Core–Shell (W@WO3) Nanostructure to Improve Electrochemical

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Core-Shell (W@WO) Nanostructure to Improve Electrochemical Performance Khabibulla Abdullin Abdulaevich, Zhanar Kanievna Kallkozova, Aiymkul Markhabayeva, Robin Dupre, Md Moniruddin, and Nurxat Nuraje ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01869 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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ACS Applied Energy Materials

Core-Shell (W@WO3) Nanostructure to Improve Electrochemical Performance Khabibulla Abdulaevich Abdullin1*, Zhanar Kanievna Kalkozova1, Aiymkul Alikhanovna Markhabayeva 1

1,2,

Robin Dupre2, Md Moniruddin2, and Nurxat Nuraje2*

National Nanotechnology Laboratory of Open Type (NNLOT), Kazakh National University, Almaty, 050012 Kazakhstan 2 Texas

Tech University, Lubbock, TX, 79409 USA

*: corresponding author, [email protected]; [email protected] Key words: tungsten oxide; core-shell tungsten oxide; specific capacitance; aerosol approach; templated approach; reduced tungsten oxide Abstract Two different approaches were developed to fabricate core-shell tungsten and tungsten oxide (W@WO3) nanostructures in combination with a hydrogen reduction technique. One is a combination of aerosol and pyrolysis approaches which produce spherical tungsten oxide nanoparticles with a hexagonal crystal structure. The other is a combination of templating and impregnating approaches which lead to two-dimensional structures of tungsten oxide with a monoclinic crystalline structure. Subsequent hydrogen reduction lead to the formation of coreshell W@WO3 nanostructures with two different tungsten metallic phases. Structural and morphological characterizations were performed using X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The electrochemical performance of W@WO3 electrodes synthesized by two different approaches was studied using cyclic voltammetry, impedance spectroscopy and galvanostatic charge-discharge methods. The capacity of core-shell W@WO3 nanostructures can reach 148 F g-1 compared with 21 F g-1 at 0.43 A g-1 for the WO3 sample, which is also higher than the reported literature value. It is shown that the W@WO3 nanostructures have significantly better electrochemical performance than WO3 alone.

Introduction Supercapacitors are a promising energy storage device due to their excellent characteristics such as high power density, fast charge/discharge rate and long cyclic life [1-2]. Carbon materials including activated carbon, carbon nanotubes, graphene, etc. have been extensively studied as active electrode materials for supercapacitors due to their high specific 1 ACS Paragon Plus Environment

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area, chemical and thermal stability, and high electrical conductivity

[3].

The combination of

both charge storage mechanisms - a double dielectric layer and Faraday reversible process— significantly increases the capacitance. Nowadays, composite capacitors consisting of carbon and redox active materials have been intensively explored. Different types of composite electrodes of tungsten oxide and carbon materials including carbon cloth [4-5], carbon aerogel [6-7],

and graphene [8-9] have been reported. Various transition oxides are used as redox active

materials, including RuO, MnO2, V2O5, etc [10]. Among them, tungsten oxide is also considered as a good electrode candidate material for an energy storage device

[5, 11-15]

since its oxidation state, structure and properties can be

easily controlled. For example, Bi et al studied electrochemical performance of tungsten trioxide monohydrate nanosheets on FTO glass substrates and found 43.30 mF/cm2 of areal capacitance

[16].

Lee et al. also reported improved pseudocapacitive performance (20.3 F/g)

for tungsten oxide nanoplates via changing oxidation state

[17].

Also, the electrochemical

performance has been improved when the film thickness or the dimensions of the WO3 nanoparticles in the electrode are reduced [12]. However, regarding the pure tungsten oxide as electrode materials, few studies have been performed to improve average specific capacitance via fabricating various nanostructures and different treatments. Therefore, to increase specific capacitance of tungsten oxide our strategy is to apply core-shell nanostructure of tungsten and tungsten oxides as electrode materials, which not only provides a better conductivity, but also higher capacitance based on the hypothesis that nanostructured high surface materials with better conductivity improve electrochemical performance of tungsten oxide. We have investigated the core-shell nanostructures of W and WO3 (aka W@WO3) as electrode materials since metal tungsten provides conductivity and nanoscale tungsten oxide provides a high capacity. To fabricate the core-shell structure of tungsten and tungsten oxide (W@WO3),

two different morphologies of WO3 were

synthesized via two different synthesis approaches. One is a combination of aerosol and pyrolysis (aka an aerosol-pyrolysis) approaches, while the other is a combination of template and impregnating approaches (aka a template-impregnating approach). Metallic tungsten core particles were obtained by further reducing these oxides in a hydrogen environment. Our experimental results showed that the core-shell structure of metallic tungsten and tungsten oxide, W@WO3, demonstrated better electrochemical performance than pure tungsten oxide electrodes.

