Selective Etching Quaternary MAX Phase toward Single Atom Copper Immobilized MXene (Ti3C2Clx) for Efficient CO2 Electroreduction to Methanol

Multi-mechanism antitumor/antibacterial effects of Cu-EGCG self-assembling nanocomposite in tumor nanotherapy and drug-resistant bacterial wound infections

Chemotherapy and surgery stand as primary cancer treatments, yet the unique traits of the tumor microenvironment hinder their effectiveness. The natural compound epigallocatechin gallate (EGCG) possesses potent anti-tumor and antibacterial traits. However, the tumor's adaptability to chemotherapy due to its acidic pH and elevated glutathione (GSH) levels, coupled with the challenges posed by drug-resistant bacterial infections post-surgery, impede treatment outcomes. To address these challenges, researchers strive to explore innovative treatment strategies, such as multimodal combination therapy. This study successfully synthesized Cu-EGCG, a metal-polyphenol network, and detailly characterized it by using synchrotron radiation and high-resolution mass spectrometry (HRMS). Through chemodynamic therapy (CDT), photothermal therapy (PTT), and photodynamic therapy (PDT), Cu-EGCG showed robust antitumor and antibacterial effects. Cu+ in Cu-EGCG actively participates in a Fenton-like reaction, generating hydroxyl radicals (·OH) upon exposure to hydrogen peroxide (H2O2) and converting to Cu2+. This Cu2+ interacts with GSH, weakening the oxidative stress response of bacteria and tumor cells. Density functional theory (DFT) calculations verified Cu-EGCG's efficient GSH consumption during its reaction with GSH. Additionally, Cu-EGCG exhibited outstanding photothermal conversion when exposed to 808 nm near-infrared (NIR) radiation and produced singlet oxygen (1O2) upon laser irradiation. In both mouse tumor and wound models, Cu-EGCG showcased remarkable antitumor and antibacterial properties.

Heteroatom Immobilization Engineering toward High-Performance Metal Anodes

Heteroatom immobilization engineering (HAIE) is becoming a forefront approach in materials science and engineering, focusing on the precise control and manipulation of atomic-level interactions within heterogeneous systems. HAIE has emerged as an efficient strategy to fabricate single-atom sites for enhancing the performance of metal-based batteries. Despite the significant progress achieved through HAIE in metal anodes for metal-based batteries, several critical challenges such as metal dendrites, side reactions, and sluggish reaction kinetics are still present. In this review, we delve into the fundamental principles underlying heteroatom immobilization engineering in metal anodes, aiming to elucidate its role in enhancing the electrochemical performance in batteries. We systematically investigate how HAIE facilitates uniform nucleation of metal in anodes, how HAIE inhibits side reactions at the metal anode-electrolyte interface, and the role of HAIE in promoting the desolvation of metal ions and accelerating reaction kinetics within metal-based batteries. Finally, we discuss various strategies for implementing HAIE in electrode materials, such as high-temperature pyrolysis, vacancy reduction, and molten-salt etching and anchoring. These strategies include selecting appropriate heteroatoms, optimizing immobilization methods, and constructing material architectures. They can be utilized to further refine the performance to enhance the capabilities of HAIE and facilitate its widespread application in next-generation metal-based battery technologies.

MXene Synthesis and Carbon Capture Applications: Mini-Review

MXenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, have garnered significant attention due to their remarkable potential for energy storage, electrocatalysis, and gas separation applications. The fabrication processes of MXene involve building up the MXene structure from constituent elements and the selective elimination of M-A bonds from the precursor MAX. However, considerable efforts are still required to design and develop efficient MXene-based technologies. This review article aims to briefly analyse the synthesis methods employed for MXene production, ranging from direct synthesis and conventional chemical wet etching approach to the more recent molten salt etching technique. The review highlights the advancements made in achieving precise control over the terminal groups, which is paramount for tailoring the properties of MXenes for specific applications. Furthermore, the potential of MXene-based materials for carbon capture applications, particularly in developing advanced adsorbents, is emphasized. The in-depth examination of MXene synthesis techniques and their implications for carbon capture applications provides a solid foundation for developing and optimizing these promising materials.

Proton-Coupled Electron Transfer on Cu2O/Ti3C2Tx MXene for Propane (C3H8) Synthesis from Electrochemical CO2 Reduction

Electrochemical CO2 reduction reaction (CO2RR) to produce value-added multi-carbon chemicals has been an appealing approach to achieving environmentally friendly carbon neutrality in recent years. Despite extensive research focusing on the use of CO2 to produce high-value chemicals like high-energy-density hydrocarbons, there have been few reports on the production of propane (C3H8), which requires carbon chain elongation and protonation. A rationally designed 0D/2D hybrid Cu2O anchored-Ti3C2Tx MXene catalyst (Cu2O/MXene) is demonstrated with efficient CO2RR activity in an aqueous electrolyte to produce C3H8. As a result, a significantly high Faradaic efficiency (FE) of 3.3% is achieved for the synthesis of C3H8 via the CO2RR with Cu2O/MXene, which is ≈26 times higher than that of Cu/MXene prepared by the same hydrothermal process without NH4OH solution. Based on in-situ attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and density functional theory (DFT) calculations, it is proposed that the significant electrocatalytic conversion originated from the synergistic behavior of the Cu2O nanoparticles, which bound the *C2 intermediates, and the MXene that bound the *CO coupling to the C3 intermediate. The results disclose that the rationally designed MXene-based hybrid catalyst facilitates multi-carbon coupling as well as protonation, thereby manipulating the CO2RR pathway.

Molten Salt Derived MXenes: Synthesis and Applications

About one decade after the first report on MXenes, these 2D early transition metal carbides or nitrides have become among the best-performing materials in electrode applications related to electrical energy storage devices and power-to-fuels conversion. MXenes are obtained by a top-down approach starting from the appropriate 3D MAX phase that undergoes etching of the A-site metal. Initial etching procedures are based on the use of concentrated HF or the in situ generation of this highly corrosive and poisonous reagent. Etching of the MAX phase is one of the major hurdles limiting the progress of the field. The present review summarizes an alternative, universal, and easily scalable etching procedure based on treating the MAX precursor with a Lewis acid molten salt. The review starts with presenting the current state of the art of the molten salt etching procedure to obtain or modify MXene, followed by a summary of the applications of these MXene samples. The aim of the review is to show the versatility and advantages of molten salt etching in terms of general applicability, control of the surface terminal groups, and uniform deposition of metal nanoparticles, among other features of the procedure.

