RuO2 thin films electrodeposited on polystyrene nanosphere arrays

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RuO thin films electrodeposited on polystyrene nanosphere arrays: growth mechanism and application to supercapacitor electrodes Dajung Hong, and Sanggyu Yim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00829 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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RuO2 thin films electrodeposited on polystyrene nanosphere arrays: growth mechanism and application to supercapacitor electrodes

Dajung Hong and Sanggyu Yim* Department of Chemistry, Kookmin University, Seoul, 02707, South Korea

Abstract Two-dimensionally (2D) arrayed polystyrene (PS)/ruthenium oxide (RuO2) core/shell nanospheres are successfully prepared by the electrodeposition of RuO2 nanoparticles on a hexagonal close-packed PS monolayer. This nanosphere structure is entirely different from the structure previously reported for other transition metal oxides electrodeposited on the PS nanosphere arrays. The different growth behaviour is analysed and a possible deposition mechanism is proposed based on the morphological evolution and photoelectron spectroscopy measurements. As an electrode for supercapacitors, this 2D arrayed nanosphere structure exhibits superior capacitive properties such as significantly large areal capacitance, tight binding with current collectors and retarded saturation of the capacitance, compared to a planar RuO2 film electrode.

Keywords: supercapacitors, ruthenium oxide, electrodeposition, polystyrene, deposition mechanism. * Tel:

+82-2-910-4734

Fax: +82-2-910-4415 E.mail: [email protected]

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1. Introduction Pseudocapacitor electrodes based on transition metal oxides (TMOs) have attracted growing attention because they deliver superior power and capacitance compared to the carbon-based electrode materials used in current electrochemical double layer capacitors (EDLCs) [1-3]. Among TMOs, ruthenium oxide (RuO2) has exceptional capacitive properties, including reported high specific capacitance (900 – 1400 F g-1), excellent proton transfer, and superior reversible redox transition, and is accordingly one of the most widely studied materials for the application [4,5]. Thin film RuO2 electrodes are generally prepared using an electrodeposition technique because of its ability to produce nanostructured electrodes with arbitrary geometries and low energy consumption [6]. However, when fabricated in this way, only a very thin surface layer of the film can participate in the charge storage process because of its dense morphology. In addition, the films are easily detached from the current collector substrates [7,8]. This detachment problem can partly be solved by using polymeric binders, but the addition of these binders leads to an inevitable deterioration in device properties [4,9-12]. In order to overcome the limitations of film-type electrodes, nanostructured RuO2 electrodes based on nanosheets [13], nanotubes [14], nanorods [15,16], nanopillars [17] and nanofibers [18] have been proposed. Although the introduction of these nanostructures has been reported to improve the electrode’s capacitive properties to some extent, the techniques are currently unsuitable for practical use because of their complicated and expensive nano-processes. Recently, we reported a relatively simpler and low-cost technique for fabricating two-dimensionally (2D)-arrayed manganese oxide (MnO2) nanostructures by electrodepositing MnO2 on a 2D-arrayed polystyrene (PS) nanosphere monolayer template and subsequently extracting the PS nanospheres [19]. The electrodeposited


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MnO2 infiltrated the interstitial spaces between the PS nanospheres from the bottom, and hence a MnO2 nanostructure with a 2D-arrayed hemispherical bowl-shaped surface could be obtained after the PS extraction. The increased surface area of the nanostructured MnO2 electrode led to an enhancement in specific capacitance up to 714 F g-1, which was approximately 28 % larger than the value of a planar film electrode. Similar growth was observed for cobalt oxide (Co3O4) films electrodeposited on 2D-arrayed PS spheres [20]. The hemispherical bowl-shaped Co3O4 electrode also exhibited noticeable enhancement of cycling performance and discharge capacity in lithium ion batteries. In this work, we adopted this nanostructuring technique to fabricate periodically arrayed RuO2 nanostructure electrodes. However, it was observed that the growth behaviour of the RuO2 layers being electrodeposited on the PS template was entirely different from that observed for the previously reported metal oxides. The RuO2 layer grew selectively on the PS surfaces, rather than on the surface of the current collectors. Consequently, PS/RuO2 core/shell nanospheres were obtained instead of the expected bowl-shaped nanostructures. The fabricated RuO2 electrodes consisting of these 2Darrayed nanospheres showed significant improvements in areal capacitance compared to a planar RuO2 film electrode. In addition, in the current work the fabricated electrodes considerably retarded both capacitance saturation and film detachment. A growth mechanism for the electrodeposited RuO2 layer is proposed based on its morphological evolution and photoelectron spectroscopy measurements.

