From negligible to giant electrocaloric effect of poly(vinylidene fluoride)

Domain: physics
Published: 2025-12-17
Editorial outcome: major_revision

Abstract
Poly(vinylidene fluoride) (PVDF), the most widely used ferroelectric polymer, typically exhibits a negligible electrocaloric effect (ECE) due to its symmetrical molecular configuration and bulky ferroelectric domains. PVDF-based ferroelectric terpolymers with giant ECE are expensive and have a narrow temperature window (Tw). Here, we report that PVDF films prepared by solid-phase pressure forming (PVDF-SPF) show a giant ECE of up to 12.8 K at 25 °C and a wide Tw of 60 °C. PVDF films underwent significant plastic deformation and extensive crystal fragmentation, forming loose and tiny β crystal grains under 30 MPa solid-phase pressure. The giant ECE primarily originates from orientation entropy changes of molecular chains associated with the reversible transition between loose and compact β crystals, as well as switching entropy changes of polar dipoles in tiny grains. Given the availability of PVDF, this work opens a route for the development of scalable solid-state cooling techniques.

Introduction
Our society heavily relies on cooling technologies, such as air conditioning, with the traditional vapor compression cycle being the dominant method. However, this technology suffers from low energy efficiency, large volume, and high complexity. Furthermore, there is an intensifying environmental concern regarding the ozone-depleting and greenhouse gas-emitting refrigerants utilized in vapor compression systems1. Solid-state cooling technologies based on caloric effects offer promising alternatives with superior energy efficiency and zero emissions2. Caloric effects can be classified into magnetocaloric3,4, mechanocaloric5–9 and electrocaloric effect10–12 according to the action of external fields. Electrocaloric cooling shows potential for applications in human thermoregulation, battery thermal management, energy harvesting and refrigeration13. To date, refrigerants with both high ECE and low cost remain a significant technical challenge for the large-scale application of electrocaloric cooling.

Entropy changes stemming from conformational transitions and dipole orientations are the main contributors to the ECE temperature drop (ΔTECE) in polymers14. PVDF-based ferroelectric terpolymers, such as poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)], are well-studied relaxor ferroelectric terpolymers in electrocaloric. The large volume and physical pinning effect of the comonomers in the terpolymer restrict the ferroelectric domains to the nanoscale, creating polar nanoregions. The nanoregions facilitate filed-induced relaxor ferroelectric to ferroelectric conformation transitions, resulting in a slim hysteresis loop15,16. The non-polar phase of the terpolymer transform into the polar β phase when an electric field was applied, confirming that its ECE is primarily attributable to the structural phase transition11. The reversible reorientation of the molecular chain conformation contributes to the large entropy and temperature change of terpolymers17.

Despite the superior ferroelectricity of PVDF, it exhibits negligible ECE at room temperature due to bulky and compact ferroelectric domains, which result from its symmetrical and regular molecular configuration. To improve its electrical performance, strategies such as repeated pressing-and-folding have been employed to extend polymer chains multidirectionally, thereby boosting dielectric energy storage capability18. In addition, high-power ultrasonication was utilized to induce a hard-to-soft transition, increasing the piezoelectric coefficient d31 in PVDF19. The presence of oriented amorphous fraction structures has been reported to greatly improve the piezoelectric response of PVDF20. In polymers, the introduction of a small number of double bonds can generate numerous loosely correlated polar entities in a high-entropy state, which also favor the ECE11. This suggests that such loosely correlated intermediate phases with ordered conformations may significantly contribute to the polarization behavior. In general, the morphology of polymers is influenced by both chain structure and processing techniques21,22. We believe that optimized forming techniques can restrict the ferroelectric domains of PVDF to tiny grains with loosely assembled chains. Given that the dipole moment of the PVDF monomer is higher than that of the comonomers in PVDF-based terpolymer23,24, the maximum polarization (Pmax) of PVDF is consequently higher. Therefore, PVDF has the potential to exhibit giant ECE if the bulky polar dipoles are decoupled.

Here, we first performed uniaxial stretching for the PVDF films prepared by solvent casting (PVDF-C), achieving a stretched PVDF film (PVDF-S) with 96% β crystal. This process increased the long-range order while minimizing the initial dipole switching entropy, resulting in an entropic storage effect. The entropic storage effect boosted the piezoelectricity of PVDF but inhibited the ECE25. Subsequently, the PVDF-S underwent significant plastic deformation and extensive crystal fragmentation during solid-phase pressure forming (SPF), a pressure-forming method below the melting temperature of materials, to form PVDF-SPF film. The SPF technology exhibited a super entropic effect, effectively releasing the entropy stored during the stretching process. The variable-angle attenuated total reflection infrared (VA-IR) spectra revealed the internal orientation of the β phase. The atomic force microscopy with infrared spectroscopy (AFM-IR) images proved the decoupling of long-range order in the molecular chains of PVDF-SPF. In situ high-voltage infrared spectra confirmed the reversibility of dipole flipping, and in situ high-voltage 2D wide-angle X-ray diffraction (2D-WAXD) curves illustrated the reversible transition between loose and compact β crystals. The PVDF-SPF achieved a giant ΔTECE of 12.8 K at 25 °C and 200 MV m−1 (breakdown field strength, Eb = 617 MV m−1), surpassing PVDF-C by 20 times. The PVDF-SPF displayed a wide Tw of 60 °C and a high Young’s modulus (Y) of 1.7 GPa, demonstrating excellent overall performance and application potential.

Results
Solid-phase pressure formed PVDF for ECE
The PVDF-C is dominated by α and γ spherulites with large and weakly polar paraelectric dipole domains26 (Fig. 1a), which is mainly used as dielectric materials27. The PVDF-C demonstrates a slender hysteresis loop28 with a Pmax generally around 2 μC cm−2 at 200 MV m−1. Upon uniaxial stretching for PVDF-C, the α and γ spherulites can transform into fibrous β crystals with electroactivity29, which are mainly used as piezoelectric materials with a piezoelectric coefficient25 typically between 20 and 35 pC N−1. However, the ECE of these piezoelectric materials is generally poor. To achieve a giant ECE in PVDF, it is essential to maximize the β phase to increase the density of field-induced dipoles. Uniaxial stretching of PVDF-C to a strain of up to 1400% (Fig. 1b) yielded PVDF-S with approximately 96% β phase (Supplementary Note 1), forming fibrous β crystals with bulky and compact ferroelectric dipoles. Subsequently, the PVDF-S was gradually pressurized to 30 MPa at 85 °C by a hot press machine with adhesive gaskets for primary SPF. At 155 °C, we applied the same pressurization method for secondary SPF. The processes cause the film to undergo significant plastic deformation along the transverse direction (Fig. 1d) and extensive β crystal fragmentation, destroying the long-range ordered structure and forming loose and tiny β grains (Fig. 1c). The β phase content of PVDF-SPF was reduced to 94% (Supplementary Fig. 1).

The PVDF-C showed a negligible ΔTECE of 0.6 K. The PVDF-S exhibited a limited ΔTECE of 1.9 K, predicting a small amount of crystal fragmentation in the film by high-draw ratio stretching. The PVDF-SPF demonstrated a significantly higher ECE, with a maximum ΔTECE of 12.8 K at 25 °C and 200 MV m−1 (Fig. 1e and Supplementary Figs. 3 and 5a). This result was cross-validated using an infrared thermometer. Due to lower sampling rates of the infrared method, the measured temperature change was reduced (Supplementary Fig. 5). A high-resolution infrared thermal imager visually captured the temperature variations of the PVDF-SPF during a single electrocaloric cycle, confirming the giant electrocaloric effect (Fig. 1f and Supplementary Movie 1).

To investigate the intrinsic relationship between ECE and crystal morphology, we employed VA-IR30 to characterize the orientation and distribution of the β phase in PVDF membranes prepared by three processing methods (Supplementary Fig. 2). VA-IR is a characterization technique that captures conformational information across all azimuthal angles by rotating the film sample. This enables in situ visualization of the spatial distribution of molecular conformations within the film. The PVDF-C primarily contained the short-γ phase16 (at 833 cm−1), which displayed weak electroactivity and showed strong vibrational signals at all angles (Fig. 1g). Almost no intensity of the long-β phase16 (at 1275 cm−1) was observed. The PVDF-S revealed a strong long-β phase signal at all angles, while the short-β phase16 (at 837 cm−1) presented a pronounced signal near 0° (Fig. 1h). This observation indicated that the molecular chains in the PVDF-S possessed predominant regular long-range ordered structure with limited short-range ordering. The longer all-trans conformation necessitates a higher electric field strength for alignment with the field direction31, which impedes dipole orientation and reduces the switching entropy change during flipping.

The PVDF-SPF presented a distinct β phase distribution compared to the PVDF-S. The long-β phase became discontinuous, while the short-β phase became continuous at all angles, indicating an interruption of long-range order (Fig. 1i). The spectral width of the electroactive short-β phase increased, suggesting a greater variety of microscopic polar states and a more disordered short-β phase orientation32. Additionally, across the entire angle, the difference between the maximum and minimum intensities of the short-β phase decreased, suggesting to a more uniform distribution. Thus, the number of microscopic polar states in the PVDF-SPF was dramatically increased, which enhanced the dipole switching entropy change (ΔSdip) and positively impacted the ECE of the system.

Reversible transition in β crystals
To facilitate a more direct observation of the polar conformational distribution in the three films, we characterized the samples using AFM-IR33. An infrared wavelength of 1275 cm−1, associated with the all-trans conformation of PVDF, was selected for analysis34. The topographies of the films were measured, and the nanoscale infrared images were normalized to more clearly delineate the β phase distribution (Supplementary Fig. 6).

The surface morphology of PVDF-C exhibited spherical aggregation (Supplementary Fig. 6a), with a quite weak intensity of the all-trans conformation and only discernible in specific regions (Fig. 2a). In contrast, the PVDF-S displayed a clear molecular chain orientation, as evidenced by stretching marks on the surface (Supplementary Fig. 6b). The oriented molecular chains aggregated, reaching a width of approximately 730 nm (Fig. 2b). The infrared amplitude for all-trans conformation was observably enhanced, indicating strong long-range coupling. Despite the presence of oriented molecular chains in the PVDF-SPF, the long-range coupling was disrupted. The aggregation width of the short all-trans conformation was reduced to 150 nm (Fig. 2c), with a more uniform distribution and increased orientational randomness. This result was highly consistent with the measurements obtained from VA-IR (Fig. 1i). The peak intensities of the infrared amplitude for the PVDF-S and PVDF-SPF were 0.64 mV and 0.6 mV, respectively (Supplementary Fig. 6d), without a significant decrease in the molecular chain polarity during SPF. Additionally, Young’s modulus (Y) was characterized by AFM (Supplementary Fig. 7), with values of 1.1, 2.1, and 1.7 GPa for PVDF-C, PVDF-S, and PVDF-SPF, respectively. High modulus could notably improve the mechanical performance and fatigue durability of polymer electrocaloric devices.

Moreover, we employed high-voltage infrared spectroscopy (Supplementary Fig. 8) to analyze the evolution of internal polar conformation in the PVDF-SPF17,35. After applying 200 MV m−1 to PVDF-SPF, the infrared absorption of both the long-β phase and the short-β phase weakened (Fig. 2d). Given the high β phase content (94%) in PVDF-SPF, further increase in its concentration under the electric field is considered negligible. The reduction in absorption intensity is primarily attributed to the reorientation of polar dipoles along the field direction, which decreases their vibrational component along the direction of the infrared electric vector36,37 (Supplementary Fig. 9). This reorientation also leads to a decrease in dipolar entropy (Supplementary Note 3). The initial spectrum of PVDF-SPF was nearly identical to that after high-voltage removal, confirming the reversibility of the dipole flipping. Subsequently, high-voltage 2D WAXD characterization was performed (Fig. 2g)36. The peak position of PVDF-SPF shifted from 20.15° to 20.48°, decreasing the interchain spacing of the β crystal (Fig. 2e). The dispersion of the Debye ring reflects the orientational disorder among β crystals. A broad and diffuse ring indicates more random orientation, corresponding to a more disordered system with higher entropy. In contrast, a sharp and concentrated ring suggests increased orientational order and lower entropy. Based on this observation, we propose that changes in the orientational order of crystal domains may represent an additional and previously overlooked entropy contribution, namely the β phase orientation entropy change (ΔSorient) (detailed derivation in Supplementary Note 3). The degree of chain compactness, the orientation of molecular chains, and the quantity of β grains all affect the microstate number in the crystal morphology, leading to the corresponding entropy, known as β phase orientation entropy. The ΔSorient can be calculated using Boltzmann’s entropy equation:

ΔSorient = kB (N ln ΔΩD + ln ΔΩP) (1)

where kB is the Boltzmann constant, ΔΩD, ΔΩP and N represented the changes in microstates due to interchain spacing (D), chain orientation (P) and β grain quantity (N), respectively. The broadening of 837 cm−1 in the VA-IR characterization (Fig. 1i) indicated that the initial PVDF-SPF had a large microstate number in the short-β phase. AFM-IR images (Fig. 2c) showed that lower β phase orientation corresponded to greater randomness. After applying a high voltage, the molecular chains in PVDF-SPF became more compact (Fig. 2f), resulting in a decrease in D, accompanied by an increase in relative crystallinity (Supplementary Fig. 10). The narrowed diffraction rings (Fig. 2g) indicated an increase in P. These changes significantly reduced the microstate number of the system, leading to a decrease in entropy.

Upon removal of the electric field, an entropy increase occurred. Moreover, a tinier crystal size implies a larger N, with small changes in ΔΩD and ΔΩP leading to a great ΔSorient. The minor shaded areas signified that the transition from loose to compact β crystal resulted in a negligible loss (Fig. 2f). As with ΔSdip, the orientation entropy was reversible under the electric field.

Electrical responses and structural analysis
We performed various electrical and structural characterizations on PVDF-C, PVDF-S and PVDF-SPF. The PVDF-C exhibited a slim polarization-electric field hysteresis loop (P-E loop), with a Pmax of only 1.86 μC cm−2 at 200 MV m−1 (Fig. 3a), insufficient for providing adequate switching entropy. The PVDF-S showed a fat P-E loop (Fig. 3b), with a high Pmax of 9.83 μC cm−2 and a large remanent polarization (Pr) of 5.85 μC cm−2 at 200 MV m−1, indicative of piezoelectricity38. However, the permanent dipoles with high density, which are proportional to large Pr, failed to follow electric field variations, sacrificing both ΔSdip and the ECE. In contrast, the PVDF-SPF displayed a distinctly different P-E loop (Fig. 3c and Supplementary Fig. 11) with a high Pmax of 9.13 μC cm−2 and a low Pr of 1.12 μC cm−2 at 200 MV m−1, favoring a high ΔSdip and a giant ECE. The unipolar P-E loop efficiency of PVDF-SPF was much higher than that of PVDF-S and comparable to that of the linear dielectric PVDF-C (Fig. 3d), explaining the excellent reversibility of ECE cycling for PVDF-SPF. This was further confirmed by the large electrostriction of up to 5% under 200 MV m−1 (Supplementary Fig. 13). When the field exceeded 250 MV m−1, dipole alignment approached saturation, and the polarization increase slowed markedly. The Pr remained nearly constant, suggesting that the reversible polarization component was nearly exhausted, and the material behavior became more linear (Supplementary Fig. 12).

To gain a more profound comprehension of the effects of processing temperature on the structure of PVDF-SPF, we examined the crystalline morphology using 2D-WAXD (Supplementary Fig. 14). The PVDF-SPF subjected only to low-temperature SPF at 85 °C (PVDF-SPF85) exhibited the same 2θ angle of 20.52° (β crystals, 200/110 plane39) as PVDF-S, but with a dramatically broader full width at half maximum. The crystallite size reduced to 6.22 nm (Supplementary Fig. 14f). In addition, the crystalline melting enthalpy of PVDF-SPF85 (54.8 J g−1) also be lower than that of PVDF-S (61.3 J g−1) (Supplementary Fig. 15c). The above results show that low-temperature SPF mainly crushed larger β crystals, decoupled polar dipoles and increase ΔSdip.

After further high-temperature SPF at 155 °C, the 2θ angle of PVDF-SPF decreased from 20.52° (PVDF-SPF85) to 20.16° without new peaks (Fig. 3e). The interchain spacing increased to 4.41 Å (Supplementary Fig. 14f). The DSC melting range of PVDF-SPF expanded from 13.2 °C (PVDF-SPF85) to 17.1 °C (Fig. 3f and Supplementary Fig. 15d). These results demonstrated that high-temperature SPF increased the polydispersity of the β crystals and expanded the molecular chain spacing within the β crystals, creating ΔSorient from loose to compact β crystals.

We then conducted measurements of the dielectric properties of the films. Both stretching and SPF methods increased the dielectric constant (k) of PVDF films, with PVDF-SPF exhibiting the highest k of 19.2 at 1 kHz (Supplementary Fig. 16a). The stretching process enhanced the number of polar dipoles and improved electroactivity. The SPF process decoupled larger polar dipoles, facilitating dipole flipping and increasing the k. Additionally, the PVDF-SPF achieved a higher Eb (617 MV m−1) (Supplementary Fig. 16b), making it suitable for higher voltage ECE devices.

