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. 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 off oxygen molecules from water.
Improving the way hydrogen energy is generated is key to its emergence as a viable fuel source. Most hydrogen production today occurs through heating steam and methane, but that relies heavily on fossil fuels and produces carbon emissions.
There is a push toward ‘green hydrogen’ which uses more environmentally friendly methods to generate hydrogen. Simplifying the water-splitting reaction is a key part of that effort.
Researchers have been investigating the possibility of using solar energy to generate hydrogen since the early 1970s. The inability to find materials with the combination of properties needed for a device that can perform the key chemical reactions efficiently has kept it from becoming a mainstream method, however.
“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, this created another challenge. The electrons and holes created by absorption of sunlight in silicon must be able to move easily across the silicon dioxide layer. This usually requires the silicon dioxide layer to be no more than a few nanometres, which reduces its effectiveness in protecting the silicon absorber.
The key to solving this issue came through a method of creating electrically conductive paths through a thick silicon dioxide layer that can be performed at low cost and scaled to high manufacturing volumes.
To get there, Yu and his team used a technique first deployed in the manufacturing of semiconductor electronic chips. By coating the silicon dioxide layer with a thin film of aluminium and then heating the entire structure, arrays of nanoscale ‘spikes’ of aluminium that completely bridge the silicon dioxide layer are formed.
These can then 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 at a separate electrode and exhibits considerable stability over extended periods.
Because the techniques employed to create these devices are commonly used in manufacturing of semiconductor electronics, the researchers predict it should be easy to scale for mass production.
Going forward, the team will work to improve the efficiency of the oxygen portion of water-splitting by increasing the reaction rate. The researchers’ next major challenge is then to 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.”
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