The correction for electrode oxidation during the anodic estimation of adsorbate coverage on smooth Pt electrodes

Abstract

Summary “Reduced CO 2 ” adsorbed on smooth Pt electrodes from I M H 2 SO 4 solutions at 40° has been used as a model system to investigate the anodic charging methods of estimating adsorbates. Three methods have been compared: the usual “direct” methood and two variation of the previously-developed current-reversal method. In the direct method, the charge passed to O 2 -evolution during an anodic galvanostatic transient increases with decrease of the current density, i a , both in the presence of adsorbate (Q A ) and for the background electrolyte (Q B ). As is often found, the difference (Q A -Q B ) is independent of i a (I-500 mA/r. cm 2 ). This suggests complete elimination of electrode oxidation effects and correct estimation of the adsorbate, Q ads . Current reversal, with low i a to oxidize the adsorbate, and high i c to estimate the “end point” of total electrode cleanliness from the adsorbate, is an independent method of adsorbate analysis because no electrode oxidation occurs. Use of this method with i a −100 μA/r. cm 2 shows that Q ads is equal to (Q A -Q B ) and that the direct method is appropriate for adsorbate analysis. The current-reversal technique is more tedious but more accurate than the direct method. The current-reversal technique was also investigated for higher values of i a (10–20 mA/r.cm 2 ) where electrode oxidation occurs before reversal. By using appropriate capacity data, good elimination of electrode oxidation may be made by direct subtraction of the charge found for oxide reduction. The agreement with the current-reversal method without electrode oxidation is within experimental error but there is apparently a small systematic over-correction for electrode oxidation. This is not sufficient to vitiate estimates of Q ads , and allows use of the current-reversal technique in the presence of complicating effects from solution reactions during the anodic stripping. The useful ranges of experimental application of these different methods for estimating adsorbate coverage are discussed.