"This discovery improves the stability of continuous flow lithium-mediated ammonia synthesis from 10 hours to 300 hours and maintains an ammonia selectivity of 64±1%. For the Nature paper of which he is the first author, Dr. Li Shaofeng of the Technical University of Denmark said.
Figure | Li Shaofeng (Source: Li Shaofeng)
In fact, in the field of lithium-mediated ammonia synthesis, Li Shaofeng and his team not only set new records in terms of stability and ammonia production, but also made a major breakthrough in the proportion of ammonia distribution in the gas phase.
That's a cumulative production of 4.6 grams of ammonia over a 300-hour period, with 98% of the ammonia distributed in the gas phase.
The electrochemical reduction of nitrogen to produce ammonia has potential application value in the fields of renewable energy storage, hydrogen energy storage, fertilizer production, chemical production, and combustion power generation. Specifically:
First, it can be used for renewable energy storage.
Electrochemical ammonia synthesis can be used to produce ammonia from nitrogen and green hydrogen using electricity from renewable sources (e.g., wind, solar).
This ammonia can be used as a storage medium for renewable energy, releasing hydrogen for power generation when needed.
Second, it can be used for hydrogen energy reserves.
Ammonia is a source of liquid hydrogen that is easy to transport and store. Through the electrochemical synthesis of ammonia, ammonia can be produced as a portable and high-density carrier of hydrogen for the release of hydrogen when needed for use in fuel cells or other hydrogen energy applications.
Third, it can be used in fertilizer production.
Ammonia is one of the main raw materials for chemical fertilizers, and traditional ammonia preparation methods often rely on fossil fuels and have high carbon emissions.
Electrochemical ammonia synthesis can provide a more environmentally friendly alternative and reduce carbon emissions from fertilizer production.
Fourth, it can be used in chemical production.
Ammonia, as an important raw material for the synthesis of nitrogen-containing chemicals such as nitric acid and urea, has a wide range of applications in the chemical field.
Through electrochemical synthesis of ammonia, it can provide a more environmentally friendly and sustainable source of ammonia, which can promote the sustainable development of some chemical production.
Fifth, it can be used in ammonia internal combustion engines.
Compared with traditional oil-fired internal combustion engines, ammonia internal combustion engines produce only water and nitrogen as by-products, which have a lower carbon footprint.
This is in line with the strict international requirements for ship emissions and helps to reduce the environmental impact of ships.
Electrochemical ammonia synthesis can be combined with renewable energy sources to provide sustainable and environmentally friendly ammonia fuel for ammonia combustion engines.
The invention of synthetic ammonia: marking the beginning of the era of artificial nitrogen fixation technology
According to reports, the invention of synthetic ammonia marks that mankind has entered the era of revolutionary artificial nitrogen fixation technology. This innovation ended the history of humanity's total dependence on natural nitrogen fertilizers and brought a boon to human development.
Currently, about 50% of the world's food production relies on ammonia-related fertilizers. However, the current ammonia industry generally uses the Haber-Bosch process, which requires the synthesis of ammonia under high temperature and high pressure conditions.
Despite the huge amount of ammonia produced by this method, about 2.1 tonnes of CO2 is released for every 1 tonne of ammonia produced, equivalent to 1.3% of total annual global emissions. In addition, this process alone accounts for 1% of the world's total annual energy consumption.
Therefore, in the context of increasing energy and environmental pressures, academia and industry have been looking for green and sustainable synthetic ammonia production paths.
Electrochemical ammonia synthesis is considered to be a green, low-energy ammonia synthesis pathway that is expected to replace the traditional Haber-Bosch process. Lithium-mediated nitrogen reduction (Li-NRR) has been traced back to 1930 as one of the reliable pathways for the electrochemical synthesis of ammonia at room temperature[1].
The reaction process can be divided into three steps:
Firstly, the electrochemical reduction of lithium ions to lithium metal is carried out.
Secondly, lithium metal activates inert nitrogen to produce lithium nitride;
Finally, lithium nitride is protonated by the proton shuttle agent to produce ammonia while releasing lithium ions.
Similar to lithium-metal batteries, lithium-mediated ammonia synthesis involves the critical step of reducing lithium ions to lithium metal and forming a solid-electrolyte interphase (SEI) layer that conducts ions but is electronically insulated.
The difference is that in lithium-mediated ammonia synthesis, after the solid electrolyte interface layer is formed, nitrogen needs to react with the metal lithium through the solid electrolyte interface layer to produce lithium nitrogen compounds, and then further combine with protons to form ammonia.
Therefore, the solid electrolyte interface layer is a key factor in determining the selectivity and stability of lithium-mediated ammonia synthesis.
For the solid electrolyte interface, it can not only determine the selectivity and reaction rate of ammonia synthesis by controlling the relative diffusion rate of Li+, H+, and N2 in the solid electrolyte interface, but also improve the system stability by avoiding excessive decomposition of the electrolyte.
In 2019, since the team of Professor Ib Chorkendorff of Li Shaofeng proved the reliability of lithium-mediated ammonia synthesis through isotope quantification experiments [2], many scientific research teams around the world have conducted in-depth research on this topic.
