Highly temperature dependent mass-transport parameters for ORR in Nafion® 211

Abstract

Nafion® 211 differs from previous versions of Nafion in that the membrane is cast from a dispersion rather than being melt-extruded. As such, the water sorption properties are different, as is the rate of increase in water content with temperature. Kinetic and mass-transport parameters for dispersion-cast Nafion® 211 were determined using solid-state electrochemistry in the temperature range 30–70 °C, 100% relative humidity, and 30 psi oxygen pressure. Exchange current densities, Tafel slopes, and transfer coefficients for ORR in Nafion® 211, are similar to those observed in Nafion® 117; mass-transport parameters are not. At 30 °C and 100% RH oxygen solubility and the diffusion coefficient is determined to be 1.16 × 10 −5 mol cm −3 and 1.13 × 10 −6 cm 2 s −1, respectively. Oxygen permeability at 30 °C (1.28 × 10 −11 mol cm −1 s −1) is lower than in Nafion® 117 (5.31 × 10 −11 mol cm −1 s −1) by factor of 4, while at T > 60 °C the permeability of Nafion® 211 increases significantly to values higher than Nafion® 117, and is correlated with the increase in water content and hydration number ( λ) with temperature.