Characteristic electrochemical features of nanoparticles and microemulsions

Abstract

Research on energy to replace fossil fuels has been one of the most important topics for decades to address the problems of depleting fossil fuels and increasing environmental pollution. Despite numerous efforts, the energy consumption is steadily increasing globally and most of the energy needs are still being met by fossil fuels. Therefore, research on new energy conversion technologies and storage devices has become very important. One of the important requisites for fabricating in energy conversion devices is choosing an appropriate catalytic material for the electrochemical reaction. Recently, much research has been conducted on the synthesis and application of nanoparticles for improving the efficiency of such catalyst materials. The physical and chemical properties of nanoparticles are different from those of bulk materials. They have a very large surface area compared to their volume, and their electrochemical activity is superior to bulk materials because of their dominant surface properties. However, the electrochemical properties of nanoparticles and their causes are still not fully understood. Energy storage devices are also attracting attention from another perspective. In general, energy is consumed in the form of electricity, however, the supply and consumption of electrical energy are not always balanced. In order to effectively solve these problems, an electric energy storage system needs to be developed that stores the remaining electricity and draw it out when required. Among the various energy storage technologies, redox flow battery is attracting attention because of its ability to easily control the power and capacity. Typical Redox flow batteries include Br-polysulfide and Zn-Br, which involve the redox reactions of Br-/Br2. However, problem with these systems is that the electrochemically generated Br2 passes through the separator and causes self-discharge. To prevent this, quaternary ammonium bromides that capture Br2 and form microemulsions are usually added to the systems. Recently, it has been reported that this microemulsions participate not only in capturing Br2 but also in electrochemical reactions, however the mechanism is not clear. As mentioned above, small particles such as nanoparticles and microemulsions participate in electrochemical reactions in energy conversion and storage devices. However, many studies focused on material aspects and did not fully explain the role of such small particles and their electrochemical properties in electrochemical reactions. This dissertation describes two case studies to investigate the effect of the fundamental electrochemical characteristics of such small particles on energy conversion and storage devices. First, the oxidation stability of gold nanoparticles in an electrochemical environment was investigated. Using a diblock copolymer, we synthesized different arrays of nanoparticles, which can uniformly control the size and interparticle spacing to the nanometer scale on the substrate electrode. Then, we observed the change in the oxidation reactivity according to the size and spacing of the nanoparticles. The surface coverage, total amount of gold deposited on the substrate, also changed when the size and spacing parameters were adjusted. Experimental and theoretical results showed that the surface coverage is more influential than diameter on the oxidation reaction under experimental conditions. Secondly, It is studied how the bromine complexing agent electrochemically forms emulsions and how the emulsions participate in the Br-/Br2 half reaction with respect to redox flow batteries. The electrochemical properties of microemuslion was investigated by acquiring chronoamperometric signals and confocal spectroscopic images simultaneously. The experimental results show that microemulsions heterogeneously generate on the electrode surface and the adsorbed emulsions increase the oxidation current. Furthermore, a current spike in the chronoamperogram was generated by a collision of emulsion. It is proved that the formation and collision of the emulsion had a great influence on the Br- oxidation. These results show a new understanding of the underlying mechanisms through which small particles that affect energy conversion and storage devices, and are expected to provide new directions to the research and development of energy conversion technologies and storage devices.