More than a decade of research on the electrocaloric (EC) effect has resulted in EC materials and EC multilayer chips that satisfy a minimum EC temperature change of 5 K required for caloric heat pumps1,2,3. However, these EC temperature changes are generated through the application of high electric fields4,5,6,7,8 (close to their dielectric breakdown strengths), which result in rapid degradation and fatigue of EC performance. Here we report a class of EC polymer that exhibits an EC entropy change of 37.5 J kg−1 K−1 and a temperature change of 7.5 K under 50 MV m−1, a 275% enhancement over the state-of-the-art EC polymers under the same field strength. We show that converting a small number of the chlorofluoroethylene groups in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer into covalent double bonds markedly increases the number of the polar entities and enhances the polar–nonpolar interfacial areas of the polymer. The polar phases in the polymer adopt a loosely correlated, high-entropy state with a low energy barrier for electric-field-induced switching. The polymer maintains performance for more than one million cycles at the low fields necessary for practical EC cooling applications, suggesting that this strategy may yield materials suitable for use in caloric heat pumps.
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This work was supported by the National Key R&D Program of China (2020YFA0711500 and 2020YFA0711503), the National Natural Science Foundation of China (52076127, 51776119, 11974239, 31630002, 51877132, 52003153 and 51972028), the Natural Science Foundation of Shanghai (20ZR1471700) and the China Postdoctoral Science Foundation-funded project 2019M661479. X.Q. acknowledges the support by the Prospective Research Program at Shanghai Jiao Tong University (19X160010008), the Oceanic Interdisciplinary Program of Shanghai Jiao Tong University (project number SL2020MS009), the Student Innovation Center and the Instrumental Analysis Center at Shanghai Jiao Tong University. X.C. and Q.M.Z. acknowledge the support of a seed grant from the Pennsylvania State University MRI-IEE, USA. L.-Q.C. acknowledges the support by the Donald W. Hamer Foundation through the Hamer professorship at Penn State. We thank L. Jiang and B. Zhu for assistance with measurements of energy-dispersive X-ray spectroscopy and attenuated total reflection Fourier-transform infrared spectroscopy. Access to the HFBS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. The DFT calculations were performed at the Oak Ridge Leadership Computing Facility, supported by DOE contract DE-AC05-00OR22725. We thank N. Li from the BL19U2 beamline, X. Miu from the BL16B1 beamline and Y. Yang at the BL14B beamline of Shanghai Synchrotron Radiation Facility for the help with synchrotron X-ray measurements. L.H. thanks the Innovation Program of the Shanghai Municipal Education Commission. H.Q. and J.B. thank B. Zhang and W. Lu for extensive discussions. X.Q. thanks S. Lin for support with simulation software, and X. Yao, Q. Wang and G. Meng for inspiring discussions. Certain commercial material suppliers are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
X.Q. and S. Zheng are inventors on a provisional patent application related to the described work. The other authors declare no competing interests.
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Extended data figures and tables
The breakdown strength was characterized at Shanghai Juter High Voltage Electrical & Equipment Co. Ltd. under a DC voltage at a rate of 200 V s−1. The electrode diameter was 1 mm and the film thickness was 6 μm. After introducing 0.4 mol% of the double bonds, the Weibull breakdown strength increased from 350 to 395 MVm−1. As the -CH = CF- bond content increased to 0.6%, 0.8%, and 2.0%, the Weibull breakdown strength decreased to 390, 375, alnd 370 MVm−1. As a result, the maximum cycling field can be extended to 80 MVm−1 for TD-0.6%. 50 MVm−1 is about 14% of the Eb of the polymer.
a, Simulated temperature changes of the MLC with and without the epoxy (0.5 μm thick) and un-electrode margin (with 1 cm x 2 cm total area and un-electroded margin = 0.5 mm on each side) as inactive materials. b, Schematics of the numerical model composing of 9 layers of EC polymer and 10 layer of epoxy. The electrode thickness for the polymer is below 0.05 μm (negligible), and we considered the epoxy layer that mechanically stabilize the interfaces between each layers of MLCs. Hence, the total passive part in the MLC will be less than 15%, causing about 15% reduction of ΔT (and ΔS).
Extended Data Fig. 3 Field-dependent EC-induced entropy changes of the base terpolymer and the modified terpolymer with different mole percentages of CFE reduction.
EC-induced entropy changes of the base terpolymer and the modified terpolymers with different mole percentages of CFE reduction (i.e., DB concentration ranges from 0, 0.4%, 0.6% and 2.0%). (n ≥ 3, points are centred on the mean and the bars indicate the s.d., each sample of TD-0.6% in the EC heat flux measurement was verified by independent measurements by an infrared camera, see Supplementary Sections 1.6 and 1.7).
