the charge discharge process. The electrochemical cycling stability of AgNW-Cd-2 was recorded for up to 2000 cycles. This study
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was conducted by recording CVs at 10 mVs-1. As shown in Fig. 7(f), AgNW-Cd-2 exhibited 6.9 % loss of capacitance retention after 1000 cycles, and only 3.3 % loss for the subsequent 1000 cycles. That is, a total 89.8 % capacitance retention was achieved after 2000 cycles, suggesting
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the stable electrochemical performance of the AgNW-Cd-2 electrode. Conclusions
AgNW@CdS nanostructured electrodes were synthesized via a facile dip coating and
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SILAR technique. The results showed that the loading of CdS nanoparticles on AgNWs strongly affect the electrochemical properties of AgNWs@CdS. This makes the transformation of
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AgNWs into Ag2S, which boosts the electrochemical performance of the resulting electrode. The electrode developed with two SILAR cycles of CdS showed the highest areal capacitance of 2662 and 810 mFcm-2 at 10 mVs-1 and 45 mA, respectively. The AgNW-Cd-2 electrode has adequate cycling stability over 2000 cycles. These results suggest that this simple and easy
Acknowledgments
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process can be adopted for other metal sulfides.
This work was supported by the National Research Foundation of Korea (NRF-
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Figure captions Fig. 1 Schematic representation of the synthesis of CdS coated Ag nanowires.
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Fig. 2 XRD pattern of AgNWs@CdS. Fig. 3 XPS of AgNW-CdS, high resolution spectra of (a) Ag 3d, (b) Cd 3d (c), and (f) S 2p. Fig. 4 SEM images of AgNW-Cd-1 (a,b), AgNW-Cd-2 (c, d), AgNW-Cd-3 (e, f) and AgNW-
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Fig. 5 TEM (a), HRTEM (b,c) and SAED pattern (d) of AgNW. TEM (e), HRTEM (f, g), SAED
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pattern (h) and EDS element mapping Silver (i), Cadmium (j) and Sulfur (k) of AgNW-Cd-2. Fig. 6 N2 adsorption–desorption isotherm and corresponding pore-size distribution curve (inset) of AgNW-Cd.
Fig. 7 CV curves of (a) CdS and all synthesized electrodes at 10 mVs-1, (b) AgNW-Cd-2 at
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different scan rate, (c) Nyquist plots, Inset: Magnified Nyquist plots of all electrodes (d) Chargedischarge of all the electrodes at 45 mA (e) Ragone plot, Inset: Charge discharge of AgNW-Cd-2 at different applied currents; (f) capacitance retention and specific capacitance versus number of
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Research Highlights
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Date: 09/05/2018
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CdS coated Ag nanowires on Ni foam was prepared by chemical route.
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The Ag nanowires convert into Ag2S during the deposition of CdS. The AgNW@CdS electrode showed a high areal capacitance of 2662 mFcm-2.
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The positive synergistic effect of Ag nanowires and CdS is observed.
S0925-8388(18)32745-2
DOI:
10.1016/j.jallcom.2018.07.244
Reference:
JALCOM 46960
To appear in:
Journal of Alloys and Compounds
Received Date: 1 June 2018 Revised Date:
19 July 2018
Accepted Date: 21 July 2018
Please cite this article as: D.S. Patil, S.A. Pawar, J.C. Shin, Alteration of Ag nanowires to Ag/Ag2S nanowires@CdS core-shell architectures for electrochemical supercapacitors, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.07.244. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Alteration of Ag nanowires to Ag/Ag2S nanowires@CdS core-shell architectures for electrochemical supercapacitors Dipali S. Patil*, Sachin A. Pawar, Jae Cheol Shin*
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Department of Physics, Yeungnam University, Gyeongsan, Gyeongbuk 38541, South Korea. *Corresponding Author Emails: [email protected], [email protected] Abstract
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Silver nanowire@cadmium sulfide (AgNW@CdS) core-shell nanostructured electrodes were synthesized in two steps. Initially, a layer of AgNWs was coated onto Ni foam by drop
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casting. CdS was then deposited onto the pre-coated Ni foam by a successive ionic layer adsorption and reaction (SILAR) method. This process converted the Ag nanowires to Ag2S, which was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The effects of the CdS layer thickness on AgNWs on the electrochemical properties of the nanostructure were
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examined by varying the number of SILAR cycles. The resulting AgNWs@CdS electrode exhibited a high areal capacitance of approximately 2662 mFcm-2 at 10 mVs-1 and 810 mFcm-2 at 45 mA applied current.
