Excellent Energy Storage of Sandwich-Structured PVDF-Based

Nov 23, 2017 - Synopsis. Environmentally friendly PVDF/CFO@BZT−BCT NFs−PVDF/PVDF composite possesses high energy storage due to CoFe2O4@BZT−BCT ...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 403−412

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Excellent Energy Storage of Sandwich-Structured PVDF-Based Composite at Low Electric Field by Introduction of the Hybrid CoFe2O4@BZT−BCT Nanofibers Qingguo Chi,*,†,‡,§ Tao Ma,† Yue Zhang,† Qingguo Chen,*,† Changhai Zhang,† Yang Cui,† Tiandong Zhang,† Jiaqi Lin,† Xuan Wang,† and Qingquan Lei† †

Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, 52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China ‡ State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, 4 second Section on Jianshe North Road, Chengdu, Sichuan 610054, P. R. China § Key Laboratory of Functional Materials Physics and Chemistry, Ministry of Education, Jilin Normal University, 1301 Haifeng Street, Siping, Jilin 136000, P. R. China S Supporting Information *

ABSTRACT: The high-performance energy-storage dielectric capacitors are increasingly necessary for the development of miniaturization, integration, and multifunctionality of electronic devices. Here, we describe a new strategy of a sandwich-structured polymer-based dielectric composite with inorganic fillers of semiconductor@perovskite hybrid fibers, and this novel dielectric composite possesses an excellent energy-storage performance at low electric field. It is of crucial importance to achieve hybrid nanofibers of 0.5Ba(Zr0.2Ti0.8)O3− 0.5(Ba0.7Ca0.3)TiO3 nanofibers (BZT−BCT NFs) deposited by CoFe2O4 nanoparticles (CFO; hereafter, CFO@BZT−BCT NFs in short). Herein, the BZT−BCT NF ceramic has the typical perovskite structure and large dielectric constant, which is used as the ceramic-support for CFO. Meanwhile, the semiconductor of CFO works as the electron donor to offer a great interfacial polarization for the improvement of overall dielectric constant of the composite. Remarkably, the trilayer structure is composed of outer poly(vinylidene fluoride) (PVDF) layer to improve the breakdown strength and middle CFO@BZT−BCT NFs−PVDF nanocomposite to enhance the dielectric properties. The BZT−BCT NFs were prepared by electrospinning, and then the CFO@BZT−BCT NFs were gained by hydrothermal method. Furthermore, the BZT−BCT NFs and the CFO@BZT−BCT NFs were modified by polydopamine (PDA). Finally, the sandwich-structured composites were gotten by a typical process of solution-casting and hot-pressing. The influences of fillers’ volume fraction and type on the performances of composites have been systematically investigated. The PVDF/CFO@BZT−BCT NFs−PVDF/PVDF composite (P/CFO@BZT−BCT NFs−P/P) exhibits an outstanding energy density performance under a low electric field. The trilayer structure composite with an optimized content of nanofibers possesses an excellent dielectric performance (dielectric constant ∼20.1 at 100 Hz) and great energy-storage performance (electric displacement ∼10.7 μC/cm2, discharged energy density ∼11.3 J/cm3, and efficiency ∼55.5% at a low electric field of 350 kV/mm). This work paves the way for potential applications in integrated electronic devices. KEYWORDS: Poly(vinylidene fluoride) (PVDF), Dielectric material, Electric polarization, Dielectric properties, Energy density

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INTRODUCTION

At the same time, electrochemical supercapacitors hold a medium energy-storage density and power density, but also have a complex structure, a low operating voltage, a large leakage current, and short cycle time.2,3 As compared to batteries and electrochemical supercapacitors, dielectric capacitors possess the advantage of fast charge/discharge rate, long cycle time, and environmental friendliness, which are also

The questions of sustainability, energy, and environment are the undoubted challenges in any period of development of human civilization. Nowadays, to build a harmonious home, researchers need to solve the key issues of how to effectively store energy, reduce energy loss, and mitigate the environmental burden. Energy storage is an important part of energy development research, which generally includes batteries, electrochemical supercapacitors, and dielectric capacitors. Batteries usually possess a great energy density, but they have a small power density and are also harmful to the environment.1 © 2017 American Chemical Society

Received: August 3, 2017 Revised: October 24, 2017 Published: November 23, 2017 403

DOI: 10.1021/acssuschemeng.7b02659 ACS Sustainable Chem. Eng. 2018, 6, 403−412

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ACS Sustainable Chemistry & Engineering

