Hexagonal MBene (Hf2BO2): A Promising Platform for the Electrocatalysis of Hydrogen Evolution Reaction

Environmental and biomedical applications of 2D transition metal borides (MBenes): recent advancements

Recently, interest has surged in the environmental and biomedical applications of two-dimensional transition metal borides, commonly referred to as MBenes. These materials have emerged as promising candidates for energy storage devices, such as batteries and supercapacitors. Additionally, MBenes have shown remarkable catalytic activity due to their high surface area and tunable electronic properties. They exhibit significant promise in various catalytic applications, particularly in nitrogen reduction reactions (NRRs), electrocatalytic conversion of nitrogen oxides, and several electrochemical reactions such as the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). Notably, MBenes have shown great potential in water treatment and pollutant removal applications, such as desalination and water purification. Their high water permeability, ion selectivity, and excellent stability make them suitable for efficient water treatment processes. On the other hand, MBenes are emerging as versatile materials with significant potential in various biomedical applications, particularly in biosensing, cancer therapy, and the treatment of neurodegenerative diseases. However, several challenges hinder their practical implementation in biomedical and environmental fields. One significant issue is the scalability of synthesis methods; producing MBenes in large quantities while maintaining high purity and uniformity is often complex and costly. Moreover, the stability of MBenes and their composites under different environmental and biological conditions raises concerns, as they may undergo degradation or lose their functional properties over time, which could limit their long-term effectiveness. Additionally, there is a need for comprehensive toxicity assessments to ensure the safety of MBenes in biomedical applications, particularly when interacting with human tissues or biological systems. This review aims to systematically investigate the environmental and biomedical applications of MBenes and their composites, emphasizing their unique characteristics and potential roles in addressing pressing global challenges. Furthermore, the review will identify and discuss the existing challenges and limitations in the operational performance of MBenes and their composites, providing a critical assessment of their current state in various applications.

Nanocrystalline transition metal tetraborides as efficient electrocatalysts for hydrogen evolution reaction at the large current density

While great efforts have been made to improve the electrocatalytic activity of existing materials toward hydrogen evolution reaction (HER), it is also importance for searching new type of nonprecious HER catalysts to realize the practical hydrogen evolution. Herein, we firstly report nanocrystalline transition metal tetraborides (TMB4, TM=W and Mo) as an efficient HER electrocatalyst has been synthesized by a single-step solid-state reaction. The optimized nanocrystalline WB4 exhibits an overpotential as low as 172 mV at 10 mA/cm2 and small Tafel slope of 63 mV/dec in 0.5 M H2SO4. Moreover, the nanocrystalline WB4 outperforms the commercial Pt/C at high current density region, confirming potential applications in industrially electrochemical water splitting. Theoretical study reveals that high intrinsic HER activity of WB4 is originated from its large work function that contributes to the weak hydrogen-adsorption energy. Therefore, this work provides new insights for development of robust nanocrystalline electrocatalysts for efficient HER.

MXene Manipulating the Electronic and Photoelectric Properties of a Fullerene-Layered Heterojunction

An in-depth study of the substrate effect is crucial for optimizing and designing the performance of two-dimensional (2D) materials in practical applications. Fullerene monolayers (FMs), a new pure carbon system successfully prepared recently, have prompted renewed interest in the question of whether FMs might be exploited to create carbon-based functional materials with improved performance. Here, the electronic structure of a MXene-supported FM was investigated by first-principles calculations. Various band offset types, including types I, II, and III, exist in the FM/M2X heterostructures, which are determined by the energy level arrangement of individual layers. Interestingly, strain also plays an important role in the band offset of the FM/M2X heterostructures. From the selection of a specific substrate and introduction of proper strain in the substrate, the desired band structure can be obtained. Our results offer profound physical insights into the mechanism of electronic structure tuning of FM by substrates.

Discovery of Bimetallic Hexagonal MBene Mo2ErB3T2.5 (T = O, F, and Cl)

Exfoliation from quaternary hexagonal MAB (h-MAB) phases has been suggested as a method for producing 2D in-plane ordered MBenes (i-MBenes) with the general formula (M'2/3M″1/3)2AB2. However, experimental realization of defect-free i-MBenes has not been achieved yet due to the absence of a suitable parent quaternary h-MAB phase. In this study, a machine learning (ML) model is used to predict the stability of 15771 quaternary h-MAB phases generated by considering 33 transition metals for the M site and 16 p-block elements for the A site. Out of these compounds, only 195 are identified as potentially stable. Subsequent high-precision first-principles calculations confirm that 47 of them exhibit both thermodynamic and dynamic stability. Their potential for exfoliation into bimetallic i-MBenes is investigated by bonding analysis. Leveraging these theoretical insights, a bimetallic i-MBene is successfully synthesized, namely 2D Mo2ErB3T2.5 (T = F, Cl and O). Further experimental scrutiny reveals its excellent performance for the hydrogen evolution reaction (HER), highlighting the application potential of bimetallic i-MBenes.