Extended Data Fig. 4 Heat flow signals of EC polymers during the heating and cooling process from the differential scanning calorimetry.
a, base terpolymer, b, TD-0.4%, c, TD-0.6% and d, TD-2.0%.
Extended Data Fig. 5 Rotation energy of carbon-carbon bond in PVDF single chains before (black) and after (red) a CFE group was replaced by a covalent double bond.
a, Rotation energy of the C-C bond adjacent to the Cl atom (on the left of the DB after the CFE reduction), and b, Rotation energy of the C-C bond on the other side of the CFE (DB) group. The simulation was carried out by MM2 force field simulation through ChemOffice (version 19.0). The chain was defined as 5VDF + 1CFE + 5VDF and was relaxed in TGTG’ conformation under zero kelvin. A covalent double bond was in-situ introduced by cancel a HCl of the CFE.
Extended Data Fig. 6 Comparison of the temperature-dependent permittivity between the extrinsic blends and intrinsically modified terpolymer.
The blends were composed of terpolymer P(VDF-TrFE-CFE) and copolymer P(VDF-TrFE) 65/35 mol%. The ferroelectric copolymer was intentionally added to generate a larger ECE by inducing internal fields and providing more polar entities. The composition of blends is noted as TC followed by the percentage of the terpolymer in the blends, e.g., TC90 refers to a polymer blend with 90 mol% terpolymer in the matrix. As the composition of ferroelectric copolymer increases, the dielectric material exhibits higher transition temperatures and stronger polar correlations as shown by the slope of blue arrows, compared with those of the TD series.
Extended Data Fig. 7 Relation between the maximum polarization and the electrocaloric effect of polar materials.
a, Schematic of microscopic ferroelectric-domain switching induced polarization change which does not generate electrocaloric effect. ECE is directly related to the electric field induced polarization change. In ferroelectrics and dielectric in general, the efficiency of inducing electrocaloric entropy change from polarization change can vary over a great range. As an example, macroscopic polarization domain switching, as illustrated here, although generates large polarization change, does not contribute to electrocaloric effect. b, Summarized P-E loops for the base terpolymer and TD series under 120 MVm−1 and 10 Hz. c, Ratio of the β coefficient of modified polymeric EC materials to their respective base polymers16,28,38,39, the inset presents values of the β coefficient of the enhanced EC polymeric materials reported in the literature and this work. As shown by Pirc et al31, ΔS = P2 ln(Ω)/(ε0Θ), where Ω is the number of polar entities (number of polar-states) which can be accessed by dipoles, ε0 is vacuum permittivity, and Θ is an effective Curie constant, which is directly related to the polar correction in a ferroelectric material. Our results reveal that a large ECE can be achieved (at low fields) by engineering a high-entropy dielectric material which possesses a large β coefficient, and hence a large ECE may not require a material to have a large polarization.
a, In-situ heat flux directly recorded during cycling under 80 MVm−1. b, Initially measured EC entropy change compared to that after 61200 cycles under different electric fields.
Extended Data Fig. 9 Comparison of the EC fatigue performances when the base terpolymer and the TD-0.6% were generating the same amount of ECE.
a, Cyclic performance of TD-0.6% before and after 106 cycles, at 50 MVm−1. The integrated peak areas, which represent the total heating and cooling signals, were similar to each other. b, The base terpolymer film experienced a series of breakdown after 50 cycles under 150 MVm−1.
Extended Data Fig. 10 Comparison of hysteresis losses between the base terpolymer and the TD-0.6% high entropy polymer when they were generating the same amount of ECE.
a, Integrated hysteresis loss in percentage. The inset presents PE loops of the base terpolymer under 100 MVm−1 and TD-0.6% under 50 MVm−1, at which the two polymers induced similar ECE. b, Hysteresis heat loss for the base terpolymer and TD-0.6% when they induce the same amount of entropy change of 40 and 70 Jkg−1K−1, respectively.
This file contains four sections and additional references, and includes Supplementary Figs. 1–28 and Tables 1–7. Section 1. Characterization of EC polymers; Section 2. Phase-field simulation; Section 3. DFT calculation of energy barriers in phase transitions; Section 4. Simulation of EC heat pumps using EC polymers.
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Qian, X., Han, D., Zheng, L. et al. High-entropy polymer produces a giant electrocaloric effect at low fields. Nature 600, 664–669 (2021). https://doi.org/10.1038/s41586-021-04189-5
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