Structure-Performance Descriptors and the Role of the Axial Oxygen Atom on M–N4–C Single-Atom Catalysts for Electrochemical CO2 Reduction

Engineering the regulation strategy of active sites to explore the intrinsic mechanism over single‑atom catalysts in electrocatalysis

The development of efficient and sustainable energy sources is a crucial strategy for addressing energy and environmental crises, with a particular focus on high-performance catalysts. Single-atom catalysts (SACs) have attracted significant attention because of their exceptionally high atom utilization efficiency and outstanding selectivity, offering broad application prospects in energy development and chemical production. This review systematically summarizes the latest research progress on SACs in five key electrochemical reactions: hydrogen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, and oxygen evolution reaction. Initially, a brief overview of the current understanding of electrocatalytic active sites in SACs is provided. Subsequently, the electrocatalytic mechanisms of these reactions are discussed. Emphasis is placed on various modification strategies for SAC surface-active sites, including coordination environment regulation, electronic structure modulation, support structure regulation, the introduction of structural defects, and multifunctional site design, all aimed at enhancing electrocatalytic performance. This review comprehensively examines SAC deactivation and poisoning mechanisms, highlighting the importance of stability enhancement for practical applications. It also explores the integration of density functional theory calculations and machine learning to elucidate the fundamental principles of catalyst design and performance optimization. Furthermore, various synthesis strategies for industrial-scale production are summarized, providing insights into commercialization. Finally, perspectives on future research directions for SACs are highlighted, including synthesis strategies, deeper insights into active sites, the application of artificial intelligence tools, and standardized testing and performance requirements.

Enhanced CO2 electrochemical reduction on single-atom catalysts with optimized environmental, central and axial chemical ambient

Single-atom catalysts (SACs) have received significant research interests for electrocatalytic CO2 reduction reaction (CO2RR) to produce valuable chemicals. Designing optimal SACs for CO2RR is a great challenge because of the strong scaling relationship among the many carbon-containing intermediates. In this study, we designed high-performance SACs, breaking the scaling relationship through changing environmental nonmetals, central atoms and axial nonmetals together via a series of density functional theory (DFT) calculations. After screening through configuration stabilities, CO and CO2 adsorption energy, limiting potential of H2, product adsorption energy, limiting potential of products, energy barrier of C-C coupling process and AIMD simulations, we finally observed ten optimal SACs (Ti-N4-B, Ti-N4-Si, Ti-CN3-Si, Ti-CN2O(1)-S, Sc-C2NO(1)-B, Sc-C2NO(1)-Si, Ti-BCN2(2)-N, Sc-CN2O(3)-Si, Ru-C2NO(3)-C and Ti-BONC-C) after considering 4311 possible configurations with high activity and selectivity for HCOOH, CH4 and C2H6O formation. Among them, Ti-N4-B, Ru-C2NO(3)-C, and Sc-C2NO(1)-B have the lowest overpotentials for producing HCOOH, CH4, and C2H6O with UL of -0.2 V, -0.29 V, and -0.51 V, respectively. Subsequently, electronic analysis is implemented to provide a more comprehensive explanation at the electronic level for the enhanced CO2RR performance of the discovered SACs. Our research demonstrates that the performance of SACs on CO2RR can be significantly enhanced and altered by the combination of environmental nonmetals, central atoms, and axial nonmetals in a rational design. Importantly, it also establishes a design principle for the rapid screening of prospective catalysts for CO2RR with high activity and selectivity.

Breaking efficiency Limits in solar water Splitting: Ferroelectric CuInP2Se6 with Ni Single-Atom cocatalysts for enhanced carrier separation and activity

Photocatalytic water splitting represents a promising approach for sustainable energy production, yet its practical implementation remains hindered by insufficient light absorption, rapid charge recombination, and inadequate catalytic efficiency. This study proposes a two-dimensional ferroelectric Ni2/CuInP2Se6 that addresses these fundamental limitations through rational material design. Through systematic screening of ABP2X6 materials (A=Cu; B = In/Cr; X=S/Se), the ferroelectric CuInP2Se6 monolayer emerges as an optimal substrate due to its exceptional stability, appropriate band alignment, and superior light absorption coefficient. Subsequent selection of Ni single-atom cocatalysts from nine transition metals reveals optimal hydrogen evolution (ηHER = 0.09 V) and oxygen evolution (ηOER = 0.37 V) overpotentials, achieving remarkable solar-to-hydrogen efficiency (24.63 %). Crucially, the ferroelectric polarization-induced built-in electric field enables spatial separation of photogenerated carriers while modulating the Ni d-band center, which effectively regulates the adsorption strength of reaction intermediates, facilitating the adaptive optimization of distinct reaction. The proposed strategy not only demonstrates the viability of 2D ferroelectric materials for photocatalytic applications but also establishes a general framework for designing high-performance photocatalysts through coupled polarization engineering and single-atom catalysis.

