Enhancing Magnetic Ordering in Two-Dimensional Metal-Organic Frameworks via Frontier Molecular Orbital Engineering.

Two-dimensional (2D) metal-organic frameworks (MOFs) have promise for use in lightweight permanent magnets in contrast to inorganic solid- or molecule-based magnets, but the realization of 2D MOF magnets with a high ordering temperature is limited by the typically weak spin exchange interactions. Here, we have proposed a frontier molecular orbital engineering strategy for modulating magnetism in 2D MOFs. It shows that the magnetic ground state can be mediated by two intra-atomic spin exchange pathways in organic ligands, akin to the Bloch and Heisenberg models, depending on the shape of the frontier orbitals of the organic ligands. By engineering the shape of the lowest unoccupied molecular orbital (LUMO) via chemical hydrogenation, we achieved a nearly 11-fold increase in the ordering temperature. In particular, a quantitative analysis shows that the ordering temperature increases linearly with the orbital delocalization index of the ligands' LUMO. This work suggests a general frontier orbital engineering approach for modulating the spin exchange interaction in 2D MOF magnets.

Precise Synthesis of Dual-single-atom Electrocatalysts through Pre-coordination-directed in situ Confinement for CO2 Reduction.

Dual-single-atom catalysts (DSACs) are the next paradigm shift in single-atom catalysts because of the enhanced performance brought about by the synergistic effects between adjacent bimetallic pairs. However, there are few methods for synthesizing DSACs with precise bimetallic structures. Herein, a pre-coordination strategy is proposed to precisely synthesize a library of DSACs. This strategy ensures the selective and effective coordination of two metals via phthalocyanines with specific coordination sites, such as -F- and -OH-. Subsequently, in-situ confinement inhibits the migration of metal pairs during high-temperature pyrolysis, and obtains the DSACs with precisely constructed metal pairs. Despite changing synthetic parameters, including transition metal centers, metal pairs, and spatial geometry, the products exhibit similar atomic metal pairs dispersion properties, demonstrating the universality of the strategy. The pre-coordination strategy synthesized DSACs shows significant CO2 reduction reaction performance in both flow-cell and practical rechargeable Zn-CO2 batteries. This work not only provides new insights into the precise synthesis of DSACs, but also offers guidelines for the accelerated discovery of efficient catalysts.

Rechargeable Seawater-Based Chloride-Ion Batteries Enabled by Covalent Surface Chemistry in MXenes.

Rechargeable aqueous chloride-ion batteries (ACIBs) using Cl- ions as charge carriers represent a promising energy-storage technology, especially when natural seawater is introduced as the electrolyte, which can bring remarkable advantages in terms of cost-effectiveness, safety, and environmental sustainability. However, the implementation of this technology is hindered by the scarcity of electrodes capable of reversible chloride-anion storage. Here, we show that a Ti3C2Clx MXene with Cl surface terminations enables reversible Cl- ion storage in aqueous electrolytes. Further, we developed seawater-based ACIBs that show a high specific capacity and an exceptionally long lifespan (40000 cycles, more than 1 year) in natural seawater electrolyte. The pouch-type cells achieve a high energy density (50 Wh Lcell-1) and maintain stable performance across a broad temperature range (-20 to 50 °C). Our investigations reveal that the covalent interaction between Cl- ions and Cl-terminated MXene facilitates Cl- ion intercalation into the MXene interlayer, promoting rapid ion migration with a low energy barrier (0.10 eV). Moreover, this MXene variant also enables the reversible storage of Br- ions in an aqueous electrolyte with a long cycle life. This study may advance the design of anion storage electrodes and enable the development of sustainable aqueous batteries.

Ultra-Thin RuIr Alloy as Durable Electrocatalyst for Seawater Hydrogen Evolution Reaction.

The development of efficient, high-performance catalysts for hydrogen evolution reaction (HER) remains a significant challenge, especially in seawater media. Here, RuIr alloy catalysts are prepared by the polyol reduction method. Compared with single-metal catalysts, the RuIr alloy catalysts exhibited higher activity and stability in seawater electrolysis due to their greater number of reactive sites and solubility resistance. The RuIr alloy has an overpotential of 75 mV@10 mA cm-2, which is similar to that of Pt/C (73 mV), and can operate stably for 100 hours in alkaline seawater. Density functional theory (DFT) calculations indicate that hydrogen atoms adsorbed at the top sites of Ru and Ir atoms are more favorable for HER and are most likely to be the reactive sites. This work provides a reference for developing highly efficient and stable catalysts for seawater electrolysis.