However, most studies have used batch reactors and used sacrificial proton sources.
In 2023, Prof. Ib Chorkendorff's team successfully improved the operational stability of the mobile electrolyzer and solved the problem of reactant mass transfer limitation by developing a highly stable and highly active hydrogen oxidation catalyst.
Under the conditions of room temperature and pressure, the continuous electrochemical synthesis of ammonia was realized through the coupling of nitrogen reduction and hydrogen oxidation, and the selectivity of ammonia production reached 61%[3].
However, none of the previously reported continuous flow electrolyzers could operate stably for more than 10 hours, and the ammonia production was only at the milligram level.
In this study, Li Shaofeng and his team studied the effect of solvents on the stability of lithium-mediated ammonia synthesis and found the key factors that lead to poor stability.
They found that tetrahydrofuran, a cyclic ether solvent (THF), which has been widely used since 1993, was the main cause of the problem.
Based on this, they proposed for the first time the design criteria for the development of new solvents, and found that high-boiling chain ether solvents are more suitable for lithium-mediated ammonia synthesis than low-boiling cyclic ether solvents.
Hanging in the balance: how to achieve high selectivity at industrial-grade current densities
It is understood that when Li Shaofeng first joined the team, his first research direction was to verify whether lithium-mediated synthetic ammonia can obtain high selectivity at industrial-grade current density.
Before he began his research, some important progress had been made in the field of lithium-mediated ammonia synthesis.
For example, ammonia production can be as selective as 69%[4] and current densities can reach 100 mA per square centimeter[5].
However, the electrode area of the 69% ammonia selectivity obtained in the above work is only 0.012 square centimeters, and the current density is only about 20 milliamps per square centimeter, which is less scalable.
In addition, although a current density of 100 mA per square centimeter has been studied, the selectivity for ammonia production is only 13%. How to achieve high selectivity at industrial-grade current densities is still up in the air.
In order to solve this problem, he and his team proposed a design strategy for the micro-nano structure of the electrode meter interface.
By changing the diffusion rate of lithium ions in the solid electrolyte interface and using a hierarchical porous electrode, the selectivity and rate of the reaction were controlled simultaneously.
The LiF-rich solid electrolyte interface layer not only significantly improves selectivity by decreasing the diffusion rate of the Li+ bulk phase, but also enables uniform deposition of lithium metal by increasing the Li+ surface migration rate, thereby improving system stability.
For graded porous electrodes, it can effectively increase the reaction rate by increasing the electrochemical specific surface area of the electrode.
According to Li Shaofeng, he and his team have achieved ammonia synthesis from nitrogen by electrochemical reduction of ampere current density for the first time in the world, and the ammonia selectivity at a current density of 1 ampere per square centimeter has reached 71 ± 3%, and the ammonia production rate is more than an order of magnitude higher than the highest reported value in the field of lithium-mediated ammonia synthesis at that time.
(Source: Li Shaofeng)
In response to this achievement, a professor at the Georgia Institute of Technology in the United States wrote a special article [6], which pointed out that the work of Li Shaofeng et al. achieved the first lithium-mediated nitrogen reduction reaction at industrial-grade current density.
"However, we are also well aware that although this work has conceptually demonstrated that lithium-mediated ammonia can achieve high selectivity at industrial-grade current densities in a high-voltage single-chamber electrolyzer, there are still many challenges in how to carry out efficient continuous production. Li Shaofeng said.
Among them, the primary challenge is to extend lithium-mediated ammonia synthesis from a high-pressure single-chamber electrolyzer to an atmospheric flow cell to solve the problem of sacrificial proton source and limited mass transfer.
While carrying out the above work, Dr. Fu Xianbiao, a colleague in Li Shaofeng's team, has also made important progress in the atmospheric pressure flow electrolyzer.
By using a highly stable and active hydrogen oxidation catalyst in a mobile electrolysis cell, they realized the coupling of nitrogen reduction and hydrogen oxidation under normal temperature and pressure, and could carry out continuous electrochemical ammonia synthesis with a selectivity of 61% for ammonia production [3].
However, there are still some challenges to lithium-mediated ammonia synthesis in this mobile cell, such as the inability to operate stably for more than 10 hours, the ammonia production is only milligram level, and about half of the ammonia is distributed in the organic electrolyte.
Therefore, the next important challenge for the team was how to significantly improve the stability of lithium-mediated ammonia synthesis and obtain higher yields of gas-phase ammonia. This is because there is currently no effective means to effectively extract ammonia from liquid electrolytes.
Considering the low boiling point (66ºC) and volatile properties of tetrahydrofuran used in lithium-mediated ammonia synthesis, Li Shaofeng realized that although there are many factors affecting the stability of lithium-mediated ammonia synthesis, low-boiling solvents must be an obstacle to achieving long-term stability.
Therefore, he first studied the effect of solvents on the stability of lithium-mediated ammonia synthesis and found that tetrahydrofuran is a key cause of poor stability.
