Scalable synthesis of active, durable, low-cost, and self-supported electrodes assembled by electrocatalysts towards hydrogen and oxygen generation reactions for water electrolyzers remains a grand challenge. We develop here a facile fabrication of nickel-molybdenum carbide heterostructures embedded in large-area (100 cm²) hierarchically assembled nitrogen-enriched carbon, forming Mott-Schottky array on nickel foam (Ni-Mo₂C/NC@NF). The Ni-Mo₂C/NC array is directly applied as the bifunctional catalyst with high activity and durability in alkaline electrolyte. Particularly, an extremely low overpotential of 40 mV is needed to generate hydrogen. Density functional theory calculation revealed that the formation of Ni-Mo₂C Mott/NC Schottky interfaces enables favorable electronic structures for electrocatalytic water splitting. Besides, 3D hierarchical structure provides exposed active sites, facilitates mass and charge transfer, graphitic shells enhance stability. A symmetric electrolyzer using Ni-Mo₂C/NC@NF generates 10 mA cm⁻² at 1.59 V and operates steadily for 150 h, which even outperforms the noble metal couple, Pt/C//RuO₂ for water electrolysis. The scalability, activity and durability renders Ni-Mo₂C/NC@NF potential industrial application. The assembly-carburization strategy could be further applied for preparing Co-Mo₂C/NC@NF and Cu-Mo₂C/NC@NF.