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[Display omitted] A self-standing NiFe-LDH@CoS x /NF electrocatalyst is developed by electrodepositing amorphous CoS x onto NiFe-LDH nanosheet arrays, which shows highly efficient performance towards overall water splitting. • A novel self-standing 3D NiFe-LDH@CoS x /NF bifunctional electrocatalyst is fabricated. • Integration of HER-active CoS x and OER-effective NiFe-LDH endows dual-function activity. • The NiFe-LDH@CoS x /NF assembly requires ultralow overpotentials for both HER and OER. • Interface interactions and synergistic effect favor the both HER and OER activity. Electrocatalytical water splitting to produce hydrogen energy is a promising green technology to solve energy crisis and environmental issues. The development of highly efficient and low-cost electrocatalysts is crucial for mass hydrogen production from water splitting. We newly assemble the self-standing 3D NiFe-LDH@CoS x /NF bifunctional electrocatalysts, via the electrodeposition of amorphous CoS x on two-dimensional (2D) NiFe-LDH nanosheets which are supported by porous nickel foam (NF). The integration of HER-active electrocatalyst of CoS x with outstanding OER catalyst of NiFe-LDH guarantees the bifunctional electrocatalytic activity. Furthermore, the formation of the constructed interfaces between amorphous CoS x and NiFe-LDH nanosheets synergistically favors the electron transfer. Therefore, this novel NiFe-LDH@CoS x /NF hierarchical assembly needs ultralow overpotentials of 136 mV (for HER) and 206 mV (for OER) to afford the 10 mA cm−2 current density in alkaline medium (1 M KOH). An electrolyzer based on the NiFe-LDH@CoS x /NF produces an extremely low cell voltage (1.537 V) and good durability. This study offers a promising strategy for facile fabrication of low-cost electrocatalysts which boost the electrocatalytic performance for overall water splitting. [ABSTRACT FROM AUTHOR]