MIT Research News' Journal

Technique could enable cheaper fertilizer production

Most of the world’s fertilizer is produced in large manufacturing plants, which require huge amounts of energy to generate the high temperatures and pressures needed to combine nitrogen and hydrogen into ammonia.

MIT chemical engineers are working to develop a smaller-scale alternative, which they envision could be used to locally produce fertilizer for farmers in remote, rural areas, such as sub-Saharan Africa. Fertilizer is often hard to obtain in such areas because of the cost of transporting it from large manufacturing facilities.

In a step toward that kind of small-scale production, the research team has devised a way to combine hydrogen and nitrogen using electric current to generate a lithium catalyst, where the reaction takes place.

“In the future, if we envision how we want this to be used someday, we want a device that can breathe in air, take in water, have a solar panel hooked up to it, and be able to produce ammonia. This could be used by a farmer or a small community of farmers,” says Karthish Manthiram, an assistant professor of chemical engineering at MIT and the senior author of the study.

Graduate student Nikifar Lazouski is the lead author of the paper, which appears today in Nature Catalysis. Other authors include graduate students Minju Chung and Kindle Williams, and undergraduate Michal Gala.

Smaller scale

For more than 100 years, fertilizer has been manufactured using the Haber-Bosch process, which combines atmospheric nitrogen with hydrogen gas to form ammonia. The hydrogen gas used for this process is usually obtained from methane derived from natural gas or other fossil fuels. Nitrogen is very unreactive, so high temperatures (500 degrees Celsius) and pressures (200 atmospheres) are required to make it react with hydrogen to form ammonia.

Using this process, manufacturing plants can produce thousands of tons of ammonia per day, but they are expensive to run and they emit a great deal of carbon dioxide. Among all chemicals produced in large volume, ammonia is the largest contributor to greenhouse gas emissions.

The MIT team set out to develop an alternative manufacturing method that could reduce those emissions, with the added benefit of decentralized production. In many parts of the world, there is little infrastructure for distributing fertilizer, making it expensive to obtain fertilizer in those regions.

“The ideal characteristic of a next-generation method of making ammonia would be that it’s distributed. In other words, you could make that ammonia close to where you need it,” Manthiram says. “And ideally, it would also eliminate the CO₂ footprint that otherwise exists.”

While the Haber-Bosch process uses extreme heat and pressure to force nitrogen and hydrogen to react, the MIT team decided to try using electricity to achieve the same effect. Previous research has shown that applying electrical voltage can shift the equilibrium of the reaction so that it favors the formation of ammonia. However, it has been difficult to do this in an inexpensive and sustainable way, the researchers say.

Most previous efforts to perform this reaction under normal temperatures and pressures have used a lithium catalyst to break the strong triple bond found in nitrogen gas molecules. The resulting product, lithium nitride, can then react with hydrogen atoms from an organic solvent to produce ammonia. However, the solvent typically used, tetrahydrofuran, or THF, is expensive and is consumed by the reaction, so it needs to be continually replaced.

The MIT team came up with a way to use hydrogen gas instead of THF as the source of hydrogen atoms. They designed a mesh-like electrode that allows nitrogen gas to diffuse through it and interact with hydrogen, which is dissolved in ethanol, at the electrode surface.

This stainless steel, mesh structure is coated with the lithium catalyst, produced by plating out lithium ions from solution. Nitrogen gas diffuses throughout the mesh and is converted to ammonia through a series of reaction steps mediated by lithium. This setup allows hydrogen and nitrogen to react at relatively high rates, despite the fact that they are usually not very soluble in any liquids, which makes it more challenging to react them at high rates.

“This stainless steel cloth is a way of very effectively contacting nitrogen gas with our catalyst, while also having the electrical and ionic connections that are needed,” Lazouski says.

Splitting water

In most of their ammonia-producing experiments, the researchers used nitrogen and hydrogen gases flowing in from a gas cylinder. However, they also showed that they could use water as a source of hydrogen, by first electrolyzing the water and then flowing that hydrogen into their electrochemical reactor.