2. Experimental 2.1. Fabrication of PS/RuO2 nanosphere electrodes


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A commercially available aqueous suspension of PS nanospheres with chlorinated functional groups on the surface (4 % w/v, 0.6 µm) was purchased from Interfacial Dynamics Co. The hexagonal close-packed PS monolayer was formed and transferred onto a thoroughly cleaned indium tin oxide (ITO)-coated glass substrate using a scooping transfer technique [21]. Prior to electrodeposition cycles, the PScovered substrate was placed in a 10 mM ruthenium chloride hydrate (RuCl3·xH2O, Sigma-Aldrich) aqueous solution for 30 min. The electrodeposition of RuO2 was then performed using a three-electrode system in which the substrate, platinum plate and Ag/AgCl (in saturated KCl aqueous solution) were used as a working electrode, counter electrode and reference electrode, respectively. The electrodeposition was carried out by cycling the potential between -0.2 and 1.2 V in the aqueous RuCl3·xH2O solution at a scan rate of 50 mV s-1. For comparison, planar RuO2 film electrodes were also fabricated using the same electrodeposition procedures on bare ITO-coated glass substrates without PS nanospheres. FE-SEM images of the surfaces of these planar RuO2 electrodes are shown in the Supporting Information (Figure S1). After deposition, the electrodes were washed with distilled water and annealed at 90 °C for 1 h to remove residues and enhance capacitive performance.

2.2. Characterization The nanostructure and surface morphology of the fabricated RuO2 electrodes were characterized by field emission scanning electron microscopy (FE-SEM, JSM7410F, JEOL Ltd.). The electron binding energies of the electrodeposited RuO2 were confirmed by x-ray photoelectron spectroscopy (XPS, Escalab 220i-XL). The electrochemical properties of the electrodes were evaluated in 0.5 M aqueous H2SO4


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solution at room temperature using a cyclic voltammeter (ZIVE SP2, WonATech) over the potential range from 0.3 to 0.8 V at various scan rates from 10 to 100 mV s-1.

3. Results and discussion The morphological evolution of the RuO2-electrodeposited PS arrays was examined by FE-SEM, and representative surface SEM images after various numbers of electrodeposition cycles are shown in Figure 1. Compared to the predominantly smooth PS surfaces before potentials were applied (See Supporting Information, Figure S2), the uneven surface morphology produced by one cycle of electrodeposition (Figure 1a) indicates that the nucleation of RuO2 progresses rapidly during the initial stage of the electrodeposition process. An even rougher surface morphology and some RuO2 particles bridging the PS nanospheres could be observed after three cycles of electrodeposition (Figure 1b). Continuous deposition led to a gradual increase in surface roughness due to the increasing number of RuO2 nanoparticles on the PS surface. In contrast, the deposition of RuO2 particles on the

Figure 1. Surface FE-SEM images of RuO2 nanoparticles electrodeposited on a monolayer of hexagonal close-packed PS nanospheres. The number of electrodeposition cycles is (a) 1, (b) 3, (c) 5, (d) 10, (e) 20 and (f) 50.


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surface of the ITO substrates could barely be observed during the entire electrodeposition process. The selective growth of RuO2 layers on the PS surface is more apparently displayed in the cross-sectional FE-SEM images shown in Figure 2. The gradual growth of the RuO2 thin films on the PS nanospheres and the absence of RuO2 on the ITO substrates can both be clearly observed. The elemental SEM mapping results for Ru atoms also supports the preferential growth of RuO2 films on the PS nanospheres (See Supporting Information, Figure S3a). Electrodeposition onto hexagonal close-packed PS nanospheres has previously been reported as a simple method for fabricating 2D-arrayed metal oxide nanostructures, such as MnO2 and Co3O4 nanostructures [19,20]. In both cases,

Figure 2. Cross-sectional FE-SEM images of RuO2 nanoparticles electrodeposited on a monolayer of hexagonal close-packed PS nanospheres. The number of electrodeposition cycles is (a) 1, (b) 10, (c) 20 and (d) 50. Insets in (b)-(d) represent the magnified images of the marked area in each figure.


