Electrocatalytic Hydrogenation of N2 to NH3 by MnO: Experimental and Theoretical Investigations

High-Entropy Metal Nitride Embedded in Concave Porous Carbon Enabling Polysulfide Conversion in Lithium-Sulfur Batteries

The practical implementation of lithium-sulfur batteries is severely hindered by the rapid capacity fading due to the solubility of the intermediate lithium polysulfides (LiPSs) and the sluggish redox kinetics. Herein, high-entropy metal nitride nanocrystals (HEMN) embedded within nitrogen-doped concave porous carbon (N-CPC) polyhedra are rationally designed as a sulfur host via a facile zeolitic imidazolate framework (ZIF)-driven adsorption-nitridation process toward this challenge. The configuration of high-entropy with incorporated metal manganese (Mn) and chromium (Cr) will optimize the d-band center of active sites with more electrons occupied in antibonding orbitals, thus promoting the adsorption and catalytic conversion of LiPSs. While the concave porous carbon not only accommodates the volume change upon the cycling processes but also physically confines and exposes active sites for accelerated sulfur redox reactions. As a result, the resultant HEMN/N-CPC composites-based sulfur cathode can deliver a high specific capacity of 1274 mAh g-1 at 0.2 C and a low capacity decay rate of 0.044% after 1000 cycles at 1 C. Moreover, upon sulfur loading of 5.0 mg cm-2, the areal capacity of 5.0 mAh cm-2 can still be achieved. The present work may provide a new avenue for the design of high-performance cathodes in Li-S batteries.

Electrocatalysis in deep eutectic solvents: from fundamental properties to applications

Electrocatalysis stands out as a promising avenue for synthesizing high-value products with minimal environmental footprint, aligning with the imperative for sustainable energy solutions. Deep eutectic solvents (DESs), renowned for their eco-friendly, safe, and cost-effective nature, present myriad advantages, including extensive opportunities for material innovation and utilization as reaction media in electrocatalysis. This review initiates with an exposition on the distinctive features of DESs, progressing to explore their applications as solvents in electrocatalyst synthesis and electrocatalysis. Additionally, it offers an insightful analysis of the challenges and prospects inherent in electrocatalysis within DESs. By delving into these aspects comprehensively, this review aims to furnish a nuanced understanding of DESs, thus broadening their horizons in the realm of electrocatalysis and facilitating their expanded application.

Design of Single-Atom Catalysts for E lectrocatalytic Nitrogen Fixation

The Electrochemical nitrogen reduction reaction (ENRR) can be used to solve environmental problems as well as energy shortage. However, ENRR still faces the problems of low NH3 yield and low selectivity. The NH3 yield and selectivity in ENRR are affected by multiple factors such as electrolytic cells, electrolytes, and catalysts, etc. Among these catalysts are at the core of ENRR research. Single-atom catalysts (SACs) with intrinsic activity have become an emerging technology for numerous energy regeneration, including ENRR. In particular, regulating the microenvironment of SACs (hydrogen evolution reaction inhibition, carrier engineering, metal-carrier interaction, etc.) can break through the limitation of intrinsic activity of SACs. Therefore, this Review first introduces the basic principles of NRR and outlines the key factors affecting ENRR. Then a comprehensive summary is given of the progress of SACs (precious metals, non-precious metals, non-metallic) and diatomic catalysts (DACs) in ENRR. The impact of SACs microenvironmental regulation on ENRR is highlighted. Finally, further research directions for SACs in ENRR are discussed.

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.

Study on the effect of Ce-Cu doping on Mn/γ-Al2O3 catalyst for selective catalytic reduction in NO with NH3

A series of Mn/γ-Al₂O₃, Mn-Cu/γ-Al₂O₃, Mn-Ce/γ-Al₂O₃ and Mn-Ce-Cu/γ-Al₂O₃ catalysts were prepared by equal volume impregnation. The denitrification effects of the different catalysts were studied by activity measurement, X-ray diffraction, Brunauer, Emmett, and Teller surface area tests, Scanning electron microscopy, H₂-temperature programmed reduction and Fourier-transform infrared spectroscopy. The experimental results show that Ce and Cu are added to a Mn/γ-Al₂O₃ catalyst as bimetallic additives, which weakens the interaction between Mn and the carrier, improves the dispersion of MnO_x on the surface of the carrier, improves the specific surface area of the catalyst, and improves the reducibility. Mn-Ce-Cu/γ-Al₂O₃ catalyst reaches a maximum conversion of 92% at 202 °C. Also, the addition of the auxiliary metals promotes the reaction mechanism to a certain extent, and the addition of Ce especially promotes the conversion of NO-NO₂, which is conducive to the production of intermediate products that promote the NH₃-SCR reaction.

