Ammonia is a vital ingredient in the fertilizers that sustain global agriculture, but it comes with a huge environmental cost. The Haber-Bosch process, which combines nitrogen and hydrogen to make ammonia, consumes about 2% of the world’s energy supply, and its hydrogen feedstock is made by steam reforming methane at high temperature and pressure, producing significant CO2 emissions. An electrochemical system that relies on an unusual electrolyte could now point the way to a more sustainable form of ammonia production (ACS Energy Lett. 2018, DOI: 10.1021/acsenergylett.8b00487).
Chemists have long sought alternatives to the Haber-Bosch process that could use solar or wind power to reduce nitrogen electrochemically at ambient temperature and pressure with the help of a catalyst. But these electrochemical cells have struggled with poor efficiency because nitrogen is not very soluble in water, so protons in the aqueous electrolyte are reduced to hydrogen gas instead. The result is “a process that makes hydrogen and a tiny bit of ammonia along with it,” says Douglas R. MacFarlane of Monash University.
Some researchers have surmounted this hurdle by eschewing the iron-based catalyst used in Haber-Bosch in favor of a catalyst with a lower affinity for protons, such as gold nanorods. But MacFarlane is tackling the problem by swapping the aqueous electrolyte for an ionic liquid—a salt that cannot crystallize at ambient conditions and so exists in liquid form. Last year, he and colleagues developed a cell using a fluorinated ionic liquid called 1-butyl-1-methypyrrolidinium tris(pentafluoroethyl)trifluorophosphate. Nitrogen is about 20 times as soluble in this ionic liquid as in water, and while the protons necessary to produce ammonia are delivered via water vapor along with nitrogen, there are none in the electrolyte, slashing the extent of the competing reaction to hydrogen (Energy Environ. Sci. 2017, DOI: 10.1039/c7ee02716h). This cell achieved a 60% Faradaic efficiency, meaning 60% of the current applied went to produce ammonia, far surpassing the approximately 10% Faradaic efficiency of the most-efficient systems using heterogeneous electrocatalysts under ambient conditions. But the rate of the reaction itself—the amount of ammonia produced per unit time—was about a tenth that of other systems because the high viscosity of the ionic liquid slowed down the reacting molecules.
Now, he and his team have blended an aprotic fluorinated solvent, 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, with the ionic liquid to lower the electrolyte’s viscosity while still retaining high nitrogen solubility and limiting proton reduction. The researchers also increased the accessible surface area of their iron-oxide-based catalyst by growing it into nanorods on carbon fiber paper. The result is a system with 32% Faradaic efficiency and a reaction rate about ten times higher than last year’s test system.
Shelley D. Minteer, a chemist at the University of Utah who is designing bioelectrocatalysis systems that use nitrogen-fixing enzymes, says she is impressed by the study. “The majority of times, scientists are focused on the catalyst and not thinking about the importance of rationally designing the electrolyte,” she says. “This clearly shows the importance of the electrolyte in this very difficult reaction.” Bioelectrocatalysis systems match the efficiency of the new system but have slower reaction rates by orders of magnitude, she says.
MacFarlane and his colleagues plan to spin out the system from Monash University to bring it from a small, lab-scale reactor to commercial scale. Australia’s abundant potential for solar and wind energy is driving interest in ammonia production systems run by large solar or wind farms, MacFarlane says. Though toxicity is often a concern when using fluorinated solvents, MacFarlane says that the closed system they have designed would prevent release of the electrolyte into the environment during its use.