Self-Assembled Bi2MoO6 Nanopetal Array on Carbon Spheres toward

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Self-assembled Bi2MoO6 nano-petal array on carbon spheres towards enhanced supercapacitor performance Kunda Samdani, Jeong Hwa Park, Dong Woo Joh, and Kang Taek Lee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03988 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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ACS Sustainable Chemistry & Engineering

Self-assembled Bi2MoO6 nano-petal array on carbon spheres towards enhanced supercapacitor performance

Kunda J. Samdani, Jeong Hwa Park, Dong Woo Joh, and Kang Taek Lee*

Department of Energy Science and Engineering, DGIST, Daegu 42988, Republic of Korea

*Corresponding Author: Prof. Kang Taek Lee Email : [email protected], Address: DGIST, 333, Techno jungang-daero, Hyeonpung-myeon, Dalseong-gun, Daegu 42988, Republic of Korea, Tel : +82-53-785-6430, Fax : +82-53-785-6409 ORCID : (Kang Taek Lee) 0000-0002-3067-4589

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Abstract The rational design and exploration of the metal oxide-carbon composite are greatly desired for enhanced supercapacitor application. Herein, we develop a novel Bi2MoO6 and carbon sphere hybrid material as a supercapacitor electrode via a simple solvothermal process. The microstructural analysis of the carbon sphere@Bi2MoO6 reveals that the 10 nm thick Bi2MoO6 nano-petals are consistently anchored on the carbon spheres surface, forming a 3-dimensional nano-architecture. The carbon sphere@Bi2MoO6 electrode displays an excellent specific capacitance of 521.42 F g-1 at 1 A g-1, which is one of the best values of any reported Bi2MoO6based electrodes to date. Moreover, this hybrid electrode can accumulate total charge as high as 2083 C g-1, which is consistent with high capacitance. The all-solid-state symmetric supercapacitor device exhibited the specific capacitance of 26.69 F g-1, along with ~80 % of capacitance retention after 10000 cycles. The superior supercapacitor performance of the carbon sphere@Bi2MoO6 electrode is primarily due to the hierarchical nano-architecture of Bi2MoO6, its promotion of redox reactions, and the presence of highly conductive carbon spheres at cores, which provides pathways for rapid electron transfer. These results highlight feasibility of the carbon sphere@Bi2MoO6 hybrid material as a highly propitious electrode for supercapacitor applications.

Key words: Carbon sphere; Bi2MoO6; Self-assembly; Supercapacitor; Nanocomposite


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Introduction In the last few decades, the supercapacitor is emerging as one of the sustainable and favorable energy storage devices for many applications, such as electric vehicles, mobile devices, cameras, and energy backup systems. In comparison with the battery, the supercapacitor can provide high gravimetric power density (up to 10 kW/kg), and long cycle life (300,000 cycles) along with additional advantage of low cost and environmental friendliness.1 Despite its high power density, the supercapacitor still requires higher energy density and stability levels for practical power source applications. To boost the energy density of the supercapacitor, researchers have developed various materials of binary metal oxides with multiple oxidation states including Bi2WO6,2 CoMoO4,3 NiCo2O4,4 and CoFe2O4.5 Recently, bismuth and molybdenum-based metal oxides have attracted attention as supercapacitor electrode materials. The Bi2MoO6 is an Aurivillius-type oxide (Bi2An-1BnO3n+3, where n=1) that has both perovskite-like and fluorite-like blocks,6 and has been widely studied due to its outstanding intrinsic properties e.g. dielectric nature, catalytic activity, and luminescence.7 Recently, Yu et al.8 prepared a Bi2MoO6 microsphere via a simple hydrothermal method and obtained a supercapacitor performance of 182 F g-1 at a current density of 1 A g-1. Additionally, Yesuraj and Suthanthiraraj9 reported the octahedron-like bismuth molybdate possessing a capacitance value of 258 F g-1 at 2 A g-1, which was synthesized by the DNA-mediated sonochemical method. However, as yet, the performance of these supercapacitors are far below that required for practical applications. Metal oxide-based supercapacitors also suffer performance degradation due to their poor electronic conductivity, structural instability, and dissolution into the electrolyte. To overcome these issues, the use of a variety of combinations of metal oxides and electron-conducting materials have been investigated, including heteroatom doping into the metal oxide and synthesis of metal



