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Poly(vinylidene fluoride) Flexible Nanocomposite Films with Dopamine Coated Giant Dielectric Ceramic NanopowdersBa(Fe0.5Ta0.5)O3 for High Energy Storage Density at Low Electric Field Zhuo Wang, Tian Wang, Chun Wang, Yujia Xiao, Panpan Jing, Yongfei Cui, and Yongping Pu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08664 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Poly(vinylidene fluoride) Flexible Nanocomposite Films with Dopamine Coated Giant Dielectric Ceramic Nanopowders-Ba(Fe0.5Ta0.5)O3 for High Energy Storage Density at Low Electric Field Zhuo Wang*, Tian Wang, Chun Wang, Yujia Xiao, Panpan Jing，Yongfei Cui and Yongping Pu

*School of Materials Science and Engineering, Shaanxi University of Science & Technology, 710021 Xi’an, China.

AUTHOR INFORMATION

Corresponding Author

(*Zhuo Wang) E-mail: [email protected].

KEYWORDS

BFT@DA/PVDF nanocomposite films• hole defect• interfacial polarization• energy storage density

1

ACS Paragon Plus Environment


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ABSTRACT

Ba(Fe0.5Ta0.5)O3/poly(vinylidene fluoride) (BFT/PVDF) flexible nanocomposite films are fabricated by tape-casting using dopamine (DA) modified BFT nanopowders and PVDF as matrix polymer. After surface modification by DA layer with a thickness of 5 nm, the interfacial couple interaction between BFT and PVDF is enhanced, resulting in less hole defects at the interface. Then the dielectric constant (ε'), loss tangent (tan δ) and AC conductivity of nanocomposite films are reduced. Meanwhile, the value of reduced dielectric constant (∆ε') and strength of interfacial polarization (k) is introduced to illustrate the effect of DA on dielectric behavior of nanocomposite films. ∆ε' can be used to calculate the magnitude of interfacial polarization and the strength of dielectric constant contributed by interface can be expressed as k. Most importantly, energy storage density and energy storage efficiency of nanocomposite films with a small BFT@DA filler content of 1 vol % at low electric field 150 MV/m are enhanced by about 15 % and 120 % respectively after DA modification. The high energy storage density 1.81 J/cm3 is obtained in the sample. This value is much larger than the reported polymer-based nanocomposite films. In addition, the outstanding cycle and bending stability of the nanocomposite films make it a promising candidate for future flexible portable energy devices.

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1. INTRODUCTION Electrostatic capacitors have been widely applied in electronic devices and play an important role in advanced pulsed power generation, high-frequency inverters, etc, due to its fast charge and discharge speeds and high power density.1–3 Polymer capacitors with high energy storage density are attracting increasing fundamental interests in recent year.4,5 At present, the energy storage density of most commercially available electrostatic capacitors is 1-2 J/cm3.6 However, this density can only be achieved under a high electric field. For instance, the energy storage density of biaxially oriented polypropylenes (BOPP) is 1.8 J/cm3 at an electric field as high as 440 MV/m.7 The disadvantages of the high electric field lie in two reasons. On the one hand, the reliability of capacitor is a great challenge. When capacitors are applied under high electric field, the probability of failure will significantly increase. On the other hand, the applications of electrostatic capacitors may be substantially limited by the high electric field required, making them not viable choices for a number of electronic devices operated under low voltage.8,9 Therefore, it is urgent to develop a novel material component system for high energy storage density under a low electric field. In general, the energy storage density of dielectric materials is expressed as: U = ∫ EdP

(1)

and for linear dielectrics, it can be derived from: U = 1/ 2ε0ε ' E2

(2)

where U is the energy storage density, E is the electric field, P is the polarization, ε0 is the vacuum dielectric constant (8.85×10-12 F/m), ε' is the dielectric constant of materials, respectively.10,11 Obviously, the energy storage density is dependent on E and ε'. Pure polymer materials are easy to 3



