TiO2 Hollow Spheres

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Design and Synthesis of Hierarchical SiO2@C/TiO2 Hollow Spheres for High Performance Supercapacitors Ying Zhang, Yan Zhao, Shunsheng Cao, Zhengliang Yin, Li Cheng, and Limin Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08776 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017

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ACS Applied Materials & Interfaces

Design and Synthesis of Hierarchical SiO2@C/TiO2 Hollow Spheres for High Performance Supercapacitors

Ying Zhang,1 Yan Zhao,1 Shunsheng Cao,*1, Zhengliang Yin,1 Li Cheng,1 and Limin Wu*2,3

1

School of Materials Science and Engineering, Institute for Energy Research, Jiangsu

University, Zhenjiang 212013, China. 2Department of Materials Science, Fudan University, Shanghai200433, China. 3Collaborative Innovation Center of Novel Organic Chemical Materials of Hubei Province, College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, China

*

Corresponding authors: [email protected] (S. Cao), [email protected] (L. Wu)

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ABSTRACT: TiO2 has been widely investigated as electrodes materials due to its long cycle life and good durability, but the relatively low theoretical capacity restricts its practical application. Herein, we design and synthesize a novel hierarchical SiO2@C/TiO2 (HSCT) hollow spheres via a template-directed method. This unique HSCT hollow spheres combine both advantages from both TiO2 such as cycle stability and SiO2 with high accessible area and ionic transport. Especially, the existence of C layer is able to enhance the electrical conductivity. The SiO2 layer with porous structure can increase the ions diffuction channels and accelerate the ions transfer from the outer to the inner layers. The electrochemical measurements demonstrate that the HSCT hollow sphere-based electrode manifests a high specific capacitance of 1018 F/g at 1 A/g which is higher than hollow TiO2 (113 F/g) and SiO2/TiO2 (252 F/g) electrodes, and substantially higher than those of all the previously reported TiO2-based electrodes. Keywords: Hierarchical structure; Hollow spheres; SiO2@C/TiO2; High capacity; Supercapacitors


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INTRODUCTION Supercapacitors have been widely considered as one of the most potential electrode

materials because they hold a higher power density in comparison with batteries as well as higher energy density than traditional capacitors.1-4 Since the electrochemical performance of supercapacitors greatly depend on their electrodes, numerous effort has been devoted recently to construct new electrode materials with tailorable structures and surface properties.5-8 Owing to their enhanced energy density than carbon materials as well as higher electrochemical stability than polymer materials, transitional metal oxides such as MnO2, Nb2O5, Co3O4 and Ni(OH)2 have attracted much attention.9-16 For example, MnO2 has been widely investigated as an electrode material because of its high theoretical specific capacitance (Cs, ≈1400 F g−1), unfortunately its intrinsically poor electrical conductivity (10−6∼10−5 S cm−1) prevents it from achieving high electrochemical performance.9,13,17 In comparison with MnO2, TiO2 exhibits higher electrical conductivity (10−5∼10−2 S cm−1)

9,18

and lower volume expansion during

charging/discharging process, longer cycle life and better durability, driving the construction of TiO2 electrode materials as high performance energy storage devices.18-20 The main challenge is the relatively low theoretical capacity of TiO2 (1206 F/g) restricts its further practical application because of its wide electronic bandgap (~3.2 eV) and its high resistivity would lead to strong internal resistance of the charge-storage devices.20-23 To work out these issues, persistent efforts have been focused on modifying pure TiO2 by introducing oxygen vacancy (Ti3+ sites), doping hetero-atoms, or combining carbon materials, to efficiently improve the electrical conductivity.18,19,24,25 Nonetheless, the TiO2-based electrode materials reported so far, e.g., H-TiO2/NG-B hybrids electrode,22 carbon nanotubes/TiO2 nanoparticles,26 3 ACS Paragon Plus Environment


