Introduction

Piezoelectricity, as one of the key capabilities of ferroelectrics, plays a crucial role in broad uses of energy harvesting1,2,3,4,5,6, transducer7,8,9,10, sensing11,12,13, catalysis14,15,16, etc. Since the piezoelectricity of quartz crystals discovered by Curie brothers in 188017, extensive studies and related applications have been long dominated by perovskite oxides represented by barium titanate (BTO)18,19 and lead zirconate titanate (PZT)20,21,22,23 due to their outstanding properties including high piezoelectric coefficients, stability, and electricity generation capabilities. The growing demand for flexible and wearable devices has prompted the exploitation on composite materials combining flexible polymer substrates and inorganic ceramic piezoelectrics, to address their issue of mechanical rigidity24,25,26,27. However, their structural disparities create new thorny problems like phase separation, non-uniform stress distribution, and limited composite ratios that severely impede device performance.

Compared to the mismatch between inorganic ceramics and polymer substrates, the relative softness and compatibility of organic-inorganic hybrid materials bring great prospects for composite piezoelectric substances28,29,30,31,32,33,34. One of the notable advantages is their ability to form intermolecular forces with polymer matrices to address issues like phase separation35. But for a long time in the past organic-inorganic hybrid piezoelectric materials generally suffer from the defect of weak piezoelectric response. TMCM-CdCl336,37,38 and its solid solution (TMFM)0.26(TMCM)0.74CdCl3 (TMFM = trimethylfluoromethyl ammonium [Me3NCH2F]+) and (TMCM = trimethylchloromethyl ammonium [Me3NCH2Cl]+)39, whose piezoelectric performance can be on par with those of inorganic ceramics BTO and PZT respectively. These attributes ignite people’s enthusiasm for exploration on composite piezoelectric substances based on organic-inorganic hybrid materials40,41,42,43,44. For example, Bu et al45. reported a kind of composite piezoelectric material, which is composed of polydimethylsiloxane (PDMS) and (BTMA)2CoBr4 (BTMA = benzyltrimethyl ammonium), achieving short-circuit current of 4.24 μA, and power density of 11.72 μW cm−2 when the mass fraction of molecular ferroelectrics is 10 wt%. Furthermore, Li et al35. reported energy harvesting devices made by the composite films of TMCM-CdCl3 and PDMS, showing a high power density up to 115.2 µW cm−2. Despite of this progress, there are two significant issues with such dense polymer structures that cannot be ignored: the first aspect is the limited doping proportion of hybrid piezoelectric materials that usually unevenly dispersed, while excess will be detrimental to flexibility; another is the uneven stress distribution and low-stress absorption efficiency under external forces. These would be reflected in the final devices that exhibit unfavorable fatigue resistance and limited performance output.

Promising solution would be found in porous material like thermoplastic polyurethane (TPU) sponge, taking the advantages of light-weight, cost-effective, corrosion resistance, excellent flexibility, and stretchability46,47,48. Due to the presence of the porous structure, compared to dense materials, TPU-polymer substrate shows superior compressibility, response stability, and large composite ratios49,50,51, enabling better device performance output as demonstrated in [C(NH2)3]ClO4-TPU array52. In this study, we have constructed a porous composite piezoelectric material by incorporating soft polyurethane and molecular ferroelectric materials (TMCM-CdCl3) instead of a simple mixture of the two systems. COMSOL finite element analysis reveals that the three-dimensional interconnected skeleton structure of porous thermoplastic polyurethane plays a vital role in absorbing mechanical energy, endowing flexible piezoelectric composites with high power density, sensitivity, and fatigue resistance. Under periodic external stimulation, the flexible piezoelectric composites with the molecular ferroelectric content reach 50% exhibit remarkable output voltage (103 V), current (42 μA), and power density (636.9 µW cm−2). This power density surpasses poly(vinylidene fluoride) (PVDF), a typical flexible piezoelectric material, by more than 2000 times53, setting a new standard for organic-inorganic hybridized molecular ferroelectric composites. This outstanding power density output effect holds significant promise for advancing the application of molecular ferroelectric materials in the realm of flexible piezoelectric devices.

