Currently most fuel cells operate with hydrogen as the fuel, and require a reforming step upstream of the fuel cell to convert a hydrocarbon or alcohol fuel into hydrogen. The efficiency of the entire process would be improved if the fuels could be reacted directly in the fuel cell with no pretreatment. One of the difficulties with reacting fuels other than hydrogen at high temperatures over most solid-oxide fuel cell (SOFC) anode catalysts is the propensity for carbon formation.

We have prepared nickel/yttria-stabilized zirconia (Ni/YSZ) anodes and examined these anodes before and after exposure to various feeds including hydrogen, dry methane, humidified methane, methanol, and ethanol, using scanning electron microscopy, electron impedance spectroscopy, temperature-programmed oxidation, and temperature-programmed hydrogenation.

The reaction of methane over Ni/YSZ anodes results in carbon that dissolves into the Ni structure at 1073 K. As the current density increases, the amount of carbon decreases and becomes more reactive for removal with oxygen. Regeneration with oxygen, however, destroys the microstructure through particle disintegration and expansion of the anode because of the formation of nickel oxide. Hydrogen can be used to remove some, if not all, carbon deposits, provided that the concentration of hydrogen in the regeneration stream is sufficiently high. Increasing the hydrogen content of the feed during operation through, for example, recycle from the anode exhaust, reduces the amount of carbon deposition and the extent to which the carbon dissolves into the Ni structure.