The electrochemical CO₂ reduction reaction (CO₂RR) represents a promising approach to upgrade CO₂ into chemical feedstocks and fuels. Nafion ionomer, composed of hydrophobic Teflon backbones and hydrophilic sulfonic acid side chains, is the most widely used additive for preparing catalyst layers (CLs) for CO₂ electrolysis. Typically, the Nafion ionomer serves as a dispersant for catalyst mixing and provides conduction channels for cation transport within the CL. However, the effect of the Nafion ionomer on selectivity and activity of CO₂ electrolysis remains unknown. Here, we choose commercial Cu nanoparticles as the model catalyst and control the structure of the CL by tuning the content of Nafion and solvent used for depositing the catalyst onto carbon paper. The structural differences in CLs, probed by scanning electron microscopy (SEM) and grazing-incidence small angle X-ray scattering (GISAXS), results in significant variations of Faradaic efficiencies (e.g., 20%-82% for hydrogen and 12%-55% for ethylene) of products during 5 h chronopotentiometry tests. To better understand reaction pathways, operando Raman and FTIR-ATR spectroscopies are utilized to probe changes of local environments (pH and adsorption of intermediates) at the surface. This work shows that in addition to the critical role of Nafion ionomer for improved dispersion and stability of catalyst, optimization of its content and the solvent system within the catalyst ink is fundamentally required to realize the full potential of CO₂RR.