Results

Design strategy of flexible porous piezoelectric composite

The primary challenge lies in the phase separation between the piezoelectric component and the flexible substrate during composite processing, attributed to the weak intermolecular forces between the two. The unique organic and inorganic hybrid structure of TMCM-CdCl3 ferroelectric molecules enhances their organic compatibility. Molecular dynamics simulations (LAMMPS) were conducted to validate their advantages in composite processing with flexible polymer substrates (TPU), as depicted in Fig. 1a. The simulation results demonstrate a robust interaction between TMCM-CdCl3 and TPU, ensuring close molecular chain integration and uniform dispersion, thus mitigating the phase separation issue. However, a persistent challenge in piezoelectric material utilization remains: how to efficiently convert mechanical energy into electrical energy to achieve higher output power density. Porous materials inherently offer superior mechanical energy absorption efficiency due to their flexible porous structure characteristics, as depicted in Fig. 1c. Leveraging these features, a porous composite strategy is proposed in this study (Fig. 1), aiming to enhance the output power density of flexible piezoelectric materials significantly. The detailed fabrication method is shown in the Supplemental Information (Supplementary Fig. 1).

Fig. 1: Design strategy of flexible porous piezoelectric material in this work.
figure 1

a Molecular dynamics simulation of super-strong intermolecular forces between thermoplastic polyurethane (TPU) and [Me3NCH2Cl]CdCl3 (TMCM-CdCl3) materials. b Porous composite piezoelectric materials were prepared by TMCM-CdCl3 and TPU. c Advantages of porous materials, high mechanical energy absorption efficiency.

Full size image

Structural characterizations of TMCM-CdCl3/TPU porous composites

The freeze-drying method was employed to synthesize porous TPU, showcasing remarkable porosity and exceptional compressibility. Even under multiple successive rapid large strain compressions, the porous composite piezoelectric material retains its elasticity, as illustrated in Fig. 2a. Notably, the outstanding porosity properties of the porous TPU enable it to accommodate a higher proportion of the piezoelectric component, thereby enhancing the piezoelectric output performance while retaining exceptional flexibility. Furthermore, TMCM-CdCl3, characterized by a large piezoelectric coefficient (d33) of 220 pC N−1, was synthesized via solvent evaporation as the piezoelectric source. Leveraging this, a series of TMCM-CdCl3/TPU porous composites with varying mass percentages (30%, 40%, 50%, 60%, and 70%) were synthesized to explore the impact of different mass ratios of TMCM-CdCl3 on the morphology and piezoelectric performances of the composites.

Fig. 2: Structural characterizations of TMCM-CdCl3/TPU porous composites.
figure 2

a Optical photographs (scale bar: 1 cm) of a representative TMCM-CdCl3/TPU porous composites, showing excellent compression and recovery reversibility. b Packing view of the crystal structure of TMCM-CdCl3. c XRD patterns of TMCM-CdCl3, the TPU sponge, and the TMCM-CdCl3/TPU hybrid. d FE-SEM micrograph (scale bar: 10 µm) of the 50 wt% TMCM-CdCl3 hybrid sponge. e–j EDS mapping (scale bars: 50 µm) of a 50 wt%TMCM-CdCl3/TPU hybrid sponge.

Full size image

The structure of TMCM-CdCl3, illustrated in Fig. 2b, depicts Cd2+ coordinating with six Cl- ions to form octahedrons. These CdCl6 octahedrons share face arrangements, creating one-dimensional CdCl3 chains, with the organic component TMCM occupying the spaces within these chains. Powder X-ray diffraction (PXRD) analysis revealed characteristic diffraction peaks corresponding to both TPU and TMCM-CdCl3 in the porous composites with different mass ratios, confirming the successful integration of ferroelectric molecules into the highly porous TPU matrix (Fig. 2c). Moreover, PXRD analysis demonstrated an increase in peak intensities of the ferroelectric molecules as the mass ratio of TMCM-CdCl3 increased from 30 wt% to 70 wt%. Field Emission Scanning Electron Microscopy (FE-SEM) was utilized to observe the cross-section of the porous composites. Figure 2d exhibits electron microscopy patterns of the composite with 50 wt% TMCM-CdCl3, revealing pore sizes ranging from 20 to 30 µm within the TPU matrix, with ferroelectric crystals distributed throughout these pores. At lower proportions of ferroelectric crystals, the pore size remains uniform, and the ferroelectric molecules are evenly dispersed throughout the porous TPU matrix. However, as the proportion of TMCM-CdCl3 increases, the pore-like structure of the TPU matrix becomes disrupted, resulting in inconsistent pore sizes and localized aggregation of ferroelectric molecules, leading to poor dispersion (Supplementary Fig. 2). Energy dispersive spectroscopy (EDS) mapping (Fig. 2e–j) indicates the presence of five main elements: C, O, Cl, N, and Cd, uniformly distributed in the skeleton, confirming the presence of TMCM-CdCl3 crystals on the TPU skeleton. The chemical composition of these elements remains unaffected by the presence of TPU. FE-SEM and EDS mapping results affirm the successful recombination of TMCM-CdCl3 in TPU, with TPU maintaining a uniform pore size at lower proportions of ferroelectric crystals.