Biologically templated formation of Cobalt-Phosphide-Graphene hybrids with charge redistribution for efficient hydrogen evolution.

Developing highly efficient and sustainable hydrogen evolution reaction (HER) electrocatalysts is important for the practical application of emerging energy technologies. The spherical structure and phosphorus-rich properties of Chlorella can facilitate the construction of comparable transition metal phosphide electrocatalysts. Here, a microorganism template strategy is proposed to construct a cobalt-phosphide-graphene hybrid. Chlorella can absorb metal ions, and the generated rough spherical nanoparticles are uniformly distributed around the reduced graphene oxide nanosheets. This designed catalyst has comparable HER performance in acidic electrolytes and needs an overpotential of only 153 mV at a current density of 10 mA cm-2. The experimental and density functional theory results imply that the charge redistribution between Co2P and pyrrole-N is the key factor in enhancing the HER activity. The induced electron aggregation at the N and P sites can serve as a key active site for absorbing the adsorbed hydrogen atom intermediate to accelerate the HER process, contributing to the active sites of Co2P- and pyrrole-N-doped carbon with 0 eV hydrogen adsorption free energy. This work provides a broad idea for synthesizing advanced catalysts by a biological template approach, facilitating the innovative integration of biology and emerging electrochemical energy technologies.

Stabilizing the oxidation state of catalysts for effective electrochemical carbon dioxide conversion.

In the electrocatalytic CO2 reduction reaction (CO2RR), metal catalysts with an oxidation state generally demonstrate more favorable catalytic activity and selectivity than their corresponding metallic counterparts. However, the persistence of oxidative metal sites under reductive potentials is challenging since the transition to metallic states inevitably leads to catalytic degradation. Herein, a thorough review of research on oxidation-state stabilization in the CO2RR is presented, starting from fundamental concepts and highlighting the importance of oxidation state stabilization while revealing the relevance of dynamic oxidation states in product distribution. Subsequently, the functional mechanisms of various oxidation-state protection strategies are explained in detail, and in situ detection techniques are discussed. Finally, the prevailing and prospective challenges associated with oxidation-state protection research are discussed, identifying innovative opportunities for mechanistic insights, technology upgrades, and industrial platforms to enable the commercialization of the CO2RR.

Heterogeneous-Structured Molybdenum Diboride as a Novel and Promising Anode for Lithium-Ion Batteries.

With the development of electric vehicles, exploiting anode materials with high capacity and fast charging capability is an urgent requirement for lithium-ion batteries (LIBs). Borophene, with the merits of high capacity, high electronic conductivity and fast diffusion kinetics, holds great potential as anode for LIBs. However, it is difficult to fabricate for the intrinsic electron-deficiency of boron atom. Herein, heterogeneous-structured MoB2 (h-MoB2) with amorphous shell and crystalline core, is prepared by solid phase molten salt method. As demonstrated, crystalline core can encapsulate the honeycomb borophene within two adjacent Mo atoms, and amorphous shell can accommodate more lithium ions to strengthen the lithium storage capacity and diffusion kinetics. According to theoretical calculations, the lithium adsorption energy in MoB2 is about -2.7 eV, and the lithium diffusion energy barrier in MoB2 is calculated to be 0.199 eV, guaranteeing the enhanced adsorption capability and fast diffusion kinetic behavior of Li+ ions. As a result, h-MoB2 anode presents high capacity of 798 mAh g-1 at 0.1 A g-1, excellent rate performance of 183 mAh g-1 at 5 A g-1 and long-term cyclic stability for 1200 cycles. This work may inspire ideas for the fabrication of borophene analogs and two-dimensional metal borides.

Turning Nonmagnetic Two-Dimensional Molybdenum Disulfides into Room-Temperature Ferromagnets by the Synergistic Effect of Lattice Stretching and Charge Injection.

Exploring two-dimensional (2D) room-temperature magnetic materials in the field of 2D spintronics remains a formidable challenge. The vast array of nonmagnetic 2D materials provides abundant resources for exploration, but the strategy to convert them into intrinsic room-temperature magnets remains elusive. To address this challenge, we present a general strategy based on surface halogenation for the transition from nonmagnetism to intrinsic room-temperature ferromagnetism in 2D MoS2 based on first-principles calculations. The derived 2D halogenated MoS2 are half-semimetals with a high Curie temperature (TC) of 430-589 K and excellent stability. In-depth mechanistic studies revealed that this marvelous nonmagnetism-to-ferromagnetism transition originates from the modulation of the splitting as well as the occupation of the Mo d orbitals by the synergy of lattice stretching and charge injection induced by the surface halogenation. This work establishes a promising route for exploring 2D room-temperature magnetic materials from the abundant pool of 2D nonmagnetic counterparts.

