Engineering unsymmetrically coordinated Cu-S1N3 single atom sites with enhanced oxygen reduction activity

Strengthened d-p Orbital Hybridization through Asymmetric Coordination Engineering of Single-Atom Catalysts for Durable Lithium-Sulfur Batteries

Although single-atom catalysts (SACs) have been largely explored in lithium-sulfur (Li-S) batteries, the commonly reported nonpolar transition metal-N₄ coordinations only demonstrate inferior adsorption and catalytic activity toward shuttled lithium polysulfides (LiPSs). Herein, single Fe atoms with asymmetric coordination configurations of Fe-N₃C₂-C were precisely designed and synthesized as efficient immobilizer and catalyst for LiPSs. The experimental and theoretical results elucidate that the asymmetrically coordinated Fe-N₃C₂-C moieties not only enhance the LiPSs anchoring capability by the formation of extra π-bonds originating from S p orbital and Fe d_x²-y²/d_xy orbital hybridization but also boost the redox kinetics of LiPSs with reduced Li₂S precipitation/decomposition barrier, leading to suppressed shuttle effect. Consequently, the Li-S batteries assembled with Fe-N₃C₂-C exhibit high areal capacity and cycling stability even under high sulfur loading and lean electrolyte conditions. This work highlights the important role of coordination symmetry of SACs for promoting the practical application of Li-S batteries.

Nested order-disorder framework containing a crystalline matrix with self-filled amorphous-like innards

Solids can be generally categorized by their structures into crystalline and amorphous states with different interactions among atoms dictating their properties. Crystalline-amorphous hybrid structures, combining the advantages of both ordered and disordered components, present a promising opportunity to design materials with emergent collective properties. Hybridization of crystalline and amorphous structures at the sublattice level with long-range periodicity has been rarely observed. Here, we report a nested order-disorder framework (NOF) constructed by a crystalline matrix with self-filled amorphous-like innards that is obtained by using pressure to regulate the bonding hierarchy of Cu12Sb4S13. Combined in situ experimental and computational methods demonstrate the formation of disordered Cu sublattice which is embedded in the retained crystalline Cu framework. Such a NOF structure gives a low thermal conductivity (~0.24 W·m-1·K-1) and a metallic electrical conductivity (8 × 10-6 Ω·m), realizing the collaborative improvement of two competing physical properties. These findings demonstrate a category of solid-state materials to link the crystalline and amorphous forms in the sublattice-scale, which will exhibit extraordinary properties.

Ru-Co Pair Sites Catalyst Boosts the Energetics for the Oxygen Evolution Reaction

Manipulating the coordination environment of the active center via anion modulation to reveal tailored activity and selectivity has been widely achieved, especially for carbon-based single-atom site catalysts (SACs). However, tuning ligand fields of the active center by single-site metal cation regulation and identifying the effects on the resulting electronic configuration is seldom explored. Herein, we propose a single-site Ru cation coordination strategy to engineer the electronic properties by constructing a Ru/LiCoO₂ SAC with atomically dispersed Ru-Co pair sites. Benefitting from the strong electronic coupling between Ru and Co sites, the catalyst possesses an enhanced electrical conductivity and achieves near-optimal oxygen adsorption energies. Therefore, the optimized catalyst delivers superior oxygen evolution reaction (OER) activity with low overpotential, the high mass activity of 1000 A g_oxide ^-1 at a small overpotential of 335 mV, and excellent long-term stability. It also exhibits rapid kinetics with superior rate capability and outstanding durability in a zinc-air battery.

Atomic-Level Pt Electrocatalyst Synthesized via Iced Photochemical Method for Hydrogen Evolution Reaction with High Efficiency

In heterogeneous catalysis, metal particle morphology and size can influence markedly the activity. It is of great significance to rationally design and control the synthesis of Pt at the atomic level to demonstrate the structure-activity relationship toward electrocatalysis. Herein, a powerful strategy is reported to synthesize graphene-supported platinum-based electrocatalyst, that is, nanocatalysts with controllable size can be prepared by iced photochemical method, including single atoms (Pt-SA@HG), nanoclusters (Pt-Clu@HG), and nanocrystalline (Pt-Nc@HG). The Pt-SA@HG exhibits unexpected electrocatalytic hydrogen evolution reaction (HER) performances with 13 mV overpotential at 10 mA cm^-2 current densities which surpass Pt-Clu@HG and Pt-Nc@HG. The in situ X-ray absorption fine structure spectroscopy (XAFS) and density functional theory (DFT) calculations determine the Pt-C₃ active site is linchpin to the excellent HER performance of Pt-SA@HG. Compared with the traditional Pt-N_x coordination structure, the pure carbon coordinated Pt-C₃ site is more favorable for HER. This work opens up a new way to adjust the metal particle size and catalytic performance of graphene at a multiscale level.

Macro/Micro-Environment Regulating Carbon-Supported Single-Atom Catalysts for Hydrogen/Oxygen Conversion Reactions

Single-atom catalysts (SACs) have attracted tremendous research interest due to their unique atomic structure, maximized atom utilization, and remarkable catalytic performance. Among the SACs, the carbon-supported SACs have been widely investigated due to their easily controlled properties of the carbon substrates, such as the tunable morphologies, ordered porosity, and abundant anchoring sites. The electrochemical performance of carbon-supported SACs is highly related to the morphological structure of carbon substrates (macro-environment) and the local coordination environments of center metals (micro-environment). This review aims to provide a comprehensive summary on the macro/micro-environment regulating carbon-supported SACs for highly efficient hydrogen/oxygen conversion reactions. The authors first summarize the macro-environment engineering strategies of carbon-supported SACs with altered specific surface areas and porous properties of the carbon substrates, facilitating the mass diffusion kinetics and structural stability. Then the micro-environment engineering strategies of carbon-supported SACs are discussed with the regulated atomic structure and electronic structure of metal centers, boosting the catalytic performance. Insights into the correlation between the co-boosted effect from the macro/micro-environments and catalytic activity for hydrogen/oxygen conversion reactions are summarized and discussed. Finally, the challenges and perspectives are addressed in building highly efficient carbon-supported SACs for practical applications.

