A sustainable solution to produce fertilisers probed at the ESRF

More than 180 million tons of ammonia, a crucial chemical used to make synthetic fertilizers, are produced annually. Currently, ammonia is made using a process called Haber-Bosch, which relies on fossil fuels and requires high temperature and pressure. However, researchers are working on a very promising, low-carbon, electrochemical alternative route. 

This alternative, called lithium-mediated nitrogen reduction reaction (Li-NRR), was first discovered in 1930, almost a century ago, but hardly anyone explored it further. “Five years ago, we decided to focus on Li-NRR as the only reliable strategy for electrochemical ammonia production from elemental nitrogen”, explains Ib Chorkendorff, professor from DTU and corresponding author of the paper.

Tracking reaction as it happens

The Li-NRR process aims to activate nitrogen using electrochemically plated lithium. A critical role in determining the efficiency plays the solid electrolyte interphase (SEI) layer formed on the plated lithium.

Now the team from DTU and Stanford University, together with the ESRF, has carried out operando investigations on ID31 using Grazing Incidence Wide-Angle X-ray Scattering (GI WAXS) to shed light on the SEI layers and reaction intermediates.

The experiments used metallic lithium to make nitrogen molecules split. They used LiBF4 as the electrolyte salt, instead of the typically used one (LiClO4), which formed lithium fluoride (LiF) and lithium ethoxide (LiEtO). These species play a crucial role in regulating proton transport to the plated lithium, influencing the Li-NRR performance.

Additionally, the study revealed LiH and LiNxHy-species as reaction intermediates, shedding light on the competing pathways that lead to both undesired and desired products.

The restructuring of the copper single crystal substrate also inferred an interaction with plated lithium, potentially impacting Li-NRR performance.

EBS photon flux key in the study

“Probing this kind of interphases operando is usually very difficult, but thanks to EBS and the huge amount of photons we can get, we were able to probe this really thin layer composed with light elements (hence elements that interact weakly with X-rays) while operating the cell”, explains Valentin Vinci, post-doctoral researcher at the ESRF and co-author of the publication.  

One of the key challenges of the experiment was the cleanliness of the copper surface. “It needs to be extremely clean if we want to get the right data, so the surface had to be cleaned between each experiment and the cell assembled under Argon atmosphere”, says Vinci. “We are already preparing an optimized protocol for future experiments with this team”, he adds.

 “These findings pave the way for designing suitable SEI layers for Li-NRR systems with optimal performance”, explains Niklas Deissler, first author of the publication. “This doesn’t mean that this technology will be in the market tomorrow, but it certainly helps us to have a better idea of what goes on during the reaction and how we can improve it for the future”, he concludes.

Reference:

N. H. Deissler, et al., Energy Environ. Sci., 2024, DOI: 10.1039/D3EE04235A.

Text by Montserrat Capellas Espuny