Recycling Carbon Dioxide as Fuel Could Lead to Zero-Emissions Flight Travel

The process could make electric planes irrelevant.

Flickr / Jason O'Halloran

Air travel may be a convenient way to see the world, but its growing carbon footprint has given some travelers pause in recent years. A creative new solution could help reduce the environmental impact of air travel, though.

In a study published Monday in the journal Nature Energy, researchers from Stanford University and the Technical University of Denmark share new findings that could help the aviation industry reach carbon neutrality. By chemically converting carbon dioxide, they propose a strategy that would not only eliminate airplanes’ CO^^2 emissions but also create a way to recycle the waste product into jet fuel.

Some travelers, like teenage environmental activist Greta Thunberg, have chosen to forego air travel altogether in an effort to decrease their environmental impacts. But if this technology is widely adopted, it could put a significant dent in the impact airplanes have on the global climate. The aviation industry accounts for 2 percent of global CO^^2 emissions annually — equaling roughly 859 million tonnes — and a 2018 report from the International Air Transport Association predicted that passenger numbers will double from present rates to 8.2 billion a year by 2037.

Air travel accounts for about 2 percent of the world's carbon dioxide emissions each year, and this contribution is expected to increase in each coming year.

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Study co-author William Chueh, Ph.D., an associate professor of materials science and engineering at Stanford, said this technique could help reduce the CO^^2 contributions of air travel without creating new side effects.

“We showed we can use electricity to reduce CO^^2 into [carbon-monoxide] with 100 percent selectivity and without producing the undesired byproduct of solid carbon,” he said in a press release.

While this study is not the first to propose capturing and converting CO^^2 — in fact the practice itself is generally referred to as carbon capture — the researchers propose a new, more effective way to equip airplanes for carbon capture by introducing a chemical compound called cerium oxide.

In this carbon capture process cerium oxide acts a catalyst to jumpstart the reaction that will strip CO^^2 of its extra oxygen and prepare the resultant CO to be converted into synthetic fuel, like jet fuel, later on by simply introducing an additional hydrogen. The use of air travel emissions to potentially create new jet fuel would help the aviation industry create a carbon-neutral fuel cycle.

Researchers fabricate cerium oxide electrodes for the carbon capture process.

Mark Golden

Previous research has shown some success using nickel and silver as catalysts instead, but those metals can be susceptible to solid carbon build-up through the conversion process, which eventually shortens the lifespan of the fuel cells.

To determine the effectiveness of using cerium oxide instead, the team created two fuel cells — one using a standard nickel electrode and the other a cerium oxide one. They saw that the cerium oxide electrode remained more stable than the nickel-based one during the conversion process . In turn, this stability helped the cerium oxide-based cell perform longer.

“This remarkable capability of ceria has major implications for the practical lifetime of CO2 electrolyzer devices,” DTU researcher and study co-author Christopher Graves said in the press release. “Replacing the current nickel electrode with our new ceria electrode in the next generation electrolyzer would improve device lifetime.”

Unlike recent proposals for electric planes, the researchers say that this process would enable the aviation industry to maintain and use its current fueling infrastructure, but in a more effective and environmentally conscious way.

Abstract: High-temperature CO^^2 electrolysers offer exceptionally efficient storage of renewable electricity in the form of CO and other chemical fuels, but conventional electrodes catalyse destructive carbon deposition. Ceria catalysts are known carbon inhibitors for fuel cell (oxidation) reactions; however, for more severe electrolysis (reduction) conditions, catalyst design strategies remain unclear. Here we establish the inhibition mechanism on ceria and show selective CO^^2 to CO conversion well beyond the thermodynamic carbon deposition threshold. Operando X-ray photoelectron spectroscopy during CO^^2 electrolysis—using thin-film model electrodes consisting of samarium-doped ceria, nickel and/or yttria-stabilized zirconia—together with density functional theory modelling, reveal the crucial role of oxidized carbon intermediates in preventing carbon build-up. Using these insights, we demonstrate stable electrochemical CO^^2 reduction with a scaled-up 16 cm^2 ceria-based solid-oxide cell under condi-tions that rapidly destroy a nickel-based cell, leading to substantially improved device lifetime.
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