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be applied under a high electric field, but its low dielectric constant becomes a great weakness for energy storage applications.12 In this condition, ceramic/polymer nanocomposite films emerge as the times required. Utilizing the properties of both, polymers exhibit high breakdown strength, while the ceramics have a high dielectric constant.13 In recent years, a large number of polymers have been used to prepare polymer nanocomposites, such as: polystyrene (PS),14 poly(ethylene terephthalate) (PET),15 polymethyl methacrylate (PMMA),16 polyimide (PI),17,18 etc. However, poly(vinylidene fiuoride) (PVDF) has received much attention due to that its dipoles are in one direction in all-trans β-PVDF, giving rise to polar behavior.19 And it has a dielectric constant of up to 10.20 Meanwhile, ceramic nanopowders with high dielectric constant, such as: BaTiO3 (BT),21–25 Ba0.6Sr0.4TiO3 (BST)26 and Pb(Zr,Ti)O3 (PZT)27 are introduced into the polymer matrix to form polymer nanocomposites. Nevertheless, the value of dielectric constant of the nanocomposites (usually ε'＜50) is limited by the dielectric constant of ceramic fillers, which is closely associated with the energy storage density of nanocompoites at low electric field. For example, Yu K et al. have investigated that the dielectric constant of PVDF-based nanocomposites at 1 kHz increased from 10 to 40 after blending with 50 vol % of BT nanopowders (ε'BT＜ 2×103).28 Thus, it is essential to develop a new filler of PVDF-based nanocomposites. Previous works of our group have revealed that the dielectric constant of Ba(Fe0.5Nb0.5)O3 (BFN)/PVDF nanocomposites is as much as 65 with 50 vol % of BFN nanopowders (ε'BFN＞104).29–31 Compared with BFN, Ba(Fe0.5Ta0.5)O3 (BFT) ceramics have not only giant dielectric constant but also low loss tangent and excellent frequency stability (Figure S1).32 The remarkable dielectric properties of BFT ceramics make BFT nanopowders an excellent candidate of the filler in ceramic/polymer nanocomposites. 4


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As nanopowders with giant dielectric constant are added into polymers, agglomerated charges at hole defects between nanoparticles and polymer are quite serious. Hence, surface modification of nanopowders is extremely essential in nanocomposites. Meanwhile, the surface modification of nanopowders by coupling agents has become a focus in relative research recently, such as hydrogen peroxide (H2O2),33 polyvinylpyrrolidone (PVP),34 aminopropyl triethoxy silane (KH550),35 fluoroalkyl acrylate.36 Although the surface modification leads to a sharp drop in dielectric constant, it can achieve a low loss tangent at the same time.37 And it will be of benefit to improve energy storage efficiency to obtain a high energy storage density at low electric field. Dopamine (DA) as binding agents for surface modification can be synthesized from lots of living species.38 It is reported that DA performs greatly for coating ceramic surfaces.39–43 Up to now, more and more attentions have been paid on improving the dielectric constant when the giant dielectric constant fillers are added into polymers. It is due to that polymers added giant dielectric constant fillers result in seriously agglomerated charges at hole defects between nanoparticles and polymers, which lead to the sharp decline in breakdown strength of nanocomposites. Thus, these kinds of nanocomposites are not considered to use in energy storage density. Nevertheless, coupling agent can establish bindings with nanoparticles and polymers to reduce the agglomerated charges. Giant dielectric constant ceramic/polymer nanocomposites with high energy density under low electric field can be possible for applications. In this contribution, we report a general strategy for improving energy storage density under low electric field in polymer nanocomposite films. The nanocomposite films, consisting of polymer matrix (PVDF), nanopowders fillers (BFT) and coupling agent (DA) are prepared via tape-casting. The crystal structure, microstructure, mechanical properties and dielectric performance of BFT@DA/PVDF 5



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nanocomposite films are discussed in detail. We hope to open the doors to improve the applications of regarding giant ceramic nanopowders as the filler.