oxide-carbon composite. In addition, carbonaceous materials can further enhance supercapacitive performances by minimizing the diffusion length of ions (macro-pores), facilitating ion transport or charge storage (meso-pores), and increasing charge accommodation (micro-pores). Carbonaceous materials including carbon nanotubes (CNTs), graphene, carbon nanofibers, carbon nanosheets, and carbon spheres (CSs) are believed to be promising candidates for enhancing supercapacitor performance.10-13 Ma et al.14 prepared Bi2MoO6 nanosheet arrays on a carbon cloth, but observed inferior supercapacitor performance (27 F g-1 at 1 A g-1) compared to that on Ni foam (38 F g-1 at 1 A g-1). Thus, the supercapacitor performance and conductivity of Bi2MoO6-carbon composites remains obscure. To magnify the performance of the Bi2MoO6-based electrode, we propose a hybrid CS@Bi2MoO6 material with a hierarchical nanostructure. We expected the utilization of CSs helps to promote the electrochemical performance of Bi2MoO6 owing to its excellent electronic transport properties. In addition, CSs possess an extra feature, i.e., an inner cavity, which can serve as an ‘ion reservoir’ for electrolyte ions. Thus, it was expected that this cavity can holds the electrolyte ions inside and release them during the charging/discharging cycles, resulting in facilitating effectiveness of electroactive catalysts.15 Herein, we report the synthesis of a hierarchical architecture of CS@Bi2MoO6 via a simple solvothermal method. The physicochemical and morphological properties of the developed materials were investigated. Furthermore, the supercapacitor performance of the CS@Bi2MoO6 composite was studied in a three-electrode system. We observed that the CS@Bi2MoO6 composite possessed superior supercapacitor performance (521.42 F g-1 at a current density of 1 A g-1) compared to that of the pure Bi2MoO6 (322.85 F g-1 at a current density of 1 A g-1). Thus, our study


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demonstrates the high feasibility of using the novel nanostructured CS@Bi2MoO6 as an effective supercapacitor electrode.

Experimental section Materials synthesis To synthesize CSs, an appropriate amount of glucose (9.0 g) was added into 50 ml of distilled water under magnetic stirring, and this solution was transferred into a stainless steel autoclave. The autoclave was then transferred to an oven, which was maintained at 180 °C for 24 h, and then allowed to cool to room temperature. We then collected and washed the dark brown precipitates with deionized (DI) water and ethanol to remove any unreacted glucose, and dried the obtained sample at 90 °C overnight. Next, CS@Bi2MoO6 composite was synthesized using a simple solvothermal method. The bismuth nitrate, sodium molybdate, potassium hydroxide, poly (vinylidene fluoride) (PVDF), Nmethylpyrrolidone (NMP), and carbon black were purchased from Alfa Aesar. Stoichiometric amounts of Bi(NO3)3·5H2O and Na2MoO4·2H2O were added in 50 mL of ethanol under continuous magnetic stirring to get the transparent solution. At the same time, appropriate amount of synthesized CSs were dispersed in 10 mL of ethylene glycol. Then the two solutions were mixed together by stirring for 30 min, and then were transferred to a stainless steel autoclave. The autoclave was later moved to an oven and kept for 24 h, at 180 ºC, and then cooled to the room temperature. The obtained sample was filtered, and dried at 90 ºC in air. Using the same method, Bi2MoO6 powders without the addition of CSs were also synthesized.

Characterization



The surface morphologies, particle size, and selective surface area (SAED) of the assynthesized materials were analyzed using scanning electron microscopy (SEM, S4800, Hitachi Ltd.), and transmission electron microscopy (TEM, HF-3300, Hitachi Ltd.). The crystallographic phase of the synthesized material was analyzed using X-ray diffraction (XRD, Mini Flex 600, Rigaku). Brunauer-Emmett-Teller (BET) surface area analysis was carried out using a surfacearea analyzer (ASAP 2020) via N2 adsorption–desorption. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific) analysis was performed to characterize the chemical composition of the powders. The unit cell structure of the Bi2MoO6 was drawn using a software VESTA based on standard XRD data (JCPDS card no. 021-0102).

