Influences of aniline, carbazole, indole, and pyrrole monomers and polymers on formic acid oxidation at Pt electrodes

Abstract

The effects of aniline, carbazole, indole, and pyrrole on formic acid oxidation at Pt electrodes are compared with those of their corresponding polymers. While thin films of polyaniline, polycarbazole, polypyrrole, and polyindole all promote the direct oxidation (i.e. the pathway that does not involve carbon monoxide as an intermediate) of formic acid at low potentials on Pt, only aniline exhibits a similar effect when added to the formic acid solution. At +0.25V vs. SCE, current enhancements over bare Pt by factors of 4.0, 3.0, 3.4, 5.2, 6.8, respectively, were observed. Interestingly, drop coating of a Pt electrode with carbazole also promoted the direct pathway, although adsorption of carbazole from solution only inhibited formic acid oxidation. The promotion of the direct pathway by aniline adsorbed from solution followed a dependence on surface coverage that suggests inhibition of poisoning by adsorbed carbon monoxide. It is argued that the diversity of these effects indicates that the mechanism by which the direct pathway becomes favoured does not involve any specific chemical functionality of the modifier. However, it must be dependent on the orientation of the modifier relative to the Pt surface. A recent literature model, in which the direct pathway is shown to be favoured by disruption of the H-bonding network of water at the water/metal interface, is invoked to explain this. According to this model, the reaction pathway is determined by the orientation of the formic acid molecule as it adsorbs, which can be changed by other adsorbed species (formate in the model) that create a local hydrophobic region. It is suggested that the modifiers used here can play a similar role.