Catalytic Activity of Alkali Metal Cations for the Chemical Oxygen Reduction Reaction in a Biphasic Liquid System Probed by Scanning Electrochemical Microscopy

Abstract

Chemical reduction of dioxygen in organic solvents for the production of reactive oxygen species or the concomitant oxidation of organic substrates can be enhanced by the separation of products and educts in biphasic liquid systems. Here, the coupled electron and ion transfer processes is studied as well as reagent fluxes across the liquid|liquid interface for the chemical reduction of dioxygen by decamethylferrocene (DMFc) in a dichloroethane‐based organic electrolyte forming an interface with an aqueous electrolyte containing alkali metal ions. This interface is stabilized at the orifice of a pipette, across which a Galvani potential difference is externally applied and precisely adjusted to enforce the transfer of different alkali metal ions from the aqueous to the organic electrolyte. The oxygen reduction is followed by H2O2 detection in the aqueous phase close to the interface by a microelectrode of a scanning electrochemical microscope (SECM). The results prove a strong catalytic effect of hydrated alkali metal ions on the formation rate of H2O2, which varies systematically with the acidity of the transferred alkali metal ions in the organic phase.

Ionosomes: Observation of Ionic Bilayer Water Clusters

“…This trend is also in line with well-known facts: Li + is a good kosmotrope (i.e., a water structure maker), Na + is only a marginal kosmotrope, and K + and NH 4 + are chaotropes (i.e., water structure breakers) . Interestingly, this effect was also observed for biphasic oxygen reduction reactions. , …”

“…46 Interestingly, this effect was also observed for biphasic oxygen reduction reactions. 39,47 From an energetic perspective, an ionosome functions as an ionic capacitor that stores energy in an electric field. The capacitance can be approximated with the simple formula for a spherical capacitor with a dielectric shown in eq 1: (1)…”

Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction at Polarizable Liquid|Liquid Interfaces

“…Strongly acidic hydrated Li + cation, [96] transferred from an alkaline solution can be also a source of protons participating in reaction (4). This phenomenon was recognised for other alkali metal cations and a correlation between the efficiency of H 2 O 2 generation at DCE|H 2 O interface with DMFc as electron donor and hydration enthalpy of alkali metal cation was found [97] . As formal ion transfer potentials of proton and lithium cation across liquid|liquid interface are very high (for DCE|H 2 O interface are equal 0.549 and 0.591 V respectively) addition of Lewis base – dodecylaniline as lipophilic proton carrier to organic phase decreases this barrier [41] .…”

“…This phenomenon was recognised for other alkali metal cations and a correlation between the efficiency of H 2 O 2 generation at DCE j H 2 O interface with DMFc as electron donor and hydration enthalpy of alkali metal cation was found. [97] As formal ion transfer potentials of proton and lithium cation across liquid j liquid interface are very high (for DCE j H 2 O interface are equal 0.549 and 0.591 V respectively) addition of Lewis base -dodecylaniline as lipophilic proton carrier to organic phase decreases this barrier. [41] A correlation between DMFc + formation efficiency and aniline derivative basicity was found.…”

Unraveling the Link between Catalytic Activity and Agglomeration State with Scanning Electrochemical Microscopy and Atomic Force Microscopy

“…Another approach is using local probe techniques for studying a few amount of materials deposited on a substrate. Particularly, scanning electrochemical microscopy in redox competition mode (RC-SECM) is increasingly used for the local investigation of catalyst electroactivity. − This technique offers the advantage to study the material at a much lower loading than with classical voltammetry. For instance, loadings between 4 and 50 μg·cm –2 are investigated in this paper, while RDE studies of PGM-free materials usually consider loadings around 600 μg·cm –2 . , Working with low loadings enables to use less additives such as a binder and thus to access the intrinsic activity of the material.…”