New evaluation method for the effectiveness of platinum/carbon electrocatalysts under operating conditions

To facilitate the development of high performance polymer electrolyte fuel cells (PEFCs), we have proposed a new evaluation method for the effectiveness of Pt (EfPt) under actual PEFC operating conditions. We also have examined the dependence of the values of EfPt on experimental parameters, including the ionomer content in the catalyst layer. The EfPt parameter is defined as the ratio of mass activity (MA) in the membrane-electrode assembly (MEA) to the maximum mass activity (MAmax) at high potential, as estimated by use of the channel flow double electrode (CFDE) technique as an ideal case of complete ionic contact with the Pt surface. For the ionomer content in the catalyst layer (Nafion/carbon ratio, N/C), we found a distinct increase of EfPt with increasing N/C, indicating a beneficial effect of the ionic contact and connectivity of the ionic conduction path within the catalyst layer, even though the apparent utilization of platinum (UPt) was nearly constant. Typical EfPt values for the mid-range of N/C ratios were found to only reach ca. 12.5% for pure O2 and Tcell = 65 °C. Furthermore, the EfPt values in air only reached ca. 7.5%. It must be emphasized that there is still much room for the improvement of EfPt.