Theoretical Study On The Sulfur Tolerance Mechanism Of Yttria-stabilized Zirconia

Due to their many advantages in terms of high efficiency, fuel adaptability, all-solid state, modularassembly and low pollution, the solid oxide ruel cells (SOFCs) as a prospective power generation systemhave attracted much attention．SOFCs have been proven useful in industrial, residential, and transportapplications in the past and present, and continue to be developed and improved upon to meet growingenergy demands. However, the YSZ-based anode (Ni/YSZ) for SOFCs is highly susceptible to deactivation(poisoning) by contaminants commonly encountered in readily available fuels, especially sulfur-containingcompounds, which is one of major problems of SOFCs. Using the first-principles method based on densityfunctional theory (DFT) and the Vienna Ab-initio Simulation Package (VASP) package with the projectoraugmented wave (PAW) potential and the Perdewe Burkee Ernzerhof (PBE) functional, in combinationwith the ab initio atomistic thermodynamics method, we study the mechanism for the sulfur tolerant of theYSZ exposure to H2S.1. The adsorption of H2S on YSZ (111) surfaceThe adsorption and dissociation of H2S on the yttria-stabilized zirconia (YSZ)(111) surface arestudied using the first-principles methods. It is found that H2S and SH species are bound weakly on theYSZ (111) surface, while the S atom is strongly bound and stably anchored on the O atop of the YSZsurface with the formation of the SO2-fragment. The nudged elastic band (NEB) calculations show that theformation of SH+H from the first dissociation of H2S is very easy, while the presence of a co-adsorbed Hwould inhibit the further dissociation of SH. In contrast, the hydrogenation of the adsorbed sulfur is rathereasy. It is concluded that H could inhibit the formation of sulfur, thus the sulfur poisoning of the YSZsurface would be prevented by co-adsorbed hydrogen.2. The adsorption of H2S on YSZ+O (111) surfaceThe strudies from the first-principles method based on DFT show that there exist both molecularadsorption and dissociative adsorption modes for the H2S and SH adsorbed on the YSZ+O (111) surface.For S adsorption, there exist two types of stable species, namely, the SO and the hyposulfite (SO22), whichwill block the active sites for fuel oxidation and result in the poisoning of the YSZ+O surface. The dissociation of H2S is very easy with low energy barriers (~0.5eV), and the dissociative S atoms may resultin the poisoning of the YSZ+O surface. In addition, using the ab initio atomistic thermodynamics method,the surface regeneration or de-sulfurization process of a sulfur-poisoned (i.e. sulfur covered) YSZ+O (111)surface is studied. It is concluded that by introducing oxidizing reagents (e.g. O2and H2O), the adsorbedatomic sulfur can be oxidized to SO2and removed from the YSZ+O surface3. Adsorption and oxidation of SO2on the YSZ surfaceThe interaction of SO2with the YSZ (111) and the YSZ+O (111) surfaces is investigated using thefirst-principles method based on DFT. It is found that SO2is adsorbed either as a molecule or a SO23species with new S–O bonds to a surface oxygen on the YSZ (111) surface. In addition, there exist otherspecies, e.g., SO3and SO24on the very active YSZ+O (111) surface. All the formed SOxspecies (with x=24) are strongly bound to the surfaces and are of a poisoning nature for the YSZ surfaces. Using the abinitio atomistic thermodynamics method, we present a detailed analysis on the stability of theSO2YSZ/YSZ+O system as a function of the ambient conditions, such as temperature and SO2partialpressure. For example, at the same ambient conditions, the YSZ+O surface is more susceptible to sulfurpoisoning than the YSZ surfaceThe results from analyzing the sulfurization process of the YSZ (111) upon exposure to H2S and theregeneration mechanism of the sulfur-poisoned surface would be helpful for understanding and furtherexploring sulfur tolerance properties of the YSZ surface under SOFCs operating conditions and searchingfor sulfur tolerant anodes. Through detecting the oxidized species and analyzing of their mutualtransformation in the ambient conditions for SO2adsorption on the YSZ (111) surface, we reach betterunderstanding on the sulfur poisoning mechanism of the YSZ anode of SOFCs.