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 (La0.75Sr0.25Cr0.50Mn0.50O3-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 SOFC3, 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 < LSCM < LSCMCo < LSCMNi. Whereas the total oxidation of CH4 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.
1. Toebes ML, Bitter JH, van Dillen AJ, de Jong KP. Catal Today 2002 Nov 1;76(1):33-42.
2. Tao SW, Irvine JTS. Nat Mater 2003 May;2(5):320-3.
3. van den Bossche M, McIntosh S. J Catal 2008 Apr 25;255(2):313-23.
4. Bruce MK, van den Bossche M, McIntosh S. J Electrochem Soc 2008;155(11):B1202-9.
5. van den Bossche M, Matthews R, Lichtenberger A, McIntosh S. J Electrochem Soc;157(3):B392-9.
6. Wan J, Zhu JH, Goodenough JB. Solid State Ion 2006 May;177(13-14):1211-7.
7. Kim G, Lee S, Shin JY, Corre G, Irvine JTS, Vohs JM, Gorte RJ. Electrochem Solid State Lett 2009;12(3):B48-52.
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