X-SEED aims at developing an innovative alkaline membrane-less electrolyzer that works at supercritical water conditions (>374°C; >220 bar) generating high-quality H2 at pressures over 200 bar. This technology maximizes energetic efficiency, improves circularity, and enhances lifetime, resulting in a more competitive green H2 production. X-SEED validates results at laboratory scale (TRL4) for a single cell and a 5-cell stack. Novel catalysts and electrodes are designed, synthesized, and characterized to ensure high efficiencies. Multiscale modeling and cell design ensure laminar fluid flows, allowing H2 and O2 separation without a membrane. Supercritical conditions and membrane-less configuration reduce the electrochemical work required to generate H2 (as interface resistances across cell components are decreased) and increase system lifetime. This results in an improved voltage and energy efficiency (42 kWh/kg H2), current density (> 3 A/cm2), H2 production rate and robustness (degradation rate < 1%/1000h). X-SEED also integrates circularity and sustainability assessments in decision-making, limiting the use of critical raw materials (below 0.3 mg/W) and using wastewaters both for catalyst production and as a possible electrolyte for the supercritical electrolyser. X-SEED consortium possess extensive technical knowledge and experience in key enabling technologies and areas. These will be utilized to realize multiphysics models of cell and stack (DTU, SNAM, IDN, PMat), manufacture and select the best catalyst and electrodes (LEITAT, PMAT, IDN), and design the cell, the stack, and the test bench to validate the supercritical electrolyzer at a laboratory scale (IDN, PMat, SNAM). In conclusion, X-SEED project's relevance and added value extend beyond the technological dimension: it will accelerate the H2 ecosystem, supporting Europe in meeting climate targets and maintaining its leadership position as a technological developer, producer, and exporter of green energy