Effect of electrode Pt loading on the oxygen reduction reaction evaluated by rotating disk electrode and its implication on the reaction kinetics

The oxygen reduction reaction (ORR) activity was evaluated over film electrodes prepared from different Pt catalysts using rotating-disk electrode (RDE) and rotating-ring-disk electrode (RRDE) systems. The ORR activity a s a function of Pt loading using the same catalyst was examined first, and it is concluded to be strongly influenced by mass transfer limitation. Two types of mass transfer limitation were identified. One is the external diffusion as described by the rotating- speed-dependent Levich equation. The other one is the internal diffusion within the electrode in which the electrode thickness is the governing parameter. The electrode thickness is closely related to the catalyst loading of the electrodes. After corrections of these mass transfer limitations, all the tested Pt catalysts under the conditions of this study show similar specific ORR activity with a Tafel slope near -120 mV/dec This result suggests that the ORR over Pt catalysts is a structure-insensitive reaction and the overall electrode performance can be limited by the mass transfer.