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The concerted catalysis throughout multiple elementary steps is key to impel sluggish hydrogen evolution reaction (HER). Herein, NiMo/Mo 2 N/NC(500-R) array is constructed by embedding densely packed NiMo/Mo 2 N heterojunctions into nanorod skeleton of nitrogen-rich derived carbons (NC), then followed by electrochemical surface reconstruction. The great electrochemical surface area of resultant electrocatalyst implies fully-exposed concerted diatomic-ensembles. The ensemble effect of unique heteronanojunction induces charge polarizations between Mo and Ni species, gives pseudocapacitance comparable to supercapacitors, and pumps mass transfers to refresh entire HER process. The exceptional activity is demonstrated by ultra-low overpotentials of 5.8 and 200.6 mV respectively at 10 and 1000 mA cm−2, the impressive durability is indicated by lasting alkaline HER for 1000 h at 1 A cm−2. Moreover, the performances in water-alkali electrolyzer outperform most reported catalysts. This work presents valuable insights into concerted catalysis and designing efficient and durable electrocatalysts by ensemble effect from different metal heterojunctions for commercial applications. [Display omitted] • Well-separated heteronanojunctions impart the enhanced intrinsic activity to array. • Ensemble effect refreshes the concerted diatomic sites by charge polarizations. • Electrocatalyst delivers ultra-low overpotentials in alkaline hydrogen evolution. • Striking durability makes electrocatalyst commercializable in hydrogen production. • In-depth insights offer guidance to design efficient and durable electrocatalysts. [ABSTRACT FROM AUTHOR]