Electrochemical measurement The electrochemical performances of the samples were measured in a standard three-electrode setup, in which Pt and Ag/AgCl (KCl-saturated) electrodes were the counter and reference electrodes, respectively. The Ni-foam substrate was cleaned with dilute HCl and DI water, prior to use as a current collector. Moreover, the electrode ink was prepared by mixing the assynthesized materials, carbon black, and poly(vinylidene fluoride) (PVDF) at a mass ratio of 8 : 1 : 1 dispersed in N-methylpyrrolidone (NMP). The cleaned Ni-foam (1 cm2) was pasted using the prepared ink and dried in an oven at 80 °C overnight. Further, electrochemical measurements, including cyclic voltammetry (CV), galvanostatic charging and discharging (GCD), and electrochemical impedance spectroscopy (EIS) measurements, were performed by a potentiostat (VMP-300, Bio-Logic) in the 6 M KOH aqueous electrolyte at ambient temperature. For twoelectrode system, an all-solid-state symmetric supercapacitor device was prepared. The thin film of PVA-KOH was used as a separator as well as an electrolyte in the symmetric cell. The film was 6 ACS Paragon Plus Environment

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prepared by mixing KOH (0.75 g) and PVA (1.5 g, MW of 85,000 - 124,000) in DI water, further heated at 90 °C for 1 h. The symmetric supercapacitor device was fabricated with two CS@Bi2MoO6 electrodes, and a PVA-KOH thin film there between.

Results and discussion Morphology and microstructure characterization Fig. 1 shows a schematic illustration of the synthetic strategy of both the Bi2MoO6 and the hierarchical CS@Bi2MoO6 composite. It is well known that crystalline orthorhombic Bi2MoO6 is created by alternating (Bi2O2)n2n+ and perovskite layers, such as (MoO4)n2n-.16 In this synthesis process, ethylene glycol (EG) can act as a coordination agent. The reaction system is weakly acidic, so the smaller crystallites gradually are dissolved. Then, Bi2O22+ and MoO42- ions are generated under solvothermal conditions.17 Furthermore, Bi2O22+ ions subsequently attach to the anionic species (MoO42-) to gradually convert into nucleation sites. These nucleation sites then tend to attach each other forming a plate like structure of Bi2MoO6. During the reaction, the nano-plates can be fully grown and randomly arranged, as shown in Fig. 1 (upper line). For the synthesis of composite, we used CSs as templates to grow Bi2MoO6 nano-petals. The formation of CS@Bi2MoO6 can be explained by the heterogeneous nucleation and subsequent orientation of the crystal growth. It is well known that the surface of CS is rich in oxygenic functional groups (i.e. C=O, COOH, and OH), which lead to form metal-CSs precursor by electrostatic adsorption. 18

During the composite synthesis probably the Bi2O22+ ions become attached to the oxygenic

functional group through electrostatic attraction, and subsequently the anionic species (MoO42-) to (CS-O-Bi2O22+), to form nano-sized nucleation sites. As the reaction progresses, these nanocrystal