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graphene/ALD TiO2 electrode,27 and carbon/mesoporous TiO2,28 are still not up to the mark of high performance energy storage devices. One of the possible ways is to devise novel nano-/microscale heterostructures, such as hierarchical hollow structures. On one hand, the unique structure may enhance the surface areas and create more active sites for better electrolyte infiltration. On the other hand, the viod spaces can be serve as a soft medium to hinder volume changes and facilitate the adsorption of ions and the fast intercalation as well as de-intercalation of active species by shortening the transportation route.5,29 Although SiO2 has been widely synthesized into hollow, mesopore and hierarchical structures as load and delivery systems, supports for catalysts with large spcific area and voids, etc,30-32 SiO2-based composites are almost impossible to be used as electrodes due to their dielectric nature and low capacitive contribution.33-35 In this study, however, we successfully design and synthesize a kind of novel hierarchical SiO2@C/TiO2 (HSCT) hollow spheres. Because the HSCT spheres effectively integrate SiO2, carbon, TiO2 and hierarchical nano-architectures, they can not only exhibit a higher specific surface area for ions transport, but also exploit the synergetic properties of components and provide an efficient diffusivity path of active materials, resulting in faster kinetics, more efficient contacts with the electrolyte ions, as well as more electroactive sites in comparison with pure TiO2.7,9 Moreover, the hierarchical HSCT structure even enables new function of the electrode. For example, the inner SiO2 shell can act as an ion reservoir, while carbon species can improve electronic conductivity for supporting electrochemically active materials. Accordingly, the HSCT hollow sphere-based electrode achieves a remarkable Cs (1018 F/g at 1 A/g), far beyond those of all of the previously reported TiO2-based supercapacitors electrode materials and some other typical electrodes for supercapacitors. 4 ACS Paragon Plus Environment

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EXPERIMENTAL SECTION

Synthesis of cationic polystyrene spheres/silica microspheres (CPS/SiO2). The hybrid microspheres were prepared using allyltrimethoxysilane (ATS) as functional monomer. The as-prepared CPS were dispersed in ethanol system, and then added quickly by tetraethyl orthosilicate (TEOS) and ammonia. This mixture was stirred at 50oC for about 6 hours in the presence of ATS, preparing the CPS/SiO2 spheres. Preparation of hierarchical SiO2@C/TiO2. The synthesized CPS/SiO2 were further polymerized with styrene and 2-(methacryloyloxy) ethyltrimethylammonium chloride (DMC), forming CPS/SiO2/CPS spheres, which were further coated by tetrabutyl titanate (TBT) under the ammonia to prepare the outer TiO2. After calcination at 450oC in air (closed system) to remove CPS, HSCT were obtained. Preparation of supercapacitor electrodes. Hollow TiO2, HST, HSCT and activated carbon (AC) were used as active materials. A homogeneous slurry of active material (Hollow TiO2, HST, HSCT or AC), polytetrafluoroethylene and acetylene black with a mass ratio of 7:2:1 were prepared with a trace amount of water as dispersing agent. After that, the mixture were pressed onto a 10 mm×25 mm Ni foam under a pressure of 10 MPa, and then dried at 100oC for 4 h. About 4.5 mg/cm2 of active materials were loading on the surface of Ni foam. Assembly of asymmetric supercapacitor. The AC electrode (negative electrode) and the HSCT electrode (positive electrode) were assembled into a full cell with 1 M KOH as the electrolyte. Electrochemical masurements. All the electrochemical properties of the electrodes were evaluated by using a standard three-electrode system at 25oC where Hg/HgO electrode and Pt foil (1×1cm2) were used as reference electrode and counter electrode, respectively. 1M aqueous KOH was used as the electrolyte. Asymmetric 5 ACS Paragon Plus Environment


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supercapacitor was assembled by integrating as-prepared products positive electrode with AC negative electrode. A CHI 660B electrochemical workstation was employed to measure the cyclic voltammetry (CV), galvanostatic charging/discharging (GCD) curves and electrochemical impedance spectroscopy (EIS). EIS measurements were operated in the frequency range from 1 MHz to 0.01 Hz with applying 5 mV AC amplitude. The specific capacitance is calculated by the equation: C=

I∆ t m∆ V

where m is the mass of the active material (g), I is the discharge current (A), ∆V is the potential window (V), C is the speciﬁc capacitance (F/g) and ∆t is the discharge time (s). To keep charge balance and achieve the maximum capacitance, the mass ratio of active materials were calculated to be 1:3 according to the following equation:36, 37 m + C _ × ∆V _ = m_ C + × ∆V +

Where m is the mass, and ∆V is the voltage range for positive electrode (+) and negative electrode (−) electrodes, respectively. The relationship between energy densites (E) and power densities (P) were calculated as follows according to galvanostatic discharge measurement: 38 E=

1 CV 2 2 × 3.6

P=

E × 3600 t

RESULTS AND DISCUSSION


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Synthesis and characterization of HSCT hollow spheres: Scheme 1 briefly describes our design thought and fabrication process. The cationic monodisperse polystyrene (CPS)/SiO2 core−shell colloidal spheres are synthesized through electrostatic interaction between CPS and SiO2 derived from the sol−gel reaction of tetraethyl orthosilicate (TEOS) and some amount of allyltrimethoxysilane (ATS), and further employed as the templates for polymerization with styrene and a cationic DMC, and then TiO2 shells using the same process.31, 32 After removal of the two CPS templates, hierarchical SiO2@C/TiO2 hollow spheres are obtained.

