Multimetallenes and their electrocatalytic applications

Abstract

As a core driving force for energy conversion and storage, electrochemical catalytic technology has an irreplaceable strategic position in realizing the goal of carbon neutrality. However, it is still limited by bottlenecks such as low catalytic activity, selectivity and stability. In this context, multimetallenes have become a new platform to overcome the above bottlenecks by virtue of their ultra-high specific surface area, tunable electronic properties, and synergistic effects of multiple components brought about by atomic-level thickness. However, the current research hotspots still focus on monometallene systems, and the development of multimetallenes is limited by the core problems such as compositional segregation due to the differences in the reduction kinetics of the multivariate precursors, the difficulty of controllable synthesis of high-entropy systems, and the unknown mechanism of the dynamic evolution of the active sites under the complex working conditions, which seriously restrict their practical application performance under high current density and harsh conditions. This review systematically compiles the latest progress in this field: first, focusing on the innovation of controllable synthesis strategies, detailing the methods of surface energy-regulated reduction growth, hard template-limited domain thermally driven co-reduction, and self-template-assisted step-by-step alloying, aiming at solving the problem of multimetallene homogeneity and realizing precise construction; second, in-depth discussion of the optimization of multi-dimensional structural modification, covering the defect engineering, lattice strain modulation, polyalloying, non-metallene doping, heterogeneous interface design, surface functionalization, etc., to finely regulate the electronic and geometric structures of the materials and significantly enhance the reaction kinetics and durability; finally, in terms of application expansion, we focus on the breakthroughs of its performance in high-current density scenarios, such as oxygen reduction, all-water decomposition, small-molecule electro-oxidation/coupling, and CO₂ reduction. The aim of this paper is to provide comprehensive guidance for the design of highly efficient and stable multimetallene-based electrode materials, to promote their large-scale application in high-performance membrane electrode devices and integrated energy systems, to accelerate the industrialization of electrochemical conversion technologies, and to help achieve the goal of carbon neutrality.

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1. Introduction

Two-dimensional multimetallene materials, as new atomic-thickness catalysts, show unique advantages in the field of energy catalysis due to their tunable electronic structure and multimetallene synergistic effect.^1–3 Compared to monometallene systems, polyalloying has the potential to endow materials with a wider variety of active sites, greater resistance to toxicity, and wider pH adaptability, while an ultrathin two-dimensional morphology significantly improves atomic utilization and mass transfer efficiency.^4,5 However, the precise synthesis of multimetallenes faces multiple challenges: strong metal bonding makes it difficult to achieve atomic-level dispersion by conventional exfoliation methods, and the differential reduction kinetics of multiple precursors are prone to compositional segregation.^6,7

To address the synthesis bottleneck, the surface energy-modulated reduction and growth strategy dynamically reduces the surface energy of the target crystal surface through the selective adsorption of gas molecules (e.g., CO) on specific crystal surfaces (e.g., Pd(111)) to induce two-dimensional extension of the atoms and matches the difference in the decomposition temperatures of the polyprecursors (e.g., low-temperature-preferred reduction for Pd(acac)₂ and stepwise embedding for Ir₄(CO)₁₂ by synergizing the oleylamine (OAM) reductant with CO gas to achieve step-by-step reduction and embedded alloying, avoiding component segregation) to achieve stepwise reduction and embedding alloying and circumvent component segregation. The hard template domain-limited thermally driven reduction strategy precisely constrains the metal growth paths with the lattice-matching properties of inorganic salt templates (e.g., KBr), and realizes the multivariate synchronous co-reduction by matching the thermal decomposition temperatures of the precursors (e.g., the decomposition of Ru/Ga acetylacetonate salts occurs at 280–300 °C); the self-template-assisted step-by-step reduction and diffusion alloying strategy makes use of the self-assembled network of precursors such as a cyano-gel (e.g., RhCl₃ Co(CN)₆ cyanogel), coordinating the atomic-level homogeneous mixing of metals with significant reduction potential differences (e.g., Rh³⁺/Co³⁺) through ligand pre-fixation and stepwise catalytic reduction mechanisms.^8–10

These breakthroughs have led to comprehensive performance enhancements of multimetallenes in energy electrocatalytic systems: high intrinsic activity and durability in the oxygen reduction reaction (ORR); low overpotentials and high current densities in the hydrogen/oxygen exchange reactions (HER/OER); breakthroughs in the selectivity bottleneck of the C–C bond breaks in small molecule electro-oxidation (alcohols, formic acids, and formaldehydes); successful reactions and pathway synergies and energy–matter cogeneration in electrocatalytic coupling systems; low voltage drive and long-term stable operation in total water decomposition applications; and the development of a new energy–substance coupling system. Path synergy and energy–matter cogeneration; and low voltage drive and long-term stable operation for total water decomposition applications.^11,12 In this paper, we systematically discuss multimetallenes from the three dimensions of methodological breakthroughs in their synthesis, structural functionalization and electrocatalytic applications (Fig. 1), and with reference to the development history of multimetallenes (Fig. 2), look forward to their cross-applications in the fields of energy, environmental remediation, etc., so as to provide theoretical guidance and technological routes for the development of new-generation high-efficiency electrocatalysts.^13,14

Fig. 1 Schematic representation of the topics covered in the review of advances in multimetallene catalyst research.

Fig. 2 Schematic diagram summarizing the historical development of multimetallenes.

2. Methods for the preparation of multimetallenes

2.1. Reduced growth strategies for surface energy modulation

This strategy realizes the atomic-level thickness control of multimetallenes by selectively adsorbing specific crystal surfaces through dynamic template molecules (e.g., CO), which significantly reduces the surface energy of the target crystal surfaces, thus thermodynamically driving the preferential growth of metal atoms along the two-dimensional direction of extension. As shown in Fig. 3, the core breakthrough lies in regulating the differential reduction kinetics of polyprecursors: by matching the precursor decomposition temperature and stepwise reduction mechanism, the problem of component segregation can be avoided, and the precise construction of polyalloys can be realized.¹⁵

Fig. 3 Schematic diagram of the multimetallene synthesis strategy.

A typical representative is the PdIr bimetallene system developed by Guo's team, which uses oleylamine (OAM) as the reduction medium to reduce palladium acetylacetonate (Pd(acac)₂) and tetrairidium dodecacarbonyl (Ir₄(CO)₁₂) precursors in a stepwise manner, combining with the selective adsorption on the (111) crystal plane of the pyrolyzed CO from W(CO)₆, to achieve the stepwise reduction and two-dimensional domain-limited growth of the Pd and Ir atoms. The CO molecules generated by the pyrolysis of W(CO)₆ are preferentially adsorbed on the Pd(111) crystalline surface, which significantly reduces the surface energy of this crystalline surface and effectively inhibits the atomic stacking in the vertical direction.¹⁶ To address the challenge of differential reduction kinetics of multiple precursors, this strategy achieves synergistic regulation through a stepwise reduction and alloying mechanism: precursors with differential thermal decomposition temperatures are selected; Pd(acac)₂ requires chemical reduction, whereas Ir₄(CO)₁₂ is decomposed by low-temperature thermal decomposition, which results in the preferential nucleation of Pd to form an initial nanosheet template within 10 min, followed by the gradual embedding of Ir⁰ atoms into the Pd lattice to form the uniform PdIr alloy structure (Fig. 4a). This process cleverly overcomes the kinetic barrier of conventional high-temperature reduction of Ir³⁺ (>200 °C) and ensures that Pd and Ir atoms are synergistically assembled under mild conditions (150 °C), avoiding the problem of component segregation. The experimental data show that the 8 h reaction at 150 °C yields curved nanosheets with a thickness of ∼1.0 nm and an Ir alloying ratio of 23 at%, with an electrochemically active surface area (ECSA) as high as 127.5 ± 10.8 m² g⁻¹, which is significantly better than that of conventional PdIr nanoparticles (38.9 ± 5.0 m² g⁻¹). Synchrotron EXAFS analysis confirms that the capping effect of CO induces the expansion of the Pd–Pd(Ir) bond length from 0.274 nm to 0.277 nm, generating a 1.1% lattice strain, which optimizes the free energy of the hydrogen intermediate adsorption through the downward shift of the d-band center (−1.7 eV → −2.6 eV), resulting in the lowering of the HER overpotential (34 mV@10 mA cm⁻²) compared to that of Pt/C 56%.

Fig. 4 (a) Low-magnification HAADF-STEM image of the PdIr bimetallene. (b) TEM image of Pd₅₀W₂₇Nb₂₃/C. The inset in (b) is the vertical part of as-prepared Pd₅₀W₂₇Nb₂₃/C; the scale bar is 20 nm. (c) Digital photographs before (left) and after (right) gelatinization of the K₃[Co(CN)₆]–RhCl₃ cyanogel. (d) The molecular structure of the K₃[Co(CN)₆]–RhCl₃ cyanogel. (e) Proposed model for a bubbling CO-induced gelation method.

This strategy is further extended in the construction of trimetallene systems. Taking PdWNb trimetallene as an example,⁶ preferential adsorption of CO released from the decomposition of W(CO)₆ on the (111) crystal surface in acetic acid medium guides the two-dimensional extension while synergizing with the stepwise reduction kinetics of NbCl₅ for W/Nb atomic-level doping and porous structure construction (Fig. 4b). In response to the core challenge of the reduction rate mismatch of multiple precursors (Pd(acac)₂, W(CO)₆, and NbCl₅), the system is dynamically regulated by the differential design of precursor decomposition temperature: W(CO)₆ is pyrolyzed at 150 °C to generate CO and W⁰ atoms, whereas NbCl₅ needs to be converted to Nb⁰ gradually by the action of the reducing agent, which is inversely related to the fast-reducing Pd precursor. The stepwise alloying process (Pd preferential nucleation within 10 min and Nb/W continuous embedding within 8 h) effectively avoids the problem of component segregation. The continuous adsorption of CO on the (111) facet (2000 cm⁻¹ characteristic peak) effectively reduces the surface energy of this crystalline facet and maintains the advantage of the two-dimensional growth path to form a 1.5 nm-thick porous structure. By precisely controlling the reaction time, the W/Nb atomic ratio is dynamically regulated (Nb content tends to be close to 0 at 10 min, and reaches 27 at% W/23 at% Nb after 8 h), which synchronously optimizes the pore structure and electronic environment. X-ray photoelectron spectroscopy (XPS) analysis showed that the C1 pathway selectivity for ethanol oxidation was enhanced to 55.5% (7.7-fold higher than that of conventional Pd/C) by promoting C–C bond breaking through strong d–p orbital hybridization. This stepwise reduction–adsorption synergistic mechanism was successfully extended to the Mo/Ta system, and the PdWMo/PdWTa trimetallene showed a similar porous structure with high C1 selectivity (43.7%/35.0%), which verified the universality of the surface energy modulation strategy in the multifaceted system.

The dynamic gel network strategy innovatively combines the template effect of CO with solvent coordination for ultrafast assembly.⁷ CO bubble perturbation induces the formation of [Pd₄(CO)₄(OAc)₄] intermediates from Pd(acac)₂ and W(CO)₆ in a 50 °C acetic acid system.¹⁷ and self-assembly of single-atom W-doped Pd metallene aerogels (SA W–Pd MAs) in 1 hour (Fig. 4c and d). This strategy synchronizes the reduction kinetic differences of the multiplexed precursors through the dual function of CO bubbles (reducing agent and structure-directing agent): the rapid decomposition of W(CO)₆ at low temperatures (50 °C) releases the W atoms, whereas Pd(acac)₂ accelerates the reduction to Pd⁰ clusters under the perturbation of CO, and the two form a dynamic intermediate through the bridging of the acetic ligands to achieve the atomic-level assembly and doping. CO bubble perturbation induces the self-assembly of [Pd₄(CO)₄(OAc)₄] intermediates, synchronizing the rapid co-reduction of Pd(acac)₂ and W(CO)₆ with in situ doping of monoatomic W. Atomic force microscopy (AFM) characterization shows that its thickness is only 1.21 nm, and the three-dimensional pore structure enhances the ethanol oxidation mass activity (5.29 A mg⁻¹) by a factor of 3.1 compared with that of Pd/C. The synergistic mechanism effectively circumvents the problem of component segregation in the conventional multifunctional system: the low-temperature decomposition property of W(CO)₆ (<100 °C) is highly compatible with the mild reduction kinetics of Pd(acac)₂ (50 °C), and the CO bubble perturbation further accelerates the mass-transfer process, which ensures that the W atoms are embedded in situ in the form of monoatoms (W content of 0.2 wt%) and dispersed homogeneously in the Pd crystal lattice. This dynamically balanced reduction–assembly process provides a universal solution for the differential kinetic modulation of multimetallene precursors.

In the synthesis system for the surface energy-regulated growth strategy, the synergistic control of the surface energy of specific crystalline surfaces regulated by dynamic templates (e.g., CO) through selective adsorption and the reduction kinetics of multiple precursors is the core mechanism for the controllable preparation of multimetallenes.^18,19 Through the facet-selective adsorption of CO molecules and the ligand-mediated atomic assembly process, this strategy successfully achieves two-dimensional directional growth from bimetallene to ternary and quaternary alloy systems. Essentially, it enables the preferential deposition and ductility of atoms on the target crystalline facet (e.g., Pd(111) facet) by reducing the surface energy of the target facet, restricts the three-dimensional stacking of atoms through the facet-selective adsorption of CO molecules, and regulates the reduction order and rate of multiple precursors to realize the synchronous or stepwise co-reduction of polymetals. Future research can further explore the dynamic interaction mechanism between template molecules and different metal precursors, and develop in situ characterization techniques to analyze the interfacial evolution of the 2D growth process.^20,21

2.2. Thermally driven reduction strategies for hard stencil-limited domains

In the synthesis of pure metal multimetallenes, the hard template method has found relatively little use, but recent studies have shown that inorganic salt templates (e.g., KBr, etc.) have been used to modulate the morphology and component distributions of metallenes, with the advantage that the well-defined crystal structure of the templates can precisely modulate the lattice orientation and morphology of the metallene features.²² Taking KBr crystals as an example, our group dissolved Ru(acac)₃, Ga(acac)₃, and KBr in a mixed ethanol/water solvent, and then dried to obtain a mixed powder.²³ In this mixed powder, precursor molecules of Ru and Ga (acetylacetone salts) are in close contact with the KBr template. By precisely matching the kinetic properties of the thermal decomposition of Ru(acac)₃ (decomposition temperature ∼300 °C) and Ga(acac)₃ (decomposition temperature ∼280 °C), the two precursors achieve synchronous reduction (Ru³⁺ → Ru⁰, Ga³⁺ → Ga⁰) during the heating process, which can effectively avoid the problem of component segregation caused by the difference of reduction potentials in the traditional multifunctional system. The mixed powders were heated to specific temperatures (285 °C or 360 °C) under an air atmosphere. During the heating process, Ru(acac)₃ and Ga(acac)₃ undergo simultaneous thermal decomposition and reduction, and the released Ru and Ga atoms are restricted to diffuse, aggregate, and grow together on the surface of the KBr template. The KBr template not only provides a dynamic two-dimensional confined interface, but also promotes the homogeneous mixing of Ru/Ga atoms by lowering the atomic diffusion barrier to realize the atomic-level co-assembly. KBr provides a dynamic two-dimensional confined interface at high temperature, which guides the self-assembly of the reduced metal atoms into the ultrathin lamellar-structured metallene.

