Bottling the Sun

Sossina Haile's research brings the world closer to liquid energy fueled by the sun

It’s not living. But it is breathing.

The solar reactor in Sossina Haile’s laboratory is respiring oxygen. And with its every breath, the world comes another step closer to bringing the vision of liquid solar fuels to life.

“My lab does not have the total energy solution, but we do have a couple pieces of it,” says Haile, Walter P. Murphy Professor of Materials Science and Engineering. “I can give you two components that will help you get to the end.”

Those two components are hydrogen and carbon monoxide, which are basic ingredients for making fuels that can store solar energy. Haile can make both on demand.

Harnessing the Power

Although the sun’s energy is inarguably an enormous resource, researchers have long experienced difficulty harnessing it.

Solar cells, which have advanced significantly in recent years, still have major drawbacks. Cloudy days and nighttime ensure that sunlight is not always available. Even when the sun does shine, solar cells are inherently restricted to capturing only a small portion of the solar light spectrum. Possibly the most serious drawback of all, researchers have yet to find an ideal way to store energy captured by solar cells for later use. Even the best batteries have limited capacities and eventually self-discharge.

Solar fuels, on the other hand, do not share solar cells’ more serious shortcomings. In particular, they can be stored as liquid for later use. And if sunlight is collected in the form of thermal energy, the resulting solar fuel uses its entire spectrum.

“The benefit of a liquid fuel is that storing it is easy,” Haile says. “We store liquid all the time and could easily store solar fuels with our existing infrastructure.”

The key to Haile’s work lies in a ceramic material called cerium dioxide, or “ceria” for short. At ultrahigh temperatures, ceria exhales oxygen while its structure remains intact. Employing a giant, parabola-shaped mirror, a solar reactor focuses intense sunlight onto the ceria. Using the entire solar spectrum, the method reaches temperatures up to 1500 degrees Celsius, causing the ceria to release its oxygen.

“If you have ever used a magnifying glass to start a fire,” Haile says, “then you have used the same technique on a smaller scale.” As the ceria cools, it dislikes the empty areas left in its structure from the exhaled oxygen atoms and wants to breathe them back in. At this point, Haile’s team steams the ceria, causing it to inhale the oxygen from the water, leaving hydrogen behind. The team then collects the hydrogen, a precursor to liquid fuel.

Giving Energy to CO2

Haile’s group can perform the same process with carbon dioxide. If the gas is pumped near the ceria as it cools down, ceria will steal an oxygen atom to leave behind carbon monoxide. Researchers can then react hydrogen with carbon monoxide to make methane, a primary component of natural gas.

“Carbon dioxide is the end product of burning,” she says. “So it does not have any energy content. We can convert it into something that carries energy and is useful.”

This could potentially crack two parts of the carbon crisis with one solution: carbon dioxide could be collected from the air and turned into a clean liquid fuel that does not contribute to climate change. Haile says that while capturing carbon dioxide from the air is very difficult, if not impossible, using carbon dioxide from coal-fired power plants would be compatible with her process.

While I’m motivated to make a real difference to society, I also want to make sure to do the fundamental science. A technical solution may be great today, but there will always be a better invention tomorrow. So it’s absolutely critical to advance underlying scientific principles that explain what we are able to achieve. This sort of insight is essential for enabling the inventions of the more distant future.
—Sossina Haile, Walter P. Murphy Professor of Materials Science and Engineering

Next Steps

Haile’s team is now exploring the use of a different class of ceramics as the backbone of this research, which could produce more hydrogen more rapidly. She is also working to perfect a fuel cell that uses a unique electrolyte created in her laboratory. The fuel cell fills in the second half of the energy cycle, using solar fuel to produce electricity on demand. For both projects, she hopes that combining science and engineering will produce devices that will have societal impact—and advance the materials science field for future generations.

“While I’m motivated to make a real difference to society, I also want to make sure to do the fundamental science,” Haile says. “A technical solution may be great today, but there will always be a better invention tomorrow. So it’s absolutely critical to advance underlying scientific principles that explain what we are able to achieve. This sort of insight is essential for enabling the inventions of the more distant future."