Molecular dynamics simulation

To investigate the intermolecular forces between TMCM-CdCl3 molecules and TPU, molecular dynamics simulations were conducted using the large-scale atomic molecular massively parallel simulator (LAMMPS)54. The simulations employed universal force field (UFF), applicable to almost all elements of the periodic table55. The simulation system comprised 10 TPU chains, each with 10 repeating units, positioned above TMCM-CdCl3 (Fig. 3a), with a 1:1 mass ratio. The system’s energy was minimized using the conjugate gradient algorithm to achieve equilibrium structure. Subsequently, the system was equilibrated under the NVT ensemble for 1000 ps at 293 K, with a time step of 0.5 fs, and temperature controlled by a Nosé-Hoover thermostat. The interaction energy between the TPU and the TMCM-CdCl3 was calculated based on the following Eq. (1):

$${{{\boldsymbol{E}}}}_{{{{\bf{interaction}}}}}={{{\boldsymbol{E}}}}_{{{{\bf{total}}}}}-\left({{{\boldsymbol{E}}}}_{{{{\bf{TPU}}}}}+{{{\boldsymbol{E}}}}_{{{{{\bf{TMCM}}}}}-{{{{\bf{CdCl}}}}}_{{{\bf{3}}}}}\right)$$
(1)

where \({{{{\boldsymbol{E}}}}}_{{{{\bf{interaction}}}}}\), \({{{{\boldsymbol{E}}}}}_{{{{\bf{TPU}}}}}\) and \({{{{\boldsymbol{E}}}}}_{{{{{\bf{TMCM}}}}{{{\boldsymbol{-}}}}{{{\bf{CdCl}}}}}_{{{{\boldsymbol{3}}}}}}\) are the potential energies of the TPU/TMCM-CdCl3 nanocomposite, the TPU, and the TMCM-CdCl3, respectively. Figure 3b, c and d shows the changes of the potential energies of the TMCM-CdCl3, the TPU, and the TPU/TMCM-CdCl3 nanocomposite during simulations, respectively. The potential energies of the TMCM-CdCl3, and the TPU and the TPU/TMCM-CdCl3 nanocomposite approach some certain values, indicating that all the three systems reach the equilibrium states at the end of the simulations. Then, the values of the potential energies of the systems at the end of the simulations were used to calculate the \({{{{\boldsymbol{E}}}}}_{{{{\bf{interaction}}}}}\) based on the above equation, and the corresponding results were listed in Supplementary Table 1. Supplementary Table 1 proved that the value of \({{{{\boldsymbol{E}}}}}_{{{{\bf{interaction}}}}}\) is −21023.05 Kcal·mol−1. That is, a strong interaction exists between the TPU chains and the TMCM-CdCl3 due to the presence of the hydrogen bonds and polar interactions between the TPU chains and the TMCM-CdCl3. As shown in the Supplementary Movie 1, during simulations the TMCM-CdCl3 molecules gradually mix with the TPU chains, also demonstrating that there are strong interactions between the TPU chains and the TMCM-CdCl3.

Fig. 3: Molecular dynamics simulation.
figure 3

a The Snapshot of the beginning and end of the initial simulation system including the TPU chain and TMCM-CdCl3. b–d The changes of the potential energies of the TMCM-CdCl3, the TPU, and the TPU/TMCM-CdCl3 nanocomposite during simulations. e Potential energy between TPU/TMCM-CdCl3.

