Scientists increase the stable time of synthetic ammonia by 30 times, which can

2024-07-20

This discovery has increased the stability of continuous flow lithium-mediated ammonia synthesis from 10 hours to 300 hours, while maintaining an ammonia selectivity of 64 ± 1%. Dr. Shaofeng Li from the Technical University of Denmark, who served as the first author of the Nature paper, stated this.

In fact, in the field of lithium-mediated ammonia synthesis, Dr. Li and his team have not only set new records in terms of stability and ammonia production, but also achieved significant breakthroughs in the distribution ratio of ammonia in the gas phase.

That is, within 300 hours, a total of 4.6 grams of ammonia were produced, with 98% of the ammonia distributed in the gas phase.

The electrochemical reduction of nitrogen to synthesize ammonia has potential application value in the fields of renewable energy storage, hydrogen energy storage, fertilizer production, chemical production, and combustion power generation. Specifically:

Firstly, it can be used for renewable energy storage.Electrochemical synthesis of ammonia can utilize electricity from renewable energy sources (such as wind power and solar energy) to produce ammonia from nitrogen and green hydrogen.

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This ammonia can serve as a storage medium for renewable energy, releasing hydrogen when needed for power generation.

Secondly, it can be used for hydrogen energy storage.

Ammonia is a liquid hydrogen source that is easy to transport and store. Through electrochemical synthesis of ammonia, a portable and high-density carrier of hydrogen can be produced, which can be released when needed for fuel cells or other hydrogen energy applications.

Thirdly, it can be used in the production of fertilizers.Ammonia is one of the main raw materials for chemical fertilizers, and traditional methods of ammonia production typically rely on fossil fuels, which have high carbon emissions.

 

Electrochemical synthesis of ammonia can provide a more environmentally friendly alternative, reducing the carbon emissions in the production of chemical fertilizers.

 

Fourthly, it can be used for the production of chemical products.

 

Ammonia, as an important raw material for the synthesis of nitrogen-containing chemical products such as nitric acid and urea, has a wide range of applications in the chemical industry.

 

Through electrochemical synthesis of ammonia, a more environmentally friendly and sustainable source of ammonia can be provided, which can promote the sustainable development of some chemical product production.Its fifth advantage is its applicability to ammonia internal combustion engines.

Compared to traditional fuel internal combustion engines, the byproducts of combustion in ammonia internal combustion engines are only water and nitrogen, which have lower carbon emissions.

This meets the strict international requirements for ship emissions, helping to reduce the environmental impact of ships.

Electrochemical synthesis of ammonia can be combined with renewable energy, thereby providing sustainable and environmentally friendly ammonia fuel for ammonia internal combustion engines.The Invention of Synthetic Ammonia: Marking the Beginning of the Era of Artificial Nitrogen Fixation Technology

It is reported that the invention of synthetic ammonia marks the beginning of a revolutionary era of artificial nitrogen fixation technology for humanity. This innovation has put an end to the history of human reliance on natural nitrogen fertilizers, bringing good news to human development.

At present, about 50% of global food production depends on ammonia-related fertilizers. However, the current industrial synthesis of ammonia generally adopts the Haber-Bosch process, which requires the synthesis of ammonia under high temperature and high pressure conditions.

Although this method produces a huge amount of ammonia, about 2.1 tons of carbon dioxide are released for every ton of ammonia produced, accounting for 1.3% of the global annual emission volume. In addition, the annual energy consumption of this process alone accounts for 1% of the global annual total energy consumption.

Therefore, under the increasing pressure of energy and the environment, academia and industry have been looking for a green and sustainable path for the production of synthetic ammonia.Electrochemical synthesis of ammonia is considered a green and energy-efficient pathway for ammonia production, with the potential to replace the traditional Haber-Bosch process. Lithium-mediated nitrogen reduction reaction (Li-NRR), as one of the reliable pathways for electrochemical synthesis of ammonia at room temperature, dates back to as early as 1930[1].

The reaction process can be divided into three steps:

Firstly, lithium ions are electrochemically reduced to metallic lithium;

Secondly, metallic lithium activates the inert nitrogen gas to produce lithium nitride;

Lastly, lithium nitride is protonated through a proton shuttle agent to produce ammonia, while releasing lithium ions.Similar to lithium metal batteries, the lithium-mediated synthesis of ammonia also involves the reduction of lithium ions to metallic lithium and the formation of a solid electrolyte interface (SEI, solid-electrolyte interphase) layer that is conductive to ions but insulating to electrons, which is a key step.

In the case of lithium-mediated ammonia synthesis, after the formation of the solid electrolyte interface layer, nitrogen gas needs to react with metallic lithium through the solid electrolyte interface layer to produce lithium-nitrogen compounds, which then further combine with protons to form ammonia.

Therefore, the solid electrolyte interface layer is a key factor that determines the selectivity and stability of lithium-mediated ammonia synthesis.

