Understanding the role of gas diffusion electrodes in steering the CO₂ electroreduction pathway

Electrochemical CO₂ reduction reaction (CO₂RR) offers a promising route for converting CO₂ into value-added chemicals. GDEs are pivotal for pushing CO₂RR toward industrial competitiveness, yet the GDE macro- and nano-structural parameters and their relationship with CO₂RR performance remain unclear. Here, we experimentally quantified the intrinsic and extrinsic properties of Cu-based GDEs (28BC, 22BB, 39BB, 36BB) and their CO₂RR performance, integrating this with mass-transport simulations. From this, we constructed a GDE structure–CO₂RR activity map that reveals two operating windows: A high-current-density (HCD) onset region (−1.50 V (vs. SHE)) at which product selectivity is governed by surface roughness which drives the re-adsorption of CO and further reduction to oxygenates, and competition for surface adsorbed hydrogen. Deeper in the HCD regime (−1.63 V (vs. SHE)), optimal activity requires balancing CO₂ transport with surface adsorbed hydrogen coverage, exemplified by one of the GDEs (22BB), whose high roughness and low microporous-layer porosity deliver the highest intrinsic rates for both hydrocarbon and oxygenate pathways while suppressing hydrogen evolution. These findings identify roughness and porosity as the primary, tunable levers for steering Cu-GDE product selectivity, provide actionable design rules for next-generation CO₂ electrolyzers and important mechanistic insights.