(134c) Multiscale Modeling of the H2 Oxidation Reaction At the Ni/YSZ Interface in the Presence and Absence of Sulfur
Understanding the mechanism of sulfur poisoning at the three-phase boundary (TPB) of Ni/YSZ is essential for identifying specific mitigation solutions against degradation and the rational design of sulfur tolerant anodes for solid oxide fuel cells. In the present study, we used ab initio methods and microkinetic modeling techniques to investigate the oxidation reaction of hydrogen fuel at the Ni/YSZ interface in the presence and absence of adsorbed sulfur species. The oxidation mechanism of H2 at the Ni/YSZ interface has been investigated considering the following pathways: (i) H-spillover pathway, (ii) OH-migration pathway, and (iii) O-migration pathway. In all of these pathways H2 adsorbs dissociatively on Ni and reacts with an oxygen atom at the interface which leads to the formation of H2O and a surface vacancy. In order to complete the catalytic cycle, we also considered the migration of bulk oxygen to the surface followed by the bulk vacancy being filled by an oxygen atom from the cathode. Analysis of a microkinetic model containing all three pathways led to the following conclusions: a) If we assume that the cathode reaction is fast and thus used the cathode oxygen partial pressure (PO2 = 0.21 atm) for the oxygen addition process to the bulk YSZ oxygen vacancy, the apparent activation barrier for all three pathways are within the range of reported experimental values (100-135 kJ/mol). However, the highest rate (by four orders of magnitude) is predicted for the O-migration pathway. In this pathway, Campbell’s degree of rate control analysis indicates that the bulk oxygen diffusion in YSZ is the rate determining process at lower temperatures (900-1100 K), while hydrogen spillover to the migrated oxygen at the interface becomes rate-limiting at higher temperatures (>1200 K). b) If we consider the cathode reaction to be sluggish and thus lowered the oxygen partial pressure to 10-10 atm in the bulk of YSZ, the rate for the O-migration pathway decreases dramatically and the H-spillover pathway becomes dominant with the bulk oxygen diffusion being rate-limiting at lower temperatures (<1100 K). The effect of sulfur on this process has been evaluated by investigating all three pathways in the presence of an adsorbed sulfur atom at the interface. The strong adsorption of sulfur at the interface decreases the availability of active sites for H-adsorption. Furthermore, it inhibits the migration of oxygen or –OH species to the interfacial Ni atoms due to lateral sulfur interaction effects which significantly increase the apparent activation barrier.