| Solid oxide fuel cells (SOFCs) are very promising electrochemical devices that convert chemical energy directly into electricity energy. Anode is an important component for a solid oxide fuel cell (SOFC). The most common SOFC anode material is Ni-based anode, which has a good electrochemical performance with pure H2 as fuel. However, this anode is readily fouled by carbon deposition and sulfur poisoning when operated on natural gas. In order to overcome this problem, various attempts have been made to develop alternative anode materials which can resist carbon deposition and/or tolerate sulfur contamination in hydrocarbon fuel conditions. In this paper, A2FeMoO6-δ(A = Ca, Sr and Ba), Sr2-xSmxMgMoO6-δ(SSMM, 0≤x≤0.8) and Sr2-xBaxMgMoO6-δ(SBMM, x=0.0, 0.5, 1.0, 2.0) materials were used as the SOFCs anodes and synthesized by a solid-state reaction and/or a sol-gel method. During the whole cell fabrication process, both the anode and cathode were fired in nitrogen atmosphere to prevent re-oxidation and decomposition of A2FeMoO6-δanode.Polycrystalline samples of A2FeMoO6-δ(A = Ca, Sr and Ba) were prepared by a solid-state reaction. At room temperature, A2FeMoO6-δcompounds crystallize in a monoclinic, tetragonal and cubic structure for A = Ca, Sr and Ba. The weak peak at around 880 cm-1 observed in the Raman spectra is identified as traces of AMoO4. XPS confirms the coexistence of Fe2+-Mo6+ and Fe3+-Mo5+ electronic configurations. Moreover, a systematic shift from Fe2+/3+-Mo6+/5+ to Fe2+-Mo6+ configuration increases with increasing A-site cation size. A2FeMoO6-δsamples present distinct electrical properties in H2, which can be attributed to the different degree of degeneracy of Fe2+-Mo6+ and Fe3+-Mo5+ configurations. Ca2FeMoO6-δis unstable in nitrogen atmosphere, while Sr2FeMoO6-δand Ba2FeMoO6-δare stable up to 1200 oC. The thermal expansion coefficients of Sr2FeMoO6-δand Ba2FeMoO6-δare very close to that of La0.9Sr0.1Ga0.8Mg0.2O3-δ(LSGM). The performance of single-cell with a 300μm thick LSGM electrolyte, double-perovskite SmBaCo2O5+x cathode and A2FeMoO6-δanodes, follows the sequence: Ca2FeMoO6-δ< Ba2FeMoO6-δ< Sr2FeMoO6-δ. The maximum power densities of the cell with Sr2FeMoO6-δanode achieve 831 mW cm-2 in dry H2 and 735 mW cm-2 in commercial city gas at 850°C, respectively.Polycrystalline samples of Sr2-xSmxMgMoO6-δ(SSMM, 0≤x≤0.8) were prepared by a sol-gel method. The X-ray diffraction (XRD) patterns have confirmed that the impurities appeared in the samples SSMM when x≥0.6, and the ordering of B/B' sites decresed with increasing the content of Sm. Raman spectra have also confirmed the same results. XPS analysis showed that introduction of a small amount of Ba2+ in Sr2MgMoO6-δenhanced the concentration of Mo5+/Mo6+ redox couple, thus resulting in high conductivity and good catalytical activity.Among these SSMM compositions, all the oxides exhibit polaronic conduction in H2. The electrical conductivity increases greatly with Sm substitution for Sr, and reaches the maximum value at x=0.6. For the close relationship between electrical conductivity and Mo5+/Mo6+ redox couple, the amount of Mo5+/Mo6+ redox couple also increased with Sm substitution for Sr. To prevent phase segregations and re-oxidation of the samples, the cell was fabricated under nitrogen atmosphere for the first time. The X-ray diffraction (XRD) patterns have confirmed that the double perovskites SSMM (x=0.4) is stable at 1100 oC in nitrogen atmosphere, and no phase segregation occurs. Therefore, the cell fabrication under nitrogen atmosphere is feasible. With a 300-um-thick La0.9Sr0.1Ga0.8Mg0.2O3-δ(LSGM) as electrolyte and SmBaCo2O5+x as cathode, the SSMM (x=0, 0.2, 0.4, 0.6 and 0.8) anodes show maximum power densities of 692, 581, 858, 905 and 671 mW cm-2 at 850 oC in H2. For the impurities appeared in SSMM (x≥0.6), the sample SSMM (x=0.4) is considered to be the best choice for the anode of a SOFC. SSMM (x=0.4) anode also exhibited notable power output in coal gas and the maximum power density reached 726 mW cm-2 at 850 oC. The thermal expansion coefficient of SSMM with x=0.4 has also been measured and the value is 13.5×10-6 K-1 under nitrogen atmosphere, which is quite close to that of LSGM electrolyte under the same condition.Polycrystalline samples of Sr2-xBaxMgMoO6-δ(SBMM, x=0.0, 0.5, 1.0, 2.0) were also prepared by a sol-gel method. Raman spectra showed that incorporation of Ba at relatively lower concentration weakens the average metal-oxygen bond energy, which facilitates oxygen ion mobility in the lattice structure. The electrical conductivity increased with Substitution of Ba for Sr, and reaches the maximum value at x=0.5. XPS analysis showed that substitution of Ba for Sr enhaced the concentration of Mo5+/Mo6+ redox couple, and the concentrations of Mo5+ and Mo6+ ions are 45.7% and 54.3%, respectively. The single cell with SBMM (x=0.5) anode exhibits excellent electrochemical performance in H2 and coal gas, and the maximum power density values can reach 903 and 745 mW cm-2 at 850 oC, repectively. Thermal expansion measurement indicated that with increasing the content of Ba the thermal expansion coefficient (TEC) tends to increase. The TEC values for SBMM with x=0.0, 0.5, 1.0, 2.0 are 13.4, 14.1, 14.3, 14.6×10-6 K-1, respectively. Clearly the TEC increases with increasing the content of Ba. This can be attributed to the increase of oxygen vacancies.
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