Controlling Proton and Electron Transfer Rates to Enhance the Activity of an Oxygen Reduction Electrocatalyst

Abstract

An electrochemical approach is developed that allows for the control of both proton and electron transfer rates in the O reduction reaction (ORR). A dinuclear Cu ORR catalyst was prepared that can be covalently attached to thiol-based self-assembled monolayers (SAMs) on Au electrodes using azide-alkyne click chemistry. Using this architecture, the electron transfer rate to the catalyst is modulated by changing the length of the SAM, and the proton transfer rate to the catalyst is controlled with an appended lipid membrane modified with proton carriers. By tuning the relative rates of proton and electron transfer, the current density of the lipid-covered catalyst is enhanced without altering its core molecular structure. This electrochemical platform will help identify optimal thermodynamic and kinetic parameters for ORR catalysts and catalysts of other reactions that involve the transfer of both protons and electrons.

Brønsted Acid Scaling Relationships Enable Control Over Product Selectivity from O2 Reduction with a Mononuclear Cobalt Porphyrin Catalyst

“…Efforts to control ORR product selectivity with molecular catalysts include diverse catalyst classes, including Fe, − Cu, , and Mn − complexes. New catalyst designs commonly feature multinuclear metal complexes ,,− ,,, or implement other approaches to control the relative rates of electron − and/or proton transfer ,,,, as a means to facilitate O–O cleavage and avoid H 2 O 2 generation. Elucidation of the factors that contribute to product selectivity are complicated, however, by the manner in which these catalysts are studied.…”

Four-Electron Electrocatalytic O2 Reduction by a Ferrocene-Modified Glutathione Complex of Cu

“…We view using ferrocene to accelerate electron transfer as a complementary approach to previous studies in which the rates of proton transfer were decreased to dinuclear Cu ORR catalysts using membranes ,. In these studies, the slower proton transfer rates are thought to decrease the rates of protonating the Cu–O–O–Cu intermediate, thus avoiding the production of H 2 O 2 (red box).…”

“…44 We view using ferrocene to accelerate electron transfer as a complementary approach to previous studies in which the rates of proton transfer were decreased to dinuclear Cu ORR catalysts using membranes. 43 , 45 In these studies, the slower proton transfer rates are thought to decrease the rates of protonating the Cu−O−O−Cu intermediate, thus avoiding the production of H 2 O 2 (red box). Here, with a ferrocenemodified Cu ORR catalyst, the opposite approach is taken whereby faster electron transfer rates favor O−O bond breaking (blue box), which also avoids H 2 O 2 production and yields a catalyst that selectively produces H 2 O.…”

Influence of pH and Proton Donor/Acceptor Identity on Electrocatalysis in Aqueous Media

“…21,75 An elegant way of controlling the rate of proton transport based on lipid-modified self-assembled monolayers (SAMs) was reported by Barile and co-workers. 76 Building on earlier work by Gewirth and co-workers, 77 they used thiol-based SAMs to link a molecular ORR catalyst to an Au electrode and blanketed the assembly with a lipid membrane. By adjusting the length of the SAM linkers, they controlled the distance between electrode and catalyst, which provides control over the electron-transfer rate.…”

“…An elegant way of controlling the rate of proton transport based on lipid-modified self-assembled monolayers (SAMs) was reported by Barile and co-workers . Building on earlier work by Gewirth and co-workers, they used thiol-based SAMs to link a molecular ORR catalyst to an Au electrode and blanketed the assembly with a lipid membrane.…”