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Electric double layers are crucial to energy storage and electrocatalytic device performance. While double layer formation originates in electrostatic interactions, electric double layer properties are governed by a balance of both electrostatic and entropic driving forces; favorable ion-surface electrostatic interactions attract counterions to charged surfaces to compensate, or "screen," potentials, but the confinement of these same ions from a bulk reservoir to the interface incurs an entropic penalty. Here, we use a dicationic imidazolium ionic liquid and its monovalent analogue to explore how cation valence and entropy influence double layer formation and electrochemical reactivity using CO2 electroreduction as a model reaction. We find that divalent and monovalent cations display similar CO2 reduction kinetics but differ vastly in steady-state reactivity due to rapid electrochemically induced precipitation of insulating dicationic (bi)carbonate films. Using in situ surface-enhanced Raman scattering spectroscopy, we find that potential-dependent reorientation occurs at similar potentials between the two ionic liquids, but the introduction of a covalent link in the divalent cation imparts a more ordered double layer structure that favors (bi)carbonate precipitation. In mixed monovalent-divalent electrolytes, we find that the divalent cations dominate interfacial properties by preferentially accumulating at surfaces even at very low relative concentrations. Our findings confirm that ion entropy plays a key role in modulating local electrochemical environments and highlight how double layer properties are very sensitive to the properties of counterions that pay the lowest entropic penalty to accumulate at interfaces. Overall, we illustrate that ion entropy provides a new knob to tune reaction microenvironments and unveil how entropy plays a major role in modulating electrochemical reactivity in mixed ion electrolytes.
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Linking Electric Double Layer Formation to Electrocatalytic Activity
Electric double layers form at electrode-electrolyte interfaces and often play defining roles in governing electrochemical reaction rates and selectivity. While double layer formation has remained an active area of research for more than a century, most frameworks used to predict electric double layer properties, such as local ion concentrations, potential gradients, and reactant chemical potentials, remain rooted in classical Gouy-Chapman-Stern theory, which neglects ion-ion interactions and assumes non-reactive interfaces. Yet, recent findings from the surface forces and electrocatalysis communities have highlighted how the emergence of ion-ion interactions fundamentally alters electric double layer formation mechanisms and interface properties. Notably, recent studies with ionic liquids show that ionic correlations and clustering can substantially alter reaction rates and selectivity, especially in concentrated electrolytes. Further, emerging studies suggest that electric double layer structures and dynamics significantly change at potentials where electrocatalytic reactions occur. Here, we provide our perspective on how ion-ion interactions can impact electric double layer properties and contribute to modulating electrocatalytic systems, especially under conditions where high ion concentrations and large applied potentials cause deviations from classical electrolyte theory. We also summarize growing questions and opportunities to further explore how electrochemical reactions can drastically alter electric double layer properties. We conclude with a perspective on how these findings open the door to using electrocatalytic reactions to study electric double layer formation and achieve electrochemical conversion by engineering electrode-electrolyte interfaces.
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- Award ID(s):
- 2237311
- PAR ID:
- 10479779
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- ACS Catalysis
- ISSN:
- 2155-5435
- Page Range / eLocation ID:
- 16222 to 16239
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acscatal.3c04255