This simultaneous thermal decomposition/reduction using precursors with similar decomposition/reduction temperatures in a confined space provided by a hard template is an effective method for the co-reduction of multiple metals and the formation of alloy nanostructures.

2.3. Self-template-assisted stepwise reduction and the diffusion alloying strategy

Self-template-assisted stepwise reduction and diffusion alloying are synthetic methods for the in situ formation of structure-directed motifs by the precursors themselves during the reaction process. The strategy does not require the introduction of an external templating agent, but rather the spontaneous formation of intermediates with a specific morphology using chemical coordination or self-assembly behavior among the precursors, which provides spatial constraints for the subsequent directional growth of metal atoms. This endogenous template mechanism effectively avoids the complex process of exogenous template removal and shows unique advantages in the controlled preparation of 2D metallenes.^24,25

In the preparation of a Rh–Co alloyed bimetallene, the researchers innovatively developed a dynamic self-templating strategy based on cyanogel self-assembly.¹⁶ RhCl₃ and K₃[Co(CN)₆] form a three-dimensional networked cyanogel via [RhCl₃]–[Co(CN)₆]³⁻ coordination during heating. This cyanogel has a unique two-dimensional structural unit, and its layers are connected by metal–cyano coordination bonds as shown in Fig. 4e, which provides an ideal template substrate for subsequent two-dimensional domain-limited growth of metal atoms. At high temperatures, the unsaturated carbon atoms in the cyanogel act as reducing agents. In response to the significant reduction potential difference between Rh³⁺ (+0.758 V vs. RHE) and Co³⁺ (−0.28 V), the strategy achieves kinetic synergy by a mechanism of ligand pre-fixation and stepwise catalytic reduction: the rigid ligand network of the cyanogel atomically locks Rh³⁺/Co³⁺ at the neighboring sites; even if Rh³⁺ is preferentially reduced to metallene Rh nuclei, its spatial position is still restricted by the cyano-skeleton, which effectively circumvents the problem of component segregation in the traditional stepwise reduction. Critically, the standard reduction potential of Rh³⁺ species (+0.758 V vs. RHE) is higher than that of Co³⁺ (−0.28 V vs. RHE), and thus Rh³⁺ is preferentially reduced to metallene Rh nuclei. These newly formed Rh nuclei then acted as in situ catalytic centers, significantly accelerating the reduction of the surrounding Co³⁺ species by lowering the reduction activation energy (kinetic enhancement of ∼3-fold), and synchronizing Co⁰ atom generation and diffusion. In this process, the RhCl₃–K₃[Co(CN)₆] cyanogel effectively acts as a self-template, providing a two-dimensional backbone for growth. The newly generated metallene Co atoms subsequently diffuse into the preformed Rh nanosheets driven by thermodynamic factors, ultimately forming Rh–Co alloy nanosheets. The high-temperature environment (220 °C) further strengthens the atomic interdiffusion barriers, resulting in the deep embedding of Co atoms into the Rh lattice to form a homogeneous alloy (Rh/Co = 83 : 17), and thermodynamically stable interfacial fusion. The simultaneous atomic diffusion process prompts the embedding of Co atoms into the Rh lattice, resulting in the formation of a folded bimetallene with a thickness of only 1.0 nm, whose atomically thin-layer property originates from the two-dimensional domain-limiting effect of the cyanogel template. This synergistic mechanism provides a universal alloying pathway for polymeric systems with significant differences in reduction potentials.²⁶

The self-templating strategy also exhibits unique structure-oriented advantages in the synthesis of a two-dimensional RhNi bimetallene. In this study, a RhCl₃–K₂[Ni(CN)₄] cyanogel with a two-dimensional layered structure was generated in situ by the cyano-gelation reaction of RhCl₃ with K₂[Ni(CN)₄], and its polymer properties provided a natural template for subsequent alloy growth.²⁷ Freeze-dried SEM images showed that the cyanogel consisted of a large number of layered units, and its Tyndall effect confirmed the stability of the colloid. During thermal reduction at 220 °C, the two-dimensional structural units of the cyano-gel guided the directional arrangement of metal atoms to form an ultrathin RhNi bimetallene (RhNi BMLs). The time-dependent structural evolution showed that Rh³⁺ was preferentially reduced to Rh nanocrystalline nuclei, followed by gradual diffusion of Ni²⁺ into the lattice interstitials, which self-assembled into nanoflower structures constructed by cross-linked nanosheets via anisotropic growth. Notably, the RhNi BMLs themselves were synthesized via a cyanogel self-template reduction strategy. In this process, the RhCl₃–K₂[Ni(CN)₄] cyanogel with intrinsic two-dimensional structural units acts as a template. During high-temperature reduction, the Rh³⁺ species (standard reduction potential +0.758 V vs. RHE) was reduced preferentially to the Ni²⁺ species (−0.34 V vs. RHE) to form the initial Rh nucleus. Subsequently, Ni²⁺ species were reduced on or near the pre-formed Rh nucleus. The newly reduced Ni atoms subsequently diffuse into the Rh lattice driven by thermodynamic factors, resulting in the formation of RhNi alloy bimetallene structures with a Rh/Ni atom ratio close to 1 : 1. This self-templating strategy, which does not require externally added surfactants, utilizes the lamellar domain-limiting effect intrinsic to cyano-gels to achieve an ultrathin structure with a thickness of 1.6 nm and a lattice spacing (0.216 nm) intermediate between those of Rh(111) and Ni(111), which confirms the Rh–Ni alloying effect. The RhNi@Rh BMLs formed after acid etching still maintain the nanoflower morphology with a thickness of only 1.65 nm as shown by AFM, and the spherical aberration-corrected-high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) reveals a 1.0 nm-thick Rh-skin layer on the surface, which highlights the key role of the self-templating strategy in the structural construction of the complex core–shell structure. The strategy realizes the synergy of high specific surface area (86.3 m² g⁻¹) and stabilized active sites through the spatially confined domains of the material's own chemical components, providing an ideal two-dimensional reaction interface for acidic nitrate electroreduction.

Future research can further explore the self-assembly chemistry of multimetallene precursors and analyze the dynamic evolution of gel network-metal growth in conjunction with in situ characterization techniques, providing a new paradigm for the precise construction of complex systems such as high-entropy alloys.

3. Modification methods for multimetallenes

The modification strategies for multimetallenes focus on multidimensional modulation: defect and strain engineering to optimize the electronic structure through lattice distortion;^28,29 alloying and doping to break the single component limitation and balance activity and stability;³⁰ surface functionalization to introduce functional groups or nanoparticles to expand catalytic selectivity;³¹ and heterostructure design utilizing interfacial charge and strain to synergistically reduce reaction energy barriers. These strategies synergistically (Fig. 6) promote the performance breakthrough of multimetallenes in the fields of efficient catalysis, energy conversion, etc., and are expected to realize the precise customization of material functions in the future in combination with intelligent design.^32,33

3.1. Defect engineering strategies

Defect engineering is an effective means to enhance the electrocatalytic performance of multimetallenes. Zhang et al. prepared a defect-enriched PdMo metallene (D-PdMo) by KBr oxidative etching (Fig. 5a), which utilizes Br⁻/O₂ oxidation for selective etching of the metal crystalline surfaces to form 1–20 nm nanoholes (Fig. 5b).³⁴ High-resolution transmission electron microscopy (HRTEM) showed that D-PdMo has significant lattice distortion and localized amorphization features, with its (111) crystalline facet spacing expanding from 0.21 nm to 0.23 nm. This defective structure not only significantly increases the electrochemically active area (0.178 cm²), but also causes modulation of the position of the d-band center (−2.57 eV vs. −2.78 eV). Optimizing the adsorption behavior of dopamine molecules, the detection sensitivity was increased to 0.029 μM. XPS analysis showed that the defects led to a positive shift of the Pd 3d binding energy and an increase in the content of oxidation state Pd, which further enhanced the catalytic activity.

Fig. 5 (a) Schematic of the synthesis of PdMo and D-PdMo metallenes. (b) TEM image of the D-PdMo metallene. (c) Schematic diagram of the synthesis of the PtIr hetero-metallene.

Fig. 6 Schematic diagram of the modification method of multimetallenes.

In the future, machine learning can be used to predict the regulation law of defect configuration on electronic states, and guide the construction of multiple defect systems; we need to further explore the integrated application of defect engineering in membrane electrode devices, and promote the leap and upgrading of multimetallene olefins from laboratory catalysts to industrial-grade energy devices.^35,36

3.2. Contingency engineering strategy

Strain engineering optimizes the electronic state of the material surface and the intermediate adsorption behavior by modulating the magnitude and direction of the metallene lattice distortion. The application of this strategy in multimetallene systems has demonstrated significant catalytic performance enhancement.

Taking a PtIr heterogeneous metallene as an example (Fig. 5c),³⁷ high-resolution TEM analysis shows that the loading of Pt nanoclusters leads to a significant lattice expansion effect on the Ir metallene substrate. In the palladium/iridium (PtIr) interface region, the (111) crystal plane spacing expands from 0.21 nm in pure Ir to 0.221 nm, creating a tensile strain of 1.1%. This lattice distortion was verified by the XPS binding energy shift (Ir 4f orbitals negatively shifted by 0.23 eV) and Bader charge analysis, confirming the mechanism of the strain-induced downward shift of d-band centers, which effectively optimizes the dissociation process of water molecules, and decreases the HER overpotential to 31 mV@10 mA cm⁻², which is significantly superior to that of the pure Ir metallene (233 mV) and commercial Pt/C (51 mV). This work reveals, for the first time, the enhancement mechanism of the tensile strain generated at the noble metal heterointerface on the catalytic performance of electrolytic water, which breaks through the single modulation mode of compressive strain in the conventional alloying strategy.

This strain modulation strategy complements the Pd-based and Rh-based systems of the previous studies. In the PdWM trimetallene,³⁸ the synergistic doping of W and Nb induced a 1.1% compressive strain in the Pd lattice, leading to a downward shift of the center of the d-band by 0.78 eV, which significantly weakened the strength of CO intermediate adsorption, whereas the lattice compressive strain of the RhNi@Rh bimetallene (the shortening of Rh–Rh bond lengths to 2.65 Å) optimized the *NH₂ adsorption energy through the surface Rh-skin structure. Comparatively, it is found that the tensile strain of the PtIr system mainly acts on the carrier metal (Ir), while the compressive strain of the Pd, Rh system concentrates on the active metal (Pd, Rh), and this difference originates from two different strain generation mechanisms: heterogeneous interface formation and alloying. Notably, the PtIr system achieves dual electronic structure-geometry optimization through the synergistic effect of interfacial charge transfer (Pt → Ir) and lattice strain, providing a new design dimension for strain engineering.

Current strain modulation mainly relies on alloying or doping-induced lattice distortion, but precise control of the strain magnitude and direction is still a challenge. In the future, it is necessary to combine the in situ characterization technology to reveal the strain evolution law under dynamic working conditions, and explore new methods.

3.3. Doping strategy

Doping strategy accurately regulates the electronic structure and lattice strain of multimetallenes through the introduction of heterogeneous atoms as a key means to break the bottleneck of catalytic activity and stability.³⁹ This strategy induces local charge redistribution, optimizes intermediate adsorption energies, and strengthens structural stability while retaining the intrinsic advantages of 2D materials through a differential doping mechanism of hydrogen, carbon, and phosphorus elements.⁴⁰

For example, hydrogen doping induces lattice expansion to lower the reaction energy barrier. Wang's team prepared a hydrogen-doped PdCu metallene (PdCuH metallene) by alloying and hydrogen doping, which significantly enhanced its ORR and formic acid oxidation reaction (FOR) performance in direct formic acid fuel cells (DFFCs) (Fig. 7a).⁴¹ The researchers first synthesized a PdCu alloy metallene using wet chemistry, with the introduction of copper to optimize the electronic structure of palladium, modulation of the position of the d-band center, and reduction of its adsorption energy for reaction intermediates (e.g., *CO and *O) (Fig. 7b), thus enhancing the catalytic activity. On this basis, a hydrogen doping strategy was further employed to embed H atoms into the PdCu alloy lattice via a high-temperature reaction using N,N-dimethylformamide (DMF) as the hydrogen source, which triggers lattice stretching strain and reduces the electron density. This process not only enlarges the lattice spacing of the metallene (e.g., the crystal plane spacing of (111) increases from 0.225 nm for PdCu to 0.230 nm for PdCuH), but also significantly weakens the CO adsorption energy through the d-band centers moving further away from the Fermi energy level (−1.836 eV for PdCuH and −1.695 eV for PdCu) (from −1.766 eV to −1.958 eV), thus substantially improving the resistance to CO poisoning. The synergistic effect of alloying and hydrogen doping not only increased the ECSA from 40.26 m² g⁻¹ for PdCu to 47.47 m² g⁻¹ for PdCuH, but also led to an ORR mass activity up to 12.4 times that of Pt/C (Fig. 7c), and a FOR specific activity up to 3.2 times that of Pd/C (Fig. 7d). Density Functional Theory (DFT) calculations further verified the mechanism of electronic structure modulation and revealed the central role of alloying and hydrogen doping in reducing the binding energy of intermediates and optimizing the reaction pathways.

Fig. 7 (a) Schematic illustration of the synthesis process of the PdCuH metallene. (b) The CO adsorption energy of the PdCuH metallene and PdCu metallene. (c) Mass activities and specific activities at 0.9 V (vs. RHE) for different samples. (d) ECSA-normalized CV curves of different samples in 1 M KOH + 1 M HCOOK at a scan rate of 50 mV s⁻¹. (e) Schematic illustration of the synthetic process of the C-doped Pd₉₅Mo₅ bimetallene. The enlarged schematic enclosed by the yellow rectangle shows the relative positions of the Mo atom and its surrounding C atoms. (f) High-resolution TEM image taken from a single C-doped Pd₉₅Mo₅ bimetallene.

Carbon doping opens up new paths for the electronic structure and stability modulation of metallenes through covalent bonding mechanisms. Taking the PdMo bimetallene as an example, the research team used the solvothermal method combined with the gradient annealing technique (Fig. 7e) to precisely embed carbon atoms into the molybdenum lattice sites (Fig. 7f), forming a stable Mo–C covalent bond with a bond length of 2.08 Å.⁴² This strong covalent interaction significantly improves the electrochemical stability of the molybdenum component: after 5000 cycles under the harsh oxidizing conditions of 0.9 V vs. RHE, the retention rate of Mo atoms reaches up to 96.7%, which is three times higher than that of the undoped system, and fundamentally solves the problem of solubility loss of non-precious metal components in the catalytic process. Synchrotron X-ray absorption spectroscopy further reveals that carbon doping induces a 1.3% compressive strain in the Pd lattice and synergistically optimizes the adsorption behavior of oxygen intermediates through the electron transfer of Pd → Mo–C (XPS binding energy shift of 0.38 eV), which results in a breakthrough of the ORR mass activity to 7.013 A mg⁻¹, which reaches 42 times that of the commercial Pt/C, with a positive shift of the half-wave potential by 52 mV. Density-functional theory calculations show that the introduction of carbon atoms drives the d-band center of Mo downward by 0.15 eV, decreasing the *OOH desorption energy barrier from 0.82 eV to 0.61 eV, while increasing the 4e⁻ reaction path selectivity to over 98% (H₂O₂ yield <2%). This strategy provides a universal solution for the development of highly stable and low-cost bimetallene catalysts through the dual roles of covalent bond anchoring and electronic structure redistribution, especially in systems containing easily oxidizable metals (e.g., Mo, Fe, and Co), and the performance limit can be further explored in the future by tuning the carbon-doped sites.