2. EXPERIMENTAL SECTION 2.1. Materials. Oxalic acid dehydrate ((COOH)2•2H2O), iron nitrate nonahydrate (Fe(NO3)3•9H2O),

barium

nitrate

(Ba(NO3)2),

ammonium

hydroxide

(NH4OH),

N,N-dimethylformamide (DMF) were supplied from Sinopharm Chemical Reagent Co., Ltd. Tantalum chloride (TaCl5) and dopamine (DA) hydrochloride were purchased from Alfa Aesar Co., Ltd. Poly(vinylidene fluoride) (PVDF, Mw=534,000) was obtained from Sigma-aldrich Co., LLC. 2.2. Preparation of the BFT nanopowders. BFT nanopowders were prepared by an oxalate co-precipitation route. First, TaCl5 was dissolved in ethanol, (COOH)2•2H2O, Fe(NO3)3•9H2O and Ba(NO3)2 were dissolved in deionized water, respectively. Next, the solutions were completely mixed under stirring. Then, the pH value of the solution was adjusted to 10.0 by NH4OH. After centrifugation and drying, the powder was calcined at 1000 oC for 2 h. 2.3. Surface modification of BFT nanopowders. In order to add hydroxyls on the surface of BFT nanopowders, the nanopowders were dispersed in a mixture of deionized water and ethanol. After drying, BFT nanopowders were dispersed in 0.01M of dopamine hydrochloride and stirred for 10 h at 60 oC to acquire the core-shell structured BFT@DA nanopowders. 2.4. Preparation of the BFT@DA/PVDF nanocomposite films. For the fabrication of the nanocomposite films, the BFT@DA nanopowders and PVDF were dispersed into DMF and ultrasonically treated for 2 h. After stirring for 12 h, the stable suspension was casted on a glass substrate and dried at 100 oC for 12 h for the evaporation of DMF. Subsequently, the films were heat treated at 200 oC for 7 min followed by quenching in cold water rapidly. Finally, the flexible 6


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films were peeled from the glass substrate and dried at 50 oC overnight. The thickness of the final nanocomposite films was about 15-25 µm. The schematic diagram of the fabrication of BFT@DA/PVDF nanocomposite films is shown in Figure 1.

Figure 1. Schematic diagram for modified mechanism of core-shell structured BFT@DA/PVDF nanocomposites. 2.5. Characterization. XRD patterns of samples were collected on an X-ray diffractometer (Rigaku D/max-2200PC). Surface modification of BFT nanopowders could be characterized by TEM (FEI TE). The surface composition of samples was also examined by FTIR (Bruker VECTOR-22) and XPS (Thermo Fisher K-Alpha). The cross-sectional morphology of the nanocomposite films was obtained by using SEM (Hitachi S-4800). Before the test, the films were fractured in liquid nitrogen, and sputtered with a uniform gold layer. The mechanical properties of the nanocomposite films were performed on a tensile tester (Baoda 1036PC). The dielectric constant and loss tangent of the nanocomposite films were obtained by an impedance analyzer (Agilent E4980A) at the frequency range from 100 Hz to 2000 kHz. Polarization-electric field (P-E) loops and leakage current were obtained by using a ferroelectric test system (Radiant Premier II). Prior to measurement, the gold electrodes were sputtered on both sides of the films. 3. RESULTS AND DISCUSSION 3.1. Microstructure and morphology of BFT nanopowders. The BFT nanopowders are 7



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examined using SEM to observe the morphology and size dispersion of BFT particles. Figure 2(a) shows that the nanometer scale BFT nanopowders are prepared by oxalate coprecipitation method. The particle size ranges from 60 nm to 130 nm and the average size is 90.90 nm.

