Oxygen permeability of cast ionomer films from chronoamperometry on microelectrodes

The paper presents a method for determination of diffusion coefficient and solubility of oxygen in cast polymer electrolyte from chronoamperometry of oxygen on polymer electrolyte coated platinum disk ultramicroelectrodes. The approach is based on numerical fitting of measured currents to the known but not previously used for polymer electrolytes equation derived by Shoup and Szabo. The method was applied to cast films of a novel polymer electrolyte 6F-40. As opposed to Nafion®, cast 6F-40 films do not undergo interfacial restructuring, i.e., they retain their original morphology under selected temperature and humidity conditions, which allows for accurate determination of both parameters from measurements for a range of electrode radii and film thicknesses. It is demonstrated that the Shoup and Szabo equation satisfactorily describes measured current transients for shorter oxygen reduction times, i.e., when the diffusion field in the thin polymer film can be regarded as semiinfinite. The accuracy of the diffusion coefficient and solubility determinations was not measurably affected by the product water and the observed systematic changes of the fit quality in various time domains were attributed to approximate character of the fitting equation.