Advances in MXene surface functionalization modification strategies for CO2 reduction

MXenes, 2D transition metal carbides and nitrides, show great potential in electrocatalytic CO2 reduction reaction (ECO2RR) applications owing to their tunable structure, abundant surface functional groups, large specific surface area and remarkable conductivity. However, the ECO2RR has a complex pathway involving various reaction intermediates. The reaction process yields various products alongside a competitive electrolytic water-splitting reaction. These factors limit the application of MXenes in ECO2RRs. Therefore, this review begins by examining the functionalized modification of MXenes to enhance their catalytic activity and stability via the regulation of interactions between carriers and the catalytic centre. The review firstly covers the synthesis methods and characterisation techniques for functionalized MXenes reported in recent years. Secondly, it presents the methods applied for the functionalized modification of carriers through surface loading of single atoms, clusters, and nanoparticles and construction of composites. These methods regulate the stability, active sites, and metal-carrier electronic interactions. Finally, the article discusses the challenges, opportunities, pressing issues, and future prospects related to MXene-based electrocatalysts.

Challenges and Opportunities for Single-Atom Electrocatalysts: From Lab-Scale Research to Potential Industry-Level Applications

Single-atom electrocatalysts (SACs) are a class of promising materials for driving electrochemical energy conversion reactions due to their intrinsic advantages, including maximum metal utilization, well-defined active structures, and strong interface effects. However, SACs have not reached full commercialization for broad industrial applications. This review summarizes recent research achievements in the design of SACs for crucial electrocatalytic reactions on their active sites, coordination, and substrates, as well as the synthesis methods. The key challenges facing SACs in activity, selectivity, stability, and scalability, are highlighted. Furthermore, it is pointed out the new strategies to address these challenges including increasing intrinsic activity of metal sites, enhancing the utilization of metal sites, improving the stability, optimizing the local environment, developing new fabrication techniques, leveraging insights from theoretical studies, and expanding potential applications. Finally, the views are offered on the future direction of single-atom electrocatalysis toward commercialization.

Single-Atom Cu Nanozyme-Loaded Bone Scaffolds for Ferroptosis-Synergized Mild Photothermal Therapy in Osteosarcoma Treatment

The rapid multiplication of residual tumor cells and poor reconstruction quality of new bone are considered the major challenges in the postoperative treatment of osteosarcoma. It is a promising candidate for composite bone scaffold which combines photothermal therapy (PTT) and bone regeneration induction for the local treatment of osteosarcoma. However, it is inevitable to damage the normal tissues around the tumor due to the hyperthermia of PTT, while mild heat therapy shows a limited effect on antitumor treatment as the damage can be easily repaired by stress-induced heat shock proteins (HSP). This study reports a new type of single-atom Cu nanozyme-loaded bone scaffolds, which exhibit exceptional photothermal conversion properties as well as peroxidase and glutathione oxidase mimicking activities in vitro experiments. This leads to lipid peroxidation (LPO) and reactive oxygen species (ROS) upregulation, ultimately causing ferroptosis. The accumulation of LPO and ROS also contributes to HSP70 inactivation, maximizing PTT efficiency against tumors at an appropriate therapeutic temperature and minimizing the damage to surrounding normal tissues. Further, the bone scaffold promotes bone regeneration via a continuous release of bioactive ions (Ca2+, P5+, Si4+, and Cu2+). The results of in vivo experiments reveal that scaffolds inhibit tumor growth and promote bone repair.

Structural Design of Single-Atom Catalysts for Enhancing Petrochemical Catalytic Reaction Process

Petroleum, as the "lifeblood" of industrial development, is the important energy source and raw material. The selective transformation of petroleum into high-end chemicals is of great significance, but still exists enormous challenges. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites, promise a broad application in petrochemical processes. Herein, the research systematically summarizes the recent research progress of SACs in petrochemical catalytic reaction, proposes the role of structural design of SACs in enhancing catalytic performance, elucidates the catalytic reaction mechanisms of SACs in the conversion of petrochemical processes, and reveals the high activity origins of SACs at the atomic scale. Finally, the key challenges are summarized and an outlook on the design, identification of active sites, and the appropriate application of artificial intelligence technology is provided for achieving scale-up application of SACs in petrochemical process.

Self-Assembled MXene@Fluorographene Hybrid for High Dielectric Constant and Low Loss Ferroelectric Polymer Composite Films

Modern electrical applications urgently need flexible polymer films with a high dielectric constant (εr) and low loss. Recently, the MXene-filled percolative composite has emerged as a potential material choice because of the promised high εr. Nevertheless, the typically accompanied high dielectric loss hinders its applications. Herein, a facile and effective surface modification strategy of cladding Ti3C2Tx MXene (T = F or O; FMX) with fluorographene (FG) via self-assembly is proposed. The obtained FMX@FG hybrid yields high εr (up to 108 @1 kHz) and low loss (loss tangent tan δ = 1.16 @ 1 kHz) in a ferroelectric polymer composite at a low loading level (the equivalent of 1.5 wt % FMX), which is superior to its counterparts in our work (e.g., FMX: εr = 104, tan δ = 10.71) and other studies. It is found that the FG layer outside FMX plays a critical role in both the high dielectric constant and low loss from experimental characterizations and finite element simulations. For one thing, FG with a high F/C ratio would induce a favorable structure of high β-phase crystallinity, extensive microcapacitor networks, and abundant interfacial dipoles in polymer composites that account for the high εr. For another, FG, as a highly insulating layer, can inhibit the formation of conductive networks and inter-FMX electron tunneling, which is responsible for conduction loss. The results demonstrate the potential of a self-assembled FMX@FG hybrid for high εr and low loss polymer composite films and offer a new strategy for designing advanced polymer composite dielectrics.

