New System for Turning Seawater into Hydrogen Fuel

Stanford University, CA

A representation of the team’s bipolar membrane system that converts seawater into hydrogen gas. (Image: Nina Fujikawa/SLAC National Accelerator Laboratory)

Researchers at the DoE’s SLAC National Accelerator Laboratory and Stanford University with collaborators at the University of Oregon and Manchester Metropolitan University have found a way to tease hydrogen out of the ocean by funneling seawater through a double-membrane system and electricity. The design successfully generated hydrogen gas without producing large amounts of harmful byproducts. The results, published in Joule, could help advance efforts to produce low-carbon fuels.

“Many water-to-hydrogen systems today try to use a monolayer or single-layer membrane. Our study brought two layers together,” said Adam Nielander, associate staff scientist, SUNCAT Center for Interface Science and Catalysis, a SLAC-Stanford joint institute. “These membrane architectures allowed us to control the way ions in seawater moved in our experiment.”

Many attempts to make hydrogen gas start with fresh or desalinated water, but those methods can be expensive and energy intensive. Treated water is easier to work with because it has fewer chemical elements or molecules floating around. However, purifying water is expensive, requires energy, and adds complexity to devices, the researchers noted. Another option — natural freshwater — also contains many impurities that are problematic for modern technology, in addition to being a more limited resource on the planet, they added.

To work with seawater, the team implemented a bipolar membrane system and tested it using electrolysis, a method that uses electricity to drive ions, or charged elements, to run a desired reaction. They started the design by controlling the most harmful element to the seawater system, chloride, said Joseph Perryman, a SLAC and Stanford postdoctoral researcher.

“There are many reactive species in seawater that can interfere with the water-to-hydrogen reaction, and the sodium chloride that makes seawater salty is one of the main culprits,” Perryman said. “In particular, chloride that gets to the anode and oxidizes will reduce the lifetime of an electrolysis system and can actually become unsafe due to the toxic nature of the oxidation products that include molecular chlorine and bleach.”

The bipolar membrane in the experiment allows access to the conditions needed to make hydrogen gas and mitigates chloride from getting to the reaction center.

“We are essentially doubling up on ways to stop this chloride reaction,” Perryman said.

An ideal membrane system performs three primary functions: separates hydrogen and oxygen gases from seawater; helps move only the useful hydrogen and hydroxide ions while restricting other seawater ions; and helps prevent undesired reactions. Capturing all three of these functions together is hard, and the team’s research is targeted toward exploring systems that can efficiently combine all three of these needs.

Specifically in their experiment, protons passed through one of the membrane layers to a place where they could be collected and turned into hydrogen gas by interacting with a negatively charged electrode, or cathode. The second membrane in the system allowed only negative ions, such as chloride, to travel through.

As an additional backstop, one membrane layer contained negatively charged groups that were fixed to the membrane, which made it harder for other negatively charged ions, like chloride, to move to places where they shouldn’t be, said co-author Daniela Marin. The negatively charged membrane proved to be highly efficient in blocking almost all of the chloride ions in the team’s experiments, and their system operated without generating toxic byproducts like bleach and chlorine.

The work could also help scientists design stronger membranes for other applications, such as for producing oxygen gas.

“There is also some interest in using electrolysis to produce oxygen,” Marin said. “Understanding ion flow and conversion in our bipolar membrane system is critical for this effort, too. Along with producing hydrogen in our experiment, we also showed how to use the bipolar membrane to generate oxygen gas.”

The next step is to improve the electrodes and membranes by building them with materials that are more abundant and easily mined.

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