Theoretical investigations of two-dimensional intrinsic magnets derived from transition-metal borides M3B4 (M = Cr, Mn, and Fe)

Two-dimensional (2D) magnetic materials with high critical temperatures (T C ) and robust magnetic anisotropy energies (MAE) hold significant potential for spintronic applications. However, most of 2D magnetic materials are derived from the van der Waals (vdW) layered bulks, which greatly limits the synthesis of 2D magnetic materials. Here, 2D M3B4 (M = Cr, Mn, and Fe; B = Boron), derived from hexagonal and orthorhombic M3AlB4 phases by selectively etching Al layers, was studied for its structural stability, electronic structure, and magnetic properties. By utilizing ab initio calculations and Monte Carlo simulations, we found that the orthorhombic Cr3B4 shows ferromagnetic (FM) metal and possesses an in-plane magnetic easy axis, while the remaining hexagonal and orthorhombic M3B4 structures exhibit antiferromagnetic (AFM) metals with a magnetic easy axis which is perpendicular to the two-dimensional plane. The critical temperatures of these 2D M3B4 structures are found to be above the 130 K. Notably, the ort-Mn3B4 possesses highest T C (~600 K) and strongest MAE (~220 µeV/atom) among these borides-based 2D magnetic materials. Our findings reveal that the 2D M3B4 compounds exhibit much better resistance to deformation compared to M2B2 MBenes and other 2D magnetic materials. The combination of high critical temperature, robust MAE, and excellent mechanical properties makes 2D Mn3B4 monolayer exhibits a favorable potential for spintronic applications. Our research also sheds light on the magnetic coupling mechanism of 2D M3B4, providing valuable insights into its fundamental characteristics.

The Quest for Two-Dimensional MBenes: From Structural Evolution to Solar-Driven Hybrid Systems for Water-Fuel-Energy Generation and Phototherapy

The field of 2D materials has advanced significantly with the emergence of MBenes, a new material derived from the MAX phases family, a novel class of materials that originates from the MAX phases family. Herein, this article explores the unique characteristics and morphological variations of MBenes, offering a comprehensive overview of their structural evolution. First, the discussion explores the evolutionary period of 2D MBenes associated with the several techniques for synthesizing, modifying, and characterizing MBenes to tailor their structure and enhance their functionality. The focus then shifts to the defect chemistry of MBenes, electronic, catalytic, and photothermal properties which play a crucial role in designing multifunctional solar-driven hybrid systems. Second, the recent advancements and potentials of 2D MBenes in solar-driven hybrid systems e.g. photo-electro catalysis, hybrid solar evaporators for freshwater and thermoelectric generators, and phototherapy, emphasizing their crucial significance in tackling energy and environmental issues, are explored. The study further explores the fundamental principles that regulate the improved photocatalytic and photothermal characteristics of MBenes, highlighting their promise for effective utilization of solar energy and remediation of the environment. The study also thoroughly assesses MBenes' scalability, stability, and cost effectiveness in solar-driven systems. Current insights and future directions allow researchers to utilize MBenes for sustainable and varied applications. This review regarding MBenes will be valuable to early researchers intrigued with synthesizing and utilizing 2D materials for solar-powered water-energy-fuel and phototherapy systems.

Interlayer Entropy Engineering Inducing the Symmetry-Broken Layered Oxide Cathodes to Activate Reversible High-Voltage Redox Reaction

