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

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.