Full size image

Characterization of piezoelectric properties

To assess the piezoelectric output performance of the porous composites, sandwich-structured piezoelectric devices were fabricated with a diameter of 1.5 cm and a thickness of 0.5 cm. Copper foils served as electrodes on the top and bottom surfaces of the porous composites. These devices underwent testing where an 80 N force was applied at a frequency of 10 Hz. The induced voltage signals were carefully observed and recorded using a signal collector, as shown in Supplementary Fig. 3. The device composed of 50 wt% TMCM-CdCl3/TPU exhibited optimal piezoelectric performance, yielding a maximum open-circuit voltage (Voc) and short-circuit current (Isc) of up to 103 V and 42 µA, respectively (Fig. 4a and  c)56. Polarity switching tests confirmed the piezoelectric nature of the devices, demonstrating positive signals in the forward connection and reversed signals in the reverse connection (Fig. 4a,  c and Supplementary Fig. 4). To provide a detailed analysis, magnified plots of the voltage signals (Fig. 4b and d) illustrated both positive and negative signals for each compression and release process, the piezoelectric mechanism is shown in the Supplementary Fig. 4. The voltage signals were solely attributed to TMCM-CdCl3, evidenced by a blank control using pure porous TPU without the piezoelectric source. The voltage output of the devices increased with the mass fraction of TMCM-CdCl3, albeit with stability issues observed at mass fractions of 60 wt% and 70 wt%. (Fig. 4e) This instability resulted from disruptions in the pore size distribution of the TPU matrix as a higher content of ferroelectric molecules led to uneven dispersion and stress transfer blockage. Despite these challenges, the devices maintained stability over 10, 000 cycles without significant voltage decrease (Supplementary Fig. 5). And the cross-sectional images of composite before and after the cycle have no obvious damage, as shown in Supplementary Fig. 9. Due to its outstanding piezoelectric output performance and stability, devices with 50 wt% TMCM-CdCl3 were selected for all subsequent tests.

Fig. 4: Piezoelectric energy harvesting performance of TMCM-CdCl3/TPU composite materials.
figure 4

a, b Variation of open circuit voltage over time in forward and reverse connections and detailed magnified images (Including piezoelectric device model). c, d Variation of short circuit current over time in forward and reverse connections and detailed magnified images (Including piezoelectric device model). e Open circuit voltage of piezoelectric composites with different contents. f Comparison of power density of the piezoelectric composite materials with previously reported works27,35,57,58,59,60,61. g Direct lighting of 36 white LEDs without bridge rectifiers and capacitors.

Full size image

Considering the practical application scenarios, the power output density feature becomes crucial. The 50 wt% TMCM-CdCl3/TPU composites were subjected to testing under resistances ranging from 1 to 30 MΩ. Subsequently, the power density was calculated using the Eq. (2):

$${{{\boldsymbol{P}}}}={{{\boldsymbol{V}}}}^{{{\boldsymbol{2}}}}/{{{\boldsymbol{R}}}}\times {{{\boldsymbol{S}}}}$$
(2)

where V represents the voltage, R is the corresponding external load resistance, and S denotes the stimulus area. With a resistive load of 5 MΩ, the device generated a maximum instantaneous power density of 636.9 µW cm−2 (Supplementary Fig. 6). Moreover, we also tested the power density at different temperature and pressure loads, as shown in Supplementary Fig. 8. Notably, this power density is up to hundreds of times higher than what has been previously reported for molecular ferroelectric composites with organic-inorganic hybridization, even surpassing some flexible composites prepared from inorganic ceramics (Fig. 4f)27,35,57,58,59,60,61. Also, more detailed power density comparison data is listed in the supplemental information (Supplementary Tables 2 and 3).

