A panoramic view on lithium-mediated electrochemical dinitrogen reduction reaction

The Haber-Bosch process is the industrial approach for NH₃ production today, which must be operated at energy-intensive high temperatures and pressures. The reduction of dinitrogen (N₂) by electrocatalysis offers an alternative way for NH₃ production at ambient conditions and a variety of electrocatalysts have been studied over the past few years.

However, even the best catalytic system reported could only get unsatisfied performance (such as the selectivity and production rate of NH₃) due to the inertness of N₂. The lithium-mediated N₂ reduction reaction (Li-eN₂RR) has been found to be a promising route to promote electrochemical NH₃ synthesis.

In this regard, a group of researchers has summarized the reaction mechanisms, the catalysts developed, and the electrolytes involved based on the most recent research progress of Li-eN₂RR. They also point out the challenges and possible resolving strategies in the Li-eN₂RR.

This could provide a panoramic view of the related field and facilitate the development of Li-eN₂RR for green NH₃ production. Sun et al. published their review in Industrial Chemistry & Materials.

"The electrocatalytic reduction of N₂ for the production of NH₃ has been the subject of extensive research, which has been comprehensively reviewed by numerous scholars with expertise in the field," said corresponding author Zhenyu Sun, a professor at Beijing University of Chemical Technology.

"These reviews provide detailed insights into the catalytic performance and mechanisms involved, which can be of great assistance to researchers. However, the NH₃ production rate is still low, far behind the requirement of industrial application. The lithium-mediated N₂ reduction reaction (Li-eN₂RR) holds great potential for the production of NH₃ under ambient conditions."

"Therefore, we provide this comprehensive overview to summarize the recent progress of Li-eN₂RR. This review demonstrates the basic aspects of Li-eN₂RR including the common composition of electrolytes, suppression of hydrogen evolution, reaction mechanism, the reported electrocatalysts, and the challenges and prospects, which can be helpful to new individuals in this field as well as serve as a valuable resource for researchers, policymakers, and industry professionals alike."

The rate-determining step in the synthesis of NH₃ is often the adsorption and activation of N₂. However, due to the inertness of N₂, it is extremely hard to break the N≡N triple bond. In the Li-eN₂RR process, Li can be used as a mediator to fix N₂ as Li₃N, which subsequently converts to NH₃.

"There have been different viewpoints on the reaction mechanism of Li-eN₂RR. We provide detailed analysis of the reported mechanisms," Sun said.

Three different mechanisms, including chemical N₂ splitting and chemical protonation, N₂ activation and protonation through an associative mechanism, and chemical N₂ splitting and electrochemical protonation were introduced with their respective reaction steps.

The section about the reaction mechanisms of Li-eN₂RR demonstrates the different routes of N₂ activation and hydrogenation to NH₃ production. "Through the different mechanisms, we can have a comprehensive understanding of Li-eN₂RR, which can guide the catalytic system design."

The rational design of electrocatalysts is crucial for Li-eN₂RR. There have been some strategies for the engineering of the electrodes to get enhanced performance.

"The electrocatalysts together with their design strategies were categorized based on the metal species involved," Sun said.

"Different kinds of electrocatalysts including noble metal catalysts such as Ru, Ag, Au; nonprecious metal catalysts such as copper-based materials, molybdenum, lithium-liquid alloy-salt, stainless steel cloth; and non-metal catalysts such as carbon-based materials were comprehensively overviewed. It was discussed in detail about their performance and the catalytic active sites in Li-eN₂RR, which may enlighten the design of the future catalysts towards efficient NH₃ production."

Another important part of Li-eN₂RR is the electrolytes. According to Zhenyu Sun, more attention should be given to the study of electrolytes in electrocatalytic N₂ reduction considering the significance of electrolytes for the reaction and for N₂ dissolution.

Some Li-containing electrolytes have been developed for Li-eN₂RR, yet more efforts still need to be devoted to further improving the NH₃ yield. This requires the synergistic work between the electrolytes and the electrocatalysts as well as the study of their interface.

"The primary objective of this review is to provide readers with a clear understanding of the current research progress of Li-eN₂RR, which is still in its early stage yet promising for electrocatalytic NH₃ production. We also highlight the challenges and propose strategies to overcome them. We hope it is helpful to promote the development of Li-eN₂RR towards green NH₃ production," Sun said.