GAS CONVERSION IMPEDANCE - A TEST GEOMETRY EFFECT IN CHARACTERIZATIONOF SOLID OXIDE FUEL-CELL ANODES

The appearance of an extra are in impedance spectra obtained on high p erformance solid oxide fuel cell (SOFC) anodes is recognized when expe riments are conducted in a test setup where the working and reference electrodes are placed in separate atmospheres. A simple continuously s tirred tank reactor (CSTR) model is used to illustrate how anodes meas ured with the reference electrode in an atmosphere separate from the w orking electrode are subject to an impedance contribution from gas con version. The gas conversion impedance is split into a resistive and a capacitive part, and the dependences of these parameters on gas compos ition, temperature, gas flow rate, and rig geometry are quantified. Th e fuel gas flow rate per unit of anode area is decisive for the resist ivity, whereas the capacitance is proportional to the CSTR volume of g as over the anode. The model predictions are compared to actual measur ements on Ni/yttria stabilized zirconia cermet anodes for SOFC. The co ntribution of the gas conversion overpotential to de current-voltage c haracteristics is deduced for H-2/H2O and shown to have a slope of RT/ 2F in a Tafel plot.