Favorable Moderate Adsorption of Polysulfide on FeNi3 Intermetallic Compound Accelerating Conversion Kinetics for Advanced Lithium-Sulfur Batteries

Sluggish conversion kinetics of polysulfides during discharge and the severe shuttle effect significantly hinder the practical application of lithium-sulfur (Li-S) batteries. In this work, the lattice engineering strategy of Fe hybridization is employed to manipulate the bulk phase spacing of FeNi3 (space group Pm3m) intermetallic compounds to adjust the 3d electronic structure, optimizing the adsorption of polysulfides, thereby accelerating the catalytic conversion. As a result, FeNi2.25@OC achieves favorable moderate adsorption toward polysulfides. Due to the larger number of electrons occupying the lowest occupied molecular orbital of Li2S4, the S-S bonds are weakened and broken. Temperature-dependent experiments confirm that FeNi2.25@OC exhibits the lowest activation energy and can effectively accelerate the catalytic conversion of polysulfides. The Li-S cell assembled with FeNi2.25@OC modified PP separator delivers a high initial discharge specific capacity of 1219.5 mAh g-1 at 0.2 C. Even at a high sulfur loading of 6.06 mg cm-2 and lean electrolyte conditions (6 µL mg-1), it can cycle stably for 60 cycles.

Digital Descriptors in Predicting Catalysis Reaction Efficiency and Selectivity

Accurately controlling the interactions and dynamic changes between multiple active sites (e.g., metals, vacancies, and lone pairs of heteroatoms) to achieve efficient catalytic performance is a key issue and challenge in the design of complex catalytic reactions involving 2D metal-supported catalysts, metal-zeolites, metal-organic catalysts, and metalloenzymes. With the aid of machine learning (ML), descriptors play a central role in optimizing the electrochemical performance of catalysts, elucidating the essence of catalytic activity, and predicting more efficient catalysts, thereby avoiding time-consuming trial-and-error processes. Three kinds of descriptors─active center descriptors, interfacial descriptors, and reaction pathway descriptors─are crucial for understanding and designing metal-supported catalysts. Specifically, vacancies, as active sites, synergize with metals to significantly promote the reduction reactions of energy-relevant small molecules. By combining some physical descriptors, interpretable descriptors can be constructed to evaluate catalytic performance. Future development of descriptors and ML models faces the challenge of constructing descriptors for vacancies in multicatalysis systems to rationally design the activity, selectivity, and stability of catalysts. Utilization of generative artificial intelligence and multimodal ML to automatically extract descriptors would accelerate the exploration of dynamic reaction mechanisms. The transferable descriptors from metal-supported catalysts to artificial metalloenzymes provide innovative solutions for energy conversion and environmental protection.

Fundamentals and Perspectives of Positively Charged Single-Metal Site Catalysts for CO2 Electroreduction

Single-atom catalysts (SACs) show superior efficiency in electrocatalytic carbon dioxide reduction, a key stage in achieving carbon neutrality. Atomically dispersed single-metal sites of SACs are invariably in a positive valence state; namely, they are positively charged single-metal sites (PCSSs). The PCSS catalysts generally possess a distinctive and asymmetric electronic structure, which enables the activation of linear carbon dioxide molecules and stabilizes miscellaneous intermediates during electrocatalysis. Herein, this review summarizes the manner in which the coordination environment, neighboring atoms or groups, and the interaction with the substrate modulate the distinctive electronic properties of PCSSs. Additionally, we overview the recently reported theoretical and experimental advances in terms of structure-performance relationship. Furthermore, we emphasize the previously underappreciated durability of positively charged single-metal sites in CO2 reduction. Finally, we discuss several pending issues and potential breakthroughs of PCSSs for CO2 reduction.

Edge-doped substituents as an emerging atomic-level strategy for enhancing M-N4-C single-atom catalysts in electrocatalysis of the ORR, OER, and HER

M-N4-C single-atom catalysts (MN4) have gained attention for their efficient use at the atomic level and adjustable properties in electrocatalytic reactions like the ORR, OER, and HER. Yet, understanding MN4's activity origin and enhancing its performance remains challenging. Edge-doped substituents profoundly affect MN4's activity, explored in this study by investigating their interaction with MN4 metal centers in ORR/OER/HER catalysis (Sub@MN4, Sub = B, N, O, S, CH3, NO2, NH2, OCH3, SO4; M = Fe, Co, Ni, Cu). The results show overpotential variations (0 V to 1.82 V) based on Sub and metal centers. S and SO4 groups optimize FeN4 for peak ORR activity (overpotential at 0.48 V) and reduce OER overpotentials for NiN4 (0.48 V and 0.44 V). N significantly reduces FeN4's HER overpotential (0.09 V). Correlation analysis highlights the metal center's key role, with ΔG*H and ΔG*OOH showing mutual predictability (R2 = 0.92). Eg proves a reliable predictor for Sub@CoN4 (ΔG*OOH/ΔG*H, R2 = 0.96 and 0.72). Machine learning with the KNN model aids catalyst performance prediction (R2 = 0.955 and 0.943 for ΔG*OOH/ΔG*H), emphasizing M-O/M-H and the d band center as crucial factors. This study elucidates edge-doped substituents' pivotal role in MN4 activity modulation, offering insights for electrocatalyst design and optimization.

