Simultaneous oxygen reduction and methanol oxidation on a carbon-supported Pt catalyst and mixed potential formation-revisited

Abstract

The mutual effects of the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) occurring simultaneously on a Pt/Vulcan fuel cell catalyst in O 2-saturated 0.5 M H 2SO 4 solution containing 0.1 or 0.01 M methanol and the formation of a mixed potential were studied using differential electrochemical mass spectrometry (DEMS). This allows to separately monitor the partial reaction rates via CO 2 formation (MOR) and O 2 consumption (ORR) and determine the partial Faradaic currents for the individual reactions. While the influence of methanol and of the MOR lead to significant changes in the ORR characteristics, e.g., a shift of the cathode potential by about 0.3 V in 0.1 methanol, the mutual effects on the reaction characteristics of the two partial reactions are much smaller. A fit of the sum of the partial currents to the measured net current results in electrons yield of 8.3 electrons (0.1 M methanol solution) and 6.3 electrons (0.01 M methanol solution) per CO 2 molecule for the MOR, compared to values of 7.5 and six electrons in the absence of O 2, indicating an O 2-induced increase in incomplete methanol oxidation by-products. Slight deviations from a simple additive superposition of the ORR and MOR currents, the ‘classic’ picture for mixed potential formation, are mainly caused by O 2-induced changes in the MOR pathways, together with small changes in the O 2 transport, while the ORR selectivity, predominant oxygen reduction to water, is maintained at relevant potentials.