Composite electrodes were prepared by adding 40 wt % La 0.8 Sr 0.2 FeO 3 ͑LSF͒ into porous yttria-stabilized zirconia ͑YSZ͒ and their performance was studied as a function of time and calcination temperature. X-ray diffraction ͑XRD͒ patterns of the LSF-YSZ composites indicated an expanded lattice parameter after calcination above 1523 K, suggesting that Zr reacted with the LSF to form a Zr-doped perovskite; but XRD provided no evidence for reaction between LSF and YSZ after calcination at 1373 K or after operation for 1000 h at 973 K and 700 h at 1073 K. A composite of 40 wt % La 0.8 Sr 0.2 Fe 0.9 Zr 0.1 O 3 in YSZ showed reasonable performance at 973 K, with an area-specific resistance ͑ASR͒ of 0.22 ⍀ cm 2 . Based on symmetric-cell measurements, electrodes calcined at 1123 K showed an initial ASR of 0.13 ⍀ cm 2 at 973 K but this increased linearly with time to 0.55 ⍀ cm 2 after 2500 h at 973 K. However, the ASR depended strongly on current density, decreasing dramatically under both anodic and cathodic polarization. Electrodes calcined at 1373 K showed an ASR of 2.5 ⍀ cm 2 at 973 K but this value also decreased dramatically under polarization. Scanning electron microcopy images demonstrate that aging at 973 K and calcination at 1373 K cause significant sintering of the LSF. It is therefore suggested that deactivation is caused by morphological changes, rather than solid-state reactions, with a dense layer of LSF forming over the YSZ substrate.

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There would be significant advantages to having anodes for solid oxide fuel cells (SOFC) that were capable of directly utilizing hydrocarbon fuels. Because conventional Ni-based anodes catalyze the formation of carbon fibers, new anode compositions are required for this application, but most of the materials that have been proposed exhibit either limited thermal stability or poor electrochemical activity. In this paper, we will describe two strategies for the development of new anodes with improved performance. The first strategy involves the use of bimetallic compositions with layered microstructures. In the bimetallic anodes, one metal is used for thermal stability while the other provides the required carbon tolerance. The second strategy involves separating the anode into two layers: a thin functional layer for electrocatalysis and a thicker conduction layer for current collection. With this approach, the functional layer can be optimized for catalytic activity and, if it is thin enough, requires minimal conductivity. Examples are shown for each of these approaches and possible future directions are outlined. AbstractThere would be significant advantages to having anodes for solid oxide fuel cells (SOFC) that were capable of directly utilizing hydrocarbon fuels. Because conventional Ni-based anodes catalyze the formation of carbon fibers, new anode compositions are required for this application, but most of the materials that have been proposed exhibit either limited thermal stability or poor electrochemical activity. In this paper, we will describe two strategies for the development of new anodes with improved performance. The first strategy involves the use of bimetallic compositions with layered microstructures. In the bimetallic anodes, one metal is used for thermal stability while the other provides the required carbon tolerance. The second strategy involves separating the anode into two layers: a thin functional layer for electrocatalysis and a thicker conduction layer for current collection. With this approach, the functional layer can be optimized for catalytic activity and, if it is thin enough, requires minimal conductivity. Examples are shown for each of these approaches and possible future directions are outlined.

Li2SnS3 is a fast Li+ ion conductor that exhibits high thermal stability (mp ∼750 °C) as well as environmental stability under ambient conditions. Polycrystalline Li2SnS3 was synthesized using high-temperature, solid-state synthesis. According to single-crystal X-ray diffraction, Li2SnS3 has a sodium chloride-like structure (space group C2/c), a result supported by synchrotron X-ray powder diffraction and 119Sn Mössbauer spectroscopy. According to impedance spectroscopy, Li2SnS3 exhibits Li+ ion conductivity up to 1.6 × 10–3 S/cm at 100 °C, which is among the highest for ternary chalcogenides. First-principles simulations of Li2SnS3 and the oxide analogue, Li2SnO3, provide insight into the basic properties and mechanisms of the ionic conduction. The high thermal stability, significant lithium ion conductivity, and environmental stability make Li2SnS3 a promising new solid-state electrolyte for lithium ion batteries.

A proposal that high solid oxide fuel cell (SOFC) anode performance can be achieved by using a very thin, catalytically active functional layer, with a noncatalytic conduction layer, has been tested. An anode impedance of 0.26 Ω cm 2 was obtained at 973 K in humidified H 2 using a Ag-paste conduction layer and a 12 µm thick functional layer made from 1 wt % Pd and 40 wt % ceria in yttria-stabilized zirconia. Replacing the Ag paste with a 100 µm layer of porous La 0.3 Sr 0.7 TiO 3 (LST) had minimal effect on cell performance. The anode concept is flexible and should allow various materials to be used in the functional and the currentcollector layers.

The properties of solid oxide fuel cell ͑SOFC͒ anode functional layers prepared by impregnation of ceria and catalytic metals into porous yttria-stabilized zirconia ͑YSZ͒ have been examined for operation at 973 K. By varying the thickness of the functional layer, the conductivity of the ceria-YSZ composite was determined to be only 0.015-0.02 S/cm. The initial performance of anodes made with ceria loadings of 40 or 60 wt % were similar but the anodes with lower loadings lost conductivity above 1073 K due to sintering of the ceria. The addition of dopant levels of catalytic metals was found to be critical. The addition of 1 wt % Pd or Ni decreased the anode impedances in humidified H 2 dramatically, while the improvement with 5 wt % Cu was significant but more modest. Pd doping also decreased the anode impedance in dry CH 4 much more than did Cu doping; however, addition of either Pd or Cu led to similar improvements for operation in n-butane. Based on these results, suggestions are made for ways to improve SOFC anode functional layers.

