Cation Exchange Ionomer-Based Microenvironment Control of Gas Diffusion Electrode for Efficient CO₂ Electroreduction

Electrochemical CO₂ reduction reaction (CO₂RR) where CO₂ is converted into a high-value-added compound can be a great strategy for carbon neutrality. Gas diffusion electrode (GDE) which allows the formation of triple phase boundary (TPB) of catalyst, electrolyte, and CO₂ gas has been widely applied for CO₂RR. It results in overcoming the CO₂ solubility limitation in double phase boundary (DPB) of electrolyte and catalyst, and thus increases the CO₂ availability. However, the volume of TPB where the reaction occurs is much smaller than the volume of DPB. Therefore, it is important to increase the volume of TPB for CO₂RR activity. Cation exchange ionomer enables us to control the volume of TPB. The hydrophobic chains of ionomer that play a role as gas channel and hydrophilic ionic groups that form a water matrix increase the volume of TPB in GDE-based systems.
Here, we verify the effect of ionomer on CO₂RR performance in 1 M KOH electrolyte by controlling CO₂/H₂O ratio and CO₂ availability near GDE-based Cu catalyst surface. To distinguish the effect of each part of ionomer, we control the variables of ionomer; (1) Controlling the length of side chain with the application of Nafion as long side chain (LSC) ionomer and Aquivion as short side chain (SSC) ionomer, (2) Fixation of equivalent weight (EW). Compared to the maximum H₂ partial current density (J_H2) of 71.3 mA/cm² at bare Cu, Nafion decreases the maximum J_H2 to 11.5 mA/cm² while Aquivion decreases it to 26.4 mA/cm². On the other hand, the maximum CO partial current density (J_CO) is 75.1 mA/cm² at Nafion whereas it is 24.1 mA/cm² at Aquivion. Nafion and Aquivion also improve the maximum C₂H₄ partial current density (J_C2H4) to 169.5 and 173.3 mA/cm², respectively. Based on the characterization of ionomers in GDE, we found that LSC ionomer can induce a higher CO₂/H₂O ratio than SSC ionomer and increase CO₂ availability near the active site. Also, we assume that it affects the CO₂RR selectivity by tuning the local OH^- concentration. Improved OH^- formation by high current density CO₂RR at ionomer-augmented catalysts affects the surface charge density that can enhance CO₂RR selectivity. Therefore, the key factor of ionomer effect on CO₂RR performance enhancement is regulation of the water concentration near the catalyst surface which impacts the CO₂ availability and ion conductivity through this water matrix. We expect that this work will suggest a more accurate understanding of the mechanism of ionomer effect on CO₂RR and strategy for optimizing the microenvironment of catalyst for efficient CO₂RR.