Enhancing C≥2 product selectivity in electrochemical CO2 reduction by controlling the microstructure of gas diffusion electrodes†

We fabricate polymer-based gas diffusion electrodes with controllable microstructure for the electrochemical reduction of CO₂, by means of electrospinning and physical vapor deposition. We show that the microstructure of the electrospun substrate is affecting the selectivity of a Cu catalyst, steering it from H₂ to C₂H₄ and other multicarbon products. Specifically, we demonstrate that gas diffusion electrodes with small pores (e.g. mean pore size 0.2 μm) and strong hydrophobicity (e.g. water entry pressure >1 bar) are necessary for achieving a remarkable faradaic efficiency of ∼50% for C₂H₄ and ∼75% for C_≥2 products in neutral 1M KCl electrolyte at 200 mA cm⁻². We observe a gradual shift from C₂H₄ to CH₄ to H₂ during long-term electrochemical reduction of CO₂, which we ascribe to hygroscopic carbonate precipitation in the gas diffusion electrode resulting in flooding of the Cu catalyst by the electrolyte. We demonstrate that even with minimal electrolyte overpressure of 50 mbar, gas diffusion electrodes with large pores (mean pore size 1.1 μm) lose selectivity to carbon products completely, suddenly, and irreversibly in favor of H₂. In contrast, we find that gas diffusion electrodes with small pore size (mean pore size 0.2 μm) and strong hydrophobicity (water entry pressure ∼5 bar) are capable of resisting up to 1 bar of electrolyte overpressure during CO₂RR without loss of selectivity. We rationalize these experimental results in the context of a double phase boundary reactivity, where an electrolyte layer covers the Cu catalyst and thus governs local CO₂ availability. Our results emphasize the pivotal role of microstructure and hydrophobicity in promoting high C_≥2 product selectivity and long-term stability in CO₂RR flow cells.