Boiling and hydrogen evolving systems both exhibit N-shaped curves, which include peak points known as critical heat flux

(CHF) and critical current density (CCD). Since the CCD represents the maximum manageable current density, it would be

a tentative obstacle to improving the hydrogen generation rate using a water electrolysis. However, studies explaining the

mechanism of or modeling the critical current density (CCD) are scarcely performed. In contrast, substantial efforts have

been made to develop the CHF models in the nuclear engineering field, resulting in well-accepted CHF correlations. Based

on the analogous N-shaped curves in the two systems, the present study explores the CCD in hydrodynamic perspectives

by adopting CHF models. The critical superficial velocities of gas (bubbles), where CHF or CCD occurs are introduced

to compare the bubble generation rate adjacent to the surface. The result shows that the critical superficial velocity in the

hydrogen evolving experiment is about 100 times smaller than that predicted by the CHF correlation. It seems that remarkably

higher active nucleation site density in the hydrogen evolving system attributes the discrepancy. In a phenomenological

standpoint, the critical number of surrounding bubbles limiting the gas generation rate was estimated as six in the hydrogen

evolving system with a maximum error of 8.07%, which is similar to estimate of five in the boiling system. It is concluded

that the CCD phenomenon is governed by the hydrodynamic behavior of the bubbles adjacent to the surface similar to that

of the CHF. This work also insists that the key parameter governs CCD is active nucleation site density, while CHF can be

predicted well by physical properties of fluid solely.