Structure/performance relations for Ni/yttria-stabilized zirconia anodes for solid oxide fuel cells

The relations between morphology and electrochemical performance of Ni/yttria-stabilized zirconia (YSZ) anodes are investigated. Four types of anodes are prepared on YSZ electrolyte three-electrode pellets. A fine cermet of 0.5-1 mu m particles, a coarse cermet of 2-3 mu m particles, a porous Ni-paste anode, and a Ni-felt anode. The anodes are characterized by impedance spectroscopy at open-circuit potential, and the electrode relevant part (polarization resistance, Rp) Of the spectra is identified and investigated. The active thickness of the fine cermet anode is demonstrated to be about 10 mu m and is believed to relate to the conductivity of the YSZ network. In the temperature range 850-1000 degrees C, Rp exhibits an apparent activation energy which increases with coarseness of the anode. No significant dependence on p(H2) (0.01-0.97 atm) at 1000 degrees C is observed. A dependence on p(H2O) of about R-p proportional to p(H2O-1/2) is found. Physical transport limitations are suggested as possible causes for the observed anode polarization. (C) 2000 The Electrochemical Society. S0013-4651(99)06-003-9. All rights reserved.