Hydrogen from seawater

New electrolysis system enables efficient water splitting with salt water

So far, electrolysis systems for water splitting need purified drinking water - a scarce commodity. But in the future, the abundantly available seawater could be used for hydrogen production. © RusN / iStock
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Salt water instead of drinking water: Researchers have developed an electrolysis system that enables water splitting even with unpurified seawater - and thus hydrogen production from the sea. A special coating protects the electrodes from corrosion. In the test, the system coupled with a solar cell achieved an efficiency of almost twelve percent - it is thus as efficient as electrolysis systems with purified water.

Hydrogen is considered an environmentally friendly fuel - for example, for vehicles or fuel cell aircraft. At the same time, this gas would be a good way to buffer excess electricity from solar or wind power plants. The electricity generated by the systems supplies electrodes that decompose the water electrochemically into oxygen and hydrogen. Even now, the first systems for solar water splitting achieve efficiencies of 19 percent and novel electrodes even allow electrolysis with a 1.5 volt battery.

The problem of corrosion

But there is a catch: so far, the electrolysis systems only work with pure drinking water - and that is scarce in many regions of the world. "Using the water splitting to store a substantial amount of global energy would therefore create significant water supply problems, " said Yun Kuang of Standford University and his colleagues.

One solution would be to simply use seawater for the electrolysis - for there is enough of that on earth. But that has failed so far because the salt water eats the electrodes of such systems relatively quickly. The chloride ions of the salt water corrode the anode within a few hours. This process is faster, the higher the applied voltage. As a result, previous attempts to perform an electrolysis with seawater, hardly practical.

Negative protective layer

But Kuang and his team have now developed a system that can withstand the salt water despite high voltage. The key to this was the development of a special coating for the anode. Its electrode consists of a nickel foam, which is surrounded by a layer of nickel sulfide. In turn, researchers applied a layer of nickel-iron hydroxide to this layer. display

The clou: During the electrolysis, the nickel sulphide forms a protective layer of negatively charged sulphate and carbonate molecules. Because the destructive chloride ions are also negatively charged, they are repelled. At the same time, however, this layer does not interfere with the function of the electrode. "These in situ generated passivating layers are responsible for the high resistance to corrosion, " explain the researchers.

Stable for more than a thousand hours

How well this protection works, showed first tests with seawater from the San Francisco Bay: "Our system worked continuously for more than a thousand hours without obvious decay phenomena, " report Kuang and his team. And even in tests with a higher salt content than in seawater, the corrosion remained: "The electrolysis ran so more than a thousand hours stable and without corrosion, " the researchers.

Prototype of the new electrolysis system in the glass vessel is seawater. H. Dai, Yun Kuang, Michael Kenney

But what about the performance? To test this, the scientists operated their electrolysis system with different voltages - first at 2.12 volts and a current density of 400 milliamps per square centimeter, then 2.75 milliamps and a voltage of 2.75 volts. In this test, they coupled the electrolytic cell directly to a solar cell and this cell remained stable.

"The impressive thing about it was that we worked with it in areas of tension that are commonplace in classic systems today, " says Kuang's colleague Michael Kenney. "However, we could have set a tension record in the division of seawater."

Efficiency as with purified water

Positive also: The efficiency of this solar-powered hydrogen production was 11.9 percent, as the researchers report. "This is similar to similar systems that use purified water." Kuang and his colleagues believe that their technology could open new avenues for the development of seawater electrolysis systems. "This gives us the opportunity to use the vast seawater resources of the earth as an energy source, " they say.

So far, the system is just a laboratory-scale prototype, but for companies, it's easy to scale the principle and mass-produce such electrodes, Kuang and his team say. "You just have to integrate these components into existing electrolysis systems, that would be pretty quick, " says co-author Hongjie Dai of Stanford University. "Because we do not have to start from zero - rather 80 or 90 percent." (Proceedings of the National Academy of Sciences, 2019: doi: 10.1073 / pnas.1900556116)

Source: Stanford University

- Nadja Podbregar