Oxygen Reduction Kinetics on a Platinum RDE Coated with a Recast Nafion Film

Abstract

Measurements of currents of the oxygen reduction reaction (ORR) at a Pt RDE coated with a recast Nafion film yield the permeability of through such films. Combined with results from linear potential scanning experiments on similar electrodes, these measurements also allow the separate evaluation of the concentration of and its diffusion coefficient in such films. Similar recast Nafion films have been used successfully to promote protonic access to catalyst sites in fuel cells. Accordingly, the maximum film thickness which would allow adequate oxygen flux through such a layer is evaluated. Measurements in 85% at room temperature show that a very significant enrichment of (e.g., by a factor of 20) in a region close to the electrode can be achieved by coating the electrode with a recast Nafion film. This result is expected from the high solubility of in Nafion. However, the measured resulting enhancement of the kinetic ORR currents was found to be only a factor of two. The interpretation suggested for the lack of direct proportionality between average concentration near the electrode and ORR kinetic current density is that enhanced local ORR rates are possible only at a limited number of free Pt sites that are immediately adjacent to a surface region covered by the hydrophobic phase. Contamination by organics of lower molecular weight originating from the Nafion film may contribute to site‐blocking. With increase in temperature the enrichment effect becomes less pronounced, because the solubility of in 85% increases with temperature while in Nafion it decreases.