Long-Term Stability of Seawater Acidification and Its Effect on the Formation of Mg(OH)2 Films with a Hierarchical Porous Structure in Bipolar Membrane-Based Direct Seawater Electrolysis

This is the first report discussing the long-term stability (1000 h) of direct seawater electrolysis (DSWE) in relation to seawater acidification and inorganic precipitation. Unlike general DSWE, in which inorganic precipitates are formed at the cathode surface due to a local pH increase caused by the hydrogen evolution reaction (HER), bipolar membrane-based DSWE acidified natural seawater to pH 3, suppressing the formation of inorganic precipitates and inducing a positive shift in the equilibrium potential for the HER. Because the acidified seawater became alkaline after 300 h, periodic seawater replacement was suggested as a method of maintaining seawater acidification for 1000 h, during which a Mg(OH) 2 film with a thickness of 1 mm or more was formed at the cathode surface. Using Brunauer–Emmett–Teller (BET) analysis, water vapor sorption, and electrochemical impedance analysis, it was confirmed that the Mg(OH) 2 film has a hierarchical mesoporous structure and high affinity for water, which maintained mass transport. The unique properties of the Mg(OH) 2 film under seawater acidification contributed to a lower rate of increase in the cathodic potential than that under seawater alkalization, where very thin inorganic deposits were formed.