A little over 100 years ago, humankind learned how to take nitrogen from the atmosphere (where it is plentiful) and turn it into ammonia that can be used as source of fertilizer for growing food. That chemical process, known as nitrogen fixation, has allowed huge increases in crop production and a subsequent boom in human populations fed by those crops.
Nearly all artificial nitrogen fixation is done with what is known as the Haber–Bosch process, which uses a metal catalyst to combine gaseous nitrogen and hydrogen into ammonia, at high pressures and temperatures. Ammonia fixed through this process is estimated to be responsible for growing crops that feed half the world's population.
But there is another large source of nitrogen fixation: bacteria that live in soil, which fix nitrogen at normal atmospheric temperatures and pressures. In recent decades, researchers searching for sustainable agriculture practices have looked to these microbes as inspiration for developing nitrogen-fixation processes that are easier to conduct and more environmentally friendly than the energy-intensive Haber-Bosch process. Now, a team at Caltech led by Jonas Peters, Bren Professor of Chemistry and director of the Resnick Sustainability Institute, has made a breakthrough that increases the efficiency of one of these low-temperature and low-pressure processes, further opening the door to greener fertilizer, and even the production of zero-carbon fuels.
In a paper appearing in the August 31 issue of the journal Nature, the team outlines how they have reduced a major inefficiency in an earlier nitrogen-fixation process developed in Peters's lab. That process uses electricity and a specialized catalyst in a solution to combine nitrogen with hydrogen. Though that catalyst offered proof of principle, the reaction needed to take place at cold temperatures, and a significant portion of the electrical current was wasted creating hydrogen gas, which then bubbles away unused. This unwanted effect is called the hydrogen evolution reaction (HER), and its mitigation has been an important goal of Peters's nitrogen-fixation research.
"Devising clever approaches to convert electron/proton currency to highly desired products other than hydrogen is a major goal in catalysis research, especially in the context of ammonia synthesis and solar fuels," Peters says.
The new research shows how HER can be reduced through the use of a cobalt-based co-catalyst that mediates the reaction and prevents the creation of hydrogen gas. The presence of this co-catalyst allows the primary catalyst to efficiently generate ammonia at room temperature, while requiring less voltage than the earlier process to do so.
"Our strategy was to devise a scheme where a co-catalyst mediator successively shuttles a temporarily stored proton and electron—effectively a hydrogen atom—to a separate catalyst that binds and fixes nitrogen," Peters says. "The key is to pass off the stored hydrogen atom before it can combine with itself to produce hydrogen."
Though the process they have developed is not yet practical for real-world applications, the researchers say it is an important step forward in the development of nitrogen-fixation methods that are less energy-intensive. That means fertilizer production could be conducted with solar or wind power in parts of the globe that do not have reliable electrical grids. It could also make ammonia as fuel for vehicles an economic reality. Ammonia used in this way would not generate any climate change-inducing carbon dioxide.
"Ammonia is needed for fertilizer, but is also promising as a storable high-energy density fuel. Nitrogen makes up about 80 percent of the atmosphere, and along with water and sunlight, is essentially available in infinite supply," Peters says. "The trick is to unlock the chemistry that will enable efficient combination of these reagents for a more sustainable future."
The paper describing the work is titled: "Tandem electrocatalytic N2 fixation via proton coupled electron transfer." In addition to Peters, co-authors include: Pablo Garrido-Barros, senior postdoctoral scholar research associate; Joseph Derosa, Arnold O. Beckman Postdoctoral Fellow in Chemical Sciences; Matthew J. Chalkley (PhD '20) of UC San Francisco.
Funding for the research was provided by Dow Next Generation Educator Funds and Instrumentation Grants, Caltech's Resnick Water and Environment Laboratory (WEL), the U.S. Department of Energy, and the National Institutes of Health.