Coordination Configuration Research Articles

Recent progress of iron-based electrocatalysts for nitrogen reduction reaction

• xyz • Tuning electronic structure improves Fe-based catalytic performance • Coordination effect and synergistic effect simultaneously module Fe-based catalysts • Combining DFT and experiments is a promising method to advance electrocatalysts. Ammonia is a critical feedstock for modern industry and agriculture, which is also a promising carbon-free energy carrier. Electrochemical nitrogen reduction reaction (ENRR) is a sustainable and efficient approach to synthesize ammonia under ambient reaction conditions. ENRR relies on the development of highly efficient electrocatalysts. So far various strategies have been applied to enhance the performance of catalysts including tuning the electronic structure, binding strength, coordination configuration as well as the introduction of multiple active sites and defects. In this review, the recent progress of Fe-based electrocatalysts and their coordination effect, synergistic effect and defect effect for nitrogen reduction reaction have been summarized. Our critical review focuses on the discussion of theoretical advancement, the performance origin of ENRR activity and selectivity of Fe-based electrocatalysts.

Engineering the First Coordination Shell of Single Zn Atoms via Molecular Design Strategy toward High-Performance Sodium-Ion Hybrid Capacitors.

Atomically dispersed Zn moieties are efficient active sites for accelerating the electrode kinetics of carbons for sodium-ion hybrid capacitors (SIHCs), but the low utilization and symmetric configuration of Zn single-atom greatly hamper the Na ion storage capability. Herein, a molecular design strategy is employed to synthesize high-density Zn single atoms with asymmetric Zn-N3 S coordination embedded in nitrogen/sulfur codoped carbon (Zn-N3 S-NSC). The key to this strategy lies in the Zn power-catalyzed condensation of trithiocyanuric acid molecules to generate S-doped g-C3 N4 , which can in situ coordinate with Zn sources to form Zn-N3 S moieties during pyrolysis. By virtue of the highly exposed Zn-N3 S moieties, Zn-N3 S-NSC presents ultrahigh reactivity, efficient electron transfer, and decreased ion diffusion barriers for SIHCs, rendering an impressive energy density of 215Whkg-1 and a maximum power density of 15625 Wkg-1 . Moreover, the pouch cell displays a high capacity of 279mAhg-1 after 4000cycles. This work provides a new avenue for the regulation of the coordination configuration of single metal atoms in carbons toward high-performance electrochemical energy technologies at the molecular level.

Atomically Precise Dinuclear Ni2 Active Site-Modified MOF-Derived ZnO@NC Heterojunction toward High-Performance N2 Photofixation

The development of heterogeneous catalysts with atom-dispersed active sites is essential to facilitate nitrogen (N2) activation for the N2 reduction reaction (NRR). However, it remains a major challenge to tune the coordination configuration of the metal centers to further accelerate the activation kinetics. Herein, an atomically precise dinuclear Ni2 site-modified metal–organic framework (MOF)-derived ZnO@NC heterojunction (ZnO@NC-Ni2) was developed for effective N2 photofixation under mild conditions. Moreover, advanced structural characterization indicates that the most active N-coordinated bimetallic site configurations are Ni2–N6, where two Ni1–N4 moieties are shared with two N atoms. Theoretical calculations further demonstrate that the binuclear Ni2 active sites of ZnO@NC-Ni2 could adjust the N2 adsorption configuration as the side-on bridging adsorption mode (denoted as “*N≡N*”), while the single metal Ni1 sites of ZnO@NC-Ni1 tend to form a terminal adsorption configuration with N2 (“*N≡N” type). As a result, the unique electronic structure of binuclear Ni2 active sites in ZnO@NC-Ni2 tends to proceed as an associative alternating pathway, thereby decreasing the activation energy barrier of the reaction procedure and favoring the photocatalytic NRR. The present study provides a perspective to probe the relationship between the coordination architecture of earth-abundant metal active centers and NRR activities.

