A Stanford University team of researchers have demonstrated a new way of separating hydrogen and oxygen gas from seawater using electricity. Existing water-splitting methods rely on highly purified water, which is a precious resource and costly to produce.
“To power cities and cars, you would need so much hydrogen it is not conceivable to extract it from purified water,” says Hongjie Dai, a chemistry professor in Stanford’s School of Humanities and Sciences. “We barely have enough water for our current needs in California.”
Hydrogen is an appealing option for fuel because it doesn’t emit carbon dioxide when combusted, according to Dai. Burning hydrogen creates only water and should ease worsening climate change problems if widely used as a fuel.
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.
As a concept, splitting water into hydrogen and oxygen with electricity (also known as electrolysis) is a simple and old idea. A power source connects to two electrodes placed in water. When power is turned on, hydrogen gas bubbles out of the negative end (the cathode) while breathable oxygen emerges at the positive end (the anode).
But negatively charged chloride in seawater salt corrodes the anode, limiting the device’s operational life. Dai and his team wanted to find a way to stop the anode from breaking down and reducing the device’s efficiency.
The researchers discovered if they coated the anode with layers rich in negative charges, the layers repelled chloride and slowed the anode’s decay. So, 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, while 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 Stanford graduate student. “The whole electrode falls apart into a crumble,” Kenney says. “But with this layer, it lasts more than a thousand hours.”
Previous attempts to split seawater for hydrogen fuel had run low amounts of electric current, because corrosion forms at higher currents. But the Stanford team sent up to 10 times more electricity through their multi-layer device, which let it generate hydrogen faster from seawater. “I think we set a record on the current to split seawater,” Dai says.
The team members conducted most tests in controlled laboratory conditions, where they could regulate the amount of electricity entering the device. 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 electrolysis methods that use purified water.
Looking back, Dai and Kenney can see the simplicity of their design. “Now that the basic recipe is figured out for electrolysis with seawater, the new method will open doors for increasing the availability of hydrogen fuel powered by solar or wind energy.”
In the future, this new method could be used for tasks beyond generating energy. The process also creates breathable oxygen, so divers or submarines could take devices into the ocean and generate oxygen without having to surface for air.
In terms of transferring the technology, “one could just use these elements in existing electrolyzer systems and that could be pretty quick,” Dai says. “It’s not like starting from zero—it’s more like starting from 80 or 90%.”