Theoretical Study of Electrocatalytic CO2 Reduction Mechanism on Typical MXenes under Realistic Conditions

MXenes are a revolutionary class of two-dimensional materials that have been recently demonstrated to exhibit promising capability of electrocatalytic CO2 reduction reaction (CO2RR) in theory and experiment. In electrocatalytic reactions, the active phases, the mechanism, and the performance can be greatly influenced by electrochemical conditions such as applied electrode potential, pH, and electrolyte. Therefore, in this first-principles study, the stable surface structures of three typical MXenes (V2C, Mo2C, and Ti3C2) with variation of electrocatalytic conditions were determined by the Pourbaix phase diagrams. Additionally, the reaction mechanism for CO2RR toward C1 products was investigated based on the thermal dynamically stable phases. The computation revealed that surfaces of all three MXenes are dominated by H* termination throughout the practical CO2RR electrochemical condition ranges. Meanwhile, the bicarbonate ions, which serve as the major electrolyte in CO2RR, show thermal dynamic unfavorability to adsorb on the surfaces. Among the three types of MXenes, V2CH exhibits higher activity in generating CO and HCOOH through the CO2RR, while Mo2CH exhibits higher activity in producing HCHO, CH3OH, and CH4. This comprehensive study provides crucial insights into the mechanism of electrocatalytic CO2RR on MXenes under realistic electrochemical conditions.

Loading of Single Atoms of Iron, Cobalt, or Nickel to Enhance the Electrocatalytic Hydrogen Evolution Reaction of Two-Dimensional Titanium Carbide

The rational design of advanced electrocatalysts at the molecular or atomic level is important for improving the performance of hydrogen evolution reactions (HERs) and replacing precious metal catalysts. In this study, we describe the fabrication of electrocatalysts based on Fe, Co, or Ni single atoms supported on titanium carbide (TiC) using the molten salt method, i.e., TiC-FeSA, TiC-CoSA, or TiC-NiSA, to enhance HER performance. The introduction of uniformly distributed transition-metal single atoms successfully reduces the overpotential of HERs. Overpotentials of TiC-FeSA at 10 mA cm-2 are 123.4 mV with 61.1 mV dec-1 Tafel slope under acidic conditions and 184.2 mV with 85.1 mV dec-1 Tafel slope under alkaline conditions, which are superior to TiC-NiSA and TiC-CoSA. TiC samples loaded with transition-metal single atoms exhibit high catalytic activity and long stability under acidic and basic conditions. Density functional theory calculations indicate that the introduction of transition-metal single atoms effectively reduces the HER barrier of TiC-based electrocatalysts.

Modulating the Structure and Composition of Single-Atom Electrocatalysts for CO2 reduction

Electrochemical CO2 reduction reaction (eCO2 RR) is a promising strategy to achieve carbon cycling by converting CO2 into value-added products under mild reaction conditions. Recently, single-atom catalysts (SACs) have shown enormous potential in eCO2 RR due to their high utilization of metal atoms and flexible coordination structures. In this work, the recent progress in SACs for eCO2 RR is outlined, with detailed discussions on the interaction between active sites and CO2 , especially the adsorption/activation behavior of CO2 and the effects of the electronic structure of SACs on eCO2 RR. Three perspectives form the starting point: 1) Important factors of SACs for eCO2 RR; 2) Typical SACs for eCO2 RR; 3) eCO2 RR toward valuable products. First, how different modification strategies can change the electronic structure of SACs to improve catalytic performance is discussed; Second, SACs with diverse supports and how supports assist active sites to undergo catalytic reaction are introduced; Finally, according to various valuable products from eCO2 RR, the reaction mechanism and measures which can be taken to improve the selectivity of eCO2 RR are discussed. Hopefully, this work can provide a comprehensive understanding of SACs for eCO2 RR and spark innovative design and modification ideas to develop highly efficient SACs for CO2 conversion to various valuable fuels/chemicals.

Recent advancement and key opportunities of MXenes for electrocatalysis

MXenes are promising materials for electrocatalysis due to their excellent metallic conductivity, hydrophilicity, high specific surface area, and excellent electrochemical properties. Herein, we summarize the recent advancement of MXene-based materials for electrocatalysis and highlight their key challenges and opportunities. In particular, this review emphasizes on the major design principles of MXene-based electrocatalysts, including (1) coupling MXene with active materials or heteroatomic doping to create highly active synergistic catalyst sites; (2) construction of 3D MXene structure or introducing interlayer spacers to increase active areas and form fast mass-charge transfer channel; and (3) protecting edge of MXene or in situ transforming the surface of MXene to stable active substance that inhibits the oxidation of MXene and then enhances the stability. Consequently, MXene-based materials exhibit outstanding performance for a variety of electrocatalytic reactions. Finally, the key challenges and promising prospects of the practical applications of MXene-based electrocatalysts are briefly proposed.

Dual-Atom Co/Ni Electrocatalyst Anchored at the Surface-Modified Ti3C2Tx MXene Enables Efficient Hydrogen and Oxygen Evolution Reactions

Dual-atom catalytic sites on conductive substrates offer a promising opportunity for accelerating the kinetics of multistep hydrogen and oxygen evolution reactions (HER and OER, respectively). Using MXenes as substrates is a promising strategy for depositing those dual-atom electrocatalysts, if the efficient surface anchoring strategy ensuring metal-substrate interactions and sufficient mass loading is established. We introduce a surface-modification strategy of MXene substrates by preadsorbing L-tryptophan molecules, which enabled attachment of dual-atom Co/Ni electrocatalyst at the surface of Ti3C2Tx by forming N-Co/Ni-O bonds, with mass loading reaching as high as 5.6 wt %. The electron delocalization resulting from terminated O atoms on MXene substrates, N atoms in L-tryptophan anchoring moieties, and catalytic metal atoms Co and Ni provides an optimal adsorption strength of intermediates and boosts the HER and OER kinetics, thereby notably promoting the intrinsic activity of the electrocatalyst. CoNi-Ti3C2Tx electrocatalyst displayed HER and OER overpotentials of 31 and 241 mV at 10 mA cm-2, respectively. Importantly, the CoNi-Ti3C2Tx electrocatalyst also exhibited high operational stability for both OER and HER over 100 h at an industrially relevant current density of 500 mA cm-2. Our study provided guidance for constructing dual-atom active metal sites on MXene substrates to synergistically enhance the electrochemical efficiency and stability of the energy conversion and storage systems.