seeds are expected to convert into Bi2MoO6 nano-petals and to uniformly self-assemble on the surface of the CSs (Fig.1, lower line). Fig. 2a-b show typical SEM images of the as-synthesized CS@Bi2MoO6 with a hierarchical architecture of CSs adorned with nanoscale Bi2MoO6-petals. These nano-petals grow vertically on the CS surfaces, and become interconnected, which is similar to the schematic diagram of the expected nanocomposite structure shown in Fig. 1. We also observed that the Bi2MoO6 nano-petals on CSs were very thin with an average thickness of ~ 10 nm. This type of interconnected 3D nano-architecture can facilitate effective electron transport at the electrode/electrolyte interface. 19 A single CS veiled with Bi2MoO6 nano-petals can be seen in inset of Fig. 2a. In addition, Fig. S1 shows an SEM image of bare CSs, which have smooth surfaces and an average diameter of ∼ 400 nm. For comparison purposes, the Bi2MoO6 without CSs was also synthesized. Fig. 2c shows that the Bi2MoO6 has a plate-like morphology due to its random growth during the solvothermal reaction. Closer observation of the Bi2MoO6 (Fig. 2d) confirms that the diameter of the unevenly distributed nano-plate is about 100 - 300 nm with a thickness of 20–30 nm. Fig. 3 depicts the TEM analysis of the CS@Bi2MoO6 nano-petal composite. Fig. 3a clearly shows the presence of CSs at the cores, surrounded by Bi2MoO6 nano-petals, which is good in agreement with the SEM observations as shown in Fig. 2a-b. The magnified TEM image confirms the formation of very thin nano-petals of Bi2MoO6 (Fig. 3b), which can provide a sufficient number of electrochemically active sites for a surface redox reaction, thus a high pseudo-capacitance.20 Fig. 3c displays an HR-TEM image of CS@Bi2MoO6 with two lattice spacings of 0.25 and 0.37 nm corresponding to the (022) and (111) crystal planes of orthorhombic Bi2MoO6 (JCPDS No.021010), respectively. The polycrystalline nature of CS@Bi2MoO6 was confirmed by the SAED pattern with presence of some bright spots and rings (Fig. 3d). In comparison, TEM images of


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Bi2MoO6 synthesized without CSs (Fig. S2) show a randomly distributed nano-plate morphology with a lattice spacing of 0.31 nm, which is consistent with the (131) planes of orthorhombic Bi2MoO6 (Fig. S2c). The bright spots in the SAED patterns confirm the polycrystalline nature of the Bi2MoO6 nano-plates (Fig. S2d). Fig. 4a illustrates the crystal structure of Bi2MoO6. In this structure, four corners of each MoO6 octahedra help to form the MoO42− slabs in contact with the edges of the adjacent MoO6 octahedra in the same plane. On the other hand, the Bi3+ ions possess a pyramidal coordination in the Bi2O22+ layer.21 Fig. 4b shows the XRD patterns of CS@Bi2MoO6 and Bi2MoO6, both of which have the distinctive peaks at 28.16°, 32.40°, 35.93°, 46.51°, 55.42° and 58.46° that can be indexed to the (131), (200), (151), (202), (331), and (262) planes of the orthorhombic Bi2MoO6 (JCPDS no. 021-0102), respectively. The additional peak of the CS@Bi2MoO6 at 23.51° corresponds to the (002) plane of amorphous carbon, which confirms the presence of carbon in the composite. The individual XRD spectrum for the CSs is also shown in Fig. S3. The specific surface area and pore size distribution of the Bi2MoO6 nano-plates and CS@Bi2MoO6 nano-petal composite materials were evaluated by BET measurements. As depicted in Fig. 4c-d, the Bi2MoO6 nano-plates show a type-IV isotherm, whereas the CS@Bi2MoO6 nano-petal composites show a type-V isotherm with a type-H3 hysteresis loop. This difference in the type of isotherms may be due to the sheet-like morphology of Bi2MoO6 on the CS surface, which gives rise to a slit-shaped mesoporous structure, as shown in Fig. 3. The specific surface area, pore volume, and pore diameter of the Bi2MoO6 nano-plates and CS@Bi2MoO6 nano-petal composites are summarized in Table S1. We observed that the CS@Bi2MoO6 nano-petal composites possessed > 7 times larger surface area (37.1 m2 g1

) compared to that of the Bi2MoO6 nano-plates (5.01 m2 g-1). In this case, the Bi2MoO6 nano-

plates had a wider average pore diameter than that of CS@Bi2MoO6 nano-petals. On the other