Regulating Au coverage for the direct oxidation of methane to methanol.

The direct oxidation of methane to methanol under mild conditions is challenging owing to its inadequate activity and low selectivity. A key objective is improving the selective oxidation of the first carbon-hydrogen bond of methane, while inhibiting the oxidation of the remaining carbon-hydrogen bonds to ensure high yield and selectivity of methanol. Here we design ultrathin PdxAuy nanosheets and revealed a volcano-type relationship between the binding strength of hydroxyl radical on the catalyst surface and catalytic performance using experimental and density functional theory results. Our investigations indicate a trade-off relationship between the reaction-triggering and reaction-conversion steps in the reaction process. The optimized Pd3Au1 nanosheets exhibits a methanol production rate of 147.8 millimoles per gram of Pd per hour, with a selectivity of 98% at 70 °C, representing one of the most efficient catalysts for the direct oxidation of methane to methanol.

Two-Dimensional Transition Metal Boron Cluster Compounds (MBnenes) with Strain-Independent Room-Temperature Magnetism.

Two-dimensional (2D) metal borides (MBenes) with unique electronic structures and physicochemical properties hold great promise for various applications. Given the abundance of boron clusters, we proposed employing them as structural motifs to design 2D transition metal boron cluster compounds (MBnenes), an extension of MBenes. Herein, we have designed three stable MBnenes (M4(B12)2, M = Mn, Fe, Co) based on B12 clusters and investigated their electronic and magnetic properties using first-principles calculations. Mn4(B12)2 and Co4(B12)2 are semiconductors, while Fe4(B12)2 exhibits metallic behavior. The unique structure in MBnenes allows the coexistence of direct exchange interactions between adjacent metal atoms and indirect exchange interactions mediated by the clusters, endowing them with a Néel temperature (TN) up to 772 K. Moreover, both Mn4(B12)2 and Fe4(B12)2 showcase strain-independent room-temperature magnetism, making them potential candidates for spintronics applications. The MBnenes family provides a fresh avenue for the design of 2D materials featuring unique structures and excellent physicochemical properties.

Heterojunction Engineering of Multinary Metal Sulfide-Based Photocatalysts for Efficient Photocatalytic Hydrogen Evolution.

Photocatalytic hydrogen evolution (PHE) via water splitting using semiconductor photocatalysts is an effective path to solve the current energy crisis and environmental pollution. Heterojunction photocatalysts, containing two or more semiconductors, exhibit better PHE rates than those with only one semiconductor owing to the altered band alignment at the interface and stronger driving force for charge separation. Traditional binary metal sulfide (BMS)-based heterojunction photocatalysts, such as CdS, MoS2 , and PbS, demonstrate excellent PHE performance. However, the recently developed multinary metal sulfide (MMS)-based photocatalysts possess favorable chemical stability, tunable band structure, and flexible element compositions, and have considerable potential to realize higher PHE rates than those of BMSs. In this review article, the mechanism of PHE is first elucidated and then various single and heterojunction MMS-based photocatalysts and their charge transfer behaviors and PHE performances are systematically summarized. A perspective on potential future research directions in this field is concluded.

Multimetallic Single-Atom Catalysts for Bifunctional Oxygen Electrocatalysis.

Multimetallic alloys have demonstrated promising performance for the application of metal-air batteries, while it remains a challenge to design multimetallic single-atom catalysts (MM-SACs). Herein, metal-C3N4 and nitrogen-doped carbon are employed as cornerstones to synthesize MM-SACs by a general two-step method, and the inherent features of atomic dispersion and the strong electronic reciprocity between the multimetallic sites have been verified. The trimetallic FeCoZn-SACs and quatermetallic FeCoCuZn-SACs are both found to deliver superior oxygen evolution reaction and oxygen reduction reaction activity, respectively, as well as outstanding bifunctional durability. Density functional theory calculations elucidate the crucial contribution of Co sites of FeCoCuZn-SACs to the efficient catalysis of both the ORR and the OER. More importantly, Zn-air batteries with FeCoCuZn-SACs as cathodic catalysts exhibit a high power density (252 mW cm-2), high specific capacity (817 mAh gZn-1), and considerable stability (over 225 h) for charging-discharging processes. This work provides a visual perspective for the advantages of MM-SACs toward oxygen electrocatalysis.

Understanding the Bifunctional Trends of Fe-Based Binary Single-Atom Catalysts.

