The resting-state of the Pt-black electrode in acid solution and the structure of the adsorption layer. Coulometric and electrochemical quartz crystal microbalance measurements

摘要

The properties of Pt-electrodes in 1 M HClO_4 solution, in air, were studied both in the resting-state and mild oxidation conditions, within the potential range of 1000 mV and 1600 mV. Cyclic voltammetry (CV) and cathodic stripping voltammetry (CSV) were used for coulometric determination of the amount of O-containing species adsorbed on the electrode surface during the oxidation-reduction reactions. Simultaneously, the mass of the adsorption layer was measured in situ, with the electrochemical quartz crystal microbalance (EQCM). The surface of the Pt-black electrode in the resting-state is covered with OH~*-species (apparently a ML), which are spontaneously generated by immersion in the aqueous solution without any polarization potential. The surface of the Pt smooth electrode is covered uppermost by a sub-monolayer of oxygen, in similar conditions. The resting-state of the Pt-black electrode is characterized by a reproducible value of the rest potential, E_r = 1040 ± 5 mV (average of 10 samples) and a reduction peak in CV mode at E'_p = 791 ± 7 mV (average of 10 samples). In anodic polarization conditions, PtOH~* is oxidized to PtO~*, in CV mode, and to Pt(O_2)~*, in the potentiostatic mode. The reduction peak of PtO~* in CV mode is E"_p = 694 ± 9 mV (average of 10 samples), indicating a stronger bond to the surface of the electrode. The yielding of Pt(O_2)~* is not fast enough to accommodate the increase of the oxidation potential by finite sweep rate of the CV mode at 10 mV s~(-1). Therefore this oxidation level could not be achieved in CV mode. The prolonged oxidation in the potentiostatic mode results in the coverage of the surface of Pt-black electrode with a monolayer of (O_2)~*-species.