Studies on electrochemical reaction pathways of methanol electro oxidation on nanoscale fuel cell catalysts
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In situ infrared spectroscopy and cyclic voltammetry were used to study the electrochemical reaction pathways of methanol oxidation on nanoscale carbon supported platinum (C/Pt, 10% Pt) and platinum ruthenium (C/PtRu, 30% Pt, 15% Ru) fuel cell catalysts. A temperature controlled electrochemical cell and electrodes that contained a built-in temperature sensor were utilized to probe surface electrochemistry of the catalysts from ambient temperatures to 70° C. Initial experiments were conducted on bulk Pt to help interpret results in experiments with nanocatalysts. Methanol dissociative adsorption studies on bulk Pt in 0.1 M HCIO4 and 0.1 M 13CHsOH in 0.1 M HCIO4 helped attribute bands from 2065-2080 cm"^ to C-0 stretching modes of CO molecules coordinated to single Pt atoms in an atop coordination arrangement. An increase in band intensity was observed when potentials (reported in volts measured with respect to a reversible hydrogen electrode, VRHE) were stepped positive up to 0.5 VRHE. These intensities began to diminish when potentials approached the oxidation levels for CO to CO2 above 0.5 VRHE- Catalyst inactivation data derived from the experiments on bulk Pt directed research into probing surface electrochemistry of methanol oxidation on the nanoscale catalysts. Formic acid and methanol electro oxidation studies were carried out on the C/Pt and C/PtRu nanocatalysts from ambient to 70° C. The C/Pt 10% loading catalyst was not as active as bulk Pt, as indicated by a comparison of current densities for methanol oxidation at 60° C and by temperature dependent shifts in the oxidation potential for rapid methanol oxidation. Greater attention to thermal and electrochemical pretreatment of the nanoscale catalyst, in order to remove surface contamination would likely help to increase nanocatalyst activity. Experiments on the PtRu nanoscale catalysts show that they had comparable behavior as that of their bulk counterparts. The PtRu nanocatalysts were more active for CO2 formation than bulk PtRu or C/Pt, but adsorbed CO on the catalyst surface was difficult to probe. Pretreatment procedures and inherent surface properties of C/PtRu are expected to affect electrochemical responses for methanol oxidation compared to bulk materials. A better understanding of fuel cell catalysts can help speed up the commercialization of direct methanol fuel cells.