Theoretical Study On The Sulfur Poisoning And Carbon Deposition Tolerance Mechanism Of Nickel /Yttria-Stabilized Zirconia

Solid oxide fuel cells(SOFCs) is promising alternative energy sources, which have advantages of high operating efficiency, fuel flexibility, low emissions and relatively low cost. However, there are several challenges concerning the SOFC research. Little is known about the complex interfacial electrochemistry and thermochemistry, and it is also difficult to diagnose problems and optimize cell performance. Therefore, physics-based models are needed to better understand the underlying mechanisms of SOFCs, e.g, sulfur poisioning, coking of the traditional anode of SOFCs, Nickel/Yttria-stablized zirconia(Ni/YSZ). The present thesis addresses four important aspects of the theoretical modeling of SOFCs.1. The Ni/YSZ and Ni/YSZ+O and their sulfur poisoning mechanismsThe Ni/YSZ and Ni/YSZ+O models are builded to study the sulfur poisoning mechanisms. It is found that the adsorbed sulfur does not favor to be located at the stoichiometric Ni/YSZ interface. Instead, it diffuses much easily away from the interface due to the repulsion between the formed S- and the interface O2-. With the formation of O vacancy at the Ni/YSZ interface, the diffusion direction of sulfur is reversed. The adsorbed S- diffuses back to the Ni/YSZ interface and is oxidized to S2- and trapped at the oxygen vacancy. Moreover, the trapped sulfur is very difficult to be removed by the fuel(e.g., H2). Therefore the trapped sulfur blocks the pathway of the oxygen ion transfer. As a result, the oxygen ion transfer resistance would be increased and the SOFC performance would drop. It is found that the extra O in YSZ weakens the S adsorption at the interface of Ni/YSZ+O; the extra O in YSZ improves the diffusion of S out of the vacancy of Ni/YSZ+O; the Ni/YSZ+O can help to alleviate the sulfur poisoning as compared with the Ni/YSZ.2. The Ni-Sn/YSZ and the mechanisms for its high resistance to sulfur poisoningModels with Sn dopant or adsorbate are proposed. It is found that the triple phase boundary(TPB) model of the Ni/YSZ system with Sn dopant in Ni can to some extent restrain the diffusion of sulfur from the Ni part to the interface O vacancy by forcing the sulfur atom to diffuse along a longer path, which increases the time for sulfur remaining at the Sn doped Ni surface and allows the O ion to diffuse to the O vacancy at the interface. The results give a detailed dynamic picture of the mechanism of the high tolerance to sulfur poisoning of the Ni/YSZ anode at the TPB after the pre-exposures to metallic tin vapor.3. Resistance to sulfur poisoning of the Au-, Ag- or Cu-doped Ni/YSZThe effects of IB metals(Gold, Silver, and Copper) dopants at the TPB on the resistance to sulfur poisoning of the Ni/YSZ are studied using the first-principles method based on density functional theory. It is found that the Au dopant prefers to be at the neighbor of the oxygen vacancy site(denoted as NiAu-d/YSZ) while the Ag, Cu dopants tend to be located at the top Ni layer, which have little effects on the sulfur adsorption at the interface oxygen vacancy site. Compared with Ni/YSZ, the NiAu-d/YSZ can not only weaken the sulfur adsorption at the interface oxygen vacancy site, but also restrain the diffusion of sulfur to the interface oxygen vacancy. Instead, the adsorbed S at the oxygen vacancy is more easily to diffuse out of the interface oxygen vacancy site. The doping Au in Ni at the neighbor of the interface oxygen vacancy site would be a good way to increase the resistance to sulfur poisoning of the Ni/YSZ anode.4. The effects of H2, CO or H2 O containing fuels on the carbon deposition of the Ni/YSZThe the carbon deposition on the Ni/YSZ caused by the H2, CO or H2 O containing fuels is studied. The results show that 1) compared to the pure Ni(111) surface, the CH fragment can easily diffuse and be trapped at interface O vacancy site. The trapped CH can dissociate to C and H atoms much with lower barrier; 2) CO containing fuels may cause carbon deposition easily at the interface oxygen vacancy. The existence of H facilitates the dissociations of CO at the interface oxygen vacancy. 3) When the H2 O amount is low, the dissociated H2 O reacts with the interface C forming CHO. The dissociation process of CHO to CH and O is more favorable as compared with that of CHO to CO and H, which is the main reason for the low efficiency of carbon removal from the TPB of Ni/YSZ by adsorbed water.The studies of the mechanism of the sulfur poisoning and carbon deposition as well as their tolerance mechanism of the Ni based alloy/YSZ would be helpful for understanding and further exploring sulfur and coking tolerance properties of the Ni/YSZ under SOFCs operating conditions.