Solar power used to create hydrogen fuel from seawater

Stanford scientists have devised a way to generate hydrogen fuel from seawater using solar power, an advance that may help pave the way for a novel sustainable source of energy.The findings, published in the journal Proceedings of the National Academy of Sciences, demonstrate a new way of separating hydrogen and oxygen gas from salt water. The technology could also 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. Existing water-splitting methods rely on highly purified water, which is a precious resource and costly to produce. Theoretically, to power cities and cars, "you need so much hydrogen it is not conceivable to use purified water," said Hongjie Dai, professor at Stanford University in US.

Hydrogen is an appealing option for fuel because it does not emit carbon dioxide, Dai said. Burning hydrogen produces only water and should ease worsening climate change problems. Researchers showed proof-of-concept with a demo, but the researchers will leave it up to manufacturers to scale and mass produce the design.

As a concept, splitting water into hydrogen and oxygen with electricity -- called electrolysis -- is a simple and old idea: a power source connects to two electrodes placed in water. When power turns on, hydrogen gas bubbles out of the negative end -- called the cathode -- and breathable oxygen emerges at the positive end -- the anode.

However, negatively charged chloride in seawater salt can corrode the positive end, limiting the system's lifespan. Researchers wanted to find a way to stop those seawater components from breaking down the submerged anodes. They 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 sulphide, 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 sulphide 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, said Michael Kenney, a graduate student in the Dai lab.

"The whole electrode falls apart into a crumble. But with this layer, it is able to go more than a thousand hours," Kenney said. Previous studies attempting to split seawater for hydrogen fuel had run low amounts of electric current, because corrosion occurs at higher currents. Researchers 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.