A common greenhouse gas could be repurposed in an efficient and environmentally friendly way with an electrolyzer that uses renewable electricity to produce pure liquid fuels.

The catalytic reactor developed by the Rice University lab of chemical and biomolecular engineer Haotian Wang uses carbon dioxide as its feedstock and—in its latest prototype—produces highly purified and high concentrations of formic acid.

Formic acid produced by traditional carbon dioxide devices needs costly and energy-intensive purification steps, Wang said. The direct production of pure formic acid solutions will help to promote commercial carbon dioxide conversion technologies.

The method is detailed in Nature Energy.

Wang and his group pursue technologies that turn greenhouse gases into useful products. In tests, the new electrocatalyst reached an energy conversion efficiency of about 42%. That means nearly half of the electrical energy can be stored in formic acid as liquid fuel.

Wang and his group pursue technologies that turn greenhouse gases into useful products. In tests, the new electrocatalyst reached an energy conversion efficiency of about 42%. That means nearly half of the electrical energy can be stored in formic acid as liquid fuel.

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"Formic acid is an energy carrier," Wang said. "It's a fuel-cell fuel that can generate electricity and emit carbon dioxide—which you can grab and recycle again.

"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," he said. "That's currently a big challenge for hydrogen fuel-cell cars."

Two advances made the new device possible, said lead author and Rice postdoctoral researcher Chuan Xia. The first was his development of a robust, two-dimensional bismuth catalyst and the second was a solid-state electrolyte that eliminates the need for salt as part of the reaction.