Highly Ordered Metal Oxide Nanorods inside Mesoporous Silica

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Highly Ordered Metal Oxide Nanorods Inside Mesoporous Silica Supported Carbon Nanomembranes: High Performance Electrode Materials for Symmetrical Supercapacitor Devices Jian Zhi, Sheng Deng, Youfu Wang, and Aiguo Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01230 • Publication Date (Web): 27 Mar 2015 Downloaded from http://pubs.acs.org on March 31, 2015

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The Journal of Physical Chemistry

Highly Ordered Metal Oxide Nanorods inside Mesoporous Silica Supported Carbon Nanomembranes: High Performance Electrode Materials for Symmetrical Supercapacitor Devices Jian Zhi, Sheng Deng, Youfu Wang, Aiguo Hu* Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, 200237, China.

ABSTRACT Highly ordered metal oxide nanorods (MnO2, SnO2, NiO) inside mesoporous silica supported carbon nanomembranes have been applied for electroactive materials to fabricate symmetrical supercapacitors. Maximum specific capacitance of the obtained cells reaches up to 964 F g–1 in aqueous electrolyte with energy density of 33.5 Wh kg−1 for a 1 V voltage window, which are among the highest values in two electrodes supercapacitor cells employing similar metal oxide/carbon materials. This high performance is attributed from the synergic effect of the conductive carbon nanomembrane and well-ordered pseudocapacitive metal oxide nanorods.

Keywords: Capacitance, Aqueous electrolyte, Energy Density, Surface Area, Mesopore

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Introduction Along

with

urgent

energy demands

from

next-generation

electrical

vehicles

to

electromechanical microsystems, more and more scientific works have focused on novel materials of electrodes for energy storage devices. Among various kinds of power source devices, electrochemical capacitors, also widely known as supercapacitors, have attracted great interests owing to their fast charging-discharging rates, superior cycle life, and up to ten times more power densities than conventional batteries.1-2 Moreover, ECs are the key parts in complementing fuel cells in all-electric vehicles using green and recyclable energy media.3 According to the mechanism of charge storage, supercapacitors are usually identified into two kinds: electrical double-layer capacitors (EDLCs) and pseudocapacitors (also called Faradaic supercapacitors).2 The former ones, normally employing carbon materials builds up electrical charge interface between electrode and electrolyte. The latter ones utilize reversible and fast surface reactions to storage charges.4-5 The specific capacitance of pseudocapacitor is generally 5-10 times higher EDLCs. Pseudocapacitive transition metal oxides, such as MnO2, NiO, SnO2 and Co3O4 have been recognized as ideal materials for supercapacitor electrodes.6-12 Nonetheless, their practical applications, especially for two electrode supercapacitor cells, are still limited by their poor cycling life and low energy density (generally less than 10 Wh kg-1), mostly because of their poor chemical stability and low electrical conductivity.13-15 These issues encouraged us to design new electrode materials from these pseudocapacitive metal oxides. Electrochemical performance of various materials containing metal oxides/carbon in two electrode

symmetrical

or

asymmetrical

MnO2/graphene//MnO2/graphene,17 Fe3O4/graphene//Fe3O4/graphene19

devices,

such

as

MnO2/CNT//MnO2/CNT,16 oxide,18

Ni(OH)2/graphene//graphene and

SnO2/CNT//MnO2/CNT20

have

been

evaluated.


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Attributing to the incorporation of the carbon materials, the electrical conductivity of these hybrid electrodes was increased in a large scale. It is widely acknowledged that capacitance and stability of pseudocapacitive materials are strongly relied on the order degree of nanostructures. Nanomaterials with highly ordered structures are proved to be quite desirable in application of energy storage, due to their high surface area and short ionic-diffusion path.21-22 However, electrochemical performance in above mentioned cases are still restricted because of their disordered structures, resulting in relatively lower specific capacitance and energy density based on totally active mass in the whole cell (normally below 500 F g-1 and 20 Wh kg-1, respectively). Up to now, it remains a great challenge to fabricate highly ordered metal oxide pseudocapacitive materials on carbon supports for two electrode supercapacitor devices. Recently, we developed a novel

porous conductive support with highly ordered

microstructures by the formation of a three dimensional (3D) carbon nanomembranes on the surfaces of mesoporous silica SBA-15 (denoted as SS-CNM) to prepare high-performance supercapacitor.23 The thin and strong carbon nanomembranes of these SS-CNMs endow them high porosity as a class of ideal conducting supports. In this work, three kinds of metal oxides, MnO2, SnO2 and NiO, are in-situ grown inside the channels of SS-CNMs. Based on the spatial confinement of SS-CNM, highly ordered metal oxide nanorods (denoted as HOMORs) were fabricated inside the channels of SS-CNM. Such HOMORs@SS-CNM hybrids not only exhibit high pseudocapacitive behavior thanks to the HOMORs, but also possess the capability of fast charge transportation. As a result, supercapacitor electrode composed of the HOMORs@SSCNM is expected to exhibit high energy density without sacrificing power density, as well as excellent cycling life. High specific capacitance of 964 F g

−1

is obtained in HOMORs@SS-


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CNM symmetrical cell.