Temperature-dependent ECE behavior
It was crucial to explore the effective Tw of PVDF-SPF. To this end, we systematically study the dielectric response of three films using variable-temperature dielectric spectroscopy (Supplementary Fig. 17). Compared to PVDF-C and PVDF-S, the k of PVDF-SPF increased with temperature (Fig. 4a), indicating higher polarizability and stronger electroactivity under an electric field. At low temperatures (approximately −30 °C), both k and dielectric loss increased significantly (Supplementary Fig. 18a, d). This behavior resembles the polarization enhancement mechanism reported previously40. The structure in PVDF-SPF, referred to as loose β crystals (similar to intermediate phases), can greatly improve the reversible ECE. In contrast to terpolymers (Supplementary Fig. 18c, f), no dielectric peak was observed in PVDF-SPF at room temperature, suggesting the absence of conformational transitions41,42. The drop of k around 90 °C (Fig. 4a) illustrates structural relaxation and partial melting (Supplementary Fig. 15a), which may seriously reduce the ECE of PVDF-SPF.

Variable-temperature infrared spectra show a clear red shift in the characteristic peak of the long-β phase with increasing temperature, indicating weakened interchain dipole-dipole interactions (Supplementary Fig. 19a, c). Its absorbance gradually decreases and drops sharply above 80 °C, reflecting the strong temperature sensitivity of the long-β phase content. In contrast, the peak position and absorbance of the short-β phase remain nearly unchanged below 80 °C, which demonstrates its great thermal stability (Fig. 4b and Supplementary Fig. 19b). However, once the temperature exceeds 80 °C, a rapid decrease in the short-β phase is observed. This behavior indicates that 80 °C serves as a critical threshold for the structural stability of the short-β phase. Below this temperature, the β phase in PVDF-SPF remains stable without conformational transformation41. The structural evolution of β crystals was directly observed via variable-temperature 2D-WAXD (Supplementary Fig. 20). Around 80 °C, both crystal size and relative crystallinity exhibit inflection points. This indicates the β crystals undergo considerable thermal expansion accompanied by partial disintegration, which leads to a rapid decrease in relative crystallinity (Fig. 4c). A weak endothermic peak in the DSC curve also reflects this transition (Supplementary Fig. 15a). Before the collapse, the increased interchain spacing43 favors the ΔSorient.

In the range of 20–80 °C, the polarization difference (ΔP = Pmax − Pr) gradually increased (Fig. 4d and Supplementary Fig. 22). The resulting increase in ΔSdip led to a total ECE entropy change (ΔSECE) of 52.5 J kg−1 K−1 at 80 °C (Fig. 4e). This enhancement was attributed to tensileresidual stress (~15 MPa) introduced during the SPF process (Supplementary Fig. 23), which stabilized the loose β crystals and enabled a large ΔSorient below 80 °C. However, the ΔTECE of PVDF-SPF gradually decreased to 10.8 K at 80 °C (Supplementary Fig. 24a), due to the rapid increase in specific heat capacity (Supplementary Fig. 25) offsetting the gain in ΔSECE. Above 80 °C, the internal stress generated was gradually released. This induced molecular chain rearrangement and disorientation, along with partial melting of the loose β crystals. At high temperatures, the excessive thermal motion of molecular chains could not follow the electric field variation, and ΔSorient disappeared. At this point, the PVDF-SPF presented a high Pr and hysteresis loss (Supplementary Fig. 21), lowering the ΔSdip and ECE. Once the structure becomes unstable and collapses, the electrocaloric effect drops rapidly to nearly zero. The ΔSECE versus temperature curve of PVDF-SPF completely echoed the above variable-temperature tests, revealing an effective Tw of 60 °C for PVDF-SPF.

Considering the practical requirements for electrocaloric polymers, we selected five parameters: ΔTECE, Tw, Eb, Y, and Pmax as evaluation criteria (Supplementary Table 1). PVDF-SPF exhibited a large ΔTECE (12.8 K) and a high Pmax (9 μC cm−2), demonstrating significant potential for electrocaloric performance tuning. Its high Y (~1700 MPa) is expected to enhance the structural stability and operational durability of electrocaloric devices (Supplementary Fig. 26). Compared with previously reported electrocaloric materials (Supplementary Table 2), PVDF-SPF outperforms commercially available terpolymers in ΔTECE, ΔSECE, and Tw, highlighting its notable advantage in overall property balance (Fig. 4f). These results provide useful guidance for optimizing the trade-off between performance and fabrication in the design of future electrocaloric materials.

Methods
Materials and general film preparation
PVDF powder (Solef® 5130) was purchased from Solvay S.A. with weight-average molecular weights (Mw) of 1000 kg mol−1. N, N-dimethylformamide (DMF, 99.9%) was obtained from Aladdin Biochemical Technology Co., Ltd. A 13 wt.% PVDF solution was prepared by dissolving 10 g of PVDF in 67 g of DMF. The solution was vigorously stirred for 12 h, then cast onto a smooth glass plate using a thick blade. The glass plate was placed in a vacuum oven at 80 °C for 24 h to obtain the PVDF-C film. The PVDF-C was cut into rectangular strips and stretched to a strain of 1400% at 85 °C using an electromechanical test system (Model E44, MTS Systems Co., Ltd.) to obtain the PVDF-S film.

Solid-phase pressure forming
The PVDF-S was clamped using adhesive gaskets (PTFE with single-sided tape, Taizhou Chenguang Plastic Industry Co., Ltd.) and placed in a hot press (R-3201, Wuhan Qien Science & Technology Development Co., Ltd.) at 85 °C. The pressure was gradually applied to 30 MPa within 2 min. Cooling water was then circulated to bring the film to room temperature, resulting in the PVDF-SPF film. This film was subsequently subjected to the same process at 155 °C to obtain the PVDF-SPF film. The process of solid-phase pressure forming primarily involves transverse expansion of the film, with slight change in the original stretching direction (Supplementary Fig. 18).

Microstructure characterization
Fourier transform infrared spectroscopy (FTIR, Spectrum two, PerkinElmer Inc.) was employed to characterize the conformational composition of the samples with attenuated total reflection (ATR) accessory. The infrared spectra of the samples were collected using the ATR accessory with incident light parallel to the original stretching direction. For variable-angle infrared spectroscopy (VA-IR), the sample was placed at the center of the protractor, and infrared spectra were obtained in ATR mode at 15° intervals (Supplementary Note 3). For variable-temperature infrared spectroscopy, the sample was positioned on a custom temperature-controlled stage. The heating rate was 1–2 °C min−1, with measurements taken at 5 °C intervals. The temperature-controlled stage consisted of a temperature controller (TCM1030, Chengdu Yexian Technology Co., Ltd.), a polyimide heating plate (0402828, Yancheng Zhenglong Electric Heating Technology Co., Ltd.), and a platinum resistor (PT1000, Heraeus), which was positioned near the sample to monitor the temperature accurately. 2D wide-angle X-ray diffraction (Xeuss 3.0, Xenocs S.A.) with Cu/Kα radiation (wavelength 1.54189 Å) was performed to characterize the crystalline structure of the samples, with a pixel size of 75 μm. For variable-temperature WAXD, the temperature was controlled using a cooling-heating stage (HFSX350, Linkam), with a heating rate of 1–2 °C min−1 and measurements taken at 5 °C intervals. Thermal analysis was characterized using differential scanning calorimeter (DSC) (DSC25, TA Instruments) with a heating/cooling rate of 10 °C min−1. The conformational distribution and morphology of samples were conducted using tapping mode with pixel scan of 256 × 256 by atomic force microscopy equipped with infrared spectroscopy (AFM-IR, Dimension Icon IR, Bruker). The samples were excited with infrared light at 1275 cm−1 and scanned simultaneously. The Young’s modulus of the samples was measured using the same AFM.

Functional properties characterization
Permittivity spectroscopy was evaluated using a precision LCR meter (E4980A, Keysight Technologies) equipped with a temperature-controlled stage (same as the stage utilized in variable-temperature infrared spectroscopy). The heating rate was maintained at 1–2 °C min−1, with measurements taken at 2 °C intervals. Unipolar and bipolar ferroelectric hysteresis loops were measured by a ferroelectric tester (CPE1901, PolyK Technologies) with a modified Sawyer-Tower circuit at a frequency of 10 Hz. The electrostriction of PVDF-SPF was measured using a high-voltage amplifier equipped with a laser interferometer (SP-S120, SIOS Meßtechnik GmbH). DC breakdown strength was tested at a rate of 500 V s−1. Weibull analysis with two parameters was used to study the characteristic breakdown strength (Eb) of the samples18,

P(E) = 1 − exp (− (E/Eb)β) (2)

where E is the breakdown field of fifteen samples measured during the ferroelectric hysteresis test, Eb is the characteristic breakdown strength at 63.2% probability of breakdown, P(E) is the cumulative probability of breakdown, and β is the parameter related to film reliability. Unless otherwise stated, silver electrodes were sputtered on both sides of the films for all electrical measurements. The upper and lower electrodes had diameters of 6 mm and 8 mm, respectively, and their centers were aligned.

In-situ FTIR and 2D-WAXD characterization
For In situ high-voltage infrared spectroscopy (HV-IR), the sample surfaces were coated with carbon nanotubes (CNTs, Suzhou Tanfeng Technology Co., Ltd.). The film was positioned on a custom high-voltage sample stage (Supplementary Fig. 7), and an external high-voltage source (TCM6000i, Dalian Teslaman Technology Co., Ltd.) was used to apply the electric field. Spectra were measured in transmission mode. For in situ high-voltage 2D wide-angle X-ray diffraction (2DHV-WAXD), silver electrodes were sputtered on both sides of the film by a magnetron sputtering system (GVC-2000, Gevee Instruments Co., Ltd.). The film was then placed on the high-voltage sample stage, with other procedures following the same protocol as the HV-IR measurement. The thickness of the samples was limited to 10 μm.

Estimation of interchain distance and crystallite size
The interchain distance was estimated using Bragg’s law,

nλ = 2d sin θ (3)

where n is the diffraction order, λ is the X-ray wavelength, θ is the diffraction angle, and d is the interchain spacing. The crystallite size was calculated using the Scherrer equation

Lhkl = kλ / (β cos θ) (4)

where Lhkl represents the crystallite size perpendicular to the (hkl) plane, λ is the X-ray wavelength, θ is the Bragg angle, and β is the full width at half maximum (FWHM) of the specified peak44. The dimensionless shape factor k = 0.89 was adopted in this work.

ECE measurement
The ECE of the samples was captured using a heat flux sensor (HFS-5, OMEGA Engineering Inc.). A high-voltage amplifier (EEL1102.05, Electrical Energy Limited LLC) provided the required voltage, and data were collected using an oscilloscope (DHO914, RIGOL Technologies Co., Ltd.). The heating and cooling of the samples were directly observed using a high-precision infrared thermal imager (Testo SE & Co. KGaA). The time-resolved electrocaloric temperature change was measured using an infrared thermometer (CTlaser LT, Optris GmbH). Before infrared measurement, a layer of carbon nanotubes was sprayed onto the silver electrode surface to enhance the emissivity of film. The emissivity of the film (ε = 0.85) was measured using a dual-band emissometer (IR-2, Shanghai Institute of Technical Physics). The film thickness used for ECE measurement was approximately 10–15 μm. Detailed information is available in Supplementary Note 2.