Thermal Enhancement of Luminescence for Negative Thermal Expansion in Molecular Materials

Overcoming thermal quenching is an essential issue in the practical application of luminescent materials. Herein, we found that negative thermal expansion (NTE) can achieve the thermal enhancement of luminescence in molecular materials based on three metal-organic frameworks CuX-bpy (X = Cl, Br, I; bpy = 4,4'-bipyridine). All complexes exhibit NTE on the c-axis, and the strongest NTE leads to a contraction of the Cu...Cu distance in CuCl-bpy, which further intensifies the luminescence emission. This phenomenon indicates the existence of thermally enhanced charge transfer. Moreover, the origin of the distinction in charge transfer attributed to the different valence states of the copper is investigated through the combined studies of X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and density functional theory calculations. This research provides a new approach to modulating the luminescence thermal enhancement by NTE.

Rational coordination regulation in carbon-based single-metal-atom catalysts for electrocatalytic oxygen reduction reaction

Single-metal-atom catalysts (SMACs) have garnered extensive attention for various electrocatalytic applications, owing to their maximum atom-utilization efficiency, tunable electronic structure, and remarkable catalytic performance. In particular, carbon-based SMACs exhibit optimal electrocatalytic activity for the oxygen reduction reaction (ORR) which is of paramount importance for several sustainable energy conversion and generation technologies, such as fuel cells and metal-air batteries. Despite continuous endeavors in developing various advanced carbon-based SMACs for electrocatalytic ORR, the rational regulation of coordination structure and thus the electronic structure of carbon-based SMACs remains challenging. In this review, we critically examine the role of coordination structure, including local coordination structure (i.e., metal atomic centers and the first coordination shell) and extended local coordination structure (i.e., the second and higher coordination shells), on the rational design of carbon-based SMACs for high-efficiency electrocatalytic ORR. Insights into the relevance between coordination structures and their intrinsic ORR activities are emphatically exemplified and discussed. Finally, we also propose the major challenges and future perspectives in the rational design of advanced carbon-based SMACs for electrocatalytic ORR. This review aims to emphasize the significance of coordination structure and deepen the insightful understanding of structure-performance relationships.

Tracking the Oxygen Dynamics of Solid-Liquid Electrochemical Interfaces by Correlative In Situ Synchrotron Spectroscopies

ConspectusOxygen-involved electrocatalytic processes, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), are central to a series of advanced modern energy and conversion technologies, such as water electrolyzers, fuel cells, and CO₂ reduction or N₂ fixation devices. A comprehensive and in-depth understanding of the charge transfer and energy conversion process that ubiquitously occurs over solid-liquid electrochemical interfaces during oxygen electrocatalysis is crucial for understanding the key essence of oxygen-related electrochemistry. The huge challenges for dynamic studies over solid-liquid interfaces during oxygen electrocatalysis lie in the all-embracing electrochemical processes of the catalytic reactions, associated with both structural and reactive intermediates evolution on the electrode surface, and in the significant influence of the aqueous environments of electrolytes used. Hence, overcoming these challenges intrinsically calls for a great cooperation of multiple cutting-edge in situ technologies. Synchrotron radiation (SR) X-ray absorption fine structure (SR-XAFS) spectroscopy is highly sensitive to the local atomic structure of nanomaterials, and SR-based Fourier transform infrared (SR-FTIR) spectroscopy features unique molecular fingerprint identification to determine active species on the surface of electrodes. One can imagine that the correlative in situ SR-XAFS/FTIR spectroscopic investigations will potentially provide sufficient, reliable, and complementary information at the atomic/molecular level to depict vivid and comprehensive "dynamic movies" of solid-liquid electrochemical interfaces during oxygen electrocatalysis, which will help effectively promote/simplify the complicated screening process of advanced oxygen electrocatalysts for efficient high-energy-density energy systems.In this Account, starting with some fundamentals of SR-based spectroscopic technologies, tips for obtaining high-quality SR-XAFS and SR-FTIR spectroscopy results during the electrocatalytic process are comprehensively specified. Subsequently, the latest research achievements of dynamic investigations mainly from our group based on in situ SR-XAFS and/or SR-FTIR spectroscopies will be systematically scrutinized and properly emphasized in detail, where the currently attractive metal-organic-framework (MOF) nanomaterials and single-atom catalysts (SACs) are selected as the main object of research. Moreover, the vital contributions of correlative in situ SR-XAFS/FTIR studies on new discoveries of the dynamic evolution of solid-liquid interfaces during oxygen electrocatalysis are highlighted. In particular, our pioneering research found that the potential-dependent dynamically coupled oxygen formed in the precatalytic stage was a very useful promoter in SACs to promote efficient OER kinetics under acidic conditions. In addition, the in situ generated metastable Ni₁-N₂ centers with more structural degrees of freedom in SACs could potentially facilitate the fast 4e^- ORR kinetics. This Account is anticipated to stimulate broad interest in dynamic explorations in various catalytic processes of interest in the material science and electrochemistry communities using correlative SR-based technologies.

Theory-Guided Regulation of FeN4 Spin State by Neighboring Cu Atoms for Enhanced Oxygen Reduction Electrocatalysis in Flexible Metal-Air Batteries

Iron, nitrogen-codoped carbon (Fe-N-C) nanocomposites have emerged as viable electrocatalysts for the oxygen reduction reaction (ORR) due to the formation of FeN_x C_y coordination moieties. In this study, results from first-principles calculations show a nearly linear correlation of the energy barriers of key reaction steps with the Fe magnetic moment. Experimentally, when single Cu sites are incorporated into Fe-N-C aerogels (denoted as NCAG/Fe-Cu), the Fe centers exhibit a reduced magnetic moment and markedly enhanced ORR activity within a wide pH range of 0-14. With the NCAG/Fe-Cu nanocomposites used as the cathode catalyst in a neutral/quasi-solid aluminum-air and alkaline/quasi-solid zinc-air battery, both achieve a remarkable performance with an ultrahigh open-circuit voltage of 2.00 and 1.51 V, large power density of 130 and 186 mW cm^-2 , and good mechanical flexibility, all markedly better than those with commercial Pt/C or Pt/C-RuO₂ catalysts at the cathode.

