Immobilized Organic Cations Augment CO Electroreduction via Molecular Vibration Modulation

The catalyst–ionomer heterojunction (CIH) determines the activity and product selectivity of electrochemical CO₂ reduction to multicarbon (C₂₊) products. However, the prevailing explanation of the CIH’s role is limited to early stage steps, such as CO₂ diffusion and activation. Evidence of ionomers’ impacts on CO, whose adsorption energy typically serves as a descriptor of activity toward C₂₊ products, is absent. Here, we compare CO-to-C₂₊ electroreduction performance on Cu catalysts modified by polynorbornene ionomers with different immobilized cation groups. We found that imidazolium cations offer the highest activity for C₂₊ products, including a Faradaic efficiency (FE_C2+) of 83 ± 2% and a partial current density of 584 ± 34 mA cm^–2 at a 2.8 V full-cell potential. This phenomenon is linked to the modulation of the vibrational frequency of linearly bound *CO on Cu, stemming from the charge transfer between the organic cation and the adsorbed *CO. It weakens the CO bond and facilitates carbon–carbon coupling, augmenting CO reduction to C₂₊ products.