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

摘要

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 H_2SO_4 and H_2SO_4 (0.06-0.92 M) + CH_3OH (0.10-1.00M) solutions at 25.0-45.0℃. CO oxidation showed an irreversible behaviour with an adsorption control giving an exchange current density of 2.3 x 10~(-6) A cm_(-2) and a Tafel slope of 113mVdec_(-1) (α = 0.52) at 25.0℃. 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 x 10~(-6)Acm~(-2) and a Tafel slope of 199mVdec~(-1) (α = 0.30) at 25.0℃. 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 kJmol~(-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, RuO_xH_y, which enhances the process.