Cyclic voltammetric and EQCM study of the behaviour and stability of Pt-Ir deposits and Pt-Ir wire electrodes for methanol electrooxidation in acid solution
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The electro-oxidation of methanol at Pt-Ir and Pt deposits, and at Pt, Ir, and Pt-Ir wires in sulfuric acid solution was studied by cyclic voltammetry (CV), and in the case of Pt-Ir and Pt electrodeposits, also with the electrochemical quartz crystal microbalance (EQCM). The effect of repetitive potential cycling on the Pt-Ir deposits, in the absence and presence of methanol, was also investigated by EQCM. Ir did not increase the activity of Pt for methanol oxidation, nor there was a significant effect of potential cycling on the activity of Pt-Ir electrodes. EQCM and CV results showed that potential cycling produced a sintering of the Pt-Ir codeposits without loss of Ir to the solution. XPS analyses of cycled Pt-Ir/Ti electrodes showed that in the absence of methanol the lr/Pt ratio decreased with cycling to one half of its value for the fresh deposit if the upper potential limit was 0.86 V vs. SCE, but increased by 30% if the said limit was increased to 1.1 V, showing the growth of an Ir oxide layer. Quite unexpectedly, cycling up to 1.1 V in the presence of methanol completely eliminated Pt(II) species from the surface, and decreased very much the fraction of lr(IV) species.