A three-dimensional multiphysics model was developed to investigate the coupled transport and electrochemical behavior of a solid oxide electrolysis cell (SOEC) with a 45 cm² active area and a realistic geometry based on an experimentally established cell configuration. The model simultaneously accounts for ionic and electronic charge transport, Butler–Volmer electrochemical kinetics, multicomponent gas transport using the Maxwell–Stefan formulation, and gas flow through free and porous regions using the Brinkman approach. The main contribution of this study lies in combining full-area three-dimensional modeling of a realistic 45 cm² cell, coupled multicomponent transport on both electrode sides, and independent consistency checks of current, species, and total-mass conservation

with Faraday-law consistency. The results show that increasing current density intensifies steam consumption and hydrogen production on the fuel-electrode side while increasing oxygen enrichment on the oxygen-electrode side. The three-dimensional fields reveal pronounced spatial non-uniformities in gas velocity, species distribution, and electrochemical reaction rate across the active area. At 0.9 A cm⁻², the predicted cell voltage agreed with the reference value within about 4%, while current, species, and total-mass balances showed deviations below 0.5%. Agreement between outlet compositions and Faraday-law predictions further confirms the physical consistency of the model.