2, Manufacturing Systems Ltd., Auckland, , New Zealand
Many reactions central to energy conversion processes such as the O2 reduction reaction (ORR), the O2 evolution reaction, and the CO2 reduction reaction involve the transfer of both protons and electrons. We have developed electrochemical platforms that allow for enhanced physiochemical control at the electrode-electrolyte interface and that enable us to quantitatively modulate the kinetics of proton and electron transfer to electrocatalysts. The platforms consist of a molecular electrocatalyst appended to a self-assembled monolayer (SAM) via azide-alkyne click chemistry, which is subsequently covered by a proton-permeable membrane. By changing the length of the alkyl SAM, the kinetics of electron transfer to the catalyst can be controlled. Altering the permeability of the membrane through the use of proton carriers allows us to tune the kinetics of proton transfer to the catalyst.
Enhancing selectivity is a grand challenge for ORR catalysts, which must exclusively produce water in fuel cells. CO2 reduction catalysts must also be selective and should not produce undesirable side products such as H2. We demonstrate that the selectivity of a Cu-based ORR catalyst can be significantly improved by controlling proton and electron transfer rates. Under normal conditions, this non-precious metal ORR catalyst produces ~10% deleterious H2O2 side product. However, with properly regulated proton and electron transfer rates, the catalyst produces exclusively water. We also discuss how modulating proton and electron transfer rates in the membrane-modified electrochemical platform affects the selectivity of molecular CO2 reduction catalysts such as metal porphyrins.