dielectric materials. For the first point, what matters is that the multilayer structure can be chosen to reduce breakdown probability and reinforce breakdown strength of the composite due to the redistribution of internal electric field.32 Another effective measure is that the one-dimensional inorganic fillers can be chosen to combine with the polymer matrix to reduce the doping content of fillers and to twist the growth of electrical treeing at low electric field.29 For a more important step, the surface modification for highly dielectric ceramics can be employed to improve the compatibility between the ceramic and the polymer, which is beneficial to avoid aggregation of nanofillers and to relieve the dielectric difference.31 These clever ideas provide us with more highlights. Inspired by the above-mentioned literature, we have carried out the following research work in this paper. A novel strategy of sandwich-structured PVDF-based dielectric nanocomposite with one-dimensional 0.5Ba(Zr0.2Ti0.8)O3−0.5(Ba0.7Ca0.3)TiO3 nanofibers (BZT−BCT NFs) decorated by CoFe2O4 nanoparticles was proposed in the present study. The hybrid nanofibers combining the high-polarization perovskite BZT− BCT fibers and the semiconductor of CoFe2O4 nanoparticles were prepared by the electrospinning technique and hydrothermal synthesis. The introduction of CoFe2O4 nanoparticles onto the surface of BZT−BCT fibers was intended to increase the number of carriers to further improve the polarizability and interfacial polarization of the composite under the low applied voltage. Simultaneously, the trilayer dielectric composite was achieved by the typical solution-casting and the hot-pressing process. The purpose of the design of a sandwich structure for the composite is to build the redistribution of internal electric field among each layer, which can reinforce the breakdown strength of the composite. The polydopamine (PDA), which worked as the modifier for the inorganic filler, could improve the compatibility and avoid the aggregation of the hybrid nanofibers in the composite. Meanwhile, the influences of content and type of the nanofillers on the performances of composites were systematically investigated. This present work is expected to pave the way for preparation of a polymer-based dielectric material with excellent energy-storage performance at a low electric field for energy-saving sustainability in the modern electrical and electronic industry.

widely used in pulsed power devices. However, energy-storage density of dielectric capacitors is relatively low.4−6 Therefore, how to further improve the energy-storage density of dielectric capacitors is the main problem for the development of dielectric capacitors. Polymer-based dielectric composite has attracted worldwide attention for potential applications in future flexible and integrated electronic devices.7−9 In particular, the dielectric composite with high energy storage, large dielectric constant, and low dielectric loss under low electric field is increasingly required in modern high-performance power electronic systems.10−12 Many researchers have carried out a series of experimental and theoretical studies on this field to explore or optimize an ideal high-energy-storage dielectric material.13−18 A typical formula can be given to express the relationship of energy-storage performance with the electric field strength and intensity of polarization for the nonlinear dielectric materials as 0

Ue = ∫ E dD, where Ue, E, and D represent the energyD max

storage density, the electric field strength, and the electric displacement, respectively. D can be described as D = P + εrε0E, in which P and εr mean the electric polarization and the relative permittivity, respectively. Therefore, it can be seen clearly from these expressions that the electric polarization, dielectric constant, and electric field strength significantly influence the energy-storage performance of dielectric materials. At present, the research on the energy-storage dielectric materials with the PVDF-ferroelectric-polymer has attracted much more considerable interest. Compared with the commercial biaxially oriented polypropylene (εr ∼ 2.2, Ue ∼ 2 J/cm3),19 the energy density of the PVDF with γ-phase [transtrans-trans-gauche (TTTG) chain conformation] exhibits as high as 6 J/cm3 at 350 kV/mm, and the dielectric constant of PVDF is up to 8 at 100 Hz.15−17,20 Obviously, the energy density and the electric field strength of γ-phase PVDF is greater than that of inorganic dielectric ceramics under a low voltage,21,22 but the dielectric constant of PVDF is much lower than that of inorganic ceramics. More remarkably, the γ-phase can be transformed from β-phase by a quenching process,20,23 which offers inspiration for the preparation of PVDF-based dielectric composite. Recently, the dielectric nanocomposite, achieved by introduction of the inorganic fillers into one kind of polymer matrix, possesses the wondrous performances as compared to any single component of the filler or the matrix.24−26 Meanwhile, the dielectric properties and the energy-storage performance of the composite can be tailored by adding fillers with different structure and types.27 However, the discharged energy density of polymer-based nanocomposite is not apparently improved because of the deteriorated breakdown strength by the introduction of inorganic fillers.28−30 Moreover, the large doping content of fillers damages the comprehensive performance of the dielectric materials, because some voids, pores, and other macrostructural imperfections are induced into the matrix. In addition, agglomeration of nanoparticles can happen and cause the rapid accumulation of internal carriers, which are harmful to the reinforcement of the energy-storage performance of materials.8,31 In addition, another barrier in the way of enhancing overall performance is to relieve the difference in dielectric property between the matrix and the fillers. There is still much work to be done around the abovementioned issues. Fortunately, three promising approaches can be carried out to improve the energy-storage performance of

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EXPERIMENTAL SECTION

Materials. The poly(vinylidene fluoride) (PVDF) was provided by Shanghai 3F New Material Co., Ltd., China. The polyvinylpyrrolidone (PVP) was purchased from Alfa Aesar. Dopamine hydrochloride and tris(hydroxymethyl)-aminomethane (Tris) were offered by Aladdin. The Ba(OH)2·8H2O, Ca(OH)2, CoCl2·6H2O, and FeCl3·6H2O were obtained from Tianjin TIANLI Chemical Reagents, Ltd. All other chemicals were supplied by Sinopharm Chemical Reagent Co., Ltd., and all the chemicals were analytical grade and were used as received without further purification. Preparation of CFO@BZT−BCT Nanofibers. The BZT−BCT nanofibers were prepared by the sol−gel and electrospinning method, which can be seen in the Supporting Information. The CoFe2O4@ BZT−BCT NFs (hereafter, CFO@BZT−BCT NFs in short) were produced by the sol−gel, electrospinning, and hydrothermal synthesis method. To make the hybrid nanofibers, BZT−BCT nanofibers were first achieved by the above-mentioned method. Then, the BZT−BCT nanofibers were dissolved in 1 g/mL NaOH solution for ultrasonic dispersion to obtain the suspension A solution. Afterward CoCl2· 6H2O and FeCl3·6H2O powders were dissolved in deionized water and stirred for 30 min to obtain the mixed solution with mole ratio of Fe3+/Co2+ = 2:1, as B solution. Finally, the solution B was slowly dropped into the suspension A, and the mixed solution was 404

DOI: 10.1021/acssuschemeng.7b02659 ACS Sustainable Chem. Eng. 2018, 6, 403−412

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Figure 1. Schematic preparation process of CoFe2O4@BZT−BCT NFs.