Gram-scale synthesis and unraveling the activity origin of atomically dispersed Co-N4O sites toward superior electrocatalytic oxygen reduction

Exploring highly efficient nonprecious metal-based single-atom catalysts (SACs) toward the electrocatalytic oxygen reduction reaction (ORR) is critical for the sustainable development of ORR-related energy conversion and storage systems. However, the scalable synthesis, delicate regulation of the coordination environment and molecular-level elucidation of the electrocatalytic mechanism remain challenging. Herein, we report a facile gram-scale synthesis of atomically dispersed Co sites anchored on N-doped carbon nanofibers (noted as Co-SA@N-CNFs) via a reliable predesigned phenolic resin-mediated strategy for efficient oxygen reduction electrocatalysis. The local coordination configuration of the single-atomic Co sites is proposed as the Co-N4O moiety with one O atom in the axial direction perpendicular to the Co-N4 plane. Theoretical calculations uncover that, compared with the common Co-N4 single sites, the formation of Co-N4O configuration is beneficial to reduce the reaction energy barrier, adjust the bond length between the metal sites and the intermediates, and also increase the electric conductivity. Therefore, the Co-SA@N-CNFs demonstrated distinguished ORR activity, outstanding electrochemical stability and methanol tolerance in KOH electrolyte. Furthermore, when assembled in liquid and flexible solid-state rechargeable zinc-air batteries (ZABs), the Co-SA@N-CNFs-equipped ZABs exhibited higher power densities, larger specific capacities and extraordinary cycling performance, compared with the Pt/C-based ZABs. The simple and robust methodology for the mass production of SACs and the engineered coordination environment for performance optimization inspire the future design of a wide range of SACs for energy devices.

Water-soluble copper-based simulated enzyme: Biomimetic synthesis and activities in vitro

Two copper(II) complexes were obtained by solvothermal reaction induced by isonicotinic acid ligand through the study of natural simulated enzymes. The activity of superoxide dismutase (SOD) 1 and 2 was higher than that of most Cu-SOD mimics, and they have the activity of catechol analogues. It is found that SOD activity and catecholase activity are directly affected by the coordination configuration of the central metal atom and the ability of the complex to accept electrons. It is also responsible for the excellent enzyme activity of these two Cu-based complexes. The results of MTT assay show that complexes 1 and 2 have no significant inhibitory effect on cell proliferation. It has good application potential in the clinical biomedical field in the future.

Sb2S3-templated synthesis of sulfur-doped Sb-N-C with hierarchical architecture and high metal loading for H2O2 electrosynthesis

Selective two-electron (2e−) oxygen reduction reaction (ORR) offers great opportunities for hydrogen peroxide (H2O2) electrosynthesis and its widespread employment depends on identifying cost-effective catalysts with high activity and selectivity. Main-group metal and nitrogen coordinated carbons (M-N-Cs) are promising but remain largely underexplored due to the low metal-atom density and the lack of understanding in the structure-property correlation. Here, we report using a nanoarchitectured Sb2S3 template to synthesize high-density (10.32 wt%) antimony (Sb) single atoms on nitrogen- and sulfur-codoped carbon nanofibers (Sb-NSCF), which exhibits both high selectivity (97.2%) and mass activity (114.9 A g−1 at 0.65 V) toward the 2e− ORR in alkaline electrolyte. Further, when evaluated with a practical flow cell, Sb-NSCF shows a high production rate of 7.46 mol gcatalyst−1 h−1 with negligible loss in activity and selectivity in a 75-h continuous electrolysis. Density functional theory calculations demonstrate that the coordination configuration and the S dopants synergistically contribute to the enhanced 2e− ORR activity and selectivity of the Sb-N4 moieties.

Cobalt single atom anchored on N-doped carbon nanoboxes as typical single-atom catalysts (SACs) for boosting the overall water splitting

Single-atom catalysts (SACs) are considered one of the promising strategies to achieve efficient energy conversion, due to their advantage of both maximum atomic utilization and minimum catalyst cost. However, finding a balance between increasing the atom loading and preventing the agglomeration of metal single atoms is a current research hotspot. Herein, we have developed single-atom cobalt embedded in N-doped carbon nanoboxes as high-efficiency bifunctional electrocatalysts for overall water splitting in alkaline/acidic electrolytes. Due to the reliable metal-nonmetal bonds between the Co single atom and the substrate carbon, where the uniformly dispersed Co atoms could be effectively and stably riveted on the carbon nanoboxes, and the as-prepared electrocatalyst possesses high mass loading of single Co atoms (∼10.2 wt%). In addition, the systematic X-ray absorption fine structure (XAFS) and density functional theory (DFT) calculations were performed to further investigate the relationship between coordination number, configuration and electrocatalytic properties of Co single atoms and N atoms. Based on the above results, the presence of Co atoms induces the formation of pyrr-N and possesses Co@CNB-N4 configuration with HER overpotential of 45 mV, comparable to Pt/C (20 wt%). As for OER, Co@CNB-N4 still has satisfactory catalytic performance, superior to the benchmark catalyst RuO2. Thus, this work builds a bridge to understand the impact of metal single atoms and substrate configuration on catalytic performance and opens a door to the successful synthesis of SACs with high loading of non-precious metal atoms, high atomic utilization and electrocatalytic activity.

