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

Abstract

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.

Keywords: 3D printing; cocrystals; energy harvesting; ferroelectricity; piezoelectricity.

Figure 1

(a) The molecular structure of (p-TEA)(p-TEAH)·PF₆ at 100 K (the disordered F atoms are omitted for clarity). (b) The view of the zigzag packing of (p-TEA)(p-TEAH)·PF₆ along the a-axis (including the disordered F atoms). (c) The view of two-dimensional hydrogen bonding N–H···F interactions in (p-TEA)(p-TEAH)·PF₆ along the ab-plane. (d) The d_norm mapped Hirshfeld surface analysis of (p-TEA)(p-TEAH)·PF₆ (including the disordered F atoms) from its crystal structure at 100 K showing the various interactions present in it.

Figure 2

(a) The SHG profile of (p-TEA)(p-TEAH)·PF₆ and its comparison with standard KDP, obtained upon irradiation with 800 nm femtosecond laser pulses. (b) PXRD profiles of (p-TEA)(p-TEAH)·PF₆ upon exposure to various humidity conditions showing its high stability.

Figure 3

(a) The ferroelectric behavior of (p-TEA)(p-TEAH)·PF₆ showing the rectangular P–E hysteresis loop at 298 K. (b) The fatigue test showing the retention of polarization of (p-TEA)(p-TEAH)·PF₆ up to 10⁶ cycles at 298 K.

Figure 4

PFM-derived (a) amplitude and (b) phase images of (p-TEA)(p-TEAH)·PF₆. (c) The visualization of a single crystal of (p-TEA)(p-TEAH)·PF₆ on the drop-casted thin film along with the PFM tip and its 3D-topography image. (d) The PFM amplitude-bias and phase-bias “butterfly” and “hysteresis” loops of (p-TEA)(p-TEAH)·PF₆.

Figure 5

(a) Photographs of a 10 wt % (p-TEA)(p-TEAH)·PF₆-PCL composite film showing its flexibility, as demonstrated for stretching, bending, twofold bending, and rolling operations. (b) FE-SEM images of the 10 and 15 wt % (p-TEA)(p-TEAH)·PF₆-PCL composite films. (c) Stacked PXRD profiles of (p-TEA)(p-TEAH)·PF₆-PCL films and their comparison with the diffraction patterns of as-synthesized (p-TEA)(p-TEAH)·PF₆. (d) Open-circuit peak-to-peak voltages (V_OC-PP) of (p-TEA)(p-TEAH)·PF₆-PCL composite devices. The shifted time axis provided here is a guide to the eyes. (e) Frequency-dependent permittivity for all (p-TEA)(p-TEAH)·PF₆-PCL composite films. (f) The obtained output signals upon forward and reverse connection of the electrode contacts to the oscilloscope. (g) The peak voltage drop and power density (PD) values of the 10 wt % (p-TEA)(p-TEAH)·PF₆-PCL composite device under different external load resistances. The inset depicts the maximum PD obtained at 1 MΩ resistance.

Figure 6

(a) Schematic showing the filament preparation and FDM printing of a (p-TEA)(p-TEAH)·PF₆-PCL composite slab. (b) Pictures of the computerized and as-made 3DP-Gy slabs with dimensions. (c) FE-SEM image of the 3DP-Gy composite slab. (d) Picture of the as-made 3DP-Gy device. (e) Measured V_OC-PP and calculated I_PP for the 3DP-Gy device (the shifted time axis provided here is a guide to the eye). (f) Calculated power density and voltage drop profile of 3DP-Gy with a range of load resistances. The inset depicts the maximum PD obtained at 1 MΩ resistance. (g) The cyclic stability tests of 10 wt % (p-TEA)(p-TEAH)·PF₆-PCL and 3DP-Gy devices showing the retention of V_OC-PP up to 10000 cycles. (h) The RH-dependent V_OC-PP of the 3DP-Gy device. (i) The reproducibility of V_OC-PP of 10 wt % (p-TEA)(p-TEAH)·PF₆-PCL and 3DP-Gy devices after 6 months.

Figure 7

(a) The circuit diagram of the full-wave four-diode bridge rectifier circuit utilized for capacitor charging and LED lighting experiments. (b) The voltage accumulated in a 22 μF capacitor by utilizing the 3DP-Gy. Inset: the image of the lighted green LED by using the charge stored in the 22 μF capacitor.