Recent studies have confirmed that phosphorus atom doping can significantly expand the lattice spacing of bimetallenes and modulate the position of the d-band centers to optimize the adsorption–desorption equilibrium of intermediates. Liu et al. prepared a phosphorus doped PdMo bimetallene (P–PdMo) by wet chemistry, and their unique 2D curved ultrathin structure (thickness ∼1.2 nm) with partially amorphous features exposes a rich set of active sites.⁴³ Synchrotron X-ray absorption spectroscopy (XAS) and DFT calculations indicate that the doping of P atoms extends the lattice spacing of Pd from 0.225 nm in PdMo to 0.238 nm, inducing a further downward shift of the d-band center of Pd from −1.64 eV to −1.86 eV. This electronic modulation significantly weakens the adsorption energy of CO intermediates (ΔG_CO from −2.24 eV in Pd to −1.84 eV) and promotes the complete oxidation pathway of alcohol molecules in alkaline media by optimizing OH adsorption (ΔG_OH = −0.26 eV). In the ethanol oxidation reaction (EOR), P–PdMo exhibited a mass activity of 4.95 A mg Pd⁻¹, which was a 1.7-fold enhancement over undoped PdMo, and the activity retention rate was 94.6% after a 12 h constant potential test. In addition, the material also exhibits excellent performance in the ORR, with a half-wave potential (E_1/2 = 0.88 V) that exceeds that of commercial Pt/C (0.85 V) and a mass activity of 1.25 A mg Pd⁻¹ (2.4 times that of Pt/C). The phosphorus doping strategy breaks through the trade-off between activity and stability in conventional bimetallene systems through the synergistic effect of atomic-level electron redistribution and lattice strain effect, providing a new idea for designing multifunctional electrocatalysts. Compared with the hydrogen/carbon doped systems (e.g., PdCuH, PdMo–C) in previous studies, phosphorus doping exhibits more remarkable non-metal-to-metal orbital hybridization properties, and its wide pH adaptability (from the alkaline AOR to acidic HER) further expands the scenarios for the application of metallenes in energy conversion devices.⁴⁴

Differential doping mechanisms for hydrogen, carbon, and phosphorus reveal rich possibilities for nonmetallene modulation. Three types of strategies exhibit differentiated structural effects: H doping mainly achieves electronic modulation through lattice expansion; C doping realizes dual regulation of geometric constraints and electron transfer by virtue of covalent bonds; and P doping combines lattice expansion with non-metallic orbital hybridization to achieve broad-spectrum adaptability. In terms of stability, C doping fixes non-precious metal components through strong covalent bonds, and its anti-dissolution ability is significantly superior to that of H/P doping; P doping, due to its strong electronic modulation characteristics (with a d-band center shift of 0.22 eV), is more suitable for scenarios requiring in-depth optimization of reaction pathways.

In the future, we can further explore the coupling effect of multi-element synergistic doping, analyze the dynamic evolution of doping sites by combining with in situ dynamic characterization, and develop universal synthesis methods to realize the precise construction of high-entropy doping systems, so as to promote the development of multimetallene olefin catalysts from laboratory innovations to industrial-scale applications.^45,46

3.4. Alloying strategy

The alloying strategy is a central means to optimize the catalytic performance of multimetallenes through synergistic electronic effects and lattice modulation of multimetallenes.⁴⁷ This strategy utilizes orbital hybridization and charge redistribution between different metals to precisely regulate the electronic structure of the active site and effectively reduce the energy barrier of key reaction steps.⁴⁸

The alloying strategy has become a central means to optimize the catalytic performance of multimetallenes through the electronic synergy and lattice modulation of multicomponents. In the PtTeAu trimetallene system (Fig. 8a), the synergistic interaction between Te and Au exhibits a unique electronic regulation mechanism:¹² the partial replacement of Te atoms with Au through dynamic electrical substitution reaction (Fig. 8b and c) significantly suppressed the competitive HER activity of the Pt site by 47%, shifted the d-band center of Pt upward by 0.32 eV, optimized the adsorption strength of *NO intermediates (ΔG_*NO decreased by 0.18 eV), and ultimately achieved a 96.3% NH₃ faradaic efficiency in the nitrate electroreduction reaction and 3.499 mg h⁻¹ mg cat⁻¹ mass activity, a 6.1-fold enhancement over that of commercial Pt black. This effect stems from the precise modulation of the electronic structure of Pt by Au atoms, which highlights the key role of alloying in reconstructing the intrinsic properties of the active site by weakening the adsorption of toxic intermediates and accelerating the reaction kinetic pathway.

Fig. 8 (a) Schematic illustration of Au-NC (gold nanocrystal)/PtTeAu-ML synthesis. (b) TEM image, (c) HRTEM image and size distribution histogram of Au-NCs/PtTeAu-MLs.

The alloying strategy not only enhances the intrinsic activity of the material through the electronic synergy and lattice regulation of multimetallenes, but also strengthens the structural stability through the mutual constraints between the components. The future development needs to integrate the innovation of synthesis methodology and the analysis of the dynamic mechanism, and to push the multimetallene from laboratory innovation to use as industrial catalytic materials.^49,50

3.5. Surface functionalization strategies

The surface functionalization strategy of multimetallenes significantly enhances their catalytic performance and stability through multiple synergistic effects. In the study by Miao et al. the surface modification of copper-based multimetallenes (CuZn, CuSn, CuNi, etc.) not only maintains the synergistic antimicrobial effect of each metal component, but also significantly improves the dispersion of the material.⁵¹ The experimental data showed that the modified multimetallene suspension could be stabilized in the physiological environment for more than 72 h. Moreover, the killing efficiency of the CuZn metallene against MRSA (methicillin-resistant *Staphylococcus aureus*) at a concentration of 1 μg mL⁻¹ reached 99.8%, which was 2.3-fold higher than that of the unmodified sample. This surface functionalization treatment enhanced the Fenton-like reaction activity by modulating the electron transfer at the multimetallene interface, resulting in a 40% increase in the conversion efficiency of the Cu⁺/Cu²⁺ redox pair.

Surface functionalization of multimetallenes also modulates catalytic performance by optimizing metal–ligand interactions. It is shown that in multimetallene systems such as CuZn/CuSn, the surface-modified poly(ethylene glycol) chains not only provide a spatial site-blocking effect, but also modulate the position of the d-band centers through the coordination of oxygen atoms with metal sites. XPS analysis showed that the binding energy of Cu 2p_3/2 was shifted to the high energy direction by 0.3 eV and the binding energy of Zn 2p_3/2 was decreased by 0.2 eV after the functionalization treatment, and this electron redistribution enabled multimetallene olefins to catalyze the decomposition of H₂O₂ to produce –OH more efficiently in the antimicrobial process. In the animal experiments, the rate of wound healing in the functionalized multimetallene olefin treatment group was 1.8 times faster than that of the monometallene control group, and the accumulation of metal ions in the major organs was reduced by 60–75%, confirming that the surface functionalization enhanced the synergistic effect of multimetallenes while controlling the biotoxicity effectively.⁵²

The strategy significantly enhanced the catalytic performance and stability by precisely modulating the interfacial chemical environment of the multimetallene. In the future, we can synergistically inhibit active-site poisoning through ligand spatial site resistance and electronic effects; use in situ spectroscopy to track the structural evolution of the functional layer in real time and establish a predictive model for interfacial stability; and promote the integrated application of functionalized multimetallenes in membrane electrode devices to break through the bottleneck of wide pH adaptability and material transport.^53,54

3.6. Heterogeneous structure design strategy

The construction of 0D/2D or 2D/2D heterogeneous interfaces can accelerate charge separation and thus enhance the catalytic performance of multimetallenes. For example, Chen's team significantly enhanced the performance of the nitrate electroreduction reaction (NO₃RR) by designing porous PtTeAu metallene heterostructures (Au-NCs/PtTeAu-MLs) modified with gold nanocrystals (Au-NCs).¹² In the study, ultrathin PtTe metallenes (PtTe-MLs) were first synthesized by liquid-phase chemical reduction, followed by the replacement of Te atoms with Au atoms using Galvanic Replacement Reaction (GRR) and the generation of Au nanocrystals on the surface of the metallene. This heterostructure design not only preserves the two-dimensional nanosheet properties of the metallene, but also significantly modulates the electronic structure of the Pt sites by introducing Au nanocrystals and Te atoms. The introduction of Te atoms inhibited the high activity of the HER at the Pt site and reduced the reaction path competing with the NO₃RR; while the introduction of Au atoms further modulated the d-band center position of Pt and optimized the adsorption behavior of the reaction intermediates during the NO₃RR. DFT calculations showed that the introduction of Au atoms shifted the d-band centers of the PtTeAu metallene upward and enhanced the reactants’ adsorption by the Pt site, while weakening the adsorption of the product and making the desorption of the reaction intermediate easier, thus improving the selectivity and activity of the NO₃RR. In addition, the localized surface plasmon resonance effect of Au nanocrystals further promoted the kinetic process of the NO₃RR under light conditions, resulting in a NH₃ yield of 4.684 mg h⁻¹ mg cat⁻¹ under light conditions, which was significantly higher than that under dark conditions. The experimental results showed that the Au-NCs/PtTeAu-MLs exhibited excellent performance in the NO₃RR, with an NH₃ yield of 3.499 mg h⁻¹ mg cat⁻¹ and a faradaic efficiency (FE) of 96.3%, which was much higher than those of commercial Pt black catalysts. The heterostructure also exhibited good electrochemical stability and long-term durability, with the NH₃ yield and FE value remaining stable during six consecutive cycling tests and long-term tests of up to 20 h. The heterostructure was designed to provide an efficient method for the modification of multimetallenes. By introducing Au nanocrystals and Te atoms, not only the electronic structure of the multimetallene can be modulated, but also its catalytic performance and stability in the NO₃RR can be significantly enhanced.⁵⁵

Heterostructure design significantly optimizes the catalytic performance of multimetallenes by accurately constructing 0D/2D or 2D/2D interfaces and exploiting the synergistic effect of interfacial charge redistribution and strain. In the future, in situ characterization technology can be developed to analyze the reconfiguration behavior of interfacial atoms in real time and establish a prediction model by combining with machine learning; at the same time, a new lattice coherent growth process can be developed to realize the precise assembly of heterogeneous interfaces at the atomic level, which will promote the integration of these catalysts in membrane electrode devices.^44,56

4. Electrocatalytic applications of multimetallenes

4.1. Electrocatalytic oxygen reduction reaction

The highly active sites and strain effects of multimetallenes make them highly efficient ORR catalysts. The PdMo bimetallene shows excellent performance in the alkaline ORR. Guo's team has developed a new Pd-based metallene, PdMo bimetallene, which consists of palladium (Pd) and molybdenum (Mo) alloys in the form of highly curved sub-nanometer-thick metal nanosheets (Fig. 9a).⁵⁷ This unique structure (Fig. 9b) not only provides an extremely high electrochemically active surface area (138.7 m² g⁻¹ Pd), but also achieves efficient utilization at the atomic level. The PdMo bimetallene/carbon (PdMo bimetallene/C) catalyst exhibited a mass activity of 16.37 A mg⁻¹ Pd at 0.9 V relative to the reversible hydrogen electrode (RHE) with a scan rate of 20 mV s⁻¹ (for recording the oxygen reduction reaction (ORR) polarization curve) in a 0.1 M KOH alkaline electrolyte at room temperature, which is 78 and 327 times higher than those of commercial Pt/C and Pd/C catalysts, respectively. In addition, the mass activity of PdMo bimetallene/C showed almost no decay after 30 000 potentiostatic cycles, demonstrating excellent stability. DFT calculations reveal a high-performance mechanism for the PdMo bimetallene (Fig. 9c): alloying effects, strain effects induced by the bending geometry, and quantum size effects together modulate the electronic structure and optimize the binding strength of the oxygen intermediates. Among them, the alloying effect plays the most important role. In addition, PdMo bimetallene/C not only performs well in the ORR (Fig. 9d), but also exhibits good activity and durability in the OER, making it a promising bifunctional oxygen electrocatalyst (Fig. 9e). Based on these properties, PdMo bimetallenes have also achieved remarkable results in their application as air electrode catalysts in rechargeable zinc–air and lithium–air batteries. For example, a zinc–air battery with PdMo bimetallene/C as the cathode exhibited a specific capacity as high as 798 mA h g⁻¹ Zn and an energy density of 1043 Wh kg⁻¹ Zn, which is much higher than that of the battery using IrO₂ + Pt/C as the catalyst.

Fig. 9 (a) High-magnification HAADF-STEM and (b) AFM image of the PdMo bimetallene. Electrocatalytic performance of PdMo bimetallene/C, Pd metallene/C, and the commercial catalysts Pt/C and Pd/C. (c) Height profiles of the PdMo bimetallene. ORR polarization curves (d) and a comparison of the mass and specific activities (e) of the stated catalysts in 0.1 M KOH at 0.9 V versus RHE.

In addition, Pt-based multimetallenes also show significant advantages in the ORR by virtue of their atomic-thickness two-dimensional structure, high specific surface area and abundant low-ligand active sites. Compared with conventional Pt nanoparticle catalysts, Pt-based multimetallenes optimize the adsorption strength of oxygen intermediates through a quantum size effect and a surface strain effect, which significantly enhance the intrinsic activity and atom utilization. The PtTe₂ metallene prepared by Chen's team via a topological reduction method exhibits excellent ORR performance in acidic medium.⁵⁸ It achieved a mass activity of 296.8 mA mg⁻¹, which was 9.7 times higher than those of commercial IrO₂ catalysts, and the activity retention was as high as 98% after 126 h of continuous operation at a high current density (50 mA cm⁻²). Theoretical calculations show that the two-dimensional layered structure of PtTe₂ enhances the d–p orbital hybridization by shortening the Pt–O bond lengths and significantly reduces the energy barriers of oxygen intermediates (e.g., *O and *OH), thus accelerating the reaction kinetics.

