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.

Atomic Design of Alkyne Semihydrogenation Catalysts via Active Learning

Alkyne hydrogenation on palladium-based catalysts modified with silver is currently used in industry to eliminate trace amounts of alkynes in alkenes produced from steam cracking and alkane dehydrogenation processes. Intensive efforts have been devoted to designing an alternative catalyst for improvement, especially in terms of selectivity and catalyst cost, which is still far away from that as expected. Here, we describe an atomic design of a high-performance Ni-based intermetallic catalyst aided by active machine learning combined with density functional theory calculations. The engineered NiIn catalyst exhibits >97% selectivity to ethylene and propylene at the full conversion of acetylene and propyne at mild temperature, outperforming the reported Ni-based catalysts and even noble Pd-based ones. Detailed mechanistic studies using theoretical calculations and advanced characterizations elucidate that the atomic-level defined coordination environment of Ni sites and well-designed hybridization of Ni 3d with In 5p orbital determine the semihydrogenation pathway.

Multi-d Electron Synergy in LaNi1-x Cox Ru Intermetallics Boosts Electrocatalytic Hydrogen Evolution

Despite the fact that d-band center theory links the d electron structure of transition metals to their catalytic activity, it is yet unknown how the synergistic effect of multi-d electrons impacts catalytic performance. Herein, novel LaNi1-x Cox Ru intermetallics containing 5d, 4d, and 3d electrons were prepared. In these compounds, the 5d orbital of La transfers electrons to the 4d orbital of Ru, which provides adsorption sites for H*. The 3d orbitals of Ni and Co interact with the 5d and 4d orbitals to generate an anisotropic electron distribution, which facilitates the adsorption and desorption of OH*. The synergistic effect of multi-d electrons ensures efficient catalytic activity. The optimized LaNi0.5 Co0.5 Ru has an overpotential of 43mV at 10 mA cm-2 for alkaline electrocatalytic hydrogen evolution reaction. Beyond offering a variety of new electrocatalysts, this work reveals the multi-d electron synergy in promoting catalytic reaction.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Chemical Potential-Modulated Ultrahigh-Phase-Purity Growth of Ultrathin Transition-Metal Boride Single Crystals

Two-dimensional (2D) transition-metal borides (TMBs) are especially expected to exhibit excellent performance in various fields among electricity, superconductivity, magnetism, mechanics, biotechnology, battery, and catalysis. However, the synthesis of ultrathin TMB single crystals with ultrahigh phase purity was deemed extremely challenging and has not been realized till date. That is because TMBs have the most kinds of crystal structures among inorganic compounds, which possess generous phase structures with similar formation energies compared with other transition-metal compounds, attributing to the metalloid and electron-deficient characteristics of boron. Herein, for the first time, we demonstrate a chemical potential-modulated strategy to realize the precise synthesis of various ultrahigh-phase-purity (approximately 100%) ultrathin TMB single crystals, and the precision in the phase formation energy can reach as low as 0.01 eV per atom. The ultrathin MoB2 single crystals exhibit an ultrahigh Young's modulus of 517 GPa compared to other 2D materials. Our work establishes a chemical potential-modulated strategy to synthesize ultrathin single crystals with ultrahigh phase purity, especially those with similar formation energies, and undoubtedly provides excellent platforms for their extensive research and applications.

Increasing the Distance of Adjacent Palladium Atoms for Configuration Matching in Selective Hydrogenation

Precisely tailoring the distance between adjacent metal sites to match adsorption configurations of key species for the targeted reaction pathway is a great challenge in heterogeneous catalysis. Here, we report a proof-of-concept study on the atomically sites-tailored pathway in Pd-catalyzed acetylene hydrogenation, i.e., increasing the distance of adjacent Pd atoms (d_Pd-a-Pd ) for configuration matching in acetylene semi-hydrogenation against coupling. d_Pd-a-Pd is identified as a structural descriptor for describing the competitiveness for reaction pathways, and the increased d_Pd-a-Pd prefers the semi-hydrogenation pathway due to simultaneously promoted C₂ H₄ desorption and the destabilized transition state of the C₂ H₃ * coupling. Spectroscopic, kinetics and electronic structure studies reveal that increasing d_Pd-a-Pd to 3.31 Å delivers superior selectivity and stability due to energy matching and appropriate hybridization of Pd 4d with In 2s and, especially, 2p orbitals.