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Compared to other metal oxide/carbon electrode materials,

HOMORs@SS-CNM cell exhibited a high energy density of 33.5 Wh kg-1, showing the application potential of such composite ordered materials on energy storage. In addition, the strong mechanical strength and toughness of SS-CNM support limits the volume change of the electroactive materials during the course of charge-discharge,3, 24 rendering the HOMORs@SSCNM composites good cycling stability (8.2% capacitance decay after 10000 cycles).

Experimental section Materials. Mn(NO3)2 (50% aqueous solution), SnCl2•2H2O, Ni(NO3)2•6H2O and other chemicals were commercially available. SBA-15 supported CNM (SS-CNM) was synthesized though the procedures in previously reported article. 25 Preparation of HOMORs@SS-CNM. 20 mg of SS-CNM was added into 1 mL of DMF solutions containing Mn(NO3)2 (or SnCl2•2H2O, Ni(NO3)2•6H2O) at different concentrations. The mixed solutions were sonicated for 1 h and stirred overnight. After drying in at 100 °C, the obtained powders were calcinated at 250 °C with the bubbling of air for 6 h to give the corresponding HOMORs@SS-CNM composites with different HOMORs contents (24 wt%, 50 wt%，66 wt%, 80 wt%, calculated from the total concentration of precursor salt in the hybrid materials). HOMORs@SBA-15 was synthesized through the same procedure, employing SBA-15 instead of SS-CNM. Preparation of HOMORs nanorods. HOMORs@SS-CNM samples (MnO2 nanorods@SSCNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM ) were heated at 550 °C in air


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atmosphere for 3 h. To remove SBA-15 matrix, the powders after calcination was soaked in aqueous NaOH solution. The final product was collected by filtration. Characterization. To study the morphology of all the related samples, a transmission electron microscope (TEM, JEOL, JEM-2010) equipped with energy-dispersive X-ray analysis and scanning electron microscope (HITACHI S-4800) were employed. XRD measurements were performed by X0 Pert Pro, Philips instrument using Cu Ka radiation. The measurement of nitrogen adsorption-desorption were performed by ASAP2010 instrument. Electrochemical Tests. Electrochemical measurements were carried out by employing a special two electrode setup where a carbon paper coated with electroactive samples served as working electrodes. Electrochemical data was recorded on a CHI 660D electrochemical workstation in aqueous electrolyte solution (1 M Na2SO4 for MnO2 nanorods@SS-CNM, 6M KOH for SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM, respectively). The fabrication of working electrodes was frequently stated in previously reported literatures.26-27 Two pieces of the working electrodes were immersed into the electrolyte, and sealed with Parafilmtm to fabricate the two-electrode cells. Before each test, the cells were degassed for several times under vacuum. The reversible reactions of MnO2, SnO2 and NiO in the redox process are shown as follows:8, 28 MnO2 + M++e- ←→ MnOOM (M=Li, Na, K) SnO2 + M++e- ←→ SnOOM (M=Li, Na, K) NiO + OH- ←→ NiOOH + eSpecific capacitance was calculated using the equation Cg = 2(IΔt)/(mΔV ).29 In this equation, Δt represents full discharge time (s), I represents current (A), m represents the mass of materials in each electrode (g), ΔV is the potential window. Energy density (Ed) and power density (Pd)


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were calculated from equations E = (1/8)CgΔV2 and Pd = Ed /Δt .30 Cg is the value obtained from above mentioned equation, while ΔV and Δt represent the potential change and the time after a full discharge.

Results and Discussion The SS-CNM supports were synthesized by self-assembly enediyne monolayers on the surface of SBA-15 followed by Bergman cyclization and carbonization.31 Transmission electron microscopy (TEM) images of the SS-CNMs were shown in Figure S1. Clearly, SS-CNMs exhibit highly ordered structures, maintaining a rod like structure, similar with the SBA-15 template, as revealed from the SEM image (Figure S2). Even after the silica framework was etched off, the mesoporous channels of carbon membranes remained intact.25 The synthetic process of the hybrid HOMORs@SS-CNM is illustrated in Figure 1. Highly ordered MnO2, SnO2 and NiO nanorods were simply loaded inside SS-CNM channels from the oxidation of corresponding metal salt precursors at 250°C.