Data availability
All data are available from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The article entitled ‘From negligible to giant electrocaloric effect of poly(vinylidene fluoride)’ by Y. Zhang et al investigates the enhancement of the electrocaloric effects (ECE) in poly(vinylidene fluoride) (PVDF) films through solid-phase pressure forming (SPF), and achieved a seemingly giant ECE of deltaT = -14.1 K near 25degC within a wide temperature window of 60 Kelvins. The figures of merit attractively surpass various existing electrocaloric materials. Most notably, while double hysteresis in the bipolar P(E) underpinning large ECEs is normally understood in the framework of phase transitions between a low-field antiferroelectric/paraelectric phase and a high-field ferroelectric phase, the double hysteresis in PVDF-SPF is attributed to another route via decoupling the long-range ferroelectric order. An ‘amorphous beta phase’ could be readily induced into crystalline/ferroelectric beta phase, during which large entropy change associated with polar orientation could be harnessed.\nThe work is thus scientifically and technically novel, and suitable for publications on Nature Communications if the following concerns on methodological weakness and data inconsistence could be addressed.\nI suggest the authors to add in the supplementary section a schematic illustration of how VA-IR works and how the data for PVDF-SPF relates to the illustration in Fig 1a-c for easier understanding of the readers.\nTo properly measure the ECE using an IR imager, the authors should provide time-resolved thermal profiles, alike of what has been demonstrated for the heat flux method in Fig. S3a. The authors should also present cross-verification between the heat flux method and the IR imager method for deltaT at different electric field near room temperature (or at various temperatures).\nIt’s also better to consolidate the ECE by showing calibration of the IR imager. Comparison of the temperature of CNT-sprayed surface measured by IR imager and the temperature of the heat controller should be plotted in the vicinity of room temperature, to show that emission of the CNT-sprayed surface is reliable and free from interference of low-emission metallic electrode surface and background noise.\nThe authors observed that ‘the infrared absorption of both the long-beta phase (1275 cm-1) and the short-beta phase (837 cm-1) weakened’, while the electric field application is supposed to increase the fraction of long-beta phase, and ideally enhances absorption at 1275 cm-1. What leads to the decrease?\nThe authors noted that the drastic decrease of ECEs at 90 degC is attributed to the crystallization of the amorphous beta phase, presumably into the normal beta phase since large remanent polarization is observed above 90 degC. But there’s inconsistence between such a claim and the fact that no signals for crystallization, e.g. peaks, were observed in their DSC near 90 degC. The inconsistence also exists for temperature-dependent FT-IR data, where the authors observed that both short-beta phase and long-beta phase decreased uniformly with rising temperature, instead of noticing a trade-off between the normal and amorphous beta phase.\nThe claim that there’s a drop in dielectric constant at 90 degC relating to this crystallization is also not solid, given that this dielectric drop near 90 degC has been seen in the PVDF-C and PVDF-S samples which would not contain the amorphous beta-phase.\nThere’s also inconsistence between the polarization data and the thermal data. In Fig. S16, either the remnant polarization or the maximal polarization for PVDF-SPF increases with temperature, and it seems dP/dT is positive for all fields. Since the Maxwell relation defines that dP/dT = dS/dE, an inverse effect (deltaS>0 for field application, and hence field application generates temperature drop) instead of the observed conventional effect should arise. And such an inverse effect should be the most substantial above 90 degC given the larger dP/dT values. All these lead to a significant discrepancy against the thermal measurements presented in the rest of this work.", "reviewer_2": "In this study, the authors present pressed PVDF films that achieve an EC temperature change of up to 14.1 K at 25 °C and a temperature window of 60 °C. The authors claim that the PVDF films underwent substantial plastic deformation and extensive crystal fragmentation under 30 MPa solid-phase pressure, resulting in the formation of small, loosely packed beta crystal grains. It is always interesting to see the highly reversible polar responses from the conventional ferroelectric pvdf, the manuscript is therefore interesting. However, I still have some questions and comments that are related to the validation, novelty, and significance of the work.\n1. The authors claimed that \"forming loose and tiny β crystal grains\", and \"The giant ECE primarily originates from orientation entropy changes of molecular chains associated with the reversible transition between loose and compact β crystals\", as if the tiny and compact polar structures leading to the large ECE is the key finding. However, recent reports in PVDF-based polymers have already demonstrated the point (see ref. 11). From a broader picture, it is well known that the smaller and compact polar structures would lead to a high dipolar response (see Nature,2017, 546, 391). Therefore, the interesting part of this work should be how to generate these polar structures \"in PVDF\". To this point, the ref. 40 (Nat. Commun., 2019, 10, 4535) should be cited in a clearer position in this manuscript compared to its current position. The ref. 40 utilized a similar hot press process to reduce the size of the polar structures, which is directly related to this work and should be discussed in the beginning of the manuscript. I have no idea why the authors cite the papers but are not willing to discuss their true findings.\n2. From the data of the heat sensor, the pressed PVDF is too lossy, as we observed that a constant heat was generated under the electric field. The amount of heat generated exceeds the heat absorbed during the removal of the electric field. This significant heating effect can accumulate over time, making the material unsuitable for practical applications in refrigeration devices. The authors should provide a solution to reduce the heating effect significantly before discussing the possibility for practical use of the material. Or the authors should argue that the heating is insignificant in application.\n3. The heat measurement of the cooling effect is questionable. Anything above the baseline should not be integrated into the cooling effect without further evidence. The authors should work carefully on the measurement. Even without any cooling effect from the ECE, the heating curve will reduce to the baseline due to the convection. Once the field is removed, how much area of the signal integration (parts above the baseline) is due to the convection or heat absorption of ECE is not clear. It is highly possible that the reported ECE is overrated.\n4. The analysis of economic refrigeration capacity is unnecessary and misleading. At the current stage, EC technology, including the material and device, has not been deployed in any market. Only a handful of companies have the raw polymer powder for sale. Therefore, the price of the relaxor ferroelectric polymer is overly high because there is no market. However, the energy cost to produce monomers TrFE and CFE is similar to the VDF. The price of the relaxor ferroelectric polymer should not be too far from the PVDF unless the authors prove otherwise with the data from the fuorine chemical industry. The current high price is temporary and can be affected by many factors. According to Arkema, once there is a market, the price of their polymers will drop significantly. Also, the refrigeration capacity is field-dependent. There is no standard for how much field was applied in this comparison (Supplementary Table II).\nSince there is no commercially available electrocaloric device, there is no point in introducing this value of economic refrigeration capacity. The \"high\" ERC is shortsighted and immature, indicating the lack of investigation of this work.", "reviewer_3": "This work reports giant ECE of up to 14.1 K at 25 °C from PVDF films prepared by solid-phase pressure forming (PVDF-SPF). If experimentally substantiated, the work would represent an important progress in the field. The manuscript extensively describes the structural characterization results and rationalizes these to high ECE. Additional experimental data are required on the characterization of the ECE and direct measurement of the adiabatic temperature change. Once these are furnished, the work would become acceptable for publication.\n1. Provide experimental data of the temperature change measured with IR camera, and how the ECE varies with sample information such as different thicknesses. Thicker films produce larger heat flux and thus minimizes the impact by parasitic loss. Also provide charts to show the temperature change with time during sinusoidal or square wave voltage scans.\n2. Does the PVDF-SPF film show any electrostrictive behavior (e.g. expanding in film area) when subjected to the electric field?\n3. The authors state that the giant electrocaloric effect in PVDF-SPF originates from the entropy change associated with the reversible reorientation of the tiny β-crystal grains. This core statement of the paper is against the prevailing point of view stating that the non-polar to polar phase transition generates the largest entropy change and is the origin of the giant electrocaloric effect. The paraelectric phases also have larger interchain spacings compared to that of β-phase. The chain orientation of the paraelectric phases can also be very random. Therefore, the discussions from line 178 to 196 are not convincing enough.\n4. In figure 2d, why do peaks for long β-phase (1275 cm-1) and short β-phase (837 cm-1) both get weakened, but there is no intensity increase in any characteristic peak for the newly formed compact β-phase at 200MV/m? In fact, the compact β-phase is still β-phase. Is there any characteristic peak in the full FTIR spectrum for the compact β-phase formed at a high field?\n5. Figure 2e shows the adiabatic temperature change (ΔT) versus the field strength for the PVDF-SPF film. The ΔT is calculated from the data measured by a heat flux sensor. The curve does not show any platform but a large increasing rate even if the field is increased up to 200MV/m, and the breakdown strength of 617 MV/m is still far above the maximum test field. Does this mean that ΔT still has a large room to be increased if the field is higher than 200MV/m?\n6. The 14.1K of ΔT at 200MV/m calculated from the heat flux sensor measurement is a very large number. How does this compare to the ΔT measured using an IR camera under the same condition?\n7. Figure 2e shows that the ΔT of PVDF-SPF is almost negligible at 50-80MV/m. To get a ΔT (at least 3-4 K) for practical cooling applications, the field needs to be at least 100MV/m which is not ideal for cooling device development. How does the ΔT scale with film thickness? From the material standpoint, provide potential approaches to obtain large ΔT at <100 MV/m.\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_5": "This paper presents an enhanced electrocaloric effect (ECE) in poly(vinylidene fluoride) (PVDF) homopolymer subjected to solid-state pressure forming. Given the low cost and wide availability of PVDF homopolymer, the reported findings are noteworthy and have the potential to stimulate further research of ECE in PVDF homopolymer. I recommend the publication of this paper.\nHowever, I have concerns regarding the high Joule heating observed, as shown in Supplementary Figures 2(c) and 3(a), which may limit the practical applications of the reported material for electrocaloric cooling devices. Despite this limitation, I feel this work will inspire future research of ECE on PVDF homopolymer that may lead to practical (and low cost) electrocaloric cooling materials. The authors may consider (optional) adding one or two sentences in Supplementary Information about this Joule heating when discussing Supplementary Figures 2(c) and 3(a)."}, "author_response": "Response to the Reviewers’ Comments\nFrom negligible to giant electrocaloric effect of poly(vinylidene fluoride)\n(NCOMMS-25-05580-T)\nDear Reviewers:\nWe thank you for your handling and suggestions about our manuscript. And we\nappreciate you for providing positive feedback and a thorough evaluation of our study.\nWe have fully revised our manuscript and have addressed all the reviewers' comments,\nas well as added new analyses and discussions to further strengthen our work.\nThe major revisions, experiments, and new analyses we have undertaken are\nsummarized below and discussed in detail in the point-by-point responses. All changes\nin the manuscripts were highlighted in red in the revised manuscript text files.\nWe would appreciate your time and further consideration of our revised manuscript\nfor publication in Nature Communications.\nBest regards\nReviewer #1 (Remarks to the Author):\n1. I suggest the authors to add in the supplementary section a schematic illustration of\nhow VA-IR works and how the data for PVDF-SPF relates to the illustration in Fig 1a-\nc for easier understanding of the readers.\nResponse: Thank you for your valuable suggestion. In fact, the use of polarized infrared\nlight to investigate molecular orientation is a well-established technique. When using\nan attenuated total reflectance (ATR) accessory, the incident infrared light is directed\nonto the interface between the sample and the ATR crystal. The sample absorbs part of\nthe infrared light, and the reflected beam carries vibrational information of the internal\nmolecular groups. We utilized the reflective polarization effect of the ATR crystal1,\nthrough which the reflected infrared light at the sample surface becomes polarized. It is\nknown that infrared light comprises p- and s-polarized components, both of which are\nperpendicular to the direction of incidence (Fig. R1a). Among them, p-polarized light\nlies in the plane of incidence (defined by the directions of the incident and reflected\nbeams), while s-polarized light is perpendicular to this plane. Due to the reflective\npolarization effect of the ATR crystal, the reflected infrared beam is dominated by p-\npolarized light rather than s-polarized light. This leads to enhanced absorbance when\nthe dipole moment vibration direction is parallel to the incidence plane\n[Characterization of Solid Surfaces. (eds. Kane, P. F. & Larrabee, G. B.) 215-245\n(Springer US, Boston, MA, 1974)]. The variable angle attenuated total reflection\ninfrared spectroscopy (VA-IR) method is based on the above principle, with an\nadditional rotatable protractor installed on the film sample, enabling the collection of\ninfrared spectra over the range of -90° to 90° (Fig. R1b).\nIn poly(vinylidene fluoride) (PVDF), the vibrational mode at 1275 cm-1 is assigned\nto the coupled symmetric stretching vibration of CF and C-C groups, with the dipole\n2\nmoment vibration direction oriented perpendicular to the chain backbone (stretching\ndirection) [Acs Macro Lett. 1, 428-431 (2012)]. This implies that only a portion of\ndipoles with vibration directions parallel to the incidence plane contribute to the\ninfrared absorption in the long-β phase at 0°. At 90°, almost all dipole moment vibration\ndirections become parallel to the incidence plane, resulting in maximum absorbance for\nthe long-β phase (Fig. R1c). In contrast, the vibration mode near 837 cm-1 corresponds\nto the coupling of CH rocking and CF asymmetric stretching vibration, with the dipole\n2 2\nmoment vibration direction parallel to the chain backbone [J. Macromol. Sci., Part\nB:Phys. 40, 517-527 (2001)]. As a result, the short-β phase exhibits maximum\nabsorbance at 0°, whereas the absorbance nearly vanishes at 90° (Fig. R1d).\nFig. R1. Schematic diagram of the variable angle attenuated total reflection infrared\n(VA-IR) spectroscopy test principle. a, Front view of the infrared spectroscopy setup\nequipped with an ATR accessory. b, ATR accessory with a rotatable protractor. The film\nsample is attached to the lower surface of the protractor, with the opposite side in\ncontact with the ATR crystal for infrared measurement. The relationship between the\ndipole moment vibration direction (not dipole direction) of the film sample and the\nincidence plane at 0° and 90° of c, 1275 cm-1 and d, 837 cm-1. 0° refers to the angle at\nwhich the incident infrared light is parallel to the chain backbone direction (original\nstretching direction), while 90° indicates that the incident infrared light is perpendicular\nto the chain backbone direction (stretching direction).\nTo help readers better understand the differences in internal structure between PVDF-\nC, PVDF-S and PVDF-SPF after solid-phase pressure forming, we have added the\ncorresponding relationships between crystal structures and dipole states in the\nschematic diagram (Fig. R2). PVDF-C is primarily composed of the γ phase, which\ntends to form spherulites and thus exhibits large and weakly polar paraelectric dipole\ndomains (Fig. R2a). PVDF-S is almost entirely composed of the β phase; after\nmechanical drawing, it forms fibrous β crystals with bulky, compact ferroelectric\ndipoles (Fig. R2b). PVDF-SPF also mainly contains the β phase; however, it consists\nof tiny and loose β crystals, in which dipoles are more spatially dispersed and decoupled\n(Fig. R2c). This structural feature enhances entropy change under an applied electric\nfield.\nFig. R2. Electrocaloric effect and β phase distribution in solid-phase pressure formed\nPVDF. a, b, c, Schematic diagrams of (a) PVDF-C (casted), (b) PVDF-S (stretched),\nand (c) PVDF-SPF (solid-phase pressure formed). The enlarged images show the\ncrystal morphology and dipole states within the films. PVDF-C contains spherulites\nwith large and weakly polar paraelectric dipole domains. PVDF-S exhibits fibrous or\nextended-chain β crystals with bulky and compact ferroelectric dipoles. PVDF-SPF\nconsists of tiny and loose β crystals, with spatially dispersed and decoupled dipoles,\nwhich favor enhanced EC entropy change. d, Longitudinal (original stretching direction)\nand transverse strains of the films during the stretching and solid-phase pressure\nforming process. e, ΔT of the PVDF films measured by a heat flux sensor at 25 °C.\nECE\nf, Infrared thermal images of PVDF-SPF during application and removal of electric\nfield at room temperature. Before 200MV m-1 is removed, the film temperature slightly\nexceeds room temperature due to Joule heating. g, h, i, VA-IR contour maps of (g)\nPVDF-C, (h) PVDF-S, and (i) PVDF-SPF, normalized at 875 cm-1. The contour lines\nrepresent the same absorbance levels (solid lines: maximum absorbance, dotted lines:\n80% of the maximum absorbance). 0° refers to the angle at which the incident infrared\nlight is parallel to the original stretching direction.\nTo address this comment, we first removed original Supplementary Note 3 from the\nSupplementary Information and added Fig. R1 as Supplementary Figure 2 to provide a\nmore detailed explanation of the VA-IR principle and measurement procedure. In the\nrevised version, we explicitly defined 0° as the angle at which the incident infrared light\nis parallel to the original stretching direction, and we adjusted the measurement range\nto -90° to 90°. In addition, the vertical axes of the VA-IR contour maps in Fig. 1g-i were\ncorrected accordingly in manuscript. Furthermore, we updated the schematic diagrams\nin Fig. 1 by adding the corresponding relationships between crystal structures and\ndipole states, to help readers better understand the correlation between the structure and\nperformance of PVDF-SPF.\nRevised Main Text: “VA-IR is a characterization technique that captures\nconformational information across all azimuthal angles by rotating the film sample.\nThis enables in situ visualization of the spatial distribution of molecular conformations\nwithin the film.”