Green ammonia synthesis using CeO2/RuO2 nanolayers on vertical graphene catalyst via electrochemical route in alkaline electrolyte

The electrochemical synthesis of ammonia at ambient temperature and pressure has the potential to replace the conventional process for the production of ammonia. However, the low ammonia yield and poor long-term stability of catalysts for the synthesis of ammonia hinders the application of this technology. Herein, we endeavored to tackle this challenge by synthesizing 3-D vertical graphene (VG) on Ni foam via a one-step, low-temperature plasma process, which offered high conductivity and large surface area. Subsequently, the vertical graphene on Ni foam was loaded with nanolayers of ruthenium oxide (RuO₂, ∼2 nm) and cerium oxide (CeO₂, <20 nm) nanoparticles via magnetron sputtering. The incorporation of nanoparticle layers (RuO₂ and CeO₂/RuO₂) on VG significantly increased the NH₃ yield in KOH electrolyte. Finally, the performance and long-term stability of this composite material were successfully demonstrated by the addition of CeO₂/RuO₂ nanolayers on the VG electrocatalyst. The catalyst achieved an excellent performance with a high ammonia synthesis yield of 50.56 μg mg_{total cat.}^-1 h^-1 (1.11 × 10^-10 mol cm^-2 s^-1) during the performance evaluation period of 36 h. This observation was also verified by density functional theory calculation, where CeO₂ exhibited the best catalytic performance compared to RuO₂ and pristine graphene.

Single atom catalysis for electrocatalytic ammonia synthesis

This review points out major challenges and outlook of NH3 synthesis via SACs. Summarizing the deficiencies of existing research can help researchers to continuously innovate and improve, and explore new research approaches.

Emerging two-dimensional nanomaterials for electrochemical nitrogen reduction

Ammonia (NH₃) is essential to serve as the biological building blocks for maintaining organism function, and as the indispensable nitrogenous fertilizers for increasing the yield of nutritious crops. The current Haber-Bosch process for industrial NH₃ production is highly energy- and capital-intensive. In light of this, the electroreduction of nitrogen (N₂) into valuable NH₃, as an alternative, offers a sustainable pathway for the Haber-Bosch transition, because it utilizes renewable electricity and operates under ambient conditions. Identifying highly efficient electrocatalysts remains the priority in the electrochemical nitrogen reduction reaction (NRR), marking superior selectivity, activity, and stability. Two-dimensional (2D) nanomaterials with sufficient exposed active sites, high specific surface area, good conductivity, rich surface defects, and easily tunable electronic properties hold great promise for the adsorption and activation of nitrogen towards sustainable NRR. Therefore, this Review focuses on the fundamental principles and the key metrics being pursued in NRR. Based on the fundamental understanding, the recent efforts devoted to engineering protocols for constructing 2D electrocatalysts towards NRR are presented. Then, the state-of-the-art 2D electrocatalysts for N₂ reduction to NH₃ are summarized, aiming at providing a comprehensive overview of the structure-performance relationships of 2D electrocatalysts towards NRR. Finally, we propose the challenges and future outlook in this prospective area.

Two-dimensional transition metal borides as high activity and selectivity catalysts for ammonia synthesis

In comparison to defect/doping induced activity in materials, transition metal borides with exposed metal atoms, large specific surface area, and high active site density show advantages as durable and efficient catalysts for specific electrochemical reactions. In this work, ReB₂ for N₂ reduction reaction (NRR) for ammonia (NH₃) with a record-low limiting potential of U_L = -0.05 V and high Faraday efficiency (FE) of 100% is screened out from a new class of TMB₂. It is concluded that high pressure/temperature is favorable to N₂ adsorption and kinetic barrier minimization; the maximal turnover frequency (TOF) at 700 K and 100 bar is 1.24 × 10^-2 per s per site, which is comparable to that of the benchmark Fe₃/Al₂O₃ catalysts, achieving an extremely fast reaction rate. In addition, crystal orbital Hamilton population (COHP) of *N₂ reveals the intrinsic origin of N₂ activation by analyzing the d-2π* interactions, and integrated COHP could be a quantitative descriptor to describe the N₂ activation degree. It is evident that our results not only identify an efficient NRR electrocatalyst in particular, paving the way for sustainable NH₃ production, but also explain the chemical and physical origin of the activity, advancing the design principle for catalysts for various reactions in general.