Figure 2. Relationship between dielectric properties and frequency and the energy-storage performance of the sandwich-structured P/x vol % BZT− BCT NFs−P/P (x = 0, 1, 3, 5, and 7) composite with different filler contents at room temperature. (a1) Dielectric constant. (a2) Dielectric loss. (b1) Discharged energy density. (b2) Energy efficiency. continuously stirred for 20 min to obtain a homogeneous reaction solution. The homogeneous mixture solution was slowly transferred into the 50 mL stainless steel autoclave with the Teflon liner and was kept at a temperature of 180 °C for about 12 h. Then, the reactor was cooled to room temperature; finally, the CFO@BZT−BCT NF hybrids are obtained after centrifuging and washing with distilled water to the neutral pH, and then drying at 80 °C for 8 h. The schematic preparation process of CoFe2O4@BZT−BCT NFs can be seen in Figure 1. Surface Modification for BZT−BCT NFs or CFO@BZT−BCT NFs. For fibers possessing an excellent dispersion and compatibility, the PDA was coated on the surface of nanofibers.31 Tris−HCl buffer agent was first dissolved in deionized water at a concentration of 2 mg/mL (pH = 8.5). A 0.1 g portion of PDA was dissolved in the last solution to obtain the PDA solution. Then, the BZT−BCT NFs or CFO@BZT−BCT NFs were immersed into the above PDA solution

in the mass ratio of dopamine to modified nanofibers 1:10, and stirred vigorously at room temperature for 12 h. Then, BZT−BCT NFs or CFO@BZT−BCT NFs modified by PDA were gained by being centrifuged, washed, and dried in a vacuum oven at 80 °C for 12 h. Preparation of Sandwich-Structured Dielectric Composites. The preparation of monolayer BZT−BCT NFs/PVDF or CFO@ BZT−BCT NFs/PVDF composite can be seen in the Supporting Information. Typically, the sandwich-structured dielectric composite was fabricated by hot-pressing. First, the outer monolayer was PVDF, and middle monolayer was the BZT−BCT NFs/PVDF or the CFO@ BZT−BCT NFs/PVDF composite. After that, these three layers were stacked together, followed by hot-pressing at 170 °C for 10 min. Then, the hot sandwich-structured films were cooled to room temperature quickly. Finally, the sandwich-structured PVDF/x vol % BZT−BCT NFs−PVDF/PVDF and PVDF/x vol % CFO@BZT−BCT NFs− PVDF/PVDF (hereafter, P/x vol % BZT−BCT NFs−P/P and P/x vol % CFO@BZT−BCT NFs−P/P in short; x = 0, 1, 3, 5, and 7) 405

DOI: 10.1021/acssuschemeng.7b02659 ACS Sustainable Chem. Eng. 2018, 6, 403−412

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Figure 3. (a) XRD of BZT−BCT NFs and CFO@BZT−BCT NFs. (b) TEM of CFO@BZT−BCT NFs. (b1) High-resolution TEM image of CFO. (b2) Partial enlarged TEM of CFO@BZT−BCT NFs and (b3) CFO@BZT−BCT NFs. (b4) EDS of CFO@BZT−BCT NFs.

from ∼9.7 of the P/P/P film to ∼16.9 of the P/7 vol % BZT− BCT NFs−P/P nanocomposite. These results indicate that the large aspect ratio of BZT−BCT NFs is conducive to enhance the dielectric property of the composite, which possesses the greater polarization and higher dielectric constant. At the same time, as can be seen from Figure 2a2, there is no great difference in the dielectric loss for the composites with different BZT−BCT NF doping contents; when the measured frequency is lower than 3 × 104 Hz, the loss tangent value is less than 0.05. In particular, the dielectric loss tangent is less than 0.10 at 1 MHz. The reason for this phenomenon is that the BZT−BCT NFs block the movement of internal electronic and space-charge conduction in composites; meanwhile, the high-aspect-ratio nanofibers hinder the rotation of the organic chain in the matrix and inhibit the charge-carrier movement and the formation of a conductive path to reduce the conduction loss of the composite. Moreover, with the increase of the volume fraction of BZT−BCT NFs, the dielectric loss does not change too much. It is worth mentioning that the distinctive feature is the enhancement of dielectric constant with a lower dielectric loss. For more intuitive analysis of the energy-storage performance of the composite, the relationship between the energystorage performance and the electric field of the composite is obtained by calculation, and the discharged energy density (Figure 2b1) and energy efficiency (Figure 2b2) versus the electric field of the sandwich-structured P/x vol % BZT−BCT NFs−P/P (x = 0, 1, 3, 5, and 7) at 10 Hz under a low applied voltage are given in Figure 2b. Moreover, the relationship between the electric displacement and the electric field of the sandwich-structured P/x vol % BZT−BCT (x = 0, 1, 3, 5, and 7) at a low electric field of 350 kV/mm is presented and discussed in Figure S3 in the Supporting Information, which shows that the electric displacement of the composite increases from ∼4.1 μC/cm2 for the P/P/P composite to ∼5.8 μC/cm2 for the P/7 vol % BZT−BCT NFs−P/P composite. Furthermore, from Figure 2b1, the energy density of the P/x vol % BZT−BCT NFs−P/P composite is improved with the increase of electric field strength; however, the energy efficiency decreases slowly as the external electric field increases. The energy density of composite also increases gradually with the increased content of fillers at the same field. With the content

