Oxidation of H2, CH4, and CO Molecules at the Interface between Nickel and Yttria-Stabilized Zirconia: A Theoretical Study Based on DFT

Activation of fuel molecules (H-2, CH4, CO) at the anode triple phase boundary (TPB) of a solid oxide fuel cell, modeled by a Ni (nickel)/YSZ (yttria-stabilized zirconia)/fuel interface is investigated using density functional theory. We demonstrate that, by employing A initio calculations, it is possible to elucidate the mechanisms of electronic charge transfer and current generation as a result of electrochemical oxidation of fuel in the anode TPB. Moreover, we show that an oxygen-enriched YSZ surface (YSZ+O) of the Ni/YSZ cermet is significantly less active toward oxidation Of fuel molecules than an oxygen-enriched YSZ surface in the absence of Ni, which is explained by partial saturation of the valence of an extra oxygen atom of the Ni/YSZ cermet. On the other hand, the chemical activity of the Ni part of the cermet is close to that of the infinite Ni Surface, even in the proximity to the YSZ. The YSZ+O surface of the cermet is found generally inert, as adsorption of H-2, and CH4 is associated with high kinetic barriers, whereas CO adsorption is a more favorable, although endothermic reaction. The oxidation of H-2 and CO on Ni after oxygen spillover from the oxide is exothermic, and CH4 oxidation is mildly endothermic. It is also shown that transfer of hydrogen atoms from Ni to YSZ (hydrogen spillover reactions) with subsequent water formation is another possible scenario of hydrogen oxidation oil the YSZ+O surface.