Application of macrolayer dryout model for the critical current density of water electrolysis

Electrochemical hydrogen evolving system such as the water electrolysis has the upper operation limit called critical current density (CCD) when the hydrogen film was formed on the surface as the vapor film in the critical heat flux (CHF) condition. This paper tried to apply one of the well-accepted CHF models, macrolayer dryout model to elucidate CCD phenomenon, which is a novel attempt to predict the CCD using the CHF model based upon the hydrodynamic nature of the phenomena. Hydrogen bubble behaviors at around the CCD were observed and compared with those of the CHF. At high nucleate bubble regime, presence of the macrolayer in the hydrogen evolving system was confirmed. Above this layer, hydrogen bubble cluster is generated and detached from the surface. At the CCD point, the nucleate bubbles adjacent to the surface started to coalescence and subsequently formed a hydrogen film. Based on the photographic analyses, the thickness of the macrolayer and the hovering time of the hydrogen bubble cluster were estimated. Application of these parameters to the existing macrolayer dryout model predicts the CCD well within 4.37%. It is concluded that the hydrodynamics of the CCD and the CHF are quite analogous.