Oxygen reduction at the pt/carbon black-polyimide ionomer interface

We have investigated the oxygen reduction reaction (ORR) at the interface of Pt nanoparticles dispersed on carbon black (Pt/CB) with a sulfonated polyimide ionomer layer (SPI-8). The electrochemical experiments were carried out in air-saturated 0.1 M HClO₄ aqueous solution at 25 °C via the rotating ring disk electrode technique. Slow-sweep (5 mV s^-1) hydrodynamic voltammetry yielded accurate estimates of the activity of the Pt catalysts under steady state conditions. It was found that the ORR was purely kinetically controlled provided that the polyimide ionomer layer covering the Pt/CB catalysts was thinner than 0.05 μm (threshold thickness) for higher potentials. When thicker than 0.05 μm, the oxygen diffusion limitation through the ionomer was non-negligible for potentials lower than 0.8 V vs the reversible hydrogen electrode, RHE. The threshold thickness was approximately half of that for a similar perfluorinated ionomer (Nafion)-coated Pt/ CB (Nafion-Pt/CB) electrode. The hydrogen peroxide yield, P(H₂O ₂), was lower than 0.6% of the overall ORR current in the 0.7-+0.8 V vs RHE range for the SPI-8-Pt/CB electrodes, which was somewhat lower than that for the Nafion-Pt/CB ones. In particular, P(H₂O₂) was negligibly low (<0.2%) when the polyimide ionomer layer was thinner than 0.05 μm. The low P(H₂O₂) is favorable for fuel cell applications, since hydrogen peroxide is regarded as one of the major factors responsible for the degradation of fuel cell constituent materials such as the ionomer binder in the catalyst layers, the ionomer membrane itself, and the carbon support.