Electrocatalytical Properties of Perovskites for SOFC Anodes

The materials in the series La_0.75Sr_0.25Cr_1-xMn_xO_3-δ have recently been shown effective for the direct utilization of methane fuel in Solid Oxide Fuel Cells (SOFCs). A critical aspect of such hydrocarbon fueled SOFCs is the electrocatalytic activity of the anode material and how this is influenced by fuel cell operating conditions. 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 pO₂ 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 pO₂ is O(10^-20 - 10^-24) atm. The low anode pO₂ leads to a lower equilibrium lattice oxygen stoichiometry (3-δ). In this work we measure catalytic activity under SOFC performance and utilize this information to interpret the performance of both anode supported SOFC and thin-film electrolyte supported model anode systems.

We describe a novel pulse reactor approach to measuring the methane oxidation activity of La_0.75Sr_0.25Cr_1-xMn_xO_3-δ 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 anode supported direct methane SOFCs - a strong link between changing catalytic activity and non-linear SOFC performance with increasing current density is demonstrated. Finally, additional insights in to the overall anode mechanism are provided by a model fuel cell system with a 500nm thick dense LSCM anode film and lithographically patterned electrical contacts. This simplified electrode geometry allows separation of the contributions from bulk ion-electron transport and surface reaction to the electrochemical impedance spectrum.