Electrochemical approaches to the formation of Au nanostructures for efficient $CO_2$ reduction catalysts

Abstract

Technology to convert carbon dioxide ($CO_2$) into value-added chemical feedstock has extensively attracted much attention as the way to reduce atmospheric concentration of $CO_2$, which is one of the major greenhouse gases, by creating carbon neutral cycle. In particular, there is a great interest in conversion of $CO_2$ to carbon monoxide (CO) because CO can be used as a raw material for producing syngas, ethylene, and moreover multi-carbon oxygenates such as high-order alcohols. To selectively convert $CO_2$ to CO, however the use of a proper catalyst is essential due to thermodynamically stable nature of $CO_2$. Gold (Au) is a well-known $CO_2$ reduction catalyst with high selectivity to CO production. However, because a conventional flat Au catalyst still requires high overpotential for efficient $CO_2$ reduction, the use of planar Au catalyst itself is relatively inefficient in terms of economic view. Meanwhile, since nanostructured Au exhibited much more improved $CO_2$ reduction properties than planar Au catalyst, many efforts to form Au nanostructures have been recently reported. However, most works are focusing on demonstration of the nanostructure surface properties-related $CO_2$ reduction activity, while there is a lack of study on the developing principle of catalyst to enable not only efficient but also long-term stable $CO_2$ reduction activity. In this dissertation, we aim to present an effective strategy to form Au nanostructures for not only highly efficient and selective but also long-term stable $CO_2$ reduction catalysts. Specifically, we show that Au nanostructures can be formed via a facile yet reproducible electrochemical treatment composed of oxidation and reduction sequence, and provide the simple pathway to tune morphology and composition of Au nanostructures through systematic control of electrochemical process parameters. Then, we systematically investigate the correlations between $CO_2$ reduction catalytic activity and the Au nanostructures whose morphology and composition are controlled by the electrochemical treatment. In Chapter 1, previous works on the formation of Au nanostructure are introduced. Therein, we present the characteristics of Au nanostructures formed by various methods and their $CO_2$ reduction catalytic activities. Chapter 2 covers the electrochemical process to form morphology-controlled Au nanostructures. In addition, the principle for effective catalyst design that can enable efficient $CO_2$ conversion into CO is suggested. Chapter 3 presents a strategy to overcome the low stability of developed Au nanostructure catalyst, and finally Chapter 4 proposes the design principles of photoelectrode system for efficient and selective photoelectrochemical (PEC) $CO_2$ reduction.