Improvement of the critical current density of alkaline water electrolysis based on the hydrodynamic similarity between boiling and water electrolysis

•Critical current density (CCD) of alkaline water electrolysis was improved by honeycomb porous plate (HPP).•Capillary suction and path separation of gas-liquid effects are necessary for the CCD improvement.•CCD and boiling critical heat flux (CHF) showed similar trends with the thickness of HPP.•Honeycomb cooling method to improve the boiling CHF was confirmed to be applied to enhance the water electrolysis CCD. Since water electrolysis is a pollution-free and economical method to produce hydrogen which has a great potential as an energy carrier, it is imperative to improve the performance of water electrolysis for efficient hydrogen production. Supposing that the analogy between boiling and water electrolysis holds, the next interesting question is whether the water electrolysis performance can also be significantly improved by using a method that improves the heat transfer coefficient and the critical heat flux (CHF). The critical current density (CCD), which is the upper operation limit of water electrolysis, is considered to be one of the keys for effective hydrogen generation. However, the study on the enhancement of CCD has rarely been conducted. It is known that the boiling CHF can be improved by a honeycomb porous plate (HPP) due to two effects, capillary force and path separation of gas-liquid. In this work, we applied a cooling method using an HPP which has succeeded in improving the boiling CHF to alkaline water electrolysis. For the first time, we have succeeded in improving the CCD to approximately 1.3 times (CCD: 6.6 A/cm2) compared to without capillary force case (CCD: 5.1 A/cm2). As regards the mechanism, the effect of the liquid supply by capillary action and the improvement of the bubble discharge by separating the gas–liquid flow path using the honeycomb structure of the HPP play an important role in increasing the CCD.