Copper Single-Atom Catalysts-A Rising Star for Energy Conversion and Environmental Purification: Synthesis, Modification, and Advanced Applications

Future renewable energy supply and green, sustainable environmental development rely on various types of catalytic reactions. Copper single-atom catalysts (Cu SACs) are attractive due to their distinctive electronic structure (3d orbitals are not filled with valence electrons), high atomic utilization, and excellent catalytic performance and selectivity. Despite numerous optimization studies are conducted on Cu SACs in terms of energy conversion and environmental purification, the coupling among Cu atoms-support interactions, active sites, and catalytic performance remains unclear, and a systematic review of Cu SACs is lacking. To this end, this work summarizes the recent advances of Cu SACs. The synthesis strategies of Cu SACs, metal-support interactions between Cu single atoms and different supports, modification methods including modification for carriers, coordination environment regulating, site distance effect utilizing, and dual metal active center catalysts constructing, as well as their applications in energy conversion and environmental purification are emphatically introduced. Finally, the opportunities and challenges for the future Cu SACs development are discussed. This review aims to provide insight into Cu SACs and a reference for their optimal design and wide application.

Functional MXenes: Progress and Perspectives on Synthetic Strategies and Structure-Property Interplay for Next-Generation Technologies

MXenes are a class of 2D materials that include layered transition metal carbides, nitrides, and carbonitrides. Since their inception in 2011, they have garnered significant attention due to their diverse compositions, unique structures, and extraordinary properties, such as high specific surface areas and excellent electrical conductivity. This versatility has opened up immense potential in various fields, catalyzing a surge in MXene research and leading to note worthy advancements. This review offers an in-depth overview of the evolution of MXenes over the past 5 years, with an emphasis on synthetic strategies, structure-property relationships, and technological prospects. A classification scheme for MXene structures based on entropy is presented and an updated summary of the elemental constituents of the MXene family is provided, as documented in recent literature. Delving into the microscopic structure and synthesis routes, the intricate structure-property relationships are explored at the nano/micro level that dictate the macroscopic applications of MXenes. Through an extensive review of the latest representative works, the utilization of MXenes in energy, environmental, electronic, and biomedical fields is showcased, offering a glimpse into the current technological bottlenecks, such asstability, scalability, and device integration. Moreover, potential pathways for advancing MXenes toward next-generation technologies are highlighted.

Instructive Synergistic Effect of Coordinating Phosphorus in Transition-Metal-Doped β-Phosphorus Carbide Guiding the Design of High-Performance CO2RR Electrocatalysts

Developing efficient electrocatalysts for the CO2 reduction reaction (CO2RR) is the key and difficult point to alleviate energy and climate issues. The synergistic catalytic effects between metal and nonmetal elements have gained attention for the design of the CO2RR electrocatalysts. The realization of this effect requires a suitable combination of metal and nonmetal elements, as well as the support of suitable substrates. Based on this, the transition-metal-doped β-phosphorus carbide (TM-PC) (TM = 4d and 5d transition metals except Tc) catalysts are designed, and their structures, electronic properties, and CO2RR catalytic performances are studied in depth via first-principle calculations. The strong bonding ability and high reactivity brought by the moderate electronegativity and abundant electrons and orbitals of phosphorus are the key to the excellent catalytic performance of TM-PCs. Coordinating phosphorus atoms improve the catalyst activity in two ways: (1) regulating the electron transfer of the TM active site, and (2) acting as the active site and changing the reaction mechanism. With the participation of coordinating P atoms, the "relay" of active sites reduces the limiting potential values for the reduction from CO2 to CH4 catalyzed by Cr-PC and Mo-PC by 0.27 and 0.23 V, respectively, compared with pathways where only the TM atom is the active site, reaching -0.55 and -0.63 V, respectively. Regarding the coordinating P atom as the second active site, Cr-PC and Mo-PC can catalyze the production of CH3CH2OH at limiting potential values of -0.54 and -0.67 V, respectively. This study demonstrates the dramatic enhancement of catalytic activity caused by suitable nonmetal coordinating atoms such as P and provides a reference for the design of high-performance CO2RR electrocatalysts based on metal-nonmetal coordinating active centers.

Atomic Cu Sites Engineering Enables Efficient CO2 Electroreduction to Methane with High CH4/C2H4 Ratio

Electrochemical reduction of CO2 into high-value hydrocarbons and alcohols by using Cu-based catalysts is a promising and attractive technology for CO2 capture and utilization, resulting from their high catalytic activity and selectivity. The mobility and accessibility of active sites in Cu-based catalysts significantly hinder the development of efficient Cu-based catalysts for CO2 electrochemical reduction reaction (CO2RR). Herein, a facile and effective strategy is developed to engineer accessible and structural stable Cu sites by incorporating single atomic Cu into the nitrogen cavities of the host graphitic carbon nitride (g-C3N4) as the active sites for CO2-to-CH4 conversion in CO2RR. By regulating the coordination and density of Cu sites in g-C3N4, an optimal catalyst corresponding to a one Cu atom in one nitrogen cavity reaches the highest CH4 Faraday efficiency of 49.04% and produces the products with a high CH4/C2H4 ratio over 9. This work provides the first experimental study on g-C3N4-supported single Cu atom catalyst for efficient CH4 production from CO2RR and suggests a principle in designing highly stable and selective high-efficiency Cu-based catalysts for CO2RR by engineering Cu active sites in 2D materials with porous crystal structures.

