Porous flexible molecular-based piezoelectric composite achieves milliwatt output power density

The study addresses the challenge of deploying molecular ferroelectrics—despite their strong intrinsic piezoelectricity comparable to perovskite oxides like BTO and PZT—for high-density energy harvesting and flexible, wearable devices. Conventional ceramic–polymer composites suffer from mechanical rigidity and, when combined with polymers, exhibit phase separation, non-uniform stress distribution, and limited filler ratios that restrict performance. Organic–inorganic hybrid ferroelectrics offer improved compatibility with polymers but have historically shown weak piezoelectric responses. Recent advances with TMCM-CdCl3 and related solid solutions demonstrate piezoelectric properties on par with leading inorganic ceramics, motivating composite strategies. The research proposes leveraging a porous thermoplastic polyurethane (TPU) matrix to (i) increase the loading of molecular ferroelectric filler, (ii) enhance stress transfer and absorption, and (iii) improve energy conversion efficiency, thereby achieving high power density in a flexible piezoelectric composite.

Prior work in piezoelectric energy harvesting has been dominated by inorganic perovskite ceramics such as BTO and PZT due to high piezoelectric coefficients and stability. To address rigidity, composites pairing flexible polymers with inorganic ceramics have been explored but face phase separation and stress transfer issues that limit performance and filler ratios. Organic–inorganic hybrid ferroelectrics improve compatibility with polymers through intermolecular interactions; however, many exhibited weak piezoelectric responses historically. Notable recent examples include PDMS composites with (BTMA)2CoBr4 delivering 4.24 µA and 11.72 µW cm−2 at 10 wt% loading, and TMCM-CdCl3/PDMS films achieving up to 115.2 µW cm−2. Despite progress, dense polymer matrices limit doping and stress absorption, causing unstable dispersion and low energy conversion. Porous TPU substrates have shown benefits in compressibility, response stability, and higher composite ratios, improving device output as evidenced by prior TPU-based arrays. These insights motivate a porous composite design using TMCM-CdCl3 within a TPU sponge to mitigate phase separation, enhance stress transfer, and boost power density.

Materials and crystal growth: TMCM-CdCl3 single crystals were synthesized via solution methods. (Chloromethyl)trimethylammonium chloride was prepared by reacting trimethylamine (30% aqueous) with dichloromethane in acetonitrile at room temperature for 24 h, then combined with cadmium chloride (50 mmol each) in deionized water and slowly evaporated to yield TMCM-CdCl3 crystals.

Porous composite synthesis and poling: Porous TPU matrices were fabricated by a freeze-drying process. TPU was dissolved in dioxane with heat, followed by addition of deionized water to form a precursor. TMCM-CdCl3 crystals were introduced and thoroughly dispersed. Pre-freezing caused dioxane to solidify and separate first, then water solidified, establishing an ordered porous architecture. Freeze-drying sublimated ice and dioxane to form an interconnected porous skeleton. Composites with 30–70 wt% TMCM-CdCl3 were prepared. Samples were electrically poled for 12 h at 3 kV mm−1.

Device fabrication and testing: Sandwich-structured disk devices (diameter 1.5 cm, thickness 0.5 cm) were assembled with copper foil (or conductive tape) electrodes and encapsulated in PET films. Devices were compressed by a linear motor at 80 N and 10 Hz while voltage and current were recorded using a signal collector. Output power density was evaluated across external loads from 1 to 30 MΩ using P = V^2/(R×S), where S is the stimulus area. Long-term stability was assessed over 10,000 cycles. Ultrasonic sensing performance was evaluated in a water tank with varying sensor positions relative to an ultrasonic source.

Structural and materials characterization: PXRD confirmed the coexistence and increasing intensity of TMCM-CdCl3 peaks with higher loading; FE-SEM imaged pore morphology (20–30 µm pores at 50 wt% loading) and dispersion; EDS mapping verified uniform distributions of C, O, Cl, N, and Cd on the TPU skeleton. Pure porous TPU served as a blank control.

Simulations: Molecular dynamics (LAMMPS) using the UFF force field modeled interactions between 10 TPU chains (10 repeat units each) and TMCM-CdCl3 at a 1:1 mass ratio. Energy minimization (conjugate gradient) preceded NVT equilibration at 293 K for 1000 ps with a 0.5 fs timestep (Nosé–Hoover thermostat). Interaction energy was computed as Einteraction = Etotal − (ETPU + ETMCM−CdCl3). COMSOL finite element simulations modeled stress distributions for a porous Weaire–Phelan skeleton versus a solid block (unit cell 80×80×80 µm3) under applied pressures of 0.017, 0.45, 1.3, and 11.3 MPa, with one face loaded and the opposite fixed.

• Porous TPU enabled high filler loading (optimal at 50 wt% TMCM-CdCl3) with efficient stress absorption and uniform dispersion at lower loadings; excessive loading (≥60 wt%) disrupted pore structure and stability.
• The best-performing device (50 wt% TMCM-CdCl3/TPU) delivered an open-circuit voltage of 103 V and short-circuit current of 42 µA under 80 N, 10 Hz loading.
• Maximum instantaneous power density reached 636.9 µW cm−2 at a 5 MΩ load; volumetric density reported as 1273.9 µW cm−3. This is over 2000 times higher than typical PVDF-based flexible piezoelectrics and surpasses many hybrid molecular and even some inorganic ceramic composite benchmarks.
• Devices maintained stable performance over 10,000 cyclic compressions without significant voltage decay; cross-sectional morphology showed no obvious damage after cycling.
• Molecular dynamics indicated strong intermolecular interactions between TPU and TMCM-CdCl3 with an interaction energy of −21023.05 kcal·mol−1, supporting compatibility and mitigated phase separation via hydrogen bonding and polar interactions.
• COMSOL simulations showed porous structures amplify internal stress relative to solid blocks under identical applied loads (e.g., under 11.3 MPa, porous average internal stress ≈13.35 MPa vs. bulk ≈−2.10 MPa), facilitating effective mechanical-to-electrical energy conversion.
• Demonstrations: Directly lit 36 series-connected white LEDs without rectifiers/capacitors. In underwater ultrasonic sensing, porous composites produced ≈3 V at lateral positions and ≈5 V near the central source, enabling spatial detection across a water tank.

The work addresses core barriers to high-performance flexible piezoelectric energy harvesting with molecular ferroelectrics—namely low mechanical robustness, phase separation in polymer composites, and inefficient stress transfer. Strong TPU–TMCM-CdCl3 intermolecular interactions (validated by MD) promote homogeneous dispersion and stable interfaces, alleviating phase separation. The porous TPU skeleton enhances compressibility, stress transfer, and stress amplification (confirmed by COMSOL), which together translate mechanical stimuli into higher electric output. Experimentally, the composite achieves 103 V, 42 µA, and 636.9 µW cm−2 under periodic loading, outperforming PVDF-based systems by >2000× and rivaling or surpassing many inorganic-based flexible composites. Durability over 10,000 cycles and functionality in ultrasonic detection underscore practical relevance for energy harvesting and self-powered sensing applications.

By integrating a high-performance molecular ferroelectric (TMCM-CdCl3) within a porous TPU matrix, the study demonstrates a flexible composite that attains milliwatt-level power density on a per-area basis, with excellent stability and direct device demonstrations (LED lighting, underwater ultrasonic detection). The combination of molecular-level compatibility and a stress-amplifying porous architecture provides a feasible pathway for translating molecular ferroelectrics into real-world flexible energy harvesters and sensors. Future work can build on these insights to optimize pore architecture and filler ratios, explore broader families of molecular ferroelectrics and polymer matrices, and develop scalable device architectures for wearable electronics and aquatic sensing networks.

At higher TMCM-CdCl3 loadings (≥60 wt%), the porous TPU structure becomes disrupted, leading to non-uniform pore sizes, localized aggregation, hindered stress transfer, and unstable electrical output. This underscores a trade-off between maximizing filler content and maintaining structural integrity and dispersion. Additionally, while performance and cycling stability are strong, results are demonstrated on relatively thick samples (0.5 cm) under controlled loading conditions; translation to thinner, highly flexible formats and diverse real-world mechanical stimuli remains to be validated.