Identification of FeN4 as an Efficient Active Site for Electrochemical N2 Reduction

Ultrafast joule-heating-assisted O, N dual-doping of unfunctionalized carbon enhances Ru nanoparticle-catalyzed hydrogen production

The development of a rapid and convenient strategy to regulate the surface microenvironment of inert carbon supports, along with the physicochemical properties of their supported metal nanoparticles, is essential for enhancing catalytic performance. In this study, we describe a straightforward and efficient solid-state microwave method that utilizes a household microwave oven to achieve the co-doping of oxygen and nitrogen in unfunctionalized carbon black (ONCB) using urea as a nitrogen source. The microwave solid-state treatment of commercial carbon black (CB) with urea not only introduces a significant number of heteroatomic functional groups but also substantially increases the pore size and pore volume of the matrix. These enhancements facilitate the uniform growth and dispersion of ultrafine Ru nanoparticles on the surface of ONCB. Consequently, the Ru/ONCB catalyst provides abundant catalytic active sites and mass transfer channels, thereby improving catalytic performance for hydrogen evolution from ammonia borane hydrolysis (ABH). The turnover frequency of Ru/ONCB for ABH reaches 4529 ± 238 min-1 (determined based on Ru dispersion), surpassing a range of analogues and many previously reported carbon-supported Ru catalysts. This study presents a simple and rapid strategy to regulate the surface microenvironment of unfunctionalized carbon support, thereby enhancing the catalytic performance of its supported metal nanoparticles for catalytic hydrogen generation.

Electronic Descriptor to Identify the Activity of SACs for E-NRR and Effect of BF3 as Electrolyte Ion

Electrochemical nitrogen reduction reaction (e-NRR) is an eco-friendly alternative approach to generate ammonia under ambient conditions, with very low power supply. But, developing of an efficient catalyst by suppressing parallel hydrogen evolution reaction as well as avoiding the catalysts poisoning either by hydrogen or electrolyte ion is an open question. So, in order to screen the single atom catalysts (SACs) for the e-NRR, we proposed a descriptor-based approach using density functional theory (DFT) based calculations. We investigated total 24 different SACs of types TM-Pc, TM-N3C1, TM-N2C2, TM-NC3 and TM-N4, considering transition metal (TM). We have considered mainly BF3 ion to understand the role of electrolyte and extended the study for four more electrolyte ions, Cl, ClO4, SO4, OH. Herein, to predict catalytic activity for a given catalyst we have tested 16 different electronic parameters. Out of those, electronic parameter dxz↓ occupancy, identified as electronic descriptor, is showing an excellent linear correlation with catalytic activity (R2=0.86). Furthermore, the selectivity of e-NRR over HER is defined by using an energy parameter ▵G*H-▵G*NNH. Further, the electronic descriptor (dxz↓ occupancy) can be used to predict promising catalysts for e-NRR, thus reducing the efforts on designing future single atom catalysts (SACs).

Repairable body-centered cubic Fe0 anchoring on porous hollow nitrogen-doped carbon spheres with adjusting electron distribution for efficient electrocatalytic ammonia synthesis

Electrocatalytic nitrogen reduction reaction (ENRR) is a promising and efficient method for ammonia production. However, ENRR is restricted by the adsorption and activation of N2. Herein, an efficient nitrogen reduction reaction (NRR) electrocatalyst loaded with zero valent iron (ZVI) particles onto porous nitrogen-doped carbon (NC) hollow spheres is reported. The optimal Fe@10N3C-950 exhibits excellent performance with high ammonia (NH3) yield (152.28 µg h-1 mgcat-1) and Faradaic efficiency (FE, 54.55 %) at - 0.3 V (versus reversible hydrogen electrode, vs. RHE). Bader charge shows that the adsorbed N2 acquires more electrons from Fe sites with body-centered cubic (BCC) structure to better activate N2. Moreover, i-t experiments are performed before electrocatalytic NH3 production to effectively eliminate the effect of oxidation on ZVI and thus, maintain high ENRR activity for Fe@10N3C-950. Theoretical calculations indicate that nitrogen doping not only reduces the Gibbs free energy of rate determining step (RDS), but the BCC-structured Fe can also decrease the energy barriers of N2 activation and RDS.

