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Proton transfer dynamics control the mechanism of O2 reduction by a non-precious metal electrocatalyst

Abstract

Many chemical and biological processes involve the transfer of both protons and electrons. The complex mechanistic details of these proton-coupled electron transfer (PCET) reactions require independent control of both electron and proton transfer. In this report, we make use of lipid-modified electrodes to modulate proton transport to a Cu-based catalyst that facilitates the O2 reduction reaction (ORR), a PCET process important in fuel cells and O2 reduction enzymes. By quantitatively controlling the kinetics of proton transport to the catalyst, we demonstrate that undesired side products such as H2O2 and O2 arise from a mismatch between proton and electron transfer rates. Whereas fast proton kinetics induce H2O2 formation and sluggish proton flux produces O2, proton transfer rates commensurate with O–O bond breaking rates ensure that only the desired H2O product forms. This fundamental insight aids in the development of a comprehensive framework for understanding the ORR and PCET processes in general.

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Figure 1: Schematic of a HBM that controls the thermodynamics and kinetics of protons and electrons.
Figure 2: Electrochemistry of an O2 reduction catalyst under varying regimes of proton transfer kinetics.
Figure 3: Control of proton transfer kinetics alters the pathways and product speciation of O2 reduction.
Figure 4: Kinetic isotope effects.
Figure 5: Regulation of proton transfer kinetics induces mechanistic changes of O2 reduction, which dictates the selectivity of products.

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Acknowledgements

E.C.M.T. acknowledges a Croucher Foundation Scholarship. C.J.B. acknowledges a National Science Foundation Graduate Research Fellowship (NSF DGE-1144245) and a Springborn Fellowship. We thank the US Department of Energy (DE-FG02-95ER46260) for support of this research. This work was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, which are partially supported by the US Department of Energy (DE-FG02-07ER46453 and DE-FG02-07ER46471). We acknowledge J. Varnell for providing PANI-Fe-C, E. Barile for help with artwork, and R. Gennis, T. Rauchfuss, K. Suslick and R. Nuzzo for insightful discussions.

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E.C.M.T., C.J.B., S.C.Z., A.H. and A.A.G. designed the experiments. E.C.M.T., C.J.B. and N.A.K. performed the experiments. Y.L. synthesized BTT. E.C.M.T., C.J.B., Y.L., J.P.G., S.C.Z., A.H. and A.A.G. wrote the paper. E.C.M.T., C.J.B., N.A.K., J.P.G., A.H. and A.A.G. analysed the data. All authors discussed the results and commented on the manuscript.

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Correspondence to Andrew A. Gewirth.

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The authors declare no competing financial interests.

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Tse, E., Barile, C., Kirchschlager, N. et al. Proton transfer dynamics control the mechanism of O2 reduction by a non-precious metal electrocatalyst. Nature Mater 15, 754–759 (2016). https://doi.org/10.1038/nmat4636

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