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however, the metal oxide films grew on the substrate surface as well, leading to a 2Darrayed hemispherical bowl-shaped nanostructure, as schematically illustrated in the left part of Figure 3a. In contrast, during the electrodeposition of RuO2 in this work, 2D-arrayed PS/RuO2 core/shell nanospheres were obtained due to the selective growth of RuO2 films on the PS surfaces (right part of Figure 3a). This is probably due to differences in the precursor materials and the respective growth mechanisms of the metal oxide films. For example, for the MnO2 films, divalent manganese compounds such as MnSO4 and Mn(CH3COO)2 were generally used as the precursor in the electrodeposition process, and the overall reaction during the process was known to be [22-24]; Mn 2+ + 2H 2 O → MnO 2 + 4H + + 2e −

(1)

Although there is some debate about the reaction mechanism, it is generally accepted that the deposition proceeds through hydroxyl- or hydrated-intermediates such as

Figure 3. (a) Schematic illustration of the PS nanosphere-aided fabrication of (left) MnO2 [19] and Co3O4 [20] bowl-shaped nanostructure in previously studied, and (right) RuO2 nanospheres in this study. (b) Schematic illustration of the fabrication process of the PS/RuO2 nanospheres, indicating the formation of Ru–Cl bonds on the Cl–terminated PS surface prior to the electrodeposition of RuO2 nanoparticles.


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MnOOH, [Mn(H2O)6]4+ and Mn(OH)4 [24-26]. For the electrodeposition of RuO2, RuCl3 is the most widely used precursor, and the overall reaction is known to be [27]; Ru 3+ + 4OH - → RuO 2 ⋅ 2H 2 O + e −

(2)

In this case, the deposition mechanism has rarely been reported, and hence remains poorly understood. The preferential growth of the RuO2 layer on the PS surface in this work probably occurred because the RuO2 deposition is initiated by the bonding of Ru atoms to Cl atoms on the Cl-terminated PS surface. The RuCl3 dissolved in the aqueous solution can be converted into Ru(OH) δCl3-δ by reaction with water molecules [28]; RuCl 3 ⋅ xH 2 O + H 2 O → Ru(OH) δ Cl 3-δ ⋅ xH 2 O + δH + + δCl −

(3)

where δ denotes the amount of coordinated chlorides replaced by hydroxyl groups. The δ value was reported to be approximately 0.45 immediately after dissolving RuCl3·xH2O in water [28]. An Ru-Cl bond will form on the Cl-terminated PS surface through this reversible replacement process in the initial stages of the electrodeposition. In order to identify the chemical environments and oxidation states of the Ru atoms during electrodeposition, XPS measurements were carried out. The XPS spectra of Ru 3d and O1s core-level electrons are shown in Figure 4a and 4b, respectively. The bottom spectra were taken after immersing the PS-covered ITO substrate in the RuCl3 aqueous solution for 30 min prior to electrodeposition. Two peaks centred at 281.6 eV and 284.5 eV in Figure 4a correspond to the binding energies of Ru 3d5/2 and 3d3/2, respectively. These results are consistent with previously reported values of chloro or chlorohydroxo Ru(III) core electrons [29-31]. After one cycle of electrodeposition, the characteristic Ru(IV)O2 peak at 280.3 eV appeared, which is in good agreement with the rapid nucleation of RuO2 layers


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observed in the FE-SEM measurements (Figure 1a). The binding energy of this Ru(IV)O2 electron is 1.3 eV lower than that of the chlorinated Ru(III) electron in spite of the higher oxidation states. This is because an oxygen atom has significantly weaker electron affinity than the chlorine atom. The 1.0 – 2.2 eV lower binding energies of RuO2 compared to RuCl3 have been previously reported in the literature [31,32]. As the electrodeposition proceeded, the intensity of the RuO2 peak increased rapidly and became dominant after two cycles of electrodeposition. After 4 cycles, the shape of the spectra was almost unchanged, and was also identical to the spectrum of the planar RuO2 films. The position of the 3d5/2 peak gradually shifted to 280.7 eV at 20 cycles of electrodeposition, which is consistent with the value of a planar RuO2 thin film. The RuO2 peaks are broader and more asymmetric than the initial Ru(III)Clx peaks, indicating various chemical environments and multiple oxidation states. The peaks at 530.3 eV and 531.5 eV in an O 1s region (Figure 4b) can be assigned as a