Synergy of Bi2 O3 and RuO2 Nanocatalysts for Low-Overpotential and Wide pH-Window Electrochemical Ammonia Synthesis

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is still seriously impeded by the inferior NH₃ yield and low Faradaic efficiency, especially at low overpotentials. Herein, we report the synthesis of nano-sized RuO₂ and Bi₂ O₃ particles grown on functionalized exfoliated graphene (FEG) through in situ electrodeposition, denoted as RuO₂ -Bi₂ O₃ /FEG. The prepared self-supporting RuO₂ -Bi₂ O₃ /FEG hybrid with a Bi mass loading of 0.70 wt% and Ru mass loading of 0.04 wt% shows excellent NRR performance at low overpotentials in acidic, neutral and alkaline electrolytes. It achieves a large NH₃ yield of 4.58±0.16 μg_NH3  h^-1  cm^-2 with a high Faradaic efficiency of 14.6 % at -0.2 V versus reversible hydrogen electrode in 0.1 M Na₂ SO₄ electrolyte. This performance benefits from the synergistic effect between Bi₂ O₃ and RuO₂ which respectively have a fairly strong interaction of Bi 6p orbitals with the N 2p band and abundant supply of *H, as well as the binder-free characteristic and the convenient electron transfer via graphene nanosheets. This work highlights a new electrocatalyst design strategy that combines transition and main-group metal elements, which may provide some inspirations for designing low-cost and high-performance NRR electrocatalysts in the future.

Reduction of N2 to NH3 by TiO2-supported Ni cluster catalysts: a DFT study

Electrochemical techniques for ammonia synthesis are considered as an encouraging energy conversion technology to efficiently meet the challenge of nitrogen cycle balance. Herein, we find that TiO2(101)-supported Ni4 and Ni13 clusters can serve as efficient catalysts for electrocatalytic N2 reduction based on theoretical calculations. Electronic property calculations reveal the formation of electron-deficient Ni clusters on the TiO2 surface, which provides multiple active sites for N2 adsorption and activation. Theoretical calculation identifies the strongest activated configuration of N2* on the catalysts and confirms the potential-limiting step in the nitrogen reduction reaction (NRR). On Ni4-TiO2(101), N2* → NNH* is the potential-limiting step with a very small free energy increase (ΔG) of 0.24 eV (the corresponding overpotential is 0.33 V), while on Ni13-TiO2(101) the potential-limiting step occurs at NH* → NH2* with ΔG of 0.49 eV (the corresponding overpotential is 0.58 V). Moreover, the Nin-TiO2(101) catalyst, especially Ni13-TiO2(101), involves in a highly selective NRR even at the corresponding NRR overpotential. This work will enlighten material design to construct metal oxide supported transition metal clusters for the highly efficient NRR and NH3 synthesis.

Emulsion-template synthesis of mesoporous nickel oxide nanoflowers composed of crossed nanosheets for effective nitrogen reduction

A novel emulsion-template synthesis approach was developed for the preparation of nickel oxide nanoflowers (NiO-NFs) composed of crossed mesoporous nanosheets. The interface assembly process was regulated by tuning the dosage of NH3·H2O, resulting in the tunability of thickness and size of mesoporous NiO nanosheets. The as-prepared NiO-NFs were characterized by field emission scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller analysis, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). The results indicate that NiO-NFs have a mesopore size of about 9.5-15 nm and a crossed nanosheet thickness of about 12.4-50 nm. XPS results demonstrated that all NiO-NF samples consisted of Ni2+ and Ni3+. Electrochemical nitrogen reduction reaction (NRR) measurements revealed that NiO-NF-3.0 showed an optimal NRR performance of NH3 yield and faradaic efficiency (16.16 μg h-1 mg-1cat. and 9.17% at -0.4 V vs. RHE) in 0.1 M Na2SO4. Interestingly, NiO-NF-3.0 also displayed the highest Ni3+ content, which correlates with the order of electrochemical NRR performance. This can be attributed to the fact that Ni3+ promotes the electropositivity of NiO-NFs, resulting in more facile adsorption of N2 gas than Ni2+, and leading to enhanced electrocatalytic properties. These enhanced NRR performances are comparable or superior to those of reported noble-metal catalysts. This study provides a novel method for the fabrication of low-cost metal oxide nanomaterials that allows the construction of electrochemical NRR catalysts to meet the needs of industrial production. Also, it provides a new approach to improve the electrochemical properties by increasing the content of high-valent metal ions in a metal oxide.

