Neutron scattering study paves way for electrochemistry for carbon-neutral ammonia

Scientists at Stanford University and the Department of Energy’s Oak Ridge National Laboratory are turning air into fertilizer without leaving a carbon footprint. Their discovery could provide a much-needed solution to help achieve global carbon neutrality goals by 2050.

Published in the journal Energy and environmental sciencesThe study describes a sustainable electrochemical – rather than chemical – process to produce ammonia, a key ingredient in nitrogen fertilizer.

Essentially, researchers used neutron scattering to understand how cycling an electric current when converting nitrogen to ammonia, also known as the nitrogen reduction reaction, increases the amount of ammonia produced. This process could potentially allow farmers to convert nitrogen, the most abundant element in our atmosphere, into ammonia-based fertilizer without emitting carbon dioxide.

“Ammonia is essential to the food supply for most of the world’s population,” said Sarah Blair, a former doctoral student at Stanford’s Center for Interface Science and Catalysis who now works at the National Renewable Energy Laboratory in Colorado as a as a postdoctoral researcher. “As the world’s population continues to grow, we need sustainable ways to produce fertilizer, especially as warming intensifies.”

Industrial fertilizers allow farmers to grow more food on less land. Yet the primary method of creating industrial ammonia for more than a century, the Haber-Bosch process, accounts for nearly 2 percent of all carbon dioxide emissions because of the fossil fuels it requires.

Two percent may not seem like a lot, but we’re adding carbon dioxide to the atmosphere faster than the planet can absorb it, and every effort counts to reduce that number. The Haber-Bosch process produces about 500 million tons of carbon dioxide each year, which would require the equivalent of almost all of the federal land in the United States to absorb and store.

The study results could also help scientists understand other processes for producing carbon-neutral ammonia for other applications. These could include recycling or recovering fertilizer runoff before it enters waterways and producing ammonia at seaports to refuel ships. Global shipping produces an additional 3% of global carbon dioxide emissions, and the burning of fossil fuels represents the largest source of carbon dioxide from human activity.

“You can’t improve the design of something if you don’t know how it already works,” Blair said. “Neutrons help science evolve by illuminating at the atomic level certain systems that are impossible to study otherwise.”

Blair used a glove box in the experiments, which required close collaboration and careful design on Doucet’s part so that the project could make the most of the limited beam time. Credit: Geneviève Martin/ORNL, US Department of Energy

Blair and Mat Doucet, senior neutron scattering scientist at ORNL, conducted their neutron experiments on the spallation neutron source’s liquid reflectometer instrument. Their goal was to understand the effect of cycling an electric current on the formation of the solid-electrolyte interface, or SEI, in a nitrogen reduction reaction system that produces ammonia using lithium as a mediator.

Understanding SEI formation is the key to not only unlocking the science behind electrochemical ammonia production, but also to producing better batteries. The study also marks the first use of neutron-based techniques to observe the formation of an SEI layer during this particular electrochemical conversion.

In addition, a unique new neutron technique, time-resolved reflectometry, emerged from the study. This technique allows scientists to slice neutron data into increments of a few seconds, capturing more detail, much like watching a movie frame by frame. Initially, Blair and Doucet thought that the electrochemical changes they observed occurred gradually. However, using the new technique, they found that the changes occurred in much shorter time intervals.

“Processes that appear linear might not be linear at all when you look closely,” Doucet said. “Arriving at this structure as a function of time is the hardest part. The technique we developed for this experiment allowed us to do just that.”

SNS discoveries lay the foundation of knowledge necessary for technological innovations that improve people’s daily lives. The technique developed by Blair and Doucet opens new possibilities in electrochemistry for SNS users.

Hanyu Wang, an ORNL instrument scientist who also works closely with SNS users, said: “These time-dependent experiments will attract scientists who study separation chemistries. »

Jim Browning, leader of ORNL’s Neutron Reflectometry Group, added: “Their approach can answer many questions regarding separation chemicals, batteries, and a range of different areas of interest, such as the production of energy, energy storage and energy conservation.