A highly accelerated stress test (HAST) has been developed to generate local stressful conditions that are representative of those in automotive fuel cell stacks. Using a 50-cm 2 cell cycled between 0.05 and 1.2 A/cm 2 with a low inlet RH in the co-flow configuration, the HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the outlets. Conducting HASTs using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a robust membrane containing both a mechanical support and a chemical stabilizing additive, the result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer. One of the key challenges facing the commercialization of automotive fuel cells is the development of membrane electrode assemblies (MEAs) that can meet durability targets. In proton exchange membrane (PEM) fuel cells, the PEM serves to conduct protons from the anode to the cathode of the fuel cell while simultaneously insulating electronic current from passing across the membrane as well as preventing crossover of the reactant gases, H 2 and O 2 . State-of-the-art PEM fuel cells for high power density operation utilize perfluorosulfonic acid (PFSA) membranes that are typically no more than 25 μm thick. To be viable for automotive applications, these membranes must survive 10 years in a vehicle and 8000 hours of operation including transient operation with start-stop and freeze-thaw cycles. Fuel cells cannot operate effectively when gas can permeate the membrane through microscopic pinholes or excessively reduced thickness. Ultimately, fuel cells can fail because such excessive crossover leaks develop and propagate within the polymer membranes. Fuel cells can also fail if electronic current passes through the membranes and causes the system to short. It is thus critical that these membranes are sufficiently robust to thinning, cracking and thermal decomposition over the range of conditions experienced during fuel cell operation. In automotive fuel cell systems, there are three primary root causes of membrane failure: (1) Chemical degradation: polymer decomposition caused by the direct attack on the polymer from radical species generated as byproducts or side reactions of the fuel cell electrochemical reactions; (2) Mechanical degradation: membrane fracture caused by cyclic fatigue stresses imposed on the membrane via hygrothermal fluctuations in a constrained cell; and (3) Thermal degradation: membrane degradation caused by ohmic heating thr...

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The 180 isotopic oxygen exchange of the oxo (yl) oxygens of dr-V02+(aq) is too rapid to measure by static methods. Studies using the rapid formation of [V02(C204)2]3", followed by [Co(en)3]3+ precipitation, revealed complete oxygen exchange during the conversions. Studies on the yl-oxygen exchange of c/r-[V02(C204)2]3' found the rate to be less and the rate equation to be rate = fc0[VO2(C2O4)2]3" + fch[H+][V02(C204)2]3", where A:0 = 0.31 ± 0.1 s"1 and *h = (4.7 ± 1) X 103 s"1 M"1 at 0 °C and a pH <6. Above this pH a chemical modification occurs and the observed rate increases with pH.An impurity, presumably V(IV), also increases the rate. It can be removed with traces of H4I06", which oxidizes it to V(V). V02+(aq) is rapidly reduced and complexed by excess NCS" to VCKNCSls3", which exchanges its yl oxygen slowly.Competition between lsO-solvent exchange and NCS" reduction allows an estimation of the intrinsic exchange rate of V02+(aq). At 0 °C the r1/2 of 0.15 s was estimated (A: = 4.7 s"1).

The effects of aging temperature and time on the oxygen storage capacity (OSC) of Pt-Rh-promoted Ce0.75Zr0.25O2 solid solutions were measured and correlated with the BET surface area and noble metal (NM) surface area in the catalysts. The NM surface area is better correlated with OSC than is with the BET surface area. On a practical level, our results demonstrated that, even when operating at 900°C with alternating oxidizing and reducing conditions, these materials deactivate slowly with a near t-1 time dependence. Deactivation rates for these catalysts are dependent on the NM loading with the highest loaded catalysts deactivating roughly half as fast as the lowest loaded catalysts. As the aging temperature is increased from 900°C to 1000°C, the deactivation rate becomes two to four2-4 times higher for all three properties (BET surface area, NM surface area and OSC). The lowest NM loaded samples are more sensitive to aging temperature than the highest loaded ones