A Single-Atom Fe-N-C Catalyst with Ultrahigh Utilization of Active Sites for Efficient Oxygen Reduction

Fe-N-C single-atom catalysts (SACs) are emerging as a promising class of electrocatalysts for the oxygen reduction reaction (ORR) to replace Pt-based catalysts. However, due to the limited loading of Fe for SACs and the inaccessibility of internal active sites, only a small portion of the sites near the external surface are able to contribute to the ORR activity. Here, this work reports a metal-organic framework-derived Fe-N-C SAC with a hierarchically porous and concave nanoarchitecture prepared through a facile but effective strategy, which exhibits superior electrocatalytic ORR activity with a half-wave potential of 0.926 V (vs RHE) in alkaline media and 0.8 V (vs RHE) in acidic media while maintaining excellent stability. The superior ORR activity of the as-designed catalyst stems from the unique architecture, where the hierarchically porous architecture contains micropores as Fe SAC anchoring sites, meso-/macro-pores as accessible channels, and concave shell for increasing external surface area. The unique architecture has dramatically enhanced the utilization of previously blocked internal active sites, as confirmed by a high turnover frequency of 3.37 s^-1 and operando X-ray absorption spectroscopy analysis with a distinct shift of adsorption edge.

Tailoring the microenvironment in Fe-N-C electrocatalysts for optimal oxygen reduction reaction performance

Fe-N-C electrocatalysts, comprising FeN4 single atom sites immobilized on N-doped carbon supports, offer excellent activity in the oxygen reduction reaction (ORR), especially in alkaline solution. Herein, we report a simple synthetic strategy for improving the accessibility of FeN4 sites during ORR and simultaneously fine-tuning the microenvironment of FeN4 sites, thus enhancing the ORR activity. Our approach involved a simple one-step pyrolysis of a Fe-containing zeolitic imidazolate framework in the presence of NaCl, yielding a hierarchically porous Fe-N-C electrocatalyst containing tailored FeN4 sites with slightly elongated Fe-N bond distances and reduced Fe charge. The porous carbon structure improved mass transport during ORR, whilst the microenvironment optimized FeN4 sites benefitted the adsorption/desorption of ORR intermediates. Accordingly, the developed electrocatalyst, possessing a high FeN4 site density (9.9 × 1019 sites g-1) and turnover frequency (2.26 s-1), delivered remarkable ORR performance with a low overpotential (a half-wave potential of 0.90 V vs. reversible hydrogen electrode) in 0.1 mol L-1 KOH.

Carbon-based material-supported single-atom catalysts for energy conversion

In recent years, single-atom catalysts (SACs) with unique electronic structure and coordination environment have attracted much attention due to its maximum atomic efficiency in the catalysis fields. However, it is still a great challenge to rationally regulate the coordination environments of SACs and improve the loading of metal atoms for SACs during catalysis progress. Generally, carbon-based materials with excellent electrical conductivity and large specific surface area are widely used as catalyst supports to stabilize metal atoms. Meanwhile, carbon-based material-supported SACs have also been extensively studied and applied in various energy conversion reactions, such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Herein, rational synthesis methods and advanced characterization techniques were introduced and summarized in this review. Then, the theoretical design strategies and construction methods for carbon-based material-supported SACs in electrocatalysis applications were fully discussed, which are of great significance for guiding the coordination regulation and improving the loading of SACs. In the end, the challenges and future perspectives of SACs were proposed, which could largely contribute to the development of single atom catalysts at the turning point.

Dual-Metal Active Sites Mediated by p-Block Elements: Knowledge-Driven Design of Oxygen Reduction Reaction Catalysts

In this study, the oxygen reduction reaction (ORR) process of dual-metal active site catalysts (FeMN₆-Gra, M = Mn, Ni, Co, or Cu) mediated by p-block elements was investigated using density functional theory calculations. The obtained results demonstrate that, in most cases, the B-doped FeMN₆-Gra (M = Mn, Ni, Co, or Cu) catalysts exhibit higher catalytic performance than their undoped counterparts. Among the investigated catalysts, FeNiN₆-Gra doping by B modulates the adsorption strength of the metal center on the oxygen-containing intermediates, showing the largest increase in the onset potential (from 0.66 to 0.94 V). Importantly, we found a new law that B-doping affects the total charge of the metal adsorption site and the four surrounding N atoms and that there is a linear relationship between the total charge and the Gibbs free energy. Transition state analysis shows that the energy barrier of the thermodynamic rate-determining step (*OH hydrogenation to H₂O) in the FeNiN₆B1-Gra-catalyzed ORR process is 0.17 eV, which is smaller than that of the FeNiN₆-Gra-catalyzed process (0.28 eV). Overall, the results demonstrate that B-doping can improve the activity of FeMN₆-Gra catalysts and provide a new method for the future development of efficient electrocatalysts.

