Ferroelectric Polymer Nanocomposites with Complementary

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Ferroelectric Polymer Nanocomposites with Complementary Nanostructured Fillers for Electrocaloric Cooling with High Power Density and Great Efficiency Guangzu Zhang,†,‡ Baoyan Fan,†,§,‡ Peng Zhao,∥,‡ Zhaoyao Hu,† Yang Liu,§ Feihua Liu,§ Shenglin Jiang,† Sulin Zhang,∥ Honglang Li,⊥ and Qing Wang*,§ †

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ∥ Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ⊥ Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China §

S Supporting Information *

ABSTRACT: The exploration of electrocaloric cooling is of great importance to address the environmental and energyefficiency issues in the currently available refrigeration technologies. Although pronounced electrocaloric effect (ECE) has been demonstrated in ferroelectric materials, it is a necessary but far from sufficient condition to achieve substantial cooling. For instance, the narrow operation temperature windows and limited thermal conductivities of ferroelectric materials pose challenging obstacles for ferroelectric materials to realize high cooling power density and great cooling efficiency. In this work, we present polymer nanocomposites with multiple nanostructured fillers, including barium strontium titanate nanowires (BST NWs) with systematically varied Curie temperatures and boron nitride nanosheets (BNNSs). The introduced BST NWs effectively enhance EC strength and significantly extend the operating temperature so that giant ECE is achieved at relatively low electric fields in a wide temperature range. Meanwhile, it is found that BNNSs form an electrically insulating and thermally conductive network in the nanocomposites, resulting in remarkable enhancements in dielectric breakdown strength and thermal conductivity. As validated by the finite element simulations, the synergistic integration of multiple components with complementary functionalities, such as BST NWs and BNNSs, in the ferroelectric polymer renders the nanocomposites with unprecedented high cooling power densities and great cooling efficiencies. Coupled with the facile processability of polymers and lead-free nature of electroactive ceramics, the polymer nanocomposites unleash the immense potential of ECE for environmentally friendly and highly efficient cooling applications. KEYWORDS: polymer nanocomposites, nanowires, boron nitride nanosheets, electrocaloric effect, ferroelectricity

1. INTRODUCTION Cooling is ubiquitous in the contemporary society and contributes largely to the global energy consumption. For example, air conditioners, owned by two-thirds of homes in the United States, consume more than 5% of all the electricity generated in the nation.1 However, by contrast with the rapid evolution in electronics, no revolution improvement in cooling efficiency has been achieved since the first artificial refrigeration was demonstrated by W. Cullen in 1755.2 Today, more than 90% commercial cooling facilities, for example, refrigerators and © XXXX American Chemical Society

air conditioners, are based on the mechanical compression− expansion technology, which has dominated the refrigeration field for more than one century with the cooling efficiencies (η, represented by the ratio of the absorbed heat versus total input work, η = Q/W) around 3.5.3 More problematically, fluorinated coolants, such as hydrochlorofluorocarbons and hydrofluorReceived: January 13, 2018 Accepted: February 23, 2018

A

DOI: 10.1021/acsaem.8b00052 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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severely reducing the actual cooling performance in spite of strong ECE.32 In this contribution, we present the solution-processable ferroelectric polymer nanocomposites consisting of multiple nanostructured inorganic fillers, including barium strontium titanate (BST) nanowires (NWs) with systematically varied Curie temperatures (TC) and boron nitride nanosheets (BNNSs). It is found that the EC nanocomposites display strong and stable ECE over a wide temperature range from 0 to 60 °C under both relatively low and high electric fields, which is attributable to the introduction of multiple BST NWs possessing large electric polarization and optimized phase transition behavior, and BNNSs with high breakdown strength. Concomitantly, the incorporated BNNSs greatly improve the thermal conductivity of the polymer nanocomposites. Finite element simulations validate the substantial improvement in cooling power density and cooling efficiency of the polymer nanocomposites in comparison to pristine ferroelectric polymers as a result of the significantly upgraded ECE and superior thermal conductivity.