\n(Page 6, lines 135-138)\n2. To properly measure the ECE using an IR imager, the authors should provide time-\nresolved thermal profiles, alike of what has been demonstrated for the heat flux method\nin Fig. S3a. The authors should also present cross-verification between the heat flux\nmethod and the IR imager method for deltaT at different electric field near room\ntemperature (or at various temperatures).\nResponse: Thank you for your valuable suggestion. We acknowledge the omission of\ntime-resolved electrocaloric temperature change (ΔT) curves of PVDF-SPF measured\nby infrared thermal imaging. In response, we have supplemented the time-resolved IR\ntemperature profiles of PVDF-SPF under various electric fields and compared them\nwith the results obtained from the heat flux sensor (Fig. R3b and R3c). It was found\nthat the ΔT values recorded by the infrared thermometer were lower than those obtained\nusing the heat flux sensor. Upon careful investigation, we identified the following\nfactors that may contribute to the lower signals observed in the IR measurements:\n(1) The sampling rate of the IR thermometer (~100 ms) is significantly lower than\nthat of the heat flux sensor (~10 ms), which may hinder the accurate capture of the\ntransient electrocaloric response of PVDF-SPF.\n(2) To enhance the surface emissivity of the sample, a layer of carbon nanotubes\n(CNTs) was sprayed onto the silver electrodes. However, under the current\nexperimental conditions, we were unable to precisely control the CNT’s coating\nthickness (~1μm) in a way that both increases emissivity and minimizes thermal signal\nloss. An excessively thick CNT layer may partially absorb the released heat of the\nsample, thereby reducing the maximum temperature change measured by the IR\nthermometer. This phenomenon has also been reported in previous articles [Nat.\nCommun. 15, 6670 (2024); Nature 640, 924-930 (2025)].\nFig. R3. ECE performance of the PVDF-SPF. a, Time-resolved heat flux signal of\nPVDF-SPF. b, Time-resolved infrared thermometer signal of PVDF-SPF. c,\nComparison of the temperature change measured by the two methods. d, Electrocaloric\nentropy change of the three films.\nMoreover, we have included IR-based ΔT measurements of PVDF-SPF under\nvarious temperatures and electric fields (Fig. R4b). The trends obtained from the IR\nmeasurements are consistent across different conditions.\nFig. R4. a, Temperature change of PVDF-SPF measured by the heat flux sensor at\ndifferent temperatures. b, Temperature change of PVDF-SPF measured by the infrared\nthermometer at different temperatures.\nTo address this comment, we have added the electrocaloric temperature change (ΔT)\nof PVDF-SPF measured by the infrared thermometer under different electric fields at\nroom temperature (Supplementary Figure 5b), as well as a comparative plot showing\nthe results obtained from both measurement methods (Supplementary Figure 5c). In\naddition, temperature-dependent electrocaloric ΔT curves measured by the IR\nthermometer have been included in Supplementary Figure 24. Corresponding\nexplanatory content has also been added to the manuscript.\nRevised Main Text: “This result was cross-validated using an infrared thermometer.\nDue to lower sampling rates of the infrared method, the measured temperature change\nwas reduced (Supplementary Fig. 5b-c).”\n(Page 6, lines 128-130)\n3. It’s also better to consolidate the ECE by showing calibration of the IR imager.\nComparison of the temperature of CNT-sprayed surface measured by IR imager and the\ntemperature of the heat controller should be plotted in the vicinity of room temperature,\nto show that emission of the CNT-sprayed surface is reliable and free from interference\nof low-emission metallic electrode surface and background noise.\nResponse: Thank you for your valuable suggestion. To ensure the accuracy of\ntemperature measurements, all samples were coated with a calibrated CNT layer with\nan emissivity of 0.85 before the infrared imager (Fig. R5a-g). The emissivity was pre-\ndetermined using a dual-band emissivity measurement instrument (IR-2, Shanghai\nInstitute of Technical Physics). As shown in Fig. R5a-g, when the samples were placed\non a heat controller at different temperatures, the surface temperatures recorded by the\ninfrared imager were consistently in agreement with those measured by the\nthermocouple (Fig. R5h), confirming the reliability of the infrared measurements. In\naddition, we also measured the surface temperature of silver electrodes without the\nCNT coating. It is well known that glossy metallic surfaces typically exhibit low\nemissivity values (<0.3); therefore, the use of an incorrect emissivity setting prevents\naccurate measurement of the surface temperatures of both the silver electrodes and the\nmetallic heating plate.\nFig. R5. Verification of the emissivity of the electrocaloric (EC) film. a-g, Comparison\nbetween the surface temperature of the EC film and the actual temperature of the heat\ncontroller in the range of 20-80 °C. The insets show photographs of the EC films: from\nleft to right are the thermocouple, the EC film with silver electrodes only, and the EC\nfilm coated with a CNT layer on the silver electrodes. h, Temperature measurement of\nCNT coating and silver electrode surfaces by IR imager at an emissivity of 0.85. i,\nComparison of surface temperature errors (T ) for CNT coating and silver electrode\nError\nregions relative to the heat controller.\nTo address this comment, we included an emissivity verification experiment for the\nsurface of the electrocaloric (EC) film, which has been provided as Supplementary\nFigure 4. A detailed explanation of the emissivity verification has been added in\nSupplementary Note 2.\n4. The authors observed that ‘the infrared absorption of both the long-beta phase (1275\ncm-1) and the short-beta phase (837 cm-1) weakened’, while the electric field application is supposed to increase the fraction of long-beta phase, and ideally enhances\nabsorption at 1275 cm-1. What leads to the decrease?\nResponse: Thank you for your thoughtful question. In relaxor ferroelectric polymers\nsuch as P(VDF-TrFE-CFE), it has been reported that under an applied electric field, the\ninfrared absorption of both the long-β phase (1275 cm-1) and short-β phase (840 cm-1)\nbecomes enhanced [Adv. Funct. Mater. 34, 2314705 (2024)]. When the external field is\napplied, the G conformation vanishes and a long T sequence is formed. The dipoles of\nT3 in T3G tend to be aligned with the field, and the three Ts will be switched as an\nentity. As a result, the G conformation vanishes, and the adjacent T3s units align to form\na longer-range all-T conformation. This phenomenon is attributed to the dominance of\nthe T3G sequence in the terpolymer. In contrast, in PVDF system, a uniaxial drawing\nwith a high strain of approximately 1400% was first applied, which promoted the\nformation of long T sequences with a content of about 96%. The subsequent SPF\nprocess did not significantly reduce the proportion of long T sequences. Therefore,\nwhen an electric field is applied, no further increase in the β phase is observed,\nindicating that almost no phase transition involving the formation of a “new β phase”\noccurs in PVDF-SPF. Only dipole reorientation and alignment with the electric field\ncan be observed. In transmission-mode infrared spectroscopy, the contribution of\nvibrational modes with dipole moments oriented perpendicular to the polymer chain\n(i.e., along the film thickness direction) is limited, due to the mismatch with the electric\nfield vector of the incident IR light, which lies parallel to the film surface. As a result,\nthe corresponding absorption bands exhibit reduced intensity. This behavior has also\nbeen confirmed in previous studies [Macromolecules 54, 2334-2352 (2021);\nMacromolecules 45, 1954-1962 (2012)].\nTo address this comment, we have added the corresponding explanation in the\nmanuscript and included new references.\nRevised Main Text: “Given the high β phase content (94%) in PVDF-SPF, further\nincrease in its concentration under the electric field is considered negligible. The\nreduction in absorption intensity is primarily attributed to the reorientation of polar\ndipoles along the field direction, which decreases their vibrational component along the\ndirection of the infrared electric vector (Supplementary Figure 9). This reorientation\nalso leads to a decrease in dipolar entropy (Supplementary Note 3).”\n(Page 8, lines 186-192)\n5. The authors noted that the drastic decrease of ECEs at 90 degC is attributed to the\ncrystallization of the amorphous beta phase, presumably into the normal beta phase\nsince large remanent polarization is observed above 90 degC. But there’s inconsistence\nbetween such a claim and the fact that no signals for crystallization, e.g. peaks, were\nobserved in their DSC near 90 degC. The inconsistence also exists for temperature-\ndependent FT-IR data, where the authors observed that both short-beta phase and long-\nbeta phase decreased uniformly with rising temperature, instead of noticing a trade-off\nbetween the normal and amorphous beta phase.\nResponse: Thank you for your thoughtful question. We acknowledge that key\ninformation was overlooked during the initial analysis of the DSC curves, where the\nweak endothermic feature in the 70-90 °C range was previously misinterpreted as\nbaseline fluctuation. After careful re-examination, we found a subtle endothermic peak\naround 80 °C for PVDF, rather than an exothermic crystallization signal (as shown in\nFig. R6a). For PVDF-C, this feature is almost absent at 80 °C due to its well-developed,\nnonpolar spherulitic morphology. In contrast, PVDF-S, PVDF-SPF , and PVDF-SPF\n85\nall exhibit a clear endothermic peak near 80 °C, and the peak position gradually shifts\nto lower temperatures as the processing is more intense. We propose that this behavior\noriginates from the fragmentation of large crystals into smaller crystallites during\nprocessing, which subsequently melt in this temperature region, suggesting that a\nstructural transformation is occurring inside the material. The lower melting\ntemperature further implies that the crystalline structure in PVDF-SPF is more loosely\npacked. Based on these findings, we re-examined PVDF-SPF using temperature-\ndependent 2D-WAXD and optimized the one-dimensional integration and analysis\nprocess to obtain more reliable conclusions.\nFig. R6. (a) DSC heat flow curves of the films at 20-120 °C. (b) Melting curves of the\nfilms at 120-200 °C. (c) Crystallization enthalpy and melting temperature of the films.\n(d) Melting range and melting onset temperature of the films.\nThe normalized one-dimensional integrated profiles show that as temperature\nincreases, the proportion of the amorphous phase in PVDF-SPF rises, accompanied by\na continuous decrease in relative crystallinity (Fig. R7a). Simultaneously, the\ndiffraction peak corresponding to the β crystals gradually shifts to lower angles and its\nFWHM slightly narrows (Fig. R7b), indicating a progressive increase in the interchain\nspacing within the crystalline domains. In the low-temperature range (<80 °C), the\ninterchain spacing of PVDF-SPF increases slowly and linearly with temperature; above\n80 °C, it increases significantly (Fig. R7c). This suggests that, under the influence of\ninternal stresses, the relatively loose structure of the β crystals remains stable below\n80 °C without substantial disruption. Around 80 °C, however, both the crystallite size\nand relative crystallinity exhibit inflection points, implying that the β crystals undergo\nconsiderable thermal expansion accompanied by partial disintegration, which leads to\na rapid decrease in relative crystallinity. This abrupt structural transition is also evident\nas a weak endothermic peak in the DSC curve. Before this structural collapse, the\nexpansion of the interchain spacing favors the enhancement of the electrocaloric\nentropy change; once the structure becomes unstable and collapses, the ECE drops\nsharply to zero.\nFig. R7. (a) One-dimensional integrated profiles of temperature-dependent 2D-WAXD\npatterns of PVDF-SPF. The one-dimensional integration of the 2D-WAXD data was\nobtained by integrating along the x-axis. (b) Contour plot of diffraction intensity near\n2θ = 20°. The dashed lines (---) indicate the trend of peak position with temperature. (c)\nInterchain spacing as a function of temperature. (d) Crystal size and relative\ncrystallinity as a function of temperature. The XRD crystallinity at the initial\ntemperature (20 °C) was 61.9%, slightly higher than the 52.8% estimated from the DSC\nenthalpy (theoretical enthalpy for fully crystalline PVDF is 103.4 J g-1).\nIn the temperature-dependent infrared analysis, we also overlooked important\ninformation. The characteristic peak of the long-β phase (1275 cm-1) exhibits a\npronounced redshift with increasing temperature, indicating a weakening of the\ninterchain dipole-dipole interactions, which is consistent with the expansion of the\ninterchain spacing observed in the 2D-WAXD results. Meanwhile, the absorbance of\nthe long-β phase continuously decreases as the temperature rises and drops markedly\nafter 80 °C, suggesting that the long-β phase content is highly temperature-dependent.\nIn contrast, the characteristic peak position and absorbance of the short-β phase (837\ncm-1) show only a slight decrease in the low-temperature range (<80 °C), indicating\nthat the short-β phase exhibits good thermal stability within this temperature range.\nHowever, once the temperature exceeds 80 °C, the short-β phase content also decreases\nrapidly. This implies that 80 °C is a critical threshold for the stability of the short-β\nphase, beyond which its concentration is significantly reduced.\nFig. R8. Contour plot of temperature-dependent ATR spectra near (a) 1275 cm-1 (long-\nβ phase) and (b) 837 cm-1 (short-β phase). Peak position and absorbance of (c) the long-\nβ phase and (d) the short-β phase as a function of temperature, extracted from the\ncontour plots.\nTo address this comment, we carefully reconsidered the initial analysis of the DSC\ncurves and temperature-dependent infrared spectra, identifying several important\nfeatures previously overlooked. Additional experiments were carried out on PVDF-SPF,\nincluding reanalyzing the DSC results and adding supplementary 2D-WAXD\nmeasurements. The FTIR data processing was also optimized, resulting in more reliable\ninsights into the structural evolution.\nRevised Main Text: “Variable-temperature infrared spectra show a clear redshift in the\ncharacteristic peak of the long-β phase with increasing temperature, indicating\nweakened interchain dipole-dipole interactions (Supplementary Fig. 19a and 19c). Its\nabsorbance gradually decreases and drops sharply above 80 °C, reflecting the strong\ntemperature sensitivity of the long-β phase content. In contrast, the peak position and\nabsorbance of the short-β phase remain nearly unchanged below 80 °C, which\ndemonstrates its great thermal stability (Fig. 4b and Supplementary Fig. 19b). However,\nonce the temperature exceeds 80 °C, a rapid decrease in the short-β phase is observed.\nThis behavior indicates that 80 °C serves as a critical threshold for the structural\nstability of the short-β phase. Below this temperature, the β phase in PVDF-SPF\nremains stable without conformational transformation. The structural evolution of β\ncrystals was directly observed via variable-temperature 2D-WAXD (Supplementary Fig.\n20). Around 80 °C, both crystal size and relative crystallinity exhibit inflection points.\nThis indicates the β crystals undergo considerable thermal expansion accompanied by\npartial disintegration, which leads to a rapid decrease in relative crystallinity (Fig. 4c).\nA weak endothermic peak in the DSC curve also reflects this transition (Supplementary\nFig. 15a). Before the collapse, the increased interchain spacing favors the ΔS .”\norient\n(Page 12, lines 282-298)\n6.The claim that there’s a drop in dielectric constant at 90 degC relating to this\ncrystallization is also not solid, given that this dielectric drop near 90 degC has been\nseen in the PVDF-C and PVDF-S samples which would not contain the amorphous\nbeta-phase.\nResponse: Thank you for your thoughtful question. We acknowledge that our initial\nattribution of the observed dielectric behavior to “crystallization of amorphous β phase”\nwas not accurately phrased, especially considering that a similar dielectric response is\nalso observed in the PVDF-C and PVDF-S samples. To clarify this issue, we conducted\na systematic comparison of structural and dielectric behaviors across different samples.\nBy integrating data from DSC, temperature-dependent 2D-WAXD, variable-\ntemperature ATR, and dielectric spectroscopy, we now provide a more comprehensive\ninterpretation based on microstructural evolution. First, DSC curves reveal weak\nendothermic peaks in the range of 80-90 °C for PVDF-C, PVDF-S, and PVDF-SPF\n(Fig. R6a). In PVDF-C, where the spherulitic crystals are complete and stable, this peak\nis barely visible. However, the peak becomes more pronounced and slightly shifts to\nlower temperatures in PVDF-S (after uniaxial stretching) and PVDF-SPF (after solid-\nphase pressure forming). We believe this endothermic event is not associated with\ncrystallization but rather with the melting or reconstruction of small, loosely packed\ncrystalline domains (such as short-β phase or loosely packed β crystal) generated during\nmechanical processing. This interpretation is supported by the fact that no exothermic\npeaks are present in this region indicative of new crystal formation. Second, the\ntemperature-dependent dielectric spectra of all three samples show a peak in dielectric\nconstant near 90 °C. This common feature suggests that the observed dielectric anomaly\nshould not be attributed to the transformation of a specific phase (e.g., amorphous β to\ncrystalline β), but rather to a more general thermal activation of chain segmental\ndynamics and local structural rearrangements common to PVDF-based systems [Nat.\nMater. 12, 433-438 (2013)]. Notably, PVDF-SPF exhibits significantly higher dielectric\nloss (tan δ) at the same temperature, suggesting the presence of more thermally and\nelectrically responsive local structures.\nThe enhanced dielectric response of PVDF-SPF in this temperature range can be\nattributed to the following factors:\n(1) A higher content of short-β phase or loosely packed β crystals induced by\nprocessing, which possess larger interchain spacing and lower thermal stability, making\nthem more susceptible to thermal disruption between 80-90 °C;\n(2) Increased freedom of chain segmental motion, leading to stronger polarizability\nunder the applied electric field;\n(3) A more disordered structure promotes the migration of charge carriers under high\nfields, resulting in increased conduction loss and explaining the sharp decline in\ndielectric constant above 90 °C.\nFinally, temperature-dependent 2D-WAXD and FTIR analyses confirm that\nstructural changes occur in this temperature range: interchain spacing expands, relative\ncrystallinity decreases, and β phase absorption diminishes. These observations support\nour revised interpretation that the system undergoes structural relaxation and partial\nmelting rather than crystallization or phase transition.\nTo address this comment, we systematically compared the structural evolution and\ndielectric response of different films within this temperature range. By integrating data\nfrom DSC, variable-temperature 2D-WAXD, variable-temperature FTIR, and dielectric\nspectroscopy, we proposed a more comprehensive explanation. The relevant\ndescriptions and interpretations have been revised accordingly in the manuscript.\nRevised Main Text: “Compared to PVDF-C and PVDF-S, the k of PVDF-SPF increased\nwith temperature (Fig. 4a), indicating higher polarizability and stronger electroactivity\nunder an electric field. At low temperatures (approximately −30 °C), both k and\ndielectric loss increased significantly (Supplementary Fig. 18a and 18d). This behavior\nresembles the polarization enhancement mechanism reported previously39. The\nstructure in PVDF-SPF, referred to as loose β crystals (similar to intermediate phases),\ncan greatly improve the reversible ECE.”\n(Page 12, lines 270-277)\n“The drop of k around 90 °C (Fig. 4a) illustrates structural relaxation and partial melting\n(Supplementary Fig. 15a), which may seriously reduce the ECE of PVDF-SPF.”