Redox cycling of Ni-based solid oxide fuel cell anodes: A review

The published literature relating to damage to SOFCs caused by redox cycling of Ni-based anodes is reviewed. The review covers the kinetics of Ni oxidation and NiO reduction (as single phases and as constituents of composites with yttria-stabilised zirconia, YSZ), the dimensional changes associated with redox cycling and the effect of this on the mechanical integrity and electrical performance of cells and stacks. A critical parameter is the expansion strain that is caused by oxidation. Several studies report that the first complete oxidation of a Ni/YSZ composite causes a linear expansion of the order of 1%, but the actual values vary substantially between different investigations. The oxidation strain is the result of microstructural irreversibility during the redox process and leads to strain accumulation over several redox cycles. This can cause mechanical disruption to an anode, anode support or other cell components attached to the anode. A simplified mechanical model of the stress and damage that are likely to be caused by anode expansion is proposed and applied to anode-supported, electrolyte-supported and inert substrate-supported cell configurations. This allows the maximum oxidation strain to avoid damage in each configuration to be estimated.