d-sp orbital hybridization: a strategy for activity improvement of transition metal catalysts

Orbital hybridization to regulate the electronic structures and surface chemisorption properties of transition metals has been extensively investigated for searching high-performance catalysts toward various reactions. Unlike conventional d-d hybridization, the d-sp hybridization interaction between transition metals and p-block elements could result in surprising electronic properties and catalytic activities. This feature article highlights the recent progress in the development of high-performance transition metal-based catalysts through the extraordinary d-sp hybridization strategy, particularly for energy-related electrocatalytic applications. We start by giving an introduction of fundamental concepts associated with electronic structures of transition metal catalysts, including the Sabatier principle, d-band theory, electronic descriptor, as well as the comparison of d-d hybridization and d-sp hybridization strategies. Then, we summarize the theoretical and experimental advances in d-sp hybridization catalysts, including p-block element-doped metal catalysts, intermetallic catalysts and supported metal catalysts, with emphasis on the important roles of d-sp hybridization in tuning catalytic performances. Finally, we present existing challenges and future development prospects for the rational design of advanced d-sp hybridization catalysts.

Boron-induced activation of Ru nanoparticles anchored on carbon nanotubes for the enhanced pH-independent hydrogen evolution reaction

As a promising dopant, electron deficient B atom not only tunes the electronic structure of electrocatalysts for improving their intrinsic catalytic activities, but also combines with hydroxy radical as strong adsorption sites for accelerating the water dissociation during the hydrogen evolution reaction (HER). In this paper, we report an electrocatalyst based on boron-modified Ru anchored on carbon nanotubes (B-Ru@CNT) that shows impressive HER activity in acidic and alkaline media. The boron-rich closo-[B₁₂H₁₂]^2- borane was selected as a moderately strong reductant for the in situ reduction of a Ru salt, which yielded B-doped Ru nanoparticles. The experimental and theoretical results indicate that the incorporation of B not only weakens the Ru-H bond and downshifts the d-bond centre of Ru from the Fermi level by reducing the electron density at Ru but also accelerates the water dissociation reaction by providing B sites, which strongly adsorb OH* intermediates, and nearby Ru sites, which act as sites for the adsorption of the H* intermediate, thus boosting the HER performance and enhancing the HER kinetics. As a result of the tuning of the electronic structure via B doping, B-Ru@CNT showed excellent HER performance, yielding overpotentials of 17 and 62 mV at a current density of 10 mA cm^-2 in alkaline and acidic solutions, respectively. These results indicate that our synthetic method is a promising route to B-doped metallic Ru with enhanced pH-independent HER performance.

