Nowadays, the tendency of renewable energy sources to increase their contribution to the global energy network is observed, but this will require the existence of highly efficient energy storage technologies to make possible a decarbonized energy scenario. Hydrogen technologies such as fuel cells and electrolyzers have already demonstrated the potential to be implemented as a solution for clean and efficient power generation and chemical storage. The dual functionality implies the energy production from hydrogen in the fuel cell mode operation and hydrogen generation by water electrolysis in the reversed (electrolyzer) mode of the device. In comparison with other technologies of fuel cells and electrolyzers (PEM and AEL), Solid Oxide Cells (SOCs) present the highest efficiencies due to the kinetic and thermodynamic advantages related to their operation at high temperatures (700-900ºC). Despite the advantages, currently applied SOC fabrication procedure involves multiple intermediate steps and the use of various manufacturing techniques, such as tape-casting, screen-printing, manual stacking, joining, sealing, etc., which makes the process more expensive and time-consuming. Recently, the capabilities of additive manufacturing (AM) technologies to improve significantly the fabrication process of functional ceramics, in particular, for SOC application, were demonstrated. AM allows freedom of design which enhances the performance of the SOC-based device. Moreover, AM increases the manufacturing speed and productiveness, while the waste material is reduced. As a result, 3D printing provides the opportunity to produce a monolithic SOC stack in a single-step process with a combination of different AM technologies.

For this final goal, hybrid 3D printing technology based on stereolithography (SLA) and robocasting (direct ink writing, DIW) has been developed. The hybridization of the 3D printing approach allows manufacturing of the different SOC functional layers with different microstructures, such as dense SOC electrolyte (SLA) or porous electrodes (DIW), combined in a single 3D printed part. In particular, the technique uses yttria-stabilized zirconia (8 mol.%, 8YSZ) for the electrolyte fabrication by SLA and deposition of electrodes (LSM-YSZ, NiO-YSZ) and interconnectors by DIW. Thus, the elaborated hybrid multimaterial 3D printing technology makes possible the production of SOC stack in a one-step process. However, the compatibility of different 3D printed SOC components, especially during the sintering post-printing steps is essential to reach the final device goal and has to be carefully investigated. In the present work, the 3D printing fabrication steps and the results of a complete characterization of the obtained cells are discussed with a special focus on the issue of the co-sintering. 

Acknowledgements:
The authors acknowledge the financial support of the Government of Catalonia, the Secretariat for Universities and Research of the Ministry of Business and Knowledge of the Government of Catalonia and the European Social Fund. (2021 SGR 00750 and 2021 FI_B 00984). The research leading to these results has received the funding on the frame of the RETOS call for the 3D-Progress (PID2019-107106RB-C31) and the HyFLHi Project (PDC2021-121131-C21/AEI/10.13039/501100011033), and co-financed by the European Union “NextGenerationEU”/PRTR.