\n(Page 12, lines 279-281)\n7.There’s also inconsistence between the polarization data and the thermal data. In Fig.\nS16, either the remnant polarization or the maximal polarization for PVDF-SPF\nincreases with temperature, and it seems dP/dT is positive for all fields. Since the\nMaxwell relation defines that dP/dT = dS/dE, an inverse effect (deltaS>0 for field\napplication, and hence field application generates temperature drop) instead of the\nobserved conventional effect should arise. And such an inverse effect should be the\nmost substantial above 90 degC given the larger dP/dT values. All these lead to a\nsignificant discrepancy against the thermal measurements presented in the rest of this\nwork.\nResponse: Thank you for your thoughtful question. We believe that the fundamental\nreason for this discrepancy lies in the presence of significant residual stress within the\nPVDF-SPF film. When internal stress (σ) exists in a material, the interchain spacing\nwill vary depending on the orientation of the crystal planes relative to the stress\ndirection, thereby causing a shift in the X-ray diffraction angle 2θ according to Bragg’s\nlaw. By employing the side-inclination method and measuring the 2θ angle of the\n(110)/(200) crystal planes at different ψ angles, the transverse residual stress σ can be\nquantitatively determined [Practical Residual Stress Measurement Methods (ed.\nSchajer G. S.) 139-161 (John Wiley & Sons, Ltd, West Sussex, 2013)]. The stress can\nbe calculated using the following equation [Nano Energy 72, 104662 (2020); J. Mater.\nSci.:Mater. Electron. 32, 1341-1368 (2021)]:\n𝐸 𝜋 ∂2𝜃\n𝜎 = − cot𝜃 (Eq.R1)\n2(1+𝜈)180 0∂sin2𝜓\n𝐸 𝜋\n𝐾 = − cot𝜃 (Eq.R2)\n2(1+𝜈)180 0\n∂2𝜃\n𝑀 = (Eq.R3)\n∂sin2𝜓\n𝜎 = 𝐾𝑀 (Eq.R4)\nHere, E is Young's modulus of PVDF-SPF, and v is the Poisson's ratio. K is the stress\nconstant, and M is the slope obtained from the linear fitting of 2θ versus sin2ψ. Based\non our measurements, the tensile residual stress in PVDF-SPF was estimated to be\napproximately 15 MPa. This stress originates from the orientation-induced strain during\nsolid-phase pressure forming, which significantly alters the crystalline structure and\npolarization behavior, resulting in a non-equilibrium metastable state.\nFig. R9. a β crystal diffraction peaks measured at different ψ angles. b Linear fitting of\nextracted 2θ values versus sin2ψ to obtain the stress constant (K) and slope (M). The X-\nrays were incident along the original stretching direction, and the transverse residual\nstress in the crystalline region of the film was measured using the side-inclination\nmethod. Negative values indicate tensile stress, while positive values indicate\ncompressive stress in the crystalline domains.\nIt is important to note that the Maxwell relation is derived under the assumption of\nthermodynamic equilibrium and quasi-static processes, without considering the\ninfluence of mechanical stress or structural non-equilibrium. In our case, the\npolarization variation with temperature (dP/dT) reflects not only thermal activation but\nalso stress relaxation and structural rearrangement. Therefore, dP/dT no longer provides\nan accurate representation of the true entropy change. We also observed a marked\nincrease in dielectric loss above 90 °C, indicating enhanced leakage and conductive\nlosses under high electric fields. These effects contribute to the polarization but do not\nlead to reversible entropy change, ultimately diminishing the ECE performance. In\nsummary, due to the stress-dominated and non-equilibrium nature of the PVDF-SPF\nsystem, the applicability of the Maxwell relation for quantitative analysis of its ECE\nbehavior may be limited.\nTo address this comment, we conducted residual stress analysis of PVDF-SPF using\nXRD. The stress originated from the SPF process and resulted in a structurally non-\nequilibrium state. As a result, it does not accurately represent the thermodynamic\nentropy change defined by the Maxwell relation. We have revised the relevant\ndescriptions and explanations in the manuscript.\nRevised Main Text: “In the range of 20-80 °C, the polarization difference (ΔP)\ngradually increased (Fig. 4d and Supplementary Fig. 22). The resulting increase in ΔS\ndip\nled to a total ECE entropy change (ΔS ) of 52.5 J kg-1 K-1 at 80 °C (Fig. 4e). This\nECE\nenhancement was attributed to tensile residual stress (~15 MPa) introduced during the\nSPF process (Supplementary Fig. 23), which stabilized the loose β crystals and enabled\na large ΔS below 80 °C.”\nECE\n(Page 13, lines 299-304)\nReviewer #2 (Remarks to the Author):\nIn this study, the authors present pressed PVDF films that achieve an EC temperature\nchange of up to 14.1 K at 25 °C and a temperature window of 60 °C. The authors claim\nthat the PVDF films underwent substantial plastic deformation and extensive crystal\nfragmentation under 30 MPa solid-phase pressure, resulting in the formation of small,\nloosely packed beta crystal grains. It is always interesting to see the highly reversible\npolar responses from the conventional ferroelectric pvdf, the manuscript is therefore\ninteresting. However, I still have some questions and comments that are related to the\nvalidation, novelty, and significance of the work.\n1. The authors claimed that \"forming loose and tiny β crystal grains\", and \"The\ngiant ECE primarily originates from orientation entropy changes of molecular chains\nassociated with the reversible transition between loose and compact β crystals\", as if\nthe tiny and compact polar structures leading to the large ECE is the key finding. However,\nrecent reports in PVDF-based polymers have already demonstrated the point (see ref. 11).\nFrom a broader picture, it is well known that the smaller and compact polar structures\nwould lead to a high dipolar response (see Nature,2017, 546, 391). Therefore, the\ninteresting part of this work should be how to generate these polar structures \"in PVDF\".\nTo this point, the ref. 40 (Nat. Commun., 2019, 10, 4535) should be cited in a clearer\nposition in this manuscript compared to its current position. The ref. 40 utilized a similar\nhot press process to reduce the size of the polar structures, which is directly related to\nthis work and should be discussed in the beginning of the manuscript. I have no idea why\nthe authors cite the papers but are not willing to discuss their true findings.\nResponse: Thank you for your thoughtful question. We fully agree that the references\nyou suggested are closely related to the topic of this work. Based on your suggestion,\nwe further clarify the differences between our work and these studies in terms of\nresearch subject, processing pathway, and underlying physical mechanism. Ref. 11\n[Nature 600, 664-669 (2021)] introduced high-entropy modifications to a terpolymer\nsystem by partially converting chlorofluoroethylene groups into covalent double bonds.\nThis significantly increased the number of polar entities and enlarged the interfacial\narea between polar and non-polar phases. The resulting polar regions exhibited a\nloosely correlated high-entropy state, which reduced the energy barrier for electric-\nfield-induced switching and thereby enhanced the ECE performance under low fields.\nThe reference you mentioned [Nature 546, 391-395 (2017)] proposed a slush-like polar\nnanodomain model based on molecular dynamics simulations. It revealed that such\nstructures originate from complex local dynamics in perovskite oxide single-crystal\nrelaxors, emphasizing the importance of small domain sizes and low energy barriers in\npromoting strong polarization responses. Ref. 40 [Nat. Commun. 10, 4535 (2019)]\nemployed a repeated pressing-folding strategy to extend the polymer chains in multiple\ndirections using commercial PVDF. This process is analogous to biaxial stretching and\nrequires multiple mechanical cycles to induce a nearly 98% β phase content. The\nresulting structure is isotropic, relatively loose, and has a slightly lower crystallinity\n(~38%). While this microstructure favors high energy storage density, the lower\ncrystallinity limits the amount of active crystalline regions that contribute to entropy\nchange during dipole switching, which is less advantageous for ECE applications.\nOur work proposes and systematically demonstrates a simplified strategy that\ncombines uniaxial pre-orientation with solid-phase pressure forming. Uniaxial\nstretching up to 1400% strain served as a pre-orientation process that effectively aligned\nthe molecular chains, yielding a highly oriented structure with approximately 96% β\nphase content. Subsequent hot pressing at 85 °C and 155 °C induced lateral expansion\nand disruption of the crystalline domains, resulting in a structure characterized by high\ncrystallinity (52.8%), loosely packed, and tiny β crystals. These loosely and compactly\npacked β crystals are capable of undergoing reversible transitions under an applied\nelectric field, accompanied by significant dipolar switching entropy and orientation\nentropy changes. Such structural dynamics greatly enhance the ECE while maintaining\nhigh reversibility and operational stability. This approach not only provides a simpler\nand more efficient processing route but also reveals the morphological evolution of\nloose β crystal under electric fields, offering improved structural tunability and strong\npotential for electrocaloric applications.\nTo address this comment, we have strengthened the explanation of the processing\nmethod and structural regulation mechanism of Ref. 40 in the Introduction, and\nsystematically compared its similarities and differences with our work to further clarify\nthe distinctive features and innovations of this study.\nRevised Main Text: “To improve its electrical performance, strategies such as repeated\npressing-and-folding have been employed to extend polymer chains multidirectionally,\nthereby boosting dielectric energy storage capability.”\n(Page 3, lines 67-69)\n2. From the data of the heat sensor, the pressed PVDF is too lossy, as we observed that\na constant heat was generated under the electric field. The amount of heat generated\nexceeds the heat absorbed during the removal of the electric field. This significant\nheating effect can accumulate over time, making the material unsuitable for practical\napplications in refrigeration devices. The authors should provide a solution to reduce\nthe heating effect significantly before discussing the possibility for practical use of the\nmaterial. Or the authors should argue that the heating is insignificant in application.\nResponse: Thank you for your valuable suggestion. We agree that a significant\nirreversible heating effect under an electric field could impact the material's suitability\nfor practical applications in cooling devices. In our ECE measurements, the PVDF-SPF\nfilms were approximately 10 μm thick and cut into 15×15 mm squares. Silver electrodes\nwith diameters of 6 mm and 8 mm were deposited concentrically on both surfaces and\nconnected using conductive silver paste and fine silver wires. Under typical testing\nconditions (with a high electric field applied for only 6 seconds), the inherently high\nelectrical resistance of PVDF generally prevents notable Joule heating or dielectric\nbreakdown. However, we did observe that a small number of samples exhibited\nasymmetric temperature behavior, where the heat released during field application\nexceeded the heat absorbed during field removal. We attribute this to possible factors\nsuch as localized contact resistance at the electrode, nonuniform silver paste coating or\nundetected local breakdowns. Therefore, we fully acknowledge your concern that\nparasitic heating effects must be carefully identified and eliminated in electrocaloric\nmeasurements.\nFig. R10. Time-resolved heat flux signal of PVDF-SPF.\nTo address this comment, we conducted repeated measurements on several other\nsamples from the same batch. The results showed that most samples did not exhibit\nsimilar overheating behavior, confirming that the previously reported data reflected an\nanomalous case. Accordingly, we have removed the abnormal data from the revised\nmanuscript and updated the electrocaloric temperature change curves and associated\ntemperature values to better represent the intrinsic ECE behavior of the PVDF-SPF\nfilms. Based on the ratio of Q /Q , the contribution of Joule heating to the total\nJoule heating\nentropy change was estimated. Under an electric field of 200 MV m-1, the Joule heat\nloss accounted for approximately 15-18% of the total entropy change. Therefore, we\nhave added a brief discussion in the main text regarding the potential sources of heating,\nand suggested strategies such as electrode optimization, improved interface bonding,\nand voltage reduction to further suppress undesirable thermal effects in future\napplications.\nRevised Main Text: “Before 200 MV m-1 is removed, the film temperature slightly\nexceeds room temperature due to Joule heating. Optimizing electrode design,\nimproving interface bonding, and reducing operating voltage could effectively suppress\nthis effect.”\n(Page 26, lines 592-595)\n3. The heat measurement of the cooling effect is questionable. Anything above the\nbaseline should not be integrated into the cooling effect without further evidence. The\nauthors should work carefully on the measurement. Even without any cooling effect\nfrom the ECE, the heating curve will reduce to the baseline due to the convection. Once\nthe field is removed, how much area of the signal integration (parts above the baseline)\nis due to the convection or heat absorption of ECE is not clear. It is highly possible that\nthe reported ECE is overrated.\nResponse: Thank you for your valuable suggestion. We fully agree with your point that,\nwhen quantifying the ECE cooling based on heat flux sensor signals, it is essential to\ndistinguish between the actual cooling contribution from the ECE and the baseline\nreturn caused by natural convection after the field is removed. As you pointed out, even\nin the absence of any ECE, the heat flux signal may decay toward the baseline due to\nconvection. Therefore, integrating any portion of the signal above the baseline could\nindeed lead to an overestimation of the ECE.\nFig. R11. Calibrating the heat flux sensor using the heat flux signal. (a) Applied pulse\nvoltage and heat flux signal. (b) Joule heat and integral area fitting. Five different pulse\nvoltage values were set, and the corresponding Joule heat was measured and linearly\nfitted. (c) ECE signal of PVDF-SPF measured by the heat flux sensor under 200 MV\nm-1. The red shaded area represents the Joule heating of the film during field application,\nwhile the blue shaded area indicates the electrocaloric cooling upon field removal.\nTo address this comment, we carefully re-evaluated our data processing method. After\nperforming repeated measurements on multiple samples, we revised the integration\napproach according to your suggestion. Specifically, in the recalculation of the ECE,\nwe strictly followed your recommendation by integrating only the portion of the heat\nflow signal that falls below the baseline after the electric field is removed, excluding\nany area above the baseline during the decay process. This adjustment allows for a more\naccurate estimation of the electrocaloric cooling. As a result, the maximum ECE\ntemperature change under 200 MV m-1 has been corrected from 14.1 K to 12.8 K, and\nthe corresponding isothermal entropy change has been revised from 53.7 J kg-1 K-1 to\n48.6 J kg-1 K-1. The relevant figures and data have been updated in the revised\nmanuscript, and the rationale for this correction has been further explained in the main\ntext.\n4. The analysis of economic refrigeration capacity is unnecessary and misleading. At\nthe current stage, EC technology, including the material and device, has not been\ndeployed in any market. Only a handful of companies have the raw polymer powder\nfor sale. Therefore, the price of the relaxor ferroelectric polymer is overly high because\nthere is no market. However, the energy cost to produce monomers TrFE and CFE is\nsimilar to the VDF. The price of the relaxor ferroelectric polymer should not be too far\nfrom the PVDF unless the authors prove otherwise with the data from the fuorine\nchemical industry. The current high price is temporary and can be affected by many\nfactors. According to Arkema, once there is a market, the price of their polymers will\ndrop significantly. Also, the refrigeration capacity is field-dependent. There is no\nstandard for how much field was applied in this comparison (Supplementary Table II).\nSince there is no commercially available electrocaloric device, there is no point in\nintroducing this value of economic refrigeration capacity. The \"high\" ERC is\nshortsighted and immature, indicating the lack of investigation of this work.\nResponse: Thank you for your valuable suggestion. We agree with your concern that,\ngiven the absence of commercially available electrocaloric devices, a quantitative\ncomparison based on material cost and “economic refrigeration capacity” lacks a\nconsistent standard. As the current price of relaxor ferroelectric polymers is affected by\nlimited production and market demand, emphasizing ERC at this stage could lead to\nmisinterpretation of the material’s actual performance. Therefore, we acknowledge that\nthis metric should not be highlighted as a core feature of the study. In addition, data on\nthe applied electric field have been added to Supplementary Table 2 to facilitate a better\ncomparison of the electrocaloric performance among different materials.\nSupplementary Table 2. The economic refrigeration capacity of the electrocaloric\nmaterials have been reported.\nReported works\nΔTECE ΔS ΔE Tw RC Cost ERC\nRef\n(K) (J kg-1 K-1) (MV m-1) (K) (J kg-1) (USD kg-1) (J USD-1)\nP(VDF-TrFE) 68/32 mol.% 2.5 8 70 15 120 5000 0.024 13\nTer-DB 7.5 37.5 50 20 750 10000 0.075 10\nP(VDF-TrFE-CFE) nanowire\n2.5 12.6 50 60 756 10000 0.076 14\narrays\nP(VDF-TrFE) 55/45 mol.% 12 70 120 10 700 5000 0.14 12\nIrradiated P(VDF-TrFE) 6 22.5 160 35 787.5 5000 0.16 15\nP(VDF-TrFE-CFE) 8 42.1 150 60 2526 10000 0.25 16\nInt-biased blends 14.4 72 100 40 2880 9500 0.30 17\nTPD 20 100 100 50 5000 10000 0.50 9\nBST/PCBM/P(VDF-TrFE-\n18 95.5 150 80 7640 14000 0.55 16\nCFE)\nBZT 4.5 8 14.5 35 280 265 1.06 18\nKNN-STO 1.9 3.6 15.9 40 144 63 2.29 19\nBT 7.1 10.1 80 40 404 71 5.69 20\nThis\nPVDF-SPF 12.8 48.6 200 60 2916 30 97.2\nwork\nTo address this comment, we have revised the structure and focus of the fourth section\nin the manuscript. First, the section title has been changed from “Economic\nrefrigeration capacity” to “Temperature-dependent ECE behavior”. This new title better\nreflects the section’s focus on how temperature affects the dielectric response,\npolarization state, ferroelectric hysteresis, and electrocaloric performance of PVDF-\nSPF. We have also added detailed analyses of the temperature-induced evolution of\npolarization and structural features, such as the shift of infrared characteristic peaks and\nthe change in XRD diffraction intensities, to elucidate the relationship between ECE\ntemperature range and the structural behavior of loose β crystallites.\nFig. R12. Temperature-dependent properties and electrocaloric performance of PVDF-\nSPF. a, Temperature-dependent dielectric spectra of PVDF-SPF. b, Temperature\ndependence of the wavenumber and absorbance of the infrared characteristic peak\n(~837 cm-1) corresponding to the short-β phase. c, Crystal size and relative crystallinity\nas a function of temperature. The dashed lines (---) represent linear fits. A noticeable\nchange in slope is observed around 80 °C, indicating a structural transformation. d,\nTemperature dependence of the polarization difference (ΔP). ΔP reflects the number of\nreversibly aligned dipoles before and after the application of the electric field, thereby\nindirectly indicating the entropy change associated with the ECE. e, ΔS at different\ntemperatures. ΔS increases gradually with temperature, but drops sharply beyond\nECE\n~80 °C due to partial melting and structural relaxation of the crystalline regions. f,\nComparison of the economic refrigeration capacity (ERC) of PVDF-SPF in this work\nwith representative electrocaloric materials reported in the literature, providing a\npreliminary assessment of its potential for practical applications.