Future research needs to combine defect engineering, heterostructure design and in situ characterization techniques to further optimize its electronic structure and interfacial properties, and to promote its practical application in energy devices such as proton exchange membrane fuel cells.^59–62

4.2. Electrocatalytic hydrogen evolution reaction

In the field of electrocatalytic hydrogen reaction, multimetallenes have attracted much attention due to their unique two-dimensional structure and electronic modulation ability, among which PdIr bimetallenes have demonstrated breakthroughs in the acidic medium HER. Wang's team constructed heterojunction structures of Pd nanoparticles loaded with an Ir metallene by a one-step co-reduction method (Fig. 10a–c) and optimized the reaction kinetic pathway using the intermetallene HER.⁶⁰ In this design, the small Fermi energy level difference between Pd and Ir (ΔΦ = 0.76 eV) induces H⁺ to preferentially adsorb to the Pd site and then rapidly migrate toward the Ir carrier, shifting the rate-determining step (RDS) from Volmer adsorption to Tafel desorption (Fig. 10d), significantly reducing the reaction energy barrier. Theoretical calculations showed that the charge redistribution at the PdIr interface resulted in a downward shift of the d-band center and the free energy of hydrogen adsorption (ΔG_Hads ≈ −0.19 eV) was close to the ideal value, which effectively balanced the process of H⁺ adsorption and H₂ desorption (Fig. 10e and f). In 0.5 M H₂SO₄, the catalyst exhibited excellent performance at industrial-scale current densities: only 69 mV and 133 mV overpotentials were required to reach 500 and 1000 mA cm⁻², respectively, with Tafel slopes as low as 21 mV dec⁻¹, and maintained stable operation for 100 hours. This work provides a new paradigm for the design of highly efficient acid-resistant HER catalysts through the synergistic regulation of interfacial electronic engineering and the hydrogen evolution reaction, and promotes the application of multimetallenes in the scale-up of hydrogen reaction from electrolytic water. In the future, by further optimizing the ratio of the bimetallene components and interfacial structure, such heterojunction catalysts are expected to play a key role in the industrialization of green hydrogen.

Fig. 10 (a) Schematic diagram of the interfacial electron configuration in the PdIr hetero-metallene. (b) Illustration of the hydrogen spillover phenomenon and related processes on a PdIr hetero-metallene surface. E_vac = vacuum energy, E_c = conduction band, E_v = valence band, E_f-m = Fermi level of the metal, and E_f-s = Fermi level of the support. (c) HAADF-STEM and corresponding element mapping images of the PdIr hetero-metallene. (d) HER polarization curves and (e) Tafel plots for various samples. (f) Calculated hydrogen free energy curves of the HER for the PdIr hetero-metallene, Pt/C, Ir metallene and Pd NPs.

The catalytic effect of multimetallenes is not only limited to the electrolysis of water for hydrogen reaction but its unique electronic structure and surface active sites also show breakthrough potential in the field of low-temperature dehydrogenation of solid-state hydrogen storage materials. Xu et al. succeeded in lowering the starting dehydrogenation temperature of MgH₂ to 422 K by constructing a double-layer PdNi bilayer metallene, and it is the lowest operating temperature ever recorded for a magnesium-based hydrogen storage material.⁶¹ The catalyst was ball-milled to generate multi-scale active sites such as Pd/Ni single atoms, pure-phase clusters and PdNi alloy clusters in situ, among which the PdNi alloy clusters became the core sites dominating the dehydrogenation kinetics due to the optimized d-band center position (shifted downward by 0.38 eV compared with that of pure Pd). It is shown that the MgH₂–PdNi system releases 5.49 wt% hydrogen within 1 h at 523 K and achieves a high and reliable hydrogen storage capacity of 6.36 wt%, with a significantly lower dehydrogenation activation energy (73.9 kJ mol⁻¹) compared to that of pure MgH₂ (170.4 kJ mol⁻¹). Synchrotron radiation and XPS with spherical aberration-corrected electron microscopy (AC-TEM) confirmed that the PdNi alloy clusters synergistically weakened the Mg–H bonding energy through a strain effect (lattice expansion of 0.246 nm) and electron transfer (Pd³⁺/Ni²⁺ mixed valence), which reduced the H vacancy formation energy to −0.254 eV (DFT calculations), thus facilitating dissociative desorption of hydrogen molecules. In addition, the system maintains 97.9% capacity retention after 10 cycles, and its stability stems from the dynamic synergistic mechanism of multilevel active sites (single atoms, clusters, and alloys) induced by ball milling. This study reveals, for the first time, the dynamic conformational relationships of multimetallene catalysts in hydrogen storage materials at the atomic scale, which provides a new paradigm for the design of highly efficient, low-temperature integrated hydrogen storage-release systems.^26,62

Our group has recently shown that the catalytic performance of the alkaline HER can be significantly enhanced by introducing atomically dispersed Ga elements to construct an amorphous RuGa metallene (A-RuGa_0.06 metallene).²³ The material was synthesized by a thermal annealing method, and an ultrathin amorphous structure (∼1.8 nm in thickness) with abundant monometallene atomic interfaces was successfully prepared by modulating the annealing temperature (285 °C) and introducing a trace amount of Ga(acac)₃ precursor (Fig. 11a). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) analyses show that Ga forms a Ru–Ga coordination environment with Ru in an atomically dispersed form, and at the same time constitutes a monometallene atomic interface with uncoordinated Ru⁰ (Fig. 11b–d). This interface design not only shortens the transport path of the hydrogen intermediate (H*), but also optimizes the local electronic state of Ru through electronic modulation (Fig. 11e). The experimental results show that the A-RuGa_0.06 metallene exhibits excellent HER activity in 1.0 M KOH: the overpotential is only 18 mV (@10 mA cm⁻²), and the mass activity is as high as 11.8 A mg⁻¹ (@−0.1 V vs. RHE), which is much higher than that of commercial Pt/C (2.9 A mg⁻¹). Moreover, its Tafel slope was as low as 35 mV dec⁻¹, indicating a significant enhancement of the reaction kinetics (Fig. 11f). It was further revealed by in situ Raman spectroscopy and DFT calculations that the introduction of Ga lowered the water molecule adsorption and dissociation energy barrier (ΔG_H2O = 0.24 eV), while balancing the H adsorption free energy (ΔG_*H ≈ 0 eV), which is in accordance with the Sabatier's optimality principle (Fig. 11g). In an anion-exchange membrane electrolyzer, the catalyst shows potential for practical application: it achieves a mass activity of 5.2 A mg⁻¹ at 2.0 V and can be operated stably for 200 hours. This study provides new ideas for the design of highly efficient alkaline HER catalysts through the synergistic effect of amorphous structure design and monometallene interface engineering, and validates its feasibility for industrial-scale hydrogen reaction from electrolyzed water.⁶³

Fig. 11 Morphological and structural characterization of the A-RuGa_0.06 metallene. (a) Mechanism exploration flowchart. (b–d) HAADF-STEM and near atomic resolution mappings. (e) Water adsorption and dissociation Gibbs free energy. (f) Calculated Gibbs free energy diagram of the HER at an equilibrium potential. (g) Chronopotentiometry testing of the A-RuGa_0.06 metallene at 100 mA cm⁻² in the AEM electrolyser using commercial RuO₂ as the anodic catalyst (the inset shows a photograph of the AEM electrolyser). (h) The structural evolution from monometal catalysts to HEAs and from SACs to HESACs. (i) HRTEM image of PdRhMoFeMn HEMs. (j) η₁₀ (left) and mass activities (right) at an overpotential of 50 mV of these nanocatalysts in a 0.5 M H₂SO₄ electrolyte. (k) The ΔG_H* profiles on various catalytic sites of PdRhMoFeMn HEMs.

In recent years, high entropy alloys (HEAs) have attracted much attention in the field of electrocatalysis due to their unique “cocktail effect” and the synergistic advantages of multiple active sites.^49,64 Li et al. successfully synthesized sub-nanoscale PdRhMoFeMn high-entropy multimetallenes (HEMs) (Fig. 11h) by a one-pot wet chemical method with a thickness of only about 1 nm and exhibited excellent hydrogen precipitation performance under wide pH applicability.¹⁴ The material required only 6, 23, and 26 mV overpotentials to drive a current density of 10 mA cm⁻² in acidic, neutral, and alkaline electrolytes, respectively, and the mass activity (35.7 A mg⁻¹@50 mV) far exceeded those of commercial Pt/C catalysts (Fig. 11i). Through XAS and DFT calculations, the study reveals the dynamic synergistic mechanism of the multimetallene sites: in the acidic environment, Rh sites dominate the *H coupling by virtue of the optimal free energy of hydrogen adsorption (ΔG_H* ≈ −0.132 eV), whereas under the neutral/alkaline conditions, Pd sites promote the Volmer step through a low water dissociation energy barrier (0.53 eV), and Mn sites promote the Volmer step through a strong hydroxyl group adsorption (E_ads ≈ −3.05 eV) which accelerates water molecule dissociation, while the Rh site achieves efficient hydrogen desorption (Fig. 11j). In addition, the high entropy effect endows the material with excellent structural stability (negligible property decay after 10 000 cycles) and exposes abundant low-coordination active sites by suppressing atomolysis and lattice distortion (Fig. 11k).

Future research needs to focus on the in situ analysis of the dynamic synergistic mechanism of multiple sites, combine time-resolved spectroscopy and machine learning techniques to solve the mass transfer limitation and structural degradation problems under high current density, and promote the scale-up application of multimetallene catalysts in green hydrogen electrolyzers.

4.3. Electrocatalytic small molecule oxidation

4.3.1. Electrocatalytic methanol oxidation. The latest breakthrough of multimetallene-structured catalysts for methanol oxidation is reflected in the innovative design of PtRu-based ternary alloy systems. Pan et al. successfully prepared ultrathin PtRuM–O (M = Ni, Fe, Co) ternary alloy nanosheets by an ethylene glycol-mediated co-reduction method (Fig. 12a).⁶⁵ HAADF-STEM characterization reveals that the material has a typical atomic-level thickness (1.5–2.2 nm) (Fig. 12b) and in-plane lattice extension properties (0.221 nm), and that this two-dimensional ductile structure efficiently exposes the (111) crystalline active sites. XPS confirms the alloying-induced modulation of the electronic structure: the introduction of Ni positively shifts the Pt 4f binding energy by 0.38 eV and the Ru 3p binding energy negatively by 0.25 eV, creating a bimetallene charge redistribution effect that significantly enhances the activation of methanol molecule adsorption; 0.38 eV for Pt 4f and 0.25 eV for Ru 3p result in a bimetallene charge redistribution effect, which significantly enhances the adsorption activation of methanol molecules. In a 0.5 M H₂SO₄ + 1 M CH₃OH electrolyte, Pt₇RuNi₂–O nanosheets with optimized components exhibited a mass activity of 3.57 A mg⁻¹ (Fig. 12c), which was a 15.2-fold enhancement over the commercial Pt/C catalysts, and retained >95% of the initial activity for more than 5000 seconds. DFT calculations reveal its anti-CO poisoning mechanism: the Ni–O site preferentially adsorbs and dissociates water molecules to generate OH, prompting rapid oxidative desorption of CO intermediates adsorbed at the neighboring Pt site (energy base reduced by 0.32 eV). This ternary synergistic effect breaks through the kinetic limitation of traditional PtRu bimetallene catalysts in acidic medium, and provides a novel design paradigm for anode catalysts for direct methanol fuel cells.

Fig. 12 (a) TEM images of Pt₇RuNi₂–O NSs. (b) HRTEM image of Pt₇RuNi₂–O NSs. (c) Corresponding histogram of mass and specific activities of different catalysts in N₂-saturated 0.5 M sulfuric acid and 0.5 M methanol. (d) Schematic illustration of the formation mechanism of Mo1–PdPtNiCuZn SAHEANSs. (e) AFM image and (f) corresponding height profile of single Mo1–PdPtNiCuZn SAHEA NS. (g) CO-stripping curves of Mo1–PdPtNiCuZn SAHEA NSs and Pt/C catalysts recorded in N₂-saturated 1.0 M KOH. (h) Chronoamperometric tests for the MOR at 0.77 V vs. RHE of Mo1–PdPtNiCuZn SAHEA NSs, Mo1–PdPtNiCuZn SAHEA NPs, PdPtNiCuZn HEA NPs and Pt/C.

Compared to polyol oxidation systems,^65,66 methanol molecules are more likely to form strongly adsorbed CO intermediates on the catalyst surface, which puts higher demands on the catalyst's resistance to poisoning. To address this challenge, a joint team from Tsinghua University and Princeton University innovatively introduced a single-atom modification strategy into a high-entropy alloy system (Fig. 12d) and constructed Mo single-atom-anchored PdPtNiCuZn nanosheets (Fig. 12e).⁸ This material realizes a triple synergistic protection mechanism at the atomic scale (Fig. 12f): firstly, the high-entropy alloy matrix decomposes the conventional Pt–Pt continuum active site into an isolated single-atom configuration through atomic-level mixing of the five principal elemental metals, which radically reduces the CO formation probability by 83% (confirmed by DFT calculations); secondly, the surface-modified Mo single-atom acts as an electron donor and injects electrons into the neighboring Pt sites by injecting electrons, which shifts the d-band center of Pt downward from −2.15 eV to −2.39 eV (XANES characterization), weakening the CO adsorption energy from −1.98 eV to −1.32 eV; more importantly, the 4.4% intrinsic tensile strain inside the material (confirmed by the geometrical phase analysis) promotes the adsorption activation of OH– (Fig. 12g), and reactive oxygen species with 43% coverage were formed at a potential of 0.6 V, realizing a rapid cycle of *CO + *OH → *COOH detoxification reaction. This multistage protection system enabled the catalyst to exhibit a record mass activity of 24.55 A mg⁻¹ Pt in the 1.0 M KOH + 1.0 M CH₃OH system, and maintained 92.7% of the initial activity after 10 000 cycles (Fig. 12h), which breaks through the bottleneck of the traditional catalyst's performance degradation in long-term operation.

4.3.2. Electrocatalytic ethanol oxidation. Similarly, in direct ethanol fuel cells (DEFCs), the multiple electron transfer characteristics of the anode EOR present multiple challenges for catalyst design: simultaneous C–C bond breaking is required to enhance C1 pathway selectivity (complete oxidation to CO₂), accelerate intermediate desorption to maintain active site stability, and optimize electron transport to reduce the reaction overpotential.^67–69 Multimetallene materials provide a new way to break through the performance bottleneck of traditional catalysts by virtue of their two-dimensional ultrathin structures with high atom exposure, tunable electronic states and strain effects.

In the EOR, the RhPt bimetallene exhibits excellent C–C bond breaking ability and anti-poisoning properties through the electronic synergistic effect between Rh–Pt atoms and the intrinsic advantages of the two-dimensional structure. As an example, the RhPt bimetallene synthesized by the solvothermal method (thickness ∼1.8 nm) forms unique Rh–Pt atom pair active sites on its surface, which synergistically optimizes the reaction paths through the strong C–O adsorption property of Rh and the electron-rich state of Pt.⁷⁰ XPS analysis showed that the introduction of Rh shifted the d-band center of Pt downward by 0.15 eV, which significantly weakened the adsorption energy of CO intermediates (ΔG_CO decreased from −2.1 eV to −1.6 eV), while enhancing the dissociative adsorption of ethanol molecules (in situ FTIR showed a 3.2-fold enhancement in the intensity of C–O vibrational peaks). In 1 M KOH medium, the catalyst achieved a mass activity of 8.7 A mg⁻¹ (@0.8 V vs. RHE) and a breakthrough in C1 pathway selectivity to 58%, a 2.3-fold enhancement over that of the pure Pt metallene. DFT calculations further reveal that the Rh–Pt interface reduces the reaction energy barrier from 0.92 eV to 0.48 eV and accelerates the oxidation of CH₃CO intermediates to CO₃²⁻ (desorption energy reduced by 0.35 eV) by shortening the C–C bond breaking transition state distance (1.54 Å → 1.21 Å). In addition, its ultrathin two-dimensional structure imparts high stability (>85% retention of activity after 5000 cycles) attributed to the strain enhancement of the Pt lattice by Rh (lattice distortion <1.2%) and atomic migration suppressed by two-dimensional domain-limiting effects. This study provides a paradigm for the “electron-geometry” modulation of bimetallenes and promotes the practicalization of the complete oxidation pathway for ethanol fuel cells. The above study reveals a differentiated synergistic mechanism between electron transfer and lattice strain in the multimetallene system: for the ethanol complete oxidation pathway dominated by C–C bond breaking, this “reaction pathway-synergistic mechanism” conformational correlation provides a theoretical framework for the targeted design of multimetallene catalysts.

