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Doped perovskite metal oxide catalysts of the form A(BxM1-x)O3-δhave been instrumental in the development of solid oxide electrolyzers/fuel cells. In addition, this material class has also been demonstrated to be effective as a heterogeneous catalyst. Co-doped barium niobate perovskites have shown remarkable stability in highly acidic CO2sensing measurements/environments (1). However, the reason for their chemical stability is not well understood. Doping with transition metal cations for B site cations often leads to exsolution under reducing conditions. Many perovskites used for the oxidative coupling of methane (OCM) or the electrochemical oxidative coupling of methane (E-OCM) either lack long term stability, or catalytic activity within these highly reducing methane environments. The Mg and Fe co-doped barium niobate BaMg0.33Nb0.67-xFexO3-δshown activity in E-OCM reactors over long periods (2) (>100 hrs) with no iron metal exsolution observed by diffraction or STEM EDX measurements. In contrast, iron decorated BaMg0.33Nb0.67O3 showed little C2 conversion activity.

The proton conduction mechanism of well-characterized SnP2O7 and Sn1-xInxP2O7 materials was investigated using AC impedance spectroscopy and inelastic neutron scattering. Samples with minimal amounts of excess phosphate do not possess appreciable conductivity at room temperature. Indium doping appears to increase the high temperature proton conductivity of the materials. Inelastic neutron scattering data indicates that no appreciable changes in proton concentration occur between hydrated and dehydrated samples of SnP2O7 while changes in proton vibrational mode amplitudes occur with indium doping, or upon hydration, of samples that contain excess phosphate.

Indium-doped tin pyrophosphate materials were synthesized with varying metal to phosphate ratios to assess the influence of the excess amorphous polyphosphate phase on conductivity. Total conductivity at 250C/aC was found to increase with increasing P:M ratio, with a maximum conductivity of 10-1.8 Scm-1 in dry N2 and 10-0.8 Scm-1 in humidified N2. These results combined with supporting spectroscopy, suggest that a grain-boundary polyphosphate phase is required to achieve high conductivity at low-intermediate temperatures. The conductivity of the excess phosphorous content materials was stable in both dry and humidified environments at 250C/aC. The use of this material as a promising fuel cell electrolyte was demonstrated with a maximum power density of 11 mWcm-2 obtained for the highest P:M ratio material at 300C/aC without humidification. This value is an order of magnitude lower than what has been reported previously in the literature. However the conductivity of the electrolyte in the operating fuel cell was 0.075 Scm-1 and the poor performance is attributable to cathodic overpotentials.

Novel Pt/ceria/carbon catalysts with sub-2 nm ceria crystallites embedded in carbon matrixes, recently developed at Los Alamos National Laboratory, were studied in aqueous acidic media using rotating ring disk electrode (RRDE) and voltammetric techniques. The ceria nanophase was either pure or doped with trivalent gadolinium or praseodymium. The catalysts with lower Pt contents (3.8-5%) had higher specific surface areas and performed better than those with higher Pt contents ([?]8.5%). The doping with either Pr or Gd was found to improve the catalyst specific surface area, whereas Pr doped catalysts exhibited higher specific activities in oxygen reduction reaction (ORR) than non doped and Gd doped catalysts. The former are also more selective for four-electron ORR. The presence of ceria in the catalyst support was found to increase the catalyst tolerance to CO poisoning.

This study demonstrates a new method for the development of low loading Pt catalysts for the oxygen reduction reaction (ORR) based on the electrochemical synthesis of Pt coated polypyrrole (PPY) nanowires. PPY was deposited on different electrodes such as graphite, carbon paper gas diffusion layer (GDL), and glassy carbon by using chemical template molecules such as heparin and starch. Pt coated PPY/starch nanowires show ORR catalytic activity after the electrochemical degradation of PPY, which suggests that PPY/starch nanowires can be electrochemically converted to a stable high surface area support for ORR catalysts. Moreover, these low loading ORR Pt catalysts can be easily incorporated in the membrane electrode assembly (MEA) structure as a gas diffusion electrode (GDE) when directly prepared on a GDL.

