Intrinsically Conductive and Cu-Functionalized Polymer-Composite Membranes as Gas Diffusion Electrodes for CO2 Electroreduction.

We introduced a new class of gas diffusion electrodes (GDEs) with adjustable pore morphology. We fabricated intrinsically conductive polymer-composite membranes containing carbon filler, enabling a pore structure variation through film casting cum phase separation protocols. We further selectively functionalized specific pore regions of the membranes with Cu by a NaBH₄-facilitated coating strategy. The as-obtained GDEs can facilitate the electrochemical CO₂ reduction reaction (CO₂RR) at Cu active sites that are presented inside a defined and electrically conductive pore system. When employing them as free-standing cathodes in a CO₂ flow electrolyzer, we achieved >70 % Faradaic efficiencies for CO₂RR products at up to 200 mA/cm². We further demonstrated that deposition of a dense Cu layer on top of the membrane leads to obstruction of the underlying pore openings, inhibiting an excessive wetting of the pore pathways that transport gaseous CO₂. However, the presentation of Cu inside the pore system of our novel membrane electrodes increased the C₂H₄/CO selectivity by a factor of up to 3 compared to Cu presented in the dense layer on top of the membrane. Additionally, we found that gaseous CO₂ could still access Cu in macropores after wetting with electrolyte, while CO₂RR was completely suppressed in wetted nm-scale pores.