Ultrafast Discharge and High-Energy-Density of Polymer

Research Article pubs.acs.org/journal/ascecg

Ultrafast Discharge and High-Energy-Density of Polymer Nanocomposites Achieved via Optimizing the Structure Design of Barium Titanates Zhongbin Pan,† Lingmin Yao,‡ Jiwei Zhai,*,† Ke Yang,† Bo Shen,† and Haitao Wang† †

School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China School of Physics and Electronic Engineering, Guangzhou University, Guangzhou 510006, China

‡

S Supporting Information *

ABSTRACT: Electrostatic capacitors have been applied into high-power-density pulsed power systems comprising moderate energy density and ultrafast charging/ discharging in the order of magnitude with a few milliseconds. In this article, different BaTiO3 nanostructures (e.g., nanoparticles, nanofibers, nanotubes, core−shell structure nanofibers) were synthesized and used to prepare polymer composite films in the poly(vinylidene fluoride) (PVDF) matrix. The effects of BaTiO3 nanostructures on the dielectric constant, dielectric loss, alternating current conductivity, breakdown strength, energy density, and mechanical properties of nanocomposites were investigated systematically. The largest dielectric constant of 17.14 was found in the 4 vol % BaTiO3 nanotube/PVDF nanocomposites. The BaTiO3@Al2O3 nanofiber/PVDF nanocomposites show the highest energy density of 12.37 J cm−3 at 450 MV m−1, ultrafast discharge speed (0.32 μs), and good mechanical properties. This article could open up a convenient and effective way for practical application of power pulsed capacitors by tuning the filler nanostructure into polymer nanocomposites. KEYWORDS: Polymer nanocomposites, Nanostructure, Dielectric properties, Energy density, Capacitors

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Appropriate selection of materials with large εr and high Eb. (ii) Stabilization of nanofillers in a polymer matrix for homogeneous mixing. (iii) Optimization of material processing in order to improve morphology of dielectric films and minimize loss during energy extraction.11−15 Experimental practice and computational suggested that onedimensional (1D) materials (e.g., nanowires, nanofibers, nanorods, and nanotubes) have larger aspect ratios than zerodimensional (0D) materials (e.g., nanoparticles).16−21 The large aspect ratio has two advantages. First, the dielectric constant of nanocomposites could be more significantly enhanced by introducing a large-aspect-ratio filler at much lower dosage ( BT@Al2O3 NFs > BT NFs > BT NPs, which indicates the nanostructures of the fillers significantly influence the εr of their nanocomposites. The εr of the nanocomposites is calculated by the Maxwell−Garnett model as follows:8 εEff = ε1 + ε1

ε2 − ε1 c Σ 3 j = x , y , z ε1 + Nj(ε2 − ε1) c

1 − 3 Σj = x , y , z ε

Nj(ε2 − ε1)

1+

ax 2 az 2

is the eccentricity.