Highly Dispersed Co-, N-, S-Doped Topological Defect-Rich Hollow Carbon Nanoboxes as Superior Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries

Rechargeable Zn-air batteries have received extensive attention due to their use of nontoxic materials, safety, and high energy density. However, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air electrode of Zn-air batteries both suffer from slow kinetics, limiting their commercialization development. Herein, we prepared Co, N, and S co-doped hollow carbon nanoboxes (Co-N/S-CNBs) rich in topological defects using polyphenylene sulfide (PPS) as a sulfur-rich carbon source. Critically, by utilizing the self-propagating high-temperature synthesis (SHS), PPS can avoid melting, while simultaneously enabling the catalyst to take on a unique hollow structure. Additional post-treatment to introduce Co and N atoms as active centers further increases the defect sites and microporous structures of the catalyst. Under alkaline electrolytes, the Co-N/S-CNBs enabled Zn-air batteries to exhibit excellent bifunctional catalytic activity for both ORR and OER, surpassing commercial catalysts. Chemical analysis showed that the cracking loss of small molecules from PPS during pyrolysis is the main reason for the formation of topological defects, where the defect sites act as active centers to enhance the catalytic performance. Overall, this work provides new insights into the mechanism of how defects are formed in such a catalyst, as well as shows how a high-performance bifunctional electrocatalyst can be utilized for practical Zn-air batteries.

Regulating the Spin State of FeIII Enhances the Magnetic Effect of the Molecular Catalysis Mechanism

Aqueous-phase oxygen evolution reaction (OER) is the bottleneck of water splitting. The formation of the O-O bond involves the generation of paramagnetic oxygen molecules from the diamagnetic hydroxides. The spin configurations might play an important role in aqueous-phase molecular electrocatalysis. However, spintronic electrocatalysis is almost an uncultivated land for the exploration of the oxygen molecular catalysis process. Herein, we present a novel magnetic Fe^III site spin-splitting strategy, wherein the electronic structure and spin states of the Fe^III sites are effectively induced and optimized by the Jahn-Teller effect of Cu²⁺. The theoretical calculations and operando attenuated total reflectance-infrared Fourier transform infrared (ATR FT-IR) reveal the facilitation for the O-O bond formation, which accelerates the production of O₂ from OH^- and improves the OER activity. The Cu₁-Ni₆Fe₂-LDH catalyst exhibits a low overpotential of 210 mV at 10 mA cm^-2 and a low Tafel slope (33.7 mV dec^-1), better than those of the initial Cu₀-Ni₆Fe₂-LDHs (278 mV, 101.6 mV dec^-1). With the Cu²⁺ regulation, we have realized the transformation of NiFe-LDHs from ferrimagnets to ferromagnets and showcase that the OER performance of Cu-NiFe-LDHs significantly increases compared with that of NiFe-LDHs under the effect of a magnetic field for the first time. The magnetic-field-assisted Cu₁-Ni₆Fe₂-LDHs provide an ultralow overpotential of 180 mV at 10 mA cm^-2, which is currently one of the best OER performances. The combination of the magnetic field and spin configuration provides new principles for the development of high-performance catalysts and understandings of the catalytic mechanism from the spintronic level.

Computational Screening of Single-Metal-Atom Embedded Graphene-Based Electrocatalysts Stabilized by Heteroatoms

Metal-N-doped carbon is a promising replacement for non-precious-metal catalysts such as Pt for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). Although these materials have relatively good catalytic activity and are cost-effective, they still have lower ORR activity than Pt, and so improving their performances is greatly required. In this study, high-throughput screening was employed based on density functional theory (DFT) calculations to search for good candidate catalysts with a transition metal atom coordinated by heteroatoms (B, N, S, O, and P) embedded in a graphene structure. In addition, coordinating a transition metal with two types of heteroatom dopants in a graphene structure was also considered. We calculated the binding energies of ORR intermediates on metal-heteroatom-based graphene structures because they are known to play a key role in ORR. Based on our results, the new group of electrocatalysts imparts excellent ORR activity for PEMFCs, and we suggest that our approach provides useful insight into exploring other promising candidate catalysts.

Theoretical Predictions, Experimental Modulation Strategies, and Applications of MXene-Supported Atomically Dispersed Metal Sites

Atomically dispersed metal sites (ADMSs) attract immense attention because they can be used in the fields of energy and environmental protection as they are characterized by high atomic utilization efficiency and exhibit high activity. Various supports for anchoring isolated metal atoms are developed to construct ADMSs characterized by highly stable and well-defined structures. This can be achieved by increasing the number of anchoring sites and reinforcing metal-support interactions. MXenes, a new series of 2D nanomaterials, exhibit promising potential in stabilizing isolated metal atoms because of their large specific surface areas and unique surface properties. The high conductivity and hydrophilicity of MXenes can be attributed to the nature of surface functionalization and the properties of tunable structures of the materials. Benefiting from these excellent properties, MXenes can find their applications in various fields. Herein, the precise characterization methods that can be followed to study ADMSs, the construction of MXene-supported ADMSs using theoretical predictions, and experimental modulation strategies are summarized, and their corresponding applications in electrocatalysis, organocatalysis, and advanced battery systems are systematically illustrated. It is hoped that this review will provide insights that can be used for the further development of MXene-supported ADMSs.

Simultaneously Integrate Iron Single Atom and Nanocluster Triggered Tandem Effect for Boosting Oxygen Electroreduction

Atomically nitrogen-coordinated iron atoms on carbon (FeNC) catalysts are emerging as attractive materials to substitute precious-metal-based catalysts for the oxygen reduction reaction (ORR). However, FeNC usually suffers from unsatisfactory performance due to the symmetrical charge distribution around the iron site. Elaborately regulating the microenvironment of the central Fe atom can substantially improve the catalytic activity of FeNC, which remains challenging. Herein, N/S co-doped porous carbons are rationally prepared and are verified with rich Fe-active sites, including atomically dispersed FeN₄ and Fe nanoclusters (FeSA-FeNC@NSC), according to systematically synchrotron X-ray absorption spectroscopy analysis. Theoretical calculation verifies that the contiguous S atoms and Fe nanoclusters can break the symmetric electronic structure of FeN₄ and synergistically optimize 3d orbitals of Fe centers, thus accelerating OO bond cleavage in OOH* for improving ORR activity. The FeSA-Fe_NC @NSC delivers an impressive ORR activity with half-wave-potential of 0.90 V, which exceeds that of state-of-the-art Pt/C (0.87 V). Furthermore, FeSA-Fe_NC @NSC-based Zn-air batteries deliver excellent power densities of 259.88 and 55.86 mW cm^-2 in liquid and all-solid-state flexible configurations, respectively. This work presents an effective strategy to modulate the microenvironment of single atomic centers and boost the catalytic activity of single-atom catalysts by tandem effect.