Ambient electrosynthesis of urea with nitrate and carbon dioxide over a CuRu alloy catalyst

Urea synthesis under mild conditions starting from the electrocatalytic coupling of carbon dioxide (CO2) and nitrate represents a promising alternative experimentally to conquer the huge energy consumption in the industrial Haber-Bosch process. Herein, an electrocatalyst consisting of CuRu alloy nanoparticles on carbonized cellulose (CuRu-CBC) is designed and realizes the urea yield rate of 394.85 ± 16.19 μg h-1 mgcat-1 and an ultrahigh faradaic efficiency (FE) of 68.94 ± 3.05% at -0.55 V (vs. RHE) under ambient conditions. Further XAS analyses indicated that the favored internal electron transfer between Cu and Ru dual active sites significantly improved the C-N coupling activity. Various characterizations, including in situ ATR-SEIRAS and DEMS analysis highlighted the favorable generation of key intermediates *CO and *NH, making CuRu-CBC a promising catalyst for urea synthesis.

Electrocatalytic nitrogen reduction to ammonia at low potential using a phenalenyl-based iron(III) complex

In recent years, electrochemical nitrogen reduction reaction (ENRR) has emerged as a promising alternative for ammonia production in a clean and energy-efficient manner. We reported the remarkable performance of a transition metal-based electrocatalyst, Fe(III)(PLY)3 (where PLY-H = 9-hydroxyphenalenone), for electrochemical NRR. In an acidic electrolyte, Fe(PLY)3 catalyst demonstrates remarkable performance, achieving a high faradaic efficiency (FE) of 43.4% and an impressive ammonia (NH3) yield rate of 99.7 μg h-1 mgcat-1 at -0.2 V compared to a reversible hydrogen electrode (RHE).

Electrocatalytic nitrogen reduction to ammonia by atomically precise Cu6 nanoclusters supported on graphene oxide

The electrocatalytic nitrogen reduction reaction (NRR) enables the production of ammonia by the use of renewable energy, providing a direct method for nitrogen fixation. Nevertheless, the NRR process under ambient conditions is often impeded by inertness of N2 and the occurrence of hydrogen evolution as a byproduct in aqueous electrolytes, resulting in a diminished reaction rate and reduced efficiency. In this study, we synthesized Cu6(SMPP)6 nanoclusters (Cu6 NCs for short) and immobilized them on graphene oxide (GO) to investigate their electrocatalytic nitrogen reduction reaction (ENRR) using an H-cell setup. The GO-supported Cu6 NCs exhibit enhanced catalysis with a high NH3 yield rate of 4.8 μg h-1 cm-2 and a high faradaic efficiency up to 30.39% at -1.1 V. Quantum chemistry calculations reveal that the Cu6S6 cluster on GO support facilitates the N2 adsorption and NN bond activation with a surmountable energy barrier for the potential-determining step (N2* → NNH*).

Modulating the electronic structure of ion phthalocyanine-based molecular catalysts for electrocatalytic nitrogen reduction: a DFT study

The highly localized Fe d orbital in ion phthalocyanine (FePc)-based molecular catalysts significantly hinders their electrocatalytic nitrogen reduction reaction (eNRR) performance. Herein, we theoretically designed a series of FePc-based molecules with adjacent metal phthalocyanine sites to form an asymmetric delocalized electronic structure on Fe centers, promoting the catalytic activity and lowering the overpotential of the eNRR, as well as suppressing the hydrogen evolution reaction (HER) side reaction.

Theoretical exploration of the nitrogen fixation mechanism of two-dimensional dual-metal FeTM@GY (TM = Fe, Mo, Co, and V) electrocatalysts

In contrast to the energy-consuming Haber-Bosch process, ammonia synthesis by electrocatalysis under ambient conditions is an efficient and environmentally friendly method. In this work, through first principles calculations, the potential of four dual-atom FeTM (TM = Fe, Mo, Co, and V) anchored graphyne (FeTM@GY) as efficient nitrogen reduction reaction (NRR) catalysts is systematically investigated. Among them, FeMo@GY is the most promising, with excellent NRR catalytic activity, high ability to suppress the competing hydrogen evolution reaction (HER), and good stability. Moreover, NRR prefers the maximum pathway with the calculated onset potentials of -0.27 V for FeMo@GY. This work not only suggests that FeMo@GY holds great promise as an efficient, low-cost, and stable dual-atom catalyst for NRR but also further provides a guiding idea for the design of efficient NRR catalysts.

