How do you split water to create energy? Ask Cork researchers

19 Feb 2016

Researchers from three countries have teamed up to drag water-splitting solar energy production one step closer to reality. What will that reality be? A whole lot cleaner, that’s what.

“It’s a fascinating area,” begins Tyndall’s Dr Paul Hurley. “Splitting water, using the hydrogen to create clean fuel, or methane and methanol, it’s getting so advanced.”

He’s chatting to me after a photovoltaic project between the Tyndall National Institute in Cork, Queen’s University in Belfast and Stanford University in California just broke the record for generating a voltage from a solar cell submerged in water.

Why split water?

What’s it all about? Well, there’s a bit of a race on at the moment to find the best way to create energy from water. Not just pouring water into a car’s petrol tank, though, rather using the sun to split water into its two components of oxygen and hydrogen, using the latter to provide the cleanest of clean energy.

“The holy grail of water splitting using solar cells is that you put it into water, and just use solar energy to split the water molecules. The aim is to get hydrogen reliably without applying any voltage,” he says.

It’s not about dropping a tiny gadget into a full bathtub and powering your house, it’s more about creating a structure to contain the water, and using it as a type of battery.

Leading the race, at least in one regard, is Tyndall, after Hurley saw a paper published from Stanford in 2011.

Back then, US researchers were wondering how to deal with corrosion underwater. When solar cell structures are submerged (the simplest way to achieve split water) the anode generates oxygen, which oxidises the semiconductor it’s in contact with.

A harsh environment

“It’s a really harsh environment,” says Hurley. “You’ve got to protect the anode, and the cathode, if it’s also a semiconductor.” Alongside that, you’ve to work out how to generate voltage, purely from the power of sunlight.

In 2011, Stanford’s Prof Paul McIntyre came up with a novel way of counteracting this. He insulated the semiconductor by adding an extremely thin layer of titanium dioxide to the anode part of the solar cell.

A problem emerged, though. The thicker the film – which was just two nanometers – the less voltage was generated through the silicon parts. The thinner the film, the quicker the semiconductor corroded.

Underneath the insulating film, there was iridium on top as the oxygen catalyst and below that there was n-type silicon. This is where Hurley, who has worked with McIntyre before, stepped in.

Doped with boron

While at Stanford, being shown these results, he asked why they only used n-type silicon. Why not use a p-n junction below the film? They didn’t expect much difference, but Hurley disagreed.

“Se we developed a p-n junction. There was a thin layer, about 0.4 of a micron at the surface. We heavily doped the p-type with boron. We sent that over and thought it would get a bit better.

“The idea was to create just a little more photovoltage than achieved with one type of silicon, but when we sent it to Stanford for testing, they found it was much better, and in fact it broke the record for the voltage produced by this type of anode.”

It’s a complicated record, and an even more complicated project, but one which is certainly making waves (not sorry) around the world in 2016.

Tyndall, Stanford, Queens and beyond

A few weeks ago, Berkeley researchers produced a paper on what they call ‘nanowires’, which can be used to split water using solar power. Around the same time, Cornell University researchers wrote of something similar with ‘nanorods’. South Korean researchers are even using gold to help with the process, in what has been a whirlwind few weeks of publications in the field.

“Nanorods and nanowires are pretty much the same thing,” says Hurley, “although I’ve not heard of the gold one … but there are homes in Thailand that are powered by something similar to what we’re all trying.”

The field is divided by those looking to achieve water splitting with devices kept outside of the water, but linked by wires, and those who submerge their structure as a simpler process.

Simpler, by the way, in execution. Hardly simple in trying to get to that stage. It is the submerged variants that have Tyndall, Queens and Stanford researchers’ full focus.

Breaking the record, again

But back to the record, which, one would hope, will be blown out of the water (not sorry) pretty soon. That’s because Hurley and his colleagues are still a good bit below the required 1.23 volts needed to make this in some way sustainable.

It is giving off 0.6 volts, not enough to sustain water splitting, but the research team has plans to get above 2 volts, which Hurley reckons would be the best target.

There are options around this, like moving away from silicon, but Hurley’s enthusiasm for this “amazing, absolutely incredible material” means he’s not likely to stretch to far beyond silicon’s reach.

“We only worked on one component of the solar cell and generated good voltage. There are other components to be looked at. It’s cumulative. When we work out a process for all the components we won’t be far off.”

Hurley and his colleagues’ work is published in Nature Materials.

Main image via Shutterstock

Gordon Hunt was a journalist with Silicon Republic