Nano-materials employment in energy harvesting and storage devices

Abstract

The present thesis focuses on the development of a new generation of miniature electronic devices by employing nano-scale materials. Specifically, ZnO nanowire arrays were investigated to increase the conversion efficiency of energy harvesting devices and graphene nano-platelets employed to enhance supercapacitors' energy storage capability. The results obtained in this work pave the way to the possibility of conceiving novel autonomous devices integrating both energy units. The present thesis has been structured in five chapters. A first introduction chapter reviews the pros and cons of renewable energies against the conventional ones produced from fossil fuels as well as their impact on the modern societies. The theoretical background on vibration energy harvesting and electrochemical energy storage is provided. Vibration energy harvesting mechanism relies on piezoelectric phenomena, where a pressure applied on a piezoelectric material turns ultimately into energy. Instead, supercapacitors store large quantity of energy for time unit by high surface material dielectric polarization. In this chapter, the reasons why ZnO nanowire arrays and graphene nano-platelets were considered are introduced. The second chapter presents promising methods to synthesize piezoelectric ZnO nano-materials prior their integration into energy harvesting devices. Since the highest piezoelectric properties of the ZnO-crystal are along its c-axis, the most suitable growing methods were selected to tailor the crystal's unit-cell best orientation. In this chapter physical and chemical growing methods are reported. Physical vapor deposition (PVD) was used to grow ZnO thin film, then employed as a seed layer for the growth of 1D-ZnO nanowires by chemical methods in a second step. ZnO nanowires were synthesized either with or without a nanoporous template by: i) electrochemical deposition (ECD), and ii) hydrothermal technique. The fundamental process parameters to tailor the chemical growth are reported as well as the morphological and microstructural characterization of the structures fabricated. In the third chapter, the characteristics of the energy harvesting device fabricated from the piezoelectric ZnO nanostructures are reported. Piezoresponse force microscopy was initially used to measure the d33 piezoelectric coefficient of the ZnO nanostructures fabricated fairly matching the theoretical expectations. Finally, this chapter reports the energy harvested by the devices fabricated, measured by connecting an external resistive load to it: a maximum energy harvested equal to 2 μJ/cm2 was found. The fourth chapter focuses on nano-scale graphene based materials for supercapacitors' electrodes. Specifically, the synthesis and the characterization of the graphene nano-platelets used in this work is described. XRD and Raman spectroscopy were used to distinguish pure graphite from graphene, BET and SEM to measure its specific surface area and morphology. To determine the graphene's properties functional to the application thermogravimetric analysis (TGA) was carried out. To identify the types of oxygen groups present in the graphene materials, the corresponding Fourier Transform Infrared spectra (FTIR) were recorded and their contribution in rGO was examined by X-Ray photoelectron spectroscopy (XPS) analysis. Overall this chapter reviews the relevant analysis to be performed in candidate materials for fabrication of supercapacitor electrodes. The fifth chapter discusses the fabrication of supercapacitor electrodes made with the graphene nano-platelets previously described as well as the methods for their electrochemical characterization. As being the standard of the energy storage industry, cyclic voltammetry (CV) and constant current charge and discharge experiments were carried out for capacitance estimation. The electrochemical characteristics of the device were then linked to the properties of the graphene nano-materials employed. All measurements were done in a full-scale electrochemical cell mimicking a real supercapacitor device. The results suggest that mechanically exfoliated graphene nano-platelets (GNP) best perform among the variety of materials investigated