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

Abstract

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 H₂-3 mol% H₂O and by more than 1/3 in CH₄-3 mol% H₂O at temperatures ranging from 650 to 800 °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 °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 °C and 200 mA cm⁻², which ensured a higher and more stable electrochemical performance than that of the Ni–YSZ anode.