A representation of the bipolar membrane system that can convert seawater into hydrogen gas. Image: Nina Fujikawa/SLAC National Accelerator Laboratory.
A representation of the bipolar membrane system that can convert seawater into hydrogen gas. Image: Nina Fujikawa/SLAC National Accelerator Laboratory.

Seawater’s mix of hydrogen, oxygen, sodium and other elements makes it vital to life on Earth. But that same complex chemistry has made it difficult to extract hydrogen gas from seawater for clean energy uses.

Now, researchers at the US Department of Energy's SLAC National Accelerator Laboratory and Stanford University, together with collaborators at the University of Oregon and Manchester Metropolitan University in the UK, have found a way to tease hydrogen out of the ocean using a double-membrane system and electricity. Their innovative design proved successful in generating hydrogen gas without producing large amounts of harmful by-products. The results of their study, reported in a paper 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,” explained Adam Nielander, an associate staff scientist with the 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.”

Hydrogen gas is a low-carbon fuel currently used in many ways, such as to run fuel-cell electric vehicles and as a long-duration energy storage option – one that can store energy for weeks, months or longer – for electric grids.

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 less stuff – chemical elements or molecules – floating around. But purifying water is expensive, requires energy and adds complexity. Another option, natural freshwater, also contains a number of impurities that are problematic for modern technology, in addition to being a more limited resource than seawater.

To work with seawater, the team implemented a bipolar, or two-layer, membrane system and tested it with electrolysis, a method that uses electricity to drive a chemical reaction. According to Joseph Perryman, a SLAC and Stanford postdoctoral researcher, they started their design by controlling the most harmful element in seawater – chloride.

“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 polymer membrane used in the experiment allows access to the conditions needed to make hydrogen gas and also helps prevent chloride from getting to the reaction center. “We are essentially doubling up on ways to stop this chloride reaction,” Perryman said.

The ideal membrane system would perform three primary functions: separate hydrogen and oxygen gases from seawater; move only the useful hydrogen and hydroxide ions while restricting the migration of other seawater ions; and help to prevent undesired reactions. The team’s research is targeted toward exploring systems that can efficiently combine all three of these functions.

In their latest system, protons, which are positive hydrogen ions, pass through one of the membrane layers to a place where they can be collected and turned into hydrogen gas by interacting with a negatively charged electrode. The second membrane in the system allows only negative ions, such as chloride, to travel through.

As an additional backstop, one membrane layer contains negatively charged groups fixed to the membrane, which makes it harder for other negatively charged ions, like chloride, to move to places where they shouldn’t go, said Daniela Marin, a Stanford graduate student in chemical engineering and co-author of the paper. The negatively charged membrane proved to be highly efficient at blocking almost all of the chloride ions in the team’s experiments, and their system could operate without generating toxic by-products like bleach and chlorine.

Along with designing a seawater-to-hydrogen membrane system, the study also provided a better general understanding of how seawater ions move through membranes. This knowledge can help scientists design stronger membranes for other applications, such as 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.”

Next, the team plans to improve their electrodes and membranes by fabricating them with materials that are more abundant and easily mined. This design improvement could make the electrolysis system easier to scale up to the size needed for generating hydrogen for energy-intensive activities, like the transportation sector.

The researchers also hope to take their electrolysis cells to SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), where they can study the atomic structure of the catalysts and membranes using the facility’s intense X-rays.

“The future is bright for green-hydrogen technologies,” said Thomas Jaramillo, professor at SLAC and Stanford and director of SUNCAT. “The fundamental insights we are gaining are key to informing future innovations for improved performance, durability and scalability of this technology.”

This story is adapted from material from SLAC National Accelerator Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.