Ammonia, as a significant chemical for fertilizer production and also a promising energy carrier, is mainly produced through the traditional energy-intensive Haber–Bosch process. Recently, the electrocatalytic N2 reduction reaction (NRR) for ammonia synthesis has received tremendous attention with the merits of energy saving and environmental friendliness. To date, the development of the NRR process is primarily hindered by the competing hydrogen evolution reaction (HER), whereas the corresponding strategies for inhibiting this undesired side reaction to achieve high NRR selectivity are still quite limited. Furthermore, for such a complex reaction involving three gas–liquid–solid phases and proton/electron transfer, it is also rather meaningful to decouple and summarize the current strategies for suppressing H2 evolution in terms of NRR mechanisms, kinetics, thermodynamics, and electrocatalyst design in detail. Herein, on the basis of the NRR mechanisms, we systematically summarize the recent strategies to inhibit the HER for a highly selective electrocatalytic NRR, focusing on limiting the proton- and electron-transfer kinetics, shifting the chemical equilibrium, and designing the electrocatalysts. Additionally, insights into boosting the NRR selectivity and efficiency for practical applications are also presented in detail with regard to the determination of ammonia, the activation of the N2 molecule, the regulation of the gas–liquid–solid three-phase interface, the coupled NRR with value-added oxidation reactions, and the development of flow cell reactors.

Strategies to suppress hydrogen evolution for highly selective electrocatalytic nitrogen reduction: challenges and perspectives

To restore the natural nitrogen cycle (N-cycle), artificial N-cycle electrocatalysis with flexibility, sustainability, and compatibility can convert intermittent renewable energy (e.g., wind) to harmful or value-added chemicals with minimal carbon emissions. The background of such N-cycles, such as nitrogen fixation, ammonia oxidation, and nitrate reduction, is briefly introduced here. The discussion of emerging nanostructures in various conversion reactions is focused on the architecture/compositional design, electrochemical performances, reaction mechanisms, and instructive tests. Energy device advancements for achieving more functions as well as <i>in situ</i>/<i>operando</i> characterizations toward understanding key steps are also highlighted. Furthermore, some recently proposed reactions as well as less discussed C–N coupling reactions are also summarized. We classify inorganic nitrogen sources that convert to each other under an applied voltage into three types, namely, abundant nitrogen, toxic nitrate (nitrite), and nitrogen oxides, and useful compounds such as ammonia, hydrazine, and hydroxylamine, with the goal of providing more critical insights into strategies to facilitate the development of our circular nitrogen economy. 

https://www.sciopen.com/article_pdf/1531201054523838466.pdf

Recent advances in nanostructured heterogeneous catalysts for N-cycle electrocatalysis

Electrochemical N2 reduction reactions (NRR) and the N2 oxidation reaction (NOR), using H2 O and N2 , are a sustainable approach to N2 fixation. To date, owing to the chemical inertness of nitrogen, emerging electrocatalysts for the electrochemical NRR and NOR at room temperature and atmospheric pressure remain largely underexplored. Herein, a new-type Fe-SnO2 was designed as a Janus electrocatalyst for achieving highly efficient NRR and NOR catalysis. A high NH3 yield of 82.7 μg h-1 mgcat. -1 and a Faraday efficiency (FE) of 20.4 % were obtained for NRR. This catalyst can also serve as an excellent NOR electrocatalyst with a NO3 - yields of 42.9 μg h-1 mgcat. -1 and a FE of 0.84 %. By means of experiments and DFT calculations, it is revealed that the oxygen vacancy-anchored single-atom Fe can effectively adsorb and activate chemical inert N2 molecules, lowering the energy barrier for the vital breakage of N≡N and resulting in the enhanced N2 fixation performance.

