Electroreduction of Dioxygen on Polycrystalline Platinum in Alkaline Solution II. Platinum Surface Modified by Hydrogen Evolution

Abstract

In contrast with what has been reported for acid media, the disk polarization curves obtained by the rotating ring-disk electrode (RRDE) technique reported in the literature for the reduction of O 2 on a smooth polycrystalline platinum (pc-Pt) surface in alkaline solutions show different shapes. [1][2][3][4][5][6][7][8][9][10][11] However, with only one exception (as far as we know), the corresponding ring curves have practically the same overall shape, indicating that the same reduction mechanism operates within the potential range explored. [2][3][4][5][6][7][8][9][10][11] The exception to this behavior was reported by Nekrasov and Myuller 1 and was observed for rotation rates greater than 700 rpm. In these experimental conditions, a maximum of the ring current intensity was found at Ϸ100 mV (hereafter, the potential values are referred to the reversible hydrogen electrode) whereas the more general behavior corresponds to a rising ring current intensity at this potential and to a maximum in the 850-750 mV range. The polarization curves above 700 rpm reported by Nekrasov and Myuller 1 also show a departure from the more frequent, more or less perfect, sigmoidal shape; in fact, a current intensity maximum was found at about 550 mV and a minimum in the 50-100 mV range.These differences in the polarization curves may result from differences in the reduction mechanism or in the state of the electrode surface, as was demonstrated by the extensive work done by the Frumkin school in the sixties and early seventies. However, the real meaning of state of the electrode surface remains to be clarified, and the data in the more recent literature dealing with O 2 reduction on smooth polycrystalline platinum still refer to a platinum surface to which a proprietary pretreatment that is said to assure experimental reproducibility was applied. We decided, therefore, to investigate the Nekrasov platinum surface (N-Pt). The results obtained are reported in this paper. ExperimentalTwo different pc-Pt disk/pc-Pt ring rotating electrodes (RRDE) were used in this work as working electrodes. (i) One of them, with a disk geometric area, A, of 0.1642 cm 2 and a ring collection efficiency, N, of 0.2202, was supplied by Pine Instruments. Its disk surface is mentioned below as surface A or surface C, according to the experimental conditions then indicated. (ii) The other RRDE was made by Oxford Electrodes with N ϭ 0.1995 and A ϭ 0.385 cm 2 ; its disk surface is referred to below as surface B. In both cases, the applied ring potential was 1.08 V. The electrodes were polished with diamond paste to a 0.25 m finish, sonicated in an ultrasonic bath, and carefully cleaned with AnalaR ethanol and Milipore-Q water.Two types of electrochemical cells were used. (i) In the experiments with argon-saturated hydrogen peroxide solutions, a standard three-compartment cell with a Pt-mesh auxiliary electrode separated from the electrolyte solution by a glass frit. (ii) In all the other experiments, a two-compartment cell with a cylindrical, Pt-mesh auxiliary...