The falling cost of solar and wind-generated electricity, along with the pressing need to cut carbon dioxide emissions through the burning of fossil fuels, has dramatically increased research interest in hydrogen gas production.
While there are many ways to do this, one that is of particular interest to scientists is a method that uses seawater and renewable electricity, but it’s currently not very economically viable due to the high costs of ion exchange membranes and the need to remove salt from seawater.
Now, a team of Penn State researchers say that they’ve integrated water purification technology into a new proof-of-concept design for a seawater electrolyser, which uses an electric current to break apart the oxygen and hydrogen in water molecules. This ‘seawater splitting’ method, as the scientists call it, could make it easier to turn wind and solar energy into storable, portable fuel. The Penn State scientists have published their work in Energy & Environmental Science.
Le Shi, postdoctoral researcher; and Professor Bruce Logan (pictured left to right) operate their newly-designed seawater electrolyser. Image Credit: Tyler Henderson, Penn State.
Despite how freely available and abundant seawater is, it’s not used all that much for water splitting. This is because, unless the seawater is desalinated before entering the electrolyser, the chloride ions in that water will turn into chlorine gas. This degrades the equipment and seeps into the environment.
To get around this problem, the Penn State team’s new method utilises relatively inexpensive and commercially available membranes developed for reverse osmosis (RO), which replaces the ion-exchange membrane in order to selectively transport the favourable ions. This membrane separates the reactions that occur near to two submerged electrodes—a positively charged anode and a negatively charged cathode—with an applied electric field.
When the power is turned on, water molecules begin splitting at the anode, releasing hydrogen ions and creating oxygen gas. The hydrogen ions then pass through the membrane to combine with the electrons at the cathode, ultimately forming hydrogen gas.
“The idea behind RO is that you put a really high pressure on the water and push it through the membrane and keep the chloride ions behind,” says Professor Bruce Logan, from the Environmental Engineering department at Penn State.
This is all carried out while keeping out salt anions and cations, thus desalinating the seawater. With the RO membrane inserted, seawater is kept on the cathode side while the chloride ions are kept from reaching the anode. This prevents the production of chlorine gas.
Penn State’s diagram that reflects how ion movement is affected by a reverse osmosis membrane versus a cation-exchange membrane. Image Credit: Logan Research Group, Pennsylvania State University.
Keeping a High Electrical Current
“RO membranes inhibit salt motion, but the only way you generate current in a circuit is because charged ions in the water move between two electrodes,” Logan said.
Since the larger ions’ movement is restricted by the reverse osmosis membrane, the researchers needed to find out whether there were enough protons moving through the membrane to maintain a high electrical current. “We had to prove that we 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,” he added.
During the researchers’ experiments, they tested two commercially available RO membranes and two cation-exchange membranes (namely ion-exchange membranes that enable the movement of all positively charged ions). Each were tested for the following:
- Membrane resistance to ion movement
- The amount of energy required to complete reactions
- Hydrogen and oxygen gas production
- Interaction with chloride ions
- Membrane deterioration
The researchers found that the two RO membranes yielded very different results, with one performing well in comparison to the cation-exchange membranes. The researchers can’t currently explain why this was the case, but are focused on learning more. “We do not know exactly why these two membranes have been functioning so differently,” says Logan, “but that is something we are going to figure out.”
A close-up of Pennsylvania State University’s seawater electrolyser. Image Credit: Tyler Henderson, Penn State.
Meanwhile, Logan remains hopeful, saying that the seater splitting method of the electrolyser (pictured above) “can work” and that the Pennsylvania State University researchers’ work will help realise the “holy grail” of producing hydrogen by combining seawater, wind, and solar energy found in offshore environments.