Mass-Transport Characteristics of Oxygen at Pt/Anion Exchange Ionomer Interface

This work quantifies the mass-transport characteristics of O-2 at Pt/anion exchange ionomer by determining the solubility and diffusion coefficient of O-2 in Tokuyama AS-4 anion exchange ionomer film. Electrochemical methods such as linear sweep voltammetry and chronoamperometry are performed on rotating disk electrode to determine these parameters. The diffusion process in AS-4 ionomer film is found to be Fickian in nature; that is, it is shown that the diffusion coefficient is independent of concentration and film thickness by calculating the diffusion coefficient at various film thicknesses and rotation speed of electrode. In comparison with Nafion ionomer, the magnitude of diffusion coefficient in anion exchange ionomer is found to be one order higher, and solubility is found to be one order lower. However, the overall permeability, defined by product of diffusion coefficient and solubility is similar to Nafion ionomer film. In addition, durability studies of anion exchange ionomer-coated Pt disk shows electrochemical stability of anion ionomer in alkaline media. With these studies, it is shown that O-2 transport at the Pt/anion exchange ionomer interface is not expected to limit alkaline membrane fuel-cell cathode performance.