Cr3C2 Nanoparticle-Embedded Carbon Nanofiber for Artificial Synthesis of NH3 through N2 Fixation under Ambient Conditions

Active sites engineering on FeNi alloy/Cr3C2 heterostructure for superior oxygen evolution activity

Exploring high active electrocatalysts for oxygen evolution reaction (OER) is of great significance for a sustainable hydrogen economy. The development of non-precious transition metals, with sufficient active sites and ample intrinsic activity, remains a challenge. Herein, a new type of FeNi-Cr3C2 heterostructure anchored on carbon sheets (FeNi-Cr3C2@C) was reported, which can effectively catalyze OER with swift kinetics and outstanding intrinsic activity. The introduced Cr3C2 phase not only serves as a support material but also effectively suppresses the thermal coarsening of FeNi alloy nanoparticle. The FeNi-Cr3C2@C displays a robust OER activity with a low overpotential of 283 mV at the current density of 10 mA cm-2, a high turnover frequency value of 1.69 s-1 at the overpotential of 300 mV (10 times higher than that of FeNi@C) and good stability in alkaline media. Density functional theory calculations (DFT) calculations show that Cr3C2 can facilitate the generation of electron-rich region at the Ni site in FeNi alloys as an active site, exhibiting an optimized adsorption behavior toward oxygen intermediate species with regard to decreased thermodynamic energy barriers. Our work opens up a promising path to modulate the electrocatalytic active sites using inexpensive and durable Cr3C2 for electrochemical catalytic reactions.

Metal-Based Electrocatalysts for Selective Electrochemical Nitrogen Reduction to Ammonia

Ammonia (NH3) plays a significant role in the manufacture of fertilizers, nitrogen-containing chemical production, and hydrogen storage. The electrochemical nitrogen reduction reaction (e-NRR) is an attractive prospect for achieving clean and sustainable NH3 production, under mild conditions driven by renewable energy. The sluggish cleavage of N≡N bonds and poor selectivity of e-NRR are the primary challenges for e-NRR, over the competitive hydrogen evolution reaction (HER). The rational design of e-NRR electrocatalysts is of vital significance and should be based on a thorough understanding of the structure-activity relationship and mechanism. Among the various explored e-NRR catalysts, metal-based electrocatalysts have drawn increasing attention due to their remarkable performances. This review highlighted the recent progress and developments in metal-based electrocatalysts for e-NRR. Different kinds of metal-based electrocatalysts used in NH3 synthesis (including noble-metal-based catalysts, non-noble-metal-based catalysts, and metal compound catalysts) were introduced. The theoretical screening and the experimental practice of rational metal-based electrocatalyst design with different strategies were systematically summarized. Additionally, the structure-function relationship to improve the NH3 yield was evaluated. Finally, current challenges and perspectives of this burgeoning area were provided. The objective of this review is to provide a comprehensive understanding of metal-based e-NRR electrocatalysts with a focus on enhancing their efficiency in the future.

Shedding Light on the Role of Chemical Bond in Catalysis of Nitrogen Fixation

Ammonia (NH₃ ) and nitrates are essential for human society because of their widespread utilization for producing medicines, fibers, fertilizers, etc. In recent years, the development on nitrogen fixation under mild reaction conditions has attracted much attention. However, the very low conversion efficiency and ambiguous catalytic mechanism remain the major hurdles for the research of nitrogen fixation. This review aims to clarify the role of chemical bond in catalytic nitrogen fixation by summarizing and analyzing the recent development of nitrogen fixation research. In detail, the atomic-scale mechanism of nitrogen fixation reaction, the various methods to improve the nitrogen fixation performance, and the computational investigation of nitrogen fixation are discussed, all from a chemical bond perspective. It is hoped that this review could trigger more profound pondering and deeper exploration in the field of catalytic nitrogen fixation.

Monodisperse Cu Cluster-Loaded Defective ZrO2 Nanofibers for Ambient N2 Fixation to NH3

Electrocatalytic nitrogen reduction to ammonia has attracted increasing attention as it is more energy-saving and eco-friendly. For this endeavor, the development of high-efficiency electrocatalysts with excellent selectivity and stability is indispensable to break up the stable covalent triple bond in nitrogen. In this study, we report monodisperse Cu clusters loaded on defective ZrO₂ nanofibers for nitrogen reduction under mild conditions. Such an electrocatalyst achieves an NH₃ yield rate of 12.13 μg h^-1 mg_cat.^-1 and an optimal Faradaic efficiency of 13.4% at -0.6 V versus the reversible hydrogen electrode in 0.1 M Na₂SO₄. Density functional theory calculations reveal that the N₂ molecule was reduced to NH₃ at the Cu active site with an ideal overpotential. Meanwhile, the interaction between bonding and antibonding of the Cu-N bond promotes activation of N₂ and maintains a low desorption barrier.