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Experimental Section: General strategy for Fabrication of core-shell nanostructures of Tungsten and tungsten oxide (W@WO3): In this core-shell nanostructure (W@WO3), tungsten is the core and tungsten oxide is the shell. A two-step synthesis approach was applied to synthesize W@WO3 core-shell nanomaterials. In the first step, tungsten oxide was produced independently via two different synthesis approaches. The resulting tungsten oxide nanostructures were further reduced to metal tungsten under a hydrogen atmosphere, and subsequent heat was applied to generate a tungsten oxide shell layer on the surface tungsten metal core. Fabrication of tungsten oxide nanomaterials: In the first synthesis, tungsten oxide nanoparticles were formed by pyrolysis of an aerosol consisting of 0.01 M ammonium metatungstate ((NH4)6H2W12O40×H2O) aqueous solution. The aerosol was generated by an ultrasonic piezoelectric atomizer equipped with air compressor to provide air as the transport gas. Generated aerosol drops were directed into a vertical tube furnace via gas stream. Tungsten oxide nanoparticles were formed by pyrolysis in the hot zone of the oven and collected by a cylindrical electrostatic filter heated to ~ 200°C to prevent the water vapor condensation. After synthesis, the powders were easily separated from the walls of the filter. A typical output of the product was 2 grams per hour. The tungsten oxide nanomaterials obtained by this method are expressed as WO3-A. In the second method, tungsten oxide nanoparticles were formed from a precursor solution using a fibrous matrix, which was a natural cellulose in the form of ashless filter paper or cotton wool. The matrix was impregnated with an 0.02 M aqueous solution of ammonium metatungstate hydrate, then it was squeezed and dried in a vacuum. Then, the sample was heat treated at 350 °C in air with a ramping rate of 1°C/min. The heat treatment was performed at a temperature range of 350-700°C for 1 hour. A typical output of the product was 0.2 grams per gram of cellulose matrix. Herin, the tungsten oxide nanomaterials obtained by this method are expressed as WO3-B. Fabrication of core-shell nanostructures of Tungsten and tungsten oxide (W@WO3): Resulting tungsten oxide nanomaterials from the above two synthesis approaches were further reduced in a quartz furnace under a hydrogen environment. Briefly, the powder was placed in an alumina crucible and heated in a stream of hydrogen at temperature range of 400-700 °C for 1 hour. Subsequently, the crucible was cooled in a stream of hydrogen gas to room temperature. Samples were left in the oven overnight to form a thin native oxide film. Tungsten nanoparticles obtained by hydrogen reduction was denoted as W@WO3. If the air 3 ACS Paragon Plus Environment


was injected immediately after annealing, then tungsten oxide peaks are present in XRD pattern. X-ray diffraction (XRD) was performed on the above synthesized nanomaterials using a MiniFlex Rigaku diffractometer in CuKα radiation. The Raman spectra were obtained using a Ntegra Spectra (NT-MDT) spectrometer with 473 nm excitation. The surface morphology was studied using a field emission scanning electron microscope Hitachi S-4300. Measurements of electrochemical characteristics, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) measurements were obtained using the Elins P-40X potentiostat with the FRA-24M impedance measurement module in a three-electrode system. Reference and counter electrode were Ag/AgCl and Pt, respectively, with a 0.5 M H2SO4 solution used as the electrolyte. To prepare the working electrode, 0.02 g powder (samples WO3 or W@WO3) were placed in 2 ml ethanol and sonicated in an ultrasonic bath to obtain a homogeneous slurry. The as-prepared slurry was then applied to a stainless steel mesh. The dried mesh was pressed at a pressure of ~108 Pa and sonicated again in ethanol. The mass of the sample was determined as the difference between the mass of obtained electrode and the initial substrate mass. The area of prepared electrode is 1cm2. Results and discussion Based on our hypothesis, this W@WO3 core-shell nanostructure improves the electrochemical performance of tungsten oxide. Two different W@WO3 nanostructures were synthesized according to the above synthesis approaches described. To get the two different W@WO3 nanostructures, first two different approaches were applied to produce tungsten oxides with two different morphologies. One was an aeorosol-pyrolysis technique and the other was a template-impregnating approach with heat treatment. Figure 1 shows the surface morphologies of tungsten oxide nanomaterials produced by the two methods. The first method produced spherical WO3 particles with a smooth surface, whereas the second method generated 2D porous structures.