\nAs for the ERC values, they are no longer emphasized as a key finding. Instead, they\nare presented briefly in the discussion as a supplementary reference for potential\napplication prospects, without any overstated claims such as “the highest ERC.” We\nbelieve these revisions help clarify the scientific contribution of this work and highlight\nits value in structural regulation and electrocaloric mechanism analysis, without\nintroducing misleading economic interpretations.\nRevised Main Text: “The PVDF-SPF integrates the low cost of ceramic electrocaloric\nmaterials with the high ΔT of terpolymers, achieving an ERC value of 97.2 J USD-\nECE\n1 (Fig. 4f). Although the ERC is influenced by uncertainties such as raw material prices,\nprocessing scale, and the lack of a mature ECE device market, it is not intended for\ndirect economic assessment. Nevertheless, this metric may provide useful guidance for\noptimizing the trade-off between performance and preparation in the design of future\nelectrocaloric material.”\n(Page 14, lines 322-328)\nReviewer #3 (Remarks to the Author):\nThis work reports giant ECE of up to 14.1 K at 25 °C from PVDF films prepared by\nsolid-phase pressure forming (PVDF-SPF). If experimentally substantiated, the work\nwould represent an important progress in the field. The manuscript extensively\ndescribes the structural characterization results and rationalizes these to high ECE.\nAdditional experimental data are required on the characterization of the ECE and direct\nmeasurement of the adiabatic temperature change. Once these are furnished, the work\nwould become acceptable for publication.\n1. Provide experimental data of the temperature change measured with IR camera,\nand how the ECE varies with sample information such as different thicknesses.\nThicker films produce larger heat flux and thus minimizes the impact by parasitic\nloss. Also provide charts to show the temperature change with time during sinusoidal or\nsquare wave voltage scans.\nResponse: Thank you for your valuable suggestion. We agree that characterizing the\nECE solely through indirect parameters (such as entropy change calculated from heat\nflux sensors) is insufficient. Therefore, we have supplemented our work with an\ninfrared thermometer to directly validate the cooling capability of the films. In addition,\nwe compared the electrocaloric temperature change of samples with different\nthicknesses to preliminarily investigate the influence of thickness on parasitic thermal\nlosses.\nFig. R13. a, Time-resolved infrared thermometer signal of PVDF-SPF. b,\nElectrocaloric cooling (ΔT ) of PVDF-SPF with different thicknesses under different\nc\nelectric fields.\nTo address this comment, we employed a high-precision infrared thermometer to\nmeasure the electrocaloric cooling of PVDF-SPF films inside a temperature-controlled\noven. Considering that the infrared thermal imager is too bulky to be placed inside the\noven, we instead used a smaller and faster-responding IR thermometer, which enabled\nrepeated measurements in a stable thermal environment. The results show that under an\nelectric field of 200 MV m-1, the PVDF-SPF films exhibit a significant temperature\ndrop, confirming their electrocaloric performance. Moreover, we conducted a\npreliminary comparison of samples with different thicknesses (up to approximately\n30 μm) and found that thicker films indeed generate greater heat flux, which can partly\nmitigate the influence of parasitic thermal loss. This trend is also consistent with\nprevious literature [Adv. Mater. 35, 2209181 (2023)]. The maximum film thickness we\ncould obtain was limited to ~30 μm, mainly due to the initial thickness of the cast films\n(typically less than 300 μm), as well as the load-bearing limit of the force sensor during\nthe stretching process in solid-phase pressure forming.\nRegarding the time-dependent voltage input during measurement, we have now\nincluded the corresponding waveforms in both the time-resolved heat flux signal and\ninfrared-thermometer signal plots. A square-wave voltage was applied to simulate the\nquasi-adiabatic conditions of the electrocaloric process. These data have been added to\nthe revised manuscript to enhance the completeness and credibility of our conclusions.\n2. Does the PVDF-SPF film show any electrostrictive behavior (e.g. expanding in film\narea) when subjected to the electric field?\nResponse: Thank you for your thoughtful question. We indeed overlooked the\ninvestigation of the electrostrictive behavior of PVDF-SPF under an applied electric\nfield. To solve this, we have conducted electrostriction measurements. The results\nindicate that PVDF-SPF exhibits an out-of-plane strain of approximately -5% under an\nelectric field of 200 MV m-1, suggesting that dipoles can undergo reversible\nreorientation in response to the field.\nIt should be noted that the \" expanding in film area\" mentioned in your comment\nlikely refers to the transverse strain. However, due to limitations in the current\nexperimental setup, it is very hard to accurately measure the in-plane strain of the films\nat this stage. We will further improve the relevant testing methods in future studies to\nb to\nbetter reveal the complete electrostrictive behavior.\nFig. R14. Electrostrictive strain of PVDF-SPF along the thickness direction under an\nelectric field of 200 MV m-1.\nTo address this comment, we measured the out-of-plane electrostriction of PVDF-SPF\nusing a laser interferometer (SP-S120, SIOS Meßtechnik GmbH) equipped with a high-\nvoltage amplifier, under an electric field of 200 MV m-1 at 10 Hz. The corresponding\ntest conditions have been added to the revised manuscript in the “Functional properties\ncharacterization” section.\nRevised Main Text: “This was further confirmed by the large electrostriction of up to\n5% under 200 MV m-1 (Supplementary Fig. 13).”\n(Page 10, lines 237-239)\n3. The authors state that the giant electrocaloric effect in PVDF-SPF originates from\nthe entropy change associated with the reversible reorientation of the tiny β-crystal\ngrains. This core statement of the paper is against the prevailing point of view stating\nthat the non-polar to polar phase transition generates the largest entropy change and is\nthe origin of the giant electrocaloric effect. The paraelectric phases also have larger\ninterchain spacings compared to that of β-phase. The chain orientation of the\nparaelectric phases can also be very random. Therefore, the discussions from line\n178 to 196 are not convincing enough.\nResponse: Thank you for your valuable suggestion. Entropy is fundamentally a\nmeasure of the number of accessible microstates in a system. In ECE, the application\nof an electric field reduces the number of microstates, resulting in entropy decrease and\na corresponding temperature rise under adiabatic conditions; conversely, removing the\nelectric field leads to entropy increase and cooling. For terpolymers, the entropy change\nassociated with the ECE can mainly originate from two mechanisms:\n(1) Dipole switching entropy: A common source of entropy change in ferroelectric\npolymers, referring to the alignment of dipoles along the applied electric field, leading\nto a decrease in configurational disorder.\n(2) Conformational transition entropy: Arising from phase transitions between nonpolar\nand polar crystalline structures, such as the transformation from T3G segments to all-\ntrans (T) sequences.\nAccording to previous work [Adv. Funct. Mater. 34, 2314705 (2024)], terpolymer\nfilms enriched in T3G sequences exhibit pronounced ECE. In this process, the G units\nin the T3G chain can be converted into T units under an electric field, forming extended\nT-sequences aligned with the field direction. It has also been reported that only a small\nfraction of the terpolymer chains exist in the T3G conformation, as evidenced by weak\nT3G peaks in FTIR spectra. Moreover, in each T3G sequence, only the G unit\n(accounting for one-fourth of the T+G sequence) is capable of transforming. These\nfindings suggest that although the conformational phase transition from a paraelectric\nto a polar phase contributes to the entropy change, it is not the sole determinant. The\ncontribution from dipole switching should not be neglected in real systems.\nAs the reviewer pointed out, paraelectric phases typically have larger interchain\nspacings and greater conformational disorder. However, they also tend to form large\ndomains (approximately 20-30 nm). In our work, we aimed to promote crystal\ndecoupling and enhance entropy change by generating β crystals with moderately\nexpanded interchain spacing (~4.4 Å) and significantly reduced crystal size (~6 nm).\nAdditionally, we observed that the degree of Debye ring dispersion in high-field 2D-\nWAXD reflects the orientational disorder among β crystals. A broad and diffused ring\nsuggests a more randomized orientation of β crystals, corresponding to a more\ndisordered system and higher entropy. In contrast, a concentrated Debye ring indicates\nan increase in orientational order and a corresponding decrease in entropy. Based on\nthis, we propose that the variation in the orientational order of crystal domains may\nitself represent an additional and previously overlooked source of entropy change. On\nthis basis, we developed a preliminary theoretical framework of β-phase orientation\nentropy and presented an initial mathematical derivation in the manuscript as a\nsupplementary entropy mechanism. We plan to further investigate this potential\nmechanism through a combination of experimental data and theoretical simulations in\nour future work.\nTo address this comment, we have clarified in the revised manuscript that the\nelectrocaloric entropy change in PVDF-SPF originates primarily from two mechanisms:\ndipole switching entropy and β phase orientation entropy. Based on the changes in\nDebye ring dispersion observed in the 2D-WAXD patterns, we propose that the\nreduction in orientational disorder among β-phase crystals corresponds to an entropy-\ndecreasing process. The derivation and physical significance of β phase orientation\nentropy have been added to the manuscript as an extension of the existing theoretical\nframework.\nRevised Main Text: “The dispersion of the Debye ring reflects the orientational disorder\namong β crystals. A broad and diffuse ring indicates more random orientation,\ncorresponding to a more disordered system with higher entropy. In contrast, a sharp and\nconcentrated ring suggests increased orientational order and lower entropy. Based on\nthis observation, we propose that changes in the orientational order of crystal domains\nmay represent an additional and previously overlooked entropy contribution, namely\nthe β phase orientation entropy change (ΔS ) (detailed derivation in Supplementary\norient\nNote 3).”\n(Page 9, lines 196-202)\n4. In figure 2d, why do peaks for long β-phase (1275 cm-1) and short β-phase (837 cm-\n1) both get weakened, but there is no intensity increase in any characteristic peak for\nthe newly formed compact β-phase at 200MV/m? In fact, the compact β-phase is\nstill β-phase. Is there any characteristic peak in the full FTIR spectrum for the\ncompact β-phase formed at a high field?\nResponse: Thank you for your thoughtful question. As reported in previous work [Adv.\nFunct. Mater. 34, 2314705 (2024)], both the long-β phase (1275 cm-1) and short β-phase\n(837 cm-1) absorption intensities in relaxor ferroelectric polymers such as P(VDF-TrFE-\nCFE) are enhanced upon applying an electric field. This enhancement arises because\nthe G conformations can be converted into T conformations under the field, forming\nlonger T sequences. In particular, the three T units in each T3G segment polarize\ncooperatively, eventually aligning as long-range all T sequences in the direction of the\nelectric field. This process reflects the dominance of T3G sequences in the terpolymer’s\nresponse. In contrast, the PVDF-SPF films studied here were pretreated via uniaxial\nstretching with a high strain of up to ~1400%, inducing a high content of long T chains\nand yielding a β phase fraction of approximately 96%. This level remained largely\nunchanged during the subsequent SPF process. Therefore, under an applied electric\nfield, the polarization response in PVDF-SPF involves the reorientation of pre-existing\nβ-phase dipoles, without the formation of new polar phases, which is distinct from the\nfield-induced phase transitions observed in terpolymers.\nFig. R15. Top-view schematic of the infrared transmission measurement of PVDF-SPF\nunder an electric field of 200 MV m-1. Characteristic absorption peaks of specific\nfunctional groups appear in the infrared spectrum when the dipole moment vibration\ndirection lies in the incident plane.\nAdditionally, we wish to clarify the influence of the FTIR measurement mode. In the\ntransmission-mode FTIR setup used for PVDF-SPF, the infrared electric vector lies\nperpendicular to the direction of propagation. When the direction of dipole moment\nvibration lies within the incident plane, characteristic absorption peaks corresponding\nto specific groups appear in the infrared spectrum. After applying the electric field, β-\nphase dipoles tend to align with the field direction (i.e., parallel to the direction of\ninfrared light propagation), thereby reducing their projection along the direction of the\nIR electric vector. As a result, the corresponding peak intensity decreases. The short-β\nphase shows a similar trend. This behavior contrasts with that of terpolymers, where\nfield-induced β phase formation significantly increases the polar phase content and\nenhances absorption intensity, effectively masking the reduction caused by dipole\nreorientation. This behavior has also been confirmed in previous studies\n[Macromolecules 54, 2334-2352 (2021); Macromolecules 45, 1954-1962 (2012)]. It\nshould be noted that, the term “compact β-phase” refers to a state of more tightly\naligned dipoles within the β-phase under electric field, rather than the formation of a\nnew crystalline phase. Therefore, no distinct characteristic peaks corresponding to this\ncompact β-phase have been observed in the full FTIR spectrum.\nTo address this comment, we have included this clarification in the revised manuscript\nto explain that the observed IR response is attributed to dipole reorientation rather than\na phase transition. This distinction helps differentiate the behavior of PVDF-SPF from\nthat of relaxor terpolymers and highlights the structural origin of the electrocaloric\nresponse in our system.\nRevised Main Text: “Given the high β phase content (94%) in PVDF-SPF, further\nincrease in its concentration under the electric field is considered negligible. The\nreduction in absorption intensity is primarily attributed to the reorientation of polar\ndipoles along the field direction, which decreases their vibrational component along the\ndirection of the infrared electric vector (Supplementary Fig. 9).”\n(Page 8, lines 186-191)\n5. Figure 2e shows the adiabatic temperature change (ΔT) versus the field strength for\nthe PVDF-SPF film. The ΔT is calculated from the data measured by a heat flux sensor.\nThe curve does not show any platform but a large increasing rate even if the field is\nincreased up to 200MV/m, and the breakdown strength of 617 MV/m is still far above\nthe maximum test field. Does this mean that ΔT still has a large room to be increased if\nthe field is higher than 200MV/m?\nResponse: Thank you for your thoughtful question. In our follow-up experiments, we\nsupplemented the electrocaloric cooling data obtained under higher electric fields with\nsimultaneous measurements of the corresponding unipolar hysteresis loops. The results\nindicate that the maximum polarization (P ) of the PVDF-SPF film exhibits a two-\nmax\nstage trend, with a rapid increase at low fields followed by gradual saturation at higher\nfields. In the low-field region (<250 MV m-1), a large number of short-β phase structures\nwithin the PVDF-SPF film can almost fully reorient along the applied field. The initial\npolarization increase is primarily attributed to the activation of these loose and tiny β\ncrystals. As the field exceeds 250 MV m-1, the dipole alignment approaches saturation,\nand the rate of polarization growth slows down significantly. This saturation is further\nexacerbated by the limited structural flexibility between short-β phase regions and\namorphous chain segments, resulting in a “locked” microstructure that cannot respond\nfurther to the field. This trend is also reflected in the behavior of the remanent\npolarization (P). P represents the non-reversible component of the polarization that\nr r\nremains after the external field is removed. In the low-field region, as the field increases,\nmore short-β phase regions are aligned, and a small portion of dipoles remain oriented\nafter field removal, leading to a slight increase in P. However, above 250 MV m-1, P\nr r\nremains nearly constant, indicating that the reversible polarization component has been\nnearly exhausted and the material begins to behave more like a linear dielectric.\nFig. R15. a, Unipolar hysteresis loops of PVDF-SPF under electric fields ranging from\nElectrocaloric cooling measured by a heat flux sensor.\nTo address this comment, we have added additional ΔT data under fields higher than\nc\nc\nMV m-1, and further increasing the field leads to only a marginal enhancement in\ncooling. This suggests that the material's polarization response is approaching\nsaturation in this field range, and thus, the potential for further ΔT improvement is\nc\nlimited. Although the breakdown strength of the PVDF-SPF film is still far above the\nmaximum applied field, its effective polarization no longer increases significantly with\nfield strength. Therefore, we believe that the practical working limit for the\nelectrocaloric performance of PVDF-SPF lies in the 200-250 MV m-1 range. To fully\nexploit the electrocaloric performance of PVDF-SPF without applying an excessively\nhigh voltage, the operating electric field was set to 200 MV m-1.\nRevised Main Text: “When the field exceeded 250 MV m-1, dipole alignment\napproached saturation, and the polarization increase slowed markedly. The P remained\nr\nnearly constant, suggesting that the reversible polarization component was nearly\nexhausted, and the material behavior became more linear (Supplementary Fig. 12).”\n(Page 11, lines 239-242)\n6. The 14.1K of ΔT at 200MV/m calculated from the heat flux sensor measurement is\na very large number. How does this compare to the ΔT measured using an IR camera\nunder the same condition?\nResponse: Thank you for your thoughtful question. We acknowledge that the original\nmanuscript did not provide a detailed characterization of the electrocaloric temperature\nchange (ΔT ) measured using an IR camera or infrared thermometer for the PVDF-SPF\nc\nfilm. We have supplemented time-resolved ΔT curves obtained using the IR\nc\nthermometer under various electric fields with a systematic comparison of the results\nmeasured by the heat flux sensor. The comparison reveals that, under identical\nconditions, the ΔT values recorded by the IR thermometer are lower than those\nc\nobtained from the heat flux sensor. Through further analysis, we have identified the\nfollowing primary factors that may contribute to this discrepancy:\n(1) Difference in temporal resolution: The sampling interval of the IR thermometer\nis approximately 100 ms, whereas the heat flux sensor has a response time of around\n10 ms. Since the electrocaloric response of PVDF-SPF occurs within a short timescale\n(a few tens of milliseconds) [Ferroelectrics 405, 26-31 (2010)], the IR thermometer\nmay fail to accurately capture the instantaneous peak temperature change, resulting in\nan underestimation of ΔT .\nc\n(2) Thermal shielding caused by the CNT coating: To enhance the surface emissivity\nfor infrared detection, a layer of carbon nanotubes (CNTs) was sprayed onto the silver\nelectrodes. Although CNTs effectively improve emissivity, the coating thickness (~1\nμm) could not be precisely controlled under the current experimental conditions. An\nexcessively thick CNT layer may partially absorb the released heat from the film,\nleading to a lower apparent surface temperature measured by the IR thermometer.