Several reactions crucial to the design of renewable energy devices such as the O₂ evolution reaction, the O₂ reduction reaction, and the CO₂ reduction reaction require the transfer of multiple protons. These reactions can occur via different mechanisms depending upon the relative rates of these proton transfer events with respect to other steps such as electron transfer. We have developed electrode architectures that enable control over proton transfer kinetics to electrocatalysts such that the selectivity of catalysts is improved. The base of these platforms contains either a metal-centered catalyst attached to an inert electrode with a self-assembled monolayer (SAM) or are simply metal electrodes. The catalysts are then covered with polymer membranes or lipid monolayers modified with aliphatic proton carriers. The kinetics of proton transfer to the catalyst are controlled by changing the hydrophobicity of the membrane. By altering the identity of the SAM, which can be done using a modular azide-alkyne click chemistry approach, electron transfer kinetics to the catalyst can also be modulated.

A large challenge in designing catalysts for both the O₂ and CO₂ reduction reactions is selectivity. We demonstrate that the selectivity of a Cu-based O₂ reduction catalyst can be improved by controlling proton transfer rates such that the catalyst produces only water. The same Cu catalyst generates about 10% H₂O₂ side product when proton transfer is not controlled. We also discuss how altering proton transfer kinetics affects the selectivity of CO₂ reduction catalysts. In particular, a Nafion overlayer on a Cu electrode enables 88% Faradaic efficiency for CH₄ production via the stabilization of metal-bound CO intermediates.

Hanqing Pan

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