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Cetyltrimethylammonium bromide (CTAB) has been used to enhance the selectivity of CO2 electrochemical reduction. Traditionally, this selectivity was attributed to repulsion of water molecules due to a CTAB self-assembled monolayer, which forms under negative potential and disassembles at positive voltage due to electrostatic repulsions. In this report, using in operando interface sensitivity sum frequency generation spectroscopy, we investigated the self-assembly behavior of CTAB across a broad electrochemical potential range. We observed that CTAB molecules form a stable monolayer at the Stern layer over the entire potential scan, even when the electrodes are positively charged. Rather than disassembling, the CTAB molecules reorient themselves to balance the electrostatic interactions and the non-covalent hydrophobic effects, the latter being the primary driving force maintaining the monolayer at a positive potential. This finding contrasts the traditional view that CTAB monolayers are absent when the electrodes are positively charged, indicating a stable and ordered monolayer with respect to the electrostatic repulsions at liquid/electrode interfaces. The balance between non-covalent and electrostatic interactions offers a facile and reversible electrochemical method to control the local environment and dominating interactions at the Stern layer of the electrode surface, thus providing a means for engineering a micro-electrochemical environment.
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This content will become publicly available on March 5, 2026
Quantifying Stern layer water alignment before and during the oxygen evolution reaction
While water’s oxygen is the electron source in the industrially important oxygen evolution reaction, the strong absorber problem clouds our view of how the Stern layer water molecules orient themselves in response to applied potentials. Here, we report nonlinear optical measurements on nickel electrodes held at pH 13 indicating a disorder- to- order transition in the Stern layer water molecules before the onset of Faradaic current. A full water monolayer (1.1 × 1015 centimeter−2) aligns with oxygen atoms pointing toward the electrode at +0.8 volt and the associated work is 80 kilojoule per mole. Our experiments identify water flipping energetics as a target for understanding over- potentials, advance molecular electrochemistry, provide benchmarks for electrical double layer models, and serve as a diagnostic tool for understanding electrocatalysis.
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- Award ID(s):
- 2153191
- PAR ID:
- 10514266
- Publisher / Repository:
- American Association for the Advancement of Science
- Date Published:
- Journal Name:
- Science advances
- Volume:
- 11
- ISSN:
- 2375-2548
- Page Range / eLocation ID:
- eado8536
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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