Seawater electrolysis provides a desirable pathway for large-scale production of green hydrogen without dependence on freshwater. However, its practical application is seriously hindered by low energy efficiency and insufficient durability because of the detrimental chlorine electro-oxidation and complex components of seawater. Herein, 2D MoN/Co₂N hybrid nanosheets grown on 1D Cu nanowires and integrated on 3D Cu foam electrodes (Co/Mo–N–C/Cu) were fabricated for seawater splitting, which could enable the alkaline seawater electrolyzer assembled with these hierarchically structural electrodes to achieve a current density of 100 mA cm⁻² at a low voltage of 1.70 V, along with good stability due to the synergistic effect among the MoN/Co₂N hybrid nanosheets, unique 1D/2D/3D hierarchical structure, and the N-doped carbon protection layer. Theoretical calculations demonstrated that strong electron transfer occurred at heterogeneous interfaces, which could mediate the adsorption and desorption of intermediates, thus reducing the reaction energy barriers.