Electron Transfer in Microemulsion-Based Electrolytes

Abstract

The use of flowing electrochemical reactors, for example, in redox flow batteries and in various electrosynthesis processes, is increasing. This technology has the potential to be of central significance in the increased deployment of renewable electricity for carbon-neutral processes. A key element of optimizing efficiency of electrochemical reactors is the combination of high solution conductivity and reagent solubility. Here, we show a substantial rate of charge transfer for an electrochemical reaction occurring in a microemulsion containing electroactive material is loaded inside the nonpolar (toluene) subphase of the microemulsion. The measured rate constant translates to an exchange current density comparable to that in redox flow batteries. The rate could be controlled by the surfactant, which maintains partitioning of reactants and products by forming an interfacial region with ions in the aqueous phase in close proximity. The hypothesized mechanism is evocative of membrane-bound enzymatic reactions. Achieving sufficient rates of electrochemical reaction is the product of an effort designed to establish a reaction condition that meets the requirements of electrochemical reactors using microemulsions to realize a separation of conducting and reactive elements of the solution, opening a door to the broad use of microemulsions to effect controlled electrochemical reactions as steps in more complex processes.

A Theoretical Framework for the Electrochemical Characterization of Anisotropic Micro‐Emulsions**

“…Additionally, when considering a mixture of a polar and a non‐polar solvent as electrolyte, it is straightforward to envision that in direct proximity to a non‐polar electrode surface (e. g. a simple glassy carbon electrode), the local concentration of the non‐polar phase (and species) will be enhanced. This scenario will be somewhat similar to the recently presented theory of a thick‐film redox‐layer coated electrode, [18] however, complicated by a more or less smooth transition between non‐polar and polar phases which was experimentally observed by Peng [9] from neutron scattering experiments on a hydrophobic silane modified electrode immersed into a ME electrolyte. Owing to this particular anisotropy of the liquid phase, it might be considered that the mass‐transfer of the electrochemically active species in the electrolyte will become a spatially dependent quantity too.…”

“…To overcome these drawbacks, micro‐emulsions (MEs), i. e. a thermodynamically stable mixture of a polar and a non‐polar phase have been proposed as ORFB electrolytes recently [9] . Since MEs can simultaneously act as a polar and a non‐polar solvent, they feature that different oxidation states of the redox species can be dissolved.…”

Distinguishing Adsorbed and Deposited Ionomers in the Catalyst Layer of Polymer Electrolyte Fuel Cells Using Contrast-Variation Small-Angle Neutron Scattering

“… 38 The short-range order in Nafion was expressed by the T–S model, which is applicable for microemulsion systems. 39 where Δρ denotes the SLD difference, f d the domain volume fraction, k /2π the inverse of the characteristic for the domain size, and ξ the correlation length. The correlation function is Next, we combined eqs 9 and 5 and fitted the three partial scattering functions again.…”

A 2.7 V Aqueous Supercapacitor Using a Microemulsion Electrolyte**

“…Therefore, this large extension of the window can be attributed to the formation of an electrode electrolyte interphase (EEI) on the hydrophobic glassy carbon surface (Figure 2). It has been observed previously that the hydrophobic tail of the surfactant arranges itself on the hydrophobic electrode surface [13] . In this surfactant layer the oil phase is concentrated due to interactions between the non‐polar tail of the surfactant and the non‐polar oil phase.…”

“…Recently water‐in‐salt electrolytes have become popular [4–7] but other strategies also exist such as choosing materials with high over‐potentials for the water splitting reactions [8,9] or altering the mass balance of the two electrodes [10–12] . Microemulsions have been used to perform electrochemistry in the past, [13–16] however they have never been pursued in supercapacitors. Microemulsions are thermodynamically stable mixtures of two immiscible solvents, usually water and and ‘oil’, where oil is used to refer to any sufficiently hydrophobic liquid.…”