The as-reported doping entropy engineering of electrode materials that are usually realized by the sharing of multiple metal elements with the metal element from the lattice body, potentially has three shortages of stringent synthesis conditions, large active element loss, and serious lattice distortion. Herein, an interlayer entropy engineering of layered oxide cathodes is proposed, where the multiple metal ions are simultaneously intercalated into the same interlayer sites, thus avoiding the three shortages. Concretely, a novel interlayer medium-entropy V2O5 ((MnCoNiMgZn)0.26V2O5∙0.84H2O) is successfully constructed by a one-step hydrothermal method. The interlayer medium-entropy effect is revealed to be that five metal ions pre-intercalation induces the local symmetry-broken [VO6] octahedra in bilayer V2O5, thus activating the reversible high-voltage redox reaction, inhibiting the layer slip and following phase transformation by its pinning effect, and enhancing the charge transfer kinetics. As a result, the medium-entropy cathode realizes the trade-off between specific capacity and structural stability with a discharge capacity of 152 mAh g-1 at 0.1 A g-1 after 100 cycles, and a capacity retention rate of 98.7% at 0.5 A g-1 after 150 cycles for Li+ storage. This engineering provides a new guideline for the rational design of high-performance layered oxide cathodes.

h-MBenes: Promising Two-Dimensional Material Family for Room-Temperature Antiferromagnetic and Hydrogen Evolution Reaction Applications

Recently, a new class of two-dimensional (2D) hexagonal transition-metal borides (h-MBenes) was discovered through a combination of ab initio predictions and experimental studies. These h-MBenes are derived from ternary hexagonal MAB (h-MAB) phases and have demonstrated promising potential for practical applications. In this study, we conducted first-principles calculations on 15 h-MBenes and identified four antiferromagnetic metals and 11 electrocatalysts for the hydrogen evolution reaction (HER). Notably, the h-MnB material exhibited a remarkable Néel temperature of 340 K and a high magnetic anisotropy energy of 154 μeV/atom. Additionally, the hydrogen adsorption Gibbs free energies (ΔGH*) for h-ZrBO, h-MoBO, and h-Nb2BO2 are close to the ideal value of 0 eV, indicating their potential as electrochemical catalysts for HER. Further investigations revealed that the electronic structure, Néel temperature, and HER activity of the studied h-MBenes can be tuned by applying biaxial strains. These findings suggest that h-MBenes have wide-ranging applicability in areas such as antiferromagnetic spintronics, flexible electronic devices, and electrocatalysis, thereby expanding the potential applications of 2D transition-metal borides.

Establishing theoretical landscapes for identifying basal plane active sites in MBene toward multifunctional HER, OER, and ORR catalysts

Exploring multifunctional electrocatalysts to realize efficient hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) is urgently desired for developing novel renewable energy storage and conversion technologies. However, integrating these three merits in one single catalyst remains a big challenge due to the difficulty in balancing the adsorption strengths of multiple reaction intermediates. Herein, through first-principles calculations, we systematically investigated the electrocatalytic activity of M2B2, M3B4, and M4B6 type MBenes (M = Cr, Mn, Fe, Co, and Ni) for multifunctional HER, OER, and ORR. The results indicate that most of the investigated MBenes show outstanding catalytic activity for HER with hydrogen adsorption Gibbs free energy close to the optimal value (0 eV). Thereinto, Ni2B2 and Co3B4 MBenes can be promising multifunctional HER/OER/ORR electrocatalysts, and Fe3B4 MBene is expected to be a promising bifunctional electrocatalyst for HER/ORR. Especially, Ni2B2 MBene is even better than the benchmark RuO2 catalyst with ultralow low overpotentials of 0.26 and 0.30 V for OER and ORR, respectively. Then, we proposed that the overpotentials of OER/ORR can be well described by the varied ΔGOH* on MBene, which has been further illuminated through the d-band center and charge transfer analysis. Importantly, new scaling relations between the adsorption energies of OOH* and O* on MBenes have been established, where ΔGOOH* and ΔGO* possess different slopes versus ΔGOH*, allowing the significantly lower overpotentials of OER and ORR to be achieved. This work provides not only promising multifunctional HER/OER/ORR electrocatalysts but also new scaling relations to achieve the rational design of MBene-based electrocatalysts.

Computational determination of a graphene-like TiB4 monolayer for metal-ion batteries and a nitrogen reduction electrocatalyst

