INVESTIGATION OF STRUCTURAL, DIELECTRIC AND MECHANICAL PROPERTIES OF Ta2O5/PVDF BASED NANOCOMPOSITES FOR POTENTIAL FLEXIBLE ENERGY HARVESTING AND STORAGE DEVICES

Abstract

With increasing demand of energy, the emission of carbon is increasing in our environment and also the limited resources of fossil fuels have brought the green and renewable energy generation on high demand. The rapid expansion of wireless sensor networks, the Internet of Things (IoT), and next-generation wearable electronics has generated an unprecedented demand for sustainable, independent flexible power sources. Traditional battery reliant systems are increasingly constrained by finite lifespans, frequent maintenance requirements, and environmental concerns. Consequently, the development of self-powered technologies capable of harvesting ubiquitous ambient mechanical energy, such as human motion, wind, and fluid flow has emerged as a fundamental scientific necessity. Among various energy harvesting paradigms, flexible piezoelectric and triboelectric nanogenerators (PENGs and TENGs) have drawn significant attention due to their high efficiency, flexibility, and broad material applicability. However, a critical limitation of these nanogenerators is the inherently irregular and intermittent nature of ambient mechanical stimuli, which prevents continuous device operation. To address this overarching challenge, this research systematically investigates the development of advanced and optimised nanofillers, their integration into flexible polymer nanocomposites, and their ultimate application in nanogenerators and self-charging piezo-supercapacitors to achieve stable, uninterrupted power supplies. The initial aim of this research focuses on the synthesis and optimization of materials with good dielectric properties, specifically tantalum pentoxide (Ta2O5) prepared via hydrothermal method. The structural, optical, and dielectric characteristics were vii thoroughly investigated across a broad temperature range (80 K to 400 K) and frequency spectrum (20 Hz to 2 MHz). X-ray diffraction (XRD) confirmed the formation of a highly crystalline orthorhombic phase in the calcination temperature range of 800-1200 °C. Further, the effect of sintering temperature on the dielectric properties of Ta2O5 has been studied. Dielectric evaluations demonstrated that increasing the sintering temperature significantly enhanced the material‟s dielectric behaviour, yielding a dielectric constant of 20 and a remarkably low dielectric loss (<0.025) at room temperature. This enhancement is attributed to the thermally activated orientational and interfacial polarization associated with increased grain size in the Ta2O5 nanoparticles, establishing them as highly viable alternative dielectric nanofillers for flexible nanocomposites. Following the fundamental nanofiller synthesis, flexible nanocomposite thin films were prepared by reinforcing a polyvinylidene fluoride (PVDF) polymer matrix with the synthesized Ta2O5 nanoparticles at different concentration via a facile drop casting technique. Impedance spectroscopy revealed substantial dielectric improvements in the nanocomposite films. Specifically, the nanocomposite film containing an optimal 1.25 wt% of Ta2O5 nanoparticles exhibited a dielectric constant of approximately 17 at room temperature, scaling up to 40 at 120 °C at a 1 kHz frequency. This represents a massive improvement over pristine PVDF, maintained alongside a relatively low dielectric loss (<0.3). Concurrently, mechanical evaluations have been carried out to check the changes in the mechanical properties of the nanocomposite films. The mechanical analysis confirmed that the addition of Ta2O5 nanofillers enhanced the tensile strength of the optimized nanocomposite film. Further, the nanocomposite films were used to fabricate flexible piezoelectric nanogenerators (PENG). The PENG viii device with 1.25 wt% showed the highest voltage output ~ 56 V with a power density of ~ 0.100 mW/cm2 at 1M load resistance. To further maximize the piezoelectric energy harvesting efficiency, novel two dimensional heterostructure, MXene@Ta2O5 have been synthesized. XRD and Photoelectron Spectroscopy confirmed the successful synthesises of heterostructure. MXene@Ta2O5 has reinforced into the PVDF matrix at varying concentrations using a drop-casting method. That is further, used to fabricate flexible PENGs. The reinforcement of the PVDF matrix with an optimal 1.25 wt% of MXene@Ta2O5 yielded remarkable enhancements in dielectric properties and maintained the tensile strength (~26.7 MPa). Under repetitive palm-induced tapping, the optimized PENG device generated a superior output voltage of ~75 V i.e, 2.5 times greater than that of pristine PVDF. It also delivered a maximum power density of ~0.125 mW/cm2. This exceptional piezoelectric output is driven by a synergistic factor: (i) the highly polarizable nature of Ta2O5 increases the net dipole concentration; (ii) the negatively terminated surfaces (–F, –OH, –O) of the large-surface-area MXene sheets strongly interact with the CH2–CF2 dipoles of the PVDF matrix, forcing electroactive β-phase alignment; and (iii) intense interfacial polarization maximizes charge accumulation. The practical application of the optimized PENG was demonstrated by illuminating commercial LEDs and charging different capacitors. It also exhibited extreme voltage stability over 500 continuous mechanical cycles without structural degradation. Moreover, the film that generated the best piezo output voltage i.e, at 1.25 wt% concentration has been used to fabricate triboelectric nanogenerator (TENG). The effect of varying dimensions of nanogenerator on its voltage output has also been analysed. TENG based on MXene@Ta2O5/PVDF achieved almost fivefold voltage ix output as compared to the pristine PVDF due to the contact electrification electrostatic induction. The fabricated TENG also exhibited extreme voltage stability over 500 continuous mechanical stimuli without any structural or output degradation. Furthermore, to resolve the voltage fluctuations inherent to mechanical harvesting, the fabricated nanogenerator was tested for its supercapacitor configurations. It showed good areal capacitance and energy density of ~7.05 mF/cm2 and 3.175 Wh/cm2 at scan rate of 5mV/s, respectively that shows its potential towards self-charging piezo supercapacitors. By utilizing materials with dual energy harvesting and storage capabilities, a fully functional self-charging piezo-supercapacitor system can be prepared. This coupled device can effectively buffer intermittent voltage spikes, transitioning fluctuating ambient mechanical stimuli into a regulated, continuous electrical output. The prepared MXene@Ta2O5/PVDF based nanogenerator demonstrates a highly stable and efficient material solution for the uninterrupted operation of next-generation flexible and self-powered electronics.