Catalyst Particle Density Controls Hydrocarbon Product Selectivity in CO₂ Electroreduction on CuOx

A key challenge of the carbon dioxide electroreduction (CO2RR) on Cu‐based nanoparticles is its low faradic selectivity toward higher‐value products such as ethylene. Here, we demonstrate a facile method for tuning the hydrocarbon selectivities on CuO x nanoparticle ensembles by varying the nanoparticle areal density. The sensitive dependence of the experimental ethylene selectivity on catalyst particle areal density is attributed to a diffusional interparticle coupling that controls the de‐ and re‐adsorption of CO and thus the effective coverage of CO ad intermediates. Thus, higher areal density constitutes dynamically favored conditions for CO re‐adsorption and *CO dimerization leading to ethylene formation independent of pH and applied overpotential.