The measurements and theoretical calculations of dielectric constant are consistent. Remarkably, the BT NT nanocomposites show the highest dielectric constant of 17.14 and dielectric loss of 0.039 at 1 kHz (Table 1). The highest dielectric constant might mainly be ascribed to large Maxwell− Wagner−Sillars (MWS) interface polarization and space charge polarization, which both can be explained from two aspects: (i) internal interface polarization and space charge polarization in the BT NTs and (ii) external interface polarization and space charge polarization between BT NTs and PVDF matrix. The dielectric loss of BT NT nanocomposites could also result from two aspects: (i) internal electric conduction in the BT NTs and (ii) external electric conduction through accumulation or percolation of BT NTs in the PVDF matrix. Remarkably, the BT@Al2O3 NFs/PVDF nanocomposites show the lowest dielectric loss of 0.021 at 1 kHz (Table 1), which can be explained below. First, the nanocomposites may have the lowest MWS interfacial polarization after the introduction of moderating εr Al2O3 in between the fillers and polymeric matrix, within which the electric-field concentration could be mitigated.45,46 Second, Al2O3 acting as an insulated layer effectively confines the charge carrier movement within the space between the fillers and polymer matrix.47,48 Third, the fillers have good distribution and strong interface adhesion in the PVDF matrix through chemical bond. These results indicate that the interfacial surface area and aspect ratio of the nanofillers both play important roles in defining the dielectric behavior of the nanocomposites. To illustrate the thermal stability of pristine PVDF and BT nanoinclusion nanocomposites, we investigated the dielectric properties of nanocomposites as a function of temperature at different frequencies (1, 10, 100, and 1000 kHz) (Figure 5). The εr and tan δ vary in similar trend at all measured frequencies. The BT nanoinclusion/PVDF nanocomposites possess larger dielectric constant (attributed to dipolar and interfacial polarizations induced by the BT nanoinclusion filler) and lower dielectric loss (attributed to fewer molecular dipoles with the reduction of PVDF matrix) than pristine PVDF.49 The peaks of εr and tan δ of all nanocomposites gradually shift to higher temperature with the frequency increasing from 1 to 1000 kHz, which is a typical thermal relaxation. A relaxation process related to glass transition (often called α relaxation) should be ascribed to local micro-Brownian motions of segments in the main chains.50,51 Compared with the pristine PVDF, the peak of dielectric loss shifts to lower temperature after the introduction of BT nanoinclusions at the same frequency, which implies occurrence of the MWS interface polarization and an increase of trap density.52,53 To understand further the effect of the fillers on the MWS interfacial polarization relaxation of pristine PVDF and BT nanoinclusion/PVDF nanocomposites, the frequency-depend-

Figure 4. Dielectric constant (a), loss tangent (b), and AC conductivity (c) of nanocomposites filled with 4 vol % BT NPs, BT NTs, BT NFs, or BT@Al2O3 NFs.

Samples

1−

Nj(ε2 − ε1)

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DOI: 10.1021/acssuschemeng.7b00080 ACS Sustainable Chem. Eng. 2017, 5, 4707−4717

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. Dielectric properties of nanocomposites filled with bare PVDF (a), 4 vol % BT NPs (b), 4 vol % BT NTs (c), 4 vol % BT NFs (d), and 4 vol % BT@Al2O3 NFs (e) as a function of temperature at different frequencies.

ence of imaginary electric modulus (M″ =

ε )29,50,54 ε′ 2 + ε 2

of Al2O3 incorporated to the filler−matrix induces charge distribution, so the charges incline to other regions rather than being blocked on the interface. The experimental data suggest that the interphase and a Al2O3 insulator could effectively confine the mobility of free electrons and excessive current percolation, decreasing the leakage current and dielectric loss, which contributes to enhancing Eb and Ue. The BT NT/PVDF nanocomposites have the largest Ea. A large Ea indicates the fillers−matrix interface can accumulate more charge, increasing the leakage current and dielectric loss and thereby decreasing the breakdown strength and energy density. Electric dielectric breakdown, one type of material failure, was analyzed by using the Weibull distribution function.58 By contrast, the BT@Al2O3 NFs/PVDF nanocomposites exhibit the largest Eb of 450 MV m−1 (Figure 7b), which can be explained from three aspects. (i) The incorporation of a moderating-dielectric-constant Al2O3 interphase in between the BT NFs and PVDF matrix could decrease the local electric field. (ii) The Al2O3 insulation layer could effectively confine the mobility of free electrons and excessive current percolation, decreasing the leakage current and dielectric loss. (iii) The addition of dopamine effectively promotes the homogeneous filler distribution and the strong interfacial adhesion in the polymer matrix. Hence, the data of β indicate the reduction of defect quantity or size improves the Eb. Moreover, the strong interfaces can localize electrons, ions, and polymer chains, which provide more stable potential energy states and thereby