Modulating the Electronic Structure of Cobalt in Molecular Catalysts via Coordination Environment Regulation for Highly Efficient Heterogeneous Nitrate Reduction

Ammonia (NH3) is pivotal in modern industry and represents a promising next-generation carbon-free energy carrier. Electrocatalytic nitrate reduction reaction (eNO3RR) presents viable solutions for NH3 production and removal of ambient nitrate pollutants. However, the development of eNO3RR is hindered by lacking the efficient electrocatalysts. To address this challenge, we synthesized a series of macrocyclic molecular catalysts for the heterogeneous eNO3RR. These materials possess different coordination environments around metal centers by surrounding subunits. Consequently, electronic structures of the active centers can be altered, enabling tunable activity towards eNO3RR. Our investigation reveals that metal center with an N2(pyrrole)-N2(pyridine) configuration demonstrates superior activity over the others and achieves a high NH3 Faradaic efficiency (FE) of over 90 % within the tested range, where the highest FE of approximately 94 % is obtained. Furthermore, it achieves a production rate of 11.28 mg mgcat -1 h-1, and a turnover frequency of up to 3.28 s-1. Further tests disclose that these molecular catalysts with diverse coordination environments showed different magnetic moments. Theoretical calculation results indicate that variated coordination environments can result in a d-band center variation which eventually affects rate-determining step energy and calculated magnetic moments, thus establishing a correlation between electronic structure, experimental activity, and computational parameters.

Interstitial Boron-Modulated Porous Pd Nanotubes for Ammonia Electrosynthesis

Electrochemical conversion of nitrogen into ammonia at ambient conditions as a sustainable approach has gained significant attention, but it is still extremely challenging to simultaneously obtain a high faradaic efficiency (FE) and NH3 yield. In this work, the interstitial boron-doped porous Pd nanotubes (B-Pd PNTs) are constructed by combining the self-template reduction method with boron doping. Benefiting from distinctive one-dimensional porous nanotube architectonics and the incorporation of the interstitial B atoms, the resulting B-Pd PNTs exhibit high NH3 yield (18.36 μg h-1 mgcat.-1) and FE (21.95%) in neutral conditions, outperforming the Pd/PdO PNTs (10.4 μg h-1 mgcat.-1 and 8.47%). The present study provides an attractive method to enhance the efficiency of the electroreduction of nitrogen into ammonia by incorporating interstitial boron into porous Pd-based catalysts.

Development of Electrochemical Cells and Their Application for Spatially Resolved Analysis Using a Multitechnique Approach: From Conventional Experiments to X-Ray Nanoprobe Beamlines

Real (electro)catalysts are often heterogeneous, and their activity and selectivity depend on the properties of specific active sites. Therefore, unveiling the so-called structure-activity relationship is essential for a rational search for better materials and, consequently, for the development of the field of (electro-)catalysis. Thus, spatially resolved techniques are powerful tools as they allow us to characterize and/or measure the activity and selectivity of different regions of heterogeneous catalysts. To take full advantage of that, we have developed spectroelectrochemical cells to perform spatially resolved analysis using X-ray nanoprobe synchrotron beamlines and conventional pieces of equipment. Here, we describe the techniques available at the Carnaúba beamline at the Sirius-LNLS storage ring, and then we show how our cells enable obtaining X-ray (XRF, XRD, XAS, etc.) and vibrational spectroscopy (FTIR and Raman) contrast images. Through some proof-of-concept experiments, we demonstrate how using a multi-technique approach could render a complete and detailed analysis of an (electro)catalyst overall performance.

Recent Advances in Engineering of 2D Materials-Based Heterostructures for Electrochemical Energy Conversion

2D materials, such as graphene, transition metal dichalcogenides, black phosphorus, layered double hydroxides, and MXene, have exhibited broad application prospects in electrochemical energy conversion due to their unique structures and electronic properties. Recently, the engineering of heterostructures based on 2D materials, including 2D/0D, 2D/1D, 2D/2D, and 2D/3D, has shown the potential to produce synergistic and heterointerface effects, overcoming the inherent restrictions of 2D materials and thus elevating the electrocatalytic performance to the next level. In this review, recent studies are systematically summarized on heterostructures based on 2D materials for advanced electrochemical energy conversion, including water splitting, CO2 reduction reaction, N2 reduction reaction, etc. Additionally, preparation methods are introduced and novel properties of various types of heterostructures based on 2D materials are discussed. Furthermore, the reaction principles and intrinsic mechanisms behind the excellent performance of these heterostructures are evaluated. Finally, insights are provided into the challenges and perspectives regarding the future engineering of heterostructures based on 2D materials for further advancements in electrochemical energy conversion.

