Scott Paulson

Effect of Sintering Temperature on Microstructure, Chemical Stability, and Electrical Properties of Transition Metal or Yb-Doped BaZr0.1Ce0.7Y0.1M0.1O3−δ (M = Fe, Ni, Co, and Yb)

Perovskite-type BaZr0.1Ce0.7Y0.1M0.1O3-δ (M = Fe, Ni, Co and Yb) (BZCY-M) oxides were synthesized using the conventional solid-state reaction method at 1350-1550 oC in air in order to investigate the effect of dopants on sintering, crystal structure, chemical stability under CO2 and H2S, and electrical transport properties. The formation of the single-phase perovskite-type structure with an orthorhombic space group Imam was confirmed by Rietveld refinement using powder X-ray diffraction (PXRD) for the Fe, Co, Ni and Yb-doped samples. The BZCY-Co and BZCY-Ni oxides show a total electrical conductivity of 0.01 and 8 × 10-3 Scm-1 at 600 oC in wet H2 with an activation energy of 0.36 and 0.41 eV, respectively. Scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX) revealed Ba and Co rich secondary phase at the grain-boundaries, which may explain the enhancement in the total conductivity of the BZCY-Co. However, ex-solution of Ni at higher sintering temperatures, especially at 1550 oC, decreases the total conductivity of the BZCY-Ni material. The Co and Ni dopants act as a sintering aid and form dense pellets at a lower sintering temperature of 1250 oC. The Fe, Co and Ni-doped BZCY-M samples synthesized at 1350 oC show stability in 30 ppm H2S/H2 at 800 oC, and increasing the firing temperature to 1550 oC, enhanced the chemical stability in CO2 / N2 (1: 2) at 25-900 oC. The BZCY-Co and Ni compounds with high conductivity in wet H2 could be considered as possible anodes for intermediate temperature solid oxide fuel cells (IT-SOFCs).

Surface and bulk study of strontium-rich chromium ferrite oxide as a robust solid oxide fuel cell cathode

A novel Co-free cathode, La0.3Sr0.7Fe0.7Cr0.3O3−δ (LSFCr-3), exhibiting the desired combination of high electrical conductivity, physical and chemical stability, and electrocatalytic activity, was systematically investigated for SOFC applications. Its excellent performance is attributed primarily to the presence of Cr, which was found to be predominant in the 3+ and 4+ oxidation states in the LSFCr-3 bulk, thus likely maintaining a 6-fold coordination with oxygen anions. This, in turn, causes disorder in the oxygen vacancy sub-lattice, stabilized by the Fe ion–oxygen tetrahedra. However, on the surface of the LSFCr-3 oxide, Cr is primarily in the 6+ state, together with some Cr3+/Cr4+, even at 700 °C. Cr6+ can only be tetrahedrally coordinated by oxygen anions, resulting in a large concentration of oxygen vacancies on the LSFCr-3 surface, with a surface exchange coefficient and oxygen ionic conductivity of ca. 10−5 cm s−1 and ca. 10−2 S cm−1, respectively, at 700–800 °C. The use of LSFCr-3 as the cathode in a Ni–Ce0.8Sm0.2O2−δ (SDC) anode-supported single solid oxide fuel cell in 3% H2O–H2/air gave a maximum power density of 0.81 W cm2 at 750 °C, which is superior to that of similar cells in which La0.6Sr0.4Fe0.8Co0.2O3−δ, a previously well studied material, was used as the cathode.

Activation of H2 oxidation at sulphur-exposed Ni surfaces under low temperature SOFC conditions

Ni-YSZ (yttria-stabilized zirconia) cermets are known to be very good anodes in solid oxide fuel cells (SOFCs), which are typically operated at 700-1000 °C. However, they are expected to be increasingly degraded as the operating temperature is lowered in the presence of H2S (5-10 ppm) in the H2 fuel stream. However, at 500 to 600 °C, a temperature range rarely examined for sulphur poisoning, but of great interest for next generation SOFCs, we report that H2S-exposed Ni-YSZ anodes are catalytic towards the H2 oxidation reaction, rather than poisoned. By analogy with bulk Ni3S2/YSZ anodes, shown previously to enhance H2 oxidation kinetics, it is proposed that a thin layer of Ni sulphide, akin to Ni3S2, is forming, at least at the triple point boundary (TPB) region under our conditions. To explain why Ni3S2/YSZ is so active, it is shown from density functional theory (DFT) calculations that the O(2-) anions at the Ni3S2/YSZ TPB are more reactive towards hydrogen oxidation than is O(2-) at the Ni/YSZ TPB. This is accounted for primarily by structural transformations of Ni3S2 during H2 oxidation, rather than by the electronic properties of this interface. To understand why a thin layer of Ni3S2 could form when a single monolayer of sulphur on the Ni surface is the predicted surface phase under our conditions, it is possible that the reaction of H2 with O(2-), forming water, prevents sulphur from re-equilibrating to H2S. This may then promote Ni sulphide formation, at least in the TPB region.

Cr-Substituted La0.3Sr0.7FeO3-δ Mixed Conducting Materials as Potential Electrodes for Symmetrical SOFCs

of Cr-substituted La0.3Sr0.7FeO3-� SOFC electrode materials were investigated in this work, primarily for application in symmetrical solid oxide fuel cells. Cr substitution, giving La0.3Sr0.7Fe1-xCrxO3-�(LSFC, x = 0, 0.2, 0.3), remarkably improves the phase stability of La0.3Sr0.7FeO3-� materials in reducing atmospheres and their redox tolerance in varying atmospheres. Symmetrical half-cells employing La0.3Sr0.7Fe0.8Cr0.2O3-� on La0.8Sr0.2Ga0.8Mg0.2O3-�(LSGM) electrolyte supports exhibit electrode polarization resistances (RP) of as low as 0.25 �•cm 2 in air and 0.45 �•cm 2 in humidified H2 at 750 o C. Maximum power densities of 0.30 W/cm 2 (800 o C) and 0.22 W/cm 2 (750 o C) were obtained for a symmetrical single cell using humidified H2/air. The excellent anodic and cathodic electrochemical performance of LSFC is thought to be