CO₂ Conversion Enhancement by Hydrophobic Cation Additives Is Nearly Exclusive at the Undercoordinated Sites of Metal Surfaces

Modifying the composition of the interfacial liquid, the region in contact with a charged solid, has emerged as a complementary strategy to enhance CO₂ electroconversion kinetics by creating a more favorable catalytic environment. This can be accomplished by using hydrophobic cation additives like the cetyltrimethylammonium. Here, we demonstrate that the enhancement effects of cetyltrimethylammonium are highly confined to undercoordinated metal sites. Advanced surface characterization and quantitative analysis with the polycrystalline Ag catalyst reveal that cetyltrimethylammonium enhances the CO₂-to-CO turnover frequency of undercoordinated sites by nearly 2 orders of magnitude, while the rest of the other sites exhibit only a 2-fold increase. Ultimately, in the presence of cetyltrimethylammonium, the intrinsic activity of undercoordinated Ag sites surpasses that of the rest by more than 3 orders of magnitude. Since these sites constitute only a small fraction of the total surface and are highly susceptible to blockage by other species, even nanomolar concentrations of impurity metals in solution can completely nullify the overall enhancement effects, as observed for both CO and formate production with Ag and Sn as catalysts, respectively. These findings highlight the significant site-dependent nature of environmental adjustments near catalytic surfaces, an aspect previously overlooked, and the need for a deeper understanding of the interplay between surfaces and near-surface environments to further advance electrocatalytic systems for CO₂ and beyond.