The overall system is small enough to sit on a lab benchtop, but it could be scaled up to produce larger quantities of ammonia by connecting many modules together, Lazouski says. Another key challenge will be to improve the energy efficiency of the reaction, which now is only about 2 percent, compared to 50 to 80 percent for the Haber-Bosch reaction.

“We have an overall reaction that finally looks favorable, which is a big step forward,” he says. “But we know that there’s still an energy loss problem that needs to be solved. That will be one of the major things that we want to address in future work that we’ll undertake.”

In addition to serving as a production method for small batches of fertilizer, this approach could also lend itself to energy storage, Manthiram says. This idea, which is now being pursued by some scientists, calls for using electricity produced by wind or solar energy to power ammonia generation. The ammonia could then serve as a liquid fuel that would be relatively easy to store and transport.

“Ammonia is such a critical molecule that can wear many different hats, and this same method of ammonia production could be used in very diverse applications,” Manthiram says.

The research was funded by the National Science Foundation and the MIT Energy Initiative Seed Fund. Prior research which was foundational for the present work was supported by MIT’s Abdul Latif Jameel Water and Food Systems Lab (J-WAFS).

Study: Life might survive, and thrive, in a hydrogen world

As new and more powerful telescopes blink on in the next few years, astronomers will be able to aim the megascopes at nearby exoplanets, peering into their atmospheres to decipher their composition and to seek signs of extraterrestrial life. But imagine if, in our search, we did encounter alien organisms but failed to recognize them as actual life.

That’s a prospect that astronomers like Sara Seager hope to avoid. Seager, the Class of 1941 Professor of Planetary Science, Physics, and Aeronautics and Astronautics at MIT, is looking beyond a “terra-centric” view of life and casting a wider net for what kinds of environments beyond our own might actually be habitable.

In a paper published today in the journal Nature Astronomy, she and her colleagues have observed in laboratory studies that microbes can survive and thrive in atmospheres that are dominated by hydrogen — an environment that is vastly different from Earth’s nitrogen- and oxygen-rich atmosphere.

Hydrogen is a much lighter gas than either nitrogen or oxygen, and an atmosphere rich with hydrogen would extend much farther out from a rocky planet. It could therefore be more easily spotted and studied by powerful telescopes, compared to planets with more compact, Earth-like atmospheres.

Seager’s results show that simple forms of life might inhabit planets with hydrogen-rich atmospheres, suggesting that once next-generation telescopes such as NASA’s James Webb Space Telescope begin operation, astronomers might want to search first for hydrogen-dominated exoplanets for signs of life.

“There’s a diversity of habitable worlds out there, and we have confirmed that Earth-based life can survive in hydrogen-rich atmospheres,” Seager says. “We should definitely add those kinds of planets to the menu of options when thinking of life on other worlds, and actually trying to find it.”

Seager’s MIT co-authors on the paper are Jingcheng Huang, Janusz Petkowski, and Mihkel Pajusalu.

Evolving atmosphere

In the early Earth, billions of years ago, the atmosphere looked quite different from the air we breathe today. The infant planet had yet to host oxygen, and was composed of a soup of gases, including carbon dioxide, methane, and a very small fraction of hydrogen. Hydrogen gas lingered in the atmosphere for possibly billions of years, until what’s known as the Great Oxidation Event, and the gradual accumulation of oxygen.

The small amount of hydrogen that remains today is consumed by certain ancient lines of microorganisms, including methanogens — organisms that live in extreme climates such as deep below ice, or within desert soil, and gobble up hydrogen, along with carbon dioxide, to produce methane.

Scientists routinely study the activity of methanogens grown in lab conditions with 80 percent hydrogen. But there are very few studies that explore other microbes’ tolerance to hydrogen-rich environments.

“We wanted to demonstrate that life survives and can grow in an hydrogen atmosphere,” Seager says.

