Water in the Electrical Double Layer of Ionic Liquids on Graphene

Qianlu Zheng, Zachary A.H. Goodwin, Varun Gopalakrishnan, Alexis G. Hoane, Mengwei Han, Ruixian Zhang, Nathaniel Hawthorne, James D. Batteas, Andrew A. Gewirth, Rosa M. Espinosa-Marzal

Research output: Contribution to journalArticlepeer-review

Abstract

The performance of electrochemical devices using ionic liquids (ILs) as electrolytes can be impaired by water uptake. This work investigates the influence of water on the behavior of hydrophilic and hydrophobic ILs─with ethylsulfate and tris(perfluoroalkyl)trifluorophosphate or bis(trifluoromethyl sulfonyl)imide (TFSI) anions, respectively─on electrified graphene, a promising electrode material. The results show that water uptake slightly reduces the IL electrochemical stability and significantly influences graphene's potential of zero charge, which is justified by the extent of anion depletion from the surface. Experiments confirm the dominant contribution of graphene's quantum capacitance (CQ) to the total interfacial capacitance (Cint) near the PZC, as expected from theory. Combining theory and experiments reveals that the hydrophilic IL efficiently screens surface charge and exhibits the largest double layer capacitance (CIL∼80 μF cm-2), so that CQgoverns the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (CIL∼6-9 μF cm-2), which governs Cintalready at small potentials. An increase in the total interfacial capacitance is observed at positive voltages for humid TFSI-ILs relative to dry ones, consistent with the presence of a satellite peak. Short-range surface forces reveal the change of the interfacial layering with potential and water uptake owing to reorientation of counterions, counterion binding, co-ion repulsion, and water enrichment. These results are consistent with the charge being mainly stored in a ∼2 nm-thick double layer, which implies that ILs behave as highly concentrated electrolytes. This knowledge will advance the design of IL-graphene-based electrochemical devices.

Original languageEnglish (US)
Pages (from-to)9347-9360
Number of pages14
JournalACS Nano
Volume17
Issue number10
DOIs
StatePublished - May 23 2023

Keywords

  • capacitance
  • electrical double layer
  • force measurements
  • graphene
  • ionic liquids

ASJC Scopus subject areas

  • General Engineering
  • General Materials Science
  • General Physics and Astronomy

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