Initial Anodic Growth of Oxide Film on Platinum in 2N    H 2 SO 4 under Galvanostatic, Potentiostatic, and Potentiodynamic Conditions: The Question of Mechanism

Abstract

Potentiodynamic, galvanostatic, and hybrid galvano‐potentiostatic experiments were conducted in O2 saturated 2N H2SO4 on a polished platinum disk electrode. The results show conclusively that for coverage between one and three monolayers the law of anodic growth of oxide film on platinum is precisely the same regardless of the mode of growth (i.e., potentiostatic, galvanostatic, or potentiodynamic). A correspondence is analytically established between galvanostatic and potentiodynamic data, and this correspondence is verified experimentally. Galvanostatic and potentiodynamic data both show that the growth of the oxide film cannot be explained by any mechanism for which false(∂V/∂q)normali is independent of i (e.g., adsorption). The high‐field ionic conduction model accounts for the major features of all the data. A comparison of galvanostatic and hybrid galvano‐potentiostatic experiments shows that there is no aging of the film over a time interval, normalΔt , comparable to the duration of the experiments false(20sec≤normalΔt≤150secfalse) . The effect of “holding” (i.e., interrupting the potential sweep and maintaining a constant potential for a time) during potentiodynamic experiments is explained entirely in terms of the potentiostatic growth of the oxide film during the holding period.

A model for anodic hydrous oxide growth at iridium

“…Thus, one must conclude that several layers of some other form of oxide (the inner oxide) were produced and reduced back to the metal during each of these cycles. This inner oxide growth is presumably similar to oxide growth at Pt [32][33][34][35][36] and probably occurs by a place-exchange mechanism [32]. A difference from the behaviour of Pt is that no well-defined reduction peak can be obtained for the inner oxide at Ir.…”

A Study of the Transition from Oxide Growth to O 2 Evolution at Pt Electrodes in Acid Solutions

“…The dependence of V on q calculated in this way in the nonlinear Vii region is quite different from that in the initial linear Vii region and can be represented by Eq. [8], rather than by Eq. [2] OV --m = 270 VC-l-cm ~ ~ f (/disk, q) [8] aq…”

“…Once O~ evolution becomes the predominant reaction, its rate as a function of q and V can be derived from the V/q relationships represented by Eq [8] and [9] and shown in Fig. 5 and 6.…”

“…Following this region, the electrode potential increases fairly linearly with time (Fig. 1) with the current being used for further growth of the oxide film (8). Eventually at higher potentials, oxygen evolution begins and soon becomes the major electrode reaction.…”

“…Also, when io2 >> iog, as in the second apparently linear V/q region, dV/dq must also become virtually constant and equal to m (Eq. [8]).…”