hand, the CSs had a surface area of 18.80 m2 g-1 (Fig. S4). This result clearly shows that the characteristic structure of the self-assembled Bi2MoO6 nano-petals on the CS surface effectively increased the active surface area of the electrode. The Bi2MoO6 nano-plate and the CS@Bi2MoO6 nano-petal composite were further analyzed by XPS to determine their surface chemistries. The wide full scan XPS spectrum of the CS@Bi2MoO6 reveals the existence of Bi (18.16 %), Mo (5.78 %), O (46.13 %), and C (29.93 %) elements (Fig.5a). The characteristic binding energy values of 157.66 eV for Bi 4f7/2 and 162.96 eV for Bi 4f5/2 indicate a trivalent oxidation state for bismuth (Bi3+) (Fig. 5b).22 Fig. 5c shows the binding energies of Mo 3d3/2 and 3d5/2 located at 230.83 and 233.90 eV respectively, which suggests that Mo6+ exists in the samples.23 The peaks for Bi 4f and Mo 3d in the XPS spectra (Fig. 5b-c) of the CS@Bi2MoO6 nano-petal composite were slightly shifted toward higher binding energies than those of the Bi2MoO6 nano-plate, due to the strong interactions between carbon and Bi2MoO6 as similarly reported by Tian et al.24 In Fig. 5d, the peaks at 529.05, and 530.20 eV are related to Bi-O (lattice O), and Mo-O, repectively, whereas, the additional peak at 532.04 eV in the CS@Bi2MoO6 corresponds to the single oxygen bond with carbon (C-O).25 Fig. 5e shows the C 1s spectra of the CS@Bi2MoO6 nano-petal composite. Since the CS surfaces are rich in oxygen-containing functional groups, the three subpeaks at 284.16, 285.35, and 287.19 eV were observed after deconvolution, which evidently show the presence of C=C, C–O, and O=C–O bonds, respectively.26 Electrochemical study The electrochemical performances of the hierarchical CS@Bi2MoO6 nano-petal and Bi2MoO6 nano-plate electrodes were measured in a standard three-electrode system, from −0.2 to 0.6 V. The aqueous KOH (6 M) was used as the electrolyte. Fig. 6a depicts the CV curves of both electrodes at a scan rate of 50 mV s-1. These CV curves show well-defined redox peaks due to the


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conversion of Bi metal to Bi (III) being mediated by OH- ions from the electrolyte, which indicates the pseudo-capacitance behavior.27 We also observed that the CS@Bi2MoO6 exhibited enhanced capacitive current density compared to that of the Bi2MoO6 electrode, which is mainly attributed to its petal-like morphology with high specific surface area, as well as the presence of highly conductive CSs. It was observed that the CV curves of the Bi2MoO6 and CS@Bi2MoO6 electrodes retained their shapes even at a high scan rate, indicating the fast diffusion of the electrolyte into the electrode (Fig. S5a and S5c). On the other hand, an additional oxidation peak at a slow scan rate was observed for the CS@Bi2MoO6 composite electrode, which is likely attributable to few adsorbed BiO2− ionic species at the electrode/electrolyte interface.28 Nevertheless, we believe that the major oxidation current originated from the dissolution of BiO2− into the electrolyte. Fig. 6b shows the GCD curves for both the Bi2MoO6 nano-plate and CS@Bi2MoO6 nano-petal electrodes at a current density of 1 A g-1. An additional shoulder peak occurring in the GCD curve of the CS@Bi2MoO6 electrode is consistent with the CV analysis at 5 mV s-1 (Fig. S5c). The specific capacitance (Cs) of CS@Bi2MoO6 electrode in a three-electrode system was determined by GCD curves with utilization of following equation:29 & × ∆)

C# = ∆* ×+

………… (1)

where I is discharge current, Dt is the discharge time, m is mass of the active material, and DV is the voltage decrease during discharge. The maximum specific capacitance values at 1 A g-1 of the CS@Bi2MoO6 electrode (521.42 F g-1) was enhanced by ~ 62% compared to that of the single Bi2MoO6 electrode (322.85 F g-1). Moreover, the nanocomposite CS@Bi2MoO6 exhibited superior supercapacitor performance to that of previously reported any Bi2MoO6-based electrode materials as shown in Fig. 6c. In addition, Fig.S5b and S5d show the GCD curves for both electrodes at various current densities. 11 ACS Paragon Plus Environment


Furthermore, the charge stored on the surface of both electrodes was determined using a Trasatti plot.33 Based on the intercept of the linear plot of 1/q vs. υ1/2 (Fig. 6d), the total amount of charge stored (qtotal) was estimated. In addition, the intercept of the linear plot of q vs. υ-1/2 was used to evaluate the outer surface charge (qouter) of the electrode material (Fig. 6e). The resulting qtotal values in the single Bi2MoO6 and hybrid CS@Bi2MoO6 electrodes were 1169 and 2083 C g1

