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

Abstract

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 (∆G) for this pathway is compared to ∆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.

Chemical Descriptors of Yttria-Stabilized Zirconia at Low Defect Concentration: An ab Initio Study

“…YSZ is a key component of the SOFC anode where it forms an interface with a catalytic metal and a gas phase. The interface region is referred to as the anode triple phase boundary (TPB) [3][4][5][6][7][8] . There is a growing interest in developing predictive models of the physical and chemical properties of YSZ and its surfaces in order to model the chemistry occurring at the anode TPB.…”

First-principles study of oxygen coverage effect on hydrogen oxidation on Ni(111) surface

“…Most of the papers are discussed on the dissociation of H 2 O to OH and H [10][11][12][13]16,22], as a one of the steps of steam reforming reaction on Ni surface. Limited papers are reported about the oxidation reaction on Ni surface assuming the SOFC anode [21,[31][32][33][34][35]. However, the chemical reaction analysis on the clean Ni surface is main target in the previous papers.…”

The sulfur poisoning of the nickel/oxygen-enriched yttria-stabilized zirconia

“…So we select the Ni/YSZ þ O system with an extra oxygen in the V2 site as the model of Ni/ YSZ þ O shown in Fig. 1(b), which is in agreement with the Ni/ YSZ þ O model used in Shishkin and Ziegler's works [27,32]. The charge on the Ni part in Ni/YSZ þ O decreases by 1.04 e as compared with that in the Ni/YSZ based on the Bader charge [43] analysis.…”

“…The O 2À ions supplied to the intrinsic vacancy of YSZ originate from the electrochemical reaction of O 2 at an electrode, which depends on the actual SOFC fuel gas operation and polarization conditions in the anode polarization measurement [24e26]. It is found that the favored hydrogen oxidation reaction process on Ni/YSZ is water formation on an O atom at the interface and this is also the most facile hydrogen oxidation reaction process on Ni/(YSZ þ O) [27].…”