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A square-wave potentaat anodizing procedure to obtain noble metal surfaces with both a controlled roughness and a reproducable electrochemacal response is presented. The influence of the charactenstacs of the square-wave potentaal perturbauon are systematxcally stu&ed. The mo&ficataon of the surface takes place through the formation of a hydrated platinum oxade layer wluch is later electroreduced. The mechamsm of growth of the hydrated oxide layer under both square-wave potentml perturbation and potentaostatac conditions ~s critically &scussed m terms of a complex sandwich-type structure.

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The effect of the hydrogen diffusion on the hydrogen electrode reaction has been studied and rigorous kinetic expressions for the Tafel-Heyrovsky-Volmer mechanism has been derived. The analysis of the dependences of the current density ( j) on overpotential (Z), particularly oriented to the hydrogen oxidation reaction (hor), leads to the following main results: (i) in the Tafel-Volmer (TV) route, the current density reaches a maximum value ( j max ) less or equal to the limiting diffusion current density (j L ); (ii) j max is always equal to j L for the HeyrovskyVolmer (HV) route; (iii) in the simultaneous occurrence of both routes (THV), the current density always reaches the j L value, although in the range of overpotentials of applied interest (0 r Z/V r 0.6) the j max value, characteristic of the TV route, can also be obtained.The Levich-Koutecky plots have also been analysed and it has been demonstrated that the j(Z) dependence for the hor under activated control cannot always be obtained from these plots.

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