Nonprecious Metal Borides: Emerging Electrocatalysts for Hydrogen Production

ConspectusThe development of highly active noble-metal-free catalysts for the hydrogen evolution reaction (HER) is the focus of current fundamental research, aiming for a more efficient and economically affordable water-splitting process. While most HER catalysts are studied only at the nanoscale (small particle size and high surface area), metal borides (MBs) are mostly studied in bulk form. This offers a unique opportunity for designing highly efficient and nonprecious HER MBs electrocatalysts based on structure-activity relationships, especially because of their rich compositional and structural diversity.In this Account, we focus on the importance of boron and its substructures in achieving extraordinary HER performances and the importance of using structure-activity relationships to design next-generation MBs electrocatalysts. Studying the Mo-B system, we found that the HER activity of molybdenum borides increases with increasing boron content: from Mo₂B (no B-B bonds in the structure, least active) to α-MoB and β-MoB (zigzag boron chains, intermediate activity) and MoB₂ (planar graphene-like boron layer, most active). Density functional theory (DFT) calculations have shown that the (001) boron layer in hexagonal MoB₂ (α-MoB₂) is the most active surface and has similar HER activity behavior like the benchmark Pt(111) surface. However, puckering this flat boron layer to the chair-like configuration (phosphorene-like layer) drastically reduces its activity, thereby making the rhombohedral modification of MoB₂ (Mo₂B₄ or β-MoB₂) less active than α-MoB₂. This discovery was then further supported by studies of the Mo-W-B system. In fact, the binary WB₂, which also contains the puckered boron layer, is also less active than α-MoB₂, despite containing the more active transition metal W, which performs better in elemental form than Mo. To design a superior catalyst, the more active W was then stabilized in the hexagonal α-MoB₂ structure through the synthesis of α-Mo_0.7W_0.3B₂ ,which indeed proved to be a better HER electrocatalyst than α-MoB₂. Using the isoelectronic Cr instead of W led to the α-Cr_1-xMo_xB₂ solid solution, the HER activity of which followed unexpected canonic-like (or volcano-like) behavior that perfectly matched that of the c lattice parameter trend, thereby producing the best catalyst α-Cr_0.4Mo_0.6B₂ that outperformed Pt/C at high current density, thus underscoring the effectiveness of the structure-activity concept in designing highly active catalysts. This concept was further applied to the V-B system, leading to the discovery of an unexpected boron chain dependency of the HER activity that ultimately led to the prediction of new V_xB_y catalysts and their crystal structures and overpotentials. Finally, reducing the particle sizes of all of these bulk crystalline catalysts is also possible and offers an even greater potential as demonstrated for nanoscale a-MoB₂ and VB₂. Nevertheless, these crystalline nanomaterials are still highly agglomerated due to the high temperature required for their synthesis, thus the synthesis of highly dispersed MBs is an urgent goal that will enable the fulfillment of their extraordinary potential in the future.

Crystal phase-selective synthesis of intermetallic palladium borides and their phase-regulated (electro)catalytic properties

Three Pd–B intermetallics, namely, Pd6B, Pd5B2 and Pd2B, are synthesized, and their catalytic activities toward electrochemical water splitting and Suzuki coupling reactions are strongly associated with the Pd(d)–B(s,p) orbital hybridization.

Nanostructured Metal Borides for Energy-Related Electrocatalysis: Recent Progress, Challenges, and Perspectives

The discovery of durable, active, and affordable electrocatalysts for energy-related catalytic applications plays a crucial role in the advancement of energy conversion and storage technologies to achieve a sustainable energy future. Transition metal borides (TMBs), with variable compositions and structures, present a number of interesting features including coordinated electronic structures, high conductivity, abundant natural reserves, and configurable physicochemical properties. Therefore, TMBs provide a wide range of opportunities for the development of multifunctional catalysts with high performance and long durability. This review first summarizes the typical structural and electronic features of TMBs. Subsequently, the various synthetic methods used thus far to prepare nanostructured TMBs are listed. Furthermore, advances in emerging TMB-catalyzed reactions (both theoretical and experimental) are highlighted, including the hydrogen evolution reaction, the oxygen evolution reaction, the oxygen reduction reaction, the carbon dioxide reduction reaction, the nitrogen reduction reaction, the methanol oxidation reaction, and the formic acid oxidation reaction. Finally, challenges facing the development of TMB electrocatalysts are discussed, with focus on synthesis and energy-related catalytic applications, and some potential strategies/perspectives are suggested as well, which will profit the design of more efficient TMB materials for application in future energy conversion and storage devices.