Trimetallene PtPdNi nanosheets (1.4 nm thick) developed by Guo's team exemplify early applications of multimetallene materials.⁹ Through the Ni doping-induced downward shift of the center of the d-band of the Pt–Pd alloy, the material successfully attenuates the strong adsorption of the toxic intermediates (Fig. 13a), and exhibits 7.7-fold and 5.4-fold mass activities of commercial Pt/C catalysts in ethylene glycol oxidation reaction (EGOR) and glycerol oxidation reaction (GOR) (Fig. 13b). The ultrathin two-dimensional structure not only exposes more than 90% of the surface active sites, but also gives the material excellent bifunctional properties: it also achieves 7.7 times the mass activity of the commercial Pd/C in the ORR without significant degradation after 10 000 cycles (Fig. 13c). This study validates the synergistic mechanism of alloy component modulation and 2D structural advantages, and lays the foundation for more complex material designs (Fig. 13d and e).

Fig. 13 (a) TEM images of ultrathin Pt₃₂Pd₄₈Ni₂₀ NSs. (b) HRTEM image of the ultrathin Pt₃₂Pd₄₈Ni₂₀ NSs. (c) Mass activities of different catalysts at −0.1 V (vs. Ag/AgCl electrode). (d) Mass activities of different catalysts at 0.9 V versus RHE. (e) ORR polarization curves of the ultrathin Pt₃₂Pd₄₈Ni₂₀ NS catalyst before and after 10 000 cycles between 0.4 and 1.0 V versus RHE. (f) HRTEM image of Pd₅₀W₂₇Nb₂₃. (g) Histogram of specific activity and mass activity. (h) Faradaic efficiency of as-prepared PdWNb/C in different reaction pathways at 0.77 V versus RHE. (i) CVs of Pd₅₀W₂₇Nb₂₃/C before and after 3000 cycles.

On this basis, trimetallenes further promote the high efficiency of the EOR and achieve the breakthrough of complete oxidation pathway (C1) selectivity due to their multicomponent synergistic effect and structural tunability. For example, the porous PdWM (M = Nb, Mo, Ta) trimetallenes (Fig. 13f) synthesized by our group via wet chemistry significantly optimize the catalytic performance through the synergistic interaction of the two-dimensional folded structure and the multimetallene.³⁸ Among them, the Pd₅₀W₂₇Nb₂₃/C catalyst showed excellent performance in alkaline medium (Fig. 13g and h): the mass activity was up to 15.6 A mg Pd⁻¹, which was 12 times higher than that of commercial Pt/C; and the C1 selectivity reached 55.5%, which was much better than those of the traditional Pd-based catalysts (<14.1%) and bimetallene systems (14.1%). Mechanistic studies show that the introduction of W and Nb enhances the performance through a dual synergistic mechanism: W and Nb optimize the electronic structure of Pd (d-band center shifted downward by 0.78 eV) and enhance the ethanol adsorption (in situ FTIR shows a significant increase in the intensity of C–O vibrational peaks); the highly oxidized state of Nb acts as a strong oxygenophilic site, accelerating the oxidative desorption of CO and –CHx intermediates, and simultaneously decreasing the C–C bond breaking energy barrier from 0.87 eV to 0.51 eV for pure Pd (verified by DFT calculations), which drives the 12-electron complete oxidation pathway (CO₃²⁻ characteristic peak intensity is 2.3 times higher than that of the bimetallic system). In terms of stability, the catalyst showed a high activity retention of 69.9% after 3000 cycles and a lattice distortion of <1.5%, and its excellent resistance to poisoning was attributed to the fast mass transfer effect of the porous network and the stabilizing effect of W/Nb on the electronic structure (Fig. 13i). In addition, by replacing the M component with Mo or Ta, a trimetallene with high activity (14.8–13.3 A mg Pd⁻¹) and high selectivity (43.7%–35.0%) can be similarly prepared, verifying the universality of the synthesis strategy.

Our group successfully prepared a porous Pd₅₉W₈Rh₁₉Bi₁₄ trimetallene by a wet chemical method.⁷¹ For the first time, the three strategies of C–C bond breaking (Rh), OH adsorption (Bi) and electronic effect (W) were integrated in a single catalyst system. The material exhibits a mass activity of 16.70 A mg⁻¹ in a 1 M KOH electrolyte, a 9.1-fold enhancement over that of commercial Pd/C, and a C1 selectivity of 65.41% (0.87 V vs. RHE). By comparing the ternary systems (PdW/Rh: 53.97%; PdW/Bi: 48.13%), it was found that the introduction of Rh reduced the C–C bond breaking energy barrier to 0.51 eV (DFT calculations), whereas Bi significantly enhanced the antitoxicity by accelerating the oxidative desorption of *CO/*CHx intermediates (CO oxidation potential negatively shifted by 0.15 V) (Fig. 14a and b; 32% retention of activity after a 20 000 s stability test). The doping of elemental W, on the other hand, shifted the d-band center of Pd downward by 0.4 eV through the electron transfer effect, weakening the adsorption strength of the toxic intermediates (Fig. 14c and d). The mesoporous structure of this tetrametallene (BET specific surface area of 107.4 m² g⁻¹) synergized with the ultrathin thickness (∼2 nm) to promote reactant mass transfer and active site exposure. In situ FTIR spectroscopy confirmed that Rh sites specifically adsorbed ethanol's C–O bond (peak intensity decay at 1045 cm⁻¹) and produced a characteristic CO₂ peak (2350 cm⁻¹) (Fig. 14e), while Bi adsorbed ethanol by promoting OH– adsorption (CO₃²⁻/CH₃COO-peak intensity ratio at 1413 cm⁻¹ was enhanced by 3-fold), which realized the rapid removal of C1 intermediates. By precisely integrating the functional complementarity of the multimetallene sites with the mass transfer advantage of the porous structure, this work successfully breaks through the performance bottleneck of traditional catalysts in complex oxidation reactions.⁷²

Fig. 14 (a) TEM image of the Pd₅₉W₈Rh₁₉Bi₁₄ metallene. (b) The XRD pattern of the Pd₅₉W₈Rh₁₉Bi₁₄ metallene. (c) Pd 3d XPS spectrum of different electrocatalysts. (d) The TEM image of PtRuRhCoNi NWs. (e) In situ FTIR spectrum of the EOR on commercial Pt/C. (f) CV curves before and after 2000 cycles of PtRuRhCoNi NWs/C. (g) The energy change of ethanol oxidation.

While metallene materials continue to push the limits of EOR performance, the introduction of HEAs opens up a new dimension in the rational design of catalysts. Recent studies have combined the high-entropy effect with ultra-thin structures, such as the PtRuRhCoNi high-entropy alloy metallene (HEA-NWs) recently reported by our group;⁷³ its atomic-level thickness (∼1.6 nm) with a quintuple synergistic effect significantly optimizes the C–C bond breaking kinetics. The material exhibited a mass activity of 7.68 A mg⁻¹ in alkaline medium and achieved 78% C1 selectivity at 0.7 V vs. RHE – the highest value reported to date. In situ FTIR and DFT calculations reveal that the synergistic interaction of Rh and Ru sites lowers the C–C bond breaking energy barrier (0.42 eV) (Fig. 14g), while Co/Ni induced lattice strain (1.8%) accelerates the oxidative desorption of *CO intermediates via a d-band center downshift (−3.73 eV), and its ultrafine nanowire structure confers superior stability (activity retention >96.8% after 2000 cycles) (Fig. 14f). Compared with the previously discussed trimetallene (e.g., the PdWM system), the high-entropy alloy system suppresses elemental segregation through the effect of the component entropy increase and maintains the dynamic equilibrium of active sites in a complex electrochemical environment, which provides a new paradigm for the development of fully oxidized catalysts with both high activity and high stability.⁷⁴

In EOR studies, the evolution of multimetallene catalysts presents a clear technology iterative path: from the bimetallene RhPt system to optimize C–O adsorption through electronic synergy, to the trimetallene PdWM system to reduce the C–C bond-breaking energy barrier to 0.51 eV by using lattice strain, and then to the multifunctional synergistic mechanism of the trimetallene PdWRhBi system, having a mesoporous structure and an ultrathin property (∼2 nm). The mesoporous structure and ultrathin property (∼2 nm) significantly promote the mass transfer and active site exposure; finally, the quintuple synergism is realized in the high-entropy alloy PtRuRhCoNi. In the future, it is expected to further optimize the active microenvironment by modulating the short-range ordered structure of high-entropy alloys to promote the practical application of direct ethanol fuel cells (DEFCs).^75–77

4.3.3. Electrocatalytic formic acid oxidation. In direct formic acid fuel cells (DFAFCs), the efficiency of the formic acid oxidation reaction (FAOR) directly determines the output performance of the cell. Guo's team designed a bent PdIr bimetallene to achieve a breakthrough in the performance of the FAOR by modulating the electronic orbitals through lattice strain.¹⁶ The material is characterized by a curved structure of atomic-scale thickness (∼1.0 nm), forming concave micro-regions on the surface and generating a lattice tangential strain of 2.3%. Experiments show a mass activity of 2.7 A mgPd + Ir⁻¹ (0.5 V vs. RHE), which is 4.8 times higher than that of commercial Pd/C, a negative shift of the onset potential by 128 mV compared to that of the control, and an electrochemically active surface area of up to 127.5 m² g⁻¹. Theoretical calculations reveal that lattice strains, by modulating the coulombic correlation potential of the 4d orbitals of the Pd by shifting the center of the d-band downward by 0.18 eV, would result in *HCOO adsorption energy optimized to −1.25 eV, while the 5d orbital electron transfer of Ir reduces the OH dissociation energy barrier by 0.35 eV. This synergistic effect not only accelerates the formic acid dehydrogenation pathway (HCOO → CO₂), but also inhibits the reaction of toxic intermediates (e.g., CO) by stabilizing adsorbed oxygen atoms at the Ir site. Stability tests showed that the activity retention of the material was enhanced by 3.2 times compared that with Pd/C after continuous operation, verifying the enhanced effect of the strain structure on the anti-poisoning ability of the catalyst.

This study fully demonstrates the great potential of multimetallene materials, especially the systems finely tuned by strain engineering, in addressing the key challenges (activity, selectivity, and stability) of the FAOR, which can further develop the next generation of highly efficient and durable DFAFC anode catalysts.^78,79

4.3.4. Electrocatalytic formaldehyde oxidation. Formaldehyde (HCHO) is a highly toxic volatile organic compound (VOC), and its efficient electro-oxidation is of great significance for environmental and health monitoring. Zhang et al. broke through the inefficiency and toxicity-prone difficulties of traditional Pd-based catalysts in the formaldehyde oxidation reaction (FOR) by an innovative design of a two-dimensional Cr-doped Pd metallene (Cr–Pdene).⁸⁰ Characterized by an ultrathin structure (thickness ∼1.34 nm) and uniform doping of Cr atoms, the material significantly optimizes the electronic structure of Pd through strong electronic interactions between Cr and Pd, shifting the d-band center downward by 0.08 eV, which diminishes the adsorption energy of the toxic intermediate CO (ΔE_CO reduced by ∼0.3 eV). In situ Fourier transform infrared spectroscopy (FT-IR) and DFT calculations showed that Cr–Pdene preferentially adsorbs OH– species on its surface and efficiently scavenges CO intermediates via a synergistic pathway of CO + *OH → CO₂ + H⁺, which enhances the CO₂ selectivity to more than 95%, far exceeding that of the unadulterated Pd metallene (<70%).

This anti-poisoning property enabled Cr–Pdene to exhibit a mass activity of 2.17 A mg⁻¹ in alkaline media (0.8 V vs. RHE), a 5.3-fold enhancement over that of commercial Pd/C catalysts, and an activity retention of over 78% after 5000 cycles. Based on this, the researchers developed a flexible electrochemical sensor to achieve highly sensitive detection of gas-phase formaldehyde over a wide range (0.2–50 ppm; detection limit of 72 ppb) and exhibit excellent selectivity (no response to interferences such as NH₃, ethanol, etc.).

The sensor has been successfully integrated into wireless sensing networks and handheld devices, and its reliability has been verified in real-world scenarios such as indoor air quality monitoring and food formaldehyde residue detection.⁸¹ This work not only provides a synergistic strategy of “electronic regulation and anti-poisoning” for the design of formaldehyde oxidation catalysts, but also promotes the leap of multimetallene materials from basic catalytic research to intelligent sensing applications, and provides a new paradigm for the innovation of environmental pollutant monitoring technology.

4.4. Electrocatalytic hydrogen oxidation reaction

The application of multimetallene materials in the hydrogen oxidation reaction (HOR) has attracted much attention in recent years, especially in alkaline anion-exchange membrane fuel cells (AEMFCs), where their catalytic performance directly affects the cell efficiency. Guo's team successfully synthesized palladium-based pentametallenes with a hexagonal close-packed (hcp) structure via a Pt-induced autocatalytic deposition method (PMene–Ru_0.18).¹⁷ Isolated Ru–O₃ sites are uniformly dispersed on the surface, providing synergistic catalytic active sites for the HOR. The material exhibited excellent HOR performance in 0.1 M KOH: the mass activity (11.5 A mg⁻¹) and exchange current density (1.0 mA cm⁻²) reached 32.5 and 5.2 times that of commercial Pt/C, respectively, and the activity retention was 91% after 5000 cycles. The study revealed by in situ Raman spectroscopy and DFT that the enhanced ligand effect of the hcp structure enhances the adsorption of hydroxyl (OH) and water (H₂O), whereas the Ru–O₃ site weakens the binding energy of the HOR intermediates (H₂OH and H₂O), thus optimizing the reaction pathway and lowering the energy barrier (0.45 eV). In addition, PMene–Ru_0.18 lost only 19% of its current density in H₂ containing 1000 ppm CO, and its anti-poisoning performance was significantly better than those of conventional PtRu catalysts. This study provides new ideas for the design of efficient and durable alkaline HOR catalysts through a multimetal synergistic and surface oxygen species modulation strategy.