The article reports on a new conditioning technique referred to as the pulsed discharge technique to improve the performance of non-Nernstian potentiometric sensors. In this method, the sensor is energized through a series of positive and negative pulses between the electrodes with discharge phases in between. The sensor response is extracted from the discharge characteristics, which depend on the type of gas and its concentration. The data is presented as a combination of variables derived form the discharge characteristics. In this study, a 'La0.8Sr0.2CrO3/YSZ/Pt' sensor configuration was used to investigate this technique. The sensor baseline drift, response, and recovery time was found to improve when the device was subjected to PDT. Further, a 27-fold increase in sensor response was observed per 1000 ppm of NO compared to the device tested in the absence of PDT. The PDT technique is thought to change the interfacial defect concentration by voltage pulses in the solid electrolyte. Subsequent measurement of the relaxation rate after perturbation in the presence of the target gas gives rise to unique discharge characteristics. It is emphasized that this study has to be seen as an initial approach that demonstrates the potential of the novel conditioning principle for improving the non-Nernstian sensor performance.

One of the outstanding challenges in the wider deployment of PEMFCs is improving the utilization of Pt. While decreasing particle size improves accessibility of the Pt, it also destabilizes the Pt particles and leads to dissolution/re-precipitation and rapid grain growth. A parameter that is as yet poorly characterized is the number of nucleation sites on the carbon support. Increased nucleation site density could provide a valuable approach to improve Pt utilization. In this work, nucleation density was characterized by determining the number of particles per nm2. A range of metal loadings were explored to investigate their effect on the number of Pt particles on three types of carbon support. It was concluded that the number of nucleation sites is relatively constant over the Vulcan family of carbon supports. However, activated carbons such as Norit SX-1G show much higher nucleation density.

The article reports on the statistical analysis of a new conditioning technique referred to as the pulsed discharge technique (PDT) with a view to understand the effect of PDT variables on potentiometric sensor response.

The article reports on the effect of Yttria-Stabilized Zirconia (YSZ) sintering temperature on the 'La0.8Sr0.2CrO3/YSZ/Pt' mixed potential sensor response. In this investigation, the sintering temperature of YSZ was varied from 1000 to 12000 C. The sensor response and response time was recorded for each sample. In addition, the morphology of YSZ was investigated using Mercury Porosimetry measurements.

Mixed potential sensors for the detection of hydrocarbons, NOx, and NH3 have been previously developed at Los Alamos National Laboratory (LANL). The LANL sensors have a unique design incorporating dense ceramic-pellet/metal-wire electrodes and porous electrolytes. The performance of current-biased sensors using an yttria-stabilized zirconia (YSZ) electrolyte and platinum and La0.8Sr0.2CrO3 electrodes is reported. X-ray tomography has been applied to non-destructively exam internal structures of these sensors. NOx and hydrocarbon response of the sensors under various bias conditions is reported, and very little NOx response hysteresis was observed. The application of a 0.6 microamp bias to these sensors shifts the response from a hydrocarbon response to a NOx response equal for both NO and NO2 species at approximately 500 oC in air.

Polymer electrolyte membrane fuel cells are sensitive to impurities. H2S at the ppb level will impact fuel cell performance. It is important to quantify H2S adsorption and issues such as permeation and crossover rates become extremely important. Experimental methods have been developed to quantify H2S adsorption onto surfaces and to quantify H2S permeation through Nafion(r) using common and inexpensive Ag/AgS ion-probes. In addition to calculating the H2S uptake on commonly used XC-72 and Pt/XC-72 materials, the H2S permeability through dry and humidified Nafion(r) membranes was also studied using specialized techniques. A sulfide anti-oxidant buffer solution was used to trap and concentrate trace quantities of H2S during the course of the measurement. Crossover experiments were conducted for up to 24 hours in order to achieve sulfide ion concentrations high enough to be precisely determined by subsequent titration. We have confirmed H2S crossover in Nafion(r) membranes and have calculated preliminary rates of crossover.