Progress in the Synthesis Process and Electrocatalytic Application of MXene Materials

With their rich surface chemistry, high electrical conductivity, variable bandgap, and thermal stability, 2D materials have been developed for effective electrochemical energy conversion systems over the past decade. Due to the diversity brought about by the use of transition metals and C/N pairings, the 2D material MXene has found excellent applications in many fields. Among the various applications, many breakthroughs have been made in electrocatalytic applications. Nevertheless, related studies on topics such as the factors affecting the material properties and safer and greener preparation methods have not been reported in detail. Therefore, in this paper, we review the relevant preparation methods of MXene and the safer, more environmentally friendly preparation techniques in detail, and summarize the progress of research on MXene-based materials as highly efficient electrocatalysts in the electrocatalytic field of hydrogen precipitation reaction, nitrogen reduction reaction, oxygen precipitation reaction, oxygen reduction reaction, and carbon dioxide reduction reaction. We also discuss the technology related to MXene materials for hydrogen storage. The main challenges and opportunities for MXene-based materials, which constitute a platform for next-generation electrocatalysis in basic research and practical applications, are highlighted. This paper aims to promote the further development of MXenes and related materials for electrocatalytic applications.

Insight on Atomically Dispersed Cu Catalysts for Electrochemical CO2 Reduction

Electrochemical CO2 reduction (ECO2R) with renewable electricity is an advanced carbon conversion technology. At present, copper is the only metal to selectively convert CO2 into multicarbon (C2+) products. Among them, atomically dispersed (AD) Cu catalysts have received great attention due to the relatively single chemical environment, which are able to minimize the negative impact of morphology, valence state, and crystallographic properties, etc. on product selectivity. Furthermore, the completely exposed atomic Cu sites not only provide space and bonding electrons for the adsorption of reactants in favor of better catalytic activity but also provide an ideal platform for studying its reaction mechanism. This review summarizes the recent progress of AD Cu catalysts as a chemically tunable platform for ECO2R, including the atomic Cu sites dynamic evolution, the catalytic performance, and mechanism. Furthermore, the prospects and challenges of AD Cu catalysts for ECO2R are carefully discussed. We sincerely hope that this review can contribute to the rational design of AD Cu catalysts with enhanced performance for ECO2R.

Charge-orbital synergistic engineering of TM@Ti3C2O1-xBx for highly selective CO2 electrochemical reduction

Inspired by MXene nanosheets and their regulation of surface functional groups, a series of Ti3C2-MXene-based single TM atom electrocatalysts with a doped boron (B) atom (TM@Ti3C2O2-xBx, TM is V, Cr, Mn, Fe, Co or Ni, x = 0.11) are proposed for achieving a high performance catalytic CO2 reduction reaction (CO2RR). The results reveal that the doped B atom involves in the adsorption reaction of CO2 molecules and CO intermediates in the CO2RR. The TM-to-C and B-to-C π-back bonding contribute to the activation of the CO2 molecules and CO intermediates in the CO2RR. Enough electrons from the single TM atom and B atom occupied orbitals can be injected into the CO2 molecules and *CO intermediates through direct bonding interactions, which effectively alleviates the difficulty of the first hydrogenation reaction step and further helps CO reduction towards CH4. The calculated values of ΔG for the first hydrogenation reaction and the formation of *CHO on Ti3C2O2-xBx are significantly smaller than those of other single-atom catalysts (SACs). Fe@Ti3C2O2-xBx is found to have the highest electrocatalytic activity with a limiting potential of ∼0.40 V and exhibits a high selectivity for obtaining CH4 through the CO2RR compared with the hydrogen evolution reaction. This work is expected to open a research path for engineering the charge-orbital state of the innate atoms of a substrate based on mechanistic insights, which guides the rational design of highly selective MXene-based CO2RR electrocatalysts.

Combining First-Principles Modeling and Symbolic Regression for Designing Efficient Single-Atom Catalysts in the Oxygen Evolution Reaction on Mo2CO2 MXenes

In this study, we address the significant challenge of overcoming limitations in the catalytic efficiency for the oxygen evolution reaction (OER). The current linear scaling relationships hinder the optimization of the electrocatalytic performance. To tackle this issue, we investigate the potential of designing single-atom catalysts (SACs) on Mo2CO2 MXenes for electrochemical OER using first-principles modeling simulations. By employing the Electrochemical Step Symmetry Index (ESSI) method, we assess OER intermediates to fine-tune the activity and identify the optimal SAC for Mo2CO2 MXenes. Our findings reveal that both Ag and Cu exhibit effectiveness as single atoms for enhancing OER activity on Mo2CO2 MXenes. However, among the 21 chosen transition metals (TMs) in this study, Cu stands out as the best catalyst for tweaking the overpotential (ηOER). This is due to Cu's lowest overpotential compared to other TMs, which makes it more favorable for the OER performance. On the other hand, Ag is closely aligned with ESSI = ηOER, making the tuning of its overpotential more challenging. Furthermore, we employ symbolic regression analysis to identify the significant factors that exhibit a correlation with the OER overpotential. By utilizing this approach, we derive mathematical formulas for the overpotential and identify key descriptors that affect the catalytic efficiency in the electrochemical OER on Mo2CO2 MXenes. This comprehensive investigation not only sheds light on the potential of MXenes in advanced electrocatalytic processes but also highlights the prospect of improved activity and selectivity in OER applications.

MXene Derivatives for Energy Storage and Conversions

Associated with the rapid development of 2D transition metal carbides, nitrides, and carbonitrides (MXenes), MXene derivatives have been recently exploited and exhibited unique physical/chemical properties, holding promising applications in the areas of energy storage and conversions. This review provides a comprehensive summarization of the latest research and progress on MXene derivatives, including termination-tailored MXenes, single-atom implanted MXenes, intercalated MXenes, van der Waals atomic layers, and non-van der Waals heterostructures. The intrinsic relationship between structure, properties, and corresponding applications for MXene derivatives are then emphasized. Finally, the essential challenges are addressed and perspectives for the MXene derivatives are also discussed.