Binary single-atom catalysts (BSACs) have demonstrated fascinating activities compared to single atom catalysts (SACs) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Notably, Fe SACs is one of the most promising ORR electrocatalysts, and further revealing the synergistic effects between Fe and other 3d transition metals (M) for FeM BSACs are very important to enhance bifunctional performance. Herein, density functional theory (DFT) calculations are first adapted to demonstrate the role of various transition metals on the bifunctional activity of Fe sites, and a notable volcano relationship is established through the generally accepted adsorption free energy that ΔG* OH for ORR, and ΔG* O -ΔG* OH for OER, respectively. Further, ten of the atomically dispersed FeM anchored on nitrogen-carbon support (FeM-NC) are successfully synthesized with typical atomic dispersion by a facile movable type printing method. The experimental data confirms the bifunctional activity diversity of FeM-NC between the early- and late- transition metals, agrees very well with the DFT results. More importantly, the optimal FeCu-NC shows the expected performance with high ORR and OER activity, thereby, the assembled rechargeable zinc-air battery delivers a high power density of 231 mW cm-2 , and an impressive stability that can be stably operated over 300 h.

Ultrathin Nitrogen-Doped Carbon Encapsulated Ni Nanoparticles for Highly Efficient Electrochemical CO2 Reduction and Aqueous Zn-CO2 Batteries.

Electrochemical CO2 reduction reaction (CO2 RR), powered by renewable electricity, has attracted great attention for producing high value-added fuels and chemicals, as well as feasibly mitigating CO2 emission problem. Here, this work reports a facile hard template strategy to prepare the Ni@N-C catalyst with core-shell structure, where nickel nanoparticles (Ni NPs) are encapsulated by thin nitrogen-doped carbon shells (N-C shells). The Ni@N-C catalyst has demonstrated a promising industrial current density of 236.7 mA cm-2 with the superb FECO of 97% at -1.1 V versus RHE. Moreover, Ni@N-C can drive the reversible Zn-CO2 battery with the largest power density of 1.64 mW cm-2 , and endure a tough cycling durability. These excellent performances are ascribed to the synergistic effect of Ni@N-C that Ni NPs can regulate the electronic microenvironment of N-doped carbon shells, which favor to enhance the CO2 adsorption capacity and the electron transfer capacity. Density functional theory calculations prove that the binding configuration of N-C located on the top of Ni slabs (Top-Ni@N-C) is the most thermodynamically stable and possess a lowest thermodynamic barrier for the formation of COOH* and the desorption of CO. This work may pioneer a new method on seeking high-efficiency and worthwhile electrocatalysts for CO2 RR and Zn-CO2 battery.

Synthesis of Phase Junction Cadmium Sulfide Photocatalyst under Sulfur-Rich Solution System for Efficient Photocatalytic Hydrogen Evolution.

Photocatalyst with excellent semiconductor properties is the key point to realize the efficient photocatalytic hydrogen evolution (PHE). As a representative binary metal sulfide (BMS) semiconductor, cadmium sulfide (CdS) possesses suitable bandgap of 2.4 eV and negative conduction band potential, which has a great potential to realize efficient visible-light PHE performance. In this work, CdS with unique cubic/hexagonal phase junction is facilely synthesized through a sulfur-rich butyldithiocarbamate acid (BDCA) solution process. The results illustrate that the phase junction can efficiently enhance the separation and transfer of photogenerated electron-hole pairs, resulting in an excellent PHE performance. In addition, the sulfur-rich property of BDCA solution leads to the absence of additional sulfur sources during the synthesis of CdS photocatalyst, which greatly simplifies the fabrication process. The optimal PHE rate of the BDCA-synthesized phase junction CdS photocatalyst is 7.294 mmol g-1  h-1 and exhibits a favorable photostability. Moreover, density function theory calculations indicated that the apparent redistribution of charge density in the cubic/hexagonal phase junction regions gives a suitable hydrogen adsorption capacity, which is responsible for the enhanced PHE activity.

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

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

Two-Dimensional Quaternary Transition Metal Sulfide CrMoA2S6 (A = C, Si, or Ge): A Bipolar Antiferromagnetic Semiconductor with a High Néel Temperature.