Figure 4. XPS spectra of (a) Ru 3d and (b) O 1s level for the 20 cycle electrodeposited planar RuO2 film and PS/RuO2 nanospheres after various numbers of electrodeposition cycles.


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bridged oxygen (Ru–O–Ru) and hydroxyl oxygen (Ru–OH), respectively [33-35], indicating the transformation of Ru–OH into RuO2 at the initial stage of the electrodeposition process. Overall, it can be concluded that Cl–Ru–OH bonds initially form on the PS surface before applying the electric potential, and then the deposition of RuO2 layers is carried out during the subsequent electrodeposition process. In order to confirm this deposition mechanism, the electrodeposition of RuO2 was also carried out on 2D-arryayed carboxyl (COOH)-terminated PS nanospheres using the same deposition procedure. Figures 5a and 5b show surface and cross-sectional FE-SEM images taken after 20 cycles of RuO2 electrodeposition on the monolayer of hexagonal close-packed COOH-terminated PS nanospheres. The surface image shows that RuO2 particles were deposited irregularly on the PS surface. RuO2 growth on the ITO substrate can apparently be observed in the cross-sectional image shown in Figure 5b. The absence of preferred deposition on the PS surfaces supports our hypothetical mechanism, in which Ru atoms initially bond with Cl atoms on the Clterminated PS surface prior to RuO2 electrodeposition. The electrochemical performances of the PS/RuO2 nanosphere electrode and the RuO2 planar film electrode fabricated with various cycles of electrodeposition were

Figure 5. (a) Surface and (b) cross-sectional FE-SEM images of RuO2 nanoparticles electrodeposited on COOH–terminated PS nanospheres. The number of electrodeposition cycles is 20.


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estimated using cyclic voltammetry (CV) measurements within a potential range from 0.3 to 0.8 V, and the results are shown in Figure 6a and 6b, respectively. All CV contours were obtained at a scan rate of 10 mV s-1, and almost rectangular and symmetric shapes represent the reversibility of the redox transition of RuO2. Figure 6c shows plots of the areal capacitances of both types of electrodes as a function of the number of electrodeposition cycles. The areal capacitance, Careal, was calculated by the following equation, C areal =

∫ JdV

∆V (dV / dt )

(4)

where J (mA cm-2) is the current density in the CV curve, ∆V (V) is the potential window, and dV/dt (mV s-1) is the scan rate. The capacitance of the nanostructured

Figure 6. Cyclic voltammograms of (a) PS/RuO2 nanosphere and (b) planar RuO2 film electrodes. The scan rate was 10 mV s-1 for both samples. (c) Plots of the areal capacitance as a function of the number of RuO2 electrodeposition cycles. (d) Plots of areal capacitance retentions as a function of the scan rate.


