The anode efficiency in methanol direct fuel cells—A chronoamperometric approach

Abstract The methanol oxidation reaction has been studied on polycrystalline platinum surfaces, cathodically treated platinum and platinum surfaces modified by a second (or a third) metal deposited spontaneously in 0.50 M methanol + 0.5 M sulphuric acid using chronoamperometry. The electrochemical behaviour of methanol on the three types of electrodes shows that the initial oxidation rate is higher on the cathodically treated electrodes, whereas it depends on the nature of the second spontaneously deposited metal; silver shows inhibition and ruthenium and osmium exhibit a catalytic behaviour for the same 20% amount of surface metal ad-atoms. Analysis of the methanol chronoamperometric data show that the steady-state current, recorded after 3000 s, is larger for the cathodic treated surfaces than for bare platinum. Moreover, the initial slope of the current versus time plot is positive for both surfaces, that is, the competition between carbon monoxide formation and methanol first oxidation occurs. On the other hand, methanol oxidation on platinum modified by spontaneously deposited metals always exhibits negative signs in the initial current versus time slopes, independent of the nature of the metal ad-atom. A theoretical analysis of the kinetic laws for methanol electrocatalytic oxidation is presented. The fitting of the chronoamperometric data with the mathematical model, allows the discussion of methanol decomposition, carbon monoxide oxidation and direct methanol oxidation on the different surfaces. The time dependent expression of the surface coverage by methanol adsorbates is also derived and the characteristic values of the reaction are calculated for the different platinum electrodes. The potential dependences of parameters directly related to the electrochemical rate constants of the each step were also evaluated.