Physical and electrochemical characterization of a Cu-based oxygen reduction electrocatalyst inside and outside a lipid membrane with controlled proton transfer kinetics

Abstract

In this manuscript, we analyze the electrochemical behavior of a self-assembled monolayer (SAM) of a Cu-based O2 reduction catalyst. We construct a hybrid bilayer membrane (HBM) by appending a lipid monolayer on top of the SAM and control proton flux to the catalyst by incorporating an alkyl phosphate proton carrier in the lipid layer of the HBM. The HBM platform is interrogated using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) experiments performed as a function of solution pH indicate that the Cu(I)/Cu(II) couple of the catalyst without lipid involves the transfer of one proton per electron. With lipid, however, the number of protons transferred per electron for the Cu(I)/Cu(II) decreases and varies with pH depending upon the structural integrity of the lipid layer. Upon adding the proton carrier to the lipid, both the number of protons transferred during the Cu(I)/Cu(II) redox event and the integrity of the lipid layer increase. Furthermore, we report the pH dependence of the O2 reduction reaction (ORR) as mediated by the Cu catalyst inside the HBM with proton carrier. Taken together, these results provide mechanistic insight into an electrochemical platform that is broadly useful in studying proton-coupled electron transfer (PCET) reactions.