composite was obtained. The typical thickness of the outer layer is around 7 μm, and the thickness of the middle layer is ∼10 μm. Characterization. X-ray diffraction (XRD) analysis was performed with a PANalytical Empyrean instrument, and all data were analyzed through HighScore Plus software. Field emission scanning electron microscope (FE-SEM) tests were carried out through a Hitachi SU8020 Uhr instrument. Transmission electron microscopy (TEM) images were obtained under an FEI TECNAI2-12 instrument. Aluminum electrodes with diameter 3 and 25 mm were evaporated on two sides of the composites for the electrical measurements. Dielectric properties were collected using a broadband impedance analyzer of GmbH Novocontrol Alpha-A in the frequency range from 100 Hz to 1 MHz. Dielectric breakdown strength tests were conducted by using precision dc power supply test equipment YDZ-560. Electric displacement−electric field (D−E) loops were acquired over a Radiant Premier II ferroelectric test system at a frequency of 10 Hz. Finite element simulation was performed with the COMSOL Multiphysics software.

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RESULTS AND DISCUSSION

A series of sandwich-structured PVDF/BZT−BCT−PVDF/ PVDF (P/BZT−BCT NFs−P/P in short, hereafter) composites were prepared by solution-casting and hot-pressing. For an investigation into the crystalline structure of the P/BZT−BCT NFs−P/P nanocomposites with different volume percents, the X-ray diffraction (XRD) patterns of the films are presented in Figure S1 in the Supporting Information, which indicate that the major phase in the matrix of the sandwich-structured composite films is a mixture of nonpolar γ-, α-, and polar βphase. Obviously, the crystalline structure of PVDF is not influenced by introducing the BZT−BCT nanofibers. Meanwhile, all other diffraction peaks appearing in Figure S1 could be indexed to the BZT−BCT nanofibers, the pure perovskite structure of ABO3,33 and when volume percent of the BZT− BCT nanofibers introduced into the matrix is larger, the peak of the BZT−BCT nanofibers that can be found is more obvious. Meanwhile, the SEM of cross-sectional morphology for the sandwich-structured P/x vol % BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite (Figure S2 in the Supporting Information) can been observed and analyzed in the Supporting Information. Figure 2a1,2 shows the dielectric properties of the sandwichstructured P/BZT−BCT NFs−P/P nanocomposites with different volume contents of BZT−BCT NFs, separately. It is worth noting that the dielectric constant is sharply increased 406

DOI: 10.1021/acssuschemeng.7b02659 ACS Sustainable Chem. Eng. 2018, 6, 403−412

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ACS Sustainable Chemistry & Engineering of filling phase of BZT−BCT NFs increasing from 0 to 7 vol %, the discharged energy density of the composite also increases. While x is 0, 1, 3, 5, and 7, the discharged energy density is 4.1, 5.2, 6.0, 6.8, and 7.4 J/cm3 at the low electric field of 350 kV/ mm, respectively. The introduction of the BZT−BCT NFs has a higher polarization intensity, so that the discharged energy density of the composite increases. Figure 2b2 shows that the energy efficiency of the composite maintains a high level (≥48.2%) in the low electric field range 25−350 kV/mm. From the figure, it can also be found that the energy efficiency decreases obviously with the increase of electric field strength. Meanwhile, the energy efficiency also decreases obviously as the content of BZT−BCT NFs increases. The reduction of the energy efficiency for the composite can be ascribed to the following two aspects. First, the imperfections appear in the matrix under a larger filling phase content (such as 7 vol %), which causes the energy efficiency of composites to decrease faster under the high electric field. Second, the BZT−BCT NFs have excellent ferroelectric properties, which lead to the increase of the hysteresis loss. According to the previously reported literature,34−36 the growth of slightly conductive nanoparticles on the surface of inorganic fillers can enhance the dielectric and polarization performance of the composite under a low volume fraction, which can make the composite maintain a high breakdown strength and a good processing characteristic. On this basis, the CFO@BZT−BCT NFs were designed and synthesized here. XRD and transmission electron microscopy (TEM) have been employed to analyze the crystal structure, morphology, and components of hybrid inorganic nanofillers. Figure 3a shows the XRD diffraction patterns of BZT−BCT NFs and CFO@BZT−BCT NFs. The characteristic diffraction peaks of the BZT−BCT in the hybrid of CFO@BZT−BCT NFs can be accurately indexed as pure perovskite structure, and there is no other impurity phase, indicating that the crystal structure of BZT−BCT NFs is unchanged during the hydrothermal reaction of CFO grown on the surface of BZT−BCT NFs. It is worth noting that the characteristic diffraction peak (311) that appeared slightly at 2θ = 35.4° is a typical spinel crystal structure of CFO from the XRD pattern,37−41 indicating that a small amount of CFO was combined with BZT−BCT NFs successfully. However, the characteristic diffraction peak of CFO was weak because of the low relative content of CFO nanoparticles and the X-ray fluorescence by using the Cu target to characterize the CFO. To obtain the microstructure information, such as the size of CFO and BZT−BCT NFs, the distribution state, and the growth morphology of CFO on the surface of BZT−BCT NFs, the TEM and energy dispersive spectrometry (EDS) of CFO@ BZT−BCT NFs have been investigated and given in Figure 3b. It can be seen from Figure 3b1 in the high-resolution figure that the diameter of the CFO nanoparticle dispersed on the surface of BZT−BCT NFs is about 20 nm, and the CFO nanoparticles have the characteristic diffraction plane of (311), which is consistent with the XRD results. From Figure 3b2,3, it can be seen that the diameter of BZT−BCT NFs is about 300 nm, and the length is about 5 μm, which indicates that the CFO@BZT− BCT NFs has a larger ratio of length and diameter. Meanwhile, the surface of BZT−BCT NFs is very smooth, and the CFO nanoparticles on the surface of BZT−BCT NFs are only a small local area cluster, because the CFO nanoparticles possess a larger surface area and surface energy resulting in slight