ocarbons, that damage the Earth’s ozone layer or accelerate global warming are indispensable components of the conventional compression−expansion cooling systems.2−4 Among various mechanisms that are currently exploited for the next-generation cooling technologies, the electrocaloric effect (ECE), that is, the reversible thermal changes of an electrically polarizable material upon the application and removal of an electric field (E), is particularly promising because it is environmentally friendly and enables the realization of all solid-state compact cooling devices with a high cooling efficiency.4−7 After decades of intensive efforts, pronounced improvements on ECE in terms of isothermal cooling energy density (Q), adiabatic change of temperature (ΔT), isothermal change in entropy (ΔS), and EC strength represented by the values of |Q|/|ΔE|, |ΔT|/|ΔE|, and |ΔS|/|ΔE| have been achieved in a variety of ferroelectric materials including single crystals, ceramic bulks, thin films, and polymers.8−23 More recently, to combine the unique characteristics of inorganic and organic ferroelectric materials, such as large EC strength of ferroelectric ceramics and high breakdown strength of ferroelectric polymers, the ferroelectric ceramicpolymer nanocomposites have been developed as a new class of EC materials.24 It is, however, important to note that the EC properties mentioned above are necessary but far from sufficient conditions for dielectric refrigeration to achieve high cooling power density and efficiency. As schematically shown in Figure S1, the Ericsson cycle that consists of two isothermal and two iso-field processes has been proposed for practical EC cooling operation.25 The Carnot efficiency (ηC) is the highest cooling efficiency that can be attained in a cooling system, that is, ηC = TL/(TH − TL), where TL and TH are the temperatures of cold and hot sides, respectively, of the regenerator. For the Ericsson cycle to achieve an efficiency close to the Carnot efficiency, the two iso-field entropy-temperature curves have to be parallel with each other; in other words, the |ΔS| of process III (removal of electric field) at TL needs to be equal to |ΔS| of process I (application of electric field) at TH (Figure S1).25 Therefore, it is crucial for ferroelectric materials to exhibit stable ECE over the whole operating temperature range with the aim of reaching high cooling efficiencies. However, as ECE is correlated to the field-induced change of electric dipole entropy, it is most effective only within a narrow temperature range in the vicinity of the ferroelectric-paraelectric phase transition of ferroelectric materials.26−28 To address the limitation of currently available EC materials with a narrow operating temperature window, a cascaded configuration has been intricately designed for EC devices as illustrated in Figure S2, in which each EC element is operated in their respective effective temperature range between the serially connected regenerators.29 In addition, it is well-accepted that the cooling power density of EC refrigerators can be improved by increasing the operating frequency without compromising cooling efficiency in the ideal case that the EC apparatuses are provided with an infinite thermal conductivity. In practice, nevertheless, the obtained EC refrigerating in the materials would be partially or even fully offset by the heat generated during the entropy-decrement process when operating at a high frequency, resulting in significant reductions in both cooling power capacity and cooling efficiency in comparison to the ideal cases.30,31 This is because the limited thermal conductivity hinders the heat flow in the EC materials and between the material-device interfaces,

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ba1−xSrxTiO3 NWs. The Ba1−xSrxTiO3 NWs with different Ba:Sr ratios (60:40, 67:33, and 75:25) were prepared with a two-step hydrothermal method.33,34 The precursor of Na2Ti3O7 NWs was fabricated by the first-step hydrothermal reaction. NaOH (Fisher, ACS, 99%) aqueous solution (10 M, 100 mL) with 0.25 M TiO2 (Sigma-Aldrich, ACS, 99%) powders were added into the autoclave (PARR 4748A) and then heated to 200 °C for 48 h. After it was cooled down to ambient temperature, the yielded Na2Ti3O7 NWs were collected, washed, and dried at 60 °C overnight. In the second step, the Na2Ti3O7 NWs and 0.2 M Ba(OH)2·8H2O + Sr(OH)2·8H2O (Aldrich 98% and Aldrich 95%, respectively) aqueous solution were sealed in the autoclave and heated at ∼80 °C for 12 h, converting the sodium titanate NWs to BST NWs by diffusing Ba2+ and Sr2+ ions into the precursor. The obtained BST NWs were then washed and dried at 60 °C overnight. 2.2. Preparation of BNNSs. Eight grams of hexagonal boron nitride powder (Aldrich, 98%) were dispersed in ∼400 mL of N,Ndimethylformamide (DMF, Sigma-Aldrich, ACS, 99.8%) and then sonicated with a tip-type sonication for 48 h. Subsequently, the suspension was centrifuged at 3000 rpm for 30 min, and the supernatant was collected and centrifuged again at 10 000 rpm for another 20 min to precipitate the BNNSs. The precipitated BNNSs were dried under vacuum at 70 °C for 12 h. 2.3. Fabrication of the Nanocomposites. P(VDF-TrFE-CFE) (62.3/29.9/7.8 mol %) was first dissolved in DMF (4 wt %) and stirred overnight to form a homogeneous solution. Thereafter, the BST NWs and BNNSs were added into the solution with required compositions (Tables S1 and S2) and ultrasonicated (tip-type sonication) for 10 min. The mixtures were casted onto glass plates and dried at 60 °C for 16 h. Thereafter, the films were peeled off from the glass, heated at 90 °C and annealed at 105 °C for 12 h in a vacuum oven. 2.4. Characterization. X-ray diffraction was obtained using PANalytical Xpert pro MPD (Cu Kα radiation). Microstructures of the fillers were observed by Hitachi S-4800 field emission scanning electron microscope (FE-SEM) and Joel JEM-2001F transmission electron microscopy (TEM). TA Instrument Q100 differential scanning calorimeter was used to scan the DSC curves with a heating rate of 10 °C min−1. The thermal conductivity of the samples was measured by using Hot Disk thermal constants analyzer (TPS 2200). Instron 5866 with a 200 N load cell was used to test Young’s modulus of the specimens. For electrical properties measurement, the broadtemperature dielectric spectra were conducted by a Hewlett-Packard LCR (4284A) with a Delta Design oven. Hewlett-Packard 4140B pA meter/voltage source connected with KEPCO BOP 1000 M amplifier B

