Structural and Electrochemical Stability of Ni-Infiltrated Anodes for SOFC Applications
Creators: name: Keyvanfar, Parastoo
Publisher: name: Graduate Studies
Year: 2017
Subjects: subject: Chemistry--Physical
subject: solid oxide fuel cell
subject: Ni/YSZ anode
subject: infiltration
subject: nanoparticles

description: The long-term durability of solid oxide fuel cells (SOFCs) is a key challenge that continues to limit the commercial implementation of these devices. Ni/yttria stabilized zirconia (YSZ) composites are the most commonly used SOFC anode materials, exhibiting very good chemical stability and electrocatalytic activity for fuel oxidation. However, these anodes suffer from several challenges, including problematic volume changes when Ni is inadvertently exposed to air at high temperatures, causing Ni to form NiO, which is then reduced back to Ni when the fuel flow is resumed (‘redox cycling’). However, the Ni phase then has an altered morphology that can generate internal stresses and cause cells to crack. In order to overcome the redox cycling problem, the use of infiltration techniques is being investigated, where Ni solutions are dispersed into a pre-sintered YSZ scaffolds, followed by reduction to form well-dispersed Ni nanoparticles. By using this technique, lower Ni contents are sufficient to reach the electrical percolation limit (leaving more room for Ni particles to change shape), due to the higher active surfaces area of these infiltrated Ni particles as compared to those produced by conventional techniques. Moreover, the use of infiltration techniques facilitates the addition of sulphur and coke resistant materials, along with Ni, to enhance the carbon and sulphur tolerance of Ni/YSZ anodes. However, a key problem is that the nano-sized catalytic particles tend to sinter and agglomerate at SOFC operating temperatures, resulting in another source of performance degradation. In this thesis work, various approaches were investigated in order to improve the long-term stability of infiltrated Ni/YSZ anodes in both tubular and planar cell configurations, all at SOFC operating temperatures. In one direction, a combination of wetting and non-wetting infiltration solutions were used, resulting in a very good distribution of Ni nanoparticles in the YSZ scaffold and giving a stable performance over 7 days of testing at 800 °C in H2/3%H2O. Another novel approach, in which NiO was dissolved into the YSZ solid-state and then ex-soluted as Ni nanoparticles to help nucleate the deposition of the infiltrated Ni phase was also found to be effective in enhancing anode stability. Furthermore, the co-infiltration of a second phase (either YSZ or gadolinia doped ceria (GDC)) along with Ni was also examined in this work. It was found that infiltration of the YSZ scaffold with Ni + GDC precursor solutions (with a Ni:GDC weight ratio of 8) improved the anode activity by ~ 60%, while no degradation was observed for anodes infiltrated with Ni + YSZ precursor solutions (Ni:YSZ weight ratio of 15). Additionally, the porous YSZ scaffold microstructure was also found to influence the long-term stability of infiltrated Ni/YSZ anodes. It was shown that a YSZ microstructure with smaller pores (100-300 nm) and good pore interconnectivity is more suitable for Ni infiltration, as the infiltrated Ni particles do not sinter as much as those deposited in porous YSZ scaffolds containing larger pores (4-5 µm).