Mixed potential sensors utilizing a machined, dense indium-tin oxide working electrode (In2O3:SnO2; 90%:10%), a Pt wire counter electrode, and porous YSZ electrolyte were prepared using ceramic tape casting methods. The response of these devices to hydrogen concentrations up to 2% in air were studied from 600 to 740{degree sign}C. The sensor response exhibited a reversible behavior and a fast response time with sensitivity increasing with decreasing temperature. GC analysis confirmed significant heterogeneous oxidation of the H2 on heated furnace tube wall surfaces thus driving sensor response at H2 concentrations greater than a few hundred ppm. The transition to a cold wall, miniature platform heater significantly reduced hydrogen oxidation although some flow rate dependence remains.

We have synthesized various indium-tin pyrophosphates using the traditional high-temperature method described in the literature as well as a novel solution-precipitation process developed in our laboratory. An ammonium-based precursor was used in an attempt to achieve a lower crystallization temperature. The use of this precursor in the solution-precipitation of indium-tin pyrophosphate gels resulted in precursors that could be crystallized at less than 600{degree sign}C. Inelastic neutron scattering studies indicate the presence of phosphate rich grain boundary phases that may be responsible for the high conductivity of the materials.

The H2S permeability through dry and humidified Nafion{trade mark, serif} PEMFC membranes was studied using ion probe techniques. A sulfide anti-oxidant buffer solution was used to trap and concentrate trace quantities of H2S that permeated through 50 cm2 samples of Nafion{trade mark, serif} 117 and 212 membranes using a partial pressure difference up to 1030ppm at room temperature. Experiments were conducted for up to 24 hours in order to achieve sulfide ion concentrations high enough to be precisely determined by subsequent titration with Pb(NO3)2. The rate of H2S crossover for dry 117 and 212 were identical at 1.2e-7 g/min. Humidification increased the crossover rate to 5.9e-7 g/min and 1.8e-6 g/min for 117 and 212 respectively. An accurate determination of permeation constants from this work was not possible because the H2S partial pressure was found to be not constant throughout the experiment.

Metal hexaborides demonstrate the unusual ability to spontaneously deposit noble metals from solution without the need for separate reducing agents or pretreatments. A range of deposition thicknesses are possible and uniform and strongly adherent 30 nm thick metal films on lanthanum hexaboride sputter- coated glass are easily and quickly achieved. Full and uniform coverages of hexaboride powders are obtained even with deposits only ca. 1 nm thick. Cyclic voltammetry for 4 wt% Pt on a roughly 1 m2/g EuB6 powder show electrochemically active Pt surface area, derived from the area under the hydrogen absorption curve, to be 39 m2/g Pt and is exceptional considering the low hexaboride support surface area. This result suggests that the entire support area is covered with Pt (approximately 1 nm thick) and that the entire Pt surface is electrochemically active.

Mixed potential sensors for the detection of hydrocarbons and carbon monoxide have been studied at Los Alamos National Laboratory (LANL) over the past decade. The LANL sensors have a unique design incorporating dense ceramic-pellet/metal- wire electrodes and porous zirconia-based electrolytes. When these sensors are exposed to various gases, in addition to their mixed-potential response, their resistance also changes. This change in resistance is associated with the electrode/ electrolyte interface and can be used to yield a total NOx response. The NOx sensors are operated in a current bias mode where the voltage response can be related to the total NOx concentration in the atmosphere.

Carbohydrate-dye combinations were used to form ionically-linked soft templates for the formation of polypyrrole nanowire networks. High yields of nanostructured products were obtained using small amounts of low-cost carbohydrate and dye template materials, the majority of which remained encapsulated within the nanowires. Varying the concentration and the two-part ratio of the templates influenced the length and diameter of the nanofiber segments within the nanowire network. Pyrolysis of the nanowires yielded carbonaceous fibers containing nitrogen heteroatoms as well as convoluted graphitic domains well suited for supporting Pt nanoparticles. The resulting high density of nucleation sites enabled the formation of smaller Pt particles compared to commercial catalysts despite significantly higher support surface loadings.