Mass transport and kinetics of electrochemical oxygen reduction at nanostructured platinum electrode and solid polymer electrolyte membrane interface

Abstract

Oxygen reduction reaction (orr) at nanostructured Pt electrode in a flooded polymer electrolyte membrane fuel cell environment has been investigated using a nanoporous Pt–Nafion membrane composite microelectrode by means of steady-state voltammetry and chronoamperometry. The interfacial mass transport of dissolved oxygen is characterized by comparable diffusion coefficients and lower concentrations as compared with literature data obtained with a humidified membrane. The exchange current densities measured at the nanoporous Pt and membrane interface are higher than those reported for the orr in acidic solutions or at polycrystalline Pt and Nafion membrane interface, indicating the improvement of the orr kinetics. Increasing temperature substantially improves the orr kinetics and accelerates the diffusion of oxygen, as expected by their Arrhenius behavior. At the nanoporous Pt and membrane interface, the Tafel plot exhibits an unusual slope of around 240 mV dec−1 at high overpotentials. This Tafel slope doubling the value of 120 mV dec−1 normally reported for the orr in acidic media and at the polycrystalline Pt and membrane interface is a signature of non-uniform polarization of the nanoporous Pt electrode on the membrane which origins have been discussed.