Effect of the Modifier Structure on the Performance of Barium Titanate

Oct 12, 2015 - Surface modification on ceramic fillers is of interest to help improve their compatibility in ceramic/polymer nanocomposites and, if po...
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Effect of the Modifier Structure on the Performance of Barium Titanate/Poly(vinylidene fluoride) Nanocomposites for Energy Storage Applications Yujuan Niu, Yuanyuan Bai, Ke Yu, Yifei Wang, Feng Xiang, and Hong Wang* School of Electronic and Information Engineering and State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT: Surface modification on ceramic fillers is of interest to help improve their compatibility in ceramic/polymer nanocomposites and, if possible, to control the influence of modifiers on the performance of the nanocomposites. In this paper, four kinds of small-molecule modifiers were chosen to treat the surface of BT nanoparticles, and the PVDF-based nanocomposites filled with the modified BT nanoparticles were prepared. The influences of modifiers on compatibility, permittivity, breakdown strength and polarization have been systematically investigated in order to identify the optimal surface modifier to enhance the energy density of the nanocomposites. Due to different structures (including type, number, and position of functional groups in molecules), the modifiers show different effects on the permittivity of the nanocomposites, while the breakdown strengths are all significantly improved. Consequently, the discharged energy densities of nanocomposites modified by 2,3,4,5-tetrafluorobenzoic acid and phthalic acid increase 35.7% and 37.7%, respectively, compared to BT/PVDF, indicating their potential as high energy density capacitors. KEYWORDS: modification, energy storage, nanocomposites, BaTiO3, poly(vinylidene fluoride)

1. INTRODUCTION Capacitors are one of the vital energy storage devices used in the modern electrical and electronics industry.1−4 Among the various capacitors (i.e., supercapacitors, electrochemical capacitors, and dielectric capacitors), dielectric capacitors have irreplaceable advantages such as that they offer unbeatable high charge and discharge speed (millisecond or microsecond) and an almost unlimited cycle life, so they are desired for energy storage applications in various power circuits and systems when high power delivery or uptake rate is essential.5,6 In a dielectric material, the electric energy density is limited to κEb2/ 2, where κ is the permittivity of the material and Eb is the breakdown strength. The traditional dielectric materials for capacitor applications include ceramics and polymers. Although ceramics feature a very high permittivity due to their large polarization,7 their applications are largely limited because of their low breakdown strength and poor manufacturability. Polymers possess high breakdown strength, but they have low permittivity.8 Nanocomposites comprising polymer matrix and ceramic fillers have become a strenuous topic of research for exploring their dielectric properties for energy storage applications.9−12 The idea underlying this nanocomposite approach is to integrate the complementary elements, such as the high breakdown strength of the polymer matrix and the high permittivity from the ceramic particles, for a substantially enhanced energy density.13,14 © XXXX American Chemical Society

A number of experimental and theoretical studies have been carried out to develop nanocomposites with high energy density through improving the permittivity and breakdown field.15 However, the high surface energy of the nanoparticle fillers usually leads to agglomeration and phase separation from the polymer matrix, which results in a decreased breakdown strength of the nanocomposites. How to solve the problems of the dispersibility of nanoparticles in polymers matrix and the compatibility between them is a long-standing challenge in developing nanocomposite materials. Surface modification for nanoparticles is a useful method to facilitate their dispersion in polymer matrix. Modification means introduction of organic coatings onto the surface of inorganic fillers, which can be carried out by utilizing physical and chemical interactions between the filler and the modifier. Normally modifier consists of two major components. One is a functional group, such as −OH, −NH2, −NR3z+, −COOH, −COO−, −SO3H−, −SO32−, and −PO43−, which helps the anchoring of the modifiers to the particles surface through hydroxyl and electrostatic bonds. The other is a solvable macromolecular chain, such as polyolefin, polyester, polyacrylate, and polyether, which is appropriate to be dispersed in Received: August 13, 2015 Accepted: October 12, 2015

