The Solvent Effect on H₂O₂ Generation at Room Temperature Ionic Liquid|Water Interface

Abstract

H2O2 is a versatile chemical and can be generated by the oxygen reduction reaction (ORR) in proton donor solution in molecular solvents or room temperature ionic liquids (IL). We investigated this reaction at interfaces formed by eleven hydrophobic ILs and acidic aqueous solution as a proton source with decamethylferrocene (DMFc) as an electron donor. H2O2 is generated in colorimetrically detectable amounts in biphasic systems formed by alkyl imidazolium hexafluorophosphate or tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids. H2O2 fluxes were estimated close to liquid|liquid interface by scanning electrochemical microscopy (SECM). Contrary to the interfaces formed by hydrophobic electrolyte solution in a molecular solvent, H2O2 generation is followed by cation expulsion to the aqueous phase. Weak correlation between the H2O2 flux and the difference between DMFc/DMFc+ redox potential and 2 electron ORR standard potential indicates kinetic control of the reaction.

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

“…[42,90] The observed solvent effect on H 2 O 2 flux close to liquid j liquid, detected by SECM indicates that ORR is not controlled by transport of electron donor towards liquid j liquid interface as no correlation with reciprocal viscosity of the solvent is observed (Figure 6). [93] However, there is an indication of kinetic control of 2e ORR at IL j H 2 O interface as expected from Butler-Volmer equation (Figure 7), when a relatively high concentration of DMFc is kept by electrochemical recycling. [93] One can design a biphasic system in a form of carbon paste with 2-nitrophenyloctyl ether (NPOE) or IL as a binder in contact with an aqueous electrolyte solution [101,102] Then, mechanical stability of organic phase is secured and is electrically conductive due to dense network of percolation paths formed by carbon particles as in carbon paste electrode (CPE).…”

“…[93] However, there is an indication of kinetic control of 2e ORR at IL j H 2 O interface as expected from Butler-Volmer equation (Figure 7), when a relatively high concentration of DMFc is kept by electrochemical recycling. [93] One can design a biphasic system in a form of carbon paste with 2-nitrophenyloctyl ether (NPOE) or IL as a binder in contact with an aqueous electrolyte solution [101,102] Then, mechanical stability of organic phase is secured and is electrically conductive due to dense network of percolation paths formed by carbon particles as in carbon paste electrode (CPE). [103] As carbon particles are covered by NPOE or IL there is a short distance between carbon and liquid j liquid interface.…”

“…[110] Other approach to electrogeneration of electron donor is emplyment of carbon paste with viscous organic phase (NPOE [91] or hydrophobic IL. [92,93] It brings mechanical stability to the organic phase and allows to decrease its volume providing favorable conditions for electrochemical recycling. However, this strategy may not be useful for HER, because of difficult removal of oxygen from organic binder.…”

“…[42] The latter mechanism cannot be excluded for ORR at IL j H 2 O interface, in the absence of common anion in both phases. [92,93] The kinetics of ORR depends on the redox potential of the electron donor and on the solvent. For example, reactions ( 6) and ( 7) are faster than analogous reactions with weaker electron donor À 1,1'-dimethylferrocene (DFc).…”

“…Therefore, one can electrochemically recycle electron donor. [91][92][93] and increase few times H 2 O 2 generation efficiency (reaction (4) (see Figure 6).…”