Figure 1a manifests a clear difference between the dark solid edge and the pale center, suggesting that all the HSCT have a monodisperse hollow architecture. The typical SEM image also displays that the as-prepared HSCT has uniform and intact spheres (Figure S1). A gap between inner SiO2 and outer TiO2 can be found from the higher magnification TEM image in Figure 1b, which can be further confirmed by the scanning TEM (STEM) image (Figure 1c). The high resolution TEM (HRTEM) image of HSCT sphere clearly indicates that the lattice spacing is 0.34 nm and 0.25 nm (Figure S2a), which can be regarded as the (110) and (101) planes of anatase TiO2, respectively. Moreover, from the corresponding SAED pattern of HSCT, the (101),(004), (200), (105), and (211) planes can be detected, further confirming the existence of anatase TiO2 (Figure S2b). The Energy dispersive X-ray spectroscopy (EDX) mapping in Figure 1d demonstrates that Si signal constructs the inner shell of the sample, whereas the Ti signal is mostly present in the outer layer of HSCT spheres, suggesting the successful integration of SiO2 and TiO2 components. Interestingly, C signal is dispersed throughout the sample, which would facilitate improving the electronic conductivity of HSCT hollow spheres as electrode materials.



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X-ray diffraction (XRD) spectra were used to determine the crystal structure of SiO2@C/TiO2 hollow spheres (Figure S3). Six distinctive reflection peaks at 25.4o (101), 37.9o (004), 48.0o (200), 53.8o (105), 54.9o (211) and 62.8o (204) are ascribed to the anatase of TiO2 (JCPDS: No 21-1272),39 indicating a successful formation of anatase crystals by calcination at 450oC. Moreover, the diffraction peaks of the HSCT hollow spheres are sharp and intense, suggesting the highly crystallinity of outer TiO2 layer. X-ray photoelectron spectroscopy (XPS) was used to investigate the change of surface bonding of TiO2 spheres induced by SiO2 and C species, and the electronic valence band position of HSCT hollow spheres. As shown in Figure 2a, the elements of Si, O, Ti, and C can be obviously identified with the binding energies of Si2p, O1s, Ti2p, and C1s electrons, further confirming that the as-prepared HSCT spheres contain SiO2, TiO2, and C species. Figure 2b shows the normalized Ti2p core level XPS spectra of HSCT and control sample, commerically available pure TiO2. Compared to the two peaks of pure TiO2 centered at 463.8 and 458.2 eV for Ti2p1/2 and Ti2p3/2,39,40 the HSCT sampleis positively shifted by 0.35 eV (458.55 eV) for Ti2p3/2, and by 0.4 eV (464.2 eV) for Ti2p1/2 binding energy. Obviously, the positive shift of binding energies can be ascribed to the strongly interaction between TiO2 and SiO2 as well as C species, implying lattice distortion of TiO2 in HSCT.39,41 The two peaks can be resolved into two Gaussian peaks (Figure 2c). The binding energy peaks (464.7 and 458.7 eV) can be indexed to the 2p1/2 and 2p3/2 core levels of Ti4+, respectively, while the two peak energies at 463.6 and 458.2 eV are consistent with the characteristic Ti 2p1/2 and Ti 2p3/2 peaks of Ti3+,18,42 respectively. Because the HSCT hollow spheres are not doped by other elements, so the possible defect states are responsible for the existence of oxygen vacancies and surface hydroxyl groups on 8 ACS Paragon Plus Environment

