Effects of 3% Steam in Air on Lscf-6428 Cathodes during 1000 h SOFC Tests

Cells with LSCF-6428 cathodes were tested at 650, 700, 750, and 800°C in dry and humid cathode air.  At each temperature, six cells were operated at a constant current approximating an operating voltage of 800 mV.  Of the six cells, two were tested in dry cathode air during the entire 1000 hour test, two were tested in humid cathode air during the entire test, and two were tested in cathode air in which the humidity level was alternated between dry and humid air every ~250 hours. The dry air had a dew point of -80°C and the moist (~3% water) condition was achieved by bubbling the dry air through water at room temperature.  This experiment was repeated a second time, providing a total of four 1000 h cell tests under each combination of atmosphere and temperature exposure.  It was found that the effect of humidity on cell degradation changes from detrimental at lower temperature to beneficial at higher temperature somewhere between 700°C and 750°C.  Triple exponential decay equations were fit to the operating voltage as a function of time over each continuous segment of data.  This made it possible to calculate the instantaneous degradation rate as a function of time over the entire test duration based on the derivatives of the best fit curves.  Nano-sized nodules were found on the surfaces of the LSCF cathode grains in lower temperature tests whereas they were not present in the cells tested at higher temperatures.