Multimetallene materials show breakthrough in alkaline hydroxide reaction;⁸² theoretical calculations and experimental validation reveal the key roles of hydroxyl adsorption enhancement and electronic microenvironment regulation at the interface of the multimetallene, laying a material foundation for the development of highly efficient and durable anion-exchange membrane fuel cell catalysts.^83,84

4.5. Electrocatalytic coupling reaction

Electrochemical coupling reaction realizes efficient energy–matter cogeneration through multi-reaction synergy, and its core lies in the bifunctional design of catalysts.⁸⁵ Multimetallene materials are ideal carriers for coupled catalytic systems due to the intrinsic advantages of the two-dimensional structure (high specific surface area, tunable electronic states and surface strain effects).⁸⁶

Taking the nitrate reduction (NO₃-RR) coupled EOR as an example, the RhCu M-tpp (tetraphenylporphyrin ligand-modified rhodium–copper heterogeneous metallene) heterogeneous metallene was developed by Fan's team and they constructed a molecule–metal relay catalytic mechanism (Fig. 15a): on the cathode side, its precise modulation of the Rh active site by p–d orbital hybridization resulted in an ammonia selectivity of 84.8% (−0.1 to −0.4 V vs. RHE window) for the NO³⁻ → NH₃ conversion pathway (Fig. 15b); on the anode side, the tpp-modified Cu site of the organic ligand efficiently catalyzed the conversion of ethanol → acetic acid, and synchronously provided electrons to maintain the zinc–nitrate/ethanol.⁸⁷ The reversible cycle of the battery was maintained. This bi-directional catalysis not only achieves an energy efficiency of 84.2% (Fig. 15c), but also co-produces ammonia and ammonium acetate, two high-value chemicals. Expanding to the nitrogen reduction (NRR)-coupled ORR system, metallenes can be engineered to simultaneously optimize the kinetics of *NH₂ adsorption and *OOH dissociation by surface strain engineering, lowering the nitrogen/oxygen dual reduction overpotential to 0.32 V. Theoretical simulations reveal that the charge redistribution induced by heterogeneous interfaces significantly reduces the barriers to the formation of the *COOH and *NH₂ intermediates, validating the key role of metallenes in the reconfiguration of the coupled reaction pathways. The rhodium–copper (RhCu) alloy metallene achieves efficient energy–matter cogeneration in a coupled nitrate reduction-ethanol oxidation system with 84.8% ammonia selectivity and simultaneous generation of high-value-added ammonium acetate through heterogeneous interface design and charge redistribution. Theoretical simulations reveal that the interface engineering can significantly reduce the formation barriers of key intermediates (*COOH and *NH₂), providing a new paradigm of electronic structure optimization for multi-reaction co-catalysis.⁸⁸

Fig. 15 (a) Schematic illustration of the synthetic procedure of RhCu M-tpp and the working principle of assembled Zn–nitrate/ethanol batteries. (b) FEs of NH₃ yield rates on RhCu M-tpp, RhCu M, Rh M, and Cu NPs under different applied potentials. (c) Galvanostatic discharge profiles of Zn nitrate/ethanol batteries with RhCu M-tpp cathodes from OCV to 0.005 V (vs. Zn²⁺/Zn) at 0.1 and 0.5 mA cm⁻², respectively. Inset: A digital photograph showing the constructed battery device, along with the measurement of OCV. (d) HAADF-STEM image, (e) AFM image and corresponding height profile (inset) of the HEA–PdCuAgBiInene. (f) Potential-dependent FE, (g) potential-dependent yield rate comparison over HEA–PdCuAgBiInene, PdCuAgene and Pdene. (h) TEM image of the HEA–PdCuAgBiInene. (i) Schematic illustration of the proposed electrosynthesis C₆H₁₁NO mechanism over HEA–PdCuAgBiInene. (j) Stability test of HEA–PdCuAgBiInene at −0.9 V vs. Ag/AgCl.

In addition, high entropy alloy metallenes show unique advantages in electrocatalytic coupling reaction through the synergistic effect of multiple components and electronic structure modulation.⁸⁹ Taking the research of Wang's team as an example, the high-entropy alloy PdCuAgBiIn metallene (HEA–PdCuAgBilnene) (Fig. 15d) developed by his team has successfully realized the C–N coupling reaction of cyclohexanone (C₆H₁₀O) with nitrite (NO₂⁻) through the p–d orbital hybridization strategy;^90,91 the nylon-6 precursor cyclohexanone oxime (C₆H₁₁NO) was efficiently synthesized with a material thickness of about 1.05 nm (5–6 atomic layers) (Fig. 15e). The catalyst induced a non-traditional p–d orbital hybridization effect by introducing multiple alloying of d-block metals (Pd, Cu, and Ag) with p-block metals (Bi, In), which significantly optimized the electronic density of states in the active site (Fig. 15f). Among them, the electron-enriched property of the Pd site inhibited the excessive hydrogenation side reaction of the intermediate NH₂OH by modulating the hydrogen adsorption kinetics, while enhancing the adsorption capacity for cyclohexanone and promoting the directed coupling of NH₂OH with C₆H₁₀O*. The catalyst achieved 47.6% faradaic efficiency (Fig. 15g) and nearly 100% product selectivity (Fig. 15h) under mild conditions, and exhibited excellent long-term stability (Fig. 15i) under the synergistic effect of the porous structure (Fig. 15j). This achievement not only provides an efficient catalytic platform for complex organic-electrical synthesis, but also reveals the core mechanism of multimetallenes in the reconfiguration reaction pathway, which is the multi-level modulation of “electronic structure–intermediate adsorption–reaction selectivity” through the synergism of components and orbital hybridization. This provides a paradigm for the development of new coupling catalysts. In the future, through the expansion of multimetallene combinations and interfacial engineering, such materials are expected to realize a wider range of applications in C–X (X = N, O, S) bond construction and the synthesis of high-value-added chemicals. Multimetallenes exhibit multidimensional modulation advantages in electrocatalytic coupling reactions, and the high-entropy alloying strategy significantly optimizes the catalytic performance through multimetallene synergistic effects and p–d orbital hybridization. These materials provide new ideas for complex organic-electrochemical synthesis by reconfiguring the electronic state density of the active site and balancing the kinetics of intermediate adsorption and desorption.

In the field of electrocatalytic coupling and coupling reactions, multimetallene olefin materials show a revolutionary potential due to their unique two-dimensional structure and tunable electronic properties. In the future, through the optimization of high-entropy alloy components, dynamic interface engineering and in situ mechanism analysis, multimetallenes are expected to achieve more efficient and sustainable catalytic applications in the carbon and nitrogen cycles, energy–matter cogeneration, etc., and to promote the in-depth fusion of green chemistry and clean energy technologies.^92,93

4.6. Electrocatalytic total water decomposition reaction

Trimetallenes also exhibit excellent bifunctional catalytic performance in all-water decomposition due to its multicomponent synergistic effect and the intrinsic advantage of a two-dimensional structure.⁹⁴ Taking the NiFeRu trimetallene as an example, its synergistic kinetic optimization of the HER and OER is achieved through the precise tuning of lattice strain engineering and the electronic structure. The research team synthesized the NiFeRu trimetallene with a thickness of about 1.2 nm by a solvothermal method.⁹⁵ Its unique bending structure with a ternary alloying effect induced a lattice compressive strain of 2.1%, and the downward shift of the d-band center and charge redistribution phenomenon were confirmed by XRD crystal plane spacing analysis and XPS binding energy shift (0.42 eV). In the total water decomposition test (1.0 M KOH), the catalyst required only 28 mV for the HER overpotential and 228 mV for the OER overpotential at 10 mA cm⁻², which were superior to those of commercial Pt/C and IrO₂. The two-electrode system drove the 10 mA cm⁻² current density with only 1.48 V voltage, and the activity was stabilized for 200 h at a high current density of 100 mA cm⁻² with activity. The attenuation of activity was less than 3% after 200 h of stable operation at a high current density of 100 mA cm⁻². Mechanistic studies show that compressive strain reduces the OER decisive step (*O → *OOH) energy barrier from 1.52 eV to 1.21 eV by enhancing the hybridization of O 2p orbitals and metal d-orbitals at the Ni–Fe sites, whereas the introduction of Ru optimizes the free energy of H adsorption (ΔG_H = −0.03 eV), and Fe/Ni synergism significantly accelerates the OH– desorption, resulting in a low Tafel slope of 39 mV dec⁻¹. In addition, the two-dimensional porous structure (BET specific surface area of 89.7 m² g⁻¹) effectively promotes bubble desorption, and the increase in overpotential is 57% lower than that of the bulk alloy at a high current density (500 mA cm⁻²).

High-entropy alloy metallenes also show superior electrocatalytic performance in the field of all-water decomposition. Taking the high-entropy alloy IrPdRhMoW metallene as an example, it achieves efficient and stable bifunctional catalysis through the design of an amorphous/crystalline phase interface in synergism with multicomponents, and the simultaneous optimization of the HER/OER kinetics under acidic conditions.⁹⁶ Our group synthesized a five-membered high-entropy alloy metallene by a low-temperature oil phase method (IrPdRhMoW HEA); its ultrathin structure (∼1.9 nm) (Fig. 16a and b) with an amorphous/crystalline phase interface significantly optimized the synergistic kinetics of the HER (Fig. 16c) and OER (Fig. 16d and e). The catalyst exhibited HER and OER overpotentials as low as 15 mV and 188 mV, respectively, at 10 mA cm⁻² in 0.5 M H₂SO₄, which were far superior to those of commercial Pt/C and RuO₂. In two-electrode total hydropyrolysis tests, a voltage of only 1.48 V was required to drive a current density of 10 mA cm⁻² (Fig. 16f); 1.59 V drives 100 mA cm⁻², which is the best voltage reported so far under high current density, and the performance degradation is negligible after 100 hours of continuous operation at 100 mA cm⁻² (current density retention >95%). DFT calculations revealed that the amorphous region reduces the proton adsorption energy (ΔG_H⁺) of the HER and the decisive velocity step energy barrier (*O → *OOH, 1.41 eV → 0.18 eV) of the OER by regulating the electronic density of states of the metal sites (e.g., the self-equilibrium effect of the Pd-4d and Ir-5d orbitals), which results in efficient and stable bifunctional catalysis. This study not only verifies the applicability of a high-entropy alloyed multimetallene in extreme acidic environments, but also provides a material basis for the design of self-supported electrolytic devices.^97–99

Fig. 16 (a) HAADF-STEM image of IrPdRhMoW/C. (b) HER polarization curves. (c) Overpotentials at 10 mA cm⁻² of IrPdRhMoW/C and Pt/C. (d) OER polarization curves. (e) Overpotentials at 10 mA cm⁻². (f) The i–t curve of IrPdRhMoW/C.

In the field of total water decomposition, a breakthrough is needed in the dynamic synergistic mechanism of bifunctional catalysts and the bottleneck of large-scale applications. Both metallene systems exhibit a high voltage efficiency (1.48 V, efficiency ≈83%) at a current density of 10 mA cm⁻², benefiting from the mass transfer advantages of the two-dimensional structure and the synergistic mechanism of multiple metals. However, at a high current density (100 mA cm⁻²), the increase in the overpotential of IrPdRhMoW in acidic medium is 57% lower than that of NiFeRu in an alkaline environment, and its amorphous/crystalline phase interface design significantly enhances the proton transport kinetics. In terms of energy consumption control, both reduce the reaction energy barrier through lattice strain (e.g., the OER energy barrier of IrPdRhMoW is ≈0.18 eV). Nevertheless, the acidic tolerance of high-entropy alloys endows IrPdRhMoW with better long-term stability (performance decay <5% after 100 h vs. decay <3% of NiFeRu under alkaline conditions after 200 h). Combined with the interface electron self-balance effect that reduces invalid energy consumption, IrPdRhMoW has the potential of lower long-term operation and maintenance energy consumption in industrial-grade electrolyzers.

In the future, we should deeply integrate the in situ characterization technology to analyze the dynamic coordination behavior and charge transfer paths of multi-metal sites in high-entropy systems; we should focus on promoting the integrated innovation of membrane electrode devices, solving the problems of mass transfer blocking and structural stability under high current density, and promoting the practical deployment of multimetallene catalysts in industrial electrolysis tanks, so as to provide long-lasting and stable solutions for clean energy systems.¹⁰⁰

5. Challenges and prospects

Multimetallene materials have made substantial breakthroughs in the field of electrocatalysis by virtue of their unique atomic-level two-dimensional structure and the synergistic effect of multiple metals. Innovations in synthesis methods, such as the surface energy regulation strategy, hard template restricted domain and self-template-assisted stepwise alloying, have successfully overcome the problems of reduction kinetic differences and component bias in the multimetallene precursors, and realized the precise construction from a bimetallene (PdIr) to high-entropy system (PdRhMoFeMn). Through the synergistic optimization of defect engineering, lattice strain regulation, non-metallene doping (H/C/P) and heterogeneous interface design, the intrinsic activity and stability of the materials have been significantly enhanced, and the materials have shown excellent performance in key reactions, such as the ORR, HER/OER, small-molecule oxidation, and total water decomposition.^101,102

Despite the remarkable results, the field still faces deep challenges. In the synthesis of high-entropy systems (e.g., metallenes above five elements), the dynamic collaboration mechanism of multiple atoms in thermodynamic non-equilibrium has not yet been clarified, which restricts the orderly assembly of complex lattices; the lack of in situ characterization data to support the dynamic reconfiguration behaviors of the active sites under the working environment restricts the precise analysis of the catalytic pathways; moreover, the lack of interfacial adaptation between nano-sized active sites and macroscopic devices (e.g., membrane electrodes) leads to a significant attenuation of mass transport and electron conduction efficiency in large-scale applications, making it difficult to meet the requirements of industrial-grade energy systems. In addition, the lack of interfacial compatibility between nanoscale active sites and macroscopic devices (e.g., membrane electrodes) leads to a significant degradation of the material transport and electron conduction efficiency in large-scale applications, which makes it difficult to meet the demands of industrial-scale energy systems.^103–105

Future research needs to be pushed forward along three dimensions: first, the development of in situ dynamic template technology, combined with ligand pre-fixation and gradient reduction strategies, to realize the atomic-level precision synthesis of high-entropy metallenes; second, the integration of time-resolved in situ spectroscopy and machine learning simulations to reveal the orbital hybridization laws and dynamic synergistic mechanisms of multicomponents in the reaction; and lastly, the innovative design of heterogeneous dimensional interfaces and reactors through heterogeneous dimensional interface engineering and innovative design of reactors such as photovoltaic and photonic systems. Finally, through heterogeneous dimensional interface engineering and innovative reactor design (e.g. photoelectric coupling catalytic system), we will optimize the mass transfer pathway and energy transfer efficiency, and promote the practical application of multimetallenes in zinc–air batteries, nitrate reduction coupling systems, and green hydrogen electrolyzers, so that we can provide catalytic solutions for the conversion of clean energy with both high efficiency and stability.^106–110

Author contributions

L. W. and J. L. supervised the review writing process. J. L. conceived the theme and framework of the review. N. L. designed the review's structural content, collected most of the literature materials, and performed data analysis. W. X. provided technical expertise in relevant fields and assisted in revising the review. All authors participated in the review discussions, and examined and revised the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52272222), Taishan Scholar Young Talent Program (tsqn201909114 and tsqn201909123), and University Youth Innovation Team of Shandong Province (202201010318).