Bipolar antiferromagnetic semiconductors (BAFSs) make up a class of spintronic materials, holding great promise for the manipulation of spin-polarized currents simply upon application of a voltage gate, but only a few two-dimensional (2D) BAFSs with a high Néel temperature (TN) have been reported. Here, we report a family of magnetic quaternary MM'A2S6 (M = V, Cr, Mn, or Fe; M' = Nb, Mo, Tc, or Ru; A = C, Si, Ge, or Sn) nanosheets by isovalent alloying layered transitional metal trisulfides (MAS3) based on first-principles calculations. Our results show that 2D CrMoA2S6 (A = C, Si, or Ge) nanosheets are BAFSs with band gaps ranging from 1.89 to 2.23 eV. Among them, 2D CrMoC2S6 has the highest TN of 556 K with robust magnetism against carrier doping and external in-plane strain due to a strong delocalization superexchange interaction between the Cr3+ and Mo3+ cations. This study establishes that CrMoC2S6 is an ideal prototype platform for realizing electric control of spin polarization in 2D materials.

Ruthenium Complex of sp2 Carbon-Conjugated Covalent Organic Frameworks as an Efficient Electrocatalyst for Hydrogen Evolution.

It is still a great challenge to explore hydrogen evolution reaction (HER) electrocatalysts with both lower overpotential and higher stability in acidic electrolytes. In this work, an efficient HER catalyst, Ru@COF-1, is prepared by complexation of triazine-cored sp2 carbon-conjugated covalent organic frameworks (COFs) with ruthenium ion. Ru@COF-1 possesses high crystallinity and porosity, which are beneficial for electrocatalysis. The large specific surface area and regular porous channels of Ru@COF-1 facilitate full contact between reactants and catalytic sites. The nitrogen atoms of triazines are protonated in the acidic media, which greatly improve the conductivity of Ru@COF-1. This synergistic effect makes the overpotential of Ru@COF-1 about 200 mV at 10 mA cm-2 , which is lower than other reported COFs-based electrocatalysts. Moreover, Ru@COF-1 exhibits exceptionally electrocatalytic durability in the acidic electrolytes. It is particularly stable and remains highly active after 1000 cyclic voltammetry cycles. Density functional theory calculations demonstrate that tetracoordinated Ru-N2 Cl2 moieties are the major contributors to the outstanding HER performance. This work provides a new idea for developing protonated HER electrocatalysts in acidic media.

Enhanced Curie Temperature of Two-Dimensional Cr(II) Aromatic Heterocyclic Metal-Organic Framework Magnets via Strengthened Orbital Hybridization.

Two-dimensional (2D) metal-organic frameworks (MOFs) with room-temperature magnetism are highly desirable but challenging due to the weak superexchange interaction between metal atoms. For this purpose, strengthening the hybridization between metal ion and organic linkage presents an experiment-feasible chemical solution to enhance the Curie temperature. Here, we report three 2D Cr(II) aromatic heterocyclic MOF magnets with enhanced Curie temperature by bridging Cr(II) ions with pyrazine, 1,4-diphosphinine, and 1,4-diarsenin linkers, i.e., Cr(pyz)2, Cr(diphos)2, and Cr(diarse)2, and using first-principles calculations. Our results show that Cr(pyz)2, Cr(diphos)2, and Cr(diarse)2 are ferrimagnetic semiconductors. In particular, the Curie temperature of Cr(pyz)2 is estimated to be about 344 K and could be enhanced to 512 and 437 K in Cr(diphos)2 and Cr(diarse)2 by strengthening the hybridization between Cr ions and organic linkers via d-π* direct exchange interaction. This study presents a prototype to obtain room-temperature magnetism in 2D Cr(II)-based MOF magnets for nanoscale spintronics applications.

Orbital Design of Two-Dimensional Transition-Metal Peroxide Kagome Crystals with Anionogenic Dirac Half-Metallicity.

Assembling p orbital ferromagnetic half-metallicity and a topological element, such as a Dirac point at the Fermi level, in a single nanomaterial is of particular interest for long-distance, high-speed, and spin-coherent transportation in nanoscale spintronic devices. On the basis of the tight-binding model, we present an orbital design of a two-dimensional (2D) anionogenic Dirac half-metal (ADHM) by patterning cations with empty d orbitals and anions with partially filled p-type orbitals into a kagome lattice. Our first-principles calculations show that 2D transition-metal peroxides h-TM2(O2)3 (TMO3, TM = Ti, Zr, Hf), containing group IVB transition-metal cations [TM]4+ bridged with dioxygen anions [O2]8/3- in a kagome structure, are stable ADHMs with a Curie temperature over 103 K. The 2/3 filled π* orbitals of dioxygen anions are ferromagnetically coupled, leading to p orbital ferromagnetism and a half-metallic Dirac point right at the Fermi level with a Fermi velocity reaching 2.84 × 105 m/s. We proposed that 2D h-TM2(O2)3 crystals may be extracted from ABO3 bulk materials containing 2D TMO3 layers.