(326f) Stability and Catalytic Properties of La0.75Sr0.25Cr0.50Mn0.40T0.10O3-δ (T = Co, Fe, Mn, Ni or V) for Solid Oxide Fuel Cell Anodes

Solid Oxide Fuel Cells (SOFCs) are a promising method for the efficient generation of electricity from conventional fuels and biofuels, and they bypass the need for H₂ production or extensive hydrocarbon fuel pretreatment. State-of-the-art SOFC anodes consisting of a Ni phase and an yttria-stabilized zirconia (YSZ) phase, have high performance in H₂ fuel, but promote the formation of graphite on the anode reaction sites during operation with hydrocarbon fuels¹. This leads to reduced power output and mechanical failure of the cell and prevents the long-term use of hydrocarbons on this type of anode. As a result, there is a need for different anode materials.

The perovskite lanthanum chromate, substituted with strontium on the lanthanum site and with manganese on the chromium site (La_0.75Sr_0.25Cr_0.50Mn_0.50O_3-δ, LSCM), is a likely candidate to replace the Ni/YSZ cermet. It is stable in anode conditions² and it does not show significant amounts of carbon formation during fuel cell operation. The oxidation of CH₄ on LSCM was recently reported³ to occur through a modified Mars-van Krevelen mechanism, with the rate and selectivity depending on Mn concentration and oxygen content of the material. Impedance measurements performed on working fuel cells showed that electrochemical performance increases with increasing oxygen ion transport through the anode⁴, confirming that the reaction rate indeed depends on the oxygen content of the catalyst.

The catalytic activity of LSCM with 50% mol% Mn was found to be 4.11 x 10^-3 mmol CO₂ m^-2 s^-1 at 900°C, which needs to be improved for use in SOFC^{3, 5}. To do this, researchers have mixed up to 5 wt% of separate metallic phases with LSCM. Costly metals such as Pt⁶, Pd and Rh⁷ indeed raised fuel cell performance, but also raised the production costs. Other phases, such as metallic Ni are expected to be unstable during long-term operation.

In this study we report on the synthesis, stability, catalytic activity and electrochemical performance of LSCM compounds with a small amount (20 mol%) of Mn substituted with cheap transition metals, such as Co, Fe, Ni and V. All materials, apart from the vanadium-doped LSCM were synthesized as phase-pure perovskites using a sol-gel method and were found to be stable up to 800°C in dry CH₄. At higher temperatures, in the same reducing atmosphere, a Ruddlesden-Poepper impurity was detected in XRD. Although no metal or metal oxides were detected at this point, there is indirect evidence of the exsolution of Ni and Co from the LSCM lattice. Catalytic measurements performed in a pulse-type experiment³ at 750°C showed that CH₄ reaction rates increased for LSCMFe 4 to CO₂ on LSCM and LSCMFe continuously decreased with oxygen stoichiometry of the catalyst, CO₂ production on LSCMCo and LSCMNi initially decreased and then increased again after ~ 0.1 to 0.18 mol of lattice oxygen had reacted. This second CO₂ production peak was attributed to the liberation of oxygen due to the reduction of Co(II) and Ni(II) to their respective metallic phases. Carbon formation was of the same order of magnitude for all compositions. The increased CO₂ production rate on LSCMCo and LSCMNi will be further investigated using electrochemical measurements on fuel cells with LSCMCo and LSCMNi anodes, and holds promise for the design of an affordable, stable and catalytically active anode.

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