agglomeration. As shown in Figure 3b3, the CFO nanoparticles discretely grow on the BZT−BCT NFs surface, and no obvious aggregation can be found in a large area, which also indicates that the CFO does not form a conductive channel among the BZT−BCT NFs. As illustrated in Figure 3b4, the peaks of O, Ba, Ti, Fe, Co, Ca, and Zr seven elements appear in the EDS of CFO@BZT−BCT NFs, which further confirms that CFO@ BZT−BCT NFs have been successfully prepared. Figure 4a presents the XRD patterns of sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7)

Figure 4. (a) XRD and (b) SEM of the sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite: (a1) 2θ = 10−100°, (a2) 2θ = 15−38°, (b1) x = 1, (b2) x = 3, (b3) x = 5, and (b4) x = 7.

composite. According to Figure 4a2, the diffraction peaks of γ (020), γ (110), and γ (022) of the PVDF matrix after quenching exist at 18.5°, 20.1°, and 26.8°, respectively. The existence of γ-phase can reduce the residual displacement (Dr) and increase the maximum displacement (Dmax), which is helpful to enhance the energy density (Ue) of the composite. It can also be seen that the (100) diffraction peak of the nonpolar α-phase appears at 17.9°, and the (200) diffraction peak of the ferroelectric β-phase appears at 20.7° in the quenched PVDF matrix. The results indicate that the phase in the sandwichstructured composite is mainly the γ-phase and a small amount of nonpolar α-phase and polar β-phase. Furthermore, each 407

DOI: 10.1021/acssuschemeng.7b02659 ACS Sustainable Chem. Eng. 2018, 6, 403−412

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Figure 5. Relationship between dielectric properties and frequency and the energy-storage performance of the sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite with different filler contents at room temperature: (a1) dielectric constant, (a2) dielectric loss, (b1) energy-storage efficiency, and (b2) energy efficiency.

Figure 5a1,2 is the dependence of dielectric constant and dielectric loss on the frequency for the sandwich-structured P/ CFO@BZT−BCT NFs−P/P composite with different volumes of CFO@BZT−BCT NFs in the middle layer. As shown in Figure 5a1, the dielectric constant of the composite increases from ∼12.1 for P/1 vol % CFO@BZT−BCT NFs−P/P to ∼20.1 for the P/5 vol % CFO@BZT−BCT NFs−P/P composite at 100 Hz, which is attributed to the fact that the BZT−BCT NFs possess a higher dielectric constant and a good polarization. In addition, the enhancement of dielectric constant of hybrids CFO@BZT−BCT NFs is induced by the semiconductor CFO nanoparticles. Furthermore, the introduction of a small amount of CFO can increase the number of electrons and charges, which are conducive to increasing the quantity of dipoles and improving the polarizability for the composite. More importantly, the introduced CFO and BZT−BCT NFs for the hybrid CFO@ BZT−BCT NFs enhance the interfacial polarization of the composite. However, the dielectric constant of the composite decreases to ∼18.3 when the filling content increases to x = 7. The uniformly dispersed CFO@BZT−BCT NFs can be used as a miniature capacitor. The amount of microcapacitance reasonably increases when the content of BZT−BCT NFs increases. Therefore, the composites exhibit a greater dielectric constant.42,43 However, the phenomenon of agglomeration is obvious at a larger volume fraction, so the capacitances are

diffraction peak of the BZT−BCT NFs can be clearly indexed in Figure 4a, and the diffraction peak intensity gradually increases with the increasing content of fillers. The characteristic peak (311) of spinel crystal CFO with weak intensity appears at 2θ = 35.4° in the composite,37−41 indicating that a small amount of CFO successfully combined with the BZT−BCT NFs. The formed CFO@BZT−BCT NFs have been introduced into the PVDF matrix for constructing the sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/ P (x = 1, 3, 5, and 7) composite. Obviously, the introduction of CFO@BZT−BCT NFs does not change the crystal structure of the PVDF matrix. Figure 4b gives the SEM of cross-sectional morphology for the P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) sandwich-structured composite. As can be seen from Figure 4b, the thickness of the outer PVDF film is about 7 μm, and the thickness of the middle CFO@BZT−BCT NFs−PVDF layer is about 10 μm for the sandwich-structured P/x vol % NFs−P/P (x = 1, 3, 5, and 7) dielectric composite. It is important that the sandwich-structured composite has no obvious interface between the outer PVDF and the middle CFO@BZT−BCT NFs−PVDF layer, indicating that the trilayer is well-combined. Meanwhile, the dopamine-modified CFO@BZT−BCT NFs have good compatibility with the PVDF matrix; no obvious gaps or imperfections can be found, which may be beneficial for improving the performance of the composite. 408