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(a)

(b)

(c)

(d)

Figure 1. SEM images of as synthesized free particles: (a) WO3-A annealed at 6000C, (b) WO3-A reduced in a hydrogen atmosphere at 4000C, (c) WO3-B annealed at 6000C, and (d) WO3-B reduced in a hydrogen atmosphere at 4000C. In Fig 2a, the XRD spectra shows the WO3-A crystal structure transformation from hexagonal to monoclinic along with calcination temperatures in the aerosol-pyrolysis method. No crystalline WO3 peaks appeared at 350 0C. Interestingly, XRD peaks show the hexagonal crystalline structure of h-WO3 (JCPDS card No. 01-085- 2459) for the samples synthesized at 400°C. The average crystallite size for these tungsten oxide nanomaterials was estimated from XRD data, using the well-known Scherrer formula [18-19]. Pyrolysis of aerosol is a simple and efficient way to obtain a hexagonal WO3 phase, which offers higher pseudocapacitance relative to WO3 [20-21] due to lattice structure features. The h-WO3 samples synthesized at Tsyn= 400 °C have a crystallite size of ~35 nm. It was observed that increasing synthesis temperature from 400 to 500oC leads to a rapid decrease in crystallite size for the hexagonal (metastable) WO3 phases. This indicates that the formation of h-WO3 is suppressed at higher temperatures. The estimated size of the hWO3 crystallites is about 7 nm at Tsyn= 500oC, and weak reflexes of the stable m-WO3 5 ACS Paragon Plus Environment


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modification (JCPDS card No-01-083-0950, monoclinic space group P21/n) appeared. The m-WO3 becomes the main phase at Tsyn= 600-700°C (Fig. 2a). The main XRD peaks of WO3-B powders (Fig. 2b) obtained by a templateimpregnating method indicated the formation of a monoclinic tungsten oxide structure (JCPDS card No 01-083-0950). In addition, three distinct h-WO3 peaks also appear at angles of 28.2, 36.7 and 39.3 degrees which can be attributed to h-WO3. The half-width of the XRD lines decreases with increasing synthesis temperature. The estimated sizes of WO3 crystallites are ~ 7 nm for a sample obtained at 400°C and about 30 nm for a sample obtained at 700°C.

(a)

(b)

Figure 2. (a) XRD patterns of the WO3-A samples and (b) WO3-B samples synthesized at the temperatures from 350 to 700oC After successfully fabricating two different morpholgies of tungsten oxide, these tungsten oxide nanomaterials were further reduced to metallic W by annealing in a hydrogen atmosphere. XRD studies (Fig. 3a) have shown that the W phase begins to appear at 500oC, and annealing at 550oC for 1 hour leads to creation of mainly metallic tungsten phase and monoclinic WO3 (JCPDS card No. 01-086-0134). Calcinatiation at 600oC for 1 hour results in the metallic tungsten beta phase (beta-W). Fig. 3a shows XRD patterns for reduced tungsten oxide WO3-A and WO3-B obtained at Tsyn = 400-650 °C and at 600 °C. The sample WO3-A synthesized at Tsyn = 400 °C with hexagonal WO3, after reduction, mainly consists of the metastable tungsten beta-W phase (JCPDS card No 00-047-1319).


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a

b

Figure 3. (a) XRD patterns of reduced tungsten oxide WO3-A (1 and 2) and WO3-B (3 and 4) obtained at Tsyn = 400-650 ° C and reduced at 600°C (b) and the estimated crystallite size of the tungsten nanoparticles WO3 (A and B) versus the synthesis temperature at a fixed reduction temperature of 600°C The portion of the h-WO3 phase decreases with increasing temperature of synthesis (Fig. 2a), and the beta-W content in the WO3-A sample decreases (Fig. 3a). Therefore, the reduction of h-WO3 at low temperatures leads to the formation of metastable beta-W. On the other hand, WO3-B samples which consist of the m-WO3 phase (Fig. 2.b) are reduced to form stable tungsten (Fig. 3a). The dependence of the estimated tungsten nanoparticle crystallite size on synthesis temperature at a fixed reduction temperature of 600°C is shown in Fig. 3b. After extended heat treatment in air, the tungsten oxides reduced in a hydrogen atmosphere showed tungsten oxide peaks in addition to tungsten metal peaks, indicating the formation of a core-shell structure of W@WO3 (Figure S1). Figure 4 shows the Raman spectra of the obtained WO3-A samples. The main peaks in the sample synthesized at 700°C (at 807, 715, 328 and 272 cm-1) follow trends for known mWO3 Raman modes [19]. The Raman peaks broadened at low synthesis temperature, indicating the formation of a small crystallite size which is in good agreement with our XRD results. The WO3 Raman peaks shift left as synthesis temperature decreases. The appearance of the band at 940-960 cm-1 can be associated with the presence of a hydrogenated WO3-H2O layer on the surface; the formation of a hexagonal h-WO3 phase can also take place [22].



