Platinum electrodes surface diffusion

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

In order to assess the role of the platinum surface structure and of CO surface mobility on the oxidation kinetics of adsorbed CO, we carried out chronoamperometry experiments on a series of stepped platinum electrodes of [n(l 11) x (110)] orientation [Lebedeva et al., 2002c]. If the (110) steps act as active sites for CO oxidation because they adsorb OH at a lower potential than the (111) terrace sites, one would expect that for sufficiently wide terraces and sufficiently slow CO diffusion, the chronoamperometric transient would display a CottreU-hke tailing for longer times owing to slow diffusion of CO from the terrace to the active step site. The mathematical treatment supporting this conclusion was given in Koper et al. [2002]. 

The polarization curve is obtained step by step, at every potential until obtention of a steady-state value. The polarization curve must be identical during forward or backward potential scan. If not, either the steady state has not been obtained, or, more frequently, the surface of the electrode has been modified by the electrochemical reaction. Covering the platinum electrode by a Nafion film reduces the limiting current 71( by the addition of a supplementary diffusion resistance, depending on the thickness of the Nafion film (Figures 1.13 and 1.14). 

Similar processes for producing conducting polymeric films of benzene and its derivatives had been studied earlier [2-4], Necessary conditions for the successful realization of these processes are the use of a platinum electrode and a polar solvent in the presence of catalysts (Lewis acids) and thermostatting of the reactor at -75°C. A poly(para)phenylene polymerizate of the linear structure H-(-C6FLr)n-H with the degree of polymerization n, which varies between 3 and 16, is formed. Forced convection of monomeric molecules facilitates the polymerization reaction in the diffusion layer near the electrode and the formation of a dense film on the electrode surface and prevents the formation of poly(para)phenylene in the bulk. 

Whilst it is agreed that electron transport within polymer supported ferrocene involves ferrocene-ferrocenium electron hopping and requires no participation from the organic framework, recent studies of electron transfer to a substrate in solution indicate that this need not be mediated by iron(II/III) hopping. Thus X-ray photoelectron spectroscopy gave no evidence for unexposed platinum on an electrode coated with polyvinylferrocene, but scannii electron microscopy revealed channels in the polymer layer. It was concluded that the reactant could either diffuse through such channels or pinholes to the electrode surface, or indeed diffuse through the polymer matrix. These possibilities are illustrated in equation (33). 

Upon application of a certain potential across the platinum electrode and a reference electrode inserted in the soil, oxygen is reduced at the platinum surface. The electric current flowing between the electrodes is proportional to the rate of oxygen reduction which in turn is related to the rate of oxygen diffusion to the electrode. The oxygen diffusion rate (ODR) is calculated from the measured electric current according to the following equation. 

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