Evaluation of ionomer coverage on Pt catalysts in polymer electrolyte membrane fuel cells by CO stripping voltammetry and its effect on oxygen reduction reaction activity

A quantitative analytical method for estimating the ionomer coverage on Pt surfaces in an electrode for polymer electrolyte membrane fuel cells (PEMFCs) was developed and applied to three sample electrodes with different catalyst supports: non-porous (Vulcan®) and porous (Ketjen black®(Ketjen), and Cnovel®) carbons. The method is based on the measurement of an electrochemical active surface area (ECSA) of ionomer-covered Pt surfaces by CO stripping voltammetry under fluorocarbon-fluid filled conditions. The estimated ionomer (Nafion) coverages were 60±7% for Ketjen and 57.4±0.3% for the Cnovel, which were significantly lower than those for Vulcan, 90±2%. The platinum-area-specific activities (SAs) for oxygen reduction reaction (ORR) of the electrodes were also measured, and as results, the SAs with the porous carbons were 1.7-1.9 times as high as those with the non-porous carbon. From these values, the activity ratio of ionomer-covered Pt over Pt in water vaper in the electrodes was estimated by extrapolating the data with the line of the linearly weighted-averages of specific activities of covered and non-covered areas as the SAs for the electrodes with certain ionomer coverages. The activity ratio was estimated to be 0.22, which is reasonably explained by the value in a past study with the rotating disk electrode technique, 0.28. The results indicate the significant ORR suppressions by ionomer.