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

Ionic cocrystals with hydrogen bonding can form exciting materials with enhanced optical and electronic properties. We present a highly moisture-stable ammonium salt cocrystal [CH 3 C 6 H 4 CH(CH 3 )NH 2 ][CH 3 C 6 H 4 CH(CH 3 )NH 3 ][PF 6 ] ( ( p -TEA)( p -TEAH)·PF 6 ) crystallizing in the polar monoclinic C 2 space group. The asymmetry in ( p -TEA)( p -TEAH)·PF 6 was induced by its chiral substituents, while the polar order and structural stability were achieved by using the octahedral PF 6 - anion and the consequent formation of salt cocrystal. The ferroelectric properties of ( p -TEA)( p -TEAH)·PF 6 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, ( p -TEA)( p -TEAH)·PF 6 exhibits a large electrostrictive coefficient ( Q 33 ) value of 2.02 m 4 C -2 , higher than those found for ceramic-based materials and comparable to that of polyvinylidene difluoride. Furthermore, the composite films of ( p -TEA)( p -TEAH)·PF 6 with polycaprolactone (PCL) polymer and its gyroid-shaped 3D-printed composite scaled-up device, 3DP- Gy , 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- Gy device. Remarkably, no loss in voltage outputs was observed for the ( p -TEA)( p -TEAH)·PF 6 devices even after exposure to 99% relative humidity, showcasing their utility under extremely humid conditions.