The outstanding power density of the device sparked further exploration into its potential applications. With such remarkable power density, the best-performing device was capable of directly driving 36 series-connected white LEDs without the need for external capacitors and bridge rectifiers (Fig. 4g). The notable piezoelectric output power of porous composites can be attributed to two main factors. Firstly, molecular dynamics calculations revealed the presence of intermolecular forces between the piezoelectric component TMCM-CdCl3 and the TPU matrix, resulting in strong compatibility between the two materials and effectively mitigating phase separation issues. Additionally, the porous TPU skeleton structure possesses advantages such as better stress transfer and stress amplification, enabling more effective absorption and transfer of mechanical energy to the piezoelectric element, thereby further enhancing the overall performance of the composite.

COMSOL finite element simulation

To further validate the advantages of porous materials and their impact on the electric generation effect, COMSOL simulations were conducted comparing traditional film materials with porous materials (as described in this study) (Fig. 5). During the modeling process, the porous piezoelectric composite material was represented using a “Weaire-Phelan” structural array62, which forms a three-dimensional interconnected skeleton structure, while the traditional film material was modeled as a solid block structure. To simplify the calculation process, a unit structure of 80 × 80 × 80 µm³ was created. The models of both materials are depicted in Fig. 5a. Considering the varied stress scenarios in practical material applications, stress levels ranging from 0.017 MPa to 11.3 MPa were simulated for the porous material. Figure 5b illustrates that as the applied stress increases, the average stress of the porous material exceeds that of the solid bulk material, indicating that the ultra-high porosity of the porous material amplifies the applied stress. Specifically, under stress of 0.017 MPa, the average stress distribution of the porous material is ~0.02 MPa, compared to 0.003 MPa for the solid bulk material. Under lower stress conditions, the stress distribution is more uniform in both porous and bulk materials, with the average stress in the former exceeding that in the latter. Under a stress of 11.3 MPa, the average internal stress of the bulk material is ~2.10 MPa, while the porous material exhibits an average stress of 13.35 MPa, ~6.4 times higher. Moreover, numerous regions within the porous material experience a maximum stress value of 15 MPa. Therefore, the in-situ porous design strategy employed in this study enables more effective absorption and transfer of mechanical energy to the piezoelectric components within the porous materials, resulting in an outstanding power generation effect.

Fig. 5: COMSOL finite element simulation.
figure 5

a Model of porous composite of TMCM-CdCl3/TPU and TPU bulk material. b Results of COMSOL finite element simulation of Von Mises stress for porous material and block material with different stresses applied (0.017 MPa, 0.45 MPa, 1.3 MPa, 11.3 MPa).

Full size image

Piezoelectric ultrasonic detection

Taking advantage of the superior ultrasonic absorption properties of porous materials (Fig. 6a and b), the porous piezoelectric composite materials developed in this study find application in underwater ultrasonic detection, as depicted in Fig. 6. Multiple porous composite piezoelectric materials form a sensing system capable of receiving ultrasonic waves across various areas of the water body. These waves are then converted into electrical signals for transmission to a signal collection tower, enabling ultrasonic detection across different water environments. The porous piezoelectric material is positioned within a water tank, with the ultrasonic source located in the tank’s center, as depicted in Supplementary Fig. 7. Due to varying distances from the ultrasonic emission source, the electrical signals generated by the porous piezoelectric material differ in magnitude across the left, middle, and right water regions, as depicted in the Fig. 6c, d, and e. Positioned on the left and right water regions, the porous piezoelectric material consistently outputs voltage signals of approximately 3 V, derived from the absorption of ultrasonic waves. However, when positioned closer to the ultrasonic vibration source in the center of the tank, the voltage generated significantly increases to around 5 V. Upon relocation to the right region, the electrical signal attenuates to approximately 3 V with fluctuations. This fluctuation may be attributed to water movement within the tank, leading to variations in ultrasonic transmission and subsequent signal attenuation. Analysis of the electrical signals across the three water regions facilitates comprehensive ultrasonic detection of the entire water area.

Fig. 6: The application of underwater ultrasonic wave detection.
figure 6

a, b The model diagram and the voltage diagram of ultrasonic detection of TMCM-CdCl3/TPU composite film and TMCM-CdCl3/TPU porous composite, respectively. c The schematic illustration of the self-powered sensor network system for underwater ultrasonic wave detection in different regions. d–f The voltage diagram of TMCM-CdCl3/TPU porous composite in the left, middle, and right regions of the water tank respectively.