A

DOI: 10.1021/acsami.5b07486 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Process of Nanoparticle Modification and Incorporation into the Polymer Matrix

different media.16 So far, a series of modifiers have been used to improve the dispersion of metal oxide nanoparticles in polymers,17−22 such as silanes,23 carboxylic acids,24 and phosphonic acids.25,26 Among the various commonly used modifiers, phosphonic acids are especially promising for surface modifications on various oxides including ITO,27 TiO2,28 ZrO2,29 Al2O3,30 and BaTiO3,31 since they form robust monolayers without the need to resort to cross-linking, as is common, for example, in silane surface modification.32−34 Although widely implemented in research, many available modifiers have their own limitations in practical use. For example, they hardly diffuse into the agglomerates owing to their macromolecular chain feature and do not have chemical specificity to the polymer matrix. In the past, the research only focused on the requirement for chemical specificity to the surface of nanoparticles, rarely taking the compatibility between modifier and the polymer matrix into account. In fact, we can adjust the functional group in the modifier molecular chain to improve its compatibility with polymer matrix and develop a kind of simple, effective, and targeted modifier. Because residual modifier can lead to high leakage current and high dielectric loss in nanocomposites, approaches to bind surface modifiers to nanoparticles via robust chemical bonds are highly desirable. Learning from the past literature, the research of carboxylic acids as modifiers for ceramic nanoparticles is relatively less. The main reason is that the coverage level of them binding to the surface of metal oxides is small.35 However, many results show that in nanocomposites high adsorption level of modifier will significantly lower the permittivity or lead to high leakage current and dielectric loss.16 On the other hand, adjusting the number of carboxyl groups in the modifier molecule can change

the coverage level of them on the surface of metal oxides. In this contribution, we report a series of novel small-molecule surface modifiers, 2,3,4,5-tetrafluorobenzoic acid, 4(trifluoromethyl)phthalic acid, tetrafluorophthalic acid, and phthalic acid (F4C, F3C2, F4C2, and C2 for short), which have low molecule weight and simple structure and belong to carboxylic acids. We studied the influence of function groups in modifier on the nanocomposites’ performance. The aim is to provide further insight into the relationship between the performance of nanocomposites and the choice of modifier. The work reported here significantly expands our previous family of surface modifiers,24,35 showing even wider tailoring on functional group.

2. EXPERIMENTAL SECTION 2.1. Materials. The solvents acetone, butanone, and ethanol were purchased from Letai Co., China. PVDF powder was obtained from Shanghai 3F New Materials Ltd., China. The surface modifiers were purchased from Alfa Co., China. BT nanoparticles with an average size of about 100 nm were purchased from Sinocera Co., China. All solvents and chemicals were used as received. 2.2. Preparation of Modified BT Nanoparticles (m-BT for Short). BT nanoparticles were added into an ethanol/water (95/5, v/ v) solution with ultrasonic for 30 min, and then the modifier was added. The mixture was ultrasonicated for 10 min, followed by stirring at 80 °C for 1 h. The nanoparticles were separated via centrifugation and rinsed repeatedly with excess ethanol/water solvent, then dried overnight under vacuum at 80 °C to get the modified BT nanoparticles. Scheme 1 summarizes the modification of nanoparticle and the preparation of the nanocomposite process. 2.3. Fabrication of the m-BT/PVDF Nanocomposite Films. An amount of 2 g of PVDF powder was dissolved in 15 mL of mixed solvent composed of 50% acetone and 50% butanone. The modified B

DOI: 10.1021/acsami.5b07486 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Comparison of the Physical Properties of the Four Modifiers

a

Calculated based on the Mr of grafting modifier and the weight loss of nanoparticles, considering each BT nanoparticle as a sphere with d = 100 nm. displacement−electric field (D-E) hysteresis loops measured by TF analyzer 2000 system (aix ACCT, Aachen, Germany).

BT nanoparticles were dispersed into the PVDF solution with different volume fractions, and the suspensions were ball-milled for 4 h. After ball-milling, the homogeneous suspension was tape-casted onto the PET substrate. The nanocomposite films on the substrates were standing at 80 °C for 30 min to evaporate the solvent followed by thermal treating at 200 °C for 1 h and then immediately quenched by ice−water. The thickness of the obtained nanocomposite films was 15−20 μm. 2.4. Characterization. Fourier transform infrared (FTIR) (Bruker Tensor27) spectroscopy differential scanning calorimetry (DSC) were carried out in N2 atmosphere, and thermogravimetric analyses (TGA) (NETZSCH STA449C) were carried out in air atmosphere for BT and modified BT nanoparticles. The microstructure of the nanocomposites was observed by a scanning electron microscopy (SEM, JSM-6460, JEOL, Tokyo, Japan). For electric measurement, gold electrodes with thickness of about 50 nm were sputtered on both sides of the film samples. Dielectric measurement covering a frequency range from 1 kHz to 10 MHz was carried out using an impedance analyzer (Agilent 4294A, Palo Alto, CA) at room temperature. The energy storage property was evaluated through the dielectric