Figure 1. Illustration of the preparation of HOMORs@SS-CNM supercapacitor electrode materials.


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Figures 2a-c show TEM images of MnO2 nanorods@SS-CNM, SnO2 nanorods @SS-CNM and NiO nanorods@SS-CNM with the corresponding HOMORs content 66 wt%. Similar to SSCNM, such hybrid materials also showed highly ordered structures, while the dark regions indicated HOMORs nanorods encapsulated inside the SS-CNM. Energy dispersive X-ray spectrometry analysis demonstrated that MnO2, SnO2 and NiO nanorods were successfully introduced into the SS-CNM hosts (Figure S3). No obvious metal oxide particles agglomerated in the surrounding of SS-CNM, indicating that most metal oxides are successfully incorporated inside the mesopores of the matrix.

32

To verify the structure of HOMORs, the SS-CNM was

removed by burning off the carbon membrane under air atmosphere and etching off the silica framework in aqueous NaOH.33 TEM images of the HOMORs are presented in the inset images of Fig. 2a-c. Nanorods with 6–7 nm in diameter are regularly formed, confirming that the growth of HOMORs takes place inside the channels of the SS-CNM. The diameter of these HOMORs is similar to that of Co3O4 nanomaterials, employing mesoporous silica as the template.34 The XRD patterns of MnO2 nanorods@SS-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM (Figure 2d) show a large peak at the degree of 15-25°, originated from the silica of SS-CNM. Other peaks can be indexed to the crystalline MnO2, SnO2 and NiO according to the JCPDS card No. 44-0141, 41-1445 and 69-2901. The average diameters of HOMORs were calculated to be 6.7 nm according to the Scherrer equation. TEM test also support this prediction. The XRD pattern of the MnO2 nanorods after removal of SS-CNM is also shown in Figure S4. It is clear that after 550 °C heating, all the peaks are fully indexed with MnO2 (44-0141), no phase transfer to other manganese oxide e.g. Mn3O4 or Mn2O3 happened in this sample.


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Figure 2. TEM images of MnO2 nanorods@SS-CNM (a), SnO2 nanorods@SS-CNM (b) and NiO nanorods@SS-CNM (c). Insets of figure (a-c): The images of HOMORs nanorods after removal of the host. (d) XRD pattern of MnO2 nanorods@-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM. The mesoporous feature of three HOMORs@SS-CNM samples (66 wt% HOMORs contents) is further confirmed by nitrogen adsorption-desorption measurements (Fig. 3a). The curves exhibit prominent characteristic of type-IV isotherms with a clear hysteresis loop in the P/P0 range of 0.4−1.0, indicating that there are large amount of mesopores in the frameworks. Corresponding pore characteristics (Table S1) showed that MnO2 nanorods@SS-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM exhibited a surface area of 156 , 127 and 209 m2g-1, with a pore volume of 0.21, 0.16 and 0.31 cm3 g-1 respectively, smaller than those of as synthesized SS-CNM, The difference in surface area and pore size of these three samples are


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owing to their distinct shrinkage extent during the oxidation process.35 The pore diameter of MnO2 nanorods@SS-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM are narrowly dispersed and respectively centered in 3.61, 3.72 and 3.92 nm, smaller than the value for SS-CNM(5.2 nm), clearly confirming the occupation of HOMORs inside the channels.

Figure 3. (a) Nitrogen sorption isotherms and (b) pore size distributions of MnO2 nanorods@SSCNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM.


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Figure 4. (a) CV curves of MnO2 nanorods@SS-CNM, pristine MnO2 nanorods and MnO2 nanorods@SBA-15. (b) CV response of MnO2 nanorods@SS-CNM at various scan rates (5, 10, 30 and 50 mV s-1). (c) Linear voltage versus time profiles in the galvanostatic charge-discharge curves of the MnO2 nanorods@SS-CNM. (d) Impedance Nyquist plots of MnO2 nanorods@SSCNM, pristine MnO2 nanorods and MnO2 nanorods@SBA-15. Inset is a magnification of the high frequency region. From the unique hybrid mesoporous structure and pseudocapacitance of the embedded HOMORs, HOMORs@SS-CNM composites are expected to show low resistance to electron diffusions and excellent electrical conductivity. Figure 4a shows the CV curves for MnO2 nanorods@SS-CNM, pristine MnO2 nanorods and MnO2 nanorods@SBA-15 in 1 M Na2SO4 at a


