The Reduction Process of a NiO/YSZ Anode for Intermediate Temperature Solid Oxide Fuel Cells

The reduction behavior of a sintered NiO/YSZ anode used for intermediate temperature solid oxide fuel cells was studied by hydrogen temperature-programmed reduction (H-2-TPR). The reduction process of the NiO/YSZ anode in the cell was in situ monitored by open circuit voltage (OCV) and electrochemical impedance spectroscopy (EIS). H-2-TPR results show that the higher sintering temperature of the NiO/YSZ anode results in a slower reduction of NiO to metallic Ni. However, when the sintering temperature is elevated to 1500 degrees C, the reduction of sintered NiO/YSZ anode powder instead becomes easier. The higher NiO content in the anode leads to the more rapid reduction of the corresponding anode powder. The above H2-TPR results can be attributed to the combined effects of the growth up of NiO particles and the interface separation between NiO and YSZ caused by the anode sintering. The variation of OCVs reveals that for the cells, the anode with higher NiO content has a slower reduction process, which can be ascribed to the retarding effect of excessive H2O produced during the initial reduction period. It was found from the EIS results that the 50% NiO/YSZ anode has a most stable reduction process, whereas for the cells with 30 % and 70 % NiO/YSZ anodes, both the polarization resistances gradually increase after experiencing an initial decrease for a short period. The cell polarization resistance with 30 % NiO/YSZ anode keeps no change any more after reduction for 600 min, whereas the cell polarization resistance with 70 % NiO/YSZ anode increases continuously.