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Figure 1. (a−c) SEM images of the BST1, BST2, and BST3 NWs, respectively. (d) Cross-section SEM image of the P(VDF-TrFE-CFE)-BST1/2/3 nanocomposites. (e) XRD patterns of P(VDF-TrFE-CFE) and BST1, BST2 and BST3 NWs. (f) XRD patterns of the nanocomposites with different BST compositions. where ρ is the density and c the specific heat of the material, both are assumed as constant and isotropic throughout the material. Because any heat transfer between the structure and the air is assumed prohibited, the temperature field is a function of z only (Figure S4) and time t, T = T (z, t). The problem thus can be reduced to onedimensional thermal-structural analysis, which describes the governing equation as

was employed to measure the electrical resistivity of the samples. The electric hysteresis loops were obtained by the modified Sawyer−Tower circuit. The dielectric breakdown strength was tested on a TREK 610E with a voltage ramp of 500 V s−1. Two-parameter Weibull statistic was used to calculate the breakdown strength: P(E) = 1 − exp[−(E/Eb)β], in which P(E) is the cumulative probability of electric breakdown, E is the tested breakdown field, Eb is the scale data characterized breakdown strength that corresponds to a failure probability of ∼63%, and β is the slope that evaluates the measured data. The ECE was characterized by the heat flux sensor (RdF P/N 27134-3) and the details were presented in the previous report.24 The cooling efficiency is used to evaluate how much cooling energy generated by a unit volume of EC material (Q) versus the consumed electrical work (WE) in one operating cycle.15 As schematically shown in Figure S3, the electric energy charged into a unit volume of the material to generate ECE is equal to the cyan area of the electric hysteresis loop. In this work, the electric works consume by the unit-volume EC materials were calculated according to the tested electric hysteresis loops shown in Figures S12 and S16. 2.5. Finite Element Simulation. In this section, a single unit of a solid-state chip/film structure is considered without any geometrical restriction. To have a deep insight into the influence of thermal conductivity on the performance of EC cooling, only heat conduction is taken into account and the model excludes any other form of heat transfer such as thermal convection and radiation or any thermal strain and expansion. Under these conditions, conduction takes place within the boundaries of a film by the diffusion of its internal energy. The temperature within the body, T, is given in units of degrees Celsius or Kelvin. Its variation in space defines the temperature gradient vector, ∇T, with units of K/m. The heat flux vector, q, is define by Fourier’s Conduction Law, as the thermal conductivity, k, times the negative of the temperature gradient q = − k ∇T