, respectively. The high qtotal value for the CS@Bi2MoO6 electrode is consistent with its high

supercapacitor performance (521.42 F g-1). Fig. 6f further shows that inner charges stored (qtotal qouter) in the Bi2MoO6 and CS@Bi2MoO6 electrodes were as high as 92 and 88 %, respectively. This result suggests that the pseudo-capacitive mechanism is dominant for storing charge in the both electrode materials. Fig. 7 schematically illustrates a possible pseudo-capacitor mechanism of the hybrid CS@Bi2MoO6 nanocomposite. The faradic redox reaction of bismuth molybdates in an alkaline electrolyte is similar to that of bismuth oxide.34 In an aqueous KOH electrolyte, OH¯ ions are expected to react with the Bi2MoO6 and undergo redox reactions as follows;28

Bi. MoO2 + 2OH 6 ↔ 2BiO6 . + H. MoO8

....................... (2)

.6 6 Bi O6 . + e ↔ BiO.

...................... (3)

6 6 2H. O + 3BiO.6 . ↔ 2BiO. + 4OH + Bi

...................... (4)

On the other hand, Chi et al. reported that Mo atoms in Bi2MoO6 showed no evidence of involvement in the electron transfer process, and would contribute only negligibly to the capacitance.3 In the case of the CS@Bi2MoO6 nano-petal composite, the CS surface, on which the self-assembled Bi2MoO6 nano-petals were firmly anchored, would provide paths for rapid electron


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transfer. Moreover, due to the hierarchical nano-architecture, the exceptionally high surface area of the CS@Bi2MoO6 nano-petals probably made active materials fully available during the redox reactions. Additionally, the open spaces between two adjacent nano-petals could act as reservoirs for electrolyte ions, and help to avoid swelling and shrinking of the active materials during the charging and discharging cycles. Hence, the aforementioned physicochemical characteristics of the hybrid CS@Bi2MoO6 nanocomposite electrode collectively resulted in the superior supercapacitor performance. The electrochemical performance of both electrodes was further investigated using EIS analysis. Fig. 8a shows the resultant Nyquist plots, in which the semi-circle loop in the highfrequency region reflects the charge transfer resistance, and the straight line in the low-frequency region corresponds to the Warburg constant. The equivalent series resistance (ESR) of the CS@Bi2MoO6 electrode at the electrode/Ni foam interface (0.58 Ω) was ~ 60% lower than that of the Bi2MoO6 sample (0.89 Ω). The smaller ESR value of the hybrid CS@Bi2MoO6 electrode indicates the low internal resistance, and good ion response in the low frequency region reveals the reversible adsorption and desorption of ions due to the sheet like structure.35 The charge transfer resistance (Rct) for CS@Bi2MoO6 (0.15 Ω) was less than that of the Bi2MoO6 nano-plate electrode (0.23 Ω), which might facilitate fast redox reactions. In addition, a long-term stability test was performed on both samples for up to 10000 cycles at a current density of 10 A g-1. After 10000 cycles, the CS@Bi2MoO6 and Bi2MoO6 electrodes showed capacitance retentions of 70 and 42 %, respectively (Fig. 8b). Fig. S6 shows the microstructures of both electrodes following the long-term cycling test, indicating no observable changes from the original morphologies. These results demonstrate the excellent physicochemical stability of the CS@Bi2MoO6 nanocomposite under redox cycle conditions. To know the contribution of CS in supercapacitor performance of



