Potential‐Modulated Ion Distributions in the Back‐to‐Back Electrical Double Layers at a Polarised Liquid|Liquid Interface Regulate the Kinetics of Interfacial Electron Transfer

Abstract Biphasic interfacial electron transfer (IET) reactions at polarisable liquid|liquid (L|L) interfaces underpin new approaches to electrosynthesis, redox electrocatalysis, bioelectrochemistry and artificial photosynthesis. Herein, using cyclic and alternating current voltammetry, we demonstrate that under certain experimental conditions, the biphasic 2‐electron O2 reduction reaction can proceed by single‐step IET between a reductant in the organic phase, decamethylferrocene, and interfacial protons in the presence of O2. Using this biphasic system, we demonstrate that the applied interfacial Galvani potential difference Δowφ provides no direct driving force to realise a thermodynamically uphill biphasic IET reaction in the mixed solvent region. We show that the onset potential for a biphasic single‐step IET reaction does not correlate with the thermodynamically predicted standard Galvani IET potential and is instead closely correlated with the potential of zero charge at a polarised L|L interface. We outline that the applied Δowφ required to modulate the interfacial ion distributions, and thus kinetics of IET, must be optimised to ensure that the aqueous and organic redox species are present in substantial concentrations at the L|L interface simultaneously in order to react.

. Summary of the reduction half-reactions of (i) various aqueous, organic or interfacial O2 or proton reduction reactions and (ii) various organic electron donor species, described in Figures 1a and b, and their associated standard redox potentials (expressed versus the standard hydrogen electrode (SHE)) as a function of pH.
Reduction Half-Reactions Standard redox potentials (V) pH Ref.
DcMFc +,TFT + e -⇌ DcMFc TFT [a] [ DcMFc + /DcMFc 0 ] SHE TFT 0.107 n/a [60] PMFc +,TFT + e -⇌ PMFc TFT   [a] Note that the redox potential of the DcMFc + /DcMFc redox couple is barely affected by the solvent compositions and can be considered constant in the mixed solvent region. Thus, DcMFc should be considered a superior redox couple for studying solvent effects on the thermodynamics of electron transfer S2 reactions at aqueous|TFT interfaces. [54] In contrast, the standard redox potentials of DiMFc and PMFc may be affected by the composition of the mixed solvent layer.  Table S2. Summary of the interfacial electron transfer (IET) reactions for biphasic ORRs (2eor 4epathways) with various organic electron donors, described in Figure 1c and Figure S2a, and their associated standard Galvani IET potentials ( o w  IET 0 ) as a function of pH.  Table S3. Summary of the IET reactions for biphasic reduction of aqueous (H3O + ) or interfacial [H + …TB -] protons with various organic electron donors, described in Figure 1d and Figure S2b, and their associated standard Galvani IET potentials ( o w  IET 0 ) as a function of pH.   Table S1)   Tables S2 and S3, respectively. Figure S3. CVs obtained in the presence (solid) and absence (dashed) of 500 M DiMFc at pH 0.55, 7.00 and 11.87, respectively. All CVs were obtained at a scan rate of 20 mV·s -1 using Electrochemical Cells 1 (for pH 0.55), 2 (for pH 7.00) and 3 (for pH 11.87), respectively, under aerobic, ambient conditions (see Scheme 2). The compositions of the aqueous and organic phases for each electrochemical cell are further noted in each panel. Figure S4. CVs obtained in the presence (solid) and absence (dashed) of 500 M PMFc at pH 0.55, 7.00 and 11.87, respectively. All CVs were obtained at a scan rate of 20 mV·s -1 using Electrochemical Cells 1 (for pH 0.55), 2 (for pH 7.00) and 3 (for pH 11.87), respectively, under aerobic, ambient conditions (see Scheme 2). The compositions of the aqueous and organic phases for each electrochemical cell are further noted in each panel.      Figure S10. (b) The slope of the total w curve is the differential capacitance.

IET reaction Standard Galvani IET potential (V) pH
The description of the dissociation of sulfuric acid into protons (subscript H), bisulfate ions (subscript 1) and sulfate ions (subscript 2) has to take into account the strong interactions between the ions. The dissociation equilibrium of bisulfate ions can be described by  Table S4. A list of aqueous and organic soluble redox species that could fulfil the roles of the redox species in the majority of the panels shown in Figure 10. This table is not an exhaustive list, for example excluding photo-induced biphasic IET reactions, and simply contains representative examples.  Figure S11. Plot of the 1 H NMR spectrum of pentamethylferrocene. Figure S12. Plot of the 13 C{ 1 H} NMR spectrum of pentamethylferrocene. Figure S13. Plot of the 1 H-13 C{ 1 H} HMBC spectrum of pentamethylferrocene.

Supporting references
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