The team's experimental water-splitting apparatus. Image: Cockrell School of Engineering, The University of Texas at Austin
By using processes from semiconductor manufacturing and combining silicon materials, researchers found a way to efficiently split oxygen from water.
Our rain, oceans and rivers have long tempted scientists with the prospect of clean hydrogen energy. To date, most attempts to use these resources have failed because splitting water well uses too much energy, and trying to do it at a low cost usually results in poor performance.
New research from the University of Texas at Austin has found a novel way to solve one half of this difficult equation, using sunlight to efficiently split oxygen molecules from water in a low-cost manner.
Improving the way hydrogen energy is generated is key if it is to become viable fuel source. Most hydrogen production today relies heavily on fossil fuels and so produces carbon emissions.
There is a push toward green hydrogen, using more environmentally friendly methods to generate this energy.
Researchers have been investigating the possibility of using solar energy to generate hydrogen since the early 1970s. However, the inability to find materials with the properties needed for a device that can perform the key chemical reactions efficiently has kept this method from becoming mainstream.
‘This conflict can be resolved’
“You need materials that are good at absorbing sunlight and, at the same time, don’t degrade while the water-splitting reactions take place,” said Edward Yu, a professor in the Cockrell School’s Department of Electrical and Computer Engineering at the university.
“It turns out materials that are good at absorbing sunlight tend to be unstable under the conditions required for the water-splitting reaction, while the materials that are stable tend to be poor absorbers of sunlight.
“These conflicting requirements drive you toward a seemingly inevitable trade-off, but by combining multiple materials – one that efficiently absorbs sunlight, such as silicon, and another that provides good stability, such as silicon dioxide – into a single device, this conflict can be resolved.”
However, it can also create another challenge. Researchers said the electrons and holes created by the absorption of sunlight in silicon must be able to move easily across the silicon dioxide layer. This requires the layer to be no more than a few nanometres, which reduces its effectiveness in protecting the silicon absorber.
To solve this issue, Yu and his team used a technique deployed in the manufacturing of electronic chips. They coated the silicon dioxide layer with a thin film of aluminium and heated the entire structure, so arrays of nanoscale ‘spikes’ of aluminium formed across the silicon dioxide layer.
Researchers said these can easily be replaced by nickel or other materials that help catalyse the water-splitting reactions.
When illuminated by sunlight, the devices can efficiently oxidise water to form oxygen molecules while also generating hydrogen.
Because the techniques employed are commonly used in manufacturing semiconductors, the researchers predict it should be easy to scale for mass production.
The team will now work to improve the efficiency of the oxygen portion of water-splitting by increasing the reaction rate. They will then move on to the other half of the equation.
“We were able to address the oxygen side of the reaction first, which is the more challenging part,” said Yu. “But you need to perform both the hydrogen and oxygen evolution reactions to completely split the water molecules, so that’s why our next step is to look at applying these ideas to make devices for the hydrogen portion of the reaction.”
