Development of highly-active catalysts for the oxidation of hydrocarbon fuels in direct Solid Oxide Fuel Cell anodes

Solid Oxide Fuel Cells (SOFCs) are promising candidates for the efficient conversion of chemical energy to electrical energy. Fuel is oxidized at the anode, typically a nickel/yttria-stabilized zirconia (Ni/YSZ) composite. This anode material has several limitations, and other materials such as the perovskite La0.75Sr0.25Cr0.5Mn 0.5O3-delta (LSCM) and the double perovskite Sr2MgMoO 6-delta (SMMO) are being researched to replace Ni/YSZ.;In this work, the factors limiting the performance of LSCM and SMMO have been investigated. A pulse-type reactor was successfully developed to measure the differential hydrocarbon reaction rates on anode catalysts under conditions mimicking an operating SOFC. Rates were found to be catalytically limited at high oxygen stoichiometries for LSCM and SMMO.;Hydrocarbon reaction rates on LSCM and SMMO were consistent with a Mars-van Krevelen type reaction mechanism, with Mn and Mo as the active centers for the redox reaction. With decreasing oxygen stoichiometry, the product selectivity of the reaction changed from the desired total oxidation to partial oxidation and cracking. For SMMO, temperature also played a significant role, with total oxidation the preferred reaction at low temperatures. The change in selectivity with oxygen content was also seen in a working SOFCs with LSCM anodes.;In terms of absolute CH4 reaction rates, SMMO compounds had the lowest rates, 1 to 2 orders of magnitudes smaller than rates on LSCM. This is suggested to be due in part to the limited number of active centers in the SMMO lattice. CH4 reaction rates on LSCM are limited as well, compared to LSCM containing a small amount of Pt. The stability of the methane molecule likely makes it hard to crack the hydrocarbon, slowing down overall reaction kinetics.;CH4 reaction rates on LSCM were enhanced with the partial substitution of Mn with Co or Ni, while the product selectivity remained the same. Ni and Co are likely present on the surface, where they catalyze the cracking of CH4, followed by oxidation at the Mn centers. Even with the addition of Co and Ni though, reaction rates are lower than the rates desired for use in high-performance SOFCs, emphasizing the need to research alternative anode materials.