at high

temperatures were studied, as presented in Figure 6a−e. The interfacial polarization relaxation peaks (f max) in the nanocomposites move to high frequency with the temperature rise. The movement of f max could be ascribed to the charges blocked on the crystal/amorphous boundaries.50 Clearly, the nanocomposites have considerably lower relaxation intensity than the pristine PVDF. The result indicates that the incorporation of BT nanoinclusions could prevent the space charges from aggregating on the crystal/amorphous boundaries.55 The reciprocal of temperature (1000/T)-dependent Arrhenius plots (ln( f max)) of PVDF and BT nanoinclusion/PVDF nanocomposites are shown in Figure 6f and expressed as follows:56,57 ln fmax = ln f0 −

Ea kT

where f max, f 0, Ea, and k denote the peak maximum frequency of M″, pre-exponential factor, activation energy, and Boltzmann constant, respectively. The calculated Ea of pristine PVDF, BT NP/PVDF, BT NT/PVDF, BT NF/PVDF, and BT@Al2O3 NF/PVDF nanocomposites are 85.4, 87.48, 92.6, 84.89, and 80.35 kJ mol−1, respectively. The BT@Al2O3 NF/PVDF nanocomposites have the lowest Ea, indicating the lowest energy is required for space charge flow barriers and accumulated in the lamellar-crystal/interlamellar-amorphous boundaries of polymer matrix.50 In other words, the insulation 4712


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Figure 6. Frequency-dependent imaginary electric modulus of nanocomposites filled with bare PVDF (a), BT NPs (b), BT NTs (c), BT NFs (d), and BT@Al2O3 NFs (e) (dosage = 4 vol %); (f) temperature (1000/T)-dependent Arrhenius plots (ln( f max)) of pristine PVDF and BT nanoinclusion/PVDF nanocomposites.

nanocomposites (Figure 7b,c). This could be ascribed to the highest Dmax and Drem of BT NT/PVDF nanocomposites. It is noticed that the BT@Al2O3 NFs/PVDF nanocomposites show the highest energy density of 12.37 J cm−3 at 450 MV m−1 and relative high efficiency (η) of 63.7% (Figure 7b,c). The highest Ue could be attributed to the large polarization (8.39 μC cm−3) and high Eb (450 MV m−1). As for the high η, one reason is that the incorporation of a buffer layer and Al2O3 interphase reduced the leakage current and dielectric loss, and the fillers dispersed well and interfacial adhered strong in the matrix. Another important reason is a γ-PVDF phase was formed during the annealing and quenching.16 For the comparison of Ue and Eb among PVDF nanocomposits (Table S3), the value of our Ue and Eb are larger than most of the previous reported at the similar conditions. For instance, Shen et al. reported the Ue was 20 J cm−3 at 646 MV m−1 loading with 3 vol % BT@TO NFs and remarkably, at the relatively low electric field of 450 MV m−1, the Ue was about 10 J cm−3. Additionally, the mechanical properties of the electrostatic capacitor materials play an impartment role in practical applications. As shown in Figure 8a, as compared to pristine PVDF matrix, the tensile strains of BT NPs, BT NFs, and BT@ Al2O3 NFs nanocomposites only change very slightly. This

reduce the probability of breakdown. Interestingly, the BT NT/ PVDF nanocomposites show the lowest electric breakdown strength (250 MV m−1) and β (7.4) (Figure 7b). The lowest β indicates the presence of more microstructure defects in this nanocomposite film, which may be ascribed to special hollow structure of BT NTs. The lowest electric breakdown strength might be ascribed to the highest dielectric loss and AC conductivity compared with other BT nanoinclusion/PVDF nanocomposites. A polarization test uncovers the dielectric properties at high applied fields. The typical electric displacement (D)-dependent electric field (E) is shown in Figure S9. At the same applied electric field (250 MV m−1), the BT NT/PVDF nanocomposites show the highest Dmax (6.73 μC cm−3, attributed to the largest dielectric constant) and Drem (3.12 μC cm−3, attributed to the highest dielectric loss and AC conductivity) compared to other BT nanoinclusion nanocomposites (Figure 7a). However, the high Drem reduces the discharge energy, which is then computed from the D−E loops. Up to 250 MV m−1, the BT NT/PVDF nanocomposites have the largest energy density of 4.72 J cm−3 (U = ∫ E·dD) as well as the U lowest efficiency of 39.8% (η = U + eU ) among all the e