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KEYWORDS
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Silver nanowires, CdS, supercapacitor.
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Introduction Supercapacitors or ultracapacitors, also known as electrochemical capacitors, are the most attractive energy storage device because of their intermediate energy density and power
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density. Supercapacitors can be classified into two types: electric double layer capacitors (EDLC) and pseudo-capacitors [1]. In pseudo-capacitors, the capacitance is due to faradaic reactions in the nanosize thick region at the electrode-electrolyte interface. Therefore, a large electrochemical
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interface is favorable for enhancing the electrochemical performance of the supercapacitor. In this direction, several efforts have been made to develop different electrode materials with a
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range of nanostructures to improve the energy and power density [2-7]. The electrochemical performance is improved significantly by having the least resistance between the electroactive material and current collector [8, 9].
One dimensional nanostructures, such as nanorods, nanowires, and nanotubes, are
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interesting candidates that possesses high surface area and also deliver short diffusion paths to ions, leading to high charge/ discharge rates. In this way, silver nanowires (AgNWs) with high metallic conductivity, high aspect ratio, mechanical stability, and excellent anti-oxidation
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property have potential applications as current collector materials [10, 11]. Recently, a conducting network of AgNWs has been used as a core material for metal oxides, metal sulfides,
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and polymers to enhance their conductivity and capacity, and has been used as an excellent electrode in energy storage. Yuksel et al. developed core@shell AgNWs@MoO2 (Specific capacitance, 500.7 Fg-1 at 0.25 Ag-1) [12] and AgNWs@Ni(OH)2 (Specific capacitance, 1165.2 Fg-1 at 3 Ag-1) [13] nanostructures for supercapacitor electrodes. They showed that the synergy of highly conductive AgNWs and high capacitive MoO2 or Ni(OH)2 assists in ion and electron transportation and boosts the electrochemical properties of the resulting electrode. In addition,
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polypyrrole was grown onto the surface of the AgNWs and its electrochemical stability (90 % after 10000 cycles) and electrochemical properties (Specific capacitance, 509 Fg-1; energy density, 4.27 Whkg-1; power density, 60.7 Wkg-1, at 0.31 Ag-1) were improved [14]. In a
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previous report, AgNWs@Cu2S core@shell nanostructures were synthesized by simple and inexpensive drop casting followed by successive ionic layer adsorption and reaction (SILAR) methods and its electrochemical performance (Specific capacitance, 707 Fg-1, at 10 mVS-1;
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energy density, 10.01 Whkg-1; power density, 25.33 Wkg-1, at 0.2 mA) was studied [15]. Chen et al. reported the direct growth of Fe2O3 on the surface conductive networks of AgNWs on a
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coffee filter as a current collector that showed good electrochemical performance (specific capacitance, 287.4 F g-1; energy density 64.6 W h kg-1; power density 18 kW kg-1 at 1 mAcm-2) [16]. Liu et al. fabricated AgNW/graphene composites on polyethylene terephthalate (PET) using an electrophoresis method to improve the conductivity of the graphene electrode. They
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achieved the highest areal capacitance of 7.6 mFcm-2 at 100 mVs-1 with 82.6% capacitance retention after 2500 cycles [17].
In recent efforts on electrode material for supercapacitors, nanostructured metal sulfides
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(NiS, CoS, Cu2S, CdS, Bi2S3, and SnS2) have been used widely as a new class of energy storage material because of their excellent electrochemical properties [18, 19]. Among these, CdS
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exhibits attractive properties, such as good electronic conductivity, high theoretical capacitance (1675 Fg-1), lower cost, natural abundance, and environmental stability. These encouraging properties of CdS lead to potential applications in supercapacitors. Few reports on CdS-based electrode materials for supercapacitors are available and there is still scope to improve their capacitive performance. Xu et al. prepared porous CdS on nickel foam using a one-step hydrothermal method, which showed a specific capacitance of 909 Fg-1 at 2 mAcm-2 [20]. The
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three dimensional reduced graphene oxide/needle-like CdS hydrogel synthesized by a hydrothermal process displayed a specific capacitance of 300 Fg-1 at 5 mVs-1 [21]. Recently, CdS was used as a shell material for the nanostructured Ni3S2 [22] and Co3O4 [23] to improve their
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electrochemical properties.