Figure 2. (a) SEM image of pristine BFT nanopowders, the inset is the particle size distribution; (b) TEM image of BFT@DA nanopowders, the inset is the large view; (c) XRD patterns, (d) FT-IR spectra and (e, f) XPS spectra of BFT and BFT@DA nanopowders. The inset in (c) shows the schematic diagram of crystal structure of BFT. 3.2. Microstructure and morphology of surface modification of BFT nanopowders. As shown in the TEM image of BFT@DA nanopowders (Figure 2(b)), nano-sized BFT particles are surrounded by the DA coatings with a uniform thickness of 5 nm. The XRD patterns of pristine 8


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BFT nanopowders and BFT nanopowders modified by DA are given in Figure 2(c). The diffraction peaks assigned to BFT nanopowders at 31o (100), 45o (200), and 55o (211) belongs to —

cubic perovskite structure with space group Pm3m and the crystal structure of BFT is given in the inset of Figure 2(c).44 The peaks of secondary phases are not found, neither super-lattice reflections nor reflection splitting are detected. After DA modification, the XRD pattern is similar with that of pristine BFT. Figure 2(d) shows the FT-IR transmittance spectra of pristine BFT and BFT@DA nanopowders. The peak at 588 cm-1 can be assigned to the vibrations of Fe-O and Nb-O bonds, which is generated from the cubic perovskite structure of BFT. After surface modification, the peaks at 1279 cm-1, 1495 cm-1 and 1630 cm-1 can be assigned to aromatic amine C-N stretching vibrations, aromatic C-C stretching vibrations and N-H bending vibrations, respectively, which demonstrates the existence of aromatic and amido groups from DA.40 To further characterize the surface modification, XPS spectra is shown in Figure 2(e) and (f). The existence of N 1s signal proves the presence of nitrogen-containing groups after DA modification, which is originated from DA. The O 1s (O-H) signal is converted into the O 1s (O-C) signal after DA modification. It is suggested that DA is bonded with BFT particles through (BFT)-O-C bonding.40 3.3. Microstructure, morphology and mechanical properties of BFT@DA/PVDF nanocomposite films. The density of BFT/PVDF and BFT@DA/PVDF nanocomposite films is shown in Figure 3(a). The theoretical density increases linearly with the increase of the volume fraction of BFT, and the real density of nanocomposite films show the same tendency. However, compared with BFT/PVDF, BFT@DA/PVDF nanocomposite films exhibit the higher densities, illustrating BFT@DA/PVDF nanocomposite films are denser. It proves that there are less hole defects after DA modification. The defects between ceramic nanopowders and polymer should 9



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have a dominant influence on the dielectric properties of the composite materials.45 The insets in Figure 3(a) show the macroscopic images of the BFT@DA/PVDF nanocomposite film with 1 vol % BFT@DA naopowders. It is found that the high-quality nanocomposite film shows good transparence and flexibility.

Figure 3. (a) Density and (b) XRD patterns of BFT/PVDF and BFT@DA/PVDF nanocomposite films, the insets in (a) shows the macroscopic images of BFT@DA/PVDF nanocomposite films with 1 vol % BFT; Cross-sectional SEM images of (c) BFT/PVDF and (d) BFT@DA/PVDF nanocomposite films with 7 vol % BFT, respectively, the insets in (c) and (d) show the corresponding films at low magnification; (e, f) mechanical properties of BFT/PVDF and BFT@DA/PVDF nanocomposite films. 10


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Figure 3(b) presents the XRD patterns of BFT/PVDF and BFT@DA/PVDF nanocomposite films. In addition to the characteristic peaks of perovskite type belonged to BFT, the peaks assigned to PVDF are observed. The strong peaks at 2θ = 16.7° (100), 18.5° (020), 20.0° (110) and 26.7° (021) are corresponded to α-PVDF, the peak at 2θ = 20.8° (110, 200) is corresponded to β-PVDF.46 PVDF exists in two different crystalline forms, which are depended on the preparation conditions, such as: solvent, method of casting, melt temperature, stretching of films and annealing conditions.47 Compare to the peaks of BFT/PVDF, the intensity of these peaks get stronger after DA modification, which indicates a higher crystallinity. With the volume fraction of BFT increasing, the intensity of PVDF peaks shows an evident reduction, which can be ascribed to the shielding effect from high intensity of diffraction peaks in BFT.46 Figure 3(c) and (d) present cross-sectional SEM images of the PVDF-based nanocomposite films containing 7 vol % of BFT/PVDF and BFT@DA/PVDF, respectively. The insets in Figure 3(c) and (d) show that the average thickness of nanocomposite films is about 20 µm. It can be observed that BFT nanopowders are mostly dispersed well in PVDF without serious aggregation. Compared with the cross-sectional morphology of nanocomposite films before DA modification, the SEM micrograph of BFT@DA/PVDF exhibits a whole different morphology. BFT nanopowders are exposed outside before DA modification and many hole defects can be observed near the interface between BFT nanoparticles and PVDF. In contrast, BFT nanopowders are buried inside after DA modification and no obvious hole defects are noticed. These differences should be related to the surface modification with DA and key factor DA playing its role is the couple interaction between BFT and PVDF. Before DA modification, the loose contaction between inorganic BFT nanoparticles and organic PVDF results in the more hole defects. It is easy to 11