composite, we have analyzed the CS for the same in three-electrode system. Fig. S7a shows the quasi-rectangular shape CV curves obtained for CS at various scan rates (5 to 200 mV s-1), confirming the presence of various function groups on the surface of CS. Furthermore, the GCD measurements carried out at different current densities (Fig. S7b) shows the highest capacitance value of 62.55 F g-1 at a current density of 1 A g-1 (Fig. S7c). In the Nyquist plot, slightly high value of the EIS spectra crossing with the Z′ axis suggested moderate contact resistance (Fig. S7d). Whereas, in the low frequency region the presence of vertical line represented a good conductivity.35 In addition, CS electrode shows 70 % retention in specific capacitance after 10000 cycles at a current density of 5 A g-1 (Fig. S7e). Overall, Although, the supercapacitor performance of the CS itself is slightly low, when the CS combined with Bi2MoO6, it boosted the specific capacitance as well as the stability of the composite compared to that of the Bi2MoO6 nano-plates. Fig. 9a shows a schematic diagram of the assembled CS@Bi2MoO6//CS@Bi2MoO6 symmetric device with PVA-KOH thin film. The potential window was optimized for the symmetric device by carrying out CV measurements in the potential range from 0.5 to 0.9 V (Fig. 9b). The CV curves at the various scan rates as well as at various potentials show the perfect EDLC behavior and retention in shape at the high scan rate (Fig.9c), shows the good rate capability of the symmetric device. The electrochemical property of the symmetric device with the CS@Bi2MoO6 electrode was further evaluated by the GCD measurement at various current densities as shown in Fig. 9d. The GCD curves with the linear profile revealed the good capacitive characteristics of the device. The specific capacitance values of the device (Cdevice) were assessed from the charging/discharging curves by using the Eq(5) as follows:29 & × ∆)

C?@= = 2 X ∆* ×+

………………… (5)


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where I is the discharge current, Dt is the discharge time, m is the mass of the active materials in one working electrode, and DV is the potential voltage. The symmetric device delivered high specific capacitance of 26.69 F g-1 at a current density of 0.25 A g-1 (Fig. 9e). In addition, Fig. S8 shows a Nyquist plot of the all-solid-state symmetric device using the CS@Bi2MoO6 electrode. The intercept of the EIS spectra on real axis was at 5.3 Ω, indicating very small internal resistance. The small semi-circle in the high frequency region shows the fast charge transport between electrode and electrolyte. Furthermore, the energy density and power density were calculated for the symmetric device using following equations: E =

CDEFGHE × I*J . L

P = ∆)

…………………. (6) …………………… (7)

where V is the operating voltage, Cdevice is the total capacitance of the device, and Dt is the time for a full discharge. The energy density was highest as 10.8 Wh kg-1 at a power density of 410 W kg1

. On the other hand, the maximum power density of 2280 W kg-1 was achieved at the energy

density of approximately 2 Wh kg-1. As compared in a Ragone plot in Fig. S9, the obtained energy density values of the CS@Bi2MoO6 electrode based supercapacitor are superior to the some of the previously reported symmetric and asymmetric devices e.g. RGO//RGO (3.10 Wh kg-1),36 RCDGO/Mn3O4// RCDGO/Mn3O4 (8.70 Wh kg-1),37 CNP/MnO2//CNP/MnO2 (4.19 Wh kg-1),38 CNT//CNT (4.48 Wh kg-1),39 and NiO//C (4.00 Wh kg-1),40 and are comparable with that of the GO/NDC//GO/NDC (11.84 Wh kg-1),41 and CuCo2O4@CuCo2O4//AC (17.94 Wh kg-1).42 Moreover, the symmetric device showed a high cycling stability of about 78.90 % capacity retention after 10000 cycles even at a high current density of 1 A g-1 (Fig. 9f). This result indicates high potential of our nanostructured CS@Bi2MoO6 materials in practical supercapacitor applications. 15 ACS Paragon Plus Environment


Conclusion In this study, we developed a novel hybrid CS@Bi2MoO6 nanocomposite material via a facile solvothermal method. The microstructural analysis results revealed that the synthesized CS@Bi2MoO6 powder had a hierarchical nanostructure consisting of self-assembled Bi2MoO6 nano-petals (~10 nm thick) surrounding a CS at the core. In comparison, Bi2MoO6 synthesized without CSs formed a randomly arranged plate-like structure. The surface area of the CS@Bi2MoO6 nano-petal composite (37.1 m2 g-1) was > 7 times larger than that of the Bi2MoO6 nano-plates (5.01 m2 g-1). Accordingly, a high specific capacitance of the hybrid CS@Bi2MoO6 electrode (521.42 F g-1) was achieved at 1 A g-1, which was ~60 % higher than that of the pure Bi2MoO6 electrode. This remarkable improvement in supercapacitor performance is due to the characteristic and hierarchical structure of Bi2MoO6, which leads to rapid redox reactions, and the presence of the highly conductive CS cores that provide pathways for fast electron transfer. Moreover, the CS@Bi2MoO6 composite electrode exhibited a capacitance retention of ~ 70 % even after 10000 cycles, indicating its good stability. The fabricated all-solid-state symmetric device using the CS@Bi2MoO6 electrode exhibited the high specific capacitance value of 26.69 F g-1 and the maximum energy density of 10.80 Wh kg-1, along with ~ 80 % retention in specific capacitance after 10000 cycles. Hence, this study demonstrates that the use of the developed hybrid CS@Bi2MoO6 electrode is highly feasible for supercapacitor applications.

