II. Mechanisms of Elevated Temperature Methanol Electro-oxidation and Poisoning on Pt/C-Nafion Catalyst Layers

Half-cell measurements using fuel cell catalysts at elevated temperatures were made in a pulsed reactant electrochemical flow cell. Kinetics of methanol and the poisoning adlayer electro-oxidation were investigated on 20% Pt/Vulcan C-Nafion catalyst layers at 95°C. Transient measurement of the adlayer coverage during methanol electro-oxidation at 0.30 and 95°C showed that the surface reached a steady-state adlayer coverage after 30 s. Almost no change in adlayer coverage or current occurred from 30 to 300 s, indicating that steady-state conditions were maintained at 95°C. With increasing potential from 0.30-0.40 in 0.015 M methanol, the steady-state CO coverage decreased from 0.44 to 0.28 monolayers, while the steady-state current increased from 3.3 to 29 mA/mg Pt. The kinetics of CO adlayer oxidation were determined independently over this potential range for adlayers prepared by repeated, 60 s periods of methanol oxidation. The initial rates of adlayer electro-oxidation were 80-90% of the steady-state methanol electro-oxidation rates throughout the potential range. This indicates high selectivity through the CO adlayer at 95°C. Poisoning adlayer electro-oxidation occurred with faster kinetics on Pt(110) facets relative to Pt(100) facets. Dissociative adsorption of methanol at 0.05 and 95°C was also studied. The adlayer coverage reached 0.36 monolayers after a 60 s exposure to 0.015 M methanol, indicating that the Pt/C electrocatalyst reacts with methanol under these conditions.