Alternate Storage of Opposite Charges in Multisites for High-Energy-Density Al-MOF Batteries

The limited active sites of cathode materials in aluminum-ion batteries restrict the storage of more large-sized Al-complex ions, leading to a low celling of theoretical capacity. To make the utmost of active sites, an alternate storage mechanism of opposite charges (AlCl₄ ^- anions and AlCl₂ ⁺ cations) in multisites is proposed herein to achieve an ultrahigh capacity in Al-metal-organic framework (MOF) battery. The bipolar ligands (oxidized from 18π to 16π electrons and reduced from 18π to 20π electrons in a planar cyclic conjugated system) can alternately uptake and release AlCl₄ ^- anions and AlCl₂ ⁺ cations in charge/discharge processes, which can double the capacity of unipolar ligands. Moreover, the high-density active Cu sites (Cu nodes) in the 2D Cu-based MOF can also store AlCl₂ ⁺ cations for a higher capacity. The rigid and extended MOF structure can address the problems of high solubility and poor stability of small organic molecules. As a result, three-step redox reactions with two-electron transfer in each step are demonstrated in charge/discharge processes, achieving high reversible capacity (184 mAh g^-1 ) and energy density (177 Wh kg^-1 ) of the optimized cathode in an Al-MOF battery. The findings provide a new insight for the rational design of stable high-energy Al-MOF batteries.

A Bioinspired Five-Coordinated Single-Atom Iron Nanozyme for Tumor Catalytic Therapy

Single-atom nanozymes (SAzymes) represent a new research frontier in the biomedical fields. The rational design and controllable synthesis of SAzymes with well-defined electronic and geometric structures are essential for maximizing their enzyme-like catalytic activity and therapeutic efficacy but remain challenging. Here, a melamine-mediated pyrolysis activation strategy is reported for the controllable fabrication of iron-based SAzyme containing five-coordinated structure (FeN₅ ), identified by transmission electron microscopy imaging and X-ray absorption fine structure analyses. The FeN₅ SAzyme exhibits superior peroxidase-like activity owing to the optimized coordination structure, and the corresponding catalytic efficiency of Fe-species in the FeN₅ SAzyme is 7.64 and 3.45 × 10⁵ times higher than those in traditional FeN₄ SAzyme and Fe₃ O₄ nanozyme, respectively, demonstrated by steady-state kinetic assay. In addition, the catalytic mechanism is jointly disclosed by experimental results and density functional theory studies. The as-synthesized FeN₅ SAzyme demonstrates significantly enhanced antitumor effect in vitro and in vivo due to the excellent peroxidase-like activity under tumor microenvironment.

A General Strategy for Engineering Single-Metal Sites on 3D Porous N, P Co-Doped Ti3C2TX MXene

Two-dimensional (2D) MXenes have been developed to stabilize single atoms via various methods, such as vacancy reduction and heteroatom-mediated interactions. However, anchoring single atoms on 3D porous MXenes to further increase catalytic active sites and thus construct electrocatalysts with high activity and stability remains unexplored. Here, we reported a general synthetic strategy for engineering single-metal sites on 3D porous N, P codoped Ti₃C₂T_X nanosheets. Through a "gelation-and-pyrolysis" process, a series of atomically dispersed metal catalysts (Pt, Ir, Ru, Pd, and Au) supported by N, P codoped Ti₃C₂T_X nanosheets with 3D porous structure can be obtained and serve as efficient catalysts for the electrochemical hydrogen evolution reaction (HER). As a result of the favorable electronic and geometric structure of N(O), P-coordinated metal atoms optimizing catalytic intermediates adsorption and 3D porous structure exposing the active surface sites and facilitating charge/mass transfer, the as-synthesized Pt SA-PNPM catalyst shows ∼20-fold higher activity than the commercial Pt/C catalyst for electrochemical HER over a wide pH range.

Engineering the Local Atomic Environments of Indium Single-Atom Catalysts for Efficient Electrochemical Production of Hydrogen Peroxide

The in-depth understanding of local atomic environment-property relationships of p-block metal single-atom catalysts toward the 2 e^- oxygen reduction reaction (ORR) has rarely been reported. Here, guided by first-principles calculations, we develop a heteroatom-modified In-based metal-organic framework-assisted approach to accurately synthesize an optimal catalyst, in which single In atoms are anchored by combined N,S-dual first coordination and B second coordination supported by the hollow carbon rods (In SAs/NSBC). The In SAs/NSBC catalyst exhibits a high H₂ O₂ selectivity of above 95 % in a wide range of pH. Furthermore, the In SAs/NSBC-modified natural air diffusion electrode exhibits an unprecedented production rate of 6.49 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M KOH electrolyte and 6.71 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M PBS electrolyte. This strategy enables the design of next-generation high-performance single-atom materials, and provides practical guidance for H₂ O₂ electrosynthesis.

Atomically Defined Undercoordinated Copper Active Sites over Nitrogen-Doped Carbon for Aerobic Oxidation of Alcohols

Selective aerobic oxidation of alcohols offers an attractive means to address challenges in the modern chemical industry, but the development of non-noble metal catalysts with superior efficacy for this reaction remains a grand challenge. Here, this study reports on such a catalyst based on atomically defined undercoordinated copper atoms over nitrogen-doped carbon support as an efficient, durable, and scalable heterogeneous catalyst for selective aerobic oxidation of alcohols. This catalyst exhibits extremely high intrinsic catalytic activity (TOF of 7692 h^-1 ) in the oxidation of cinnamyl alcohol to afford cinnamaldehyde, along with exceptional recyclability (at least eight cycles), scalability, and broad substrate scope. DFT calculations suggest that the high activity derives from the low oxidation state and the unique coordination environment of the copper sites in the catalyst. These findings pave the way for the design of highly active and stable single atom catalysts to potentially address challenges in synthetic chemistry.