3. RESULTS AND DISCUSSION 3.1. Characterization of Modified BT Nanoparticles and Nanocomposites. Table 1 presents the comparison of the physical properties of the four modifiers. Figure 1 shows the FTIR spectra of BT and m-BT nanoparticles. The spectrum of BT nanoparticles exhibits no water characteristic band due to drying process before test. After treatment by modifiers, the BT nanoparticles have more surface oxygen functional groups. The previous literature shows that when the carboxylic acid bonds with metal atom, the carboxy group will convert into carboxylate group.36 Meanwhile the characteristic bands of CO stretching vibration (νCO) will disappear, then present two new characteristic bands CO symmetrical stretching vibration (νsCO) and CO asymmetrical stretching vibration (νasCO) at lower wavenumber, and the νasCO band often has two or three peaks.35,37 As shown in Figure 1, the spectra of mC

DOI: 10.1021/acsami.5b07486 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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there are exothermic peaks for m-BT nanoparticles on the DSC curve (Figure 2b), which presents the degradation process of modifier from BT surfaces.36 Figure 2a also shows a small weight loss between 100 and 200 °C, corresponding to an endothermic peak on the DSC curve (Figure 2b) for F3C2BT and F4C2BT nanoparticles, which may result from the absorbed water on the surface of them.39 The fracture surface morphology of the nanocomposites filled with pristine BT and m-BT is investigated by SEM (Figure 3). As shown in the SEM images of m-BT/PVDF

Figure 1. FTIR spectra of BT and the surface modified BT with modifiers.

BT nanoparticles have three characteristic bands; they are attributed to the νsCO, νasCO, and C−F stretching vibration (νC−F). The results indicate that there are chemical bonds between the modifiers and the BT nanoparticles. Due to the influence of substituent in the modifier molecule, the positions of every characteristic band for different modifiers are not quite the same. For example, since the four fluorinated substituents on the aromatic ring in modifiers F4C and F4C2 are strong electron-withdrawing groups, the peaks of νsCO (at 1586 cm−1) for F4CBT and F4C2BT nanoparticles move to higher wavenumber compared with that of C2BT (at 1530 cm−1). In modifier F3C2, the three fluorinated substituent is on the branched chain of aromatic ring, so the ability to attract electron becomes weaker than in F4C and F4C2. And then the wavenumber of νsCO (at 1556 cm−1) for F3C2BT nanoparticles is lower than that of F4CBT and F4C2BT but still higher than that of C2BT. Figure 1 also shows another characteristic band of νC−F at about 1100 cm−1.26 Due to the impact of other functional groups and the different positions of fluorinated substituent to the modifier molecules, the wavenumber of νC−F for different m-BT nanoparticles is different. Figure 2 shows TG and DSC curves of BT and m-BT nanoparticles, which are collected in air atmosphere. As shown in Figure 2a, there are significant weight losses for m-BT nanoparticles above 300 °C. Due to only having one carboxyl group in modifier F4C, the weight loss of F4CBT is only 0.8% at temperature range from 310 to 470 °C. This explains why the peaks of infrared spectrum (Figure 1) are so small for F4CBT. The original BT nanoparticles also have a negligible weight loss in the temperature range 350−450 °C, which is derived from little bonded hydroxyl group on the surface of the BT nanoparticles.38 Corresponding to the TG plot change,

Figure 3. Morphology of PVDF nanocomposites filled with pristine BT and surface modified BT with modifiers: (a) 30% F4CBT; (b) 30% F3C2BT; (c) 30% F4C2BT; (d) 30% C2BT; (e) 30% BT.

nanocomposites (Figure 3a, F4CBT/PVDF; Figure 3b, F3C2BT/PVDF; Figure 3c, F4C2BT/PVDF; Figure 3d, C2BT/PVDF), the nanoparticles are uniformly embedded in the polymer matrix and well compatible with the PVDF matrix without phase separation. But in Figure 3b and Figure 3c, there are little air voids in the nanocomposites, which may be due to the nanoparticles F3C2BT and F4C2BT being hygroscopic (Figure 2), and the presence of water on the surface of them

Figure 2. (a) TG and (b) DSC curves of BT and surface modified BT nanoparticles measured in air atmosphere. D

