AMMONIA FROM A REVERSE FUEL CELL

AMMONIA, NH₃, is used primarily as fertilizer, at an annual expenditure of $60 billion worldwide. It’s estimated that at least half the nitrogen in the human body today is traceable to synthetic ammonia. Its technology, the Haber-Bosch process, dates from 1909 and is one of the world’s biggest, dirtiest, and most time-honored industrial processes.

The article “Liquid Sunshine,” by Robert F. Service, is in the July 13, 2018, issue of Science, the weekly magazine of the American Association for the Advancement of Science. Service writes, “Ammonia made from sun, air, and water could turn Australia into a renewable energy superpower.”

The technology that might achieve this involves something akin to running a fuel cell backwards. A conventional fuel cell uses hydrogen and the air’s oxygen to produce electricity. A reverse fuel cell, as its name suggests, is powered by electricity and can transform water and the air’s nitrogen into ammonia.

The reverse fuel cell would use (preferably renewable) electricity to transform water and nitrogen into ammonia. This and the following images from Science, July 13, 2018.

Douglas MacFarlane, a chemist at Monash University, suburban Melbourne, Australia, has devised such a reverse fuel cell gizmo. In a sense, he says, “This is breathing nitrogen in and breathing ammonia out.” And, he observes, “Liquid ammonia is liquid energy.” Produce the electricity renewably through wind or solar power, and “It’s the sustainable technology we need.”

Ammonia could have multiple uses.

Ammonia production would still be important to agriculture. However, particularly with production sustainability, it would have other potential uses. Ammonia can be the source of energy in electrical production. And, through “cracking,” ammonia’s hydrogen can be released and used to power vehicles and other fuel cell applications.

The ammonia molecule consists of a central nitrogen atom and three hydrogen atoms. At room temperature and pressure, it is a colorless gas with a characteristic pungent smell. It can be transported or stored as a liquid with only modest cooling, to –28 degrees Fahrenheit. Image by Ben Mills.

Today’s Haber-Bosch ammonia production is fairly efficient, at around 60 percent in terms of energy in versus energy out. However, the process involves extremely high temperature and pressure. It consumes about two percent of the world’s energy—and generates one percent of the world’s total CO₂.

A MacFarlane reverse fuel cell transforms electricity, water, and nitrogen into ammonia.

Compared with the Haber-Bosch process, MacFarlane’s reverse fuel cell operates under less extreme conditions. Its efficiency is greater; it has no CO₂ byproduct; however, it’s still not optimal in its current state of the art.

Service writes, “The result has since improved to 70 percent, MacFarland says—but with a tradeoff. The ionic liquid in his fuel cell is goopy, 10 time more viscous than water. Protons have to slog their way to the cathode, slowing the rate of ammonia production.”

Different techniques of ammonia production are being investigated by other researchers. “The community is still trying to figure out what direction to go,” says Lauren Greenlee, chemical engineer at the University of Arkansas in Fayetteville. Grigorii Soloveichik, at the U.S. Department of Energy’s Advanced Research Projects Agency-Energy, concurs, “To make [green] ammonia is not hard. Making it economically on a large scale is hard.”

However, with a goal this high—of sustainable, transportable, and efficient energy—the research continues. ds

© Dennis Simanaitis, SimanaitisSays.com, 2018