Polymeric-Metal Oxide Nanostructures As Future Materials for Energy Applications

Abstract

One of the most important directions of global research centers are studies on issues related to the acquisition, conversion and energy storage. Currently, more and more work is devoted to research on nanostructured materials as building blocks of batteries and electrochemical capacitors. Electrochemical capacitors (ECs) are energy storage devices which can find applications ranging from portable electronics to hybrid vehicles and large scale power generations. The ECs offer a number of desirable properties, such as fast charging (within seconds), reliability, long-term cycling (more than 500.000 cycles), and the ability to deliver 10 times more power than batteries. Moreover ECs can be used to recovery energy from repetitive processes (descending elevators, braking cars), due to their fast charging rate. Electrochemical capacitors may be divided into two main categories based on their energy storage mechanism: electric double-layer capacitors (EDLCs) and supercapacitors. EDLCs store electrical charge in a double layer at the interface between the electrode and electrolyte. On the other hand, supercapacitors are devices that yield two different capacitive behaviors: electric double-layer capacitance and super-capacitance related to a redox reaction at the interface between an electrolytic solution and electrode surface [1]. As super-capacitive materials metal oxides and conducting polymer have been extensively studied in the past decades [2]. Conducting polymers, such as polypyrrole (PPy), polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT), combine advantages of organic polymers with electronic properties of semiconductors, and hence are attractive materials for the use in data storage media and photovoltaic cells. They also can be used as electrochromic devices, chemical sensors and biosensors, logic or switching elements and supercapacitors [3]. Furthermore, NiO-based nanostructures and thin films have been extensively used as electrode materials for lithium-ion batteries and fuel cells, electrochromic films, gas sensors, and electrochemical supercapacitors. Since NiO is cheaper than RuO2, environmentally benign, and easy to process using a variety of methods, it deserves considerable research activities toward high-performance electrochemical supercapacitor applications [3]. An important parameter characterizing supercapacitors is their active surface – a reaction that is a source of electric current and accumulation of the charge in a form of the electric double layer occurs at the electrolyte/electrode phase boundary. Excellent materials, suitable for fabrication of supercapacitors, seem to be polymeric and metal transition oxide nanowire arrays. It is reasonable to believe that one-dimensional nanostructures can reduce diffusion resistance of electrolytes in rapid charge/discharge processes. Furthermore, nanowires have high conductivity. In the search for new materials, composites play an important role. As a multicomponent system, composites combine the properties of their constituents, which make it possible to overcome the drawbacks of individual components. Therefore, the aim of this research was to synthesize and examine a material exhibiting a high capacitance. The first step of this work included the synthesis of nanostructured anodic aluminum oxide (AAO) by anodization aluminum foil in an oxalic acid solution. The second stage was devoted to the electrochemical reduction of the barrier layer at the bottom of pores. Then, DC electrodeposition of polymeric (e.g., PPy, PEDOT, PANI) nanowires directly in the pores of the AAO template was done. After the AAO template removal, the nanowire array was modified by NiO nanoparticles. The structural features of the hybrid polymeric-metal oxide nanowires were investigated by means of Field Emission Scanning Electron Microscopy (FE-SEM). The chemical composition of hybrid polymeric-NiO nanowires was evaluated by Energy-Dispersive X-ray Spectroscopy (EDAX). The surface states of the hybrid polymeric-NiO nanowire array was investigated by X-ray photoelectron spectroscopy (XPS). The hybrid polymeric-NiO nanowire arrays were examined by cyclic voltammetry, galvanostatic charge–discharge tests and electrochemical impedance spectroscopy in order to determine their capacitance values, optimal working potential range and charge propagation. The discussion will also include the cyclability tests and the capacitance retention during an accelerated ageing procedure.