\nSimilar thermal shielding effects have also been reported in previous studies [Nat.\nCommun. 15, 6670 (2024); Nature 640, 924-930 (2025)].\nFig. R16. ECE performance of the PVDF-SPF. a, Time-resolved heat flux signal of\nPVDF-SPF. b, Time-resolved infrared thermometer signal of PVDF-SPF. c,\nComparison of the temperature change measured by the two methods.\nTo address this comment, we have further provided ΔT data obtained via the IR\nc\nthermometer at different temperatures and electric field strengths in the Supplementary\nInformation (Supplementary Fig. 5). Although the absolute temperature changes\nmeasured by the IR thermometer are slightly lower, the observed field-dependent trends\nare consistent with those obtained using the heat flux sensor. Corresponding\nexplanations have also been added to the main text and figure captions.\nRevised Main Text: “This result was cross-validated using an infrared thermometer.\nDue to lower sampling rates of the infrared method, the measured temperature change\nwas reduced (Supplementary Fig. 5b and 5c).”\n(Page 6, lines 128-130)\n7. Figure 2e shows that the ΔT of PVDF-SPF is almost negligible at 50-80MV/m. To\nget a ΔT (at least 3-4 K) for practical cooling applications, the field needs to be at least\n100MV/m which is not ideal for cooling device development. How does the ΔT scale\nwith film thickness? From the material standpoint, provide potential approaches to\nobtain large ΔT at <100 MV/m.\nResponse: Thank you for your thoughtful question. We agree that the electrocaloric\ntemperature change (ΔT ) of PVDF-SPF under low electric fields (e.g., 50-80 MV m-1)\nc\nis relatively limited, which to some extent restricts its potential for low-voltage-driven\ncooling device applications. One of the primary reasons for this limitation is the\nrelatively strong interchain interactions within the crystalline domains of PVDF-SPF.\nWhen the polymer chains within a crystal are tightly packed and highly rigid, it\nbecomes difficult to achieve significant reversible dipole reorientation even under\nincreasing electric fields, thereby suppressing the resulting ΔT . Although our solid-\nc\nphase pressure forming (SPF) process has successfully transformed the β-phase crystals\ninto a looser structure with crystal sizes reduced to approximately 6 nm. This size is\nclose to the lower limit, where each crystal may contain only a few polymer chains\nalong the thickness direction. This “isolated crystalline domain” configuration is\nbeneficial for reducing interdomain coupling and enhancing polarizability, but it still\nleaves room for improving chain-level dipolar flexibility under low electric fields.\nWe believe that the key to enhancing the electrocaloric response at low electric fields\nlies in further weakening the interchain interactions within the crystalline regions,\nthereby increasing the freedom of dipole orientation in weak fields. This could be\nachieved by moderately increasing the interchain spacing (e.g., via fluorinated\nmonomer copolymerization or radiation-induced defects), or by increasing the distance\nbetween highly polar units (e.g., through the incorporation of disulfonyl alkyl side\nchains with a C3 spacer [Science 389, 69-72 (2025)]), while maintaining a small crystal\nsize.\nIn addition, to enhance the effective field strength acting on the crystalline regions,\nwe suggest re-designing the microstructure from the perspective of macroscopic\ncapacitor distribution. PVDF-based materials typically consist of crystalline and\namorphous domains, which can be simplified as a series connection of two capacitive\ncomponents with different dielectric constants. Since the dielectric constant of the\ncrystalline region is generally higher than that of the amorphous region, the external\nelectric field tends to concentrate in the amorphous phase, reducing the effective field\napplied to the crystals. Therefore, optimizing the degree of crystallinity, constructing\ninterfaces with large polarization contrast, or introducing interfacial polar layers can\nhelp redistribute the internal electric field more effectively toward the crystalline\nregions. This redistribution facilitates stronger dipolar reorientation and enhances the\nelectrocaloric cooling performance even under low-voltage conditions.\nTo address this comment, we plan to further explore interchain-weakening molecular\nstrategies and multiscale structural designs that could enhance the low-field\nelectrocaloric performance. Relevant literature discussions and preliminary design\nconcepts have been added to the revised manuscript and Supplementary Information.\nReviewer #4 (Remarks to the Author):\nThis article introduces a novel method utilizing poly(vinylidene fluoride) (PVDF)\nthrough solid-phase pressure forming (SPF) technology to enhance its electrocaloric\neffect (ECE). The technique undergoes significant plastic deformation and extensive\ncrystal fragmentation in PVDF films, forming loose and tiny β-phase grains under 30\nMPa solid-phase pressure. The remarkable ECE primarily originates from the entropy\nchange due to dipole swithing within molecular chains and the orientation entropy\nchange caused by reversible transitions between loose and compact β crystals. A high\nelectrocaloric temperature change of 14.1 K and a broad operational temperature\nwindow of 60 °C are achieved at room temperature. Overall, the work demonstrates a\nhighly novel approach. I recommend addressing several issues before acceptance of this\npaper.\n1. In the introduction part, the author declared they are obtaining more “extended-chain\nbeta crystal”, however, WAXD result showed the crystalline size only 6.22 nm, I\nrecommend the author to verify by SAXS, or another way is to delete the incorrect\ncalling of “ECC”.\nResponse: Thank you for your valuable suggestion. We agree with the reviewer’s\nassessment regarding the crystal size. We also realize that referring to the structure as\nan “extended-chain β crystal” could be confusing, considering the crystalline size of\nonly 6.22 nm observed by WAXD. We intended to highlight the formation of more\nextended all-trans chain conformations during original stretching, rather than\nconventional extended-chain crystals with much larger crystalline sizes. We also\nrecognize that the molecular chain length is not equivalent to the crystalline size, and\ntherefore this expression is indeed inaccurate.\nTo address this comment, we have removed the terms “extended-chain β crystal” and\n“ECC” from the revised manuscript and clarified the relevant content to avoid potential\nambiguity.\nRevised Main Text: “Upon uniaxial stretching for PVDF-C, the α and γ spherulites can\ntransform into fibrous β crystals with electroactivity, which are mainly used as\npiezoelectric materials with a piezoelectric coefficient typically between 20 and 35 pC\nN-1.”\n(Page 5, lines 109-112)\n2. In the in situ 2D-WAXD experiments, changes in β-crystal size and interchain\nspacing are clearly reported. Have the authors also quantified any change in overall\ncrystallinity? If so, what is the magnitude of this change under applied electric field,\nand what physical processes—such as field-induced recrystallization or partial\nmelting—might drive it?\nResponse: Thank you for your thoughtful question. We acknowledge that the\nquantitative analysis of crystallinity from the in situ 2D-WAXD data under the electric\nfield was missing in the original manuscript. As shown in Fig. 2e, the diffraction\nintensity of the β crystal becomes more pronounced after applying the electric field,\nwhile the scattering intensity in the amorphous region (around 18.5°) slightly decreases.\nBased on the circular integral intensity profiles of the 2D-WAXD patterns, we found\nthat the overall crystallinity increases by approximately 4% under the applied field.\nThis suggests that some amorphous chain segments became more ordered and\ntransformed into loosely packed β crystal structures. This transformation is not a\nconventional recrystallization process, as the temperature remains constant during the\napplication of the electric field, and no new diffraction peaks are observed in the WAXD\npatterns. Instead, we believe it results from field-induced conformational reordering of\nchain segments, particularly within the interfacial transition regions formed after solid-\nphase pressure forming. In these regions, the disparity in interchain spacing between\ncrystalline and amorphous zones is significantly reduced. As a result, some chain\nsegments exhibit quasi-crystalline behavior and respond more readily to the applied\nfield by forming polar-ordered structures, which enhances the polarization response.\nFig. R17. Change in circular integral crystallinity of PVDF-SPF before and after\napplying the electric field. The shaded area in the curve represents the loss associated\nwith the reversible β crystal transition, similar to a hysteresis loop. The circular integral\ncrystallinity is obtained by integrating the 2D-WAXD pattern over the full 360°\nazimuthal angle. This amplifies the contribution from the amorphous regions and leads\nto an underestimated crystallinity. The crystallinity of PVDF-SPF was calculated based\non the DSC crystallization melting enthalpy value of 54.6 J g-1, resulting in a\ncrystallinity of 52.8 % (theoretical enthalpy for fully crystalline PVDF is 103.4 J g-1).\nTo address this comment, we have added a quantitative analysis of the crystallinity\nchange before and after electric field application in the revised manuscript. The relevant\ndiscussion has been included in the “Reversible transition in β crystals” section.\nRevised Main Text: “After applying a high voltage, the molecular chains in PVDF-SPF\nbecame more compact (Fig. 2f), resulting in a decrease in D, accompanied by an\nincrease in relative crystallinity (Supplementary Fig. 10).”\n(Page 9, lines 213-215)\n3. The dielectric constant of all three films (PVDF-SPF, PVDF-C, and PVDF-S) was\nmeasured in range of 0 to 140 °C. It is reasonable to see the decrease after 80 °C,\nhowever, I recommend the authors to carry out dielectric constant measurement to\nbelow the glass transition (i.e., -40°C), It seems that the high dielectric constant is\noriginal from the orientation of the amorphous phase, i.e., the orientated amorphous\nfraction (OAF).\nResponse: Thank you for your valuable suggestion. We have added the temperature-\ndependent dielectric spectra of PVDF-SPF films in the range of -80 °C to 30 °C. The\nresults show a significant increase in both the dielectric constant and loss near the glass\ntransition temperature (approximately -30 °C). This phenomenon resembles the\npolarization enhancement mechanism proposed in previous work [J. Mater. Chem. C 9,\n894-907 (2021)], which is attributed to the formation of an oriented amorphous fraction\n(OAF). Therefore, we believe that a similar OAF-like structure may also form in the\nPVDF-SPF films. However, it should be noted that the primary focus of the referenced\nstudy was the enhancement of piezoelectric performance, while our work centers on\nimproving the ECE. These two properties rely on distinct dipole behaviors: the\npiezoelectric effect depends more on a stable and highly oriented dipole alignment,\nwhereas the ECE requires dipoles to respond reversibly to an applied electric field.\nAlthough we observed enhanced dielectric response and low-temperature\npolarization in the PVDF-SPF samples, which implies the possible presence of a certain\nlevel of order within the amorphous structure, we tend to regard it as an “intermediate\nphase” structure [J. Polym. Sci., Part B: Polym. Phys. 32, 2289-2306 (1994)]. This\nhypothesis requires further validation through detailed microstructural\ncharacterizations. As this study primarily aims to clarify the relationship between\nstructure and ECE performance, we did not elaborate on the OAF mechanism. We\nbelieve that this direction holds substantial potential for future investigations,\nparticularly in exploring the influence of OAF-like structures on ECE responses.\nFig. R18. a, Dielectric constant and b, dielectric loss spectra of PVDF-SPF in the range\nof -80 °C to 30 °C.\nTo address this comment, we have added the above discussion into the revised\nmanuscript under the “Temperature-dependent ECE behavior” section. This addition\nintroduces the concept of “intermediate phases” and briefly compares their polarization\nbehavior with that reported in previous studies.\nRevised Main Text: “At low temperatures (approximately −30 °C), both k and dielectric\nloss increased significantly (Supplementary Fig. 18a and 18d). This behavior resembles\nthe polarization enhancement mechanism reported previously. The structure in PVDF-\nSPF, referred to as loose β crystals (similar to intermediate phases), can greatly improve\nthe reversible ECE.”\n(Page 12, lines 272-277)\n4. Again, if the above assumption is true, then the OAF should also significantly\nenhance the ECE performance, the authors should discuss this effect in the manuscript.\nResponse: Thank you for your valuable suggestion. We agree that, as the reviewer\nsuggested, if the high dielectric constant originates from the oriented amorphous\nfraction (OAF), this structure may also contribute positively to the ECE performance.\nPrevious studies have shown that OAF structures can be introduced through processing\nmethods such as stretching and electrical poling [Mater. Horiz. 9, 1992-1998 (2022)],\nand they have a significant influence on polarization behavior and electromechanical\nperformance. In future work, we plan to conduct detailed structural characterizations to\nfurther verify the specific role of OAF in enhancing ECE properties.\nTo address this comment, we have added relevant background information in the\nIntroduction section of the revised manuscript to discuss the potential contribution of\nOAF structures to polarization behavior and field-induced responses. We believe that\nthis addition helps to better highlight the potential role of OAF structures in enhancing\nECE performance. The changes in the revised manuscript are as follows:\nRevised Main Text: “In addition, high-power ultrasonication was utilized to induce a\nhard-to-soft transition, increasing the piezoelectric coefficient d in PVDF19. The\n31\npresence of oriented amorphous fraction structures has been reported to greatly improve\nthe piezoelectric response of PVDF20. This suggests that such “intermediate phases”\nwith ordered conformations may significantly contribute to the polarization behavior.”\n(Page 3, lines 69-74)\nI co-reviewed this manuscript with one of the reviewers who provided the listed\nreports. This is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nReviewer #5 (Remarks to the Author):\nThis paper presents an enhanced electrocaloric effect (ECE) in poly(vinylidene fluoride)\n(PVDF) homopolymer subjected to solid-state pressure forming. Given the low cost\nand wide availability of PVDF homopolymer, the reported findings are noteworthy and\nhave the potential to stimulate further research of ECE in PVDF homopolymer. I\nrecommend the publication of this paper.\nHowever, I have concerns regarding the high Joule heating observed, as shown in\nSupplementary Figures 2(c) and 3(a), which may limit the practical applications of the\nreported material for electrocaloric cooling devices. Despite this limitation, I feel this\nwork will inspire future research of ECE on PVDF homopolymer that may lead to\npractical (and low cost) electrocaloric cooling materials. The authors may consider\n(optional) adding one or two sentences in Supplementary Information about this Joule\nheating when discussing Supplementary Figures 2(c) and 3(a).\nResponse: Thank you for your valuable suggestion. We fully understand the impact of\nJoule heating on the performance of ECE cooling devices, especially under high electric\nfields, which may indeed limit the practical application of the material. In our ECE\nmeasurements, the PVDF-SPF films had a thickness of approximately 10 μm and were\ncut into 15 × 15 mm squares. Silver electrodes with diameters of 6 mm and 8 mm were\nuniformly deposited on both surfaces, and conductive silver paste with fine silver wires\nwas used for electrical connection. Under typical testing conditions (with the entire\nECE cycle lasting ~12 seconds and the high electric field applied for only 6 seconds),\nthe intrinsically high resistivity of PVDF generally prevented noticeable Joule heating\nor dielectric breakdown. However, we did observe that a few samples exhibited\nasymmetric thermal behavior, where the heat released during field application was\ngreater than the heat absorbed during field removal. We attribute this to possible factors\nsuch as non-uniform electrode coating, local poor contact of the silver paste, or\nundetected local degradation. Therefore, we fully acknowledge that the Joule\nheating effect raised in your comment must be carefully identified and minimized\nduring ECE measurements.\nFig. R19. a, Time-resolved heat flux signal of PVDF-SPF. b, ECE signal of PVDF-SPF\nmeasured by the heat flux sensor under 200 MV m-1. The red shaded area represents\nthe Joule heating of the film during field application, while the blue shaded area\nindicates the electrocaloric cooling upon field removal. Mild Joule heating was\nobserved under high electric fields, possibly caused by leakage currents associated with\nenhanced polarization, which may slightly reduce the electrocaloric cooling. This issue\ncould be alleviated through further optimization of material processing or device design.\nTo address this comment, we repeated the measurements on several other samples\nfrom the same batch. The results showed that most samples did not exhibit similar\noverheating behavior, confirming that the previously reported data reflected an isolated\nanomaly. As a result, we have removed the anomalous data from the revised manuscript\nand updated the ECE temperature change curves and associated temperature values to\nbetter represent the intrinsic ECE behavior. In addition, we have added a brief\ndiscussion on potential thermal sources in both the main manuscript and the\nSupplementary Information. Strategies such as electrode optimization, improved\ninterface bonding, and voltage reduction are proposed to help suppress undesired\nthermal effects in future applications.\nRevised Main Text: “Optimizing electrode design, improving interface bonding, and\nreducing operating voltage could effectively suppress this effect.”\n(Page 26, lines 594-595)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have provided substantial additional experiments to support their claim. The revised interpretations resolve the methodological and consistency issues I raised.\nThe consistent contrast in temperature changes measured by the heat flux sensor and IR camera via the highly emissive surface is very interesting – it suggests a systematic discrepancy between the two methods, which the authors attributed to the difference in the frame rate. If this claim were true, it would then lead to two important implications: 1) the cooling power within those tens of milliseconds time in difference would be large and thus promising for heat pump applications; 2) such large cooling power may now only be consumed by the inactive mass of the current film geometry, and thus the effective temperature change for useful heat transfer would be small. Therefore, for future development of EC heat pumps with large cooling powers and real-world relevance, it would be important to study EC materials as working bodies with large active thermal mass, presumably in the form of multilayer capacitors, either in planar [Appl. Phys. Lett. 102, 122904 (2013)] or roller configurations.\nI therefore suggest the publication of this manuscript, and encourage the authors to consider my comment above for further improvements.", "reviewer_2": "The revised manuscript has addressed some of my concerns. However, the phenomenon of \"loosely packed beta phase enhances the ECE\" was not first presented by this work. This work identified another method that could achieve a similar polar structure to enhance the ECE in PVDF (it is good enough). The authors should acknowledge the contribution of the prior work (Ref.11) and focus the report on their own innovations. Therefore, it is these authors who made me quote this in Ref.11, \"The large number of newly formed, small-sized polar entities adopted a loosely correlated, high-entropy state, as suggested by DSC and XRD results.