(Invited) Mechanical Challenges in up-Scaling SOEC
Authors: Hendriksen Peter Vang , Khajavi Peyman , Tong Xiaofeng , Frandsen Henrik Lund , Chen Ming 
Published: December 2020

For solid oxide fuel cells there are numerous applications calling for units in the power range from 10-50 kW range, for instance for domestic CHP units and APUs for mobile applications. The situation is different for solid oxide electrolysis cells. Here, the most important applications (energy storage or production of synthetic fuels) are characterized by requiring large scale – in the 10 – 100 MW range. Also the electrolysis applications typically require even longer operational life-times than SOFCs.
In this paper we shall discuss some recent achievements in improving both cell toughness/reliability and durability.
In thin-electrolyte cell designs the fuel electrode (a porous mixture of Ni and ZrO2) is typically designed to be the mechanically carrying component. Its mechanical properties are strongly dependent on the amount of porosity [1] and for a given porosity the strength/toughness is dictated by the properties of the zirconia [2]. To maximize toughness, the zirconia chosen is typically a tetragonal composition like “3YSZ” (3 mol% Y2O3 doped zirconia) which because of the transformation toughening mechanism is much tougher than fully stabilized cubic zirconia.
To facilitate manufacture of larger foot-print SOEC cells we have investigated a series of Ce-Y-co-doped zirconias with the aim to identify compositions superior to the currently preferred 3Y-composition. Fifteen compositions have been characterized with respect to phase stability [3] and a subset of 6 of them with respect to mechanical properties. A composition with 1.5%CeO2 and 2.25%Y2O3 substitution in ZrO2 was found to increase the achievable toughness of a porous NiO/Zirconia support by 30% [4] whilst maintaining hydrothermal stability, which may enable a scale up in footprint by a factor of ~15 for a constant risk of failure.
Besides a discussion of how to improve reliability by optimizing the zirconia composition it will be presented how it, in a mechanical perspective, can be advantageous to handle cells in reduced state rather than as presently preferred in oxidized state. Introducing a reduction step converting the NiO to Ni as one of the last steps in the cell manufacture further has the advantage that the porous Ni/YSZ support/electrode lends itself well to modification by infiltration. Recently, it has been shown that the degradation of Ni/YSZ fuel electrode during electrolysis operation can be strongly reduced by Ce-oxide infiltration [5,6]. Recent progress along this line of work shall be presented.
Reducing a full cell with both electrodes or a cell, with a porous CGO-backbone infiltration layer for the oxygen electrode to be, is feasible [7], but this requires careful control of the reduction conditions not to damage the cell due to “expansion on reduction” [8] of any of the CGO parts of the cell. This phenomenon and its relation to the lattice defects will be discussed highlighting some specific works clarifying the latter [8,9].

References

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311 (2016), 1-12.
[3] Peyman Khajavi, Yu Xu, Henrik Lund Frandsen, Jérôme Chevalier, Laurent Gremillard, Ragnar Kiebach, Peter Vang Hendriksen, 2019 In review Ceramics International.
[4] Peyman Khajavi, Peter Vang Hendriksen; Jérôme Chevalier; Laurent Gremillard; Henrik Lund Frandsen, 2019, Submitted to Acta Materialia.

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