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fracture at contacting interfaces in liquid nitrogen. After DA modification, C-O-C bonds are formed between BFT nanoparticles and DA. The N-H…F hydrogen bonds are formed between PVDF and DA. DA can establish bindings with BFT nanoparticles and PVDF, which results in less hole defects between BFT nanoparticles and PVDF. Furthermore, more hole defects are observed in BFT/PVDF nanocomposite films accounts for its lower density obtained (Fig. 3(a)). To characterize the flexibility of the nanocomposite films, mechanical properties of BFT/PVDF and BFT@DA/PVDF are characterized and shown in Figure 3(e) and (f). The tensile strength and elongation at break decrease with the increasing of volume fraction of BFT. More BFT nanoparticles added into PVDF mean more interfaces formed and will induce more defects, which leads to the deterioration of mechanical properties. In addition, compared with BFT/PVDF, BFT@DA/PVDF nanocomposite films show higher tensile strength and larger elongation at break. This is attributed to the excellent coupling action of DA, establishing strong bindings between BFT nanoparticles and PVDF. 3.4. Dielectric constant, loss tangent, AC conductivity and P-E loops of BFT@DA/PVDF nanocomposite films. The dielectric properties of BFT/PVDF and BFT@DA/PVDF nanocomposite films are shown in Figure 4. As can be observed from Figure 4(a) and (b), frequency dependence of dielectric constant and loss tangent in two kinds of nanocomposite films show a same tendency. The dielectric constant of all nanocomposite films slowly decreases with the increase of frequency at 100 Hz-10 kHz. It is believed that the decrease of dielectric constant is mainly caused by the reduction in Maxwell-Wagner-Sillars interfacial polarization.48 It is originated from the regions of accumulated trap charges induced by defects in nanocomposite films. As the frequency rotation increasing, the interfacial polarization begins to fall behind the 12


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frequency of the AC electric field, which leads to the decreasing of interfacial polarization. Thus, the dielectric constant decreases with increasing frequency. However, in the higher frequency range above 10 kHz, a distinct decrease with the frequency increasing presents, which is due to the reduction in dipolar orientation polarization.19 Dipoles of the PVDF respond to the alternating external field, resulting in orientation polarization in nanocomposite films. As with interfacial polarization, this kind of polarization lags behind the change of frequency, which is the reason why the dielectric constant decreases in high frequency range. Moreover, the loss tangent decreases at first and then increases with increasing frequency. The reduced dielectric loss in the frequency range of 100 Hz-10 kHz is considered as interface relaxation polarization loss.37 The later increase of the loss tangent is ascribed with the dipolar relaxation polarization of PVDF, which is related to the micro-Brownian motion of the whole chain, known as α relaxation.49 Compared with the results of dielectric constant, the dielectric loss tangent of the all nanocomposite films shows a same result. Figure 4(c) and (d) present the comparison of the dielectric constant and loss tangent of BFT/PVDF and BFT@DA/PVDF nanocomposite films with different volume fractions of BFT at 100 Hz. It can be observed that both of dielectric constant and loss tangent continuously increases with the increase of BFT content. The highest dielectric constant is 19.4 at a small filler content of 7 vol % BFT, which is about 2 times higher than pure PVDF. The significantly enhanced dielectric constant with such a small filler content is on account of the giant dielectric constant of the BFT and the high interfacial polarization between nano-sized BFT and PVDF.34,50 Compared with nanocomposite films with pristine BFT nanopowders, BFT@DA/PVDF show a lower dielectric constant and loss tangent at the same filler content. It can be attributed to the reduced interfacial 13



