Call it the greenest of green energies. Scientists have long tried to use just Sun and water to generate energy, a bit like plants do when they photosynthesize. But the process—which involves using sunlight to split water molecules—has been too inefficient to be commercially viable. A new advance may change that.

Previous attempts to use the Sun’s energy to split water molecules have faced multiple problems. The process requires energetic photons to cut the bonds between water’s hydrogen and oxygen atoms. Shorter wave, and thus more energetic, photons from ultraviolet and visible light can accomplish the task. But the Sun’s infrared photons, which comprise roughly 50% of those that reach Earth, aren’t energetic enough.

Solar water splitters try to get around this with two strategies. The first, and most efficient, involves using a device called a photoelectrochemical cell (PEC). These are a bit like batteries, with two electrodes dunked in a liquid electrolyte. One electrode acts like a mini–solar cell, absorbing sunlight and using the energy to generate electrical charges. Those charges are then fed to catalysts on the electrodes to split water molecules and generate hydrogen gas at one electrode and oxygen gas at the other.

The best PECs can convert nearly one-quarter of sunlight’s energy into hydrogen fuel. But they require the use of corrosive electrolytes that quickly tear apart the light-absorbing semiconductor.

A second strategy, called a monolithic photocatalytic cell, does away with the batterylike setup and simply dunks a light-absorbing semiconductor in water. The semiconductor absorbs sunlight and generates electrical charges that are fed to catalytic metals on its surface that then split water molecules. But because the resulting hydrogen and oxygen are generated right next to each other, they can readily react with one another, reforming water.

That has limited the efficiency of these photocatalytic water splitters to converting only about 3% of the Sun’s incoming energy into usable hydrogen fuel. One workaround could be to simply make the semiconductors larger, like conventional solar panels. But semiconductors capable of splitting water are far more expensive than standard silicon solar panels, making that option too costly.

So, in the new study, researchers led by Zetian Mi, a chemist at the University of Michigan, Ann Arbor, tweaked their photocatalytic equipment. Above their setup they placed a lens about the size of a typical house window. This focused the sunlight to a 100-fold smaller area, allowing them to reduce the size, and cost, of their water-splitting semiconductor. The intense sunlight then generated electrical charges in the semiconductor that were passed to nanosize metal catalysts peppered on top, which then carried out the water-splitting reactions.

Mi’s team also raised the temperature of the water being split to 70°C, which prevented most of the hydrogen and oxygen gases from reacting with one another to reform water. The latest iteration of their device uses not only the visible and ultraviolet photons able to split water, but also the less energetic infrared photons.

The combined changes enabled the scientists to convert 9.2% of the Sun’s energy into hydrogen fuel, roughly three times more than previous photocatalytic setups, they report today in Nature.

“This is quite an accomplishment,” adds Peidong Yang, a chemist at the University of California, Berkeley, whose team helped pioneer photocatalytic water splitting 20 years ago but who was not involved with the current work. Todd Deutsch, a water-splitting expert at the National Renewable Energy Laboratory, adds that the efficiency is now within striking distance of the 10% target likely needed to make these devices commercially viable.

Still, the new setup faces commercial challenges, Deutsch says. It produces a potentially explosive mixture of hydrogen and oxygen gases, for example. A commercial version would have to separate those gases, he notes, adding to the cost.

If such devices eventually do make it into the market, engineers would likely need to create massive solar water-splitting farms to generate enough hydrogen to power vehicle fleets, industrial furnaces, and commercial fuel cells that could convert the hydrogen into electricity to be fed to the grid. That day remains distant, Mi notes.

Still, given that photocatalytic water-splitting cells are simpler to design than PECs, that should make them far cheaper to mass produce, Mi says. In addition, he notes, the new setup also works well, though somewhat less efficiently, with seawater, a cheap and inexhaustible resource. Being able to convert seawater cheaply into carbon-free fuel would truly be the ultimate in green energy.