The Effect of Platinum Loading and Surface Morphology on Oxygen Reduction Activity

The catalytic activity of Pt catalysts towards the oxygen reduction reaction (ORR) was investigated on a catalyst system developed by thermally induced chemical deposition of Pt on carbon. The use of this deposition method made it possible to prepare a practical catalyst system with various Pt loadings on the support. Increasing the Pt loading caused a change in the Pt surface morphology which was confirmed by transmission electron microscopy (TEM) and CO stripping voltammetry measurements. The occurrence of a low and high-potential CO oxidation peak suggested the presence of Pt agglomerates and Pt nanoparticles, respectively. An increase in Pt loading lead to a subsequent decrease in the electrochemical surface area (ECSA, m(2) (Pt)/g(Pt)) as the platinum surface transitioned from isolated platinum nanoparticles to platinum agglomerates. The specific activity was found to increase with increasing Pt loadings, while the mass activity decreased with loading. The mass and specific activity data from this study was found to follow a 'master curve' obtained by the comparison of normalised activities from various different studies in the literature. Pt selectivity was also affected by Pt loading and hence Pt surface morphology. At low Pt loadings, i.e. large interparticle distances, the amount of H2O2 produced was significantly higher than for high Pt loadings. This confirms the presence of a 'series reaction pathway' and highlights the importance of the H2O2 desorption-readsorption mechanism on Pt nanoparticles and the ultimate role of Pt interparticle distance on the ORR mechanism.