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polarization. After DA modification, the strong couple interaction between BFT and PVDF is formed, and the hole defects are eliminated (Fig. 3(d)). Thus, the interfacial polarization originated from hole defects disappears, which lead to the reduction of dielectric constant and loss tangent in nanocomposite films. To better understand the effect of DA on dielectric behavior in nanocomposite films, Figure 4(e) presents the frequency dependence of reduced dielectric constant (∆ε') in BFT@DA/PVDF nanocomposite films with different volume fractions of BFT, which is calculated as:

∆ε ' = ε 'BFT / PVDF − ε 'BFT @ DA/ PVDF

(3)

where ε'BFT/PVDF and ε'BFT@DA/PVDF are the dielectric constant of two kinds of nanocomposites, respectively. It can be seen that the value of ∆ε' increase with an increase in filler content at the low frequency, which is due to that the more interfaces between BFT and PVDF are modified by DA at higher filler content. As we known, the dielectric constant of a polymer nanocomposite depends on the individual dielectric constant of BFT and PVDF along with different filler content and interactions among them.49 The dielectric constant of a nanocomposite originated from individual dielectric constant of fillers and polymer is invariable before and after DA modification. Thus, the reason for the differences is the different interactions among them. The intimate interaction between them is established after DA modification, which causes the difference in dielectric constant. Because of the more interfaces in higher filler content of BFT, the difference in dielectric constant is more pronounced. Additionally, the value of ∆ε' remains stable up to 10 kHz. When the frequency continues to increase, the value of ∆ε' tends to be the same and approximates to zero. It is related to the disappearance of interfacial polarization, which is consistent with the results of Figure 4(a) and (b). Overall, by this method, the magnitude of dielectric constant 14


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originated from interfacial polarization can be well calculated.

Figure 4. Frequency dependence of dielectric constant and loss tangent of (a) BFT/PVDF and (b) BFT@DA/PVDF nanocomposite films; comparison of (c) dielectric constant and (d) loss tangent of the BFT/PVDF and BFT@DA/PVDF nanocomposite films; (e) frequency dependence of the value of ∆ε' and (f) k with different volume fractions of BFT in BFT@DA/PVDF nanocomposite films. Figure 4(f) presents the strength of interfacial polarization (k) in BFT@DA/PVDF nanocomposite films with different volume fractions of BFT at 100 Hz, which is calculated as: k=

∆ε ' Ai

(4) 15



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where Ai is the interfacial area between BFT and PVDF in nanocomposite films. And Ai is calculated as:

Ai =

6 × d × S ×V

φ

(5)

where d is the thickness of nanocomposite films, S is the electrode area during testing the dielectric constant, V is the volume fraction of BFT, φ is the mean size of BFT particles. It can be seen that the value of k increases with an increase in filler content, which can be explained by the phenomenon that more hole defects are emerged in higher filler content. In other words, the more filler content of BFT, the looser bond between BFT and PVDF. Thus, small filler content and modified ceramic powders are completely necessary to get the strong bond between them. In a word, the strength of the dielectric constant contributed by interface can be well calculated by this method. The AC conductivity of the BFT/PVDF and BFT@DA/PVDF nanocomposite films with different volume fractions of BFT at 100 Hz is shown in Figure 5(a). It can be clearly seen that the conductivity of all nanocomposite films remains less than 10-8 S/m, which proved that all films still remain insulating because of the existence of PVDF.51 Meanwhile, the conductivity increases with an increase in filler content. The reason is that the average interparticle distance of BFT decreases exponentially with an increase in filler concentration.37 The comparison between the AC conductivity of nanocomposite films filled with different BFT fillers reveals the effects of surface modification on the AC conductivity behavior. As shown, the nanocomposite films with BFT nanopowders modified by DA at the same loading content show a lower AC conductivity. It can be explained that DA prevents the formation of conductive paths at the interface between BFT and PVDF. In order to probe the evolution, conduction schematic of the nanocomposite films is shown 16