ρc

∂T = ∇(k∇T ) ∂t

(3)

where Q(z) is the heat generation, which can be ignored here because there is no heat source in the chip/film structure. Other than all material properties needed for a steady thermal condition, a typical transient heat transfer scenario also requires initial conditions to describe the beginning state and boundary conditions for later times. The finite element simulation is based on ABAQUS. Table S3 lists all material properties and parameters used in the simulation. Transient heat transfer simulation in time requires initial conditions to describe the beginning state and boundary conditions for later times. The thermal sink is located at the bottom of the module and its thermal conductivity is assumed to be infinite. It is treated as a surface boundary condition during each transit thermal conduction analysis. Hence, the temperature at the bottom surface has been set equal to a fixed value, T0 as room temperature (25 °C), throughout the whole simulation scenario. Any heat radiation or convection on all other surfaces is prohibited. Additionally, a single cooling cycle can be seen as a series of two substeps for ABAQUS, each has a prescribed temperature field due to ECE. After each step, the resulting temperature field can be modified according to the volumetric heat generation or absorption due to the EC effect, which is taken as the initial condition (a prescribed temperature field) for the next step. After several cycles have been taken on, the resulting temperature field is compared with the step right before, to determine whether the whole process has reached to a steady state (Figure S5).

(1)

3. RESULTS AND DISCUSSION To achieve giant ECE with a wide operating temperature range, poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), (P(VDF-TrFE-CFE), 62.3/29.9/7.8 mol %) terpolymer, a ferroelectric relaxor with a broad phase-transition temperature

The governing equation, which relates the temperature change to the heat flux, can be written as ρc

∂T ∂T ⎛⎜ ∂T ⎞⎟ k = + Q (z) ∂t ∂z ⎝ ∂z ⎠

(2) C

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Figure 2. (a) ΔS and (b) its temperature stability of P(VDF-TrFE-CFE) and the P(VDF-TrFE-CFE)-BST binary composites at 50 MV m−1.

m−1. Apparently, substantial improvement in ECE has been achieved in the binary polymer nanocomposites in comparison to that of the neat terpolymer. For example, under an applied electric field of 50 MV m−1, the binary nanocomposite with 6 vol % BST1 exhibits a sizable ECE including a Q of 8.1 MJ m−3, ΔT of 3.2 °C, and ΔS of 29.1 kJ m−3 K−1 at an operation temperature of around 15 °C (Figures 2 and S8), which is more than 3 times that of P(VDF-TrFE-CFE) and also surpasses the ECE of the most recently developed P(VDF-TrFE-CFE)/ P(VDF-TrFE) polymer blends.38 The remarkable enhancement of ECE in the polymer nanocomposites is ascribable to the comparably higher dielectric constant and electric polarization of BST than those of the polymer matrix. The introduced BSTs would create a strong local electric field at the nanofiller− polymer interfacial area to promote the dipole polarization of P(VDF-TrFE-CFE).39,40 In addition, BST fillers interact with the polymer chains and alter their crystallization behavior. As summarized in Figure S11, BST NWs increase the crystallinity and decrease the crystal size of P(VDF-TrFE-CFE), which respectively gives rise to a high electric polarization in the ferroelectric polymer and allows the polymer long chains to be easily flipped under a relatively low electric field.41 Since ECE stems from the entropy change of electric dipole polarization, the facilitation of dipole flipping directly results in higher ECE observed in the binary nanocomposites. As substantiated by Figure S11, the relative dielectric permittivity increases from ∼50 of the pristine terpolymer to ∼60 of the nanocomposite with 6 vol % BST NWs at ambient temperature. Correspondingly, a similar increment in the electric polarization has been observed in the P(VDF-TrFE-CFE)-BST binary nanocomposites (Figure S12). The decisive role of BST NWs on the ECE of the nanocomposites is further manifested by the shift of the maximum Q, ΔT, and ΔS toward high temperatures with increasing TC of the nanofillers. As clearly seen in Figures 2 and S8−S10, with the introduction of individual BST1, BST2, and