Fabrication of an Au25 -Cys-Mo Electrocatalyst for Efficient Nitrogen Reduction to Ammonia under Ambient Conditions

Electrocatalysts for efficient production of ammonia from nitrogen reduction reaction (NRR) under ambient conditions are attracted growing interest in recent years, which demonstrate a great potential to replace the Haber-Bosch method which suffers the problems of the huge energy consumption and massive CO₂ production. In this work, a novel electrocatalyst of Au₂₅ -Cys-M is fabricated for NRR under ambient conditions, with transition metal ions (e.g., Mo⁶⁺ , Fe³⁺ , Co²⁺ , Ni²⁺ ) atomically decorated on Au₂₅ nanoclusters via thiol bridging. The Au₂₅ -Cys-Mo catalyst exhibits the highest Faradaic efficiency (26.5%) and NH₃ yield (34.5 µg h^-1  mg_cat ^-1 ) in 0.1 m HCl solution. X-ray photoelectron spectroscopy analysis and high angle annular dark field image-scanning transmission electron microscopy characterization reveal that the electronic structure of Mo is optimized by forming the structure of Au-S-Mo and Mo acts as active sites for activating the nitrogen to promote the electrochemical production of ammonia. This work provides a new insight into the precise fabrication of efficient NRR electrocatalysts.

Tailoring the d-Band Center of Double-Perovskite LaCoxNi1-xO3 Nanorods for High Activity in Artificial N2 Fixation

The d-band center of a catalyst can be applied for the prediction of its catalytic activity, but the application of d-band theory for the electrocatalytic nitrogen reduction reaction (eNRR) has rarely been studied in perovskite materials. In this work, a series of double-perovskite LaCo_xNi_1-xO₃ (LCNO) nanorods (NRs) were synthesized as models, where the d-band centers can be modulated by changing the stoichiometric ratios between Co and Ni elements. Experimentally, the LCNO-III NRs (x = 0.5) attained the highest faradic efficiency and NH₃ yield rate among various LCNO NRs. This result matches well with the finding from theoretical calculations that LCNO-III has the most positive d-band center (ε_d = -0.96 eV vs Fermi level), thus confirming that LCNO-III shows the strongest adsorption ability for N₂ molecules (adsorption energy value of -2.01 eV) for the subsequent N₂ activation and reduction reactions. Therefore, this work proposes a general rule to adopt for developing novel catalysts (especially perovskite-based catalysts) for substantially increasing the eNRR activity by modulating the corresponding d-band centers.

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.

Electrochemical nitrogen reduction: recent progress and prospects

Ammonia is one of the most useful chemicals for the fertilizer industry and is also promising as an important energy carrier for fuel cell application, and is currently mostly produced by the traditional Haber-Bosch process under high temperature and pressure conditions. This energy-intensive process is detrimental to the environment due to the dependence on fossil fuels and the emission of significant greenhouse gases (such as CO2). Ammonia production via the electrochemical nitrogen reduction reaction (ENRR) has been recognized as a green sustainable alternative to the Haber-Bosch process in recent years. Current ENRR research mainly focuses on the catalyst for ammonia selective production and the enhancement of faradaic efficiency at high current density; however, these have not been explored well due to the unavailability of highly efficient and cheap catalysts. Herein, this review provides information on the ENRR process along with (i) theoretical background, (ii) experimental methodology of the electrocatalytic process and (iii) computational screening of promising catalysts. The impact of active sites and defects on the activity, selectivity, and stability of the catalysts is deeply understood. Furthermore, we demonstrate the mechanistic understanding of the ENRR process on the surface of catalysts, with the aim of boosting the improvement of the ENRR activities. The ammonia detection methods are also summarized along with thorough discussion of control experiments. Finally, this review highlights prevailing problems in existing ENRR methods of ammonia production along with technical advancements proposed to address these issues and concludes with comments on opportunities and future directions of the ENRR process.