Identification of Fenton-like active Cu sites by heteroatom modulation of electronic density

The Fenton-like process based on peroxymonosulfate (PMS) has been widely investigated and recognized as a promising alternative in recent years for the degradation of persistent organic pollutants. However, the sluggish kinetics of PMS activation results in prohibitive costs and substantial chemical inputs, impeding its practical applications in water purification. This work demonstrates that tuning the electronic structure of single-atom sites at the atomic level is a powerful approach to achieve superior PMS activation kinetics. The Cu-based catalyst with the optimized electronic structure exhibits superior performance over most of the state-of-the-art heterogeneous Fenton-like catalysts, while homogeneous Cu(II) shows very poor activity. This work provides insights into the electronic structure regulation of metal centers and structure–activity relationship at the atomic level.

Developing heterogeneous catalysts with atomically dispersed active sites is vital to boost peroxymonosulfate (PMS) activation for Fenton-like activity, but how to controllably adjust the electronic configuration of metal centers to further improve the activation kinetics still remains a great challenge. Herein, we report a systematic investigation into heteroatom-doped engineering for tuning the electronic structure of Cu-N₄ sites by integrating electron-deficient boron (B) or electron-rich phosphorus (P) heteroatoms into carbon substrate for PMS activation. The electron-depleted Cu-N₄/C-B is found to exhibit the most active oxidation capacity among the prepared Cu-N₄ single-atom catalysts, which is at the top rankings of the Cu-based catalysts and is superior to most of the state-of-the-art heterogeneous Fenton-like catalysts. Conversely, the electron-enriched Cu-N₄/C-P induces a decrease in PMS activation. Both experimental results and theoretical simulations unravel that the long-range interaction with B atoms decreases the electronic density of Cu active sites and down-shifts the d-band center, and thereby optimizes the adsorption energy for PMS activation. This study provides an approach to finely control the electronic structure of Cu-N₄ sites at the atomic level and is expected to guide the design of smart Fenton-like catalysts.

Boosting Electrochemical Oxygen Reduction Performance of Iron Phthalocyanine through Axial Coordination Sphere Interaction

Precise regulation of the electronic states of catalytic sites through molecular engineering is highly desired to boost catalytic performance. Herein, a facile strategy was developed to synthesize efficient oxygen reduction reaction (ORR) catalysts, based on mononuclear iron phthalocyanine supported on commercially available multi-walled carbon nanotubes that contain electron-donating functional groups (FePc/CNT-R, with "R" being -NH₂ , -OH, or -COOH). These functional groups acted as axial ligands that coordinated to the Fe site, confirmed by X-ray photoelectron spectroscopy and synchrotron-radiation-based X-ray absorption fine structure. Experimental results showed that FePc/CNT-NH₂ , with the most electron-donating -NH₂ axial ligand, exhibited the highest ORR activity with a positive onset potential (E_onset =1.0 V vs. reversible hydrogen electrode) and half-wave potential (E_1/2 =0.92 V). This was better than the state-of-the-art Pt/C catalyst (E_onset =1.00 V and E_1/2 =0.85 V) under the same conditions. Overall, the functionalized FePc/CNT-R assemblies showed enhanced ORR performance in comparison to the non-functionalized FePc/CNT assembly. The origin of this behavior was investigated using density functional theory calculations, which demonstrated that the coordination of electron-donating groups to FePc facilitated the adsorption and activation of oxygen. This study not only demonstrates a series of advanced ORR electrocatalysts, but also introduces a feasible strategy for the rational design of highly active electrocatalysts for other proton-coupled electron transfer reactions.

Robust MOF-derived carbon-supported bimetallic Ni-Co catalysts for aqueous phase hydrodeoxygenation of vanillin

Currently, rapidly increasing consumption of fossil resources has propelled the upgrading of biomass as an alternative and sustainable technology to produce important chemicals and bio-oils. In this regard, the rational design of low-cost and robust supported metal-based catalysts that exhibit excellent catalytic hydrodeoxygenation (HDO) performance for the conversion of biomass is quite necessary. Herein, we developed hierarchical flower-like nitrogen-doped carbon layer-coated bimetallic Ni-Co nanoparticles, which were distributed over the carbonaceous matrix (Ni_xCo@NC@C), via a metal-organic framework (MOF) ZIF-67 precursor approach, assisted by the etching of Ni²⁺ ions, hydrothermal treatment together with glucose, and following carbonization processes. The as-fabricated Ni₃Co@NC@C catalyst bearing a 3 : 1 Ni/Co molar ratio showed a superior catalytic HDO activity towards aqueous phase HDO of vanillin to other bimetallic NiCo catalysts with different Ni/Co molar ratios under mild reaction conditions, along with a 100% selectivity to 2-methoxy-4-methylphenol at a full vanillin conversion, despite its smaller number of exposed metallic sites. It was revealed that over the Ni₃Co@NC@C catalyst, the surface abundant defective oxygen vacancies and electron-rich Co⁰ species were conducive to the adsorption and activation of vanillin and the reaction intermediate, thereby giving rise to the outstanding catalytic activity. Moreover, for Ni₃Co@NC@C, the adequate protection effect of surface carbon layers, as well as the unique hierarchical flower-like microstructure, could significantly inhibit the leaching of active metal species in the reaction medium, thereby leading to high structural stability. The present findings afford a promising strategy for constructing low-cost and robust carbon-supported bimetallic catalysts for the HDO of lignin-derived derivatives.