For the solid electrolyte interface layer, it can not only determine the selectivity and reaction rate of ammonia synthesis by controlling the relative diffusion rates of Li+, H+, and N2 in the solid electrolyte interface, but also improve system stability by avoiding excessive decomposition of the electrolyte.

In 2019, since the team of Professor Ib Chorkendorff, where Li Shaofeng is located, demonstrated the reliability of lithium-mediated ammonia synthesis through isotope quantification experiments [2], multiple research teams around the world have conducted in-depth research on this topic.However, most studies have used batch reactors and have employed sacrificial proton sources.

In 2023, Professor Ib Chorkendorff's team successfully improved the operational stability of flow electrolysis cells and resolved the issue of mass transfer limitations for reactants by developing highly stable and active hydrogen oxidation catalysts.

Under ambient temperature and pressure conditions, continuous electrochemical synthesis of ammonia was achieved through the coupling of nitrogen reduction and hydrogen oxidation, with an ammonia selectivity of 61%[3].

However, previously reported continuous flow electrolysis cells could not operate stably for more than 10 hours, and the amount of ammonia produced was only at the milligram level.

In this study, Li Shaofeng and his team investigated the impact of solvents on the stability of lithium-mediated ammonia synthesis and identified the key factors leading to poor stability.They found that: since 1993, the widely used cyclic ether solvent tetrahydrofuran (THF) is the main cause of this problem.

Based on this, they proposed for the first time the design guidelines for the development of new solvents, and found that compared with low boiling point cyclic ether solvents, high boiling point linear ether solvents are more suitable for lithium-mediated synthesis of ammonia.

Pending: How to obtain high selectivity under industrial-grade current density

It is understood that when Li Shaofeng first joined the team, his first research direction was to verify whether lithium-mediated synthesis of ammonia could obtain high selectivity under industrial-grade current density.Before he began his research, the academic community had already made some significant progress in the field of lithium-mediated ammonia synthesis.

For instance, the selectivity of ammonia production could reach up to 69%[4], and the current density could reach 100 milliamperes per square centimeter[5].

However, in the aforementioned work, the electrode area that achieved 69% ammonia selectivity was only 0.012 square centimeters, and the current density was only around 20 milliamperes per square centimeter, exhibiting poor scalability.

Additionally, although some studies have achieved a current density of 100 milliamperes per square centimeter, their ammonia production selectivity was only 13%. The issue of how to achieve high selectivity under industrial-grade current densities remains unresolved.

In response to this problem, he and his team proposed a strategy for the design of micro-nano structures at the electrode interface.By altering the diffusion rate of lithium ions at the solid electrolyte interface and employing hierarchical porous electrodes, both the selectivity and rate of the reaction can be simultaneously regulated.

A solid electrolyte interphase layer rich in LiF can not only significantly improve selectivity by reducing the bulk diffusion rate of Li+, but also achieve uniform deposition of lithium metal by enhancing the surface migration rate of Li+, thereby improving system stability.

For hierarchical porous electrodes, they can effectively increase the electrochemical specific surface area of the electrode, which in turn can effectively improve the reaction rate.

According to Li Shaofeng, he and his team were the first in the world to achieve electrochemical reduction of nitrogen to synthesize ammonia at an ampere-level current density, and the ammonia selectivity at a current density of 1 A per square centimeter reached 71 ± 3%, with the ammonia production rate being more than an order of magnitude higher than the highest reported value in the field of lithium-mediated ammonia synthesis at that time.

In response to this achievement, a professor from the Georgia Institute of Technology in the United States wrote a special article to comment on it [6]. The article pointed out that the work of Li Shaofeng and others was the first to achieve lithium-mediated nitrogen reduction reactions at an industrial-level current density."However, we are also acutely aware that, despite the work conceptually proving that lithium-mediated ammonia synthesis can achieve high selectivity under industrial current densities in a high-pressure single-chamber electrolyzer, there are still many challenges in how to carry out efficient continuous production," said Li Shaofeng.

The primary challenge is to expand lithium-mediated ammonia synthesis from a high-pressure single-chamber electrolyzer to an atmospheric pressure flow electrolyzer, addressing the sacrificial proton source and mass transfer limitations.

While conducting the aforementioned work, Dr. Fu Xianbiao, a colleague in Li Shaofeng's team, has also made significant progress in the atmospheric pressure flow electrolyzer.

By using a highly stable and active hydrogen oxidation catalyst in the flow electrolyzer, they achieved the coupling of nitrogen reduction and hydrogen oxidation under ambient temperature and pressure conditions, and were able to perform continuous electrochemical ammonia synthesis, with an ammonia selectivity of 61%[3].