and the largest ECE in the reported ferroelectric polymers,5,16,35 was selected as the matrix for the polymer nanocomposites. BST NWs, a lead-free normal ferroelectric material, were chosen as the nanofillers since BST is not only environmental friendly but also has been demonstrated a larger enhancement of ECE of ferroelectric polymer than many leadcontaining ferroelectric ceramics, such as lead magnesium niobate-lead titanate (PMN-PT).36 Moreover, it is known that geometry of nanofillers plays an important role in determining the dielectric properties, breakdown strength, and consequently, ECE of polymer nanocomposites.37 Compared to 0D nanoparticles, 1-D NWs are advantageous for their greater enhancements in polarization and polarization change vs temperature, and much less reduction in breakdown strength of polymer matrix.37 Through the control of the diffusion contents of Ba2+ and Sr2+, that is, Ba/Sr molar ratio = 60/40, 67/33, and 75/25, the BST NWs with different Tc of 6, 30, and 55 °C (Figure S7a) were obtained, which are denoted as BST1, BST2, and BST3, respectively. The successful synthesis of BST NWs was corroborated by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). Representative XRD patterns of perovskite phase are shown in Figure 1e, where all the diffraction peaks are exclusively assigned to the BST phase structure without any implication of incompletely reacted precursor or crystalline byproducts such as BaCO3, SrCO3, and TiO2. As illustrated in Figure 1a−c, the prepared BST NWs with various Ba/Sr ratios have an average diameter of ∼200 nm and a mean length of ∼8 μm. Different components of BST NWs with various Ba:Sr ratios were mixed with P(VDF-TrFE-CFE) (Table S1). For ECE characterization, Q was directly measured by using a heat flux sensor on cooling while ΔS and ΔT were deduced from Q = TΔS = Cp·ΔT, where T is the environmental temperature and Cp is the heat capacity of the EC material.5 Figures 2 and S8− S10 present the ECE of the P(VDF-TrFE-CFE)-BST nanocomposites under the electric fields ranging from 25 to 75 MV D

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ACS Applied Energy Materials BST3 with continuously increased Tc into the polymer, the best ECE observed in the resulting binary nanocomposites appears at around 15, 40, and 60 °C, respectively. This distinct temperature-dependent behavior can be rationalized by the basic mechanisms of ECE, which is associated with the temperature dependent polarization and related entropy change among the dipole states in response to the applied electric field.4,5 It is known that the increase of temperature near TC lead to a sharp decrease of polarization, and hence, a high |∂P/ ∂T| (the change of polarization vs temperature) in BST.42,43 Owing to the interfacial coupling effect,44 the temperaturesensitive polarization of BST dominates the ferroelectric characteristics of P(VDF-TrFE-CFE), and consequently, produces the maximized ECE in the nanocomposites within a narrow temperature range around its TC. Interestingly, the temperature stability of ECE in the nanocomposites can be effectively ameliorated by simultaneously using multiple fillers with different TC. When the single BST NW fillers (6 vol %) were replaced by BST1 (3 vol %) and BST3 (3 vol %), the ECE of the P(VDFTrFE-CFE)-BST1/3 composite is stable from 0 to 25 °C and then drops around 30 °C, which is in the middle of the respective TC values of BST1 and BST3. As also presented in Figures 2 and S8−10, the ECE becomes much more stable versus temperature when triple fillers, that is, BST1 (2 vol %), BST2 (2 vol %), and BST3 (2 vol %), were added in the nanocomposites together. As plotted in Figure S10, for instance, with an applied electric field of 75 MV m−1, the nanocomposite exhibits a remarkable ECE-temperature stability (represented by the values of |Q/Q@25 °C| × 100%, |ΔT/ ΔT@25 °C| × 100%, and |ΔS/ΔS@25 °C| × 100%, respectively) of >90% in the temperature range from 0 to 60 °C, which is significantly superior to the single-composition BST filled nanocomposites with a temperature stability of ∼50%. Compared to the peak values of the ECE of the nanocomposites containing single-component BST NWs, only a small decrease (i.e., ∼ 10%) in Q, ΔT, and ΔS have been observed in the multiple BST-doped composites. Similar trend is found in the nanocomposites at the electric fields of 25 and 50 MV m−1 as well. Note that relaxor 0.9Pb(Mg1/3Nb2/3)O30.1PbTiO3 (PMN−PT) ceramic nanoparticles have been utilized in the EC polymer composites to improve the ECE and the temperature stability.36 However, the temperature stability of ECE found in the P(VDF-TrFE-CFE)/PMN−PT composite is below 80%. Also, the enhancement of PMN−PT on the ECE of the terpolymer is obviously inferior to that of the multiple BST NWs. For instance, at an electric field of 50 MV m−1, the P(VDF-TrFE-CFE)/PMN−PT composite shows a ΔT of ∼2 °C, only 60% that of the P(VDF-TrFE-CFE)-BST1/ 2/3. It is worth mentioning that the majority of highperformance ferroelectric ceramics for EC applications, including lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT) and PMN−PT,8,45−49 to name but a few, contain the environmentally hazardous element of lead, which further highlights the importance of the strategy reported herein. Although dramatic improvements in both ECE and its temperature stability have been demonstrated, the key drawback of the ferroelectric organic−inorganic nanocomposites is the sharp decrease of dielectric breakdown strength compared to the pristine ferroelectric polymer, for example, 350 MV m−1 of P(VDF-TrFE-CFE) vs 300 MV m−1 of the P(VDF-TrFE-CFE)-BST1/2/3 nanocomposites, as determined

