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Significantly Enhanced Energy Density in Nanocomposite Capacitors Combining the TiO2 Nanorod Array with Poly(vinylidene fluoride) Lingmin Yao, Zhongbin Pan, Shaohui Liu, JiWei Zhai, and Haydn H.D. Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09265 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016

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ACS Applied Materials & Interfaces

Significantly Enhanced Energy Density in Nanocomposite Capacitors Combining the TiO2 Nanorod Array with Poly(vinylidene fluoride) Lingmin Yaoa, Zhongbin Panb, Shaohui Liub, Jiwei Zhaib*, Haydn H.D. Chena* a

Institute of Applied Physics and Materials Engineering, Faculty of Science and

Technology, University of Macau, Macao SAR 999078, China b

Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education,

Functional Materials Research Laboratory, School of Materials Science & Engineeri g,Tongji University, 4800 Caoan Road, Shanghai 201804, China.

ABSTRACT A novel inorganic/polymer nanocomposite, using 1-dimensional TiO2 nanorod array as fillers (TNA) and poly(-vinylidene fluoride) (PVDF) as matrix, has been successfully synthesized for the first time. A carefully designed process sequence includes several steps with the initial epitaxial growth of highly oriented TNA on the fluorine-doped tin oxide (FTO) conductive glass. Subsequently, PVDF is embedded into the nanorods by spin coating method followed by annealing and quenching processes. This novel structure with dispersive fillers demonstrates a successful compromise between the electric displacement and breakdown strength, resulting in a dramatic increase in the electric polarization which leads to a significant improvement on the energy density and discharge efficiency. The nanocomposites with various height ratios of fillers between the TNA and total film thickness were investigated by

a

Faculty of Science and Technology, University of Macau Macau SAR, China. E-mail address: [email protected] (Haydn

H.D. Chen) b

School of Materials Science & Engineerig,Tongji University, 4800 Caoan Road, Shanghai 201804, China. E-mail address:

[email protected] (Jiwei, Zhai)

ACS Paragon Plus Environment


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us. The results show that nanocomposite with 18% height ratio fillers obtains maximum increase in the energy density (10.62 J cm-3) at a lower applied electric field of 340 MV m-1. And it also illustrates a higher efficiency (>85%) under the electric field less than 100 MV m-1. Even when the electric filed reached 340 MV m-1, the efficiency of nanocomposites can still maintained at ~ 70%. This energy density exceeds most of the previously reported TiO2-based nanocomposite values at such a breakdown strength, which provides another promising design for the next generation of dielectric nanocomposite material, by using the highly oriented nanorod array as fillers for the higher energy density capacitors. Additionally, the finite element simulation has been employed to analyze the distribution of electric fields and electric flux density to explore the inherent mechanism of the higher performance of the TNA/PVDF nanocomposites.

KEYWORDS: Energy density, Capacitors, Nanorod arrays, Titanium oxides, Nanocomposites, Poly(vinylidene fluoride)


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1. INTRODUCTION Owing to the ever-increasing concerns of environmental problems caused by the exhaust of fossil fuels, research of energy storage system using the sustainable and renewable resources has attracted considerable attention. Among the various energy storage

systems,

the

dielectric

capacitors

with

the

merit

of

ultrafast

charging-discharging ability have become an important technology which would greatly benefit the high performance power electronics used in military power system, hybrid electric vehicles and some potable electronics

1-4

. However, compared with

some electrochemical energy storage such as the lithium ion battery and supercapacitor, the current dielectric capacitor suffers from the relatively lower energy density. In order to realize the large scale application of dielectric capacitors, it is critical to find solutions to significantly enhance its energy density. In general, the energy density in dielectric capacitor can be written in the form (1)

=

where E is the applied electric field, and D is the electric displacement. Specially, when it comes to the linear dielectrics, the total energy Ue =

, where

E, , denote the breakdown strength, the relative dielectric constant and the vacuum permittivity, respectively. In consideration of the fact that is proportional to the square of the electric field, organic polymers with higher breakdown strength as well as lower dielectric loss initially play a dominant role in dielectric capacitors. Furthermore, the formability and easy processability are helpful for the polymer-based dielectric capacitors to be made into various shapes and forms, readily embedded into the electronic devices, hence enabling the continuing miniaturization of electronic devices. However, the practical polymer dielectric materials, such as the biaxially oriented polypropylene (BOPP) film and polypropylene (PP), are impeded by its



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inherent lower dielectric permittivity (generally Tg. This phenomenon becomes more obvious in higher frequency (1MHz). The shift of the dielectric relaxation peak towards a lower temperature and the reduced dielectric loss tangent at higher frequency are indicative of interface polarization interaction and an increased trap density caused by the incorporation of TO nanorod array into the PVDF 12. The electric breakdown strength was determined using the Weibull analysis after testing 12 samples with various filler height ratios (inserted in Fig.6d). The average breakdown strength could be obtained by the / intersect of the fitting line. The breakdown strength of the bare PVDF is 410 MV m-1. When the height ratio reaches 0.18, the value still could maintained at 344 MV m-1 though it decreases in other nanocomposites with various height ratios. The energy density was calculated according by the aforementioned integration of Equation 1. Figure 6 compares the (D-E) loops (a) before breakdown and energy density (b) of the TNA/PVDF nanocomposite and bare PVDF. Actually, the incorporation of TO nanorod array with


