Electrochemistry of proton exchange membranes

Kinetic and mass transport parameters are determined for the oxygen reduction reaction at the interface between a platinum microelectrode and three different proton exchange membranes: perfluorosulfonic acid based membranes (Nafion ® 117), sulfonated α,β,β-trifluorostyrene-co-substituted-α,β,β-trifluorostyrene copolymer membranes (BAM®), and sulfonated styrene-(ethylene-butylene)-styrene triblock copolymers (DAIS®). The membranes differ in their chemical structure, morphology, equivalent weight (EW), and water content.; These materials are investigated over a range of temperatures (303–343 K) and pressures (2–5 atm.), at 100% relative humidity, using a solid-state electrochemical cell. Slow-sweep voltammetry yields limiting current and exchange current density data, while chronoamperometry allows the determination of diffusion coefficients and solubility parameters, for the different conditions specified. This information is used to calculate activation energies for the reduction process and for diffusion of oxygen in the membranes, as well as the enthalpies of dissolution. The diffusion coefficients of oxygen (D _b) increase with temperature, while its solubility (C_b) decreases. All the membranes exhibit a Henry's Law dependence for the limiting current of the oxygen reduction reaction with increasing oxygen pressure. The limiting current and the diffusion coefficient of oxygen decreases with increasing equivalent weight, while the solubility of oxygen decreases due to the corresponding decrease in water content.; The water sorption properties of a series of BAM® (EW 407–735 g/mol) and DAIS® (EW 585–1062 g/mol) membranes were compared to those of Nafion® 117. For both series of membranes, the water content increases with decreasing equivalent weight. Distinct differences in water sorption under different relative humidity conditions were found within a series, depending on the equivalent weights. Upon boiling, the dimensions of BAM® and Nafion ® membranes increase, concurrent with a greater uptake of water.; Since maximizing the water contents of membranes is reported to maximize proton conductivity, a series of BAM® (EW 407–735 g/mol) and DAIS® (EW 585–1062 g/mol) membranes possessing different equivalent weights were studied using the technique of a.c. impedance spectroscopy in conjunction with a custom-made gold-plated coaxial probe. Conductivity measurements were made in the transverse direction (along the plane of the membrane). For the BAM® membranes (EW 407 to 735 g/mol) the water contents ranged from 26 to 85 wt% while the specific conductivity ranged from 0.05 to 0.17 S/cm. The water content decreased with increasing EW while the conductivity peaked in the mid equivalent weight range. The effect of temperature (303–353 k) on proton conductivity was determined for Nafion® 117 (EW 1100 g/mol) and BAM® (EW 407 g/mol). The transverse conductivity increases with temperature and follows an analogous trend with respect to the diffusion of oxygen as a function of temperature. Both trends are believed to be related to the increase in the water content of the membranes.