Electrochemical Proton-Coupled Electron Transfer of an Osmium Aquo Complex: Theoretical Analysis of Asymmetric Tafel Plots and Transfer Coefficients

摘要

Electrochemical proton-coupled electron transfer (PCET) atnmetal-solution interfaces is significant in a broad range of energyndevices, including photovoltaics and solar fuel cells. Althoughnhomogeneous PCET reactions have been studied extensively,1-6nelectrochemical PCET reactions have only recently started to attractnwidespread interest.6-8 A notable example is PCET of an osmiumnaquo complex attached to a self-assembled monolayer (SAM) onna gold electrode, as depicted in Figure 1. Analysis of the cyclicnvoltammetry data for this process indicates that this reaction occursnby a concerted PCET mechanism,9,10 as supported by a hydrogendeuterium kinetic isotope effect (KIE) of approximately 2 fornpH > 4.9 Kinetic analysis of the reaction pathways suggests thatnwater and OH- are not viable proton acceptors and that the terminalncarboxylate (CO2n-) groups on the long-chain thiols in the SAMnare probable proton acceptors.10 The pKa ≈ 5 of the terminal COOHngroups is consistent with the appearance of the KIE at pH > 4. ThenTafel plots are asymmetric, and the cathodic transfer coefficient atnzero overpotential, RPCET(0), deviates from the standard value ofnone-half.9