Formaldehyde formed in the electrochemical oxidation of methanol was measured in order to assess its importance as a reaction intermediate and potential source of efficiency loss in direct methanol fuel cells. Formaldehyde generated from 15 mM methanol in 0.1 M HClO4 at fixed potentials with a small volume electrolysis arrangement was determined with a sensitive fluorescence assay. The formaldehyde yields approached 30% of the total electrolysis charge at 0.2−0.3 V (vs Ag/AgCl). The percentages dropped at more positive potentials, as other oxidation pathways became dominant. However, formaldehyde production continued to increase with potential, maximizing near 0.5 V. This study demonstrates that formaldehyde, which is often not detectable with modern in situ analysis techniques, can be produced during methanol electrochemical oxidation in significant amounts. A fluorescence assay specific for formaldehyde is suggested for use in parallel with in situ measurements.

The formation of formaldehyde during methanol electrochemical oxidation on supported Pt and Pt−Ru catalysts was investigated. While on solid platinum electrodes, the formaldehyde yields from methanol oxidation are near 30% at low potentials; the yields fall below 2% for methanol electrochemical oxidation on carbon-supported catalysts in Nafion. The lower formaldehyde yields, which result from more complete methanol oxidation, are believed to arise from the ability of partial oxidation products to be transported to an array of active catalyst sites dispersed within the three-dimensional network of the Nafion film.