Iron-Doped MoO3 Nanosheets for Boosting Nitrogen Fixation to Ammonia at Ambient Conditions

Nitrogen can be electrochemically reduced to produce ammonia, which supplies an energy-saving and environmental-benign route at room temperature, but high-efficiency catalysts are sought to reduce the reaction barrier. Here, iron-doped α-MoO₃ nanosheets are thus designed and proposed as potential catalysts for fixing N₂ to NH₃. The α-MoO₃ band structure is intentionally modulated by the iron doping, which narrows the band gap of α-MoO₃ and turns the semiconductor into a metal-like catalyst. Oxygen vacancies, generated by substituting Mo⁶⁺ for Fe³⁺ anions, are beneficial for nitrogen adsorption at the active sites. In 0.1 M Na₂SO₄, the Fe-doped MoO₃ catalyst reached a high faradaic efficiency of 13.3% and an excellent NH₃ yield rate of 28.52 μg h^-1 mg_cat^-1 at -0.7 V versus reversible hydrogen electrode, superior to most of the other metal-based catalysts. Theoretical calculations confirmed that the N₂ reduction reaction at the Fe-MoO₃ surface followed the distal reaction path.

Exploration and Investigation of Periodic Elements for Electrocatalytic Nitrogen Reduction

High demand for green ecosystems has urged the human community to reconsider and revamp the traditional way of synthesis of several compounds. Ammonia (NH₃ ) is one such compound whose applications have been extended from fertilizers to explosives and is still being synthesized using the high energy inhaling Haber-Bosch process. Carbon free electrocatalytic nitrogen reduction reaction (NRR) is considered as a potential replacement for the Haber-Bosch method. However, it has few limitations such as low N₂ adsorption, selectivity (competitive HER reactions), low yield rate etc. Since it is at the early stage, tremendous efforts have been devoted in understanding the reaction mechanism and screening of the electrocatalysts and electrolytes. In this review, the electrocatalysts are classified based on the periodic table with heat maps of Faraday efficiency and yield rate of NH₃ in NRR and their electrocatalytic properties toward NRR are discussed. Also, the activity of each element is discussed and short tables and concise graphs are provided to enable the researchers to understand recent progress on each element. At the end, a perspective is provided on countering the current challenges in NRR. This review may act as handbook for basic NRR understandings, recent progress in NRR, and the design and development of advanced electrocatalysts and systems.

Current Progress of Electrocatalysts for Ammonia Synthesis Through Electrochemical Nitrogen Reduction Under Ambient Conditions

Ammonia, one of the most important chemicals and carbon-free energy carriers, is mainly produced by the traditional Haber-Bosch process operated at high pressure and temperature, which results in massive energy consumption and CO2 emissions. Alternatively, the electrocatalytic nitrogen reduction reaction to synthesize NH3 under ambient conditions using renewable energy has recently attracted significant attention. However, the competing hydrogen evolution reaction (HER) significantly reduces the faradaic efficiency and NH3 production rate. The design of high-performance electrocatalysts with the suppression of the HER for N2 reduction to NH3 under ambient conditions is a crucial consideration for the development of electrocatalytic NH3 synthesis with high FE and NH3 production rate. Five kinds of recently developed electrocatalysts classified by their chemical compositions are summarized, with particular emphasis on the relationship between their optimal electrocatalytic conditions and NH3 production performance. Conclusions and perspectives are provided for the future design of high-performance electrocatalysts for electrocatalytic NH3 production. The Review can give practical guidance for the design of effective electrocatalysts with high FE and NH3 production rates.

Efficient Electrocatalytic Nitrogen Fixation on FeMoO4 Nanorods

Electroreduction of N₂ represents a promising technique for ambient NH₃ synthesis, but exploring efficient electrocatalysts for nitrogen reduction reaction (NRR) remains a key challenge. Herein, we reported our experimental and theoretical findings that FeMoO₄ could be a new candidate for effective and durable NRR in neutral solution. The developed FeMoO₄ nanorods exhibited a fascinating NRR activity with an NH₃ yield of 45.8 μg h^-1 mg^-1 (-0.5 V) and a Faradaic efficiency of 13.2% (-0.3 V). Mechanistic studies disclosed that Fe and Mo synergistically promoted the N₂ adsorption and accelerated the electron transfer on FeMoO₄, whereas the unsaturated 3-fold coordinated Mo (Mo_3c) sites served as the main active centers for stabilizing the key *N₂H intermediate and reducing the reaction energy barrier.

Bioinspired Electrocatalyst for Electrochemical Reduction of N2 to NH3 in Ambient Conditions

Industrial ammonia production depends heavily on the traditional Haber-Bosch method at the expense of CO₂ emissions and large energy consumptions. Artificial fixation of nitrogen to ammonia is therefore regarded as a promising path to yield ammonia in energy-saving conditions. However, a competent electrocatalyst is highly desired, owing to the extremely stable bond of N≡N. In this work, we report Fe₂(MoO₄)₃ nanoparticles as a non-noble-metal electrocatalyst, inspired by nitrogenase enzymes for electrochemically converting nitrogen into ammonia, which achieves a Faradic efficiency of 9.1% and an excellent NH₃ yield of 18.16 μg h^-1 mg^-1 cat in 0.1 M sodium sulfate at -0.6 V vs reversible hydrogen electrode. Also, it has a better ammonia yield rate of 20.09 μg h^-1 mg^-1 cat in 0.1 M hydrochloric acid. Moreover, this noble-metal-free catalyst exhibits a unique reaction process selectivity and stability compared with the other catalysts working in harsh conditions. The specific reaction processes are analyzed by density functional theoretical calculations to gain insights into the nitrogen reduction reaction (NRR) by this catalyst.