Three-Phase Electrolysis by Gold Nanoparticle on Hydrophobic Interface for Enhanced Electrochemical Nitrogen Reduction Reaction

Electrochemical nitrogen reduction reaction (NRR) provides a facile and sustainable strategy to produce ammonia (NH3) at ambient conditions. However, the low NH3 yield and Faradaic efficiency (FE) are still the main challenges due to the competitive hydrogen evolution reaction (HER). Herein, a three-phase electrocatalyst through in situ fabrication of Au nanoparticles (NPs) located on hydrophobic carbon fiber paper (Au/o-CFP) is designed. The hydrophobic CFP surface facilitates efficient three-phase contact points (TPCPs) for N2 (gas), electrolyte (liquid), and Au NPs (solid). Thus, concentrated N2 molecules can contact the electrocatalyst surface directly, inhibiting the HER since the lowered proton concentration and overall enhancing NRR. The three-phase Au/o-CFP electrocatalyst presents an excellent NRR performance with high NH3 yield rate of 40.6 µg h-1 mg-1 at -0.30 V and great FE of 31.3% at -0.10 V versus RHE (0.1 m Na2SO4). The N2-bubble contact angle result and cyclic voltammetry analysis confirm that the hydrophobic interface has a relatively strong interaction with N2 bubble for enhanced NRR and weak electrocatalytic activity for HER. Significantly, the three-phase Au/o-CFP exhibits excellent stability with a negligible fluctuation of NH3 yield and FE in seven-cycle test. This work provides a new strategy for improving NRR and simultaneously inhibiting HER.

Lithium Iron Oxide (LiFeO2) for Electroreduction of Dinitrogen to Ammonia

Electrochemical nitrogen fixation offers a promising route for sustainable NH₃ production, while the rational design of effective and durable electrocatalysts is urgently required for an effective nitrogen reduction reaction (NRR) process. Herein, we explore lithium iron oxide (LiFeO₂) as a potential NRR catalyst. The developed LiFeO₂/reduced graphene oxide (rGO) delivered a combination of both a high NH₃ yield (40.5 μg h^-1 mg^-1) and high Faradaic efficiency (16.4%), exceeding those of nearly all the previously reported Li- and Fe-based catalysts. Theoretical computations showed that Fe and Li atoms on the LiFeO₂ (111) facet synergistically activated N₂ while Fe atoms served as the key active centers. Meanwhile, the undesired HER can be well impeded on both Fe and Li atoms to enable a high NRR selectivity.

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.

A Pd/MnO2 Electrocatalyst for Nitrogen Reduction to Ammonia under Ambient Conditions

Electrochemical ammonia synthesis, which is an alternative approach to the Haber–Bosch process, has attracted the attention of researchers because of its advantages including mild working conditions, environmental protection, and simple process. However, the biggest problem in this field is the lack of high-performance catalysts. Here, we report high-efficiency electroreduction of N2 to NH3 on γ-MnO2-supported Pd nanoparticles (Pd/γ-MnO2) under ambient conditions, which exhibits excellent catalytic activity with an NH3 yield rate of 19.72 μg·mg−1Pd h−1 and a Faradaic efficiency of 8.4% at −0.05 V vs. the reversible hydrogen electrode (RHE). X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization shows that Pd nanoparticles are homogeneously dispersed on the γ-MnO2. Pd/γ-MnO2 outperforms other catalysts including Pd/C and γ-MnO2 because of its synergistic catalytic effect between Pd and Mn.

Iron oxides nanobelt arrays rooted in nanoporous surface of carbon tube textile as stretchable and robust electrodes for flexible supercapacitors with ultrahigh areal energy density and remarkable cycling-stability

We report a significant advance toward the rational design and fabrication of stretchable and robust flexible electrodes with favorable hierarchical architectures constructed by homogeneously distributed α-Fe₂O₃ nanobelt arrays rooted in the surface layer of nanoporous carbon tube textile (NPCTT). New insight into alkali activation assisted surface etching of carbon and in-situ catalytic anisotropic growth is proposed, and is experimentally demonstrated by the synthesis of the Fe₂O₃ nanobelt arrays/NPCTT. The Fe₂O₃/NPCTT electrode shows excellent flexibility and great stretchability, especially has a high specific areal capacitance of 1846 mF cm⁻² at 1 mA cm⁻² and cycling stability with only 4.8% capacitance loss over 10,000 cycles at a high current density of 20 mA cm⁻². A symmetric solid-state supercapacitor with the Fe₂O₃/NPCTT achieves an operating voltage of 1.75 V and a ultrahigh areal energy density of 176 µWh cm⁻² (at power density of 748 µW cm⁻²), remarkable cycling stability, and outstanding reliability with no capacity degradation under repeated large-angle twisting. Such unique architecture improves both mechanical robustness and electrical conductivity, and allows a strong synergistic attribution of Fe₂O₃ and NPCTT. The synthetic method can be extended to other composites such as MnO nanosheet arrays/NPCTT and Co₃O₄ nanowire arrays/NPCTT. This work opens up a new pathway to the design of high-performance devices for wearable electronics.