from the exponential-Weibull statistics (Figure 3). The reduction in breakdown strength is due to the inhomogeneous

Figure 3. (a) Weibull plots and (b) breakdown strength of the P(VDF-TrFE-CFE)-BST binary and P(VDF-TrFE-CFE)-BNNSs-BST ternary nanocomposites.

field distribution as a result of the dielectric contrast between the fillers and matrix as well as structure defects in the multicomponent heterogeneous structures.39,40,50 High breakdown strength is necessary for EC materials to exhibit large ECE at high electric fields (as indicated by the Belov−Goryaga theory, the ECE is proportional to the square of electric fields in ferroelectric polymers51−53) and also enables reliable operation of EC materials and devices.4,5 To overcome the breakdown limitation in the ferroelectric nanocomposites, boron nitride nanosheets (BNNSs), a two-dimensional crystal with a wide band gap of ∼6 eV and a high breakdown strength of 800 MV m−1,54 were incorporated into the polymer matrix together with BST NWs to form the P(VDF-TrFE-CFE)BNNSs-BST ternary nanocomposites (Table S2, Figures 4a−c and S13). As summarized in Figure 3b, the ternary nanocomposites with 8 vol % BNNSs possess a considerably high breakdown strength of ∼470 MV m−1, which not only is 150% that of the binary nanocomposites but also outperforms pure P(VDF-TrFE-CFE). Polymeric materials are known to fail mainly through the avalanche breakdown and electromechanical deformation under the applied fields.55,56 As depicted in Figures S14 and S15, the introduction of BNNSs markedly decreases the electric conductivity and simultaneously strengthens the Young’s modulus of the nanocomposites, impeding both the electrical and electromechanical breakdown of the nanocomposites when subjected to a high electric field. With high breakdown strength, the nanocomposites can operate efficiently at both relatively low and high electric fields. It is found that the P(VDF-TrFE-CFE)-BNNSs-BST1/2/3 E