The volcanic relationship of model phthalocyanine molecular catalysts in the CO2 reduction reaction

We have constructed a series of model metal phthalocyanines (MPc) for the carbon dioxide reduction reaction (CO2RR), constructed a volcano relationship through density functional theory (DFT) and experiments, and obtained cobalt phthalocyanine (CoPc) at the apex. The volcano diagram is conducive to the screening of catalysts and has a guiding role in the design of catalysts.

Graphene-based single-atom catalysts for electrochemical CO2 reduction: unraveling the roles of metals and dopants in tuning activity

Discovering electrocatalysts that can efficiently convert carbon dioxide (CO2) to valuable fuels and feedstocks using excess renewable electricity is an emergent carbon-neutral technology. A single metal atom embedded in doped graphene, i.e., single-atom catalyst (SAC), possesses high activity and selectivity for electrochemical CO2 reduction (CO2R) to CO, yet further reduction to hydrocarbons is challenging. Here, using density functional theory calculations, we investigate stability and reactivity of a broad SAC chemical space with various metal centers (3d transition metals) and dopants (2p dopants of B, N, O; 3p dopants of P, S) as electrocatalysts for CO2R to methane and methanol. We observe that the rigidities of these SACs depend on the type of dopants, with 3p-coordinating SACs exhibiting more severe out-of-plane distortion than 2p-coordinating SACs. Using CO adsorption energy as a descriptor for CO2R reactivity, we narrow down the candidates and identify seven SACs with near-optimal CO binding strength. We then elucidate full reaction mechanisms towards methane and methanol generation on these identified candidates and observe highly dopant-dependent activity and rate-limiting steps, divergent from conventional mechanistic understanding on metallic surfaces, calling into question whether previous design principles established on metals are directly transferrable to SACs. Consequently, we find that zinc embedded in boron-doped graphene (Zn-B-C) is a highly active catalyst for electrochemical CO2R to C1 hydrocarbons. Our work reveals the opportunities of tuning SAC reactivity via engineering dopants and metals and highlights the importance of re-elucidating CO2R reaction mechanisms on SACs towards unearthing new design principles for SAC chemistry.

Improving Nitric Oxide Reduction Reaction Activity of TMN4-C Model Catalysts by Axial Atom Coordination

In comparison with the conventional four-nitrogen coordinated transition metal (TMN4), we clarify that the electrochemical nitric oxide reduction reaction (NORR) activity can be significantly improved by axially coordinating nonmetal atoms (O, F, Cl) over the metal sites. In light of an electron-withdrawing effect, the axial fifth ligand disrupts the electron distribution symmetry and regulates the local electronic structure of the metal active center. It subsequently moderates the TM-NO interaction and thus enhances the activity. In particular, MnN4O-C, FeN4O-C, CoN4O-C, and CoN4F-C are identified as promising NORR catalysts with ultralow limiting potential (UL) of -0.07, -0.07, -0.07, and -0.05 V, respectively. In addition, the axial atom can also passivate the competing hydrogen evolution reaction (HER), increasing the selectivity toward NH3 formation. It therefore can be concluded that the present work affirms a novel strategy for the rational design of advanced electrocatalysts, highlighting the significance of optimal metal-ligand match and the coordination microenvironment tuning of the active centers.

Modification of metals and ligands in two-dimensional conjugated metal-organic frameworks for CO2 electroreduction: A combined density functional theory and machine learning study

Electrochemical carbon dioxide reduction reaction (CO2RR) is a promising technology to establish an artificial carbon cycle. Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) with high electrical conductivity have great potential as catalysts. Herein, we designed a range of 2D c-MOFs with different transition metal atoms and organic ligands, TMNxO4-x-HDQ (TM = Cr∼Cu, Mo, Ru∼Ag, W∼Au; x  = 0, 2, 4; HDQ = hexadipyrazinoquinoxaline), and systematically studied their catalytic performance using density functional theory (DFT). Calculation results indicated that all of TMNxO4-x-HDQ structures possess good thermodynamic and electrochemical stability. Notably, among the examined 37 MOFs, 6 catalysts outperformed the Cu(211) surface in terms of catalytic activity and product selectivity. Specifically, NiN4-HDQ emerged as an exceptional electrocatalyst for CO production in CO2RR, yielding a remarkable low limiting potential (UL) of -0.04 V. CuN4-HDQ, NiN2O2-HDQ, and PtN2O2-HDQ also exhibited high activity for HCOOH production, with UL values of -0.27, -0.29, and -0.27 V, respectively, while MnN4-HDQ, and NiO4-HDQ mainly produced CH4 with UL values of -0.58 and -0.24 V, respectively. Furthermore, these 6 catalysts efficiently suppressed the competitive hydrogen evolution reaction. Machine learning (ML) analysis revealed that the key intrinsic factors influencing CO2RR performance of these 2D c-MOFs include electron affinity (EA), electronegativity (χ), the first ionization energy (Ie), p-band center of the coordinated N/O atom (εp), the radius of metal atom (r), and d-band center (εd). Our findings may provide valuable insights for the exploration of highly active and selective CO2RR electrocatalysts.