In this study, AgNWs@CdS core shell nanostructured supercapacitor electrodes were synthesized by dip coating followed by SILAR methods. An interaction was observed between
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the AgNWs and sulfur during the SILAR process, leading to the formation of Ag2S, which again contributes to the pseudo-capacitance. To examine the effects of the CdS thickness on the
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electrochemical performance, the number of SILAR cycles was varied from one to four cycles. All the synthesized electrodes were characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance. The investigations showed that the CdS thickness had a significant impact on the electrochemical performance of AgNWs@CdS.
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Experimental Details
Commercially available AgNWs with a mean diameter ≈ 30 nm and length ≈ 15 µm were purchased from CNVISION CO., LTD. (Seoul, South Korea). For the synthesis of
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AgNWs@CdS core-shell nanostructure, initially, diluted AgNWs (AgNWs:ethanol = 1:3) were deposited uniformly on Ni foam using a dip coating technique. The formation of a shell (CdS)
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layer on the pre-coated AgNWs was developed using the SILAR technique. Briefly, the precoated AgNWs Ni foam was first dipped into 0.5 M (Cd(NO3)2 in methanol for 60 s and then rinsed with methanol for 60 s. Subsequently, it was dipped into a sodium sulfide nonahydrate (Na2S.9H2O) solution in methanol:water (7:3) for 60 s, and rinsed again with methanol for 60 s. This process is called a single SILAR cycle. The number of SILAR cycles is varied (1, 2, 3 and 4)
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and the corresponding electrodes are called AgNW-Cd-1, AgNW-Cd-2, AgNW-Cd-3, and AgNW-Cd-4, respectively. X-ray diffraction (XRD (PANalytical)) was carried out using Cu Kα radiation. X-ray
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photoelectron spectroscopy (XPS, K-alpha, Thermo Scientific, UK) was performed to determine the surface chemical composition. The surface morphology of the films was examined by field emission scanning electron microscopy (FE-SEM, S-4800 HITACHI, Ltd., Japan). Elemental
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mapping and the surface morphology were analyzed by high resolution transmission electron microscopy (HRTEM, JEOL JEM 2100F with Cs-corrected STEM). The electrochemical
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measurements, such as CV, electrical impedance spectroscopy (EIS), and galvanostatic charge/discharge were performed in a 3M KOH electrolyte in a conventional three-electrode arrangement consisting of a graphite counter electrode and a saturated calomel electrode (SCE), as the reference electrode, on a ZIVE SP5 electrochemical workstation (WonAtech).
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Results and discussion
Fig.1 presents a schematic diagram of the synthesis of AgNW@CdS core-shell nanostructures. The AgNW@CdS core-shell nanostructure was formed by dip coating followed
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by the SILAR process. First, the dip coating technique was used for the uniform deposition of AgNWs onto the Ni foam. Subsequently, the SILAR process was used for the formation of CdS
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(shell), in which dipping of the pre-coated Ni foam in a cationic precursor (Cd(NO3)2 leads to the adsorption of Cd2+ ions and later dipping in an anionic precursor (Na2S.9H2O) leads to the adsorption of S2- ions [24]. The reaction on the pre-coated Ni foam leading to the formation of CdS is as follows [25]:
+ → ------------------- (1)
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XRD was performed to examine the crystal structure of the synthesized electrode. Fig. 2 presents the XRD pattern of AgNW-Cd-2 on stainless steel. In this figure, XRD peaks at 44.31°, 50.23°, 64.43°, 74.17° and 81.87° 2θ were observed for the electrode due to the stainless steel
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substrate, which are marked by ‘∆’. There are few XRD peaks observed at 44.30°, 64.58°, and 77.29° values of 2θ were assigned to the (200), (220), and (311) planes of Ag crystals,
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respectively [(JCPDS card no. 4-0783] [26]. The XRD peaks detected at 26.42°, 30.44°, 43.94°, 51.76°, 63.82°, 80.75°, and 86.53° of 2θ were assigned to the (111), (200), (220), (311), (400),
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(422), and (511), planes of the cubic phase of CdS, respectively. These XRD peaks are in good agreement with the JCPDS card no. 03-065-2887. In addition, several XRD peaks were observed from the AgNW-Cd-2 electrode. All these XRD peaks can be indexed to the planes of monoclinic Ag2S, (Fig.2) from JCPDS card no. 01-089-3840. This suggests that the AgNWs strongly affected during the deposition of CdS by SILAR. When AgNW-coated Ni foam was
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inserted into the sulfur precursor, there was an interaction between AgNWs and sulfur ions, resulting in the formation of Ag2S.
