Zinc oxide crystals grown by the seeded chemical vapor transport method have been annealed in zinc vapor at 1100 °C for 30 min. These thermochemical reduction treatments produce a deep red coloration in the crystals and increase their n-type electrical conductivity. Electron paramagnetic resonance (EPR), optical absorption, and Hall measurements were used to monitor changes in the crystals. After an anneal, an intense optical absorption band is present that extends from the band edge out to approximately 550 nm, and the EPR signal near g=1.96 (due to shallow donors and/or conduction-band electrons), the free-carrier absorption, and the Hall electron concentration are all larger. Hydrogen was not present during these anneals, thus leaving oxygen vacancies and zinc interstitials as candidates for the added donors. Neutral oxygen vacancies are produced at high temperature by the additive-coloration mechanism, and are responsible for the broad near-edge absorption band. The observed increase in the number of free carriers is a result of either (1) the formation of zinc interstitials or (2) having the ground state of the neutral oxygen vacancy near the conduction band.

Metal-organic frameworks (MOFs) are an emerging class of microporous, crystalline materials with potential applications in the capture, storage, and separation of gases. Of the many known MOFs, MOF-5 has attracted considerable attention because of its ability to store gaseous fuels at low pressure with high densities. Nevertheless, MOF-5 and several other MOFs exhibit limited stability upon exposure to reactive species such as water. The present study quantifies the impact of humid air exposure on the properties of MOF-5 as a function of exposure time, humidity level, and morphology (i.e., powders vs pellets). Properties examined include hydrogen storage capacity, surface area, and crystallinity. Water adsorption/desorption isotherms are measured using a gravimetric technique; the first uptake exhibits a type V isotherm with a sudden increase in uptake at ∼50% relative humidity. For humidity levels below this threshold only minor degradation is observed for exposure times up to several hours, suggesting that MOF-5 is more stable than generally assumed under moderately humid conditions. In contrast, irreversible degradation occurs in a matter of minutes for exposures above the 50% threshold. Fourier transform infrared spectroscopy indicates that molecular and/or dissociated water is inserted into the skeletal framework after long exposure times. Densification into pellets can slow the degradation of MOF-5 significantly, and may present a pathway to enhance the stability of some MOFs.

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a b s t r a c tNi-YSZ cermet is commonly used as the anode of a solid oxide fuel cell (SOFC) because it has excellent electrochemical performance, not only in hydrogen fuel, but also in a clean blended synthetic coal syngas mixture (30% H 2 , 26% H 2 O, 23% CO, and 21% CO 2 ). However, trace impurities, such as phosphine (PH 3 ), in coalderived syngas can cause degradation in cell performance [J.P. Trembly, R.S. Gemmen, D.J. Bayless, J. Power Sources 163 (2007) 986-996]. A commercial solid oxide fuel cell was exposed to a syngas with 10 ppm PH 3 under a constant current load at 800 • C and its performance was evaluated periodically using electrochemical methods. The central part of the anode was exposed directly to the syngas without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the impurity PH 3 caused a significant loss of the Ni-YSZ anode electrochemical performance and an irreversible Ni-YSZ structural modification. Ni 5 P 2 was confirmed to be produced on the cell surface as the dominant nickel phosphorus phase.

Proton exchange membrane fuel cells (PEMFCs) offer an attractive zero-emission mobile power source. However, the requirement of excessive platinum group metal (PGM) catalysts to facilitate the sluggish oxygen reduction reaction (ORR) in PEMFCs has prevented their widespread adoption. Despite tremendous progress in catalyst development with greatly increased catalytic activities, the reduction of PGM loading in practical PEMFCs remains a challenge. The ORR in PEMFCs occurs at a catalyst-electrolyte-gas three-phase interface, with multi-faceted challenges involving the activity of the catalysts, available active sites, and concerted transport of the reactants (oxygen, protons) to and removal of the product (water) from the active sites. The reduction of PGM loading reduces the number of catalytic sites, requiring a higher reaction rate on each site to sustain the overall power output, which in turn necessitates faster delivery of the reactants to and removal of the products from each active site. A desirable interface must allow efficiently feeding oxygen and protons to the catalytic sites without starving the reaction and must allow timely removal of water to avoid interface flooding. Herein we report the design of the three-phase microenvironment in PEFMCs by tailoring the interactions between the carbon supports and the electrolyte ionomers. We show that the carbon surface with 2.4% oxygen interacts with the ionomers through both its hydrophilic and hydrophobic regions, creating favorable transport paths for rapid delivery of both oxygen and protons, and timely removal of water. Such an elaborated interfacial design allows reducing costly platinum catalysts while maintaining state-of-the-art performance. For the first time we demonstrate PEMFCs with all key ORR catalyst performance metrics, including mass activity, rated power and durability, surpassing the U.S. DOE targets.