Electrochemical CO₂ reduction (eCO₂R) is a very promising way to convert detrimental CO₂ emissions into sustainable fuels and chemicals, and move us closer to a net zero or net negative carbon economy_.¹ To compete with traditional fuel/chemical production routes, electrocatalysts needs to be highly active, selective towards a desired product, and stable. Thanks to the use of gas diffusion electrodes (GDEs) that provide gaseous CO₂ close to the electrocatalyst, high activities can be achieved_.^2,3 However, product selectivity as well as stability still need to be improved before applications. An emerging strategy to drive product selectivity and improve stability is to control the local environment surrounding an electrocatalyst, i.e., the concentration of the products and reagents therein.⁴

In this work, we achieve such control by tuning the mass transport of the eCO₂R reactants, namely CO₂ and H₂O, to the reaction sites by varying pore size and hydrophobicity of polymeric GDE substrates coated with Ag or Cu electrocatalysts.^5,6 We show that for GDEs with large pore size and weak hydrophobicity, a deeper penetration of water/ aqueous electrolyte takes place, yielding, in the Ag case, poor selectivity towards CO and a significant production of H₂. In contrast, GDEs with small pore size prevent this local flooding of the electrocatalyst, thus yielding a greatly improved Faradaic efficiency towards CO (up to 95% at 100 mA/cm² in neutral electrolyte), minimal H₂ evolution, and remarkable long-term stability (97% of initial CO selectivity retained after > 40 h).⁵ In the Cu case, we observe a strong enhancement in C₂H₄ selectivity when employing GDE substrates with small pores and strong hydrophobicity (up to 50% Faradaic efficiency in neutral electrolyte at 200 mA/cm² for > 3 h).⁶ Both CO₂ and CO reduction experiments on Cu GDEs indicate the presence of mass transport limitations for the gaseous reactants in all GDEs, however this limitation is less severe for GDEs with small pore sizes. We rationalize these findings with the presence of an electrolyte layer covering the electrocatalyst, forcing the electrocatalytic reaction to take place at double phase boundaries. Under these conditions, CO₂ mass transport limitations are governed by the extent of electrolyte penetration within the GDE and can thus be controlled by acting on the substrate.^5,6 In addition to fundamental understanding, our results also provide a scalable strategy to improve product selectivity and stability of GDEs for eCO₂R.