Figure 4. Raman spectra of the WO3-A samples synthesized at 350-700°C To test our idea for improving the electrochemical performance of tungsten oxide, fabricated core-shell W@WO3 nanostructures were further studied as electrode materials in parallel with WO3 nanomaterials by CV, EIS, and GCD measurements. Typical CV curves at a scan rate 0.54 V s-1 are presented in Figure 5a.


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Figure 5. (a) Cyclic voltammograms (CV) of a blank electrode, WO3-A, and W@WO3A samples at a scan rate of 0.54 V s-1 in 0.5 M H2SO4 aqueous solution, (b) Plot of specific capacity versus potential scan rate for WO3 and W@WO3 electrodes As seen in Figure 5a, the CV curves exhibit asymmetric loops for all scan rates, indicating the presence of both EDLC (electro-chemical double layer capacitor) and faradic processes.The asymmetry increases with increasing scanning rates. This suggests the occurrence of oxidation-reduction reactions involving protons, as suggested earlier

[20].The

specific capacitance is calculated from cyclic voltammetry curves using the following formula:

where Cs is specific capacitance, m is the mass of deposited material, ν is the scan rate, Vmax-Vmin is the potential window, I(V) is the current, and integration is performed over one CV cycle. The plot of Cs versus scan rate is shown in Fig. 5b. The Cs increases significantly with an increased scan rate since at low scanning speed, H+ ions diffuse deep inside the electrode, resulting in a high specific capacitance. The maximum Cs calculated from the CV curves is 272 F g-1 for the W@WO3 sample and 56.8 F g-1 for the WO3 sample at a scan rate of 2 mV s-1. Thus, the specific capacitance of the electrode, which according to the XRD data consists of metallic tungsten, is almost 5 times higher than the capacity of the WO3 electrode.The higher capacity of W@WO3 samples compared to WO3 is confirmed by GCD measurements for the current range of 0.43-14 A g-1, as shown in Figure 6. Specific capacitance of the electrodes is calculated using the formula:

where C is specific capacitance (F g-1), I (A) is discharge current, m (g) is the mass of active material, Δt is total discharge time, and ΔV is the potential drop during discharging. The capacity of core-shell W@WO3 nanostructures at 0.43 A g-1 current can reach 148 F g-1 compared with 21 F g-1 for sample WO3 (Fig. 6a). The obtained specific capacitance is higher than for previously reported articles

[8,

17,

23-25].

Both core-shell W@WO3

nanostructures obtained from the two synthetic methods provided similar results (Figure 5 and Figure S2). 9 ACS Paragon Plus Environment


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(a)

(b)

(c)

(d)

Figure 6. (a) The GCD curves of WO3 and W@WO3 electrodes at a current density of 0.43 A g-1, the inset shows the plot of Cs versus charge current densities and (b) Nyquist plots of WO3 and core-shell W@WO3 nanostructures at frequencies 0.004 Hz to 105 Hz; the inset indicates the Nyquist plots at high frequency region; (c) cycling performances of the Re-WO3 sample at current density of 6 A g-1 over 1200 cycles, inset is the GCD curves during 10-th cycle (1) and 1200-th cycle (2), (d) comparison of specific capacity values for electrode materials consisting of W@WO3, WO3, and sole tungsten oxide literature values [17]. EIS measurements have also been carried out to provide a further comparison of the WO3 and core-shell W@WO3 nanostructures electrodes. Fig. 6b and Figure S3 show the Nyquist plots for WO3 and core-shell W@WO3 nanostructures. The plots consist of a semicircle at the higher frequency region and a nearly straight line at the lower frequency region. The semicircle is associated with faradic charge transfer resistance and the straight line is ascribed to the ion diffusion process. The core-shell W@WO3 nanostructures have a smaller semicircle than the WO3 sample, so it has a lower charge transfer resistance. Figure 6c shows the cyclic performance of Re-WO3 electrode at a current density of 6 −1 A g . The capacity of the Re-WO3 electrode increased by ~5% after 1200 galvanostatic charge - discharge (GCD ) cycles. The inset of Figure 6c shows the corresponding GCD 10 ACS Paragon Plus Environment

