Hydrogen Oxidation at the Ni/Yttria-Stabilized Zirconia Interface: A Study Based on Density Functional Theory

A new mechanism for hydrogen oxidation on the nickel/yttria-stabilized zirconia (Ni/YSZ) interface is proposed based on density functional theory. The new mechanism involves oxidation of hydrogen by the oxygen atoms that are bound to both nickel and zirconium (or yttrium) at the interface. The free energy change (Delta G) for this pathway is compared to Delta G for reaction steps of previously proposed oxidation mechanisms involving spillover of oxygen from YSZ to nickel or hydrogen spillover from nickel to YSZ. For all mechanisms, we consider both a stoichiometric (YSZ) surface as well as an oxygen enriched YSZ surface (YSZ+O) where in the latter case a vacant site is filled by an oxygen atom transferred from the YSZ bulk. The release of water as the final product in hydrogen oxidation is facilitated at high temperatures by entropy. The difference between the current and previous mechanisms is that for the hydrogen oxidation now we only consider the involvement of oxygen atoms that are bound to both nickel and zirconium (or yttrium). In previous studies we only considered oxygen atoms that initially were bound to zirconium (or yttrium) only.