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[Display omitted] • By optimizing the internal electronic structure and exposing more active sites. • The synergistic effect of heterogeneous atoms and oxygen defects. • The unique hexagonal morphology is more favorable to the exposure of active sites. • The highly active and stable bifunctional cobalt oxide-based catalysts. Undesirable conductivity and insufficient electrocatalytic activity are the current bottlenecks of cobalt tetroxide from achieving efficient water electrolysis. Herein, by optimizing the internal electronic structure and the controlled design of the morphology, we design a unique hexagonal Mn-Co 3 O 4 -200 nanostructure that exhibits superior water electrolysis performance with low overpotentials and excellent long-term durability under strongly alkaline conditions, outperforming many other previously reported cobalt oxide-based compounds. Theoretical combined with characterization analysis indicates that introduced Mn atoms induce an increase in the high spin-orbital occupancy of Co2+ to optimize free energy of the OOH*→O 2 step, which excites the catalytic activity of the Co sites, resulting in a substantial increase in the catalytic activity of the Mn-Co 3 O 4. Furthermore, we briefly summarize the catalytic activity variation pattern of several catalysts with different microscopy morphologies and innovatively propose a more intuitive means to evaluate their exposed active areas. Compared with other morphologies, the special hexagonal morphology with abundant pores and large effectively exposed catalytic surface area can promote the exposure rate of active centers, mass transfer of electrolytes and diffusion of hydrogen and oxygen in the electrocatalytic process, thus accelerating the rate of electrocatalytic reactions. [ABSTRACT FROM AUTHOR]

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]