Electrochemical CO2 Reduction Using Porous Cu Electrodes with Polymer Addition
Authors: Seki Yoshitomo , Kumagai Hiromu , Sugiyama Masakazu , Minegishi Tsutomu 
Published: December 2024

Greenhouse gas emissions and following global climate change problems are the most important human-being facing issues. The electrochemical CO2 reduction reaction (CO2RR) is one of the most promising processes to obtain value-added hydrocarbons from carbon dioxide (CO2) and water (H2O), and the process is being actively investigated as an alternative way to conventional hydrocarbon production by fossil resources. Among the electrocatalysts for CO2RR, copper (Cu) species are unique and most studied material group because of their capability of producing C2+ species, such as ethylene (C2H4), ethanol (C2H5OH) and propanol, through forming C-C bonding. Especially C2H4 is the most highly demanded and profitable product[1]. Porous structure of electrodes is reported to be advantageous to CO2RR because it enables enrichment of reactants, CO2 molecules, and intermediates[2–5]. Polytetrafluoroethylene (PTFE), a hydrophobic polymer, has been reported to be beneficial to increase the FE towards C2+ products[6] and suppressing competitive reaction, hydrogen production[7,8]. In this study, we prepared PTFE modified porous Cu electrodes and investigated the influence of each factor and their synergistic effect on CO2RR activity. As a result, the introduction of porous structure increased FE to C2H4 (FE(C2H4)) and suppressed both carbon monoxide (CO) and methane (CH4) production. Further modification by PTFE enhanced FE(C2H4) and improved durability significantly.
Porous Cu electrode was prepared by de-alloying process[9]. First, Cu and Al metals were co-sputtered to form CuAl alloy on gas-diffusion layer (GDL, MFK-A, Mitsubishi Chemical) coated with micro porous layer (MPL). The samples were etched using 5 wt.% hydrochloric acid for approximately 20 minutes to remove Al from the alloy and obtained porous Cu electrodes. CO2RR test was carried out in flow cell with gas-diffusion electrode (GDE), the electrocatalyst coated GDL, as a cathode under applied current density of 300 mA cm-2. Both gas phase products and liquidous products were quantified by gas chromatography (GC). Structural characterizations using scanning electron microscope (SEM) confirmed that porous Cu structure was successfully formed, and porous Cu electrodes exhibited higher FE(C2H4) of 48%, and lower FE toward CO (FE(CO)) of 6% and FE toward CH4 (FE(CH4)) of less than detection limit, while smooth Cu electrode showed FE(C2H4) of 45%, FE(CO) of 12%, FE(CH4) of 0.6%, respectively. However, FE(C2H4) started to decrease 6 hours after the start of the reaction and decreased to 12% after 24 hours reaction. This could be caused by GDE soaking to electrolyte. To overcome these issues, we next investigated PTFE added porous Cu electrodes. PTFE was introduced into the porous Cu layer by electrostatic spray[10].The introduction of PTFE further improved FE(C2H4) from 48% to 55% and demonstrated over 80% of FE towards valuable C2+ products at 300 mA cm-2. PTFE introduction enhanced durability and resulted in FE(C2H4) of 55% for over 24 hours.
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