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XPS was used to confirm the formation of CdS on the surface of the AgNWs. Fig. 3 (a) presents a high resolution spectrum of the Ag 3d region, which displays two prominent peaks at
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368.08 and 374.08 eV analogous to Ag 3d5/2 and Ag 3d3/2, respectively, suggesting that Ag exists either in metallic form or as Ag2S [27, 28]. Fig. 3 (b) presents the high resolution spectra of the Cd 3d region, which clearly defines the two distinct peaks. The XPS peaks of Cd 3d located at 405.28 eV and 411.98 eV were assigned to Cd 3d5/2 and Cd 3d3/2, respectively, with an energy separation of 6.70 eV. These peak positions correspond to the Cd2+ cations of CdS crystal structure. For S 2p, two distinct peaks were observed at 161.38, 162.38 eV. The peaks at 161.38
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and 162.38 eV were attributed to S2p3/2 and S2p1/2, respectively, indicating S2- species in CdS. The results suggest that CdS had been deposited successfully on the AgNW-coated Ni foam. The synthesized samples were examined further for their morphological and
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microstructural features by SEM, TEM and energy dispersive X-ray spectroscopy (EDS) mapping. Fig. 4 shows SEM images of all the samples at different magnifications. For AgNWCd-1, the deposition of a thin non-uniform layer of CdS was clearly seen on the surface of
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AgNWs. Furthermore, for the deposition of a second SILAR cycle of CdS (AgNW-Cd-2), the well covered, rough appearance with interconnected CdS nanoparticles is clearly visible.
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Subsequently, as the number of SILAR cycles was increased (AgNW-Cd-3, AgNW-Cd-4), the aggregation of CdS was observed within the network of AgNWs.
Fig. 5 presents TEM and HRTEM images of the AgNWs and AgNWs-CdS sample. A straight and smooth surface was observed for AgNWs, whereas the Ag NWs-CdS sample
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showed rough CdS nanoparticles. The HRTEM images from Fig. 5 clearly show that the spacing between the lattice fringes (d) is 0.238 nm for both the samples, which belongs to the (111) lattice plane of the AgNWs [29]. Furthermore, for AgNW-Cd2, the lattice spacings were
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approximately 0.33 and 0.67 nm, which corresponds to the (200) lattice plane of CdS [30] and the (010) lattice plane of Ag2S, respectively [31]. These outcomes again specify the formation of
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Ag2S during the deposition of CdS. The silver, cadmium, and sulfur distribution on AgNW-Cd2 was evaluated by quantitative EDS mapping. As shown in Fig.5, the green, red and yellow regions in the image are the silver, cadmium, and sulfur-containing regions, respectively, revealing the distribution of CdS nanoparticles on the surface of AgNWs. The Brunauer-Emmett-Teller (BET) surface area of sample AgNW-Cd is performed. Fig.6 shows the nitrogen adsorption and desorption isotherms and the corresponding pore size
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distribution curve (inset in Fig.6) of sample AgNW-Cd. The isotherm of the sample is characteristic of type IV with a hysteresis loop observed in the range of 0.7 - 1.0 P/P0, indicating
and the pore size distribution range from 20 to 90 Å.
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the existence of mesoporous structure. The BET surface area of the AgNW-Cd is 49.74 m2g-1,
To examine the potential applications of the as-synthesized electrodes as an electrode for supercapacitors, electrochemical characterization, such as cyclic voltammetry, charge discharge,
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and EIS were carried out. Fig. 7 (a) shows the CV curves of all the synthesized samples and pure CdS recorded within a potential window of -0.2 to 0.6 volts vs. SCE at 10 mVs-1 in 3M KOH.