Anion Modulation of Pt-Group Metals and Electrocatalysis Applications

Pt-group metal (PGM) electrocatalysts with unique electronic structures and irreplaceable comprehensive properties play crucial roles in electrocatalysis. Anion engineering can create a series of PGM compounds (such as RuP₂ , IrP₂ , PtP₂ , RuB₂ , Ru₂ B₃ , RuS₂ , etc.) that provide a promising prospect for improving the electrocatalytic performance and use of Pt-group noble metals. This review seeks the electrochemical activity origin of anion-modulated PGM compounds, and systematically analyzes and summarizes their synthetic strategies and energy-relevant applications in electrocatalysis. Orientation towards the sustainable development of nonfossil resources has stimulated a blossoming interest in the design of advanced electrocatalysts for clean energy conversion. The anion-modulated strategy for Pt-group metals (PGMs) by means of anion engineering possesses high flexibility to regulate the electronic structure, providing a promising prospect for constructing electrocatalysts with superior activity and stability to satisfy a future green electrochemical energy conversion system. Based on the previous work of our group and others, this review summarizes the up-to-date progress on anion-modulated PGM compounds (such as RuP₂ , IrP₂ , PtP₂ , RuB₂ , Ru₂ B₃ , RuS₂ , etc.) in energy-related electrocatalysis from the origin of their activity and synthetic strategies to electrochemical applications including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), N₂ reduction reaction (NRR), and CO₂ reduction reaction (CO₂ RR). At the end, the key problems, countermeasures and future development orientations of anion-modulated PGM compounds toward electrocatalytic applications are proposed.

Asymmetrically strained hcp rhodium sublattice stabilized by 1D covalent boron chains as an efficient electrocatalyst

Intermetallic rhodium boride (RhB) comprising an asymmetrically strained hcp Rh sublattice is synthesized. The covalent interaction of interstitial boron atoms is found to be the main contributor to the generation of asymmetric strains and the stabilization of the hcp Rh sublattice. In addition, RhB is identified as a hydrogen-evolving eletrocatalyst with Pt-like activity, because the Rh(d)-B(s,p) orbital hybridization induces an optimized electronic structure.

Germanium-regulated adsorption site preference on ruthenium electrocatalyst for efficient hydrogen evolution

A magnesiothermic reduction route has been presented to synthesize phase-pure germanides that are not readily available traditionally. The obtained ruthenium germanide (RuGe) serves as an efficient non-Pt electrocatalyst for hydrogen evolution, and its intrinsic activity is very close to that of Pt. Our combined theoretical and experimental study demonstrates that the remarkable performance is derived from the germanium-induced change in hydrogen site preference from hollow to efficient Ru top sites.

Critical Review of Platinum Group Metal-Free Materials for Water Electrolysis: Transition from the Laboratory to the Market : Earth-abundant borides and phosphides as catalysts for sustainable hydrogen production

To combat the global problem of carbon dioxide emissions, hydrogen is the desired energy vector for the transition to environmentally benign fuel cell power. Water electrolysis (WE) is the major technology for sustainable hydrogen production. Despite the use of renewable solar and wind power as sources of electricity, one of the main barriers for the widespread implementation of WE is the scarcity and high cost of platinum group metals (pgms) that are used to catalyse the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Hence, the critical pgm-based catalysts must be replaced with more sustainable alternatives for WE technologies to become commercially viable. This critical review describes the state-of-the-art pgm-free materials used in the WE application, with a major focus on phosphides and borides. Several emerging classes of HER and OER catalysts are reviewed and detailed structure‐property correlations are comprehensively summarised. The influence of the crystallographic and electronic structures, morphology and bulk and surface chemistry of the catalysts on the activity towards OER and HER is discussed.

Multiple crystal phases of intermetallic tungsten borides and phase-dependent electrocatalytic property for hydrogen evolution

Four stoichiometric W-B intermetallic phases, including W2B, WB, WB2 and WB3, are synthesized, and their hydrogen-evolution electrocatalytic properties and electronic structures are investigated comparatively. The electrocatalytic activity for the hydrogen evolution reaction is found to first increase from W2B to WB2 and then decrease; and this activity trend can be rationalized based on their different degrees of hybridization between d orbitals of W and sp orbitals of B.

Intermetallic borides: structures, synthesis and applications in electrocatalysis

This review summarizes structural features and recent synthesis methods of structurally ordered intermetallic borides, and the theoretical–experimental advances in the emerging boride-catalyzed reactions.