Properties of Pt electrodes investigated by the Electrochemical Quartz Crystal Microbalance




Wang, Tao

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The Electrochemical Quartz Crystal Microbalance (EQCM) was used as the main investigation tool coupled with other conventional electrochemical methods to study the electrocatalytic properties of polycrystalline Pt electrodes, including two separate projects. The first project studied the early stage of oxide film formation on the Pt surfaces and the inhibition of the catalytic properties by the oxide film. The inhibition of the fast electrode reaction of small molecules by the growth of oxide film allows those molecules to be used as probes for the nature of the oxide film. The hydrogen oxidation current, jox calculated by differencing the cyclic voltammetry currents with and without H₂ present showed a characteristic plateau-to-plateau profile, which implies a transition from the free Pt surface to the Pt surface completely covered by oxide film. This method allows determination of the onset potential for oxide formation and also the critical potential where a full monolayer of oxide is formed. This method applies to other fast surface reactions such as oxygen reduction reaction (ORR), and the results are enhanced by forced convection in the rotating disk electrode (RDE) experiments. The initial oxidation species was identified by charge and EQCM frequency analysis. Our results support the formation of a species with stoichiometry Pt₂O, for example, with an oxygen atom in the bridging position between two adjacent Pt atoms. In the second project, the stability of the Pt electrodes in acid media with Ag⁺ present was investigated. A substantial frequency drift (8.3 Hz cycle⁻¹, or 44 ng cm⁻² cycle⁻¹) was observed during Ag electrodeposition and stripping on the bare polycrystalline Pt surface. Cyclic voltammograms in pure HClO₄ solution showed nearly no frequency drift while the addition of 10⁻³ mol L⁻¹ Ag⁺ resulted in an immediate and characteristic frequency drift. The frequency drift appeared to be consistent with loss of material from the electrode surface and the ICP-MS detected a maximum Pt concentration of 2.3×10⁻⁶ mol L⁻¹ in solution due to Pt dissolution. The Pt concentration calculated from the EQCM frequency drift matched the ICP-MS results. This allowed the EQCM for direct investigation of Pt dissolution at different system temperatures, sweep rates, and potential ranges. The much higher rate of dissolution with Ag present than that in pure HClO₄ solution can be explained by the formation of Pt-Ag alloy during Ag underpotential deposition and the co-dissolution of Pt and Ag.



Electrochemistry, Pt dissolution, surface coverage, electrodeposition, HOR, ORR, Pt electrodes, oxide film, EQCM