As an emerging two-dimensional (2D) material, the TiB₄ monolayer possesses intrinsic advantages in electrochemical applications owing to its graphene-like structure and metallic characteristics. In this work, we performed density functional calculations to investigate the electrochemical properties of the TiB₄ monolayer as an anode material for Li/Na/K ion batteries and as an electrocatalyst for the nitrogen reduction reaction (NRR). Our investigation reveals that Li/Na/K ions could be steadily adsorbed on the TiB₄ monolayer with moderate adsorption energies, and tended to diffuse along two adjacent C-sites with lower energy barriers (0.231/0.094/0.067 eV for Li/Na/K ions) compared to the currently reported transition-metal boride monolayers. Furthermore, a N₂ molecule can be spontaneously captured by the TiB₄ monolayer with a negative Gibbs free energy (-0.925 eV and -0.326 eV for end-on and side-on adsorptions, respectively), hence provoking a conversion into NH₃ along the most efficient reaction pathway (i.e., N₂* → N₂H* → HNNH* → H₂NNH* → H₃NNH* → NH* → NH₂* → NH₃*). In the hydrogenation process, the TiB₄ monolayer exhibits much higher catalytic activity for the NRR as compared with other electrocatalysts, which should be attributed to the spontaneous achievement (ΔG < 0) at all hydrogenation reaction steps except the potential-determining step. Moreover, the TiB₄ monolayer exhibits higher selectivity toward the NRR than the hydrogen evolution reaction. Our work advances the mechanistic understanding on the electrochemical properties of the TiB₄ monolayer as an anode material for metal-ion batteries and as a NRR electrocatalyst, and provides significant guidance for developing high-performance multifunctional 2D materials.

Engineering p-Band Center of Oxygen Boosting H+ Intercalation in δ-MnO2 for Aqueous Zinc Ion Batteries

In aqueous zinc ion batteries (ZIBs), the H⁺ intercalation possesses superior electrochemical kinetics with excellent rate capability, however, precisely modulating H⁺ intercalation has been still challenging. Herein, a critical modification of pre-intercalating metal ions in the MnO₂ interlayer (M-MnO₂ ) with controllable p-band center (ϵ_p ) of O is reported to modulate the H⁺ intercalation. The modulation of metal-O bond type and covalency degree on the average charge of O atom results in optimized ϵ_p and H⁺ adsorption energy for M-MnO₂ , thus promoting the balance between H⁺ adsorption and desorption, which plays a determinant role on H⁺ intercalation. The optimized Cu-MnO₂ delivers superior rate capability with the capacity of 153 mAh g^-1 at a high rate of 3 A g^-1 after 1000 cycles. This work demonstrates that ϵ_p could be a significant descriptor for H⁺ intercalation, and tuning ϵ_p effectively increases H⁺ intercalation contribution with excellent rate capability in ZIBs.

A Rising 2D Star: Novel MBenes with Excellent Performance in Energy Conversion and Storage

As a flourishing member of the two-dimensional (2D) nanomaterial family, MXenes have shown great potential in various research areas. In recent years, the continued growth of interest in MXene derivatives, 2D transition metal borides (MBenes), has contributed to the emergence of this 2D material as a latecomer. Due to the excellent electrical conductivity, mechanical properties and electrical properties, thus MBenes attract more researchers' interest. Extensive experimental and theoretical studies have shown that they have exciting energy conversion and electrochemical storage potential. However, a comprehensive and systematic review of MBenes applications has not been available so far. For this reason, we present a comprehensive summary of recent advances in MBenes research. We started by summarizing the latest fabrication routes and excellent properties of MBenes. The focus will then turn to their exciting potential for energy storage and conversion. Finally, a brief summary of the challenges and opportunities for MBenes in future practical applications is presented.

M4 B6 X6 as a New Family of High-Efficient Electrocatalysts: The Role of Surface Reconstruction in Water Oxidization

Searching for highly-efficient electrocatalysts for water splitting has been greatly endowed due to the huge demand for green energy sources. Two-dimensional (2D) materials are widely explored for the purpose because of their unique physical and chemical properties, abundant active sites, and easy fabrication. Here, we present a new family of 2D M₄ B₆ X₆ (2D Boridenes) and investigate their physical and chemical properties for their potential applications into electrocatalysis based on first-principles calculations. We demonstrate that 2D M₄ B₆ X₆ (M=Cr, Mo, and W; X=O and F) are dynamically, thermodynamically, and mechanically stable, and show intriguing electronic and catalytic properties. Importantly, we find that M₄ B₆ O₆ are intrinsically active for oxygen evolution reaction (OER). Our results demonstrate that: (1) the adsorbate-escape mechanism dominates the OER process with a low overpotential of 0.652 V on Cr₄ B₆ O₆ ; (2) the partial surface-oxidization can improve the catalytic performance of M₄ B₆ F₆ dramatically; and (3) the surface reconstruction greatly affects the OER performance of M₄ B₆ X₆ . Our findings illustrate that the surface reconstruction is critical to the OER activity, which may provide a new strategy on the design of 2D materials for electrocatalysis and offer theoretical insight into the catalytic mechanism.