An ability to convert carbon dioxide into energy using only the power of the sun, as plants do through photosynthesis, would be a monumental breakthrough in green energy research. More and more we are seeing promising strides in this area, the latest of which is the work of scientists at the University of Central Florida (UCF), who have come up with synthetic material that turns visible light from the sun into solar fuels, sucking harmful CO2 out of the air in the process.

The prospect of artificial photosynthesis is a hugely exciting one, and it has inspired scientists to pursue this potential environmental panacea from all angles. We have seen a number of artificial leaves that seek to recreate the energy-harvesting abilities of the real thing, along with more outside-the-box approaches such as hybrid energy systems and photoelectrochemical cells inspired by moth eyes. But still the goal remains out of reach.

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In the view of the UCF team, one of the key challenges centers on the different types of sunlight that can be used to kick off the necessary chemical process, which breaks CO2 down into organic materials that can be used for fuel. The sun's ultraviolet (UV) rays have enough energy to kick off this reaction in some common materials, such as titanium oxide, but the trouble is UV rays only represent around four percent of the sunlight that hits the Earth.

Rays in the visible range, on the other hand, make up the majority of sunlight that reaches the planet. The problem with those is that there are not many materials that can detect these light colors and trigger the reaction. The ones that do, such as platinum, rhenium and iridium, are scarce and expensive, which means the approach is not cost-effective.

So how do you kick off this chemical process using both visible light and affordable materials? Led by assistant professor Fernando Uribe-Romo, the UCF chemists took common titanium and added some organic molecules they hoped would serve as a light-harvesting antennae – a way of bestowing visible-light-capturing abilities on humble titanium.

The titanium and molecules, called N-alkyl-2-aminoterephthalates, were arranged as a metal-organic framework (MOF). MOFs are porous, sponge-like structures with large surface areas that can absorb gases into their tiny pores. This has seen scientists seek to use them in a variety of ways, including building better batteries, developing detectors for nerve agents and capturing carbon, as is the case here.

The molecules also have the ability to be customized to absorb specific colors of light. In this instance, the team designed it to absorb blue light, and then put together a blue LED photoreactor that imitates the sun's blue wavelength to put their new material to the test.

The photoreactor resembled a tanning bed, and by feeding measured amounts of carbon dioxide into the glowing cylinder, the team was able to see if the material within kicked off the chemical process. It did, breaking the CO2 into two reduced forms of carbon, formate and formamides, which are two types of solar fuel.

"This work is a breakthrough," said Uribe-Romo. "Tailoring materials that will absorb a specific color of light is very difficult from the scientific point of view, but from the societal point of view we are contributing to the development of a technology that can help reduce greenhouse gases."

The team now wants to try and tweak the approach, by making adjustments to the MOF to see if it can respond to other wavelengths of visible light. This could boost the efficiency and create greater amounts of reduced carbon (fuel), while sucking greater amounts of CO2 out of the air at the same time.

As is the case with most of these lab-grown artificial synthesis success stories, if the approach can be replicated on a large scale it would be a game-changer. The team points to rooftop tiles as one example of where such a material could be used, cleaning the air and powering homes at the same time. Or perhaps it could be used closer to the source.

"The idea would be to set up stations that capture large amounts of CO2, like next to a power plant," says Uribe-Romo. "The gas would be sucked into the station, go through the process and recycle the greenhouse gases while producing energy that would be put back into the power plant."

The work was published in the Journal of Materials Chemistry A.

Source: University of Central Florida

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