Precise electronic structure modulation on MXene-based single atom catalysts for high-performance electrocatalytic CO2 reduction reaction: A first-principle study

Electrocatalytic CO₂ reduction reaction (CO₂RR) to CO is a logical approach to achieve a carbon-neutral cycle. In this work, a series of Ti₂CO₂ and O vacancy containing Ti₂CO₂ MXene-based transition metal (TM) single atom catalysts (SACs), including TM-Ti₂CO₂ and TM-O_v-Ti₂CO₂, are explored for high-performance CO₂RR. Sc/Ti/V/Cr-Ti₂CO₂ and Ni-O_v-Ti₂CO₂ are screened out with limiting potential (U_L) more positive than -0.50 V. Ni-O_v-Ti₂CO₂ is a candidate catalyst for CO₂RR to CO, considering its activity with U_L of -0.27 eV, and the selectivity relevant to hydrogen evolution reaction and HCOOH production. Meanwhile, a novel activity descriptor of TM-Ti-O group valence state is proposed according to that TMs work in synergy with coordinated Ti and O atoms and a level of around 0.64 e^- benefits to CO₂RR. This work highlights oxygen vacancy containing Ti₂CO₂-based Ni SAC as a promising catalyst for CO₂RR, and provides a feasible electronic structure design principle for guiding the design of MXene-based SACs for CO₂RR.

Progress and challenges of emerging MXene based materials for thermoelectric applications

To realize sustainable development, more and more countries forwarded carbon neutrality goal. Accordingly, improving the utilization efficiency of traditional fossil fuel is an effective strategy for this great goal. Keeping this in mind, developing thermoelectric devices to recover waste heat energy resulted in the consumption process of fuel is demonstrated to be promising. High performance thermoelectric devices require advanced materials. MXenes are a kind of 2D materials with a layered structure, which demonstrate excellent thermoelectric performance owing to their unique physical, mechanical, and chemical properties. Also, substantial achievement has been gained during the past few years in synthesizing MXene based materials for thermoelectric devices. In this review, the mainstream synthetic routes of MXene from etching MAX were summarized. Significantly, the current state and challenges of research on improving the performance of MXene based thermoelectrics are explored, including pristine MXene and MXene based composites.

Electrocatalyst Microenvironment Engineering for Enhanced Product Selectivity in Carbon Dioxide and Nitrogen Reduction Reactions

Carbon and nitrogen fixation strategies are regarded as alternative routes to produce valuable chemicals used as energy carriers and fertilizers that are traditionally obtained from unsustainable and energy-intensive coal gasification (CO and CH₄), Fischer-Tropsch (C₂H₄), and Haber-Bosch (NH₃) processes. Recently, the electrocatalytic CO₂ reduction reaction (CO₂RR) and N₂ reduction reaction (NRR) have received tremendous attention, with the merits of being both efficient strategies to store renewable electricity while providing alternative preparation routes to fossil-fuel-driven reactions. To date, the development of the CO₂RR and NRR processes is primarily hindered by the competitive hydrogen evolution reaction (HER); however, the corresponding strategies for inhibiting this undesired side reaction are still quite limited. Considering such complex reactions involve three gas-liquid-solid phases and successive proton-coupled electron transfers, it appears meaningful to review the current strategies for improving product selectivity in light of their respective reaction mechanisms, kinetics, and thermodynamics. By examining the developments and understanding in catalyst design, electrolyte engineering, and three-phase interface modulation, we discuss three key strategies for improving product selectivity for the CO₂RR and NRR: (i) targeting molecularly defined active sites, (ii) increasing the local reactant concentration at the active sites, and (iii) stabilizing and confining product intermediates.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO₂ reduction reaction (CO₂RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO₂RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO₂RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO₂RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO₂RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Recent Advances and Perspectives of Lewis Acidic Etching Route: An Emerging Preparation Strategy for MXenes

Since the discovery in 2011, MXenes have become the rising star in the field of two-dimensional materials. Benefiting from the metallic-level conductivity, large and adjustable gallery spacing, low ion diffusion barrier, rich surface chemistry, superior mechanical strength, MXenes exhibit great application prospects in energy storage and conversion, sensors, optoelectronics, electromagnetic interference shielding and biomedicine. Nevertheless, two issues seriously deteriorate the further development of MXenes. One is the high experimental risk of common preparation methods such as HF etching, and the other is the difficulty in obtaining MXenes with controllable surface groups. Recently, Lewis acidic etching, as a brand-new preparation strategy for MXenes, has attracted intensive attention due to its high safety and the ability to endow MXenes with uniform terminations. However, a comprehensive review of Lewis acidic etching method has not been reported yet. Herein, we first introduce the Lewis acidic etching from the following four aspects: etching mechanism, terminations regulation, in-situ formed metals and delamination of multi-layered MXenes. Further, the applications of MXenes and MXene-based hybrids obtained by Lewis acidic etching route in energy storage and conversion, sensors and microwave absorption are carefully summarized. Finally, some challenges and opportunities of Lewis acidic etching strategy are also presented.

Recent advances in the synthesis and electrocatalytic application of MXene materials

MXenes are a class of two-dimensional materials with a graphene-like structure, which have excellent optical, biological, thermodynamic, electrical and magnetic properties. Due to the diversity resulting from the combination of transition metals and C/N, the MXene family has expanded to more than 30 members and been applied in many fields with broad application prospects. Among their applications, electrocatalytic applications have achieved many breakthroughs. Therefore, in this review, we summarize the reports on the preparation of MXenes and their application in electrocatalysis published in the last five years and describe the two main methods for the preparation of MXenes, i.e., bottom-up and top to bottom synthesis. Different methods may change the structure or surface termination of MXenes, and accordingly affect their electrocatalytic performance. Furthermore, we highlight the application of MXenes in the electrocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and multi-functionalization. It can be concluded that the electrocatalytic properties of MXenes can be modified by changing the type of functional groups or doping. Also, MXenes can be compounded with other materials to produce electronic coupling and improve the catalytic activity and stability of the resulting composites. In addition, Mo₂C and Ti₃C₂ are two types of MXene materials that have been widely studied in the field of electrocatalysis. At present, research on the synthesis of MXenes is focused on carbides, whereas research on nitrides is rare, and there are no synthesis methods meeting the requirements of green, safety, high efficiency and industrialization simultaneously. Therefore, it is very important to explore environmentally friendly industrial production routes and devote more research efforts to the synthesis of MXene nitrides.