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Figure 4. (a) Permittivity and (b) dielectric loss of the BT/PVDF and m-BT/PVDF nanocomposites measured at 1 kHz at room temperature.

results in agglomeration. 40 By use of the ball-milling technology,41 the nanocomposites filled with pristine BT nanoparticles (Figure 3e) also show good distribution of the filler in PVDF matrix, but the compatibility between them is poor. Although there are some −OH groups bonded on the pristine BT surface, its effectiveness is limited; large amounts of the BT nanoparticles are still found to be free from the matrix. The results indicate that the modifiers introduced onto the surface of BT nanoparticles can significantly improve the compatibility of the nanocomposites, and meanwhile the m-BT nanoparticles should not be easy absorbents of water, which will affect the performance of the nanocomposites. 3.2. Dielectric Properties of the Nanocomposites. The permittivity and dielectric loss of the nanocomposites filled with BT and m-BT nanoparticles are shown in Figure 4. As shown in Figure 4a, the permittivity increases as the filler volume fraction increases to 40%. After that, further addition of filler particles leads to smaller growth or even a gradual decrease in the permittivity, which can be attributed to the voids and interfacial defects induced by agglomeration between excess fillers.1 The permittivity of the m-BT/PVDF nanocomposites is lower than that of the nanocomposites filled with pristine BT particles. As filler volume fraction increases, the differences of permittivity between them become larger. We believe that the reduction of the permittivity in films containing m-BT is due to the improved interface between the organic and inorganic phases. This can be explained based on the presence of peripheral dense modifiers with high electron affinities, which can promote strong dipole interplay with the functional groups in the polymer leading to improved dispersion of the nanoparticles in the polymer matrix and generating negative influences on the interfacial polarization.42 Furthermore, charge trapping by surface modifiers minimizes possible charge conduction pathways in the film, thus reducing the space charge polarization. The dielectric loss of m-BT/PVDF nanocomposites is little higher than that of BT/PVDF, which is mainly due to the modifiers on the interface between BT and PVDF matrix having an impact on the relaxation from interfacial polarization. It is noteworthy that the dielectric loss (Figure 4b) is almost independent of the filler content, indicating a minimized agglomeration of the filler in the nanocomposites.13 Next with 30% of the filling ratio as an example, the influence of the modifier with different structure on the properties of nanocomposites has been studied. It is well-known that compared with the interfacial polarization, dipole polarization is a dominant factor in determining the permittivity of the nanocomposites. As shown in Figure 5, the diminution of the permittivity is different for the nanocomposites modified by

Figure 5. Dielectric constant and loss tangent of the nanocomposites filled with 30% BT and m-BT/PVDF nanoparticles measured at 1 kHz at room temperature.

different modifiers. The nanocomposites F4CBT/PVDF have higher permittivity than other m-BT/PVDF nanocomposites, which can be ascribed to the modifier F4C with one carboxylic functional group getting the lowest grafting density (Table 1),43 and higher grafting density will lead to lower permittivity of the composite.44 On the other hand, modifiers F3C2, F4C2, C2 with two carboxylic functional groups not only get higher grafting density but also cannot rotate after bonding with the metal atom from the surface of BT nanoparticles. So the intensity of dipole polarization will be weakened, and the permittivity of the nanocomposites will be lower.45 The fluorine functional groups in the modifiers F3C2 and F4C2 can introduce Maxwell−Wagner−Sillars (MWS) interfacial polarization and space charge polarization to the nanocomposites.46 Meanwhile the presence of water also can have additional affects on the permittivity of the nanocomposites besides the dispersion issues.40 So the nanocomposites F3C2BT/PVDF and F4C2BT/PVDF have higher permittivity than C2BT/ PVDF. Compared with BT/PVDF nanocomposites, the dielectric losses of m-BT/PVDF nanocomposites are a little higher, and the order is F4CBT/PVDF > F3C2BT/PVDF > F4C2BT/PVDF > C2BT/PVDF (as shown in Figure 5), which is consistent with the affect trend of the crystallinity degree to dielectric loss of the nanocomposites (as shown in Table 2).47,48 As the improvement of crystallinity is beneficial to reduce the dielectric loss caused by relaxation from dipole orientation polarization,49 the differences of dielectric loss among the m-BT/polymer composites are mainly caused by the change in the relaxation from orientation polarization, while the higher dielectric loss of m-BT/polymer composites compared to BT/PVDF is largely due to the change in relaxation from interfacial polarization. The dielectric loss of the nanocomposites C2BT/PVDF is higher than that of the nanoE