\" And currently, the narrative still seems to suggest it is they who discovered this phenomenon. And when they cite the ref. 11, there is no discussion on this phenomenon. The scientific origin of this work lies in achieving the polar structure \"in PVDF\". Without correctly acknowledging the contribution of prior work, this manuscript can be misleading to potential readers about its true significance.\nFrom the revision, I still cannot understand why 97.2 J USD-1 is a meaningful value. The value only means something if the EC device is available for purchase in the market. If the authors still want to argue this is outstanding, they should compare it with that of the gaseous refrigerant that is currently available on-shelf.\nIn addition, the Joule heating is still significant. If 97.2 J USD-1 is outstanding, how about the additional cost of the additional size of heat sink that has to be there due to the additional heat generated and accumulated during each cycle? It will be a misleading point in this field from the device's point of view. I would suggest the authors not to discuss this, or they have to give a full picture of the economic view of EC technology, which is not really necessary for highlighting the key innovation of this work.", "reviewer_3": "The following is my perspective on the author’s response to reviewer #3. Right now, my primary concern lies in the large discrepancy between ΔTcs measured with the heat flux sensor and those measured with the IR thermometer. The ΔTc measured with the heat flux sensor is even more than three times that measured with the IR thermometer under the same field. Frankly speaking, it is very unlikely that the device’s capturing frequency and the emissivity can account for this significant difference. In fact, a film with a temperature change of 14.2 K (now has been changed to 12.8 K) far exceeds the requirements for the commercialization of this solid-state cooling material (assuming that the stability issue is neglected). It is very good to present the comparison of ΔTcs measured with the two different methods. However, a statement of an exact 12.8 K temperature change might not be suitable.", "reviewer_4": "The author has fully addressed my concerns, thus I recommend the manuscript to be published in Nat. Commun.", "reviewer_5": "The authors have addressed my question, and I suggest accepting this paper."}, "author_response": "Response to the Reviewers’ Comments\nFrom negligible to giant electrocaloric effect of poly(vinylidene fluoride)\n(NCOMMS-25-05580-A)\nDear Reviewers:\nThank you again for your valuable comments and suggestions on our manuscript. We\nhave carefully considered all the comments. The main revisions include additional\nexperiments, supplementary analyses, and adjustments to relevant expressions, which\nare explained in detail in the point-by-point responses. All changes are highlighted in\nred in the revised manuscript.\nBest regards\nReviewer #1 (Remarks to the Author):\nThe consistent contrast in temperature changes measured by the heat flux sensor and\nIR camera via the highly emissive surface is very interesting – it suggests a systematic\ndiscrepancy between the two methods, which the authors attributed to the difference in\nthe frame rate. If this claim were true, it would then lead to two important implications:\n1) the cooling power within those tens of milliseconds time in difference would be large\nand thus promising for heat pump applications; 2) such large cooling power may now\nonly be consumed by the inactive mass of the current film geometry, and thus the\neffective temperature change for useful heat transfer would be small. Therefore, for\nfuture development of EC heat pumps with large cooling powers and real-world\nrelevance, it would be important to study EC materials as working bodies with large\nactive thermal mass, presumably in the form of multilayer capacitors, either in planar\n[Appl. Phys. Lett. 102, 122904 (2013)] or roller configurations.\nResponse: We sincerely thank the reviewer for the in-depth analysis and constructive\ncomments on our work. We agree with the reviewer’s inference that the instantaneous\ncooling power could be significant. Still, in the current film structure, it is likely\ndissipated by the inactive thermal mass (such as the electrode mass and the high-\nemissivity coating), thereby reducing the effective temperature change. The focus of\nthis study was to reveal the structural origin and response process of the large\nelectrocaloric effect in homopolymer PVDF. For this reason, the instantaneous cooling\npower was not further quantified. We consider the reviewer’s suggestion on future\ninvestigations with working bodies of larger effective thermal mass (such as multilayer\ncapacitors, roller configurations, electrostatic-driven devices, and self-regenerative\ndevices) to be very important for the practical application of electrocaloric heat pumps.\nReviewer #2 (Remarks to the Author):\n1. The revised manuscript has addressed some of my concerns. However, the\nphenomenon of \"loosely packed beta phase enhances the ECE\" was not first presented\nby this work. This work identified another method that could achieve a similar polar\nstructure to enhance the ECE in PVDF (it is good enough). The authors should\nacknowledge the contribution of the prior work (Ref.11) and focus the report on their\nown innovations. Therefore, it is these authors who made me quote this in Ref.11, \"The\nlarge number of newly formed, small sized polar entities adopted a loosely correlated,\nhigh-entropy state, as suggested by DSC and XRD results.\" And currently, the narrative\nstill seems to suggest it is they who discovered this phenomenon. And when they cite\nthe ref. 11, there is no discussion on this phenomenon. The scientific origin of this work\nlies in achieving the polar structure \"in PVDF\". Without correctly acknowledging the\ncontribution of prior work, this manuscript can be misleading to potential readers about\nits true significance.\nResponse: We thank the reviewer for the careful examination of the relevant statements\nin our manuscript. The reviewer has demonstrated profound insight into the scientific\norigin of our work. We acknowledge the important contribution of Ref.11, which\nreported the formation of small-sized polar entities in a loosely correlated and high-\nentropy state. This discovery is highly significant for understanding the giant\nelectrocaloric effect and has also inspired our work. We admit that this finding and its\nscientific value were overlooked in our original description. Therefore, in the revised\nmanuscript, we have explicitly acknowledged the contribution of Ref.11 and adjusted\nthe related statements. These changes aim to help readers more clearly understand the\ncore viewpoint of this work, namely that a similar polar structure can be achieved in\nhomopolymer PVDF through a different method to enhance the electrocaloric effect.\nIn addition, we have supplemented the discussion of this finding when citing Ref.11 to\navoid potential misunderstanding of the scientific significance of this work.\nTo address this comment, we have supplemented the third paragraph of the\nIntroduction in the revised manuscript, where previous attempts to modulate the dipolar\nresponse of polymers are discussed. Simultaneously, we have highlighted the key\nfindings of Ref.11 and their relevance to our work, thereby clearly distinguishing the\ncontributions of previous studies from the innovations of the present work.\nRevised Main Text: “The presence of oriented amorphous fraction structures has been\nreported to greatly improve the piezoelectric response of PVDF20. In terpolymers, the\nintroduction of a small number of double bonds can generate numerous loosely\ncorrelated polar entities in a high-entropy state, which also favors the ECE11. This\nsuggests that such loosely correlated “intermediate phases” with ordered conformations\nmay significantly contribute to the polarization behavior.”\n(Page 4, lines 71-77)\n2. From the revision, I still cannot understand why 97.2 J USD-1 is a meaningful value.\nThe value only means something if the EC device is available for purchase in the market.\nIf the authors still want to argue this is outstanding, they should compare it with that of\nthe gaseous refrigerant that is currently available on-shelf.\nResponse: We thank the reviewer for the comments on our statement regarding the\neconomic refrigeration capacity. We understand the reviewer’s concern that the value\nof “97.2 J USD-1” lacks practical significance. After further consideration, we agree\nthat this value is not a generally applicable evaluation metric. Therefore, in the revised\nmanuscript, we have removed this value and the related economic comparison to avoid\npotential misunderstanding. The focus is now placed on the structural origin and\nphysical mechanism of the electrocaloric effect in the material.\nFig. R1 Temperature-dependent properties and electrocaloric performance of PVDF-\nSPF. a Temperature-dependent dielectric constant of PVDF-SPF. b Temperature\ndependence of the wavenumber and absorbance of the infrared characteristic peak\n(~837 cm-1) corresponding to the short-β phase. c Crystal size and relative crystallinity\nas a function of temperature. The dashed lines (---) represent linear fits. A noticeable\nchange in slope is observed around 80 °C, indicating a structural transformation. d\nTemperature dependence of the polarization difference (ΔP). ΔP reflects the number of\nreversibly aligned dipoles before and after the application of the electric field, thereby\nindirectly indicating the entropy change associated with the ECE. e ΔS of PVDF-\nECE\nSPF at different temperatures. ΔS increases gradually with temperature, but drops\nECE\nsharply beyond ~80 °C due to partial melting and structural relaxation of the crystalline\nregions. f Comparison of the ΔT , ΔS , and T of PVDF-SPF with representative\nECE ECE w\nelectrocaloric materials reported in the literature, providing a preliminary evaluation of\nits potential for practical applications.\nTo address this comment, we have replaced the original comparison of economic\nrefrigeration capacity with a comparison of temperature change, entropy change, and\ntemperature window. At the same time, the statements regarding economic refrigeration\ncapacity have been removed from the revised manuscript.\nRevised Main Text: “Notably, an outstanding refrigeration capacity per unit cost of 97.2\nJ USD-1 is achieved in PVDF, surpassing other electrocaloric materials. Given the\navailability of PVDF, this work opens a new route for the development of scalable solid-\nstate cooling techniques.”\n(Page 2, lines 33-36)\nRevised Main Text: “The PVDF-SPF displayed a wide T of 60 °C and a high Young's\nw\nmodulus (Y) of 1.7 GPa, demonstrating excellent overall performance and application\npotential. The economic refrigeration capacity (ERC, refrigeration capacity per unit\ncost) of the PVDF-SPF was 97.2 J USD-1, indicating strong economic practicality and\npromising potential for energy saving and cost reduction.”\n(Page 5, lines 100-105)\nRevised Main Text: “Considering the practical requirements for electrocaloric polymers,\nwe selected five parameters: ΔT , T , E , Y, and P as evaluation criteria. PVDF-\nECE w b max\nSPF exhibited a large ΔT (12.8 K) and a high P (9 μC cm-2), demonstrating\nECE max\nsignificant potential for electrocaloric performance tuning. Its high Y (~1700 MPa) is\nexpected to enhance the structural stability and operational durability of electrocaloric\ndevices (Supplementary Fig. 26). Compared with previously reported electrocaloric\nmaterials (Supplementary Table 2), PVDF-SPF outperforms commercially available\nterpolymers in ΔT , ΔS , and T , highlighting its notable advantage in overall\nECE ECE w\nproperty balance (Fig. 4f).”\n(Page 14, lines 318-326)\n3. In addition, the Joule heating is still significant. If 97.2 J USD-1 is outstanding, how\nabout the additional cost of the additional size of heat sink that has to be there due to\nthe additional heat generated and accumulated during each cycle? It will be a\nmisleading point in this field from the device's point of view. I would suggest the\nauthors not to discuss this, or they have to give a full picture of the economic view of\nEC technology, which is not really necessary for highlighting the key innovation of this\nwork.\nResponse: We thank the reviewer for the insightful analysis and constructive\nsuggestions regarding the economic refrigeration capacity. We agree with the reviewer\nthat the additional heat generated could require a larger heat sink, which would increase\nthe cost, potentially making the value of “97.2 J USD-1” misleading at the device level.\nTherefore, in the revised manuscript, we have removed the discussion of economic\nrefrigeration capacity. We have instead focused on the electrocaloric effect of the\nmaterial itself, as well as its structural origin and physical mechanism, thereby\nhighlighting the core innovation of this work.\nReviewer #3 (Remarks to the Author):\nThe following is my perspective on the author’s response to reviewer #3. Right now,\nmy primary concern lies in the large discrepancy between ΔTcs measured with the heat\nflux sensor and those measured with the IR thermometer. The ΔTc measured with the\nheat flux sensor is even more than three times that measured with the IR thermometer\nunder the same field. Frankly speaking, it is very unlikely that the device’s capturing\nfrequency and the emissivity can account for this significant difference. In fact, a film\nwith a temperature change of 14.2 K (now has been changed to 12.8 K) far exceeds the\nrequirements for the commercialization of this solid-state cooling material (assuming\nthat the stability issue is neglected). It is very good to present the comparison of ΔTcs\nmeasured with the two different methods. However, a statement of an exact 12.8 K\ntemperature change might not be suitable.\nResponse: We thank the reviewer for the careful consideration and insightful attention\nto this scientific issue. We acknowledge that there exists a discrepancy between the\nelectrocaloric temperature change (ΔT ) measured by the heat flux sensor and that\nECE\nobtained with the infrared thermometer. Previous studies have reported that the\ntemperature change measured by the heat flux sensor is generally higher than that\nobtained by an infrared thermometer, including an infrared thermal imager [Nat.\nCommun. 15, 6670 (2024)]. This difference may primarily arise from two factors: (i) to\nenhance surface emissivity, a high-emissivity carbon coating was applied to the sample\nsurface. Since this coating itself has no EC response, it introduces some thermal loss;\n(ii) the limited sampling frequency of the infrared thermometer, which is insufficient to\nfully capture the instantaneous process. Therefore, accurately determining the\ninstantaneous temperature change (ΔT ) of polymer films under an electric field\nInst\nremains a challenge at present.\nAccording to Wien’s displacement law, the infrared emission of an object at room\ntemperature is mainly concentrated in the 8-14 μm wavelength range. However, metal\nelectrodes such as silver exhibit a low emissivity in this range (approximately 0.02-\n0.05). Therefore, to accurately measure the ECE, it is necessary to coat the electrode\nsurface with a carbon layer to enhance the surface emissivity (ε = 0.853). Previous study\nhas shown that, in EC terpolymers coated with black paint, the temperature change\ninduced by the ECE can be transferred to the coating within 0.5 ms. Consequently, an\ninfrared thermal imager with a sampling frequency exceeding 2 kHz is required to\ncapture the maximum temperature change [J. Materiomics. 9, 256-260 (2023)].\nFor this work, the heat released or absorbed by the PVDF film must be conducted\nthrough the PVDF layer to the surface of the carbon coating in order to be detected by\nthe infrared thermometer. The ECE of the sample was characterized by recording the\ntemperature change of the carbon coating. For simplicity, the interfacial thermal\nresistance between the PVDF film and the high-emissivity carbon coating was\nneglected. The temperature change induced by the ECE was assumed to be\ninstantaneous and uniformly distributed across the thickness of PVDF film. According\nto Newton’s law of cooling, the surface temperature change of the carbon coating can\nbe expressed as:\n𝑡\n𝑇(𝑡)= 𝑇 +(𝑇 −𝑇 )𝑒− 𝜏 (Eq.R1)\n∞ 0 ∞\n𝑇(𝑡)−𝑇 = (𝑇 −𝑇 )𝑒− 𝜏 𝑡 = 𝛥𝑇 = 𝛥𝑇 𝑒 − 𝜏 1 𝑓 (Eq.R2)\n∞ 0 ∞ 𝐸𝐶𝐸 𝐼𝑛𝑠𝑡\n𝜌𝑉𝐶 𝜌𝐿𝐶\n𝜏 =\n𝑝\n=\n𝑝\n(Eq.R3)\nℎ𝐴 ℎ𝑛\nwhere T(t) denotes the temperature of the carbon coating at time t, T is the ambient\n∞\ntemperature (room temperature), and T is the instantaneous temperature at the onset of\n0\ncooling. f = 1/t represents the sampling frequency, and τ is the thermal time constant of\ncooling. m = ρV denotes the effective mass of the carbon coating, C is the specific heat\np\ncapacity, h is the convective heat transfer coefficient, and A is the heat exchange surface\narea. L is the thickness of the carbon coating, and n denotes the number of surfaces\nexposed to the environment, with n = 1 for single-side exposure. To obtain ΔT , the τ\nInst\nof the carbon coating was first calculated. The ρ and C of the carbon coating were 1200\np\nkg m-3 and 1500 J kg-1 K-1, respectively, and the L was approximately 1.0 ± 0.2 μm.\nDuring the ECE test, the film was placed in a forced-air oven at 25 °C, with h set to 20\nW m-2 K-1 to represent mild convective conditions. Substituting these parameters into\nEq. R3 yielded τ = 0.090 ± 0.018 s. A new high-sampling-rate infrared thermometer\n(CTlaser LTF, Optris GmbH) was used, with an adjustable sampling frequency ranging\nfrom 10 to 100 Hz. The ECE of PVDF films was measured at sampling frequencies of\n10, 20, 50, and 100 Hz. Based on the range of τ values, the experimental data were\nfitted using Eq. R2 to derive ΔT (Fig. R2).\nInst\nFig. R2 Electrocaloric temperature change of PVDF-SPF under 200 MV m-1 as a\nfunction of sampling frequency. The dashed line represents the fitted function.\nFig. R2 shows that at 2 kHz, ΔT reaches 10.4 ± 2.6 K, close to the temperature change\nInst\nmeasured by the heat flux sensor (12.8 K). The infrared thermometer used in the main\ntext operates at a fixed sampling frequency of 10 Hz. This inherent frequency ensures\nminimal measurement error, guaranteeing the reliability and reproducibility of the data.\nAdditionally, this frequency is close to the frame rate of the infrared thermal imager (9\nHz, Supplementary Video), enabling effective cross-validation between the two\ninfrared measurement techniques.\nTo address this comment, we added the results of the electrocaloric temperature\nchange as a function of sampling frequency in Supplementary Fig. 5e, along with the\ncorresponding explanation and clarification.\nReviewer #4 (Remarks to the Author):\nThe author has fully addressed my concerns, thus I recommend the manuscript to be\npublished in Nat. Commun.\nResponse: We sincerely appreciate the reviewer’s positive evaluation and\nrecommendation for publication.\nReviewer #5 (Remarks to the Author):\nThe authors have addressed my question, and I suggest accepting this paper.\nResponse: We sincerely appreciate the reviewer’s positive evaluation and\nrecommendation for publication."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have addressed my concerns and now this paper can be published in Nature Communications."}, "author_response": "Response to the Reviewers’ Comments\nFrom negligible to giant electrocaloric effect of poly(vinylidene fluoride)\n(NCOMMS-25-05580-B)\nDear Reviewers:\nWe appreciate you for providing positive feedback and a thorough evaluation of our\nwork. We appreciate you for accept this work for publication.\nBest regards\nReviewer #3 (Remarks to the Author):\nThe authors have addressed my concerns and now this paper can be published in Nature\nCommunications.\nResponse: We thank you so much for the thorough review and comments on our study."}]

Decision letter