Energy storage and conversion in aqueous system

Abstract

Research on efficient energy storage and conversion is essential to meet the increasing global energy demand. Electrical energy produced by renewable energy sources can be stored in energy storage devices or converted to chemical energy using energy conversion devices. Aqueous electrolytes are more environmentally friendly than non-aqueous electrolytes that have been studied in the past, and they are cost competitive because they do not require complicated processes. However, the aqueous environment has problems of its intrinsic limited operating voltage and the damage of electrodes because of the dissolved oxygen and water. To solve these problems, it is very important to design a material having high activity and improved stability in the aqueous system. In this thesis, I designed materials that can be applied to energy storage and conversion devices in the aqueous system. In addition, the in-situ analyses were developed to verify the mechanisms when the materials are operated in the aqueous system. In Chapter 2, nickel and iron solid solution oxide was synthesized and their characteristics as a water oxidation catalyst were investigated. Nickel-iron solid solution oxide has a distorted structure due to the aliovalent species. It was found that the structural specificity improves the activity of oxygen evolution. From this result, it confirmed the close relationship between the structural specificity and the oxygen evolution activity of the material. In Chapter 3, the surface modified lithium cobalt oxide, which is applicable to aqueous and non-aqueous electrolytes, was studied. The lithium cobalt oxide is degraded very rapidly due to side reactions between the electrolyte and the active material when the lithium cobalt oxide is operated up to high voltage condition or in an aqueous electrolyte. In this study, lithium cobalt oxide with spinel oxide surface was synthesized by the off-stoichiometric synthesis and confirmed the improved electrochemical properties. In Chapter 4, the layered double hydroxide was applied as a cathode material for zinc batteries. Also, the interlayer distance and crystallinity of layered double hydroxides were controlled by introducing various anions. This study verified the influence of the crystallinity of the material on the electrochemical performance in the aqueous environment. In Chapter 5, the graphene oxide was synthesized and its performance was confirmed as a CO$_2$ adsorbent. As a result, it was confirmed that a very high selectivity of graphene oxide was obtained. It was originated from combining CO$_2$ affinity of the abundant functional group and N$_2$-phobicity due to large pore. This study suggests the design principle of CO$_2$ adsorbent with high selectivity.