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This work aims at developing an innovative renewable energy storage solution, based on reversible Solid Oxide Cell (rSOC) technology. That is to say, one system optimized to operate either in electrolysis mode (SOEC) to store excess electricity to produce H 2 , or in fuel cell mode (SOFC) when energy needs exceed local production, to produce electricity and heat again from H 2 or any other fuel locally available. Firstly, work focused on optimization of the different layers constituting the single SOC cell to reach high initial performance applying state-of-the-art materials as previously reported [1]. Secondly, the initially highest performing cells were selected for long-term reversible SOFC/SOEC single cell tests. Thirdly, these cells were integrated in a stack design optimized for reversible operation at high degrees of H 2 and H 2 O utilization. The long-term single cell tests showed significant degradation in galvanostatic test periods during electrolysis but not in fuel cell mode prior to starting the reversible test operation while the degradation diminished during the subsequent rSOC operation of the cells operating at 700 C, +0.6 and -1.2 A/cm 2 in SOFC and SOEC modes respectively, at fuel utilization (FU) up to 80% in both modes. Electrochemical impedance spectroscopy analyses and post-mortem SEM investigations of tested single cells reveal that the fuel electrodes degraded significantly during the longterm single cell tests. Furthermore, long-term stack tests were conducted on 5-cell stacks, integrating both reference cells and optimised cells. The long-term stack tests were conducted applying different switches between SOFC and SOEC modes. Initially long duration tests (100h each mode) were performed in a mixture of 50% H 2 O and 50% H 2 to see the effect of the polarisation only. The alternating cycle SOEC/SOFC was repeated over a 1800 h testing period. Then stack switched daily from SOEC mode

Stacks dedicated to rSOC operation require improvements as compared to stacks dedicated to purely SOEC or SOFC mode. Starting from an electrolysis stack, improvements have been performed in the European project REFLEX, mainly to enhance reactants distribution, reduce pressure drops, integrate new cells specifically developed as part of REFLEX project, and finally integrate larger cells to reduce stack and system cost and footprint. For easier handling, mechanical connection to the system was optimized. Long-term degradation tests were performed both for reference and optimized cells within two 5-cell stacks. A full size 25-cell stack was assembled integrating optimized connections to gases lines, specific stack clamping system and internal electrical insulation required for stack integration into REFLEX modules. For prospective reason enlarged cells were produced and integrated within first a 5-cell stack, and then a 25-cell stack. Finally, stability of performance along pre-serial manufacturing process was checked for 20 stacks before their delivery for integration into REFLEX modules.