Enhancing Sulfur Redox Conversion of Active Iron Sites by Modulation of Electronic Density for Advanced Lithium-Sulfur Battery

Despite lithium-sulfur (Li-S) batteries possessing ultrahigh energy density as great promising energy storage devices, the suppressing shuttle effect and improving sulfur redox reaction (SROR) are vital for their practical application. Developing high-activity electrocatalysts for enhancing the SROR kinetics is a major challenge for the application of Li-S batteries. Herein, single-molecule iron phthalocyanine species are anchored on the N and P dual-doped porous carbon nanosheets (Fe-NPPC) via axial Fe-N coordination to optimize the electronic structure of active centers. The Fe-NPPC can promote the catalytic conversion of polysulfides by modulation of the electronic density in active moieties, endowing the Li-S battery with a high reversible capacity of 1023 mAh g-1 at 1 C as well as an ultralow capacity decay of 0.035% per cycle over 1500 cycles. Even with a high sulfur loading of 7.1 mg cm-2 , the Li-S battery delivers a high areal capacity of 4.8 mAh cm-2 after 150 cycles at 0.2 C. With further increasing the sulfur loading to 9.2 mg cm-2 , an excellent areal capacity of up to 9.3 mAh cm-2 is obtained at 0.1 C.

Atomic Insights into Synergistic Nitroarene Hydrogenation over Nanodiamond-Supported Pt1 -Fe1 Dual-Single-Atom Catalyst

Fundamental understanding of the synergistic effect of bimetallic catalysts is of extreme significance in heterogeneous catalysis, but a great challenge lies in the precise construction of uniform dual-metal sites. Here, we develop a novel method for constructing Pt1 -Fe1 /ND dual-single-atom catalyst, by anchoring Pt single atoms on Fe1 -N4 sites decorating a nanodiamond (ND) surface. Using this catalyst, the synergy of nitroarenes selective hydrogenation is revealed. In detail, hydrogen is activated on the Pt1 -Fe1 dual site and the nitro group is strongly adsorbed on the Fe1 site via a vertical configuration for subsequent hydrogenation. Such synergistic effect decreases the activation energy and results in an unprecedented catalytic performance (3.1 s-1 turnover frequency, ca. 100 % selectivity, 24 types of substrates). Our findings advance the applications of dual-single-atom catalysts in selective hydrogenations and open up a new way to explore the nature of synergistic catalysis at the atomic level.

Tuning the Site-to-Site Interaction of Heteronuclear Diatom Catalysts MoTM/C2N (TM = 3d Transition Metal) for Electrochemical Ammonia Synthesis

Ammonia (NH₃) synthesis is one of the most important catalytic reactions in energy and chemical fertilizer production, which is of great significance to the sustainable development of society and the economy. The electrochemical nitrogen reduction reaction (eNRR), especially when driven by renewable energy, is generally regarded as an energy-efficient and sustainable process to synthesize NH₃ in ambient conditions. However, the performance of the electrocatalyst is far below expectations, with the lack of a high-efficiency catalyst being the main obstacle. Herein, by means of comprehensive spin-polarized density functional theory (DFT) computations, the catalytic performance of MoTM/C₂N (TM = 3d transition metal) for use in eNRR was systematically evaluated. Among the results, MoFe/C₂N can be considered the most promising catalyst due to its having the lowest limiting potential (-0.26 V) and high selectivity in the context of eNRR. Compared with its homonuclear counterparts, MoMo/C₂N and FeFe/C₂N, MoFe/C₂N can balance the first protonation step and the sixth protonation step synergistically, showing outstanding activity regarding eNRR. Our work not only opens a new door to advancing sustainable NH₃ production by tailoring the active sites of heteronuclear diatom catalysts but also promotes the design and production of novel low-cost and efficient nanocatalysts.

Accelerating ammonia synthesis in a membraneless flow electrolyzer through coupling ambient dinitrogen oxidation and water splitting

An electrochemical approach for ammonia production is successfully developed by coupling the anodic dinitrogen oxidation reaction (NOR) and cathodic hydrogen evolution reaction (HER) within a well-designed membraneless flow electrolyzer. The obtained reactor shows the preferential yield of ammonia over nitrogen oxides on the vanadium nitride catalyst surface. At an applied oxidation potential of 2.25 V versus the reversible hydrogen electrode (vs RHE), a promoted ammonia production rate and Faradaic efficiency (FE) were obtained with 9.9 mmol g^-1 h^-1 (0.029 mmol cm^-2 h^-1) and 4.8%, respectively. Besides, the negative affection of ammonia contamination is efficiently alleviated. Density functional theory calculations revealed that the thermodynamic energy needed to produce ammonia (-0.63 eV) is far lower than that of producing nitrogen oxide (0.96 eV) from hydrogenated nitrogen oxides [∗N₂OH] splitting, confirming the coupling of NOR and HER.