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scan rate of 0.01 V s-1. The CV loops of all the three samples are close to a rectangular shape, no obvious redox peak is found, which is due to the continuous redox reaction of these kinds of electroactive materials.2 The rectangular area of CV loop for MnO2 nanorods@SS-CNM is significantly larger than the other two samples, indicating effective solution infiltration and fast charge transportation in such composite material.36 The CV measurement in the cases of SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM samples were also performed (Figure S5). Rectangular shapes of the CV loops, which are usually shown in the two electrode cells,37-40 further indicated the ideal capacitive behavior of these samples. The rate capability of MnO2 nanorods@SS-CNM was measured by CV curves at different scan rates, as shown in Figure 4(b). With increasing of the scan rate, the rectangular area also increased, exhibiting good capacitive behavior. The CV curves still retain symmetrical shapes with little distortion even at scan rate of 0.1 Vs−1, indicating quickly transportation of charged ions between the interface of electrode and electrolyte.41 Figure 4c depicts the galvanostatic curves of the MnO2 nanorods@SS-CNM electrodes with different current densities. The triangle shape charge-discharge characteristics represent ideal capacitive behaviors of the fabricated device. Similar galvanostatic charge-discharge curves were found for SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM samples with current density of 2 Ag-1 (Figure S6). In order to compare the electrical resistance and charge transportation ability of the three MnO2 nanorods@SS-CNM, MnO2 nanorods and MnO2 nanorods@SBA-15 electrodes, electrochemical impedance technology was employed. Figure 4d showed the Nyquist plot of the corresponding samples. All the electrodes show a straight and vertical line in the low frequency region, typically characteristic of capacitive behavior. The real axis intercept represents the equivalent series resistance (ESR), as shown in the inset of Figure 4d. The ESR values of MnO2


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nanorods@SS-CNM, MnO2 nanorods and MnO2 nanorods@SBA-15 are 3.7, 4.5, 8.2 Ω, respectively. The diameter of semicircle in high frequency area represents the Faradaic chargetransfer resistance.30 These three materials showed charge-transfer resistance of 1.9, 4.5, and 9.2 Ω, respectively. The decreased ESR and Faradaic charge transfer resistances values of MnO2 nanorods@SS-CNM compared with other control materials proved its effective diffusion and migration charged ions in one charge/discharge circle. The well dispersed small size MnO2 nanorods inside the channels of SS-CNM can not only shorten the electron path-length, but also strengthen the contact with CNM walls, decreasing the interfacial resistance between the electrode and electrolyte. Figure 5a-c show the curves of specific capacitance as a function of current density for all the measured samples. Obviously, HOMORs@SS-CNM derived electrodes showed the highest specific capacitance. The value can achieve up to 964 Fg-1 for MnO2 nanorods@SS-CNM, and 745, 620 Fg-1 in the cases of SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM, respectively, which are much higher than the values of their corresponding compared samples. Above comparisons corroborate the critical roles of combining CNM inside SBA-15 hosts to improve the electrochemical performance of two electrode cells.


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Figure 5. Curves of specific capacitance versus varied current density of MnO2 nanorods@SSCNM (a), SnO2 nanorods@SS-CNM (b) and NiO nanorods@SS-CNM (c). (d) Cycle life of MnO2 nanorods@SS-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM electrode materials at a scan rate of 0.1 V s-1. The amount of HOMORs loaded in the SS-CNM can significantly affected the total capacitance. Figure S7 depicted specific capacitance of the three HOMORs@SS-CNM samples with 24, 50, 66, and 80 wt% metal oxides contents measured at the current density of 2 A g-1. Obviously, HOMORs@SS-CNM with 66 wt% nanorods content showed the highest specific capacitance. The better dispersion of HOMORs in SS-CNM framework at 66 wt% concentration than other loading contents may be the reason for this trend. With the respect of higher HOMORs contents, the synergic effect of high surface area and good electrical conductivity of


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SS-CNM were not completely aroused because of the agglomeration of nanoparticles outside SSCNM (Figure S8 shows TEM images of the three HOMORs@SS-CNM samples with 80 wt% nanorods contents; Agglomeration of small nanoparticles outside of SS-CNM is clearly seen). Electrochemical stability of the three HOMORs@SS-CNM samples at scan rate of 0.1 Vs-1 is shown in Fig. 5d. With the increasing of the cycle number, the value of specific capacitance decreased gradually. However, the specific capacitance values still remained at 91.8%, 90.5% and 90.3 % of the initial value after 10000 cycles in the samples of MnO2 nanorods@SS-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM respectively, showing the long cycling life of the fabricated supercapacitors.

