Scanning Electrochemical Microscopy Study of H2O2 Byproduct during O2 Reduction at Pt/C-Nafion Composite Cathode

In this study, we investigated the effect of a Nafion ionomer on H 2 O 2 byproduct generation during O 2 reduction at a Pt/C-Nafion composite electrode, which is used as the cathode of practical polymer electrolyte fuel cells. The generated H 2 O 2 was detected by scanning electrochemical microscopy (SECM). First, O 2 reduction voltammetry was conducted using a rotating ring-disk electrode (RRDE) with the Nafion-coated Pt disk electrode. The results show that the diffusion-limited current density of O 2 reduction and the amount of H 2 O 2 generated decrease with increasing thickness of the Nafion coating layer. A decrease in diffusion-limited current density is also obtained using Nafion-coated Pt microdisk electrodes. However, SECM measurement using these Pt microdisk electrodes as a generator indicates that increasing the thickness of the layer suppresses the amount of H 2 O 2 generated, despite the amount of O 2 consumption slightly decreasing. Subsequently, another SECM measurement was carried out using a porous microelectrode packed with the Pt/C-Nafion composite having 0, 13.4, or 28.6 wt % Nafion ionomer. The obtained results demonstrate that an increase in the Nafion ionomer content leads to a drastic decrease in the amount of generated H 2 O 2 without a decrease in O 2 consumption. In a semiquantitative analysis of H 2 O 2 generation during O 2 reduction, the smallest percentage of H 2 O 2 generated is obtained for the Pt/C-Nafion composite with 28.6 wt % Nafion ionomer (0.1%). According to this result, increasing the content of the Nafion ionomer in the Pt/C-Nafion composite is highly effective for suppressing the generation of H 2 O 2 .