Electronically and Geometrically Modified Single-Atom Fe Sites by Adjacent Fe Nanoparticles for Enhanced Oxygen Reduction

Fe-N-C materials exhibit excellent activity and stability for oxygen reduction reaction (ORR), as one of the most promising candidates to replace commercial Pt/C catalysts. However, it is challenging to unravel features of the superior ORR activity originating from Fe-N-C materials. In this work, the electronic and geometric structures of the isolated Fe-N-C sites and their correlations with the ORR performance are investigated by varying the secondary thermal activation temperature of a rationally designed NC-supported Fe single-atom catalyst (SAC). The systematic analyses demonstrate the significant role of coordinated atoms of SA and metallic Fe nanoparticles (NPs) in altering the electronic structure of isolated Fe-N-C sites. Meanwhile, strong interaction between isolated Fe-N-C sites and adjacent Fe NPs can change the geometric structure of isolated Fe-N-C sites. Theoretical calculations reveal that optimal regulation of the electronic and geometric structure of isolated Fe-N-C sites by the co-existence of Fe NPs narrows the energy barriers of the rate-limiting steps of ORR, resulting in outstanding ORR performance. This work not only provides the fundamental understanding of the underlying structure-activity relationship, but also sheds light on designing efficient Fe-N-C catalysts.

UiO-66(Zr)-derived t-zirconia with abundant lattice defect for remarkably enhanced arsenic removal

Zirconium oxide (ZrO₂) exhibits great potential in the remediation of arsenic-polluted water. In this study, tetragonal zirconium oxide (t-ZrO₂) with high lattice defects was facilely fabricated by regulating the Zr-metal-organic framework (MOF) (UiO-66) with sodium acetate modulator and examined to adsorb arsenic from water. Benefitting from the synergistic effects of mesopores structure and lattice defect, t-ZrO₂ exhibited ultrahigh adsorption capacity and faster kinetics towards both arsenate (As(V)) and arsenite (As(III)). The Langmuir adsorption capacity for As(V) and As(III) of 147.5 mg g^-1 and 352.1 mg g^-1 on t-ZrO₂ in exothermic process, respectively, significantly outperforming reported counterparts in literature (generally ≤100 mg g^-1). The faster adsorption kinetic of both As(III) and As(V) on t-ZrO₂ is defined favorably by the pseudo-second-order model over a wide pH (3-11). Furthermore, arsenic is mainly captured by t-ZrO₂ via forming Zr-O-As bonds through occupying coordinatively unsaturated zirconium atoms adsorption sites revealed by the X-ray photoelectron spectroscopy (XPS) spectrum and Fourier-transformed infrared (FTIR) spectra analysis. This study offers a new strategy for designing ultrahigh performance Zr-MOF-derived adsorbents for capturing arsenic.

Altering Ligand Fields in Single-Atom Sites through Second-Shell Anion Modulation Boosts the Oxygen Reduction Reaction

Single-atom catalysts based on metal-N₄ moieties and anchored on carbon supports (defined as M-N-C) are promising for oxygen reduction reaction (ORR). Among those, M-N-C catalysts with 4d and 5d transition metal (TM_4d,5d) centers are much more durable and not susceptible to the undesirable Fenton reaction, especially compared with 3d transition metal based ones. However, the ORR activity of these TM_4d,5d-N-C catalysts is still far from satisfactory; thus far, there are few discussions about how to accurately tune the ligand fields of single-atom TM_4d,5d sites in order to improve their catalytic properties. Herein, we leverage single-atom Ru-N-C as a model system and report an S-anion coordination strategy to modulate the catalyst's structure and ORR performance. The S anions are identified to bond with N atoms in the second coordination shell of Ru centers, which allows us to manipulate the electronic configuration of central Ru sites. The S-anion-coordinated Ru-N-C catalyst delivers not only promising ORR activity but also outstanding long-term durability, superior to those of commercial Pt/C and most of the near-term single-atom catalysts. DFT calculations reveal that the high ORR activity is attributed to the lower adsorption energy of ORR intermediates at Ru sites. Metal-air batteries using this catalyst in the cathode side also exhibit fast kinetics and excellent stability.

N-Doped Graphene Supported Cu Single Atoms: Highly Efficient Recyclable Catalyst for Enhanced C-N Coupling Reactions

Heterogenization of homogeneous catalysis through supported single-atom catalysts (SACs) provided a feasible solution to recycling catalysts while keeping its efficiency in chemical synthesis. In this work, Cu SACs anchored on N-doped graphene (Cu SACs/NG) were prepared and first used for C-N coupling reactions. During the preparation, Cu-N-C structures, including Cu-N₄ moieties, were formed in a one-step pyrolysis method. As-prepared Cu SACs/NG exhibited excellent catalytic activity toward C-N coupling reactions with a broad scope of substrates and showed outstanding performance of recycling. Compared with Cu nanoparticles (Cu NPs/NG), the advantages of single-atom catalysts were validated via experimental and theoretical calculations. The enhanced performances were attributed to increasing the number of active sites and increasing the intrinsic activity of each active site. This work provides an alternative synthetic strategy for fabricating atomically dispersed SACs and represents a significant advance for coupling reactions.

Popcorn-like Co3O4 Nanoparticles Confined in Three-Dimensional Hierarchical N-doped carbon nanotubes Networks as Highly Efficient Trifunctional Electrocatalyst for Zinc-Air Batteries and Water Splitting Device

A novel unique popcorn-like three-dimensional (3D) hierarchical structural electrocatalyst is synthesized by the pyrolysis of ZIF-8/ZIF-67 and polyacrylonitrile fibers composites, where popcorn-like Co3O4 nanoparticles coated with nitrogen-doped amorphous carbon anchor...

