Further Insight into the Oxygen Reduction Reaction on Pt Nanopartlcles Supported on Spatially Structured Catalytic Layers

摘要

In this work, we study the oxygen reduction reaction (ORR) on the catalytic layers composed of Pt nanoparticles supported on vertically aligned carbon nano-filaments by using the rotating ring disk electrode (RRDE) method. The preparation method allows varying independently and in a wide range two characteristics of the catalytic layer: its thickness and the Pt coverage. The present work focuses on the comparison of the RRDE data in strongly (H2SO4) and weakly (HClO4) adsorbing electrolytes. It is shown that, in both electrolytes, a decrease of the number of Pt sites leads to an increased H2O2 detection at the ring of the RRDE in the potential interval from 0.2 to 0.8 V vs. reversible hydrogen electrode. The analysis of the experimental data and the kinetic modeling suggests that the H2O2 formation at potentials above the H_(UPD) is an intrinsic process of the ORR on Pt rather than a result of the site blocking by a (bi)sulfate or surface oxide. The results are consistent with a dual-path ORR mechanism, which operates in both electrolytes and comprises a "series" pathway involving H2O2 as an intermediate species and a "direct" pathway occurring through the dissociation of O2 (or HO2). The switching between the two pathways is strongly potential-dependent. Below ca. 0.6 V, the ORR preferentially occurs via the H2O2-mediated pathway,while, in the potential interval between ca. 0.8 V and the onset of the ORR, the "direct" path is dominating. The results show that the "direct" pathway and the re-adsorption of H2O2 in the "series" pathway are strongly affected by the oxide formation and (bi)sulfate adsorption, which, depending on the number of available surface sites, might lead to an increase of the H2O2 yield.