Tetrahydrofuran not only has a low boiling point and volatile characteristics, but is also prone to ring-opening polymerization, which severely limits the long-term stability of lithium-mediated ammonia synthesis.
To solve this problem, he and his team proposed design guidelines and requirements for new solvents:
(1) Ensure the solubility of lithium salt in the solvent to ensure that the electrolyte has sufficient ionic conductivity and is conducive to lithium deposition.
(2) The solvent must be compatible with lithium metal and proton shuttle agents to ensure efficient transport of protons from the anode hydrogen oxidation reaction.
(3) The solid electrolyte interface layer induced by solvent on the gas diffusion electrode needs to be uniform and compact, so as to promote the generated ammonia to enter the gas phase with nitrogen more easily, so as to achieve the purpose of easy separation.
(4) The solvent must have the characteristics of high boiling point and difficult polymerization to avoid volatilization and polymerization of the solvent.
Subsequently, through a systematic evaluation of the effects of various chain and cyclic ether solvents on lithium-mediated ammonia synthesis, he found that high-boiling chain ether solvents, especially diethylene glycol dimethyl ether (boiling point of 162ºC), are excellent solvents for lithium-mediated ammonia synthesis.
Not only does the solvent have difficult-to-polymerize properties, but it also helps to form a compact solid electrolyte interface layer on the gas diffusion electrode, which improves the distribution of ammonia in the gas phase and ensures long-term stability of the electrolyte.
(Source: Li Shaofeng)
"There is nowhere to find the iron shoes, and it takes no effort to get them"
In the study, in the process of trying to use different solvents, Li Shaofeng took many detours, including buying a lot of high-boiling carbonate solvents, but the results were not satisfactory.
After trying a variety of solvents, he began to wonder why THF solvents are widely used, and what is so special about it other than its low boiling point?
Later, it was found that tetrahydrofuran is a typical cyclic ether solvent, and there are many similar ether solvents, including cyclic and chain-like.
So he started experimenting with a chain ether solvent (ethylene glycol dimethyl ether) that the lab had been purchasing for several years, and unexpectedly found that it could achieve ammonia selectivity close to that of tetrahydrofuran.
Moreover, its boiling point is higher than that of tetrahydrofuran, which can be described as "stepping on iron shoes without finding a place, and it takes no effort to get it".
He also encountered some difficulties in expanding the chain ether solvent from ethylene glycol dimethyl ether to diethylene glycol dimethyl ether with high boiling point.
In the beginning, the ammonia selectivity was low at about 35%, only about 30% of the ammonia was distributed in the gas phase, and the stability was difficult to achieve for more than 24 hours.
"We are very confused about this reason. By comparing the electrochemical data with different solvents, I found that after switching the cyclic test from galvanostatic mode to an open-circuit voltage, the voltage of the working electrode responded completely differently to time. Li Shaofeng said.
When using high-boiling chain ether solvents, the voltage responds more slowly to time. This may be related to the high boiling point of the solvent itself, its effect on the proton carrier's ability to provide protons, and the formation of the solid electrolyte interface layer in which the solvent participates.
By digging deeper into this detail, he found that the ammonia performance of this high-boiling chain ether solvent could be significantly improved by using a controlled potentiometric cycling strategy.
最终，相关论文以《长期连续氨电合成》（Long-term continuous ammonia electrosynthesis）为题发在 Nature[7]。
李少锋是第一作者，丹麦科技大学简斯· K.诺尔斯科夫(Jens K. Nørskov)教授和伊布·乔肯多夫(Ib Chorkendorff)教授担任共同通讯作者。
Figure | Related papers (source: Nature)
The power of cooperation is far greater than going it alone
The European patent for this work has now been granted to the Danish company NitroVolt for the development of a small-scale electrochemical ammonia synthesis system.
The team is also looking forward to the continuous production of synthetic ammonia at industrial current density to promote the industrialization of this technology.
It is also reported that Professor Ib Chorkendorff has published 6 research papers including Science and 2 Nature as corresponding authors.
Impressively, the average time from submission to acceptance for these 6 Science papers was less than 3 months.
Of the 8 papers, 4 were experimental and theoretical research papers, with the team of Prof. Jens K. Nørskov providing guidance and support on theoretical calculations.
It is worth mentioning that the collaboration between Ib Chorkendorff and Professor Jens K. Nørskov dates back to the 90s of the 20th century.
"Their decades-long close collaboration on experimentation and theory provides the perfect illustration of scientific collaboration, proving that collaboration can be far more powerful than working alone. Li Shaofeng concluded.
Resources:
1.Helv. Chim. Acta 1930, 13, 1228
2.Nature 2019, 570, 504
3.Science 2023, 379, 707
4.Science 2021, 372, 1187
5.ACS Energy Lett. 2022, 7, 36
6.ACS Energy Lett. 2022, 7, 4132
7.Li, S., Zhou, Y., Fu, X.et al. Long-term continuous ammonia electrosynthesis. Nature (2024). https://doi.org/10.1038/s41586-024-07276-5
Operation/Typesetting: He Chenlong