(A) gas phase electrolyzer for $CO_2$ reduction reaction

Abstract

According to Intergovernmental Panel on Climate Change (IPCC), carbon dioxide is considered the main reason for global warming. As global warming problems become deteriorated, electrochemical carbon dioxide reduction reaction ($CO_2RR$) has received attention that is promising route for storing intermittent renewable electrical energy in the form of valuable carbon-based fuels and chemicals while reducing the $CO_2$ in the atmosphere. Catalysts is essential for $CO_2RR$ due to stable characteristics of $CO_2$ molecules. The products of $CO_2RR$ depends on the kind of catalysts. Gold and silver is the representative catalysts for producing CO and copper is the representative catalysts that can produce methane, methanol, ethylene and ethanol and so on. Many researchers have used liquid phase electrolyzer that is dissolved $CO_2$ gas in electrolyte, however, the current density of $CO_2RR$ is limited by the transport limit of $CO_2$ reactant because the solubility of $CO_2$ gas in aqueous solution is very low (33 mM at 25 $^\circ C$, 1 atm). In this thesis, we constructed a gas phase electrolyzer that is not affected by the solubility of $CO_2$ gas in aqueous solution. We verified that the gas phase electrolyzer shows better performance compared to liquid phase electrolyzer using commercial silver powder. Moreover, we achieved over than 300 mA/$cm^2$ of CO partial current density with higher than 90 % CO selectivity by employing Au25 clusters to the gas phase electrolyzer. Then, we established solar-powered gas phase electrolyzer using Au25 clusters and heterogeneous with intrinsic thin layer (HIT) cells. We achieved 7.61 % of solar to CO conversion efficiency through the solar-powered gas phase electrolyzer. At last, we tested our solar-powered gas phase electrolyzer at dilute $CO_2$ streams and recognized that 40 % of $CO_2$ in gas streams shows similar solar to CO conversion efficiency with concentrated CO2 streams.