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After the deposition of CdS on the Ag nanowires, remarkable change was observed in the shape and an area of the CV curve. The CV curves indicate the number of redox peaks for the CdScoated AgNWs electrodes, suggesting that the capacitance is due mainly to the pseudo-capacitive behavior of the active materials. One pair of redox peaks was attributed to the
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oxidation/reduction of CdS, whereas other corresponds to the oxidation/reduction of Ag nanowires and Ag2S. The oxidation of CdS takes place at 0.32 V, whereas CdS reduction occurs at 0.24 V. The oxidation of Ag nanowires takes place at 0.15 V, whereas Ag reduction occurs at -
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0.09 V. The electrochemical reversible oxidation/reduction reactions of Ag2S occurs at 0.32 V (overlapping of oxidation potential of CdS) and 0.13 V (reduction potential), respectively. The
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electrochemical behavior of AgNW@CdS in the KOH electrolyte can be described by the following reversible electrochemical reactions: + ↔ + --------- (1)
↔ +
--------- (2)
S + ↔ + --------- (3)
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As the number of SILAR cycles of CdS was varied from one to two, there was drastic enhancement in the CV current. A further increase in the number of SILAR cycles resulted in a decrease in the current of CV. This was attributed to the morphological features of the
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synthesized electrodes, where AgNWs coated with two CdS SILAR cycles showed a wellcovered uniform coating of CdS nanoparticles over the AgNWs. The areal capacitances and densities of each film were calculated using the equations as mentioned in the supporting
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information. The calculated areal capacitances for AgNW-Cd-1, AgNW-Cd-2, AgNW-Cd-3, and AgNW-Cd-4 were 1012, 2662, 1388, and 616 mFcm-2, respectively, at 10 mVs-1. As the number
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of CdS SILAR cycles was varied from one to two, the areal capacitance increased from 1012 to 2662 mFcm-2; with further increases in the SILAR cycles there was a decrease in areal capacitance. Initially, with increasing number of CdS SILAR cycles, there is an effective interaction between the AgNWs@CdS and electrolyte due to the uniform distribution of CdS
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nanoparticles onto the surface of AgNWs. On the other hand, a further increase in the number of CdS SILAR cycles led to the aggregation of CdS onto the AgNWs surface, which interrupted the interaction between the electrolyte and active materials, restricting the pseudo-capacitive
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contribution from Ag2S. The highest specific capacitance 1901 Fg-1 (with deposited mass ~ 1.4 mg) is obtained for AgNW-Cd-2 at 10 mVs-1. This value was greater than those of previously
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reported electrodes, such as porous CdS (909 Fg-1) [20], reduced graphene oxide/CdS (300 Fg-1) [21], Ag2S nanowires (268.4 Fg-1) [32] and graphene/Ag2S (1063 Fg-1) [33]. Fig. 7 (c) presents the Nyquist plots of all the electrodes synthesized over the frequency
range of 106–10−1 Hz. All Nyquist plots consisted of a small distorted semicircle in the high frequency region and a straight line in the low frequency region. Very small arc diameter was associated with the rapid ion diffusion in the synthesized electrode. The intercept of the curve
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with the real impedance axis at very high frequency region demonstrates the equivalent series resistance (ESR). From the magnified Nyquist plots, an ESR of 0.61, 0.51, 0.40, 0.49 and 0.64 Ω was achieved for CdS, AgNW-Cd-1, AgNW-Cd-2, AgNW-Cd-3, and AgNW-Cd-4, respectively.
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The AgNW-Cd-2 electrode denotes the lower ESR (0.40 Ω) compared to the other electrodes (CdS, AgNW-Cd-1, AgNW-Cd-3, and AgNW-Cd-4), indicating the more conducting behavior of the electrode, which is beneficial for the supercapacitor.
