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

Abstract

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.

Absence of CO2 electroreduction on copper, gold and silver electrodes without metal cations in solution

“…The knowledge that CO 2 can only be reduced in the presence of a metal cation interacting with the CO 2 intermediate has important implications. From the system-design point-of-view, research efforts should focus on finding species that might have an even larger stabilizing effect on CO 2 than that of Cs + , such as large multivalent cations 43 or surfactants 7 . Additionally, from the reaction modelling point-of-view, our results highlight that an accurate understanding of the electrochemical activation of CO 2 on copper, silver and gold must involve cations and water.…”

“…We can instead estimate the energy of a cation at the OHP at U = 0 V versus SHE from our AIMD simulations, as we specifically inserted an alkali metal at the OHP, which donates its valence electron to the solvation layer (equation (7); the gold surface is represented by the symbol *, and the solvation layer is indicated by 'solv'). Thus, we can calculate the thermodynamic driving force for each cation to accumulate at the OHP as in equation (8).…”

“…To date, in the majority of the studies, alkali metals were employed and the activity trend found on different electrodes is Cs + > K + > Na + > Li + (refs. [5][6][7][8][9][10][11][12][13] ). Even though this trend is consistent in different research works, diverging theories have been proposed to explain through which molecular mechanism cations affect CO 2 reduction.…”

Effect of Electrolyte Composition and Concentration on Pulsed Potential Electrochemical CO2 Reduction

“…Recognizing that improvements in electrocatalytic processes rely on better control over the solid‐liquid interface, increasing efforts have been directed at understanding the ‘soft’ side of the solid‐electrolyte interface, namely the chemistry of adsorbed species and the electrolyte environment that exist near the catalytic active sites ( i. e ., the ‘hard’ side) [11–18] . Previous studies have illustrated that the concentration and composition of the electrolyte impacts the eCO 2 R pathways in two important ways [19] .…”

Hydrophobic perfluoroalkane modified metal‐organic frameworks for the enhanced electrocatalytic reduction of CO2

“…[46][47][48][49][50][51] Nevertheless, the selectivity of the reported MOF electrocatalysts toward the CO 2 RR is still far from the commercial standards because the competitive HER is usually occurred in aqueous electrolyte. It is believed that the local environment of the active centers including coordination structure and electrolyte environment, [52][53][54][55][56] plays an important role in adjusting the catalytic performances of the CO 2 RR. As we know, most of the pores in MOFs are hydrophilic, which are more favorable for the contaction between active centers and aqueous electrolyte, thus leading to intensive HER.…”