Electrochemical CO2 reduction (CO2R) provides the opportunity to mitigate our fossil-carbon dependence, by using CO2 as an alternative source to produce value-added chemicals. Especially multi-carbon (C2+) products are industrially of high value and hence a promising candidate for CO2R products. Copper is uniquely capable of producing these value-added C2+ products. Currently, the application of CO2R technology is hampered by low selectivity and rates of C2+ products. Ionomers, or ion-conducting polymers, are used in the catalyst layer to optimize the local reaction environment. In particular Nafion, a cation exchange ionomer, has proven to promote C2+ formation, but the mechanism behind it remains unclear. For this thesis, we focused on elucidating the role of the Nafion ionomer in altering product distribution on copper. A planar Cu electrode was used, onto which the Nafion was drop-casted. Atomic force microscopy was used to observe the restructuring of the ionomer during CO2R, while attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS) enabled us to observe adsorbed chemical intermediate species during electrochemical CO2R. Flow cell experiments were performed to study the effect of the ionomer on product distribution and activity. The addition of Nafion significantly increased formation towards ethylene, due to stabilization of the atop-CO intermediate. The ionomer layer underwent restructuring during CO2R, where it is expected that the hydrophilic domain of the ionomer takes over the surface interactions from the hydrophobic backbone, due to electrowetting of the catalyst. With the insights gained in this thesis we elucidated the interactions between the ionomer and catalyst during electrochemical CO2R, which relates to the fundamental understanding required for designing advanced catalyst layers in the gas diffusion electrode.