Researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University with collaborators at the University of Oregon and Manchester Metropolitan University have developed a seawater-resilient bipolar membrane electrolyzer. The design proved successful in generating hydrogen gas without producing large amounts of harmful byproducts.

The results of their study, published in Joule, could help advance efforts to produce low-carbon fuels.

Generation of H₂ and O₂ from untreated water sources represents a promising alternative to ultrapure water required in contemporary proton exchange membrane-based electrolysis. Bipolar membrane-based devices, often used in electrodialysis and CO₂ electrolysis, facilitate impure water electrolysis via the simultaneous mediation of ion transport and enforcement of advantageous microenvironments. Herein, we report their application in direct seawater electrolysis; we show that upon introduction of ionic species such as Na⁺ and Cl⁻ from seawater, bipolar membrane electrolyzers limit the oxidation of Cl⁻ to corrosive OCl⁻ at the anode to a Faradaic efficiency (FE) of 0.005%, while proton exchange membrane electrolyzers under comparable operating conditions exhibit up to 10% FE to Cl⁻ oxidation. The effective mitigation of Cl⁻ oxidation by bipolar membrane electrolyzers underpins their ability to enable longer-term seawater electrolysis than proton exchange membrane assemblies by a factor of 140, suggesting a path to durable seawater electrolysis.

—Marin et al.

The team started their design by controlling the most harmful element to the seawater system—chloride—said Joseph Perryman, a SLAC and Stanford postdoctoral researcher.

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

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. 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.

—Joseph Perryman

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.

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, which were the positive hydrogen ions, 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. 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, such as chloride, to move to places where they shouldn’t be, said Daniela Marin, a Stanford graduate student in chemical engineering and co-author. 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 such as bleach and chlorine.

Along with designing a seawater-to-hydrogen membrane system, the study also provided a better general understanding of how seawater ions moved through membranes, the researchers said. This knowledge could help scientists design stronger membranes for other applications as well, such as producing oxygen gas.

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

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 catalysts and membranes using the facility’s intense X-rays.­­

This project is supported by the US Office of Naval Research; the Stanford Doerr School of Sustainability Accelerator; the DOE’s Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division through the SUNCAT Center for Interface Science and Catalysis, a SLAC-Stanford joint institute; and the DOE’s Energy Efficiency and Renewable Energy Fuel Cell Technologies Office.

Resources

• D.H. Marin, J.T. Perryman et al. (2023) “Hydrogen production with seawater-resilient bipolar membrane electrolyzers”, Joule doi: 10.1016/j.joule.2023.03.005