(Invited) On Degradation Mechanisms of Ni-YSZ Fuel Electrodes in Solid Oxide Cells

The solid oxide cell (SOC) is reversible. It has about equally good performance both in solid oxide fuel cell (SOFC) and in solid oxide electrolyzer cell (SOEC) mode. The classical Ni-YSZ cermet SOC fuel electrode has an excellent initial performance provided that it has a good structure in terms of particle size of both Ni and YSZ, and a suitable porosity with sufficient contact between Ni-Ni, YSZ-Ni, YSZ-YSZ particles, and in absence of certain impurities such as silica and sulfur. The essential entity of the Ni-YSZ electrode is the length of the three phase boundary (3pb) between the three phases of Ni electron conductor, YSZ oxide ion conductor, and H2-H2O gas, which have electronic contact to the main Ni electrode, ionic contact to the bulk YSZ electrolyte, and access to the the main gas atmosphere, respectively. Nano-particular Ni is an excellent electrocatalyst for the reduction of H2O to H2 + O2- and for the oxidation. It seems that it is generally accepted that at operation temperatures (above 650 °C and above) the initial nano-sized Ni particles, which are in electric contact with each other, will over time continue to sinter into larger and larger Ni particles until all the Ni has become one dense body or the growth of the Ni particles has been blocked by particles of another phase like YSZ. Thus, the relative high mobility of Ni continues to pose a problem towards the lifetime of a highly functional Ni-YSZ electrode. A most serious type of degradation of Ni-YSZ electrodes in solid oxide electrolysis cells (SOEC) seems to be the one coupled to Ni-migration, which is regarded as an important obstacle for the commercialization of SOEC. Post mortem scanning electron microscopy investigations of the degraded Ni-YSZ electrode reveal that a thin (up to 3 – 5 µm) zone of the original nano-structured active Ni-YSZ cathode has been significantly been depleted in Ni. This degradation is clearly driven by the electrochemical polarization of the Ni-YSZ electrode [1], but the mechanism is highly discussed. Several researchers have the hypothesis that the Ni depletion is a result of Ni migration in a gradient in Ni/YSZ interface energy, see e.g. [2,3]. However, a review of relevant literature points out that this hypothesis cannot explain several reported clear experimental results, whereas our hypothesis based on Ni+ ions from Ni particles, which have lost electrochemical contact, migrate as NiOH across the YSZ particles to new active 3pbs formed further away from the bulk electrolyte. This hypothesis may qualitatively explain all reported results from steam electrolysis cells and H2 fuel cells so far [1]. However, our hypothesis does not directly explain the similar migration of Ni away from the YSZ bulk electrolyte in case of CO2 electrolysis. Therefore, a revision of our hypothesis will be presented. The essential change is simply to propose that it is the Ni+-ion that migrates in the YSZ-surface layer as either NiOH in case of steam electrolysis, or as “Ni2CO3” in case of CO2 electrolysis. A monolayer of “Ni2CO3” on a YSZ surface is not assumed to be any kind of crystalline Ni2CO3, but rather a layer of adsorbed Ni+ and “CO3 1-“ in which CO3 2- has one of its oxygen atoms incorporated into the YSZ surface crystalline structure which is formed by reaction between CO2 from gas and an unsaturated surface oxygen with a “dangling” electron with similarity to CO2 reduction on ceria [4]. Furthermore, the possible role of SiO2 impurities in the loss of contact between Ni and YSZ at negatively polarized Ni will be presented as another revision of the hypothesis together with further new details. References M.B. Mogensen et al., Fuel Cells, (2021), 1–15; DOI: 10.1002/fuce.202100072, and references therein. M. Trini et al., Acta Materialia, 212 (2021) 116887; DOI: 10.1016/j.actamat.2021.116887. L. Rorato et al., J. Electrochem. Soc., 170 (2023) 034504, DOI: 10.1149/1945-7111/acc1a3, and references therein. E.M. Sala et al., Phys. Chem. Chem. Phys. (2023), DOI: 10.1039/d2cp05157e.