Ni pattern electrodes with well-defined triple phase boundary (TPB) lengths
were prepared in order to investigate the reaction mechanisms at solid oxi
de fuel cell (SOFC) anodes. The anode microstructures were stable during th
ermal treatment and electrochemical measurements. Electrochemical impedance
spectroscopy was used to study the influence of the overpotential, of the
gas atmosphere, of the temperature, and of the pattern geometry on the elec
trochemical behavior of SOFC anodes. It is found that the reaction kinetics
are dominated by one main process. This process is thermally activated wit
h an activation of energy of E-A = 0.88 +/- 0.04 eV. At overpotentials high
er than 300 mV, a second process becomes relevant. The partial pressure of
water in the fuel gas atmosphere has a catalytic effect on the anode perfor
mance, whereas variations of the patrial pressure of hydrogen in the fuel g
as atmosphere have no significant influence on the electrode behavior. A mo
del was established in order to explain the catalytic effect of water. The
direct proportionality between the relaxation frequency and the TPB length
suggests a TPB limitation of the anode kinetics. (C) 2001 The Electrochemic
al Society.