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TiO2.18,40,42 The O1s energy levels for HSCT are reported to be located at 530.05 (bulk oxygen bound in the TiO2 lattice) and 532.41eV,43 above the valence band of pure TiO2 (Figure 2d), respectively, further indicating the existence of oxygen vacancies (Ti3+ sites).18,22,40 Figure 2e reveals the C1s spectra of the HSCT spheres, which can be fitted to three peaks at 284.87, 286.02 and 288.75 eV, suggesting that three different chemical forms for carbon exist in the HSCT sample. The peak around 284.87eV is attributed to the residual elemental carbon produced by calcination, which would effectively improve electronic conductivity and speed up large strain from the reaction of the HSCT as electrode material with ions, leading to increased specific capacity.5,44 Compared with the Si2p binding energy of pure SiO2 sample, the Si2p binding energy for the HSCT spheres shows a clear negative shift (0.35 eV) (Figure 2f). Not surprisingly, such a change of surface bonding is mainly attributed to the strong interaction between SiO2 and TiO2. Thus it can be reasonably envisioned that SiO2 also plays a role in improving the specific capacities of TiO2 due to its ions channels and high surface area.33 For the sake of comparison, hollow TiO2 and hollow SiO2/TiO2 (HST, without the second CPS layer) were also synthesized using the same procedure as HSCT. The nitrogen adsorption/desorption isotherms of the hollow TiO2, HST and HSCT show that both HST and HSCT hold IV hysteresis loops (Figure S4), suggesting a typical mesoporous structure. The measured surface areas are 17.6, 91.1 and 116.4 m2 g−1 for hollow TiO2, HST and HSCT, respectively, indicating significantly increased specific surface area for HSCT. Such hierarchical hollow structures with plenty of active sites and ions transport channels may largely enhance the specific capacitance of TiO2-based electrodes.



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Electrochemical properties of HSCT hollow spheres as supercapacitor electrodes: Figure 3a exhibits the CV curves of HSCT as electrodes at scan rates from 2 mV/s to 10 mV/s within the potential range of 0.0 to 0.6 V. All the CV curves exhibit pairs of cathodic and anodic peaks, which presents an idea faradaic pseudocapacitance characteristic. These redox peaks are attributed to the surface or subsurface intercalation and deintercalation cationic process as follows:22 TiO2 + 2H2O + exK+

Ti(OH)3 + OH-

+ yTiO2 + e-

Kx(TiO2)y

With the increase of scan rates, the oxidation/reduction peaks shift to a much widersides, which should be resulted from the increase of the internal diffusion resistance in the pseudoactive material.45 The detailed relationship of the peak current intensity (Ipeak) and scan rates (ν) can be described in Figure S5. The I

peak

of HSCT

exhibits a strong linear dependence on the square root of scan rate (ν1/2), suggesting the diffusion-limited redox reactions. In addition, the slope of the plot is closely related to the diffusion coefficient of the electrolyte ions within electrode and the charge transfer ability at electrode-electrolyte interfaces, which means the process is controlled by electrochemical process. Therefore, this special structure can effectively accommodate the large volume change and release the stress caused by rapid charging/discharging process. In order to prove the capacitive contribution of nickel foam in alkaline solution, the comparison of CV curves of nickel foam and HSCT were measured at the same scan rate under the same condition in Figure S6. As can be seen that the current collector’s contribution can be negligible. Moreover, the largely different redox peak positions between Ni foam and HSCT further proves that nickel foam has a rather limited contribution to the whole capacitance of the composite material. Comparing the CV 10 ACS Paragon Plus Environment

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curves of hollow TiO2- and HST-based electrodes, one can see the as-synthesized HSCT electrode manifests the largest redox current density (Figure 3b), implying more efficient utilization of the electro-active components. Figure 3c presents the GCD curves of the hollow TiO2, HST and HSCT electrodes at various current densities within a potential range of 0 to 0.45 V. Obviously, a sudden potential drop (internal resistance) and a slow potential decay (the capacitive characteristics of the electrode) can be observed from the discharge curves. As expected, the HSCT-based electrode holds much longer discharging time and exhibits substantially larger current density than hollow TiO2 electrode (Figure 3d). Moreover, the current density of the HSCT electrode is also higher than that of the HST electrode. These should be mainly attributed to the enlarged surface area and the increased ionic transport from TiO2 layer to the inner layers via the carbon species between inner SiO2 and outer TiO2.5 Although SiO2 has negligible capacitance contribution compared to TiO2 based electrodes,35 the Cs were still calculated based on the whole mass of SiO2 and TiO2 since the former has indirect contribution to electrochemical properties such as its abundant of ions channels and high surface area. Galvanostatic measurements indicate that the HSCT electrode exhibits the longest discharge time (Figure 3d) among these electrodes. At 1A/g, the Cs (1018 F/g) of HSCT is 9 times higher than hollow TiO2 electrode (113 F/g) and 4 times higher than HST electrode (252 F/g) as demonstrated in Figure 3e, and also considerably larger than those of the all previously reported TiO2-based electrodes materials and some others typical supercapacitor electrodes (Table S1).2,9,22,28,46-49 The CV curves of HSCT electrode have been given from 1st cycle to 4000 cycles (Figure S7a). The result proves that the electrode keep about 78% capacitance retention. Besides, the good cycle stability can also be verified from the GCD curves after 1000 cycles at 1 A/g (Figure S7b). The HSCT electrode can still 11 ACS Paragon Plus Environment


