Switching CO₂ Reduction Selectivity on Cu via Organic Cation Regulation of the Electric Double Layer

Electrochemical CO₂ reduction (CO₂R) offers a promising route to mitigate the pressing carbon emissions and facilitate renewable energy storage. Here, we report a facile electrolyte engineering strategy to modulate the electrical double layer (EDL) at the electrode/electrolyte interface, steering CO₂R toward selective formate production on Cu. By introducing cationic surfactants (Dodecyltrimethylammonium bromide, DTAB) into the electrolyte, we achieve an 82% selectivity for formate on a Cu-based foam electrode at -0.9 V versus RHE. Through detailed kinetic analysis and in situ spectroscopy, we attribute this enhancement to the organic cation DTA⁺. Specifically, DTA⁺ reduces the interfacial charge-transfer resistance for CO₂R, promotes the adsorption and stabilization of formate intermediates, and disrupts the hydrogenbond network by increasing the fraction of free interfacial water. We further demonstrate the practical relevance of this approach in a zero-gap cell and achieve a formate selectivity of 88% at a current density of 300 mA cm^-2, with stable operation for 120 h at 100 mA cm^-2. This organic-cation regulation strategy offers a feasible route to tune interfacial charge-transfer dynamics and intermediate adsorption, thereby enhancing CO₂R reactivity toward a single product.