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conduction that can lead to dielectric breakdown.53 On the other hand, the modifiers serve to reduce nanoparticle aggregation formation which offers weak links for dielectric breakdown.54 Figure 6b shows the relative ordering of E0 obtained from the nanocomposites filled with different m-BT is as follows: C2BT > F4CBT > F4C2BT > F3C2BT. Due to the hygroscopicity, there is some water on the surface of F4C2BT and F3C2BT nanoparticles, which makes the separation of them more difficult, leads to agglomeration, and produces porosity in the nanocomposites.39 The electric breakdown strength of air voids is quite low (3 MV/m), which results in inhomogeneous local electric field and significant reduction of the electric breakdown strength of the nanocomposites.55 What is beyond our expectation is that the nanocomposites filled with C2BT nanoparticles have the highest characteristic breakdown strength, and the value can reach 288 MV/m when the volume fraction is 30%. The main reason is probably that the modifier C2 is without fluorinated substituent in the modifier molecule, which will result in ionic or dc conduction and decreases the electric breakdown strength.56 Being ferroelectric in nature, the PVDF matrix could be polarized significantly under a high electric field, giving rise to a deviation from the linear behavior of the electric displacement versus the electric field for the PVDF-based nanocomposites.57 So the discharged energy densities of the nanocomposites are calculated from each displacement hysteresis loop by the integral

Table 2. Data from Differential Scanning Calorimetry (DSC) of the Composites filler

crystallization temperature (°C)

melting temperature (°C)

ΔH (J/g)

degree of crystallinity (%)a

BT F4CBT F3C2BT F4C2BT C2BT

129.8 131.2 129.1 130.5 129.7

159.0 167.6 159.8 160.5 159.4

11.5 14.0 13.5 12.6 11.6

12.3 14.9 14.4 13.4 12.3

a The degree of crystallinity is defined as the ratio between the melting enthalpy of the material under study and the melting enthalpy of totally crystalline material.47

composites filled with other m-BT nanoparticles, which may be due to that there is no fluorinated substituent in the modifier molecule, and then the interfacial interactions between C2BT nanoparticles and PVDF matrix are weaker than the others. 3.3. Energy Storage Performance of the Nanocomposites. Breakdown strength is a critical parameter of dielectric materials, which denotes the highest electrical field that can be applied to the films without losing their insulating property.50 The electric breakdown strength of the nanocomposites is analyzed with a two-parameter Weibull distribution function, ⎡ ⎛ ⎞β⎤ E P(E) = 1 − exp⎢ −⎜ ⎟ ⎥ ⎢⎣ ⎝ E0 ⎠ ⎥⎦

(1)

Ue =

where P(E) is the cumulative probability of electric failure, E is experimental breakdown strength, E0 is a scale parameter, regarded as the characteristic breakdown strength, that is also the breakdown strength at the cumulative failure probability of 63.2%, and β is the shape parameter, a parameter characterizing the data scattering degree.51,52 As shown in Figure 6a, the Weibull plots show the breakdown probability distribution of the nanocomposites. The characteristic breakdown strength E0 for nanocomposites filled with BT and m-BT nanoparticles is shown in Figure 6b. It is worth noting that the breakdown strength of the nanocomposites filled with m-BT nanoparticles is greatly enhanced compared with that of the nanocomposites filled with pristine BT nanoparticles. In m-BT/PVDF nanocomposites, the modifiers act as effective passivation layers, reducing the concentration of ionizable hydroxyl groups on the nanoparticle surface, thereby minimizing the concentration and mobility of charge carriers normally associated with the surface. This also reduces leakage currents and sources of runaway

∫ E dD

(2)

where E is the electric field and D is the electric displacement.24,25 So except breakdown strength, the maximum displacement and remnant polarization also have an important effect on the discharged energy density of the nanocomposites. Figure 7 shows the D-E loops of the nanocomposites filled with BT and m-BT nanoparticles at 200 MV/m. The maximum displacement of the nanocomposites filled with BT nanoparticles is 8.0 μC/cm2, which is higher than that of nanocomposites filled with m-BT nanoparticles. The results indicate that the modifiers form a passivation layer at the surface of BT nanoparticle, obstruct the formation of dielectric channels, and weaken the intensity of dipole polarization. Due to the lowest surface coverage, the nanocomposites F4CBT/ PVDF have the highest maximum displacement in the nanocomposites m-BT/PVDF. On the other hand, the modifier C2 without side functional group can reduce the Maxwell− Wagner−Sillars (MWS) interfacial polarization and space