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electrodes was larger than that of the planar electrodes at all deposition cycles. This is certainly due to the advantage of nanostructuring, which can provide considerably increased contact area between the electrode and electrolyte. One more interesting thing to note is the retarded saturation of the areal capacitance of the nanostructured electrodes. While the areal capacitance of the PS/RuO2 nanosphere electrode was almost proportional to the amount of deposited RuO2 at least up to the 50 cycles of electrodeposition, that of the planar RuO2 film electrode decreased after 30 electrodeposition cycles. This is probably because the RuO2 planar film electrode was easily detached from the substrate. The inset in Figure 6c shows a photograph of the RuO2 planar film electrode taken after 40 electrodeposition cycles where some parts of the film were peeled off the substrate surface. Figure 6d shows plots of areal capacitance retention for the two types of electrodes fabricated with 30 electrodeposition cycles, as a function of scan rates. For the planar RuO2 electrode, the areal capacitance of 33.7 mF cm-2 at a scan rate of 10 mV s-1 dropped to 26.4 mF cm-2 at a scan rate of 100 mV s-1, and hence the retention was 78.5 %. In the case of the PS/RuO2 nanostructured electrode, however, the retention was 59.5 %, from 65.9 mF cm-2 at 10 mV s-1 to 39.2 mF cm-2 at 100 mV s-1. This result is contrary to a recently reported enhancement in the voltammetric response of the nanostructured MnO2 electrode [19]. This is probably due to a difference in the electrical conductivity of the electrode materials. The electrical conductivity of MnO2 was reported to be 10-5 – 10-6 S cm-1 [36]. For this poorly conducting material, the increase in surface area and reduced thickness of the electrode produced by nanostructuring is crucial for the efficient access of ions and transfer of charge carriers to the current collectors. In contrast, since the RuO2 electrode has a significantly higher electrical conductivity of approximately 1 S cm-1 [37], nanostructuring would not produce as


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great an impact on rapid voltammetric response. Rather, the interstitial spaces between the PS nanospheres could act as a resistance to the diffusion of ions, lowering the voltammetric response at high scan rates. A symmetric two-electrode cell based on the PS/RuO2 nanosphere arrays was also assembled. For both electrodes, 20 cycle-electrodeposited RuO2 on the arrayed PS layer was used. The deposited amount of RuO2 for each electrode was approximately 27.4 µg cm-2, estimated by quartz crystal microbalance. The capacitive properties of the fabricated symmetric cell were examined by the galvanostatic charge-discharge (GCD) measurements at various current densities (Figure 7a). The areal and specific capacitance calculated from the discharge curve at the current density of 0.5 mA cm-2 were 14.1 mF cm-2 and 258 F g-1, respectively. The energy density of the symmetric two-electrode cell was also calculated to be 35.8 Wh kg-1 using the following formula, E=

5C sp (∆V )

36

2

(5)

where Csp (mA g-1) is the specific capacitance obtained from the galvanostatic discharge curve and ∆V (V) is the applied potential window. The cyclability of the PS/RuO2 symmetric cell was examined by applying continuous GCD cycles at a fixed

Figure 7. (a) Galvanostatic discharge curves measured at various current densities and (b) Csp retention as a function of the number of GDC cycles at a current density of 1 mA cm-2 of the symmetric two-electrode cell based on the PS/RuO2 nanosphere electrode.


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current density of 1 mA cm-2. Figure 7b represents the Csp retention as a function of the number of cycles. The large retention of 84.3 % after 500 charge-discharge cycles indicates a superior long-term stability of the supercapacitor based on the PS/RuO2 nanosphere electrodes.

4. Conclusion Nanostructured RuO2 electrodes for supercapacitors were successfully fabricated by electrodepositing RuO2 nanoparticles onto a monolayer of hexagonal close-packed PS nanospheres. In contrast to the previously reported hemispherical bowl-shaped nanostructures of MnO2 and Co3O4 electrodes fabricated using the same nanostructuring technique, an entirely different nanostructure, i.e. PS/RuO2 core/shell nanospheres, was obtained. This difference was caused by the preferential deposition of RuO2 on the PS surface, which was initiated by the formation of Ru–Cl bonds on the surface. A plausible deposition mechanism was proposed based on the morphological evolution and XPS studies, and is supported by the observation of nonselective deposition on chlorine-absent PS nanospheres. The areal capacitance of the nanostructured PS/RuO2 electrode was found to be significantly larger than that of the planar RuO2 film electrode, indicating the merits of the nanostructures, i.e. a larger contact area and shorter diffusion path for ions. The electrodeposition process also considerably retarded saturation of the areal capacitance as well as detachment from the current collectors with the increasing number of electrodeposition cycles. The results presented here demonstrate the potential of the 2D arrayed PS/RuO2 nanospheres for application as highly efficient pseudocapacitor electrodes.

Acknowledgement


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This work was supported by National Research Foundation of Korea (NRF) Grant (No. 2016R1A5A1012966 and 2017R1A2B4012375) funded by the Korean Government.


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RuO2 thin films electrodeposited on polystyrene nanosphere arrays

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