Copper-based catalysts for the electrochemical reduction of carbon dioxide: progress and future prospects

There is an urgent need for the development of high performance electrocatalysts for the CO₂ reduction reaction (CO₂RR) to address environmental issues such as global warming and achieve carbon neutral energy systems. In recent years, Cu-based electrocatalysts have attracted significant attention in this regard. The present review introduces fundamental aspects of the electrocatalytic CO₂RR process together with a systematic examination of recent developments in Cu-based electrocatalysts for the electroreduction of CO₂ to various high-value multicarbon products. Current challenges and future trends in the development of advanced Cu-based CO₂RR electrocatalysts providing high activity and selectivity are also discussed.

Pushing the Performance Limit of Cu/CeO2 Catalyst in CO2 Electroreduction: A Cluster Model Study for Loading Single Atoms

Pushing the performance limit of catalysts is a major goal of CO₂ electroreduction toward practical application. A single-atom catalyst is recognized as a solution for achieving this goal, which is, however, a double-edged sword considering the limited loading amount and stability of single-atom sites. To overcome the limit, the loading of single atoms on supports should be well addressed, requiring a suitable model system. Herein, we report the model system of an ultrasmall CeO₂ cluster (2.4 nm) with an atomic precise structure and a high surface-to-volume ratio for loading Cu single atoms. The combination of multiple characterizations and theoretical calculations reveals the loading location and limit of Cu single atoms on CeO₂ clusters, determining an optimal configuration for CO₂ electroreduction. The optimal catalyst achieves a maximum Faradaic efficiency (FE) of 67% and a maximum partial current density of -364 mA/cm² for CH₄, and can maintain high CH₄ FE values over 50% in a wide range of applied current densities (-50 ∼ -600 mA/cm²), exceeding those of the reported catalysts.

Recent progress of theoretical studies on electro- and photo-chemical conversion of CO2 with single-atom catalysts

The CO2 reduction reaction (CO2RR) into chemical products is a promising and efficient way to combat the global warming issue and greenhouse effect. The viability of the CO2RR critically rests with finding highly active and selective catalysts that can accomplish the desired chemical transformation. Single-atom catalysts (SACs) are ideal in fulfilling this goal due to the well-defined active sites and support-tunable electronic structure, and exhibit enhanced activity and high selectivity for the CO2RR. In this review, we present the recent progress of quantum-theoretical studies on electro- and photo-chemical conversion of CO2 with SACs and frameworks. Various calculated products of CO2RR with SACs have been discussed, including CO, acids, alcohols, hydrocarbons and other organics. Meanwhile, the critical challenges and the pathway towards improving the efficiency of the CO2RR have also been discussed.

Synergistically boosting the anchoring effect and catalytic activity of MXenes as bifunctional electrocatalysts for sodium-sulfur batteries by single-atom catalyst engineering

MXene based sulfur hosts have attracted enormous attention in room temperature sodium-sulfur (RT Na-S) batteries due to their strong affinity towards soluble sodium polysulfides (NaPSs). However, their electrocatalytic performance needs further improvement. Here, a series of single non-noble transition metal (TM = Fe, Co, Ni, and Cu) atoms anchored on Ti₂CS₂ (TM@Ti₂CS₂) were proposed as bifunctional sulfur hosts for Na-S batteries. The results testify that the introduction of TMs dramatically enhanced the chemical interaction between sulfur-containing species and Ti₂CS₂, which is attributed to the co-formation of TM-S and Na-S covalent bonds. Importantly, compared with pristine Ti₂CS₂, the sulfur reduction reaction (SRR) is thermodynamically more favorable on TM@Ti₂CS₂. In addition, the incorporation of Fe, Co, and Ni atoms is also conducive to promoting the dissociation of Na₂S. The density of states (DOS) results suggest that TM@Ti₂CS₂ maintains metallic conductivity during the whole charge and discharge process. Overall, constructing single atom catalysts is an effective strategy to further improve the electrochemical performance of MXene based sulfur hosts for Na-S batteries.

Tunable Structured MXenes With Modulated Atomic Environments: A Powerful New Platform for Electrocatalytic Energy Conversion

Owing to their rich surface chemistry, high conductivity, tunable bandgap, and thermal stability, structured 2D transition-metal carbides, nitrides, and carbonitrides (MXenes) with modulated atomic environments have emerged as efficient electrochemical energy conversion systems in the past decade. Herein, the most recent advances in the engineering of tunable structured MXenes as a powerful new platform for electrocatalytic energy conversion are comprehensively summarized. First, the state-of-the-art synthetic and processing methods, tunable nanostructures, electronic properties, and modulation principles of engineering MXene-derived nanoarchitectures are focused on. The current breakthroughs in the design of catalytic centers, atomic environments, and the corresponding structure-performance correlations, including termination engineering, heteroatom doping, defect engineering, heterojunctions, and alloying, are discussed. Furthermore, representative electrocatalytic applications of structured MXenes in energy conversion systems are also summarized. Finally, the challenges in and prospects for constructing MXene-based electrocatalytic materials are also discussed. This review provides a leading-edge understanding of the engineering of various MXene-based electrocatalysts and offers theoretical and experimental guidance for prospective studies, thereby promoting the practical applications of tunable structured MXenes in electrocatalytic energy conversion systems.

Recent Advances in Heterogeneous Electroreduction of CO2 on Copper-Based Catalysts

Facing greenhouse effects and the rapid exhaustion of fossil fuel, CO2 electrochemical reduction presents a promising method of environmental protection and energy transformation. Low onset potential, large current density, high faradaic efficiency (FE), and long-time stability are required for industrial production, due to economic costs and energy consumption. This minireview showcases the recent progress in catalyst design and engineering technology in CO2 reduction reaction (CO2RR) on copper based-catalysts. We focus on strategies optimizing the performance of copper-based catalysts, such as single-atom catalysts, doping, surface modification, crystal facet engineering, etc., and reactor design including gas diffusion layer, membrane electrode assembly, etc., in enhancing target electroreduction products including methane, methanol, ethylene, and C2+ oxygenates. The determination of the correlation and the developed technology might be helpful for future applications in the industry.

