Researchers have integrated water purification technology into a new proof-of-concept design for a sea water electrolyzer that uses an electric current to split apart the hydrogen and oxygen in water molecules. This new method for “sea water splitting” could make it easier to turn wind and solar energy into a storable and portable fuel.
Despite the abundance of sea water, it is not commonly used for water splitting. Unless the water is desalinated prior to entering the electrolyzer — an expensive extra step — the chloride ions in sea water turn into toxic chlorine gas, which degrades the equipment and seeps into the environment. To prevent this, the researchers inserted a thin, semipermeable membrane originally developed for purifying water in the reverse osmosis (RO) treatment process. The RO membrane replaced the ion-exchange membrane commonly used in electrolyzers.
In an electrolyzer, sea water would no longer be pushed through the RO membrane but contained by it. A membrane is used to help separate the reactions that occur near two submerged electrodes — a positively charged anode and a negatively charged cathode — connected by an external power source. When the power is turned on, water molecules start splitting at the anode, releasing tiny hydrogen ions called protons and creating oxygen gas.
The protons then pass through the membrane and combine with electrons at the cathode to form hydrogen gas. With the RO membrane inserted, seawater is kept on the cathode side and the chloride ions are too big to pass through the membrane and reach the anode, averting the production of chlorine gas.
But in water splitting, other salts are intentionally dissolved in the water to help make it conductive. The ion-exchange membrane, which filters ions by electrical charge, allows salt ions to pass through. The RO membrane does not. With the movement from the bigger ions restricted by the RO membrane, the researchers needed to see if there were enough tiny protons moving through the pores to keep a high electrical current. They proved that they could get a high amount of current through two electrodes when there was a membrane between them that would not allow salt ions to move back and forth.