Kinetic analysis of carbon monoxide and methanol oxidation on high performance carbon-supported Pt–Ru electrocatalyst for direct methanol fuel cells

The kinetic parameters of carbon monoxide and methanol oxidation reactions on a high performance carbon-supported Pt–Ru electrocatalyst (HP 20% 1:1 Pt–Ru alloy on Vulcan XC-72 carbon black) have been studied using cyclic voltammetry and rotating disk electrode (RDE) techniques in 0.50 M H2SO4 and H2SO4 (0.06–0.92 M) + CH3OH (0.10–1.00 M) solutions at 25.0–45.0 °C. CO oxidation showed an irreversible behaviour with an adsorption control giving an exchange current density of 2.3 × 10−6 A cm−2 and a Tafel slope of 113 mV dec−1 (α = 0.52) at 25.0 °C. Methanol oxidation behaved as an irreversible mixed-controlled reaction, probably with generation of a soluble intermediate (such as HCHO or HCOOH), showing an exchange current density of 7.4 × 10−6 A cm−2 and a Tafel slope of 199 mV dec−1 (α = 0.30) at 25.0 °C. Reaction orders of 0.5 for methanol and −0.5 for proton were found, which are compatible with the consideration of the reaction between Pt–CO and Ru–OH species as the rate-determining step, being the initial methanol adsorption adjustable to a Temkin isotherm. The activation energy calculated through Arrhenius plots was 58 kJ mol−1, practically independent of the applied potential. Methanol oxidation on carbon-supported Pt–Ru electrocatalyst was improved by multiple potential cycles, indicating the generation of hydrous ruthenium oxide, RuOxHy, which enhances the process.

Research highlights▶ The kinetic analysis of methanol oxidation on Pt–Ru/C shows the role of adsorbed CO. ▶ The porosity of carbon in Pt–Ru/C restricts the diffusion of soluble intermediates. ▶ The complete methanol oxidation is favoured on carbon-supported Pt–Ru. ▶ The methanol adsorption on Pt–Ru follows a Temkin isotherm. ▶ High-performance Pt–Ru/C is stable in front of methanol oxidation.