Study of Complete Methanol Depletion in Direct Methanol Fuel Cells

Abstract

The aim of this study is to gain a deeper and more detailed insight into the corrosion processes during complete methanol depletion under direct methanol fuel cell operation. Three characteristic, consecutive phases during methanol depletion have been identified, with oxygen evolution and carbon corrosion as dominating anode reactions. Irreversible aging of membrane electrode assemblies is caused by corrosion of Ru and the carbon support of the anode catalyst layers. The latter process leads to an irreversible performance loss up to 84%, an active surface loss up to 62% and a thinning of the anode catalyst layer. For a better understanding of the aging processes, an anode impedance analysis was carried out using a transmission line model based on primary & secondary pores. The fitting analysis revealed a strong increase of the resistances associated with processes in the anode catalyst layer, including methanol oxidation kinetics, proton conductivity and mass transport. Impedance data and pore size distributions show a particularly strong degradation of (small) primary pores in the last phase of methanol depletion, suggesting a degradation process starting in the larger pores and continuing in the smaller pores.