Tandem Electrocatalytic Nitrate Reduction to Ammonia on MBenes

We demonstrate the great feasibility of MBenes as a new class of tandem catalysts for electrocatalytic nitrate reduction to ammonia (NO₃ RR). As a proof of concept, FeB₂ is first employed as a model MBene catalyst for the NO₃ RR, showing a maximum NH₃ -Faradaic efficiency of 96.8 % with a corresponding NH₃ yield of 25.5 mg h^-1  cm^-2 at -0.6 V vs. RHE. Mechanistic studies reveal that the exceptional NO₃ RR activity of FeB₂ arises from the tandem catalysis mechanism, that is, B sites activate NO₃ ^- to form intermediates, while Fe sites dissociate H₂ O and increase *H supply on B sites to promote the intermediate hydrogenation and enhance the NO₃ ^- -to-NH₃ conversion.

Creating Highly Active Iron Sites in Electrochemical N2 Reduction by Fabricating Strongly-Coupled Interfaces

Electrochemical N_c reduction has been regarded as one of the most promising approaches to producing ammonia under mild conditions, but there are remaining pressing challenges in improving the reaction rate and efficiency. Herein, an unconventional galvanic replacement reaction is reported to fabricate a unique hierarchical structure composed of Fe₃ O₄ -CeO₂ bimetallic nanotubes covered by Fe₂ O₃ ultrathin nanosheets. Control experiments reveal that CeO₂ species play the essential role of stabilizer for Fe²⁺ cations. Compared with bare CeO₂ and Fe₂ O₃ nanotubes, the as-obtained Fe₂ O₃ /Fe₃ O₄ -CeO₂ possesses a remarkably enhanced NH₃ yield rate (30.9 µg h^-1 mg_cat ^-1 ) and Faradaic efficiency (26.3%). The enhancement can be attributed to the hierarchical feature that makes electrodes more easily to contact with electrolytes. More importantly, as verified by density functional theory calculations, the generation of Fe₂ O₃ -Fe₃ O₄ heterogeneous junctions can efficiently optimize the reaction pathways, and the energy barrier of the potential determining step (the *N₂ hydrogenates into *N*NH) is significantly decreased.

Harnessing Nickel Phthalocyanine-Based Electrochemical CNT Sponges for Ammonia Synthesis from Nitrate in Contaminated Water

Electrochemical reduction of nitrate to ammonia is of great interest in water treatment with regard to the conversion of contaminants to value-added products, which requires the development of advanced electrodes to achieve high selectivity, stability, and Faradaic efficiency (FE). Herein, nickel phthalocyanine was homogeneously doped into the fiber of a carbon nanotube (CNT) sponge, enabling the production of an electrode with high electrochemical double-layer capacitance (C_DL) and a large electrochemically active surface area (ECSA). The as-prepared NiPc-CNT sponge could achieve 97.6% nitrate removal, 88.4% ammonia selectivity, and 86.8% FE at a nitrate concentration of 50 mg-N L^-1 under an optimized potential of -1.2 V (vs Ag/AgCl). Meanwhile, the ammonia selectivity could be further improved at the high nitrate concentration. Density functional theory calculations showed that the exposure of Ni-N₄ active sites could effectively suppress the hydrogen evolution reaction and dinitrogen generation, enhancing the ammonia selectivity and Faradaic efficiency. Overall, this work sheds light on the conversion of nitrate to ammonia on the metal phthalocyanine-based electrode, offering a novel strategy for managing nitrate in wastewater.

Oxygen Functionalization-Induced Charging Effect on Boron Active Sites for High-Yield Electrocatalytic NH3 Production

Ammonia has been recognized as the future renewable energy fuel because of its wide-ranging applications in H₂ storage and transportation sector. In order to avoid the environmentally hazardous Haber-Bosch process, recently, the third-generation ambient ammonia synthesis has drawn phenomenal attention and thus tremendous efforts are devoted to developing efficient electrocatalysts that would circumvent the bottlenecks of the electrochemical nitrogen reduction reaction (NRR) like competitive hydrogen evolution reaction, poor selectivity of N₂ on catalyst surface. Herein, we report the synthesis of an oxygen-functionalized boron carbonitride matrix via a two-step pyrolysis technique. The conductive BNCO₍₁₀₀₀₎ architecture, the compatibility of B-2p_z orbital with the N-2p_z orbital and the charging effect over B due to the C and O edge-atoms in a pentagon altogether facilitate N₂ adsorption on the B edge-active sites. The optimum electrolyte acidity with 0.1 M HCl and the lowered anion crowding effect aid the protonation steps of NRR via an associative alternating pathway, which gives a sufficiently high yield of ammonia (211.5 μg h^-1 mg_cat^-1) on the optimized BNCO₍₁₀₀₀₎ catalyst with a Faradaic efficiency of 34.7% at - 0.1 V vs RHE. This work thus offers a cost-effective electrode material and provides a contemporary idea about reinforcing the charging effect over the secured active sites for NRR by selectively choosing the electrolyte anions and functionalizing the active edges of the BNCO₍₁₀₀₀₎ catalyst.

