Rate expression for electrochemical oxidation of methanol on a direct methanol fuel cell anode

The kinetics of methanol oxidation at PtRu catalyst was investigated on a membrane electrode assembly (MEA) in a cyclone flow cell. Catalyst characterisation was done by means of cyclic voltammetry and CO stripping. The influence of the flow rate (10-20 l/h), the methanol concentration (0.03-3 M) and the temperature (22-61 °C) was checked. The reaction order with respect to methanol was found to vary from zero (low potential, high concentrations) to close to unity (high potentials), where limiting currents are recorded. A model for the anodic oxidation of methanol, based on a bifunctional mechanism was presented, which includes methanol adsorption on the Pt sites, formation of OHads on the Ru sites and surface reaction between these adsorbed species. For Langmuir adsorption conditions it predicted reaction order, Tafel slopes and activation energies in qualitative agreement with the experimental observations. The kinetic parameters were determined by fitting the experimental curves. Some modifications of the model were tested as well - Sips adsorption conditions on the Pt sites; Frumkin adsorption conditions; diffusion through the back diffusion layer and the Nafion membrane. No significant improvement has been achieved, although Frumkin conditions seem to describe better the anodic behaviour outside of the limiting current region. From these simulations it is difficult to decide whether the reaction between adsorbed methanol and OHads depends on potential or not.