Copper is among the most studied electrocatalyst for CO2 conversion due to its remarkable ability to form high-order carbon products. However, controlling factors that lead to high carbon product selectivity remains a major hurdle to fundamental scientific understanding. In this work, we investigate the utility of cationic surfactants to modify the selectivity of Cu foil in electrocatalytic CO2 reduction (CO2RR). We demonstrate that cetyltrimethylammonium bromide (CTAB) significantly suppresses the hydrogen evolution reaction (HER), a competitive parallel reaction to CO2RR. In addition, high surfactant concentrations and long alkyl chain lengths enhance the selectivity for CO2RR in NaHCO3 solutions. Importantly, electrochemical impedance spectroscopy is used to monitor the evolution of the electrode–electrolyte interface in the presence of CTAB under varying experimental conditions. On the basis of our extensive electrochemical characterization, we propose that cationic surfactants accumulate in the electrochemical double layer and effectively lower the available proton sources for HER. Our approach provides a facile strategy to modify the surface reactivity of metal electrodes with broad implications for electrocatalysis.

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 (CO 2 RR) 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 CO 2 RR. 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 CO 2 RR 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 C 2 H 4 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 smallmolecule activation.

Surfactants modulate interfacial processes. In electrochemical CO 2 reduction, cationic surfactants promote carbon product formation and suppress hydrogen evolution. The interfacial field produced by the surfactants affects the energetics of electrochemical intermediates, mandating their detailed understanding. We have used two complementary toolsvibrational Stark shift spectroscopy which probes interfacial fields at molecular length scales and electrochemical impedance spectroscopy (EIS) which probes the entire double layerto study the electric fields at metal−surfactant interfaces. Using a nitrile as a probe, we found that at open-circuit potentials, cationic surfactants produce larger effective interfacial fields (∼−1.25 V/nm) when compared to anionic surfactants (∼0.4 V/nm). At a high bulk surfactant concentration, the surface field reaches a terminal value, suggesting the formation of a full layer, which is also supported by EIS. We propose an electrostatic model that explains these observations. Our results help in designing tailored surfactants for influencing electrochemical reactions via the interfacial field effect.

The presence of cetyltrimethylammonium bromide (CTAB) near the surface of a Cu electrode promotes the electrochemical reduction of CO2 to fuels. CTAB increases the CO2 reduction rate by as much as 10× and decreased the HER rate by 4×, leading to ∼75% selectivity toward the reduction of CO2. Surface enhanced infrared absorption spectroscopy (SEIRAS) was used to probe the effects of CTAB adsorption on the structure of interfacial water and CO2 reduction intermediates. HER suppression was found to arise from the displacement of interfacial water molecules from CTAB adsorption within the double layer. The enhanced CO2 reduction rate can be correlated to an increased population of atop-bound CO and the emergence of a low frequency atop-CO band. These results unravel the role of additives in improving CO2-to-fuels electrocatalysis and establishing this as a powerful methodology for directing product selectivity.

Metrics & MoreArticle Recommendations CONSPECTUS:The electrochemical conversion of carbon dioxide to value-added chemicals provides an environmentally benign alternative to current industrial practices. However, current electrocatalytic systems for the CO 2 reduction reaction (CO 2 RR) are not practical for industrialization, owing to poor specific product selectivity and/or limited activity. Interfacial engineering presents a versatile and effective method to direct CO 2 RR selectivity by fine-tuning the local chemical dynamics. This Account describes interfacial design strategies developed in our laboratory that use electrolyte engineering and porous carbon materials to modify the local composition at the electrode− electrolyte interface.Our first strategy for influencing surface reactivity is to perturb the electrochemical double layer by tuning the electrolyte composition. We approached this investigation by considering how charged molecular additives can organize at the electrode surface and impact CO 2 activation. Using a combination of advanced electrochemical techniques and in situ vibrational spectroscopy, we show that the surfactant properties (the identity of the headgroup, alkyl chain length, and concentration) as well as the electrolyte cation identity can affect how surfactant molecules assemble at a biased electrode. The interplay between the electrolyte cations and the surfactant additives can be regulated to favor specific carbon products, such as HCOO − , and suppress the parasitic hydrogen evolution reaction (HER). Together, our findings highlight how molecular assemblies can be used to design selective electrocatalytic systems.In addition to the electrolyte design, the local spatial confinement of reaction intermediates presents another strategy to direct CO 2 RR selectivity. We were interested in uncovering the role of porous carbon-supported catalysts toward selective carbon product formation. In our initial study, we show that carbon porosity can be optimized to enhance C 2 H 4 and CO selectivity in a series of Cu catalysts embedded in a tunable carbon aerogel matrix. These results suggested that local confinement of the active surface plays a role in CO 2 activation and motivated an investigation into probing how this phenomenon can be translated to a planar Cu electrode.Our findings show that carbon modifiers facilitated surface reconstruction and regulated CO 2 diffusion to suppress HER and improve the C 2−3 product selectivity. Given the ubiquity of carbon materials in catalysis, this work demonstrates that carbon plays an active role in regulating selectivity by restricting the diffusion of substrate and reaction intermediates. Our work in tuning the composition of the electrochemical double layer for increased CO 2 RR selectivity demonstrates the potential versatility in boosting catalytic performance across an array of catalytic systems.