DOI: 10.1021/acssuschemeng.7b02659 ACS Sustainable Chem. Eng. 2018, 6, 403−412

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Figure 6. (a) Weibull distribution and (b) finite element simulation of the sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) dielectric composite: (b1) x = 1, (b2) x = 3, (b3) x = 5, and (b4) x = 7.

illustrated in Figure 5b1, the displacement polarization of the composite increases with CFO@BZT−BCT NFs increasing at the same electric field, which is good for enhancing the discharge energy density. However, when there are more CFO@BZT−BCT NFs added into the composite, the numbers of imperfections produced in the matrix are larger, and there are more formed conductive paths, which are not conducive to enhance the electric displacement; then, the breakdown strength of the composite can be deteriorated. Therefore, the breakdown strength of the P/7 vol % CFO@BZT−BCT NFs− P/P dielectric composite decreases significantly. Figure 5b2 shows the energy efficiency of all composites. Clearly, the energy efficiency of P/1 vol % CFO@BZT−BCT NFs−P/P composite is 48.8%, which is lower than that of P/x vol % CFO@BZT−BCT NFs−P/P (x = 3, 5, and 7). Meanwhile, the energy efficiency of the composite decreases with the applied electric field strength increasing. Although the introduction of CFO@BZT−BCT NFs improves the electric displacement intensity of polarization, simultaneously, the imperfections, hysteresis loss, and conduction loss can be induced by the high doping content of inorganic fillers, resulting in the reduction of electric field strength, which is not conducive to the discharge energy density and energy efficiency. Figure 6a is the two-parameter Weibull distribution of the sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite. The ln(−ln(1−P(E))) and ln E can be gained by the breakdown strength test, and the intercept and slope of the curve are represented by the Eb and β. The Weibull distribution statistical parameters and calculation of breakdown strength of the sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite are given in Table S1 in the Supporting Information. Hence, the breakdown strength of sandwich-structured P/x vol % CFO@ BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) is 418, 380, 372, and 304 kV/mm, respectively. On the basis of the analysis of the breakdown strength of the composite, the distribution of the internal field strength for the sandwich structure is investigated deeply. The P/x vol % CFO@BZT−BCT NFs−P/P sandwich-structured (x = 1, 3, 5, and 7) composite with different hybrid nanofiber doping contents in the middle layer exhibits different breakdown strength, which stems from the redistribution of local electric field in each layer of the sandwich-structured composite. The

difficult to be connected effectively, which leads the dielectric constant being reduced. From Figure 5a2, we can see that the dielectric loss tangent of the composite is less than 0.04 when the measured frequency is lower than 4 × 104 Hz; when the frequency is higher than 4 × 104 Hz, the dielectric loss tangent of the P/x vol % CFO@ BZT−BCT NFs−P/P composite increases with the increasing frequency, because the dipoles cannot follow the shift of electric field at a higher frequency, so the dipole relaxation and the relaxation loss occur.44 Meanwhile, there is no obvious difference of dielectric loss for the composite with different CFO@BZT−BCT NF doping contents. In summary, compared with those of the P/x vol % BZT−BCT NFs−P/P sandwich-structured (x = 0, 1, 3, 5, and 7) composite, the dielectric properties of the sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite obviously increase, when the BZT−BCT NFs are optimized to the CFO@BZT−BCT NFs. Figure 5b shows the relationship between the energy performance of the sandwich-structured P/x vol % CFO@ BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite and the electric field at 10 Hz under a low applied voltage. Meanwhile, the relationship between the electric displacement and the electric field of the sandwich-structured P/x vol % CFO@ BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite is presented and discussed in Figure S4 in the Supporting Information, which shows that the maximum electric displacement of the P/x vol % CFO@BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite is 6.5 μC/cm2 (at 350 kV/mm), 8.2 μC/ cm2 (at 350 kV/mm), 10.7 μC/cm2 (at 350 kV/mm), and 8.3 μC/cm2 (at 295 kV/mm), respectively. From Figure 5b, the discharge energy density of a series of P/x vol % CFO@ BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) increases, but the energy efficiency decreases with the external electric field increasing. It can be seen from Figure 5b1 that the magnitude of applied electric field and discharge energy density of the composite are obviously different for the composite with different CFO@BZT−BCT NF doping contents. For instance, the discharge energy density of the composite increases first and then decreases with CFO@BZT−BCT NFs increasing. The maximum discharge energy density of P/x vol % CFO@ BZT−BCT NFs−P/P (x = 1, 3, 5, and 7) composite is 6.6 J/ cm3 (350 kV/mm), 8.0 J/cm3 (350 kV/mm), 11.3 J/cm3 (350 kV/mm), and 7.4 J/cm3 (295 kV/mm), respectively. As 409

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ACS Sustainable Chemistry & Engineering Table 1. Local Electric Field of Every Layer Calculated by the Capacitive Voltage Divider composite P/1 P/3 P/5 P/7

vol vol vol vol

% % % %

CFO@ CFO@ CFO@ CFO@

BZT−BCT BZT−BCT BZT−BCT BZT−BCT

NFs−P/P NFs−P/P NFs−P/P NFs−P/P

d1 (μm)

d2 (μm)

εb

E1 (kV/mm)

E2 (kV/mm)

Em (kV/mm)

E1/E2

6 6 7 7

10 11 9 10

12 25 36 20

452 533 519 384

376 213 144 192

418 380 372 304

1.2 2.5 3.6 2.0

redistribution of local electric field happens much more obviously in the composite with much greater dielectric difference between the outer and middle layers. Figure 6b is the finite element simulation of the local electric field redistribution for the P/x vol % CFO@BZT−BCT NFs−P/P sandwich-structured (x = 1, 3, 5, and 7) composite. As described in the figure, the local electric field of the middle layer is smaller because of the larger dielectric constant (electric field lines are sparser); oppositely, the local electric field of the outer PVDF layer is greater because of the smaller dielectric constant (electric field lines are denser). Combined with the analysis of Table 1, we can get the detailed electric field distribution. The sandwich structure can be regarded as three kinds of capacitors connected in series, and the local electric field redistribution of each layer can be obtained according to the series capacitance partial pressure formula as follow:45 E1 =

E2 =

Figure 7. Comparison of energy density of the sandwich-structured P/ 5 vol % CFO@ BZT−BCT NFs−P/P composite and the related reported composites.

