It is highly challenging to develop efficient and low-cost catalysts to meet stringent requirements on high current density for industrial water electrolysis application. We developed sea coral-like NiCo2O4@(Ni, Co)OOH heterojunctions, synthesized based on an epitaxial in-grown method using poly(ethylene glycol) (PEG) as a template, and explored its as efficient electrocatalyst for water-splitting. A two-electrode based alkaline electrolyzer was fabricated using NiCo2O4@(Ni, Co)OOH|| NiCo2O4@(Ni, Co)OOH, which achieved a current density value of 100 mA.cm−2 with a low potential of 1.83 V and high current density approached 600 mA.cm−2 at potential of 2.1 V along with a strong stability. These are superior to most reported data for the electrocatalysts operated at high current densities. In-situ calculations based on density function theory reveal that the occurrence of water-splitting on the NiCo2O4@(Ni, Co)OOH heterojunction surface. First-principles molecular dynamics simulation reveals that the stretching vibrations of metallic bonds of NiCo2O4@(Ni, Co)OOH heterojunctions open the hydrogen bonds of water. Understanding the mechanism of water-splitting at the heterojunction from in-situ theoretical calculations is helpful to develop new generation industrial catalysts.