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We present an electrodeposition strategy to construct heterostructures nickel selenide (NiSe) and carbon nanotubes (CNTs), namely NiSe@CNTs, to tune the electronic structure of the Ni sites, thus stimulating the hidden catalytic potential of the Ni sites. As expected, the optimized NiSe@CNTs exhibits unprecedentedly ultra-high activity for HER (27 mV at 10 mA cm−2) and OER (145 mV at 100 mA cm−2) and outstanding long-term durability (730 h). Based on the systematic characterization and DFT results, the Ni sites with increased electron density induced by π-electron off-domain of CNTs not only greatly optimize the H* adsorption kinetics but also reduce the energy barrier of the *O→*OOH step, demonstrating the enhanced intrinsic catalytic activity. Furthermore, the introduced CNTs not only avoid aggregation of nanoparticles but also reduce particle size, thus increasing the number of exposed Ni active sites. This study provides guidance for the exploration of catalysts with ultra-high activity and durability. [Display omitted] • The ultra-high activity for HER (27 mV at 10 mA cm-2) and OER (145 mV at 100 mA cm-2) and outstanding long-term durability (730 h). • Internal and external means to enhance catalytic activity. • The interfacial effect between NiSe and CNTs in the heterostructure exacerbates the further π-electrons delocalization of CNTs. [ABSTRACT FROM AUTHOR]