LIMITATIONS IN THE HYDROGEN OXIDATION RATE ON Ni/YSZ ANODES

摘要

The impedance of Ni and Ni/YSZ cermet anodes in H_2/H_2O on YSZ electrolyte pellets was examined by impedance spectroscopy in a three electrode setup where concentration polarization was prevented. The polarization resistance R_P was found to depend on pH_2O~(-0.25)at 850°C, and to decrease with increasing pH_2 at high pH_2. An H/D isotope effect was found to be negligible at 1000°C, and to increase with decreasing temperature. Poisoning of the Ni surface by 35 ppm H_2S caused a 60% increase in R_P, irrespective of temperature, anode type and passage of current. Addition of small amounts of Mn to the anode appeared to be able to reduce R_P by a factor of 2 at 850 and 1000°C. The dominant limiting process is suggested to change gradually from transport of protons at lower temperatures to an unidentified process at higher temperatures.