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a homogenous dispersion and highly orientation in the polymer matrix would induce a significant electrical polarization and moderated breakdown strength, thus contributing to the final higher energy density of the nanocomposite. For the nanocomposite with h0.18, h0.25 and h0.36, the maximum polarization can reach to 8.57, 7.91, 7.85 µC cm-2, respectively, but the value is 3.82 µC cm-2 for the PVDF. Under the electric field of 150, 250, 300 and 400 MV m-1, the electric displacement of nanocomposites in h0.18 increases to 2.73, 5.70, 7.30 and 8.57 µC cm-2, respectively, but this value of bare PVDF with the similar condition is 1.46, 2.5, 3.0 and 3.82 µC cm-2, respectively (Figure 6c). The highly oriented nanorod array might play a dominant role in the contribution to the dramatic increase of the polarization besides the interfacial polarization in the binary nanocomposite. Consequently, the maximum discharge energy density, a more important parameter to characterize the effective energy storage capability of dielectric material, increases from 3.02 J cm-3 for bare PVDF to 10.62 J cm-3 for TNA/PVDF nanocomposite with h0.18 at the 340 MV m-1, respectively (Figure 6b). A comparison of the discharge energy density as a function of electric field is illustrated in Figure 6b. The maximum discharge energy density of nanocomposites with h0.36 is 2.2 J cm3 at 100 MV m-1 and that value is 4.1 J cm3 for h0.25 at 200 MV m-1. But the nanocomposite with h0.18 could obtain the energy density as high as 10.62 J cm-3 at the breakdown strength of 340 MV m-1, which is almost 3.5 times than the magnitude of PVDF under the same condition. Particularly, to the best of the authors’ knowledge, the energy density of 10.62 J cm-3 is by far the highest value achieved for similar condition in TO-based nanocomposites. Now the majority of the reported nanocomposite that could obtain higher energy density >10 J cm-3 relies strongly on the two following aspects rather than the high dielectric constant: Ⅰ) the outstanding higher breakdown strength and some even exceed the value of polymer itself; Ⅱ) the



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polymer generally used in the experiments itself is characterized by the higher energy density. For instance, Nan9 demonstrated that the BTO@TO nfs could reach a discharged energy density of ≈20 J cm-3 at the enhanced electric breakdown strength of 646 MV m-1. And the energy density is about 8 J cm-3 at 400 MV m-1. Another higher energy density value of 21.2 J cm–3 also has been obtained in a ferroelectric copolymer nanocomposite, which generally benefits from polymer itself and the total loading of nanoparticles is ≈27 wt.% (17–25 J cm-3 could be reached for P(VDF-CTFE) manufactured via melt-stretching and extrusion-blowing processes) 32. Though Tang26 has reported that the energy density of nanocomposite with 7.5 vol.% TiO2 could reach 12.4 J cm-3 at 450 MV m-1, it also should be noted that this value relies strongly on the larger breakdown strength ( 450 MV m-1). Furthermore, the PVDF employed in their experiments exhibited by itself an excellent performance (about 10 J cm-3 at the 450 MV m-1). Particularly, the maximum displacement in our nanocomposite reached at a value as high as 8.57 µC cm-2 at 340 MV m-1, almost the same as the value of 450 MV m-1 in Tang’s report. Even compared with the other inorganic/polymer nanocomposite, the value of our energy density and the increment, though is not the highest so far, exceed most of the previous studies in similar conditions. The discharge efficiency η of a capacitor is another important factor in the practical application(η =

123456 768 173298 9:95; 78 173298

). The TNA/PVDF nanocomposites

with h0.18 illustrates a higher efficiency (>85%) under the electric field less than 100 MV m-1. Even when the electric filed reached 340 MV m-1, the efficiency of nanocomposites can still maintained at ~ 70%. However, efficiency of PVDF decreased dramatically to 48% at the 340 MV m-1. Consideration of the practical application in pulsed power source, the TNA/PVDF nanocomposite was evaluated by


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charge-discharge test using similar circuit in the literature (Figure 7)