Altering Ligand Microenvironment of Atomically Dispersed CrN4 by Axial Ligand Sulfur for Enhanced Oxygen Reduction Reaction in Alkaline and Acidic Medium.

Because of the instability and Fenton reactivity of non-precious metal nitrogen-carbon based catalyst when processing the oxygen reduction reaction (ORR), seeking for electrocatalysts with highly efficient performance becomes very highly desired to speed up the commercialization of fuel cell. Herein, chromium (Cr)-N4 electrocatalyst containing extraterrestrial S formed axial S1 -Cr1 N4 bonds (S1 Cr1 N4 C) is achievedviaan assembly polymerization and confined pyrolysis strategy. Benefiting from the adjusting coordination configuration and electronic structure of the metal center through axial coordination,S1 Cr1 N4 Cexhibits enhanced the intrinsic activity (half-wave potential (E1/2 ) is 0.90 Vversus reversable hydrogen electrode, RHE) compared with that of CrN4 C and Pt/C catalysts. More notably, the catalyst is almost inert in catalyzing the Fenton reaction, and thus shows the high stability. Density functional theory (DFT) results further reveal that the existence of axial S atoms in S1 Cr1 N4 C moiety has the better ORR activity than Cr1 N4 C moieties. The axial S ligand in S1 Cr1 N4 C moiety can break the electron localization around the planar Cr1 N4 active center, which facilitated the rate-limiting reductive release of OH* and accelerated overall ORR process. The present work opens up a new avenue to modulate the axial ligand type of the single-atoms (SAs) active center to enhance intrinsic SAs performances.

Recent progress in heteroatom doping to modulate the coordination environment of M–N–C catalysts for the oxygen reduction reaction

This review focuses on the effects of heteroatom doping on the coordination configuration and electronic structure of M–N–C catalysts for ORR from three aspects: coordinated atoms, environmental atoms, and axial coordinated atoms.

The state of Fe3+ in the C–F–A–S–H system with varying Fe/Si and Ca/Si ratios

Synthetic CaO–Fe2O3–Al2O3–SiO2–H2O (C–F–A–S–H) gels with Fe/Si and Ca/Si ratios in the ranges 1/8–1/4 and 1.0–2.0, respectively, are investigated to reveal the coordination, location, and doping configuration of Fe3+.

Convergent paired electrosynthesis of dimethyl carbonate from carbon dioxide enabled by designing the superstructure of axial oxygen coordinated nickel single-atom catalysts

A dual-channel superstructured Ni single-atom catalyst with a unique axial oxygen coordination configuration was controllably constructed and affords a preeminent performance for convergent paired electrosynthesis of dimethyl carbonate from CO2.

First principles study of structural and electron properties in scorodite: the bulk and surface.

Scorodite (FeAsO4·2H2O) is an ideal material for the fixation of arsenic that has attracted considerable research interest in recent decades. However, the position of the H atom in the scorodite crystal structure, water molecular configuration, surface morphology, and chemical state of the surface atoms have not been reported. In this work, density functional theory (DFT) is used to optimize the scorodite crystal structure, and the atomic bonding is analyzed. At the same time, a surface model is constructed to calculate the configuration and electronic structure of the surface atoms for different coordination groups. The results show that the tetrahedral [AsO4] and octahedral [FeO4(2H2O)] groups in the scorodite crystal structure have good stability(geometry configuration), and the covalent bond strength between the As atom and the bridged oxygen atom (Ob) is greater than that between the Ob atom and the Fe atom. The water molecules in the crystal structure do not seriously deform and ionize. The configuration of the water molecules remains stable through electrostatic interactions (Ow-Fe) and hydrogen bonding (H-Ob). The Fe atoms on the surface of scorodite can coordinate with OH and H2O, while the As atoms can only form a stable coordination with OH. When an Fe atom on the surface coordinates with two H2O atoms, the Fe atom will shrink to the inside of the bulk. With the increase in the hydroxylation number of the Fe atom, the bonding strength between the Fe atom and the Ob atom decreases. Different surface configurations do not affect the stability(geometry configuration) of the [AsO4] structure. In addition, the surface water molecular layer has a very weak effect on the surface coordination configuration. By contrast, in the surface configuration of the (W + OH) structure, the change in the surface atomic layer spacing is the smallest.

Modulating local electronic structure enhances superior electrochemical activity in Li-rich oxide cathodes

The electrochemical activity and stability of Li2MnO3are affected synchronously by lattice vacancies and local coordination configuration.