loss

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Figure 7. Maximum polarization (Dmax) and remnant polarization (Drem) (a), Weibull distribution (b), energy density (c), and efficiency (d) of pristine PVDF and BT nanoinclusion/PVDF nanocomposites.

Al2O3 NFs compared with 0D BT NPs show larger tensile stress (14.77, 28.67, 36.0 vs 11.57 MPa) and higher tensile modulus (695.3, 954.8, 1155.5 vs 472.6 MPa) (Figure 8b) because the former three have large aspect ratios. More importantly, the BT@Al2O3 NF nanocomposites exhibit the largest tensile stress and tensile modulus, which could be explained by two reasons. (i) The well-dispersion and strong interfacial adhesion of the fillers through chemical bonds restrict the mobility of the BT@Al2O3 NFs in the polymer matrix;59−63 (ii) The tensile modulus of Al2O3 is larger than BT and polymer matrix.64 For the actual applied pulsed power systems, the power density (P) and discharge energy density (W) of the pristine PVDF and BT nanoinclusion/PVDF nanocomposites capacitors were further evaluated by using an RLC circuit (Figure9 and S10). P and W of a capacitor were calculated as follows:65−67 P=

⎞ dW d ⎛1 dV = ⎜ CV 2⎟ = CV (t ) = V (t )I (t ) ⎠ dt dt ⎝ 2 dt

= I 2(t )R(t ) W=

Figure 8. Tensile strain dependence of tensile stress (a) and tensile stress and tensile modulus (b) of the BT nanoinclusion nanocomposites and pristine PVDF.

∫0

t

P dt =

∫0

t

I 2(t )R(t )dt

where C is the capacitance of a sample, R is the resistance of the circuit, V is the voltage of the sample, and t is time. The discharge rate is also primarily driven by the RCL time constant and thus is highly dependent on the load resistance. As shown in Figure 9a,b, the BT NT/PVDF nanocomposites have the largest power density (2.01, 1.25, 1.48, 1.90, and 1.53 MW· cm−3, respectively) and discharge energy density (3.04, 1.56, 2.39, 2.73, and 2.45 J·cm−3, respectively) compared with pristine PVDF, BT NP/PVDF, BT NF/PVDF, and BT@Al2O3 NF/PVDF nanocomposites at 200 MV m−1. More notably, its discharge energy density (Figure 9b) is lower than that

might be caused by the improvement of interfacial filler−matrix interactions via dopamine. It is noteworthy that the BT NT/ PVDF nanocomposites have the lowest tensile strain among all nanocomposites, because the special hollow structure leads to the formation of more microstructure defects. All BT nanoinclusion/PVDF nanocomposites have larger tensile moduli than pristine PVDF films (Figure 8b), which is due to the contribution of BT nanoinclusion. Additionally, the nanocomposites filled with 1D BT NTs, BT NFs, and BT@ 4714


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Figure 9. Power density (a) and discharge energy density (b) of the pristine PVDF and BT nanoinclusion/PVDF nanocomposites as functions of time; power density (c) and discharge energy density (d) of the BT@Al2O3 NF/PVDF nanocomposite capacitors as functions of time. The load resistor R0 is 200 Ω and the electrical field is 400 MV m−1.

