pH‐Sensitive and Self‐Regenerative Honeycomb Polymer Membrane‐Electrode Assembly for CO ₂ Reduction: Shifting Selectivity toward C ₂ Molecules

The electrochemical CO₂ reduction reaction (CO₂RR) selectivity toward C₂ molecules presents several challenges, ranging from the chemical nature of catalysts to the hydrophobization of electrodes. In this study, a simple and versatile concept is presented based on a honeycomb polymer membrane-electrode assembly (HMEA). By covering copper foil electrodes with a PMMA-b-P4VP porous pH-sensitive polymer membrane, the electrode selectivity in the CO₂RR shifts to C₂ molecules. It is reported that both the bio-inspired honeycomb structure of the polymer and its pH-sensitivity lead to a Cassie-Baxter behavior in KHCO₃ aqueous electrolyte, which limits the hydrogen evolution reaction and concentrates the CO₂ and intermediates in the vicinity of the electrode surface. A systematic and drastic shift in selectivity is observed when using the porous polymer, toward formate at potentials below 0.9 V_RHE, and toward ethanol and ethylene for potentials above this value. After 24 h of electrocatalysis, the selectivity and efficiency of the HMEA can be restored by simple drying at room temperature. The versatility of the HMEA concept is also extended to silver foil electrodes, showing the decrease of hydrogen production and the shift of the electrode selectivity in the CO₂RR toward carbon monoxide, formate, and ethanol.