Full size image

Discussion

This work successfully designs and fabricates a flexible piezoelectric material with ultra-high output power by leveraging the excellent organic compatibility of molecular ferroelectric materials and the unique properties of porous materials, including flexibility, stress transfer, and stress amplification. Experimental data reveal a continuous and uniform phase distribution of TMCM-CdCl3 and TPU in porous materials, particularly when the content reaches 50%. Molecular dynamics calculations indicate strong intermolecular forces between TMCM-CdCl3 and TPU, affirming their good compatibility. COMSOL finite element simulation further confirms that the three-dimensional interconnected porous skeleton structure of TPU facilitates stress transfer and amplification, thereby enhancing piezoelectric properties. The fabricated composite material, incorporated into an energy-harvesting device, demonstrates remarkable output voltage (103 V), current (42 μA), and power density (636.9 µW cm−2) under periodic external stimulation. This achievement, surpassing the performance of PVDF flexible piezoelectric membranes by 2000 times, represents a new benchmark in composite materials based on TMCM-CdCl3. The insights gained from this work contribute to the future design of flexible molecular ferroelectric composites with superior energy harvesting properties.

Methods

Materials

Thermoplastic polyurethane purchased (TPU) from Ningbo Heju New Material Technology Co., LTD. All reagents were purchased from commercial suppliers and used without further purification.

Crystal growth

First, equal amounts of trimethylamine (30% water) and dichloromethane were reacted in acetonitrile at room temperature for 24 h to synthesize (chloromethyl) trimethyl ammonium chloride36. Then, 50 mmol of (Chloromethyl)trimethylammonium chloride and 50 mmol of Cadmium chloride were dissolved in deionized water. The solution was then slowly evaporated to obtain the single crystal TMCM-CdCl3.

Porous composite piezoelectric materials synthesis

The porous composite piezoelectric materials were prepared by freeze-drying method. Firstly, a precursor solution was prepared by dissolving polyurethane in a dioxane solution by heat, followed by the addition of ionized water. Secondly, a previously synthesized TMCM-CdCl3 crystal is introduced as a voltage source and thoroughly stirred to ensure a homogeneous distribution. The third step involved pre-freezing treatment, during which the cooling process caused dioxane to solidify first and separate from the polyurethane and water phase, followed by the Solidification of deionized water. This is the key to the formation of highly ordered porosity. Finally, freeze-drying was conducted, leading to the sublimation of ice and dioxane, ultimately forming an interconnected pore structure, Then, the porous composite piezoelectric materials were polarized for 12 h at a field intensity of 3 kV/mm.

Device fabrication

A series of piezoelectric energy harvesting devices with sandwich structures were prepared, the different proportions circular porous piezoelectric composite material with a diameter of 1.5 cm and a thickness of 0.5 cm as the piezoelectric active material. The upper and lower interfaces are covered with copper sheets or conductive tape as electrodes, and finally, the devices are encapsulated in commercial PET films.

Finite element calculation

The COMSOL solid mechanics module was used to describe the skeleton cell stress. The cell size was 80 × 80 × 80 µm3 and the porous skeleton was inserted inside. The pressure on one end of the skeleton cell is 0.017 MPa, 0.45 MPa, 1.3 MPa, and 11.3 MPa, and the other end is fixed. The remaining surfaces are free boundaries. The same method was used to calculate the stress of the solid block material, whose cell size was also 80 × 80 × 80 µm3. The pressure of one end of the skeleton cell was 0.017 MPa, 0.45 MPa, 1.3 MPa, and 11.3 MPa, and the other end was fixed. The remaining surfaces are free boundaries.

Characterizations

X-ray diffraction (XRD) patterns of porous piezoelectric composites with different proportions and pure TMCM-CdCl3 crystals were recorded by Bruker D8 Advance Cu-K radiation diffractometer (XRD, Bruker, Germany). The morphology of the porous composite piezoelectric materials was observed by field emission scanning electron microscope (FE-SEM, America). The porous piezoelectric composite was polarized by a high-voltage polarization device (MPD 10 kV). In the energy harvesting test, a signal collector (9004) was used to monitor the open-circuit voltage and short-circuit current between the two electrodes on the sample under repeated compression induced by a linear motor (MS-1000).