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Fig. 7 (d) presents the galvanostatic charge/discharge curves of all the synthesized electrodes recorded at an applied current 45 mA. The deviation of the charge/discharge curves
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from the linear voltage-time relation suggested the purely pseudocapacitive charge storage mechanism of the AgNWs@CdS electrodes. The AgNWs coated with two SILAR cycles of CdS showed a considerably longer discharge time than the other electrodes, which was well coordinated with the results obtained from CV. AgNW-Cd-1, AgNW-Cd-2, AgNW-Cd-3, and
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AgNW-Cd-4 showed areal capacitances (areal energy densities) of 436 mFcm-2 (213 mWhcm-2), 810 mFcm-2 (397 mWhcm-2), 538 mFcm-2 (263 mWhcm-2), and 373 mFcm-2 (183 mWhcm-2), respectively, at 45 mA. Fig. 7(b) and inset of 7(e) presents the CVs and galvanostatic
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charge/discharge curves of the AgNW-Cd-2 electrode recorded at different scan rates and current densities. The areal capacitance of AgNW-Cd-2 was 764 mFcm-2 at a higher scan rate of 100
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mVs-1, indicating the good rate capability of the optimized electrode. The Ragone plot for AgNW-Cd-2 at different applied currents is as shown in Fig. 7(e). Higher energy density of 397 mWhcm-2 with power density of 31.5 Wcm-2 is achieved for AgNW-Cd-2 at an applied current of 45 mA. Energy density decreases with increasing applied current, since at higher current the only outer layers of active materials can able to contribute to the charge discharge process. The electrochemical cycling stability of AgNW-Cd-2 was recorded for up to 2000 cycles. This study
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was conducted by recording CVs at 10 mVs-1. As shown in Fig. 7(f), AgNW-Cd-2 exhibited 6.9 % loss of capacitance retention after 1000 cycles, and only 3.3 % loss for the subsequent 1000 cycles. That is, a total 89.8 % capacitance retention was achieved after 2000 cycles, suggesting
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the stable electrochemical performance of the AgNW-Cd-2 electrode. Conclusions
AgNW@CdS nanostructured electrodes were synthesized via a facile dip coating and
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SILAR technique. The results showed that the loading of CdS nanoparticles on AgNWs strongly affect the electrochemical properties of AgNWs@CdS. This makes the transformation of
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AgNWs into Ag2S, which boosts the electrochemical performance of the resulting electrode. The electrode developed with two SILAR cycles of CdS showed the highest areal capacitance of 2662 and 810 mFcm-2 at 10 mVs-1 and 45 mA, respectively. The AgNW-Cd-2 electrode has adequate cycling stability over 2000 cycles. These results suggest that this simple and easy
Acknowledgments
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process can be adopted for other metal sulfides.
This work was supported by the National Research Foundation of Korea (NRF-
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ultraviolet irradiation catalyzed by phosphomolybdic acid and their antibacterial properties, New J. Chem., 37 (2013) 2179. [27] S. R. Ferrá-González, D. Berman-Mendoza, R. García-Gutiérrez, S. J. Castillo, R.
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Ramírez-Bon, B. E. Gnade, M. A. Quevedo-López, Optical and structural properties of CdS thin films grown by chemical bath deposition doped with Ag by ion exchange, Optik 125 (2014).
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Ag2S Ag core–shell structures, Applied Surface Science 288 (2014) 158. [32] N Nair and B. R. Sankapal, Cationic-exchange approach for conversion of two
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dimensional CdS to two dimensional Ag2S nanowires with an intermediate core–shell
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Figure captions Fig. 1 Schematic representation of the synthesis of CdS coated Ag nanowires.
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Fig. 2 XRD pattern of AgNWs@CdS. Fig. 3 XPS of AgNW-CdS, high resolution spectra of (a) Ag 3d, (b) Cd 3d (c), and (f) S 2p. Fig. 4 SEM images of AgNW-Cd-1 (a,b), AgNW-Cd-2 (c, d), AgNW-Cd-3 (e, f) and AgNW-
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Cd-4 (g, h) at different magnifications.
Fig. 5 TEM (a), HRTEM (b,c) and SAED pattern (d) of AgNW. TEM (e), HRTEM (f, g), SAED
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pattern (h) and EDS element mapping Silver (i), Cadmium (j) and Sulfur (k) of AgNW-Cd-2. Fig. 6 N2 adsorption–desorption isotherm and corresponding pore-size distribution curve (inset) of AgNW-Cd.
Fig. 7 CV curves of (a) CdS and all synthesized electrodes at 10 mVs-1, (b) AgNW-Cd-2 at
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different scan rate, (c) Nyquist plots, Inset: Magnified Nyquist plots of all electrodes (d) Chargedischarge of all the electrodes at 45 mA (e) Ragone plot, Inset: Charge discharge of AgNW-Cd-2 at different applied currents; (f) capacitance retention and specific capacitance versus number of
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Research Highlights
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Date: 09/05/2018
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CdS coated Ag nanowires on Ni foam was prepared by chemical route.
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The Ag nanowires convert into Ag2S during the deposition of CdS. The AgNW@CdS electrode showed a high areal capacitance of 2662 mFcm-2.
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The positive synergistic effect of Ag nanowires and CdS is observed.