Michael van den Bossche

Insights Into the Fuel Oxidation Mechanism of La0.75Sr0.25Cr0.5Mn0.5O3 − δ SOFC Anodes

Solid oxide fuel cells (SOFCs) were fabricated by applying reproducible thin, dense La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3-δ (LSCM) anode and La 0.8 Sr 0.2 MnO 3-δ (LSM) cathode films to yttria-stabilized zirconia (YSZ) electrolytes by ultrasonic spray pyrolysis. A gold grid was applied to the anode film by lithography and plating, enabling adequate electrical contact while maintaining an open surface area for reaction. Alignment of the two electrode films and correct placement of the reference electrode enabled the separation of the anode and cathode impedance responses. Although the open-circuit voltage (OCV) of the LSCM|YSZ|LSM fuel cell at 700°C with humidified H 2 fuel was close to that predicted by the Nernst equation, the anode polarization resistance was large (23 Ω · cm 2 ). The addition of Pd to the anode film led only to a small decrease in polarization resistance, indicating that oxygen ion conductivity was the limiting factor for H 2 fuel. The OCV for CH 4 was close to that when an inert gas (He) was supplied to the anode. This OCV increased to 0.87 V upon addition of Pd to the LSCM film surface. This indicates that the bare LSCM film is limited by a low CH 4 activation activity.

The Influence of Current Density on the Electrocatalytic Activity of Oxide-Based Direct Hydrocarbon SOFC Anodes

We have studied the performance of Lao 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3-δ (LSCM )-Cu-yttria-stabilized zirconia anodes for the direct utilization of hydrocarbon fuels in a solid oxide fuel cell (SOFC). A significant decrease in anode polarization resistance and an increase in impedance peak frequency was observed with increasing the cell current density for H 2 , CH 4 , and C 4 H 10 fuel at both 973 and 1073 K. This was interpreted by considering an increase in LSCM lattice oxygen stoichiometry and a corresponding increase in electrocatalytic oxidation activity with increasing oxygen flux to the anode. A series of catalytic measurements indicates that, at a low current density (low oxygen stoichiometry), the anode reaction mechanism is dominated by hydrocarbon cracking. At a higher current density (higher oxygen stoichiometry), the reaction mechanism is dominated by total oxidation of hydrocarbon fuels on the LSCM surface to form CO 2 and H 2 O. This change in mechanism is confirmed by measurement of cell open-circuit voltage as a function of fuel, CO 2 , and H 2 O partial pressure and by analysis of the SOFC product stream. The increase in oxidation activity is attributed to an increase in nonequilibrium oxygen, relative to the anode gas, within the LSCM phase.

On the methane oxidation activity of Sr2(MgMo)2O6-δ: a potential anode material for direct hydrocarbon solid oxide fuel cells

Sr2(MgMo)2O6−δ and materials of nominal compositions with increased and decreased Mo content were synthesized and their stability and catalytic activity towards CH4 oxidation characterized under solid oxide fuel cell (SOFC) anode conditions. Sr2(MgMo)2O6−δ was synthesized as a double perovskite. Materials with increased Mo content formed a mixture of the double perovskite Sr2MgMoO6−δ and the single perovskite SrMoO3, after synthesis in H2, with the amount of SrMoO3 increasing with increasing Mo content. SrMoO3 is unstable in air and slowly oxidized to the scheelite-structured impurity SrMoO4. All compositions decomposed to Mo, MgO and SrO in dry 20% CH4/N2 at temperatures ≥ 850 °C. CH4 reaction rates were catalytically limited with measured rates significantly lower than for La0.75Sr0.25Cr0.5Mn0.5O3-δ. Rates increased by 1–2 orders of magnitude when 2 wt-% of Pt was added to the surface.

Wind Estimation in the Lower Atmosphere Using Multirotor Aircraft

AbstractUnmanned aerial vehicles are increasingly used to study atmospheric structure and dynamics. While much emphasis has been on the development of fixed-wing unmanned aircraft for atmospheric investigations, the use of multirotor aircraft is relatively unexplored, especially for capturing atmospheric winds. The purpose of this article is to demonstrate the efficacy of estimating wind speed and direction with 1) a direct approach using a sonic anemometer mounted on top of a hexacopter and 2) an indirect approach using attitude data from a quadcopter. The data are collected by the multirotor aircraft hovering 10 m above ground adjacent to one or more sonic anemometers. Wind speed and direction show good agreement with sonic anemometer measurements in the initial experiments. Typical errors in wind speed and direction are smaller than 0.5 and 30°, respectively. Multirotor aircraft provide a promising alternative to traditional platforms for vertical profiling in the atmospheric boundary layer, especially...

Spatiotemporal Variability of Surface Meteorological Variables During Fog and No-Fog Events in the Heber Valley, UT; Selected Case Studies From MATERHORN-Fog

We investigated the spatiotemporal variability of surface meteorological variables in the nocturnal boundary layer using six automatic weather stations deployed in the Heber Valley, UT, during the MATERHORN-Fog experiment. The stations were installed on the valley floor within a 1.5 km × 0.8 km area and collected 1-Hz wind and pressure data and 0.2-Hz temperature and humidity data. We describe the weather stations and analyze the spatiotemporal variability of the measured variables during three nights with radiative cooling. Two nights were characterized by the presence of dense ice fog, one night with a persistent (‘heavy’) fog, and one with a short-lived (‘moderate’) fog, while the third night had no fog. Frost-point depressions were larger preceding the night without fog and showed a continued decrease during the no-fog night. On both fog nights, the frost-point depression reached values close to zero early in the night, but ~5 h earlier on the heavy-fog night than on the moderate-fog night. Spatial variability of temperature and humidity was smallest during the heavy-fog night and increased temporarily during short periods when wind speeds increased and the fog lifted. During all three nights, wind speeds did not exceed 2 m/s. The temporal variability of the wind speed and direction was larger during the fog nights than during the no-fog nights, but was particularly large during the heavy-fog night. The large variability corresponded with short-lived (5–10 min) pressure variations with amplitudes on the order of 0.5 hPa, indicating gravity wave activity. These pressure fluctuations occurred at all stations and were correlated in particular with variability in wind direction. Although not able to provide a complete picture of the nocturnal boundary layer, our low-cost weather stations were able to continuously collect data that were comparable to those of nearby research-grade instruments. From these data, we distinguished between fog and no-fog events, successfully quantified spatiotemporal variations in surface properties during these events, and detected gravity waves.

Representativeness of wind measurements in moderately complex terrain

We investigated the representativeness of 10-m wind measurements in a 4 km × 2 km area of modest relief by comparing observations at a central site with those at four satellite sites located in the same area. Using a combination of established and new methods to quantify and visualize representativeness, we found significant differences in wind speed and direction between the four satellite sites and the central site. The representativeness of the central site wind measurements depended strongly on surface wind speed and direction, and atmospheric stability. Through closer inspection of the observations at one of the satellite sites, we concluded that terrain-forced flows combined with thermally driven downslope winds caused large biases in wind direction and speed. We used these biases to generate a basic model, showing that terrain-related differences in wind observations can to a large extent be predicted. Such a model is a cost-effective way to enhance an area’s wind field determination and to improve the outcome of pollutant dispersion and weather forecasting models.