Magnetic Interacted Interaction Effect in MXene Skeleton: Enhanced Thermal-Generation for Electromagnetic Interference Shielding

With the rapid advancements of portable and wearable equipment, high-efficiency electromagnetic interference (EMI) shielding materials are highly entailed to eliminate radiated electromagnetic pollution. Herein, by assembling hexagonal SrFe₁₂ O₁₉ flakes into a Ti₃ C₂ T_x MXene/MWCNT substrate, a magnetized Ti₃ C₂ T_x -based film is successfully fabricated by a facile filtration approach. Carbon nanotubes are used as isolation agents to realize the submicroscopic dispersion of MXene and SrFe₁₂ O₁₉ . The obtained MXene/MWCNTs/SrFe₁₂ O₁₉ film shows a high electrical conductivity of 438 S cm^-1 and an excellent EMI shielding effectiveness of 62.9 dB in X-band at a thickness of only 40 µm. Benefiting from a strong magnetic response ability and an expanded magnetic coupling space, hexagonal SrFe₁₂ O₁₉ sheets can efficiently consume incident magnetic field energy by domain wall migration and the ferromagnetic resonance effect. Boosted EMI shielding performance can be achieved by improving the magnetic loss in the Ti₃ C₂ T_x MXene/MWCNTs/SrFe₁₂ O₁₉ film, preventing the secondary reflection of electromagnetic waves. Meanwhile, magnetized MXene-based films display the freestanding and flexible features and are suitable for installation in electric devices. It is anticipated that this strategy offers new ideas for designing EMI shielding films and in broadening potential utility of MXene-based materials.

Recent progress of Bi-based electrocatalysts for electrocatalytic CO2 reduction

To mitigate excessively accumulated carbon dioxide (CO₂) in the atmosphere and tackle the associated environmental concerns, green and effective approaches are necessary. The electrocatalytic CO₂ reduction reaction (CO₂RR) using sustainable electricity under benign reaction conditions represents a viable way to produce value-added and profitable chemicals. In this minireview, recent studies regarding unary Bi electrocatalysts and binary BiSn electrocatalysts are symmetrically categorized and reviewed, as they disclose high faradaic efficiencies toward the production of formate/formic acid, which has a relatively higher value of up to 0.50 $·per kg and has been widely used in the chemical and pharmaceutical industry. In particular, the preparation methodologies, electrocatalyst morphologies, catalytic performances and the corresponding mechanisms are comprehensively presented. The use of solid-state electrolytes showing high economic prospects for directly obtaining high-purity formic acid is highlighted. Finally, the remaining questions and challenges for CO₂RR exploitations using Bi-related electrocatalysts are proposed, while perspectives and the corresponding strategies aiming to enhance their entire catalytic functionalities and boost their performance are provided.

Roles of Metal Ions in MXene Synthesis, Processing and Applications: A Perspective

With a decade of effort, significant progress has been achieved in the synthesis, processing, and applications of MXenes. Metal ions play many crucial roles, such as in MXene delamination, structure regulation, surface modification, MXene composite construction, and even some unique applications. The different roles of metal ions are attributed to their many interactions with MXenes and the unique nature of MXenes, including their layered structure, surface chemistry, and the existence of multi-valent transition metals. Interactions with metal ions are crucial for the energy storage of MXene electrodes, especially in metal ion batteries and supercapacitors with neutral electrolytes. This review aims to provide a good understanding of the interactions between metal ions and MXenes, including the classification and fundamental chemistry of their interactions, in order to achieve their more effective utilization and rational design. It also provides new perspectives on MXene evolution and exfoliation, which may suggest optimized synthesis strategies. In this respect, the different effects of metal ions on MXene synthesis and processing are clarified, and the corresponding mechanisms are elaborated. Research progress on the roles metal ions have in MXene applications is also introduced.

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.

Modulating the Local Coordination Environment of Single-Atom Catalysts for Enhanced Catalytic Performance in Hydrogen/Oxygen Evolution Reaction

Single-atom catalysts (SACs) hold the promise of utilizing 100% of the participating atoms in a reaction as active catalytic sites, achieving a remarkable boost in catalytic efficiency. Thus, they present great potential for noble metal-based electrochemical application systems, such as water electrolyzers and fuel cells. However, their practical applications are severely hindered by intrinsic complications, namely atom agglomeration and relocation, originating from the uncontrollably high surface energy of isolated single-atoms (SAs) during postsynthetic treatment processes or catalytic reactions. Extensive efforts have been made to develop new methodologies for strengthening the interactions between SAs and supports, which could ensure the desired stability of the active catalytic sites and their full utilization by SACs. This review covers the recent progress in SACs development while emphasizing the association between the regulation of coordination environments (e.g., coordination atoms, numbers, sites, structures) and the electrocatalytic performance of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The crucial role of coordination chemistry in modifying the intrinsic properties of SACs and manipulating their metal-loading, stability, and catalytic properties is elucidated. Finally, the future challenges of SACS development and the industrial outlook of this field are discussed.

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.

Single-Atom Pt Anchored on Oxygen Vacancy of Monolayer Ti3C2Tx for Superior Hydrogen Evolution

Two-dimensional (2D) MXene-loaded single-atom (SA) catalysts have drawn increasing attention. SAs immobilized on oxygen vacancies (O_V) of MXene are predicted to have excellent catalytic performance; however, they have not yet been realized experimentally. Here Pt SAs immobilized on the O_V of monolayer Ti₃C₂T_x flakes are constructed by a rapid thermal shock technique under a H₂ atmosphere. The resultant Ti₃C₂T_x-Pt_SA catalyst exhibits excellent hydrogen evolution reaction (HER) performance, including a small overpotential of 38 mV at 10 mA cm^-2, a high mass activity of 23.21 A mg_Pt^-1, and a large turnover frequency of 23.45 s^-1 at an overpotential of 100 mV. Furthermore, density functional theory calculations demonstrate that anchoring the Pt SA on the O_V of Ti₃C₂T_x helps to decrease the binding energy and the hybridization strength between H atoms and the supports, contributing to rapid hydrogen adsorption-desorption kinetics and high activity for the HER.