Acknowledgement This work was supported by the Global Frontier R&D on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning,


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Korea (2014M3A6A7074784). This work was also supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (20174030201590).

Appendix A. Supporting information Supporting information is available free of charge on the ACS publication website. BET analysis data for all as-synthesized material (Table S1), SEM images of CS (Fig. S1), TEM images of Bi2MoO6 nano-plate (Fig. S2), XRD of CS (Fig. S3), Nitrogen adsorption−desorption isotherms of CS (Fig. S4), additional electrochemical data for Bi2MoO6 and CS@Bi2MoO6 composite (Fig. S5), SEM images after cycling stability test (Fig. S6), electrochemical data of the CS (Fig. S7), EIS spectra for all-solid-state symmetric device (Fig. S8), and Ragone plot (Fig. S9).



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Figure 1. Schematic illustration of the synthesis of Bi2MoO6 and the hierarchical CS@Bi2MoO6 composite.



Figure 2. SEM images of CS@Bi2MoO6 nano-petals composite in (a) low and (b) high magnifications, and Bi2MoO6 nano-plates in (c) low and (d) high magnifications. Inset of (a) is a single CS covered with Bi2MoO6 nano-petals.


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Figure 3. TEM images in (a) low and (b) high magnifications, (c) HR-TEM images, and (d) SAED pattern of the hybrid CS@Bi2MoO6 material.



Figure 4. (a) Crystal structure of Bi2MoO6 and (b) XRD patterns of the single Bi2MoO6 and hybrid CS@Bi2MoO6 materials. Nitrogen adsorption−desorption isotherms, and corresponding pore-size distribution plots (at the inset) for (c) Bi2MoO6, and (d) CS@Bi2MoO6.


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Figure 5. (a) XPS full scan survey and elemental XPS spectra of (b) Bi 4f, (c) Mo 3d, (d) O 1s, of CS@Bi2MoO6 and Bi2MoO6 materials, and (e) C 1s of CS@Bi2MoO6.



Figure 6. Electrochemical performances of Bi2MoO6 and CS@Bi2MoO6 electrodes. (a) CV curves at 50 mV s-1, (b) GCD curves at 1 A g-1 current density, and (c) comparison of specific capacitance with literature data, I8, II9, III30, IV14, V31, VI32. Trasatti plots of (d) 1/q vs. υ1/2 and (e) q vs. υ-1/2, and (f) charges stored in inner and outer surfaces of Bi2MoO6 and CS@Bi2MoO6 electrodes.


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Figure 7. Schematic diagram of the electrochemical mechanism for pseudocapacitance behavior of the hybrid CS@Bi2MoO6 electrode.



Figure 8. (a) EIS spectra, and b) cycling stability up to 10000 cycles of Bi2MoO6 and CS@Bi2MoO6 electrodes.


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Figure 9. a) Schematic illustrations of the assembled CS@Bi2MoO6//CS@Bi2MoO6 symmetric device, b) CV curves of different voltage windows, c) CV curves at various scan rates from 5 to 200 mVs-1, d) GCD curves at different current densities of the symmetric device, e) specific capacitance as a function of current densities, and f) cycling stability of the symmetric device.



Table Of Content (TOC)

Ultrathin Bi2MoO6 nano-petals anchored on carbon sphere by self-assembly during solvothermal synthesis shows an enhanced supercapacitor performance.


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Self-Assembled Bi2MoO6 Nanopetal Array on Carbon Spheres toward

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