Transient studies of perovskite anode catalyst for a direct CH4 Solid Oxide Fuel Cell

Introduction: The direct use of CH4 as the feed to the solid oxide fuel cell (SOFC) will allow elimination of a reformer, simplifying the SOFC system and decreasing the system cost [1]. The overall reaction on the anode catalyst in the direct CH4 SOFC is the electrochemical oxidation of CH4: CH4 + 4O CO2 + 2H2O + 8e. Due to the nature of hydrocarbon reactions at high temperature and the reducing environment of the anode chamber, an effective anode catalyst must possess oxidation activity, thermal stability, and coking resistance. Perovskites have shown the ability to oxidize CH4 while inhibiting catalyst coking [3]. The objective of this study is to investigate the catalytic reaction pathway of CH4 on a Ni anode-supported SOFC promoted with lanthanum strontium cobalt ferrite (LSCF) perovskite using transient techniques. The Ni anode-supported SOFC is selected for this study because of its robustness under SOFC operating conditions and potential for improving its coking resistance. A fundamental understanding of the reaction pathway on LSCF-promoted Ni/YSZ anodes will allow fine-tune the composition of the anode catalyst for effective use of CH4 and natural gas for electric power generation. Materials and Methods: The electrolyte disk consists of YSZ powder (Tosoh, TZ-8Y) co-axially pressed and sintered at 1450 oC to form a 100 μm thick electrolyte disk. The 25 μm LSCF/YSZ anode and 10 μm LSM/YSZ cathode were applied in a 1:1 weight ratio. Current collectors consist of Pt screens and Pt wires which were adhering onto on the anode and cathode surfaces with platinum paste (Engelhard, A3788A). The electrolyte-supported cell was attached to a ceramic tube for testing. The anode-supported SOFC consists of a 20 μm Yttria-Stabilized Zirconia (YSZ) electrolyte layer, a 10 μm Ni/YSZ interlayer anode, a 1 mm Ni/YSZ anode, a 25 μm thick LSM/YSZ cathode, and a 25 μm LSM cathode current collector [4]. LSCF was screen printed on to the sintered SOFC anode and heated to 1100 C. Testing of the electrolyte and anode-supported SOFC was carried out by heating to 800 C in flowing H2. Ar was flown to purge the system before introduction of CH4. Transient studies were carried out by step switching the inlet flow using a 4-port valve while monitoring the gas effluent responses by a mass spectrometer and the current/voltage response by a Solartron 7400E. Impedance measurements were conducted during steady state operation of the SOFC. XRD and SEM with EDS were used to characterize the anode after testing of SOFC. Results and Discussion: LSCF has been tested as the anode on an electrolyte-support SOFC. Fig. 1 illustrates the transient profiles (i.e. responses) of the concentration of the gaseous species in the SOFC effluent, current and voltage resulting from a step switch of the inlet flow from Ar to CH4 at a constant external load of 2 ohms. Fig. 1 (i) shows Ar and CH4 curves crossed at the normalized value of 0.5, indicating that a near perfect switch of the inlet flow Ar to CH4 was achieved. Fig. 1 (ii) shows that the H2 response led that of CO2 and CO, giving higher concentration of H2 than that of CO2 and CO from the SOFC effluent. CO2 emerged before CO suggesting that both CO2 and CO are produced from a parallel reaction pathway: C + 2O 2-