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Efficient bifunctional catalysts for hydrogen/oxygen evolution reactions (HER/OER) are critical for industrial application of overall water splitting. Herein, a self-supported nanostructure consisting of CoNi–P and embedded carbon dots (denoted as CoNi–P/CDs/NF) with large specific surface area has been designed and fabricated, through a phosphorization process on precursors grown on 3D porous Ni foam. The resultant electrocatalyst exhibited excellent electrocatalytic performance for both the HER (45.2 mV@10 mA cm−2) and OER (184.8 mV@20 mA cm−2) under alkaline conditions. The cell voltage of the full water splitting device assembled was only 1.50 V to achieve 20 mA cm−2 and exhibited great durability benefiting from the self-supported nanostructure. The density functional theory (DFT) calculations and XPS analysis suggest that the synergistic coupling effect between CDs and substrate CoNi–P could tune the electronic structure. Moreover, the embedded CDs increase electrical conductivity and active sites of the catalyst. Both of these and the high specific surface benefited from the 1D/3D hierarchical structure could explain the excellent electrocatalytic performance. The rational materials design of 1D/3D hierarchical structured non-noble bimetallic phosphide catalysts in this work may suggest a new strategy for designing bifunctional electrocatalysts. [ABSTRACT FROM AUTHOR]