A Janus Fe‑SnO 2 Catalyst that Enables Bifunctional Electrochemical Nitrogen Fixation

The electrochemical method of combining N2 and H2 O to produce ammonia (i.e., the electrochemical nitrogen reduction reaction [E-NRR]) continues to draw attention as it is both environmentally friendly and well suited for a progressively distributed farm economy. Despite the multitude of recent works on the E-NRR, further progress in this field faces a bottleneck. On the one hand, despite the extensive exploration and trial-and-error evaluation of E-NRR catalysts, no study has stood out to become the stage protagonist. On the other hand, the current level of ammonia production (microgram-scale) is an almost insurmountable obstacle for its qualitative and quantitative determination, hindering the discrimination between true activity and contamination. Herein i) the popular theory and mechanism of the NRR are introduced; ii) a comprehensive summary of the recent progress in the field of the E-NRR and related catalysts is provided; iii) the operational procedures of the E-NRR are addressed, including the acquisition of key metrics, the challenges faced, and the most suitable solutions; iv) the guiding principles and standardized recommendations for the E-NRR are emphasized and future research directions and prospects are provided.

Comprehensive Understanding of the Thriving Ambient Electrochemical Nitrogen Reduction Reaction.

The modern ammonia synthesis industry founded by Haber–Bosch in 1913 has successfully altered the history of food production, fed explosive population growth, and laid the foundation of heterogeneous catalysis and chemical engineering as well. However, its reliance on fossil fuels for reactant H2 production consumes 1–3% of the world's electric energy and 2–5% of the world's natural gas, accompanied by more than 400 million tons of CO2 emission annually. Making use of water as the proton source and electric energy to drive the ammonia synthesis reaction will reduce the fossil fuel consumption and CO2 emission and is thus regarded as a green and sustainable alternative to the conventional Haber–Bosch process. To date, some excellent reviews on electrochemical ammonia synthesis have been published, but most of them were organized according to the type of catalyst. A systematic summary of the performance-improving strategies of the electrocatalyst for ammonia synthesis is rarely reported and therefore highly desirable. In this review, the nitrogen reduction reaction mechanisms and recent theoretical advances are briefly outlined first. Then, strategies for both reactivity and selectivity enhancement of catalysts and catalytic systems are methodically discussed. Last, criteria for the electrochemical nitrogen reduction reaction, ammonia quantification methods, and an outlook for further research are also concluded. This review aims to provide systematic and concise guidance for the design of highly efficient and selective electrochemical ammonia synthesis systems.

Recent progress in electrochemical synthesis of ammonia from nitrogen: strategies to improve the catalytic activity and selectivity

Selective electroreduction of dinitrogen to ammonia on a molecular iron phthalocyanine/O-MWCNT catalyst under ambient conditions.

Constructing efficient catalysts for N2 reduction into value added ammonia under ambient conditions is a considerable challenge. Herein, well-defined single-site metal–organic frameworks (MOFs, M–TCPP; M = Fe, Co, or Zn) were constructed and evaluated as electrocatalysts for N2 reduction. The prepared Fe–TCPP exhibited prominent performance with a high NH3 yield of 44.77 μg h−1 mgcat.−1 and a faradaic efficiency of 16.23%, superior to that of all the reported molecular and MOF catalysts. The superior performance was ascribed to the highly effective N2 activation at the Fe site, and benefited from the overall reaction thermodynamics advantage in the key reaction step of *NNH formation. This study gives an understanding of the intrinsic activity of well-defined catalysts in the electrocatalytic N2 reduction, and provides atomic-level insights into the rational design and engineering of highly active catalysts for artificial N2 fixation.

Yu Jin

Papers

A Janus Fe‑SnO 2 Catalyst that Enables Bifunctional Electrochemical Nitrogen Fixation

Selective electroreduction of dinitrogen to ammonia on a molecular iron phthalocyanine/O-MWCNT catalyst under ambient conditions.

Selective nitrogen reduction to ammonia on iron porphyrin-based single-site metal–organic frameworks

Boosting electrocatalytic reduction of nitrogen to ammonia under ambient conditions by alloy engineering.