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takyon [soylentnews.org] writes:

Ammonia—a renewable fuel made from sun, air, and water—could power the globe without carbon [sciencemag.org]

For the past 4 years, [chemist Douglas MacFarlane] has been working on a fuel cell that can convert renewable electricity into a carbon-free fuel: ammonia. Fuel cells typically use the energy stored in chemical bonds to make electricity; MacFarlane's operates in reverse. In his third-floor laboratory, he shows off one of the devices, about the size of a hockey puck and clad in stainless steel. Two plastic tubes on its backside feed it nitrogen gas and water, and a power cord supplies electricity. Through a third tube on its front, it silently exhales gaseous ammonia, all without the heat, pressure, and carbon emissions normally needed to make the chemical. "This is breathing nitrogen in and breathing ammonia out," MacFarlane says, beaming like a proud father.

Companies around the world already produce $60 billion worth of ammonia every year, primarily as fertilizer, and MacFarlane's gizmo may allow them to make it more efficiently and cleanly. But he has ambitions to do much more than help farmers. By converting renewable electricity into an energy-rich gas that can easily be cooled and squeezed into a liquid fuel, MacFarlane's fuel cell effectively bottles sunshine and wind, turning them into a commodity that can be shipped anywhere in the world and converted back into electricity or hydrogen gas to power fuel cell vehicles. The gas bubbling out of the fuel cell is colorless, but environmentally, MacFarlane says, ammonia is as green as can be. "Liquid ammonia is liquid energy," he says. "It's the sustainable technology we need."

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[...] MacFarlane has found a way to boost efficiencies [of ammonia-based reverse fuel cells] 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.

Haber process [wikipedia.org].

Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids [rsc.org] (DOI: 10.1039/C7EE02716H) (DX [doi.org])

Rational Electrode–Electrolyte Design for Efficient Ammonia Electrosynthesis under Ambient Conditions [acs.org] (open, DOI: 10.1021/acsenergylett.8b00487) (DX [doi.org])

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