MICROELECTRODE INVESTIGATION OF OXYGEN PERMEATION IN PERFLUORINATED PROTON-EXCHANGE MEMBRANES WITH DIFFERENT EQUIVALENT WEIGHTS

Oxygen concentrations (C-b) and diffusion coefficients (D) in various proton exchange membranes were measured using chronoamperometry at mic roelectrodes. These measurements were made under conditions similar to those prevailing in proton-exchange-membrane fuel cells. Knowledge of the D and C-b parameters is essential for the determination of oxygen permeation in the catalytic layers of gas diffusion electrodes, which could be rate limiting in these low-temperature fuel cells. Furthermo re, the D and C-b values also provide the permeation rates of oxygen t hrough the bulk of the proton conducting membrane. It was found that t he concentration of oxygen increased and the diffusion coefficient dec reased with increasing equivalent weight of the membranes. These resul ts were interpreted using a model based on the microstructure of the s wollen membranes separated into two phases, one hydrophobic and the ot her hydrophilic, with distinctly different O-2-permeation properties. According to this model the relative amounts of the two phases in the membranes, caused by the different water contents, determine the O-2 s olubility and diffusion coefficient in the different membrane material s.