Monolithic Stacks: The Next Evolution for Solid Oxide Electrolysis/Fuel Cell Technology
Authors: Capotondo Federico , Pirou Steven , Sudireddy Bhaskar Reddy , Goltermann Chrisitan , Bishop Matthew Thomas , Henriksen Henrik , Abdellahi Ebtisam , Hagen Anke 
Published: December 2024

Different solutions adopted in producing Solid Oxide Cells (SOCs) have proven to be scalable and the technology is now approaching commercialization. The high energy conversion efficiency, coupled with the fuel flexibility and the reduced use of expensive catalyst materials promoted by the favorable thermodynamics at the operative temperatures, are making the technology increasingly attractive and stacks are finding applications in more fields. Large scale power-to-X plants, particularly the ones devoted to hydrogen production (Solid Oxide Electrolysis Cells – SOEC), are among the solutions that are gaining most momentum in the energy market. X-to-power (Solid Oxide Fuel Cells – SOFC) solutions such as decentralized heat and electricity production, auxiliary power systems and heavy duty transport propulsion are spreading as well, facing challenges such as power density and dynamic load operation.
Advancement in the SOC technology is opting at substituting ceramic materials with mechanical support function, in favor of metal. A lower amount of ceramic materials not only reduces the raw material costs, but also allows rapid thermal cycling without compromising the mechanical integrity of the cells – hence allowing faster start-ups and load regulations. Furthermore, our recent studies demonstrated a scalable and innovative layer integration process that allows to fabricate complex 3D multilayer structures with internal gas channels and cavities of different geometries, using conventional tape casting. The monolithic SOC stacks rising from this process, combined with the integration of metallic parts in the great majority of their volume, may achieve groundbreaking power densities and the dynamic load operation desired for the mentioned applications, while decreasing CapEX and resources necessary to produce high performing SOCs of the future.
The monolithic SOC stacks obtained within this work are made of three standard repetitive units (SRUs). Each SRU consists of a SOC backbone, composed by two porous zirconia based electrode backbones to be infiltrated with appropriate electro-catalysts, a dense thin zirconia based electrolyte, two sets of ferritic steel gas channel structures – one at the fuel side and one at the oxygen side – conferring enhanced gas diffusion to the triple phase boundaries, mechanical support and electronic conductivity and composite metal/ceramic interconnect plates connecting in series the SRUs. The total number of layers co-fired to produce monoliths within this work is 19 and the monoliths are denoted as “MNx3” (nomenclature defined for a monolith stack with 3x SRUs). The cross section is showed in Figure, where scanning electron microscopy is complemented by qualitative energy dispersive x-ray spectroscopy (EDS) to highlight the material composition of the three SRUs. This work presents the challenges and a solid framework for the solutions adopted to fabricate monolithic stacks aimed at hydrogen production, using the conventional tape casting shaping technique integrated with the innovative assembly process developed. Among the challenges, focus is given to i) the shaping of internal gas channels, ii) structural integrity during fabrication and operation and iii) the use of non-critical raw materials. Detailed electrochemical characterization is currently in progress and the results will be presented at the conference. Besides the real initial performance of the stacks at this development stage, it is speculated that SRU cells performance could get close to the ones of state of the art metal supported cells, also relying on electrocatalyst infiltration.

Figure 1