Ambient Ammonia Electrosynthesis: Current Status, Challenges, and Perspectives

Ammonia (NH₃ ) electrosynthesis from atmospheric nitrogen (N₂ ) and water is emerging as a promising alternative to the energy-intensive Haber-Bosch process; however, such a process is difficult to perform due to the inherent inertness of N₂ molecules together with low solubility in aqueous solutions. Although many active electrocatalysts have been used to electrocatalyze the N₂ reduction reaction (NRR), unsatisfactory NH₃ yields and lower Faraday efficiency are still far from practical industrial production, and thus, considerable research efforts are being devoted to address these problems. Nevertheless, most reports still mainly focus on the preparation of electrocatalysts and largely ignore a summary of optimization-modification strategies for the NRR. In this review, a general introduction to the NRR mechanism is presented to provide a reasonable guide for the design of highly active catalysts. Then, four categories of NRR electrocatalysts, according to chemical compositions, are surveyed, as well as several strategies for promoting the catalytic activity and efficiency. Later, strategies for developing efficient N₂ fixation systems are discussed. Finally, current challenges and future perspectives in the context of the NRR are highlighted. This review sheds some light on the development of highly efficient catalytic systems for NH₃ synthesis and stimulates research interests in the unexplored, but promising, research field of the NRR.

Bismuth-Based Free-Standing Electrodes for Ambient-Condition Ammonia Production in Neutral Media

Electrocatalytic nitrogen reduction reaction is a carbon-free and energy-saving strategy for efficient synthesis of ammonia under ambient conditions. Here, we report the synthesis of nanosized Bi2O3 particles grown on functionalized exfoliated graphene (Bi2O3/FEG) via a facile electrochemical deposition method. The obtained free-standing Bi2O3/FEG achieves a high Faradaic efficiency of 11.2% and a large NH3 yield of 4.21 ± 0.14 [Formula: see text] h-1 cm-2 at - 0.5 V versus reversible hydrogen electrode in 0.1 M Na2SO4, better than that in the strong acidic and basic media. Benefiting from its strong interaction of Bi 6p band with the N 2p orbitals, binder-free characteristic, and facile electron transfer, Bi2O3/FEG achieves superior catalytic performance and excellent long-term stability as compared with most of the previous reported catalysts. This study is significant to design low-cost, high-efficient Bi-based electrocatalysts for electrochemical ammonia synthesis.

A Generalized Surface Chalcogenation Strategy for Boosting the Electrochemical N2 Fixation of Metal Nanocrystals

Electrocatalytic nitrogen reduction reaction (NRR) is a promising process relative to energy-intensive Haber-Bosch process. While conventional electrocatalysts underperform with sluggish paths, achieving dissociation of N₂ brings the key challenge for enhancing NRR. This study proposes an effective surface chalcogenation strategy to improve the NRR performance of pristine metal nanocrystals (NCs). Surprisingly, the NH₃ yield and Faraday efficiency (FE) (175.6 ± 23.6 mg h^-1 g^-1 _Rh and 13.3 ± 0.4%) of Rh-Se NCs is significantly enhanced by 16 and 15 times, respectively. Detailed investigations show that the superior activity and high FE are attributed to the effect of surface chalcogenation, which not only can decrease the apparent activation energy, but also inhibit the occurrence of the hydrogen evolution reaction (HER) process. Theoretical calculations reveal that the strong interface strain effect within core@shell system induces a critical redox inversion, resulting in a rather low valence state of Rh and Se surface sites. Such strong correlation indicates an efficient electron-transfer minimizing NRR barrier. Significantly, the surface chalcogenation strategy is general, which can extend to create other NRR metal electrocatalysts with enhanced performance. This strategy open a new avenue for future NH₃ production for breakthrough in the bottleneck of NRR.