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in Figure 5(b). When the pristine BFT nanopowders are added into PVDF, numerous hole defects between inorganic BFT and organic PVDF are created in diffuse layer. Under application of an electric field, because of the different dielectric constant of BFT and PVDF, charges accumulate near the hole defects. Charges can easily transfer from one particle to another particle and give rise to conductive paths in BFT/PVDF nanocomposite films (Fig. 5(b) left). On the contrary, when BFT nanopowders modified by DA are added into PVDF, coupled BFT and PVDF result in less hole defects. Under the application of an electric field, DA prevents the motion of accumulated charges and the conductive paths are difficult to formation (Fig. 5(b) right). Thus, BFT@DA/PVDF nanocomposite films exhibit a lower AC conductivity than BFT/PVDF.

Figure 5. (a) AC conductivity of BFT/PVDF and BFT@DA/PVDF nanocomposite films with different volume fractions of BFT; (b) conduction schematic of the nanocomposite films. Figure 6 presents P-E loops of the PVDF-based nanocomposite films filled BFT/PVDF and BFT@DA/PVDF with different filler loadings under different applied fields at 10 Hz. The detailed information of remnant polarization (Pr), maximal polarization (Pm), energy storage density (U) and energy storage efficiency (η) are listed in the inserts, respectively. Compared with BFT/PVDF, BFT@DA/PVDF nanocomposite films exhibit the narrow P-E loops. The lower maximum polarization and remanent polarization are derived from DA modification, which make a great contribution to the energy storage density and energy storage efficiency of BFT@DA/PVDF 17



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nanocomposite films. The lower maximum polarization is originated from lower dielectric constant. The relationship between the two is expressed as: 49

P = (ε '− 1)ε 0 E

(6)

εo is a constant. When E is the same, a lower ε' means a smaller P. The lower ε' obtained in BFT@DA/PVDF as illustrated in Figure 4(c) account for the smaller P in these samples. Moreover, the lower remanent polarization is due to the limited mobility of charge carriers induced by DA modification. These can be supported by the decrease of AC conductivity (Figure 5(a)) and leakage current (Figure S2). Therefore, BFT@DA/PVDF nanocomposite films exhibit higher energy storage density and energy storage efficiency. Most importantly, the energy storage density and energy storage efficiency of nanocomposite films with a small BFT@DA filler content of 1 vol % at low electric field 150 MV/m are enhanced by about 15 % and 120 % respectively after DA modification. The high energy storage density 1.81 J/cm3 is obtained in the sample. For comparative analysis, the energy storage properties of some other polymer films and polymer-based nanocomposite films are reported in Table 1, including BOPP,7 PVDF,52 BaTiO3/PVDF,53 BaTiO3@SiO2/PVDF,53 and BaTiO3@TiO2/PVDF.54 Although the high breakdown strength is obtained in these works, the energy storage density of nanocomposite films in this study is significantly higher at the same electric field of 150 MV/m. Most importantly, the filler content of BFT@DA is only 1 vol %, which is lower than the filler content requirement in these works. This advantage can be reflected by the competitive price in the future industrial applications.