Optimizing Microenvironment of Asymmetric N,S-Coordinated Single-Atom Fe via Axial Fifth Coordination toward Efficient Oxygen Electroreduction

Single-atom catalysts (SACs) are attractive candidates for oxygen reduction reaction (ORR). The catalytic performances of SACs are mainly determined by the surrounding microenvironment of single metal sites. Microenvironment engineering of SACs and understanding of the structure-activity relationship is critical, which remains challenging. Herein, a self-sacrificing strategy is developed to synthesize asymmetric N,S-coordinated single-atom Fe with axial fifth hydroxy (OH) coordination (Fe-N₃ S₁ OH) embedded in N,S codoped porous carbon nanospheres (FeN/SC). Such unique penta-coordination microenvironment is determined by cutting-edge techonologies aiding of systematic simulations. The as-obtained FeN/SC exhibits superior catalytic ORR activity, and showcases a half-wave potential of 0.882 V surpassing the benchmark Pt/C. Moreover, theoretical calculations confirmed the axial OH in FeN₃ S₁ OH can optimize 3d orbitals of Fe center to strengthen O₂ adsorption and enhance O₂ activation on Fe site, thus reducing the ORR barrier and accelerating ORR dynamics. Furthermore, FeN/SC containing H₂ O₂ fuel cell performs a high peak power density of 512 mW cm^-2 , and FeN/SC based Znair batteries show the peak power density of 203 and 49 mW cm^-2 in liquid and flexible all-solid-state configurations, respectively. This study offers a new platform for fundamentally understand the axial fifth coordination in asymmetrical planar single-atom metal sites for electrocatalysis.

Size-controlled, hollow and hierarchically porous Co2Ni2 alloy nanocubes for efficient oxygen reduction in microbial fuel cells

Schematic illustration of the fabrication of CoxNiy alloy nanocubes (ANCs) and hollow CoxNiy alloy nanocubes (HANCs).

Engineering Co and Ru dual-metal atoms on nitrogen-doped carbon as highly efficient bifunctional oxygen electrocatalysts

Designing dual-metal atoms efficient bifunctional oxygen electrocatalyst by a one-step adsorption and a pyrolysis steps.

Clusters Induced Electron Redistribution to Tune Oxygen Reduction Activity of Transition Metal Single-Atom for Metal-Air Batteries

Oxygen reduction reaction (ORR) activity can be effectively tuned by modulating the electron configuration and optimizing the chemical bonds. Herein, a general strategy to optimize the activity of metal single-atom is achieved by the decoration of metal clusters via a coating-pyrolysis-etching route. In this unique structure, the metal clusters are able to induce electron redistribution and modulate M-N species bond lengths. As a result, the M-ACSA@NC exhibits superior ORR activity compared with the nanoparticles-decorated counterparts. The performance enhancement is attributed to the optimized intermediates desorption benefiting from the unique electronic configuration. Theoretical analysis reinforces the significant roles of metal clusters by correlating the ORR activity with clusters induced charge transfer. As a proof-of-concept, various metal-air batteries assembled with the Fe-ACSA@NC deliver remarkable power densities and capacities. This strategy is an effective and universal technique for electron modulation of M-N-C, which shows great potential in application of energy storage devices.

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is the vital process at the cathode of next-generation electrochemical storage and conversion technologies, such as metal-air batteries and fuel cells. Single-metal-atom and nitrogen co-doped carbonaceous electrocatalysts (M-N-C) have emerged as attractive alternatives to noble-metal platinum for catalyzing the kinetically sluggish ORR due to their high electrical conductivity, large surface area, and structural tunability at the atomic level, however, their application is limited by the low intrinsic activity of the metal-nitrogen coordination sites (M-N x ) and inferior site density. In this Perspective, we summarize the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the ORR activity. Particular emphasis is placed on the emerging strategies for regulating the electronic structure of the single metal site and populating the site density. In addition, challenges and perspectives are provided regarding the future development of single atom carbonous electrocatalysts for the ORR and their utilization in practical use.

Tuning the Ratio of Pt(0)/Pt(II) in Well-Defined Pt Clusters Enables Enhanced Electrocatalytic Reduction/Oxidation of Hydrogen Peroxide for Sensitive Biosensing

Rational design and construction of advanced sensing platforms for sensitive detection of H₂O₂ released from living cells is one of the challenges in the field of physiology and pathology. Noble metal clusters are a kind of nanomaterials with well-defined chemical composition and special atomic structures, which have been widely explored in catalysis, biosensing, and therapy. Compared with noble metal nanoparticles, noble metal clusters exhibit great potential in electrochemical biosensing due to their high atom utilization efficiency and abundant reactive active sites. Herein, Pt nanoclusters anchored on hollow carbon spheres (Pt_NCS/HCS) were successfully prepared for sensitive detection of H₂O₂. By tuning the ratio of Pt(0)/Pt(II) at different annealing temperatures, the optimized Pt_NCS/HCS-550 showed higher H₂O₂ reduction and oxidation catalytic activities than other control samples. Density functional theory calculations revealed that H₂O₂^*can be better activated and dissociated in the Pt_0II model featured with the co-existence of Pt(0)/Pt(II) and the key intermediates OOH*/OH* have a stronger interaction with the Pt_0II model. As a concept application, the electrochemical biosensing platform was successfully applied to sensitive detection of H₂O₂ released from the cells.

Stabilizing Fe-N-C Catalysts as Model for Oxygen Reduction Reaction

The highly efficient energy conversion of the polymer-electrolyte-membrane fuel cell (PEMFC) is extremely limited by the sluggish oxygen reduction reaction (ORR) kinetics and poor electrochemical stability of catalysts. Hitherto, to replace costly Pt-based catalysts, non-noble-metal ORR catalysts are developed, among which transition metal-heteroatoms-carbon (TM-H-C) materials present great potential for industrial applications due to their outstanding catalytic activity and low expense. However, their poor stability during testing in a two-electrode system and their high complexity have become a big barrier for commercial applications. Thus, herein, to simplify the research, the typical Fe-N-C material with the relatively simple constitution and structure, is selected as a model catalyst for TM-H-C to explore and improve the stability of such a kind of catalysts. Then, different types of active sites (centers) and coordination in Fe-N-C are systematically summarized and discussed, and the possible attenuation mechanism and strategies are analyzed. Finally, some challenges faced by such catalysts and their prospects are proposed to shed some light on the future development trend of TM-H-C materials for advanced ORR catalysis.