Harnessing Sunlight to Make Sustainable Fuels

Increasing energy demands and problems associated with burning fossil fuels have heightened interest in more sustainable energy sources, such as sunlight. But there are still areas where carbon-based fuel remains the standard, such as in the aviation industry. To address this need, scientists have been working to devise a way to use sunlight to generate solar-thermal heating that could then drive the chemical reactions that are needed to make jet fuel with net-zero carbon emissions.

Now, a team at Caltech that is part of a Department of Energy (DOE) Energy Innovation Hub known as the Liquid Sunlight Alliance, or LiSA, has developed such a solar-thermal heating system on a small scale and demonstrated that it can successfully drive an important reaction for jet fuel production. Completely powered by solar energy, the so-called photothermocatalytic reactor incorporates a spectrally selective solar absorber to maximize the generation of solar-thermal heating. The modular design of the reactor takes advantage of current fabrication technologies and existing silicon solar panel production infrastructure. The team has demonstrated a lab-scale operation of the reactor, and simulations show that the technology has the potential to scale up to sizes representative of commercial silicon-based thin-film technologies.

"This device demonstrates that the heat generated by abundant solar energy can be used to directly drive catalytic processes, which has normally been done using electricity or fossil fuels," says Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science, Otis Booth Leadership Chair of the Division of Engineering and Applied Science, and LiSA director.

The paper appears online and is included in this month's print edition of the journal Device. The lead author of the paper is Magel P. Su (PhD '24), who designed and fabricated the solar absorber while a graduate student in the Atwater Group.

The reactor incorporates a selective solar absorber with a multilayer design. The goal of such an absorber is to capture as much as possible of the solar spectrum while losing as little heat as possible to the surroundings. "That's very hard to accomplish with a single material, so we went with a multilayer stack," explains Caltech's Aisulu Aitbekova, an author of the new paper and a Kavli Nanoscience Institute (KNI) Postdoctoral Scholar Research Associate in Applied Physics and Materials Science. The Caltech team developed a stack of layers consisting of materials such as silicon, germanium, and gold carefully deposited atop a silver substrate. "Each layer has a specific role, but when combined together, they give you the desired output," Aitbekova says.

In this system, a quartz window at the top allows light to illuminate the solar absorber; a vacuum layer helps minimize heat losses; and the solar absorber sits at the bottom, in direct contact with the chemical reactor. The selective solar absorber achieves a calculated maximum temperature of 249 degrees C under one sun illumination and 130 degrees C under ambient operating conditions (25 degrees C, 1 atm).

The team used the generated solar-thermal heating to drive ethylene oligomerization, a chemical reaction that has traditionally relied on heat derived from the burning of fossil fuels. The oligomerization reaction, which begins with ethylene (C2H4), a hydrocarbon with two carbon atoms connected by a double bond, can be used to make longer hydrocarbon chains called alkenes, which still feature a carbon–carbon double bond. Jet fuels include a wide distribution of hydrocarbon chains, with anywhere from seven to 26 carbon atoms. In the new paper, the Caltech scientists were able to make liquid alkene products with the same range of carbon atoms using solar energy as the only driving force.

Unlike concentrated solar technology, the reactor does not require solar tracking. Solar tracking allows a solar collector, reflector, or photovoltaic panel to follow the sun during the day to maximize the absorbed solar radiation. However, solar tracking systems are more expensive than devices mounted at a fixed angle and orientation.

"We're not competing with concentrated solar technology, where you can reach up to 2,700 suns," Aitbekova says. "We're looking for a complementary technology, which can be used in areas where concentrated solar is not feasible."

In this paper, the team started with ethylene, which is currently derived from fossil fuels. But Aitbekova notes that the LiSA team recently published another paper demonstrating how to make ethylene from carbon dioxide (CO2), water, and sunlight. "So, now we show two steps: First, we use CO2, water, and sunlight to make ethylene, and then we do ethylene oligomerization. And solar energy is the only energy input to the system."

Additional authors of the paper, "A photothermocatalytic reactor and selective solar absorber for sustainable fuel synthesis," are current Caltech graduate students Matthew Salazar and Fabian J. Williams (MS '24), former Caltech postdoc Xueqian Li and former graduate student Shuoyan Xiong, postdoctoral scholar fellowship trainee Matthew Espinosa, and Caltech faculty members Jonas C. Peters and Theodor Agapie (PhD '07). Peters is Bren Professor of Chemistry and director of the Resnick Sustainability Institute; Agapie is the John Stauffer Professor of Chemistry and executive officer for chemistry. In addition to funding from the Liquid Sunlight Alliance, the project received support and infrastructure from the Kavli Nanoscience Institute at Caltech.