18


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Figure 6. P-E loops of the PVDF-based nanocomposite films filled BFT/PVDF and BFT@DA/PVDF with different filler loadings under different applied fields at 10 Hz. Table 1. Energy storage density at 150 MV/m and breakdown strength of polymer and polymer-based nanocomposite single layer films. Compositions

Energy storage density (J/cm3)

Breakdown strength (MV/m)

References

BOPP

~0.2

~650

[7]

PVDF

~0.8

~300

[52]

2 vol % BaTiO3/PVDF

~1.3

~340

[53]

2 vol % BaTiO3@SiO2/PVDF

~1.4

~340

[53]

3 vol % BaTiO3@TiO2/PVDF

~1.25

646

[54]

1 vol % BFT@DA/PVDF

1.81

200

In this work

For an energy storage material, the energy storage density just under theirs breakdown strength electric field is also an important parameter. The detailed information is shown in Table 2. Compared with before DA modification, BFT@DA/PVDF nanocomposite films exhibit higher breakdown strength, energy storage density and energy storage efficiency. Most of all, the highest energy storage density 3.52 J/cm3 is obtained in the nanocomposite films with 1 vol % of BFT@DA and its the energy storage efficiency is 58.47 % at the electric field of 200 MV/m. 19



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Table 2. Breakdown strength, energy storage density and energy storage efficiency of the PVDF-based nanocomposite films at 10 Hz. Filler content

Breakdown strength (MV/m)

Energy storage density (J/cm3)

Energy storage efficiency (%)

BFT

BFT@DA

BFT

BFT@DA

BFT

BFT@DA

1

170

200

2.10

3.52

28.38

58.47

3 5 7

110 70 50

130 80 70

0.58 0.28 0.14

0.93 0.52 0.43

21.69 19.93 9.74

46.90 33.98 41.21

3.5. Fatigue tests for applications. It is quite necessary to carry out fatigue tests for advanced applications. Because of the excellent energy storage density of 1 vol % BFT@DA/PVDF nanocomposite films at low electric field, the energy storage density of this sample as a function of cycle number and bending number is presented in Figure 7. As shown in Figure 7(a), the energy storage density of this sample is basically remaining unchanged after 50 times of recycle (ten seconds as the spacing interval each time), indicating an outstanding cycle stability for applications. Energy storage density of this sample as a function of bending number is shown in Figure 7(b). The inset shows the bending angle and test area. It also remains stable after 50 times of bending. Thus, this kind of nanocomposite film has high potential in several applications, such as electrical power systems and modern electronics.

Figure 7. Energy storage density of 1 vol % BFT@DA/PVDF nanocomposite films as a function of (a) cycle and (b) bending number. The inset of (b) shows the bending angle and test area. 20


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4. CONCLUSIONS In order to improve energy storage density of Ba(Fe0.5Ta0.5)O3/poly(vinylidene fluoride) (BFT/PVDF) nanocomposite films at low electric field, the surface of BFT nanopowders is modified by dopamine (DA). TEM results show that BFT nanopowders are surrounded by the DA coatings with a uniform thickness of 5 nm. The reduced of dielectric constant, loss tangent and AC conductivity have been achieved, which is attributed to the reduced hole defects between BFT and PVDF arising from the surface modification by DA. Meanwhile, we adopt two new parameters, reduced dielectric constant (∆ε'), strength of interfacial polarization (k), to further understand the effect of DA on dielectric behavior in nanocomposite films. The energy storage density and energy storage efficiency at low electric field have been enhanced greatly due to DA modification. An energy storage density up to 1.81 J/cm3 is achieved in the nanocomposite films with a BFT@DA filler content of 1 vol % at a low electric field of 150 MV/m while its efficiency is 60.06 %. Furthermore the high energy storage density can be maintained well even after up to 50 times cycle and bending tests. The BFT@DA/PVDF nanocomposite films with excellent energy storage performance at low electric field and good stability in this work can be applied in future portable applications. SUPPORTING INFORMATION Figure S1 shows the frequency dependence of dielectric constant and loss tangent for BFT ceramics prepared by oxalate coprecipitation method sintered at 1350 oC for 3 h. Figure S2 shows the leakage current density of pure PVDF films and PVDF-based nanocomposite films at 50MV/m. ACKNOWLEDGEMENTS 21



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This work was supported by China Postdoctoral Science Foundation (2016T90881, 2015M572516), Graduate Innovation Fund of Shaanxi University of Science and Technology, Natural Science Foundation of Shaanxi Province (2016JQ5083) and National Natural Science Foundation of China (51572160). REFERENCES (1)

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