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thermal conductivity severely hinders the heat flow in the material, most heating and cooling energy of ECE are blocked in the films after the application and removal of the electric field with an interval of 0.1 s. As a result, the yielded EC refrigerating in the EC material would be partially or even fully offset by the heat when operated with a high frequency, leading to a low cooling power density and cooling efficiency. This challenge can be addressed via rational design of the compositions of the nanocomposites, for example, introduction of BNNSs. Because of their particularly high thermal conductivity >300 W m−1 K−1 and 2-dimensional structure,64,65 BNNSs are capable of forming an effective thermal conductive network in the nanocomposites with a loading of 8 vol % as shown in Figure 4d−f. As a result, a thermal conductivity over 1.2 W m−1 K−1 is obtained in the P(VDF-TrFE-CFE)-BNNSs-BST1/2/3 ternary nanocomposites, which is 6 times that of the pristine P(VDF-TrFE-CFE) (Figure 6a) and comparable to the ferroelectric ceramics such as lead zirconate titanate with a thermal conductivity of ∼1.2 W m−1 K−1.66,67 The large increase of thermal conductivity allows a rapid heat flow in the P(VDF-TrFE-CFE)-BNNSs-BST nanocomposites. As implied by Figure 6b, in stark contrast to pristine P(VDF-TrFE-CFE), the generated EC heating and cooling energy in the P(VDF-TrFE-CFE)-BNNSs-BST nanocomposites can be absorbed unimpededly by the regenerator within the time interval of 0.1 s, allowing the ternary nanocomposites to be operated with high frequencies for high cooling power densities and great cooling efficiencies. For high volumetric cooling power density without irreversible heat loss, a rotary EC refrigerator has been designed, which consists of several pairs of turntables each of which contains ∼16 EC material elements (Figure 7a).68 According to the design of the rotary EC instrument, the typical thickness of EC material for each element is ∼200 μm, as thinner films have low overall heat capacity while thicker ones require a long period of time to complete the heat transfer.68 This design principle is applied to a variety of EC devices, for example, oscillatory and liquidus refrigerator.31,69,70 To justify the vital role of improvements in ECE and thermal conductivity on practical refrigeration, we carried out the finite element simulation to analyze the heat transfer process in the pristine polymer and the nanocomposite films with a thickness of 200 μm (Figure S21a). At the instant of withdrawing the electric field of 75 MV m−1 from the films (t = 0), ΔT of 3.9 °C, 7.5 and 7.2 °C are generated in the terpolymer, P(VDF-TrFECFE)-BST1/2/3 binary and P(VDF-TrFE-CFE)-BST1/2/3BNNSs ternary nanocomposites at room temperature (Figures S10e and S17e), respectively, followed by the heat exchange between the EC materials and the substrates. As clearly presented in Figure S21b−d, a striking difference in the temperature distribution along the direction of the heat transfer in the films has been observed. Because of their low thermal conductivities, the elimination of the temperature gradient in the pure terpolymer and its binary nanocomposite necessitates a long time span of 1 s which, notably, can be shortened by 5 times to 0.2 s in the ternary nanocomposites equipped with the BNNSs thermal conductive network. Figure S21e displays the percentage of exchanged Qc as a function of time in different EC films. The percentage of Qc transferred out of the ternary nanocomposite increases sharply after the removal of the electric field, which approaches to 95% within a short period of 0.1 s, while only ∼50% of the generated Qc in the terpolymer and its binary nanocomposite can be absorbed by the heat sink with the same time interval. The poor heat transfer perform-

Figure 4. (a) SEM and (b) TEM images of BNNSs. Inset of panel b: Electron-diffraction pattern of BNNSs, which displays a hexagonal symmetry. (c) Cross-section SEM image of the P(VDF-TrFE-CFE)BNNSs-BST1/2/3 nanocomposite. (d−f) Simulated microstructure of the P(VDF-TrFE-CFE) composites with 2, 4, and 8 vol % BNNSs, respectively. Benefiting from their unique two-dimensional nanostructure, BNNSs can construct a dense network in the nanocomposites with a loading of 8 vol %. The length of the side of the cubes is 5 μm.

nanocomposite exhibits a giant Q of 100 MJ m−3, ΔS of 310 kJ m−3 K−1, and ΔT of 35 °C at an electric field of 200 MV m−1, as shown in Figures 5c and S20, which not only surpass the pure ferroelectric polymer, but also exceed most inorganic ferroelectric materials.5,57,58 The maximum electric field in the EC measurements is deliberately set at about half to the breakdown strength of the samples, which is predicted by the theoretical calculations of dielectric materials with a lifetime of 10 years.59 Concurrently, as shown in Figures 5a and S17a and c, under a relatively low electric field of 75 MV m−1, a sizable ECE, that is, Q of 25 MJ m−3, ΔS of 70 kJ m−3 K−1, and ΔT of 8 °C, is obtained in the nanocomposites, which is ∼2 times superior to that of pristine P(VDF-TrFE-CFE), and comparable to the ECE generated in the P(VDF-TrFE-CFE)BST1/2/3 binary nanocomposites. More impressively, the P(VDF-TrFE-CFE)-BNNSs-BST1/2/3 ternary nanocomposite not only retains the outstanding temperature stability of the ECE of its binary nanocomposite but also extends the stable ECE to the relatively high electric field level of 100−200 MV m−1. As shown in Figures 5 and S17−S20, a temperatureindependent ECE with excellent temperature stability reaching up to 90% is attained in the polymer nanocomposites consisting of 8 vol % BNNSs, 2 vol % BST1, 2 vol % BST2, and 2 vol % BST3 under both the relatively low and high electric fields. The high temperature stability of ECE in the ternary nanocomposites guarantees the two iso-field entropytemperature curves of the Ericsson cooling cycle to be parallel with each other and provides the opportunity of creating EC cooling with a high efficiency close to the ηC (Figure S1). Ferroelectric polymers are well-known poor thermal conductors with thermal conductivities