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 [CH3C6H4CH(CH3)NH2][CH3C6H4CH(CH3)NH3][PF6] ((p-TEA)(p-TEAH)·PF6) crystallizing in the polar monoclinic C2 space group. The asymmetry in (p-TEA)(p-TEAH)·PF6 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 (p-TEA)(p-TEAH)·PF6 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)·PF6 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 (p-TEA)(p-TEAH)·PF6 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)·PF6 devices even
after exposure to 99% relative humidity, showcasing their utility
under extremely humid conditions.