Enhanced Output Performance of P(VDF-TrFE) Based Energy Harvesters with Controlled Dipole Moment of Solvents
Authors: Kim Jihye , Kim Sang-Woo 
Published: February 2020

Wireless sensor networks (WSN) in the form of spatially distributed sensors monitor various environmental activities such as chemical, biological, tactile, acoustic, navigational, and thermal data, and communicate the information to a central station to perform corrective measures. They can therefore greatly benefit human society in terms of healthcare, environmental sensing, and industrial monitoring. Powering the enormous number of sensor nodes in a network is a considerable challenge. Batteries however do not offer a viable solution, as they have a limited lifetime. Indeed, the number of required battery replacements could be unbelievably high. In order to circumvent the challenge, energy harvesters, which harvest the energy from the working environment of a sensor through piezoelectric, pyroelectric, and triboelectric phenomena, are proposed. Energy harvesters have therefore received enormous attention in order to provide a self-powered operation of sensors for WSNs and Internet of Things (IoT) applications. The poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)), being a ferroelectric polymer, has great promise for energy harvesting for flexible and wearable applications. In this paper, we have shown that the choice of solvent that is utilized to dissolve the polymer significantly influences its properties in terms of energy harvesting. Indeed, the P(VDF-TrFE) prepared using a high dipole moment solvent has higher piezoelectric and pyroelectric coefficients, and triboelectric property. Such improvements are the result of higher crystallinity and better dipole alignment of the polymer prepared using a higher dipole moment solvent. Tetrahydrofuran (THF), methyl ethyl ketone (MEK), dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO), at 20 °C with dipole moments of 1.75 D, 2.7 D, 3.8 D, and 4.1 D, respectively, were considered as solvents. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were carried out to investigate the crystallinity of the polymer due to the different solvents. Gel permeation chromatography (GPC) measurements were utilized to measure the relative chain length of P(VDF-TrFE). In addition, the dipole alignment of P(VDF-TrFE) with various solvents was measured using piezoelectric force microscopy (PFM). In order to investigate the solvent effect on the performance of the energy harvesters, we measured the piezoelectric and pyroelectric coefficients, and triboelectric property in terms of the contact potential difference (CPD) of the P(VDF-TrFE), using PFM and kelvin probe force microscopy (KPFM) techniques, respectively. Furthermore, in order to study the piezoelectric, pyroelectric, and triboelectric potential distribution of the various solvent-based P(VDF-TrFE)-based generators, finite element method (FEM) simulations were carried out in COMSOL. Finally, P(VDF-TrFE) based piezoelectric, pyroelectric and triboelectric generators experimentally validate that the higher dipole moment solvent significantly enhances the output performance of the energy harvesters; the improvement was 23.18 %, 81.82 % in output voltage and current, respectively, for piezoelectric generator; 39.60 %, 34.11 % in output voltage and current, respectively, for pyroelectric generator; and 65.24 % and 75.06 % in output voltage and current for triboelectric generator. In brief, the approach of utilizing high dipole moment solvent is very promising for high output P(VDF-TrFE) based wearable energy harvesters.

Figure 1