A Stanford-led team has developed a new electrolysis system to split seawater in hydrogen and oxygen. Their findings are published in an open-access paper in the Proceedings of the National Academy of Sciences. Existing water-splitting methods rely on highly purified water—a precious resource and costly to produce.

Hongjie Dai and his research lab at Stanford University have developed a prototype that can generate hydrogen fuel from seawater. (Image credit: Courtesy of H. Dai, Yun Kuang, Michael Kenney)

Electrolysis of water to generate hydrogen fuel is an attractive renewable energy storage technology. However, grid-scale freshwater electrolysis would put a heavy strain on vital water resources. Developing cheap electrocatalysts and electrodes that can sustain seawater splitting without chloride corrosion could address the water scarcity issue.

Here we present a multilayer anode consisting of a nickel–iron hydroxide (NiFe) electrocatalyst layer uniformly coated on a nickel sulfide (NiSx) layer formed on porous Ni foam (NiFe/NiSx-Ni), affording superior catalytic activity and corrosion resistance in solar-driven alkaline seawater electrolysis operating at industrially required current densities (0.4 to 1 A/cm²) over 1,000 h.

A continuous, highly oxygen evolution reaction-active NiFe electrocatalyst layer drawing anodic currents toward water oxidation and an in situ-generated polyatomic sulfate and carbonate-rich passivating layers formed in the anode are responsible for chloride repelling and superior corrosion resistance of the salty-water-splitting anode.

—Kuang et al.

Theoretically, to power cities and cars, you need so much hydrogen it is not conceivable to use purified water, said Hongjie Dai, J.G. Jackson and C.J. Wood professor in chemistry in Stanford’s School of Humanities and Sciences and co-senior author on the paper.

Dai said his lab showed proof-of-concept with a demo, but the researchers will leave it up to manufacturers to scale and mass produce the design.

Water splitting with electricity—electrolysis—is a simple and old idea: a power source connects to two electrodes placed in water. Negatively charged chloride in seawater salt can corrode the anode, however, limiting the system’s lifespan. Dai and his team wanted to find a way to stop those seawater components from breaking down the submerged anodes.

The researchers discovered that if they coated the anode with layers that were rich in negative charges, the layers repelled chloride and slowed down the decay of the underlying metal.

They layered nickel-iron hydroxide on top of nickel sulfide, which covers a nickel foam core. The nickel foam acts as a conductor—transporting electricity from the power source—and the nickel-iron hydroxide sparks the electrolysis, separating water into oxygen and hydrogen.

During electrolysis, the nickel sulfide evolves into a negatively charged layer that protects the anode. Just as the negative ends of two magnets push against one another, the negatively charged layer repels chloride and prevents it from reaching the core metal.

Without the negatively charged coating, the anode only works for around 12 hours in seawater, according to Michael Kenney, a graduate student in the Dai lab and co-lead author on the paper.

The whole electrode falls apart into a crumble. But with this layer, it is able to go more than a thousand hours.

—Michael Kenney

Previous studies attempting to split seawater for hydrogen fuel had run low amounts of electric current, because corrosion occurs at higher currents. But Dai, Kenney and their colleagues were able to conduct up to 10 times more electricity through their multi-layer device, which helps it generate hydrogen from seawater at a faster rate.

I think we set a record on the current to split seawater.

—Hongjie Dai

The team members conducted most of their tests in controlled laboratory conditions, where they could regulate the amount of electricity entering the system. But they also designed a solar-powered demonstration machine that produced hydrogen and oxygen gas from seawater collected from San Francisco Bay.

Without the risk of corrosion from salts, the device matched current technologies that use purified water.

The technology could be used for purposes beyond generating energy. Since the process also produces breathable oxygen, divers or submarines could bring devices into the ocean and generate oxygen down below without having to surface for air.

One could just use these elements in existing electrolyzer systems and that could be pretty quick. It’s not like starting from zero—it’s more like starting from 80 or 90 percent.

—Hongjie Dai

Other co-lead authors include visiting scientist Yun Kuang from Beijing University of Chemical Technology and Yongtao Meng of Shandong University of Science and Technology. Additional authors include Wei-Hsuan Hung, Yijin Liu, Jianan Erick Huang, Rohit Prasanna and Michael McGehee.

This work was funded by the US Department of Energy, National Science Foundation, National Science Foundation of China and the National Key Research and Development Project of China.

Resources

• Yun Kuang, Michael J. Kenney, Yongtao Meng, Wei-Hsuan Hung, Yijin Liu, Jianan Erick Huang, Rohit Prasanna, Pengsong Li, Yaping Li, Lei Wang, Meng-Chang Lin, Michael D. McGehee, Xiaoming Sun, Hongjie Dai (2019) “Solar-driven, highly sustained splitting of seawater into hydrogen and oxygen fuels” Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1900556116