Computational Design of Transition Metal Single-Atom Electrocatalysts on PtS2 for Efficient Nitrogen Reduction

Electrocatalytic nitrogen reduction is promising to serve as a sustainable and environmentally friendly strategy to achieve ammonia production. Single-atom catalysts (SACs) hold great promise to convert N₂ into NH₃ because of the unique molecular catalysis property and ultrahigh atomic utilization ratio. Here, we demonstrate a universal computational design principle to assess the N₂ reduction reaction (NRR) performance of SACs anchored on a monolayer PtS₂ substrate (SACs-PtS₂). Our density functional theory simulations unveil that the barriers of the NRR limiting potential step on different SAC centers are observed to be linearly correlated to the integral of unoccupied d states (UDSs) of SACs. As a result, the Ru SAC-PtS₂ catalyst with the largest number of UDSs exhibits a much lower barrier of the limiting step than those of other SACs-PtS₂ catalysts and the Ru(0001) benchmark. Our work bridges the apparent NRR activity and intrinsic electronic structure of SAC centers and offers effective guidance to screen and design efficient SACs for the electrochemical NRR process.

Synergistically Coupling Black Phosphorus Quantum Dots with MnO2 Nanosheets for Efficient Electrochemical Nitrogen Reduction Under Ambient Conditions

The electrochemical nitrogen reduction reaction (NRR) is a promising strategy of nitrogen fixation into ammonia under ambient conditions. However, the development of electrochemical NRR is highly bottlenecked by the expensive noble metal catalysts. As a representative 2D nonmetallic material, black phosphorus (BP) has the valence electron structure similar to nitrogen, which can effectively adsorb the inactive nitrogen molecule and activate its triple bond. In addition, the relatively weak hydrogen adsorption can restrict the competitive and vigorous hydrogen evolution reaction. Herein, ultrafine BP quantum dots (QDs) are prepared via liquid-phase exfoliation and then assembled on catalytically active MnO₂ nanosheets through van der Waals interactions. The obtained BP QDs/MnO₂ catalyst demonstrates admirable synergetic effects in electrochemical NRR. The monodisperse BP QDs providing major activity manifest excellent ammonia production steadily with high selectivity, which benefits from the robust confinement of the BP QDs on the wrinkled MnO₂ nanosheets with decent activity. A high ammonia yield rate of 25.3 µg h^-1 mg_cat. ^-1 and faradic efficiency of 6.7% can be achieved at -0.5 V (vs RHE) in 0.1 m Na₂ SO₄ electrolyte, which are dramatically superior to either component. The isotopic labelling and other control tests further exclude the external contamination possibility and attest the genuine activity.

Recent advancement in the electrocatalytic synthesis of ammonia

Ammonia can not only be used as an active nitrogen component of nitrogen fertilizers, fibers, explosives, etc., but also provides a high energy density and carbon free energy carrier. Currently, ammonia is industrially synthesized by the Haber Bosch process at high temperature and high pressure, which results in high energy loss and a serious greenhouse effect. The electrocatalytic nitrogen reduction reaction (NRR) is a sustainable and environmentally friendly strategy for the synthesis of ammonia. Although lots of electrocatalysts have been developed for this reaction, further breakthroughs are needed in catalytic activity, selectivity and Faraday efficiency to meet the large-scale commercial demand. In this review, the recent advance in NRR electrocatalysis is thoroughly commented on. Different kinds of electrocatalysts used in ammonia synthesis (including single atom catalysts, metal oxide catalysts, nanocomposite catalysts, and metal free catalysts) are introduced. The reaction mechanism of the NRR is discussed in detail. The structure-function relationship and efficient strategies to improve the ammonia yield are clearly discussed. The effect of the electronic structure and morphology of catalysts on the selectivity of the NRR is highlighted. The research directions and perspectives on the further development of more efficient electrocatalysts for the NRR are provided.

Atomic Modulation, Structural Design, and Systematic Optimization for Efficient Electrochemical Nitrogen Reduction

Ammonia (NH3) is a pivotal precursor in fertilizer production and a potential energy carrier. Currently, ammonia production worldwide relies on the traditional Haber-Bosch process, which consumes massive energy and has a large carbon footprint. Recently, electrochemical dinitrogen reduction to ammonia under ambient conditions has attracted considerable interest owing to its advantages of flexibility and environmental friendliness. However, the biggest challenge in dinitrogen electroreduction, i.e., the low efficiency and selectivity caused by poor specificity of electrocatalysts/electrolytic systems, still needs to be overcome. Although substantial progress has been made in recent years, acquiring most available electrocatalysts still relies on low efficiency trial-and-error methods. It is thus imperative to establish some critical guiding principles for nitrogen electroreduction toward a rational design and accelerated development of this field. Herein, a basic understanding of dinitrogen electroreduction processes and the inherent relationships between adsorbates and catalysts from fundamental theory are described, followed by an outline of the crucial principles for designing efficient electrocatalysts/electrocatalytic systems derived from a systematic evaluation of the latest significant achievements. Finally, the future research directions and prospects of this field are given, with a special emphasis on the opportunities available by following the guiding principles.