Cu-based, solid oxide fuel cell (SOFC) electrodes were modified by electrodeposition of Co. The addition of only 5-vol% Co by electrodeposition significantly improved the thermal stability compared to either Cu-ceria-YSZ, Cu-Co-ceria-YSZ, or Co-ceria-YSZ electrodes prepared only by impregnation with much higher metal loadings, demonstrating that electrodeposited metal layers form metal films with better connectivity. In the absence of Co, SEM showed structural changes in the impregnated Cu after heating to 1173 K in humidified H 2 and these changes caused large increases in the ohmic resistance of fuel cells, as measured by impedance spectroscopy. In contrast, the ohmic resistance of a cell with 13-vol% Cu, 9-vol% ceria, and 5-vol% Co increased only slightly after 48 h at 1173 K in humidified H 2 . While a Co-ceria-YSZ composite was found to form large amounts of carbon upon exposure to dry CH 4 at 1073 K for 3 h, the Co-Cu-ceria-YSZ composites did not form measurable amounts of carbon for the same conditions. XPS results for a Cu foil with a 250-nm Co film demonstrated that Cu migrates to the surface of the Co upon heating above 873 K, forming a stable Cu layer that appears to be approximately one monolayer thick. The implication of these results for the development of practical SOFC electrodes for the direct utilization of hydrocarbons is discussed. Co. The addition of only 5-vol% Co by electrodeposition significantly improved the thermal stability compared to either Cu-ceria-YSZ, Cu-Co-ceria-YSZ, or Co-ceria-YSZ electrodes prepared only by impregnation with much higher metal loadings, demonstrating that electrodeposited metal layers form metal films with better connectivity. In the absence of Co, SEM showed structural changes in the impregnated Cu after heating to 1173 K in humidified H 2 and these changes caused large increases in the ohmic resistance of fuel cells, as measured by impedance spectroscopy. In contrast, the ohmic resistance of a cell with 13-vol% Cu, 9-vol% ceria, and 5-vol% Co increased only slightly after 48 h at 1173 K in humidified H 2 . While a Coceria-YSZ composite was found to form large amounts of carbon upon exposure to dry CH 4 at 1073 K for 3 h, the Co-Cu-ceria-YSZ composites did not form measurable amounts of carbon for the same conditions. XPS results for a Cu foil with a 250-nm Co film demonstrated that Cu migrates to the surface of the Co upon heating above 873 K, forming a stable Cu layer that appears to be approximately one monolayer thick. The implication of these results for the development of practical SOFC electrodes for the direct utilization of hydrocarbons is discussed.

Ceramic anodes comprising infiltrated SrMoO 4 in porous ytttria-stabilized zirconia were investigated. Upon reduction at 1073 K, the electronically insulating SrMoO 4 phase transformed to SrMoO 3 , which has a bulk electronic conductivity of 10 3 S cm −1 under fuel cell conditions. An anode conductivity of 20 S cm −1 was achieved with a low SrMoO 4 loading of 13 vol % of the total anode. The infiltrated composite is dimensionally stable upon redox cycling, and a Pd catalyst was required to achieve good fuel cell performance. Fuel cell performance with methane was lower than with hydrogen. This lower methane performance could be due to coking.Solid oxide fuel cells ͑SOFCs͒ are a promising fuel flexible technology for clean and efficient conversion of chemical energy to electrical energy. While the fuel flexibility of SOFCs is widely considered a key benefit, it is well known that the conventional nickelyttria-stabilized zirconia ͑Ni-YSZ͒ cermet anode catalyzes the formation of deleterious carbon filaments in the presence of hydrocarbons. 1 This carbon fiber formation results in deactivation of the Ni surface, loss of Ni by metal dusting, 2 and anode fracture induced by carbon fiber growth. 3 Other drawbacks of Ni include catalytic poisoning in the presence of sulfur and low redox stability. 4,5 Replacement of Ni with an electronic conducting ceramic has generated great interest because ceramics tend to be more fuel flexible and sulfur tolerant than Ni. The target anode must offer ionic/ electronic conductivity, gas transport, and catalytic activity for fuel oxidation. Of these requirements, Ni provides both electronic conductivity and catalytic activity for the oxidation reaction. With a single metal oxide material, it is difficult to achieve high electronic conductivity and catalytic activity. Implementation of a single metal oxide to fulfill both roles would be simpler; however, incorporating two separate materials allows for independent optimization of an electronic conductor and oxidation catalyst. 6 In this paper, we focus on the development of a highly conductive ceramic anode into which any catalyst of choice can be incorporated.The target electronic conductivity for anode materials has been outlined in a review by Atkinson et al. 7 These authors have argued that the required anode conductivity under operating conditions can be relaxed to as low as 1 S cm −1 depending on cell design. The highest performing anodes are typically porous composites of the electrolyte ͑YSZ͒ and an electronic conductor. The composite conductivity of the electrode is strongly dependent on the structure. For example, the conductivity of a conventional Ni-YSZ composite is 10 3 S cm −1 , even though Ni has a conductivity of 10 6 S cm −1 . 8 Compared to conventional cosintering methods, composites formed by infiltration have the advantage of providing higher conductivity at lower volume percent ͑vol. %͒ loadings of the electronic conductor. 9 However, even with infiltrated composites, the conductivity of the composite will be at leas...