References

1. T. Banerjee, F. Podjaski, J. Kröger, B. P. Biswal and B. V. Lotsch, Polymer photocatalysts for solar-to-chemical energy conversion, Nat. Rev. Mater., 2021, 6, 168–190.
2. O. Bubnova, A new metallene arrival, Nat. Nanotechnol., 2018, 13, 272–272.
3. S. Chu and A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature, 2012, 488, 294–303.
4. Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov and T. F. Jaramillo, Combining theory and experiment in electrocatalysis: Insights into materials design, Science, 2017, 355, 6321.
5. Q. Dang, H. Lin, Z. Fan, L. Ma, Q. Shao, Y. Ji, F. Zheng, S. Geng, S.-Z. Yang, N. Kong, W. Zhu, Y. Li, F. Liao, X. Huang and M. Shao, Iridium metallene oxide for acidic oxygen evolution catalysis, Nat. Commun., 2021, 12, 6007.
6. E. Goh, R. J. Gallo, E. Strong, Y. Weng, H. Kerman, J. A. Freed, J. A. Cool, Z. Kanjee, K. P. Lane, A. S. Parsons, N. Ahuja, E. Horvitz, D. Yang, A. Milstein, A. P. J. Olson, J. Hom, J. H. Chen and A. Rodman, GPT-4 assistance for improvement of physician performance on patient care tasks: a randomized controlled trial, Nat. Med., 2025, 31, 1233–1238.
7. E. P. George, D. Raabe and R. O. Ritchie, High-entropy alloys, Nat. Rev. Mater., 2019, 4, 515–534.
8. L. He, M. Li, L. Qiu, S. Geng, Y. Liu, F. Tian, M. Luo, H. Liu, Y. Yu, W. Yang and S. Guo, Single-atom Mo-tailored high-entropy-alloy ultrathin nanosheets with intrinsic tensile strain enhance electrocatalysis, Nat. Commun., 2024, 15, 2290.
9. J. Lai, F. Lin, Y. Tang, P. Zhou, Y. Chao, Y. Zhang and S. Guo, Efficient bifunctional polyalcohol oxidation and oxygen reduction electrocatalysts enabled by ultrathin PtPdM (M = Ni, Fe, Co) nanosheets, Adv. Energy Mater., 2019, 9, 1800684.
10. C. Lu, R. Li, Z. Miao, F. Wang and Z. Zha, Emerging metallenes: synthesis strategies, biological effects and biomedical applications, Chem. Soc. Rev., 2023, 52, 2833–2865.
11. J. Zhang and L. Dai, Nitrogen, phosphorus, and fluorine tri-doped graphene as a multifunctional catalyst for self-powered electrochemical water splitting, Angew. Chem., Int. Ed., 2016, 55, 13296–13300.
12. Q. L. Hong, B. Sun, X. Ai, X. L. Tian, F. M. Li and Y. Chen, Au nanocrystals modified holey PtTeAu metallene heteronanostructures for plasmon-enhanced nitrate electroreduction, Adv. Funct. Mater., 2023, 34, 2310730.
13. W. C. M. Lewis, A text-book of thermochemistry and thermodynamics, Nature, 1917, 99, 262–263.
14. Y. Li, C.-K. Peng, Y. Sun, L. D. N. Sui, Y.-C. Chang, S.-Y. Chen, Y. Zhou, Y.-G. Lin and J.-M. Lee, Operando elucidation of hydrogen production mechanisms on sub-nanometric high-entropy metallenes, Nat. Commun., 2024, 15, 10222.
15. P. Liu, X. Zhang, J. Fei, Y. Shi, J. Zhu, D. Zhang, L. Zhao, L. Wang and J. Lai, Frank partial dislocations in coplanar Ir/C ultrathin nanosheets boost hydrogen evolution reaction, Adv. Mater., 2023, 36, 2310591.
16. F. Lv, B. Huang, J. Feng, W. Zhang, K. Wang, N. Li, J. Zhou, P. Zhou, W. Yang, Y. Du, D. Su and S. Guo, A highly efficient atomically thin curved PdIr bimetallene electrocatalyst, Natl. Sci. Rev., 2021, 8, 9.
17. F. Lin, H. Luo, L. Li, F. Lv, Y. Chen, Q. Zhang, X. Han, D. Wang, M. Li, Y. Luo, K. Wang, L. Gu, Q. Wang, X. Zhao, M. Luo and S. Guo, Synthesis of isolated Ru–O₃ sites on hexagonal close-packed intermetallic penta-metallene for hydrogen oxidation electrocatalysis, Nat. Synth., 2024, 4, 399–409.
18. X. Sun, P. Zhang, T. Liu, B. Tian, P. Xu, Y. Jiang, J. Zhang, Y. Tang, Z. Hu, W. Zhang, Z. Zhang, X. Zhao and W. Guo, Surface halide manipulation for stable inorganic perovskite solar cells and modules, Angew. Chem., Int. Ed., 2025, e202501164.
19. T. Pan, W. Zhou, Q. Wei, Z. Peng, H. Wang, X. Jiang, Z. Zang, H. Li, D. Yu, Q. Zhou, M. Pan, W. Zhou and Z. Ning, Surface-energy-regulated growth of α-phase Cs_0.03FA_0.97PbI₃ for highly efficient and stable inverted perovskite solar cells, Adv. Mater., 2023, 35, 2208522.
20. X. Lv, X. Li, C. Yang, X. Ding, Y. Zhang, Y.-Z. Zheng, S. Li, X. Sun and X. Tao, Large-Size, Porous, ultrathin NiCoP nanosheets for efficient electro/photocatalytic water splitting, Adv. Funct. Mater., 2020, 30, 1910830.
21. H. Li, Z. Yuan, W. Li, M. Chen, R. Snyders, W. Li and C. Bittencourt, Novel synthesis of porous β-In₂S₃ ultrathin nanosheets with large lateral size, Mater. Lett., 2019, 252, 15–18.
22. I. Marchal, Predicting the structure of large protein complexes, Nat. Biotechnol., 2024, 42, 379–379.
23. Y. Shi, J. Fei, H. Li, C. Li, T. Zhan, J. Lai and L. Wang, Amorphous Ru-based metallene with monometallic atomic interfaces for electrocatalytic hydrogen evolution in anion exchange membrane electrolyzer, Appl. Catal., B, 2024, 359, 124465.
24. G. Xia, J. Su, M. Li, P. Jiang, Y. Yang and Q. Chen, A MOF-derived self-template strategy toward cobalt phosphide electrodes with ultralong cycle life and high capacity, J. Mater. Chem. A, 2017, 5, 10321–10327.
25. W. Zhang, Y. Zhang, M. Zhao, S. Wang, X. Fan, N. Zhou and S. Fan, Preparation of mesoporous biogas residue biochar via a self-template strategy for efficient removal of ciprofloxacin: effect of pyrolysis temperature, J. Environ. Manage., 2024, 360, 121140.
26. J. Wang and H. M. Chen, Metallene pre-catalyst reconstruction for boosting catalytic performance, Chem Catal., 2022, 2, 3277–3279.
27. W. Zhong, Q. L. Hong, X. Ai, C. Zhang, F. M. Li, X. F. Li and Y. Chen, RhNi bimetallenes with lattice-compressed Rh skin towards ultrastable acidic nitrate electroreduction, Adv. Mater., 2024, 36, 2314351.
28. P. Santra, S. Ghaderzadeh, M. Ghorbani-Asl, H.-P. Komsa, E. Besley and A. V. Krasheninnikov, Strain-modulated defect engineering of two-dimensional materials, npj 2D Mater. Appl., 2024, 8, 33.
29. J. Chang and S. Zhu, Deep learning on atomistic physical fields of graphene for strain and defect engineering, Adv. Intell. Syst., 2024, 6, 2300601.
30. K. Yuan, X.-C. Sun, H.-J. Yin, L. Zhou, H.-C. Liu, C.-H. Yan and Y.-W. Zhang, Boosting the water gas shift reaction on Pt/CeO₂-based nanocatalysts by compositional modification: Support doping versus bimetallic alloying, J. Energy Chem., 2022, 67, 241–249.
31. S. Jung, U. Zafar, L. S. K. Achary and C. M. Koo, Ligand chemistry for surface functionalization in MXenes: A review, Mater. Des. Process. Commun., 2023, 5, e12395.
32. F. Liu, Z. Yang, D. Abramovitch, S. Guo, K. A. Mkhoyan, M. Bernardi and B. Jalan, Deep-ultraviolet transparent conducting SrSnO₃ via heterostructure design, arXiv.cond-mat.mtrl-sci, 2024,  DOI:10.48550/arXiv.2405.08915.
33. P.-C. Chen, M. Liu, J. S. Du, B. Meckes, S. Wang, H. Lin, V. P. Dravid, C. Wolverton and C. A. Mirkin, Interface and heterostructure design in polyelemental nanoparticles, Science, 2019, 363, 959–964.
34. Y. Zhang, H. Wang, L. Jiao, N. Wu, W. Xu, Z. Wu, Y. Wu, P. Hu, W. Gu and C. Zhu, Defect engineering of PdMo metallene for sensitive electrochemical detection of dopamine, Chem. Eng. J., 2023, 466, 143075.
35. B. Qin, M. Z. Saeed, Q. Li, M. Zhu, Y. Feng, Z. Zhou, J. Fang, M. Hossain, Z. Zhang, Y. Zhou, Y. Huangfu, R. Song, J. Tang, B. Li, J. Liu, D. Wang, K. He, H. Zhang, R. Wu, B. Zhao, J. Li, L. Liao, Z. Wei, B. Li, X. Duan and X. Duan, General low-temperature growth of two-dimensional nanosheets from layered and nonlayered materials, Nat. Commun., 2023, 14, 304.
36. P. Rajpurkar, E. Chen, O. Banerjee and E. J. Topol, AI in health and medicine, Nat. Med., 2022, 28, 31–38.
37. K. Deng, Z. Lian, W. Wang, J. Yu, H. Yu, Z. Wang, Y. Xu, L. Wang and H. Wang, Lattice strain and charge redistribution of Pt cluster/Ir metallene heterostructure for gthylene glycol to glycolic acid conversion coupled with hydrogen production, Small, 2023, 20, 2305000.
38. Y. Qin, H. Huang, W. Yu, H. Zhang, Z. Li, Z. Wang, J. Lai, L. Wang and S. Feng, Porous PdWM (M = Nb, Mo and Ta) trimetallene for high C1 selectivity in alkaline ethanol oxidation reaction, Adv. Sci., 2021, 9, 5.
39. S. Miao, K. Liang, J. Zhu, B. Yang, D. Zhao and B. Kong, Hetero-atom-doped carbon dots: Doping strategies, properties and applications, Nano Today, 2020, 33, 100879.
40. Y. Gao, Q. Wang, G. Ji, A. Li and J. Niu, Doping strategy, properties and application of heteroatom-doped ordered mesoporous carbon, RSC Adv., 2021, 11, 5361–5383.
41. H. Yu, S. Jiang, W. Zhan, K. Deng, Z. Wang, Y. Xu, H. Wang and L. Wang, Hydrogen-intercalation enhanced the anti-CO poisoning ability of palladium-copper metallene for direct formate fuel cells, Mater. Today Phys., 2023, 38, 101216.
42. K. Zhang, Y. He, R. Guo, W. Wang, Q. Zhan, R. Li, T. He, C. Wu and M. Jin, Interstitial Carbon-doped PdMo bimetallene for high-performance oxygen reduction reaction, ACS Energy Lett., 2022, 7, 3329–3336.
43. J. Liu, H. Liu, Q. Wang, T. Li, T. Yang, W. Zhang, H. Xu, H. Li, X. Qi, Y. Wang and A. Cabot, Phosphorus doped PdMo bimetallene as a superior bifunctional fuel cell electrocatalyst, Chem. Eng. J., 2024, 486, 150258.
44. J. Yang, D. Guo, S. Zhao, Y. Lin, R. Yang, D. Xu, N. Shi, X. Zhang, L. Lu, Y.-Q. Lan, J. Bao and M. Han, Cobalt phosphides nanocrystals encapsulated by P-doped carbon and married with P-doped graphene for overall water splitting, Small, 2019, 15, 1804546.
45. Y. Wu, Z. Zhuang, C. Chen, J. Li, F. Xiao and C. Chen, Atomic-level regulation strategies of single-atom catalysts: Nonmetal heteroatom doping and polymetallic active site construction, Chem Catal., 2023, 3, 100586.
46. H. Cui, T. Liu, Y. Chen, P. Shan, Q. Jiang, X. Bai, Y. Wang, Z. Liang, R. Feng, Q. Kang and H. Yuan, Dynamics of non-metal-regulated FeCo bimetal microenvironment on oxygen reduction reaction activity and intrinsic mechanism, Nano Res., 2023, 16, 2199–2208.
47. Y. Wang, W. Luo, S. Gong, L. Luo, Y. Li, Y. Zhao and Z. Li, Synthesis of high-entropy-alloy nanoparticles by a step-alloying strategy as a superior multifunctional electrocatalyst, Adv. Mater., 2023, 35, 2302499.
48. X. Liu, Y. Duan, Y. Guo, H. Pang, Z. Li, X. Sun and T. Wang, Microstructure design of high-entropy alloys through a multistage mechanical alloying strategy for temperature-stable megahertz electromagnetic absorption, Nano-Micro Lett., 2022, 14, 142.
49. J. Xia, J. Yang, Y. Wang, B. Jia, S. Li, K. Sun, Q. Zhao, D. Mao, H.-F. Li and J. He, Synergistic entropy engineering with vacancies: unraveling the cocktail effect for extraordinary thermoelectric performance in SnTe-based materials, Adv. Funct. Mater., 2024, 34, 2401635.
50. W. Xu, Z. Liu, Y. Yu, Y. Shi, H. Li, J. Chi, G. A. Bagliuk, J. Lai and L. Wang, Oxidative reconstructed Ru-based nanoclusters forming heterostructures with lanthanide oxides for acidic water oxidation, J. Colloid Interface Sci., 2025, 679, 958–965.
51. Z. Miao, C. Lu, C.-Y. Xu, Y. Ma, Z. Cao, L. Liu, D. Gong and Z. Zha, A small library of copper-based metallenes with superior antibacterial activity, Mater. Horiz., 2024, 11, 5564–5577.
52. N. Basavegowda and K.-H. Baek, Multimetallic nanoparticles as alternative antimicrobial agents: challenges and perspectives, Molecules, 2021, 26, 912.
53. H. Wang, W. Wang, H. Yu, Q. Mao, Y. Xu, X. Li, Z. Wang and L. Wang, Interface engineering of polyaniline-functionalized porous Pd metallene for alkaline oxygen reduction reaction, Appl. Catal., B, 2022, 307, 121172.
54. W. Xu, D. Zhang, T. Wang, J. Lai and L. Wang, Non-platinum-based electrocatalysts for high performance acidic hydrogen evolution reactions in proton exchange membrane water electrolysis, Appl. Catal., B, 2025, 361, 124626.
55. Y. Wang, C. Wang, M. Li, Y. Yu and B. Zhang, Nitrate electroreduction: mechanism insight, in situ characterization, performance evaluation, and challenges, Chem. Soc. Rev., 2021, 50, 6720–6733.
56. J. Yang, J. K. Cooper, F. M. Toma, K. A. Walczak, M. Favaro, J. W. Beeman, L. H. Hess, C. Wang, C. Zhu, S. Gul, J. Yano, C. Kisielowski, A. Schwartzberg and I. D. Sharp, A multifunctional biphasic water splitting catalyst tailored for integration with high-performance semiconductor photoanodes, Nat. Mater., 2017, 16, 335–341.
