The enhancement of methanol oxidation electrocatalysis at low and high overpotentials

• EIS results depicted two time constants.
• Between 0.40 and 0.55 V methanol dehydrogenation was the rds.
• Above 0.55 V CO-type and formiate adsorbed intermediates oxidation was the rds.
• PtRuMo/C showed the highest tolerance to methanol intermediates.

The preparation of new surfaces for anodic processes in methanol/air fuel cells has gained major attention due to the efficiency loss in the course of long-time operations. This paper proposes the use of electrochemical activated Pt/C, PtMo/C, PtRu/C, PtRuMo/C electrodes to study changes in the electrocatalytic behaviour of methanol oxidation under the potential ranges of interest for fuel cells. Electrochemical impedance spectroscopy together with polarization curves are applied to typify the interfacial behaviour of methanol electrooxidation on these activated electrodes. Impedance information discloses that these catalysts allow two distinct responses, i.e. methanol dehydrogenation as rate determining step in the low potential region (0.400-0.550 V), whereas a surface oxidation of adsorbed intermediates determining the process at high potentials (larger than 0.550 V). Moreover, we found new effects caused by molybdenum or ruthenium inclusions that are explained using the true Tafel slopes, that is, those corrected for mass transport. Thus, Tafel slopes of 0.060 V decade−1 are found for all carbon supported electrodes except for PtRu/C where the first electrochemical step as rate determining explained the experimental 0.120 V decade−1 value. The catalytic performance of carbon supported PtRuMo/C towards methanol electrooxidation showed the highest tolerance upon methanol intermediates formation.