Using a combination of simulation and experimental techniques, researchers at the University of Illinois Urbana-Champaign Material Science and Engineering were able to identify the electrical double layer structure of an ionic liquid on a series of crystalline electrodes.
The study, released Oct. 5th in the Journal of Physical Chemistry Letters, combined molecular dynamics (MD) simulations and 3D electrochemical atomic force microscopy (EC-3D-AFM) to gain a more complete understanding of the solid-liquid interface.
MatSE professor Yingjie Zhang and graduate student Kaustubh Panse, who worked with a group from the University of Texas at Austin to perform the MD simulations, recently adapted a well-known technique, AFM, to develop EC-3D-AFM, the AFM measurements performed in an electrochemical cell to study changes in surface morphology during electrochemical reactions.
Understanding the boundary between solids and liquids (the solid-liquid interface) is important to understanding how many material systems, such as batteries and supercapacitors, work. The electrical double layers (EDLs) at this interface, also called the solvation layer, refer to parallel layers of charge surrounding the solid. As Panse puts it, “EDLs are liquid molecular layers at the interface, and they form because the underlying solid affects the overlying liquid due to the interfacial effect and other interactions.”
Understanding the EDL helps scientists determine how a system works, but it’s difficult. Deducing EDL structure can be difficult due to intermolecular (between molecules) and molecule-electrode interactions.
In this research, the team used ionic liquids (a salt in the liquid state) used in various electrochemical applications such as batteries, supercapacitors and electrolysers. However, ionic liquids have not been studied as extensively as aqueous solutions. Panse says the goal was to develop a fundamental theory of how these ionic liquids are arranged at the interface. He also explains that “the process that takes place at the interface determines the overall result: if molecules lie flat along the interface, it’s possible that this can give much more capacitance than if the molecules lie perpendicular. The molecular alignment really impacts the overall capacity and the overall performance of the system.”
1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) was used as the ionic liquid system. EMIM-TFSI is a model system used in supercapacitors and energy storage devices due to its high capacitive charging window. The team studied this ionic liquid on a series of crystalline electrodes: molybdenum disulfide (MoS2) flakes and highly oriented pyrolytic graphite (HOPG). These crystalline electrodes differ in their electronic properties, meaning that if ion association is similar on both systems, these results can be attributed to EDL rather than electrode interaction. The ionic liquid and the electrodes were chosen because they are well-known and well-studied systems that allow for a fundamental study of the EDL of an ionic liquid. During the EC-3D AFM experiments, how the system behaved at different potentials was measured and compiled to get a complete picture of the EDL.
The team observed a strong association and intermolecular interaction between cations (EMIM+) and anions (TSFI–) from the MD simulations. The simulation considered all possible pairs of species in the ionic liquid (EMIM+-EMIM+TFSI–-TFSI–EMIM+-TFSI–) in the first EDL of EMIM-TFSI/MoS2. The results revealed a strong intermolecular interaction between cations (EMIM+) and anions (TFSI–), in contrast to the weaker EMIM+-EMIM+ and TFSI–-TFSI– interactions. Hence, they propose that the cation-anion association structure of the innermost layer is the key descriptor of the EDL. The simulated EDL structure of EMIM-TFSI on both electrodes was “surprisingly similar”, suggesting that electrode-specific interactions are much weaker than the effects of intermolecular interactions between the ionic species.
These theoretical predictions were confirmed with EC-3D AFM experiments. They observed a similar response across a range of systems, so “it is very likely that these descriptors can be applied to a wide range of electrodes, electrolytes and different electrochemical systems,” says Panse. “We hope that they will serve as a good rational design guide for various electrochemical systems in the future.”
Other authors of the article are Dr. Narayana R. Aluru (Professor of Mechanical and Computational Engineering & Sciences, UT Austin), Dr. Haiyu Wu (postdoc, Mechanical and Computational Engineering & Sciences, UT Austin, co-first author with Kaustubh Panse), Dr. Shan Zhou (Postdoc, Materials Science and Engineering and Materials Research Laboratory, UIUC) and Fujia Zhao (PhD student, Materials Science and Engineering and Materials Research Laboratory, UIUC)