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• The 2D porous Ni/NiMoN heterojunction nanobelts catalysts with tunable components were designed and synthesized. • The 2D porous Ni/NiMoN heterojunction have good ability for mass/charge transfer and optimized electronic structure for H* adsorption. • The Ni/NiMoN catalyst exhibits excellent HER activity and durability with low overpotential of 48 mV at 10 mA cm−2. • The electrolyzer composed of 2D Ni/NiMoN catalyst and matched OER catalyst only requires a low cell voltage of 1.47 V to attain 10 mA cm−2 with excellent durability for 96 h. • The origin of superior performance of the heterostructures is elucidated by combining density functional theory calculations and work function test etc. The design of the electrocatalysts with the plentiful heterojuctional interface and large-accessible surface is desired to boost the HER performance, but it remains a challenge. Herein, we showed the design of 2D Ni/Ni 0.2 Mo 0.8 N porous heterojunction nanobelts as effective catalyst for the HER. The intimate combination of Ni and Ni 0.2 Mo 0.8 N can make the formation of the heterojuctional interface with plentiful active sites. The 2D porous structure can provide large accessible surface with less resistance for the transmission of the electrolytes, thus promoting the mass/charge transfer. The ratio of Ni and Ni 0.2 Mo 0.8 N can be easily tuned by regulating the precursor composition, which is important to give catalysts with optimized interface and suitable pores. Density functional theory calculations reveal that the interaction between Ni and Ni 0.2 Mo 0.8 N at the heterointerface can largely improve the intermediate H* adsorption kinetics (ΔG H* = -0.07 eV). Due to the above advantages, the optimized catalysts exhibited excellent HER activity with a low overpotential of 48 mV in 1 M KOH at 10 mA cm−2 (without iR correction), which is superior to the most reported non-noble metal-based catalysts, and the long-term durability with no obvious degradation after continual operation of 48 h Notably, an electrolyzer assembled by the optimized Ni/Ni 0.2 Mo 0.8 N cathode with NiFe-LDH anode only requires a low operating voltage of 1.47 V to deliver 10 mA cm−2 in 1 M KOH, surpassing the corresponding Pt/NF||RuO 2 /NF electrolyzer (1.61 V) and most of reported electrolyzers. The 2D porous Ni/NiMoN heterojunction nanobelts were successfully obtained as highly effective hydrogen evolution electrocatalysts. Benefiting from the redistribution of electrons at the interface and the porous structure, the catalyst exhibits excellent HER activity and durability, affording low overpotential of only 48 mV at 10 mA cm−2. The Ni/NiMoN heterojunction catalyst can match well with an appropriate OER catalyst to realize overall water splitting, only requiring a cell voltage of 1.47 V to achieve 10 mA cm−2 with excellent durability for 96 h, surpassing the corresponding Pt/C/NF||RuO 2 /NF electrolyzer (1.61 V) and most of recently reported Ni/Mo-based compounds. [Display omitted] [ABSTRACT FROM AUTHOR]