Reducción electroquímica de oxigeno en electrodos de platino

F. RODRIGUEZ NIETO; M. A. PASQUALE; V. BRUNETTI; A.J.ARVIA

Medellín, Colombia

Congreso; XVIII Congreso de la Sociedad Iberoamericana de Electroquímica; 2008

Sociedad Iberoamericana de Electroquímica

Platinum supported on carbon substrate cathodes are widely used electrocatalysts in fuel cell processes. Recently, the relevance of platinum clustering in electrocatalysis was emphasized [1]. The electrcatalytic activity of these electrodes was related to the characeristics of the cluster size distribution. However, this matter is still under discussion to establish whether either cluster size and shape or cluster-cluster or cluster substrate interactions become the most relevant characteristics to be considered in architectural electrode design for attaining a high catalytic efficiency, particularly for the molecular oxygen electroreduction reaction (OERR).

This work reports the preparation of cathodes consisting of platinum electrodeposits in the range 9.6 £ q_Pt £ 96 mC cm^-2 on either HOPG or vitreous carbon substrates by applying either a repetitive triangular potential or a square wave potential cycling. The topographic features of both types of cathodes were determined from scanning tunnling microscopiy imaging. The ring-disk-rotating electrode (DRRE) technique was used to study the molecular oxygen electroreduction reaction (OEER), employing a vitreous carbon disk and a platinum ring, in aqueous 1 M sulfuric acid at room temperature.

The platinum cluster distribution of each type of electrode was evaluated from scanning tunneling imaging utilizing the SPIP software (Image Metrology, Denmark).

The OERR polarization curves in aqueous 1 M sulfuric acid under 1 atm oxygen partial pressure show an irreversible surface kinetics in the range 0.98–0.85 V, and a diffusion-controlled process for E < 0.75 V, irrespective of q_Pt. Seemingly, the onset potential for the OERR increases with the HOPG surface coverage by platinum. For platinum on vitreous carbon the cathodic Tafel slope and the quantity of H₂O₂ produced in the reaction depend on q_Pt , and to a minor extent on the potential routine used for preparing the electrodeposits.

Results indicate that the increase in the electrocatlytic activity for OEER in the acid solution is consistent with an increase in the affinity of the platinum surface for oxygen that favours a relevant role of the Pt-O interactions in the OEER reaction pathway, as it has been described recently for the OERR on nano-structures of palladium meso-particles on HOPG [1].

The adsorbed platinum on carbon implies a change in the electronic configuration of platinum atoms and clusters. For lowest values of q_Pt the rate process appears to be also influenced by substrate platinum interactions.

[1] A. Hernández Creus, Y. Gimeno, P. Díaz, L. Vázquez, S. González, R.C. Salvarezza, and  A. J. Arvia, J. Phys. Chem. B 108 (2004) 10785-10795.