Michael Van den Bossche, Chemical Engineering, University of Virginia, 102 Engineers' Way, Charlottesville, VA 22904 and Steven McIntosh, Department of Chemical Engineering, University of Virginia, 102 Engineer's Way, PO Box 400741, Charlottesville, VA 22904-4741.
The materials in the series La
0.75Sr
0.25Cr
1-xMn
xO
3-Îx have recently been shown effective for the direct utilization of methane fuel in Solid Oxide Fuel Cells (SOFCs). A critical aspect of such hydrocarbon fuelled SOFCs is the catalytic activity of the anode material; in particular, whether the dominant reaction mechanism is the desired complete oxidation of fuel or less desirable partial oxidation or steam reforming. Oxidation activity is typically measured in a flow reactor utilizing gas-phase oxygen with a
pO
2 of ~O(0.1) atm; however, in the SOFC anode, oxygen is supplied via anion transport through the oxide lattice and the gas-phase equilibrium
pO
2 is O(10
-20- 10
-24) atm. The low anode
pO
2 leads to a lower equilibrium lattice oxygen stoichiometry (3-d). In order to relate measurements of catalytic activity to SOFC performance we must replicate anode conditions.
In this work we describe a novel pulse reactor approach to measuring the methane oxidation activity of La0.75Sr0.25Cr1-xMnxO3-Îx where fuel oxidization occurs under SOFC anode (low pO2) conditions and oxygen is supplied from the oxide lattice. Each pulse consumes a small fraction of lattice oxygen. The oxidation activity and selectivity towards total oxidation is found to be a function of both lattice oxygen stoichiometry and perovskite composition. Decreasing lattice oxygen content leads to decreasing methane oxidation rate and decreasing selectivity towards total combustion. These ex-situ catalytic measurements are then utilized to interpret and enhance the performance of direct methane SOFCs - a strong link between catalytic activity and SOFC performance is demonstrated.