Solid polymer electrolyte membrane composite microelectrode investigations of fuel cell reactions. II: voltammetric study of methanol oxidation at the nanostructured platinum microelectrode|Nafion® membrane interface

Electrochemical oxidation of methanol occurring at the high surface area nanostructured platinum film|Nafion® membrane interface has been studied using voltammetry at various temperatures. The effect of mobile anions on the electrode processes usually encountered in aqueous electrolyte is avoided in this composite microelectrode configuration. Both the methanol oxidation reaction (mor) and the carbon monoxide oxidation reaction have been studied at the platinum|Nafion® membrane interface. The mor is found to be similar to that seen in aqueous acid electrolytes, although there are some interesting differences. Methanol adsorbs on platinum over the entire potential range studied. When the potential is greater than 0.3 V (RHE), a proton is stripped from the adsorbed methanol to form CH2OHad. The direct electrooxidation of methanol to CO2 commences at potentials more positive than 0.60 V vs. RHE where OHad is produced. During the negative-going scan an oxidation peak appears at potentials where the coverage of the surface hydroxyl is about 0.5. In contrast, during the positive-going scan, the peak oxidation current is seen at potentials where the hydroxyl coverage is smaller than 0.5, possibly owing to reaction intermediates blocking available methanol and hydroxyl adsorption sites. Importantly, we have found that surface sites which are not in contact with the proton-conductive Nafion® membrane participate in electrochemical reactions. The surface area accessible to the hydrogen and CO adsorption/stripping processes is about twice that available to the methanol process, suggesting that some of the intermediates in the mor have low surface diffusion coefficients compared to Had and COad. The presence of these surface diffusion processes have important implications regarding the performance of DMFC anodes and new anode electrocatalysts.