Anode-supported single-chamber SOFC for energy recovery from exhaust gases of thermal engine

International audienceSolid oxide fuel cells working 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. Hydrogen-air mixtures are not commonly used under single chamber conditions because of their high reactivity and risk of explosion. Therefore, hydrocarbons are preferentially used as fuel. 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. This forward-looking energy recovery system could be applicable to automotive vehicles as well as to plants. Yano et al. and Nagao et al. in 2008 demonstrated the feasibility of such a device by investigating a stack of 12 electrolyte supported Ni-SDC/YSZ/LSM 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 this 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 previously cut into 22.5 mm diameter discs. 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 then 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 OCV of 706 mV and a power density reaching 16mW/cm² at 500°C and R=HC/O2=0.21. These results are close to 20 mW/cm², the power density obtained at higher temperature (800°C) by Yano and Nagao during their single cell tests in a R=HC/O2=0.8 gas mixture composed of 4000 ppm of hydrocarbons (methane, ethane, propane and butane) and 5000 ppm of oxygen. In the present project, cathode materials will be compared regarding the cell performances and some improvements are in progress concerning more particularly the cell microstructure