Nafion thickness dependence of H2O2 yield during O2 reduction at Nafion/Pt microelectrode studied by scanning electrochemical microscopy

During the O2 reduction reaction (ORR) at the Pt cathode of a polymer electrolyte fuel cell (PEFC), a slight amount of H2O2 is produced as a by-product. The generated H2O2 degrades the polymer electrolyte membrane and Pt electrode due to its corrosive nature. It is reported that a relationship exists between the amount of H2O2 and an amount of ionomer used in an electrocatalyst layer. To clarify the relationship, we employed a Pt microdisk electrode so that the Nafion-coated thickness could be uniformly controlled. The H2O2 amount was then investigated with respect to the overcoated Nafion thickness. The generated H2O2 at the Nafion-coated Pt generator during the ORR is oxidized at a Pt detector of the scanning electrochemical microscope. As a result, the H2O2 detection current increased with the increasing Nafion thickness up to 0.16 μm. The current then decreased in the thicker Nafion region. The H2O2 increase at the thin Nafion/Pt can be explained by a decrease in the number of Pt-Pt pair-sites for the ORR. For the H2O2 decrease in the thicker region, a low diffusion coefficient of H2O2 in the Nafion layer makes it difficult to spread out.