Iron-Based Nanocatalysts for Electrochemical Nitrate Reduction

Nitrate has a high level of stability and persistence in water, endangering human health and aquatic ecosystems. Due to its high reliability and efficiency, the electrochemical nitrate reduction reaction (NO₃ RR) is regarded as the best available option for mitigating excess nitrate in water and wastewater, especially for the removal of trace levels of nitrate. One of the most critical factors in the electrochemical reduction are the catalysts, which directly affect the reaction efficiency of nitrate removal. Iron-based nanocatalysts, which have the advantages of nontoxicity, wide availability, and low cost, have emerged as a promising electrochemical NO₃ RR material in recent years. This review covers major aspects of iron-based nanocatalysts for electrochemical NO₃ RR, including synthetic methods, structural design, performance enhancement, electrocatalytic nitrate reduction test, and reduction mechanism. The recent progress of iron-based nanocatalysts for electrochemical NO₃ RR and the mechanism of functional advantages for modified structures are reviewed from the perspectives of loading, doping, and assembly strategies, in order to realize the conversion from pollutant nitrate to harmless nitrogen or ammonia and other sustainable products. Finally, challenges and future directions for the development of low-cost and highly-efficient iron-based nanocatalysts are explored.

Sulfur-Coordinated Transition Metal Atom in Graphene for Electrocatalytic Nitrogen Reduction with an Electronic Descriptor

The adjacent chemical microenvironment of single metal atoms in heterogeneous catalysis is crucial to their chemical activity for various catalytic processes. Here, based on first-principles calculations, 25 single transition metal atom catalysts coordinated to sulfur species embedded in graphene (TM-S₄-G-SACs) are reported for nitrogen reduction under ambient condition. It shows that nine TM-S₄-G-SACs (TM = Mo, Sc, Cr, V, W, Ti, Nb, Mn, and Re) are promising nitrogen reduction catalysts with an optimal potential of -0.425 V. Meanwhile, 18 TM-S₄-G-SACs have better catalytic activity than those with nitrogen coordination. Particularly, the catalytic activity of TM-S₄-G-SACs and the adsorption energy of intermediate NH₂* conform to a volcano-type correlation, which can be described by a universal electronic descriptor φ, defined by the electronegativity of the metal, adjacent coordinated atoms, and the valence electron occupancy. The above findings suggest the potential of sulfur-coordinated single metal atoms as electrocatalytic nitrogen reduction catalysts and an applicable descriptor to achieve optimal performance.

Rational Design of Atomic Site Catalysts for Electrocatalytic Nitrogen Reduction Reaction: One Step Closer to Optimum Activity and Selectivity

The electrocatalytic nitrogen reduction reaction (NRR) has been one of the most intriguing catalytic reactions in recent years, providing an energy-saving and environmentally friendly alternative to the conventional Haber–Bosch process for ammonia production. However, the activity and selectivity issues originating from the activation barrier of the NRR intermediates and the competing hydrogen evolution reaction result in the unsatisfactory NH3 yield rate and Faradaic efficiency of current NRR catalysts. Atomic site catalysts (ASCs), an emerging group of heterogeneous catalysts with a high atomic utilization rate, selectivity, and stability, may provide a solution. This article undertakes an exploration and systematic review of a highly significant research area: the principles of designing ASCs for the NRR. Both the theoretical and experimental progress and state-of-the-art techniques in the rational design of ASCs for the NRR are summarized, and the topic is extended to double-atom catalysts and boron-based metal-free ASCs. This review provides guidelines for the rational design of ASCs for the optimum activity and selectivity for the electrocatalytic NRR.

Graphical Abstract

Rational design of atomic site catalysts (ASCs) for nitrogen reduction reaction (NRR) has both scientific and industrial significance. In this review, the recent experimental and theoretical breakthroughs in the design principles of transition metal ASCs for NRR are comprehensively discussed, and the topic is also extended to double-atom catalysts and boron-based metal-free ASCs.

