Anode supported single chamber SOFC development for energy recovery of engine exhaust gases

National audienceSolid oxide fuel cells (SOFCs) operating in a mixed gas atmosphere (fuel and oxidant), the so-called single chamber SOFCs (SC-SOFCs), have been increasingly studied in the past few years. The absence of sealing between the two compartments provides an easier operation than a classical "two-chambers" SOFC. The single chamber configuration has several advantages over conventional SOFCs: new cell geometries, stack assembly and miniaturization of cells are more easily conceivable. These advantages open the way to new applications such as energy recovery in the exhaust gas by conversion of unburned hydrocarbons into electricity. For that purpose, cells would be embedded at the exit of the engine. Yano et al. and Nagao et al. demonstrated in 2008 the feasibility of such a device by performing a stack of 12 electrolyte supported SC-SOFCs incorporated at the exit of a scooter engine. However, optimization of the system including architecture, gas mixture and materials modification may lead to enhanced performances. In the present study, a gas mixture closer to real exhaust conditions has been selected. It is composed of hydrocarbons (HC: propane and propene), oxygen, carbon monoxide, carbon dioxide, hydrogen and water. Only oxygen content has been varied leading to different gas mixtures characterized by three ratios R=HC/O2. Concerning the cell components, a cermet composed of nickel and the electrolyte material, Ce0.9Gd0.1O1.95 (GDC) is used as anode and two cathode materials, Pr2NiO4+δ (PNO) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), have been selected. Anode support is prepared by tape casting; electrolyte material is then screen-printed on top of the green tape. Half cells are co-sintered at 1400°C for 6 hours. Cathode and gold mesh screen printing and sintering conclude cell preparation. These cells have been tested in the three gas mixtures for temperatures ranging from 400°C to 600°C. Ni-GDC/GDC/LSCF-GDC cell operation has delivered a maximum power density of 15.25 W.cm-2 at 500°C and R=HC/O2=0.21, while lower power densities were obtained for the two other ratios, R=0.44 and R=0.67. Both cathode materials were then investigated in single chamber conditions for the R=0.21 and R=0.44 ratios and LSCF was selected as the most suitable material, delivering the highest power densities. Performances were enhanced by increasingelectrolyte thickness to 30 μm, highest power value of 19.35mW.cm-2 was obtained at 500°C for R=0.21, which is close to 20 mW.cm-2, the power density obtained at higher temperature by Yano and Nagao during single cell test