Single‐atom electrocatalysts for lithium–sulfur chemistry: Design principle, mechanism, and outlook

AbstractLithium–sulfur batteries (LSBs) have been regarded as one of the promising candidates for the next‐generation “lithium‐ion battery beyond” owing to their high energy density and due to the low cost of sulfur. However, the main obstacles encountered in the commercial implementation of LSBs are the notorious shuttle effect, retarded sulfur redox kinetics, and uncontrolled dendrite growth. Accordingly, single‐atom catalysts (SACs), which have ultrahigh catalytic efficiency, tunable coordination configuration, and light weight, have shown huge potential in the field of LSBs to date. This review summarizes the recent research progress of SACs applied as multifunctional components in LSBs. The design principles and typical synthetic strategies of SACs toward effective Li–S chemistry as well as the working mechanism promoting sulfur conversion reactions, inhibiting the lithium polysulfide shuttle effect, and regulating Li+nucleation are comprehensively illustrated. Potential future directions in terms of research on SACs for the realization of commercially viable LSBs are also outlined.

Single-atom CoN4 sites with elongated bonding induced by phosphorus doping for efficient H2O2 electrosynthesis

Modification of the microenvironment of metal- and nitrogen-coordinated nanocarbons (M-N-Cs) is critical in regulating their electronic structure and thus catalytic selectivity toward the oxygen reduction reaction (ORR). Introducing heteroatoms into the carbon matrix of M-N-Cs could affect the coordination configuration and charge density of the metal centers, but it has rarely been applied to improve the ORR selectivity for H2O2 electrosynthesis. Here we show that doping phosphorus (P) atoms into the carbon substrate of a Co-N-C catalyst lengthens the Co−N bond, decreases the electron density of the Co, and weakens the adsorption strength of the key *OOH intermediate on the active sites, as demonstrated by both experimental and theoretical results. Consequently, this P-doped Co-N-C catalyst presents outstanding catalytic performance toward the 2e− ORR with an early onset potential of 0.81 V (vs. the reversible hydrogen electrode), exceptional H2O2 selectivity above 90% in a wide potential range from 0.1 V to 0.7 V (maximum value of ∼ 97% at 0.5 V) and a large turnover frequency (2.36 ± 0.15 s−1 at 0.65 V) in alkaline electrolyte, superior to almost all previously reported counterparts. Moreover, an unprecedented H2O2 production rate up to 11.2 molH2o2gcatalyst−1h−1 with long-term durability (110 h) is obtained when the catalyst is assessed as a gas diffusion layer in a practical flow cell.

Manipulating Selectivity of Hydroxyl Radical Generation by Single-Atom Catalysts in Catalytic Ozonation: Surface or Solution.

Hydroxyl radical-dominated oxidation in catalytic ozonation is, in particular, important in water treatment scenarios for removing organic contaminants, but the mechanism about ozone-based radical oxidation processes is still unclear. Here, we prepared a series of transitional metal (Co, Mn, Ni) single-atom catalysts (SACs) anchored on graphitic carbon nitride to accelerate ozone decomposition and produce highly reactive ·OH for oxidative destruction of a water pollutant, oxalic acid (OA). We experimentally observed that, depending on the metal type, OA oxidation occurred dominantly either in the bulk phase, which was the case for the Mn catalyst, or via a combination of the bulk phase and surface reaction, which was the case for the Co catalyst. We further performed density functional theory simulations and in situ X-ray absorption spectroscopy to propose that the ozone activation pathway differs depending on the oxygen binding energy of metal, primarily due to differential adsorption of O3 onto metal sites and differential coordination configuration of a key intermediate species, *OO, which is collectively responsible for the observed differences in oxidation mechanisms and kinetics.

High-quality all-inorganic CsPbI2Br thin films derived from phase-pure intermediate for efficient wide-bandgap perovskite solar cells

Mixed-halide alloying inorganic perovskite CsPbI2Br with prominent thermal stability has been considered as a promising candidate for wide-bandgap solar cells. However, the thermally driving coordination configuration evolution and phase transition during the crystallization of the perovskite CsPbI2Br were still not understood in depth, which was not conducive to the controllable preparation of high-quality CsPbI2Br thin films and the enhancement of photovoltaic performance. In this study, the phase segregation and unfavorable phase transition during the crystallization of CsPbI2Br that lead to the low quality of the perovskite thin films were investigated systematically. Moreover, the existing issue was overcome through intermediate engineering by introducing volatile organic amine acetates. Specially, the introduction of 0.5 equiv. of formamidine acetate (FAAc) in the precursor of CsPbI2Br generated a phase-pure intermediate, which facilitated the crystallization of high-quality CsPbI2Br perovskite thin films, and thus improved the device performance. As a result, the champion device achieved a power conversion efficiency (PCE) of 16.36%.