V 2d1 +

εp

d εb 2

V ε d 2 + 2 εb d1 p

(1)

meanwhile, the energy efficiency maintains at a high level (∼55.5%). This excellent performance is attributed to these following main reasons. First, a small amount of CoFe2O4 nanoparticles deposited on the surface of BZCT NFs increase the quantity of dipoles and further improve the interfacial polarization and dielectric constant of the composite. More importantly, the larger-aspect-ratio BZT−BCT NFs with low doping can improve the polarization of the PVDF/BZT−BCT NFs−PVDF/PVDF composite at low electric field, which may hinder the migration of carriers and the formation of a conductive path to reduce the conduction loss of the composite. Furthermore, the large-aspect-ratio fibers can also inhibit the electrical tree growth to enhance the energy-storage performance of the composite under low applied voltage. Remarkably, the redistribution of local electric field in each layer of the sandwich-structured composite has occurred because of the dielectric difference among three layers, which could hinder the growth of electrical treeing to reinforce breakdown strength of the composite. Finally, the compatibility between the inorganic filler and polymer matrix can be improved by the modifier of PDA to avoid the aggregation of the hybrid nanofibers in the composite. Thus, the P/5 vol % CFO@BZT−BCT NFs−P/P composite exhibits an excellent energy-storage performance as compared to other reported materials. Thus, this research has an important guiding value.

(2)

E1 is the local electric field of the outer PVDF film, and E2 is the local electric field of the middle x vol % CFO@BZT−BCT NFs−PVDF (x = 1, 3, 5, and 7) layer. V is the applied voltage on the composite; d1 is the thickness of the outer PVDF layer, and d2 is the thickness of the middle x vol % CFO@BZT−BCT NFs−PVDF (x = 1, 3, 5, and 7) layer. εp = 10.2. According to the calculation formula of series capacitor partial pressure, the numerical value of local electric field is shown in Table 1. It is found that the local electric field of each layer is different because of the redistribution of electric field caused by the various dielectric constants of each layer. The outer layer possesses a higher polarization because of the greater redistributed local electric field under a smaller applied electric field. More importantly, the middle layer possesses a weakened breakdown probability due to the lower redistributed local electric field under a smaller applied electric field. Accordingly, the middle layer with low redistributed electric field can hinder the growth of electrical treeing to improve breakdown strength of the composite.45 The introduction of a small amount of large-aspect-ratio and high-dielectric-constant CFO@BZT−BCT NFs can improve the breakdown strength of the sandwich composite, which is beneficial to further enhance the energy-storage density. Figure 7 and Table S2 (in the Supporting Information) give the comparison of energy-storage performance of the representative PVDF-based composite films.4,20,29,46−51 It can be found that the P/5 vol % CFO@BZT−BCT NFs−P/P composite in this work shows a better overall performance than most of the reported composites. For instance, when the applied electric field is 350 kV/mm, the discharged energy density reaches 11.3 J/cm3 at such a low electric field;

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CONCLUSIONS In summary, the sandwich-structured PVDF/x vol % BZT− BCT NFs−PVDF/PVDF and PVDF/x vol % CFO@BZT− BCT NFs−PVDF/PVDF dielectric composites with γ-phase have been prepared by combining electrospinning, solutioncasting, quenching, and hot-pressing. The CFO@BZT−BCT NFs prepared by hydrothermal method enhance the interfacial polarization and increase the number of dipoles in the 410

