The use of palladium and strontium in LaCrO3 perovskites for solid oxide fuel cell anodes is known to lead to improved performance, in part due to the formation of palladium at the surface under reducing conditions that can act as a catalyst, with regeneration of the catalyst possible by cycling between oxidizing and reducing conditions. Whether this cycling involves dissolution and exsolution of the palladium into the perovskite is unclear. We describe a detailed analysis of the perovskites La0.8Sr0.2Cr1−xPdxO3−δ (LSCrPd, nominal stoichiometry x=0.1 and 0.2) under reducing and oxidizing conditions. A LSCrPd perovskite was found to be the main phase, further confirmed by transmission electron microscopy. Secondary phases including metallic Pd, PdO, and La4PdO7, as well as SrCrO4, were also present. Some phases, such as PdO, SrCrO4, and La4PdO7, were no longer present following reduction while other phases such as metallic Pd and La2O3, were found in increasing amounts. When used as active solid oxide fuel cell anode layers both with and without Gd0.1Ce0.9O2−β (GDC) in La0.9Sr0.1Ga0.8Mg0.2O3−ε/La0.4Ce0.6O2 electrolyte-supported cells, SOFCs with anodes containing GDC and higher amounts of Pd demonstrated higher maximum power densities and lower anode polarization resistances compared to cells with GDC-free, lower Pd content anodes. While the Pd is important to improve the anode performance, the results indicate that cycling does not lead to simple dissolution/exsolution into the perovskite, instead many other phases are present while a limited amount of Pd was actually observed in the perovskite.

The most widely used Solid Oxide Fuel Cell (SOFC) anode materials, Ni-cermets, are susceptible to coking in hydrocarbon fuels as well as many coal- and bio-derived syngas compositions. Ni-based anodes are also poisoned by fuel impurities such as hydrogen sulfide, and can be structural vulnerable during repeated redox cycling. These problems can be largely avoided by implementing fuel processing measures, but not without sacrificing the overall power plant cost effectiveness and efficiency. A number of different conducting oxide materials have been proposed as anodes that can avoid the above-mentioned problems with Ni-based anodes. However, these oxide anodes exhibit much higher polarization resistance than anodes containing Ni or other catalytic metals. The mechanisms that limit hydrogen oxidation rates in these oxide anodes have not been determined. Several studies of chromite, titanate-manganite, chromite-manganite, and doped ceria anodes have suggested that hydrogen adsorption was a possible rate-limiting step, along with hydrogen electrochemical oxidation. Recent electrochemical impedance spectroscopy (EIS) measurements on SOFCs with SrTi0.3Fe0.7O3-δ (STF)-Gd0.1Ce0.9O2 (GDC) anode as a function of hydrogen partial pressure provided direct evidence that a H2adsorption mechanism was an important rate-limiting step in the anode reaction. Here we present new results on (La,Sr)(Ga,Mg)O3 (LSGM) electrolyte-supported SOFCs with STF and (La,Sr)(Cr,Fe)O3-δ (LSCrFe) based oxide anodes. The results show clear evidence that adsorption becomes the rate-limiting step at low temperature and low fuel hydrogen partial pressure. Figure 1 shows an example of the current-voltage results for an STF anode cell, where there is a limiting current that decreases with decreasing hydrogen partial pressure. Figure 2 shows that the limiting current decreases with decreasing temperature, indicating that it is an activated process and hence not a gas diffusion limitation. A model accounting for hydrogen adsorption and electrochemical oxidation as possible rate-limiting steps is developed and used to fit current-voltage and EIS results from the oxide anode cells. The fits, shown in Figures 1 and 2, are good for cell voltages from open circuit down to ~ 0.5 V. Possible reasons for the deviation at lower cell potentials will be discussed in the talk. It is also shown that the model is consistent with results for other oxide anodes, suggesting that the dissociative adsorption of H2can be an important rate-limiting step for various oxide anodes under SOFC operating conditions, especially lower temperature and hydrogen partial pressure. Moreover, both the experimental data and model fitting results suggest that the precipitation of metal particles at the anode surface, like Ru in LSCrFe based anodes and Ni in STF based anodes, effectively promote hydrogen adsorption. Figuer captions: Fig. 1 Current-voltage data and fitting results for LSGM electrolyte supported cell with STF anode at 800 oC versus the hydrogen partial pressure Fig. 2 Current-voltage data and fitting results for LSGM electrolyte supported cell with STF anode at various temperatures under wet H2 Figure 1

Solid oxide fuel cells with (La,Sr)(Ga,Mg)O-3 electrolytes and Sr(Ti0.3Fe0.7)O-3 anodes and cathodes yield a power density of 0.6 W/cm(2) at 0.7 V at 800 degrees C in air and humidified hydrogen. The polarization resistance values are 0.085 Omega . cm(2) for the cathodes and 0.13 Omega . cm(2) for the anodes. The cell current-voltage characteristics and the anode resistance dependence on hydrogen partial pressure (pH(2)) both indicate that adsorption limits the hydrogen oxidation process. A model is developed where dissociative hydrogen adsorption becomes an increasingly important rate-limiting step, relative to charge transfer, as temperature and pH(2) decrease. The model fits the data well for cell voltages >0.5 V, deviations at lower voltages are tentatively explained by an increase in anode oxygen content. The model also fits the electrochemical characteristics of cells with (La,Sr)(Cr,Fe)O-3 anodes and previously-reported data on other oxide anodes. (C) 2016 The Electrochemical Society. All rights reserved.

