No abstract

No abstract

Composite Nafion-based membranes containing nanosize and submicrometer-size titanium dioxide ͑TiO 2 ͒ and tin dioxide ͑SnO 2 ͒ were fabricated by recasting from Nafion solution. Proton exchange membrane fuel cell tests were conducted using these composite membranes at 80°C over a relative humidity ͑RH͒ range between 25 and 100%, using high-purity H 2 and O 2 gases. Cell tests were also conducted at 120°C and low RH. Composite membranes containing nano-TiO 2 and SnO 2 fillers showed a significant performance improvement over Nafion membranes without fillers and over composite membranes containing submicrometer-size TiO 2 and SnO 2 powders. The relative performance improvement was greater at higher temperature and low RH. The particle size ͑surface area͒ of the nanopowders plays an important role in water retention and performance of composite membranes at 80 and 120°C. The performance of SnO 2 composite membranes was slightly better than TiO 2 composite membranes for the same operating conditions.

No abstract

Samples of undoped LaMnO 3 ͑LM͒, Sr-doped LaMnO 3 ͑LSM͒, and Ti-doped LaMnO 3 ͑LTM͒ were fabricated by conventional solid-state ceramic processing methods. The objective of Ti doping was to suppress oxygen vacancy concentration/ion transport, while doping with Sr was to enhance oxygen vacancy concentration/ion transport in relation to undoped LM. Total conductivity, mainly electronic, was measured between 500 and 800°C in air. Coatings of LSM, LM, and LTM, 1 m thick, were deposited on Haynes 230 ͑H230͒ foils by sputtering. Uncoated and coated H230 samples were oxidized in air at 800°C for up to 1080 h. Oxidation kinetics of uncoated and coated samples was studied by measuring oxide scale thickness as a function of time. In all coated samples, oxide scale formed under the coating. All coatings were found to suppress oxidation kinetics. Of the materials studied, LTM was the most protective while LSM was the least protective, in accord with defect chemistry, LTM with the lowest oxygen vacancy concentration and LSM with the highest. Area specific resistance ͑ASR͒ measurements showed that LTM-coated samples exhibited the lowest ASR, while the uncoated ones exhibited the highest ASR, again in accord with defect chemistry. A 15 m LTM coating should be sufficient to ensure a 40,000 h interconnect life at 800°C. A comparison of LTM coating to Mn-Cr-spinel of earlier work shows that LTM coating is ϳ23 times more effective than spinel coating in suppressing oxidation kinetics.