Sequential and simultaneous electrodeposition of Pt–Ru electrocatalysts on a HOPG substrate and the electro-oxidation of methanol in aqueous sulfuric acid

Abstract

The catalytic activity for methanol electro-oxidation of Pt–Ru electrodeposits produced either sequentially or simultaneously on activated highly ordered pyrolytic graphite (HOPG) substrates using linear potential electrodeposition techniques was investigated. The voltammetric behavior of electrodeposits in 0.5 M aqueous sulfuric acid was compared to that of Pt electrodeposits on HOPG in the hydrogen electroadsorption–desorption and early stages of electro-oxidation potential regions. Morphological and microscopic surface characterization carried out with scanning electron microscopy (SEM) and atomic force microscopy (AFM), showed that the deposits consist of a number of aggregates. Scanning tunneling microscopy (STM) showed rounded clusters from 5 to 10 nm in diameter. For sequentially produced Pt–Ru electrodeposits, XPS and voltammetry data indicate an irregular distribution of the active materials and a non homogeneous surface consisting of Ru islands on Pt. The electrocatalytic activity of these electrodes for the oxidation of methanol in 0.5 M aqueous sulfuric acid increases by a factor of about 5 as compared to Pt. A lower electrocatalytic efficiency was found for Pt–Ru electrodes produced by simultaneous electrodeposition. For both types of electrodes, the Ru content decreased after methanol electro-oxidation, but for simultaneously formed Pt–Ru electrodes the reactivation of methanol electro-oxidation, observed in the negative potential scan, increased as the Ru content on the Pt surface decreased.