Ni–YSZ Solid Oxide Fuel Cell Anode Behavior Upon Redox Cycling Based on Electrical Characterization

Abstract

Nickel (Ni)—yttria‐stabilized zirconia (YSZ) cermets are a prevalent material used for solid oxide fuel cells. The cermet degrades upon redox cycling. The degradation is related to microstructural changes, but knowledge of the mechanisms has been limited. Direct current conductivity measurements were performed on cermets and cermets where the Ni component was removed. Measurements were carried out before, during, and after redox cycling the cermet. The cermet conductivity degraded over time due to sintering of the nickel phase. Following oxidizing events, the conductivity of the cermets improved, whereas the conductivity of the YSZ phase decreased. An improved model of the redox degradation mechanism was established based on the measurements.

Dimensional Behavior of Ni–YSZ Composites during Redox Cycling

“…The initial reduction carried out at a high temperature more than doubled the redox expansion. It is reasonable to assume that the degree of Ni sintering ͑the growth of Ni grains͒ is dependent on temperature, and according to the model of redox instability, 5,12 this reflects in the bulk redox strain upon reoxidation, in this case by a factor of about 2.…”

“…5 and 7-11. In a model proposed by Klemensø and Mogensen, 12 the irreversible redox expansion of Ni would gradually create first micro-and later macrodamage in the ceramic backbone in the form of mechanical degradation, in the worst case leading to mechanical disruption of the composite. This aspect will be further discussed elsewhere.…”

Ni-8YSZ cermet re-oxidation of anode supported solid oxide fuel cell: From kinetics measurements to mechanical damage prediction

“…When the cermet is reoxidized, it appears that the swelling of the NiO particles cannot be accommodated by the voids of the composite. Indeed, NiO particles formed from previously reduced Ni particles present a bigger volume compared to the initial state: this could be due either to the Ni phase reorganisation during reduction [12,13] or to the formation of closed porosity within the re-oxidized particles [14e16]. As a consequence, the nickel re-oxidation is accompanied by a macroscopic irreversible expansion of the composite [11,12,17].…”

In-situ redox cycling behaviour of Ni–BaZr0.85Y0.15O3−δ cermet anodes for Protonic Ceramic Fuel Cells

“…However, it is well-known that redox cycling can adversely affect the TPB length [18]. The redox behavior of state-ofthe-art Ni-YSZ anodes for oxygen-ion conducting fuel cells has been well documented in the literature [18][19][20][21] with several authors noting that the original microstructure and phase distribution cannot be restored when re-oxidation and re-reduction take place [18,20,22,23]. Moreover, previous studies have shown that the durability of anodes is critically dependent on their microstructure and the cermet phase distribution [16,24].…”