Chemists make N₂ into NH₃ in most efficient electrochemical reaction ever

Ammonia is a crucial fertilizer for the world’s food production, and now scientists have figured out how to make it with nearly 100% efficiency, using nitrogen gas and electricity (Nature 2022, DOI: 10.1038/s41586-022-05108-y). This new process could be a greener alternative to current NH_3 production processes, which consume about 1% of the world’s total energy.

Most of the NH₃ in the world is made by the Haber-Bosch process, which takes hydrogen gas made from fossil-fuel feedstocks, and ultimately combines it with N₂ to make NH₃. In 2010, the process dumped about 451 million metric tons of carbon dioxide into the air, according to the Institute for Industrial Productivity.

Alexandr Simonov, Douglas MacFarlane, and coworkers from Monash University improved on an electrochemical, lithium-mediated N₂ reduction reaction to make NH₃ at a rate 100 times as high as the rates of previous attempts. Simonov says changing the electrolyte increased the efficiency of the reaction in two ways: by increasing how much N₂ is converted to NH₃ and by improving what’s called the faradic efficiency. “It essentially means how much current is converted to the target product,” Simonov says.

In Li-mediated N₂ electroreduction, a significant portion of the current causes other reactions, including the deposition of Li metal onto the electrode inside the electrochemical device and the reductive degradation of the electrolytes, MacFarlane says. By switching to a bis(trifluoromethylsulfonyl)imide electrolyte, the team created a system that shut off unwanted side reactions with Li metal and spit out mainly NH₃. “So this is a big deal in these two aspects, not only making the process more efficient, more effective, but also making it much more stable because there is no mechanism now for it to degrade,” MacFarlane says.

The device for turning N₂ into NH₃ is small and portable, he says, which makes it ideal for farmers to have on-site. “The device doesn’t have to be Haber-Bosch, chemical plant size and producing thousands of tons a day. It can produce kilograms of NH₃ per day, which is exactly what farmers want,” MacFarlane says. In addition, scientists think NH₃ can be a renewable alternative for carbon-based fuels, as well as a way to store and move energy from one place to another. This research will likely make a stable, practical process for sustainable NH₃ production a reality.

“Reaching near 100% electron-based selectivity is something that has felt out of reach, but this study gets the field there,” says Karthish Manthiram, a chemical engineer at the California Institute of Technology. “This would have been unthinkable just a few years ago.”