Surfactant Perturbation of Cation Interactions at the Electrode-Electrolyte Interface in Carbon Dioxide Reduction

Interfacial properties at the boundary between the electrode and electrolyte have important effects on the surface reactivity in electrocatalysis. Ionic additives and electrolyte ions can serve as promoters for specific reaction pathways. The judicious addition of these charged species thus represents a rich chemical strategy for tuning the electrode-electrolyte interface to achieve high product selectivity and catalytic activity. We have previously shown that trace amounts of surfactant can efficiently suppress the hydrogen evolution reaction (HER) and promote the carbon dioxide reduction (CO2RR) toward CO and HCOO- on a polycrystalline Cu foil working electrode. The major focus of herein study is to identify the impact of a model surfactant, cetyltrimethylammonium bromide (CTAB), on the double-layer structure in the presence of different alkali metal cations during electrocatalytic CO2RR. We postulated that the alkali cations and the positively charged surfactant headgroup will compete for a position at the negatively biased Cu electrode, leading to potentially synergistic effects on the catalytic performance. Indeed, it was observed that the positively charged trimethylammonium surfactant molecules effectively displace the alkali cations and suppress HER. However, the CO2RR activity and selectivity are nearly independent with respect to the identity of the alkali cations (Li+, Na+, and K+) in the presence of CTAB. Cesium cations defy this trend, where high HCOO- activity is observed. A molecular model of the double layer is proposed where CTAB molecules are competing for a position at the outer Helmholtz plane (OHP), resulting in only a small concentration of electrolyte cation at the electrode surface in the presence of CTAB. Furthermore, we postulate that the decrease in C2H4 activity is due to interfacial hydrophobicity caused by surfactant accumulation. We expect that these fundamental understandings will lead to advanced strategies for designing efficient organic additive to modulate the double-layer structure and optimize for selective small-molecule activation.