(Invited) SOEC Upscale; Material Challenges
Authors: Hendriksen Peter Vang , Khajavi Peyman , Frandsen Henrik Lund , Wang Shu , Klitkou Morten Phan , Sudireddy Bhaskar Reddy 
Published: February 2024

To ensure the transition of the global energy system to a sustainable one, and to meet emission reduction targets of national climate policies we need in future to produce large amounts of “green” hydrogen. This will be necessary to sustain the world’s need for synthesis of “green” ammonia, “green” methanol and “green” jet fuels, but also for use in form of hydrogen as a transportation fuel and a reduction agent in steel making. Various electrolysis technologies for providing this hydrogen via an electrochemical splitting of water/steam are likely to become the backbone of this emerging sector.

Among the different electrolysis technologies, available or under development, SOEC stands out with distinct advantages related to the high temperature of operation, most markedly; 1) there is no need for precious metals in the electrodes, and 2) the achievable efficiency is higher than for the other electrolysis technologies. The electrical efficiency may exceed 90 % as the technology can be run thermo-neutrally producing no waste heat, or may even utilize waste heat if such exist for instance from a down-stream upgrading of the hydrogen to methanol or ammonia.

In view of these very significant advantages, SOEC is likely to become a key enabling technology for the above described transition. However, the SOEC technology is less mature than alkaline electrolysis and PEM-based electrolysis and is today deployed much less than the other two technologies. The largest SOEC units manufactured/operated today are in the range of ~1 MW. However, numerous scale up projects have recently been announced and the industrial stakeholders are these years investing massively to bring up production capacity and module sizes.

In this paper we shall discuss some of the challenges in upscaling SOEC and some of the materials issues to be addressed, when considering large scale global deployment over the next decades.

The inherent brittle nature of the ceramics used in the cells has so far limited the size of the cells of most stakeholders to be in the range with side-lengths from 10 to 15 cm. Ideally, in terms of upscaling module sizes, this should be much larger. This however requires great care in the manufacturing, and a tough and strong backbone material in the cell. Most stakeholders rely on stabilized zirconia to provide the structural support. Use of zirconia in the transformable (metastable) tetragonal version with 2 – 4 % Y2O3 substitution holds some advantages over the use of the fully stabilized cubic version with ~8 % Y2O3, as the former has much higher strength/toughness. We shall present our recent results on optimizing composition and processing routes to maximize toughness and strength, whilst preventing unintended spontaneous transformation to the monoclinic phase during use, and shall discuss the consequences in terms of upscaling cell foot-print.

In SOEC one does not need any precious metals (PGM-materials). However, typically the state of the art cells relies on use of several critical raw materials. Several rare earth materials (HREE/LREE) are used (Ce, Gd, Y, La) and also some critical transition metals like Co/Mn and Ni. Ni is by EU not considered critical, but is on the list of strategic materials. For large-scale deployment, it is obviously an advantage if the use of Critical Raw Materials (CRM) can be minimized.

Introducing performance improvement in terms of increased production capacity per m2 of cell or kg of material is a straightforward strategy to reduce the CRM-use of the technology and is likely a part of the strategy among all stakeholders. Endeavors to reduce the use of one particular material are also meaningful in such a materials resource perspective.

We shall here present recent results on activities to increase the competitiveness of the SOEC technology by introducing a metal-support as the structural backbone and replace the Ni/YSZ-fuel electrode with one based on a mixed conducting Ti-based oxide (Ni and Fe-doped (La,Sr)TiO3). Recently, we have found that such a material combination can in fact sustain SOEC operation for extended periods (tested over 5000 hr) at technologically relevant hydrogen production rates corresponding to current densities of 0.5 A/cm2. The results shall be presented and the possibilities to improve the titanate backbone further by compositional changes shall be discussed with special view to eliminating use of HREE/LREE and critical raw materials in general.

Finally, we shall present some recent results on how to eliminate the use of Co in the oxygen electrode. The achieved results shall be compared with other approaches adopted in literature on how to eliminate use of Co and even La in the oxygen electrode.