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with high breakdown strength and energy density utilizing boron nitride nanosheets. Energy Environ. Sci. 2015, 8, 922−931. (5) Pan, Z.; Zhai, J.; Shen, B. Multilayer hierarchical interfaces with high energy density in polymer nanocomposites composed of BaTiO3@TiO2@Al2O3 nanofibers. J. Mater. Chem. A 2017, 5, 15217−15226. (6) Chi, Q.; Ma, T.; Zhang, Y.; Cui, Y.; Zhang, C.; Lin, J.; Wang, X.; Lei, Q. Significantly enhanced energy storage density for poly(vinylidene fluoride) composites by induced PDA-coated 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 nanofibers. J. Mater. Chem. A 2017, 5, 16757−16766. (7) Chu, B.; Zhou, X.; Ren, K.; Neese, B.; Lin, M.; Wang, Q.; Bauer, F.; Zhang, Q. M. A dielectric polymer with high electric energy density and fast discharge speed. Science 2006, 313, 334−336. (8) Chi, Q.; Ma, T.; Dong, J.; Yang, C.; Yue, Z.; Zhang, C.; Xu, S.; Xuan, W.; Lei, Q. Enhanced Thermal Conductivity and Dielectric Properties of Iron Oxide/Polyethylene Nanocomposites Induced by a Magnetic Field. Sci. Rep. 2017, 7, 3072−3082. (9) Zhu, H.; Liu, Z.; Wang, F. Improved dielectric properties and energy storage density of poly(vinylidene fluoride-co-trifluoroethyleneco-chlorotrifluoroethylene) composite films with aromatic polythiourea. J. Mater. Sci. 2017, 52, 5048−5059. (10) Zhang, X.; Shen, Y.; Xu, B.; Zhang, Q.; Gu, L.; Jiang, J.; Ma, J.; Lin, Y.; Nan, C. W. Giant energy density and improved discharge efficiency of solution-processed polymer nanocomposites for dielectric energy storage. Adv. Mater. 2016, 28, 2055−2061. (11) Zhang, C.; Chi, Q.; Dong, J.; Cui, Y.; Wang, X.; Liu, L.; Lei, Q. Enhanced dielectric properties of poly(vinylidene fluoride) composites filled with nano iron oxide-deposited barium titanate hybrid particles. Sci. Rep. 2016, 6, 33508−33517. (12) Dang, Z. M.; Zheng, M. S.; Zha, J. W. 1D/2D carbon nanomaterial-polymer dielectric composites with high permittivity for power energy storage applications. Small 2016, 12, 1688−1701. (13) Chi, Q. G.; Dong, J. F.; Zhang, C. H.; Wong, C. P.; Wang, X.; Lei, Q. Nano iron oxide-deposited calcium copper titanate/polyimide hybrid films induced by an external magnetic field: toward high dielectric constant and suppressed loss. J. Mater. Chem. C 2016, 4, 8179−8188. (14) Mao, X.; Guo, W.; Li, C.; Yang, J.; Du, L.; Hu, W.; Tang, X. Low-temperature synthesis of polyimide/poly(vinylidene fluoride) composites with excellent dielectric property. Mater. Lett. 2017, 193, 213−215. (15) Meeporn, K.; Thongbai, P.; Yamwong, T.; Maensiri, S. Greatly enhanced dielectric permittivity in La1.7Sr0.3NiO4/poly(vinylidene fluoride) nanocomposites that retained a low loss tangent. RSC Adv. 2017, 7, 17128−17136. (16) Moharana, S.; Mishra, M. K.; Behera, B.; Mahaling, R. N. Enhanced dielectric properties of polyethylene glycol (PEG) modified BaTiO3 (BT)-poly(vinylidene fluoride) (PVDF) composites. Polym. Sci., Ser. A 2017, 59, 405−415. (17) Yang, M.; Zhao, H.; He, D.; Bai, J. Constructing a continuous amorphous carbon interlayer to enhance dielectric performance of carbon nanotubes/polyvinylidene fluoride nanocomposites. Carbon 2017, 116, 94−102. (18) Thakur, Y.; Zhang, B.; Dong, R.; Lu, W.; Iacob, C.; Runt, J.; Bernholc, J.; Zhang, Q. M. Generating high dielectric constant blends from lower dielectric constant dipolar polymers using nanostructure engineering. Nano Energy 2017, 32, 73−79. (19) Sharma, V.; Wang, C.; Lorenzini, R. G.; Ma, R.; Zhu, Q.; Sinkovits, D. W.; Pilania, G.; Oganov, A. R.; Kumar, S.; Sotzing, G. A. Rational design of all organic polymer dielectrics. Nat. Commun. 2014, 5, 4845−4853. (20) Shen, Y.; Shen, D.; Zhang, X.; Jiang, J.; Dan, Z.; Song, Y.; Lin, Y. H.; Li, M.; Nan, C. High energy density of polymer nanocomposites at low electric field induced by modulation of topological-structure. J. Mater. Chem. A 2016, 4, 8359−8365. (21) Furukawa, T. Ferroelectric properties of vinylidene fluoride copolymers. Phase Transitions 1989, 18, 143−211.

sandwich-structured P/x vol % CFO@BZT−BCT NFs−P/P composite under low applied voltage, which can improve the dielectric properties of the composite. Meanwhile, the large aspect ratio of BZT−BCT NFs with strong polarization can improve the dielectric and energy-storage properties of the composite at low electric field. More importantly, the middle layer of the sandwich-structured composite with a high dielectric constant induces the dielectric difference between the outer layer and the middle layer; therefore, there is the redistribution of internal electric field, which is conducive to improve the breakdown strength and the energy-storage performance. Herein, the P/5 vol % BZT−BCT NFs−P/P composite exhibits an excellent dielectric performance (the dielectric constant ∼20.1 at 100 Hz) and great energy-storage performance (the electric displacement ∼10.7 μC/cm2, the discharged energy density ∼11.3 J/cm3, and the efficiency ∼55.5% at such a low electric field of 350 kV/mm).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02659. Experimental methods, supplemental results, and additional figures including XRD patterns, SEM images, and D−E hysteresis loops (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone/Fax: +86-451-86391681. E-mail: [email protected]. (Q. Chi). *E-mail: [email protected]. (Q. Chen). ORCID

Qingguo Chi: 0000-0001-6923-337X Yue Zhang: 0000-0002-9325-3343 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the support of the National Natural Science Foundation of China (61640019), the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201601), the Youth Innovative Talents Training Plan of Ordinary Undergraduate Colleges in Heilongjiang (UNPYSCT-2016157), the Science Funds for the Young Innovative Talents of HUST (201102), and the Supported by Key Laboratory of Functional Materials Physics and Chemistry, Ministry of Education (Jilin Normal University) (2015002).

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