Electrochamical CO2 Reduction Characteristics with Various Copper-Based Alloy Plating Electrodes
Authors: Takatsuji Yoshiyuki , Kimoto Takumi , Haruyama Tetsuya 
Published: December 2024

Climate change caused by global warming has led to disasters in many countries around the world. The Sustainable Development Goals (SDGs) call for "taking specific measures against climate change." The time has come for humanity to think concretely about ways to suppress the increase in greenhouse gases, which ate thought to be the cause of global warming, and to actually implement them. Among the known greenhouse gases, carbon dioxide (CO2) accounts for about 75% and is recognized as an important target. Various efforts have already been made to reduce CO2 emissions. On the other hand, in order to reduce the amount of CO2 emitted into the atmosphere, technologies for geological storage and conversion of CO2 are indispensable. CO2 becomes a resource if it can be converted into useful industrial resources such as carbon monoxide, formic acid, or hydrocarbons in processes that utilize surplus or renewable energy. Many electrochemical reduction techniques have been studied for CO2 conversion, and Dr. Yoshio Hori has conducted a comprehensive study of electrochemical reduction of CO2 using metal electrocatalysts1). It has been clarified that the products are different depending on the metal species, and studies are underway to change the surface morphology and crystal planes of metal electrodes and to combine multiple metals in metal electrocatalysts. These previous studies were to reveal that the adsorption strength of CO2 and electrodes is based on the metal type of the electrode, and that changing the electrode metal changes the path, efficiency, and products of the electrochemical reduction reaction.

The object of this research is to take a bird's-eye view of the characteristics of CO2 electrochemical reduction using copper-based alloy plating electrodes, which we have been development. Among metal electrodes, copper, which can produce hydrocarbons, has been the most widely studied for CO2 electrochemical reduction. When a copper-cobalt electrode is prepared by alloy plating, the ratio of the hydrocarbons, methane and ethylene, changes depending on the mixing ratio of cobalt. In this copper-cobalt electrode, cobalt exists as a substituted solid solution in the copper crystal structure, and it is considered that the generation efficiency of ethylene is reduced and that of methane is increased due to the inhibition of C-C bonding.

When copper-tin electrodes are prepared by alloy plating, unlike cobalt, a solid solution or an intermetallic compound may be formed depending on the ratio of copper and tin. Since the atomic radii of tin atoms are quite different from those of copper atoms, tin atoms do not exist only in substituted solid solution as in the case of cobalt, but their crystal structure changes. The reduction properties of CO2 electrochemical reduction at a copper-tin electrode were changed. Copper produces hydrocarbons and tin produces formic acid as the main products of CO2 electrochemical reduction, while carbon monoxide is produced as the main product at a copper/tin ratio of 87:13.

Computational chemistry studies predict that the formation of hydrocarbons by copper is via the two-electron reduction product carbon monoxide. The presence of tin near the copper atoms is considered to weaken the adsorption between copper and carbon monoxide. The modeling of the adsorption and the attempt to elucidate the cause by computational science revealed that the tin atoms are weakly bonded to the reaction intermediates and act only as the active sites for the formation of formic acid. It was found that the copper atom weakens the bond with carbon monoxide due to the presence of the tin atom next to the copper atom, and that it decreases the amount of adsorption of hydrogen and carbon monoxide. It is reported that the amount of carbon monoxide adsorbed on copper atoms is important for the formation of hydrocarbons, which is stabilized by the formation of hydrogen bonds with CHO*, the reaction intermediate to hydrocarbons. These results suggest that the adsorbed carbon monoxide is destabilized, and carbon monoxide is desorbed from the copper atom, resulting in an increase in carbon monoxide as a product.