Lewis acid-dominated aqueous electrolyte acting as co-catalyst and overcoming N2 activation issues on catalyst surface

The growing demands for ammonia in agriculture and transportation fuel stimulate researchers to develop sustainable electrochemical methods to synthesize ammonia ambiently, to get past the energy-intensive Haber-Bosch process. However, the conventionally used aqueous electrolytes limit N₂ solubility, leading to insufficient reactant molecules in the vicinity of the catalyst during electrochemical nitrogen reduction reaction (NRR). This hampers the yield and production rate of ammonia, irrespective of how efficient the catalyst is. Herein, we introduce an aqueous electrolyte (NaBF₄), which not only acts as an N₂-carrier in the medium but also works as a full-fledged "co-catalyst" along with our active material MnN₄ to deliver a high yield of NH₃ (328.59 μg h^-1 mg_cat^-1) at 0.0 V versus reversible hydrogen electrode. BF₃-induced charge polarization shifts the metal d-band center of the MnN₄ unit close to the Fermi level, inviting N₂ adsorption facilely. The Lewis acidity of the free BF₃ molecules further propagates their importance in polarizing the N≡N bond of the adsorbed N₂ and its first protonation. This push-pull kind of electronic interaction has been confirmed from the change in d-band center values of the MnN₄ site as well as charge density distribution over our active model units, which turned out to be effective enough to lower the energy barrier of the potential determining steps of NRR. Consequently, a high production rate of NH₃ (2.45 × 10^-9 mol s^-1 cm^-2) was achieved, approaching the industrial scale where the source of NH₃ was thoroughly studied and confirmed to be chiefly from the electrochemical reduction of the purged N₂ gas.

Computational Insight into TM-N x Embedded Graphene Bifunctional Electrocatalysts for Oxygen Evolution and Reduction Reactions

Due to the energy crisis, development of bifunctional electrocatalysts for both oxygen evolution and reduction reactions is highly demanding. In this study, we have systematically investigated the bifunctional activity of metal (Co/Rh/Ir) and N co-doped graphene systems with varying N-dopant concentrations (TM-N _x @G, x = 0, 2, 4) using first-principles calculations. Charge transfer from the metal sites to the adsorbed intermediates and the adsorption free energy of the intermediates play important roles to help understand the potential-determining step and overpotential values for oxygen evolution reaction (OER)/oxygen reduction reaction (ORR). A dual volcano plot for all the systems using a common descriptor ΔG _OH* has been constructed. We find that the systems having ΔG _OH* values in the range of 0.40-0.70 eV can act as bifunctional electrocatalysts. Our study not only highlights the importance of metal and non-metal co-doped graphene as bifunctional catalysts but also can serve as a promising strategy for the design of efficient OER/ORR electrocatalysts.

Accelerating electrochemically catalyzed nitrogen reductions using metalloporphyrin-mediated metal-nitrogen-doped carbon (M-N-C) catalysts

Herein, a series of transition metal coordinated metalloporphyrin-mediated M-N-C catalysts with single and dual metal atoms were prepared and characterized. Interestingly, these M-N-C catalysts exhibit accelerated N₂ reduction behaviors through electrochemical catalysis. At the potential of E = -0.4V (vs. RHE), the optimum catalyst Fe_0.95Ni_0.05TPP@rGO-800 shows excellent catalytic activity, and the NH₃ yield is 22.5 μg mg_cat^-1 h^-1, which is much higher than that of its single metal counterparts alone, and the faradaic efficiency is as high as 50.7%, which is better than those of most reported catalysts. These results provide an opportunity to further explore the efficient electrochemical synthesis of NH₃ from M-N-C materials in the future.

Atomically Dispersed Uranium Enables an Unprecedentedly High NH3 Yield Rate

The low NH₃ yield rate is a grand challenge for electrocatalytic N₂ reduction to NH₃. Herein, we report the first uranium single-atom catalyst (SAC) capable of catalyzing the electrochemical N₂ reduction reaction (NRR). The uranium SAC features a low limiting potential (<0.5 V) and near-zero free energy changes for N₂ adsorption and NH₃ desorption. The integration of these merits enables the uranium SAC to afford an unprecedentedly high NH₃ yield rate, 3-4 orders of magnitude higher than that of the Ru(0001) surface, which is widely recognized as an excellent NRR electrocatalyst. Further theoretical analysis reveals that the N₂ reduction catalyzed by the uranium SAC is synergistically regulated by the d and f electrons of atomic uranium. This work proposes a promising solution (that is, atomically dispersed uranium) to the daunting challenge associated with the low NH₃ yield rate, thus enabling the scalable production of NH₃ via electrochemical N₂ reduction.