Experimental and Theoretical Insights into Single Atoms, Dual Atoms, and Sub-Nanocluster Catalysts for Electrochemical CO2 Reduction (CO2RR) to High-Value Products

Electrocatalytic carbon dioxide (CO2) conversion into valuable chemicals paves the way for the realization of carbon recycling. Downsizing catalysts to single-atom catalysts (SACs), dual-atom catalysts (DACs), and sub-nanocluster catalysts (SNCCs) has generated highly active and selective CO2 transformation into highly reduced products. This is due to the introduction of numerous active sites, highly unsaturated coordination environments, efficient atom utilization, and confinement effect compared to their nanoparticle counterparts. Herein, recent Cu-based SACs are first reviewed and the newly emerged DACs and SNCCs expanding the catalysis of SACs to electrocatalytic CO2 reduction (CO2RR) to high-value products are discussed. Tandem Cu-based SAC-nanocatalysts (NCs) (SAC-NCs) are also discussed for the CO2RR to high-value products. Then, the non-Cu-based SACs, DACs, SAC-NCs, and SNCCs and theoretical calculations of various transition-metal catalysts for CO2RR to high-value products are summarized. Compared to previous achievements of less-reduced products, this review focuses on the double objective of achieving full CO2 reduction and increasing the selectivity and formation rate toward C-C coupled products with additional emphasis on the stability of the catalysts. Finally, through combined theoretical and experimental research, future outlooks are offered to further develop the CO2RR into high-value products over isolated atoms and sub-nanometal clusters.

Applications of metal nanoclusters supported on the two-dimensional material graphene in electrocatalytic carbon dioxide reduction

Metal nanoclusters (MNCs) have been demonstrated to exhibit superior catalytic performance compared to single nanoparticles. This is attributed to their quantized electronic structure, unique geometrical stacking and abundant active sites. While the exposed metal atoms can markedly enhance the efficiency of catalysis, unfortunately, MNCs are susceptible to agglomeration, which impairs their catalytic activity and stability. Graphene is a two-dimensional material consisting of a single atomic layer formed by the hybridization of the s and p orbitals of carbon atoms. It exhibits stable physical and chemical properties and has an easily controllable structure, making it an ideal carrier for MNCs. When metal nanoclusters (MNCs) are loaded on a graphene substrate, the MNCs can form a stable binding site on the graphene substrate. Furthermore, the construction of a defective structure on the graphene substrate enables the formation of robust interactions between the metal atoms of the MNCs and the substrate, facilitating the rapid establishment of electron conduction pathways and markedly enhancing the electrocatalytic performance. This paper presents a review of the applications of metal nanoclusters supported on graphene skeletons in the field of the electrocatalytic CO2 reduction reaction (CO2RR). Firstly, we briefly introduce the reaction mechanism of the CO2RR, then we systematically discuss the synthesis strategies, properties and applications of metal nanoclusters in electrocatalytic carbon dioxide reduction from both experimental and theoretical perspectives, and lastly, we discuss the opportunities and challenges of metal nanocluster catalysts supported on carbon materials.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Valence-engineered catalysis-selectivity regulation of molybdenum oxide nanozyme for acute kidney injury therapy and post-cure assessment

The optimization of the enzyme-like catalytic selectivity of nanozymes for specific reactive oxygen species (ROS)-related applications is significant, and meanwhile the real-time monitoring of ROS is really crucial for tracking the therapeutic process. Herein, we present a mild oxidation valence-engineering strategy to modulate the valence states of Mo in Pluronic F127-coated MoO3-x nanozymes (denoted as MF-x, x: oxidation time) in a controlled manner aiming to improve their specificity of H2O2-associated catalytic reactions for specific therapy and monitoring of ROS-related diseases. Experimentally, MF-0 (Mo average valence 4.64) and MF-10 (Mo average valence 5.68) exhibit exclusively optimal catalase (CAT)- or peroxidase (POD)-like activity, respectively. Density functional theory (DFT) calculations verify the most favorable reaction path for both MF-0- and MF-10-catalyzed reaction processes based on free energy diagram and electronic structure analysis, disclosing the mechanism of the H2O2 activation pathway on the Mo-based nanozymes. Furthermore, MF-0 poses a strong potential in acute kidney injury (AKI) treatment, achieving excellent therapeutic outcomes in vitro and in vivo. Notably, the ROS-responsive photoacoustic imaging (PAI) signal of MF-0 during treatment guarantees real-time monitoring of the therapeutic effect and post-cure assessment in vivo, providing a highly desirable non-invasive diagnostic approach for ROS-related diseases.

