(Invited) Kinetic Isotope Effect As a Tool to Investigate the Oxygen Reduction Reaction on Pt-Based Electrocatalysts

Oxygen reduction reaction (ORR) mechanisms on Pt electrocatalyst was investigated with kinetic isotope effect (KIE). Specifically, the rate-determining step for ORR on dispersed Pt/C electrocatalyst and polycrystalline Pt (Pt-poly) was studied. KIE is defined as the ratio of the kinetic current measured in protonated electrolyte versus deuterated electrolyte, where KIE values larger than one indicate the proton participation in the rate-determining step. We have investigated the effect of poisoning anions on the Pt on the rate determining step by assessing the KIE in perchloric (non-poisoning) and sulfuric acid-based electrolytes. The kinetics currents were calculated using both Koutechy-Levich and Tafel analysis. A KIE of 1 was observed for high loading, Pt/C (with a 40 wt.% Pt loading) and Pt-poly, thus indicating that, on 40 wt. % Pt/C and Pt-poly, the rate determining step is proton independent. We have extended the study to various carbon black supports with specific surface area ranging from 80 to 900 m2/g. When the density of Pt nanoparticles on the surface of carbon support was high, we observed KIE to be 1. However, at lower Pt nanoparticles densities the KIE was higher than one, indicating that carbon support has some impact on the rate determining step. Figure 1