Figure 6. Ragone plots of MnO2 nanorods@SS-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM electrode materials. To further evaluate the performance of the supercapacitor devices, Ragone plots of the three HOMORs@SS-CNM samples are plotted in Figure 6 (66 wt% nanorods contents). MnO2 nanorods@SS-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM supercapacitor cells exhibit energy densities of as high as 33.5, 25.7 and 21.6 Wh kg−1 for a 1 V window voltage, which are highly competitive with previously reported symmetric and asymmetric devices based


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on similar metal oxide/carbon materials (Table S2).16-20, 42-46 More importantly, energy densities of the three samples were very stable with the incensement of the power density. Such superior performance of the three samples is attributing to the efficient charge transfer and small contact resistance between metal oxide and electrolyte originated from highly porous and conductive SSCNM hosts. The excellent rate capability of the HOMORs@SS-CNM electrodes enable the effective energy delivery at one charge-discharge circle, as well as high discharge current with negligible polarization. As a result, at relatively high power density, high energy density can be achieved in our cells. In order to deeply illustrate the cause of the rise in specific capacitance and energy density for the HOMORs@SS-CNM samples, electrochemical analysis from the work of Trasatti et al.47 was applied. The total stored charge (qt) in the whole cell can be divided into two parts: surface outer charge (qs) and inner charges (qi):

qt = qs + qi The outer charge is normally identified to be not dependent on scan rates (v), whereas the inner charge is diffusion controlled parameter. Hence, the equation of total charge stored with scan rate can be written as:

qt = qs + kv-1/2 From this equation, the value of qs can be easily obtained by plotting the qt against the reciprocal of the square root of the sweep rate (v) followed by extrapolating v to ∞ (Figure S9ac). The outer and inner charges for all the samples are shown in Figure S9d. Obviously, HOMORs@SS-CNM based cells show both higher inner and outer charges than HOMORs cells, indicating that SS-CNM has induced a better accessibility of HOMORs in the electrolyte. This study strongly proves that building of SS-CNM and HOMORs ordered network not only leads to


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more accessible charged ion transportation paths around metal oxides inside the conductive SSCNM channels, but also decrease the contact resistance between the carbon membrane and wellordered metal oxides nanorods, which is in consistent with the observed enhanced specific capacitance. Furthermore, attributing to the high values of surface area and pore volume of the HOMORs@SS-CNM as determined by nitrogen adsorption-desorption measurements (Fig. 3a), the total stored charge in three HOMORs@SS-CNM cells are also much larger than pristine HOMORs (Figure S9d), which leads to the high energy density of the fabricated devices.48 Conclusions We have reported three types of carbon nanomembrane supported highly ordered pseudocapacitive metal oxides inside carbon nanomembrane for supercapacitor electrodes. High specific capacitances (964, 745, 620 Fg−1), excellent cycling stability (less than 10% capacitance decay over 10000 circles), and high energy density (33.5, 25.7 and 21.6 Wh kg−1 within 1 V window voltage) are achieved with MnO2 nanorods@SS-CNM, SnO2 nanorods@SS-CNM and NiO nanorods@SS-CNM as electrode materials in two electrode cells. Such high performance is owing to the following two features: Firstly, the high electrical conductivity of SS-CNM serves as a highway for outer electron transportation between electrolyte ions to the electrode surfaces; Secondly, the intimate contact between the carbon membrane and well-ordered metal oxides nanorods further facilitate the charge transports in the bulk. Based on above distinct features, such series of hybrid electrode materials are expected to be promising candidates for supercapacitor devices.

AUTHOR INFORMATION Corresponding Author


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*Aiguo Hu, [email protected], Tel: +86-2164253037 ACKNOWLEDGMENT The support of National Natural Science Foundation of China (91023008), Ph. D. Programs Foundation of Ministry of Education of China (20100074110002), the Fundamental Research Funds for the Central Universities, and Shanghai Leading Academic Discipline Project (B502) is gratefully acknowledged. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1.

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Page 24 of 24

Table of Contents Graphic and Synopsis Highly ordered metal oxide nanorods inside mesoporous silica supported carbon nanomembranes: high performance electrode materials for symmetrical supercapacitor devices Jian Zhi, Sheng Deng, Youfu Wang, Aiguo Hu*


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Highly Ordered Metal Oxide Nanorods inside Mesoporous Silica

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