Global warming has become an unavoidable environmental problem as the CO₂ concentration in the atmosphere increases due to the widespread use of fossil fuels. Therefore, various studies have been conducted to reduce the concentration of CO₂. Among them, the electrochemical CO₂ reduction is promising because the harmful CO₂ can be converted into useful products such as C₁ products (CO, CH₄, CH₃OH and HCOOH) and C₂ products (C₂H₄ and C₂H₅OH). An attractive product is CO, owing to its higher market price than others and its wide application as a gas precursor for various industrial manufacturing. However, the stability of CO₂ leads to a high overpotential for activation. Thus, the development of a highly active catalysts is essential to resolve this problem. Recently, the high selectivity and activity of Au catalyst has been in the spotlight, due to its suitable binding energy of reaction intermediate for efficient CO production. To highly obtain activity and selectivity, the various investigates are being conducted to control properties, which their morphology, size, composition, facets, and oxidation states. In addition, these properties can affect intermediate adsorption, grain size and electrochemical surface area.

The conventional H-type cell using CO₂ dissolved in aqueous electrolyte have a mass-transfer problem because the cathode part is limited by the slow diffusion of CO₂ caused by low solubility. To solve this problem, the CO₂ electolyzer, by applying the zero-gap membrane electrode assembly (MEA) structure, the ohmic resistance has been minimized owing to elimination of electrolyte layer.

In conventional electrolyzer, the low solubility of CO₂ dissolved in electrolytes limits diffusion rate. As a result, the cathodic reaction rate result in mass transfer. Masstransfer of reactants leads to a low maximum current density of ~35 mA cm⁻². Therefore, recent study has focused on the development of electrolyzers using humidified gaseous CO₂ for enhancing the masstransfer of reactants.

In this study, AuCu catalysts were fabricated on carbon paper using electrodeposition method. The electrodeposition parameters such as deposition potential and precursor concentration were changed to control the atomic ratio of Au/Cu to 3.0. Trimetallic catalysts were fabricated by adding one more transition metal (e.g. In, Mo and Fe) while maintaining the atomic ratio of Au/Cu. The AuCuIn catalysts were used as cathode for electrochemical carbon dioxide conversion in H-type cell. At a certain condition, CO Faradaic efficiency of ~90 % was achieved. The crystallinity and valence band were investigated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Through valance band analysis, the d-band center was investigated. Furthermore, the AuCuIn electrode for electrolyzer was fabricated on microporous layer (MPL)-coated CP (MPL/CP). At a certain condition, CO Faradaic efficiency of ~100 % was achieved at operating cell voltage (2.8 V_cell).