Strong interaction of single-atom Pt with Cd0.5Zn0.5S: electronic structure regulation and photocatalytic hydrogen evolution promotion

Rationally designing coordination configuration of single-atom co-catalyst to boost charge separation and enhance reaction kinetics is a significant challenge. Here, a single Pt atom lattice-doped PtSA-Cd0.5Zn0.5S photocatalyst with stable Pt-S3 configuration was successfully prepared through a facile solvothermal combined with ion substitution reaction method. In particular, the crucial role of Pt single atoms in charge separation and transmission is illustrated via identifying the dynamic configurational evolution of Pt species during the catalytic process. The strong d-p orbitals hybridization of Pt-S coordination bond endows remarkable stability of lattice-doped Pt sites and simultaneously triggers the catalytic activity of S sites by regulating the surface electronic structure, increased active sites for hydrogen evolution. As a result, the lattice-doped Pt single atom proceeds efficient photocatalytic hydrogen evolution activity of PtSA-Cd0.5Zn0.5S via regulated band structure, improved charge separation, and reduced hydrogen evolution energy barrier. The PtSA-Cd0.5Zn0.5S exhibits an enhanced photocatalytic hydrogen evolution activity of 279.11 μmol·h−1, which is 84 and 3.7 times of Cd0.5Zn0.5S (3.31 μmol·h−1) and PtSA/Cd0.5Zn0.5S (Pt single atom surface modification, 75.34 μmol·h−1), respectively.

Coordination-driven chiral self-assembly: Synthesis, structures and vapor adsorption properties of Zn(II) and Ag(I) complexes derived from two helical pyridylamide ligands

Four complexes were synthesized and characterized from two helical pyridylamide ligands. A view of the geometry of these complexes discloses the self-discriminating and self-recognition process in the coordination-driven chiral assembly. • Four new complexes derived from two helical pyridylamide ligands have been synthesized and characterized. • The crystal structures of the four complexes reveal the chiral self-assembly process in the coordination reaction. • Cyclic voltammetric measurements and methanol vapor adsorptions for the complexes are investigated. Two helical pyridylamide ligands N 2 ,N 6 -bis(2-(nicotinamido)phenyl)pyridine-2,6-dicarboxamide ( L 1 ) and N 2 ,N 6 -bis(2-(isonicotinamido)phenyl)pyridine-2,6-dicarboxamide ( L 2 ) have been designed and synthesized. Four complexes, namely, {[Zn( L 1 )I 2 ]‧2CH 3 OH‧H 2 O} n ( 1 ), [Ag 2 ( L 1 ) 2 ]‧(NO 3 ) 2 ( 2 ), {[Ag( L 2 )NO 3 ]‧H 2 O} n ( 3 ), and {[Ag( L 2 ) 2 ]‧2H 2 O} n ( 4 ) were obtained via the solvent evaporation method from the reaction of the ligands with Zn(II) or Ag(I) ions respectively. The two ligands together with four complexes were structurally determined by single-crystal X-ray diffraction and additionally characterized by infrared spectroscopy, element analysis and powder X-ray diffraction. The crystal structures reveal different chiral self-assembly processes in the reaction of the helical liands with Zn(II) or Ag(I) ions. Complexes 1 , 2 and 4 exhibit as a heterochiral coordination polymer or binuclear metallamacrocycle, while complex 3 exists as a homochiral coordination polymer. Besides, the structural differences between complexes 3 and 4 show that the reaction ratio of the ligand with metal ions has obvious influences on the coordination configuration. Furthermore, thermogravimetric analysis and methanol vapor adsorption properties of the synthesized complexes have been investigated, and cyclic voltammetric measurements of complexes 2 – 4 were carried out as well.

Frontispiece: Controlled Modification of Axial Coordination for Transition‐Metal Single‐Atom Electrocatalyst

M−N−C single-atom catalysts have emerged as a novel frontier in electrocatalysis. Their catalytic behavior is optimized by the lack of metal–metal bonds and ability to adjust the coordination configuration and electronic structure of their metal–heteroatom bonds. In particular, this concept discusses the importance of the axial coordination strategy toward single atom catalyst. For more details, see the Concept by B. Wang and co-workers. (DOI: 10.1002/chem.202201471).