Takanori Tamaki

A recent rapid decline in the price of renewable energy makes electrolysis one of the key technologies to realize a sustainable society by reducing dependence on fossil fuels. In the current petrochemical industry, naphtha derived from petroleum is cracked to produce olefines such as ethylene and propylene, and then these raw materials are converted to various chemical products. CO₂ electrolysis (electrochemical CO₂ reduction) can produce ethylene that is one of the key raw materials for the chemical products. When the electricity for the electrolysis is supplied from renewable energy, CO₂ electrolysis can contribute to the sustainable society in three aspects: decreasing the dependence on fossil fuels to produce chemical products, reducing the amount of CO₂ by utilization of CO₂ as a reactant and utilizing renewable energy. In previous studies on CO₂ electrolysis, copper (Cu) has been reported to reduce CO₂ to products requiring more than two electrons, such as ethylene and methane.¹⁾ Recent growing attention to CO₂ electrolysis has resulted in many reports on nanostructural control of Cu catalysts to increase the selectivity of ethylene.²⁾ In addition to the nanostrucutral control of the catalysts, control of reaction environment around the catalysts is important for the high ethylene selectivity. In this study, we control the reaction environment around Cu catalysts for selective ethylene production by CO₂ electrolysis as shown in Fig. 1. Reaction environment was controlled by 1) the structure of a Cu-electrodeposited carbon electrode and 2) coating of an anion-conducting polymer on Cu. 1) Structure of the Cu-electrodeposited electrode. The presence or absence of the pre-treatment of carbon paper changed the thickness of the electrodeposited Cu layer in the thickness direction of the carbon paper. The electrode with the Cu-layer thickness of around 10 µm showed high ethylene selectivity (Faradaic efficiency), while hydrogen evolution was dominant when using the electrode with the Cu-layer thickness of around 170 µm. Model calculation suggested the depletion of CO₂ in the most part of the thick Cu electrode, which led to suppression of CO₂ reduction and concomitant increase in hydrogen evolution. Nanostructural control of the Cu catalysts in the thin Cu electrode further increased the ethylene selectivity. 2) Coating of the anion-conducting polymer on Cu. The anion-conducting polymer, poly(vinylbenzyl trimethylammonium chloride-co-styrene), was synthesized and coated on flat Cu electrodes. The performance for CO₂ electrolysis of the polymer-coated electrodes was evaluated in a liquid electrolyte. The polymer coating increased the selectivity of ethylene. Also, the decrease in ion conductivity of the coated polymer increased the ethylene selectivity. Model calculation suggested the increase in the local pH around the surface of Cu by coating the anion conducting polymer. These results underscore the importance of the control of reaction environment in CO₂ electrolysis. Acknowledgement Part of this work was supported by JST FOREST Program, Grant Number JPMJFR2278. References 1) Hori, Y. et al., Chem. Lett., 15, 897 (1986). 2) Nitopi, S. et al., Chem. Rev., 119, 7610 (2019). Figure 1

As phosphonic acid groups are more concentrated in the synthesized samples, their proton conductivities exhibit lower dependence on the relative humidity.