Rational Design of Earth-Abundant Catalysts toward Sustainability

Catalysis is crucial for clean energy, green chemistry, and environmental remediation, but traditional methods rely on expensive and scarce precious metals. This review addresses this challenge by highlighting the promise of earth-abundant catalysts and the recent advancements in their rational design. Innovative strategies such as physics-inspired descriptors, high-throughput computational techniques, and artificial intelligence (AI)-assisted design with machine learning (ML) are explored, moving beyond time-consuming trial-and-error approaches. Additionally, biomimicry, inspired by efficient enzymes in nature, offers valuable insights. This review systematically analyses these design strategies, providing a roadmap for developing high-performance catalysts from abundant elements. Clean energy applications (water splitting, fuel cells, batteries) and green chemistry (ammonia synthesis, CO2 reduction) are targeted while delving into the fundamental principles, biomimetic approaches, and current challenges in this field. The way to a more sustainable future is paved by overcoming catalyst scarcity through rational design.

Covalent Organic Frameworks for Electrocatalysis: Design, Applications, and Perspectives

Covalent organic frameworks (COFs) are an innovative class of crystalline porous polymers composed of light elements such as C, N, O, etc., linked by covalent bonds. The distinctive properties of COFs, including designable building blocks, large specific surface area, tunable pore size, abundant active sites, and remarkable stability, have led their widespread applications in electrocatalysis. In recent years, COF-based electrocatalysts have made remarkable progress in various electrocatalytic fields, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, nitrate reduction reaction, and carbon dioxide reduction reaction. This review begins with an introduction to the design and synthesis strategies employed for COF-based electrocatalysts. These strategies include heteroatom doping, metalation of COF and building monomers, encapsulation of active sites within COF pores, and the development of COF-based derived materials. Subsequently, a systematic overview of the recent advancements in the application of COF-based catalysts in electrocatalysis is presented. Finally, the review discusses the main challenges and outlines possible avenues for the future development of COF-based electrocatalysts.

Effect of Strain Engineering on the Spin State of the Ni-N4/C Single-Atom Catalyst and Its Consequence in Electrocatalysis

Strain engineering is an effective strategy to improve the activity of catalysts, especially for flexible carbon-based materials. Nitrogen-coordinated single atomic metals on a carbon skeleton (M-Nx/C) are of interest in catalytic electroreduction reactions due to their high activity and atomic utilization. However, the effect of strain on the structure-activity relationship between the electrochemical activity and the electronic and geometric structures of Ni-Nx/C remains unclear. Here, we found that by applying tensile strain on the Ni-N4/C, the spin state of the single atom can be changed from a low-spin to a high-spin state. Moreover, the energy gap between the highest occupied d orbital of Ni and the lowest unoccupied molecular orbital of the adsorbed species narrowed. With an increasing strain rate, the catalytic activity of O2 and CO2 electroreduction can be improved. Especially for the 2e- O2 reduction, the implicit solvent model, constant-potential method, and microkinetic model were used to verify the positive effect of suitable stretching on the catalytic activity from thermodynamic and kinetic viewpoints. This work can reveal the relationship between strain, spin state, and the catalytic activity of Ni-Nx/C.

Electronic Modulation of Doped MoS2 Nanosheets for Improved CO2 Sensing and Capture

Transition-metal dichalcogenides (TMDs) are widely used in the gas sensing field, owing to their high surface-to-volume ratio enabled by the two-dimensional (2D) structure, adjustable band gap, and high electron transfer. However, it is challenging for TMD materials to realize superior CO2 sensing, due to their weak CO2 adsorption capacity. Herein, we predict through density functional theory (DFT) calculations that rare earth metal doping is an effective strategy to boost the CO2 sensing capability of TMDs. As a proof-of-concept, we investigate and find that the introduction of rare earth metal atoms (La, Ce, Pr, or Nd) can induce lattice strain and modulate the electronic properties of MoS2. When negative charges are injected in rare earth metal doped MoS2 (R-MoS2), the 5d or 4f orbital of the rare earth metal atom in R-MoS2 can produce a stronger orbital hybridization with 2p orbitals of C and O in CO2. Therefore, the CO2 adsorption is significantly enhanced and the charge transfer is facilitated for negatively charged R-MoS2. Moreover, negatively charged R-MoS2 exhibits an excellent CO2 selectivity. Our results indicate that the rare earth metal doping and electronic modulation in 2D materials may provide a new pathway for CO2 sensing and capture.

Designing asymmetrical TMN4 sites via phosphorus or sulfur dual coordination as high-performance electrocatalysts for oxygen evolution reaction

The development ofhighly efficient oxygen evolution reaction (OER) catalysts based on more cost-effective and earth-abundant elements is of great significance and still faces a huge challenge. In this work, a series of transition metal (TM)embedding a newly-defined monolayer carbon nitride phase is theoretically profiled and constructed as a catalytic platform for OER studies. Typically, a four-step screening strategy was proposed to rapidly identified high performance candidates and the coordination structure and catalytic performance relationship was thoroughly analyzed. Moreover, the eliminating criterion was established to condenses valid range based on the Gibbs free energy of OH*. Our results reveal that the as-constructed 2FeCN/P exhibits superior activity toward OER with an ultralow overpotential of 0.25 V, at the same time, the established 3FeCN/S configuration performed well as abifunctional OER/ORR electrocatalysis with extremely low overpotential ηOER/ηORR of 0.26/0.48 V. Overall, this work provides an effective framework for screening advanced OER catalysts, which can also be extended to other complex multistep catalytic reactions.

