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

Abstract

Protic ionic liquids are promising electrolytes for fuel cell applications. They would allow for an increase in operation temperatures to more than 100 °C, facilitating water and heat management and, thus, increasing overall efficiency. As ionic liquids consist of bulky charged molecules, the structure of the electric double layer significantly differs from that of aqueous electrolytes. In order to elucidate the nanoscale structure of the electrolyte–electrode interface, we employ atomic force spectroscopy, in conjunction with theoretical modeling using molecular dynamics. Investigations of the low-acidic protic ionic liquid diethylmethylammonium triflate, in contact with a platinum (100) single crystal, reveal a layered structure consisting of alternating anion and cation layers at the interface, as already described for aprotic ionic liquids. The structured double layer depends on the applied electrode potential and extends several nanometers into the liquid, whereby the stiffness decreases with increasing distance from the interface. The presence of water distorts the layering, which, in turn, significantly changes the system’s electrochemical performance. Our results indicate that for low-acidic ionic liquids, a careful adjustment of the water content is needed in order to enhance the proton transport to and from the catalytic electrode.

Relaxation Times of Ionic Liquids under Electrochemical Conditions Probed by Friction Force Microscopy

“…This effect is regularly reported in literature and attributed to a decrease in stiffness of the layers as a function of distance from the charged substrate. [ 4,9,14 ] This reflects a slightly compressible structure and is often observed for intermediate charged surfaces, such as mica, [ 9 ] or less strong applied potentials. [ 4,14 ]…”

Microstructural probing of phosphonium-based ionic liquids on a gold electrode using colloid probe AFM

“…2 For example, the processes of energy storage and release are closely related to the EDL structure in IL-based supercapacitors, which is composed of an inner tightly bound ion layer and an outer loose layer with alternating cations and anions. [3][4][5][6][7] The formation of either a multilayer alternating cation and an anion structure or a distinctive dense counterion monolayer in ILs across the longitudinal direction to the surface has been confirmed by both experimental 4,[6][7][8][9][10][11] and theoretical [12][13][14][15][16] studies and suggested to play a crucial role in the capacitive performance at IL-electrode interfaces. 16,17 The capacitance of ILs at the electrode surface is reported to be determined by, and reflects, the microstructural changes within the innermost region of the EDL.…”

“…26 The AFMobtained contact resonance frequency provides a good measure of the contact stiffness, 27,28 reflecting on the IL microstructural changes, in which a higher contact stiffness is indicative of more ordered ion layers 29 associated with a higher rupture force. 10 Thus, in this work, we utilized a gold colloid probe to model an electrode surface to directly capture the contact resonance frequency of the ILs at the gold electrode surface at varying biased voltages, further discerning the interfacial ion microstructure at charged surfaces. We considered two ILs with cations sharing the same cation head but possessing varying chain lengths, i.e.…”

A 2023 update on the performance of ionic-liquid proton-exchange-membrane fuel cells

“…For a summary of modern studies of the IL leaching (seepage) problem, we recommend the review of Ebrahimi et al (2021), in which their Section 4 and Table 3 deals specifically with the leaching issue. For a summary of modern studies of IL effects on ORR kinetics, we recommend the short review of Wippermann et al ( 2022) and add to this the mention of a newer paper by Korte et al, which noticed, using atomic force microscopy, an altered double-layer structure near a Pt (100) surface when doping [dema][TfO] with water (Rodenbücher et al, 2021).…”