measured from the D−E loop at 200 MV m−1 (Figure 7c), which is consistent with some previous reports.68,69 Because the pulse process might finish in short time (at microsecond scale), so the materials might not be depolarized fully.70 The depolarization speed of the D−E curves reaches the nanosecond scale, which suggests the test frequency should be more than 100 kHz. The time constant τ0.9 is defined as the time when the discharged energy in the load reaches 90% of the maximum value from the discharge curve. The pristine PVDF has the fastest discharge rate of ∼0.108 μs at 200 MV m−1 compared to the other BT nanoinclusion/PVDF nanocomposites, probably because the pristine PVDF has the lowest discharge energy density at the same condition. To apply further the pulsed power capacitors at relativity high electric filed, the P and W of the BT@Al2O3 NFs/PVDF nanocomposite capacitor were further evaluated, as presented in Figure 9c,d. This BT@Al2O3 NFs/PVDF nanocomposite capacitor has a fast discharge rate of ∼0.32 μs at 400 MV m−1 (Figure 9c,d). Calculations from discharged curves show the BT@Al2O3 NFs/PVDF nanocomposite capacitor has a power density of ∼5.2 MW cm−3 and a discharge energy density of ∼7.96 J cm−3 at 0.32 μs, respectively (Figure 9c).

highest energy density of 12.37 J cm−3 at 450 MV m−1, ultrafast discharge speed (0.32 μs), and good mechanical properties. It is anticipated that the energy densities of these polymeric nanocomposites could be further improved by tuning the filler nanostructures and used for pulsed power applications.

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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.7b00080. XRD, DSC curves, TGA curves, frequency versus the εr and tan δ, D−E hysteresis of pristine PVDF and their nanocomposites; the εr and tan δ of pristine PVDF and their nanocomposites at different temperature; schematic illustration of improved coaxial electrospinning and charge−discharge experimental; Eb and Ue of the PVDF-based nanocomposites (PDF)

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AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS Flexible composite films consisting of different BaTiO 3 nanostructures (BT NPs, BT NTs, BT NFs, and BT@Al2O3 NFs) and the ferroelectric PVDF matrix were prepared. The effects of BaTiO3 nanostructures on the dielectric constant, dielectric loss, AC conductivity, breakdown strength, energy density, and mechanical properties were investigated systematically. The nanofiller/matrix compatibility and distributional homogeneity were improved by coating the nanofiller with dopamine. The largest dielectric constant of 17.14 was found in the 4 vol % BaTiO3 nanotube/PVDF nanocomposites. The BaTiO3@Al2O3 nanofiber/PVDF nanocomposites show the

*J. Zhai. E-mail: [email protected]. ORCID

Zhongbin Pan: 0000-0002-7522-5840 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China through 973-project under Grant (2015CB654601). 4715