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retains about 76% of its initial capacitance, which is consistent with the result of CV curves and further demonstrating the good cycling stability of our electrode. The EIS was carried out and the Nyquist plots are depicted in Figure 3f. The overall impedance can be reduced to the following Equation:

Z = ܴௌ +

1 ଵ

݆‫ܥݓ‬ௗ௟ + ோ

೎೟

where Z is the overall impedance, j is the imaginary unit, ߱ is the angular frequency (Hz), Rs is the solution resistance, Cdl is the double-layer capacitance, and Rct is the charge-transfer resistance. Compared with hollow TiO2 and HST electrodes, HSCT electrode shows a higher electric conductivity due to the strong interaction of SiO2, C and TiO2. Moreover, the slope of the straight line associated with HSCT electrode is larger than those of hollow TiO2 and HST electrodes and closer to the imaginary impedance axis, indicating a swift ion diffusion in electrolyte and the adsorption onto the electrode surface, which suggests a lower diffusion resistance and an ideal capacitive behavior of the HSCT hybrid electrode.50 The excellent performance of the HSCT electrode material we present here should be attributed to the following reasons: (1) HSCT occupies considerably accessible surface area (116.4 m2/g) than hollow TiO2 (17.6m2/g) and HST (82.1m2/g), which allows efficient ion intercalation/de-intercalation, promotes the electrolyte access and provide more channels for ions and electrons transfer; (2) Carbon species between inner SiO2 and outer TiO2 as shown in Figure 4a can accelerate electonic transport among TiO2 materials because of its better electrical conductivity, thus, contributing to the total capacitance; (3) The introduction of oxygen vacancy (Ti3+ sites) states is benefit for the increase of specific capacitance (Figure 2c) due to the great redox 12 ACS Paragon Plus Environment

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activity and rich polymorphism of the electrode material; (4) The void spaces and hierarchical structure on the interlayers and inner hollow spaces can not only serve as an “ion reservoir” to facilitate the transportation of KOH electrolyte ions but also withstand the volume change during the charge/discharge process, as shown in Figure 4b.

Electrochemical properties of the HSCT//AC asymmetric supercapacitor: To evaluate the possibility of the as-obtained HSCT for practical application, an asymmetric supercapacitor was further fabricated by utilizing HSCT as the anode and active carbon (AC) as the cathode. AC was used in this work because it has good electrochemical performances (Figure S8). The Cs of AC electrode was calculated from its GCD curves and exhibited specific capacitances of 163, 146, 137, 130, 125 and 121 F/g at 1, 2, 3, 4, 5 and 6 A/g, respectively. The performance isalso superior tosome of the previously reported AC-based supercapacitors.51-53 Figure 5a shows the structure scheme illustration of the asymmetric supercapacitor. The positive and negative electrodes were sandwiched with a piece of cellulose paper as the separator. The mass of the two electrodes were controlled based on the principle of charge balance at a sweeping rate of 5 mV s−1. Figure 5b illustrates the CV curves of the HSCT//AC asymmetric supercapacitor at various sweep rates with a potential window of 1.5 V. The CV curves in Figure 5b exhibits a large current response and still retains its faradaic pseudo-capacitance nature, which confirms an efficient charge storage performance. Its Cs values can be calculated to be 92.1, 73.4, 61.7, 54.1 and 47.6 F/g, as shown in Figure 5d, based on the total mass of the active materials at 0.5, 1, 2, 3 and 4 A/g, respectively, from its GCD curves (Figure 5c). Moreover, the coulombic efficiency calculated from the GCD data remain more than 96% at various current 13 ACS Paragon Plus Environment


