Enhanced electrochemical performance and carbon deposition resistance of Ni-YSZ anode of solid oxide fuel cells by in situ formed Ni-MnO layer for CH4 on-cell reforming

Two types of anode were prepared for a comparative study of their electrochemical performance and carbon deposition resistance. The first one was the Ni-YSZ cermet anode; and the other, designated as the Ni-MnO/Ni-YSZ, was the Ni-YSZ anode plus an on-cell reforming layer of Ni-MnO in situ reduced from MnNi2O4 with a microstructure of fine Ni particles embedded in MnO matrix. With the Ni-MnO layer on the top surface, open circuit polarization resistance of the Ni-YSZ anode decreased approximately by 1/2 in H-2-3 mol% H2O and by more than 1/3 in CH4-3 mol% H2O at temperatures ranging from 650 to 800 degrees C. Carbon fibers were observed in the Ni-YSZ anode, rather than in the Ni-MnO/Ni-YSZ anode, after initial impedance measurements in CH4-3 mol% H2O for 1 h. The polarization resistance of both anodes at 800 degrees C increased with time up to 6 h in CH4-3 mol% H2O due to carbon deposition; however, the carbon formed in the Ni-MnO/Ni-YSZ anode had a lower degree of graphitization. In the atmosphere of CH4-20 mol% H2O, carbon deposition was completely depressed in the Ni-MnO/Ni-YSZ anode at 800 degrees C and 200 mA cm(-2), which ensured a higher and more stable electrochemical performance than that of the Ni-YSZ anode.