A hydrogen headspace

The team took to the lab to study the viability of two types of microbes in an environment of 100 percent hydrogen. The organisms they chose were the bacteria Escherichia coli, a simple prokaryote, and yeast, a more complex eukaryote, that had not been studied in hydrogen-dominated environments.

Both microbes are standard model organisms that scientists have long studied and characterized, which helped the researchers design their experiment and understand their results. What’s more, E.coli and yeast can survive with and without oxygen — a benefit for the researchers, as they could prepare their experiments with either organism in open air before transferring them to a hydrogen-rich environment.

In their experiments, they separately grew cultures of yeast and E. coli, then injected the cultures with the microbes into separate bottles, filled with a “broth,” or nutrient-rich culture that the microbes could feed off. They then flushed out the oxygen-rich air in the bottles and filled the remaining “headspace” with a certain gas of interest, such as a gas of 100 percent hydrogen. They then placed the bottles in an incubator, where they were gently and continuously shaken to promote mixing between the microbes and nutrients.

Every hour, a team member collected samples from each bottle and counted the live microbes. They continued to sample for up to 80 hours. Their results represented a classic growth curve: At the beginning of the trial, the microbes grew quickly in number, feeding off the nutrients and populating the culture. Eventually, the number of microbes leveled off. The population, still thriving, was stable, as new microbes continued to grow, replacing those that died off.

Seager acknowledges that biologists do not find the results surprising. After all, hydrogen is an inert gas, and as such is not inherently toxic to organisms.

“It’s not like we filled the headspace with a poison,” Seager says. “But seeing is believing, right? If no one’s ever studied them, especially eukaryotes, in a hydrogen-dominated environment, you would want to do the experiment to believe it.”

She also makes clear that the experiment was not designed to show whether microbes can depend on hydrogen as an energy source. Rather, the point was more to demonstrate that a 100 percent hydrogen atmosphere would not harm or kill certain forms of life.

“I don’t think it occurred to astronomers that there could be life in a hydrogen environment,” says Seager, who hopes the study will encourage cross-talk between astronomers and biologists, particularly as the search for habitable planets, and extraterrestrial life, ramps up.

A hydrogen world

Astronomers are not quite able to study the atmospheres of small, rocky exoplanets with the tools available today. The few, nearby rocky planets they have examined either lack an atmosphere or may simply be too small to detect with currently available telescopes. And while scientists have hypothesized that planets should harbor hydrogen-rich atmospheres, no working telescope has the resolution to spot them.

But if next-generation observatories do pick out such hydrogen-dominated terrestrial worlds, Seager’s results show that there is a chance that life could thrive within.

As for what a rocky, hydrogen-rich planet would look like, she conjures up a comparison with Earth’s highest peak, Mt. Everest. Hikers attempting to hike to the summit run out of air, due to the fact that the density of all atmospheres drop off exponentially with height, and based on the dropping off distance for our nitrogen- and oxygen-dominated atmosphere. If a hiker were climbing Everest in an atmosphere dominated by hydrogen — a gas 14 times lighter than nitrogen — she would be able to climb 14 times higher before running out of air.

“It’s kind of hard to get your head around, but that light gas just makes the atmosphere more expansive,” Seager explains. “And for telescopes, the bigger the atmosphere is compared to the backdrop of a planet’s star, the easier it is to detect.”

If scientists ever get the chance to sample such a hydrogen-rich planet, Seager imagines they might discover a surface that is different, but not unrecognizable from our own.

“We’re imagining if you drill down into the surface, it probably would have hydrogen-rich minerals rather than what we call oxidized ones, and also oceans, as we think all life needs liquid of some kind, and you could probably still see a blue sky,” Seager says. “We haven’t thought about the entire ecosystem. But it doesn’t necessarily have to be a different world.”

Seed funding was provided the Templeton Foundation, and the research was, in part, funded by the MIT Professor Amar G. Bose Research Grant Program.