However, there are still some challenges in carrying out lithium-mediated ammonia synthesis in this flow electrolyzer, such as its inability to operate stably for more than 10 hours, the ammonia production is only at the milligram level, and about half of the ammonia is distributed in the organic electrolyte solution.Therefore, the next significant challenge facing the team is: how to greatly enhance the stability of lithium-mediated ammonia synthesis and obtain a higher yield of gaseous ammonia. Because there is currently no effective means to extract ammonia from the liquid electrolyte effectively.

Considering the tetrahydrofuran used in lithium-mediated ammonia synthesis has a low boiling point (66°C) and is volatile, Li Shaofeng realized that despite many factors affecting the stability of lithium-mediated ammonia synthesis, the low boiling point solvent must be an obstacle to achieving long-term stability.

Therefore, he first studied the impact of the solvent on the stability of lithium-mediated ammonia synthesis and found that tetrahydrofuran is the key reason for the poor stability.

Tetrahydrofuran not only has a low boiling point and is volatile, but it 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 new design principles and requirements for the solvent:Ensure the solubility of lithium salts in the solvent to ensure that the electrolyte has sufficient ionic conductivity and is conducive to lithium deposition.

The solvent must be compatible with metallic lithium and proton shuttle agents to ensure effective transport of protons generated by the hydrogen oxidation reaction at the anode.

The solid electrolyte interface layer formed by the solvent on the gas diffusion electrode needs to be uniform and compact to facilitate the easier entry of the generated ammonia into the gas phase with nitrogen, thus achieving easy separation.

The solvent must have a high boiling point and be difficult to polymerize to avoid evaporation and polymerization of the solvent.

Subsequently, through a systematic evaluation of the impact of various linear and cyclic ether solvents on lithium-mediated ammonia synthesis, he found that high boiling point linear ether solvents, especially diethylene glycol dimethyl ether (boiling point of 162°C), are excellent solvents for lithium-mediated ammonia synthesis.The solvent not only has the characteristic of being difficult to polymerize, but also helps to form a compact solid electrolyte interface layer on the gas diffusion electrode, thereby improving the distribution of ammonia in the gas phase and ensuring the long-term stability of the electrolyte.

"After searching high and low, success comes without effort."

During the research, in the process of trying different solvents, Li Shaofeng took many detours, including purchasing many high-boiling-point carbonate solvents, but the results were not satisfactory.

After trying various solvents, he began to ponder why tetrahydrofuran solvent is widely used. What are its special features besides having a low boiling point?It was later discovered that tetrahydrofuran is a typical cyclic ether solvent, and there are many similar ether solvents, including both cyclic and linear types.

Therefore, he began to try using a linear ether solvent (diethylene glycol dimethyl ether) that the laboratory had purchased several years ago, 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 "searching everywhere in vain, only to find it without any effort."

However, when expanding the linear ether solvent from diethylene glycol dimethyl ether to the high boiling point diethylene glycol dimethyl ether, he also encountered some difficulties.

At first, the ammonia selectivity was always low, only about 35%, with only about 30% of the ammonia distributed in the gas phase, and the stability was also difficult to exceed 24 hours."We are very puzzled by this reason," said Li Shaofeng. "By comparing the electrochemical data when using different solvents, I found that after switching the cycling test from the constant current mode to the open circuit voltage, the response of the working electrode voltage to time is completely different."

When using high-boiling chain ether solvents, the response of voltage to time is slower. This may be related to the high boiling point characteristics of the solvent itself, as well as its impact on the proton carrier's ability to provide protons, and it may also be related to the formation of the solid electrolyte interface layer involving the solvent.

By delving into this detail, he found that using a controlled potential cycling strategy can significantly improve the synthetic ammonia performance of this high-boiling chain ether solvent.

Finally, the relevant paper was published in Nature[7] with the title "Long-term continuous ammonia electrosynthesis."

Li Shaofeng is the first author, and Professor Jens K. Nørskov and Professor Ib Chorkendorff from the Technical University of Denmark serve as co-corresponding authors.The Power of Collaboration Far Exceeds Going Solo

Currently, the European patent related to this work has been granted to Denmark's NitroVolt company for the development of a small-scale electrochemical ammonia synthesis system.

The team is also looking forward to the continuous production of ammonia under industrial current density, promoting the industrialization of this technology.

It is also reported that Professor Ib Chorkendorff has published research papers, including 6 articles in Science and 2 in Nature, as the corresponding author.Impressively, the average time from submission to acceptance for these 6 Science papers was less than 3 months.

Among these 8 papers, 4 are research papers that integrate experiments with theory, where Professor Jens K. Nørskov's team provided guidance and support in theoretical calculations.

It is worth mentioning that the collaboration between Professor Ib Chorkendorff and Jens K. Nørskov can be traced back to the 1990s.

"They have conducted close cooperation in experiments and theory for decades, providing a perfect interpretation of scientific research cooperation, proving that the power of cooperation far exceeds going it alone," Li Shaofeng concluded.

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