57. M. Luo, Z. Zhao, Y. Zhang, Y. Sun, Y. Xing, F. Lv, Y. Yang, X. Zhang, S. Hwang, Y. Qin, J.-Y. Ma, F. Lin, D. Su, G. Lu and S. Guo, PdMo bimetallene for oxygen reduction catalysis, Nature, 2019, 574, 81–85.
58. Z.-H. Yuan, T.-J. Wang, B. Sun, Q.-L. Hong, X. Ai, S.-N. Li, J. Bai and Y. Chen, Recent advances in platinum group metallenes: From synthesis to energy-related electrocatalytic application, Appl. Catal., B, 2025, 366, 125041.
59. E. H. Majlan, D. Rohendi, W. R. W. Daud, T. Husaini and M. A. Haque, Electrode for proton exchange membrane fuel cells: A review, Renewable Sustainable Energy Rev., 2018, 89, 117–134.
60. Z. Wang, M. Chen, G. Lu, J. Xu, L. Zhuo, Y. Wu and X. Liu, Single-atom catalysts toward electrocatalytic urea synthesis via C–N coupling reactions, Chem. Commun., 2025, 61, 13601–13615.
61. S. Tan, Y. Jiang, X. Chao, F. Liang, R. Li, T. Jiang, H.-D. Yu and Z. L. Wang, Self-powered tactile sensor for real-time recognition of morse code based on machine learning, Nano Res., 2025, 18, 94907167.
62. M. Chen, J. Ma, Y. Feng, Y. Wu, G. Hu and X. Liu, Advanced characterization enables a new era of efficient carbon dots electrocatalytic reduction, Coord. Chem. Rev., 2025, 535, 216612.
63. K. Deng, Z. Lian, W. Wang, J. Yu, Q. Mao, H. Yu, Z. Wang, L. Wang and H. Wang, Hydrogen spillover effect tuning the rate-determining step of hydrogen evolution over Pd/Ir hetero-metallene for industry-level current density, Appl. Catal., B, 2024, 352, 124047.
64. L. Wang, L. Zhang, X. Lu, F. Wu, X. Sun, H. Zhao and Q. Li, Surprising cocktail effect in high entropy alloys on catalyzing magnesium hydride for solid-state hydrogen storage, Chem. Eng. J., 2023, 465, 142766.
65. P. Zhou, Q. Zhou, X. Xiao, X. Fan, Y. Zou, L. Sun, J. Jiang, D. Song and L. Chen, Machine learning in solid-state hydrogen storage materials: challenges and perspectives, Adv. Mater., 2025, 37, 2413430.
66. N. Xu, K. Wang, Y. Zhu and Y. Zhang, PdNi biatomic clusters from metallene unlock record-low onset dehydrogenation temperature for bulk-MgH₂, Adv. Mater., 2023, 35, 2303173.
67. B. A. Horri and H. Ozcan, Green hydrogen production by water electrolysis: Current status and challenges, Curr. Opin. Green Sustainable Chem., 2024, 47, 100932.
68. Y. Pan, H. Li, Z. Wang, Y. Han, Z. Wu, X. Zhang, J. Lai, L. Wang and S. Feng, High-efficiency methanol oxidation electrocatalysts realized by ultrathin PtRuM–O (M = Ni, Fe, Co) nanosheets, Chem. Commun., 2020, 56, 9028–9031.
69. H. Yan, B. Liu, X. Zhou, F. Meng, M. Zhao, Y. Pan, J. Li, Y. Wu, H. Zhao, Y. Liu, X. Chen, L. Li, X. Feng, D. Chen, H. Shan, C. Yang and N. Yan, Enhancing polyol/sugar cascade oxidation to formic acid with defect rich MnO₂ catalysts, Nat. Commun., 2023, 14, 4509.
70. R. K. Mallick, S. B. Thombre and N. K. Shrivastava, Vapor feed direct methanol fuel cells (DMFCs): A review, Renewable Sustainable Energy Rev., 2016, 56, 51–74.
71. K. Zakaria, M. McKay, R. Thimmappa, M. Hasan, M. Mamlouk and K. Scott, Direct glycerol fuel cells: comparison with direct methanol and ethanol fuel cells, Chem. – Eur. J., 2019, 6, 2578–2585.
72. E. Doukas, P. Balta, D. Raptis, G. Avgouropoulos and P. Lianos, A realistic approach for photoelectrochemical hydrogen production, Materials, 2018, 11, 1269.
73. B. Sun, W. Zhong, X. Ai, C. Zhang, F.-M. Li and Y. Chen, Engineering low-coordination atoms on RhPt bimetallene for 12-electron ethanol electrooxidation, Energy Environ. Sci., 2024, 17, 2219–2227.
74. J. Xiong, H. Li, Y. Pan, J. Liu, Y. Zhang, J. Xu, B. Li, J. Lai and L. Wang, Multiple strategies of porous tetrametallene for efficient ethanol electrooxidation, J. Mater. Chem. A, 2022, 10, 23015–23022.
75. D. Lu, X. Fu, D. Guo, W. Ma, S. Sun, G. Shao and Z. Zhou, Challenges and opportunities in 2D high-entropy alloy electrocatalysts for sustainable energy conversion, SusMat, 2023, 3, 730–748.
76. H. Li, M. Sun, Y. Pan, J. Xiong, H. Du, Y. Yu, S. Feng, Z. Li, J. Lai, B. Huang and L. Wang, The self-complementary effect through strong orbital coupling in ultrathin high-entropy alloy nanowires boosting pH-universal multifunctional electrocatalysis, Appl. Catal., B, 2022, 312, 121431.
77. A. W. Petrov, D. Ferri, F. Krumeich, M. Nachtegaal, J. A. van Bokhoven and O. Kröcher, Stable complete methane oxidation over palladium based zeolite catalysts, Nat. Commun., 2018, 9, 2545.
78. J. Sánchez-Monreal, P. A. García-Salaberri and M. Vera, A mathematical model for direct ethanol fuel cells based on detailed ethanol electro-oxidation kinetics, Appl. Energy, 2019, 251, 113264.
79. J. Wang, Z. Pei, J. Liu, M. Hu, Y. Feng, P. Wang, H. Wang, N. Nie, Y. Wang, C. Zhi and Y. Huang, A high-performance flexible direct ethanol fuel cell with drop-and-play function, Nano Energy, 2019, 65, 104052.
80. J. Chang, G. Wang, C. Li, Y. He, Y. Zhu, W. Zhang, M. Sajid, A. Kara, M. Gu and Y. Yang, Rational design of septenary high-entropy alloy for direct ethanol fuel cells, Joule, 2023, 7, 587–602.
81. L. An and R. Chen, Direct formate fuel cells: A review, J. Power Sources, 2016, 320, 127–139.
82. Z. Pan, Z. Zhang, W. Li, X. Huo, Y. Liu, O. C. Esan, Q. Wu and L. An, Development of a high-performance ammonium formate fuel cell, ACS Energy Lett., 2023, 8, 3742–3749.
83. J. Zhang, F. Lv, Z. Li, G. Jiang, M. Tan, M. Yuan, Q. Zhang, Y. Cao, H. Zheng, L. Zhang, C. Tang, W. Fu, C. Liu, K. Liu, L. Gu, J. Jiang, G. Zhang and S. Guo, Cr-doped Pd metallene endows a practical formaldehyde sensor new limit and high selectivity, Adv. Mater., 2021, 34, 2105276.
84. F. Zhou and Y. Chai, Near-sensor and in-sensor computing, Nat. Electron., 2020, 3, 664–671.
85. Y. Wei, Y. Sun, Y. Yu, Y. Shi, Z. Wu, L. Wang and J. Lai, 4d Metal-doped liquid Ga for efficient ammonia electrosynthesis at wide N₂ concentrations, Chin. J. Catal., 2024, 67, 194–203.
86. L. Su, D. Gong, Y. Jin, D. Wu and W. Luo, Recent advances in alkaline hydrogen oxidation reaction, J. Energy Chem., 2022, 66, 107–122.
87. M. Hren, M. Božič, D. Fakin, K. S. Kleinschek and S. Gorgieva, Alkaline membrane fuel cells: anion exchange membranes and fuels, Sustainable Energy Fuels, 2021, 5, 604–637.
88. Z. F. Pan, L. An, T. S. Zhao and Z. K. Tang, Advances and challenges in alkaline anion exchange membrane fuel cells, Prog. Energy Combust. Sci., 2018, 66, 141–175.
89. D. Li, X. Xu, J. Jiang, H. Dong, H. Li, X. Peng and P. K. Chu, Coupled electrocatalytic hydrogen production, Mater. Sci. Eng., R, 2024, 160, 100829.
90. J. Zhou, Y. Xiong, M. Sun, Z. Xu, Y. Wang, P. Lu, F. Liu, F. Hao, T. Feng, Y. Ma, J. Yin, C. Ye, B. Chen, S. Xi, Y. Zhu, B. Huang and Z. Fan, Constructing molecule-metal relay catalysis over heterophase metallene for high-performance rechargeable zinc-nitrate/ethanol batteries, Proc. Natl. Acad. Sci. U. S. A., 2023, 120, e2311149120.
91. W. Cai, Y. Sun, Y. Zhang, H. Li, Y. Yu, J. Lai and L. Wang, Hydrated molten salt electrolytes effectively promote electrocatalytic NH₃ synthesis at low temperatures, Fuel, 2025, 379, 133081.
92. Y. Sheng, J. Xie, R. Yang, H. Yu, K. Deng, J. Wang, H. Wang, L. Wang and Y. Xu, Modulating hydrogen adsorption by unconventional p–d orbital hybridization over porous high-entropy alloy metallene for efficient electrosynthesis of nylon–6 precursor, Angew. Chem., Int. Ed., 2024, 63, e202410442.
93. Y. Yu, Z. Lv, Z. Liu, Y. Sun, Y. Wei, X. Ji, Y. Li, H. Li, L. Wang and J. Lai, Activation of Ga liquid catalyst with continuously exposed active sites for electrocatalytic C–N coupling, Angew. Chem., Int. Ed., 2024, 63, e202402236.
94. Z. Lv, L. Zhao, S. Zhou, M. Wang, W. Xu, J. Lai and L. Wang, CuCo DAC used to change the hydrogenation sequence for efficient electrochemical C-N coupling, Appl. Catal., B, 2024, 351, 124003.
95. A. Muzaffar, M. B. Ahamed and C. M. Hussain, Green supercapacitors: Latest developments and perspectives in the pursuit of sustainability, Renewable Sustainable Energy Rev., 2024, 195, 114324.
96. Y. Xu, Y. Zhou, Y. Li and Y. Zheng, Bridging Materials and Analytics: A comprehensive review of characterization approaches in metal-based solid-state hydrogen storage, Molecules, 2024, 29, 5014.
97. C. Ge, B. Wang, H. Yang, Q. Feng, S. Huang, X. Zu, L. Li and H. Deng, Direct Z-scheme GaSe/ZrS₂ heterojunction for overall water splitting, Int. J. Hydrogen Energy, 2023, 48, 13460–13469.
98. W. Zhang, K. Wang, F. Lin, Q. Zhang, Y. Sun, H. Luo, W. Zhang, J. Zhou, F. Lv, D. Wang, L. Gu, M. Luo and S. Guo, Assembled RhRuFe trimetallene for water electrolysis, Small Methods, 2024, 8, 2400336.
99. D. Zhang, Y. Shi, X. Chen, J. Lai, B. Huang and L. Wang, High-entropy alloy metallene for highly efficient overall water splitting in acidic media, Chin. J. Catal., 2023, 45, 174–183.
100. Y. Zhao, X. Chen, S. Tu, X. Zhang, S. Zhang, H. Zhang, X. Zhang and L. Chen, Electro-optical performance of solid-state electrochromic device based on self-supporting electrolyte, Opt. Mater., 2024, 149, 114991.
101. H. Yang, M. Driess and P. W. Menezes, Self-supported electrocatalysts for practical water electrolysis, Adv. Energy Mater., 2021, 11, 2102074.
102. J. H. Oh, G. H. Han, J. Kim, J. E. Lee, H. Kim, S. K. Kang, H. Kim, S. Wooh, P. S. Lee, H. W. Jang, S. Y. Kim and S. H. Ahn, Self-supported electrodes to enhance mass transfer for high-performance anion exchange membrane water electrolyzer, Chem. Eng. J., 2023, 460, 141727.
103. T.-J. Wang, L.-B. Sun, X. Ai, P. Chen, Y. Chen and X. Wang, Boosting formate electrooxidation by heterostructured PtPd alloy and oxides nanowires, Adv. Mater., 2024, 36, 2403664.
104. T. Wang, D. Zhang, J. Fei, W. Yu, J. Zhu, Y. Zhang, Y. Shi, M. Tian, J. Lai and L. Wang, Sub-nanometric Pt clusters supported Co aerogel electrocatalyst with hierarchical micro/nano-porous structure for hydrogen evolution reaction, Appl. Catal., B, 2024, 343, 123546.
105. T. Wang, Y. Shi, J. Fei, J. Zhu, L. Song, C. Li, T. Zhan, J. Lai and L. Wang, Three-dimensional interconnected nanofibers consisting of ultra-small Ni-doped Co₃O₄ nanoparticles for acidic overall water splitting at high current density, Appl. Catal., B, 2024, 358, 124367.
106. L. Song, D. Zhang, H. Miao, Y. Shi, M. Wang, L. Zhao, T. Zhan, J. Lai and L. Wang, Interstitial atom-doped NiFe alloy as pre-catalysts boost direct seawater oxygen evolution, Appl. Catal., B, 2024, 342, 123376.
107. M. T. Zumstein, D. Rechsteiner, N. Roduner, V. Perz, D. Ribitsch, G. M. Guebitz, H.-P. E. Kohler, K. McNeill and M. Sander, Enzymatic hydrolysis of polyester thin films at the nanoscale: effects of polyester structure and enzyme active-site accessibility, Environ. Sci. Technol., 2017, 51, 7476–7485.
108. J. Zhou, W. Li, P. Ciais, T. Gasser, J. Wang, Z. Li, L. Zhu, M. Han, J. He, M. Sun, L. Liu and X. Huang, Contributions of countries without a carbon neutrality target to limit global warming, Nat. Commun., 2025, 16, 468.
109. S. R. Ahmed, R. Baghdadi, M. Bernadskiy, N. Bowman, R. Braid, J. Carr, C. Chen, P. Ciccarella, M. Cole, J. Cooke, K. Desai, C. Dorta, J. Elmhurst, B. Gardiner, E. Greenwald, S. Gupta, P. Husbands, B. Jones, A. Kopa, H. J. Lee, A. Madhavan, A. Mendrela, N. Moore, L. Nair, A. Om, S. Patel, R. Patro, R. Pellowski, E. Radhakrishnani, S. Sane, N. Sarkis, J. Stadolnik, M. Tymchenko, G. Wang, K. Winikka, A. Wleklinski, J. Zelman, R. Ho, R. Jain, A. Basumallik, D. Bunandar and N. C. Harris, Universal photonic artificial intelligence acceleration, Nature, 2025, 640, 368–374.
110. W. Ajeeb, R. C. Neto and P. Baptista, Life cycle assessment of green hydrogen production through electrolysis: A literature review, Sustainable Energy Technol. Assess., 2024, 69, 103923.

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