16, 34

. The

capacitor made of TNA/PVDF nanocomposite was initially charged under 100 MV m-1, and then discharged at an oscillation circuit. τ.= is defined as the time when the capacitor releases 90% of its total energy into the load. Figure 7a shows that the nanocomposites can release the energy in an extremely short time duration in the order of "μs". When the capacitor was charged at the electric field of 100 MV m-1, the released energy is ~ 0.9 J cm-3 (Figure 7b), which agreed well with the calculation result from the hysteresis loop (Figure 6b). In order to explore the reasons behind this high performance of the nanocomposites in energy density, the distribution of electric fields and electric flux density was simulated by the ANSYS (Figure 8) referring the height ratio of 0.18 . The deeper the color, the stronger the trend when follows the sequence of array. It is worthwhile to noting that weak electric fields are formed on the bottom region containing TO nanorod array when the interspace among TO nanorods is full of PVDF (Figure 8a). Actually, the electric fields at the interface between the fillers and matrix is indeed weaken than that of PVDF above nanorod, which is related to the lower dielectric contrast between the fillers and matrix. Therefore, the breakdown strength of the nanocomposites is mainly determined by the PVDF above the nanorods. Particularly, the electric flux density keeps a lower value in the most area except the edge region located by the TO nanorod (in red color of Figure 8b). Ultimately, experimental breakdown strength of the nanocomposites reduces a little when compares with the bare PVDF, but the TO nanorod array could enhance significantly the electric displacement to increase the energy density of the nanocomposites. Therefore, the reasons for higher performance of nanocomposites in energy density might include the following aspects:Ⅰ) choosing the non-ferroelectric TO as fillers so as to obtain a lower remnant polarization effectively in the whole test, thus increasing



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the effective releasable energy density; Ⅱ) the TO with quite low dielectric constant helps to improve the compatibility between the fillers and the polymer, subsequently weakening the interfacial electric field imposed on the matrix; Ⅲ) the isolated nanorod array as fillers not only has a perfect dispersion in the polymer to reduce the construction of conductive network caused by filler agglomeration at higher electric field, but also efficiently reduces the contact surface area in parallel to the electric field between the TO and the polymer. Therefore, the newly designed structure as presented in this work bears relatively high breakdown strength even though the TO has not been modified by any functional group; More importantly, the highly oriented TO nanorod array made dramatic contribution to the higher dielectric constant (~20) and thus improving the electric polarization of the nanocomposite despite of the reduction of the interface polarization.

4. CONCLUSIONS A novel designed nanocomposite, using the TO nanorod array as fillers and PVDF as matrix, was obtained for the first time. Highly oriented TO nanorod array was initially synthesized by the hydrothermal reaction. Subsequently, the polymer was embedded into the nanorod interspace and on the nanorod surface by spin-coated method followed by an annealing and quenching process. And this new structure has demonstrated a dramatically increase in the electric polarization, thus leading to significant improvement on the energy density with the maximum energy density of 10.62 J cm-3 under a breakdown strength of 340 MV m-1 with fillers of h0.18. It is around 3.5 times than that of the PVDF at the same condition. The increment of the energy density almost exceeds most of the previously reported values in similar condition for the TO-based nanocomposite. Particularly, this new method does not need any modification of the surface or some other complicated treating process.


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Considering the extremely simplicity of process, the material diversity, low-cost, more importantly the dramatically increment in the energy density of 10.62 J cm-3 at 340 MV m-1, it provides another novel potential method to synthesize the inorganic/polymer nanocomposite so as to obtain the improved energy density.

Associated content Supporting information Three ideal models for nanocomposites and temperature dependence of the dielectric constant and loss tangent of bare PVDF and nanocomposites measured from −50 to 130 °C

Acknowledgments This work was financially supported by the RDG007/FST-CHD/2012 at the University of Macau and the Ministry of Sciences and Technology of China through 973-project under Grant (2015CB654601)



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Figure 1 synthetic process of the TNA/PVDF nanocomposite Figure 2 XRD patterns of FTO (a), TO nanorod array (b), and TNA/PVDF composite with h0.18(c) Figure 3 SEM images taken from: (a) top view of the TO nanorod array; (b) cross-sectional view of the TO nanorod array; (c) cross-sectional view of the TNA/PVDF nanocomposite with h0.18; (d) the magnification image of the TNA/PVDF nanocomposite with h0.18; (e) the EDS spectrum of the TO nanorod; (f) and (g) TEM images of the TO nanorod Figure 4 The infrared spectroscopy of TNA/PVDF nanocomposite with h0.18 (red color) and the PVDF spin coated on the FTO (green color). Figure 5 Comparison of measured dielectric constant (a) and loss tangent (b) at the frequency ranging from 0.1 Hz to 1 MHz of TNA/PVDF nanocomposite and bare PVDF Figure 6 (a) Electric displacement−electric field (D−E) loops of TNA/PVDF composite and bare PVDF before breakdown strength; (b) energy density in various electric field measured at 100 Hz; (c) D-E loops of PVDF and TAN nanocomposites with h0.18; (d) Breakdown strength and maximum polarization of the TAN/PVDF nanocomposites and bare PVDF, and the inserted one is the Weibull plots of electric strength Figure 7 Charge-discharge performance of TNA/PVDF nanocomposite with h0.18 at 100 MV m-1: (a) the discharge curve; (b) the discharge energy density. Figure 8 Simulation on the distribution of electric fields (a) and electric flux density(b).



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

314x222mm (96 x 96 DPI)


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Page 31 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


314x222mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

314x222mm (96 x 96 DPI)


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Significantly Enhanced Energy Density in ... - ACS Publications

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