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densities, revealing a highly reversibility between the electrolyte and our asymmetric supercapacitor electrodes. The maximum energy density is 29 Wh/kg at a power density of 375W/kg and still maintain 14.9 Wh/kg at a highest power density of 3000 W/kg (Figure 5e). This energy density considerably exceeds those of many of the previously reported metal oxide-based

asymmetric

supercapacitors,54-57

and

carbon-based

asymmetric

supercapacitors.58-60 To validate the practical application, three asymmetric supercapacitors device assembled in series can efficiently power a round light-emitting diode indicator (5 mm diameter red, 1.8 V, 20 mA). A picture of the supercapacitor device is shown in Figure 5f. Furthermore, cycling stability experiment of the HSCT//AC device was tested to evaluate the durability at 4 A/g (Figure S9). The specific capacitance decreased rapidly at the first 1000 cycles. Then, it is gradually stabilized to over 79.4% of its original specific capacitance after 6000 cycles. This performance is also comparable to some reported Ti-based symmetric/asymmetric

supercapacitors

such

as

TiO2@MnO2

symmetric

supercapacitor (66.4% after 3000 cycles),61 and urchin-likeTiO2//AC asymmetric supercapacitor (81% after 2000 cycles).62 This result suggests that our asymmetric supercapacitor has a superior cycling stability, which may be further utilized in some fileds of energy storage and conversion.

CONCLUSION In summary, we have designed hierarchical SiO2@C/TiO2 (HSCT) hollow spheres

by a facile template-directed strategy for high-performance supercapacitor. Because of the excellent conductivity and strong synergistic effect of SiO2, C, TiO2, and the unique hierarchical structure, the HSCT-based electrode exhibits unbelievable high discharge specific capacitance (1018 F/g at 1 A/g) which considerably higher than 14 ACS Paragon Plus Environment

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those of all the previously reported TiO2-based supercapacitor electrodes even some other typical supercapacitor eletrode materials. In addition, an asymmetric pseudocapacitor based on HSCT-and AC also manifests high specific capacitance, good cycle stability (6000 cycles, 79.4% capacitance retention) and high energy density. Based on this study, it can be anticipated that high-performance and low-cost SiO2-supporting composites for electrode materials, energy storage devices such as SCs, Li-ions batteries, and solar cells, etc, may be fabricated through rationally designing the structure of SiO2 and its composites.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional figures (Figures S1−S9) of SEM, HRTEM, XRD, BET, Ipeak vs. v1/2 plot, CV curves, and cycle stability, and table. S1 (specific capacitances).

AUTHOR INFORMATION

Corresponding Authors * E-mail: [email protected] (S.C.). *E-mail: [email protected] (L.W.) Notes The authors declare no competing financial interest.

ACKOWLEDGEMENTS

Financial supports of this research from the National Key Research and Development Program of China (2017YFA0204600), National Natural Science Foundation of China (Grants 21374018 and 51673045), Jiangsu Natural Science Fund of China (No.BK20141299) and Open Foundation (K2015-14) of State Key Laboratory of Molecular Engineering of Polymers, Fudan University, are appreciated.

REFERENCE 15 ACS Paragon Plus Environment


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Scheme and Figure Captions:

Scheme 1. Schematic illustration of the hierarchical SiO2@C/TiO2 hollow spheres



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Figure 1. TEM (a), high resolution TEM (b) and STEM images (c), and EDX mapping (d) of HSCT hollow spheres


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Figure 2. (a) XPS fully scanned spectra of the HSCT sample, (b) XPS spectra of Ti2p, (c) Ti2p Gaussian peaks, (d) XPS spectra of O1s, (e) XPS spectra of C1s, and (f) XPS spectra of Si2p for HSCT sample.



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Figure 3. (a) CV curves of the HSCT electrode at different scan rates, (b) CV curves for the hollow TiO2, HST, and HSCT electrodes collected at the scan rate of 2 mV/s, (c) the galvanostatic charge-discharge curves of the HSCT electrode at various current densities, (d) the galvanostatic charge-discharge curves for the hollow TiO2, HST, and HSCT electrodes at a current density of 1 A/g, (e) comparative specificcapacitances of the hollow TiO2, HST and HSCT, and (f) EIS of the hollow TiO2, HST, and HSCT electrodes


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Figure 4. (a) EDX line scanning image (Ti, C, Si elements) of HSCT samples and (b) schematic diagram of ion and charge transfer in the HSCT electrode



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Figure 5. (a) The schematic illustration of the asymmetric supercapacitor device; (b) CV curves of the asymmetric supercapacitor at different scan rates; (c) Galvanostatic charge–discharge curves; (d) Specific capacitances and coulombic efficiency of the asymmetric supercapacitor at different current densities; (e) Ragone plots of our supercapacitor based on the full cell, compared with the literature; (f) A picture showing that three supercapacitors in series can lighten up a red LED indicator. 26 ACS Paragon Plus Environment

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Table of Contents Graphic


TiO2 Hollow Spheres

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