How do pore sizes affect the oxygen reduction reaction kinetics of platinum electrodes?

Abstract

The high overpotential of the oxygen reduction reaction (ORR) must be reduced to meet its expected role in energy conversion. Due to its high catalytic activity and stability, the electrocatalyst material has been mostly limited to expensive Pt. Therefore, much research has been conducted to reduce its loading by increasing the specific surface area and specific activity. Each enhancement is achieved by employing a porous geometry and changing the electronic structure. However, the effect of the pore size on its specific activity has not been clarified. In this study, we elucidate the relationship between the pore size and the oxygen reduction reaction kinetics by fabricating platinum model electrodes with hexagonally aligned cylindrical pores with sizes of 1.3, 1.8, and 3.0 nm. Electrochemical measurements reveal the quasi-volcano relationship between the oxygen reduction reaction kinetics and the pore size. The origin behind it is investigated by out-of-plane X-ray diffraction, CO-stripping voltammetry, angle-resolved X-ray photoelectron spectroscopy, and density functional theory calculation. It is found that the lattice constant of Pt decreases with decreasing pore size, resulting in a downward shift of the d-band centre, which has a significant effect on adsorption energy.