Atomic Force Spectroscopy on Ionic Liquids

Abstract

Ionic liquids have become of significant relevance in chemistry, as they can serve as environmentally-friendly solvents, electrolytes, and lubricants with bespoke properties. In particular for electrochemical applications, an understanding of the interface structure between the ionic liquid and an electrified interface is needed to model and optimize the reactions taking place on the solid surface. As with ionic liquids, the interplay between electrostatic forces and steric effects leads to an intrinsic heterogeneity, as the structure of the ionic liquid above an electrified interface cannot be described by the classical electrical double layer model. Instead, a layered solvation layer is present with a structure that depends on the material combination of the ionic liquid and substrate. In order to experimentally monitor this structure, atomic force spectroscopy (AFS) has become the method of choice. By measuring the force acting on a sharp microfabricated tip while approaching the surface in an ionic liquid, it has become possible to map the solvation layers with sub-nanometer resolution. In this review, we provide an overview of the AFS studies on ionic liquids published in recent years that illustrate how the interface is formed and how it can be modified by applying electrical potential or by adding impurities and solvents.

Spatially Confined Formation of Single Atoms in Highly Porous Carbon Nitride Nanoreactors

“…21−24 As the water content increases, the charge groups in ILs can be effectively separated within a certain range and can selectively adsorb or filter metal ions in aqueous solutions. 25 ILs also can tune the electronic state of the metal SAs, thus enhancing the interaction between SAs and the substrate and benefiting the stability of the SAs. 26 The platinum group metal (PGM), such as Pt, Pd, and Ir, plays a crucial role for a hydrogen evolution reaction (HER), which is closely related to the rising demand for green energy for sustainable development.…”

“…Thus, it is quite necessary to develop an effective alternative method. It has been reported that ionic liquids (ILs) offer a wide range of uses, such as a structure-regulated synthesis of metal nanoparticles and special electrolyte, due to their special properties. − As the water content increases, the charge groups in ILs can be effectively separated within a certain range and can selectively adsorb or filter metal ions in aqueous solutions . ILs also can tune the electronic state of the metal SAs, thus enhancing the interaction between SAs and the substrate and benefiting the stability of the SAs …”

“…The platinum group metal (PGM), such as Pt, Pd, and Ir, plays a crucial role for a hydrogen evolution reaction (HER), which is closely related to the rising demand for green energy for sustainable development. , Although PGMs show excellent HER activity, a large-scale production is not an economically viable option because of the high cost. Atomic utilization efficiency has emerged as an advanced indicator to evaluate the catalytic properties of catalysts, especially for PGM .…”

The Structure of the Electric Double Layer of the Protic Ionic Liquid [Dema][TfO] Analyzed by Atomic Force Spectroscopy

“…Ionic liquids are essentially salt melts consisting of bulky ions; therefore, electrostatic interactions as well as steric effects determine the arrangement of the ions in the bulk phase and at the interface to a catalytic electrode [ 9 , 10 , 11 , 12 ]. Hence, the structure of the electrolyte–electrode interface differs from that of conventional (diluted) aqueous solutions [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. In consequence, the electrochemistry determining the kinetics of the interface processes also differs [ 20 ].…”

“…Instead of a classical aqueous double-layer structure as described by Helmholtz, Gouy-Chapman, and Stern models, respectively, the interface layer of ionic liquids consists of a dense layering of alternating anion and cation layers [ 21 ]. Many experimental studies on the interface structure have been conducted using various methods, with atomic force spectroscopy (AFS) being one of the most direct of these [ 15 ]. By approaching and retracting a tip immersed in the ionic liquid to and from an electrode, a characteristic dense layering has been found with an extension of several nanometers into the bulk [ 22 ].…”