A Highly Electrostrictive Salt Cocrystal and the Piezoelectric Nanogenerator Application of Its 3D-Printed Polymer...

Ionic cocrystals with hydrogen bonding can form exciting materials with enhanced optical and electronic properties. We present a highly moisture-stable ammonium salt cocrystal [CH₃C₆H₄CH(CH₃)NH₂][CH₃C₆H₄CH(CH₃)NH₃][PF₆] ((-TEA)(-TEAH)·PF) crystallizing in the polar monoclinic C2 space group. The asymmetry in (-TEA)(-TEAH)·PF was induced by its chiral substituents, while the polar order and structural stability were achieved by using the octahedral PF₆^- anion and the consequent formation of salt cocrystal. The ferroelectric properties of (-TEA)(-TEAH)·PF were confirmed through P-E loop measurements. Piezoresponse force microscopy (PFM) enabled the visualization of its domain structure with characteristic "butterfly" and hysteresis loops associated with ferro- and piezoelectric properties. Notably, (-TEA)(-TEAH)·PF exhibits a large electrostrictive coefficient (Q₃₃) value of 2.02 m⁴ C^-2, higher than those found for ceramic-based materials and comparable to that of polyvinylidene difluoride. Furthermore, the composite films of (-TEA)(-TEAH)·PF with polycaprolactone (PCL) polymer and its gyroid-shaped 3D-printed composite scaled-up device, 3DP-, were prepared and evaluated for piezoelectric energy-harvesting functionality. A high output voltage of 22.8 V and a power density of 118.5 μW cm^-3 have been recorded for the 3DP- device. Remarkably, no loss in voltage outputs was observed for the (-TEA)(-TEAH)·PF devices even after exposure to 99% relative humidity, showcasing their utility under extremely humid conditions.