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

Abstract

In this study, we investigated the effect of a Nafion ionomer on H2O2 byproduct generation during O2 reduction at a Pt/C-Nafion composite electrode, which is used as the cathode of practical polymer electrolyte fuel cells. The generated H2O2 was detected by scanning electrochemical microscopy (SECM). First, O2 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 O2 reduction and the amount of H2O2 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 H2O2 generated, despite the amount of O2 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 H2O2 without a decrease in O2 consumption. In a semiquantitative analysis of H2O2 generation during O2 reduction, the smallest percentage of H2O2 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 H2O2.