The perovskite compounds La0.33Sr0.67Cr1-x-yFexRuyO3-delta (LSCrFeRu, x = 0.62, 0.57, and 0.47; y = 0.05, 0.14, and 0.2, respectively) were synthesized and assessed as a new type of solid oxide fuel cell (SOFC) anode in composite with Gd0.1Ce0.9O2-beta (GDC) in La0.9Sr0.1Ga0.8Mg0.2O3-epsilon/La0.4Ce0.6O2 bilayer electrolyte-supported cells. By comparing anode polarization resistance R-P,R-A values for the LSCrFeRu compounds to the either exclusively Fe- or Ru-substituted (La,Sr)CrO3-delta perovskites, the present results demonstrate that the two substituent cations work synergistically to provide further reduction in R-P,R-A from 0.290 Omega.cm(2) for La0.33Sr0.67Cr0.33Fe0.67O3-delta (LSCrFe) and 0.235 Omega.cm(2) for La0.8Sr0.2Cr0.8Ru0.2O3-delta (LSCrRu) to 0.195 Omega.cm(2) for LSCrFeRu (all measured in humidified hydrogen at 800 degrees C). These impedance results also strongly suggest that hydrogen dissociative adsorption was the rate-limiting step in the hydrogen oxidation reaction sequence for LSCrFe anodes at some of the pH(2) and temperatures measured. However, the formation of Ru nanoparticles on LSCrFeRu and LSCrRu surfaces, observed by scanning and transmission electron microscopy, appears to promote hydrogen dissociation. Substituting even small amounts of Ru into (La,Sr)(Cr,Fe)O3-delta perovskites is thus sufficient to make hydrogen electrochemical oxidation the rate-limiting step, resulting in anodes with significantly reduced R-P,R-A.

The perovskite series, La1-xSrxCr1-xFexO3-delta (x = 0.2, 0.3, 0.4, 0.5, 0.67, LSCrFe), was synthesized and examined as both single phase and LSCrFe-Gd0.1Ce0.9O2-beta (GDC) composite solid oxide fuel cell anodes in full cells with La0.9Sr0.1Ga0.8Mg0.2O3-epsilon/La0.4Ce0.6O2 bilayer electrolytes. Each anode demonstrated marked improvement in polarization resistance compared to prior studies on Fe-free La1-xSrxCrO3-delta-based anodes and in stability compared to studies on more Fe-rich compositions. Higher Fe content anodes yielded lower polarization resistances, with the x = 0.67 anodes obtaining resistances of 0.275 Omega.cm(2) for LSCrFe and 0.333 Omega. cm(2) for LSCrFe-GDC in humidified H-2 at 800 degrees C. The lower polarization resistance with increasing Fe content can be attributed to oxygen loss, which introduces significant ionic conductivity into these perovskites. Substitution of an intermediate amount of Fe and Sr into the perovskites can thus optimize anode performance.

A new composite solid oxide fuel cell anode material, SrTi1-xFexO3-delta mixed with Gd-doped ceria, was tested in La0.9Sr0.1Ga0.8Mg0.2O3-delta electrolyte-supported cells with La0.4Ce0.6O2 barrier layers and La0.6Sr0.4Fe0.8Co0.2O3 cathodes. The x = 0.7 composition had an anode polarization resistance of 0.17 Omega cm(2), at 800 degrees C in humidified H-2, much lower than the value 0.39 Omega cm(2) measured for x = 0.4 and 3.14 Omega cm(2) for x = 0. The reduced polarization resistance correlated with increased oxygen non-stoichiometry delta, which suggests that it may be related to increased ionic conductivity. Electrochemical impedance spectroscopy (EIS) measurements at 800 degrees C on the x = 0.7 anode cell showed a main response centered at similar to 1 Hz that increased with decreasing hydrogen partial pressure. The maximum power density observed was for the x = 0.7 cell - 337 mW cm(-2) at 800 degrees C in air and humidified hydrogen - and was limited mainly by the thick electrolyte.

The transport of microscale carbonyl iron powder suspensions modified with anionic homopolymers was studied in water-saturated sand columns containing well-dispersed hydrophobic sand grains. Sand grains functionalized with hexadecyltrimethoxysilane were coated with a eutectic mixture of dichlorobenzenes that was solid at -10 °C and was mixed by grinding with unmodified sand grains. The dichlorobenzene coating liquefied at the temperature of the transport experiments, and the coated grains were thus mimetic of uniform droplets of dense nonaqueous phase liquid (DNAPL) contaminants. By comparing iron particle transport in uncontaminated columns with those that contained a small fraction of DNAPL-coated sand grains, sticking coefficients for both types of grains could be estimated. The anionic polyelectrolytes tested (polyacrylate, carboxymethylcellulose, alginate, and metasilicate) all gave low particle sticking coefficients (0.004-0.05) to unmodified sand, as expected from earlier studies. However, iron particles modified with the two moderately hydrophobic polymers (carboxymethylcellulose and polyacrylate) had 30-fold higher sticking coefficients (0.40 and 0.13, respectively) to the model DNAPL surface than they did to the sand surface. In contrast, no significant difference between the two kinds of collector grains was found with the more polar polymers (metasilicate and alginate). The trend in sticking coefficients was correlated with the surface energy of the polymer-modified iron surface as measured by the static contact angle method. From these data one can conclude that the hydrophobicity of the polymer dispersant is a key factor in targeting zerovalent iron to DNAPL source zones in soil and groundwater.