Examing Proton Coupled Electron Transfer, Oxygen Reduction, and Anion Diffusion Using a Hybrid Bilayer Membrane

In this talk, we report on our work synthesizing and characterizing Cu-based catalysts for the oxygen reduction reaction.  In particular we prepared the first synthetic Cu O₂ reduction catalyst that forms a self-assembled monolayer (SAM) on Au surfaces. We embeded this Cu catalyst inside a hybrid bilayer membrane (HBM) by depositing a monolayer of lipid on the SAM. By incorporating an alkyl phosphate in the lipid layer, we develop the first HBM containing a reversible switch that gates proton transport to a molecular oxygen reduction reaction (ORR) catalyst. We envision that this strategy can give unique mechanistic insight not only to the ORR, but to all proton-coupled electron transfer (PCET) reactions.

We also use the hybrid bilayer membrane as an electrochemical platform to study anion diffusion through a lipid monolayer. We first append lipid on a self-assembled monolayer that contains a covalently-bound Cu(I)/(II) redox center. We then perform cyclic voltammetry using different anions in the bulk solution and extract thermodynamic and kinetic information about anion transport. We analyze the results using linear combinations of fundamental chemical trends, and determine that anion transport quantitatively correlates to polarity and basicity, a relationship we formalize as the lipid permeability parameter. In addition, we discuss how our findings can be interpreted according to the two leading mechanisms describing ion permeability through lipids. Our results demonstrate that anion transport in a HBM is best described by the solubility-diffusion mechanism, not the pore mechanism.