Advancing electrocatalytic reactions through mapping key intermediates to active sites via descriptors

Descriptors play a crucial role in electrocatalysis as they can provide valuable insights into the electrochemical performance of energy conversion and storage processes. They allow for the understanding of different catalytic activities and enable the prediction of better catalysts without relying on the time-consuming trial-and-error approaches. Hence, this comprehensive review focuses on highlighting the significant advancements in commonly used descriptors for critical electrocatalytic reactions. First, the fundamental reaction processes and key intermediates involved in several electrocatalytic reactions are summarized. Subsequently, three types of descriptors are classified and introduced based on different reactions and catalysts. These include d-band center descriptors, readily accessible intrinsic property descriptors, and spin-related descriptors, all of which contribute to a profound understanding of catalytic behavior. Furthermore, multi-type descriptors that collectively determine the catalytic performance are also summarized. Finally, we discuss the future of descriptors, envisioning their potential to integrate multiple factors, broaden application scopes, and synergize with artificial intelligence for more efficient catalyst design and discovery.

Unraveling the activity trends of T-C2N based Single-Atom catalysts for electrocatalytic nitrate reduction via high-throughput screening

Electrochemical nitrate reduction reaction (NO3RR) offers a cost-effective and environmentally friendly method to simultaneously yield valuable NH3and alleviate NO3-pollution under mild operating conditions.However, this complicated eight-electron reaction suffers from low selectivity and Faradaic efficiency, which highlight the importance of developing efficient catalysts, but still a critical challenge. Here, a theoretical screening is performed on transition metal-tetragonal carbon nitride (TM@T-C2N) as active and selective electrocatalysts for NO3RR, where detailed reaction mechanisms and activity origins are explored. In addition, five-step screening criteria and volcano plots enable fast prescreening among numerous candidates.We identify that V@T-C2N and Cr@T-C2N are promising candidates with low overpotentials and high selectivity and stability. In particular, a significant negative correlation between the adsorption strength ofnitrate and the Gibbs free energy for the last proton-electron coupling step (*NH2→*NH3) was existed, which is considerably advantaged to track the activity trend and reveal the origin of activity. This work provides theoretical insights into the rational design of TM-N4/C catalysts for NO3RR andpaves a valuable electrochemical screening framework for other multi-step reactions.

A feasible strategy for designing cytochrome P450-mimic sandwich-like single-atom nanozymes toward electrochemical CO2 conversion

Carbon dioxide electroreduction (CO2ER) presents a promising strategy for environmentally friendly CO2 utilization due to its low energy consumption. Single-atom nanozymes (SANs), amalgamating the benefits of single-atom catalysts and nanozymes, have become a hot topic in catalysis. Inspired by the intricate structure of cytochrome P450, we designed 81 sandwich-like SANs using Group-VIII transition metals (TMN4-S-TM'N4) and evaluated their performance in CO2ER using density functional theory (DFT). Our investigation revealed that most SANs display superior catalytic activity and improved specific product selectivity in comparison to the Cu (211) surface. Notably, IrN4-S-TMN4 (TM = Co, Rh, Pd) exhibited selective CO2 reduction to CO with remarkable limiting potentials (UL) of -0.11, -0.07, and -0.09 V, respectively, demonstrating potential as artificial CO dehydrogenases. Furthermore, RuN4-S-RuN4 exhibited formate dehydrogenase-like activity, resulting in selective production of HCOOH at a UL of -0.10 V. Machine learning analysis elucidated that the exceptional activity and selectivity of these SANs stemmed from precise modulation of electron density on sulfur atoms, achieved by varying transition metals in the subsurface. Our research not only identifies exceptional SANs for CO2ER but also provides insights into innovative methods for regulating non-bonding interactions and achieving sustainable CO2 conversion.

Benchmarking a Molecular Flake Model on the Road to Programmable Graphene-Based Single-Atom Catalysts

Single-atom catalysts (SACs) of embedding an active metal in nitrogen-doped graphene are emergent catalytic materials in various applications. The rational design of efficient SACs necessitates an electronic and mechanistic understanding of those materials with reliable quantum mechanical simulations. Conventional computational methods of modeling SACs involve using an infinite slab model with periodic boundary condition, limiting to the selection of generalized gradient approximations as the exchange correlation (XC) functional within density functional theory (DFT). However, these DFT approximations suffer from electron self-interaction error and delocalization error, leading to errors in predicted charge-transfer energetics. An alternative strategy is using a molecular flake model, which carved out the important catalytic center by cleaving C-C bonds and employing a hydrogen capping scheme to saturate the innocent dangling bonds at the molecular boundary. By doing so, we can afford more accurate hybrid XC functionals, or even high-level correlated wavefunction theory, to study those materials. In this work, we compared the structural, electronic, and catalytic properties of SACs simulated using molecular flake models and periodic slab models with first-row transition metals as the active sites. Molecular flake models successfully reproduced structural properties, including both global distortion and local metal-coordination environment, as well as electronic properties, including spin magnetic moments and metal partial charges, for all transition metals studied. In addition, we calculated CO binding strength as a descriptor for electrochemical CO2 reduction reactivity and noted qualitatively similar trends between two models. Using the computationally efficient molecular flake models, we investigated the effect of tuning Hartree-Fock exchange in a global hybrid functional on the CO binding strength and observed system-dependent sensitivities. Overall, our calculations provide valuable insights into the development of accurate and efficient computational tools to simulate SACs.

