Testing and improving the redox stability of Ni-based solid oxide fuel cells

Despite active development, solid oxide fuel cells (SOFCs) based on Ni-YSZ anodes still suffer from thermomechanical instability under conditions where the anode side is exposed to oxidising conditions at high temperature. In the first part of the paper, structures and solutions, which could improve the redox stability of Ni-YSZ anode supported SOFC's in terms of dimensional and mechanical stability are reported. Porosity is identified as a major microstructural parameter linked to the dimensional and structural stability during redox cycling. The cumulative redox strain (CRS) after three isothermal redox cycles at 850 degrees C increases by a factor of more than 20 when the as-sintered porosity of the composites is reduced from 34 to 9%. The effect of reduction and redox cycling on the Ni-YSZ anode are discussed in light of electrochemical measurements using impedance spectroscopy on symmetric cells. When the symmetric cells are reduced and redox cycled isothermally at 850 degrees C, no major change in the serial or polarisation resistance of the cell and electrodes was measured. When the cells are, after the similar initial reduction treatment, redox cycled at 650 degrees C, the serial resistance remains almost unchanged but the polarisation resistance decreased by about 60%. (C) 2009 Published by Elsevier B.V.