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curves during 10-th cycle (1) and 1200-th cycle (2). The performance test demonstrates that the GCD curves remain almost unchanged over 1200 cycles. In the high frequency region, the intercept at the real part (Z′) represents equivalent series resistance (Rs) which is the combination of ionic resistance of the electrolyte, intrinsic resistance of electroactive material and contact resistance at the interface between electrode and electrolyte.The Rs value of WO3 electrode (2.3 Ω) is smaller compared to that of coreshell W@WO3 nanostructures (4.5 ohms). Apparently, this is because of wettability of the WO electrode. At the lower frequency region, the straight line slope of the core-shell W@WO3 nanostructures electrode is higher than that of WO3, indicating that the ion diffusion resistance for the Re-WO3 electrode is lower than that of WO3. In addition, the straight lines of the Nyquist plots may be approximated by the impedance of the constant phase element (CPE) which is expressed by the equation:

where ω is the angular frequency. The “n” values for W@WO3 and WO3 samples are 0.74 and 0.57 respectively (for n=1 CPE represents pure capacitor). The Y0 values of core-shell W@WO3 nanostructures are also significantly higher than that of WO3. This indicates that the charge storage process in the core-shell W@WO3 nanostructures is more capacitive in nature than in WO3. Thus, CV, EIS, and GCD measurements demonstrate that core-shell W@WO3 nanostructures have significantly enhanced the electrochemical properties than WO3 samples synthesized by both methods used in this study. Since no significant changes in the morphology of both WO3-A and WO3-B samples were observed during the annealing process in hydrogen atmosphere, the high specific capacity of core-shell W@WO3 nanostructures relative to that of WO3 can be attributed to the presence of a thin submicron oxide film on the surface of the metallic particles and better electrical conductivity of tungsten core materials. It is known that capacity significantly increases when WO3 film thickness is decreased [12].The

high specific capacity of thin films is achieved due to a greater proportion of very

active atoms on the surface. They also contribute to a high specific capacity

[12].

In addition,

core tungsten nanomaterials also contribute to improved electrochemical performance by providing better electrical conductivity.



In all, such "metal core-oxide shell" structures can have a much lower series resistance than tungsten oxide particles because the current passing through the very thin films has tunnelling characteristics. Subsequently, it is available to get good contact with substrate. Conclusion Two different synthesis approaches were developed to produce tungsten oxide nanomaterials. One is an aerosol approach combined with a pyrolysis technique which produces hexagaonal tungsten oxide (h-WO3) which had better electrochemical characteristics than m-WO3

[22].

The other is a combination of templating and impregnating

approaches. The fabrication technique results in two dimensional nanostructures of WO3 oxide materials with monoclinic crystalline structure. The subsequent thermal reduction of two different tungsten oxides in a hydrogen atmosphere results in W@WO3 nanostructures. The reduction of hexagonal WO3 results in a predominantly metastable phase of tungsten (beta-W). Comparative studies of the electrochemical performance for the WO3 and W@WO3 electrodes using cyclic voltammetry, impedance spectroscopy and galvanostatic chargedischarge methods show that the W@WO3 nanostructures have significantly better characteristics than WO3 alone. The capacity of core-shell W@WO3 nanostructures at a current of 0.43 A g-1 can reach 148 F g-1 compared with 21 F g-1 for the WO3 sample, which is higher than the previously reported literature value[17].The high efficiency of the W@WO3 electrodes can help increase the performance of existing WO3 supercapacitors. Associated Content Supporting information is available. XRD spectra of tungsten oxide after hydrogen treatment; CV and Nyquist plot for WO3B and W@WO3 -B Acknowledgments A.Kh.A. acknowledges the financial support from the Ministry of Education and Science of the Republic of Kazakhstan, grant no. AP05130100.


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Core-shell nanostructure of Tungsten and Tungsten oxide enhanced electrochemical performance 276x163mm (96 x 96 DPI)

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Core–Shell (W@WO3) Nanostructure to Improve Electrochemical

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