The Synergistic Effect between Metal and Sulfur Vacancy to Boost CO2 Reduction Efficiency: A Study on Descriptor Transferability and Activity Prediction

Both metal center active sites and vacancies can influence the catalytic activity of a catalyst. A quantitative model to describe the synergistic effect between the metal centers and vacancies is highly desired. Herein, we proposed a machine learning model to evaluate the synergistic index, PSyn, which is learned from the possible pathways for CH4 production from CO2 reduction reaction (CO2RR) on 26 metal-anchored MoS2 with and without sulfur vacancy. The data set consists of 1556 intermediate structures on metal-anchored MoS2, which are used for training. The 2028 structures from the literature, comprising both single active site and dual active sites, are used for external test. The XGBoost model with 3 features, including electronegativity, d-shell valence electrons of metal, and the distance between metal and vacancy, exhibited satisfactory prediction accuracy on limiting potential. Fe@Sv-MoS2 and Os@MoS2 are predicted to be promising CO2RR catalysts with high stability, low limiting potential, and high selectivity against hydrogen evolution reactions (HER). Based on some easily accessible descriptors, transferability can be achieved for both porous materials and 2D materials in predicting the energy change in the CO2RR and nitrogen reduction reaction (NRR). Such a predictive model can also be applied to predict the synergistic effect of the CO2RR in other oxygen and tungsten vacancy systems.

Tailoring the catalytic activity and selectivity on CO2 to C1 products by the synergistic effect of reactive molecules: A DFT study

The conversion of CO2 to CO is one of the crucial pathways in the carbon dioxide reduction reaction (CO2RR). Iron and nitrogen co-doped carbon matrix (FeN4) is a promising catalyst for converting CO2to CO with excellent activity and selectivity. However, the reactive mechanism of CO2RR on the FeN4 catalyst is not fully unveiled. For example, it is still evasive that the obtained C1 product is methanol and/or methane instead of CO in some cases. Herein, DFT calculation is conducted to unravel the effect from both solvent molecules and intermediates as axial groups on the selectivity of C1 products in CO2RR using FeN4 catalyts. Calculation results demonstrate that the FeN4(H), FeN4(OH), FeN4(COOH), and FeN4(CO) configurations are not only beneficial to the removal of CO, but also effectively suppress the hydrogen evolution reaction, whereas the FeN4, FeN4(CO2) and FeN4(H2O) configurations are inclined to produce CH3OH and/or CH4. The mechanism studied in this work provides an inspiration of optimizing the selectivity of C1 products in CO2RR from the perspective of regulating solvent molecules and intermediates as axial groups on FeN4.

A novel tetragonal T-C2N supported transition metal atoms as superior bifunctional catalysts for OER/ORR: From coordination environment to rational design

Single-atom catalysts with particular electronic structures and precisely regulated coordination environments delivering excellent activity for oxygen-evolution reaction (OER) and oxygen-reduction reaction (ORR) are highly desirable for renewable energy applications. In this work, a novel tetragonal carbon nitride T-C2N monolayer with remarkable stability was predicted by using the RG2 method. Inspired by the well-defined atomic structures and just right N4 aperture of T-C2N substrate, the electrocatalytic performance of a series of transition metal single-atoms anchored on porous T-C2N matrix (TM@C2N) have been systematically investigated. In addition, machine learning (ML) method was employed with the gradient boosting regression GBR model to deeply explore the complex controlling factors and offer direct guidance for rational discovery of desirable catalysts. On this basis, the coordination environment of the central TM active sites has been tailored by incorporating heteroatoms. Impressively, the Co@C2N/B-C, Rh@C2N/SC and Rh@C2N/SN exhibit significantly enhanced OER/ORR activity with notably low ηOER/ηORR of 0.39/0.32, 0.26/0.35 and 0.37/0.27 V, respectively. Our work provides insights into the rational design, data-driven, performance regulation, mechanism analysis and practical application of TMNC catalysts. Such a systematic theoretical framework can also be expanded to many other kinds of catalysts for energy storage and conversion.

