Hydrogen can be made from water with light – but it’s tricky

Hydrogen made from water can be a zero-emissions fuel. But for now, it’s still expensive and difficult to make at scale, and it’s not clear which hydrogen production route will work best for the energy industry.

One possible method is called photocatalysis: using light to induce and speed up the reaction.

An international team of researchers has been working on improving a chunk of the hydrogen photocatalysis reaction.

“Photocatalysis is an emerging alternative to electrocatalysis,” says Professor Gunther Andersson, a researcher at Flinders University’s College of Science and Engineering.

“It’s not as mature a technology as electrocatalysis yet, but it’s definitely an alternative.”

Electrocatalysis is how most zero-emissions hydrogen is currently made at scale – using renewable electricity to split water (H₂O) into hydrogen (H₂) and oxygen (O₂).

Photocatalysis creates the same reaction, but instead of electricity it harnesses sunlight directly.

Like many chemical processes, it uses catalysts – substances which make reactions easier – to do the job.

“A photocatalyst is basically absorbing the light and turning the light into something which then can be used for the reaction,” explains Andersson.

“But there are a few other things needed to make it efficient. One is the so-called ‘co-catalysts’, which don’t contribute to the absorption of light, but they help the water-splitting reaction happen efficiently.”

Another problem is that, left to their own devices, any created hydrogen and oxygen gases will react together and turn back into water.

“The catalyst can work in two directions: you can split water to hydrogen and oxygen, but it likewise can do the opposite, and facilitate the reaction of hydrogen and oxygen to form water again. We call that the back reaction, and that’s what you want to avoid,” says Andersson.

One of Andersson’s collaborators has shown that a layer of a substance called chromium oxide can efficiently prevent the back reaction from happening – but it’s not perfect.

“The whole work is now about the stability of the chromium oxide layer,” says Andersson.

Andersson and colleagues have been looking into the properties of chromium-oxide layers over different catalysts and co-catalysts, while they’re being annealed (basically heated). They’ve published their findings in ACS Applied Materials & Interfaces.

The researchers showed that the stability of the chromium depends on the catalyst or co-catalyst it’s sitting on.

They also found that chromium oxide doesn’t interfere with the overall water-splitting reaction.

“World-leading photocatalysts have chromium oxide overlayers, and this work reveals new insights into the nature of the coating that could lead to improvements in future materials,” says co-author Professor Gregory Metha, a chemist at the University of Adelaide.

Researchers are working to come up with a suite of catalysts and co-catalysts that make photocatalytic hydrogen production competitive with existing methods.

“The holy grail of photocatalysis is getting to materials which absorb light in the visible part of the spectrum,” says Andersson.