Ammonia—a renewable fuel made from sun, air, and water—could...

Ammonia—a renewable fuel made from sun, air, and water—could power the globe without carbon​

At present, the yields are modest. At room temperature and pressure, the fuel cell reactions generally have efficiencies of between 1% and 15%, and the throughput is a trickle. But MacFarlane has found a way to boost efficiencies by changing the electrolyte. In the water-based electrolyte that many groups use, water molecules sometimes react with electrons at the cathode, stealing electrons that would otherwise go into making ammonia. "We're constantly fighting having the electrons going into hydrogen," MacFarlane says.

To minimize that competition, he opted for what's called an ionic liquid electrolyte. That approach allows more N₂ and less water to sit near the catalysts on the cathode, boosting the ammonia production. As a result, the efficiency of the fuel cell skyrocketed from below 15% to 60%, he and his colleagues reported last year in Energy & Environmental Science. The result has since improved to 70%, MacFarlane says—but with a tradeoff. The ionic liquid in his fuel cell is goopy, 10 times more viscous than water. Protons have to slog their way to the cathode, slowing the rate of ammonia production. "That hurts us," MacFarlane says.
To speed things up, MacFarlane and his colleagues are toying with their ionic liquids. In a study published in April in ACS Energy Letters, they report devising one rich in fluorine, which helps protons pass more easily and speeds ammonia production by a factor of 10. But the production rate still needs to rise by orders of magnitude before his cells can meet targets, set for the field by the U.S. Department of Energy (DOE), that would begin to challenge Haber-Bosch.

http://www.sciencemag.org/news/2018...ter-could-power-globe-without-carbon[/BCOLOR]

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