Turning greenhouse gas into fuel

Greenhouse gases are a major contributor to climate change. They have increased in concentration over the last century and are a very common air pollutant. Now, they may become a useful source of energy.

A catalytic reactor developed by researchers at Rice University converts carbon dioxide into pure liquid fuels in an efficient and eco-friendly manner. The prototype was created in the lab of chemical and biomolecular engineering researcher Haotian Wang. It re-purposes carbon dioxide as purified high-concentration formic acid, an energy carrier that usually requires extensive conversion to be useful. However, this technology demonstrates an efficiency of 42 percent in tests and is able to store nearly half of the input greenhouse gas energy in a liquid formic acid fuel cell.

“Formic acid is an energy carrier,” Wang said in a press release. “It's a fuel-cell fuel that can generate electricity and emit carbon dioxide — which you can grab and recycle again.” He continued, “It's also fundamental in the chemical engineering industry as a feedstock for other chemicals, and a storage material for hydrogen that can hold nearly 1,000 times the energy of the same volume of hydrogen gas, which is difficult to compress. That's currently a big challenge for hydrogen fuel-cell cars.”

Lead author and Rice postdoctoral researcher Chuan Xia made the key discovery that allows the reactor to function. He developed a two-dimensional bismuth catalyst and a solid-state electrolyte that removes the requirement of salt in the reaction.

“Bismuth is a very heavy atom, compared to transition metals like copper, iron or cobalt,” Wang explained. “Its mobility is much lower, particularly under reaction conditions. So that stabilizes the catalyst.”

Usually, the production of formic acid introduces salty water which is energy-intensive and costly to remove. The flexible use of solid electrolytes eliminates this concern, according to Wang. The process used to produce these components can be scaled up, opening the door to commercial carbon dioxide conversion technologies.

In addition, the reactor runs water through the product chamber faster than previously possible, resulting in more concentrated, and therefore more efficient, formic acid. Next-generation reactors are expected to achieve even higher concentration as they accept gas flow to bring out pure formic acid vapors.

To test their prototype, the Rice scientists worked with the Brookhaven National Laboratory using X-ray absorption spectroscopy to view formic acid production in progress. The reactor ran continuously for 100 hours with negligible degradation. Wang also thinks it could be customized to produce fuels other than formic acid, like acetic acid, ethanol, or propanol fuels.

“The big picture is that carbon dioxide reduction is very important for its effect on global warming as well as for green chemical synthesis,” Wang concluded. “If the electricity comes from renewable sources like the sun or wind, we can create a loop that turns carbon dioxide into something important without emitting more of it.”