The mechanisms for the high resistance to sulfur poisoning of the Ni/yttria-stabilized zirconia system treated with Sn vapor.

The mechanisms for the resistance to sulfur poisoning at the triple phase boundary (TPB) of the Ni/yttria-stabilized zirconia (YSZ) system treated with Sn vapor are studied using the first-principles method based on density functional theory. Models with Sn dopant or adsorbate are proposed. It is found that the 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 which sulfur remains at the Sn doped Ni surface and allows the O ion to diffuse to the O vacancy at the interface. Once the O ion diffuses to the O vacancy, it forms interface O(2-), which repels the sulfur adsorbate and eliminates the sulfur poisoning. However, as the barriers of sulfur diffusion to the vacancy are still small (0.25 eV or smaller), the Sn dopant in Ni does not efficiently eliminate the sulfur poisoning at the TPB. In contrast, the TPB model of the Ni/YSZ system with an Sn adatom on the Ni can form a physical barrier and prevent effectively sulfur diffusion to the O vacancy at the interface. The diffusion barriers are as large as 1.41 eV, which therefore eliminates the sulfur poisoning at the TPB. 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.