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(19) Pan, Z. B.; Yao, L. M.; Zhai, J. W.; Fu, D. Z.; Shen, B.; Wang, H. T. High-energy- density polymer nanocomposites composed of newly structured one-dimensional BaTiO3@Al2O3 nanofiber. ACS Appl. Mater. Interfaces 2017, 9, 4024−4033. (20) Yao, L. M.; Pan, Z. B.; Zhai, J. W.; Chen, H. H. D. Novel design of highly [110] -oriented barium titanate nanorod array and its application in nanocomposite capacitors. Nanoscale 2017, 9, 4255− 4264. (21) Varghese, J.; Whatmore, R. W.; Holmes, J. D. Ferroelectric nanoparticles, wires and tubes: synthesis, characterisation and applications. J. Mater. Chem. C 2013, 1, 2618−2638. (22) Liu, S.; Xue, S.; Zhang, W.; Zhai, J.; Chen, G. Significantly enhanced dielectric property in PVDF nanocomposites flexible films through a small loading of surface- hydroxylated Ba0.6Sr0.4TiO3 nanotubes. J. Mater. Chem. A 2014, 2, 18040−18046. (23) Liu, S.; Zhai, J.; Wang, J. W.; Xue, S. X.; Zhang, W. Q. Enhanced energy storage density in poly(vinylidene fluoride) nanocomposites by a small loading of suface-hydroxylated Ba0.6Sr0.4TiO3 nanofibers. ACS Appl. Mater. Interfaces 2014, 6, 1533−1540. (24) Zhang, Q.; Gao, F.; Zhang, C.; Wang, L.; Wang, M.; Qin, M.; Hu, G.; Kong, J. Enhanced dielectric tunability of Ba0.6Sr0.4TiO3/ Poly(vinylidene fluoride) composites via interface modification by silane coupling agent. Compos. Sci. Technol. 2016, 129, 93−100. (25) Tang, H.; Lin, Y.; Andrews, C.; Sodano, H. A. Nanocomposites with increased energy density through high aspect ratio PZT nanowires. Nanotechnology 2011, 22, 015702. (26) Yao, L. M.; Pan, Z. B.; Liu, S. H.; Zhai, J. W.; Chen, H. H. D. Significantly enhanced energy density in nanocomposite capacitors combining the TiO2 nanorod array with Poly(vinylidene fluoride). ACS Appl. Mater. Interfaces 2016, 8, 26343−26351. (27) Dang, Z. M.; Zhou, T.; Yao, S. H.; Yuan, J. K.; Zha, J. W.; Song, H. T.; Li, J. Y.; Chen, Q.; Yang, W. T.; Bai, J. Advanced calcium copper titanate/polyimide functional hybrid films with high dielectric permittivity. Adv. Mater. 2009, 21, 2077−2082. (28) He, D.; Wang, Y.; Chen, X.; Deng, Y. Core−shell structured BaTiO3@Al2O3 nanoparticles in polymer composites for dielectric loss suppression and breakdown strength enhancement. Composites, Part A 2017, 93, 137−143. (29) Pan, Z. B.; Yao, L. M.; Zhai, J. W.; Liu, S. H.; Yang, K.; Wang, H. T.; Liu, J. H. Fast discharge and high energy density of nanocomposite capacitors using Ba0.6Sr0.4TiO3 nanofibers. Ceram. Int. 2016, 42, 14667−14674. (30) Parizi, S. S.; Mellinger, A.; Caruntu, G. Ferroelectric barium titanate nanocubes as capacitive building blocks for energy storage applications. ACS Appl. Mater. Interfaces 2014, 6, 17506−17517. (31) Almadhoun, M. N.; Bhansali, U. S.; Alshareef, H. N. Nanocomposites of ferroelectric polymers with surface-hydroxylated BaTiO3 nanoparticles for energy storage applications. J. Mater. Chem. 2012, 22, 11196−11200. (32) Liu, S.; Zhai, J. Improving the dielectric constant and energy density of poly(vinylidene fluoride) composites induced by surfacemodified SrTiO3 nanofibers by polyvinylpyrrolidone. J. Mater. Chem. A 2015, 3, 1511−1517. (33) Jung, H. M.; Kang, J. H.; Yang, S. Y.; Won, J. C.; Kim, Y. S. Barium titanate nanoparticles with diblock copolymer shielding layers for high-energy density nanocomposites. Chem. Mater. 2010, 22, 450− 456. (34) Kim, P.; Doss, N. M.; Tillotson, J. P.; Hotchkiss, P. J.; Pan, M. J.; Marder, S. R.; Li, J.; Calame, J. P.; Perry, J. W. High energy density nanocomposites based on surface-modified BaTiO3 and a ferroelectric polymer. ACS Nano 2009, 3, 2581−2592. (35) Paniagua, S. A.; Kim, Y.; Henry, K.; Kumar, R.; Perry, J. W.; Marder, S. R. Surface-initiated polymerization from barium titanate nanoparticles for hybrid dielectric capacitors. ACS Appl. Mater. Interfaces 2014, 6, 3477−3482. (36) Barber, P.; Balasubramanian, S.; Anguchamy, Y.; Gong, S.; Wibowo, A.; Gao, H.; Ploehn, H. J.; Loye, H. C. Polymer composite and nanocomposite dielectric materials for pulse power energy storage. Materials 2009, 2, 1697−1733.

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