Modulating Proton and Electron Transfer Dynamics in Molecular Electrocatalysis Using Membrane-Covered Self-Assembled Monolayers

Abstract

Many reactions central to the development of renewable energy devices such as the O2 reduction reaction, the O2 evolution reaction, and the CO2 reduction reaction require the transfer of multiple electrons and protons. These reactions proceed by a variety of mechanisms depending upon the relative rates of these transfer events. We are developing electrode frameworks that enable control over proton and electron transfer kinetics to molecular catalysts such that the selectivity of catalysts can be improved. These electrodes contain a transition metal catalyst such as a Cu-triazole or an Fe-porphyrin attached via a self-assembled monolayer (SAM), which is then covered by a proton-permeable membrane. Initial studies have utilized a lipid monolayer modified with aliphatic proton carriers as the proton-permeable membrane. The kinetics of proton transfer to the catalyst are controlled by changing the quantity and identity of proton carriers in the membrane. By altering the identity of the SAM, which can be facilely done using a modular azide-alkyne click chemistry approach, the kinetics of electron transfer to the catalyst can also be controlled. One grand challenge in designing catalysts for both the O2 and CO2 reduction reactions is selectivity. We show that the selectivity of a Cu-based O2 reduction catalyst can be improved by controlling proton transfer rates such that the catalyst produces solely water. In contrast, the same Cu catalyst produces ~10% H2O2 side product when proton transfer is not regulated. We also discuss how modulating proton and electron transfer kinetics affects the selectivity of molecular CO2 reduction catalysts and the performance of Ni-based O2 evolution catalysts.