Theoretical screening of efficient single-atom catalysts for nitrogen fixation based on a defective BN monolayer

The electrocatalytic reduction of naturally abundant N2 to NH3 is an attractive approach to replace the Haber-Bosch nitrogen-fixation process that causes enormous energy consumption and greenhouse gas emissions. However, designing high-performance catalysts toward the electrocatalytic N2 reduction reaction (eNRR) remains one of the greatest challenges in this area. Herein, high-throughput screening of catalysts for the NRR among a series of transition metal atoms supported on a defective hexagonal boron nitride (h-BN) nanosheet is performed through spin-polarized density functional theory (DFT) computations. Strikingly, among the 18 candidates, the V/Tc atom anchored on a defective h-BN monolayer (V@BN and Tc@BN) showed good NRR activity with relatively low onset potentials. Particularly, V@BN was found to exhibit outstanding catalytic activity for the NRR via an enzymatic pathway with an extremely low overpotential of 0.25 V. The value is significantly lower than that on the Ru (0001) stepped surface that has the best NRR catalytic performance among bulk metal catalysts. The novel NRR activity of V@BN is attributed to the enhanced electrical conductivity due to V-doping, the "donation-backdonation" process for N2 activation, and the highly centralized spin-polarization on the V atom. This work not only provides a quite promising catalyst for the NRR but also provides new insights for the rational design of single-atom NRR catalysts.

S-Doped three-dimensional graphene (S-3DG): a metal-free electrocatalyst for the electrochemical synthesis of ammonia under ambient conditions

3D-graphene provide abundant space for N₂, and the carbon–sulfur bonds provides a continuous supply of electrons for N₂ reduction. A remarkably large NH₃ yield of 38.81 μg_NH3 mg_cat⁻¹ h⁻¹ and FE of 7.72% for N₂ reduction was obtained.

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

Industrial production of NH₃ heavily depends on the conventional Haber-Bosch process under rigorous conditions with a large amount of energy consumption and carbon emissions. Electrocatalysis exhibits an intriguing prospect for the N₂ reduction reaction (NRR) at ambient conditions. In this case, a high-efficiency and low-cost catalyst is paramount. In this letter, Cr₃C₂ nanoparticles and carbon nanofiber composite (Cr₃C₂@CNF) are proposed as a noble-metal-free NRR electrocatalyst for converting N₂ to NH₃ with an excellent selectivity. The optimal Faradic efficiency and NH₃ yield rate achieved are as high as 8.6% and 23.9 μg h^-1 mg_cat.^-1 at -0.3 V vs reversible hydrogen electrode in 0.1 M HCl, respectively. Theoretical calculations show a low reaction barrier of merely 0.53 eV in the enzymatic route for this catalyst.

Facile Preparation of Carbon Shells-Coated O-Doped Molybdenum Carbide Nanoparticles as High Selective Electrocatalysts for Nitrogen Reduction Reaction under Ambient Conditions

Electrochemical nitrogen reduction reaction (NRR) has been considered as a promising alternative to the traditional Haber-Bosch process for the preparation of ammonia (NH₃) under ambient conditions. The development of cost-effective electrocatalysts with suppressive activity for hydrogen evolution reaction is critical for improving the efficiency of NRR. Herein, oxygen-containing molybdenum carbides (O-MoC) embedded in nitrogen-doped carbon layers (N-doped carbon) can be easily fabricated by pyrolyzing the chelate of dopamine and molybdate. A rate of NH₃ formation of 22.5 μg·h^-1·mg_cat^-1 is obtained at -0.35 V versus reversible hydrogen electrode with a high faradaic efficiency of 25.1% in 0.1 mM HCl + 0.5 M Li₂SO₄. Notably, the synthesized O-MoC@NC-800 also exhibits high selectivity (no formation of hydrazine) and electrochemical stability. The moderate electron structure induced by the interaction between O-MoC and N-doped carbon shells can effectively weaken the activity of hydrogen evolution reaction and increase the faradaic efficiency of NRR. Additionally, by applying the in situ Fourier transform infrared spectroscopy, an associative reaction pathway is proposed on O-MoC@NC-800. This work provides new insights into the rational design of carbon-encapsulated metal nanoparticles as efficient catalysts for NRR at ambient conditions.