Advances on Axial Coordination Design of Single-Atom Catalysts for Energy Electrocatalysis: A Review

Single-atom catalysts (SACs) have garnered increasingly growing attention in renewable energy scenarios, especially in electrocatalysis due to their unique high efficiency of atom utilization and flexible electronic structure adjustability. The intensive efforts towards the rational design and synthesis of SACs with versatile local configurations have significantly accelerated the development of efficient and sustainable electrocatalysts for a wide range of electrochemical applications. As an emergent coordination avenue, intentionally breaking the planar symmetry of SACs by adding ligands in the axial direction of metal single atoms offers a novel approach for the tuning of both geometric and electronic structures, thereby enhancing electrocatalytic performance at active sites. In this review, we briefly outline the burgeoning research topic of axially coordinated SACs and provide a comprehensive summary of the recent advances in their synthetic strategies and electrocatalytic applications. Besides, the challenges and outlooks in this research field have also been emphasized. The present review provides an in-depth and comprehensive understanding of the axial coordination design of SACs, which could bring new perspectives and solutions for fine regulation of the electronic structures of SACs catering to high-performing energy electrocatalysis.

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.

Single-Site Metal-Organic Framework and Copper Foil Tandem Catalyst for Highly Selective CO2 Electroreduction to C2 H4

Tandem catalysis is a promising way to break the limitation of linear scaling relationship for enhancing efficiency, and the desired tandem catalysts for electrochemical CO₂ reduction reaction (CO₂ RR) are urgent to be developed. Here, a tandem electrocatalyst created by combining Cu foil (CF) with a single-site Cu(II) metal-organic framework (MOF), named as Cu-MOF-CF, to realize improved electrochemical CO₂ RR performance, is reported. The Cu-MOF-CF shows suppression of CH₄ , great increase in C₂ H₄ selectivity (48.6%), and partial current density of C₂ H₄ at -1.11 V versus reversible hydrogen electrode. The outstanding performance of Cu-MOF-CF for CO₂ RR results from the improved microenvironment of the Cu active sites that inhibits CH₄ production, more CO intermediate produced by single-site Cu-MOF in situ for CF, and the enlarged active surface area by porous Cu-MOF. This work provides a strategy to combine MOFs with copper-based electrocatalysts to establish high-efficiency electrocatalytic CO₂ RR.

Emerging materials for electrochemical CO2 reduction: progress and optimization strategies of carbon-based single-atom catalysts

The electrochemical CO₂ reduction reaction can effectively convert CO₂ into promising fuels and chemicals, which is helpful in establishing a low-carbon emission economy. Compared with other types of electrocatalysts, single-atom catalysts (SACs) immobilized on carbon substrates are considered to be promising candidate catalysts. Atomically dispersed SACs exhibit excellent catalytic performance in CO₂RR due to their maximum atomic utilization, unique electronic structure, and coordination environment. In this paper, we first briefly introduce the synthetic strategies and characterization techniques of SACs. Then, we focus on the optimization strategies of the atomic structure of carbon-based SACs, including adjusting the coordination atoms and coordination numbers, constructing the axial chemical environment, and regulating the carbon substrate, focusing on exploring the structure-performance relationship of SACs in the CO₂RR process. In addition, this paper also briefly introduces the diatomic catalysts (DACs) as an extension of SACs. At the end of the paper, we summarize the article with an exciting outlook discussing the current challenges and prospects for research on the application of SACs in CO₂RR.

Surface Co-Modification of Halide Anions and Potassium Cations Promotes High-Rate CO2 -to-Ethanol Electrosynthesis

The high-rate electrochemical CO₂ conversion to ethanol with high partial current density is attractive but challenging, which requires competing with other reduction products as well as hydrogen evolution. This work demonstrates the in situ reconstruction of KCuF₃ perovskite under CO₂ electroreduction conditions to fabricate a surface fluorine-bonded, single-potassium-atom-modified Cu(111) nanocrystal (K-F-Cu-CO₂ ). Density functional theory calculations reveal that the co-modification of both F and K atoms on the Cu(111) surface can promote the ethanol pathway via stabilization of the CO bond and selective hydrogenation of the CC bond in the CH₂ CHO* intermediate, while the single modification of either F or K is less effective. The K-F-Cu-CO₂ electrocatalyst exhibits an outstanding CO₂ -to-ethanol partial current density of 423 ± 30 mA cm^-2 with the corresponding Faradaic efficiency of 52.9 ± 3.7%, and a high electrochemical stability at large current densities, thus suggesting an attractive means of surface co-modification of halide anions and alkali-metal cations on Cu catalysts for high-rate CO₂ -to-ethanol electrosynthesis.

Microenvironment Modulation in Carbon-Supported Single-Atom Catalysts for Efficient Electrocatalytic CO2 Reduction

The electrocatalytic CO 2 reduction reaction (ECRR) becomes an effective way to reduce excess CO 2 in the air and a promising strategy to maintain carbon balance. Carbon-supported single-atom catalysts (C-SACs) is a kind of cost savings and most promising catalysts for ECRR. For C-SACs, the key to achieving efficient ECRR performance is to adjusting the electronic structure of the central metal atoms by modulating their microenvironment of the catalysts. Not only the coordination numbers and hetero-atom coordination, but also the regulation of diatomic sites have a great influence on the performance of C-SACs. This review mainly focuses on recent studies for the microenvironment modulation in C-SACs for efficient ECRR. We hope that this review can contribute readers a comprehensive insight in the current research status of C-SACs for ECRR, as well as provide help for the rational design of C-SACs with better ECRR performance.