The power of the sun, wind and sea may soon combine to produce clean-burning hydrogen
fuel, according to a team of Penn State researchers.
The team integrated water purification technology into a new proof-of-concept design for a sea
water electrolyzer, which 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.
Hydrogen is a great fuel, but we have to make it.
The only sustainable way to do that is to use renewable energy and produce it from water.
We also need to use water that people do not want to use for other things, and that would
be sea water.
So, the holy grail of producing hydrogen would be to combine the sea water and the wind and
solar energy found in coastal and offshore environment.
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 made 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.
Through a series of experiments recently published in the journal Energy & Environmental Science,
the researchers tested two commercially available RO membranes and two cation-exchange membranes,
a type of ion-exchange membrane that allows the movement of all positively charged ions
in the system.
Each were tested for membrane resistance to ion movement, the amount of energy needed
to complete reactions, hydrogen and oxygen gas production, interaction with chloride
ions and membrane deterioration.
While one RO membrane turned out to be a "dirt road," the other performed well in comparison
to the cation-exchange membranes.
The researchers are still investigating why there was such a difference between the two
RO membranes.
The researchers do not know exactly why these two membranes have been functioning so differently,
but that is something they are going to figure out.