Charge-Transfer Spectroscopy of Bisaxially Coordinated Iron(II) Phthalocyanines through the Prism of the Lever's EL Parameters Scale, MCD Spectroscopy, and TDDFT Calculations

The position of the experimentally observed (in the UV-vis and magnetic circular dichroism (MCD) spectra) low-energy metal-to-ligand charge-transfer (MLCT) band in low-spin iron(II) phthalocyanine complexes of general formula PcFeL₂, PcFeL'L″, and [PcFeX₂]^2- (L, L', or L″ are neutral and X^- is an anionic axial ligand) was correlated with the Lever's electrochemical E_L scale values for the axial ligands. The time-dependent density functional theory (TDDFT)-predicted UV-vis spectra are in very good agreement with the experimental data for all complexes. In the majority of compounds, TDDFT predicts that the first degenerate MLCT band that correlates with the MCD A-term observed between 360 and 480 nm is dominated by an e_g (Fe, d_π) → b_1u (Pc, π*) single-electron excitation (in traditional D_4h point group notation) and agrees well with the previous assignment discussed by Stillman and co-workers[ Inorg. Chem. 1994, 33, 573-583]. The TDDFT calculations also suggest a small energy gap for b_1u/b_2u (Pc, π*) orbital splitting and closeness of the MLCT₁ e_g (Fe, d_π) → b_1u (Pc, π*) and MLCT₂ e_g (Fe, d_π) → b_2u (Pc, π*) transitions. In the case of the PcFeL₂ complexes with phosphines as the axial ligands, additional degenerate charge-transfer transitions were observed between 450 and 500 nm. These transitions are dominated by a_2u (Pc + L, π) → e_g (Pc, π*) single-electron excitations and are unique for the PcFe(PR₃)₂ complexes. The energy of the phthalocyanine-based a_2u orbital has large axial ligand dependency and is the reason for a large energy deviation for B1 a_2u (Pc + L, π) → e_g (Pc, π*) transition. The energies of the axial ligand-to-iron, axial ligand-to-phthalocyanine, iron-to-axial ligand, and phthalocyanine-to-axial ligand charge-transfer transitions were discussed on the basis of TDDFT calculations.

A 3D FeOOH nanotube array: an efficient catalyst for ammonia electrosynthesis by nitrite reduction

Nitrite (NO₂^-) is a detrimental pollutant widely existing in groundwater sources, threatening public health. Electrocatalytic NO₂^- reduction settles the demand for removal of NO₂^- and is also promising for generating ammonia (NH₃) at room temperature. A nanotube array directly grown on a current collector not only has a large surface area, but also exhibits improved structural stability and accelerated electron transport. Herein, a self-standing FeOOH nanotube array on carbon cloth (FeOOH NTA/CC) is proposed as a highly active electrocatalyst for NO₂^--to-NH₃ conversion. As a 3D catalyst, the FeOOH NTA/CC is able to attain a surprising faradaic efficiency of 94.7% and a large NH₃ yield of 11937 μg h^-1 cm^-2 in 0.1 M PBS (pH = 7.0) with 0.1 M NO₂^-. Furthermore, this catalyst also displays excellent durability in cyclic and long-term electrolysis tests.

A FeCo2O4 nanowire array enabled electrochemical nitrate conversion to ammonia

Electrocatalytic nitrate (NO₃^-) reduction not only generates high-value ammonia (NH₃) but holds significant potential in the control of NO₃^- contaminants in natural environments. Here, a bimetallic FeCo₂O₄ spinel nanowire array grown on carbon cloth is proposed as an efficient electrocatalyst for the conversion of NO₃^- to NH₃ with a high faradaic efficiency of up to 95.9% and a large NH₃ yield of 4988 μg h^-1 cm^-2. Furthermore, it also exhibits excellent stability during 16 h electrolysis.

Descriptors and graphical construction for in silico design of efficient and selective single atom catalysts for the eNRR

Outline a screening protocol that uses density functional theory calculations to simultaneously optimize with respect to multiple criteria, thereby successfully identifying catalysts that are highly selective and also result in low overpotentials for ammonia production through eNRR.

Computational Screening of Single Transition−metal Atoms Anchored to g−C9N4 as Catalysts for N2 Reduction to NH3

Electrocatalytic nitrogen reduction reaction (NRR) is considered to be the most desirable strategy for ammonia production, but still faces many challenges in terms of high activity and high selectivity. Based...