Figure 6. (a) Weibull distribution of the dielectric breakdown strength of nanocomposites filled with 30% BT and m-BT/PVDF nanoparticles. (b) Variation of characteristic breakdown strength from Weibull distribution for nanocomposites filled with 30% BT and m-BT nanoparticles. F

DOI: 10.1021/acsami.5b07486 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces η=

discharge energy Ue = charge energy Ue + Uloss

(3)

where Uloss is the energy loss. The energy loss was calculated by the numerical integration of the closed area of the hysteresis loops.58,59 As shown in Figure 9, the efficiency of the

Figure 7. D-E loops of nanocomposites filled with BT and m-BT nanoparticles at 200 MV/m.

charge polarization of the nanocomposites,46 which makes the nanocomposites C2BT/PVDF have the lowest remnant polarization than other nanocomposites and able to tolerate higher voltage without breakdown. Since the modifier layers failed to produce positive influence on the material performance, the nanocomposites F3C2BT/PVDF and F4C2BT/ PVDF have lower maximum displacement and higher remnant polarization. Figure 8 shows the discharged energy density of the nanocomposites filled with BT and m-BT nanoparticles. It

Figure 9. Efficiency of the nanocomposites filled with BT and m-BT nanoparticles.

nanocomposites filled with BT naoparticles presents an increasing trend with a decrease at first as the electric field increases. When the nanocomposites filled with m-BT naoparticles, the above trend is not obvious, and the efficiencies first reducing under the low electric field tend to be smooth as the electric field increases. The efficiencies of the nanocomposites filled with F4CBT and C2BT naoparticles are much higher than those of nanocomposites filled with other m-BT nanoparticles and even higher than that of nanocomposites BT/PVDF under the low electric field. As a result of the larger remnant polarization, the nanocomposites filled with F3C2BT nanoparticles have the lowest efficiency.

4. CONCLUSIONS Through the study of the performance of the nanocomposites filled with BT nanoparticles modified by four kinds of modifiers of 2,3,4,5-tetrafluorobenzoic acid, 4-(trifluoromethyl)phthalic acid, tetrafluorophthalic acid, and phthalic acid, we found that the modifiers could significantly improve the compatibility between the filler and matrix and reduce the permittivity of the nanocomposites. Only 2,3,4,5-tetrafluorobenzoic acid and phthalic acid in the four kinds of modifiers could obviously improve the breakdown strength of the nanocomposites. At the same time the maximum polarization did not greatly decrease and the remnant polarization did not rise significantly. The results indicate that the structure of modifier, the type, number, and position of functional groups in the modifier molecule show different affects on the performance of the nanocomposites. Comprehensively concluding, 4-(trifluoromethyl)phthalic acid and phthalic acid are more suitable for the surface modification to filler particles in the nanocomposites applied for energy storage, while too easily ionized atom introduction will damage the performance of the nanocomposites.

Figure 8. Discharged energy density of the nanocomposites filled with BT and m-BT nanoparticles.

can be observed that the discharged energy density of the nanocomposites filled with m-BT nanoparticles is higher than that of nanocomposite BT/PVDF except F3C2BT/PVDF. The discharged energy densities of the nanocomposites F4CBT/ PVDF and C2BT/PVDF are 6.5 and 6.7 J/cm3, respectively, which are 35.7% and 37.7% higher than that of BT/PVDF nanocomposites. As a result of the lower breakdown strength, lower displacement, the nanocomposites filled with F3C2BT and F4C2BT nanoparticles have the lower discharged energy density. In particular, the discharged energy density of the nanocomposites filled with F3C2BT nanoparticles is lower than that of nanocomposites BT/PVDF. The results demonstrate that the choice of modifier has a great influence on the performance of the nanocomposites for energy storage. For practical applications, the nanocomposites not only need to have a high energy density but also are desired to maintain a high efficiency, since higher energy loss in capacitor will lead to more heat, which destroys the performance and reliability of the whole device. The energy storage efficiency (η) could be calculated according to the formula

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*E-mail: [email protected]. G

DOI: 10.1021/acsami.5b07486 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (Grant 2015CB654603).

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.5b07486 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX