A Metal-Free Molecular Ferroelectric With Large Piezoelectricity and Its Porous Composite for Superior Output Power Density.

The key to achieving high power density in piezoelectric materials lies in the synergistic optimization of both high piezoelectric constant (d₃₃) and piezoelectric voltage coefficient (g₃₃). Traditional inorganic ceramics offer high d₃₃ but suffer from low g₃₃, poor flexibility, and toxic-Pb content. Polymers, while flexible, are limited by their relatively low d₃₃ and insufficient stability. Molecular ferroelectrics can simultaneously offer high piezoelectricity, flexibility, and low loss, yet most reported systems contain toxic-Cd components, restricting their use in biomedical and wearable applications. Metal-free molecular ferroelectrics, replaced toxic metals while retaining the molecular ferroelectric attributes, combine environmental friendliness, and biocompatibility, making them ideal candidates for piezoelectrics. However, their piezoelectric output has traditionally been weak. Here, a novel metal-free molecular ferroelectric, [(CH₃C₆H₁₀NH₃)(18-crown-6)][ClO₄] (CH₃C₆H₁₀NH₃ = trans-4-methylcyclohexylaminium), was designed and synthesized via a methylation-assisted ring-expansion strategy, enhancing the piezoelectric performance. The material exhibits a d₃₃ of 67.42 ± 1.87 pC/N and a g₃₃ of approximately 2768.9 × 10⁻³ V m/N. When composited with thermoplastic polyurethane (TPU) into a porous architecture, the material delivers an output voltage of 120 V and a power density of 432.1 µW/cm², representing the highest performance reported among metal-free molecular piezoelectrics. This work offers a practical way to design high-performance, biocompatible piezoelectrics, creating a new platform for flexible sensors and self-powered devices.